Solar cells formed from metal halide perovskites (MHPs) have reached remarkably high power conversion efficiencies over the past several years, with nearly 26% in single junction devices and over 27% in all-MHP tandem devices. To achieve such high-efficiency tandem devices, stacking of MHPs with different bandgap energies is a necessity. Bandgap engineering in MHPs can be achieved by varying the stoichiometry of the components; for example, changing the halide ratio in CsFAPb(Br_xI_1-x)₃ can continuously tune the bandgap across a wide range, from 1.6-2.2 eV. While this halide mixing is critical towards developing tandem devices, there is also a drawback: photo-induced phase segregation occurs within these materials, in which different halides separate into iodide-rich and bromide-rich perovskite phases, embedded within the remaining well-mixed phase.

Recently, surface modification of transparent conducting oxides with self-assembled monolayers (SAMs) have emerged as novel hole transport layers (HTLs) in MHP solar cells. The presence of SAMs has been shown to mitigate defect formation at metal oxide/MHP interfaces. Additionally, SAMs benefit from their ability to bond covalently to and tune the work function of transparent electrodes, their vanishingly low parasitic absorption, and their strong dipole moments. One SAM in particular, (2-(9H-carbazol-9-yl)ethyl)phosphonic acid (i.e., 2PACz), and its derivatives have stood out as leading to the highest improvement in device efficiencies. While there have been numerous studies on the improved device performance when incorporating 2PACz-derivatives as HTLs, the interplay between charge extraction and recombination at SAM/MHP interfaces has not yet been fully explored.

In this work, using a combination of time-resolved and steady-state optical spectroscopies, we investigate hole extraction across SAM/MHP interfaces. We explore the use of 2PACz and its derivatives interfaced with MHPs of different bandgap energies. We reveal the competition between hole extraction and recombination through systematic transient absorption (TA) and time-resolved photoluminescence (PL) spectroscopic measurements. We demonstrate that certain 2PACz-derivatives can help suppress halide segregation, by monitoring the rise of the iodide-rich phase photobleach signal at longer pump-probe delay times in TA measurements, and the growth of the iodide-rich phase emission in steady-state and time-resolved PL measurements. Understanding the photophysical processes at SAM/MHP interfaces will help to facilitate more efficient MHP solar cells with greater phase stabilities.