Optimization of GaN window layer for InGaN solar cells using polarization effect

The III-nitride material system offers substantial potential to develop high-efficiency solar cells. Theoretical modeling of InGaN solar cells indicate strong band bending at the top surface of p-InGaN junction caused due to piezoelectric polarization-induced charge at the strained p-GaN window interface. A counterintuitive strained n-GaN window layer is proposed, modeled and experimentally verified to improve performance of InGaN solar cells. InGaN solar cells with band gap of 2.9 eV are grown using MOCVD with p-type and n-type strained GaN window layers, and fabricated using variable metallization schemes. Fabricated solar cells using n-GaN window layers yield superior V _OC and FF compared to those using p-GaN window layers. The V _OC's of InGaN solar cells with n-GaN window layers are further enhanced from 1.5 V to 2 V by replacing the conventional NiO_X top contact metal with Ti/Al, which also verifies the tunneling principle.