Size-dependent surface photovoltage in CdSe nanocrystal-based thin films

Abstract

Three different sizes (2.0 nm, 2.8 nm, and 4.9 nm) of highly monodisperse zinc blende CdSe nanocrystals were selected to construct heterojunctions with and without ZnO particles. To investigate photoinduced charge transfer at CdSe/ITO and CdSe/ZnO/ITO heterojunctions, surface photovoltage (SPV) and surface photovoltage spectroscopy (SPS) were employed. All the SPV responses observed in the CdSe/ITO junctions involved a higher energy band than the corresponding band-gap excitation of CdSe. Based on the SPV measurements, the photovoltage response is a function of the size of the nanocrystals, and it is generally red shifted as the diameter of the CdSe nanocrystals increases. For CdSe/ZnO/ITO heterojunctions, photovoltage responses were increased up to 10 times compared to bare CdSe. This suggests that a good alignment of energy levels between CdSe and ZnO is formed and efficient charge transfer may occur. From the studies of the SPV transient spectra of these junctions, the excitations above the band gap of ZnO generally show a fast decay rate.