Planar perovskite solar cells (PSCs) incorporating n‐type SnO2 have attracted significant interest because of their excellent photovoltaic performance. However, the film fabrication of SnO2 is limited by self‐aggregation and inhomogeneous growth of the intermediate phase, which produces poor morphology and properties. In this study, a self‐controlled SnO2 layer is fabricated directly on a fluorine‐doped tin oxide (FTO) surface through simple and rapid chemical bath deposition. The PSCs based on this hydrolyzed SnO2 layer exhibit an excellent power conversion efficiency of 20.21 % with negligible hysteresis. Analysis of the electrochemical impedance spectroscopy on the charge transport dynamics indicates that the bias voltage influences both interfacial charge transportation and the ionic double layer under illumination. The hydrolyzed SnO2‐based PSCs demonstrate a faster ionic charge response time of 2.5 ms in comparison with 100.5 ms for the hydrolyzed TiO2‐based hysteretic PSCs. The results of quasi‐steady‐state carrier transportation indicate that a dynamic hysteresis in the J–V curves can be explained by complex ionic‐electronic kinetics owing to the slow ionic charge redistribution and hole accumulation caused by electrode polarization, which causes an increase in charge recombination. This study reveals that SnO2‐based PSCs lead to a stabilized dark depolarization process compared with TiO2‐based PSCs, which is relevant to the charge transport dynamics in the high‐performing planar SnO2‐based PSCs.

Iodide-free tribromide-based perovskites, with their wide bandgap of over 2.0 eV, are highly regarded as potential candidates for a photoelectrochemical water splitting system and the topmost cell in tandem solar cell. Herein, we report on the importance of microtuning of the crystal lattice by cesium incorporation into the A-site on low temperature processed formamidinium lead tribromide (CH(NH 2 ) 2 PbBr 3 = FAPbBr 3 ) perovskite films. The partial incorporation of cesium bromide (CsBr) into the FAPbBr 3 film tunes crystal-lattice interactions, resulting in a high-purity cubic crystal system with preferred orientation. An entirely low temperature processed planar photovoltaic device assembled with FAPbBr 3 containing 8% Cs (Cs 0.08 FA 0.92 PbBr 3 ) exhibited an optimum PCE (power conversion efficiency) of 8.56% with a V oc (open-circuit voltage) of 1.516 V, which is higher than the PCE of 7.07% and V oc of 1.428 V of the FAPbBr 3 device. Photoluminescenceintensity and temporal-imaging measurements were conducted by laser scanning confocal time-resolved microscopy (LCTM), which revealed that CsBr incorporation into a FAPbBr 3 film significantly suppresses the nonradiative recombination pathways and homogenizes the spatial distribution of photoluminescence. It was visualized that the incorporation of CsBr in FAPbBr 3 directly affects the bulk defect and photoluminescence properties, which provides evidence that Cs ions surely alleviate the segregation and aggregation of ions in the perovskite film. Notably, the Cs 0.08 FA 0.92 PbBr 3 film, with a carrier lifetime of about 270 ns, exhibited a 1.37-fold longer radiative recombination time than that (210 ns) observed for the FAPbBr 3 film. Furthermore, aging experiments without encapsulation under ambient (in air for 2000 h) and severe (65 °C and 65% RH for 500 h) conditions revealed that the Cs 0.08 FA 0.92 PbBr 3 devices were more robust than the FAPbBr 3 devices.

In this study, cobalt diselenide (CoSe 2) and the composites (CoSe2@RGO) of CoSe 2 and reduced graphene oxide (RGO) were synthesized by a facile hydrothermal reaction using cobalt ions and selenide source with or without graphene oxide (GO). The formation of CoSe 2 @RGO composites was identified by analysis with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and scanning electron microscopy (SEM). Electrochemical analyses demonstrated that the CoSe 2 @RGO composites have excellent catalytic activity for the reduction of I 3-, possibly indicating a synergetic effect of CoSe 2 and RGO. As a consequence, the CoSe 2 @RGO composites were applied as a counter electrode in DSSC for the reduction of redox couple electrolyte, and exhibiting the comparable power conversion efficiency (7.01%) to the rare metal platinum (Pt) based photovoltaic device (6.77%).