Metal halide perovskite-based flexible tandem solar cells: next-generation flexible photovoltaic technology

Abstract

Flexible solar cells, which are compatible with low cost and high throughput roll-to-roll manufacturing, are specifically attractive for applications in wearable/portable electronic devices, building-integrated photovoltaics (BIPV), drones and satellites, etc. Integration of the narrow bandgap flexible solar cells, e.g., Cu(In, Ga)(S, Se)₂ solar cells, organic solar cells, or the new class of lead-tin mixed perovskite solar cells (PSCs) with wide bandgap NIR-transparent PSCs allows two sub-cells to utilize solar light with different photon energies more efficiently and therefore minimizes thermalization loss to overcome the theoretical Shockley–Queisser single-junction limit (33%). In this review, we provide an overview of the recent progress of flexible perovskite-based tandem solar cells from the perspective of the narrow bandgap bottom cell and the near-infrared (NIR) transparent top cell. In addition, we discuss the key limitations related to energy losses in the recombination layer in two-terminal (2-T) tandems and the optical losses in four-terminal (4-T) tandems. Then we outline several strategies to overcome these limitations. Finally, we provide an outlook on roll-to-roll manufacturing and device encapsulation.

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Yan Jiang

Yan Jiang is a Professor at Songshan Lake Materials Laboratory. He received his BS and PhD from Sun Yat-Sen University and Institute of Chemistry Chinese Academy of Science, respectively. Between 2015 and 2020, he worked at Okinawa Institute of Science and Technology Graduate University and Swiss Federal Laboratories for Materials Science and Technology as a postdoctoral scholar. His research focuses on next-generation high-performance energy harvesting materials and devices.

Yabing Qi

Yabing Qi is the Professor and Unit Director of the Energy Materials and Surface Sciences Unit at Okinawa Institute of Science and Technology Graduate University in Japan, and a Fellow of the Royal Society of Chemistry. He received his BS, MPhil, and PhD from Nanjing University, Hong Kong University of Science and Technology, and UC Berkeley, respectively. His research interests include surface/interface sciences, perovskite solar cells, lithium ion batteries, organic electronics, energy materials and devices (https://groups.oist.jp/emssu).

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1. Introduction

According to the global agenda by the Paris COP 21 agreement, the world is transitioning to a low-carbon and sustainable society. As a promising renewable energy source, solar photovoltaic (PV) contributed to about 2% of the total global electricity in 2016 and is expected to boost to an estimated weight of 25% by 2050.¹ The increasing demand for light-weight and flexible PV devices has become more apparent in recent years because the technology revolution and advancement of energy-consuming portable devices allow integration of the device and solar cell into a single piece. Several examples among the diverse applications of flexible solar cells are presented in Fig. 1.^2–7 In 2019, Samsung and Huawei produced flexible and foldable cell phones almost at the same time, making 2019 the first year of wide-spread commercial flexible electronics.^8,9 Flexible solar cells may hold the key to developing overall flexible self-powered portable electronics. Besides, their high-efficiency and light weight allow flexible PV devices to be used for some critical missions in space such as the utilization of solar electric propulsion for space explorations and construction of the space-based solar power (SSP), where traditional space-based PV technologies do not meet the needs from a cost perspective.^10,11

Fig. 1 Flexible solar cells and their diverse applications. (a) Solar-powered high altitude airship,² (b) solar vest,³ (c) solar recharge backpack,⁴ (d) solar leaf,⁷ (e) flexible solar panel,² (f) flexible perovskite solar cells for space application,⁵ (g) solar-powered model plane,⁷ and (h) powering a boat with printed solar.⁶

Improving the power conversion efficiency (PCE) of a flexible solar cell is an ideal way to reduce the levelized cost of electricity (LCOE) of the photovoltaic (PV) system. So far, Cu(In,Ga)(S,Se)₂ (CIGS) and amorphous silicon (a-Si:H) are the most successful flexible solar cell technologies and are dominating the flexible PV market.^12,13 With several technological breakthroughs (e.g., substrate optimization, optical and electronic losses reduction, bandgap grading and alkali post-deposition treatment) in the last three decades, flexible CIGS solar cells have reached a certified efficiency of 20.8% (0.51 cm² aperture area) (Fig. 2).^14–20 A commercial product from MiaSolé Hi-Tech Corp shows a PCE of 17.4% on an aperture area of 0.49 m².²¹ The a-Si:H solar cells have a lower efficiency (16.3% in the case of the a-Si:H/a-SiGe:H/nc-Si:H triple-junction device) but the unique deposition technique, i.e., plasma-enhanced chemical vapor deposition (PECVD), allows the solar cell to be directly fabricated on large-area substrates of up to 5.7 m².¹³ Organic solar cells (OSCs) and perovskite solar cells (PSCs) are new classes of flexible solar cell technologies holding great potential. Flexible OSCs have demonstrated a rapid technological advancement in the last two decades and reached a PCE of 14.06% (Fig. 2).^22–25 PSCs, a so-called game-changer in the PV field, have shown outstanding progress since the first study published in 2009 (Fig. 2).^26,27 Development of low-temperature processed charge transporting layers (CTLs),^28,29 utilizing advanced flexible electrode³⁰ and tuning perovskite composition³¹ have led to flexible PSCs with a stabilized PCE of 19.01% on the area of lab-scale (0.1 cm²).

Fig. 2 Efficiency of flexible single-junction solar cells and perovskite-based flexible tandems in 4-T and 2-T configurations. The data points are adapted from published papers on flexible perovskite solar cells,^{28,29,31–34} flexible CIGS solar cells,^14–20 flexible organic solar cells,^22–25 2-T flexible perovskite-perovskite tandems,³⁵ 4-T perovskite-CIGS flexible tandems,^36–38 2-T flexible perovskite-CIGS tandems³⁹ and 2-T flexible perovskite-OSC tandems.⁴⁰ The dash lines indicate the projected efficiency evolution of various perovskite-based tandems.

State-of-the-art CIGS and OSCs are narrow bandgap solar cells with absorption edges close to 1100 nm and 950 nm, respectively. High-efficiency PSCs show a tunable bandgap with an absorption edge of around 1000 nm in the case of a lead-tin mixed perovskite⁴¹ and 700 nm in the case of a mixed cation (e.g., FA⁺, MA⁺, Cs⁺) and mixed halide (e.g., I⁻, Br⁻) perovskite.⁴² Integration of the narrow bandgap flexible CIGS, OSCs, or lead-tin mixed PSCs with wide bandgap near-infrared (NIR)-transparent PSCs allows two sub-cells to utilize solar light with different photon energies more efficiently and therefore minimize thermalization losses to overcome the theoretical Shockley–Queisser single-junction limit (33%). The two sub-cells can either be fabricated independently and mechanically stacked to form a four-terminal (4-T) configuration or monolithically grown with one device on top of the other to form a two-terminal (2-T) configuration. Simulated PCEs of 4-T and 2-T tandems as a function of the bandgaps of the top and bottom cells are shown in Fig. 3. Each of these two device configurations show unique advantages, which have been reported previously.⁴³ In short, 4-T tandem solar cells present fewer processing challenges and outperform 2-T tandems in terms of PCE and annual energy yield because current matching between the two sub-cells is not required. Besides, the two sub-cells are electrically insulated, meaning that one part of the device still produces power output in case of the failure of the other sub-cell.⁴⁴ 2-T tandem, on the other hand, is simpler in structure and the constituent components, known as the balance of system (BOS), leading to a lower overall cost and possibly a lower LCOE.⁴⁵ In both tandem architectures, the optimal bandgap of the bottom absorber is slightly below 1.0 eV, corresponding to an absorption edge wavelength above 1300 nm. From the efficiency viewpoint, a flexible CIGS solar cell is the best partner for perovskite-based tandems. On the other hand, the optimal bandgap of the top cell is over 1.7 eV in the 4-T tandem and below 1.7 eV in 2-T tandems.

Fig. 3 Structure and simulated efficiency of flexible tandem solar cells. (a) 4-T tandems and (b) 2-T tandems. Reproduced with permission.⁴³ Copyright 2020, Wiley-VCH.

Although the research and development of flexible perovskite tandem solar cells are at an early stage, encouraging results have been demonstrated in several different types of combinations. By the end of 2019, the national renewable energy laboratory (NREL) reported for the first time 2-T flexible all-perovskite tandem solar cells (Fig. 2).³⁵ Integration of a nucleation layer before the recombination layer and engineering the cation composition of the wide bandgap perovskite enable the flexible all-perovskite tandems to achieve an efficiency of 21.3%, which is an over 2% absolute PCE improvement compared to the best single-junction flexible PSCs. In the same month, MiaSolé Hi-Tech Corp and Solliance Solar Research established a new world record PCE of 23% on a 4-T flexible perovskite-CIGS tandem solar cell, higher than the record efficiency of the sub-cells (Fig. 2).³⁸ In 2020, a 2-T perovskite-OSC tandem solar cell was reported by Wang and Zhu et al. by integrating a wide-bandgap perovskite (bandgap = 1.74 eV) with a narrow bandgap organic active PBDB-T:SN61C-4F (bandgap = 1.30 eV) layer. They obtained a PCE of 13.61% and a solar-to-hydrogen efficiency of 11.21% (Fig. 2).⁴⁰ It is rationally expected that more and more efficiency breakthroughs in perovskite tandems will be realized in the next few years.

While the PCE gain in the tandem configurations is a critical performance parameter, the additional weight and manufacturing cost of the component in a tandem solar cell should not be significant. The unique application conditions of flexible PV devices make lightweight and high flexibility prime criteria. Power-per-weight is a vital figure of merit, especially for the high-altitude and space applications when the weight of the solar cells plays a huge role. From this viewpoint, perovskite thin-film PV technologies show special advantages because the thickness of absorbers in devices is around or less than one micrometer. In 2015, Kaltenbrunner et al. fabricated highly flexible PSCs on transparent, commodity-scale, and ultrathin 1.4 μm-thick PET foils (Fig. 4a).⁷ The PSC has a total thickness of 3 μm, including a substrate, electrodes, and a protective encapsulating layer. Using a chromium oxide–chromium interlayer and a transparent polymer electrode, the flexible PSCs realized a stabilized 12% efficiency and a record high power-per-weight of 23 W g⁻¹ (Fig. 4b).⁷ This is three magnitudes higher than CIGS deposited on the glass substrate and two magnitudes higher than CIGS deposited on a polyimide (PI) substrate respectively. Flexible perovskite tandems have demonstrated an almost two-fold factor higher efficiency compared to such flexible PSC. A higher power-per-weight value could be obtained on perovskite-based tandems, especially in the case of the 2-T configuration. Energy payback time (EPBT), which is the time needed for a solar cell to generate the equivalent energy consumed during manufacturing the solar cell, is another important figure of merit from the cost perspective. The EPBT is determined by several factors such as device stack, manufacturing process and solar cell efficiency, etc.⁴⁶ Solution processability and low processing temperature of state-of-the-art PSCs allow an EPBT of 2–3 months, lower than all the other flexible PV technologies such as CIGS (5.6 months), CdTe (8.4–13.2 months) or OSCs (3.6–8.4 months) (Table 1).^47–49 Celik et al. reported that 2-T PSC-CIGS tandem solar cells would realize a similar EPBT value compared to CIGS once the PCE reaches 23%.⁴⁷ More interestingly, they show that PSC-PSC tandems have an EPBT of only 1–1.7 months. This is the lowest value so far reported for PV devices.⁴⁷ Flexible perovskite-based tandems use wide-bandgap near-infrared (NIR)-transparent PSCs as the top cell and/or the narrow bandgap flexible PSC as the bottom cell. In the following two sections, we discuss the criteria and the recent progress in these two sub-cells.

Fig. 4 Power-per-weight analysis. (a) Schematic diagram of the ultrathin perovskite solar cell. (b) Power-per-weight of various photovoltaic technologies. Reproduced with permission.⁷ Copyright 2015, Springer Nature.

Table 1 Energy payback time (EPBT) of various PV technologies

	CIGS	CdTe	OSC	PK	PK-CIGS (2T)	PK-PK (2T)
The data are obtained from ref. 45,47–49.
EPBT (months)	5.6	8.4–13.2	3.6–8.4	2–3	7–8(6.4)	1–1.7

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2. Flexible NIR-transparent perovskite top cell

The function of a flexible NIR-transparent perovskite top cell is to utilize the solar energy in the UV and visible region and to allow energy in the NIR region to pass through. According to the efficiency simulation, flexible NIR transparent perovskite top cells should have a bandgap above 1.55 eV and below 1.9 eV for 4-T tandems and 1.8 eV for 2-T tandems, respectively (Fig. 3). Perovskite materials offer several advantages such as tunable bandgaps covering the target wavelength range, high absorption coefficient, sharp absorption edge and low-temperature processibility, and are ideal absorbers for flexible top cells in tandem configurations.³⁶ Efficiency, flexibility and NIR-transparency are the three key parameters for flexible NIR-transparent PSCs. Flexible NIR-transparent PSCs have a structure of flexible substrate/bottom electrode/electron transport layer (ETL) or hole transport layer (HTL)/perovskite/HTL or ETL/top electrode. Depending on the sequence of ETL and HTL, the devices are either in an n-i-p or a p-i-n configuration. In the following sections, we overview the special manufacturing requirements for flexible PSCs compared to rigid ones and the recent progress of flexible NIR-transparent perovskite top cells.

2.1 Flexible substrates

The flexibility of NIR-transparent PSCs can be realized by deposition of functional layers on top of flexible and transparent substrates. So far, flexible foils such as PEN or PET are the widely used flexible substrates.^50,51 They are mainly polymeric materials and show several advantages including low cost, favorable bendability, high optical transparency, and chemical stability, etc. However, these substrates have a relatively low glass-transition (T_g) temperature (<150 °C) and tend to deform once the temperature reaches their T_g.⁵² To overcome this challenge, a series of low-temperature manufacturing processes have been developed for the deposition of every single layer in PSCs. As a result, whole low-temperature processed PSCs have been reported with a promising PCE.³⁰ On the other hand, flexible foils show a high water vapor transmission rate (WVTR) and oxygen transmission rate (OTR).^53–56 H₂O and O₂ are well-known environmental impacts causing degradation of perovskite materials.^11,57–59 Long-term operational stability of flexible NIR-transparent PSCs using flexible foils is a potential issue. Encapsulation of flexible NIR-transparent PSCs becomes a necessity. Ultra-thin glass is another flexible substrate candidate showing higher transparency, lower H₂O and O₂ permeability than the flexible foils, and can withstand an annealing temperature up to 600 °C.⁶⁰ However, the relatively high price and brittle behavior may become an issue for industrial production on a large scale.

2.2 Flexible and transparent electrodes

Optical transparency, electrical conductivity and mechanical robustness against bending, deformation, twisting and stretching are basic requirements for flexible and transparent electrodes. Transparent conductive oxides (TCOs) are the most successful electrode materials that can be deposited by industrial relevant magnetron sputtering methods.⁴³ However, several issues exist in state-of-the-art TCOs in view of flexible tandem applications, limiting the device performance and robustness. First, to obtain the best optical and electrical performance, a post-deposition-annealing treatment is usually required for TCOs. The high annealing temperature is usually not compatible with the flexible substrates.³⁶ Second, most of the TCOs developed for single-junction solar cells (e.g., ITO and FTO) are highly degenerated n-type semiconductors with a carrier density on the order of 10²¹ cm⁻³.⁶¹ Because of free-carrier intra-band absorption, significant parasitic absorptions appear in the NIR wavelength range (800–1300 nm), accounting for a total photocurrent loss of several mA cm⁻² in perovskite tandems.⁴³ Third, most of the high-performance TCOs, e.g., indium doped tin oxide (ITO) are highly crystalline materials, resulting in poor mechanical robustness. Cracks form in the TCO layers after bending the flexible PSCs at a small radius, which can cause a severe increase in the sheet resistance. In the last years, high-mobility and amorphous TCOs with low processing temperatures such as indium zinc oxide (IZO)³⁷ and hydrogenated indium oxide (IO:H)⁶² have been investigated, laying the foundation for breakthroughs in flexible NIR-transparent PSCs. Besides TCOs, electrode materials that have been utilized in other types of flexible solar cells may also be suitable for flexible NIR transparent PSCs. These electrodes include conductive polymers,⁶³ thin metallic layers,⁶⁴ a stack of a dielectric-thin metallic layer-dielectric (DTD) structure,⁶⁵ transparent carbon-based electrodes (graphene³⁰ and nanotubes⁶⁶) and silver nanowires, etc.^67,68

2.3 Recent progress in flexible NIR transparent PSCs

In 2017, Pisoni et al. employed a low-temperature vacuum-based approach to grow MAPbI₃ perovskite on a flexible substrate that is generally used to encapsulate flexible CIGS (Fig. 5a).³⁵ The flexible substrate shows an ultra-low water vapor transmission rate (WATR) and good UV blocking properties. They demonstrated the first flexible and NIR-transparent PSC (flexible foil/AZO/ZnO/C₆₀/MAPbI₃/Spiro/MoO_x/IO:H/(Ni–Al grid)) with an average transmittance of 78% between 800 and 1000 nm and a PCE of 12.2% on an area of 0.213 cm² (Fig. 5b and c). Integration with a flexible CIGS to form the 4T Pero-CIGS tandem cell resulted in a PCE of 18.2%. In 2018, Pisoni et al. developed a multi-stage approach to deposit perovskite absorbers that allow tailoring of the lead halide growth and perovskite crystallization kinetics.³⁷ They used sputtered IZO as the top and bottom electrodes and prepared a flexible PSC with a structure of flexible foil/IZO/PEIE/C₆₀/MAPbI₃/Spiro-MeOTAD/MoO_x/IZO/(Ni–Al grid) (Fig. 5d). The amorphous nature of IZO enables PSCs to obtain high mechanical robustness against bending at small radii of 6 and 4 mm. They further demonstrated a 4T PSC-CIGS flexible tandem solar cell with a PCE of 19.6%. In 2019, Hu et al. developed a conductive and robust network PEDOT:PSS electrode by combining an ionic additive-conductivity and flexibility enhancer (PEDOT:PSS:CFE) (Fig. 5g).⁶³ The electrode displays high conductivity over 4100 S cm⁻¹ and high transmittance over 85% (Fig. 5h). Using the conductive polymer as the top and bottom transparent electrodes and a mixed halide perovskite as the absorber, they fabricated a flexible semi-transparent PSC with a PCE of 12.5%, excellent mechanical flexibility, and long-term stability (Fig. 5i). In 2019, a record PCE of 21.5% on an area of 0.09 cm² was released by Solliance and MiaSolé.⁶⁹ In 2020, Solliance and MiaSolé announced a flexible 4T tandem device with a new record PCE of 23%.³⁸ So far, no details on these devices have been revealed. Li et al. showed that energy level alignment between a perovskite and charge transport layers plays a significant role in determining nonradiative recombination loss. By constructing gradient energy level alignments at both electron- and hole-selective contacts, they obtained a flexible semitransparent 1.75 eV wide-bandgap PSC with an efficiency of 15.02% and NIR transparency of around 70% (Table 2).⁷⁰

Fig. 5 Flexible NIR-transparent perovskite solar cell. (a) Schematic diagram, (b) transmittance and reflectance curves through the entire device stack, and (c) steady-state efficiency of the flexible NIR transparent PSC. (d) Cross-section SEM image, (e) transmittance, reflectance and absorptance curves, and (f) steady-state efficiency of the flexible NIR transparent PSC. (g) Configuration of the flexible PSC, (h) transmittance curve, and (i) current–voltage curve of the flexible NIR transparent PSC. The insets in (i) show photographs taken through the NIR-transparent perovskite solar cell. (a–c) Reproduced with permission.³⁶ Copyright 2017, the Royal Society of Chemistry. (d–f) Reproduced with permission.³⁷ Copyright 2018, Springer Nature. (g-i) Reproduced with permission.⁶³ Copyright 2019, Elsevier Inc.

Table 2 Summary of flexible NIR transparent PSCs

Device structure	V oc (V)	J sc (mA cm^−2)	FF (%)	η (%)	η mpp (%)	Area (cm^2)	NIR T (%)	Year	Ref.
Foil/AZO/ZnO/C60/MAPbI3/Spiro-MeOTAD/MoOx/IO:H/(Ni–Al grid)	1.08	16.1	68.5	11.9	12.2	0.285	78	2017	36
Sub/IZO/PEIE/C60/MAPbI3/Spiro-MeOTAD/MoOx/IZO/(Ni–Al grid)	1.06	18.7	68.9	13.7	14	0.27	74	2018	37
PET/n-PEDOT:PSS/FAxMA1−xPbIBr/Spiro-MeOTAD/n-PEDOT:PSS	0.99	17.3	60	10.3	NA	0.11	60-80	2018	71
(PET)/TETA-doped GR (TETA-GR)/ZnO/MAPbI3/poly-triarylamine and the PEDOT:PSS/TFSA-doped GR (TFSA-GR)/PET stacks	0.94	18.4	64.87	11.22	NA	1	<70	2018	72
PET/PEDOT:PSS:CFE/PEDOT:PSS/PVSK/PCBM/PEDOT:PSS:CFE	NA	NA	NA	NA	12.5	0.1	NA	2019	63
PET/ITO/CyDTA/SnO2/C–SnO2/perovskite/spiro-OMeTAD/MoOx/ITO.	1.186	16.35	77.5	15.02	13	0.07	70	2020	70

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In 2019, Palmstrom et al. reported an exciting result on flexible and all-perovskite monolithic tandems.³⁵ They partially substituted Cs with dimethylammonium (DMA) in mixed halide perovskite, leading to a 1.7 eV bandgap perovskite with a high and stable V_oc of 1.2 V (Fig. 6a–c). Furthermore, they implemented a 1 nm-thick poly(ethylenimine)ethoxylated (PEIE) nucleation layer that enables a conformal coating of crystallized aluminum zinc oxide recombination layer (Fig. 6d). Such a recombination layer efficiently combines the two sub-cells and prevents damage to the bottom cell when processing the top cell. The flexible and all-perovskite monolithic tandems deposited on a PEN substrate achieved a PCE of 21.4% (Fig. 6e, f). The all-perovskite flexible monolithic tandem may hold the potential for the development of high-efficiency flexible tandem solar cells with ultra-low EPBT.

Fig. 6 Stable wide bandgap PSC for flexible tandems. (a) Absorbance spectra and (b) bandgap of perovskite with different DMA ratios in the A-site. Compositions of the perovskite layers are FA_0.725(1−x)Cs_{0.275(1−x)+0.5x}DMA_0.5xPbI₃ and FA_0.725(1−x)Cs_{0.275(1−x)+0.5x}DMA_0.5xPbI_2.4Br_0.6. DMA refers to dimethylammonium iodide. (c) Current–voltage characteristics of the champion PSC (10% DMA, 20% Br). (d) Cross-section SEM image, (e) photograph and (f) current–voltage characteristics of the flexible all-perovskite tandem solar cell. (a–f) Reproduced with permission.³⁵ Copyright 2019, Elsevier Inc.

3. Progress in narrow bandgap perovskite bottom cells

3.1 Sn-based perovskite solar cell

The narrow bandgap sub-cell in a tandem solar cell absorbs lower energy photons and extends the absorption edge of a tandem device to the NIR region. A practical approach to reducing the bandgap of perovskite is to substitute Pb with Sn. Sn-based perovskites have a similar isoelectronic configuration of s²p² compared to the Pb-based perovskites but a smaller optical bandgap.⁷³ In 2014, Snaith et al. reported the first methylammonium tin iodide (CH₃NH₃SnI₃) perovskite solar cells on glass substrates with a PCE of 6.4%. The bandgap of CH₃NH₃SnI₃ is 1.23 eV, derived from the absorption spectra.⁷⁴ Key limits of Sn-based perovskite are the relative low formation energy of Sn²⁺ vacancy and the tendency of oxidization to Sn⁴⁺, resulting in a highly conductive p-type behavior.⁷⁵ Additives such as 5-AVAI,⁷⁶ SnF₂, etc.,^77,78 are developed to mitigate the Sn²⁺ oxidation issue. Wu et al. introduced Lewis base molecules in the precursor solution that can efficiently tune the crystallization kinetics. They fabricated a pinhole-free FASnI₃ absorber which showed a PCE of 10.1% in PSCs.⁷⁹ In 2019, Xi et al. developed a multichannel interdiffusion method to prepare uniform FASnI₃ films. They for the first time demonstrated flexible Sn-based PSCs on flexible polyethylene naphthalate (PEN) substrates with a PCE reaching 3.12% (Fig. 7a).⁸⁰

Fig. 7 Flexible narrow bandgap PSC. (a) Efficiency evolution of the Pb–Sn mixed PSCs on rigid substrates and a flexible FASnI₃ PSC. (b) Bandgap variation of the mixed Sn–Pb perovskites with different Cs ratios, (c) origin of the bandgap bowing in MA(Pb_1−xSn_x)I₃ from the molecular orbital point of view. (a) Reproduced with permission.⁸³ Copyright 2020, Wiley-VCH. (b) Reproduced with permission.⁹⁰ Copyright 2017, American Chemical Society. (c) Reproduced with permission.⁸² Copyright 2018, American Chemical Society.

3.2 Mixed Sn–Pb perovskite solar cell

Mixed Sn–Pb iodide perovskite alloy, e.g., MAPb_xSn_1−xI₃ is another promising narrow bandgap absorber candidate. Interestingly, the bandgap of the alloy is lower than MAPbI₃ and MASnI₃, showing a bandgap bowing effect (Fig. 7b). This makes mixed Sn–Pb perovskite a better bottom cell for perovskite tandems in view of the theoretical PCE (Fig. 3). The reason for this abnormal behavior has been investigated by various researching groups. Several explanations such as composition-induced crystal structure variations⁸¹ and short-range ordering of Pb and Sn ions⁴¹ have been proposed. Goyal et al. applied electronic structure calculations at different levels of theory to investigate the origin of the nonlinear bandgap behavior. The results indicate that the bandgap bowing effect is because of the mismatch in energy between the s and p orbitals of Pb and Sn, which forms the band edges of the alloy (Fig. 7c).⁸² The lowest bandgap of mixed Sn–Pb is around 1.2 eV for MASn_1−xPbXI₃ where x values equal to 0.25 and 0.5. With great research efforts devoted to the field, the PCE of the mixed Sn–Pb PSCs on glass substrates boosts from 10.1% in 2014 to 21.1% in 2020 (Fig. 7a and Table 3).⁸³ PEDOT:PSS, although found to cause device degradation, is the commonly used HTL to deliver high-efficiency narrow bandgap PSCs. In 2020, Chen et al. reported an exciting result by substituting PEDOT:PSS with a room temperature-processed NiO_x HTL based on nanocrystals. The NiO_x HTL shows a deeper valence band and lower trap density than high-temperature processed NiO_x, leading to a PCE of 18.77% for narrow bandgap PSCs.⁸⁴ So far flexible mixed Sn–Pb PSCs have rarely been reported because the widely used flexible substrates for the fabrication of PSCs have high WATR and OTR. It is technically challenging to prevent the oxidation of Sn²⁺ to Sn⁴⁺. Implementation of low WATR and OTR flexible substrates may hold the key to developing flexible mixed Sn–Pb PSCs.

Table 3 Summary of narrow bandgap PSCs

Device structure	Bandgap (eV)	V oc (V)	J sc (mA cm^−2)	FF (%)	η (%)	η mpp (%)	Area (cm^2)	Year	Ref.
Glass/FTO/c-TiO2/meso-TiO2/CH3NH3SnI3/Spiro-MeOTAD/Au	1.23	0.88	16.8	42	6.4	—	0.12	2014	74
Glass/ITO/PEDOT:PSS/CH3NH3SnI3/PCBM/Ag	1.23	0.57	20.68	66	7.78	—	0.09	2020	78
Glass/ITO/PEDOT:PSS/FASnI3/C60/BCP/Ag	1.4	0.64	21.83	73.9	10.32	10.1	0.09	2020	79
PEN/ITO/PEDOT:PSS/FASnI3/C60/BCP/Ag	1.4	0.31	16.07	62.6	3.12	—	0.09	2017	80
glass/ITO/PEDOT:PSS/MAPb0.85Sn0.15I3−xClx/PCBM/C60/Ag	1.4	0.77	19.5	67.0	10.1	—	0.03	2014	85
Glass/ITO/PEDOT:PSS/(FASnI3)0.6(MAPbI3)0.4/C60/BCP/Ag	1.25	0.853	28.5	72.5	17.6	—	0.085 to 0.12	2017	86
Glass/ITO/PEDOT:PSS/(FASnI3)0.6(MAPbI3)0.4/C60/BCP/Ag	1.25	0.841	29.0	74.4	18.1		0.105	2018	87
Glass/ITO/PEDOT:PSS/(FASnI3)0.6(MAPbI3)0.4/C60/BCP/Ag	1.25	0.834	30.4	80.8	20.5	20.2	0.105	2019	88
Glass/ITO/PEDOT:PSS/FA0.7MA0.3Pb0.5Sn0.5I3/C60/BCP/Cu	1.22	0.831	31.4	80.8	21.1	20.9	0.049	2019	89
Glass/ITO/NIOx/(FASnI3)0.6(MAPbI3)0.4/C60/BCP/Ag	1.25	0.82	29.6	77.2	18.77	18.4	0.04	2020	84

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4. Energy losses in the recombination junction of 2-T perovskite tandems

Recombination junction, also called interconnection or tunneling junction, is a typical structure in the 2-T tandem solar cell that physically and electrically connects the two sub-cells. Under illumination, electrons and holes are continuously generated from the two sub-cells. Photo-generated electrons transfer to the conduction band minimum and holes to the valence band maximum. Electron tunneling happens through forbidden regions into empty states, allowing the electrons and hole to recombine in the recombination junction (Fig. 8a). A typical recombination junction is a heavily doped p⁺–n⁺ junction.^91,92 Different from a p–n junction where a high resistive and wide depletion region is created, the p⁺–n⁺ junction forms a narrow depletion region with atomic width,⁹³ enabling the tunneling current to flow at a small bias voltage (Fig. 8b).

Fig. 8 Tunneling junction in a perovskite tandem solar cell. (a) Energy level diagram of a tandem solar cell connected via a recombination layer and (b) current–voltage curves at the tunneling junction with respect to a p–n junction and an ohmic contact. (c–e) Schematical drawing of perovskite-perovskite tandems. The tunneling junctions are SnO₂/Au/PEDOT:PSS in (c), ITO/PEDOT in (d) and (n⁺) C₆₀/(n) SnO_2−x in (e). (f and g) Schematical drawing and SEM images of perovskite-CIGS tandems. The tunneling junctions are ITO/PTAA in (f) and AZO/SAM in (g). (a) Reproduced with permission.¹⁰⁰ Copyright 2020, Wiley-VCH. (b) Reproduced with permission.¹⁰¹ Copyright 1974, American Chemical Society. (c) Reproduced with permission.⁹⁴ Copyright 2020, Springer Nature. (d) Reproduced with permission.⁹⁵ Copyright 2020, Springer Nature. (e) Reproduced with permission.⁹⁶ Copyright 2020, Springer Nature. (f) Reproduced with permission.⁹⁸ Copyright 2018, the American Association for the Advancement of Science. (g) Reproduced with permission.⁹⁹ Copyright 2019, the Royal Society of Chemistry.

In perovskite-based tandems, the recombination junction usually consists of ETL and HTL from the perovskite sub-cell that is not a highly-degenerated semiconductor. Direct contact of the ETL with the p⁺ layer or the HTL with the n⁺ layer may result in high contact resistivity and therefore the unbalanced electron and hole populations on two sides of the recombination junction. A substantial resistive loss will be encountered, decreasing V_oc and FF of the tandem device. A thin layer of metal or TCO is usually implemented as part of the recombination junction to aid in carrier recombination via a trap-assisted tunneling (TAT) channel.⁹³ Xiao et al. used a 20 nm-thick SnO₂ film by atomic layer deposition (ALD) as the ETL of a PSC. They thermally evaporated a 1 nm-thick Au layer between the n-type SnO₂ and the p-type PEDOT:PSS. The SnO₂/Au/PEDOT:PSS recombination junction facilitates electron–hole recombination and results in a tandem PCE of 24.8% (Fig. 8c).⁹⁴ Li et al. deposited a 130 nm-thick ITO between the ETL of C₆₀/BCP and HTL of PEDOT:PSS to construct the recombination junction, and their all-perovskite tandems show a V_oc of 1.043 V, a FF of 79.3% and a PCE of 23.3% (Fig. 8d).⁹⁵ Yu et al. developed a C₆₀/SnO_1.76 recombination junction structure. The presence of a large density of Sn²⁺ allows SnO_2−x to show ambipolar carrier transport property and forms ohmic contacts with adjacent layers from the two sub-cells (Fig. 8e).⁹⁶ Their all-perovskite tandems using the C₆₀/SnO_1.76 recombination junction structure obtained a PCE of 24.4%.

From the energy loss perspective, some critical criteria for development of recombination layers include (i) excellent out-of-plane conductivity for efficiency electron–hole recombination; (ii) limited lateral conductivity to avoid unwanted shunting paths; (iii) have a minimum optical absorption in the NIR region. With these aims, Palmstrom et al., used a highly resistive IZO to construct a recombination junction with a structure of C₆₀/PEIE/AZO/IZO/PEDOT:PSS. They reported the first flexible and all-perovskite monolithic tandems with a PCE of 21.3%.³⁵

From the manufacturing point of view, the tunneling junction physically connects the two sub-cells. Deposition of uniform and high-electric-quality charge-transport layers and absorber in the top cell relies on the suitable recombination layer. PSC-CIGS 2-T tandem is a promising flexible solar cell technology. Generally, a high-quality CIGS absorber displays a rough surface (roughness is on the hundred-nanometer level). Because the functional layers in PSCs are a few tens to hundreds of nanometers, it is a challenge to achieve the conformal deposition of these layers on top of the rough CIGS surface. Jošt et al. systematically optimized the recombination junction from ZnO/PTAA to ZnO/NiO_x and ZnO/NiO_x/PTAA, and realized a conformal coating of NiO_x on top of the rough CIGS bottom cell. The efficiency of the 2-T PSC-CIGS tandems on a glass substrate improves from 3.1% to 21.6%.⁹⁷ Han et al. developed a boron-doped ZnO(BZO)/ITO/PTAA recombination junction. The ITO layer was chemical-mechanically polished to realize a smooth surface (Fig. 8f).⁹⁸ The advancement in recombination junction design leads to a PSC-CIGS 2-T tandem solar cell with an efficiency of 22.4%. In 2019, Al-Ashouri et al. used a self-assembled monolayer (SAM) of [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) as an AZO surface modifier and eliminated PTAA HTL. The SAMs create an energetically aligned interface to the perovskite absorber with negligible non-radiative losses, resulting in a PSC-CIGS 2-T tandem with an efficiency of 23.3% (Fig. 8g).⁹⁹

5. Optical losses in the flexible 4-T perovskite tandems

Flexible 4-T perovskite tandems have three transparent electrodes and several air/flexible substrate interfaces. Parasitic absorption losses in the TCOs and broadband reflection losses at the air/flexible substrate interfaces account for a total current loss of 4–5 mA cm⁻².⁴³ Mitigation of these optical losses holds the key to enhancing the photocurrent and the overall performance of flexible 4-T tandems.

5.1 Parasitic absorption loss in transparent conductive oxides

Transparent conductive oxides are the most common electrode materials for flexible perovskite and tandem solar cells. State-of-the-art TCOs such as FTO and ITO offer superior conductivity and high transparency in the visible region (400–800 nm). However, significant parasitic absorptions appear in the NIR wavelength range (800–1300 nm), owing to free-carrier intra-band absorption (FCA). Take the commercial ITO as an example, absorptance of a 200 nm-thick ITO film reaches 18% at a wavelength of 1000 nm.⁶² Considering that the low-energy photons should pass through three TCO layers before reaching the absorber of the narrow bandgap cell, a huge absorption loss is created and transformed to heat waste. To mitigate the FCA losses while maintaining high carrier conductivity, high carrier mobility TCOs are urgently needed. In addition, these TCOs should be deposited and post-annealed at a low temperature, i.e., <150 °C to be compatible with flexible substrates.

In the last years, a series of TCO layers such as tungsten and hydrogen doped indium oxide (IWO:H), cerium and hydrogen doped indium oxide (ICO:H), zirconium-doped indium oxide (IZrO) etc., have been developed, showing high electrical conductivity and small NIR absorptance of less than 2%.¹⁰² In 2020, Jiang et al., developed an In₂O₃:H (IO:H) film deposition process by independently controlling H₂ and O₂ gas flows during magnetron sputtering.⁶² Tuning the H₂ and O₂ partial pressures allows decreasing the crystallization temperature to 130 °C, making it suitable for flexible substrates. Optical loss analysis suggests that replacing the state-of-the-art rear IZO of the PSC and front IZO of the CuInSe₂ (CIS) with the low-temperature processed IO:H electrode leads to a 1.38 mA cm⁻² short-circuit current (J_sc) gain in the bottom cell (Fig. 9a–c).

Fig. 9 Optical loss analysis. (a) Schematic diagram of the 4-T perovskite/CIS tandems with IZO or IO:H electrodes. Absorptance loss analysis of the 4-T perovskite/CIS tandems (b) using IZO and (c) using IO:H as the back electrode of PSC and the front electrode of CIS. (d) Schematical drawing of the flexible NIR transparent PSC and (e) the optical loss analysis. (a–c) Reproduced with permission.⁶² Copyright 2020, the American Chemical Society. (d and e) Reproduced with permission.³⁷ Copyright 2018, Springer Nature.

Besides, incorporation of metallization schemes is a powerful strategy that has been widely employed in thin-film solar cells such as CIGS and CdTe. A well-designed high-conductivity metal grid with minimal shadowing losses could be deposited on TCOs to enable reduced thickness while maintaining high conductivity.¹⁰³

5.2 Reflection losses at the device/air interfaces

Reflective losses at the cell/air interface account for around 10% of the total available photocurrent from the AM 1.5 solar spectrum.¹⁰³ Previous reports on reflective loss are mostly obtained on non-flexible solar cells, where the interface is an air/glass substrate. Stefano et al. investigated the reflective loss at the air/flexible PSC interfaces. The device has a structure of flexible foil/IZO/C₆₀/PEIE/MAPbI₃/Spiro-MeOTAD/MoO_x/IZO/(Ni–Al grid) (Fig. 9d).³⁷ Experimental and simulation results reveal a significant reflection loss between 450 nm and 500 nm (Fig. 9e). A total reflection loss between 380–800 nm accounts for 1.8 mA cm⁻². Several strategies have been attempted to mitigate reflection losses. (I) Deposition of an anti-reflective coating (ARC) with a refractive index intermediate between that of air and the flexible substrate. ARCs such as MgF₂ and LiF have been proven to be effective solutions in reducing reflection losses at the device/air interface. Coatings that give very low reflection over a broadband of frequencies can also be realized by the construction of multi-layer interference. (II) Application of polymer-based light management foils. Because of the retroreflective effect, these foils minimize reflection in a relatively wide wavelength range.¹⁰⁴

6. Outlook

In the last years, the significant progress in flexible electronic and portable devices has resulted in an unprecedented demand for light-weight and flexible PV devices. Several technology breakthroughs have been realized in both 4-T and 2-T perovskite-based tandem solar cells, leading to tandem PCEs surpassing single-junction flexible PV technologies. Perovskite-based flexible tandem solar cells are very likely to be the next-generation flexible photovoltaic technology. On the other hand, development of perovskite-based tandems is still at an early stage. According to the technology maturity estimation methods, e.g., technology readiness levels (TRLs), developed at the National Aeronautics and Space Administration (NASA) and modified by the European Commission (EC), flexible perovskite tandem solar cells are at a level of TRL1 and TRL2 (basic technology research). The next breakthrough in flexible perovskite tandems is to manufacture proof-of-concept demonstrators (TRL3) and validate them in independent laboratories (TRL4). So far, several main challenges lie ahead, hindering the advancement in flexible perovskite tandem solar cells. These challenges include maintaining high efficiency over large area devices, reduction of the environmental impact, and demonstration of the long-term operational stability. In the next, we provide insight into solving these challenges by development of large-area low-temperature manufacturing processes and encapsulation methods.

6.1 Large area low-temperature manufacturing

Although high-efficiency flexible perovskite tandem solar cells have been demonstrated, these devices have so far been achieved with relatively small active areas (<1 cm²). Flexible perovskite tandem mini-module (200–800 cm²) or modules (>800 cm²) have not been reported yet. Increasing the cell area to the mini-module or module level usually results in efficiency loss because of increased series resistance, dead areas at the interconnection region and more shunting paths. While resistive and dead area losses can be largely mitigated by using highly conductive and transparent electrodes and rational design of P1, P2 and P3 patterning, reducing shunting loss relies on the development of deposition methods for every layer in perovskite tandem solar cells with high uniformity and without pinholes. Many low-temperature vapor and solution-based methods that have been successfully demonstrated in rigid perovskite tandem solar cells can be used for processing flexible perovskite tandem solar cells. These methods include magnetron sputtering,^105,106 hybrid chemical vapor deposition,^107–111 atomic layer deposition,³² blade coating,¹¹² slot-die coating,^113,114 spray coating^115,116 and inkjet printing, etc.¹¹⁷

Several issues have to be considered when transferring the technology from rigid to flexible substrates. First, coefficient of thermal expansion (CTE) of flexible foil is several times and up to over a magnitude higher than the rigid substrate. Deposition recipes that result in uniform thin-film coating on rigid substrates may not work for flexible substrates (i.e., forming cracks in thin film). Post-annealing temperature and ramp up/down speeds might need to be reoptimized. Second, surface roughness of flexible substrates is usually higher than rigid ones. The thickness of functional layers may need to be finely adjusted. A demonstration of flexible tandem mini-modules processed by these low-temperature methods could be an encouraging step forward. These deposition recipes can be used as benchmarkers for further optimization to obtain large-area flexible tandem modules.

In addition, the high flexibility makes roll-to-roll (R2R) methods especially suitable for manufacturing flexible perovskite tandem solar cells with high throughput and therefore low EPBT. In 2018, Dou et al. developed a perovskite precursor ink (methylamine-charged acetonitrile) that allows for fast (∼1 s) perovskite crystallization and uniaxially oriented perovskite crystal growth. They demonstrated a uniformly R2R-coated perovskite absorber on a flexible glass substrate (25 cm × 20 m) with a PCE reaching 14.1% on 0.15 cm².¹¹⁸ In 2020, Kim et al. combined the R2R gravure-printing and anti-solvent bathing methods to fabricate flexible PSCs on PET substrates (Fig. 10a).¹¹⁹ The SnO₂ ETL, perovskite layer and the P3HT HTL are all R2R-processed at fast speeds (3–8 m min⁻¹) (Fig. 10b). The flexible PSC shows a structure of PET/ITO/SnO₂/FAPbI₃/P3HT/Au (Fig. 10c). They demonstrated a pilot-scale (100 m long roll) flexible PSCs with an efficiency of 13.8% on an area of 0.096 cm². These methods pave the way for large area and low-temperature manufacturing of flexible perovskite tandem mini-modules and modules.

Fig. 10 Roll-to-roll manufacturing. (a) Schematic diagram of the R2R processing for flexible PSCs. (b) Photograph of R2R gravure-printed PSCs. Inset is a photograph of the R2R-processed roll showing constituent layers. (c) Cross-section SEM image of the flexible PSC. (a–c) Reproduced with permission.¹¹⁹ Copyright 2020, Springer Nature.

6.2 Encapsulation for flexible perovskite tandems

Perovskite-based solar cells tend to degrade under external stresses such as moisture, heat, oxygen, illumination and vacuum, etc.¹¹ Instability of perovskite subcells is the Achilles heel hindering the long-term operational stability of the perovskite-based flexible tandems. Realization of stable perovskite-based flexible tandems relies on breakthroughs from the perovskite cell side, instead of the partner cell. In addition, high-efficiency PSCs contain ionic Pb²⁺ in the absorber, accounting for around 30% of the weight of the perovskite absorber. Potential environmental impacts entailed by perovskite-based solar cells remain a significant concern. The good news is that recent advances on encapsulation methods for PSCs may hold the key to these issues for flexible perovskite tandems.

Weerasinghe et al. investigated the effect of various encapsulation methods on the stability of the flexible PSCs.¹²⁰ They used a plastic barrier encapsulant, i.e., Viewbarriers (Mitsubishi Plastic, Inc) to partially or completely laminate the flexible PSCs (Fig. 11a) and kept the flexible PSCs under ambient conditions. The encapsulated devices show substantially improved shell-lifetime compared with the non-encapsulated devices (Fig. 11b). The partially encapsulated devices displayed a slow PCE decay at the first 450 h and a drastic decay afterward, and the completely encapsulated device maintains the initial PCE after 500 h.

Fig. 11 Encapsulation against instability and Pb leakage. (a) Schematic diagram showing a partially (top) and a completely (bottom) encapsulated flexible PSC. (b) Stability of the flexible PSCs. The devices were kept in an ambient atmosphere. (c) Encapsulation methods against Pb leakage. (d) Molecular structure of the epoxy resin. (e) Schematic diagram showing the self-healing property of the epoxy resin. (a and b) Reproduced with permission.¹²⁰ Copyright 2015, Elsevier Inc. (c–e) Reproduced with permission.¹²¹ Copyright 2019, Springer Nature.

Jiang et al. quantitatively estimated the Pb leakage from damaged perovskite solar modules with a structure shown in Fig. 11c.¹²¹ Experimental and simulation results suggest that the microcracks formed during external impact are a crucial factor determining the Pb leakage rates. They developed an epoxy resin encapsulation method that reduced the Pb leakage rate by a factor of 375 compared to the normal encapsulation method. Li et al., deposited transparent and lead-absorbing materials on both sides of the devices that can chemically absorb leaked Pb of up to 96%.¹²² Chen et al. reported an abundant, low-cost and chemically robust cation-exchange resin (CER)-based encapsulation approach that exhibits both high adsorption capacity and high adsorption rate of lead in water, and negligible effect on the solar cell efficiency.¹²³ Although these encapsulation methods are demonstrated on rigid PSCs, with some modifications they can very likely be implemented in flexible perovskite tandems.

Aiming at the development of stable and low environmental impact flexible perovskite tandem solar cells, encapsulation materials should process the following criteria, including high flexibility, low WATR and OTR, high optical transparency and low reflectivity, high mechanical robustness and the capability to stabilize the leaked Pb²⁺. In addition, suitable thin-film fabrication methods have to be developed for these advanced encapsulation materials.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

Y. J. acknowledges the funding support from the Energy Materials and Optoelectronics Unit of Songshan Lake Materials Laboratory. Y. B. Q. acknowledges the funding support from the Energy Materials and Surface Sciences Unit of the Okinawa Institute of Science and Technology Graduate University.

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