Diagnostics of thin-film silicon solar cells and solar panels/modules with variable intensity measurements (VIM)

Omid Madani Ghahfarokhi, Fanny Sculati-Meillaud, Arvind Shah
Elsevier, 2011
Article de conférence

A simple and low-cost method for analyzing amorphous silicon solar cells and modules, which have low values of the fill factor (FF), is proposed. Low fill factors can occur mainly because of 3 reasons: (a) excessive recombination due to "bad" intrinsic layers; (b) shunts and (c) very high series resistance. The method described here allows one to discriminate between (a), (b) and (c). It consists of measuring the J-V curves at different light intensities, varying typically from 0.05 to 1 sun. It has been called the "variable intensity method (VIM)". Here, one plots R-sc=partial derivative V/partial derivative J (at V=0) and R-oc=partial derivative V/partial derivative J (at J=0) as a function of J(sc). From the slope of the R-sc-J(sc) curve, one derives the "collection voltage V-coll"; from the asymptotic value of R-sc for low values of J(sc) ( < 0.1 mA/cm(2)) one obtains the "true" shunt resistance R-shunt; from the asymptotic value of R-oc for high values disc J(sc) (around 10 mA/cm(2)) one obtains the "true" series resistance R-series. This paper shows quantitatively how too low values of V-coll and of R-shunt as well as how too high a value of R-series lead to a low value of FF for both cells and panels/modules. (c) 2010 Elsevier B.V. All rights reserved.

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Self-textured zinc oxide polycrystalline films prepared by metalorganic low-pressure chemical vapor deposition combine excellent transparency, conductivity and light-scattering ability when used as electrodes for high-efficiency thin-film silicon solar cells. However, the growth of silicon layers with low defect density, which are necessary for high-performance solar cells, often requires the rough surface morphology of as-deposited zinc oxide films to be smoothened. This is usually achieved by a post-deposition argon plasma-etching treatment. We investigate here an alternative method to modify the surface morphology by changing the zinc oxide growth conditions over only the last hundreds of nanometers of the total film thickness. We discuss two types of zinc oxide cap layers, one grown with the addition of ethanol and the other with an enhanced diethylzinc (DEZ) precursor flow. We show that the presence of either type of cap layer does not alter the layer's electrical properties, but does slightly diminish the layer's light-scattering ability. The cap layer grown with ethanol leads to a more pronounced leveling of the film texture, while the high-DEZ-grown layer better preserves the sharp features of the underlying ZnO. Finally, zinc oxide films with cap layer are used as front electrodes for tandem amorphous/microcrystalline silicon solar cells, and are compared to rough films treated with an argon plasma. With rising thickness of the capping layers, the efficiency of tandem cells increases reaching values over 12%, and approaches that of cells on Ar plasma treated ZnO films. (C) 2014 Elsevier B.V. All rights reserved.

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Christophe Ballif, Laura Ding, Sylvain Nicolay

High-performance transparent conducting oxides (TCOs) have significance for optimising PV performance. The efficiency of the resulting solar cells is dependent particularly on achieving high light scattering, low resistivity and low absorption (via low free carrier absorption), in addition to suitable surface morphology for absorber growth quality. These properties have been targeted by systematic exploration of the Atmospheric Pressure Chemical Vapour Deposition (APCVD) growth parameters, in particularly the effect of the tin precursor to water ratio. Using the APCVD process F-doped SnO2 has been deposited on glass using monobutyl tin trichloride with trifluoro-acetic acid as the dopant source. Experiments established that an increased water to tin precursor ratio gave smaller surface features, along with increased electrical mobility and optical transmittance at high reactant ratios. Samples were then used in manufacture of thin film solar cells, which showed enhanced performance, in comparison to commercially available TCO CVD coated glasses, with high quantum efficiency yield. In particular, high water to tin precursor ratios led to much improved open circuit voltages, fill factors and high current densities within the cells. Tandem cells with efficiencies of 10.75% were achieved, which were higher than those produced concurrently using commercially available TCO both on-line 9.50% and off-line 10.20%. (C) 2013 Elsevier B.V. All rights reserved.

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