Aluminum Oxide Passivation Layer for Crystalline Silicon Solar Cells Deposited by Mist CVD in Open-Air Atmosphere
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Aluminum Oxide Passivation Layer for Crystalline Silicon Solar Cells Deposited by Mist CVD in Open-Air Atmosphere Toshiyuki Kawaharamura1, Takayuki Uchida2, Kenji Shibayama2, Shizuo Fujita3, Takahiro Hiramatsu4, and Hiroyuki Orita4 1 Institute for Nanotechnology, Kochi University of Technology, Kami 782-8502, Japan 2 Department of Electronic Science and Engineering, Kyoto University, Kyoto 615-8510, Japan 3 Photonics and Electronics Science and Engineering Center, Kyoto University, Kyoto 615-8520, Japan 4 Future Technology Center, Toshiba Mitsubishi-Electric Industrial Systems Corporation (TMEIC), Kobe 650-0047, Japan ABSTRACT The surface passivation of Si wafer by AlOx thin films grown by mist CVD in an open-air atmosphere was studied with a view to improving the effect of high-performance c-Si solar cells. In AlOx thin film grown at a temperature above 400°C by mist CVD, the OH bonding did not remain in the film and the breakdown field (EBD) was over 6 MV/cm. In Si wafers passivated by AlOx thin films grown by mist CVD at growth temperature above 400°C, the negative fixed charge density (Qf) at the interface was higher than 1012 cm-2 and the surface recombination velocity (Seff) was 44.4 cm/s. These results show that mist CVD, which is fundamentally an environmentally friendly technique, may be suitable for the fabrication of a passivation film on Si surfaces designed to improve the effect of high-performance c-Si solar cells. INTRODUCTION Boron-doped p-type Si with around 1 Ωcm has been commonly used as a wafer in crystal silicon (c-Si) solar cells. The latest high-performance c-Si solar cells have been developed with thinner Si wafers (< 400 μm) for reasons of economy, and hence the influence of rear surface recombination appears to be more significant [1]. Therefore, surface passivation is being widely studied in order to reduce the surface recombination velocity (Seff) in c-Si solar cells. In particular, Seff is required to be as low as 10 cm/s because the degradation of efficiency in thin wafer c-Si solar cells is apparently seen when Seff is faster than 10 cm/s [1-3]. Surface passivation with a material forming negative charges with density in the order of 1012-1013 cm-2 at the interface of p-type c-Si is one of the solutions yielding Seff as low as 10 cm/s. Aluminum oxide (AlOx) thin films can form negative charges over 1012 cm-2 at the interface of silicon [3,4]. AlOx thin films have been fabricated by many methods, such as sputtering (SPT) [5-10], metal organic chemical vapor deposition (MOCVD) [11], plasma-enhanced (PE) CVD [12,13], pulse laser deposition [14,15], atomic layer deposition (ALD) [16-23], sol-gel [24,25], and spray pyrolysis [26,27]. Among them, AlOx thin films fabricated by SPT [10], PECVD [12,13] and ALD [20-23] have shown the high-quality passivation desired for high-performance c-Si solar cells. With these fabrication methods, however, AlOx thin films are fabricated at a temperature as low as 200°C but an additional annealing process at a temperature over 400°C is mandatory in order to
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