Performance of CuGaSe 2 Solar Cells Grown By Co-Evaporation Process
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F14.21.1
Performance of CuGaSe2 Solar Cells Grown By Co-Evaporation Process
Jae Ho Yun, R.B.V.Chalapathy, Seok Ki Kim, Jeong Chul Lee, Jinsoo Song, Kyung Hoon Yoon Solar Cells Research Center, Korea Institute of Energy Research, 71-2, Jang-dong, Yuseong-gu, Daejeon, 305-343, South Korea.
ABSTRACT CuGaSe2 absorber layers were prepared by evaporating elemental Cu, Ga and Se in three stage on Molybdenum coated soda lime glass. The composition of the resultant film was studied by monitoring the substrate temperature, which decreased when a Cu-Se secondary phase was formed. As the Ga supplement increased during the third stage the void that formed in the beginning of the third stage was removed, while a small grain Ga-rich layer was formed on the surface, resulting in a Cu deficient surface. Therefore, the Voc was improved because of the enhanced surface morphology and the Jsc was reduced, due to the Ga rich layer on of the surface. Under optimal conditions, we achieved a cell performance of Voc = 780 mV, Jsc = 12.9 mA/cm2, ff = 62.5 and η = 7.3 %. INTRODUCTION Recently, improved efficiencies of up to 19.5% have been reported for Cu(InGa)Se2 (Eg~1.1 eV) solar cells[1]. Despite these success, higher-bandgap (Eg>1.5 eV) absorber materials for the solar cells are desirable because of their higher open-circuit voltage and lower current which enable better module performance and current matching in a two-terminal tandem cell[2]. The CuGaSe2 film with a band gap of 1.68 eV is an attractive absorber material for wide bandgap solar cells and an ideal partner for CuInSe2/CuGaSe2 based tandem solar cells. However, the grain size of CuGaSe2 is smaller than that of CI(G)S and its open circuit voltage is low compared to its band gap energy. In addition, a suitable buffer other than CdS must be found for electrical matching. Such limiting factors for higher efficiencies in these cells are thought to be result of defect states at the interface between the absorber and the CdS buffer layer that enhances interface recombination [3]. To date, an improved efficiency of greater than 10.2 % has been reported using the CuGaSe2 absorbers with thickness of less than 2 ㎛ and modified surfaces [2]. In this paper, we report on the deposition of CuGaSe2 thin films using a three-stage coevaporation method with controlling the deposition time of second and third stage by using substrate temperature monitoring. The in-situ composition monitoring technique by pyrometer measurement or thermocouple had already been reported [4,5]. In addition, we utilize the thermocouple to detect the substrate temperature and we achieve reproducibility for CuGaSe2 absorber composition.
F14.21.2
EXPERIMENTAL DETAILS Molybdenum (Mo) films were deposited onto 5 x 5 cm2 soda-lime glass substrates from a 99.95% pure Mo target via a DC magnetron sputtering system. The CuGaSe2 absorber layers were deposited onto Mo-coated soda-lime glass substrates via the evaporation of elemental Cu, Ga and Se from effusion cells. Figure 1 shows a schematic diagram of the substrate heat
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