Analysis of the intermediate-band absorption properties of type-II GaSb/GaAs quantum-dot photovoltaics

Abstract

Quantum-dot (QD) intermediate-band (IB) materials are regarded as promising candidates for high-efficiency photovoltaics. The sequential two-step two-photon absorption processes that take place in these materials have been proposed to develop high-efficiency solar cells and infrared (IR) photodetectors. In this work, we experimentally and theoretically study the interrelation of the absorptivity with transitions of carriers to and from the IB in type II GaSb/GaAs QD devices. Our devices exhibit three optical bandgaps with: E L =0.49 eV, E H =1.02 eV and E G =1.52 eV, with the IB located 0.49 eV above the valence band. These values are well supported by semi-empirical calculations of the QDs electronic structure. Through intensity-dependent twophoton photocurrent experiments, we are able to vary the filling state of the IB, thus modifying the absorptivity of the transitions to and from this band. By filling the IB with holes via E=1.32 eV or E=1.93 eV monochromatic illumination, we demonstrate an increase in the E L-related absorptivity of more than two orders of magnitude and a decrease in the E H-related absorptivity of one order of magnitude. The anti-symmetrical evolution of those absorptivities is quantitatively explained by a photo-induced shift of the quasi-Fermi level of the IB. Furthermore, we report the observation of a two-photon photovoltage; i. e., the contribution of sub-bandgap two-photon absorption to increase the open-circuit voltage of solar cells. We find that the generation of the twophoton photovoltage is related, in general, to the production of a two-photon photocurrent. However, while photons with energy close to E L participate in the production of the two-photon photocurrent, they are not effective in the production of a two-photon photovoltage. We also report the responsivity of GaSb/GaAs QD devices performing as optically triggered photodetectors. These devices exhibit an amplification factor of almost 400 in the IR spectral region. This high value is achieved by minimizing-via doping-the absorptivity in the IR range of the QDs under equilibrium conditions. I. INTRODUCTION

Advanced material system for the design of an intermediate band solar cell: Type-II CdTe quantum dots in a ZnCdSe matrix

“…Using either epitaxial quantum dots (QDs) or impurities one can introduce such a band 3,4 . A two step-photon process occurs from the valence band (VB) to the IB and from IB to the conduction band (CB), ultimately increasing light absorption of the solar spectrum without compromising the open circuit voltage (VOC) of the device 5 . The proof of concept for an IBSC has been realized, and it has been shown that the short circuit current can be increased by the introduction of QDs 6 .…”

Adiabatic two-step photoexcitation effects in intermediate-band solar cells with quantum dot-in-well structure

“…When only irradiated by the 784-nm LD, quasi-Fermi level is single. When the additional IR light pumps electrons from the IB into the CB, quasi-Fermi levels of the IB and the CB start splitting, and, therefore, the adiabatic process of the two-step photoexcitation recovers the output voltage 38 . In order to demonstrate the voltage recovery effect caused by two-step photoexcitation, we investigated short-circuit current, J SC , and open-circuit voltage, V OC , under the irradiation of the both sources, the 784-nm LD and the additional IR light.…”

Design and modelling of InGaP/GaSb tandem cell with embedded 1D GaAs quantum superlattice

“…In the proposed work, materials and their thickness are chosen from the previously reported literature [22, 23]. The thickness and doping concentration of 1D quantum confined superlattice are taken from experimental reports [26, 27]. The InGaP/GaSb tandem cell is designed by incorporating GaAs QS layers with a total area of 4.0384 × 10 −8 cm 2 .…”