Effects of a Bias Voltage During Hydrogenation on Passivation of the Defects in Polycrystalline Silicon for Solar Cells
- PDF / 123,061 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 21 Downloads / 274 Views
1153-A18-01
Effects of a Bias Voltage During Hydrogenation on Passivation of the Defects in Polycrystalline Silicon for Solar Cells Yoji Saito Hayato Kohata, and Hideyuki Sano Seikei University, 3-3-1 Kichijoji-Kitamachi, Musashino, Tokyo 180-8633, Japan.
ABSTRACT The short circuit current and conversion efficiency of the poly(multi)-crystalline solar cells are increased by the passivation process using hydrogen plasma. The passivation rate apparently increases at a reverse bias voltage near 0.6V during the hydrogenation process. The effects of the bias voltage on the passivation are large at the substrate temperatures between 200°C and 250°C. The phenomena are likely due to the existence of positively-ionized hydrogen, H+. The H+ ions can be accelerated from the surface into the bulk by the electric field with the negative bias. The possibility of the H+ ions in the bulk silicon has been predicted in the previous reports. The increase of the incorporated hydrogen is confirmed by IR absorption measurements. The enhanced diffusion of hydrogen induced by the reverse bias is supported by the results of spectral response characteristics of the hydrogenated solar cells.
INTRODUCTION Most of commercial solar cells are based on multi(poly)-crystalline silicon substrates. A large amount of lattice defects, however, are included in multi-crystalline silicon substrates, and the defects can degrade the electrical characteristics of the solar cells. Hydrogen passivation process is usually performed to reduce the electrically-active defect levels at temperatures above 300°C for hours. In this study, we found phenomena of the enhanced hydrogen diffusion into multi-crystalline silicon substrates by a bias voltage on the solar cells.
EXPERIMENTAL PROCEDURE Fabrication Process of Solar Cells Multi-crystalline silicon (p-type, 1.9Ωcm) substrates, which were supplied from Sharp Co., were used in this study. The substrates were cleaned with a hot alkaline solution (NH4OH:H2O2:H2O=1:1:6). The substrates were rinsed with deionized water, dipped HF solution, and rinsed again with deionized water. The substrates were then thermally oxidized in wet oxygen ambient to form the oxide films with thickness of about 500nm. The solar cells were fabricated in the followings. Phosphorus was doped by thermal diffusion to form the pn junction. The front and rear aluminum electrodes were formed by evaporation. The substrates were sintered at 420°C in nitrogen ambient to form Ohmic contacts.
Hydrogenation Process The solar cells were introduced into a conventional rf plasma system with two-paralleled electrodes, and were maintained between 200°C and 300°C. A hydrogen gas was introduced into the reaction chamber, and the pressure was maintained to be 106Pa by evacuating by a diffusion pump. The rf electric power of 10W was applied to produce atomic hydrogen. A schematic diagram of the experimental system for the hydrogenation is shown in Fig. 1. The sample was mounted onto the lower side electrode with a heater. The sample was covered by fine-meshe
Data Loading...
