Abstract
We have investigated the performance of 10-layer stacked GaSb/GaAs quantum dot (QD) and quantum ring (QR) solar cells (SCs) having a type-II band alignment. For both SCs, the external quantum efficiency (EQE) increased in the longer wavelength region beyond GaAs bandedge wavelength of λ > 870 nm due to an additive contribution from GaSb/GaAs QD or QR layers inserted in the intrinsic region of p-i-n SC structure. The EQE of GaSb/GaAs QRSC was higher than that of QDSC at room temperature and the photoluminescence intensity from GaSb/GaAs QRs was stronger compared with GaSb/GaAs QDs. These results indicate that crystal quality of GaSb/GaAs QRs is superior to that of GaSb/GaAs QDs. Furthermore, a photocurrent production due to two-step photo-absorption via GaSb/GaAs QD states or QR states, ΔEQE was measured at low temperature and the ratio of two-step absorption to total carrier extraction defined as ΔEQE / (ΔEQE + EQE), was higher for GaSb/GaAs QRSC than that of QDSC. The ratio of GaSb/GaAs QRSC exceeds 80% over the wavelength region of λ = 950 - 1250 nm. This suggests that two-step absorption process is more dominant for carrier extraction from GaSb/GaAs QR structure.
Highlights
Intermediate band solar cells (IBSCs) that utilize the photo-absorption by sub-bandgap photons have attracted significant attention as one of the means to achieve the high conversion efficiency exceeding the Shockley-Queisser limit.[1]
The PL intensity attributed to GaSb quantum ring (QR) was stronger compared with GaSb quantum dot (QD)
These results indicate that nonradiative recombination losses are decreased by forming QR structure
Summary
Intermediate band solar cells (IBSCs) that utilize the photo-absorption by sub-bandgap photons have attracted significant attention as one of the means to achieve the high conversion efficiency exceeding the Shockley-Queisser limit.[1]. There have been several studies on IB materials such as highly mismatched with using dilute nitride semiconductors,[3,4] and multi-stacked quantum nanostructures (QNs)[1,5,6,7,8,9,10] for application to IBSCs. Extensive efforts have been put into demonstrating IBSCs using III-V semiconductor QDs. In particular, investigations were centered on multi-stacked In(Ga)As/GaAs QDs grown on GaAs substrate. Investigations were centered on multi-stacked In(Ga)As/GaAs QDs grown on GaAs substrate In this material system grown on GaAs (001) substrate, a stacked structure of 400 QD layers while keeping high crystal quality has been reported.[9] Further, InGaAs QD structures grown on high-index GaAs(311)B substrate exhibit a spatially ordered in-plane structure with a six-fold symmetry.[10,11] for In(Ga)As/GaAs system, the lifetime of generated carriers within QD layers is relatively short, because of the type-I band alignment between InGaAs QD and GaAs barrier layers
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