High-aspect-ratio structures for efficient light absorption and carrier transport in InGaAs/GaAsP multiple quantum well solar cells

High-aspect-ratio (HAR) quantum well was proposed as a general design principle to overcome the trade-off problem between the light absorption and the carrier collection in multiple quantum well (MQW) solar cells. HAR-MQW structure consists of thin wells and barriers, and its fundamental strategies are: (i) Thinner wells enhance the light absorption for 1HH transition, and make it possible to absorb the same amount of light with a thinner MQW region. (ii) Thinner barriers allow the photo-generated carriers to be extracted by means of tunneling transport. (iii) The wells must be deeper 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 photo-absorption 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 be still dominated by thermionic processes due to its large effective mass, tunneling of the electrons was clearly observed, and the extraction efficiency of photo-excited electrons remained much higher than that of a normal MQW cell at forward biases.