A facile and low-temperature process to prepare planar perovskite solar cells (PSCs) has led to considerable progress in flexible solar cells toward high throughput production based on a roll-to-roll process. However, the performance of planar PSCs is still lower than that of mesoscopic PSCs using a high temperature process. Here, we report a new concept of a low temperature processed porous planar electron transport layer (ETL) inspired by a mesoporous structure for improving the performance of flexible devices. The structurally and energetically designed porous planar ETL induced the formation of a high quality perovskite and a preferred band alignment, resulting in improved charge collection efficiency in a fabricated device. Through the porous planar ETL, we achieved a power conversion efficiency (PCE) of 20.7% with a certified efficiency of 19.9% on a flexible substrate, which is the highest PCE reported to date. In addition, for the first time, we succeed in fabricating a large area flexible module with the porous planar ETL, demonstrating a PCE of 15.5%, 12.9% and 11.8% on an aperture area of 100 cm2, 225 cm2 and 400 cm2, respectively. We believe that this strategy will pave a new way for realizing highly efficient flexible PSCs.

　The hole-transporting layer is an essential component in a perovskite solar cell (PSC) and plays a key role in controlling both power conversion efficiency (PCE) and stability. Here, we report a new hole-transporting material (HTM), methoxy group-containing poly(triarylamine) (PTAA) (CH3O-PTAA), for efficient PSCs with improved thermal stability. As compared to commonly used PTAA (CH3-PTAA), CH3O-PTAA exhibits enhanced doping ability and stability under thermal stress. With CH3O-PTAA, (FAPbI3)0.85(MAPbBr3)0.15-based PSCs show high PCEs over 20%, comparable to those of CH3-PTAA devices. More importantly, better long-term thermal stability with only 3% reduction from the initial PCE (6.1% reduction on average) has been achieved for encapsulated PSCs with CH3O-PTAA than that of PSCs with CH3-PTAA under dark storage conditions (ISOS-D-3) of 85 °C and 85% relative humidity (RH) over 1000 h. Detailed studies have been conducted to reveal the strong correlation between the doping behavior of HTMs and the performance of PSCs, which provide useful guidelines for the design of new HTMs for efficient and stable PSCs.

　Driven by recent improvements in efficiency and stability of perovskite solar cells (PSCs),
upscaling of PSCs has come to be regarded as the next step. Specifically, a high-throughput,
low-cost roll-to-roll (R2R) processes would be a breakthrough to realize the commercialization of PSCs, with uniform formation of precursor wet film and complete conversion to
perovskite phase via R2R-compatible processes necessary to accomplish this goal. Herein, we
demonstrate the pilot-scale, fully R2R manufacturing of all the layers except for electrodes in
PSCs. Tert-butyl alcohol (tBuOH) is introduced as an eco-friendly antisolvent with a wide
processing window. Highly crystalline, uniform formamidinium (FA)-based perovskite formation via tBuOH:EA bathing was confirmed by achieving high power conversion efficiencies
(PCEs) of 23.5% for glass-based spin-coated PSCs, and 19.1% for gravure-printed flexible
PSCs. As an extended work, R2R gravure-printing and tBuOH:EA bathing resulted in the
highest PCE reported for R2R-processed PSCs, 16.7% for PSCs with R2R-processed SnO2/
FA-perovskite, and 13.8% for fully R2R-produced PSCs.

　Wide-bandgap perovskite solar cells (WBG PSCs) have gained attention as promising tandem partners for silicon solar cells due to their complementary absorption, superb open-circuit voltage, and an easy solution process. Recently, both their performance and stability have been improved by compositional engineering or defect passivation strategies, due to the modulation of perovskite crystal size and reduction of crystal defects. Herein, a report on the thermally induced phase control (TIPC) strategy is provided, which enables efficient and photostable WBG PSCs without compositional engineering by exploring a thermal annealing process window (100–175 °C and 3–60 min) of the WBG perovskite films. Within this window, a key annealing regime is found that produces preferred crystal orientations of lead iodide and the WBG perovskite, suppressing phase segregation and reducing charge recombination in the perovskites. The WBG PSCs (composition of FA0.75MA0.15Cs0.1PbI2Br and Eg of 1.73 eV) optimized by TIPC exhibit an excellent power conversion efficiency (PCE) of 18.60% and improved operational stability, maintaining >90% of the maximum PCE (during maximum power point tracking) without encapsulation after 12 h of operation (air mass 1.5 global irradiation in ambient air conditions) or after 500 h of operation (white light-emitting diode irradiation (100 mW cm−2) in N2 conditions).

　For semitransparent devices with n-i-p structures, a metal oxide buffer material is commonly used to protect the organic hole transporting layer from damage due to sputtering of the transparent conducting oxide. Here, a surface treatment approach is addressed for tungsten oxide-based transparent electrodes through slight modification of the tungsten oxide surface with niobium oxide. Incorporation of this transparent electrode technique to the protective buffer layer significantly recovers the fill factor from 70.4% to 80.3%, approaching fill factor values of conventional opaque devices, which results in power conversion efficiencies over 18% for the semitransparent perovskite solar cells. Application of this approach to a four-terminal tandem configuration with a silicon bottom cell is demonstrated.

　To extract charges more efficiently through charge-transporting layers (CTLs), various dopants are necessary. Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) is the most widely used dopant in electron- and hole-transporting layers. However, Li+ ions easily migrate into the perovskite and deteriorate the device performance. To address this issue, several efforts such as introducing a buffer layer have been tried, but the issue is still not fully resolved. Thus it is required to find a simple way without additional treatments. In this work, we propose a simple strategy to use defect-tolerant dopant in CTLs, sodium bis(trifluoromethanesulfonyl)imide (Na-TFSI), to improve both the efficiency and the stability of perovskite solar cells (PSCs). The PSCs with Na-TFSI for both the electron-transport layer and the hole-transport layer show the highest power conversion efficiency up to 22.4%. In addition, the device with Na-TFSI exhibited better long-term operating stability at 45 °C, maintaining >80% of the initial performance even after 500 h of continuous 1 sun illumination.

　For commercializing perovskite solar cells (PSCs), moisture-tolerant materials are required because a moisture-free environment cannot be maintained on an actual production line (large scale). Recently, PSCs with efficiency exceeding 22% have been fabricated using Li-doped mesoporous TiO2 as an electron transport layer (ETL). However, the use of Li can negatively influence device stability during the fabrication process under humid air because of its hydroscopic property. Here, we report a strategy for improving processing stability without sacrificing the power conversion efficiency (PCE) under a humid atmospheric environment by employing a mesoporous BaSnO3 as an ETL. Using the mesoporous BSO ETL, we achieved a certified efficiency of 21.3% and stabilized efficiency of 21.7%. Furthermore, the BSO-based PSCs also exhibited better processing stability than Li-doped TiO2-based PSCs under humid air. We believe that this strategy of introducing BSO into PSCs will accelerate the commercialization of PSCs.

　Perovskite solar cells (PSCs) with mesoporous TiO2 (mp-TiO2) as the electron transport material attain power conversion efficiencies (PCEs) above 22%; however, their poor long-term stability is a critical issue that must be resolved for commercialization. Herein, it is demonstrated that the long-term operational stability of mp-TiO2 based PSCs with PCE over 20% is achieved by isolating devices from oxygen and humidity. This achievement attributes to systematic understanding of the critical role of oxygen in the degradation of PSCs. PSCs exhibit fast degradation under controlled oxygen atmosphere and illumination, which is accompanied by iodine migration into the hole transport material (HTM). A diffusion barrier at the HTM/perovskite interface or encapsulation on top of the devices improves the stability against oxygen under light soaking. Notably, a mp-TiO2 based PSC with a solid encapsulation retains 20% efficiency after 1000 h of 1 sun (AM1.5G including UV) illumination in ambient air.

　Recent advances in perovskite solar cells (PSCs) have resulted in greater than 23% efficiency with superior advantages such as flexibility and solution-processability, allowing PSCs to be fabricated by a high-throughput and low-cost roll-to-roll (R2R) process. The development of scalable deposition processes is crucial to realize R2R production of flexible PSCs. Gravure printing is a promising candidate with the benefit of direct printing of the desired layer with arbitrary shape and size by using the R2R process. Here, flexible PSCs are fabricated by gravure printing. Printing inks and processing parameters are optimized to obtain smooth and uniform films. SnO2 nanoparticles are uniformly printed by reducing surface tension. Perovskite layers are successfully formed by optimizing the printing parameters and subsequent antisolvent bathing. 2,2′,7,7′-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene is also successfully printed. The all-gravure-printed device exhibits 17.2% champion efficiency, with 15.5% maximum power point tracking efficiency for 1000 s. Gravure-printed flexible PSCs based on a two-step deposition of perovskite layer are also demonstrated. Furthermore, a R2R process based on the gravure printing is demonstrated. The champion efficiency of 9.7% is achieved for partly R2R-processed PSCs based on a two-step fabrication of the perovskite layer.

　Optical films with antireflective and self-cleaning surfaces have much potential for applications in solar cells, architectural glasses, and outdoor displays. Here we demonstrate the fabrication of novel self-cleaning antireflection (AR) thin films by depositing plasma-polymerized fluorocarbon (PPFC) which is fluorinated polymer consisting carbon and fluorine formed under plasma environment on Nb2O5/SiO2/Nb2O5 (NSN) trilayers using mid-range frequency power source in a continuous roll-to-roll sputtering system. The reflectance of PPFC/NSN films with a PPFC thickness of 70 nm was 1.71% at a wavelength of 528 nm, and the PPFC/NSN films showed low reflectance in a wide range in the visible region. The PPFC/NSN AR films exhibited a water-repelling surface with water contact angle of 105° after the application of a top fluorocarbon layer with low surface energy. We tested and confirmed the AR and self-cleaning functions of the PPFC/NSN films through the incorporation to perovskite solar cells (PSC). The short-circuit current density and power conversion efficiency of PPFC/NSN/HC-PET/PSC were 20.6 mA cm−2 and 17%. By attaching a self-cleaning AR films to both side of PSC, photocurrent collection of the device was improved and applicability of protective films to PSC was demonstrated.

　Perovskite solar cells typically comprise electron- and hole-transport materials deposited on each side of a perovskite active layer. So far, only two organic hole-transport materials have led to state-of-the-art performance in these solar cells1: poly(triarylamine) (PTAA)2,3,4,5 and 2,2ʹ,7,7ʹ-tetrakis(N,N-di-p-methoxyphenylamine)-9,9ʹ-spirobifluorene (spiro-OMeTAD)6,7. However, these materials have several drawbacks in terms of commercialization, including high cost8, the need for hygroscopic dopants that trigger degradation of the perovskite layer9 and limitations in their deposition processes10. Poly(3-hexylthiophene) (P3HT) is an alternative hole-transport material with excellent optoelectronic properties11,12,13, low cost8,14 and ease of fabrication15,16,17,18, but so far the efficiencies of perovskite solar cells using P3HT have reached only around 16 per cent19. Here we propose a device architecture for highly efficient perovskite solar cells that use P3HT as a hole-transport material without any dopants. A thin layer of wide-bandgap halide perovskite is formed on top of the narrow-bandgap light-absorbing layer by an in situ reaction of n-hexyl trimethyl ammonium bromide on the perovskite surface. Our device has a certified power conversion efficiency of 22.7 per cent with hysteresis of ±0.51 per cent; exhibits good stability at 85 per cent relative humidity without encapsulation; and upon encapsulation demonstrates long-term operational stability for 1,370 hours under 1-Sun illumination at room temperature, maintaining 95 per cent of the initial efficiency. We extend our platform to large-area modules (24.97 square centimetres)—which are fabricated using a scalable bar-coating method for the deposition of P3HT—and achieve a power conversion efficiency of 16.0 per cent. Realizing the potential of P3HT as a hole-transport material by using a wide-bandgap halide could be a valuable direction for perovskite solar-cell research.

　Light-induced electron-spin-resonance (LESR) spin density in a perovskite solar cell (PSC) is ascribed to photo-induced charges accumulated at the interface of the perovskite and the hole transport layer (HTL). The LESR spin density was observed to be decreased as the temperature was lowered from room temperature to 170 K in a PSC; below ∼170 K, no LESR was found. Meanwhile, the photocurrent showed only an ∼25% decrease between room temperature and 170 K. The comparison of LESR in temperature-dependence with transient or steady photocurrents suggested that the decrease in photocurrent at the low temperatures stems mainly from trapping of delocalized holes at the interface of perovskite and HTL.