Radiation response of the fill-factor for GaAs solar cells with InGaAs quantum dot layers

The radiation response of GaAs PiN solar cells with quantum dot (QD) layers was investigated. We particularly noted the degradation of fill-factor (FF) for the QD cell. To obtain the degradation effect of the diffusion length of minority carrier, the generation current was measured with various bias voltages not under light illumination but under high-energy electron beam irradiated from the surface (HE-EBIC: High energy- electron beam induced current). The degradation of generation current due to irradiation was also obtained simultaneously since high energy electrons degrade solar cells. When sufficiently large reverse-bias voltage was applied, degradation of the generation current in the QD cell was saturated with bias voltage and only reflected in the declining diffusion length of the minority carrier. However, in the case of forward bias voltage, degradation of the generation current in the QD cell exceeded that with reverse- bias voltage. It is considered that there was a change in characteristics within the depletion region by irradiation in addition to the degradation of the diffusion length of minority carrier. The results suggest that the depletion region degrades at forward bias voltage in the QD cell post-after irradiation, which leads to serious degradation of FF.