Bulky Cations Improve Band Alignment and Efficiency in Sn–Pb Halide Perovskite Solar Cells

The commercial feasibility of perovskite solar cells (PSCs) is not guaranteed as long as lead (Pb) is present in the active material, halide perovskites. Mixed halide tin (Sn)-based alloyed perovskites with optimal band gaps ranging from 1.15 to 3.55 eV are excellent alternatives to Pb-based perovskites. In this work, we find that the addition of a bulky phenylethyl ammonium (PEA) cation in the precursor solution leads to an improved solar cell performance and optoelectronic properties. A prolonged laser exposure is found to induce a redshift in the sample absorption for the control and no shift for the PEA-added sample, as shown by transient absorption spectroscopy. Further, we show that the addition of PEA improves band alignment of the perovskite with phenyl-C61-butyric acid methyl ester, which aids in electron injection and therefore increases photocurrents in solar cells. Further, we show that PEA addition suppresses halide segregation improving material stability and recombination dynamics in the perovskite material. As a result, the PEA-containing Sn-rich PSCs exhibited a champion efficiency of 13% with an open-circuit voltage of 0.77 V and improved current–voltage hysteresis behavior. These results shed light on the importance of halide segregation and band alignment when designing lead-free PSCs.