Progress and challenges on scaling up of perovskite solar cell technology

Abstract

Since the first application of a metal halide perovskite (PVK) absorber in a solar cell, these materials have drawn a great deal of attention in the photovoltaic (PV) community, showing exceptional rapid progress in power conversion efficiency. The potential advantages of low-cost, high efficiency, easy processability, and wide range of applications make PVK solar cells (PSCs) a desirable candidate for future uptake in the PV market over traditional semiconductors such as silicon. Furthermore, PVK thin-film technology holds a concrete potential to closely approach the theoretical efficiency limit for single-junction solar cells via unique control of the optoelectronic properties. However, for a disruptive breakthrough of PVK technology from fundamental research to industry, systematic research efforts are required to unravel the poor long-term stability and to reach a reliable large area fabrication process. In this review, we examine in detail recent progress on large-scale PSCs and we discuss challenges for commercialization touching upon the following aspects: material properties, fabrication technology, and industrialization challenges. Besides, the long-term stability and efficiency of large-area PSCs as well as PVK-based two-terminal tandem devices are discussed. In addition, strategies for PSC upscaling are further studied for scalable deposition technologies. Finally, we review the most recent literature on costs and environmental assessment.

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1. Introduction

Photovoltaic (PV) technology converts solar energy into electricity. Over the last seventy years, many different absorber materials and device architectures have been developed resulting in high power conversion efficiencies (PCEs). Among them, crystalline silicon (c-Si) and so-called III–V solar cells have demonstrated high efficiencies and a mature level of development. Currently, c-Si solar cells dominate more than 90% of the PV market share¹ because of cheap raw materials, long stability, and well-established technology. However, the limitation of c-Si PV is the complex fabrication process as well as high requirements for wafer quality, which increase the fabrication cost and hinder further commercialization of high-efficiency architectures.^2–4 The promise of emerging thin-film PV technology is to cost-effectively fabricate high-quality semiconductor materials by simpler deposition processes and reducing the amount of material used.

Over the last decade, organic–inorganic metal halide perovskite (PVK) has attracted much attention in the PV research community, after Kojima et al.⁵ discovered in 2009 the ability of PVK to convert the energy carried by sunlight into electricity. The reasons behind the success of PVK can be ascribed to its exceptional electrical properties with direct bandgap and high absorption coefficient, long carrier diffusion length, tuneable bandgap by compositional engineering, and simplified deposition process. With these outstanding properties, PVK solar cells (PSCs) have been developed rapidly, demonstrating in only a decade, albeit on a sensibly smaller device area, PCEs approaching those recorded by solar cells based on c-Si.⁶

However, compared to c-Si, PVK technology still has two main drawbacks namely device stability and device upscaling. These two factors directly determine the steps toward commercialization. Pinholes and defects existing in thin films have a negative impact on photoelectric properties^7,8 and their influence becomes more severe for an effective lab-to-fab transition.⁹ To address this problem, various scalable deposition strategies are employed for each layer of the PSC to obtain high-quality films and excellent device properties. Challenges in device stability, as well as cost including environmental impact, are all crucial topics of relevance for the PV industry.

In fact, PVKs suffer from different degradation pathways related to temperature,^10,11 humidity,^12,13 composition,¹⁴ and light.^15,16 This leads to decomposition of the material and decrease of performance as time goes on. Several approaches^17–20 have been explored to prevent this reduction in performance. Recently, PVK degradation mechanisms have been reported²¹ and different materials are tested to replace less stable systems.²² Up to now, large-scale (100 cm²) PSCs show promising stabilities.^23,24 Nevertheless, there is still a long way towards commercialization, especially compared with c-Si PV. An approach to control degradation deals with tailoring the nature of the PVK lattice by using various cations²⁵ and halide anions.^26,27 Other strategies comprise functionalizing the PVK material to improve stability.²⁸

Although there are numerous challenges, the progress of PSCs towards commercialization is unprecedented in the PV community. Many technologies are demonstrated to be suitable for scalable film deposition, which can produce each layer of PSC with high film quality. With the application of various strategies, such as precursor engineering and interfacial engineering, the efficiency for large area devices has been largely improved. PSCs with an active area of more than 57 cm² have been fabricated with a certified PCE of 14.6%.²⁹

Besides, PVK is also employed as at least one of the absorbers in double and triple-junction solar cells fully based on PVK PV technology^30–32 or in combination with c-Si PV technology. For their promising PCEs, the PVK-based tandem solar cells provide a valuable economic approach to break through the Shockley and Queisser (SQ) limit³³ for single-junction devices. Currently, the world record PCE for PVK/c-Si tandem solar cell has achieved 29.8%.³⁴ Apart from efficiency, long-term stability has also been improved in the last years with the introduction of inorganic cations, such as cesium (Cs). Nowadays, large-area devices are reported to work consistently for 1000 h under the condition of 25 °C and 85% humidity with an area of 25 cm².³⁵ With further development of encapsulation technology and device optimization, the device stability aims at fulfilling the test requirements for commercialization in near future.

In this review, we discuss recent progress in large-scale PSCs and focus on the challenges for commercialization. Section 2 summarizes several high-volume manufacturing technologies which are suitable for scalable deposition of PVK film. Section 3 summarizes the challenges of up-scaling for different PVK layers. Section 4 provides an overview of efficiency and stability for large area PVK-based monolithic tandem solar cells. Additionally, to have a reasonable assessment of PVK industrialization, cost and environmental impact are discussed in Section 5. Finally, we review several companies which have started their business using PSCs for a variety of applications.

2. Progress for scalable techniques

Fabrication of uniform and pinhole-free large-area PVK films can be realized by employing suitable deposition methods, such as blading, slot die coating, spray deposition and more. In this section, we review some deposition methods enabling scalable processes for PVK film fabrication.

2.1 Solution based processes

2.1.1 Blade coating. In the blade coating process, the precursor solution is spread onto a substrate by a moving blade, forming a wet film, as shown in Fig. 1a.²⁹ The film quality depends on the properties of the substrate surface,³⁶ velocity of the blade, solvent properties (for example composition,³⁷ concentration, solvent viscosity),^38,39 annealing temperature,⁴⁰ and atmosphere. As an example, Fig. 1b illustrates the relationship between coating speed, film thickness, and solvent evaporation.²⁹ Since the first PVK film was formed with blade coating in 2015, many studies have been reported ranging from in situ observations to technical optimization.^41,42In situ grazing-incidence XRD measurements are employed for blade coating to observe the ink-to-solid phase transformation of PVK films during preparation.⁴³ The composition, solvate phase, and intermediate complexes are observed under low processing temperatures; the crystallization process takes place directly without forming intermediate phases, leading to a successful method for large-area film fabrication.⁴³ Apart from the in situ observation of PVK crystallization, various basic technical parameters are also optimized. Temperature and solvent evaporation rate are of critical importance to control the film formation.⁴⁴ Kim et al.⁴⁵ found that a slow solvent drying process could encourage the formation of large crystals, which were formed immediately after solution blading. In Mallajosyula's work,⁴⁰ a temperature-controlled blading technique was employed for the growth of large-grain PVK thin films. Apart from temperature, other parameters, like coating speed, is found to have a relationship with film thickness, which is clearly explained in Horizontal dip-coating regime.⁴⁶ Besides, a combination of blade coating and rapid thermal processing has been demonstrated to exhibit high PVK film quality.⁴⁰ Devices with a PCE over 17% for an active area of 2.7 cm² were reported.⁴⁷ The interaction between the ink and the substrate was also investigated. Dai et al.³⁶ introduced ammonium chloride as an additive into precursor solution to reduce film trap density, flexible PSCs with a record PCE of 15.86% was obtained for an aperture area of 42.9 cm² (see Fig. 1c). Similarly, Chen et al.⁴⁸ recently partially replaced dimethyl sulfoxide with solid-state carbohydrazide to avoid the formation of voids at perovskite–substrate interfaces. Based on the research mentioned above, this technique shows a high sensitivity towards temperature and towards the interaction between the ink and the substrate. Blade coating approach is flexible to combine with other methods to prepare PVK films, such as roll-to-roll setups, rapid thermal processing,⁴⁰ and sequential deposition,³⁸ which provides more possibility for the future commercial application.

Fig. 1 (a) Diagram of blade coating of PVK film²⁹ (b) the film thickness as a function of coating speed for blade coating²⁹ (c) J–V curves for flexible PSCs with a record aperture area of 42.9 cm² and PCE of 15.86% (ref. 36) (d) diagram of the spray coating process⁴⁹ (e) diagram of the magnified structure of nozzle trajectory⁴⁹ (f) J–V curves for PSCs with different active areas.⁵⁰

2.1.2 Spray coating. The spray coating technology was firstly used for PVK preparation in 2014 (ref. 51) and developed rapidly from then on with various ways of creating the spray droplets. In this process, tiny liquid droplets are formed with a nozzle and then dispersed onto a substrate, as shown in Fig. 1d and e. According to the investigation of Chen et al.,⁵² the spray process is classified into three stages: (i) atomization process, (ii) droplet flight, and (iii) film deposition. Each of them was discussed in detail from principles to parameters in ref. 52. The flying route of these tiny droplets during spray has a strong influence on the final film quality. Ishihara et al.⁵³ studied the fluid dynamics of droplets and Girotto et al.⁴⁹ described the relationship between spray velocity and precursor solution spreading capabilities. Besides, factors affecting the deposition were systematically optimized, such as precursor solution (viscosity, concentration, compositions),^54,55 substrate (wettability, roughness, temperature),^56,57 boiling point of solution,⁵⁸ distance between the nozzle and substrate.⁵⁹ Park et al.⁵⁶ prepared high-quality PVK layers by controlling the flow rate of precursor solution and the reaction temperature. As a result, larger-scale PVK film with an area of 7.5 × 7.5 cm² showed excellent photovoltaic properties and uniformity.⁵⁶ Chou et al.⁵⁰ realized a PCE of 7.01% with an aperture area up to 3 cm² by controlling precursor solution concentration and spray passes; the corresponding J–V curves are shown in Fig. 1f. Recently, Heo et al.⁶⁰ successfully prepared CsPbI_3−xBr_x sub-module by spray coating with an efficiency of 13.82% (112 cm² aperture area). A graded PVK absorber structure is created and it shows only 9% degradation after 1-sun light soaking for 1000 h.⁶⁰ All-spray coating PSCs (CTLs and PVK) have also been reported,⁶¹ and PCEs are in the range of 10–12% with an aperture area of about 1 cm². Spray coating of PVK PV shows an increasing PCE situation in the recent few years and the future experiments can concern more about low volatility controlling, new additives for PVK solution and novel transport materials.⁵⁹

2.1.3 Slot-die coating. Slot-die coating can be applied for film deposition of different layers, especially the PVK layer. During the process, the coating head is positioned close to substrate and there is a narrow slit on the coating head. Precursor solution is pumped into the coating head and is forced to flow out of the narrow slit onto the moving substrate. The film quality is extremely sensitive to the processing parameters such as the substrate temperature, wettability, moving speed, and width of the slit. Besides, the wettability, viscosity of precursor solution also influences the degree of PVK crystallinity.^62,63 In Cotella's work,⁶⁴ the preheated substrate combining with air-knife led to a temperature gradient in the wet film which plays a crucial role in controlling the crystallization. By selecting low boiling solvents and employing multifunctional additives, the processing window time can be widened. The Dutch institute Solliance, which is specialized in the roll-to-roll slot die production process of PVK layers,⁶⁵ has modified the ink with a co-solvent and additive, avoiding efficiency losses with the increase of area during the drying process. They demonstrated a 144 cm² module with a PCE of 14.5%.⁶⁶ Apart from the physical property influence of precursor solution, chemical properties such as additive^67,68 and composition^69,70 can also lead to excellent film properties. By applying additive method, Yang et al.⁷¹ prepared large-area FACs based perovskite films with a certified quasi-stabilized efficiency of 16.63% (20.77 cm²). Besides several basic parameters, models and principles are also introduced to further explain the slot die coating process. Xu et al.⁷² investigated the infiltration process of the precursor solution into a mesoporous titanium oxide (TiO₂) according to the Lucas–Washburn model.⁷³ Under the guidance of the model, coating parameters were optimized and a PCE of 12.87% was obtained with an active area of 60.08 cm².⁷²Fig. 2a and b show the device structure and corresponding J–V curves.⁷² Bu et al. prepared 65 cm² FACs-based PSCs with a certificated PCE of 19.54%. In their work, avoiding the formation of PVK intermediate complex is the key point to obtain high quality PVK film.⁷⁴ Not only for PVK film formation, but deposition of CTLs can be also realized with slot-die coating. All-slot-die coating devices with PCEs around 11% have been successfully fabricated.^75–77 Among all-slot-die coating works, Di Giacomo et al.⁷⁸ in Solliance fabricated PVK solar modules; the highest PCE exceeded 10% with a power output of 1.7 W for an area of 168.75 cm². Slot-die coating shows a great potential toward industrialization because it is one of the most used techniques for the roll-to-roll fabrication of PSCs.⁷⁹

Fig. 2 (a) Image of slot die coated PSC-based mini-module with an active area of 60.08 cm²;⁷² (b) the J–V curves of the device shown in (a);⁷² (c) image of 7 m² fully printed PSC-based modules;⁸⁰ (d) diagram of fabrication process for fully printed PSCs.⁸⁰

2.1.4 Inkjet printing. In terms of inkjet printing, precursor solution is dispersed by nozzles. The influence of some basic factors (such as substrate wettability, temperature, ink droplet wetting behavior, viscosity, and solvent evaporation rate) on PVK film quality was investigated. Properties of the precursor solution play an important role in controlling PVK film formation.⁸¹ Li et al.⁸² reported a new ink system, whose solvent was composed of n-methyl pyrrolidone (NMP) and dimethylformamide (DMF), because NMP can effectively adjust the viscosity and surface energy of the precursor solution. As a consequence, devices were fabricated with PCEs of 14.5% (4.04 cm²).⁸² As the most basic factor, temperature is also taken into consideration. In Liang's work,⁸³ inkjet printing combined with vacuum-assisted thermal annealing was employed for scaling up. High-performance PSCs based on printed MAPbI₃ were demonstrated to obtain a PCE of 13.3% with an active area of 4.0 cm².⁸³ Recently, the emerging of all-inkjet-printed PSCs accelerates the process of industrialization. Hu et al.⁸⁰ fabricated a large-area (49 cm²) PVK module that exhibited a PCE of 10.4% with a fully printable process. Besides, a 7 m² fully printed PSC-based module has been successfully prepared (see Fig. 2c and d), showing promising potential in the practical photovoltaic application.

2.2 Vacuum based processes

As an alternative to solution-based processes, vacuum deposition has been explored to grow thin-film PVK from dry sources. There are two unique advantages of vacuum-based process. Firstly, the process does not involve the use of toxic solvents which may have a negative effect on the environment. The second advantage is the conformal growth onto rough substrates allowing, for example, the use of textured substrates in devices to improve light in-coupling.⁸⁴ This opens the integration of PVK cells on textured c-Si bottom cells for further applications.^85–87

2.2.1 Thermal evaporation. In the thermal evaporation process, the precursor materials are sublimated by heating under high vacuum conditions. They are deposited onto substrates to form different material layers as shown in Fig. 3a. Therefore, this approach to fabricate PVK layers is scalable. According to the evaporation temperature of different source materials, a PVK film can be grown using two methods: co-evaporation^88,89 and sequential vacuum deposition.⁹⁰ Liu et al.⁸⁹ firstly reported PSCs with high PCEs, which were prepared by the dual-source co-evaporation method. Large area PVKs have also been reported. Borchert et al.⁹¹ applied the co-evaporation method for large-area PVK deposition. Formamidinium lead triiodide (FAPbI₃) thin films were obtained with an area of 64 cm², as shown in Fig. 3b. Up to now, MAPbI₃ layer deposited by thermal evaporation demonstrates a PCE of 18.13% with 21 cm² active area (mini-modules).⁹² However, a challenge for PVK deposition with thermal evaporation is the control of reaction between lead iodide (PbI₂) and MAI. The introduction of MAI may have an influence on chamber pressure, which will lead to fluctuations of the evaporation rate.⁹³ Therefore, hybrid approaches and multi-source depositions can be applied for PVK fabrication to avoid the MAI-induced chamber pressure fluctuations.^94,95 Feng et al.⁹⁶ successfully deposited some superior FA-based PVK films using low temperature annealing under vacuum. Compared with other solution based methods, thermal evaporation has been demonstrated to allow PVK deposition onto textured substrates,⁹⁷ and this feature provides a promising future for further applications. Sahli et al.⁹⁷ applied the sequential two-step method to deposit the PVK layer for a monolithic tandem device. The PVK film fully covered the μm-sized silicon pyramids (see Fig. 3c and d), and the tandem device achieved an efficiency as high as 25.2%.⁹⁷ Following the same approach, the same group demonstrated a PVK/PVK/c-Si monolithic triple-junction solar cell.⁹⁴ All-vacuum-processable PSCs are also obtained recently,^98,99 Lei et al. reported an all-evaporation method to achieve PCEs beyond 13% with an active area of 16 cm².¹⁰⁰ The recently reported improved results based on thermal evaporation indicate the potential application in PSCs scaling up, especially for scalability and the compatibility with textured device-Si bottom cells.

Fig. 3 (a) Schematic diagram of the co-evaporation of PVK film;⁸⁹ (b) image of 64 cm² FAPbI₃ thin film and corresponding fabrication process;⁹¹ (c) SEM image of the PVK film fully covering the silicon pyramids;⁹⁷ (d) a cross-section of the PVK/SHJ monolithic tandem solar cell.⁹⁷

2.2.2 Pulsed laser deposition. Pulsed laser deposition (PLD) is a versatile technique for stoichiometric deposition of various inorganic materials independently of vapour pressure to form thin films. The PLD configuration and deposition process are shown in Fig. 4a. Different from other vacuum-based approaches, PLD exhibits stoichiometric mass transfer of materials from the target to the substrate.¹⁰¹ This unique advantage offers a potential application for PVK film formation. PLD consists of two main stages, namely formation of the target plasma and material deposition. In the first stage, the focused laser strikes the surface of the target material for a short time, ablating material that forms a so-called plasma plume containing ions and atoms. In the second stage, these excited ions directly impinge the substrate in front of the target. Film properties can be optimized by controlling the laser parameters,¹⁰² substrate temperature,¹⁰² and chamber pressure.¹⁰³ The first deposition of MAPbI₃ film based on PLD was reported by Bansode et al.¹⁰³ in 2015. Following the same method, they successfully deposited PVK films on silicon showing good crystal quality. The corresponding SEM-based top view and cross-sectional images are shown in Fig. 4b and c,¹⁰³ respectively. From then on, several PLD-grown inorganic metal halide PVKs have been successfully prepared, showing good photovoltaic properties and film stability.^104–106 Wang et al.¹⁰⁴ fabricated a dense CsPbBr₃ film via PLD. No significant decomposition was observed when the sample was placed in a highly humid (80%) environment for 15 days.¹⁰⁴ Lead-free PVK film formation is also achieved with PLD. CsSnI₃ film was recently reported by Kiyek et al.¹⁰⁵ The thickness-optimized film shows a stabilized black phase with a sharp absorption edge. About the deposition of organic–inorganic hybrid PVKs, another method named resonance infrared matrix-assisted pulsed laser evaporation¹⁰⁷ should be mentioned, showing low laser-induced damage of organic material. A detailed description of the new pulsed laser-based method can be found in Soto-Montero's review.¹⁰⁸ Based on the film quality as well as stability mentioned above, PLD has a potential application for scalable deposition of PVKs, especially in terms of fully inorganic PVK deposition.

Fig. 4 (a) Schematic diagram of the PLD configuration and deposition process¹⁰³ (b) top view of PVK film on silicon wafer deposited with PLD¹⁰³ (c) the cross-section of the PLD PVK film¹⁰³ (d) schematic diagram of the HCVD configuration for MAI deposition onto PbI₂ coated substrates¹⁰⁹ (e) image of cells and modules prepared with HCVD approach demonstrating the up-scaling process.¹¹⁰

2.3 Hybrid chemical vapor deposition

Hybrid chemical vapour deposition (HCVD) method can avoid solvent-related complications, which are challenging to control in industrial large-scale fabrication,¹¹¹ such as fast solvent removal and wettability issues. This method combines chemical vapour deposition (CVD) and other scalable deposition methods, such as thermal evaporation¹¹² or spray coating.¹¹³ A typical PVK-based HCVD process consists of two steps: formation of inorganic film and organic/inorganic halide vapour deposition. The inorganic film is mainly deposited via thermal evaporation in reported works.^213,216 As for the CVD process, the organic halide vapour is optimized by pressure and temperature to deposit on the inorganic film, following a gas–solid reaction and converting into uniform PVK films.¹¹⁴ The HCVD configuration and vapour deposition process for MAPbI₃ are shown in Fig. 4d. Several works have reported a successful formation of scalable PVK films by combining thermal evaporation and CVD.^109,110,115 Leyden et al.¹⁰⁹ firstly prepared a planar MAPbCl₃ device via HCVD method, achieving a PCE as high as 11.8%. Following the similar approach, Leyden et al.¹¹⁰ again indicated the device upscaling process via HCVD. They fabricated MAPbI₃ based devices with a PCE of 9.5% (8.8 cm²) and obtained a PCE of 9.0% (12 cm²) for FAPbI₃ based devices, the corresponding up-scaling processes are shown in Fig. 4e. However, similarly to solution-based film formation, mixed-cations and mixed-halides play a critical role in film optoelectronic properties and stability.¹¹⁵ Therefore, mixed cations/halides are commonly introduced in the first step by co-evaporation.¹¹⁶ Differently from this approach, Luo et al.¹¹² prepared a mixed-cation PVK film by optimizing the second step. They replaced pure FAI vapour with FAI/FACl mixed vapour to optimize the vapour–solid reaction, resulting in improved phase stability. Consequently, the large-sized PSC modules (active area of 41.25 cm²) demonstrated champion PCE of 12.24%.

3. Challenges for device upscaling

The abovementioned deposition techniques are all for large area PVK film formation. As for other methods (e.g. CVD, soft-cover deposition), detailed technical parameters and models can be found somewhere else.^{113,116–118} The device PCEs, deposition approaches, and corresponding areas are summarized in Fig. 5, which clearly shows that PCEs significantly decrease for large device areas. Therefore, the upscaling of PVK PV technology still faces challenges toward commercialization. In this section, we review the most significant challenges for processing high PCE devices from lab-scale to fab-scale.

Fig. 5 Summary of PCEs as a function of the device area for PSCs prepared by different fabrication methods.^{39,57,72,80,83,98,111,113,119–122,122–142}

3.1 Precursors and absorber layer

Upscaling of PVK films emphasizes the suitable storage of precursors as well as film deposition process. For solution-based methods, the precursor solution ratio and storage may have impacts on film quality, influencing both the nucleation and the crystal growth. To address this question, many attempts have been performed to control the film morphology and photovoltaic performance starting from a non-stoichiometric precursor solution.^143,144 As an example, excess of PbI₂ can lead to higher PCEs suggesting that unreacted PbI₂ improves the crystallinity of PVK films.¹⁴⁵ The reported precursor engineering may have some differences based on different solution deposition methods, but most of them are related to introducing of additives as stabilizer or mixing solvents to optimize the precursor viscosity as well as boiling point.¹⁴⁶Table 1 briefly shows the reported requirements, approaches and corresponding solution-based deposition methods. Besides, the detailed PVK precursor solution chemistry is reviewed by Jung et al.¹⁴⁷ from fundamentals to industrialization.

Table 1 Different requirements and strategies of precursor solution preparation/storage based on different solution-based methods

Methods	Requirements	Approaches
Blade coating	Purified organic precursor^148	Reductant^149,150
	High viscosity^39	Moisture barrier^151
		Material with synergistic effects^152
Spray coating	Low viscosity^147	Co-solvents^153
	Suitable boiling point solvent^51	Low ink concentrations^54,154
		Material with synergistic effects^155
Slot die coating	—	Co-solvents^156–158
		Lewis base additive^71
		Material with synergistic effects^134,159,160
Inkjet printing	Slow solvent evaporation^161	Material with synergistic effects^82,162–164
		Co-solvents^165

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Compared with solution-based methods, vacuum deposition can exactly control the deposited amount. However, one of the drawbacks is the lack of compatibility with additive engineering, which is an excellent approach to improve crystal quality, as shown in Table 2. Furthermore, this method shows less flexibility in the number of sources that can be incorporated in the final PVK material. For the deposition process, one of the most challenging part is to form a suitable interaction between substrate and precursors for both solution and vacuum related deposition approaches.¹⁶⁶ A better interaction not only depresses formation of non-radiative centres in the interfacial, but also has an influence on PVK crystal growth.

Table 2 Comparison of solution- and vacuum-based depositions

	Advantages	Disadvantages
Solution methods	• Simple	• Solvent usage^168
	• Compatible with additive engineering^167	• Low material utilization^169
Vacuum methods	• Uniformity control on large area	• limitation in number of sources evaporated
	• Non-toxic solvent	• Long deposition process
	• Flexibility in substrate usage (low processing temperature)^170

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3.2 Charge transporting layers

For large area film fabrication, charge transporting layers (CTLs) should not only have appropriate energy alignment, thickness as well as conductivity, but also be suitable for scalable deposition. Furthermore, environmental stability, energy/time consumption, and costs, are also important aspects for up-scaling. Therefore, the selection of compatible materials and processes is important for PSC commercialization. In this section, CTLs will be briefly discussed focusing on challenges for both hole transporting layer (HTL) and electron transporting layer (ETL).

3.2.1 HTL. Organic materials, such as spiro-MeOTAD^171,172 and poly(triarylamine) (PTAA),^173,174 are widely used as hole transporting material for high-performance PSCs. Spiro-MeOTAD is the most common HTL material in lab-scale device preparation. There are some successful examples for large area device preparation with spiro-MeOTAD as HTL. Luo et al.¹¹² prepared PVK solar modules with spiro-MeOTAD, the device achieved a PCE of 12.24% and device area is 64 cm². Following the spin coating method, Yang et al.³⁹ demonstrated a four-cell module (12.6 cm²) with a stabilized efficiency output of 13.3%. Nevertheless, these pure organic HTL materials have low hole mobility, and dopants such as bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) are required to improve the device PCE. Unfortunately, the introduction of the dopants can accelerate the degradation of PVK. Therefore, spiro-MeOTAD still has a gap between lab and fab due to its cost and the negative effect of dopant. In terms of inorganic materials, such as metal oxides, they are utilized not only to improve device stability but also to simplify the deposition process in mass production. Nickel oxide (NiO_x) is a popular HTL with high material stability.¹⁷⁵ NiO_x film can be formed with a variety of approaches, such as screen printing,¹⁷⁶ spin-coating,¹⁷⁷ and spray deposition.¹⁷⁸ However, the carrier mobility in NiO_x is also not so high and the reported NiO_x based device area is only 3–5 cm².^179,180. Another commonly used metal oxide is molybdenum oxide (MoO_x).¹⁸¹ Different MoO_x film morphologies are obtained with different approaches, such as solution preparation,¹⁸² thermal evaporation,¹⁸³ blade coating,¹⁸⁴ and electrodeposition.¹⁸⁵ Thermally deposited MoO_x is commonly used in PSCs to realize energy level alignment and hole extraction.^186,187 It is generally applied together with spiro-MeOTAD as a bilayer because the interface reaction between MoO_x and PVK may accelerate the degradation of PVK.¹⁸⁸ Several other inorganic materials are also researched for HTL application, for example, CuSCN¹⁸⁹ and VO_x.^190,191 Even though small area devices based on CuSCN and VO_x have achieved excellent PV properties, PCEs drop dramatically with the increase of device area. Next to the abovementioned materials used as HTL in PSCs, some novel materials (such as TaTm:F₆-TCNNQ, TaTm) are also applied as HTL and the details can be found in ref. 192. Taking the cost and stability into consideration, inorganic materials show potential candidates for large area devices, further works focused on good film quality with high hole mobility should be achieved in near future.

3.2.2 ETL. SnO₂,^112,116 C₆₀,^91,183 and PCBM^193,194 are most frequently used as ETLs for high PCE devices because of their high carrier mobility and excellent conductivity. Those materials can also be used for large area ETL fabrication. For TiO₂,^80,195,196 several works show outstanding properties.^197,198 Keremane et al.¹⁹⁸ printed in ambient atmosphere mesoporous TiO₂ enabling 11.55% PCE (70 cm²). However, TiO₂ is reported to show a photocatalytic effect which may degrade the PVK absorber layer.¹⁹⁹ Atomic layer deposition (ALD)-based SnO₂ is also an excellent candidate demonstrating high V_oc, small J–V hysteresis, and negligible photocatalytic effect. Currently, the highest PCE of SnO₂-based PSCs has achieved 23.3% with an active area of 0.1 cm².²⁰⁰ However, SnO₂-based large-area device fabrication still needs to be further improved, because SnO₂ has a higher probability of pinhole formation during film fabrication compared with TiO₂.¹¹⁶ SnO₂ film needs to be thin while it is not easy to fully cover the substrate surface with extremely thin SnO₂ film. This problem is more critical for large area PSCs. Therefore, a trade-off between thickness and mobility needs to be carefully found. Up to now, the most successful example for SnO₂ film scaling up is by Qi's group.¹¹⁶ By precise interface engineering, they demonstrated a PCE of 10% with a designated area of 91.8 cm².¹¹⁶ PCBM is also applied for scalable deposition, which is usually in combination with other materials such as C₆₀ or BCP to form an efficient electron transporting bilayer.^134,193,194 Especially, C₆₀ and BCP can be deposited by thermal evaporation, that makes these films easy to realize also on large area substrates, making it compatible with industrialization. The reported C₆₀ and BCP based devices show PCEs of 14.6% (57.2 cm²)²⁹ and 16.4% (63.7 cm²).¹³⁵

3.3 Back electrode

Thermally or e-beam evaporated metals (Au, Ag, Al) are the mostly used materials for the back electrode in PSCs. Analogous to the metal electrode in c-Si solar cells, a scalable and industrial process for metallization is a critical aspect to address in terms of material consumption for large-area devices. Cheaper materials are needed in replacement of noble metals when looking into commercialization. Nickel (Ni) film is also applied as back electrode for scalable PVK application.²⁰¹ One interesting aspect is that degraded PVK can be washed off from a device with Ni electrodes and then fresh PVK can be reloaded.²⁰¹ This unique recycling technology shows a new way for further development of low-cost PSCs. Besides, the direct contact between a metal and the CTL or PVK may lead to additional reactions. To address this issue, TCO films (for example, indium-doped tin oxide (ITO), aluminum-doped zinc oxide (AZO), indium-doped zinc oxide (IZO)) can be inserted between metal and CTL to prevent the undesired ion immigration. Lee et al.²⁰² fabricated large-scale PVK solar modules with application of multi-layered transparent electrodes, demonstrating excellent thermal stability. Carbon has also been proposed as electrode due to several advantages, such as high stability, low cost, excellent conductivity, and environmental protection.^203,204 A large-area (70 cm²) PVK solar module with carbon electrode was fabricated, achieving a PCE of 10.74% (certified PCE 9.11%).⁶⁴ Besides, graphene is also an excellent candidate as electrode due to its good conductivity, stability and transparency.^205,206 Thus, it is commonly applied as electrode for PSC, especially for semi-transparent PSCs^207,208 or flexible PSCs.^209,210 Based on the above discussion, carbon electrodes show a potential application in PSC commercialization, especially because of their stability and low cost.

3.4 Stability of large area PVK devices

For commercialization of PSCs, device stability is a critical factor that should be taken into consideration, since long-term applications require that PSCs must be stable enough to continuously operate under outdoor conditions. However, PVK materials are sensitive to temperature, light, and humidity, leading to material decomposition and ion migration,²¹¹ finally resulting in poor device stability. This part will highlight the potential instability of PSCs and strategies on the way towards industrialization.

3.4.1 Sources of instability. The instability of PVK and degradation mechanisms have been systematically studied based on experiments and simulations.^21,212 Lab-scale devices show improved stability via different optimization steps, such as introducing inorganic cations,^111,213 surface modification and using 2-dimensional (2D) materials.^214–216 However, to make PVK competitive to c-Si, the issues related to device lifetime still need to be addressed. The instability of PVK film comes mainly from defects (ion migration),^217,218 pinholes (lattice deformation),²¹⁹ and phase transition,²²⁰ which may accelerate the degradation in presence of water,²²¹ high temperature,²²² light²²³ and electric field.²²⁴ To estimate the potential lifetime of PSCs, the International Electrotechnical Commission (IEC) is normally used based on a series of strict tests, such as UV-light, thermal cycling and damp heating.¹⁴² The long-term operation of PSCs does not only depend on stability of PVK film, but also properties of CTLs and electrode, for example, the thermal instability of spiro,²²⁵ UV-instability of TiO₂ (ref. 226) and reaction between I⁻ and Ag.²²⁷

3.4.2 Developments and strategies on long term device stability. Focusing on the instability problems mentioned in section 3.4.1, some strategies are proposed to improve the stability of large area PSCs, which can be summarized into four main parts. Firstly, component engineering. Inorganic cations such as Cs⁺ and Rb⁺ are introduced to stabilize the PVK cubic phase.^228,229 Secondly, interfacial modification is useful to not only passivate non-radiative central defects, increase resistance to moisture, but also hinder I⁻ migration.²³⁰ As an example, the use of 2D materials has been successfully extended to large-area PSC fabrication.²³¹ It was reported that 2D material-based large-area PVK solar modules (PCE 13.4% for 108 cm² and 15.3% for 82 cm²) showed excellent stability under thermal stress test at 65 °C (ISOSD2) for over 1000 h.²¹⁵ Thirdly, select other functional layer materials to replace susceptible CTLs and electrodes. Especially for the HTL, inorganic candidates such as NiO_x,²³² CuO_x²³³ and CuSCN²³⁴ are introduced to substitute organic materials. Table 3 summarizes the structure, stability, and corresponding performance of PSCs with areas larger than 5 cm². Finally, encapsulation. It is the final step of device fabrication, which provides a physical barrier protecting against various outdoor environmental factors. The company Microquanta Semiconductor announced that their encapsulated PVK module (20 cm²) showed degradation of less than 2% after a 100 kW h ultraviolet (UV) preconditioning test (6.5 times IEC standards).²³⁵ One thing need to be noted, we can observe a great variety of test conditions reported in the literature which makes a systematic comparison difficult. To solve this issue, a consensus statement²³⁶ was given by many researchers for stability assessment. Based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols, some other properties of PSCs are investigated, such as ion redistribution under electric fields, reversible degradation, and distinguishing ambient-induced degradation from other stress factors.²³⁶ This section has a summary about strategies on PSC stability towards commercialization, the corresponding detailed information can be found in ref. 142, 237 and 238–242.

Table 3 The structure (light entering from the left-hand side), performance, and stability of large area (≥5.00 cm²) PSCs

Structure	PCE (%)	Area (cm^2)	Stability	Ref.
Glass/ITO/NiOx/PVK/Nb2O5/Ag	11.20	5.00	98% for 150 min, N2 glove box, 100 °C	180
Glass/FTO/TiO2/PVK/spiro-MeOTAD/Au	14.60	12.00	96% for 1200 min, steady-state measurement by tracking the MPP in N2	111
Glass/FTO/d-TiO2/m-TiO2/PVK/WBH/P3HT/Au	16.00	24.97	85% for 1370 h, RH 85%, room temperature	35
Glass/FTO/c-TiO2/m-TiO2/ZrO2/PVK/carbon	10.40	49.00	30 days for RH 80%, 30 °C	80
Glass/FTO/c-TiO2/m-TiO2/ZrO2/PVK/carbon	10.75	70.00	95% 2000 h, RH 65–70%, T 25–30 °C	243
Glass/FTO/c-TiO2/m-TiO2/ZrO2/PVK/carbon	6.40	100.00	96% for full sun light illumination at 35 °C, 1046 h	244

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4. Scaling up of two-terminal monolithic tandem devices

To overcome the SQ limit of single-junction solar cells, the multijunction (MJ) configuration has been developed. The MJ approach is certainly not new in the PV community. In fact, it has been intensively investigated for thin-film Si^245–257 and III-V PV technologies.^258–266 It consists of two or more sub-cells that absorb light of different energies. By tuning the bandgap (E_g) of these sub-cells, better utilization of solar photons can be achieved resulting in higher PCEs. The simplest example is a so-called ‘tandem’ device consisting of two sub-cells stacked one on top of the other with engineered absorber layers. In the monolithic configuration, also known as two-terminal (2T), the sub-cells are electrically connected in series, meaning that the absorber properties must be carefully designed for ‘current matching’, namely each sub-cell should deliver an equal photocurrent. There are also other configurations, such as the four-terminal (4T),²⁶⁷ which eliminates the current matching requirement and provides only a mechanical stacking, and the three-terminal (3T),²⁶⁸ which is suitable for back-contacted bottom cell architectures.^269,270

In this context, PVK/c-Si tandem device attracts much attention because of the stable and mature c-Si technology, the bandgap compatibility between the two absorbers, and the low processing temperature of the PVK top cell. Starting from the 1.12 eV of the E_g of c-Si and using a PVK top cell of 1.70 eV, the theoretical limit of PCE for PVK/c-Si monolithic tandem device is 44.1%.²⁷¹ Considering the great variety of PVK compositions and the ease of integration of new elements in the PVK lattice, E_g can be varied in a wide range of energies from 1.2 eV (ref. 272) to 3.0 eV.^273,274 Consequently, MJ solar cells fully consisting of PVK devices^30–32 or combinations with CIGS bottom cells^275–277 have been demonstrated. Independent of the deployed hybrid technologies, MJ devices using PVKs have shown a rapid PCE increase which can contribute to the acceleration towards the commercialization of PVK devices.

This section focuses on progress and challenges for PCE and stability, which is based on large area (≥1 cm²) PVK/c-Si two-terminal monolithic tandem devices. The detailed discussion based on material, technical and device levels of small area PVK/c-Si tandem devices can be found somewhere else.^278–282

4.1 Conversion efficiency

The efficiency of tandem devices is related to the properties of bottom cell, top cell, and various anti-reflective, recombination, and contact layers. Homojunction and heterojunction silicon (SHJ) can be used as the bottom cell for both 4T^283,284 and 2T tandem devices.^97,285 SHJ solar cells are commonly chosen due to their high V_oc and PCE, which have a positive effect on the tandem device performance. Homojunction cells are also excellent candidates because they currently dominate most parts of the PV market. Up to now, works reported on efficiency improvement of large area tandem devices can be classified into three areas: PVK sub-cell optimization, recombination layer optimization and optical design. Fig. 6 shows a summary of different tandem structures, which were reported to be used for large area tandem devices.

Fig. 6 A summary of different 2T tandem structures for large area PVK-based tandem devices.^{31,97,285,289–293}

4.1.1 PVK sub-cell. For 2T large area tandem solar cells, the PVK film quality strongly determines the top cell photovoltaic properties. It is a big challenge to form high-quality large area PVK film on a textured surface to enable integration with industry-relevant textured c-Si bottom cells. Hou et al.²⁸⁶ demonstrated a solution processed PVK top cell with fully textured c-Si bottom cell by employing an μm-thick PVK layer.²⁸⁶ In recent work, a two-step sequential vacuum process was applied for PVK deposition on textured surface.²⁸⁷ Consequently, 100 cm² textured silicon substrate was perfectly covered by a PVK film via sputtering and CVD. In addition, interface modification, as well as composition engineering, have a huge influence on the whole device efficiency. Low-temperature (70 °C) slot-die coating was applied with a combination of modification strategy for top cell preparation.²⁸⁸ A PVK/textured silicon monolithic 2T tandem device achieved a PCE of 23.8% with an area of 1 cm².²⁸⁸ Besides, Cs⁺ and FA⁺ were also applied to tune the PVK bandgap and high device stability was obtained. The mixed-cation based PVK/c-Si solar cell achieved a PCE of 23.6% with an area of 1 cm².²⁸⁹ Current matching cannot be ignored to obtain a high PCE for tandem device, and the most common method is to optimize the bandgap of the top cell. The bandgap optimization can be realized by composition engineering.

4.1.2 Recombination layer. Improvements in PCE are also driven by selection and optimization of the recombination layer between the two sub-cells. A thorough analysis of many aspects related to the recombination junction for tandem devices was recently published by Bastiani et al.²⁹⁴ Most commonly, transparent conductive oxide (TCO) materials are used as recombination layers because they can fulfill both optical and electrical interconnection requirements. Up to now, recombination layers such as ITO, IZO, AZO have been generally applied for large area 2T tandem devices and excellent outcomes have been obtained. With optimization of the IZO thickness, Werner et al.²⁹⁰ achieved a stable PCE of 19.2% over an aperture area of 1.22 cm². A PCE of 22.6% (area 57.4 cm²) was achieved with a sputtered ITO recombination layer.²⁹⁵ Apart from TCO layers, doped hydrogenated nanocrystalline silicon (nc-Si:H) has also been employed as a recombination layer. For example, Sahli et al.⁹⁷ introduced a nc-Si:H recombination junction to the tandem device with an effective decrease of the parasitic absorption and optical reflection, demonstrating a certified PCE of 25.2% with an active area of 1.419 cm².⁹⁷ Following a similar approach, PCE of 18.0% and 25.1% were obtained with areas of 12.96 cm² and 1.42 cm², respectively.²⁹⁶ The use of nc-Si:H as the recombination layer for large area monolithic tandem devices has been mainly reported by EPFL until now. Another unique structure was reported by Zheng et al.:²⁹¹ the tandem device shows a recombination contact between SnO₂ and a p-doped region, as illustrated in Fig. 6. Compared with the common structure with an interfacial layer, SnO₂ serves not only as an ETL for the top cell, but also a recombination contact with the n-type silicon homojunction solar cell. As a result, PCEs of 17.6% (area 16.00 cm²) and 21% (area 4.00 cm²) were achieved.²⁹¹

4.1.3 Optical design. Optical design is an efficient approach to further improve the device's short current density (J_sc) and to achieve current matching. Currently, optical design for PVK-based tandem devices deals with three aspects: (i) light trapping, (ii) reflection losses, and (iii) parasitic absorption.²⁹⁷ Regarding light trapping, a rear-side textured bottom cell can effectively improve light trapping and increase its near-infrared spectral response.²⁹⁸ In addition, textured structures can also be applied for light management at the front side of tandem devices.^290,299,300 As an example, Jošt et al.³⁰¹ employed a textured foil on the planar front-side of a tandem solar cell, significantly improving J_sc from 17.3 mA cm⁻² to 18.5 mA cm⁻². As for the reduction of reflection losses, introducing an optical interlayer between the sub-cells can significantly decrease the reflection losses resulting from large differences in optical refractive index of Si and PVK. Mazzarella et al.³⁰² employed hydrogenated nanocrystalline silicon oxide (nc-SiO_x:H) with optimized refractive index and thickness as an optical interlayer of a tandem device resulting in 1.4 mA cm⁻² current gain with a device area of 1 cm². Besides, nc-Si:H replacing ITO as a recombination layer improves the optical properties of monolithic tandem device,²⁹⁶ shown in Fig. 6. Based on comparison with ITO, the nc-Si:H recombination junction was demonstrated to mitigate reflection at the sub-cell interface and increase the light transmittance to the bottom cell, consequently, the bottom cell photocurrent increased by more than 1 mA cm⁻².²⁹⁶ In addition, thinner front ITO can also improve light-harvesting due to a lower parasitic absorption as well as reflection. Focussing on decreasing parasitic absorption, spectrum down-conversion materials can convert high energetic photons into visible light, which can be used to avoid the parasitic absorption of CTLs for tandem devices. Zheng et al.³⁰³ employed (Ba,Sr)₂SiO₄:Eu²⁺ phosphor at the front of the monolithic tandem cells to realize PCE as high as 23.1% for an area of 4 cm². It should be noted that J_sc has been dramatically improved from 14.1 mA cm⁻² to 16.5 mA cm⁻². Besides, a thinner transparent electrode and inorganic HTL can effectively decrease the parasitic absorption and further improve the device J_sc. Current progress in terms of parameters and properties of PVK-based large area 2T tandem devices are summarized in Table 4.

Table 4 Parameters and properties of PVK-based large area 2T tandem devices. Reported results are sorted by date

PVK/silicon
Silicon	Perovskite	Top cell bandgap (eV)	Recombination layer	Efficiency (%)	Area (cm^2)	Institute	Year	Ref.
Homo-junction	MAPbI3	1.55	n^++/p^++ Si	13.7	1.00	MIT/stanford	2015	304
	MAPbI3	1.55	ZTO	16.3	1.43	EPFL/CSEM	2016	305
	CsRbFAMAPbI3−xBrx	1.62	ITO	22.5	1.00	ANU	2017	292
	MAPbI3	1.58	—	21.0	4.00	UNSW	2018	291
	MAPbI3	1.58	—	17.6	16.00	UNSW	2018	291
	(FAPbI3)0.83(MAPbBr3)0.17	1.59	—	21.8	16.00	UNSW	2018	291
	CsRbFAMAPbI3−xBrx	1.62	—	23.2	1.00	ANU	2018	285
	FAMAPbI3−xBrx	1.61	—	23.1	4.00	UNSW	2019	303
SHJ	MAPbI3	1.55	IZO	19.2	1.22	EPFL/CSEM	2015	290
	MAPbI3	1.55	IZO	20.5	1.43	EPFL/CSEM	2016	283
	CsFAPbI3−xBrx	1.63	ITO	23.6	1.00	Stanford/ASU	2017	289
	CsFAPbI3−xBrx	1.63	nc-Si:H	21.2	1.43	EPFL/CSEM	2017	296
	CsFAPbI3−xBrx	1.63	nc-Si:H	18.0	12.96	EPFL/CSEM	2017	296
	CsFAPbI3−xBrx	1.63	nc-Si:H	25.2	1.419	EPFL/CSEM	2018	97
	CsRbFAMAPbI3−xBrx	1.62	—	24.1	1.00	ANU	2018	285
	CsFAPbI3−xBrx	1.68	ITO	25.0	1.00	Stanford/ASU	2018	300
	CsFAPbI3−xBrx	1.63	nc-Si:H	25.4	1.43	EPFL	2019	306
	Cs0.05(FA0.77MA0.23)0.95Pb(I0.77Br0.23)3	1.68	ITO	29.1	1.06	HZB	2020	307
	(FA0.65MA0.2Cs0.15)Pb(I0.8Br0.2)3	1.68	ITO	26.2	1.00	KAIST	2020	308
	FA0.75Cs0.25Pb(I0.8Br0.2)3	1.67	ITO	27.1	1.00	CU-Boulder	2020	309
	(Cs0.06FA0.78MA0.16)Pb(Br0.17I0.83)3	1.64	ITO	26.3	1.43	C.H.O.S.E	2020	310
	CsxFA1−xPbIyBr1−y	1.63	ITO	22.6	57.4	CSEM/EPFL	2019	295
	MAPb(I0.75Br0.25)3	1.68	ITO	23.8	1.00	KSC	2020	288

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4.2 Stability of large area PVK-based monolithic tandem devices

Currently, there are some works based on the stability of large-area monolithic tandem devices. The stability of PVK-based monolithic tandem devices is mainly determined by the PVK sub-cell. c-Si is stable enough to operate consistently for many years in ambient conditions. Therefore, the research based on large-area tandem device stability is mainly focused on the top-cell stability improvement. According to recent studies, pinholes are among the factors responsible for device instability. The existence of pinholes leads to film instability because of a decrease in carrier lifetime and mobility.³⁰⁸ To solve this problem, Bush et al.²⁸⁹ applied a bilayer SnO₂/ZTO by ALD or pulsed-CVD deposition to prevent the formation of pinholes. The double-layer enables the device to withstand a 1000 hour damp heat test at 85 °C and 85% relative humidity. Another factor is the reaction between the electrode (Ag) and halogens in PVK. Reducing the reaction between PVK and the Ag electrode can also improve the device stability.³⁰⁹ Sahli et al.⁹⁷ pointed out that buffer layers, as well as a transparent conductive electrode, can efficiently prevent ion migration and suppress the reaction of PVK with the Ag electrode. Besides, a stable carbon or gold electrode can largely improve the device's stability. A third factor impacting stability is moisture/light/temperature-induced environmental degradation. Inorganic materials can be applied to improve thermal stability, such as Cs⁺ for PVK²⁸⁹ and NiO_x for the HTL. As for molecular water, dense buffer layers or transparent polymer layers are introduced to realize a moisture barrier.^97,303 In addition, spectrum down-conversion materials are employed to transfer high-energy photons into low energy photons, realizing a device UV stability.³⁰³Table 5 summarizes the large area device stabilities in detail. Compared to c-Si solar cells, the stability of PVK-based large area 2T tandem solar cells still needs to be improved further.

Table 5 Summary of stability for large area PVK-based 2T tandem devices sorted per device area from the smallest to the largest

Structure	PCE (%)	Area (cm^2)	Stability	Ref.
CsFAPbI3−xBrxPVK/SHJ	23.6	1.00	80% for 1000 h, 85% RH, 85 °C	289
FA0.75Cs0.25Pb(I0.8Br0.2)3PVK/SHJ	27.1	1.00	96% for 1000 hours of MPP operation at 60 °C.	309
Cs5(MA17FA83)95Pb(I83Br17)3PVK/CIGS	23.3	1.03	97% for 11 h, 40 °C, under constant illumination at MPP	311
CsFAPbI3−xBrxPVK/SHJ	25.2	1.42	90% after 270 hours under constant illumination at MPP	97
FAMAPbI3−xBrxPVK/homojunction	23.1	4.00	90% for 288 h, UV exposure	303
(FAPbI3)0.83(MAPbBr3)0.17PVK/homojunction Si	21.8	16.00	91%, for 31 days, room temperature, N2	312

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5. Other issues

5.1 Costs

PVK holds the potential for low-cost production due to the use of abundant material, low processing temperature, and simple deposition methods. The cost of modules is estimated to be around 2 US$ m⁻².³¹³ Therefore, the potentially low cost of PSCs has drawn a lot of attention in the PV market. However, the total industrialization costs, especially when the materials, the lifecycle, and energy consumption are taken into consideration, still don't have a conclusive assessment yet. This section will give a brief illustration of the industrialization cost, focusing on manufacturing cost and levelized cost of energy (LCOE).

5.1.1 Manufacturing cost. The processing cost of PVK solar modules is estimated to be 6.8 ± 1.2 US$ m⁻² based on fully printed devices with sputtering TCO and Al electrode,³¹⁴ while the processing costs of CIGS and CdTe modules are 29 and 27 US$ m⁻², respectively.^315,316 The costs mentioned above exclude the price of glass, frame, laminating film, junction-box, and testing. The lower costs for PVK solar modules originate from cheaper materials used and lower energy consumption. A detailed comparison between PVK and other PV materials for industrialization has been reported in ref. 317. About the material costs, Cai et al.³¹⁸ listed a comparison based on two representative examples of PSCs. The results are shown in Fig. 7a where the calculated costs for materials of module A (expensive example) was 0.127 US$ W⁻¹, which was higher than that of module B (cheap example) of 0.102 US$ W⁻¹. For both examples, the highest expense among the materials comes from the TCO, while the cost of the PVK layer is minor. For the processing costs, Chang et al.³¹⁹ performed a detailed cost distribution of all steps for PVK solar modules based on a fully printing method, as shown in Fig. 7b. Once again, TCO fabrication accounts for a large part of the fabricating cost.

Fig. 7 (a) Material cost analysis for two representative examples³¹⁸ (b) cost contribution of each step for PVK solar modules manufacturing process.³¹⁹ (c) The relationship between PCE, lifetime, and LCOE for PVK PV modules.³¹⁴

Cost modelling is also applied to analyse the PVK/silicon tandem PV technology. Efficiency potential and cost evaluation are accessed in terms of tandem devices and modules. Based on six demonstrated tandem structures, Chang et al.³²⁰ found that the cost barriers are in PVK part because of the expensive transporting layer materials. In particular, the use of homojunction cells with p-type wafers helps to decrease the total costs. With similar approach, Messmer et al.³²¹ has an assessment on both PCEs (optical and electrical) and costs based on four promising silicon bottom cell concepts. The detailed comparison between these structures is shown in ref. 321. All these tandem-based cost analysis points out that attentions should be directed to cheaper materials and feasible techniques in the recent future.

5.1.2 Levelized cost of energy. PV techno-economic competitiveness can be evaluated by the LCOE value. Compared with silicon solar cells,³²² PSCs still exhibit a higher LCOE because of their high capital expenditure. It is a big challenge for PSC fabrication to reach similar costs as silicon solar cells with a similar size, especially considering that the price of silicon drastically reduced over the last 39 years.³²² The relationship between LCOE, PCE and lifetime based on PVK solar modules has been discussed by Song et al.³¹⁴ and Cai et al.³¹⁸ The LCOE values ranging from 4.93 to 7.90 ¢ kW⁻¹ h⁻¹ when device can operate with a PCE of 16% for around 30 years.³¹⁴Fig. 7c summarizes the LCOE for PVK PV modules with different PCEs and lifetimes. The module lifetime strongly affects the value of LCOE. Under the condition that modules work constantly, the LCOE are 10 ¢ kW⁻¹ h⁻¹ and 6 ¢ kW⁻¹ h⁻¹ assuming different lifetimes of 10 years and around 23 years, respectively.³¹⁴ Therefore, stability improvement is an important factor for industrialization of modules related to ambient stability and encapsulation.

The LCOE of PVK/silicon tandem modules is also evaluated, normally together with a detailed comparison of silicon and PVK single junction in reported works.³²³ By applying this approach, Zafoschnig et al.³²⁴ find that PVK/silicon tandem devices have a promising application if the module has a long lifetime and the PCE can exceed 30% using low-cost industrial-scale processes in the next 5–6 years.³²⁴ Therefore, the short lifetime of PSCs (compared to more than 25 years for silicon solar cells) and low-cost processing should be especially addressed in near future.^325,326

5.2 Perovskite solar cell reproducibility

Although many PSC-related articles have reported high PCEs and excellent stabilities, these data are usually the best or average value over many cells. For the PSC industrialization, efforts based on large-area PVK fabrication need to overcome the reproducibility problem. For different fabrication techniques, device reproducibility is mentioned. Vapor-assisted growth method is indicated to have high reproducibility. Gujar et al.³²⁷ demonstrated that 35 devices (36 in total) fabricated with vapor-assisted method worked with very little fluctuation of the photovoltaic parameters (V_oc, J_sc, FF and PCE). Following a similar method, Leyden et al.³²⁸ and Borchert et al.³²⁹ also achieved high device reproducibility for large-area PSCs. In addition, spray coating⁵⁶ and slot-die coating¹³⁴ have also been discussed in detail based on large-area PSC reproducibility. Apart from the active layer, the reproducibility of CTLs is also reported.³³⁰ PSCs with e-beam evaporated NiO_x as HTL show high uniformity and reproducibility.³³¹ However, these studies emphasize the influence of the fabrication technique on reproducibility. Materials should also be taken into consideration for PSC fabrication. Odabası et al.³³² assessed the reproducibility by comparing a large number of PSCs using pooled variances (S_p²) comparing different materials and methods. The S_p² parameters are graphically reported in Fig. 8 and classified by materials that are commonly used in PSCs.³³² The reproducibility is higher for lower S_p² values. This figure provides guidance for identifying the most promising materials for PVK in PV applications. For the ETL, compact SnO₂ is an excellent candidate compared with other materials. P3HT and PTAA show smaller S_p² values compared with spiro-MeOTAD when looking at the HTL. It should be noted that HTL-free devices have also smaller S_p² values when compared to inorganic HTLs for instance. For common electrode materials reported in this study, the use of carbon results in a higher device reproducibility.

Fig. 8 The S_p² value of materials for each layer of PSCs. The data comes from ref. 332.

5.3 Life cycle assessment

A life cycle assessment (LCA) is employed to evaluate the environmental profile and potential impact of PVK solar modules due to lead toxicity and fabrication exhaust.³³³ A detailed relationship between PCE, stability and environmental impacts based on PSCs is systematically reviewed by A. Urbina.³³⁴ In his work, the author reports different materials and configurations discussing the corresponding environmental impacts.³³⁴ Ibn-Mohammed et al.³³⁵ reported on the LCA of PVK solar modules based on MAPbX₃ and CsFAPbX₃. They provide a comparison of a PVK device to other existing PV technologies based on energy consumption, environmental profile and energy payback period (EPBP).³³⁵ They pointed out that not only the toxicity of lead, but also the processing of the operating emissions and used materials should be taken into consideration when evaluating the impact on environment. Besides, greenhouse gas emissions factor (GEF) of the PSC, as well as OPV, are much higher than other PV technologies because of their short lifetime, shown in Fig. 9a.³³⁵ Therefore, long-term stability indirectly affects energy consumption. In addition, a HTL-free device is also modelled providing excellent LCA results.³³⁶Fig. 9b shows the comparison of the PSCs with commercial PV technologies. Environmental impact of global warming potential and primary energy demand are shown as GWP and PED in Fig. 9b.

Fig. 9 (a) A comparison of GEF based on different PV technologies³³⁵ (b) a comparison of environmental impacts based on different factors between PSC and other PV technologies.³³⁶

Moreover, a LCA of PVK-based tandem solar cells has also been modelled.^337–339 Tian et al.³⁴⁰ performed a LCA on the EPBP, carbon footprint, and environmental impact scores for both PVK/silicon and PVK/PVK tandem devices. They emphasized that periodic module replacement and material recycling strategies should be implemented to accelerate the application of PVK/silicon tandem devices in PV market.³⁴⁰ To avoid the trouble of module replacement, a method was proposed by Barkhouse et al.³⁴¹ in 2015. Their modelled results showed that it is important to maintain high infrared transparency and conductivity for the PVK layer after the top cell has stopped working. These properties could ensure that the underlying silicon cell can continue to work without requiring module replacement.³⁴¹ The detailed discussion and comparison of LCA on PVK/silicon and other single-junctions can be found in Leccisi's review.³⁴²

5.4 Summary of companies

With the rapid development towards PSC scaling up, a few companies have been focussing on module fabrication. Most of these companies aim to realize large-area devices (area >100 cm²) mainly using screen printing,²³⁵ roll-to-roll printing,³⁴³ inkjet-printing³⁴⁴ for the PVK film formation. Besides, some companies fabricate flexible PVK solar modules, such as Toshiba,³⁴⁵ Saule Technologies³⁴⁴ and Energy Materials Corporation.³⁴³ Oxford PV has started a manufacturing line for PVK/c-Si tandem cells that are expected to commence production in the year 2021.³⁴⁶ Detailed company information and corresponding progress are summarized in Table 6.

Table 6 Recent development of PSCs for institutions and companies

Company name	Country	Efficiency (%)	Area (cm^2)	Type of devices	Ref.
Oxford PV	United Kingdom	28.0	1	PVK/silicon tandem cells	346
		29.5	—	PVK/silicon tandem cells	352
HZB	Germany	29.8	1	PVK/silicon tandem cells	34
Greatcell solar	Australia	—	20	Focusing on the solar enablement of two substrates: glass and steel, 20 cm^2 PVK module	347
Weihua solar	China	10.6	25	PVK solar module, carbon electrode, HTL-free	348
Solliance solar research	Netherlands	14.5	144	PVK solar module	66
Microquanta semiconductor	China	17.9	227	PVK solar module	235
Solaronix	Switzerland	12.0	500	PVK solar module	349
Toshiba	Japan	11.7	703	PVK flexible solar module	345
Panasonic	Japan	16.1	802	PVK solar submodule (55 series cells)	350
Wonder solar	China	—	110 × 10^4 (3600 cm^2 for each module)	PVK PV system, screen-printed triple mesoscopic PVK solar modules	9
Saule Technologies	Poland	—	—	Inkjet-printed flexible PSCs	344
Energy materials Corporation	United States	—	—	High speed, roll-to-roll printing lines	343
Swift solar	United States	—	—	All-PVK tandem cells and modules	351

---

6. Conclusion and outlook

In this review, the developments and challenges for PSCs from lab-to-fab are systematically discussed focusing on different relevant aspects. In the past years, key material processing, scalable device fabrication, and PSC properties have been investigated in depth. However, compared with other well-established PV technologies, both the efficiency and stability of PSCs show a large gap between small-area cells and large-area modules. To achieve the goals of PSCs upscaling and further industrialization, many issues need to be addressed, including not only the efficiency, but also stability, costs, reproducibility, and fabrication techniques. At present, the main challenges involving PSCs commercialization are: (1) long term stability. Modules should ensure durable outdoor operation under operative conditions such as long-term illumination and heat; (2) cost issues. More efforts should be devoted to identify alternative low-cost raw materials for each layer composing the cell, as well as simplifying the device process; (3) further applications. Broad the PSCs application market, for example, tandem device, flexible device, and indoor PV technology. Finally, we are still at the early stage of the PSCs industrialization. In the long term, can PVK realistically become a competitive candidate in the PV market to uptake silicon technology? How the research community will be able to come up with new ideas and solutions aiming at addressing the key challenges/problems we discussed in this review, will indicate the future paths, and define the success of the emerging PVK technology.

Conflicts of interest

There are no conflicts to declare.

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