Hole transport layers regulate polaron formation in perovskite solar cells

Abstract

The mechanisms by which hole transport layers (HTLs) regulate carrier relaxation and polaron formation at perovskite interfaces remain poorly understood, despite their critical role in determining photovoltaic performance. Here, we combine transient absorption spectroscopy and time-resolved photoluminescence to elucidate the HTL-dependent carrier dynamics of metal halide perovskites. We show that Poly[bis(4-phenyl)(2,4,6-triMethylphenyl)aMine] (PTAA) promotes interfacial non-radiative recombination, while [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2-PACz) passivates interfacial defects and facilitates carrier transfer across the perovskites/HTL interface, resulting in accelerated ground-state bleach recovery. Under high excitation fluences, a second ground-state bleach (GSB2) emerges that is consistent with HTL-facilitated polaron formation, revealing an additional carrier-relaxation pathway. The associated polaronic screening suppresses electron-hole Coulomb interactions and sustains hot-carrier populations over extended timescales. These spectroscopic insights directly correlate with device performance: 2-PACz-based perovskite solar cells achieve a power conversion efficiency of 26.03% under 1-sun illumination and 27.24% under 5-sun concentration, outperforming PTAA-based counterparts. Our findings establish interfacial control of polaron formation as a design principle for simultaneously regulating carrier relaxation and photovoltaic performance in perovskite solar cells.