Dopant-free fluorene based dimers linked with thiophene units as prospective hole transport materials for Sb2S3 solar cells

Abstract

Novel dopant-free dimers comprising methoxydiphenylamine substituted fluorene derivatives and connected by central cores consisting of different numbers of thiophene moieties were synthesized and explored as hole transport materials (HTMs) in Sb₂S₃ absorber solar cells. Energy level diagrams show agreeable band offsets validating the compatibility of novel HTMs for the FTO/TiO₂/Sb₂S₃/HTM/Au solar cell with TiO₂ and Sb₂S₃ layers deposited by ultrasonic spray. X-ray photoelectron spectroscopy (XPS) study reveals the Sb 3d core level peak shift upon applying any of the HTMs on Sb₂S₃ indicating an increased electron density surrounding Sb atoms which refers to the interaction of S from electron-rich thiophene units with Sb in the absorber at the Sb₂S₃/HTM interface. It is demonstrated that application of HTMs containing diphenylamine units in their side fragments increases the cell open circuit voltage from 478 mV to 673 mV, fill factor from 46% to 56% and conversion efficiency from 1.9% to 4.5% as compared to the device without any HTM and the observed improvement can be explained by the passivation of the interfacial states. In contrast, no enhancement in device performance has been observed when applying HTMs containing triphenylamine units although strong Sb–S interaction has been detected at the Sb₂S₃/HTM interface. Quantum chemical simulation results suggest that to achieve enhanced charge selectivity by the organic HTM layer, the HOMO of the HTMs should be formed by the thiophene groups. Possible phenomena occurring at the Sb₂S₃/HTM interface are discussed providing new insights towards understanding the charge transfer at the Sb₂S₃/HTM interface.