Basic Diagnostics of Electrical and Structural Properties of Solar Cells

Abstract

In a single batch of base material for Si wafers from a single manufacturer, there are no two wafers with absolutely identical physical and material parameters. The production of a series of cells based on them in a specific technological process introduces further differences in the distribution of parameters. This results in variability of measured performance parameters of solar cells, which generally takes the form of a Gaussian distribution. A solar cell based on crystalline silicon features a large front surface area; for example, for a standard 6-inch single element produced in the photovoltaic industry, it amounts to 236 cm². During the technological process, the cell surface undergoes texturing, which increases its size by approximately 1.7 times in the case of a classic texture consisting of randomly distributed pyramids. Achieving a cell with high efficiency is determined by an internal quantum efficiency above 90%, which in turn requires that the doping concentration in the surface-near region cannot exceed 1.4 x 10²⁰ atoms/cm³. On the other hand, when using metallic contacts for the front electrode of the cell applied via screen printing with Ag paste, the doping concentration in the surface-near region cannot be lower than 10²⁰ atoms/cm³ to ensure a contact resistivity value at the level of a few  mΩ · cm². Operations carried out in the solar cell manufacturing technology, such as crystallization, doping of the base material, and particularly high-temperature processes, influence the carrier lifetime in the semiconductor in various areas of the cell. This lifetime is the sum of the components dependent on the carrier lifetime due to the processes of radiative recombination, Auger recombination, and recombination through trapping in different regions of the cell. The article presents the results of research on the electrical and structural properties of commercially available polycrystalline photovoltaic cells purchased from one of the manufacturers. The issues discussed and the investigations conducted contribute to a deeper understanding and characterization of the solar cell, while the proposed diagnostic tools pave the way for innovation by identifying problems that need to be addressed in the future, including the potential use of new materials.