Multi-band solar cell using strain-balanced quantum well of nitride semiconductors

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Aiming at the photocurrent generation on the basis of two-step photon absorption, which is a promising way of over 50% efficiency, quantum well structure was employed and its potential as a 3-level light absorbing medium. First, InGaAs/GaAsP strain-balanced quantum wells were investigated and a superlattice structure with the barriers thinner than 3 nm exhibited the photocurrent generation via 2-step photon absorption, probably due to extended carrier lifetime in the quantum confinement state owing to efficient carrier separation by tunneling transport.Next, the strain-balanced stacking technology for InGaN/AlN system was established. This material system is ideal for a obtaining a large band offset which is required for the high efficiency by two-step photon absorption. The strain-balanced InGaN/AlN stack was superior to conventional InGaN/GaN in terms of crystal quality and photoluminescenc property. Finally, the carrier extraction from the nitride quantum wells was examined.