Passivation of Defect States in Amorphous and Crystalline Si by use of Cyanide Treatment and Improvement of Solar Cell C
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Passivation of Defect States in Amorphous and Crystalline Si by use of Cyanide Treatment and Improvement of Solar Cell Characteristics Hikaru Kobayashi, Naozumi Fujiwara, Tetsushi Fujinaga, Daisuke Niinobe, Osamu Maida, and Masao Takahashi Institute of Scientific and Industrial Research, Osaka University, and CREST, Japan Science and Technology Corporation, 8-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan ABSTRACT We have developed a new method of eliminating defect states in Si. This method called cyanide treatment simply includes immersion of Si in KCN solutions followed by the rinse. The contamination by potassium ions can be completely prevented by the inclusion of 18-crown-6 in the KCN solutions (crown-ether cyanide treatment). When the crown-ether cyanide treatment was performed on intrinsic amorphous Si (a-Si) films, decreases in the photoand dark current densities with the irradiation time were completely prevented. When cyanide treatment using aqueous KCN solutions was applied to pin-junction a-Si solar cells, the conversion efficiencies measured before and after light-induced degradation became higher than those with no treatment. These improvements are attributed to the elimination of defect and defect precursor states by the reaction with cyanide ions, resulting in the formation of Si-CN bonds. From density functional calculations, Si-CN bonds are found to possess a high bond energy of 4.5 eV. Due to the high bond energy, the bonds are not ruptured by heat treatment at 800 °C and upon irradiation, resulting in the thermal and irradiation stability of the cyanide treatment.
INTRODUCTION Solar energy coming to the earth is 1.2×104 kW, which corresponds to 10,000 times of the total energy consumption all over the world. Solar cells directly convert solar energy to electrical energy without generating undesirable gases such as CO2, NOx, and SOx. Therefore, the development of large-scale solar cells can give an essential solution for the environmental problems as well as energy problems. For large-scale solar cells, several requirements should be satisfied: 1) presence of abundant amounts of solar cell materials on the earth, 2) low production cost, and 3) high energy conversion efficiency. In the present stage, Si solar cells A11.5.1 Downloaded from https:/www.cambridge.org/core. University of Pennsylvania Libraries, on 14 Apr 2017 at 03:20:30, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-715-A11.5
are the most promising type which satisfies all the requirements to some extent. The production cost of solar cells can be evaluated by "pay-back time" [1], i.e. a period in which the energy consumed for the production of solar cells is paid back by the usage of the solar cells. The pay-back times of single crystalline Si and polycrystalline Si-based solar cells are about 5 and 2 years, respectively. The pay-back time of amorphous Si (a-Si) solar cells, on the other hand, can be reduced to less than one year. This is mainly because unli
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