Carrier dynamics in type-II quantum dots for wide-bandgap intermediate-band solar cells

Abstract

Type-II quantum dots (QDs) have attracted attention for the formation of multiband solar cells based on the intermediate-band (IB) concept. The type-II confinement potential causes a spatial separation between electrons and holes, which strongly suppresses the carrier recombination in the QDs. As a result, the carrier lifetime in the QDs increases, which results in an increase in the number of photocarriers in the QDs under continuous light irradiation. This enhanced carrier number in the IB has an advantage for efficient two-step photon absorption because the probability of the second optical excitation to extract carriers from the QDs depends on the number of photocarriers in the QDs. Thus far, type-II QDs, such as GaSb/GaAs and Ge/Si QDs, have been introduced to demonstrate the operation principle of IB solar cells. In narrow-bandgap semiconductors, however, the photocarriers are extracted from the QDs by thermal excitation, which causes reduced carrier lifetime even in type-II QDs, and inefficient two-step photon absorption. In this paper, the carrier dynamics in type-II InP QDs in the wide-bandgap InGaP host are investigated by using time-resolved optical spectroscopy. The photoluminescence spectra of the InP QDs exhibit a high-energy shift with increasing excitation power density, which is a typical behavior of type-II QDs. Time-resolved photoluminescence measurements show a longer carrier lifetime in type-II InP QDs compared to that in the well-known type-I InAs QDs. Temperature dependent photoluminescence of the photoluminescence indicates that type-II InP QDs in the InGaP host are a promising candidate for realizing IB solar cells.