Citation
Ramiro Gonzalez, Iñigo and Villa, Juan and Tablero Crespo, César and Antolín Fernández, Elisa and Luque López, Antonio and Martí Vega, Antonio and Hwang, J. and Phillips, J. and Martin, A.J. and Millunchick, J.
(2017).
Analysis of the intermediate-band absorption properties of type-II GaSb/GaAs quantum-dot photovoltaics.
"Physical Review B", v. 96
;
pp. 125422-1.
ISSN 1098-0121.
https://doi.org/10.1103/PhysRevB.96.125422.
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 band gaps with: EL = 0.49 eV,
EH = 1.02 eV, and EG = 1.52 eV, with the IB located 0.49 eV above the valence band. These values are well
supported by semiempirical calculations of the QDs electronic structure. Through intensity-dependent two-photon
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 EL-related absorptivity of more than two orders of magnitude
and a decrease in the EH-related absorptivity of one order of magnitude. The antisymmetrical evolution of those
absorptivities is quantitatively explained by a photoinduced shift of the quasi-Fermi level of the IB. Furthermore,
we report the observation of a two-photon photovoltage, i.e., the contribution of subband gap two-photon
absorption to increase the open-circuit voltage of solar cells. We find that the generation of the two-photon
photovoltage is related, in general, to the production of a two-photon photocurrent. However, while photons with
energy close to EL 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.
©2017 American Physical Society
