All‐inorganic cesium lead iodide perovskites (CsPbI3) are promising wide‐bandgap materials for use in the perovskite/silicon tandem solar cells, but they easily undergo a phase transition from a cubic black phase to an orthorhombic yellow phase under ambient conditions. It is shown that this phase transition is triggered by moisture that causes distortion of the corner‐sharing octahedral framework ([PbI6]4−). Here, a novel strategy to suppress the octahedral tilting of [PbI6]4− units in cubic CsPbI3 by systematically controlling the steric hindrance of surface organic terminal groups is provided. This steric hindrance effectively prevents the lattice distortion and thus increases the energy barrier for phase transition. This mechanism is verified by X‐ray diffraction measurements and density functional theory calculations. Meanwhile, the formation of an organic capping layer can also passivate the surface electronic trap states of perovskite absorber. These modifications contribute to a stable power conversion efficiency (PCE) of 13.2% for the inverted planar perovskite solar cells (PSCs), which is the highest efficiency achieved by the inverted‐structure inorganic PSCs. More importantly, the optimized devices retained 85% of their initial PCE after aging under ambient conditions for 30 days.

The development of organic–inorganic hybrid perovskite solar cells requires critical understanding in the charge-carrier behaviors in the perovskite light absorbers and devices. Kelvin probe force microscopy (KPFM) has been applied as a powerful tool to probe the electrical potential distribution of perovskite films and devices, providing fundamental insights into their charge-carrier properties. When measuring the material photoresponses, various approaches have been employed to illuminate the samples. Here, we measured the surface potential of the layer in the regular mesoporous structure (CH3NH3PbI3/m-TiO2/c-TiO2/FTO) and inverted planar structure (CH3NH3PbI3/NiO/FTO) devices via KPFM. Effects of two representative illumination methods are comparedillumination from top, and from underneath through the transparent glass substrate. By comparing the variation in surface potential under two illumination methods, the surface potential of the perovskite-absorbing layer in a regular structure is higher than that in the inverted structure. The potential difference in two structures implies that the photogenerated charge carriers are injected to the TiO2 electron-transport layer and NiO hole-transport layer, resulting in positive charges and negative charges accumulated in the perovskite-absorbing layer. We demonstrated that the illumination direction has an impact on the surface potential measurement. For the CH3NH3PbI3/TiO2 structure, illumination from underneath facilitates a larger potential change. While for the CH3NH3PbI3/NiO structure with insensitive photoresponse in potential change, the illumination direction has a minor effect.

Tin perovskite solar cells (TPSCs) are promising for lead-free perovskite solar cells (PSCs) and have led to extensive research; however, the poor crystallinity and chemical stability of tin perovskites are two issues that prevent stable TPSCs. In this study, we outline a new process that addresses these issues by using tin(II) acetate (Sn(Ac) 2 ) in place of the conventional SnF 2 precursor additive. Compared with SnF 2 , Sn(Ac) 2 improves the crystallinity and stability of tin perovskite with fewer defects and better charge extraction. Using this process, we developed a device that has a higher external quantum efficiency for charge extraction compared with the control devices and a power conversion efficiency of 9.93%, which maintained more than 90% of its initial efficiency after 1000 h operation at the maximum power point under standard AM 1.5G solar illumination.

Tin perovskite solar cells (TPSCs) are the most promising candidates for lead-free perovskite solar cells (PSCs). However, the poor crystallization and chemical stability of Sn perovskites are the two challenging issues for further application of TPSCs. Here, we present a strategy to stabilize CH(NH 2 ) 2 SnI 3 (FASnI 3 ) perovskite enabled by an amine complex, CH 3 NH 3 I•3CH 3 NH 2 , which can hinder the major degradation issue caused by the oxidation of Sn 2+ to Sn 4+ . The resulting Sn perovskite films exhibit enhanced crystallinity and stability in comparison with those made with conventional inorganic SnF 2 additives. Finally, the device achieved a higher external quantum efficiency for charge extraction and a power conversion efficiency (PCE) of 9.53%, which maintained more than 90% of the initial efficiency after 1000 h of light soaking under the standard AM 1.5 G solar illumination.