Long Minority‐Carrier Diffusion Length and Low Surface‐Recombination Velocity in Inorganic Lead‐Free CsSnI₃ Perovskite Crystal for Solar Cells

Abstract

Sn-based perovskites are promising Pb-free photovoltaic materials with an ideal 1.3 eV bandgap.However, to date, Sn-based thin film perovskite solar cells have yielded relatively low power conversion efficiencies (PCEs). This is traced to their poor photophysical properties (i.e., short diffusion lengths (<30 nm) and two orders of magnitude higher defect densities) than Pb-based systems. Herein we reveal that melt-synthesized cesium tin iodide (CsSnI 3 ) ingots containing high quality large single crystal (SC) grains transcend these fundamental limitations. Through detailed optical spectroscopy, we uncover their inherently superior properties, with bulk carrier lifetimes reaching 6.6 ns, doping concentrations of around 4.5 × 10 17 cm -3 , and minority-carrier diffusion lengths approaching 1 µm, as compared to their polycrystalline counterparts: ~54 ps, >9.2 × 10 18 cm -3 , and 16 nm, respectively. CsSnI 3 SCs also exhibit very low surface recombination velocity of <2 × 10 3 cm.s -1 similar to Pb-based perovskites. Importantly, these key parameters are comparable to highperformance p-type photovoltaic materials (e.g., InP crystals). Our findings predict a PCE of ~23% for optimized CsSnI 3 SCs solar cells, highlighting their great potential.

Potential Substitutes for Replacement of Lead in Perovskite Solar Cells: A Review

“…Cesium‐based tin perovskite has a high hole mobility of 585 cm −1 V −1 s −1 , low exciton binding energy (180 meV) than MAPbI 3 . The melt‐synthesized CsSnI 3 ingots containing high‐quality large single crystal grains have been reported to have bulk carrier lifetime more than 6.6 ns, doping concentration of about 4.5 × 10 17 cm −3 , and minority carrier diffusion lengths approaching to 1 µm . A SPCE of 23% was predicted for optimized single crystal solar cells CsSnI 3 highlighting their great potential for use in perovskite solar cell.…”

Lead‐Free Tin‐Based Perovskite Solar Cells: Strategies Toward High Performance

“…Defects play an important role in the photovoltaic performance of Sn‐based perovskites. In addition to the deep‐level defects in these materials that may act as recombination centers, the easy oxidation of Sn 2+ and the low formation energy of Sn vacancies ( V Sn ) result in the notoriously high hole doping densities up to 10 18 –10 20 cm −1 . Consequently, Sn‐based perovskites act like conductors rather than semiconductors.…”

“…For pristine MASnI 3 , the film deposited on mesoporous TiO 2 has a diffusion length of 30 nm, whereas the value for that on a planar substrate is over 200 nm . For CsSnI 3 , the electron diffusion length increases from 16 nm in polycrystalline films to 930 nm in low‐defect single crystals . It is expected that by lowering the defect density in Sn‐based perovskite films, the charge‐carrier mobility can be enhanced and recombination rate can be reduced, and therefore the diffusion length will be adequately sizeable allowing for efficient charge‐carrier extraction.…”

All‐Inorganic Halide Perovskites for Optoelectronics: Progress and Prospects

“…The PCEs of the PSCs based on CsSnX 3 IMH perovskites was considered to have a lot of room for improvement. Wu et al claimed that the PCEs of CsSnI 3 based PSCs may reach 23% by theoretical simulation . Therefore, it is meaningful to pay more attentions into Sn‐based IMH perovskites in the future studies.…”