14 March 2016 Observation of mini-band formation in the ground and high-energy electronic states of super-lattice solar cells
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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.
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Takanori Usuki, Takanori Usuki, Kouki Matsuochi, Kouki Matsuochi, Tsubasa Nakamura, Tsubasa Nakamura, Kasidit Toprasertpong, Kasidit Toprasertpong, Atsuhiko Fukuyama, Atsuhiko Fukuyama, Masakazu Sugiyama, Masakazu Sugiyama, Yoshiaki Nakano, Yoshiaki Nakano, Tetsuo Ikari, Tetsuo Ikari, } "Observation of mini-band formation in the ground and high-energy electronic states of super-lattice solar cells", Proc. SPIE 9743, Physics, Simulation, and Photonic Engineering of Photovoltaic Devices V, 974312 (14 March 2016); doi: 10.1117/12.2214094; https://doi.org/10.1117/12.2214094
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