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 (HEAxFA1–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
(HEAxFA1–xSn0.67I2.33, 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
HEA0.4FA0.6SnI3 (HEAI = 40%) device
gave the best photovoltaic performance with JSC = 18.52 mA cm–2, VOC = 371 mV, FF = 0.562, and overall efficiency η = 3.9%
after the device was stored for a period of 340 h.