Impact of spatial separation of type‐II GaSb quantum dots from the depletion region on the conversion efficiency limit of GaAs solar cells

Abstract

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.

Ge/Si core/shell quantum dots in alumina: tuning the optical absorption by the core and shell size

“…Semiconductor core/shell quantum dots (QDs) receive strong attention due to the tunability of their electronic properties, leading to many relevant nanotechnology and optoelectronic applications [1][2][3][4][5][6][7]. Both germanium and silicon are materials frequently used in the electronic industry; in addition, they show strong confinement effects in QDs [7,8].…”

Releasing confined holes from type-II quantum dots by inelastic scattering with hot photoelectrons

“…We have already shown that intra-band QD absorption of longwavelength part of solar spectrum with photon energy being less than the energy band gap of QDs facilitates exciton disintegration in GaSb/GaAs type-II QD absorber [8]. Such intra-band absorption transfers photon energy to confined holes.…”

“…This process leads to generation of dark current in the depletion region of p-n-junctions. Therefore, embedding of smaller energy band gap materials like QDs into the depletion region inevitably increases the dark current generated in the p-n-junction [8]. Numerous experimental studies of QD solar cells demonstrated such increase of the dark current [9,10].…”

“…This enables confined holes to escape from QDs into mobile states in the valence band of absorber, which decomposes excitons into mobile electrons and holes. We have estimated that collection of such mobile carriers would increase both photocurrent and conversion efficiency limit of single junction GaAs solar cells by 50% in case of 500-sun concentrated illumination [8].…”

Delta doping and positioning effects of type II GaSb quantum dots in GaAs solar cell

“…But, electron-hole recombination in QDs results in additional dark current which reduces the open circuit voltage and keeps the conversion efficiency of QD solar cells below the Shockley-Queisser limit. Using a GaSb/GaAs type II QD absorber embedded in the p-doped region of an ideal solar cell but spatially separated from the depletion region is expected to lead to voltage preservation [2]. Meanwhile, improvements in voltage preservation through suppression of hole thermionic emission rates by n-type doping at the expense of reduction in short circuit current and by positioning of QD layers at the edge of the space charge region have been reported [3].…”