We show that commercial float glass can be used as a substrate to deposit polycrystalline silicon (poly-Si) by chemical vapor deposition from trichlorosilane at temperatures between 740 and 870 • C. By using scanning electron microscopy an average grain size lower than 0.4 μm was observed, with a columnar structure suitable for the electrical conduction in photovoltaic cells. X-ray diffraction reveals a strong (2 2 0) preferential orientation of the films, which is indicative of a low density of intra-grain defects. Atomic force microscope images reveal a conical structure, with a root mean square roughness of ∼65 nm for samples of around 3 μm in thickness. This natural texture is a positive characteristic from the point of view of light trapping. By using boron tribromide as a doping agent, degrees of doping ranging from intrinsic to clearly p-doped were obtained, as shown by dark conductivity measurements as a function of temperature. The process, the reactants and the substrate used are of low cost and proved to be adequate for direct poly-Si deposition.
We deposited polycrystalline silicon (poly-Si) thin films on commercial float glass by chemical vapour deposition from trichlorosilane at temperatures between 735 and 870 • C. The structural properties of the films were evaluated by means of scanning electron microscopy, x-ray diffraction, atomic force microscopy, reflectance in the ultraviolet region and Raman spectroscopy. The electrical characterization involved measurements of dark conductivity and photoconductivity as a function of temperature, Hall effect, ambipolar diffusion length from the steady-state photocarrier grating technique and density of defects by means of modulated photoconductivity. By using boron tribromide as a doping agent, degrees of doping ranging from intrinsic to clearly p-doped were obtained. The process, the reactants and the substrate used are of low cost, and proved to be adequate for direct poly-Si deposition, giving films of good structural and electrical properties.
Atmospheric pressure (AP) thermal CVD is used to deposit thin poly-Si films on glass substrates. Also produced are heterojunction solar cells carrying out the deposition on c-Si wafers. A batch-type hot-wall reactor, employing SiHCl 3 as a precursor, H 2 as a carrier and reaction gas, BBr 3 as a p-type doping agent, and PCl 3 as a n-type doping agent, is used. The films obtained are homogeneous and well-adhered to the substrate. Samples are structurally characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), reflectance spectroscopy in the UV-vis region, X-ray diffraction (XRD), and Raman spectroscopy (RS). The electrical characterization includes conductivity measurements as a function of temperature, and Hall effect measurements. For the p-doped samples, XRD reveals a strong (220) preferential orientation of the films, while the n-doped samples lack columnar structure or preferential orientation. RS and UV-reflectance confirm a high crystalline fraction. Dark conductivity measurements as a function of temperature show that the films can be grown intrinsic, p-type or n-type. Activation energies between 0.61 and $0 eV are obtained, with reasonable values for the carrier mobilities. For the solar cells, relatively high values of V OC ($507 mV) and J SC ($29.6 mA cm À2 ) are measured. In conclusion, these results demonstrate the feasibility of directly depositing doped poly-Si thin films on glass and c-Si substrates at intermediate temperatures, with interesting characteristics for photovoltaic applications.
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