Microcrystalline Silicon Thin-Film Solar Cells Prepared at Low Temperature
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Microcrystalline Silicon Thin-Film Solar Cells Prepared at Low Temperature Y. Nasuno, M. Kondo and A. Matsuda National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan. ABSTRACT Hydrogenated microcrystalline silicon (µc-Si:H) p-i-n solar cells have been prepared using a conventional RF plasma-enhanced chemical vapor deposition (PECVD) method at a low process temperature of 140
. The low temperature deposition of µc-Si:H has been found to be
effective to suppress the formation of oxygen-related donors that cause a reduction in open circuit voltage (Voc) due to shunt leakage. We demonstrate the improvement of Voc by lowering the deposition temperature down to 140
, while suppressing the reduction in high short circuit
current density (Jsc) and fill factor (FF). A high efficiency of 8.9% was obtained using an Aasahi-U substrate. Furthermore, by optimizing textured structures on ZnO transparent conductive oxide (TCO) substrates, an efficiency of 9.4% (Voc=0.526V, Jsc=25.3mA/cm2, FF=0.710) was obtained. In addition, relatively high efficiency of 8.1% was achieved using VHF (60MHz) plasma at a deposition rate of 12 Å/s. Thus, this low temperature deposition technique for µc-Si:H is promising for both high efficiency and high rate deposition of µc-Si:H solar cells. INTRODUCTION Hydrogenated microcrystalline silicon (µc-Si:H) has been applied extensively to thin-film solar cells [1-4] owing to its better stability and higher efficiency, compared with amorphous silicon (a-Si:H) solar cells. In µc-Si:H solar cells, since diffusion length of photo-generated carriers is very short, photo-carriers are usually collected by built-in field in the i-layer using p-i-n type solar cell structures. It has been reported that non-dope µc-Si:H often shows high dark conductivity of the order of 10-3 to 10-4 S/cm and the Fermi level position is shifted toward conduction band side [3]. This n-type character of non-dope µc-Si:H has been ascribed to the incorporation of oxygen related donors at µc-Si:H grain boundary [3,5]. It has been reported that the n-type character of µc-Si:H i-layer leads to the poor spectral response in longer wavelengthes and that the increase in shunt leakage current [3]. The shunt leakage possibly decreases open A15.5.1
circuit voltage (Voc) which limits the µc-Si:H based solar cell performance, as compared to polyor single- crystalline silicon solar cells. In improving device performances, therefore, it is of significant importance to suppress the density of oxygen related donors. Although µc-Si:H has better stability and higher efficiency than these of a-Si:H, one disadvantage is a low optical absorption coefficient caused by indirect band gap [1-4]. Therefore, an optical confinement for the efficient light absorption is an important technology. Various textured structures have been prepared by textured growth in SnO2 [6] and by wet etching process in ZnO [7]. In a-Si:H solar cells, textured SnO2 substrate (Asahi-U) [6] with optimized surface morph
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