Abstract

The talk is one of the seven talks scheduled for the “University of Cambridge – The University of Tokyo Joint Workshop on Quantum Nanostructures”, at 14:00-17:00. Significant effort and a rapid progress have been made today in modeling the device physics as well as practical demonstration of quantum dot intermediate band solar cells (QD-IBSCs). Presently, demonstration of QD-IBSCs is undergoing various development stages. The first challenge is to establish methods to fabricate high-density QDs arrays or superlattice of low defect density and long carrier lifetimes. The strain-compensated or strain-balanced growth technique by MBE can significantly improve the QDs quality and characteristics of solar cells even after stacking of > QD 100 layers in self-organized heteroepitaxy. The second effort is to increase the carrier lifetimes in IB states by controlling the recombination rates such as by using (1) a type-II QD heterostructure that allows a fast spatial separation of electrons and holes, (2) a high potential barrier preventing the thermal electron escape out of QDs, and (3) an electric field damping layer suppressing the field-assisted escape of electrons out of QDs. The last challenge is to realize partially filled IB states in order to maximize photocurrent generation by two-step IR photon absorption. For this, doping of QDs and photofilling of QDs by light concentration as well as photon confinement thin-film structure are all considered important.