Charge delocalization and separation in type II tunneling structures of CdTe and CdSe nanocrystals by surface photovoltage spectroscopy

Combinations of CdTe and CdSe nanocrystals (NCs) provide type II structures with the lowest excited energy state for electrons in CdSe and the lowest excited hole state in CdTe. This kind of energy level alignment facilitates the separation of photo-excited charge carriers in thin film devices made of colloidal CdTe and CdSe NCs even despite the organic polymers and ligands acting as barrier in between. Photoluminescence quenching in hybrid structures evidenced on the charge separation indirectly. Surface photo voltage (SPV) spectroscopy allowed us to prove accumulation of electrons in CdSe NC layers and holes in CdTe NC layers. Changing the order of the NC layers inverted the SPV signal. The onset of the spectrally resolved SPV signal was correlated to the absorption spectra of the NCs. Measurements on NC multilayers of variable thickness showed a strong increase of the SPV signal which was related both to the higher number of photo-excited charge carriers in the thicker NC layers and to the simultaneously increased average distance for charge separation. We interpret it as an evidence of delocalization of charge carriers in layered NC structures made by layer-by-layer assembly with an inter-particle distance of ∼1 nm. Type II structures demonstrated here may find future applications in photovoltaics.