Temperature and Light Intensity Dependences of Open Circuit Voltage in a-Si:H Pin Solar Cells
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TEMPERATURE AND LIGHT INTENSITY DEPENDENCES OF OPEN CIRCUIT VOLTAGE IN a-Si: H PIN SOLAR CELLS
JIN JANG * AND CHOOCHON LEE** ADept. of Physics and Research Inst. for Basic Sciences, Kyung Hee University Dongdaemoon-ku, Seoul 131, Korea ** Dept. of Physics, Korea Advanced Inst. of Science& Technology, P.O.Box 150 Chongyang, Seoul, Korea
ABSTRACT The temperature and incident light intensity dependences of open circuit voltage for hetero-and homojunction a-Si:H pin solar cells have been studied. The temperature coefficient of the open circuit voltage decreases from -2.40 2 0 to -2.67 mV/ C as the illumination intensity is decreased from 150 to 7mW/cm This is consistent with the empirically calculated open circuit voltages. The diode quality factor obtained under dark condition shows high values(l.63.0) and strong temperature dependence. On the other hand, the diode quality factor under illumination is less than 1.5 and shows slight temperature dependence. The results appear to be due to the long dielectric relaxation time compared with the carrier lifetime in the i-layer.
INTRODJUCTION In recent years, remarkable progress has been seen in both the physics and technology of hydrogenated amorphous silicon(a-Si:H) thin film solar cells as a promising photovoltaic device [i]. Since the diffusion length of minority carriers in a-Si:H is generally less than 1 pm [2], the width of depletion layer should be large to increase the collection efficiency. pin and nip type cells are the best ones to increase the conversion efficiency in unstacked a-Si:H solar cells. The relation between built-in potential(V) and open circuit voltage(Voc) for various cell structures have been found experimentally [3]: the linear increase of Voc with Vb in the lower Vb region and a saturation in the high Vb region. This overall tendency is in good agreement with that found theoretically predicted by Hack and Shur [4]. In order to obtain a high open circuit voltage it is necessary to decrease the drift/diffusion and generation/recombination currents. To decrease the drift/diffusion current we should increase the built-in potential and to decrease the generation/recombination current the density of states around the Fermi level should be decreased. In this work, the temperature and incident light intensity dependences of open circuit voltages have been measured for hetero-and homojunction a-Si:H pin solar cells, and the results are analyzed by the measurements of dark- and illuminated current voltage characteristics.
EXPERIMENT The heterojunction a-Si:H pin type diodes were fabricated by the radio frequency glow discharge decomposition of the gas mixture with ratios of SiH4 : CH4 :B H6 =1:2:0.006 for the deposition of p-layer. The p-layer was deposited onto ihe ITO coated glass substrate. The sheet resistance of ITO was 20 ohms/ square. Next, an approximately 4000 X thick undoped layer was deposited and followed by the deposition of a thin(400 % n+ a-Si:H layer. The top contact was an evaporated Al layer. In the case of fabricating homojunction pi
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