Organic Interlayers for Hole Transfer in MA-Free Mixed PB/SN Halide Perovskites for All-Perovskite Tandem Solar Cells

The efficiency of mixed lead–tin perovskite solar cells has increased rapidly, thanks to efficient passivation strategies of bulk and interfacial defects. For example, this occurs at the hole-transport layer and the perovskite interface. Here, we compare the self-assembled monolayers and multilayers (SAMs), [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) and methylphosphonic acid (MPA), to a PEDOT:PSS layer at the rear interface of a MA-free narrow band gap perovskite in single-junction (SJ) and all-perovskite tandem solar cells. PEDOT:PSS-based devices show the best power conversion efficiency of 14% in SJ and 17.2% in all-perovskite tandem architecture. By using photoluminescence and ultraviolet photoelectron spectroscopy, we show that this behavior is due to better energy alignment at the PEDOT:PSS/PK than the SAM/PK interface. However, SAMs also show lower nonradiative recombination rates at this interface. The results identify the limits of the effectiveness of 2PACz and MPA in mixed lead–tin MA-free perovskite solar cells and confirm the need for other SAMs with improved energy-level alignment while maintaining their passivating properties.