Impact of Spatial Separation of Type-II GaSb Quantum Dots from the Depletion Region on the Conversion Efficiency Limit of GaAs Solar Cells
The purpose of this work is to look for a practical structure for application of quantum dots (QD) in solar cells in order to enhance sub-band gap photon absorption. We focuse on a stack of strain-compensated GaSb/GaAs type-II QDs. We propose a novel structure with GaSb/GaAs type-II QD absorber embedded in the p-doped region of ideal solar cell, but spatially separated from the depletion region. We developed the model and used the detailed balance principle along with Poisson and continuity equations for calculating of the energy band bending along with the photocurrent and the dark current, and the conversion efficiency of the cell. Our model takes into account both single-photon and double-photon absorption as well as non-radiative processes in QDs and predicts that oncentration from 1-sun to 500-sun raises the efficiency from 30% to 50%. We showed that accumulation of charge in the QD absorber is the clue to understanding of potentially superior performance of the proposed solar cell. An attractive feature of the proposed solar cell is that QDs do not reduce the open circuit voltage but facilitate generation of the additional photocurrent to the extent that photovoltaic characteristics reduce to that of ideal IB solar cell while the efficiency meets the Luque-Marti limit. It should be noted that, although non-radiative processes like relaxation in QDs and recombination through QDs degrade photovoltaic characteristics of the proposed solar cell, its conversion efficiency is still predicted to be above the Shockley-Queisser limit by 5% to 10%. This study is an important step toward producing practical solar cells that benefit from additional photocurrent generated by sub-band gap photons.