Effect of Quantum Well on the Efficiency of Carrier Collection in InGaAs/GaAsP Multiple Quantum Well Solar Cells

Abstract

For a quantitative evaluation of the carrier transport dynamics of multiple quantum well (MQW) solar cells, carrier collection efficiency (CCE) was defined and its measurement procedure was proposed. CCE is essentially the quantum efficiency normalized to the saturation value at a reverse bias. It allows us to know whether substantial carriers are actually extracted at any bias voltage, and to uncover the bottleneck problems of carrier transport that emerge at the operation bias. The advantage of CCE analysis is that the dynamics of photoexcited carriers can be selectively examined independently of the diode characteristics of the devices if the effects of resistance are small enough. In the present study, GaAs p–i–n solar cells including various numbers of InGaAs/GaAsP MQWs with a band gap of 1.2 eV in the i-region of equal thickness were fabricated and characterized. Interfered carrier transport by increasing the well number was quantitatively and directly evaluated. With up to 30 periods of MQWs, the carriers generated, especially in the wells, were less likely to be collected than those generated in the top p-region at a moderate forward bias, but collection of both was found to be degraded severely with 40 periods of MQWs.