Photovoltaic response of InGaN/GaN multi-quantum well solar cells enhanced by inserting thin GaN cap layers

•The spectral response of InGaN/GaN MQW solar cells with different LT-cap layer thicknesses was investigated.•Inserting a 0.75 nm-thick LT-cap layer can remarkably improve photovoltaic response of InGaN/GaN MQW solar cells.•A macroscopic optical method was used to investigate the microscopic localization effect and defect density in InGaN QW.•Strong localization effect in InGaN QW is harmful for the formation of high efficiency InGaN/GaN MQW solar cells.

Structural properties and photovoltaic response of InGaN/GaN multi-quantum well (MQW) solar cells with low temperature grown GaN cap (LT-cap) layers were investigated. It is found that inserting a thin LT-cap layer (around 0.75 nm) between each InGaN quantum well (QW) and GaN quantum barrier (QB) can remarkably improve photovoltaic response of InGaN/GaN MQW solar cells. This is attributed to the increased optical absorption of InGaN QWs due to higher indium content and thicker InGaN QWs thickness when LT-cap layers are added. However, if the LT-cap layer is too thick, the localization effect is enhanced and the defect density of InGaN QW layers will increase too much. Both of them will result in a reduced photovoltaic response of InGaN/GaN MQW solar cells.