Overcoming Short-Circuit in Lead-Free CH₃NH₃SnI₃ Perovskite Solar Cells via Kinetically Controlled Gas–Solid Reaction Film Fabrication Process

Abstract

The development of Sn-based perovskite solar cells has been challenging because devices often show short-circuit behavior due to poor morphologies and undesired electrical properties of the thin films. A low-temperature vapor-assisted solution process (LT-VASP) has been employed as a novel kinetically controlled gas-solid reaction film fabrication method to prepare lead-free CH3NH3SnI3 thin films. We show that the solid SnI2 substrate temperature is the key parameter in achieving perovskite films with high surface coverage and excellent uniformity. The resulting high-quality CH3NH3SnI3 films allow the successful fabrication of solar cells with drastically improved reproducibility, reaching an efficiency of 1.86%. Furthermore, our Kelvin probe studies show the VASP films have a doping level lower than that of films prepared from the conventional one-step method, effectively lowering the film conductivity. Above all, with (LT)-VASP, the short-circuit behavior often obtained from the conventional one-step-fabricated Sn-based perovskite devices has been overcome. This study facilitates the path to more successful Sn-perovskite photovoltaic research.

Lead‐Free Hybrid Perovskite Absorbers for Viable Application: Can We Eat the Cake and Have It too?

“…PCEs of 6.4% and 5.73% were reported, respectively, and after three years they are still the record efficiencies among MASnX 3 ‐based PSCs (Figure 3c). This dramatic different situation from the lead‐based perovskite can be understood by the fact that Sn‐based perovskites are prone to self‐doping in ambient air resulting in instability and poor reproducibility 46, 47, 48, 51, 52, 53, 135…”

“…Due to the 4s lone pairs of Ge is more active than the Pb 6s lone pair, Ge 2+ is easier to be oxidized leading to metallic conductivity in the hybrid materials and short‐circuit behavior in the photovoltaic devices similar to the case of Sn‐based perovskite 40, 135, 141. However, germanium has demonstrated much less toxicity compared to lead146 and is expected to be a promising candidate in the search for Pb‐free perovskite materials.…”

“…In the case of tin perovskite, the popularly used SnF 2 additive for the elimination of the Sn vacancy could result in poor film morphology and bad device performance due to the phase separation of SnF 2 within the perovskite film 6, 135. In this regard, Lee et al58 introduced pyrazine into FASnI 3 perovskite precursors in conjunction with SnF 2 in the form of the SnF 2 ‐pyrazine complex ( Figure 10 a,b).…”

“…To improve tin perovskite surface coverage, Yokoyama et al135 developed low‐temperature vapor‐assisted solution process (LT‐VASP), a kinetically control gas‐solid reaction method, to prepare lead‐free MASnI 3 thin films. They pointed out that the substrate temperature (60−80 °C window) of preformed solid SnI 2 is very important for achieving homogeneous and high surface‐coverage perovskite films.…”

“…a) One‐step MASnI 3 ; b) VASP–MASnI 3 ; c) LT–VASP‐MASnI 3 ; Reproduced with permission 135. Copyright 2016, ACS.…”