Efficient carrier multiplication and extraction in aqueous-processed giant CdTe-CdS nanocrystal bulk heterostructures

Abstract

Auger recombination as an intrinsic mechanism for bypassing the ‘phonon bottleneck’ plays an important role in semiconductor quantum dots, which makes the possible carrier multiplication or multiple-exciton generation occurred in nanoscale. Here, we show that for aqueous-processed giant CdTe-CdS core-shell nanocrystal solids (the diameter of CdTe core is ∼20 nm, larger than its bulk exciton Bohr radius of ∼7.5 nm), it is a type-II structure with small band offsets and strong delocalization of electrons. Thus, there is an efficient carrier multiplication by Auger processes, in comparison with the exciton relaxation behaviors in reference films consisting of large CdTe quantum dots (the diameter is ∼11 nm) synthesized by an oil-phase approach. The efficient carrier extractions are further demonstrated using TiO2 and MoO3 as carrier transport layers in CdTe-CdS nanocrystal depleted bulk heterostructures, and imbalanced carrier extraction efficiencies by TiO2 and MoO3 are revealed. Our findings unravel the fundamental photophysical mechanisms for the high-efficient all-solid nanocrystal photovoltaics based on aqueous-processed giant CdTe-CdS nanocrystal solids.