Thin polycrystalline silicon films on glass substrates deposited by CVD at intermediate temperatures

BENVENUTO, A. G.; BUITRAGO, R. H.; SCHMIDT, J. A.

Valencia, España

Conferencia; 25th European Photovoltaic Solar Energy Conference; 2010

WIP - Renewable Energies

Thin film polycrystalline silicon (poly-Si) solar cells deposited on low-cost substrates may be the path to achieve the expected reduction in the prices of photovoltaic electricity. For that purpose, a careful choice of the substrate and the deposition process is essential. In this work, we use atmospheric pressure thermal chemical vapor deposition (AP-CVD) to deposit thin poly-Si films. The CVD reactor is a batch-type hot-wall reactor, employing trichlorosilane as a precursor and hydrogen as a carrier and reaction gas. We use temperatures below 860 ºC, and a commercial aluminosilicate float glass as a substrate. The process, the reagents and the substrate are of low cost, and proved to be adequate for direct poly-Si deposition. Previous works employed high temperatures – usually higher than 1000 ºC – to deposit poly-Si by AP-CVD on ceramic substrates [1]. These high temperatures impose a restriction on the materials that can be used as substrate, therefore increasing the process costs. The work presented here is a demonstration that a commercial float glass can be used to deposit poly-Si at intermediate temperatures, giving films of good structural and electrical properties for solar cells. The films that we obtain are homogeneous and well adhered to the substrate. For the glass substrate and the deposition parameters that we use, the deposition regime is seen to change from reaction-rate limited to mass-transport limited at a temperature of 800 ºC. Below this temperature, the deposition rate is limited by the surface reaction kinetics, which is thermally activated with activation energy of ~ 1.78 eV. The deposition rate increases exponentially with temperature in this range. Above 800 °C, on the other hand, the rate-limiting factor is the mass delivery of the species towards the surface. In this mass-transfer-controlled regime, a temperature-independent deposition rate of ~170 nm/min, compatible with industrial applications, is obtained for the deposition conditions that we use. Scanning electron microscopy (SEM) was used to observe the surface and the cross-section of the samples. We find 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. The average grain size, evaluated from an analysis of the SEM images, shows a slight increase with the deposition temperature, from ~250 nm for TS=750 °C to ~370 nm for TS=860 °C. The general tendency of the grain size to increase with the deposition temperature is in agreement with results from other authors [2]. Although the grain size is lower than 0.4 µm, this should not be an obstacle for obtaining good photovoltaic properties, since open-circuit voltages around 540 mV have been demonstrated even for fine-grained poly-Si with an average grain size of ~0.2 μm [3]. Atomic force microscope (AFM) images of the front surface of the samples reveal a pyramidal structure, with grain dimensions in accordance with the SEM observations. The root mean square roughness, evaluated from the AFM measurements, is ~65 nm for samples of around 3 mm in thickness. The natural texture exhibited by the front-surface is a positive characteristic from the point of view of light trapping. The morphology of these layers would allow increasing the optical path length of light within the solar cell structure. Dark conductivity measurements as a function of temperature show that the films are intrinsic, with activation energies in the range of 0.48 - 0.54 eV. This is an indication of the absence of contamination in the deposition system that we use. In conclusion, our preliminary results demonstrate the feasibility of directly depositing poly-Si thin films on glass substrates at intermediate temperatures, with characteristics that make them appropriate for photovoltaic applications.