Semitransparent organic solar cells: from molecular design to structure–performance relationships

Abstract

Organic solar cells (OSCs) have drawn tremendous interest because of their potential for low-cost solution processing and color tunability. OSCs with bulk-heterojunction structures offer an attractive pathway to efficiently utilize solar energy and have the potential to make transparent and flexible devices. The design of new materials and optimized device fabrications with visible-light transparency make this technology attractive for various applications, including greenhouses, windows, building structures, etc. In this review, we highlight the developments of fullerene-free acceptors (FFAs) for semitransparent OSCs (ST-OSCs) and device architectures, including transparent electrodes, giving special emphasis to the power conversion efficiency (PCE) and the average visible transparency (AVT).

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Kanupriya Khandelwal

Kanupriya Khandelwal is a PhD Scholar in the Department of Physics at The LNM Institute of Information Technology, Jaipur, India, under the supervision of Prof. Ganesh D. Sharma and Dr Subhayan Biswas. Her main research interests are in the optoelectronic properties of organic materials for applications in photovoltaic devices. She is currently working on the semitransparent organic solar cells and its BIPV (Building Integrated Photovoltaics) application.

Subhayan Biswas

Dr Subhayan Biswas is an associate professor in the Department of Physics at The LNM Institute of Information Technology, Jaipur, India. After completing PhD from the Indian Association for the Cultivation of Science, Kolkata, India, he did post-doctoral research in Inha University (South Korea), National Sun Yat-Sen University & National Taiwan University (Taiwan) and Toyama University (Japan). His research field is nanostructured solar cells including dye-sensitized solar cells, quantum-dot solar cells and organic solar cells.

Amaresh Mishra

Dr Amaresh Mishra is an Associate Professor at the School of Chemistry, Sambalpur University, India. He received his PhD in 2000 from Sambalpur University. After a postdoctoral stay at the University of South Florida and University of Akron from 1999–2001, he joined TIFR, Mumbai, India as a Visiting Fellow. He then moved to University of Ulm, Germany in 2005, as Alexander von Humboldt Fellow and continued there to lead the organic solar cells research group till 2015. His current research focuses on the design and development of functional organic semiconductors and hybrid materials for photovoltaic applications.

Ganesh D. Sharma

Prof. Ganesh D. Sharma is working as Senior Professor of Physics, Electronics Communication and Engineering and Dean (Research and development) at The LNM Institute of Information Technology, Jaipur (Raj.), India since Dec 2014. Prof. Sharma obtained his PhD degree from India Institute of Technology in 1985 and after than he had joined as Assistant Professor at JNV University, Jodhpur (Raj), India and subsequently as Professor. His area of Research is Organic solar cells, nanocrystalline organic–inorganic hybrid solar cells and fuel cells, organic nanomaterials for energy conversion and published more than 320 research papers. He had research collaborations with different International and National research organization.

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1. Introduction

Globally, energy is one of the crucial parameters for sustainability and socio-economic growth. At present, a significant part of the energy requirements is fulfilled by using non-renewable fossil fuels which cause irreparable damage to the environment. It is clear that the structure of energy demand is likely to change over time with the decline in use of fossil fuels and the increasing demand for renewable energy. So, exploiting clean and green renewable energy resources is necessary to fulfill the global energy demand. In this respect, solar energy as an alternative to fossil fuels has been considered as a renewable energy source to meet the energy needs of society.^1,2 Photovoltaics that can convert solar energy directly into electricity hold promise for cost-effective solar cells prepared via simple solution-processing. Among the photovoltaic technologies, organic solar cells (OSCs) have emerged as a convincing technology because of their numerous advantages, such as new molecular design, tunable device structure, fabrication on flexible substrates, and transparency. Building-integrated photovoltaic (BIPV) devices are architecturally integrated on the rooftops/facades of commercial and residential buildings as an added function to produce electricity.^3–7 Thus, the BIPV system simultaneously serves as a building envelope material to improve the aesthetic appearance due to color transparency and as a source for power generation.

Organic materials with a high absorption coefficient (∼10⁵ cm⁻¹) and a tunable bandgap have tremendous potential to be used in semitransparent solar cells (ST-OSCs) due to their variable absorption from the visible to the near-IR regions, leaving some transparent windows in the visible region.^8,9 They have the advantages of flexibility, being solution-processable and suitable for large-scale roll-to-roll fabrication.^10–13 ST-OSCs have become the stronger contender for BIPV devices and are believed to be an effective solution for balancing energy generation with visual eye comfort and a pleasant appearance.^14–18 The transparency in ST-OSCs can be maintained by reducing the thickness of the organic photoactive layers, by choosing appropriate transparent bottom or top electrodes and matching energetics, and by maintaining a high power conversion efficiency (PCE) and an average visible transmittance (AVT).¹⁹ In the past few years, the PCE of single-junction OSCs has increased significantly, up to 18%, using either binary or ternary blends (Fig. 1a).^20–24 The only difference between the opaque OSCs and ST-OSCs is the device transparency in the visible region. There is an inherent trade-off between the PCE and AVT, so a balance between these two crucial factors is necessary (Fig. 1b). The AVT of ST-OSCs using fullerene acceptors and wide bandgap donors is quite low due to the relatively broad photon harvesting in the visible-light range. Due to the diversity in organic light-absorbing materials and specifically after the development of fullerene-free acceptors (FFAs) that absorb in the visible to NIR regions, ST-OSCs can be engineered in various colors.^16,25 The NIR-absorbing FFAs play a key role to enhance the transmittance of ST-OSCs with increasing PCE. Because of their aesthetic advantages, flexibility, and low cost, ST-OSCs have become a strong contender for BIPV applications. ST-OSCs have been intensively studied in recent years, and the PCE has increased from ∼2% to 13% with outstanding AVT values for windows applications.^15,18 These improvements are due to the rapid development in high-performance photoactive materials, the judicious choice of transparent bottom and top electrodes, and to the low-temperature-processable interface layer.

Fig. 1 (a) Progress in organic solar cell efficiencies over the years. (b) Comparative plot of PCEs versus AVTs as discussed in this article and presented in Tables 1–3.

Dye-sensitized solar cells (DSSCs) are another photovoltaic system for BIPV applications and offer high transparency and AVT (>80%) at micron-level thicknesses (10–20 nm TiO₂ NPs).^26–30 However, due to the liquid nature of the materials utilized in DSSCs, poor charge transmission and leakage problems pose a hurdle to their practical use. Singh and co-workers reported a semitransparent DSSC using a NIR-absorbing squaraine-based sensitizer.³¹ Recently, Naim et al. reported a colorless transparent DSSC using a polymethine cyanine dye that achieved a PCE of 3.1% and an AVT close to 76%.³² Mini modules with an active area of 14 cm² based on organic dyes have been reported with a PCE of 8.7% and an AVT of 26%.³³ In a very short period the field of perovskite solar cells (PSCs) based on organic–inorganic hybrid metal halides has emerged as a promising source of energy-harvesting materials, which are considered to be a suitable alternative to DSSCs, with PCE values reaching over 22%.^34–36 The absorption, bandgap and thickness of perovskite films can be easily tuned to achieve visible transparency for ST-PSCs. The major difference between a ST-PSC and a ST-OSC is the photoactive layer.^37–39 The perovskite materials generally absorb strongly in the visible region, therefore, the thickness must be reduced to achieve semitransparency. Using a thin film of CH₃NH₃PbI₃ perovskite, Bolink and co-workers achieved a PCE of 6% with an AVT close to 29%.⁴⁰ Li and co-workers realized a ST-PSC by increasing the bromine content in the CH₃PbI_3−xBr_x perovskite to tune the bandgap and achieved a transparency of 20%.⁴¹ Snaith's group reported microporous layers of neutral-color CH₃NH₃PbI₃ perovskite films using a partial dewetting method, achieving PCEs of 8–4% and AVTs of 7–30%.⁴² Color-tunable ST-PSCs were also reported by tuning the optical methods, i.e., using a metal/dielectric/metal electrode without disturbing the perovskite film.^43,44 Chen et al. prepared a macroporous CH₃NH₃PbI₃ perovskite film by modifying the polystyrene microsphere template, reaching a high AVT of 36.5% with an impressive PCE of 11.7%.⁴⁵

In this review, we present a brief description of the current status in molecular design for ST-OSCs and deduce the structure–performance relationships for a BIPV or power-generating window and greenhouse applications. We further discuss the challenges and future research direction for ST-OSCs, providing ideas for new technologies.

2. Basic characterizations of semitransparent OSCs

In our daily life, there has always been a significant impact of the solar light that passes through the transparent windows of buildings and houses on the visual comfort of the human eye. Semitransparent OSCs are considered to be an effective approach to balancing the visual comfort of the eyes whilst simultaneously generating energy in glass buildings and windows. ST-OSCs also offer an aesthetic color selection that enables natural colors to be seen through windows. For optimizing ST-OSCs three critical parameters, such as the AVT, color perception transparency, and color rendering index (CRI), should be considered along with the PCE. The PCE is determined from PCE = (J_SC × V_OC × FF)/P_in, where J_SC is the short-circuit current density, V_OC is the open-circuit voltage, FF is the fill factor and P_in is the incident light power. Transparency is a very important factor in ST-OSCs and is usually assessed by calculating the AVT in the visible region from 400–780 nm. Although the AVT requirement is based on real applications, it is considered that an AVT of 25% is a minimum requirement for window applications.⁴⁶ While measuring the transmittance, the light beam should be focused within the device's active area.⁴⁷ The AVT can be calculated using the equation , where T(λ) is the transmittance spectrum, P(λ) is the human eye photopic response, and S(λ) is the solar photon flux (AM 1.5) for window applications.

Color perception is a parameter determined via the International Lighting Commission (CIE) 1931 xy chromaticity to estimate the transparency. AM1.5G and D65 are the commonly used reference light sources for estimating the color properties of ST-OSCs with the color coordinates of (0.3202,0.3324) and (0.3128,0.3290), respectively (Fig. 2). For window applications, the color coordinates of ST-OSCs are preferably close to the white point (0.333,0.333). Ameri et al. first introduced the concept of the chromaticity diagram for analyzing the color properties of ST-OSCs.⁴⁸ The color rendering index (CRI) is another parameter that measures how well a light source can expose an object's color when compared with a reference or natural source of light. The human eye has an amazing ability to adapt to color variations induced by different lighting situations. The CRI in ST-OSCs represents the degree of color rendering variance between the incident and the transmitted light, and a higher CRI means a better color-rendering ability. For power-generating window applications, a CRI value close to 100 has been approved.⁴⁹ The use of a dielectric/metal/dielectric (DMD) as the top electrode in ST-OSCs has been found to reach a CRI value close to 100 without sacrificing the performance.⁵⁰

Fig. 2 (a) Schematic diagram of the semitransparent organic solar cell, (b) solar emission spectrum and the energy fraction in the UV, visible and infrared regions, (c) corresponding color coordinates of semitransparent binary and ternary devices under AM 1.5G illumination on the Commission Internationale de l’Eclairage (CIE) 1931 chromaticity diagram, and (d) photopic spectral response of the human eye.¹⁸ Copyright 2020, Elsevier Ltd.

The objective parameter quantum utilization efficiency (QUE), defined as the sum of the external quantum efficiency (EQE) and the transmittance (QUE = EQE + T), is used to evaluate the photon utilization efficiency in ST-OSCs. Due to intrinsic losses of photons and charge carriers in ST-OSCs the QUE value should be <90% over the visible region. The light utilization efficiency (LUE) is defined as the product of the PCE and the AVT (LUE = PCE × AVT).⁶ The LUE is used to compare the performance metrics between technologies and theoretical limits of different ST-OSCs.^6,51 The trade-off between the AVT and the PCE needs to be optimized to improve the overall LUE of ST-OSCs.

3. A brief description of semitransparent OSCs

The key requirements for ST-OSCs typically include a blend layer of photoactive p-type donor and n-type electron acceptor materials, sandwiched between a front transparent electrode and a transparent reflective counter electrode. The anode and cathode interfacial buffer layers are placed between the active layer and electrodes to enhance the charge collection and charge-transport properties.^15,16 Generally, ST-OSCs differ from common OSCs only with respect to the bottom transparent electrode. The prerequisites for bottom transparent electrodes are their (i) low cost, (ii) high transparency in the visible-light range and high reflectance in the near-infrared light range, and (iii) low sheet resistance/high conductivity. For a flexible ST-OSC the electrode should be highly resistive to cracks and bending.

Organic semiconducting materials have a high molar absorption coefficient and a lower carrier mobility, so the thickness of the active layer is usually kept below 200 nm, which is sufficient for strong light absorption to bring the optical semitransparency.^3,9,16,18,25 The absorption spectra of the organic materials can also be tuned from the visible to the NIR region via bandgap engineering to obtain color-tunable ST-OSCs. In the following sections we will discuss the current developments in ST-OSCs with respect to the interfacial carrier-transport layers, photoactive materials and top and bottom electrodes. The solar cell parameters are presented in Table 1.

Table 1 Photovoltaic parameters of binary BHJ ST-OSCs with different transparent electrode materials

Photoactive layer	Cathode	Anode	V OC [V]	J SC [mA cm^−2]	FF	PCE [%]	AVT [%]	Ref.
Conductive metal oxide
P3HT:PC61BM	ITO/TiO2	MoO3/ITO	0.53	6.3	0.57	1.9	80 (>650 nm)	83
PCDTBT:PC71BM	TiOx/Al/ZAO	ITO/PEDOT:PSS	0.84	9.2	0.48	3.9	34	87
P3HT:PC61BM	ZAO	ITO/PEDOT:PSS	0.45	8.24	0.28	1.0	77 @ 700 nm	86
P3HT:PC61BM	ZAO(400 nm)/LiF/Al	ITO/PEDOT:PSS	0.59	9.35	0.61	3.4	60.7 @ 700 nm	86
P3HT:PC61BM	ITO/nc-TiO2	V2O5/Ag/V2O5	0.58	4.83	0.61	1.7	—	92
PBDB-T:ITIC	ITO/ZnO	MoO3/Ag/WO3	0.88	13.8	0.60	7.4	25.2	94
Conductive PEDOT:PSS polymer
P3HT:PC61BM	ITO/ZnO/PEIE	PEDOT:PSS	0.57	6.91	0.51	2.04	51.2	107
PTB7:PC71BM	ITO/PEIE	PEDOT:PSS–PDMS	0.72	11.97	0.44	3.75	35	109
PTB7-Th:IDTBR	ITO/ZnO	PEDOT:PSS/IL	0.98	11.35	0.57	6.32	35.4	110
P3HT:IDT-2BR	PES/PH1000/PEIE	PEDOT:PSS PH1000-T	0.84	5.93	0.58	2.88	50	112
PBDB-T-2F:Y6	PDINO/Al	PET/D-PEDOT:PSS/PEDOT:PSS (4083)	0.80	19.28	0.68	10.53	21	111
P3TI:PC71BM	ITO/Rinsed PFPA-1	PEDOT:PSS PH1000	0.69	8.08	0.60	3.35	∼50.0	105
TQ1:PC71BM	ITO/Rinsed PFPA-1	PEDOT:PSS PH1000	0.77	5.46	0.62	2.61	∼50.0	105
Metal nanowires
P3HT:PC61BM	ITO/AZO	PEDOT:PSS/Ag NWs	0.56	5.83	0.65	2.13	—	114
PBDTT-FDPP-C12:PC61BM	TiO2/Ag NW	ITO/PEDOT:PSS	0.76	9.1	0.61	4.3	62.0	121
PBDTT-SeDPP:PC61BM	TiO2/Ag NW	ITO/PEDOT:PSS	0.73	10.9	0.58	4.6	63.0	121
P3HT:Si-PCPDTBT:PC61BM	Ag NWs/AZO	PEDOT:PSS/Ag NWs	0.57	6.4	0.60	2.2	33	122
P3HT:PC61BM	ZnO/TiOx/Ag NWs	PEDOT:PSS/GO/PEDOT:PSS	0.58	8.2	0.49	2.3	55 @ 650 nm	124
PBDTT-DPP:PC61BM	TiO2/Ag NWs	ITO/PEDOT:PSS	0.77	9.3	0.56	4.0	66 @ 550 nm	125
PBTZT-stat-BDTT-8:PC61BM	PET:PEDOT:PSS:AgNW/ZnO	PEDOT:PSS:AgNW	0.73	8.9	0.58	3.8	25	129
P3HT:PC61BM derivative	Polyacrylate/Cu NWs/PEDOT:PSS (PH1000)/Y-TiO2	PEDOT:PSS(4083)/CuNWs/polyimide/PDMS	0.62	7.08	0.45	1.97	42	130
PBDB-TF:Y7	ANZnO/Al (15 nm).	ITO/PEDOT:PSS	0.83	23.6	0.70	13.76	19.09	132
Ultrathin metal
PBDTTT-C-T:PC71BM	ITO/ZnO/C60-SAM	MoO3/Ag	0.76	13.01	0.63	6.22	21.3	46
PTB7-Th:IEICO-4F	PNDIT-F3N-Br/Au (1 nm)/Ag (12 nm)	ITO/PEDOT:PSS	0.67	18.9	0.62	8.1	21.1	137
PTB7-Th/IEICO-4F	PNDIT-F3N-Br/Au (1 nm)/Ag (12 nm)	ITO/PEDOT:PSS	0.67	19.6	0.65	8.5	21.5	137
PTB7-Th:PC71BM	PF3N-2TNDI(ETL)/Ag	ITO/PEDOT:PSS	0.76	11.62	0.68	6.05	30.35	142
PTB7-Th:PC71BM	PF3N-2TNDI(ETL)/Ag	ITO/PEDOT:PSS	0.77	12.66	0.70	6.78	20.71	142
P3HT:PC71BM	LiF/Al/Ag (20 nm)	ITO/PEDOT:PSS	0.61	8.4	0.54	2.7	11	143
PCDTBT:PC71BM	LiF/Al/Ag (20 nm)	ITO/PEDOT:PSS	0.90	6.6	0.51	3.0	16	143
PBDTTT-CT:PC71BM	LiF/Al/Ag (20 nm)	ITO/PEDOT:PSS	0.82	13.8	0.46	5.2	14	143
PBDTTT-EFT:PC71BM	LiF/Al/Ag (20 nm)	ITO/PEDOT:PSS	0.84	11.0	0.61	5.6	10	143
PBDB-T-2F:PC71BM	ITO/PFN	MoO3/Au	0.96	9.4	0.63	5.7	67.0	144
PBDB-T-2F:Y6	ITO/SnO2	MoO3/Ag (15 nm)	0.83	21.56	0.72	12.88	25.6	76
PBDB-T-2F:IT-4F	ITO/SnO2	MoO3/Ag (15 nm)	0.79	19.74	0.72	11.2	26.2	76
PBDB-TF:Y6	ITO/Al(acac)3	MoO3/Ag	0.82	20.73	0.73	12.41	25.33	77
PBDB-TF:Y6	ITO/ZnO	MoO3/Ag	0.82	19.80	0.74	12.01	23.65	77
PCPDTFBT:PC71BM	ITO/ZnO	PEDOT:PSS/Ag	0.74	11.4	0.59	5.0	47.0	145
PTB7-Th:PC71BM	ITO/ZnO	MoO3/Ag	0.81	14.59	0.72	8.52	26.2	146
PBDTTT-E-T:IEICO	PFN-2TNDI-Br/Ag (14 nm)	ITO/PEDOT:PSS	0.81	14.4	0.66	7.9	23.8	147
PTB7-Th:ITVfIC	PDINO/Ag (15 nm)	ITO/PEDOT:PSS	0.74	17.54	0.63	8.21	26.4	148
PBDB-TF:ID-4Cl	PDINO/Au (15 nm)	ITO/PEDOT:PSS	0.75	13.77	0.68	6.99	43.7	149
PBN-S:IT-4F	ZnO/Au (1 nm)/Ag (20 nm)	ITO/PEDOT:PSS	0.88	15.48	0.65	9.83	32.0	150
PTB7-Th:IEICO-4F	PDIN/Au (1 nm)/Ag (11 nm)	ITO/PEDOT:PSS	0.71	18.81	0.68	9.06	27.1	151
D18:N3	PDIN/Au (1 nm)/Ag (10 nm)	ITO/PEDOT:PSS	0.83	20.9	0.74	12.91	22.49	152
PTB7-Th:IEICO-4F	PET/Ag mesh/PH 1000/ZnO	MoO3/Au (1 nm)/Ag (15 nm)	0.71	20.27	0.67	10.03	34.2	153
PTB7-Th:FOIC	ITO/ZnO	MoO3/Au (1 nm)/Ag (15 nm)	0.74	20.0	0.70	10.3	37.4	154
PTB7-Th:FOIC	ITO/PFN	MoO3/Ag (15 nm)/WO3/Pt	0.74	20.3	0.67	10.23	25.4	155
PBDB-TF:Y6	ITO/PEDOT:PSS	PDIN/Au (1 nm)/Ag (10 nm)	0.85	20.35	0.71	12.37	18.6	156
PTB7-Th:ATT-2	ZrAcac/Ag	ITO/PEDOT:PSS	0.71	18.53	0.59	7.74	37.0	157
PTB7-Th:IEICS-4F	ITO/ZnO/PFN	MoO3/Au (1 nm)/Ag (20 nm)	0.74	22.5	0.66	11.1	28.6	158
PTB7-Th:BT-CIC	ITO/ZnO	MoO3/Ag (20 nm)	0.68	18.0	0.68	8.2	26.0	159
PTB7-Th:BT-CIC	ITO/ZnO	MoO3/Ag (15 nm)	0.68	17.0	0.67	7.7	33.0	159
J52:IEICO-4Cl	PFN-Br/Au(15 nm)	ITO/PEDOT:PSS	0.73	19.6	0.59	6.37	35.1	160
PBDB-T:IEICO-4Cl	PFN-Br/Au (15 nm)	ITO/PEDOT:PSS	0.73	19.6	0.59	6.24	35.7	160
PTB7-Th:IEICO-4Cl	PFN-Br/Au (15 nm)	ITO/PEDOT:PSS	0.73	19.6	0.59	6.97	33.5	160
PTB7-Th:IEICO-4Cl	PFN-Br/Au (30 nm)	ITO/PEDOT:PSS	0.73	19.6	0.59	8.38	25.6	160
PTB7-Th:IHIC	ITO/ZnO	MoO3/Au (1 nm)/Ag (15 nm)	0.75	19.01	0.68	9.77	36.0	161
PTB7-Th:FNIC2	ITO/ZnO	MoO3/Ag (20 nm)	0.73	21.87	0.73	11.6	13.6	162
PTB7 -Th:FNIC2	ITO/ZnO	MoO3/Ag (14 nm)	0.72	19.1	0.69	9.51	20.3	162
PTB7-Th:IUIC	ITO/ZnO	MoO3/Au (1 nm)/Ag (15 nm)	0.79	18.31	70.3	10.2	31.0	163
PTB7-Th:IT-4F	ITO/ZnO	MoO3/Au (1 nm)/Ag (15 nm)	0.68	13.2	0.72	6.42	28.0	163
PFBDB-T:C8-ITIC	ITO/ZnO	MoO3/Ag/MoO3 (10/15/40 nm)	0.92	15.5	0.70	9.8	22.0	164
PBDB-T:Y14	ITO/SnO2	MoO3/Ag (15 nm)	0.79	22.5	0.72	12.67	23.7	165
PTB7-Th:BDTThIT-4F	PDIN/Au (1 nm)/Ag (10 nm)	ITO/PEDOT:PSS	0.73	15.13	0.68	7.53	24.2	190
PTB7-Th:IEICO-4F	PDIN/Au (1 nm)/Ag (10 nm)	ITO/PEDOT:PSS	0.72	19.17	0.60	8.35	25.3	190
Carbon nanotube (CNT)
/P3HT:PC61BM	ITO/ZnO	CNT	0.57	10.5	0.40	2.5	80 @ 670 nm	168
C60/ZnPc:C60	ITO/n-C60	p-BF-DPB/CNT	0.57	4.5	0.58	1.5	—	169
PTB7:PC61BM	ITO/ZnO	CNT without dopant	0.66	6.5	0.40	1.8	80 @ 400 nm	170
PTB7:PC61BM	ITO/ZnO	MoOx-Doped CNT	0.62	8.8	0.56	3.1		170
PTB7:PC61BM	ITO/ZnO	HNO3-Doped CNT	0.69	9.5	0.56	3.7		170
Graphene
P3HT:PC61BM	ITO/ZnO	GO/graphene (10 layers)	0.54	10.5	0.44	2.5	70 @ 650 nm	175
P3HT:PC61BM	ITO/ZnO	PEDOT:PSS/Au-doped graphene	0.59	10.58	0.43	2.7	—	176
PTB7:PC71BM	ZnO-NP/PEDOT:PSS/graphene	Graphene/PEDOT:PSS	0.67	12.1	0.41	3.4	40	177

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Table 2 Photovoltaic parameters of ST-OSCs based on the ternary blend and tandem structure reported so far

Photoactive layer	Cathode	Anode	V OC [V]	J SC [mA cm^−2]	FF	PCE [%]	AVT [%]	Ref.
Ternary devices
PTB7-Th:PBT1-S:PC71BM/	ZrAcac/Ag	ITO/PEDOT:PSS	0.83	15.6	0.71	9.2	20.0	188
PTB7-Th1:COi8DFIC:IEICO-4F	ITO/ZnO	MoO3/Ag (10 nm)	0.71	14.94	0.64	6.78	24.18	189
PTB7-Th1:COi8DFIC:IEICO-4F	ITO/ZnO	MoO3/Ag (15 nm)	0.71	17.35	0.67	8.23	20.78	189
PTB7-Th1:COi8DFIC:IEICO-4F	ITO/ZnO	MoO3/Ag (20 nm)	0.71	19.52	0.67	9.37	17.23	189
PTB7-Th:BDTThIT-4F:IEICO-4F	PDIN/Au/Ag	ITO/PEDOT:PSS	0.73	19.21	0.67	9.40	24.6	190
D18-Cl:Y6-1O:Y6	PDIN/Au/Ag	ITO/PEDOT:PSS	0.88	19.56	0.75	13.02	20.2	191
J52:IEICO-4F:PC71BM	PFN-Br/Ag	ITO/PEDOT:PSS	0.69	19.04	0.67	8.83	15.8	192
PCDTBT:PC71BM:ITIC	ITO/ZnO	MoO3/Ag/MoO3	0.89	8.65	0.52	4.02	39	193
PBDB-TF:Y6:DTNIF	PDIN/Ag	ITO/PEDOT:PSS	0.86	23.61	0.72	14.50	19.78	195
PBDB-TF:Y6:DTNIF	PDIN/Ag	ITO/PEDOT:PSS	0.85	22.71	0.70	13.49	22.58	195
PBDB-TF:Y6:DTNIF	PDIN/Ag	ITO/PEDOT:PSS	0.84	20.56	69.93	12.14	25.89	195
Tandem devices
TQ1:PC71BM (front)	ITO/Rinsed PFPA-1	PEDOT:PSS PH1000	1.46	6.03	0.64	5.63	∼25.0	105
P3TI:PC71BM (rear)
PIDT-phanQ:PC61BM (front)	ITO/ZnO/C60-SAM	(ICL: m-PEDOT:PSS/PEDOT:PSS PH1000/ZnO) MoO3/Ag	1.63	5.23	0.69	7.4	40.0	197
PIDT-phanQ:PC71BM (rear)
P3TEA:FTTB-PDI4 (front)	ITO/ZnO	(ICL: PEDOT:PSS/ZnO) MoO3/Au	1.73	9.62	0.63	10.5	20.0	198
PTB7-Th:IEICS-4F (rear)
PBDTT-FDPP-C12:PC61BM (front)	TiO2/Ag NW	(ICL: PFN/TiO2/PEDOT:PSS) ITO/PEDOT:PSS	1.47	8.4	0.59	7.3	30.0	121
PBDTT-SeDPP:PC71BM (Rear)
PSEHTT:IC60BA (front)	TiO2/Ag NWs	(ICL: ZnO/PEDOT:PSS) graphene mesh/PEDOT:PSS	1.62	7.62	0.64	8.02	44.9	199
PBDTT-DPP:PC71BM (rear)

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Table 3 Photovoltaic parameters of ST-OSCs based on photonic crystals reported so far

Photoactive layer	Cathode	Anode	V OC [V]	J SC [mA cm^−2]	FF	PCE [%]	AVT [%]	Ref.
PCDTBT:PC71BM	ITO/TiO2	WO3/Ag/1DPC	0.87	9.67	0.63	5.31	25.1	50
PTB7:PC71BM	BCP/Ag/LiF/[MoO3/LiF]	ITO/PEDOT:PSS	0.73	10.9	0.70	5.6	30.0	201
P3HT:PC61BM	ITO/ZnO	MoOx/(Au/Ag)/DM-6	0.56	10.4	0.55	3.2	19.0	202
PTB7:PC71BM	ITO/ZnO	MoOx/(Au/Ag)/DM-6	0.73	12.8	0.68	6.4	16.9	202
PTB7-Th:PC71BM	ITO/ZnO	MoOx/(Au/Ag)/DM-6	0.79	13.1	0.68	7.0	12.2	202
PTB7-Th:PC71BM	1DPC/Au/ZnO	MoOx/Ag/MoOx/LiF	0.73	10.7	0.68	5.3	21.4	203
PTB7-Th:IEICO-4F	PFN-Br/Ag/1DPC	ITO/PEDOT:PSS	0.72	21.9	0.69	10.83	29.5	204
P3HT:ICBA	ITO/PEI	WO3/Ag/[WO3/LiF]^pairs	0.87	8.79	0.67	5.12	24.4	205
PTB7-Th:PC71BM	ITO/PFN	MoO3/Ag/WO3(x)/[WO3/LiF]^5	0.79	17.15	0.69	9.36	14.31	206
J71:IHIC	PDINO/Au/Ag/3-DMs	ITO/PEDOT:PSS	0.81	18.54	0.57	8.48	21.5	207
PTB7-Th:IHIC	PDINO/Au/Ag/3-DMs	ITO/PEDOT:PSS	0.75	17.7	0.65	8.48	20.5	207
J71:PTB7-Th:IHIC	PDINO/Au/Ag/3-DMs	ITO/PEDOT:PSS	0.76	21.08	0.58	9.37	21.4	207

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3.1. Carrier transport layers

Carrier-transport layers (CTLs), which are placed between the transparent electrode and photoactive layer, play an essential role in the performance of ST-OSCs. A proper selection of CTLs leads to an enhanced PCE and the long-term stability of the device. In OSCs the hole-transport layer (HTL) can be separated into two categories: inorganic HTLs and organic HTLs.⁵² Although OSCs with inorganic HTLs can achieve a satisfactory PCE, the processing technique of high-temperature vacuum evaporation makes them unsuitable for large-scale commercial roll-to-roll production. Inorganic p-type transition metal oxides or metal sulphides, such as MoO₃, V₂O₅,⁵³ NiO,⁵⁴ WO₃,⁵⁵ and MoS₂,⁵⁶ are the most commonly used HTLs in OSCs. By contrast, organic HTLs show numerous advantages, including low cost, large area roll-to-roll processibility, non-toxicity, and a wide range of fabrication flexibility. Organic HTLs include conductive polymers such as PEDOT:PSS,⁵⁷ conductive polyelectrolytes,⁵⁸ and graphene oxide.⁵⁹ In inverted OSCs, ZnO,⁶⁰ AZO (aluminum-doped ZnO), ZTO (zinc tin oxide),^61,62 TiO₂,⁶³ and Cs₂CO₃⁶⁴ have been used as electron-transport layers (ETLs). The lifetime and stability of ST-OSCs have been significantly improved using a ZnMgO-modified cathode layer and thin MoO₃/Ag as the anode.⁶⁵ Cao and co-workers used ZnO/C₆₀-SAM as the ETL and the MoO₃/Au/Ag transparent anode to improve the AVT and LUE.⁶⁶

Generally, in opaque OSCs, the CTL is only used for the charge-transport process; however, besides charge transport the optical properties of CTLs are considered crucial in ST-OSCs. In ST-OSCs, ZnO and PEDOT:PSS are widely used as CTLs because of their excellent charge-carrier properties.^67–69 However, their optical properties are not sufficiently ideal for ST-OSCs.^48,70 About 30–40-nm-thick CTL films are deposited to obtain pinhole-free films, which eventually reduced their transmittance. The ZnO ETL has an AVT of around only 80–85%, which limits the transmittance of ST-OSCs. Recently, Kadam et al. introduced polyethyleneimine ethoxylated (PEIE)-doped ZnO as the ETL in flexible OSCs. The doping of PEIE causes a reduction of the work function of ZnO and passivates the interface, which results in an improved OSC performance.⁷¹ Although the PEDOT:PSS layer has a relatively high transmittance, it has a low environmental stability and shows low CRI values due to its intrinsic absorption. Further modification has been done to improve the stability and adhesive properties of PEDOT:PSS.⁷² Ozcan et al. found that the GO-doped PEDOT:PSS HTL enhances the PCE of the device by about 2.2%.⁷³ Recently, Han et al. reported a hybrid HTL (h-HTL) incorporating WO₃ and MoO₃ in an NFA-based ST-OSC.⁷⁴ The energy levels of the h-HTL can be easily controlled due to the high bond energies and electronegativities of the W and Mo atoms, resulting in strong conductivity and good film-forming capabilities. The ST-OSC module with an active area of 30 cm² fabricated via printing process using the h-HTL exhibited a PCE of 7.2% and an AVT of 22.3%. More recently, Chaudhary et al. used TiO₂ as the ETL in place of ZnO and suggested that the TiO₂ ETL possess better operational stability for BIPV devices.⁷⁵

Bai et al. employed chemically precipitated SnO₂ colloidal particles as the ETL in ST-OSCs.⁷⁶ They found that SnO₂ CTLs exhibited excellent transparency with a high reflective index. The deep blue colored ST-OSCs fabricated using a blend of PBDB-TF:Y6 bulk heterojunction (BHJ) layer and SnO₂ as the ETL and MoO₃ as the HTL achieved a high PCE of 12.88%, an AVT of 25.6%, and a CRI value of 97.6. The PBDB-TF:ITIC-4F-based ST-OSC achieved a PCE of 11.2%, an AVT of 26.22% and a CRI value of 97.5. The same group further achieved a PCE of 12.41%, an AVT of 25.33%, and a CRI value of 94.6 using aluminum(III) acetylacetonate (Al(acac)₃) as the CTL, in an ITO/Al(acac)₃/PBDB-TF:Y6/MoO₃/Ag device.⁷⁷ Al(acac)₃ showed an excellent transmittance up to 95% in the visible and near-infrared regions with a film thickness of 10 nm. By comparison, the AVT of the ST-OSC with a ZnO interlayer was 23.65% and the PCE was 12.01%. These encouraging findings suggest that the proper choice of CTL layers will significantly improve the efficiency of ST-OSCs. Some modification in CTLs is still needed to improve their optical properties for commercial applications. The utilization of CTLs (both HTLs and ETLs) in ST-OSCs is discussed in the following section along with the photoactive and electrode materials.

3.2. Photoactive layer engineering for ST-OSCs

The active layer, which consists of donor and acceptor units, plays a crucial role in ST-OSCs. These materials can be small molecules, conjugated polymers, or a combination of both. A list of polymer donors used in ST-OSCs is presented in Fig. 3. In the solar emission spectrum, the NIR region accounts for about 51% of the solar photon flux, 47% in the visible region and 2% in the UV region. Therefore, an ideal active layer should have a high transparency in the visible range with a strong light absorption in the UV and NIR regions for efficient ST-OSCs. The photoactive material must be chosen with the absorption peak located in the NIR or IR region to simultaneously improve the PCE and visible transparency.⁶ The choice of active layer materials also influences the apparent color of ST-OSCs.

Fig. 3 Chemical structures of polymer donors for ST-OSCs.

Initially, the efficiency of ST-OSCs was very low (PCE < 3%) because of the use of a wide band gap (E_g > 1.9 eV) organic polymer donor and fullerene acceptor (FA) as the active layer materials, which can only harvest the photons of the UV-visible region. Therefore, a trade-off between the PCE and visible transparency arises.⁴⁸ According to the Shockley–Queisser model, to obtain semitransparency the estimated band gap ranges from 1.36 eV to 1.12 eV (910–1100 nm). The low bandgap polymers in combination with fullerene acceptors reached PCEs of 3–7%. Later in 2017, fullerene-free acceptors emerged as NIR-absorbing molecules to improve the PCEs of ST-OSCs (Fig. 4). The device performance improved from ∼5% to 13% with AVT values ranging from 9 to 25%. Recently, Kini et al. reported the development of a photo-absorbing material for multifunctional ST-OSCs.⁷⁸ Herein, we discuss the recent transformation from fullerene-based acceptors to NIR-absorbing FFAs to develop high-performance ST-OSCs with simultaneous color tunability and transparency.

Fig. 4 Chemical structures of fullerene and fullerene-free acceptors used in ST-OSCs.

3.3. Semitransparent top electrode

In conventional OSCs, usually non-transparent highly reflective Al, Ag, Au or Pt are used as the metal electrode. Therefore, the electrode is one of the vital components in ST-OSCs. It should be transparent to obtain efficient ST-OSCs with a high average visible transmittance (AVT) with neutral color perception. The top electrodes generally transmit the light and transport charge carriers to or from the photoactive layers and supply the electric field. It is a most critical task to find a suitable transparent top electrode for ST-OSCs because it requires high transparency, high conductivity, and an appropriate work function. Many studies have been performed to develop transparent electrodes based on conducting polymers, metal oxides, graphene, carbon nanotubes (CNTs), ultrathin metal film, and metal nanowires (NWs) for ST-OSCs.⁷⁹

3.3.1. Conductive oxide-based top electrode. Generally, the most applied electrodes in solar cells are transparent conductive oxides. The excellent electrical and optical properties of indium tin oxide (ITO) make it a very common electrode for ST-OSCs as both the bottom electrode and the top electrode. The conductivity of ITO is as high as >2500 S cm⁻¹ and it has a high AVT of >90%.^80,81 High-quality ITO films (80 nm) can show transparency over 85% while maintaining charge-carrier mobilities up to 20–40 cm² V⁻¹ s⁻¹ along with a low sheet resistance of 20 Ω sq⁻¹.⁸² At the same time, intrinsic brittleness, high cost, and high toxicity of indium limit its use. The high-energy sputtering method used for the preparation of ITO also causes damage to the organic layer. Therefore, metal oxides such as V₂O₅/VO, WO₃/WO_x, MoO₃/MoO_x, and NiO/NiO_x have been employed as a HTL in inverted OSCs to prevent this unwanted damage.¹⁷ In 2009, Schmidt et al. reported a ST-OSC with sputtered ITO as the top electrode, TiO₂ as the ETL and MoO₃ as the HTL. Inverted ST-OSCs fabricated with the ITO/TiO₂/P3HT:PC₆₁BM/MoO₃/ITO structure achieved PCEs of up to 1.9% with a transmittance of 80% for wavelengths above 650 nm and a low transmittance from 350 to 650 nm due to strong absorption of the photoactive layer.⁸³ Another study by Yang and co-workers reported the lamination process to fabricate ST-OSCs with P3HT:PC₆₁BM as an active layer, as illustrated in Fig. 5. The ITO electrode was first coated with a Cs₂CO₃ buffer layer followed by an active layer coating. The second ITO electrode was coated with D-sorbitol-doped PEDOT:PSS, which works as both a HTL and electronic glue for the active layer in the lamination process. After drying, both substrates were laminated together by applying pressure at 105 °C–120 °C and a final heat treatment of ∼5–10 min. The addition of a few monolayers of Cs₂CO₃ formed a strong dipole at the Cs–ITO interface and reduced the work function of ITO. The self-encapsulated ST-OSC achieved a PCE of 3% with ∼70% transmittance in the visible region.⁸⁴

Fig. 5 (a) Schematic representation of the lamination process for device fabrication, (b) ST device, and (c) transmission curve and solar spectrum of the device.⁸⁴ Copyright 2008, John Wiley Publishers.

Aluminum (Al)-doped ZnO (AZO) was also used as the top transparent electrode for ST-OSCs.⁸⁵ The use of AZO as the top electrode required sputtering, and therefore, a protecting layer is used underneath the AZO layer. Hanisch et al. obtained a PCE of 1% using a sputtered AZO film (400 nm) as the ST top electrode, showing transmittance values of 13.0% and 77% at 500 nm and 700 nm, respectively.⁸⁶ Further optimization of the device by insertion of a layer of LiF/Al before the AZO electrode improved the PCE to 3.4% and with transmittance values of 60.7% and 9.3% at 700 nm and 500 nm, respectively. The LiF/Al layer simultaneously reduces the work function and protects the device from sputtering. Bauer et al. fabricated ST-OSCs with the ITO/PEDOT:PSS/PCDTBT:PC₇₁BM/TiO₂/Al/AZO structure, which gave a PCE of 3.9% with an AVT of 34%. The thin 4 nm Al layer prevents cracks forming due to direct contact of the AZO layer with TiO₂. The TiO₂ was used as the ETL and as a protective coating for the BHJ.⁸⁷

In 2009, Chen et al. developed a ST-OSC using dielectric/metal/dielectric or DMD (MoO₃/Ag/MoO₃) as a transparent anode. The inner ultrathin layer of MoO₃ (1 nm) works as a buffer layer to improve hole collection, while the outer layer of MoO₃ served as a light-coupling layer to enhance the optical transmittance of the device and reduce the series resistance (R_s). The ITO/nc-TiO₂/P3HT:PC₇₁BM/MoO₃/Ag/MoO₃-based device exhibited a PCE of 1.40%.⁸⁸ They further investigated the dependence of the device performance on the thickness of the outer MoO₃ layer. Wilken et al. used ultrathin Au instead of Ag embedded between the two evaporated MoO₃ layers as the anode and an Al-doped ZnO cathode to prepare ITO-free devices using a P3HT:PC₆₁BM absorber, which showed a PCE of 2% and with 60% optical transmission.⁸⁹ When MoO_x/Ag/MoO_x was incorporated as the transparent electrode in the PPT-PC₆₁BM-based ST-OSC that used ZnO as the ETL, the device generated a maximum PCE of 4.1% with a good average light transmission of ∼60% over the 400–600 nm range.⁹⁰ A PCE of 5.0% was achieved with 200-nm-thick PTB7:PC₇₁BM and a 40 nm outer MoO₃ layer in the MoO₃/Ag/MoO₃ electrode with an AVT of 18.3%.⁹¹ Other conducting oxides, like V₂O₅/Ag/V₂O₅⁹² and zinc tin oxide (ZTO)/Ag/ZTO,⁹³ were also used as top transparent electrodes, generating PCEs of 1.7% and 2.6%, respectively, with a transmittance of the electrodes of >80% from 400 to 750 nm.

Upama et al. reported a ST-OSC with an improved PCE of 7.4% (AVT = 25.2%) using the PBDB-T donor and the NIR-absorbing ITIC molecule.⁹⁴ The top transparent electrode used was MoO₃/Ag/MoO₃. The authors showed the influence of the active-layer thickness on the device performance, and the device with a 100-nm-thick layer showed the best performance. The lowering of the thickness improved the AVT but significantly lowered the PCE due to a lack of active materials for light absorption.

Recently, Liu et al. introduced bifacial ST-OSCs with tellurium oxide (TeO₂) on the top of an ultrathin Ag electrode as the optical coupling layer. The incorporation of TeO₂ requires only one additional evaporation step and provides better CRI, AVT and LUE values. With the optimized thickness of the Ag/TeO₂ (11/40 nm) electrode, the ST-OSCs with the structure of ITO/PEDOT:PSS/PBDB-TF:Y6:PC₇₁BM/PDIN/Ag/TeO₂ exhibit a PCE of 10.14% from the ITO side and 9.18% from the Ag side. The ST-OSCs had a high AVT of 27.83%, a high bifaciality factor of 90.53%, an improved LUE of 2.82%, and a CRI of 88.28. The device performance was also improved by applying a TeO₂ capping layer in PTB7-Th:IEICO-4F-based ST-OSCs, indicating that the Ag/TeO₂ back electrode shows great potential for upgrading bifacial ST-OSCs for future applications.⁹⁵

3.3.2. Conductive PEDOT:PSS polymer top electrodes. Conductive polymer electrodes are found to be attractive for ST-OSCs because of their easy preparation from aqueous solution and deposition using low-temperature printing techniques. Along with this, conducting polymers also show some advantages, like low cost, being lightweight, having good thermal stability, mechanical flexibility and low-temperature solution processability. Mainly, the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) blend has been intensively employed as a top transparent electrode for ST-OSCs.^96–98 They exhibit a relatively high transmittance of over 90%;⁹⁹ however, the detrimental factors are low electrical conductivity (σ) <1.0 S cm⁻¹ and low environmental stability. The conductivity of the PEDOT:PSS film depends on an array of factors like the organic solvent in the precursor,¹⁰⁰ the surface tension and wettability of the substrate, the boiling point of the solvent, and the processing conditions.⁵⁷ The conductivity of the film can be further increased up to 4000 S cm⁻¹ by introducing some additives such as polar solvents,¹⁰⁰ ionic liquids¹⁰¹ and strong acids.^102,103 The conductivity of aqueous-processed PEDOT:PSS films is quite high.¹⁰⁴

Tang et al. applied two polymers based on pyrazine and isoindigo (P3TI and TQ1, respectively) as donors and PC₇₁BM as an acceptor in combination with an amphiphilic conjugated polymer (PFPA-1)-modified ITO cathode and PEDOT:PSS PH1000 as the hole-collecting anode.¹⁰⁵ The P3TI:PC₇₁BM- and TQ1:PC₇₁BM-based ST-OSCs achieved PCEs of 3.35 and 2.61%, respectively, with a visible transparency of ∼50%.

Colsmann and co-workers prepared an inverted ST-OSC using a modified PEDOT:PSS bilayer top electrode. The top PEDOT:PSS layer was blended with a red-absorbing cyanine dye which reduces the work function of the PEDOT:PSS electrode. The ITO/ZnO/PCDTBT:PC₇₁BM/MoO₃/PEDOT:PSS/PEDOT:PSS-dye device displayed a PCE of 1.9% with a high transmittance of ∼40% in the visible range.¹⁰⁶

In 2019, Lee et al. reported solution-processed ST-OSCs fabricated using a PEDOT:PSS-based transparent top electrode, which showed a different average transmittance when the concentration of the active layer (P3HT and PC₆₁BM) was varied from 1.5 to 3.1 wt% using an inverted device structure of ITO/ZnO/PEIE/P3HT:PC₆₁BM/PEDOT:PSS. The highest PCE of 2% and transmittance of 51.2% were obtained with the concentration of 2.3 wt%.¹⁰⁷ Several studies have been carried out to enhance the conductivity of PEDOT:PSS for ST-OSCs.¹⁰⁰ In 2020, Kim et al. reported a highly conductive PEDOT:PSS polymer prepared from the mixture of PH 1000, AI 4083, and ethylene glycol as the additive solvent. The modified PEDOT:PSS can be used as both a HTL and an anode for all-solution-processed inverted ST-OSCs.¹⁰⁸ The ST-OSC fabricated using modified PEDOT:PSS gave a PCE of 2.04% and a transmittance of >85%.

The realization of highly conductive stretchable PEDOT:PSS films is a big challenge due to strong acid treatments involved and their poor intrinsic stretchability. Using a unique dipping-embedded transfer printing process, Fan et al. developed a highly conductive stretchable PEDOT:PSS electrode with a novel dipping-embedded transfer printing technique using mild acids.^103,109 The modified PEDOT:PSS, via treatment with 40 wt% CH₄SO₃, and a low viscous mild acid, on a quartz substrate, improved the conductivity (3470 S cm⁻¹), transparency and mechanical stability of the electrode. The OSC with a device structure of PDMS/PEDOT:PSS–FTEs/PEDOT:PSS (4083)/PBDTT-S-TT:PC₇₁BM/Ca/Al gave a PCE of 5.38%, a higher value than that of H₂SO₄-treated FETs (4.8%). The modified PEDOT:PSS film showed a high transmittance of over 90% from 380 to 800 nm.¹⁰³ The transfer process of PEDOT:PSS using PDMS was carried out by dipping the PEDOT:PSS surface into PDMS liquid, followed by the penetration and solidification of the PDMS, which was then peel off quickly due to the strong interfacial adhesion between both materials (Fig. 6).¹⁰⁹ The modified PDMS–PEDOT:PSS composite flexible film showed a high electrical conductivity of 2890 S cm⁻¹. ST-OSCs prepared with ITO-coated glass substrates/polyethyleneimine (PEIE)/PTB7:PC₇₁BM/PEDOT:PSS-PDMS showed high transparency of ∼35% in the visible region with a neutral color and PCEs of 3.75% and 3.46% when illuminated from the PDMS and ITO sides, respectively. The device performance was higher than the reference device with DMSO-doped PEDOT:PSS-PDMS (PCE = 2.86%, AVT = 31.1%).

Fig. 6 (a) Dipping-embedded transfer printing process, (b) transmittance spectra of the PEDOT:PSS–PDMS film, and (c) transmittance spectrum of the device with a photograph of the active ST-OSC (inset).¹⁰⁹ Copyright 2017, John Wiley publishers.

Lee and co-workers have shown two different printing techniques using a PEDOT:PSS/ionic liquid (IL) top electrode. First, they observed an increase in conductivity when the IL was mixed with PEDOT:PSS, but the underneath photoactive layer was oxidized during the ion exchange between PSS and the IL. Therefore, the group developed the sequential printing of PEDOT:PSS and the IL to prevent chemical doping of the photoactive layer by newly generated ion pairs due to retarded ion exchange (Fig. 7). The ST-OSC (ITO/ZnO NP/PTB7-Th:EH-IDTBR/PEDOT:PSS/IL) with sequential printing achieved a PCE of 6.32% and an AVT of 35.4% at an active layer thickness of 120 nm.¹¹⁰ A further increase in the thickness resulted in a reduction in both the AVT and PCE. The use of low bandgap materials resulted in a high visible transmittance for the 420–550 nm region.

Fig. 7 (a) Schematic representation of procedures used to manufacture PEDOT:PSS/IL composite electrodes; (top) a mixed solution of PEDOT:PSS and IL is printed and (bottom) PEDOT:PSS and IL are printed sequentially. (b) Effect of the photoactive layer's thickness on the device PCE and AVT. (c) Transmission spectrum of the device. The inset image shows an actual ST-OSC device with a PEDOT:PSS/IL top electrode.¹¹⁰ Copyright 2020, ACS.

In 2020, Song et al. prepared a flexible electrode by doping a PEDOT:PSS aqueous solution with xylitol (a polyhydroxy compound) followed by surface treatment of the film with methanesulfonic acid (MSA). The doping of PEDOT:PSS with the polyhydroxy compound (xylitol) improved the phase separation with a fiber-like interconnected network due to H-bonding interactions between the xylitol and PSS, which was further improved via the treatment with MSA. The doping enhanced the adhesion ability of PEDOT on the PET substrate. The ST-OSC with the PET/D-PEDOT:PSS/PEDOT:PSS/PBDB-TF:Y6/PDINO/Ag (15 nm) structure achieved a PCE of 10.53% and an AVT of 21%.¹¹¹ The flexible device also sustained about 1000 bending and folding cycles, still retaining 80% of the device PCE.

Wang et al. reported the first all-plastic flexible ST-OSCs with a P3HT:IDT-2BR-based non-fullerene photoactive layer.¹¹² The PEDOT:PSS modified by a layer of polyethyleneimine (PEIE) was used as the bottom electrode, and the top electrode was prepared through film transfer of PEDOT:PSS via a lamination process using a PDMS substrate. The NF ST-OSC exhibited an AVT of 50% and a PCE of 2.88% with an enhanced bending stability. The results demonstrated the possibilities of conducting polymer semitransparent top electrodes for future applications in ST-OSCs.

3.3.3. Nanowire-based top electrodes. The high transmittance, small sheet resistance, ductility, and good mechanical flexibility of metallic nanowires make them suitable candidates for transparent conductive electrodes for ST-OSCs since the size of nanowires is in the range of a few nanometers; hence, they require a small amount of materials, which can reduce the overall processing cost of ST-OSCs, which is compatible with mass area production. Commonly used metallic nanowires are gold, copper, and silver. Notably, silver nanowires (Ag NWs) have become the extensively used electrode material in OSCs. The transparent Ag NW electrode is easily processable via different solution-based methods such as spin coating, spray coating,^113–115 drop casting,¹¹⁶ lamination techniques,¹¹⁷ doctor blading¹¹⁸ and inkjet printing.¹¹⁹ However, the major drawbacks with Ag NWs are that they are easily oxidized in the presence of air, they have high porosity, high roughness, and show an inherent trade-off between transmittance and conductivity.

Krantz et al. reported a PCE of 2% for P3HT:PC₆₁BM-based inverted solar cells using spray-coated Ag NW layers as transparent top electrodes.¹¹⁴ ST-OSCs fabricated using PEDOT:PSS/Ag NW composite films formed on glass and a flexible PET substrate and with a P3HT:PC₆₁BM active layer exhibited PCEs of up to ∼4.2% and ∼3.8%, respectively.¹²⁰ Because of the composite film's lower sheet resistance, the device generated a higher fill factor on a flexible substrate. Yang and co-workers achieved decent PCE values of 4.3 and 4.6% in single-junction devices using PBDTT-FDPP-C12:PC₆₁BM and PBDTT-SeDPP:PC₆₁BM blends, respectively, using a TiO₂/Ag NW top electrode, showing a high AVT of ∼62%.¹²¹ Guo et al. used Ag NW as both the top and bottom electrodes in Si-PCPDTBT:P3HT:PC₆₁BM-based ternary devices, achieving a PCE of 2.2% and a FF of 0.63 with an AVT of 33%.¹²² The PCE and AVT were further improved to 2.9% and 41%, respectively, with pDPP5T-2:PC₆₁BM in a fully solution-processed device structure of Ag NW/PEDOT:PSS/photoactive layer/ZnO/Ag NW (Fig. 8a and b).¹²³ The thin layer of ZnO was introduced to improve the energy-level alignment and the formation of ohmic contacts between the active layer/ZnO and ZnO/Ag NWs. Yim et al. used solution-processed Ag NW and doped PEDOT:PSS as the cathode and anode, respectively, in P3HT:PC₆₁BM-based devices and achieved a PCE of ∼2.3% with a visible transparency of ∼55% beyond 650 nm (Fig. 8c and d).¹²⁴ TiO₂/ZnO and AI4083/GO were used as the cathode and anode buffer layers, respectively.

Fig. 8 (a) Photograph and (b) transmittance spectra of the semitransparent AgNW–AgNW device,¹²³ (c) optical transmittance spectrum of the device, and (d) photograph of a typical P3HT:PC₆₁BM ST-OSC with AgNW- and PH1000-based electrodes.¹²⁴ Copyright, 2014, ACS. (e) Photograph of a fully transparent organic solar cell in which the yellow and blue brackets indicate the top AgNW-based composite electrode and the bottom ITO electrode, respectively, and (f) absorption spectra of the pristine donor–acceptor materials and transmittance spectrum of the blend.¹²⁵ Copyright, 2012, ACS.

Kang et al. developed a fully spray-coated ST-OSC with Ag NWs as the top electrode and P3HT:PC₆₁BM as the active layer, exhibiting a transmittance of ∼70% beyond 650 nm with a PCE comparable to a conventional metal electrode.¹¹⁵ Chen et al. demonstrated the use of a Ag NW-based composite transparent electrode formed via the spin-coating of a 10 wt% ITO nanoparticle dispersion onto the Ag NW matrix as a conducting filler in a ST-OSC.¹²⁵ The photoactive layer was covered with a TiO₂ layer over which the Ag NW dispersed in isopropanol was spray coated. The adhesion of the Ag NW network was improved by applying a dilute solution of sol–gel TiO₂ in ethanol. The ITO/PEDOT:PSS/PBDTT-DPP:PC₆₁BM/Ag NWs device gave a PCE of 4% and with a high AVT of 61% from 400 to 650 nm (Fig. 8e and f).

Min et al. prepared small-molecule ST-OSCs by inserting ZnO and the perylene diimide derivative (PDINO) as a cathode interfacial layer between the active layer and the top Ag NW-based transparent electrode.¹²⁶ The ZnO/PDINO layer, apart from serving as a buffer layer, is also acting as a protective film for the active layer. Because of the variable absorptions of the active layers, various colorful ST devices can be prepared. The ZnO/PDINO/Ag NW electrode showed a transmittance of about 80% from 350 to 800 nm, which is similar to a pristine Ag NW or ITO electrode. The transmittance spectra of the devices with different active layers are between 30 and 55% (Fig. 9). The best device with the ITO/PEDOT:PSS/BHJ/ZnO/PDINO/Ag NW structure using BDTT-S-TR:PC₇₁BM gave a PCE of 3.62% and an AVT of 32% at 435 nm. By contrast, other active layers demonstrated higher AVTs but with lower PCEs between 2.2 and 2.5%.

Fig. 9 (a) Transmission spectra of the ITO, the pristine Ag NW film and the ZnO/PDINO/Ag NW stack along with the ST-OSCs based on small molecules, and (b) photograph of the ST devices showing the variable color transparency that is suitable for colorful power-generating windows.¹²⁶ Copyright, 2016, John Wiley publishers.

In 2016, the Colsmann group fabricated a large-area flexible ST-OSC with the PET/Ag mesh/PEDOT:PSS/ZnO/PffBT4T-2OD:PC₆₁BM:PC₇₁BM/PEDOT:PSS:Ag NWs structure using doctor-blade coating. They developed a transparent top electrode by intermixing PEDOT:PSS with Ag NWs. The PEDOT:PSS provides a homogeneous matrix for distribution of the Ag NWs. The SEM cross-section image showed the layer stacking of the device components with a homogeneous distribution of Ag NWs. The 1 cm² large-area device realized a PCE of 5.9% with an AVT of 6% and a peak transmittance of 13% at 515 nm (Fig. 10).¹²⁷

Fig. 10 (a) Schematic of device architecture, and (b) cross-sectional SEM image of a typical all-doctor-bladed ST-OSC on Ag/PET with contrast enhancement. (c) Transmittance spectrum of a typical photoactive device. The inset shows a photograph of a specific doctor-blade-coated ST-OSC (>1 cm²).¹²⁷ Copyright, 2016, John Wiley publishers.

ITO NPs have a high carrier density, which can cause parasitic absorption in Ag NW-based transparent electrodes. To overcome this issue, Beiley et al. proposed replacing the ITO NPs with ZnO NPs. The composite top electrode of Ag NW–ZnO NPs has a low sheet resistance of 14 Ω sq⁻¹, which is essential for large-scale manufacturing. The composite electrode showed an AVT of over 90%. The ST-OSCs with an active layer of PBDTTPD:PC₇₁BM exhibited a PCE of 4–5%, which has an average above bandgap transmission of 34% and a sub-bandgap transmission of 81%.¹²⁸ Koppitz et al. effectively used PEDOT:PSS–Ag NW hybrid electrodes as both top and bottom electrodes and prepared all-solution-processed ST-OSCs. The Ag NW-embedded PEDOT:PSS hybrid electrode was compressed via hot pressing after deposition to avoid any of the nanowires sticking out. The ST-OSC with a device structure of PET:PEDOT:PSS:Ag NW/ZnO/PBTZT-stat-BDTT-8:PC₆₁BM/PEDOT:PSS:Ag NW, prepared using doctor blading, gave a PCE of 3.8% with an AVT of 25%.¹²⁹

Zhai et al. reported inverted ST-OSCs with an all-copper-nanowire (Cu NW) electrode with a low surface roughness and homogeneous conductivity.¹³⁰ They used the ‘in situ polymerization method’ for decreasing the roughness of the Cu NW-based electrodes. The fabricated ST-OSCs with the device structure of polyacrylate/CuNWs/PEDOT:PSS (PH1000)/Y-TiO₂/P3HT:PC₆₁BM/PEDOT:PSS(4083)/Cu NWs/polyimide/polydimethylsiloxane (PDMS) showed an AVT of 42% with PCE values of 1.97% and 1.85% for illumination from the top and bottom sides, respectively. The Cu NW/PI top electrode was coated with a thin layer of PEDOT:PSS(4083), and D-sorbitol was added as the electronic glue for adhesion of PEDOT:PSS with the active layer.

Ma and co-workers inserted phosphomolybdic acid (PMA) and PEDOT:PSS composite materials between the active layer and the Ag NW top electrode to improve the active layer wettability. The composite layer also has good solvent resistance and improves the wetting behavior with the Ag NWs. The device ITO/ZnO/PTB7-Th:PC₇₁BM/PMA:PEDOT:PSS/Ag NW gave a PCE of 5.01% with an AVT of 50.3% when illuminated through ITO, while the device with MoO₃/Ag/MoO₃ as the transparent electrode showed a comparable PCE (5.77%) but with a lower AVT (19.5%).¹³¹ Illumination through the bottom Ag NW electrode gave a higher performance compared with the MoO₃/Ag/MoO₃ electrode.

Li and co-workers introduced Ag NWs into a ZnO film to increase the conductivity and LUE of PBDB-TF:Y7-based ST-OSCs without affecting the AVT.¹³² The LUE was improved from 2.46 to 2.63% with a PCE of 13.76% and an AVT of 19.09%. The improvement was mainly due to the increase in J_SC resulting from the high conductivity and surface plasmon resonance of the Ag NWs. This study demonstrates an efficient approach for fabricating ST-OSCs with a high LUE. Grossman and co-workers prepared a Ag NW-based anode encapsulated with graphene oxide to protect it from degradation and simultaneously increase the lifetime of inverted ST-OSCs.¹³³

These metal NW transparent electrodes have a promising future for ST-OSCs. However, a judicious choice of solvent is necessary to prevent the solvent-induced corrosion of the metal NWs or conducting polymers. The solvent used for the preparation of the electrodes could harm the carrier-transport layer and the active layer beneath it. Future research will require careful solvent selection for solution-processed transparent electrodes.

3.3.4. Ultrathin metals-based top electrode. Ultrathin metal films (UTMFs) of Au and Ag prepared via thermal evaporation are the most convenient electrodes for ST-OSCs. In inverted OSCs, ITO is generally used as the cathode and high-work-function metals, such as Au and Ag, are used as the anode. UTMFs offer a lower air sensitivity, a smooth surface, improved stability, a high work function and a high conductivity up to ∼10⁷ S m⁻¹. The thickness of the metal film should be precisely controlled to make the transparent electrode. However, there is a trade-off between conductivity and transparency because a high conductivity cannot be achieved by increasing the transparency via a reduced thickness.

ST-OSCs fabricated using a chloro-aluminum phthalocyanine (ClAlPc) or BODIPY derivative as the donor, and C₆₀ as the acceptor in a bilayer structure achieved a high AVT of >55% and a PCE of approximately 2%.^134,135 Bilayer OSCs are fabricated via the sequential deposition of donor and acceptor materials. Hany and co-workers used Ag/Alq₃ cathodes to prepare ST-OSCs and achieved a PCE of 1% and an AVT of over 65% with a maximum of 78.8% at 568 nm using a near-IR-absorbing cyanine dye donor.¹³⁶ The formation of a planar heterojunction results in insufficient donor–acceptor interfaces for exciton dissociation. So subsequently, the BHJ structure has often been used to fabricate most of the ST-OSCs. Though the BHJ structure is beneficial for obtaining high-performance ST-OSCs, it is also hard to optimize and control the phase separation in the BHJ active layer. The bulk morphology is rarely the thermodynamically stable situation at equilibrium. The bilayer structure may be able to prevent this unstable morphological problem, resulting in an improved device stability for ST-OSCs. Therefore, exploring the potential of the active layers in the bilayer structure would be fascinating.

Recently, Song et al. deposited a bilayer of the PTB7-Th/IEICO-4F-based active layer to fabricated ST-OSCs. They found that the bilayer deposition not only simplified the processing but also improved the PCE of the device compared with the BHJ-based device. They pointed out that the thickness of the IEICO-4F acceptor does not affect the visible transmittance while absorbing in the near-IR region for charge generation (Fig. 11). By optimizing the thickness of the acceptor and donor materials and using Au/Ag (1/12 nm) as the ultrathin transparent top electrode, the device exhibited a PCE of 8.5% with an AVT of 21%, which is better than the PCE of 8.1% (AVT = 21.1%) obtained for the BHJ structure.¹³⁷

Fig. 11 (a) Normalized absorption spectra of donor and acceptor units, (b) device structure of the bilayer ST-OSCs, and (c) digital photographs taken through the reference device and ST-OSCs devices.¹³⁷ Copyright 2020, ACS.

In 2006 Yang et al. used a thin Au (12 nm) layer as the top transparent electrode in inverted ST-OSCs.¹³⁸ The BHJ device with the structure ITO/Cs₂CO₃/P3HT:PC₆₁BM/V₂O₅/Au exhibited a PCE of 0.85% when illuminated through the ITO side and 0.52% by illuminating the Au electrode. The comparatively low PCE from the semitransparent Au electrode is due to the partial loss of photons via absorption and reflection.

UTMFs prepared via vapor deposition exhibited poor adherence to the underlying substrate because of their tendency to develop through the island-like Volmer–Weber nucleation pattern, leading to a rough surface morphology with a high electrical resistivity.¹³⁹ Rough surfaces generally form plasmonic resonances with visible light, thus significantly reducing the film transparency. Therefore, to reduce the surface roughness and increase the visible transparency it is important to control the wetting behavior of the metal or to insert a dielectric layer by application of a thin (0.1–1 nm) nucleation layer of metal, which shifts the percolation threshold of the thinner film and lowers the electrical resistance.^79,140 Apart from metals, solution-processed PEDOT:PSS has also been used as a viable nucleation layer. Nucleation layers can help the thermally formed ultrathin metal layer to percolate onto the underlying substrate, resulting in a better film quality.¹⁴¹

Schubert et al. demonstrated an efficient Ag top transparent electrode with a 1-nm-thick Au layer as the metal seed layer in a dielectric/metal/dielectric (DMD) multilayer structure. The Au/Ag bilayer is sandwiched between two MoO₃ layers that serve as the dielectric to increase electrode transparency and to improve the device lifetime. They observed that the transmittance and conductance of the Ag films vary due to the significant variation of the substrate wetting conditions. The transparent electrode with ultrathin 1 nm Au and 7 nm Ag exhibited a maximum transmittance of 83% at 580 nm, which is even higher compared with ITO in this spectral range. The insertion of a 1 nm Au seed layer in the Ag layer decreased the sheet resistance (R_s) from 32 Ω sq⁻¹ to 19 Ω sq⁻¹. The device with F4ZnPc:C₆₀ achieved PCEs of 4.6% and 4.7% when illuminated from the ITO and Au (1 nm)/Ag (7 nm) layer sides, respectively.¹⁴⁰

Jen et al. reported a bilayer semitransparent hybrid electrode prepared via thin Ag metal deposition on an ultra-thin fullerene-containing surfactant as the cathode interfacial layer (CIL).¹⁹ The CIL adjusts the energy-level alignment at the metal/organic interface and efficiently enables electron extraction and photocurrent generation. The fabricated ST-OSCs using a thin layer of PIDT-PhanQ:PC₇₁BM (50 nm) as the active layer and a hybrid transparent electrode containing 30 nm Ag exhibited a PCE of 5.1% with an AVT of ∼25% from 400 to 800 nm and have a good color perception that was close to white light (Fig. 12). The PCE improvement is related to an increase in the V_OC and the FF.

Fig. 12 (a) Transmission spectra of the ST-OSCs with varying thickness of the Ag electrode in the visible range. (b) Top: representation of the color coordinates of ST-OSCs on the CIE 1931 xy chromaticity diagram upon illumination with AM1.5G; and bottom: the relevant expanded part of the color space of the CIE 1931 xy diagram. Inset (in between): two digital pictures taken through the ST-OSC that has a 20-nm-thick Ag layer.¹⁹ Copyright, 2012, John Wiley publishers.

Recently, Shi et al. reported an ST-OSC with an ultra-thin Ag layer (7 nm) as the top transparent electrode and PF3N-2TNDI, a self-doped n-type polymer, as the ETL.¹⁴² The ST-OSC ITO/PEDOT:PSS/PTB7-Th:PC₇₁BM/PF3N-2TNDI/Ag structure exhibited a PCE of 6% with an AVT of 30%. Moreover, the ST-OSC exhibited a CRI approaching 100% and CIE color coordinates of (0.326,0.328). Wong et al. prepared ST-OSCs using a bilayer of an ultrathin metal film (LiF/Al/Ag) as the top transparent electrode. Blade-coated ST-OSCs based on the PBDTTT-CT:PC₇₁BM and PTB7-Th:PC₇₁BM active layers with PEDOT:PSS as the HTL gave PCEs of 5.2 and 5.6%, respectively, with an AVT between 10 and 20% for the 400–800 nm region with good color perception and high CRI of ∼90, making them suitable for building-integrated photovoltaic window applications.¹⁴³ Large-area devices with 10.8 cm² gave a PCE of 3.8 and 5.3%, respectively, for ST-OSCs using the different active layers.

Jen and co-workers prepared an inverted ST-OSC using a low band-gap polymer PBDTTT-C-T as the donor and PC₇₁BM as the acceptor (Fig. 13).⁴⁶ The complementary absorpton of the donor and acceptor materials resulted in a neutral color appearance of the device over the visible spectral region. They further showed that the AVT could be tuned from 36% to 2% by increasing the thickness of the Ag electrode from 6 to 60 nm, with PCEs ranging from 4.25 to 7.56% and with a CRI value of about 97 using a device structure of ITO/ZnO/C₆₀-SAM/PBDTTT-C-T:PC₇₁BM/MoO₃/Ag. Hou and co-workers prepared an inverted ST-OSC using the wide bandgap polymer donor PBDB-TF with a deep HOMO energy level (−5.45 eV) and the PC₇₁BM acceptor, achieving a PCE of 5.7%, a V_OC of 0.96 V, and an excellent average transmission of 67% in the visible region.¹⁴⁴

Fig. 13 Transmission spectra of ST-OSCs obtained via optical modeling (closed symbols) and from the actual devices (open symbols); (b) photograph of a flower taken with a camera; and (c) photograph taken through the bare glass, the ITO and the ST-OSCs with the Ah electrode of different thicknesses.⁴⁶ Copyright, Royal Society of Chemistry.

Jen and co-workers reported the use of a fluoro-containing polymer donor PCPDTFBT which, with the PC₇₁BM acceptor, gave a PCE of 5% and a high AVT of 47.3% using a 10 nm thin Ag on top of PEDOT:PSS as an anode. Increasing the Ag thickness to 15 nm dis not affect the PCE (5.1%); however, the AVT dramatically decreased to 39.4% for the 450–900 nm region.¹⁴⁵ Cao's group achieved the highest PCE of 6.78% in a ST-OSC using the PTB7-Th:PC₇₁BM active layer after interfacial modification with 5-nm-thick PF3N-2TNDI and a 20 nm Ag electrode.¹⁴²

Dudem et al. created a subwavelength architecture using polydimethylsiloxane (SWA-PDMS) as an anti-reflective (AR) layer via the soft imprinting lithography technique to simultaneously improve the PCE, the transparency and the color perception.¹⁴⁶ The PCE of the modified device with the AR layer was enhanced from 7.07% to 8.52%, with AVT increasing from 23.0% to 26.2%. The CRI value increased from 84% to 90% using the SWA-PDMS layer.

Cao and co-workers fabricated an ST-OSC using the PBDTTT-E-T:IEICO active layer, which showed a PCE of 7.9% and an AVT of 23.8%.¹⁴⁷ The thickness-dependent study of the Ag electrode showed that with an increase in the Ag thickness from 10 to 20 nm the device PCE increased from 6.8 to 9.0% while the AVT decreased from 28 to 17%. They further revealed that at the same AVT of ∼20% the ST-OSC with a fullerene-free acceptor achieved a higher PCE of 8.4% compared with 6.2% for a fullerene-based device, which was mainly due to the contribution of absorption from the NIR region.

Huang et al. fabricated an ST-OSC using the PTB7-Th polymer donor and ITVfIC as the acceptor, achieving a PCE of 8.21% with an AVT of 26.4% between 370 and 740 nm without any additional treatment. The thin film of ITVfIC displayed an absorption peak at 772 nm with a bandgap of 1.37 eV, thus complementing the absorption of PTB7-Th (λ_max = 690 nm).¹⁴⁸ Li et al. prepared a ST-OSC with a PBDB-TF:ID-4Cl blend achieving a PCE of 6.99%, lower than the opaque device (10.2%), and an AVT of 43.7%.¹⁴⁹ Wu et al. achieved a PCE of 9.83% with an AVT of 32% in a ST-OSC with a bluish color by employing a PBN-S:IT-4F blend and a 20-nm-thick Ag cathode. The polymer PBN-S displayed a strong absorption for the 500–700 nm region with a low-lying HOMO energy level (−5.48 eV), which is compatible with IT-4F. The opaque device with PBN-S:IT-4F gave a high PCE of 13.1% due to efficient charge separation and transport.¹⁵⁰

Hu et al. reported a smart strategy of adjusting the D : A weight ratio. Opaque and ST-OSCs were prepared by optimizing the doping ratios of PTB7-Th : IEICO-4F blends. The opaque OSCs maintained PCEs of over 11% by altering the PTB7-Th : IEICO-4F ratios from 1.4 : 1.5 to 0.8 : 1.5 (wt/wt), along with a red-shifted absorption edge of the active layers resulting from the molecular aggregation of IEICO-4F. The ST-OSCs were prepared with 1 nm Au/11 nm Ag replacing 100 nm Al as the electrode. The AVT of the ST-OSCs was increased from 23.7% to 27.1% by decreasing the PTB7-Th content in the active layers, along with a slight decrease in the PCE from 9.48% to 9.06%.¹⁵¹

In addition to decreasing the relative weight ratio of D : A, Xu et al. proposed an effective strategy of using the wide bandgap polymer donor D18, which has a small absorption full-width half-maximum (FWHM) of 118 nm, to enhance the AVT of ST-OSCs.¹⁵² The small FWHM of D18 can enable narrow photon harvesting in the visible-light range, which is promising for the preparation of high-efficiency ST-OSCs. Combining the strategy of decreasing the D : A weight ratio and optimizing the thickness of the semitransparent top electrode, a highest LUE of 2.90% with a PCE of 12.91% and an AVT of 22.49% was achieved in the D18 : N3-based ST-OSCs.

Yang's group developed flexible ST-OSCs with good visible transparency, which used visible light for the growth of plants and NIR light for generation of the current. The devices, which used PTB7-Th as the donor and IEICO-4F, F8IC and FOIC acceptors, achieved high PCEs of 10.03%, 9.26% and 8.92%, respectively, with a AVT of ∼34% which is suitable for plant-growth applications.¹⁵³ Zhan and co-workers synthesized a fused-ring octacyclic fullerene-free acceptor (FOIC), which exhibits an absorption maximum at 836 nm with an optical gap of 1.32 eV. The as-cast ST-OSC exhibited an excellent PCE of 10.3% and an AVT of 37.4% from 370 to 740 nm due to the strong absorption of the acceptor in the NIR region.¹⁵⁴ The authors further prepared a ST-OSC with a PCE of 10.2% and an AVT of 25.4% for PTB7-Th:FOIC photoactive layer, which can be reversibly switched on and off by implementation of the gasochromic WO₃/Pt back-reflector layer under hydrogen exposure. The device was also found to improve the AVT to 33.8% under the bleached state whilst still maintaining a PCE of 9.1%, suggesting a color-switchable ST-OSC.¹⁵⁵ Hu et al. achieved a PCE of 12.37% and an 18.6% AVT using the PBDB-TF:Y6 photoactive layer.¹⁵⁶ The transmittance was adjusted by manipulating the thickness of the Ag layer.

Zhu and co-workers developed an acceptor ATT-2, which shows a strong NIR absorption between 600 and 940 nm with an optical energy gap of 1.32 eV. A blend layer of PTB7-Th:ATT-2 achieved a PCE of 7.7% and a high transparency of 37% with good color-rendering properties (CRI = 94) and CIE color coordinates of (0.2805,0.3076).¹⁵⁷ Chen et al. used the PTB7-Th donor and the low bandgap IEICS-4F acceptor (E_g = 1.3 eV) and achieved a PCE improvement from 9.4 to 11.1% with an AVT of 43.2 to 28.6%, respectively, by varying the thickness of the Ag electrode from 10 to 20 nm in the MoO₃/Au/Ag electrode.¹⁵⁸ The reduction of the Ag thickness enhanced the AVT but reduced the PCE value.

Li et al. used an inverted device ITO/ZnO/PTB7-Th:BT-CIC/MoO₃/Ag, which exhibited a PCE of 7.1% with an AVT = 43%, when measured from the ITO side, and a CRI value of 91. It was shown that the increase in the Ag thickness reduced the visible transmittance (Fig. 14a); the ST-OSC with 10-nm-thick Ag is shown in Fig. 14b.¹⁵⁹

Fig. 14 (a) Optical transmittance of ST-OSCs with varying thicknesses of the Ag electrode, and (b) photograph of the PTB7-Th:BT-CIC-based ST-OSC.¹⁵⁹ (c) Transmittance spectra of ST-OSC devices with various donor materials, and (d) photograph of the ST-OSCs with an active donor layer of J52, PBDB-T, or PTB7-Th when using the IEICO-4Cl acceptor (from left to right).¹⁶⁰ Copyright, 2017, John Wiley and Sons publishers.

Hou and co-workers achieved variable-color ST-OSCs from purple, to blue to cyan when using J52, PBDB-T, and PTB7-Th as the donor and IEICO-4Cl the acceptor (PCE 6.2–6.97% and AVT of ∼35%) (Fig. 14c and d). The devices gave a PCE of around 4% when illuminated through the Au electrode.¹⁶⁰ When PTB7-Th was blended with IHIC acceptor the best ST-OSC gave a PCE of 9.77% without sacrificing the AVT (36%) with a Au/Ag top electrode.¹⁶¹ A higher PCE of 11.6% could be observed with the PTB7-Th:FNIC2 blend but at the cost of the AVT (13.6%).¹⁶² When PTB7-Th was mixed with a complementary absorbing low bandgap acceptor IUIC the ST-OSC with the ITO/ZnO/PTB7-Th:IUIC/MoO_x/Au(1 nm)/Ag structure displayed a PCE of 10.2% and an AVT of 31% with >40% from the 370–500 nm region.¹⁶³

Moreover, Heeney and co-workers used a fullerene-free acceptor C8-ITIC and the PFBDB-T donor as the absorber layer in the ST-OSC prepared using a MoO₃/Ag/MoO₃ top electrode and an ITO/ZnO bottom electrode, providing an average PCE of 9.8% and an AVT of 22%, with CIE coordinates of (0.24,0.29).¹⁶⁴ The thickness of the MoO₃/Ag/MoO₃ layer was optimized at 10/15/40 nm to give the best device performance. Luo et al. reported an efficient ST-OSC using the fused dithienothiophen[3,2-b]-pyrrolobenzotriazole-based fullerene-free acceptor Y14 absorbing from 300 to 1000 nm. The Y14-based inverted ST-OSC after additive and thermal-annealing treatment reached an excellent PCE of 12.67% whilst still maintaining a high AVT of 23.69% which is required for window applications.¹⁶⁵ The opaque device gave a PCE of 14.92%.

3.3.5. Carbon-based top electrode.
3.3.5.1. Carbon nanotubes (CNTs). Carbon nanotubes (CNTs) are one of the allotropes of carbon, which offer excellent flexibility, high conductivity, carrier mobility, chemical and mechanical stability and visible transparency. These properties make CNTs a potential candidate for the top transparent electrode.^166,167 The resistance of a CNT network depends on many factors like the length, diameter, purity, synthetic method, and metallic/semiconducting volume ratio and is governed by the contact resistance between the CNTs and their individual resistance. Besides, the surface roughness of CNT films can result in a considerable shunt resistance loss.

Xia et al. prepared CNT films via the chemical vapor deposition (CVD) method and used them as a transparent top electrode in ST-OSCs. The CNT films exhibit 80–95% transmittance for the 400–2500 nm wavelength range and a sheet resistance of 400–600 Ω cm⁻¹. The ST-OSCs were prepared with the ITO/ZnO/P3HT:PC₆₁BM/CNT structure and showed PCEs of up to 2.5% and an AVT of 60–80% from NIR to the IR region.¹⁶⁸ Kim et al. used free-standing multi-wall carbon nanotube (f-CNT) sheets as the top electrode in ZnPc:C₆₀-based ST-OSCs. The f-CNT electrode was deposited on top of the BHJ layer at room temperature using an orthogonal liquid solution-assisted self-laminating process. The ITO/n-C60/C60/ZnPc:C60/p-BF-DPB/f-CNT device generated PCEs of 1.3% or 1.0% when illuminated from the ITO or CNT sides, respectively. The CNT-based devices showed higher FF values and stability compared with devices using the Al (1 nm)/Ag (14 nm) semitransparent top electrode.¹⁶⁹ Joen et al. developed an aerosol method to synthesize a transparent CNT electrode with strong interchain interactions. The CNT films were p-doped using HNO₃via ‘sandwich transfer’, and thermally evaporated MoO_xvia ‘bridge transfer’ and fabricated using a lamination process. The doped CNT-based devices using ZnO as the ETL and PTB7:PC₇₁BM as the active layer generated PCEs of 3.7 and 3.1%, respectively, with a transmittance of 90% compared with 1.8% for the un-doped device. It was observed that the conductivity of the CNTs was increased with the doping process.¹⁷⁰

3.3.5.2. Graphene-based electrode. Graphene is the thinnest 2-D carbon allotrope, comprising sp² hybridized carbon. Graphene has all the properties for becoming a good semitransparent electrode for OSCs, such as high conductivity, high transparency (90% in NIR range), a high mechanical, chemical, and thermal stability, a low sheet resistance, and high carrier mobility. Graphene electrodes can be fabricated using various methods like chemical vapor deposition (CVD), chemical synthesis, unzipping of CNT mechanical exfoliation, and the reduction of graphene oxide (rGO).^171–174 Lee et al. reported an inverted ST-OSC with a laminated graphene top electrode without damaging the underlying active layer. For the top electrode, graphene film was deposited on top of the ITO/ZnO/P3HT:PC₆₁BM/GO device with thermal release tape followed by the removal of the tape during thermal annealing. The device achieved a maximum PCE of 2.5% with a high transparency beyond 700 nm (Fig. 15).¹⁷⁵ The transmittance and sheet resistance of the graphene layer were found to be decreased by increasing the number of graphene layers.

Fig. 15 (a) Current–voltage characteristics of the ST-OSCs that consist of various top graphene layers with light shining from the ITO side; (b) transmittance spectrum with a picture of the semitransparent device in the inset; and (c) transparency and (d) sheet resistance of graphene film layers on a glass substrate.¹⁷⁵ Copyright 2011, ACS.

Yan et al. used highly doped single-layer graphene as the top electrode in an inverted ST-OSC with the P3HT:PC₆₁BM active layer. The conductivity of graphene was found to be increased by doping with Au nanoparticles and PEDOT:PSS. The graphene film was p-doped by PEDOT:PSS due to the high work function of the latter, resulting in electron transfer from graphene to PEDOT:PSS. The optimized ITO/ZnO/P3HT:PC₆₁BM/PEDOT:PSS/graphene devices using the doped single-layer graphene electrode resulted in the best PCE of 2.7% in a 20 mm² active area.¹⁷⁶ Liu et al. fabricated ST-OSCs using graphene as both the transparent cathode and anode. In this process, the graphene film was n-doped with ZnO NPs. The devices with the modified ZnO-NP/PEDOT:PSS/graphene cathode gave the best PCE of 5.78% compared with the device with only a ZnO NP (4.89%) buffer layer due to lowering of the sheet resistance and improved FF. For all graphene electrode-based devices, the graphene anode was laminated via a graphene/PMMA film staked on a thin PDMS substrate. The graphene/PMMA/PDMS film was p-doped with PEDOT:PSS, and D-sorbitol was used to improve the adhesion on the active layer during the lamination process (Fig. 16). The device with the graphene/PEDOT:PSS/PTB7:PC₇₁BM/ZnO NP/PEDOT:PSS/graphene structure fabricated via a lamination process exhibited a PCE of up to 3.35% and an AVT of 40%.¹⁷⁷ The measurement through the modified bottom cathode side showed a slightly lower performance than with illumination through the top anode side. The device showed excellent color neutrality (0.3098,0.3306) that was close to white light.

Fig. 16 (a) Schematic representation of laminated all-graphene electrodes; (b) J–V characteristics of OSCs with all-graphene electrodes, where solid lines and dashed lines represent illumination from the anode side and the cathode side, respectively; (c) EQE spectra taken from each side of an ST-OSC with a graphene cathode modified with ZnO-NPs/PEDOT:PSS. Inset: the device's energy diagram; (d) transmittance spectra of two ST-OSCs with a graphene cathode and an ITO cathode; and (e) photograph of a ST-OSC with all-graphene electrodes.¹⁷⁷ Copyright 2015, ACS.

Graphene is also effectively used in flexible solar cells. Lin et al. reported a hybrid top electrode consisting of monolayer graphene and a metal grid on a flexible polyethylene terephthalate (PET) substrate. The electrode possesses a small sheet resistance of 22 ± 3 Ω sq⁻¹ and a high transmittance of 81.4% at 550 nm. The ST-OSCs with the ITO/TiO₂/P3HT:PC₆₁BM/PEDOT:PSS/monolayer graphene/Au grid/PET device structure achieved PCEs of 3.1% and 2.8% with illumination from the ITO side and graphene side, respectively, higher than for the reference device without the Au grid (0.8%).¹⁷⁸ Liu et al. reported a flexible OSC prepared on a polyimide substrate with a double-layer graphene top transparent anode and a P3HT:PC₆₁BM active layer, showing the maximum PCE of 3.2% and an excellent bending stability via retaining 92% of the device efficiency after 1000 bending cycles.¹⁷⁹

Shin et al. prepared a P3HT:PC₆₁BM flexible ST-OSC where both the top and bottom graphene electrodes were doped with bis(trifluoromethanesulfonyl)amide (TFSA) and triethylenetetramine (TETA), respectively. The R_s of the graphene films (∼775 Ω sq⁻¹) decreased after doping with TFSA (∼185 Ω sq⁻¹) and TETA (∼220 Ω sq⁻¹) on PET.¹⁸⁰ The electron-accepting or -donating ability of the TFSA or TETA dopant makes graphene p- or n-type, which is compatible with the anode or cathode transparent conductive electrode (TCE) for OSCs.^181,182 The p- and n-doping further resulted in modification of the work function of graphene electrodes from 4.56 eV to 4.88 and 4.49 eV, respectively. Flexible ST-OSCs (TFSA/graphene/PEDOT:PSS/P3HT:PC₆₁BM/ZnO/TETA/graphene) with PEDOT:PSS as the HTL and ZnO as the ETL showed PCEs between 3.12 and 3.30% when illuminated from the TETA- and TFSA-doped graphene sides, respectively, with a high transparency of 30–40% for the 400–600 nm region and 70% above 650 nm (Fig. 17).

Fig. 17 (a) Schematic of the device structure of ST-OSCs with doped-graphene electrodes (both cathode and anode), (b) J–V curve of the flexible device illuminated from different sides, and (c) transmittance spectrum of the ST-OSC.¹⁸⁰ Copyright, 2018, ACS.

4. Device engineering

4.1. Ternary bulk-heterojunction structure for ST-OSCs

In the past few years rapid progress has been made in the development of ternary OSCs based on fullerene-free acceptors.^183–186 The ternary strategy has been realized as an effective and simple strategy for enhancing the performance of OSCs. The active layer of a ternary structure consists of either two donors and one acceptor (D1 : D2 : A) or one donor and two acceptors (D : A1 : A2).¹⁸⁷ It is crucial that the careful selection of the active layer materials provides a broad absorption coverage, resulting in improved photon harvesting and record efficiencies for opaque OSCs. Now, ST-OSCs based on the ternary structure have also become an emerging area of research. The photovoltaic data are presented in Table 2.

Xie et al. reported ternary ST-OSCs with two polymer donors, of PTB7-Th (covering 400–800 nm, E_g = 1.59 eV) and PBT1-S (covering 300–700 nm, E_g = 2.10 eV), and PC₇₁BM as the acceptor.¹⁸⁸ The binary device using the PTB7-Th:PC₇₁BM blend gave the highest PCE of 8.0% with an AVT of 20.8%. The addition of 10 wt% of PBT1-S to the PTB7-Th:PC₇₁BM blend in the ITO/PTB7-Th:PBT1-S:PC71BM/Zr(acac)/Ag device achieved an enhanced PCE of 9.2% without sacrificing the AVT and color perception. The devices were optimized using 1,3-propanedithiol (PDT) as the solvent additive. A trade-off between the PCE and the AVT can be seen by lowering the Ag thickness from 15 nm to 5 nm.

Ma et al. prepared a ST-OSC using the PTB-Th donor and two ultra-narrow bandgap acceptors (COi8DFIC, E_g = 1.26 eV; and IEICO-4F, E_g = 1.24 eV) where the optimal concentration of IEICO-4F was maintained at 15 wt%.¹⁸⁹ The ST-OSCs were fabricated by varying the thickness of the Ag electrode from 10 to 20 nm. The thin Ag film possesses a high transmittance in the visible region and a low transmittance in the long-wavelength region, which is favorable for photon harvesting in the NIR region with FFAs. The ternary device gave an average PCE of 8.23% with an AVT of 20.78% for 370–740 nm, which is comparable to the COi8DFIC-based binary ST-OSCs (PCE = 7.2%, AVT = 20.25%).

Zhang and co-workers fabricated ternary ST-OSCs using the PTB7-Th donor and two narrow bandgap materials BDTThIT-4F and IEICO-4F as alloyed acceptors.¹⁹⁰ The IEICO-4F acceptor with a maximum at 880 nm in the NIR region was used as the third component, complementing the absorption of PTB7-Th and BDTThIT-4F. For ST-OSC fabrication, 1 nm Au/10 nm Ag was used as the top electrode, which exhibited a high transmittance in the visible region from 300 nm to 600 nm. The ST-OSCs with the device structure ITO/PEDOT:PSS/PTB7-Th:BDTThIT-4F:IEICO-4F/PDIN/Au/Ag exhibited a PCE of 9.40% and an AVT of 24.6% from 370 to 740 nm. In comparison, the binary BHJ ST-OSCs using PTB7-Th:BDTThIT-4F and PTB7-Th:IEICO-4F generated PCEs of 7.53 and 8.35% with AVT values of 24.2 and 25.3%, respectively.

Hu et al. successfully employed the ternary strategy to simultaneously enhance the PCE and the AVT of wide bandgap polymer donor-based semitransparent OPVs.¹⁹¹ They introduced the Y6 acceptor with a red-shifted absorption into the D18–Cl:Y6–1O host system. The PCE of opaque OPVs can be enhanced to a best value of 16.05% by introducing 50 wt% Y6, which is mainly due to the improved photon harvesting of the ternary layers. By increasing the Y6 content, the AVT of the active layers is gradually enhanced. The ternary blend film with 50 wt% Y6 realized an AVT of 50.1%. Using Au (1 nm)/Ag (10 nm) as the top electrode, the ST-OSC obtained a PCE of 13.02%, an AVT of 20.2% and a LUE of 2.63%.

Cao and co-workers reported an ST-OSC with the J52:IEICO-4F:PC₇₁BM ternary blend for greenhouse applications.¹⁹² The addition of PC₇₁BM suppressed the trap-assisted recombination and enhanced the charge extraction, thus improving the V_OC. The ternary ST-OSC achieved a PCE of 8.83% and an AVT of 15.8% using 20-nm-thick Ag. Further reduction of the Ag thickness to 15 and 10 nm lowered the J_SC, thus decrease the PCE to 7.75% and 5.93%, and increased the transmittance to 19.9% and 23.7%, respectively, because of less light reflection by the thinner Ag film. Furthermore, the transmission spectra matched well with the absorption of chlorophyll photoreceptors in green plants, making ST-OSCs a viable approach towards light management for plant growth via self-powered greenhouses.

Sano et al. revealed the influence of the blend ratio on the color perception and performance of ST-OSCs. Ternary ST-OSCs were fabricated with the PCDTBT:PC₇₁BM:ITIC BHJ layer sandwiched between a MoO₃/Ag/MoO₃ top anode and an ITO/ZnO cathode, where the device with an active layer ratio of 1 : 4 : 1 exhibited a PCE of 4.0%, an AVT of 39.2% and a CRI value of 97, which is close to the neutral color perception. The device with an active layer ratio of 1 : 3 : 1 showed a slight blue color, with a slightly lower PCE of 3.7%, a higher AVT of 44.8% and a lower CRI of 93 (Fig. 18).¹⁹³

Fig. 18 (a) J−V characteristics of the binary and ternary ST-OSCs with different blend ratios; (b) transmittance (T) and reflectance (R) of the active layer and ternary ST-OSCs; and (c) photographs taken through the 1 : 3 : 1 and 1 : 4 : 1 ST-OSCs. The lighter areas at the outer ends of the substrates correspond to the electrode-free active layers.¹⁹³ Copyright, 2019, ACS. (d) J−V characteristics and (e) photographs demonstrating the color tuning using different top electrodes.¹⁹⁴ Copyright, 2020, John Wiley and Sons Publishers.

The suitable choice of active layer materials and electrodes is crucial to obtaining a high PCE and keeping suitable visible transparency for energy-generating windows. Wang et al. achieved an excellent PCE of 13.1% whilst still maintaining a high AVT of 22.4% when using a PBDB-TF:Y6:BTTPC ternary blend and a 12-nm-thick Ag anode, compared with a PCE of 12.8% and an AVT of 19.3% for the binary device without BTTPC.¹⁹⁴ The two acceptors Y6 and BTTPC formed an alloy model absorbing in the NIR region, improving the transparency in the visible region. When a pair of LiF/MoO₃ bilayers on top of an ultrathin Ag electrode was used as an effective photonic reflector layer, the device generated a PCE of 12.3% whilst still maintaining a high AVT of 23.4% (Fig. 18d and e). Zheng et al. obtained the highest recorded PCE of 13.49% with a high AVT of 22.58% for ternary ST-OSCs using a complementary absorbing blend of PBDB-TF:Y6:DTNIF (a dithienonaphthalene-based acceptor) and a 15 nm Ag top electrode with good visible-light transparency (Fig. 19).¹⁹⁵ An opaque device with the same configuration generated a PCE of 16.73%. With a lower thickness of 10 nm, the device gave a PCE of 12.1% and AVT of 25.9%, while further increase in thickness resulted in an increase in PCE to 14.5% with lower AVT of 19.78%.

Fig. 19 (a and b) Normalized absorption spectra of pristine PBDB-TF, DTNIF, and Y6 films; (b) energy level diagram of the device components; and (c) photograph of the ST-OSC.¹⁹⁵ Copyright 2020, Royal Society of Chemistry.

4.2. Tandem structure for ST-OSCs

In tandem solar cells, two or more solar cells are stacked together to utilize more of the solar spectrum. The tandem strategy becomes an efficient solution by combining the different subcells together. The tandem cell approach has also been used in ST-OSCs to make them more efficient. In the tandem structure using PBDTT-FDPP-C12 in the front subcell and PBDTT-SeDPP in the rear subcell, the device achieved an improved PCE of 7.3%, a high V_OC of 1.47 V, and an AVT of 30%.¹²¹ Meiss et al. fabricated tandem ST-OSCs by stacking two subcells with complementary absorbing F₄-ZnPc and DCV6T as donors and C₆₀ as the acceptor. The tandem cell exhibits a PCE of 4.9 ± 0.2% with a 24% AVT.¹⁹⁶ Al/Ag was used as top contact, over which a 90-nm-thick Alq3 layer was used to increase the light out-coupling (Fig. 20).

Fig. 20 (a) Chemical structures of the absorber materials F4-ZnPc, DCV6T, and C₆₀; (b) schematic of the tandem solar cell; and (c) a photograph of the devices.¹⁹⁶ Copyright 2011, AIP Publishers.

The PCEs of tandem ST-OSCs with TQ1:PC₇₁BM and P3TI:PC₇₁BM as front and rear subcells demonstrated improved PCEs of 5.29% and 5.63% in parallel and series configurations, respectively. The tandem cell outperformed the PCEs of single-junction ST-OSCs by about 50% whilst still maintaining a high transmittance of ∼25%. The overall loss of photons in the ST tandem OSC due to parasitic absorption, reflection and transmission can be lower than the loss in a single-junction solar cell using the same absorber materials.¹⁰⁵ In parallel connection, the J_SC of the tandem cell is the addition of the J_SC values of two subcells, while the V_OC is the average of each subcell. However, the J_SC obtained in the series connection is constant throughout the device, and thus lower than for the parallel connection. In a series connection the V_OC is the sum of two subcells. Chang et al. used the same polymer donor PIDT-phanQ with the PC₆₁BM or PC₇₁BM acceptors in different subcells to prepare efficient tandem ST-OSCs. The device using a MoO₃/Ag top electrode achieved a PCE of 7.4% with a high AVT of 40% and a CRI approaching ∼100.¹⁹⁷

Chen et al. prepared a tandem ST-OSC using a narrow band gap fullerene-free acceptor IEICS-4F absorbing up to 1000 nm blended with the PTB7-Th polymer donor in the rear subcell. The front cell consists of a medium bandgap acceptor FTTB-PDI4 blended with the P3TEA polymer. The tandem device has a broad absorption from 300 nm to 1000 nm. The device structure of glass/ITO/ZnO/P3TEA:FTTB-PDI4/PEDOT:PSS/ZnO/PTB7-Th:IEICS-4F/MoO₃/Au was used to fabricate a series of tandem ST-OSCs, which exhibited a high PCE of 10.5% with an AVT of 20%.¹⁹⁸ Yusoff et al. used solution-processed graphene mesh as the ST anode and laminated AgNWs as the ST cathode in tandem OSCs. The front subcell was made using a wide bandgap PSEHTT:IC₆₀BA blend, while the rear subcell was made with a narrow bandgap PBDTT-DPP:PC₇₁BM blend. ST-OSCs illuminated from either the graphene mesh or laminated AgNWs sides obtained PCEs of 8.02% and 6.47%, respectively, with a visible transparency of about 45%.¹⁹⁹

4.3. Photonic crystals for ST-OSCs

Photonic crystals (PCs) are periodic dielectric nanostructures that are designed to establish the energy band structure for photons, allowing or prohibiting the transmission of photons over certain wavelength ranges. They can be applied as a one dimensional (1D) photonic crystal (1DPC), a dielectric mirror or a distributed Bragg reflector in ST-OSCs to boost their performance. The dielectric mirror is able to manage the photons and resolves the trade-off between the PCE and the AVT in ST-OSCs. When the photon energy is smaller than the photonic band gap of a 1DPC, the photons will be reflected. The operating concept of distributed Bragg reflectors and dielectric mirrors is based on the constructive interference of multilayer dielectric films. They are all made up of multilayer dielectric films with varying refractive indices. The data are presented in Table 3.

In 2012 Yu et al. used 8 pairs of high-refractive-index (WO₃)/low-refractive-index (LiF) in 1DPC, which act as a distributed reflector in ST-OSCs. The use of 1DPC improved the PCE of the P3HT:PC₆₁BM device from 2.04% to 2.58%.²⁰⁰ The AVT using 1DPC was almost zero for the 400–600 nm range, suggesting sufficient light absorption by the active layer. In 2015, the same group reported neutral colored binary ST-OSCs based on PCDTBT:PC₇₁BM by using 5 pairs of 1DPCs consisting of [MoO₃/LiF]^pair which exhibited PCE of 5.31 and AVT of 25.1% and CRI close to 97.⁵⁰

Martorell's group used a photonic crystal based on a multilayer MoO₃/LiF to trap the near-infrared and near-ultraviolet photons and to improve the device transparency in PTB7:PC₇₁BM-based OSCs.²⁰¹ The PC was grown on top of a thin Ag electrode. Use of the photonic crystal improved the J_SC by about 26% compared with the cell without the PC layer whilst still maintaining a visible transparency close to 30% and a PCE of 5.6%. The J_SC of the transparent cell (10.9 mA cm⁻²) is about 77% lower compared with the opaque cell. In 2017, Li and co-workers designed an inverted ST-OSC using the PTB7-Th:PC₇₁BM active layer and six pairs of MoO₃/LiF-based 1DPCs, which exhibited a PCE of 7.0% with an AVT of 12.2%.²⁰² When P3HT or PTB7 was used as the donor, the device PCE was reduced to 3.2% and 6.4% with an AVT of 19.0 and 16.9%, respectively.

ST-OSCs prepared using the PTB7:PC₇₁BM blend, and six layers of 1DPC, made up of alternating TiO₂ and SiO₂, were incorporated between the Au and glass substrate to maximize the current while maintaining the overall visible transparency of the OSC above 20%. An antireflection coating of MoO₃/LiF was deposited over the Ag cathode and ZnO was used as the ETL.²⁰³ The ST-OSC gave a PCE of 5.3% which is about 90% of the analogous opaque device with an AVT of 21.4%. The group of Brabec and Cao developed an optical model to design 1DPCs for high-performance ST-OSCs.²⁰⁴ Two pairs of 1DPC [MoO₃/LiF] were deposited over the thin Ag layer to improve the optical properties. The binary ST-OSC based on PTB7-Th:IEICO-4F reached a PCE of 10.83% and an AVT of 29.5%.

Yu et al. introduced five pairs of [WO₃/LiF] as a 1DPC to develop a neutral color P3HT:ICBA-based ST-OSC with high PCE and CRI values. The P3HT-based device generally absorbs in the green-light region showing a crimson color with a low CRI value. A simultaneous increase in PCE and lowering of the AVT was observed by increasing the number of 1DPCs. The strong reflection of the 1DPC levels up the transmittance spectrum induced in the weak absorption region of the P3HT:ICBA. With 5 pairs of 1DPC, the device gave a PCE of 5.12% with an AVT of 24.4%. The CRI value increased from 79 (without 1DPC) to 89 (with 5 pairs of 1DPC).²⁰⁵ Shen et al. developed a ST-OSC using 1DPC [WO₃/LiF]⁵ and the PTB7-Th:PC₇₁BM active layer, yielding a PCE of up to 9.36% and an AVT of 14.31%.²⁰⁶ The EQE spectra were enhanced in the 540 nm to 700 nm region due to the enhancement in the reflection by the WO₃/[WO₃/LiF]⁵ layer. Therefore, light management via the introduction of photonic crystals has significant importance for management of the color purity and performance of ST-OSCs.

Zhang et al. reported that the ternary neutral-colored ST-OSC with a CRI approaching 100 makes it useful for BIPV applications (as shown in Fig. 21).²⁰⁷ The device was fabricated with three layers of MoO₃/LiF as a photonic crystal or DM with the structure of ITO/PEDOT:PSS/J71:PTB7-Th:IHIC/PDINO/Au/Ag/DM. The PCE is increased from 8.93% to 9.37% compared with the reference device without the DM, while the AVT was lowered substantially from 24.4% to 21.4%. Meanwhile, the CRI of the ternary ST-OSCs is 97, with a CIE of (0.3200,0.3287) and a correlative color temperature (CCT) of 5931 K.

Fig. 21 Left: digital photographs of the J71:PTB7-Th:IHIC-based ternary ST-OSC and its glass counterpart. Right: Devices used as glass applied to windows.²⁰⁷ Copyright, 2019, John Wiley and Sons.

Li et al. developed a new photonic crystal consisting of LiF/TeO₂/LiF/TeO₂ dielectric multilayers as a transparent electrode and photon reflector.²⁰⁸ The ST-OSC achieved a PCE 7.3% and an AVT value of 29.5% suggesting excellent multifunctional ST-OSCs for window applications. They have further developed high-efficiency variable-color ST-OSCs via optical modulation using an Ag/TeO₂/Ag Fabry–Pérot electrode.²⁰⁹ The devices with the ITO/PEDOT:PSS/PBDB-TF:Y6/Bis-FIMG/Ag/TeO₂/Ag structure generated an excellent tunable color with PCEs of >14% and a maximum transmittance of up to 31%.

5. Potential applications and future perspective

Research in the field of solution-processable flexible and robust ST-OSCs has made significant progress in the past decade with the development of new materials, transparent electrode designs, interfacial layer processing and device engineering. The performance of ST-OSCs has been significantly improved, which makes them a strong contender for commercial applications such as in BIPV devices, power windows, indoor photovoltaics, automobile sun-rooves with variable color and transparency, and agrivoltaics.^78,210–212 BIPV devices using ST-OSCs act as a power-generating source as well as lighting up the building interior via the transmission of light. One of the attractive features of ST-OSCs is their variable color and transparency.^76,160 The color selection and transparency are the major focus of future technology and will depending on the end user's applications. As the technology progresses, there will be a strong requirement for color selection and quality, which can be easily tuned via bandgap modulation using an appropriate molecular design or via device optical engineering with dielectric structures.

The integration of ST-OSCs for agrivoltaics, i.e., in greenhouse-based agriculture, provides additional energy for crop growth and offers a high production rate.^213,214 The ST-OSCs for agricultural applications must be adaptable to the solar spectrum in response to plant photomorphogenesis. The light within the blue region enables vegetative leaf growth while the light within the red region facilitates flower growth.²¹⁵ Song et al. reported flexible ST-OSCs that use the PBDB-T-2F:Y6 photoactive layer absorbing in the NIR region and a modified PEDOT:PSS electrode, achieving over 10% PCE and 21% AVT for use in agricultural plant growth.¹¹¹ Therefore, the proper choice of material combinations can develop efficient ST-OSCs with longer lifetimes for power-generating greenhouse applications.^153,192 Colsmann and co-workers reported the potential application of ST-OSCs in wearable electronics, such as solar-powered sunglasses.²¹⁶ Although the applications of ST-OSCs are currently in their initial stage, they will play a key role in the future photovoltaic technology. It is also important to find out the appropriate conditions for low-temperature processing for the development of flexible ST-OSCs.

6. Conclusion

Opaque and semitransparent OSCs, as a new photovoltaic technology, have complemented silicon-based photovoltaics well, with current efficiencies reaching 18% and 13%, respectively. ST-OSCs as power-generating windows will not only generate power but will also create beautiful, pleasant and heat-controlled surroundings in buildings. By choosing the appropriate semitransparent top electrode, active layer, and device structure of a ST-OSC, a high PCE and AVT can be obtained. In this review, we focused on summarizing the recent developments in each component of the ST-OSC device, such as the top electrode, active layer materials, carrier-transport layer and device engineering. Despite the fact that the PCEs of ST-OSCs have climbed to ∼13%, they have been achieved at the expense of poor AVT values. The well-balanced PCE and AVT of semitransparent OSCs remains a major issue, and much attention should be devoted to the synthesis of organic materials, active layer design, semitransparent top electrodes, optical modeling, device engineering, and stability. The recent improvements have mainly been due to the development of fullerene-free acceptors and new polymer donors that enable simultaneously high PCE and AVT values to be maintained. Although some high-performance devices have been reported for binary and ternary devices, achieving a PCE of >12% and an AVT reaching 25%, which is the requirement for window applications, rooms for further improvement remains. New molecular designs, morphology and device structure optimizations could further enhance the efficiency and transparency for practical applications in building-integrated photovoltaics and glass windows for vehicles.

To fabricate highly efficient ST-OSCs, the materials must have a strong absorption in the near-infrared range and high transmittance in the visible-light region. In terms of that, a ternary structure can be one of the preferred compositions for improving photon harvesting of active layers in the near-infrared region. The photon-harvesting ability is also influenced by the chemical composition of the photoactive layers, especially in the long-wavelength region. The photon harvesting of active layers in the visible and near-infrared ranges may also be adjusted by changing the D : A ratio, optimizing the active layer thickness, and the addition of a bilayer structure, all of which have been shown to be effective ways of increasing the AVT of ST-OSCs.

ST-OSCs also require a transparent top electrode with high conductivity and transmittance in the visible-light region and high reflectance in the near-infrared region, in addition to the materials used in the active layers. The transmittance of the top electrode is crucial for calculating the AVT of ST-OSCs. To achieve a good balance between conductivity and transmittance, the thickness of a semitransparent electrode must be carefully optimized. Meanwhile, the optimal semitransparent top electrodes should be compatible with the bottom active layer, fabricated using simple solution processing. To achieve selective transmittance and reflection for the simultaneous improvement of the PCE and AVT values of ST-OSCs, photonic crystals, antireflection coatings, optical microcavities, and DMD structures have also been used. However, because of their complicated device structure, the cost of the ST-OSCs could increase. Some thin metal electrodes including Au, Ag, and AgNWs with high transmittance have been implemented, although their high reflectance can be unfavorable for the device performance. As a result, semitransparent top electrodes with the necessary optical, electrical, and processing properties, as well as being low cost and having high stability, should be investigated further for constructing highly efficient ST-OSCs. Other carbon-based electrode materials have also been used, but they have low transparency and conductivity, therefore limiting the performance of ST-OSCs. Many efforts are required, and include material selection, active layer optimization to improve the photon-usage efficiency, and semitransparent electrode design to concurrently enhance the PCE and AVT for efficient ST-OSCs. Finally, studies on device stability will need special attention in the future. We anticipate considerable advancements in both the electrical and optical performance in the near future. As a result, we believe that ST-OSCs, with their unique flexibility and great optical and electrical performance, have a bright future in the solar industry.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the Department of Science and Technology (DST), New Delhi (DST/TMD/SERI/D05 G), for financial support. KK thanks DST for awarding the DST-INSPIRE fellowship.

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