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.