Absorption coefficients of quantum dot intermediate band material with negligible valence band offsets

Abstract

Solar cells with quantum dot nanostructure absorbing medium have a potential to overcome single junction limit and achieve the solar energy conversion efficiency as high as 63%. The confined energy states in quantum dots can mediate the absorption of photons with energy lower than the band gap of the barrier material. Closely spaced array of quantum dots (QDs) can form a mini band due to electronic coupling of the confined states among the neighboring dots. Absorption properties of the quantum dot nanostructaures are different from that of a bulk material. For the detailed balance efficiency calculations, the absorption coefficients of the QD nanostructures are required for realistic QD structures. After finding out material combinations with negligible valence band offset for quantum dot intermediate band solar cells(QDIBSCs), present work is focused on the calculation of absorption coefficients of QD arrays. The confined electronic states are calculated with the effective mass theory for single and coupled quantum dots. The electronic coupling of the ground states of an array of quantum dots is calculated for negligible valence band offset material combinations (especially InAs dots in GaAs (0.84) Sb (0.16) matrix grown on [001] GaAs substrate). The intermediate bandwidth vs the veretical interdot separation is presented. For some suitable interedot separation, the absorption coefficients are calculated for valence band to intermediate band, Intermediate band to conduction band transitions.