Contact resistance losses occur at the interface between the silicon solar cell and the metal contact. To keep top contact losses low, the top N+ layer must be as heavily doped as possible. However, a high doping level creates other problems. If a high level of phosphorus is diffused into silicon, the excess phosphorus lies at the surface of the cell, creating a "dead layer", where light generated carriers have little chance of being collected. Many commercial cells have a poor "blue" response due to this "dead layer". Therefore, the region under the contacts should be heavily doped, while the doping of the emitter is controlled by the trade-offs between achieving a low saturation current in the emitter and maintaining a high emitter diffusion length.
In commercial screen printed solar cells the contact resistance varies across the wafer. The physics of silver paste firing are quite complicated so small differences in surface topology and local heating cause large variations in the quality of the silver-silicon bond.
The contact resistance of a solar cell can be measured in a variety of ways. One popular method is to bias the cell at the maximum power point an then measure voltage drops along the cell 1. The method can be automated to produce a map of the device showing regions of very large contact resistance.
- 1. “Mapping of contact resistance and locating shunts on solar cells using Resistance Analysis by Mapping of Potential (RAMP) techniques”, 16th European Photovoltaic Solar Energy Conference. Glasgow (United Kingdom), p. 1438, 2000.