In addition to maximizing absorption and minimizing recombination, the final condition necessary to design a high efficiency solar cell is to minimise parasitic resistive losses. Both shunt and series resistance losses decrease the fill factor and efficiency of a solar cell. A detrimentally low shunt resistance is a processing defect rather than a design parameter. However, the series resistance, controlled by the top contact design and emitter resistance, needs to be carefully designed for each type and size of solar cell structure in order to optimise solar cell efficiency.
The series resistance of a solar cell consists of several components as shown in the diagram below. Of these components, the emitter and top grid (consisting of the finger and busbar resistance) dominate the overall series resistance and are therefore most heavily optimised in solar cell design.
Resistive components and current flows in a solar cell.
The metallic top contacts are necessary to collect the current generated by a solar cell. "Busbars" are connected directly to the external leads, while "fingers" are finer areas of metalization which collect current for delivery to the busbars. The key design trade-off in top contact design is the balance between the increased resistive losses associated with a widely spaced grid and the increased reflection caused by a high fraction of metal coverage of the top surface.
Top contact design in a solar cell. The busbars connect the fingers together and pass the generated current to the external electrical contacts.