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Proceedings Paper

Observation of mini-band formation in the ground and high-energy electronic states of super-lattice solar cells
Author(s): Takanori Usuki; Kouki Matsuochi; Tsubasa Nakamura; Kasidit Toprasertpong; Atsuhiko Fukuyama; Masakazu Sugiyama; Yoshiaki Nakano; Tetsuo Ikari
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Paper Abstract

Multiple Quantum wells (MQWs) have been studied as one promising material for high-efficiency nextgeneration solar cells. However, a portion of photo-excited carriers recombine in MQWs, resulting in the degradation of cell performance. Super-lattice (SL) structures, where quantum states in neighboring quantum wells strongly couple with each other, have been proposed for the carrier collection improvement via the tunneling transport through mini-bands. Therefore, it is important to characterize mini-band formation in various types of SL structures. We examined p-i-n GaAs-based solar cells whose i layers contain 20 stacks of InGaAs/GaAsP MQW structures with 2.1-nm GaAsP barriers (thin-barrier cell), with 2.1-nm barriers and 3-nm GaAs interlayers in between GaAsP barriers and InGaAs wells (stepbarrier cell), and with 7.8-nm barriers (thick-barrier cell). We investigated the optical absorption spectra of the SL solar cells using piezoelectric photo-thermal (PPT) spectroscopy. In the thick-barrier cell, one exciton peak was observed near the absorption edge of MQWs. On the other hand, we confirmed a split of the exciton peak for the thin-barrier SL, suggesting the formation of mini-band. Moreover, in the step-barrier cell, the mini-band at the ground state disappears since thick GaAs interlayers isolate each quantum-well ground state and, instead, the mini-band formation of highenergy states could be observed. By estimating from the energy-level calculation, this is attributed to the mini-band formation of light-hole states. This can well explain the improvement of carrier collection efficiency (CCE) of the thinbarrier and the step-barrier cells compared with the thick-barrier cell.

Paper Details

Date Published: 14 March 2016
PDF: 8 pages
Proc. SPIE 9743, Physics, Simulation, and Photonic Engineering of Photovoltaic Devices V, 974312 (14 March 2016); doi: 10.1117/12.2214094
Show Author Affiliations
Takanori Usuki, The Univ. of Tokyo (Japan)
Kouki Matsuochi, Univ. of Miyazaki (Japan)
Tsubasa Nakamura, Univ. of Miyazaki (Japan)
Kasidit Toprasertpong, The Univ. of Tokyo (Japan)
Atsuhiko Fukuyama, Univ. of Miyazaki (Japan)
Masakazu Sugiyama, The Univ. of Tokyo (Japan)
Yoshiaki Nakano, The Univ. of Tokyo (Japan)
Tetsuo Ikari, Univ. of Miyazaki (Japan)

Published in SPIE Proceedings Vol. 9743:
Physics, Simulation, and Photonic Engineering of Photovoltaic Devices V
Alexandre Freundlich; Laurent Lombez; Masakazu Sugiyama, Editor(s)

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