Perovskite solar cells (PSCs) have attracted extensive research interest in the last decade due to their high power conversion efficiency (PCE) and simple solution-based fabrication process. [1,2] Evolved from dye-sensitized solar cells (DSSCs), [3] typical PSCs usually employ a mesoporous TiO 2 as the electron-transport layer (ETL), which also functions as the scaffold for depositing the perovskite absorbing layer. [4,5] Although it is criticized that the high-temperature (>450 °C) sintering process for the mesoporous TiO 2 layer makes the device manufacturing complex and energy consumptive, which also hinders the integration of PSCs with flexible substrates and electronics, such mesoscopic PSCs have been dominating the efficiency breakthroughs of PSCs from certified 14.1% in 2013 to 23.7% in 2019. [5-8] The latest 25.2% is highly possible also obtained by mesoscopic PSCs. [9] The ambipolar charge transport characteristics and long charge carrier diffusion length of lead halide perovskites offers the possibility of replacing the mesoporous ETL by a planar one, and constructing planar-structured PSCs with low-temperature (≤150 °C) processes. [10,11] For inverted (p-in) planar PSCs, there are plenty of options available for ETLs and hole-transport layers (HTLs), such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and phenyl-C61-butyric acid methyl ester (PC 61 BM), attributing to years of research on organic solar cells. [12,13] For regular (n-i-p) planar PSCs, compact TiO 2 layer was first used as the ETL, which soon aroused the attention on the anomalous hysteresis phenomenon for PSCs. [14,15] It was claimed that the low electron mobility of compact TiO 2 resulted in charge accumulations at the TiO 2 /perovskite interface and thus caused significant hysteresis. [16,17] Then, it was further found out that the electronic contact between the TiO 2 ETL and the perovskite layer played an essential role in the hysteresis behaviors of PSCs. [18] This is in agreement with the fact that the mesoporous TiO 2-based PSCs usually show much reduced hysteresis, [6,7] since the mesoscopically structured ETL can provide much larger surface area for contacting the perovskite absorber with stabilized properties. Along with the defects at the interfaces, ion migration and trap states in the perovskite layer have also been considered as the origin of the hysteresis Perovskite solar cells (PSCs) have rapidly developed and achieved power conversion efficiencies of over 20% with diverse technical routes. Particularly, planar-structured PSCs can be fabricated with low-temperature (≤150 °C) solution-based processes, which is energy efficient and compatible with flexible substrates. Here, the efficiency and stability of planar PSCs are enhanced by improving the interface contact between the SnO 2 electron-transport layer (ETL) and the perovskite layer. A biological polymer (heparin potassium, HP) is introduced to regulate the arrangement of SnO 2 nanocrystals, and induce vertically aligned crystal growth of perovski...

High-quality perovskite light harvesters and robust organic hole extraction layers are essential for achieving high-performing perovskite solar cells (PSCs). We introduce a phosphonic acid–functionalized fullerene derivative in mixed-cation perovskites as a grain boundary modulator to consolidate the crystal structure, which enhances the tolerance of the film against illumination, heat, and moisture. We also developed a redox-active radical polymer, poly(oxoammonium salt), that can effectively p-dope the hole-transporting material by hole injection and that also mitigates lithium ion diffusion. Power conversion efficiencies of 23.5% for 1-square-centimeter mixed–cation-anion PSCs and 21.4% for 17.1-square-centimeter minimodules were achieved. The PSCs retained 95.5% of their initial efficiencies after 3265 hours at maximum power point tracking under continuous 1-sun illumination at 70° ± 5°C.

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