Carbazole-based self-assembled monolayers (SAMs) have proven effective as HTLs in perovskite solar cells (PSCs). However, the energy alignment with narrow-bandgap lead-tin (Pb-Sn) perovskites is suboptimal. Here, we design four phenoxazine/phenothiazine SAMs. Despite varied energy levels, the performance remains similar, showing that the energy alignment alone cannot explain device behavior. Advanced spectroscopic measurements reveal a complex interplay of charge extraction and interfacial recombination.
Hole-selective self-assembled monolayers (SAMs) based on carbazole head groups have enabled major performance improvements of perovskite solar cells (PSCs) by eliminating parasitic absorption and nonradiative losses. However, the energy levels of the carbazole-based, commercially available SAMs poorly match the valence band maximum (VBM) of narrow-bandgap, lead-tin (Pb-Sn) perovskites, relevant for tandem applications. In this work, we expand the library of SAMs compatible with Pb-Sn PSCs by synthesizing four novel SAMs containing phenoxazine (POz) and phenothiazine (PTz) as their head groups and investigate their interaction with the Pb-Sn perovskite in detail. We obtain working devices with all SAMs, but despite significant differences between the highest occupied molecular orbital (HOMO) levels of the SAMs, the open-circuit voltage (V
OC) and fill factor (FF) across devices remains similar, suggesting that the role of energy level alignment is less relevant at this interface. Through in-depth analysis including photoluminescence quantum yield (PLQY), transient photocurrent (TPC), and combined time-resolved surface photovoltage (trSPV) and time-resolved photoluminescence (trPL) measurements, we unveil the charge extraction dynamics of these systems featuring different head groups and HOMOs. This work highlights that the SAMs’ structure affects the overall charge extraction process and provides insights into the strategies needed to maximize charge extraction for more efficient PSCs.
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