Novel Wide-Band-Gap Ag(In 1-x Ga x )Se 2 Thin Film Solar Cells
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Novel Wide-Band-Gap Ag(In1-xGax)Se2 Thin Film Solar Cells Tokio Nakada, Keiichiro Yamada, Ryota Arai, Hiroki Ishizaki and Naoomi Yamada Department of Electrical Engineering and Electronics, Aoyama Gakuin University, Sagamihara, Kanagawa, 229-8558, JAPAN ABSTRACT Ag(In1-xGax)Se2 thin films have been deposited on Mo-coated soda-lime glass substrates by the three-stage process using a molecular beam epitaxy (MBE) system. We found a remarkable decrease in the substrate temperature during the 2nd stage in which the film composition changes to a Ag excess. A single phase chalcopyrite AIGS thin film with a slightly Ag poor composition was obtained by using the temperature monitoring composition method. The cell performance of the AIGS thin film solar cell was found to strongly depend on the Ga/(In+Ga) and Ag/(In+Ga) atomic ratios. A high efficiency wide-gap (Eg=1.7eV) Ag(In0.2Ga0.8)Se2 thin film solar cell with a total-area efficiency of 9.3% (10.2% active area efficiency), Voc = 949mV, Jsc = 17.0 mA/cm2, FF = 0.577, and total area = 0.42 cm2 was achieved. The junction formation mechanism of AIGS devices is discussed based on electron beam induced current (EBIC) and scanning capacitance microscopy (SCM) analyses. INTRODUCTION Wide band gap chalcopyrite materials are required for the top cell of tandem solar cells. However, the cell performance of Cu(In1-xGax)Se2 (CIGS) thin film solar cells decreases as the band gap energy (Eg) increases above 1.3 eV [1]. In fact, the best cell efficiency for a CuGaSe2 device with a band gap energy (Eg ) of 1.68 eV remains around 10% at present [2]. We thus have investigated a novel wide-gap material, Ag(In1-xGax)Se2 (hereafter AIGS), in order to explore the possibilities of this material for use in the top cell of tandem devices. AIGS is a chalcopyrite I-III-VI2 group semiconductor with band gap energies ranging from 1.24 (for AgInSe2) to 1.83 (for AgGaSe2) [3]. The AI(G)S thin film solar cells such as p-AGS/n-CdS [4], p-AIS/n-CdS [5] and p-AIGS/n-ZnCdS [6] heterojunctions have been reported, however, efficient solar cells have not been achieved to date. We previously showed that AgInSe2 thin films deposited by molecular beam epitaxy (MBE) using high-purity source materials showed n-type conduction. Moreover, the conduction type of AIGS thin films could not be determined by thermo-probe or Hall measurements due to the extremely high resistivity of the films. In this paper, the deposition process and properties of AIGS thin films are presented, together with the cell performance as a function of film compositions. The junction formation mechanism of AIGS devices is then discussed based on data from scanning capacitance microscopy (SCM), electron beam induced current (EBIC), and secondary ion mass spectroscopy (SMS) measurements.
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AIGS DEPOSITION AIGS thin films were deposited on Mo-coated soda-lime glass substrates by the three-stage process using a molecular beam epitaxy (MBE) system. The substrate temperature was monitored using a thermocouple installed near the back s
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