| AVS 61st International Symposium & Exhibition | |
| Energy Frontiers Focus Topic | Tuesday Sessions |
| Session EN-TuP |
| Session: | Energy Frontiers Poster Session |
| Presenter: | Tetsuo Ikari, University of Miyazaki, Japan |
| Authors: | T. Ikari, University of Miyazaki, Japan A. Fukuyama, University of Miyazaki, Japan K. Nishioka, University of Miyazaki, Japan T. Aihara, University of Miyazaki, Japan H. Kuradome, University of Miyazaki, Japan K. Toprasertpong, University of Tokyo, Japan M. Sugiyama, University of Tokyo, Japan Y. Nakano, University of Tokyo, Japan |
| Correspondent: | Click to Email |
Embedding of multi quantum well (MQW) structures in an absorption layer of GaAs solar cell is a promising idea for developing higher efficient solar cell devices. Extension of the absorbing region to longer wavelength side enhances the short-circuit current. When the mini-band forms between the QW, further developing of the conversion efficiency would be expected in term of a carrier tunneling without recombination of the photo excited carriers. However, recombination at the QW interface still leads to the degradation of the efficiency. Therefore, we fabricated a strain-balanced InGaAs/GaAsP layer in the intrinsic region of the GaAs p-i-n solar cell [1]. The mini-band formation for the samples with different thickness of the barrier region from 5.3 to 1.9 nm was studied by using a photoreflectance (PR) and a surface photovoltage (SPV) spectroscopy at room temperature.
The energies at the Brillion zone edges were detected from the Kramers-Kronig analysis of the PR spectra for estimating the mini-band width as a function of barrier thickness. When the mini-band formed, the ratio of carriers escaped thermally from each QW may change. SPV spectra were, then, measured to know the carrier escaping rate and how this component affects the absorption spectra of the solar cell structures.
The observed PR spectra for the sample with thinnest barrier width of 1.9 nm show two critical energies at 1.274 and 1.312 eV. The energy difference of these critical energies was around 0.038 eV and this energy difference reduces from 0.020 to 0.009 eV for the samples with barrier thickness of 2.7 and 3.6 nm, respectively. Supposing that such energy difference shows the mini-band width, the reduction of the energy with increasing the thickness of the barrier can be understood. The theoretical estimation using a Kronig-Penny model calculation for the width suggests that the mini-band width in the QW is 70 and 0 meV for e1 and topmost hh1 sub-bands. Therefore, the observed mini-band width is smaller than expected. The reason is not clear at present. For the SPV spectra, two broad peaks were observed at 1.29 and 1.32 eV. The energy difference also decreases with increasing the thickness of the barriers. Smaller critical energies for SPV may be explained that these signals detect the maxima energy of the joint-density of states which is larger than the band edge energies. Since the carrier escaping from the QW and radiative and non-radiative recombination rates may be strongly temperature dependent, estimation of the relevant activation energies for carrier escaping from these measurements were useful for further discussion.
[1] M. Sugiyama, et al., J. Cryst. Growth 315 (2011) 1.