Hot-Carrier Extraction from Intermediate-Band Absorbers through Quantum-Well Energy-Selective Contacts

Abstract

We have revealed that a new scheme of solar cells, i.e., an intermediate-band-assisted hot-carrier solar cell (IB-HC-SC) using energy-selective contacts (ESCs) consisting of quantum wells (QWs) can be a practical solution to achieve significantly high conversion efficiency. There are three requisites unique to hot-carrier extraction for high conversion efficiency: (1) a long thermalization time of carriers in the absorber, (2) a narrow energy-selection width of the ESCs, and (3) a short equilibration time of carriers in the absorber. The use of an intermediate-band (IB) absorber relaxes the first requisite, because the two-step excitation via the IB dramatically suppresses entropy generation associated with hot-carrier extraction that is more remarkable at a shorter thermalization time. The suppression of the entropy generation allows us to use QW-ESCs to solve the issue related to the second requisite involved in practical ESCs consisting of size-distributed quantum dots. The new scheme provides limiting conversion efficiency of around 50% (0.1 sun)–65% (1000 sun) that are significantly higher than those of conventional IB solar cells, when the thermalization time of hot carriers is assumed to be 1 ns.