3Elkem Solar AS, Kristiansand, Norway. ABSTRACT. Multicrystalline solar grade silicon solar cells from metallurgical process route and small Cz-Si test ingots.
MULTICRYSTALLINE SOLAR GRADE SILICON SOLAR CELLS 1
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K. Peter , R. Kopecek , M. Wilson , J. Lagowski , E. Enebakk , A. Soiland , S. Grandum 1 International Solar Energy Research Center Konstanz, Germany 2 Semilab SDI LLC, Spectrum Boulevard, FL 33612, USA 3 Elkem Solar AS, Kristiansand, Norway
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ABSTRACT
EXPERIMENTAL
Multicrystalline solar grade silicon solar cells from metallurgical process route and small Cz-Si test ingots have been investigated to further understand the impact of compensation and related reduced majority carrier mobilities. A possible PB complex which may prevent BO2i complex formation has been discussed recently by different research groups in several contributions. Our low temperature PL measurements strongly support the existence of such a boron/phosphorous complex. The industrially applicable multicrystalline crystallisation process at Elkem Solar as well as the solar cell process at ISC Konstanz were consequently further developed to improve the cell efficiency towards 17%.
The solar cell process includes Elkem Solar in house expertise of multicrystalline ingot growth leading to wafers with large grains and low dislocation densities. The solar cell process includes wet chemical isotexturisation, POCl3 diffusion (optimized with regard to effective phosphorous gettering), screen printing of full area Al back contact and Ag front grid and firing though a PECVD SiNx passivation layer. With this process multicrystalline silicon solar cells from Elkem Solar silicon with efficiencies up to 16.8% have been achieved. To investigate the influence of compensation on physical parameters such as mobility or minority carrier lifetime, four small Cz-Si ingots were grown from poly silicon with different doping concentrations under similar conditions. The small wafers were characterised by 4-point probe resistivity measurements and the active carrier concentration was measured by Electrochemical Capacitive Voltage (ECV) technique, to achieve the effective wafer doping concentrations. From the carrier concentrations and the bulk resistivities we calculated the majority carrier mobilities. All majority carriers were holes, as only p-type regions have been investigated from the compensated test ingots. The minority carrier lifetime was measured by ยต-PCD technique. As the wafer surfaces were not passivated the resulting effective lifetimes were limited by both, the surface recombination velocity and the diffusivity of the wafers. We used this possibility to estimate the diffusivity of samples with different degree of compensation. The minority carrier diffusion length was determined on both, small test samples and full size finisched solar cells. Noncontact measurement of alternating potential (AC) surface photovoltage (SPV) is commonly used for determining the minority-carrier diffusion length L. Such a measurement is done with a transparent pick-up electrode capacitively coupled to the wafer. In the constant-photonflux SPV method employed in this study, two SPV signals are measured simultaneously using two intensity modulated monochromatic light beams slightly shifted in modulation frequency. The two beams have different wavelengths, giving different light penetration depths, z. The L value is extracted by fitting the SPV signal ratio to a theoretical expression. The AC SPV has been applied for quantitative measurement of recombination defects in crystalline Si photovoltaic wafers and solar cells, with an emphasis on light-induced degradation (LID) of lifetime, open-circuit voltage and efficiency. Finally low (T
