Enhancing photo-induced ultrafast charge transfer across heterojunctions of CdS and laser-sintered TiO2 nanocrystals

Abstract

Enhancing the charge transfer process in nanocrystal sensitized solar cells is vital for the improvement of their performance. In this work we show a means of increasing photo-induced ultrafast charge transfer in successive ionic layer adsorption and reaction (SILAR) CdS–TiO₂ nanocrystal heterojunctions using pulsed laser sintering of TiO₂ nanocrystals. The enhanced charge transfer was attributed to both morphological and phase transformations. At sufficiently high laser fluences, volumetrically larger porous networks of the metal oxide were obtained, thus increasing the density of electron accepting states. Laser sintering also resulted in varying degrees of anatase to rutile phase transformation of the TiO₂, producing thermodynamically more favorable conditions for charge transfer by increasing the change in free energy between the CdS donor and TiO₂ acceptor states. Finally, we report aspects of apparent hot electron transfer as a result of the SILAR process which allows CdS to be directly adsorbed to the TiO₂ surface.