New methods for investigation of surface passivation layers for crystalline silicon solar cells

Abstract

One of the main challenges of the c-Si PV industry is the implementation of high quality surface passivation layers. Effective surface passivation is needed in order to avoid large efficiency losses when moving towards thinner silicon wafers and to utilize the full potential of high quality Si material. To obtain a fundamental understanding of the surface recombination mechanisms and their impact on device performance, precise characterization methods and suitable physical models are of high importance. The main result of this work is the development of a new method for analyzing surface recombination for passivated Si substrates under varying surface band bending conditions: A photoluminescence imaging setup is used to measure the effective minority carrier lifetime of passivated Si wafers while applying an external bias to a metal electrode deposited onto the rear side passivation layer. The experimental measurements have been analyzed using an extended Shockley Read-Hall model with added recombination in the space charge region. This approach is shown to give valuable information about the passivation mechanisms. The measured data can be used to independently determine the fixed charge density under illumination (field-effect passivation) and the surface recombination velocity parameters for electrons and holes (chemical passivation) for a wide range of dielectric passivation layers. The results are in good agreement with both capacitance-voltage measurements and lifetime measurements after deposition of corona charges on the sample surface. A second major contribution from this work is the development of a modified version of the widely used simulation tool PC1D which runs from a command line, thus allowing for scripted simulations. This modified version of the program has also been used as a basis for a new user interface, which allows for improved visualization, multivariable analysis, optimization, fitting to experimental data and implementation of additional models. Through this work, several passivation processes have been developed and optimized, and excellent passivation quality has been demonstrated for a stack of a-Si:H and a-SiNx:H deposited by plasma enhanced chemical vapor deposition. Selected passivation layers have also been demonstrated as rear side passivation in a co-planar solar cell test structure with local rear contacts.