Journal of Non-Crystalline Solids 137&138 (1991) 1161-1164 North-Holland }OUHNA L OF NON-C STALLINE SOI DS AMORPHOUS SILICON DOUBLE STACKED SOLAR CELL USING LOW BAND GAP A-SI BOTTOM CELL Tae Gon KIM, Sung Chul KIM, Jung Mok JUN, Kyu Chang PARK, Sung-Ok KOH, Min-Koo HAN* and Jin JANG Dept. of Physics and Research Inst. for Basic Sciences, Kyung Hee University,Seoul 130-701, Korea We have studied the preparation of low band gap hydrogenated amorphous silicon(a-Si:H) and the photovoltaic applications of this material. We obtained a-Si:H film with optical band gap of 1.60 eVat the substrtate temperature of 280°C and made solar cells using this material with single and double-stacked structures. The optical gaps for top and bottom i-layers of the stacked solar cells are 1.78 and 1.60 eV, respectively. Both cells show high stability against long light illumination. 1. INTRODUCTION The conversion efficiency of small area, hydrogenated amorphous silicon(a-Si:H) solar cell was reached to 12% in 1988, but it did not change greatly after then. One of the most important breakthroughs is to obtain low defect density, low band gap amorphous silicon or silicon-germanium alloy in order to get higher collection efficiencies in the long wavelength region. Recently, Shimizu group 1-4 developed the layer by layer deposition technique to reduce the hydrogen content and concomitant optical band gap of a-Si:H. The optical gap and hydrogen content decrease at first sharply and then saturate with increasing the exposure time of atomic hydrogen. The structure of the film is maintained to be amorphous after long exposure to atomic hydrogens. The chemical annealing after deposition of a few tens of angstrom thick a-Si:H gives rise to the out-diffusion of hydrogen with some relaxation of the silicon network. However, Asano 5 reported that the long exposure to the hydrogen plasma after depositing several angstroms of a-Si:H gave rise to the change in the structure of the film into microcrystal. In the present work, we studied the deposition technique of low band gap a-Si:H with small hydrogen content and then applied this material to make amorphous silicon solar cells with single and double-stacked structures. 2. SAMPLE PREPARATION We used the remote plasma chemical vapour deposition(RP-CVD) system to make amorphous materials. The cylindrical quartz tube of diameter of 3.8 cm is connected upstream of a remote deposition chamber. Silane hydrogen or silane helium mixture is introduced into the downstream reactor and helium passes through the upstream quartz tube. The silane and hydrogen can be decomposed by the excited helium atoms. There is no plasma around the substrate holder, so that RP-CVD is a nice deposition method of a-Si:H. We have tried two kinds of experiments to reduce the band gap of a-Si:H. 3. LAYER BY LAYER DEPOSITION Shirai et al 3 obtained low band gap a-Si:H by controlling the substrate temperature and the time of exposure to atomic hydrogen using layer by layer deposition technique. The band gap decreases with increasing the substrate temperature and/or the exposure time to hydrogen radicals. We have made Si films at 330°C by layer by layer deposition technique. We varied the time of exposure of hydrogen radicals to the surface * Dept. of Electrical Engineering, Seoul National University, Kwanak-ku, Seou1151-742, Korea 0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.