Impact of P3HT hole transport layer thickness on carrier dynamics in Sb₂S₃ n–i–p planar solar cells

Abstract Planar Sb₂S₃ solar cells, typically fabricated by spin-coating precursor solutions onto TiO₂/FTO (with or without CdS) superstrates, have achieved promising power conversion efficiencies, especially when paired with suitable hole transport layers (HTLs). While doped small-molecule HTLs such as Spiro-OMeTAD commonly enable record efficiencies above 5%, conductive polymers like poly(3-hexylthiophene-2,5-diyl) (P3HT) present a compelling alternative, offering high intrinsic hole mobility without the need for extrinsic dopants that can compromise chemical stability. However, the processing of P3HT—particularly the concentration in solution and resulting film thickness—often lacks systematic optimization. Here, we address this gap by investigating how P3HT concentration influences charge transport and recombination dynamics in Sb₂S₃ solar cells. Initially, we studied isolated P3HT films to elucidate the relationship between thickness, conductivity, and morphology/structure. Subsequently, we conduct a comprehensive electrical characterization of complete devices using electrochemical impedance spectroscopy, transient photovoltage, transient photocurrent, and intensity-modulated photocurrent and photovoltage spectroscopy. These techniques probe time- and frequency-resolved charge extraction and recombination processes, which are correlated with device performance metrics from J – V measurements. Our results demonstrate that P3HT thickness variations critically impact film morphology, carrier dynamics and overall photovoltaic performance. This work provides new insights into the operation of P3HT/Sb₂S₃ solar cells, informing strategies to optimize their performance towards efficiencies comparable to those achieved with doped Spiro-OMeTAD.