Bulky Cations Improve Band Alignment and Efficiency in Sn-Based 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 bulky phenylethyl ammonium (PEA) cation in the precursor solution leads to improved solar cell performance and optoelectronic properties. A prolonged laser exposure is found to induce a redshift 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 (PCBM), which aids in electron injection and therefore increases photocurrents in solar cells. These results show that PEA addition suppresses halide segregation improving material stability, charge collection at perovskite/electron transport layer, and recombination dynamics in perovskite material. As a result, the PEA-containing Sn-rich PSCs exhibited a champion efficiency of 13% with a high open-circuit voltage of 0.77 V and improved current-voltage hysteretic behavior. These results shed light on the importance of halide segregation and band alignment when designing lead-free PSCs.