An investigation into the efficiency enhancement of strained and strain-balanced quantum well solar cells

The optical absorption of a p-i-n solar cell may be increased by incorporating quantum well layers in the intrinsic region, resulting in a larger short-circuit-current. Further, the additional quantum well absorption may be achieved without incurring a comparable loss in open-circuit-voltage; offering potential for enhanced solar power conversion efficiencies. The work reported in this thesis is concerned with the application of quantum well techniques to the GaAs cell, currently the highest efficiency single band-gap cell. In the absence of lattice matched lower band-gap materials, strained InGaAs was used as the quantum well material. An optical study of the carrier dynamics at the solar cell operating point indicates suppressed quasi-Fermi levels in the QW with respect to the bulk material, with advantageous implications for the open-circuit-voltage. However the strained quantum wells introduce misfit dislocations into the structure that dominate the recombination of the cell. The associated loss in open-circuit-voltage is shown to be too large to allow strained GaAs/InGaAs cells to realistically match GaAs in terms of power conversion efficiency. A strain-balance technique is presented, surmounting the limitations imposed by strain and is applied to a 5% InGaAs virtual substrate GaAs/InGaAs structure and a GaAs substrate GaAsP/InGaAs structure. An equivalent power conversion efficiency to GaAs is demonstrated for the GaAsP/InGaAs cell. The scope for further improvement of the GaAsP/InGaAs cell is discussed, with particular reference to light trapping. (author)Available from British Library Document Supply Centre-DSC:DXN035941 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo