Study of Recombination in Cu(In,Ga)Se2 Solar Cells by Time-Resolved Photoluminescence
- PDF / 225,686 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 56 Downloads / 174 Views
1012-Y04-06
Study of Recombination in Cu(In,Ga)Se2 Solar Cells by Time-Resolved Photoluminescence Sho Shirakata1, and Tokio Nakada2 1 Electric and Electronic Engineering, Ehime Univsersity, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan 2 Electrical Engineering and Electronics, Aoyama Gakuin University, 5-1-10 Fuchinobe, Sagamihara, Kanagawa 229-8558, Japan
ABSTRACT Time-resolved photoluminescence (TR-PL) measurements have been carried out on Cu(In1-x,Gax)Se2 (CIGS) thin films and solar cells to study the recombination of photo-generated minority carriers. Room temperature photoluminescence (PL) of both CIGS thin films and solar cells exhibited band-to-band transition for the alloy composition x of 0-0.4, and PL decay curves have been utilized as a measure of the carrier lifetime. The solar cell fabrication leads to the enhancement of band-to-band PL intensity, the reduction of defect-related PL, and the increase of the PL decay time. It is noted that the increase in the PL decay time has been observed in CIGS films after the chemical-bath deposition of the CdS buffer layer. Solar cells with high conversion efficiency are characterized by both high PL intensity and the long PL decay time. The PL decay curve of CIGS solar cell is studied in terms of the alloy composition x. For small x, PL decay times of CIGS solar cells are almost independent of the emission photon energy. For large x, the PL decay time depended on the PL emission energy. The result suggests the change of the recombination mechanism in CIGS solar cells with the change in the alloy composition x. INTRODUCTION The Cu(In1-x,Gax)Se2 (CIGS) alloys are promising for a high efficiency thin film solar cell. The maximum conversion efficiency (η) for CIGS solar cells is 19.5%, which is the highest among thin film solar cells [1]. However, η of CIGS solar cells with x>0.3 is considerably lower than that of the recorded cells with x=0.2, because the open-circuit voltage (Voc) increases at a lower rate than the band gap energies of the absorber for wide-gap CIGS solar cells (Eg>1.3 eV) corresponding to CIGS with the alloy composition x=Ga/(In+Ga)>0.5 [2, 3]. This tendency in Voc is mainly related to the recombination of photo-generated carriers. The recombination mechanisms, both the bulk recombination and the hetero-interface recombination, have not yet been well understood. In order to improve Voc, and thus increase of η, a new technique is strongly required to study the carrier recombination and the carrier dynamics in CIGS solar cells. Photoluminescence (PL) is utilized as a powerful tool for the characterization of semiconductor based on the radiative recombination of photo-excited carriers. In particular, one can study the carrier dynamics by the time-resolved PL (TR-PL), and the minority carrier lifetime can be estimated by the PL decay time of the band-to-band transition. TR-PL has been reported for thin films of CIS [4], CIGS [5] and CIGSSe [6]. However, nothing has been reported for TR-PL of CIGS solar cells, although a TR-PL of the CdTe/CdS solar ce
Data Loading...
