Absorption coefficients of GaSb/GaAs quantum dots (QDs) are calculated by the 8-band strain-dependent k· p method and Fermi’s golden rule. A more realistic but simple approach to model the QD ensemble with wetting layer is described. Effects of the QD size and density, and the GaAs spacer thickness for multi-stacked QDs on absorption characteristics are studied. Absorption spectra of the single QD, single layer of QDs, and multi-stacked QDs are presented and discussed. Interband absorption is found to be more intense than intraband absorption. The calculated absorption spectra are brought into the drift-diffusion model coupled with rate equations to determine the current density-voltage curves of the GaSb/GaAs QD solar cells, which are compared with measured data in literature for validation. The models proposed in this work are capable of predicting the short-circuit current density and open-circuit voltage of real devices, and would have the potential to investigate the impact of doping and position of the QD layers, which is necessary for intermediate band solar cell analysis and design.
Self-assembled GaSb/GaAs quantum-ring-with-dot structures (QRDSs) are the nanostructures exhibiting type-II band alignment. Each QRDS consists of both quantum ring (QR) and quantum dot (QD) parts which have their own energy levels. In this work, the carrier dynamics in the GaSb/GaAs QRDSs are explored and quantitatively described by a rate equation model which is developed from experimental photoluminescence (PL) spectra in literature. The model is comprised of the carrier transition rates and activation energies involved the transfer processes of thermal-excited carriers. The difference between the QR and QD PL intensities is also taken into account in the model. Electronic structures of a single QRDS are calculated in order to determine the overlap between electron and hole wave functions that can be used for estimating the radiative transition rates. Numerical values of the radiative and non-radiative transition rates, carrier capture and escape rates, and activation energies are presented. The PL spectra obtained from the model are consistent with the reported experimental data.
Optical absorption spectra of GaSb/GaAs quantum ring-with-dot structures (QRDSs) are calculated by the Fermi’s golden rule which the electronic states involved the optical transitions are obtained from the eight-band k · p method. The absorption spectra of multi-stacked QRDS layers show the interband and intraband transitions that are favorable to intermediate band solar cells (IBSCs). A drift-diffusion model with rate equations for a solar cell containing multi-stacked QRDS layers is formulated according to the absorption spectra. The external quantum efficiency (EQE) determined by the model with the AM1.5 solar spectrum and additional infrared light demonstrates that the confined hole states in the quantum ring (QR) parts of the QRDSs effectively function as an intermediate band. More efficient two-step photon absorption indicated by the enhancement of EQE also suggests the potential of QRDSs for the IBSCs.
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