| AVS 59th Annual International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Tuesday Sessions |
| Session EN+TF-TuA |
| Session: | Thin Film, Heterostructured, and Organic Solar Cells |
| Presenter: | T. Ikari, University of Miyazaki, Japan |
| Authors: | T. Ikari, University of Miyazaki, Japan T. Aihara, University of Miyazaki, Japan Y. Nakano, University of Miyazaki, Japan Y. Wang, University of Tokyo, Japan M. Sugiyama, University of Tokyo, Japan Y. Nakano, University of Tokyo, Japan A. Fukuyama, University of Miyazaki, Japan |
| Correspondent: | Click to Email |
Fabrication of multi quantum well (MQW) or superlattice (SL) structures embedded in an absorption layer of solar cell is a promising idea for developing higher efficient devices. This is because the quantum well can extend the absorption to longer wavelength region and enhance the short-circuit current. However, recombination centers for carriers are simultaneously generated at the boundaries, leading to the degradation of conversion efficiency. Although optical absorption and spectral response spectroscopy are usually used for investigating absorption and recombination mechanism in the solar cell, no direct technique for characterizing nonradiative recombination is presented. We have developed the piezoelectric photothermal (PPT) spectroscopy for detecting such nonradiative recombination in the QW [1]. Heat generated by a nonradiative recombination of photoexcited carriers were detected as PPT signal by using a piezoelectric transducer. In this paper, we report on the PPT spectra of InGaAs/GaAsP SL layer and show that this technique is sensitive and powerful to investigate the absorption spectra of SL. It is, then, becomes possible to discuss a recombination mechanism of the photo-excited carriers in the solar cell structure from the non-radiative transition point of view.
A strain-balanced InGaAs/GaAsP SL layer embedded into the intrinsic region of the GaAs p-i-n solar cell were prepared. The SL absorbing layer was prepared in the structure of InGaAs(3.7nm)/GaAsP(5.4nm) with 0.56-nm-thick GaAs buffer layer by MOVPE technique on the GaAs substrate [2]. PPT spectrum at 100K shows two dominant peaks. The conventional absorbance of SL and the PPT spectrum of the MQW (InGaAs(7.4nm)/GaAsP (10.8nm)) samples were also discussed for comparison. A signal from SL is more clearly observed in the PPT than the absorbance spectra. Although the step like signals accompanied with the exciton transition are well resolved for MQW samples, no step but broad peaks around 1.395 and 1.45 eV were observed for SL sample. The energies of these peaks were as expected from the calculation of the single QW without any interaction of the neighboring QW, i.e. tunneling. Since the wave function of quantized level spread into the next well for the SL structure, broad peaks were, then, observed. Although the step like density of states should appear even in the SL, drastic decrease of the PPT signal beyond the peak was observed. One possible reason is that the number of carriers that recombine nonradiatively inside the quantum well decreases by tunneling.
[1] T. Ikari, et al., Phys. Rev. B77 (2008) 125311.
[2] M. Sugiyama et al.: J. Cryst. Growth 315 (2011) 1.