Carrier Time-of-Flight Measurement Using a Probe Structure for Direct Evaluation of Carrier Transport in Multiple Quantum Well Solar Cells

Carrier transport across multiple quantum well (MQW) structures inserted in the i-region of a p-i-n diode is an important mechanism that determines the performance of MQW solar cells. We have employed a carrier time-of-flight measurement technique using a quantum-well probe to investigate the electron transport time across MQW structures. Delay of the carrier arrival time, defined as time-of-flight, caused by MQWs shows almost linear increment to the number of wells. Tunneling transport in InGaAs/GaAsP MQW structures is studied by varying the GaAsP barrier thickness. Barrier thickness of 2 nm results in extremely small electron time-of-flight, less than a hundred picoseconds per well, whereas MQW with 8-nm-thick barriers results in approximately 20 times slower transport. Furthermore, the fast time-of-flight in thin barriers shows no degradation after the insertion of GaAs interlayers that form the multistep potential. This suggests that the fast thermally assisted tunneling transport dominates the electron escape dynamics in thin-barrier InGaAs/GaAsP MQWs. The rapid carrier transport results in the significant suppression of current drop at the cell maximum power point of MQW solar cells.