Temperature coefficients in compensated silicon materials for solar cells
Abstract
The pricing of solar modules is closely connected to the rating at standard testing conditions (STC). Lately more emphasis has been put on the relationship between field performance of PV-modules and the temperature sensitivity of cells. As it turns out, a benefit inthe performance of cells made of Elkem Solar SiliconR (ESS) in high temperature, high irradiance areas relative to polysilicon cells has been showed. This benefit has been ascribedbetter temperature coefficients (TCs) in the cell parameters. In this thesis the physicalphenomena behind the TCs are investigated. Through temperature and injection dependent lifetime spectroscopy (TIDLS) measurements of wafers, and temperature dependentIV-measurements of Passivated Emitter and Rear Contact (PERC-) cells, made of bothESS and polysilicon, an experimental foundation for the understanding these phenomenais built.More importantly, the experimental results are used in the development of an advancedcell-model in an updated version of the device modeling software PC1Dmod 6.2. Themodel is verified by comparison to the measurements. Further, the independent and jointeffect of varying the material parameters (such as compensation level, effective dopinglevel and bulk lifetime) on the performance and temperature sensitivity of solar cells ofdifferent quality is investigated. In the end, a comprehensive understanding of the phenomena involved is reached, and novel knowledge relevant for compensation engineeringis acquired.The conclusion is that the most decisive material parameters for the temperature sensitivity of the measured PERC-cells is the the bulk lifetime and its temperature coefficient,and that any benefit in the TCs of ESS-cells relative to similarly rated polysilicon-cellsprobably stems from higher bulk lifetimes. However, this should not lead to the conclusion that ESS-cells in general have higher lifetimes; instead this comes down to the factthat if an ESS-cell and a polysilicon-cell are to have similar efficiencies, the ESS-cell necessarily needs to have higher lifetimes, and thus it will have better TCs too. Still, thisshould be considered in the pricing of the cells, since it might lead to relatively high performance ratios in ESS-cells, compared to polysilicon cells, especially in hot locationswith high levels of irradiation