Crystallization Kinetics in High-rate Electron Beam Evaporated Poly-Si Thin Film Solar Cells on ZnO:Al
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1245-A20-01
Crystallization kinetics in high-rate electron beam evaporated poly-Si thin film solar cells on ZnO:Al T. Sontheimer, C. Becker, C. Klimm, S. Gall, B. Rech Helmholtz Zentrum Berlin fuer Materialien und Energie, Silicon Photovoltaics, Kekuléstr. 5 12489 Berlin, Germany. ABSTRACT The microstructure and crystallization kinetics of electron beam evaporated Si on ZnO:Al coated glass for polycrystalline solar cells was studied by electron backscatter diffraction and optical microscopy at various deposition temperatures. A time dependent analysis of the dynamics of the crystallization allowed for the individual determination of growth and nucleation processes. The nucleation process of Si on ZnO:Al was found to be influenced by a variation of the deposition temperature of the amorphous Si in a critical temperature regime of 200 ˚C to 300 ˚C. The nucleation rate decreased significantly with decreasing deposition temperature, while the activation energy for nucleation increased from 2.9 eV at a deposition temperature of 300 ˚C to 5.1 eV at 200 ˚C, resulting in poly-Si which comprised grains with features sizes of several µm. INTRODUCTION A major challenge for the low-cost thin film Si solar cell production is the fast and effective deposition of Si films with excellent electronic quality which are embedded in a smart contacting scheme and efficient light trapping structure. The preparation of Si thin film solar cells with excellent electronic quality has been investigated by numerous poly-crystalline Si (poly-Si) approaches. One poly-Si concept is based on the crystallization of deposited amorphous Si (a-Si) by thermal annealing in a solid phase crystallisation (SPC) process. With plasma enhanced chemical vapor deposition (PECVD) used for the deposition of a-Si, the concept was already investigated and a conversion efficiency of 10.4 percent has been achieved [1-3]. However, the low deposition rate of 0.5 - 2 nm/s curtails the effectiveness of the poly-Si concept’s production process. A poly-Si solar cell device prepared by high-rate electron beam (ebeam) evaporation with deposition rates of 1 µm/min or even higher overcomes this obstacle. Recently, an efficiency of 6.7 percent has been achieved for e-beam evaporated poly-Si solar cells on planar glass, illustrating the technology’s potential for the fabrication of high quality Si [4, 5]. The additional implementation of ZnO:Al as a front-contact layer into the superstrate concept allows for the incorporation of light trapping structures and a realization of a smart contacting scheme. Moreover, the charge carrier mobility of Si-capped ZnO:Al is significantly improved by thermal annealing, making it the ideal transparent conductive oxide to be implemented in a poly-Si solar cell [6]. The compatibility of the front contact layer ZnO:Al with poly-Si solar cells in a superstrate configuration has been demonstrated by a systematic analysis of the photovoltaic performance at various deposition parameters [7]. The amorphous-tocrystalline phase transformation represents
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