How do crystal defects and impurities reduce solar cell efficiency?

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Diagram of a solar cell showing labeled layers, including a front electrical contact grid, antireflection layer, absorber layer, back junction layer, and back electrical contact layer, with arrows indicating light entering and terminals connecting to a circuit. — solar cell A commonly used solar cell structure. In many such cells, the absorber layer and the back junction layer are both made of the same material.

In solar cells, the conversion of light into electricity relies on the movement of electrons and holes generated by photons. For a cell to function efficiently, these charge carriers must be able to move through the semiconductor material to create an electric current.

Efficiency is reduced when these electrons and holes undergo recombination, a process where they neutralize each other and are removed from the current. While every material has an intrinsic rate of recombination, this process is significantly enhanced by crystal defects and impurities. Specifically, structural irregularities such as grain boundaries and dislocations, as well as the presence of impurities, act as sites that facilitate this recombination. By promoting the premature recombination of charge carriers, these defects prevent the electrons and holes from contributing to the electrical output, thereby lowering the overall efficiency of the solar cell.1