Photovoltaic properties and technological aspects of In1−xGaxN/Si, Ge (0<x<0.6) heterojunction solar cells

Abstract

Photovoltaic properties of n-In1−xGaxN/p-Si, Ge (IGN) heterostructures, covering the compositional range 0<x<0.6, have been evaluated by 1d device simulation, and are compared with the performance of c-Si homojunction thin film cells. Film morphology and physical properties were characterized by high-resolution transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS) and photoluminescence (PL). Best achievable cell performances under AM1.5 illumination conditions were 18% for p-Ge, and up to 27% for n-IGN/p-Si contacts, achievable under optimum cell design, materials and operation parameters. Pure InN bottom layers, exhibiting an intrinsic band gap of 0.7 eV, reveal a reduced efficiency of 2.5%. The cell efficiency is strongly affected by film quality, accounted for by variation of electron affinity, majority carrier mobility, minority carrier lifetime, film thickness and doping levels. The morphology of thin IGN and InN films deposited onto silicon and sapphire substrate material revealed granular growth, along with a high density of grain boundaries. TEM resolved the formation of a very thin homogeneous silicon nitride interlayer on silicon substrates. The electrically isolating layer almost completely suppresses the photovoltaic effect. Depth profiling of InN films deposited onto sapphire substrates by SIMS analysis indicated oxygen as the dominant material contamination. It accounts, among other effects, for a gradually increasing band gap throughout the film structure. Observed large photoluminescence broadening effects, and related short minority carrier lifetimes are most likely related to high levels of oxygen contamination and concentration of grain boundaries. Possible routes to overcome these problems are discussed.