By using absolute EL measurement, we characterize current-dependent external radiative efficiency of GaInAs/GaAsP multiple-quantum-well (MQW) single-junction solar cells with and without a back distributed Bragg reflector (DBR). We quantified the internal radiative efficiency (η
) under illuminated condition for analyzing their difference in photovoltaic performances. It was revealed that MQWs showed an advantage of improved qint compared with a bulk layer, and that a back DBR indeed improved conversion efficiency via double-pass absorption of sun light, but improvement via reduction of rear radiative emission loss toward substrate was small. Efficiency add-on via improved material quality, or qint, in the same structures was predicted.