High-Aspect Ratio Structures for Efficient Light Absorption and Carrier Transport in InGaAs/GaAsP Multiple Quantum-Well Solar Cells

The high aspect ratio (HAR) quantum well was proposed as a general design principle to overcome the tradeoff problem between light absorption and carrier collection in multiple quantum-well (MQW) solar cells. An HAR-MQW structure consists of thin wells and barriers, and its fundamental strategies are 1) thinner wells to enhance the light absorption for 1HH transition and make it possible to absorb the same amount of light with a thinner MQW region; 2) thinner barriers to allow the photogenerated carriers to be extracted by means of tunneling transport; and 3) deeper wells to obtain the same effective bandgap as thicker wells because of the stronger confinement. The enhanced absorption coefficient for HAR-MQW was proved by the measurement of both photoabsorption and the quantum efficiency at a sufficiently large reverse bias. Stronger photon absorption via 1HH transition was achieved with a smaller total thickness of the wells area. In the HAR-MQW cell, although the transport of the heavy holes was found to still be dominated by thermionic processes due to its large effective mass, tunneling of the electrons was clearly observed, and the extraction efficiency of photoexcited electrons remained much higher than that of a normal MQW cell at forward biases.