Rate limiting interfacial hole transfer in Sb2S3 solid-state solar cells

Transfer of photogenerated holes from the absorber species to the p-type hole conductor is fundamental to the performance of solid-state sensitized solar cells. In this study, we comprehensively investigate hole diffusion in the Sb2S3 absorber and hole transfer across the Sb2S3-CuSCN interface in the TiO2-Sb2S3-CuSCN system using femtosecond transient absorption spectroscopy, carrier diffusion modeling, and photovoltaic performance studies. Transfer of photogenerated holes from Sb2S3 to CuSCN is found to be dependent on Sb2S3 film thickness, a trend attributed to diffusion in the Sb2S3 absorber. However, modeling reveals that this process is not adequately described by diffusion limitations alone as has been assumed in similar systems. Therefore, both diffusion and transfer across the Sb2S3- CuSCN interface are taken into account to describe the hole transfer dynamics. Modeling of diffusion and interfacial hole transfer effects reveal that interfacial hole transfer, not diffusion, is the predominant factor dictating the magnitude of the hole transfer rate, especially in thin (< 20 nm) Sb2S3 films. Lastly, the implications of these results are further explored by photovoltaic measurements using planar TiO2-Sb2S3-CuSCN solar cells to elucidate the role of hole transfer in photovoltaic performance.