Effect of CdCl 2 Treatment Conditions on the Deep Level Density, Carrier Lifetime and Conversion Efficiency of CdTe Thin
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EFFECT OF CdCl2 TREATMENT CONDITIONS ON THE DEEP LEVEL DENSITY, CARRIER LIFETIME AND CONVERSION EFFICIENCY OF CdTe THIN FILM SOLAR CELLS A. Sandhu, K. Kobayashi, T. Okamoto1, A. Yamada1, M. Konagai1 Tokai University, Dept of Electrical Engineering, 1117 Kita-kaname, Hiratsuka-shi, Kanagawa 239-1292, Japan 1 Dept of Physical Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan ABSTRACT The effect of CdCl2 annealing conditions of glass/TCO/n-CdS/p-CdTe solar cell structures on the deep level density and carrier lifetime of the p-CdTe layer and correlation with the solar cell conversion efficiency was investigated. CdCl2 treatment was carried out at temperatures ranging from 370 to 460°C for 15 min. A clear correlation between trap density, carrier lifetime, conversion efficiency and the CdCl2 annealing conditions was observed. Un-annealed structures had a conversion efficiency of 5.7%, hole trap energy of EV+0.42eV, hole trap density of 8.71x1014cm-3, and decay lifetime of 0.15µs. The optimum CdCl2 annealing temperature was found to be 415°C for structures grown at a substrate temperature of 595°C, where the conversion efficiency, hole trap energy, hole trap density, decay lifetime were 13.4%, EV+0.44eV, 8.10x1012 cm-3 and 0.40µs, respectively. INTRODUCTION The glass/TCO/n-CdS/p-CdTe structure is a promising material system for the mass production of high efficiency solar cells having a near optimal band gap (1.44eV) and absorption coefficient. We have been successful in improving the performance of CdTe thin film solar cells and achieved conversion efficiencies of 15.3% employing a glass/ITO/CdS/CdTe/Cu-doped graphite/Ag structure [1]. The enhancement of device performance using these structures was achieved by optimizing conditions during a CdCl2 anneal process after CdTe deposition [2,3]. However, a deeper knowledge of the relationship between electronically active defects and device performance will be important in order to achieve efficiencies approaching the theoretically predicted value of about 29% [4]. In this paper, we report on the effect of CdCl2 treatment on the electrical and optical properties of CdS/CdTe solar cell structures as observed using deep level transient spectroscopy (DLTS), isothermal capacitance transient spectroscopy H8.13.1
(ICTS) and carrier lifetime measurements.
EXPERIMENTAL CdTe thin film solar cells with a glass/ITO/CVD-CdS/CSS-CdTe/Cu-doped graphite/Ag structure were characterized using DLTS, ICTS and carrier lifetime measurements. The substrate for the CdTe active layer consisted of glass (Corning 1737) with a 250-nm-thick ITO film and a 60nm thick layer of CdS. CdTe films with thicknesses between 6-8 µm were deposited by the close-space sublimation (CSS) method [1], where the substrate and source temperatures were 595 and 625°C, respectively. The deposition was carried out in an argon atmosphere at a constant pressure of 2 Torr. After deposition, the CdS/CdTe structure was coated with 0.3M CdCl2 aqueous solution and annealed at temperat
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