Pyramidal light trapping and hydrogen passivation for high-efficiency heteroepitaxial (100) crystal silicon solar cells

We report growth and characterization of heteroepitaxial silicon solar cells on sapphire to demonstrate the promise of heteroepitaxial crystal silicon (c-Si) film photovoltaics on inexpensive substrates coated with chemically inert crystalline buffer layers such as Al2O3. Our work isolates and addresses critical material and light-trapping issues that must be solved to develop film c-Si solar cells. Microscopy reveals high dislocation densities and other crystalline defects in the silicon layers, and these defects limit the unhydrogenated devices with a 1.5 μm absorber layer to below 1% sunlight-to-electricity conversion efficiency. By exposing an identical device to atomic H from a remote plasma, we demonstrate a 5.2% efficient device with dramatically improved quantum efficiency (QE) and open circuit voltage, as the minority carrier diffusion length increases from ∼1 μm to ∼4.5 μm. When we incorporate both hydrogen passivation and top surface pyramidal light trapping we further improve the QE and achieve 6.8% efficiency.