$hbox{In}_{x}hbox{Ga}_{1-x}hbox{N/GaN}$ multiple quantum-well (QW) (MQW) solar cells with $x = hbox{0.30} hbox{and} hbox{0.15}$ were characterized. The MQWs with $x = hbox{0.30}$ show deteriorated performances due to the inferior crystal qualities. At the temperatures above 200 K, the conversion efficiency $(eta)$ for $x = hbox{0.30}$ exhibits an abrupt increase led by the thermally activated carriers. Two potential origins are proposed for the hot carriers: 1) the native shallow donors in the MQWs and 2) the shallow QWs due to the compositional fluctuations. According to the distinct behavior of the device with $x = hbox{0.15}$, it is believed that the shallow QWs lead to the abrupt increase in $eta$.
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