Comparison of Electron and Hole Mobilities in Multiple-Quantum-Well Solar Cells Using a Time-of-Flight Technique

Understanding of transport dynamics of both electrons and holes in quantum-structure solar cells is essential for their structure design and performance enhancement. By applying our proposed carrier time-of-flight technique on p-on-n and n-on-p configurations, we can separately evaluate electron and hole transport across quantum structures inserted in the i-region of solar cells. Electron and hole behaviors in two sets of InGaAs/GaAsP multiple-quantum-well (MQW) solar cells with different potential barrier heights are investigated in this study. Both types of carriers in In_0.21Ga_0.79As/GaAs_0.75P_0.25MQWs show faster averaged velocities by an order of magnitude than those in In_0.21Ga_0.79As/GaAs_0.59P_0.41MQWs, which have higher potential barriers. Within the same MQW structure, the measured values of electron and hole averaged velocities are very close to each other. This results in the same order of effective mobilities of electrons and holes and the similar tendency of cell performance in p-on-n and n-on-p MQW solar cells. Holes in high-barrier InGaAs/GaAsP MQWs show nonlinear mobility, suggesting that a careful design of high-barrier MQWs is required to avoid the hole bottleneck and enhance charge carrier collection.