Coevaporation Stabilizes Tin-Based Perovskites in a Single Sn-Oxidation State

Abstract

Chemically processed methylammonium tin-triiodide (CH3NH3SnI3) films include Sn in different oxidation states, leading to poor stability and low power conversion efficiency of the resulting solar cells (PSCs). The development of absorbers with Sn [2+] only has been identified as one of the critical steps to develop all Sn-based devices. Here, we report on coevaporation of CH3NH3I and SnI2 to obtain absorbers with Sn being only in the preferred oxidation state [+2] as confirmed by X-ray photoelectron spectroscopy. The Sn [4+]-free absorbers exhibit smooth highly crystalline surfaces and photoluminescence measurements corroborating their excellent optoelectronic properties. The films show very good stability under heat and light. Photoluminescence quantum yields up to 4 × 10–3 translate in a quasi Fermi-level splittings exceeding 850 meV under one sun equivalent conditions showing high promise in developing lead-free, high efficiency, and stable PSCs.

Origins and Suppression of Sn(II)/Sn(IV) Oxidation in Tin Halide Perovskite Solar Cells

“…To avoid the adverse effects of nonperovskite components such as solvents, additives, and antisolvents on Sn(II) oxidation, evaporation processes such as singlesource evaporation, evaporation-assisted solution method, and dual-source coevaporation have recently been used in the preparation of TPSCs. [110,113,114] Redinger and co-workers prepared tin perovskite films using a dual-source coevaporation preparation process (Figure 7e). [114] Due to the absence of Sn(IV), the tin perovskite films exhibited a smooth crystalline surface and excellent photovoltaic properties.…”

“…[110,113,114] Redinger and co-workers prepared tin perovskite films using a dual-source coevaporation preparation process (Figure 7e). [114] Due to the absence of Sn(IV), the tin perovskite films exhibited a smooth crystalline surface and excellent photovoltaic properties.…”

“…Reproduced with permission. [114] Copyright 2022, American Chemical Society. [58] Copyright 2018, The Royal Society of Chemistry.…”

“…[118] In summary, optimization of the deposition process of tin perovskite films can effectively avoid Sn(II)/Sn(IV) oxidation by: i) avoiding the Sn(II) transfer process; [109] ii) prioritizing the preparation of single crystals to avoid the adverse effects of direct formation of perovskite films from halides; [112,119] iii) avoiding the participation of intermediates (solvents, additives, etc.) during the tin perovskite formation process; [110,113,114] iv) regulating the crystallization of tin perovskite films to achieve dense films to prevent water and oxygen intrusion. [58,115,116,[120][121][122][123] As summarized in Table 1, additive engineering is the most widely reported strategy due to the relatively easy operation.…”

“…e) Schematic diagram of the preparation of MASnI 3 by co-evaporation. Reproduced with permission [114]. Copyright 2022, American Chemical Society.…”