High external quantum efficiency and fill-factor InGaN/GaN heterojunction solar cells grown by $NH3$-based molecular beam epitaxy

High external quantum efficiency (EQE) p-i-n heterojunction solar cells grown by $NH3$-based molecular beam epitaxy are presented. EQE values including optical losses are greater than 50% with fill-factors over 72% when illuminated with a 1 sun AM0 spectrum. Optical absorption measurements in conjunction with EQE measurements indicate an internal quantum efficiency greater than 90% for the InGaN absorbing layer. By adjusting the thickness of the top p-type GaN window contact layer, it is shown that the short-wavelength $(<365nm)$ quantum efficiency is limited by the minority carrier diffusion length in highly Mg-doped p-GaN.