Feasibility of submonolayer ZnTe/ZnCdSe quantum dots as intermediate band solar cell material system

Intermediate band solar cells can potentially have an efficiency of ~63% under full solar concentration, but the material systems investigated until now are far from optimum and are fraught with growth related issues such as low quantum dot densities, presence of wetting layers, and strain driven dislocations. Also, incorporation of type-I quantum dots increases carrier recombination rates, resulting in inferior performance. Here, we show that a novel material system with stacked type-II ZnTe-rich submonolayer QDs embedded in ZnCdSe has close to the optimal material parameters required for an intermediate band material system. We have grown structures comprising of as many as 150 layers of QDs that are formed without wetting layers and that have a valence band offset of ~0.8 eV relative to the host with a bandgap of ~2.1 eV. We demonstrate the possibility of intermediate band formation and subsequent absorption of below bandgap photons. Additionally, these structures are expected to have longer radiative lifetimes and to suppress Auger recombinations owing to their type-II nature.