Tin-based halide perovskite materials have been successfully employed in lead-free perovskite solar cells, but the tendency of these materials to form leakage pathways from p-type defect states, mainly Sn and Sn vacancies, causes poor device reproducibility and limits the overall power conversion efficiencies (PCEs). Here, we present an effective process that involves a reducing vapor atmosphere during the preparation of Sn-based halide perovskite solar cells to solve this problem, using MASnI, CsSnI, and CsSnBr as the representative absorbers. This process enables the fabrication of remarkably improved solar cells with PCEs of 3.89%, 1.83%, and 3.04% for MASnI, CsSnI, and CsSnBr, respectively. The reducing vapor atmosphere process results in more than 20% reduction of Sn/Sn ratios, which leads to greatly suppressed carrier recombination, to a level comparable to their lead-based counterparts. These results mark an important step toward a deeper understanding of the intrinsic Sn-based halide perovskite materials, paving the way to the realization of low-cost and lead-free Sn-based halide perovskite solar cells.

Low electrical resistivity (high dark carrier concentration) of CH 3 NH 3 SnI 3 often leads to short-circuiting in solar cells, and appropriate thin-film modifications are required to ensure functional devices. The longterm durability of organic−inorganic perovskite solar cells necessitates the protection of perovskite thin films from moisture to prevent material decomposition. Herein, we report that the electrical resistivity and the moisture stability of two-dimensional (2D) Ruddlesden−Popper (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n−1 Sn n I 3n+1 perovskites are considerably improved compared to those of the three-dimensional (3D) CH 3 NH 3 SnI 3 perovskite and subsequently show the solar cell fabrication using a simple one-step spin-coating method. These 2D perovskites are semiconductors with optical band gaps progressively decreasing from 1.83 eV (n = 1) to 1.20 eV (n = ∞). The n = 3 and n = 4 members with optimal band gaps of 1.50 and 1.42 eV for solar cells, respectively, were thus chosen for in-depth studies. We demonstrate that thin films of 2D perovskites orient the {(CH 3 NH 3 ) n−1 Sn n I 3n+1 } 2− slabs parallel to the substrate when dimethyl sulfoxide solvent is used for deposition, and this orientation can be flipped to perpendicular when N,Ndimethylformamide solvent is used. We find that high-purity, single-phase films can be grown only by using precursor solutions of "pre-synthesized" single-phase bulk perovskite materials. We introduce for the first time the use of triethylphosphine as an effective antioxidant, which suppresses the doping level of the 2D films and improves film morphology. The resulting semiconducting 2D Sn-based iodide perovskite films were incorporated in solar cells yielding a power conversion efficiency of 2.5% from the Sn 4 I 13 member. From the temporal stability standpoint, the 2D Sn perovskite solar cells outperform their 3D analogs.

Achieving high open-circuit voltage (V) for tin-based perovskite solar cells is challenging. Here, we demonstrate that a ZnS interfacial layer can improve the V and photovoltaic performance of formamidinium tin iodide (FASnI) perovskite solar cells. The TiO-ZnS electron transporting layer (ETL) with cascade conduction band structure can effectively reduce the interfacial charge recombination and facilitate electron transfer. Our best-performing FASnI perovskite solar cell using the cascaded TiO-ZnS ETL has achieved a power conversion efficiency of 5.27%, with a higher V of 0.380 V, a short-circuit current density of 23.09 mA cm, and a fill factor of 60.01%. The cascade structure is further validated with a TiO-CdS ETL. Our results suggest a new approach for further improving the performance of tin-based perovskite solar cells with a higher V.

Singlet exciton fission (SF) in organic chromophore assemblies results in the conversion of one singlet exciton (S) into two triplet excitons (T), provided that the overall process is exoergic, i.e., E(S) > 2E(T). We report on SF in thin polycrystalline films of two terrylene-3,4:11,12-bis(dicarboximide) (TDI) derivatives 1 and 2, which crystallize into two distinct π-stacked structures. Femtosecond transient absorption spectroscopy (fsTA) reveals a charge-transfer state preceding a 190% T yield in films of 1, where the π-stacked TDI molecules are rotated by 23° along an axis perpendicular to their π systems. In contrast, when the TDI molecules are slip-stacked along their N-N axes in films of 2, fsTA shows excimer formation, followed by a 50% T yield.

Perovskite solar cells (PSCs) have advanced rapidly with power conversion efficiencies (PCEs) now exceeding 22%. Due to the long diffusion lengths of charge carriers in the photoactive layer, a PSC device architecture comprising an electron-transporting layer (ETL) is essential to optimize charge flow and collection for maximum performance. Here, a novel approach is reported to low temperature, solution-processed ZnO ETLs for PSCs using combustion synthesis. Due to the intrinsic passivation effects, high crystallinity, matched energy levels, ideal surface topography, and good chemical compatibility with the perovskite layer, this combustion-derived ZnO enables PCEs approaching 17-20% for three types of perovskite materials systems with no need for ETL doping or surface functionalization.