Investigation of Photocarrier Losses in Pyrite (FeS2) Film Consisting Single Crystal Nanocubes

Successful extraction of photo excited carriers from a photo active material is key towards a successful PV device. The high losses exhibited by a device with pyrite as the active layer has been noted previously but never investigated systematically to understand the photo physics and the carrier loss mechanisms to improve its performance. Here we report a detailed characterization of a film made using {100} terminated, pure pyrite single crystal nano cubes. Using ultrafast transient absorption spectroscopy we found fast carrier localization of photo excited carriers to indirect band edge and shallow trap states with characteristic decay time of 1.8 picoseconds, followed by relaxation to deep states and recombination of trapped carriers with long characteristic decay times of 50-990 nanoseconds. Its optical absorption characteristics correlate to a disordered semiconductor. Temperature dependent electrical resistivity exhibits a Mott variable range hopping (VRH) type conduction mechanism consistent with the presence of high density of defect states. An electron band model with midgap defect states is formulated that could explain all the observed phenomena.