Pump-probe experiments of multicrystalline silicon for solar cell applications
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- Institutt for fysikk 
In order to make cost effective solar cells from mc-Si materials, the negative contributions from defects and impurities should be reduced. The analysis of the photogenerated carrier properties is therefore of great importance for characterising carrier processes and hence, for improving the material performance.In this work, pump-probe measurement of a range of silicon wafers have been performed, using anultrafast laser of 800 nm wavelength and 85 fs pulses. The optical response in the samples were analysed by measuring the reflected probe beam initial transient.The purpose of this theses was to explore the use of pump-probe experiment to study carrier dynamics in mc-Si. Measurements of single c-Si samples were used as a basis for developing good experimental skills as well as achieving knowledge about carrier dynamics in c-Si. The initial Delta R/R was studied for a range of input parameters, aiming to characterise important contributions to the measurements.The effects of passivation has been studied, indicating a significant contribution to R~R. Etchingoff the passivated layer of an oxide (SiO2) wafer, showed a radically increased in pump beam reflectivity, from 9% to 32%, and a reduced DeltaR~R from 47×10-6 to 37×10-6 was be observed. Analysis has showed that incident angle may be chose such that the pump reflection loss is at a minimum for the given passivation thickness.The final results showed a R~R is in the range of (14-41)e-6 for bare c-Si, and (47-171)e-6 for passivated c-Si wafers.Ultrafast initial recovery has been observed for mc-Si samples, and attributed to trapping of carriers. Decay times in the range of 1-6 ps are deduced and trapping densities are found as (1:3 - 4:3) × 10^18 cm-3, which is in the same order as the excitation densities.A methodology for using pump-probe measurements to analyse mc-Si samples is established, and the technique is used in characterising the observed defect states, which is of great interest for improving solar cell materials.