Electron and hole transfer at metal oxide/Sb2S3/spiro-OMeTAD heterojunctions

Antimony sulfide has recently demonstrated huge potential as an absorber material in solid-state semiconductor-sensitised solar cells (SSSCs). Here, we present a transient absorption spectroscopy study of the TiO2/Sb2S3/spiro-OMeTAD heterojunctions to elucidate the key factors influencing charge photogeneration. Efficient device performance requires that photoresponse extends into the near-infrared, yet we find that the efficiency of charge separation at the TiO2/Sb2S3 interface decreases significantly as increasingly red-absorbing Sb2S3 nanocrystals are photoexcited. However, the efficiency of hole transfer to spiro-OMeTAD appears much less sensitive to shifts in the Sb2S3 absorption edge and we also observe hole transfer to spiro-OMeTAD occurring on ZrO2 substrates, where electron injection does not occur. These observations may point to the importance of the hole transfer reaction not simply as a means of regenerating the sensitiser, but rather as an integral part of the charge separation process in Sb2S3 SSSCs. The present findings should help inform strategies aimed at further increasing the efficiency of nanocrystal sensitized solar cells.