Enhanced infrared photo-response from GaSb/GaAs quantum ring solar cells

Abstract

GaAs-based solar cells containing stacked layers of nanostructured type II GaSb quantum ring solar cells are reported which show significantly enhanced infrared photo-response extending out to 1400 nm. The ring formation reduces the net strain energy associated with the large lattice mismatch making it possible to stack multi-layers without the need for strain balancing. The (1 sun) short-circuit current for a 10 layer sample is enhanced by ∼6% compared to a GaAs control cell. The corresponding open-circuit voltage of 0.6 V is close to the theoretical maximum expected from such structures.

Simulation of the enhanced infrared photoresponse of type-II GaSb/GaAs quantum ring solar cells

“…The spectral response is characterised by two broad bands that have been found to scale linearly with the number of GaSb layers introduced inside the junction. 1 Both bands have therefore been attributed to absorption by the GaSb/GaAs nano-structures, with the narrower band nearer to the GaAs absorption edge (at 1.3 eV) assigned to the wetting layer (WL) formed during the GaSb deposition. The broader band, which typically extends down to 850 meV, was ascribed to absorption by the quantum rings.…”

“…The incorporation of GaSb/GaAs quantum rings (QRs) into the junction of a GaAs solar cell has been shown to be an effective means of increasing the photocurrent of a conventional thin-film solar cell. 1 The hole confinement and the separation of the electron and hole wave-functions characteristic of type-II heterostructures (i.e., one charge carrier is localized and the QR and the other one in the barrier) leads to a range of interesting attributes, such as sub-band gap absorption and increased electron lifetimes. Although the photoluminescence (PL) properties of the various GaSb/ GaAs quantum structures have been extensively reported, 2 there still remain some inconsistencies between the observed optical transitions and the nano-structure.…”

“…14 When considering some variance in the QR height and ring width, the associated width of the energy distribution will primarily depend on the smaller dimension. Since H < DR for a variety of growth conditions and material systems, 1,15,16 the height distribution of the rings is expected to determine the inhomogeneous broadening of the hole states, with the integrated radial confinement only offsetting the mean energy.…”

“…A detailed description of the growth procedure and the device structure is given in Ref. 1. The active region of the p-i-n diode contain ten layers of GaSb/GaAs QRs, with a typical surface density of 1 Â 10 10 rings/cm 2 .…”

“…Cross-sectional transmission electron microscopy (TEM) images showed the rings to have a typical outer and inner diameter of 23 and 10 nm, respectively, with a thickness of approximately 1.7 nm. 1 These structures were processed into circular solar cells with a diameter of 1 mm using standard photolithography (see inset in Fig. 1).…”