Trends and developments in flexible solar cells: insights from a bibliometric analysis

Abstract

This review comprehensively analyzes the development, efficiency, and applications of flexible solar cells (F-SCs) over the past four decades. F-SCs, including flexible-dye-sensitized solar cells (Flexible-DSSCs), flexible-organic solar cells (Flexible-OSCs), flexible-perovskite solar cells (Flexible-PeSCs), and flexible-thin-film SCs (CIGS, CdTe, and a-Si), offer significant advantages over traditional rigid solar cells, such as mechanical flexibility, lightweight, and potential for large-area and low-cost production. This review focuses on advancements in materials and fabrication techniques of F-SCs, highlighting their impact on performance and market expansion. Notably, flexible-PeSCs have achieved high power-conversion efficiencies (PCEs), making them a focal point of current research. The integration of F-SCs into diverse applications, such as portable electronics, wearables, and building-integrated photovoltaics (BIPVs), underscores their versatility and potential to meet the growing demand for renewable energy solutions. This review explores the influence of different substrates, electrodes, and material structures on the PCE and stability of F-SCs and highlights the progress in manufacturing techniques, such as inkjet printing and screen printing. The findings emphasize interdisciplinary collaboration in advancing F-SC technologies for overcoming technical and economic barriers, concluding that continuous research and global cooperation are vital for maximizing their potential and implementation, paving the way for a sustainable energy future.

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1. Introduction

Since the 19th century, significant scientific advances have progressively revealed the potential of the sun as a source of abundant and free energy. French physicist Edmond Becquerel, specializing in optics and light, is credited with uncovering the principles of solar energy. In 1839, he discovered the photovoltaic (PV) effect, demonstrating how sunlight could be converted into electricity.¹ His experiment involved immersing platinum or gold plates in an acidic silver chloride solution within a wet electrolytic cell. When exposed to sunlight, this setup generated a weak electrical current between the electrodes.

In the late 1800s, William G. Adams and Richard E. Day published a study on the effects of sunlight on selenium,² and Charles Edgar Fritts developed a prototype solar cell similar to modern cells, consisting of a layer of selenium between glass and a thin gold wire.^3,4 Despite these innovations, progress was slow until the mid-20th century owing to a lack of interest and knowledge and dominance of fossil fuels. However, the turn of the century brought notable theoretical advances. In 1900, German theoretical physicist Max Planck introduced the quantum hypothesis, and in 1905, Albert Einstein explained the photoelectric effect, laying the foundation for understanding the interaction between light and matter.⁵ Einstein's work was built from previous experiments, including Heinrich Hertz's (1888) study of radio waves, which proved the existence of electromagnetic waves. Hertz observed that exposing metallic electrodes to ultraviolet light under a potential difference generated sparks, which was a phenomenon supporting Einstein's theory of light quanta and photoelectric effect. These theoretical advances expanded our understanding of photovoltaic technology. However, significant improvements in solar cell efficiency occurred only in the 20th century, marking the beginning of their viability as a renewable energy source.

The oil crisis of the 1970s stimulated interests and investments in alternative energy sources including solar energy. This led to significant improvements in solar cell efficiency and reductions in production costs. The efficiency of photovoltaic cells is measured by the amount of electricity generated relative to the total potential energy received. Although the initial prototype was inefficient, it laid the foundations for ongoing development that would lead to modern solar cells. Initially, progress was hindered by several factors, but recognition of the value of solar energy has grown over time, evidenced by continued research and advancements in solar technologies' applicability and efficiency.⁶

Flexible solar cells (F-SCs) emerged as a groundbreaking innovation in the photovoltaic technology field, offering several advantages over traditional rigid solar cells.⁷ This review explores the different types of F-SCs including flexible-DSSCs, flexible-OSCs, flexible-PeSCs, and flexible-thin-film SCs. The development of these technologies has expanded the domain and market of F-SCs, driven by the need for lightweight, portable, and versatile solar energy solutions.⁸

In recent years, high-flexibility photovoltaic technology has attracted attention owing to its light weight, flexibility and low cost of manufacturing.⁹ These include thin-film solar cells, perovskite solar cells, organic solar cells (OSCs) and dye-sensitized solar cells (DSSCs), each with their unique features and hurdles. Made from materials such as CdTe, CIGS and amorphous silicon, thin-film cells have the advantage of lower raw material consumption and the ability to perform well in low light.^10,11 However, its stability and the use of toxic materials remain hurdles. Perovskite solar cells have become widely known for their high efficiency and simplified production, but have proven to be thermally unstable and sensitive to moisture degradation. OSC polymer semiconductors make them lightweight, printable, and potential for use in windows and wearables. However, they have a lower efficiency and a relatively short working life.^12,13

Flexible-DSSCs utilize a dye-sensitized anode and a liquid electrolyte, offering the advantage of low-cost materials and simple fabrication processes.^14–17 Flexible-OSCs are based on organic-molecules or polymers, providing high mechanical flexibility and the potential for low-cost, large-area production.^17–19 Flexible-PeSCs have gained significant importance due to their high-PCE and the potential for low-temperature processing.^20–23 Flexible-OSCs combine the advantages of polymer materials with flexible substrates, resulting in lightweight and highly flexible devices.^24–26 Flexible-thin-film SCs, such as those based on CIGS, CdTe, and a-Si, offer the benefit of reduced material usage and the ability to be deposited on bendable, rollable, flexible substrates.^11,27–32 The expansion of the F-SC market is driven by advancements in materials science, manufacturing techniques, and the increasing demand for renewable energy sources.³³ The integration of F-SCs into various applications, from portable electronics to building-integrated photovoltaics (BIPVs), underscores their potential to revolutionize the solar energy industry.

Fig. 1A and B compare the number of articles published in the top 10 journals between 1979 and 2023. Fig. 1A illustrates the annual publication trends in several key journals in the field of flexible solar cells.

Fig. 1 (A) Annual articles published by various journals (1979–2023). (B) Comparison of the number of articles on the top 10 journals from Scopus.

ACS-Applied Energy Materials showed a significant increase starting in 2018, peaking in 2022. RSC-Advances experienced initial growth around 2012, followed by fluctuations. Solar-RRL has showed consistent growth since 2016, with a notable increase in 2022. Advanced Energy Materials has exhibited stable and consistent growth since 2010, while Advanced Materials has shown a growth trend since 2005, with a sharp increase in recent years. Advanced Functional Materials has seen steady growth since 2010, and Solar Energy has fluctuated over time but displayed a clear increase starting in 2015. Nano Energy has seen significant growth since 2013, with spikes in 2017 and 2020. The Journal of Materials Chemistry A (JMC-A) has experienced sharp growth since 2013, and Solar Energy Materials and Solar Cells (Sol. Energy Mater. Sol. Cells) has shown steady growth since 2010, with a sharp increase in recent years. ACS-Applied Materials and Interfaces has seen a continuous and significant increase since 2010.

ACS-Applied Materials & Interfaces ranks first with a total of 199 articles. When comparing the total number of articles with Sol. Energy Mater. Sol. Cells. (180 articles) over the period 1979–2023, Sol. Energy Mater. Sol. Cells. published 9.55% fewer articles, JMC-A published 16.1% fewer articles (167 articles), and Nano Energy and Solar Energy each published 49.3% fewer articles (101 articles) than the leading source, ACS-Applied Materials & Interfaces.

Fig. 1B identifies the key publishers in this field: Elsevier, the American Chemical Society, Wiley-Blackwell, and the Royal Society of Chemistry (RSC). Elsevier, with 382 articles, has the highest publication, reflecting substantial influence. The American Chemical Society, known for its prestigious journals such as ACS Applied Materials and Interfaces, and ACS Applied Energy Materials, leads in the number of individual publications, totaling 199 articles. Wiley-Blackwell maintains a strong presence with high-impact journals like Advanced Functional Materials, Advanced Materials, Advanced Energy Materials, and Solar RRL. RSC also holds significant influence, particularly through the JMC-A. These publishers are fundamental to disseminating innovative research and advancing knowledge in the field of materials for high-efficiency flexible-solar cells (Flex-SCs), playing a fundamental role in the development of sustainable technologies.³⁴

2. Methodological framework

The significant increase in publications related to Flex-SCs, as shown in Table 1, has prompted several authors to undertake comprehensive bibliometric analyses.^35–37 These analyses aimed to identify key themes, emerging trends, and the evolutionary trajectory of Flex-SC research. They explored various aspects including publication characteristics, topic areas, author collaborations, co-citations, influential journals, institutional affiliations, keyword co-occurrence, and research focus.

Table 1 Summary of the key aspects of previous review articles (1979–2023) on Flex-SCs

Author	Title/theme	Time span	Database and records	Publication search criteria
This review	Trajectories of research on the efficiency of flexible-solar cells (Flex-SCs): bibliometric insights from 1979–2023	1979–2023	Scopus: 3812 documents	Within: TITLE-ABS-KEY fields of a publication
				Language: english
				Search string: (“solar cells”) AND (“flexible”) AND (“efficiency”)
				Refined by: articles only
David, Thamyres Machado et al., 2020 (ref. 35)	Future research tendencies for solar energy management using a bibliometric analysis 2000–2019	2000–2019	Scopus: 3260 documents	Within: TITLE-ABS-KEY fields of a publication
				Search string: (“solar”) AND (“energy”) AND (“management”) AND (“photovoltaic”)
Du, Huibin, et al., 2014 (ref. 36)	A bibliographic analysis of recent solar energy literature: the expansion and evolution of a research field	1992–2011	WoS: 6670 documents	Within: TITLE-ABS-KEY fields of a publication
				Language: english
				Search string: (“solar energy”) OR (“solar energies”)
				Refined by: article only
de Paulo, A. F., 2017 (ref. 37)	Solar energy technologies and open innovation: a study based on bibliometric and social network analysis	2000–2014	WoS: 739 documents	Within: TITLE-ABS-KEY fields of a publication
				Language: english
				Search string: (“Open innovation”), (“user innovation”), (“cumulative innovation”), (“know-how trading”), (“mass innovation”), (“distributed innovation”), (“innovative cooperation”), (“collaborative innovation”)
				Refined by: article only

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Alan Pritchard³⁸ and Stephen M. Lawani³⁹ defined bibliometrics as the application of statistical and mathematical methods to the study of books and other scientific communications. Antonio-Rafael Ramos-Rodríguez et al.⁴⁰ emphasize the importance of a broad study scope for effective bibliometric analysis, noting that the approach is particularly suited to handling large datasets. Conversely, Oğuzhan Öztürk et al.⁴¹ highlighted that bibliometric analysis serves as a valuable tool for understanding and mapping scientific knowledge and evolutionary patterns within established fields, providing a solid foundation for advancing research in innovative and impactful ways.

Bibliometric methods are widely applied across various domains including engineering and industry,^42–44 biomass energy and the environment,^45,46 and renewable energy.^47–49 Over the past four years, 22 873 documents (60.5% articles, 24.3% reviews, 9.9% conference articles, 2.6% book chapters and 2.7% other types) include the term ‘bibliometric analysis' in their TITLE-ABS-KEY. This trend was identified through a search of the Scopus database using the keyword “bibliometric analysis” from 2019 to 2023.

In the field of Flex-SC research, bibliometric analysis has gained prominence, with several studies employing these methods to explore the literature and produce significant findings. Notable examples include the works by David and Thamyres Machado et al.,³⁵ Du and Huibin, et al.,³⁶ and de Paulo.³⁷ These studies utilized various techniques such as social network analysis and co-citation analysis, to uncover patterns in knowledge dissemination, identify critical research areas, and assess the evolution and impact of publications. A summary of the main aspects of these Flex-SC bibliometric reviews is presented in Table 1.

The goal of the bibliometric review is to conduct a thorough analysis of research on Flex-SCs over a span of 44 years, with a particular focus on developments from 2019 to 2023. By employing quantitative bibliometric methodologies, the review aims to address the research questions. This article intends to provide an updated and comprehensive overview of the Flex-SC field, covering its intellectual landscape, research trajectory, and key areas of interest.

Over the past four years, the structure of the article is as follows. Section 1 discusses the limitations of previous bibliometric studies and introduces the background of the Flex-SCs field. Section 2 describes the methodology employed in the study. Section 3 presents and discusses the results of the data analysis, including publication characteristics, research progress, performance, and emerging hotspots and trends. Section 4 highlights the main findings of the study. Finally, Section 5 provides a summary and final considerations, addressing the study's limitations and suggesting future directions for research in Flex-SCs.

2.1. Bibliometric analysis of Flex-SCs

Bibliometrics is a multifaceted approach that encompasses scientometrics, known for its dynamic, evaluative, and predictive nature.⁵⁰ This method performs a comprehensive quantitative analysis of science, effectively evaluating contributions across various aspects of a specific topic.^51–53 It enables detailed analysis at the level of individual scientists and institutions, as well as broader national and global perspectives. Additionally, bibliometrics helps identify research trends and topics of current, past, or future interest.⁵⁴ The bibliometric analysis in this review follows the methods described by O. Öztürk et al.⁴¹ utilizing the bibliometric studies of T. M. David et al.³⁵ and A. S. Shikoh⁵⁵ as a foundation for this investigation on Flex-SCs.

2.2. Data collection and analysis

In this review, a specific search string was used, incorporating terms such as (“solar cells”) AND (“flexible”) AND (“efficiency”), and the data were collected on March 21, 2023 through the Scopus (SCP) database. The search was conducted within the limits of titles, keywords and abstracts (TITLE-ABS-KEY), yielding 5420 documents. By restricting the language to English and considering only articles, the number of scientific articles was narrowed down to 3812, as shown in Table 2.

Table 2 Key information about Flex-SCs from 1979 to 2023

Description	Results
Main information about the data
Timespan	1979–2023
Sources (journals)	160
Total documents	5420
Annual growth rate (%)	26.30
Average citations per doc	53.25
References	124 470
Document contents
Keywords plus (ID)	14 148
Author's keywords (DE)	6783
Document types (English)
Article	3812
Conference papers	897
Reviews	320
Book chapter	117
Books	8
Others	266

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The study covers 44 years, from 1979 to 2023. The year 1979 was chosen as the starting point because it marks the identification of the first articles on Flex-SCs in this period by SCP. The “VOSviewer” software was used to create and visualize citation and collaboration density networks.⁵⁶ Scopus, a bibliographic reference database launched in 2004 by Elsevier, provided the data, which were compiled into CSV matrices for various analyses such as citation, co-citation, bibliographic coupling, overlap maps, keyword co-occurrence, co-authorship and cluster-highlights, as shown in Fig. 2.⁵⁷ From these data, detailed reports and graphs were generated, covering publications on Flex-SCs, thematic areas, collaborations between authors, co-citations, relevant journals, institutional affiliations, co-occurrence of keywords, and research focus.

Fig. 2 Toolbox used in this bibliometric study.

3. Results and discussion

3.1. Aspects of articles in F-SCs

Table 3 shows the linguistic composition of Flex-SCs publications (1979–2023). English dominates, comprising 56.5% of publications (2155) from 1979 to 2018, followed by Chinese and German (0.03% each). From 2019 to 2023, English publications dropped to 43.4% (1655), a 13.1% decline. Over the full period, English accounted for 99.87% (3810) of records, with other languages (Chinese, Ukrainian, and German) making up only 0.13%. This highlights English as the primary research language in Flex-SCs, even in non-English-speaking countries like China, Ukraine, and Germany.^58,59

Table 3 Distribution of languages in Flex-SC publications (1979–2023)^a

Language	1979–2018		2019–2023		1979–2023
	TP	%	TP	%	TP (%)
a TP: total articles published.
English	2155	56.5	1655	43.4	3810 (99.87)
Chinese	1	0.03	1	0.03	2 (0.05)
Ukrainian	0	0	2	0.05	2 (0.05)
German	1	0.03	0	0	1 (0.03)

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3.2. Expansion of scientific production in F-SCs

Between 1979 and 2023, the Scopus database recorded 5420 articles on Flex-SC research, spanning five types of documents. The most prevalent were original articles, numbering 3812, accounting for 70.3% of the total. This was followed by conference papers with 897 (16.6%), reviews with 320 (5.9%), book chapters with 117 (2.2%), and books with 11 (0.2%). Other less frequent document types totaled 263 (4.9%). Due to the predominance of original articles, these were used for subsequent analyses.

English was the dominant language, with 90.3% of works published. The data reveal an increasing trend in publications from 2009 to 2023, as shown in Fig. 3A. According to M. R. Sabour et al. 2020, there was a significant rise in Flex-SC publications indexed in Scopus, with notable growth after 2009.⁶⁰ Between 1992 and 2011, 6670 articles were published.³⁶ This number increased to 739 between 2000 and 2014³⁷ and reached 3265 from 2000 to 2019.³⁵

Fig. 3 (A) Yearly citation-articles in Flex-SCs from 1979 to 2023; and (B) articles published annually in Flex-SCs (2007–2023).

The average annual growth rate of publications since 1979 is 87 articles. A peak of 60 articles was observed in 2009, followed by steady growth until 2013 and an exponential increase after 2014. In recent years (2019–2023), the average production was 331.4 articles per year, with a 3.0% decrease from 2023 compared to 2019 for articles and 21.7% for citations. This trend reflects growing interest and the importance of topics within the Flex-SCs field.

Global crises such as environmental changes, financial instability, and political turbulence may have influenced these trends. While the number of published articles grew moderately, citations increased more significantly, indicating substantial interest and discussion in the scientific community. Based on a model with a coefficient of determination (R² = 0.970), the number of publications in this domain is expected to grow by at least 31.78% by 2030, as shown in Fig. 3B.

Fig. 4 shows the temporal evolution and percentage share of different PV technologies. Fig. 4a shows the growth trend of PeSCs, thin-film SCs, OSCs and DSSCs since 1993. PeSCs stand out with exponential growth from 2013 onwards, surpassing the other technologies. Thin-film and organic cells show a more modest increase, while DSSCs grew until 2014 but then declined. This trend suggests that PeSCs are gaining ground due to their high-efficiency and low-manufacturing cost. Fig. 4B reinforces this scenario, showing the percentage share of each technology as PeSCs (37.6%), DSSCs (22.8%), OSCs (21.9%) and thin-film SCs (17.7%). This panorama confirms the dominance of PeSCs in current research and development, while the other technologies remain relevant, but with less expressive growth. Thus, the data indicate that PeSCs emerge as the most promising technology for the future of photovoltaics.

Fig. 4 (A) Evolution of PV technology over time and (B) % share of each solar cells.

3.3. The most referenced and prolific authors

From 1979 to 2018, researchers Frederik C. Christian Krebs of Elitescience in Lyngby, Denmark, Kuo-Chuan Ho of National Taiwan University in Taipei, Taiwan, and Ayodhya Nath Tiwari of Empa-Swiss Federal Laboratories for Materials Science and Technology in Dübendorf, Switzerland, stood out as the most influential authors. Fig. 5 shows the co-citation patterns among the highly cited authors from 1979 to 2023. Among the main contributors to the total number of citations are Xiaotian Hu,^61–64 Yiwang Chen,^65–67 Yongfang Li,^68–70 Kuo-Chuan Ho,^71–73 Frederik C. Christian Krebs,^74–76 Ayodhya Nath Tiwari,^77–80 and Han-ki Kim.^81–84 Their influential work and significant contributions to the field are highlighted in their numerous highly cited publications.

Fig. 5 Analyzing highly cited authors in Flex-SC publications from 1979 to 2023 through co-citation density; each author is identified by a match corresponding to the number of cited articles mentioned to them (the colors range from yellow, representing higher density, to blue, representing lower density, offering insights into the density of restrictions related to each author).

Table 5 highlights the ten most influential authors in the area of Flex-SCs between 2019 and 2023, demonstrating the predominance of Chinese researchers, who represent 90% of the list. The remaining 10% is represented by a Japanese researcher. Ge, Z. from Ningbo Institute of Industrial Technology-CAS contributed 28 articles and 6421 citations, maintaining an h-index of 46. The h-indices of the notable contributors, shown in Table 5, range from 24 to 140, highlighting their significant impact and recognition in the field, with particular emphasis on the exceptional indices of Y. Li and Y. Song.

Table 4 Leading countries in publications on Flex-SCs from 2019 to 2023^a

Countries	TP	%	TC	TC/TP
a TP: total articles published; TC: total citations.
China	882	23.14	21 810	24.73
South Korea	224	5.88	4716	21.05
United States	177	4.64	6051	34.19
India	85	2.23	878	10.33
Hong Kong	71	1.86	2665	37.54
United Kingdom	69	1.81	1440	20.87
Japan	69	1.81	1244	18.03
Germany	69	1.81	2007	29.09
Italy	50	1.31	1127	22.54
Taiwan	48	1.26	654	13.63

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Table 5 Most productive authors in Flex-SCs from 2019 to 2023: Top 10^a

Author name	TP	TC	Institution	Country	H-index	Trends in publications and citations: annual author analysis and cumulative impact
a TP: total articles published; TC: total citations.
Hu, X.	34	5.30	Nanchang University	Nanchang, China	46
Chen, Y.	29	14.95	Jiangxi Normal University	Nanchang, China	74
Ge, Z.	28	6.42	Ningbo Institute of Industrial Technology-CAS	Ningbo, China	46
Meng, X.	23	2.22	Zhengzhou University	Zhengzhou, China	27
Song, W.	20	2.66	University of Chinese Academy of Sciences	Beijing, China	27
Li, Y.	19	44.26	Soochow University	Suzhou, China	140
Ma, W.	18	25.31	Xi'an Jiaotong University	Xi'an, China	93
Huang, Z.	18	2.32	Jiangxi Normal University	Nanchang, China	24
Song, Y.	16	21.16	Institute of Chemistry-CAS	Beijing, China	107
Minemoto, T.	16	6.03	Ritsumeikan University Biwako-Kusatsu Campus	Kusatsu, Japan	45

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3.4. The most productive nations/territories

Fig. 6A shows the map of international collaboration from 1979 to 2023, highlighting productivity in Flex-SC research. China leads with 1622 publications (42.6%), followed by South Korea with 575 (15.1%) and the United States with 526 (13.8%). In terms of the CT indicator, China is followed by the USA and South Korea. This analysis of the co-authorship map between countries indicates the intensity of collaborations in Flex-SC research. China, marked in red, stands out as a central axis of collaboration, likely due to its extensive research activity and international partnerships. Other countries such as the United States, Germany, and the United Kingdom also show strong participation, indicating well-established and active research community in Flex-SCs.

Fig. 6 (A) Analysis of international collaboration on Flex-SCs between 2019 and 2023. (B) and (C) distribution of publications on Flex-SCs by co-authoring countries from 1979 to 2023.

Fig. 6B shows the temporal distribution of publications by country from 1979 to 2023, illustrating the evolution of the number of publications over time. China exhibits clear dominance, especially from 2019 onwards, reflecting significant investments in research and development.⁵³ Other countries such as the United States, South Korea, and India also show growth, albeit on a smaller scale than China.

Fig. 6C shows the VOSviewer map, which uses nodes to represent countries and lines to indicate collaborations. China, India, and South Korea are highlighted, indicating a high density of internal and external collaborations. The larger size of the nodes for these countries suggests a greater production of publications and, potentially, a leading role in research in Flex-SC research.⁸⁵ The connections between various European and Asian countries also suggest a well-integrated network of scientific collaboration.⁸⁶

Fig. 6A–C collectively reflect the importance of international collaboration in advancing research on Flex-SCs, with China positioning itself as a leader. The evolution in the number of publications and collaboration networks demonstrates the global growth and integration of this field of study, emphasizing the need for transnational collaboration for innovations and scientific advances.

China leads significantly in all aspects, boosting the highest number of publications (882 TP) and the largest percentage of the global total (23.14%). Its citation rate per article is also impressive (21 810 TC), indicating that its works are not only abundant (24.73 TC/TP) but also highly influential.⁸⁷ South Korea shows a solid academic output (224 TP, 5.88%) with an excellent citation rate per article (4716 TC), suggesting that its research is of high quality and well-received by the scientific community (21.05 TC/TP). Although the United States has fewer publications (177 TP) than China and South Korea, it has the highest citation rate per article (34.19 TC/TP) among the listed countries. This reflects the significant influence of its research in the field of Flex-SCs.

Countries such as India (85 TP) and Hong Kong (71 TP) have fewer publications, but Hong Kong's citation rate per article is exceptionally high (37.54 TC/TP), highlighting the quality and impact of its research. The United Kingdom, Japan, Germany, Italy, and Taiwan have similar publication numbers (69 TP each for the UK, Japan, and Germany, 50 TP for Italy, and 48 TP for Taiwan) with varying citation rates (1440 TC, 1244 TC, 2007 TC, 1127 TC, and 654 TC, respectively), indicating different levels of impact within the academic community (20.87 TC/TP, 18.03 TC/TP, 29.09 TC/TP, 22.54 TC/TP, and 13.63 TC/TP, respectively).

Overall, China is the undisputed leader in Flex-SCs during the 2019–2023 period, demonstrating a strong combination of volume and impact.⁸⁸ It is evident that a country's effectiveness in Flex-SCs is measured not only by the quantity of articles published but also by their quality, as evidenced by citation rates.⁸⁹ The data in Table 4 present a diverse panorama of contributions and impacts, with both Asian and Western countries actively participating in the advancement of this scientific field.

3.5. Organizations with high scientific production

Between 1979 and 2023, studies on Flex-SCs involved 160 institutions, with an average of 23.8 articles published per institution. From 1979 to 2018, several global academic institutions stood out, with the Chinese Academy of Sciences (CAS) leading with an average of 101.6 articles per institution, followed by Peking University and other Chinese institutions.⁹⁰ As of 2019, the most productive publications, totaling 764 articles, are mainly concentrated in China, as shown in Table 6. China, led by the Chinese Academy of Sciences with 204 publications (5.35% of the global total), and followed by the Ministry of Education with 166 publications (4.35%), stands out as a main center for research in Flex-SCs, reflecting significant investment and government support in this area.

Table 6 Key institutes that worked on Flex-SCs from 2019 to 2023^a

Institutes	TP	%	Countries
a TP: total articles published.
Chinese Academy of Sciences	204	5.35	China
Ministry of Education of the People's Republic of China	166	4.35	China
University of Chinese Academy of Sciences	86	2.25	China
Soochow University	54	1.42	Taiwan
Institute of Chemistry Chinese Academy of Sciences	50	1.31	China
Nanchang University	49	1.28	China
Beijing National Laboratory for Molecular Sciences	40	1.05	China
Ningbo Institute of Industrial Technology, CAS	40	1.05	China
Xi'an Jiaotong University	38	1.0	China
Huazhong University of Science and Technology	37	0.97	China

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Institutions such as the University of Chinese Academy of Sciences, Soochow University, Institute of Chemistry Chinese-Academy of Sciences, and Nanchang University are fundamental to the advancement of knowledge in Flex-SCs. Notably, Soochow University in Taiwan, with 54 publications, is the only one outside mainland China, standing out with 1.42% of total publications. These data highlight not only Chinese leadership in research, but also Taiwan's relevant contribution, reflecting the importance of regional collaboration in Asia-Pacific.

These institutions are essential not only for the amount of research produced but also for their significant impact on academic and industrial development, demonstrating a robust concentration of expertise and resources dedicated to this area.^91–93 The concentration of research efforts in these institutions underscores the strategic importance placed on Flex-SCs within the broader context of sustainable energy solutions and technological innovation.

Fig. 7 illustrates a citation network of articles on Flex-SCs from 1979 to 2023. The size of each node in the network corresponds to the total citations (TC) each department received, highlighting the involvement of multiple departments from various universities in this research.^94,95 Notable departments include California NanoSystems Institute (University of California, USA), the Department of Materials Science and Engineering (Yonsei University, South Korea), the Beckman Institute for Advanced Science and Technology (University of Illinois, USA), the Department of Energy Conversion and Storage and the Department of Chemical and Biochemical Engineering (Technical University of Denmark, Denmark), Clarendon Laboratory, Oxford (University of Oxford Parks Road, United Kingdom), the Department of Electrical and Computer Engineering (University of Texas, USA), Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University, China), MOE Key Laboratory of Bioinorganic and Synthetic Chemistry (Sun Yat-sen University, China), the Department of Soft Matter Physics (Johannes Kepler University, Austria), CHOSE-Center for Hybrid and Organic Solar Energy (University of Rome Tor Vergata, Italy), the Department of Chemistry (University of Washington, USA), Beijing National Laboratory for Molecular Sciences (Institution of Chemistry, Chinese Academy of Sciences, China), the Department of Chemical Engineering (Kyung Hee University, South Korea), the Center for Polymers and Organic Solids (University of California, USA), Pen-Tung Sah Institute of Micro-Nano Science and Technology (Xiamen University, China), the Department of Physics and Astronomy (University of Toledo, USA), and the University of Chinese Academy of Sciences (China).

Fig. 7 TC visualization of Flex-SC articles at the institute from 1979 to 2023, where the node size indicates the total number of citations, and the color represents the average citation timeline by department.

This globalized interest in Flex-SCs underscores the growing importance of this field of study. The widespread involvement of leading institutions worldwide highlights the collaborative and interdisciplinary nature of research in flexible solar cells, driving advancements in technology and applications. The diverse geographic representation in the citation network also indicates the significant impact and recognition of research contributions from various regions, emphasizing the global commitment to the development of sustainable energy solutions.^96,97

3.6. Knowledge propagation and collaboration network

Fig. 8 highlights the cooperation network between authors from organizations involved in Flex-SC publications from 2019 to 2023, illustrating the dissemination of knowledge and academic collaboration.⁹⁸ According to Kafouros et al., firms with a higher degree of internationalization significantly enhance their innovation performance because international exposure improves their ability to capture and utilize innovative ideas effectively.⁹⁹ Similarly, a study by Zahra et al. on smaller and medium-sized enterprises found that expanding into broader international markets exposes firms to a rich network of information, which stimulates product innovation.¹⁰⁰ Research by Zivlak et al. on Chinese companies indicates that internationalization positively impacts all four types of innovation--products, process, marketing, and organizational--with direct contact with foreign customers being crucial for these benefits.¹⁰¹

Fig. 8 Mapping of publication co-authorship organizations in Flex-SCs (2019–2023).

The most prominent cluster in the cooperation network is that of China, suggesting that the country is a leader in research into Flex-SCs. Among the various participating Chinese institutions are the Ningbo Institute of Material Technology and Engineering, several Institutes of the Chinese Academy of Sciences, and other specialized research centers. China's robust national research infrastructure and significant funding in the area are evidenced by the presence of multiple institutions.¹⁰² The clusters indicate collaborations with countries such as the United States and Hong Kong, emphasizing an exchange of knowledge and resources between world leaders in innovation and technology. Partnerships with the United States are particularly notable for focusing on innovation and technological development.

The most diverse cluster involves China, South Korea, Denmark, Sweden, and Hong Kong, creating a truly global research network. The collaboration between Asia and Europe, with the participation of Denmark and Sweden, highlights the emphasis on sustainable technologies and renewable energy, which are crucial for rapid progress in overcoming the technical challenges of flexible solar cells. These collaborations tend to involve institutions that are leaders in research and development, reflecting a focus on innovation and practical applications.

The growing global interest in Flex-SCs aligns with the increasing demand for renewable and sustainable energy solutions. Flexible solar cells are particularly promising due to their ease integration into diverse environments, thanks to their flexibility and low-cost potential. These characteristics underscore the importance of flexible SCs as a dynamic and promising field, with broad implications for the future of renewable energy. Global collaboration, as illustrated in Fig. 8, is essential to overcoming technical challenges and accelerating the adoption of this emerging technology.

3.7. Entities and institutions of funding

Fig. 9 illustrates the number of articles published annually from 1979 to 2023 on Flex-SCs, funded by different entities and/or institutions. The analysis reveals funding trends and the role of various organizations in developing research on Flex-SCs over the years, providing valuable insights into the engagement of different countries and entities in supporting this research, which is essential for the future of renewable energy.^103–105 Initially, no funding was recorded between 1979 and 1998. Funding began in 1999–2001, when the National Science Foundation (NSF) funded two publications.

Fig. 9 Financing entities/institutions contributed to the growth of Flex-SCs (1979–2023).

Starting in 2010, a significant increase in support was noted from several organizations, including the NSF, the National Natural Science Foundation of China (NSFC), the National Research Foundation of Korea (NRF), the National Key Research and Development Program of China (NKPs), the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and the Chinese Academy of Sciences (CAS). In that year, the NRF, CAS, and KETEP each contributed one grant, while the NKPs and NSF supported two publications each, and the NSFC contributed eight grants. Between 2014 and 2018, the NSFC funding increased notably, jumping from 17 to 144 publications, with an increase of 747.1%. During the same period, the NRF also expanded its support from 11 to 38 publications, a 245.5% increase.

Peak funding occurred between 2022 and 2023, when NSFC funded 151 publications (a 4.9% increase) and NKPs 45 publications (a 350% increase). From 1979 to 2023, the main funders were the NSFC, with 1103 projects, followed by the NRF, with 283, and the NKPs, with 217. In 2023, these three main entities indicated that China and South Korea are the largest sponsors of this type of research. The concentration of funders in Asia, especially China and South Korea, reflects a significant regional focus on developing renewable energy technologies.^106,107

The overall growth in publications over the years highlights a growing global interest in Flex-SC research, driven by the desire to develop more sustainable and scalable energy technologies.^108–110

The increase in funding for Flex-SC-related research stems from multiple factors, spanning regulatory, economic, and technological domains, along with a growing emphasis on sustainability. Greater awareness of environmental issues such as climate change, pollution, and resource depletion has heightened efforts to understand and mitigate the environmental ramifications of human actions. Notably, collaborations between academia, industry professionals, and government bodies have proliferated, leading to an increase in Flex-SC research funding.^111–113

Table 7 highlights the top funding sponsors for research into Flex-SCs from 2019 to 2023, providing a clear view of global commitment and investment in developing more sustainable and efficient energy technologies. The efficiency of these solar cells is crucial as it plays a key role in enabling their practical application in diverse environments, from wearables to architectural integrations.^114,115

Table 7 Key funding sponsor about Flex-SCs from 2019 to 2023

Funding sponsor	TP	%	Countries
National Natural Science Foundation of China (NSFC)	699	18.32	China
National Key Research and Development Program of China (NKPs)	174	4.56	China
National Research Foundation of Korea (NRF)	160	4.19	South Korea
Fundamental Research Funds for the Central Universities (FRF-CU)	124	3.25	China
Ministry of Science, ICT and Future Planning (MSIP)	88	2.31	South Korea
China Postdoctoral Science Foundation (CPSF)	74	1.94	China
Ministry of Trade, Industry and Energy (MoTIE)	64	1.68	South Korea
Chinese Academy of Sciences (CAS)	55	1.44	China
Korea Institute of Energy Technology Evaluation and Planning (KETEP)	51	1.34	South Korea
Ministry of Science and Technology of the People's Republic of China (MSaT)	47	1.23	China

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The National Natural Science Foundation of China (NSFC) leads in funding with 699 published works, representing 18.32% of the total. This reflects strong support from the Chinese government to lead in renewable technology research. The National Key Research and Development Program of China (NKPs) and the National Research Foundation of Korea (NRF) are also significantly involved, with 174 and 160 publications, representing 4.56% and 4.19% respectively. Other important sponsors include the Fundamental Research Funds for the Central Universities (FRF-CU) and the Ministry of Science, ICT, and Future Planning (MSIP) of South Korea, each contributing a considerable share of publications and funding.

Thus, China and South Korea are the main sponsoring countries, highlighting their leading role in innovation in renewable energy technologies in Asia.¹¹⁶ The substantial investment in these countries suggests a national strategy to advance the development of clean energy technologies as an economic and environmental priority.¹¹⁷ The high percentage of funding reflects not only financial support but also encouragement of continued research and development, creating a robust ecosystem for future innovations in Flex-SCs.

The National Natural Science Foundation of China (NSFC) is the largest sponsor, both in the number of publications and in the total percentage, indicating that China is the country that sponsors the most research in this field. This not only highlights China as a global leader in flexible solar cell research but also underlines the strategic importance that China attaches to renewable energy technologies.¹¹⁸ These data are crucial for understanding how investments in research and development can drive technological innovation and contribute to global sustainability goals, as well as offering insights into how countries are positioning themselves in the global technological race for renewable energy.

3.8. The most explored areas

Fig. 10 shows the evolution of the number of publications in Flex-SCs, divided by different thematic areas over the period from 1979 to 2023. During the period 2014–2023, there was a notable increase of 38.8% in TP, in all subject areas, with the most significant growth observed in Engineering at 112.7%.^119–122 This trend underlines the growing recognition and interest in Flex-SCs.

Fig. 10 Growth of publications in Flex-SCs (1979–2023) according to subject area.

The Materials Science topic area showed the most substantial growth, going from zero publications in 1979 to 249 in 2022, averaging 59 publications per year (TP year⁻¹).^123–127 Similarly, chemistry, mirroring the trajectory of Materials Science, started with no publications and rose to 129 in 2023, at an average rate of 32.4 TP per year.^128–132 The Energy topic area began with a single publication in 1988 and has since expanded to 139 publications by 2022, averaging 29.5 TP per year.^133–137 The Flex-SC domain experienced a notable increase in 2023, reaching 322 publications, indicative of a growing interest in this field of scientific research.

The area of chemistry also shows considerable growth, especially from 2000 with one publication¹³⁸ and increased to 129 in 2023, highlighting the importance of chemical processes in improving the properties of Flex-SCs. Domains related to Environmental Science show steady growth, indicating a growing interest in integrating Flex-SCs into systems that minimize environmental impact.^139–143 The Chemical Engineering subject area has demonstrated remarkable growth, with fewer than 5 articles between 1979 and 2002,^144–146 and reaching around 68 in 2023. This growth in Chemical Engineering reflects the need for innovative and sustainable manufacturing processes.

Other specialized areas such as Multidisciplinary and Physics and Astronomy began with publications by Philip J. Ireland et al. from the United State in 1979,¹⁴⁷ and by N. Tsuya et al. from Japan in 1980,¹⁴⁸ experiencing a peak of 9 and 109 publications in 2012 and 2018, respectively. Similarly, Biochemistry, Genetics and Molecular Biology, which began with two publications in 2008,^130,149 peaked at 22 in 2023,^150–155 suggesting an interdisciplinary interest in Flex-SCs. Although growth is modest, Mathematics plays a key role in modeling and performance analysis, which began with a publication by Claudia Longo et al. from Brazil in 2002¹⁵⁶ and reached a peak of 8 in 2023,^157–161 being crucial for the development of effective technologies.

The period around 2014 marks a significant increase in all areas, coinciding with the replacement of the eight United Nations MDGs in 2015 by the 17 United Nations SDGs.^162,163 This change notably catalyzed research into Flex-SCs. Materials Science, Engineering, and Chemistry emerge as dominant contributors to publications for most of the period examined, highlighting their fundamental role in the research and development of Flex-SCs.

These trends reveal the importance of interdisciplinary collaboration in advancing Flex-SCs, highlighting the need to cross-disciplinary boundaries to address the diverse challenges associated with creating flexible, efficient, stable, durable, and low-cost solar cell solutions.^164,165 Furthermore, the accelerated growth observed in specific areas such as Materials Science (451), Engineering (286), Chemistry (247), and Energy (241), which make up 75.5% of the total publications (field of innovation), suggests emerging applications of FoF-SCs starting from rigid solar cells and covering advanced flexible-SCs such as flexible-PeSCs, flexible-OSCs, flexible-DSSCs, and flexible-thin-film SCs based on CIGS, CdTe, and a-Si:H.

Table 8 provides a detailed overview of the primary topic areas in Flex-SC research from 2019 to 2023. Flexible solar cells are considered pivotal for the next generation of solar energy technologies due to their portability, lightweight nature, and adaptability to various surfaces, advantages not offered by traditional rigid solar cells.^166,167 The efficiency of these cells is crucial for their commercial viability and environmental impact, making research into their enhancement a priority. The Material Science area, with 1170 publications (30.7% of the total), leads in research output. This underscores the importance of material innovations for Flex-SC development, as advancements in materials can significantly impact the durability and efficiency of flexible solar cells, particularly with the use of 2D materials.

Table 8 Key thematic areas in Flex-SCs research from 2019 to 2023

Subject area	TP	%
Materials science	1170	30.7
Engineering	676	17.7
Energy	633	16.6
Chemistry	577	15.1
Physics and astronomy	490	12.9
Chemical engineering	286	7.5
Environmental science	107	2.8
Biochemistry, genetics and molecular biology	74	1.9
Multidisciplinary	32	0.8
Mathematics	31	0.8

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The Engineering field, with 676 publications (17.7% of the total), plays a critical role in applying new designs and integrating innovative technologies into practical solar energy systems. The Energy area, with 633 publications (16.6% of the total), focuses on optimizing the conversion of solar energy into electrical power and its integration into larger energy systems, addressing the final application of Flex-SCs. Chemistry, with 577 publications (15.1% of the total), emphasizes the chemistry of materials and processes, which is essential for improving the energy conversion efficiency in flexible solar cells. The Physics and Astronomy domain, with 490 publications (12.9%), contributes to the fundamental understanding of the physical processes governing Flex-SC operation. Chemical Engineering and Environmental Science have a smaller focus, with 7.5% and 2.8% of publications, respectively. However, these areas are still crucial for developing sustainable manufacturing processes and the studying the environmental impact of Flex-SCs.

Fields such as Biochemistry, Genetics and Molecular Biology, Multidisciplinary, and Mathematics contribute specialized perspectives and multidisciplinary approaches to addressing specific challenges in Flex-SCs, with each having less than 2% of the total publications. Overall, the prominence of the Materials Science area highlights its central role in advancing the efficiency and applicability of Flex-SCs.¹⁶⁸ Innovations in this area have the potential to transform solar energy collection and utilization, enhancing the attractiveness of Flex-SCs for diverse applications, ranging from consumer electronics to large industrial systems.¹⁶⁹

Fig. 11 illustrates the bibliographic coupling among widely cited articles on Flex-SCs from 2019 and 2023, showcasing the interconnections between different research themes. This type of analysis helps identify promising areas for future research and potential collaborations between research groups, advancing the field of Flex-SCs. Each cluster in the figure represents a distinct thematic focus, highlighting areas of intersection and collaboration among researchers. Cluster 1: Efficiency of Flexible Organic Solar Cells^67,170–175 focuses on enhancing the efficiency of flexible organic solar cells, known for their lightweight and potential for integration on non-planar surfaces. Cluster 2: Stability and Efficiency of Flexible-Perovskite SCs^176–182 addresses the dual challenges of energy efficiency and stability in perovskite solar cells. Cluster 3: Efficiency of Flexible-Perovskite SCs^183–187 also concentrates on efficiency but does not explicitly address stability, offering a more focused perspective in optimizing performance. Cluster 4: Durability of Flexible-Perovskite SCs^188,189 examines durability, a critical factor for the practical application and commercialization of flexible-perovskite technologies. Cluster 5: Electron-Transport Layer and Interface of Flexible-Perovskite SCs^190,191 focused on interface technologies and electron transport layers, essential for improving the overall cell efficiency.

Fig. 11 Bibliographic coupling in widely cited articles to identify correlations between thematic areas in Flex-SC publications (2019–2023).

The five clusters reveal a significant emphasis on perovskite solar cells, highlighting diverse research perspectives crucial for the practical implementation of Flex-SCs: efficiency, stability, durability, and interface technologies. The prevalence of perovskite-focused clusters reflects the scientific community's considerable interest in this material, owing to their outstanding photovoltaic properties and potential to address the limitations of existing solar technologies. The correlation between these themes suggests an integrated approach to Flex-SC research, where advancements in one aspect (such as efficiency) can be complemented by improvements in others (such as stability and durability), supporting the development of more robust and efficient solar technologies.

3.9. Analysis of the main scientific journals

According to VOSviewer, 551 journals have published articles on Flex-SCs, spanning a wide range of topics within Materials Science, Engineering, Energy, and Chemistry. Key research themes include the efficiency and stability of flexible perovskite SCs, the efficiency of flexible organic SCs, the durability of flexible perovskite SCs, and advancements in the electron transport layer and interfaces for flexible perovskite SCs. Additional areas of interest encompass flexible-PeSCs, flexible-OSCs, flexible-DSSCs, and flexible-thin-film SCs, along with their various applications. Flex-SCs have gained significant interest and are increasingly adopted across diverse research fields, from flexible electronics to building-integrated photovoltaics.^192,193

Between 1979 and 2023, this study highlights the journals with the greatest impact in terms of citations and number of publications, as shown in Fig. 12. The TC per journal increased significantly over time: from 4109 citations (averaging 132.6 per year) between 1979 and 2009, to 92 025 citations (averaging 10 225 per year) from 2010 to 2018, and then to an exponential rise with 106 839 citations (averaging 21 367.8 per year) from 2019 onwards.

Fig. 12 Citations and cumulative publications of journals in the area of Flex-SCs from 1979 to 2023 using overlay visualization.

During the period from 1979 to 2018, Sol. Energy Mater. Sol. Cells. was the leading journal. However, it was surpassed in 2021 by ACS Applied Materials and Interfaces. Among the most cited and productive journals from 1979 to 2023, Nano Energy led with a citation per publication (TC/TP) ratio of 65.63, followed by JMC-A with 51.83 TC/TP, Sol. Energy Mater. Sol. Cells. with 49.3 TC/TP, and ACS Applied Materials and Interfaces with 39.73 TC/TP. From 2019 to 2023, Advanced Materials ranked first with a TC/PC ratio of 91.2, followed by Advanced Energy Materials with 57.03 TC/TP. This highlights the growing quality and relevance of these publications in the field of Flex-SCs.

Table 9 lists the top 10 journals with the highest productivity in terms of publications on Flex-SCs from 2019 to 2023. The efficiency of these cells is crucial for advancing their practical applications, including integration into portable and wearable devices, buildings, and vehicles, offering a flexible and potentially more affordable alternative to traditional rigid solar cells.

Table 9 Top 10 Journals with the highest productivity (2019–2023) in Flex-SC research^a

Journal	TP	Contribution (%)	Publisher	Quartiles	Cite score
a TP: Total Articles Published.
ACS Applied Materials and Interfaces (ACS AMI)	90	2.36	American Chemical Society	Q1	15.7
Solar RRL (SRRL)	70	1.84	Wiley-Blackwell	Q1	11.7
ACS Applied Energy Materials (ACS AEM)	67	1.76	American Chemical Society	Q1	9.5
Solar Energy (SE)	58	1.52	Elsevier	Q1	13.1
Advanced Functional Materials (AFM)	56	1.47	Wiley-Blackwell	Q1	27.9
Journal of Materials Chemistry A (JMC-A)	54	1.42	Royal Society of Chemistry	Q1	22.0
Nano Energy (NE)	48	1.26	Elsevier	Q1	29.3
Advanced Materials (AM)	45	1.18	Wiley-Blackwell	Q1	45.5
Advanced Energy Materials (AEM)	36	0.94	Wiley-Blackwell	Q1	42.6
Chemical Engineering Journal (CEJ)	35	0.92	Elsevier	Q1	21.5

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Among the journals listed, ACS Applied Materials and Interfaces stands out with 90 publications, representing 2.36% of the total publications in this field. This highlights the journal's strong focus on applied materials, which is vital for the development of Flex-SCs. It is followed by Solar RRL with 70 publications and ACS Applied Energy Materials with 67 publications, demonstrating significant interest in applied results related to renewable energy. The percentages range from 2.36% at the top to 0.92% for the Chemical Engineering Journal.

These numbers indicate a relatively balanced distribution of research among the key specialized journals. The American Chemical Society (ACS) and Wiley-Blackwell are the most prominent publishers, with ACS overseeing two major journals (ACS AMI, and ACS AEM), contributing a combined total of 4.12% of publications. Elsevier is also a major publisher, responsible for three journals (SE, NE, and CEJ), though it lacks a single dominant journal like Wiley-Blackwell's Advanced Materials, which has the highest CiteScore of 45.5.

All journals listed in Table 9 are in Quartile 1 (Q1), reflecting their high quality and impact in this research area. However, the CiteScore varies significantly, with Advanced Materials having the highest score of 45.5, indicating its exceptional influence and citation frequency in the scientific community.

Research on Flex-SCs is well distributed across several high-quality journals, underscoring the growing importance of this field.^194,195 The dominance journals focused on materials and energy applications highlight research priority areas in optimizing the efficiency and applicability of Flex-SCs. These journals are at the forefront of disseminating discoveries that have the potential to transform solar energy technologies, making them more accessible and integrable into various new environments and applications.^196,197

3.10. Analysis of the most cited articles

Table 10 presents the most cited articles in the field of Flex-SCs from 1979 to 2023, highlighting significant and impactful contributions to the scientific literature on this topic. Leading these is the work by J. Peet et al. published in Nature Materials in 2007, titled “Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols,” which has garnered 3062 citations. This article discusses increasing the PCE in photovoltaic cells from 2.8 to 5.5% by adding a small volumetric percentage of alkane dithiols in films produced by spin-casting.¹¹⁹

Table 10 Highly cited articles from 1979 to 2023

Author and year of publication	Total	Title	Journal	Keywords
Peet, J., Kim, J., Coates, N. et al., 2007	3062	Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols	Nature Materials (Springer Nature)	—
Liu, D., Kelly, T., 2014	2435	Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques	Nature Photonics (Springer Nature)	—
Blom, Paul WM, et al., 2007	2025	Device physics of polymer: fullerene bulk heterojunction solar cells	Advanced Materials (Wiley-Blackwell)	—
Frost, Jarvist M., et al., 2014	2009	Atomistic origins of high-performance in hybrid halide perovskite solar cells	Nano Letters (American Chemical Society)	Hybrid perovskite, photovoltaic, density functional theory, ferroelectric
Tan, Hairen, et al., 2017	1974	Efficient and stable solution-processed planar perovskite solar cells via contact passivation	Science (American Association for The Advancement of Science)	—
He, Zhicai, et al., 2015	1599	Single-junction polymer solar cells with high efficiency and photovoltage	Nature Photonics (Springer Nature)	—
Docampo, Pablo, et al., 2013	1489	Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates	Nature Communications (Springer Nature)	—
Chuang, Chia-Hao M., et al., 2014	1464	Improved performance and stability in quantum dot solar cells through band alignment engineering	Nature Materials (Springer Nature)	—
Sun, Yanming, et al., 2012	1449	Solution-processed small-molecule solar cells with 6.7% efficiency	Nature Materials (Springer Nature)	—
Kaltenbrunner, Martin, et al., 2012	1409	Ultrathin and lightweight organic solar cells with high flexibility	Nature Communications (Springer Nature)	—

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Following this, D. Liu and T. Kelly in their 2014 Nature Photonics publication, “Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques”, achieved 2435 citations. They studied PeSCs with a planar heterojunction structure prepared using room-temperature processing techniques, highlighting advances in the manufacture of more affordable and efficient solar cells.¹²³

Another notable study is by P. W. M. Bloom et al., published in Advanced Materials in 2007, titled “Device Physics of Polymer: Fullerene Bulk Heterojunction Solar Cells,” which received 2025 citations. This work delves into the fullerene-bulk heterojunction SCs, emphasizing their potential for large-scale power generation by requiring controlled charge carrier transport and improved photogeneration mechanisms to maximize the efficiency.¹²⁰

In 2014, J. M. Frost et al., published “Atomistic origins of high-performance in hybrid halide perovskite solar cells” in Nano Letters, which obtained 2009 citations. They investigated the superior performance of hybrid PeSCs, highlighting their structural flexibility, spontaneous electrical polarization, ferroelectric domains, high dielectric constant, and low effective mass.¹²¹ Z. He et al., in their 2015 Nature Photonics article, “Single-junction polymer solar cells with high efficiency and photovoltage,” obtained 1599 citations. The authors addressed single-junction PSCs with high efficiency, expanding possibilities for large-scale production of Flex-SCs.¹²⁴ H. Tan et al., in their 2017 Science publication, titled “Efficient and stable solution-processed planar perovskite solar cells via contact passivation,” reached a total of 1974 citations. They investigated improving efficiency and stability in low-temperature planar PeSCs using a contact the passivation technique with a colloidal TiO₂ nanocrystal film coated with chlorine, facilitating manufacturing on flexible substrates.¹⁹⁸ The article by P. Docampo et al., published in 2013 in Nature Communications, was a landmark in perovskite solar cell research, receiving a total of 1489 citations. In the study, the authors demonstrated that “Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates” achieved power conversion efficiencies of up to 10% on glass substrates and more than 6% on flexible polymer substrates. These values are comparable to those of organic photovoltaic cells, but with the advantage of high light absorption and ease of fabrication of perovskites. This research represented the first demonstration of flexible-PeSCs, establishing a new revolution for the development of portable and lightweight photovoltaic devices.¹²⁸

C.-H. M. Chuang et al., in their 2014 Nature Materials publication, received 1464 citations for their work “Improved performance and stability in quantum dot solar cells through band alignment engineering”. They discovered low-cost, flexible solar cells made from ZnO/PbS quantum dots, processed in solutions at room temperature, which achieved an efficiency of 8.55% with air stability, providing short energy return times.¹²² Y. Sun et al., in their 2012 Nature Materials article, received 1449 citations for “Solution-processed small-molecule solar cells with 6.7% efficiency”. They used a new molecular donor to obtain solution-processed small-molecule solar cells with 6.7% efficiency, showing potential to compete with polymeric counterparts.¹⁹⁹ Finally, M. Kaltenbrunner et al., in their 2012 Nature Communications publication, received 1409 citations for “Ultrathin and lightweight organic solar cells with high flexibility”. They developed ultrathin OSCs, lighter, more flexible, and ten times thinner than current technologies, with great potential for future use in electronic skins, displays, and photovoltaics.¹²⁹

These articles represent significant advances in the research and development of Flex-SCs, with considerable focus on improving their stability and efficiency. The high citation counts reflect the impact and relevance of these studies in advancing flexible solar cells. Collectively, the ten most cited articles cover a wide range of innovative techniques and fundamental discoveries in Flex-SCs, demonstrating the feasibility and potential application of these technologies in diverse contexts-from flexible electronics to building-integrated photovoltaics. The advancements promote the greater adoption of renewable energies.^200–202

Fig. 13 illustrates the five main areas of research identified within the set of most-cited articles on Flex-SCs. The first area addresses the improvement of efficiency and stability of solar cells, focusing on flexible OSCs and DSSCs. These articles explore strategies such as ternary heterojunctions and organic dye sensitization to optimize light capture and minimize energy losses, enhancing both the efficiency and the stability.^175,203,204 The second area concentrates on advances in materials and processing for flexibility. Studies in this category develop manufacturing methods that increase device flexibility while maintaining or improving efficiency, which is essential for practical applications in various environments.^170,205,206 The third area deals with innovations in interfaces and electron management. These studies investigate techniques to optimize electron management and modulate interface defects, focusing on enhancing the electrical performance and energy efficiency of perovskite solar cells.^177,207,208 The fourth area focuses on the optimization of tandem solar cells. Articles in this area play a crucial role in advancing technology for tandem solar cells, highlighting methods to address technical challenges and enhance the energy efficiency of photovoltaics systems. They represent innovative efforts to improve the solar energy capture and conversion through multilayer structures, exploring the flexibility of these configurations for more versatile and adaptable applications.^209–211 Finally, the fifth area emphasizes organic solar cell optimization. These studies aim to improve organic solar cells through the advancements in materials, manufacturing processes, and cell stability, seeking to maximize both efficiency and practical applicability.^212–214

Fig. 13 Highly cited articles from publications on Flex-SCs using overlay visualization between 2019 and 2023.

Each topic area reflects a critical aspect of Flex-SC research, from enhancing the efficiency and stability to developing new materials and manufacturing techniques that support the broader integration and application of these technologies. These studies represent significant milestones in Flex-SC literature, indicating current progress and guiding future research to overcome the remaining challenges in this emerging technological field.

Fig. 14 shows a co-citation network highlighting highly relevant articles on Flex-SCs, showcasing the connections between different studies through citation data. This network visualization identifies how various research areas in Flex-SCs are interconnected, and which themes have driven academic discussions and technological innovations. Three main thematic areas are identified, each representing sets of interconnected research crucial for advancing knowledge in this field.

Fig. 14 44 most relevant articles that cited Flex-SCs using a co-citation network.

Cluster 1, development of materials for perovskite solar cells, includes articles focusing on the development of perovskite materials and their applications in flexible-SCs.^215–220 These studies discuss advances in synthesis techniques, composition optimization, and interface improvements, which are essential for the efficiency and stability of perovskite solar cells.^221–227 The research in this cluster aims to enhance perovskite materials' performance to make them more suitable for flexible applications.

Then, there is cluster 2, development and optimization of interfaces and materials in perovskite solar cells. Articles in this cluster address innovations to improve the efficiency, stability, and applicability of perovskite solar cells, particularly in planar and flexible configurations.^228–230 These studies contribute various strategies to optimize perovskite solar cells through materials engineering, manufacturing processes, and interface optimization.^231–233 The focus here is on creating robust interfaces and high-quality materials that can sustain long-term performance in diverse environmental conditions.

Cluster 3, advances in organic solar cells, includes studies that focus on developing small molecule acceptors and new organic materials to achieve higher efficiencies and improved open-circuit voltages in organic solar cells.^{129,170,234–239} Additionally, the cluster explores tandem solar cells, which stack multiple photovoltaically active layers to maximize the solar energy capture.²⁴⁰ These advancements aim to increase the practical applicability of organic solar cells, making them more competitive with traditional solar technologies.

Each of these topic areas reflects crucial aspects of research and development in Flex-SCs, highlighting both technical progress and remaining challenges. The search for more stable and efficient perovskite materials is ongoing, particularly to address issues like degradation under environmental conditions and the toxicity of certain components. Organic solar cells are evolving with sophisticated strategies to enhance light absorption and efficiency, but still face challenges related to longevity and manufacturing scalability. Integrating flexible solar cells into commercial and industrial products requires advancements in materials and devices and considerations regarding cost, manufacturing efficiency, and compatibility with existing production processes. Understanding these connections is vital for directing future research and development efforts, ensuring that advances in one area can effectively address challenges in others.

3.11. Progression of F-SC research topics: an analysis of Co-occurrence of keywords and areas of interest

Table 11 illustrates the evolution of the most frequently used keywords in Flex-SC research from 1979 to 2023. The analysis of these terms over time highlights significant changes in research trends and technological focus in this area. The evolution of Flex-SC research over the past few decades reflects significant changes in the materials and technologies explored. Between 1979 and 1990, research was still in its early stages, focusing mainly on basic terms such as “solar cells”, “semiconducting silicon”, and “substrates”, reflecting the interest in semiconductor materials and manufacturing, with an emphasis on chemical deposition and polymers. In the period from 1991 to 2001, an expansion of research was observed to include “thin films”, “heterojunctions”, “plastic films”, and “fullerenes”, suggesting an increase in interest in new materials and structures that could improve the flexibility and efficiency of solar cells. Between 2002 and 2012, research began to emphasize alternative technologies to silicon, such as “dye-sensitized solar cells” (DSSCs) and materials such as “titanium dioxide”, seeking efficiency and new applications. During this period, the focus also shifted to improving “power conversion efficiency”, indicating a growing concern about the practical feasibility of these technologies. In the following years, between 2013 and 2018, the scenario changed dramatically with the emergence of “perovskite solar cells”, which quickly gained prominence due to their potential for high efficiency and low production costs. The term “perovskite” appeared with significant frequency, accompanied by keywords such as “electrodes”, “temperature”, and “thin films”, highlighting efforts to solve challenges such as thermal stability and device optimization. The integration of the UN Sustainable Development Goals (SDGs), especially SDG 7 (ensure reliable, sustainable, modern, and affordable access to energy for all) boosted research focused on renewable energy in the Flex-SCs area.^241,242 Global attention to these goals drove increased research into renewable energy and low-environmental-impact technologies.

Table 11 List of the top 20 author-keywords within the Flex-SC keywords from 1979 to 2023

S. no.	1979–1990		1991–2001		2002–2012		2013–2018		2019–2023
	Keywords	Count	Keywords	Count	Keywords	Count	Keywords	Count	Keywords	Count
1	Solar cells	5	Solar cells	15	Solar cells	380	Solar cells	1073	Perovskite	647
2	Semiconducting silicon-amorphous	2	Substrates	10	Conversion efficiency	235	Power conversion efficiencies	499	Perovskite solar cells	618
3	Yag lasers	1	Silicon solar cells	9	Substrates	129	Efficiency	354	Power conversion efficiencies	540
4	Solar cells	1	Amorphous silicon	7	Photoelectrochemical cells	129	Perovskite	330	Efficiency	474
5	Solar cell	1	Photovoltaic cells	6	Dye-sensitized solar cell	118	Substrates	306	Conversion efficiency	376
6	Silicon ribbon	1	Copper compounds	6	Solar energy	108	Electrodes	294	Solar cells	344
7	Silicon	1	Thin films	5	Power conversion efficiencies	105	Perovskite solar cells	276	Solar power generation	321
8	Substrates	1	Solar energy	4	Dye-sensitized solar cells	93	Dye-sensitized solar cells	266	Organic solar cells	242
9	Single-chamber deposition	1	Plastic films	4	Titanium dioxide	90	Temperature	238	Temperature	235
10	Semiconductor devices-manufacture	1	Fullerenes	4	Photovoltaic cells	87	Titanium dioxide	218	Substrates	211
11	Semiconducting silicon	1	Heterojunctions	3	Electrodes	73	Solar power generation	193	Electron transport properties	202
12	Semiconducting cadmium compounds-chemical vapor deposition	1	Fabrication	3	TiO	71	Thin films	191	Electron transport layers	167
13	Rapid quenching	1	Evaporation	3	Thin films	71	Tin oxides	189	Open circuit voltage	160
14	Polyimide film	1	Energy conversion efficiency	3	Tin	67	Conversion efficiency	181	Lead compounds	156
15	Polymers-films	1	Efficiency	3	Photovoltaic effects	60	Polymer solar cells	152	Thin films	155
16	Polymers	1	Chemical vapor deposition	3	Counter electrodes	59	Zinc oxide	148	II-VI semiconductors	147
17	Polyimide -substrates	1	Thin film devices	2	Zinc oxide	56	Organic solar cells	148	Titanium dioxide	144
18	Plastics films	1	Synthesis (chemical)	2	Flexible substrate	56	Heterojunctions	138	Dye-sensitized solar cells	136
19	Optical	1	Short circuit currents	2	Tin oxides	55	Flexible solar cells	131	Transparent electrodes	135
20	Organic polymer film substrates	1	Semiconductor junctions	2	Platinum	55	Open circuit voltage	130	Flexible solar cells	134

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From 2019 onwards, despite the challenges imposed by the COVID-19 pandemic, the dominance of perovskite technology became even more evident, with the terms “perovskite” and “perovskite solar cells” being the most mentioned. New areas of interest have emerged, including “electron transport properties”, “electron transport layers”, “open circuit voltage”, and “transparent electrodes”, showing that current challenges focus on improving charge transport, stability and transparency of devices, essential elements for the commercial viability of flexible solar cells. However, reflecting a continued focus on both conventional and unconventional technologies, from flexible electronics to building-integrated photovoltaics.^243,244 This panorama suggests that, in the future, research efforts will continue to focus on structural optimization, integration into flexible devices and production scalability, consolidating this technology as a promising alternative for renewable energy generation.

The transition to advanced technologies such as organic–inorganic Hybrid Perovskites and DSSCs, and innovative materials, like 2D Materials and Quantum Dots, is evident in Table 11. This transition reflects technological progress and the response to global sustainable energy needs, driven largely by the UN SDGs.²⁴² These objectives have promoted continuous innovation and investment in more efficient and adaptable solar energy technologies, responding to demands for sustainability and lower environmental impact.²⁴⁵

The integration of 2D materials into solar cell technologies represents an exciting frontier in photovoltaics research. These materials offer advantages such as high light absorption, mechanical flexibility, and potentially high efficiencies due to their unique quantum properties and electronic interactions.²⁴⁶ Research continues to evolve, exploiting these properties to create more efficient and durable solar cells. Advances in Flex-SCs (flexible-PeSCs, flexible-OSCs, flexible-DSSCs, and flexible-thin film SCs) have been significantly accelerated by global events and technological innovations.^10,247 The development of research platforms like Scopus and Google Scholar and global impacts of the COVID-19 pandemic stand out as key drivers.^248,249 Each of these factors has contributed uniquely to driving research and development in flexible solar cell technologies. These developments have shaped the path of research and development in the field, promoting a continuous cycle of innovation and opening up new possibilities for future applications in renewable energy.

The flexible solar cells market has maintained a 5% share of annual solar cell production. However, it is forecast to grow at a CAGR of 10% over the next decade, reaching US$ 6.1 billion by 2033 due to improved efficiency and cheaper manufacturing.²⁵⁰ Key applications include building-integrated photovoltaics, where panels are up to 90% lighter than silicon, making them ideal for facades and weak structures, and semi-transparent flexible-thin film solar cells for windows. Another application is in the Internet of Things (IoT) sector, where lightweight and flexible-thin film modules can power devices, offering a cost-effective and durable alternative to batteries.²⁵¹ In summary, the evolution of keywords in Flex-SCs research from 1979 to 2023 reflects a dynamic and rapidly advancing field, driven by technological innovations, global sustainability goals, and increasing demand for efficient, flexible, and sustainable solar energy solutions.

3.12. Outlook emerging in F-SCs: 2020-towards the future

Fig. 15 shows the evolution of research on Flex-SCs over several decades using a keyword overlay visualization. Fig. 15A–E reflect a specific period and illustrates the prevailing trends in flexible solar cell research during that time. In this early period from 1979 to 1990 (Fig. 15A), keywords like “Heterojunction solar cell” and “polyimide film” indicate a focus on basic concepts of F-SCs, emphasizing heterojunction structures and base materials such as polyimide films for initial flexibility applications. From 1991 to 2001 (Fig. 15B), the keywords “amorphous silicon”, “multi-junction”, and “energy payback time” suggest a shift towards exploring amorphous silicon and multiple junction solar cells. This period marks an advance in efficiency and a growing concern with the energy return of the materials used.

Fig. 15 Evolution of research on Flex-SCs over time: (A) 1979–1990; (B) 1991–2001; (C) 2002–2012; (D) 2013–2018; and (E) 2019–2023, using overlay visualization of keywords.

During 2022 to 2012 (Fig. 15C), the keywords “organic solar cells”, “polymer solar cell”, and “flexible dye-sensitized solar cells” highlight significant diversification. This period saw the emergence of flexible OSCs and flexible-DSSCs, reflecting innovations in materials and design for flexibility. The creation of Scopus in 2004 facilitated international dissemination and collaboration in this research. In the period from 2013 to 2018 (Fig. 15D), keywords such as “perovskite solar cells” and “counter electrode” indicate the rise of perovskite solar cells. This coincided with the global integration of the UN's SDGs, particularly SDG 7, which focuses on energy efficiency and renewable sources.^252–254 Flexible cell research began to incorporate new materials like perovskite to enhance efficiency and enable large-scale production. From 2019 to 2023 (Fig. 15E), keywords like “Perovskite solar cells”, “stability”, and “large-area cell” were most commonly used by the scientific community. The COVID-19 pandemic accelerated the search for more resilient and sustainable energy solutions, boosting research into flexible technologies such as flexible-PeSCs and flexible-OSCs.²⁵⁵ Developments in flexible solar cells suggest a promising future, where such technologies play a crucial role in sustainable energy applications. Advancements in conversion efficiency and long-term stability are expected to continue, enhancing their viability for commercial and residential use. The trend is toward greater integration of flexible SCs into building materials and wearables, expanding the usability of solar energy in new applications. Additionally, advances in manufacturing techniques, such as roll-to-roll printing, are poised to reduce costs and increase the production of flexible solar cells.^256,257

Ongoing research aims to minimize the environmental impact of flexible solar cells disposal and production, aligning with global sustainability goals. These trends indicate not only continued evolution in flexible solar cell technologies, but also reflect a shift toward a more versatile and sustainable energy future. This aligns with global needs for emission reduction and energy efficiency, reflecting a broader commitment to a greener planet.

Fig. 16 shows a network of keywords visually organized into distinct colored clusters, each representing different thematic foci within the field of 2020–2023 Flex-SCs. Cluster 1 focuses on the low-temperature processing of electronic transport layer for flexible organic and perovskite solar cells. This cluster explores manufacturing methods that enable the processing of electronic transport layers at low temperatures, which is crucial for maintaining the integrity of flexible substrates and improving the efficiency and stability of organic and perovskite solar cells.

Fig. 16 Thematic keyword cluster on Flex-SCs from 2020 onwards.

Cluster 2 delves into the Robustness and mechanical stability in flexible organic solar cells using PEDOT:PSS as a hole transport layer for application in flexible electronics. This cluster investigates the use of PEDOT:PSS as a hole-transport layer, targeting applications in flexible electronics where durability and flexibility are essential.²⁵⁸

Cluster 3 concentrates on the counter electrode for efficient dye-sensitized solar cells. This cluster is focused on developing efficient counter electrodes, a key component for increasing the energy conversion efficiency of these cells, which are particularly useful for flexible applications due to their robustness and efficiency under low-light conditions.

Cluster 4 addresses the flexible and stable PeSCs for application in flexible devices. This cluster aims to develop PeSCs that not only conform to the flexibility required by flexible devices but also offer improved stability, overcoming one of the biggest challenges of perovskites.²⁵⁹

Cluster 5 aims to maximize the efficiency of flexible solar cell using thin films and perovskite materials. This cluster focuses on applications of solar energy where efficiency and adaptability to different surfaces are crucial. It addresses the need for high-efficiency flexible solar cells that can be used in various solar energy applications.

Table 12 summarizes the main keywords and associated themes for each Flex-SC research cluster as of 2020. Analyzing these data provides a comprehensive overview of current research foci and emerging perspectives for the future of FoF-SC technologies.

Table 12 Cluster keywords for Flex-SCs between 2020 and 2023

Cluster	Main keywords	Theme
1	Organic solar cells; perovskite solar cell; electron transport layer; low temperature; flexibility	Low-temperature processing of electronic transport layer for flexible organic and perovskite solar cells
2	Mechanical robustness; mechanical stability; flexible electronics; flexible organic solar cells; power conversion efficiency; PEDOT:PSS	Robustness and mechanical stability in flexible organic solar cells using PEDOT:PSS as hole transport layer for application in flexible electronics
3	Solar cells; perovskite, flexible; dye-sensitized solar cells; efficiency; flexible substrate; counter electrode	Counter electrode for efficient dye-sensitized solar cells
4	Perovskite solar cells; flexible perovskite solar cell; stability; flexible devices; defect passivation; tin oxide	Flexible and stable perovskite solar cell for application in flexible devices
5	Flexible solar cells; perovskite; thin films; photovoltaics, high efficiency; solar energy	High-efficiency flexible solar cell from perovskite, thin films for solar energy applications

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In cluster 1, the main keywords are “organic solar cells”, “perovskite solar cell”, “electron transport layer”, “low temperature”, and “flexibility”. This cluster focuses on the development of electronic transport layers that can be processed at low temperatures, which is essential for preserving the integrity of flexible materials used in organic and perovskite solar cells.

Cluster 2 features keywords such as “mechanical robustness”, “mechanical stability”, “flexible electronics”, “flexible organic solar cells”, “power conversion efficiency”, and “PEDOT: PSS” to improve the mechanical robustness and stability of flexible organic solar cells, which is a key component for flexible electronics applications.

In cluster 3, the main keywords are “solar cells”, “perovskite”, “flexible”, “dye-sensitized solar cells”, “efficiency”, “flexible substrate”, and “counter electrode”. This cluster focuses on the development of efficient counter electrodes for dye-sensitized solar cells, which are crucial to improving the efficiency of these cells.

Cluster 4 includes keywords such as “perovskite solar cells”, “flexible perovskite solar cell”, “stability”, “flexible devices”, “defect passivation”, and “tin oxide”. This cluster aims to increase the stability of flexible perovskite solar cells by using techniques such as defect passivation and the application of tin oxide for flexible devices.

Lastly, cluster 5 focuses on keywords like “flexible solar cells”, “perovskite”, “thin films”, “photovoltaics”, “high-efficiency”, and “solar energy”. This cluster aims to develop high-efficiency flexible solar cells using perovskite thin films, with a special emphasis on solar energy applications where efficiency is a priority.

Emerging perspectives across clusters indicate a strong focus on overcoming challenges related to the stability, efficiency, and integration of flexible-SCs with other technologies. Continued innovation in materials, processing techniques, and device design promises significant advances, particularly in enhancing compatibility with ecological and sustainable manufacturing practices. Notably, there are strides being made in integrating flexible-SCs with a wide range of portable and wearable electronic devices, improving manufacturing techniques to make flexible solar cell production more scalable and less expensive,²⁶⁰ expanding the use of flexible solar cells in new areas such as architecture (smart buildings) and transport (electric vehicles).²⁶¹ These directions not only reflect ongoing technological advances but also align with global needs for more adaptable and environmentally responsible renewable energy solutions.

Within the context of Flex-SCs, several subtypes have rapidly evolved, with research highlighting significant advances and future possibilities for each technology.²⁶² Flexible-perovskite SCs have garnered attention due to their high conversion efficiency and low-cost manufacturing potential. Recent research has focused on improving the stability and durability of these solar cells under varying environmental conditions.²⁶³ The challenge is to develop robust encapsulations and defect passivation techniques that allow their application in external environments without significant degradation.^264,265 Advances in interface materials and low-temperature deposition methods could resolve these limitations, making perovskites a viable option for large-scale commercial applications.

Flexible-organic SCs are valued for their lightness and versatility in integrating with different substrates.²⁶⁶ Research has concentrated on increasing their operational efficiency and stability by utilizing new organic materials and processing methods that can extend their shelf life and effectiveness.^267,268 The exploration of new organic semiconductor materials and the optimization of multilayer cell structures could significantly expand the applicability of these technologies.^269–271

Flexible-polymer SCs offer advantages in terms of simple manufacturing and cost-effectiveness.²⁷² Recent advances include the development of polymers with greater charge mobility and stability, as well as efficient printing techniques that facilitate large-scale production. Improvements in polymer ink formulation and roll-to-roll printing techniques promise to revolutionize flexible solar energy production, making it more affordable and adaptable.²⁷³

Flexible dye-sensitized SCs are known for their ability to operate efficiently in low light conditions.²⁷⁴ Research is focused on developing more efficient and photostable dyes, as well as exploring new electrolytes and substrate materials that increase efficiency and durability.^275,276 Advances in the synthesis of new dyes and interface engineering could lead to a renaissance of dye-sensitized cells in commercial and residential applications.

Finally, flexible-thin-film SCs continue to be a popular choice for flexible applications due to their formability and decent efficiency. Research is being directed towards materials such as CIGS and CdTe to improve the conversion efficiency and robustness.²⁷⁷ Innovations in deposition methods and the development of more durable and less expensive substrates are crucial to expanding the use of flexible-thin films on a larger scale.

The outlook for flexible-SCs is promising, with each type moving toward overcoming technical and economic barriers. The expectation is that as these technologies mature, they will become fundamental to the broader implementation of renewable energy solutions in a variety of new environments and applications, driving the future into a more sustainable era.

From Table 13, it is evident that flexible substrates used in solar cells have made significant strides in terms of power conversion efficiency. Willow-glass substrates, known for their high transparency and thermal stability, hold promise for solar cell applications. However, the high cost and manufacturing complexity may limit widespread adoption.³⁰⁷ Common electrodes for Willow-glass include silver (Ag), copper (Cu), gold (Au), aluminum (Al), and platinum (Pt). The typical structure comprises layers of materials such as TiO₂, ZnO, PTB7, MoO₃, and perovskites like MAPbI₃.^308,309 The PCE for flexible solar cells with Willow-glass ranges from 4.53% to 19.72%, with advancements driven by improved active layers and electrodes.

Table 13 Type of photovoltaic (PV) cells and its substrates, electrodes, structure, and PCE

Substrates	Electrodes	Structure	PCE (%)	Ref.
Flexible glass	Ag	Willow-glass/TiO2/Ag/AZO/ZnO/PTB7:PC71BM/MoO3/Ag	6.6	278
	Cu	MgF2/Willow-glass/ITO/PTAA/MAPbI3/(C60/BCP)/Cu	19.72	179
	Au	Willow-glass/ITO/ZnO/CH3NH3PbI3/Spiro-OMeTAD/Au	12.1	279
	Au	Willow-glass/AZO/Au/AZO/c-TiO2/mp-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au	9.6	280
	Al	Willow-glass/IZO/SnO2/Cs0.04MA0.16FA0.80Pb1.04I2.6Br0.48/Spiro-OMeTAD/MoOx/Al	18.1	281
	Pt	Willow-glass/ITO/TiO2/N719:THF dye/Pt	4.53	282
	Al	Willow-glass/PEDOT:PSS/MoOx/BQR:PC71 BM/Ca/Al	8	283
	Ag	Willow-glass/ITO/PTB7:PC70BM/ZnO/MoO3/Ag	6.3	284
	Al	Willow-glass/ITO/PC70BM/MAPbI3/PEDOT:PSS/Al	11.9	284
	Ag	Willow-glass/AgNW/ZnO/PTB7-Th:PC70BM/MoO3/Ag	7.3	285

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Substrates	Electrodes	Metal foil	PCE (%)	Ref.
Copper (Cu)	Ag	Cu/CuI/CH3NH3PbI3/ZnO/Ag	12.8	286
Aluminum (Al)	AgNW	Al/MoO3/PTAA/PEI/MAPbI3/PCBM/PEI/AgNW	7.1	287
Titanium (Ti)	Gr/PDMS	Ti/TiO2/MAPbI3/PTAA/PDMS/Gr	15.0	288
	—	Ti/(DB-TiO2)/(TiO2-NW)/MAPbI3/PEDOT/ITO	13.1	289
	Ni-mesh	Ti/TiO2/Al2O3/perovskite/PEDOT:PSS/Spiro-OMeTAD/TCO/PET + Ni mesh	10.3
	—	Ti/TiO2 + CH3NH3PbI3/Spiro-MeOTAD/ITO	9.1	290
	(Ti-CNTs)	Ti/(TiO2-NT/MAPbI3)/(spiro-OMeTAD/CNTs)	8.3	291
		Ti/TiO2-flowers/N719-dye/PEDOT/ITO-PEN	6.3	292
	AgNP-TiO2	Ti-foil/AgNP-TiO2/N719dye-electrolyte/Pt/FTO-glass	4.7	293
Stainless steel (SS)	—	Steel/Ti/ITO/SnO2/PCBA/Cs0.05MA0.17FA0.83Pb 0.95I0.83Br0.17/TCTA/MoO3/ITO/MgF2	15.2	294
	—	Steel/Ti/ITO/SnO2/PCBA/Cs0.05MA0.17FA0.83Pb 0.95I0.83Br0.17/TCTA/MoO3/ITO/MgF2	14.9	294
	—	Steel/Ti/ITO/SnO2/PCBA/Cs0.05MA0.17FA0.83Pb 0.95I0.83Br0.17/TCTA/MoO3/ITO/MgF2	14.1	294
	—	Steel/Ti/ITO/SnO2/PCBA/Cs0.05MA0.17FA0.83Pb 0.95I0.83Br0.17/TCTA/MoO3/ITO/MgF2	13.8	294
	Ag	SS/Si3N4/(Cr/Au)/SnO2/MAPbI3/Spiro-OMeTAD/MoO3/Ag	14
	Au	SS/SiO2/Au/TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au	3.5	295

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Substrates	Electrodes	Polymeric	PCE (%)	Ref.
PET	Au	Au/(Spiro-OMeTAD)/MHP/SnO2/(ITO/PET)	18.1	296
	Ag	PET/ITO/HTL/PVK/PCBM/Ag	14.7	61
	Ag	PET/(PEDOT:PSS)/HTL/PVK/PCBM/Ag	19.0	61
	Cu	PET/(PEDOT:PSS)/MAPbI3/(C60/BCP)/Cu/Parylene	17.03	188
	Ag	PET/ITO/PEIE/PCE-10:IEICO-4F/MoO3/Ag	12.5	174
	Al	PET/MoOx/SWCNT/MoOx/PEDOT:PSS/PTB7:PC71BM/LiF/Al)	3.9	297
	Al	PET/(ITO/graphene)/PEDOT:PSS/PM6:Y6/PDINO/Al	14.5	298
PI	Cu	(SWNT/PI)/MoO3/PTAA/MAFAPbIBr/(C60/BCP)/Cu	15.2	299
	Al	PI/(ITO/graphene)/PEDOT:PSS/PM6:Y6/PDINO/Al	15.2	298
PEN	Au	PEN/ITO/BK-TiO2/(MAFACs)Pbl3/(Spiro-OMeTAD)/Au	13.1	300
Parylene	Ag	Parylene/Su8/ITO/ZnO/(PBDTTT-OFT:IEICO-4F:PC71BM)/MoO3/Ag	13	301
Cellulose paper (Ce-P)	Au	Wood-based Ce-P/ITO/TiO2/(FAPbI3)0.85(MAPbBr3)0.15/Spiro-OMeTAD/Au	16.8	302
	Au	Cellophane/OMO/CPTA/CH3NH3PbI3/Spiro-OMeTAD/Au	13	303
	(Cu + Au)	Paper/carbon/MAPbI3/C60/BCP/(Cu + Au)	9.1	304
	Ag	Paper/PVF/Ag/ZnO/PTB7:PCBM/PEDOT:PSS	6.4	305
	Al	Cellulose-paper/PEDOT:PSS/MAPbI3/PCBM/Al	4.3	306

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Metal foils, another type of durable substrate, offer excellent electrical conductivity and are suited for solar cells requiring high mechanical and thermal stability. Common electrodes used in metal foils include copper (Cu), aluminum (Al), titanium (Ti), and nickel mesh (Ni-mesh). The structures often integrate materials such as CuI, CH₃NH₃PbI₃, ZnO, PEDOT, and perovskites.³¹⁰ The conversion efficiencies for metal foil-based flexible solar cells vary from 3.5% to 15.2%. The combination of metal sheets with materials such as TiO₂ and perovskites has led to notable efficiency improvements, making them suitable for industrial and outdoor environments.³¹¹

Polymeric substrates, including PET, PI, PEN, parylene, and cellulose paper (Ce-P), are highly flexible and lightweight, ideal for portable and wearable devices.^312,313 Common electrodes for polymeric substrates include gold (Au), silver (Ag), copper (Cu), and aluminum (Al). These structures typically incorporate materials such as Spiro-OMeTAD, MAPbI₃, PEDOT, PCBM, PTB7, and perovskites. The power conversion efficiencies for flexible-solar cells using polymeric substrates range from 3.9% to 19.0%. Ongoing innovation in organic and hybrid materials has significantly enhanced both efficiency and flexibility, making these substrates well suited for applications ranging from wearable electronics to building-integrated photovoltaics (BIPVs). Continued advancements in materials and manufacturing processes are essential for realizing the full potential of these technologies. Such progress will drive the broader adoption of solar energy across diverse contexts, from portable gadgets to large-scale renewable energy installations.

Table 14 compares the merits and weakness of flexible substrates made from flexible-glass, metal-foil, and polymer, which have been utilized in flexible-SCs over recent decades. Flexible-glass substrates are known for their high cost, approximately $1000 per kg, due to the specialized manufacturing processes involved.³¹⁴ This high expense can limit large-scale production and increase the overall cost of technologies utilizing this type of substrate. Despite this, flexible-glass offers high transparency, which is advantageous for efficient flexible solar cells such as flexible-PeSCs, flexible-OSCs, flexible-DSSCs, and flexible-thin film SCs. However, flexible glass, while flexible when thin, has high flexural rigidity, which limits its use in some flexible applications. It also has a low coefficient of thermal expansion (CTE) of 3.2–4.0 μm mK⁻¹, ensuring dimensional stability, and exhibits excellent thermal and chemical stability, making it suitable for harsh environments.^315–318

Table 14 Merits and weaknesses of flexible substrates for application in flexible-SCs

Substrate	Merits	Weaknesses
Flexible glass	Low permeation of water vapor and oxygen (protects internal components)	High flexural rigidity, limiting flexibility
	High thermal stability (Tg > 600 °C)	Brittle when thin
	Good chemical resistance	High cost (∼$1000 per kg)
	High transparency	Moderate CTE (3.2–4.0 μm mK^−1), which may cause incompatibilities
	Low surface roughness	High density (2.23–2.40 g cm^−3), increasing weight
Metal foil	Variable cost ($1–26 per kg)	Capacitive effect may interfere with solar efficiency
	Low CTE (8–22 μm mK^−1 at 20 °C), contributing to dimensional stability	High surface roughness, making it difficult to deposit thin layers
	High chemical and thermal resistance	Opaque (not suitable for transparent solar cells)
		High density (2–8 g cm^−3), increasing weight
Polymeric	Variable cost (∼$1–110 per kg)	Low barrier against oxygen/moisture, which may compromise durabilityy
	High transparency (important for solar cells)	Low thermal stability (not suitable for high temperatures)
	Flexible and bendable	Low chemical resistance
	Lightweight (density ∼1–1.4 g cm^−3)	Dimensional instability, which may deform with variations in temperature and humidity

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Metal-foil substrates have a more varied cost ranging from $1 to $26 per kg, depending on the type of metal used. This moderate cost compared to flexible-glass makes metal-foil more feasible for large-scale production.^315–317 However, metal-foil is opaque, which is unsuitable for applications where transparency is crucial. It has limited flexibility and can be bent but is more rigid compared to polymers.^319,320 Metal-foils are thermally and chemically stable, with a moderate CTE of 8–22 μm mK⁻¹, making them durable in extreme environments.^321–324 They are also highly resistant to high temperatures and aggressive conditions. Despite their stability, metal-foils add significant weight due to their density (2–8 g cm⁻³).

Polymer substrates, including PET, PI, PEN, parylene, and cellulose paper, offer a much broader price range from $1 to $110 per kg. Lower-cost common polymers make these substrates suitable for large-scale production and low-cost technologies.^325–328 Polymeric substrates are highly transparent, which is essential for high energy conversion efficiency in flexible-OSCs. They also exhibit excellent flexibility, making them ideal for portable and foldable devices requiring rollability and flexibility.^325,329 Cellulose paper is a cheap (∼$1.5 per kg) and lightweight material, but it has limited thermal stability and low durability, especially under humid conditions. Its CTE varies widely due to the structure of the material. PET is an inexpensive option (∼$2 per kg), with moderate thermal stability (∼75 °C), good mechanical strength and high transparency. It has moderate UV resistance and good ductility, but can become brittle at very low temperatures. Its CTE is in the range of 60–70 μm mK⁻¹. PEN is more expensive (∼$8 per kg), but offers better thermal stability (∼120 °C) and higher mechanical strength compared to PET. It also has improved UV resistance and is biodegradable, with a moderate CTE of approximately 35–45 μm mK⁻¹. Parylene is an expensive polymer, but it has excellent properties such as high barrier against gases and moisture, UV resistance and high thermal stability (∼290 °C). Its CTE is relatively low (∼25 μm mK⁻¹) and its flexibility depends on the thickness used. Polyimide is the most expensive option (∼$110 per kg), but it stands out for its very high thermal stability (>380 °C), excellent mechanical and chemical resistance, and a low CTE (∼20–30 μm mK⁻¹). Its flexibility is preserved over a wide temperature range, making it an ideal choice for extreme applications. However, polymeric substrates have low thermal and chemical stability, degrading at high temperatures and in aggressive environments.³³⁰ They also have a higher CTE, which can lead to dimensional stability issues under temperature variations.^331–333 Additionally, polymeric substrates have high water vapor and oxygen permeability, which can compromise the durability of internal components unless protected by additional coatings.^334,335

Overall, flexible-glass and metal-foil substrates offer high stability and durability, with low permeability providing excellent protections for flexible solar cells. However, they come with higher sot and weight considerations. Polymeric substrates, while more cost-flexible, require additional measures to address their lower stability and higher permeability. The choice of substrate depends on the specific requirements of the application, balancing factors such as cost, flexibility, stability, and environmental protection.

The efficiency of Flex-SCs is crucial because it determines their ability to convert solar energy into electricity effectively.³³⁶ More efficient solar cells must be thinner, lighter, and more flexible, expanding their applications in areas ranging from portable devices to large surfaces without compromising aesthetics or functionality. Improving efficiency also means reducing the cost per watt of energy produced, making solar technology more competitive against other energy sources.^337,338

In recent decades, polymeric substrates such as cellulose paper (Ce-P),^339–341 polyethylene terephthalate (PET), polyethylene naphthalate (PEN), Para-xylylene (parylene), and polyimide (PI) have become increasingly preferred for flexible solar cells.^342–344 Among these, PET, PEN, and Ce-P are especially favored due to their lower cost and compatibility with various printing methods including inkjet printing, screen printing, slot-die coating, gravure printing, spin coating, spray coating, and blade coating, as shown in Fig. 17.

Fig. 17 Main polymer-based flexible substrates used in recent decades for flexible-SCs.

The detailed parameters of these substrates, shown in Fig. 17, significantly impact the efficiency, durability, and feasibility of large-scale production of flexible solar cells, influencing their application in both indoor and outdoor environments.³⁴⁵ For outdoor applications, flexible-PeSCs and flexible-OSCs are commonly integrated into items such as clothing, backpacks, and awnings to charge mobile devices.

Flexible-OSCs are used in solar windows, solar roofs on vehicles, and building facades (BIPVs) due to their high transparency and efficiency.

Indoors, flexible-DSSCs are typically utilized in portable electronic devices and sensors, such as solar-powered IoT sensors, because of their ability to function effectively under low-light conditions. Additionally, flexible-thin-film SCs are applied to consumer electronics such as electronic displays and small wearable devices, where flexibility and lightness are crucial.

Fig. 18 highlights the key aspects of flexible solar cells, including efficiency, flexibility, stability, large-scale production, and applications. The PCE of flexible solar cells has seen remarkable progress over the years. Starting at 3% in 2004,³⁴⁶ reaching 10.2% in 2014,³⁴⁷ 15.76% in 2017,³⁴⁸ 18.53% in 2019,³⁴⁹ 20.4% in 2020,³⁵⁰ 25% in 2022,¹³⁴ and 37% in 2023,³⁵¹ as shown Fig. 18A. This upward trend reflects ongoing technological advancements. High efficiency is critical for commercial viability, as it translates to greater energy generation per unit area, making solar cells more competitive with traditional energy sources. Achieving high efficiencies requires continuous research and development in materials and manufacturing processes, such as optimizing active layer deposition and electrode performance.

Fig. 18 Developmental aspects of flexible-SCs, covering (A) efficiency, (B) flexibility, (C) large-scale production, (D) stability, and (E) applications. The efficiency graph was created using the ref. 134, 143, 346–371.

Flexibility, as depicted in Fig. 18B, refers to the ability of solar cells to be bent, twisted, and roll without losing functionality. This property allows solar cells to be integrated into a variety of applications that rigid solar cells cannot accommodate, including curved surfaces, clothing, and wearable devices. Key to achieving flexibility are polymeric materials and substrates like flexible glass, which balance flexibility with efficiency and durability.

Fig. 18C outlines various large-scale production techniques including inkjet printing, screen printing, gravure printing, slot-die coating, blade coating, and spray coating. Efficient and cost-effective large-scale production is vital for commercialization. These techniques enable rapid and uniform production of flexible solar cells, with scalable methods like slot-die coating and gravure printing playing a crucial role in maintaining low production costs while increasing output volumes.

Stability, shown in Fig. 18D, involves a solar cell's resistance to environmental factors such as humidity, temperature, and UV radiation. Ensuring stability is crucial for the longevity of flexible solar cells, particularly in outdoor settings where they face harsh conditions. Employing high-quality encapsulants and materials with low permeability to water vapor and oxygen, such as flexible glass and surface-treated polymers, is essential for enhancing stability.

Fig. 18E illustrates the diverse applications of flexible solar cells, including wearable electronics, electric vehicles, building-integrated PVs, and greenhouses. Flexible solar cells offer significant versatility, enabling their use in portable and wearable devices to power mobile devices and IoT sensors. In BIPV applications, they can be integrated into windows and building facades, combining functionality and aesthetics. Successful integration into commercial products needs interdisciplinary collaboration across product design, materials engineering, and manufacturing technologies. Overall, flexible solar cells represent a unique combination of efficiency, flexibility, stability, and large-scale production viability, making them suitable for a wide range of applications. Ongoing research and development in materials, manufacturing processes, and encapsulation are essential to overcoming challenges and maximizing the potential of these technologies. The ability to integrate solar cells into various surfaces and products creates new opportunities for renewable energy generation in both indoor and outdoor environments. Manufacturing processes and encapsulation are essential to overcoming challenges and maximizing the potential of these technologies. The ability to integrate solar cells into various surfaces and products creates new opportunities for renewable energy generation in both indoor and outdoor environments.

4. Findings

This review analyzed a total of 3812 publications on flexible-SCs, with 66% of these articles published in the past four years (2019–2023). There has been a substantial increase in the number of publications since 2013, with a particularly notable surge in 2017. English is overwhelmingly the dominant language, accounting for 99.87% of all F-SC literature.

4.1. Geographical distribution

China leads in the number of publications, followed by South Korea and the United States. The geographic analysis highlights strong international collaborations, particularly between China, the United States and several European countries.

4.2. Influential authors and institutions

Key contributors in the field include researchers such as Frederik C. Christian Krebs, Kuo-Chuan Ho, and Ayodhya Nath Tiwari. Prominent institutions leading the research include the Chinese Academy of Sciences, Peking University, and several other major research centers in China.

4.3. Financing and support

The primary funding body for flexible-SC research is the National Natural Science Foundation of China (NSFC), followed by the National Research and Development Program of China (NKPs) and the National Research Foundation of Korea (NRF). A significant increase in funding from these organizations after 2010 reflects the growing global interest and investment in renewable energy technologies.

4.4. Research topics and trends

The analysis identified key research areas, including materials science, engineering, chemistry and energy. From 2014 to 2023, there has been a substantial increase in publications in these domains, driven by the global pursuit of sustainable energy solutions.

4.5. Types of substrates and efficiency

Flexible glass substrates are commonly used with electrodes such as Ag, Cu, Au and Al, and materials such as TiO₂, ZnO and perovskites, achieving efficiencies ranging from 4.53% to 19.72%. Metal-foil substrates utilize electrodes such as Cu, Al, and Ag, combined with materials such as ZnO and perovskites (e.g., CH₃NH₃PbI₃), resulting in efficiencies ranging from 3.5% to 15.2%. For polymeric substrates (PET, PI, PEN, parylene, and Ce-P), electrodes such as Au, Ag, and Al are used alongside materials including PEDOT, PCBM, and perovskites, with efficiencies varying from 3.9% to 19.0%.

4.6. Manufacturing techniques

Techniques for large-scale production of flexible solar cells include Inkjet Printing, Screen Printing, Blade Coating, Slot-die Coating, Gravure Printing, and Spray Coating. These methods facilitate efficient, high-volume production, which is essential for the commercialization of flexible solar cell technologies.

5. Conclusions

This review provided an extensive overview of the research landscape for flexible solar cells over the past 44 years. It highlights the rapid progress and shifting focus in this field, driven by advancements in materials and technology, as well as significant international collaboration and funding. The increasing efficiency and versatility of flexible solar cells underscore their potential to play a crucial role in the future of renewable energy. Continued research and development, supported by global cooperation and investment, are essential for addressing technical challenges and enhancing the practical applications of these cells across various environments, from wearable electronics to building-integrated PVs. Flexible solar cells offer unique advantages in terms of efficiency, flexibility, stability, and large-scale production viability, making them ideal for a wide range of applications. The integration into products such as wearable electronics, electric vehicles, and building-integrated photovoltaics demonstrates their potential to revolutionize the energy sector. Moving forward, interdisciplinary collaboration and sustained research efforts will be essential for overcoming existing challenges and fully realizing the potential of flexible solar technologies.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Thiago F. Santos, PhD student (conceptualization: lead; validation: lead; writing – original draft: lead). Bruno R. Carvalho (conceptualization: supporting; funding acquisition: equal; writing – review & editing: equal). Elisama V. Santos (conceptualization: supporting; funding acquisition: equal; investigation: equal; methodology: equal; validation: equal). Rui F. P. Pereira (investigation: equal; methodology: equal). Domingos F. S. Souza (investigation: equal; methodology: equal). Jose H. O. Nascimento (funding acquisition: lead; project administration: lead; resources: lead; validation: equal; writing – original draft: equal; writing – review & editing: lead).

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This research was supported by the CARREFOUR group and the Coordination for the Improvement of Higher Education Personnel – Brazil (CAPES) – Finance Code 001, as well as to the National Council for Scientific and Technological Development (CNPq) under projects number 400913/2023-2 and 404139/2022-1. Further, thanks are due to the Graduate Program in Chemical Engineering at the Federal University of Rio Grande do Norte (PPGEQ/UFRN) for providing technical assistance. B. R. C acknowledges the financial support from the Brazilian agencies CNPq, CAPES, and Brazilian Institute for Science and Technology of Carbon Nanomaterials (INCT-Nanocarbono). R. F. P. Pereira acknowledges FCT for CQ-UM base (https://doi.org/10.54499/UIDB/00686/2020) and programmatic (https://doi.org/10.54499/UIDP/00686/2020) projects and FCT-UMinho for the Assistant Research contract (https://doi.org/10.54499/2023.07994.CEECIND/CP2841/CT0013).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4se01537a

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