Elucidating Mechanistic Details of Photo-Induced Charge Transfer in Antimony Sulfide-Based p-i-n Junctions

The initial kinetics and mechanisms of photo-induced charge transfer in photovoltaic materials are critical to the operation of fabricated devices. Despite the importance of charge transfer in the picosecond to nanosecond timescales, mechanistic understanding of these events is still limited. To address this challenge, a series of p-i-n junction samples that comprises fluorine-doped tin oxide (FTO)/TiO2/ZnS/Sb2S3/P3HT layers was prepared by atomic layer deposition (ALD). ALD allows for carefully controlled film thickness in samples that enable systematic evaluation of photo-induced charge-transfer kinetics by transient absorption spectroscopy (TAS). Sb2S3 serves as the intrinsic light absorber, P3HT is the hole acceptor, and TiO2 is the electron acceptor. An extremely thin, electron-blocking layer of ZnS was deposited between Sb2S3 and TiO2 varied in thickness by ALD to create a series of 20 samples that included (1) five different ZnS thicknesses (0, 2, 5, 10, and 15 ALD cycles) and (2) four combinations of layers, always including ZnS/Sb2S3, that built up to the completed stack. These mechanistic studies confirm our proposed mechanism for photo-induced electron and hole transfer and recombination in these p-i-n junction samples and provide predictive insights into the charge-transfer processes that may be most determinant in the operation of completed devices.