| AVS 61st International Symposium & Exhibition | |
| Energy Frontiers Focus Topic | Monday Sessions |
| Session EN+EM+MN+NS+TR-MoA |
| Session: | Energy Harvesting with Nanostructures |
| Presenter: | Atsuhiko Fukuyama, University of Miyazaki, Japan |
| Authors: | A. Fukuyama, University of Miyazaki, Japan T. Ikari, University of Miyazaki, Japan K. Nishioka, University of Miyazaki, Japan T. Aihara, University of Miyazaki, Japan H. Suzuki, University of Miyazaki, Japan H. Fujii, The University of Tokyo, Japan M. Sugiyama, The University of Tokyo, Japan Y. Nakano, The University of Tokyo, Japan |
| Correspondent: | Click to Email |
Fabrication of multiple quantum well (MQWs) in an absorption layer can extend the absorption region toward a longer wavelength and enhance the short-circuit current in the solar cells. However, MQWs function as recombination centers, leading to degradation in both open-circuit voltage and fill factor. We have already reported that the increase in stack number of QW causes the degradation of carrier collection efficiency [1]. In this study, we investigate the effects of stacks number on temperature dependences of the photoluminescence (PL), photothermal (PPT) and the surface photovoltage (SPV) signals. Although the photoexcited carriers in the barrier should relax by the radiative recombination (PL), carriers can thermally escape (SPV) or non-radiatively recombine (PPT) at the same time. Therefore, the latter two methodologies give us new insights for the carrier recombination and drift through the QW.
The present strain-balanced InGaAs/GaAsP MQWs absorption layer was composed of a 7.0-nm-thick In0.25Ga0.75As well and a 10.8-nm-thick GaAs0.66P0.34 barrier. All layers were grown on an n-type GaAs substrate using metal-organic vapor phase epitaxy. We prepared different samples with MQW stack numbers of 10, 20, 30, and 40 in the i-region.
All PPT and SPV spectra showed three distinctive peaks followed by a step like function. They were decomposed into inter-subband transitions expressed by the two dimensional density of states for the QW and exciton peaks [2]. Although the PL intensity decreases with increasing the temperature, signals for PPT and SPV increases. We suppose two activation energies for the process: one is that for the carrier escape from the QW and another is for the non-radiative recombination in the QW. The three rate equations were built for PL, PPT and SPV and the temperature dependences are numerically fitted to estimate the two activation energies. As a result, we have estimated the activation energy for carrier escaping from the QW is constant as 70 meV for all samples with different stacks number. This is the same as the calculated barrier height. However, the activation energy for the non-radiative recombination increases from 6 to 49 meV for the sample with 10 and 40 stacks. This means that radiative recombination increases with increasing the stack number. The carriers thermally escape from the QW again relax into next well and may contribute to increase the radiative recombination.
[1] H. Fujii et al., Jpn. J. Appl. Phys. 51, 10ND04 (2012).
[2] M. Kondow, A. Fukuyama, and T. Ikari et al., Appl. Phys. Express 2, 041003 (2009).