Carrier extraction behaviour in type II GaSb/GaAs quantum ring solar cells

Abstract

The introduction of quantum dot (QD) or quantum ring (QR) nanostructures into GaAs single-junction solar cells has shown enhanced photo-response above the GaAs absorption edge, because of sub-bandgap photon absorption. However, to further improve solar cell performance a better understanding of the mechanisms of photogenerated carrier extraction from QDs and QRs is needed. In this work we have used a direct excitation technique to study type II GaSb/GaAs quantum ring solar cells using a 1064 nm infrared laser, which enables us to excite electron-hole pairs directly within the GaSb QRs without exciting the GaAs host material. Temperature and laser intensity dependence of the current-voltage characteristics revealed that the thermionic emission process produced the dominant contribution to the photocurrent and accounts for 98.9% of total photocurrent at 0 V and 300 K. Although the tunnelling process gives only a low contribution to the photocurrent, an enhancement of the tunnelling current was clearly observed when an external electric field was applied.

Open‐circuit voltage recovery in type II GaSb/GaAs quantum ring solar cells under high concentration

“…At low temperature (100 K), J SC for the GaSb QR cell is lower V OC recovery in type II GaSb/GaAs QR solar cells H. Fujita et al than that of the GaAs control. However, J SC starts to increase rapidly above 180 K and surpasses the GaAs control cell above 250 K. This behaviour is consistent with the observation that the photocurrent due to sub-bandgap photon absorption is dominated by thermionic emission of holes [14]. At the same time, V OC for the GaSb QR cell shows a slight change of slope, and the FF starts to decrease rapidly above 180 K, resulting in a net reduction of the efficiency.…”

“…Illumination was provided by a calibrated tungsten‐halogen light source where the maximum achievable concentration was equivalent to 19 suns. In order to clarify the behaviour of the photo‐generated carriers in the GaSb QRs, a 1064‐nm infrared laser was used to illuminate the device during the J–V measurement at room temperature . The wavelength is chosen so that it has small enough photon energy not to excite GaAs but has large enough energy to excite electron–hole pairs in the GaSb QRs.…”

“…Recently, GaSb/GaAs QR solar cells with extended infrared response up to 1400 nm have been demonstrated . Furthermore, in this system, it has also been shown that the sub‐bandgap response is limited by the thermionic emission of holes from QRs . However, in order to further improve the solar cell efficiency, a more detailed understanding of the photo‐carrier dynamics is essential, especially under high solar concentration conditions under which these solar cells will be implemented.…”

Optical absorption coefficient calculations of GaSb/GaAs quantum dots for intermediate band solar cell applications

“…Type-II nanostructures become interesting choices due to long carrier lifetime. In respect of the IBSC applications, GaSb/GaAs QDs and QRs exhibiting the type-II band alignment have been investigated [10,11,[15][16][17][18][19][20][21]. For GaSb nanostructures in the GaAs matrix, holes are confined in the GaSb region by a large VB offset, while electrons reside in the GaAs region around the nanostructures [22,23].…”

Two-dimensional simulation of GaAsSb/GaAs quantum dot solar cells

“…Recently, type-II GaSb/GaAs nanostructures have gained a lot of attention owing to their long carrier lifetime as a result of the reduced electron-hole wave function overlap [18][19][20]. Both theoretically and experimentally, GaSb/GaAs QD and QR solar cells have shown an extended spectral response and efficient carrier extraction [21][22][23][24][25][26][27][28]. Even type-II nanostructures possess a long carrier lifetime.…”