V oc Saturation Effect in High-temperature Hydrogenated Polycrystalline Silicon Thin-film Solar Cells
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Voc Saturation Effect in High-temperature Hydrogenated Polycrystalline Silicon Thin-film Solar Cells Hidayat Hidayat1, 2,*, Per I. Widenborg2, 1, and Armin G. Aberle2, 1 1 Electrical and Computer Engineering, National University of Singapore, Singapore 2 Solar Energy Research Institute of Singapore, National University of Singapore, Singapore *Electronic mail: [email protected] ABSTRACT Hydrogenation of polycrystalline silicon thin-film solar cells is performed to improve the one-sun open-circuit voltage (Voc) of the device. Voc is found to increase linearly with increasing hydrogenation temperature and then saturates. For planar and textured samples, the Voc saturates at about 340 ºC and 307 ºC respectively. The low hydrogenation temperature helps to lower thermal budget during industrial process. Arrhenius plot of Voc prior to the saturation shows that the textured samples have lower activation energies than the planar sample. The activation energies of samples 188 (planar), 788 (textured) and 888 (textured) are 1.31 eV, 0.86 eV and 0.92 eV, respectively. The lower activation energies of the textured samples could be due to the shorter diffusion thickness and the increased surface area that is exposed to the hydrogen plasma. INTRODUCTION Polycrystalline silicon (poly-Si) thin-film solar cells have the potential of achieving a conversion efficiency of more than 13% using a simple solar cell structure. The highest efficiency so far is 10.5%, achieved by CSG Solar [1]. This technology has the potential to reach low fabrication costs due to several advantages, such as the use of relatively inexpensive largearea glass substrates and monolithic series interconnection of the solar cells to form a solar module. In addition, the abundance and non-toxicity of silicon make it a promising candidate for large-volume production. However, compared to silicon wafer solar cells, the poly-Si thin-film material quality suffers from a high defect density, such as dangling bonds at the grain boundaries. One way to improve the quality of poly-Si thin-film material is to passivate these dangling bonds by diffusing hydrogen into the film, a method that has been found to improve the device performance of transistors and solar cells. This process is termed as hydrogenation. Deuterium, an isotope of hydrogen is sometimes used as well, because it is easily detected in secondary-ion mass spectroscopy (SIMS) measurements. The terms deuteration and hydrogenation are sometimes interchangeably used, as there is no significant difference found in terms of diffusion, defect passivation and defect generation [2]. Two main factors that affect the hydrogenation process are the hydrogenation temperature and hydrogenation time. It was found in the experiment done by Nickel et al. [3] that with increasing temperature, the deuterium diffuses deeper into the bulk of the film and the surface concentration of deuterium drops from 1x1021 cm-3 at 250 ºC to 2.6x1019 cm-3 at 450 ºC. Decreasing the hydrogenation time from 30 minutes to 10 minutes also reduces th
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