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.