Understanding the Limitations of Charge Transporting Layers in Mixed Lead–Tin Halide Perovskite Solar Cells

Lead–tin (Pb/Sn) mixed perovskites are considered as promising photovoltaic materials owing to their adjustable bandgap and excellent optoelectronic properties. The low‐bandgap perovskite solar cells (PSCs) based on lead–tin mixed perovskites play a critical role in the overall performance of perovskite‐based tandem devices. Nevertheless, the current record efficiencies for Pb/Sn PSCs are mostly reported in devices with p–i–n configuration rather than n–i–p, which restricts the further development of conventional perovskite‐based tandem solar cells. Herein, this work systematically investigates the influence of the interlayers on the performance of low‐bandgap PSCs by analyzing the energy losses in both n–i–p and p–i–n devices. Quasi‐Fermi level splitting (QFLS) analysis of pristine films and films covering charge extraction layers reveals that the electron transport layer/perovskite interface is dominating the VOC losses. A joint experimental–simulative approach quantitatively determines the interface defect density to be more than one order in magnitude larger for the n–i–p architecture. Among the polymeric hole transport layers investigated for n–i–p devices, poly(3‐hexylthiophen‐2,5‐diyl) (P3HT) exhibits the most favorable energy‐level alignment to Pb/Sn perovskites. These results clarify the nature of VOC losses in Pb/Sn perovskites and highlight the necessity to develop electron extraction layers with a significantly reduced interface defect density. Energy losses in Pb/Sn‐based narrow‐bandgap perovskite solar cells are comprehensively analyzed by quasi‐Fermi level splitting of perovskite/interlayer junctions and drift‐diffusion simulation of J–V curves of n–i–p and p–i–n devices. Compared with the bulk, perovskite/interlayer interface dominates nonradiative recombination loss and VOC is mainly limited at the interface of perovskite/electron transport layer.