Charge carrier dynamics and self-trapping on Sb2S3(100)

Antimony sulfide (Sb2S3) is a promising material for solar energy conversion. It consists of earth-abundant elements with a low toxicity and high stability. Furthermore, it shows suitable optical and electronic properties like a large absorption coefficient and a convenient band gap of 1.7 eV. Nevertheless, so far, the obtained power conversion efficiencies and the open circuit voltages are far below the theoretical limits and need to be increased for a viable application of Sb2S3-based cells at the large scale. To achieve this it is important to identify the dominating loss mechanism. Here time-resolved two-photon photoemission experiments are presented from the (100) surface of a cleaved Sb2S3 single-crystal to investigate the charge carrier dynamics after photoexcitation. Based on these measurements an ultrafast relaxation within below 150 fs towards the conduction band minimum is observed. This is followed by a decay within 1 ps into states, which are located in the band gap 0.06 eV and 0.44 eV below the conduction band minimum where the charge carriers have long lifetimes of 27 ps and 63 ps, respectively. Based on the energetic position, the energy width and the formation time of the lower-lying gap state a self-trapping mechanism of free charge carriers by optical phonons with a frequency of 1.44 THz is proposed, and the results are discussed in this context. These findings provide evidence that the poor performance of Sb2S3 in solar devices can be traced back to intrinsically formed traps that can hardly be avoided