Recent Advances in Low‐Bandgap, InP‐Based GaInAs/InAsP Materials and Devices for Thermophotovoltaic (TPV) Energy Conversion

Salient advances in the development of thermophotovoltaic (TPV) energy converters based on low‐bandgap, InP‐based, GaInAs/InAsP heterostructures are presented and discussed. InP‐based materials are well‐suited and advantageous for TPV converter applications. Substantial improvements in the quality of lattice‐mismatched (LMM) heterostructures have been realized through an enhanced understanding of the relaxation behavior, and associated microstructure, of InAsP compositionally graded layers and GaInAs/InAsP interfaces. Double‐heterostructure, GaInAs/InAsP test structures with bandgaps as low as 0.5 eV (1.6% lattice mismatch) have been demonstrated with exceptional low‐injection, minority‐carrier lifetimes (several μs) and large estimated diffusion lengths — comparable to those for lattice‐matched materials. The advances in material quality have contributed to a number of notable TPV device achievements. A record in‐cavity efficiency of 23.6% was reported for a 0.6‐eV, GaInAs/InAsP monolithic interconnected module. Additionally, 0.52‐eV GaInAs/InAsP TPV converters were demonstrated with near‐unity internal quantum efficiencies and reverse‐saturation current densities nearly equaling the best reported for lattice‐matched, 0.52‐eV GaInAsSb/GaSb devices. Furthermore, InP‐based, 0.74/0.63‐eV, monolithic, series‐connected, tandem TPV converters are also under development and show promising performance; an in‐cavity efficiency of 11% has been reported for preliminary devices.