The Origin of Lower Hole Carrier Concentration in Methylammonium Tin Halide Films Grown by a Vapor-Assisted Solution Process

Abstract

A low hole carrier concentration in methylammonium tin halide (MASnX3) perovskite semiconductors is a prerequisite for a nonshorting solar cell device. In-depth film characterizations were performed on MASnI3–x Br x films, fabricated by both a low-temperature vapor-assisted solution process (LT-VASP) and conventional one-step methods, to reveal the origin of the lower hole carrier concentration from films of the former approach. We found that the vaporization of CH3NH3I solid at 150 °C, the temperature at which the LT-VASP occurs, does not supply iodine to the SnX2 (X = Br, I) films. As a result, secondary phases form aside from the desired MASnX3 perovskite; the secondary phases are suggested to be SnO and Sn­(OH)2 via a proposed reaction pathway and are further supported by X-ray photoemission spectroscopy (XPS). These nonperovskite Sn2+ phases are beneficial because they assist in achieving the lower hole-doping levels in LT-VASP films. Remarkably, LT-VASP devices demonstrate improved air stability. Overall, our findings suggest that not only the commonly used SnF2 but also other divalent Sn compounds could serve as Sn vacancy suppressors. Further work on modulating the perovskite film compositions could realize more efficient and stable tin-based perovskite solar cells.

Organic–Inorganic Copper (II)‐Based Perovskites: A Benign Approach toward Low‐Toxicity and Water‐Stable Light Absorbers for Photovoltaic Applications

“…As the performance of PSCs can be improved by employing novel strategies such as using different electron/hole transport layers, we employed mesoporous zinc oxide (ZnO) as an electron transport layer and developed PSC devices with two different light absorbers (C 6 H 4 NH 2 CuBr 2 I and C 6 H 4 NH 2 CuCl 2 I). The obtained PCEs for the fabricated devices are higher compared to the reported ones (Table ) …”

“…Ma et al and Yang et al used a novel strategy to prepare high‐quality films of CH 3 NH 3 SnI 3 , and the efficiency was found to be 0.3%. Yokoyama et al employed CH 3 NH 3 SnBr 3 in Pb‐free PSCs and achieved a PCE of 0.5%. Yang et al also tried to improve the efficiency of a Sn‐based light absorber by preparing a mixed‐halide‐based perovskite (CH 3 NH 3 SnI 2.6 Br 0.4 ), but the PCE was found to be very poor.…”

Tin Halide Perovskites: Progress and Challenges

“…Notably, the stability of halide perovskites largely depends on the stability of the crystalline unit cells. For instance, the electronic properties of bulk single crystals of cubic MASnI 3 and FASnI 3 are highly sensitivity to moisture and ambient conditions with FASnI 3 showing a relatively higher tolerance to ambient conditions than MASnI 3 . Based on their crystal lattice, Sn‐based halide perovskites having MA occupying the A‐site can be soft and can exhibit tunable bandgap with changes in the anions occupying the X‐site .…”

Organic–inorganic metal halide hybrids beyond perovskites

“…LEDs (Figure 2(c)), optically pumped lasers (Figure 2(d)), etc [32][33][34]. To address the toxicity issue of Pb, 3D tin halide perovskites have been developed [35][36][37][38][39]. However, bivalent Sn 2+ could be easily oxidized to Sn 4+ , resulting in the instability of the material itself and morphological change into other species [40].…”