InGaAs/GaAsP superlattice solar cells with reduced carbon impurity grown by low-temperature metal-organic vapor phase epitaxy using triethylgallium

Abstract

In this paper, we investigated the effects of carbon incorporation on photovoltaic performance of InGaAs/GaAsP superlattice (SL) solar cells grown by low-temperature MOVPE (LT-MOVPE), which is required for stable SL growth on vicinal substrates. Using trimethylgallium (TMGa) as the gallium precursor, methyl radicals formed by its pyrolysis tend to be absorbed on the surface at low temperature, causing severe carbon incorporation and p-type background doping. High background carrier concentration flattens the band-lineup of the intrinsic region and blocks the carrier transport across the SLs, and resulted in serious degradation of photocurrent. Intentional sulfur doping to cancel out the background doping and hence to recover the built-in field greatly improved the cell performance, but was found to require very precise control of doping level to achieve an exact compensation doping condition. Use of triethylgallium (TEGa) instead of TMGa much reduced the carbon incorporation at low temperature and significantly enhanced the photocurrent extraction without sulfur doping treatment. By thinning GaAsP barriers to 3 nm to facilitate efficient tunneling transport, a 50-period SL cell with bandgap of 1.22 eV grown on 6°-miscut substrates achieved 1.13 times higher efficiency with 31% current enhancement as middle cell performance than a GaAs reference cell.