Material Control for High-Efficiency Amorphous Silicon Solar Cells
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MATERIAL CONTROL FOR HIGH-EFFICIENCY AMORPHOUS SILICON SOLAR CELLS YOSHIHIRO HISHIKAWA, MANABU SASAKI, SADAJI TSUGE, SHINGO OKAMOTO AND SHINYA TSUDA Functional Materials Research Center, SANYO Electric Co., Ltd. 1-18-13 Hashiridani, Hirakata, Osaka 573 JAPAN ABSTRACT A total-area conversion efficiency of 12.0% (100cm 2) has been achieved for a singlejunction a-Si solar cell. The film deposition rate (Rd) plays an essential role in controlling the optical and electrical properties of "device-quality" a-Si:H for high-efficiency solar cells. The properties of conventional "device-quality" a-Si:H films deposited from 100% Sill4 are primarily determined by the balance between Ts and Rd. A lower or higher deposition rate results in a-Si:H with a narrower or wider bandgap, respectively. This enables the properties of a-Si:H to be controlled independent of Ts. The controllable range of a-Si:H properties can be widened by effectively utilizing factors such as ion bombardment. For example, a high dilution of SiH 4 with H2 ( H2 / Si 4 = 10 or more) or an H 2 plasma treatment after deposition results in a-Si:H with a very wide bandgap and low defect density, which cannot be achieved by using 100% SiH 4 . Controlling the properties of a-Si:H by applying vibrational / rotational energy is also investigated. 1. INTRODUCTION Amorphous silicon (a-Si) solar cells consist of several layers: a transparent electrode, player(s), i-layer(s), n-layer(s), and interface layers between them. Although these layers are all important in the design of a-Si solar cell devices, the role of the i-layer is practically significant. Therefore, extensive efforts have been made in order to clarify the deposition mechanism of a-Si:H, and to improve the quality of a-Si:H films for solar cells[1-5]. The conversion efficiency of a solar cell is the product of the short circuit current (Isc), the open circuit voltage (Voc), and the fill factor (FF). A high optical absorption coefficient (a), especially at 700 - 800 nm, is necessary in order to improve Isc. On the other hand, the bandgap of the i-layer should preferably be wide in order to attain a high Voc. Because x at 700-800 nm usually decreases as the bandgap increases, there is a strong trade-off between the Isc and Voc of a-Si solar cells. Furthermore, the relation among the defect density (which strongly affects FF), the bandgap, and the deposition conditions, etc., is not yet well understood. Therefore, the most important tasks for improving the performance of a-Si solar cells are to clarify the mutual relationship between these properties, and to define guidelines in order to optimize them for the i-layer. In this study, techniques for controlling the properties of hydrogenated amorphous silicon (a-Si:H) are investigated based on experiments on the deposition-condition dependence of the optical, electrical and structural properties of a-Si:H. Techniques to further improve the ilayer are also proposed. 2. EXPERIMENTAL PROCEDURE a-Si films were deposited on quartz, Corning #7059 glass, and crystalline
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