Thickness-modulated InGaAs/GaAsP superlattice solar cells on vicinal substrates

Abstract

InGaAs/GaAsP superlattice (SL) is a promising narrow-gap material for III–V multi-junction solar cells on Ge. In metal-organic vapor phase epitaxy (MOVPE) of SL on vicinal substrates, the component layers tend to be undulated due to step bunching occurring at high temperature. In this paper, the effects of growth temperature and thickness modulation of the SL-region on the photovoltaic performance were investigated. Lowering the growth temperature successfully enabled epitaxy of an extremely uniform SL, from which a clear step-like absorption spectrum including sharp exciton peaks was obtained due to layer-by-layer deposition of the individual layers. Larger layer undulation at higher temperature led to poorer in-plane coverage of the InGaAs region, resulting in the reduction of both light absorption and short circuit current. The open circuit voltage, on the other hand, was higher for the cells grown at higher temperature owing to suppressed dark current as a result of reduced crystal defects. Moreover, the lateral thickness variation of the GaAsP barriers in the undulated SL allowed efficient tunnel transport through the thinner part of the barrier, and improved the carrier collection and the fill factor. By optimizing the growth temperature for SL on vicinal substrates, an N-on-P cell including 100-period SL with a bandgap of 1.21 eV achieved 1.11 times higher efficiency than a GaAs reference cell with 36% current enhancement as middle cell performance.