Control of Crystal Structures and Optical Properties with Hybrid Formamidinium and 2-Hydroxyethylammonium Cations for Mesoscopic Carbon-Electrode Tin-Based Perovskite Solar Cells

Alcohol-based bifunctional ammonium cations, 2-hydroxyethylammonium (HEA⁺), HO(CH₂)₂NH₃ ⁺, were introduced into formamidinium (FA⁺) tin-based perovskites (HEA_xFA_1-xSnI₃; x = 0-1) to absorb light in carbon-based mesoscopic solar cells. We found that HEA⁺ cations play a key role to control the crystal structures, the lattice structures altered from orthorhombic (x = 0) to rhombohedral (x = 0.2-0.4) with greater symmetry. When x was increased to 0.6-1.0, tin and iodide vacancies were formed to generate 3D-vacant perovskites (HEA_xFA_1-xSn_0.67I_2.33, x ≥ 0.6) with a tetragonal structure. Tin-based perovskites in this series were fabricated into mesoporous solar cells using one-step drop-cast (DC), two-step solvent-extraction (SE), and SE + 3% ethylenediammonium diiodide (EDAI₂) as an additive. After optimization of device performance with the SE + 3% EDAI₂ approach, the HEA_0.4FA_0.6SnI₃ (HEAI = 40%) device gave the best photovoltaic performance with J_SC = 18.52 mA cm^-2, V_OC = 371 mV, FF = 0.562, and overall efficiency η = 3.9% after the device was stored for a period of 340 h.