Definition of Contact Resistance
Ohmic contacts connect the photovoltaic absorber from the two sides of the charge selective contact of the solar cell with metal leads connecting to external load. Barriers can be formed at metal semiconductor interfaces based on the metal work function, the electron affinity, inter-facial defects and doping of the semiconductor. Extra high densities of interface defects can cause Fermi level pinning. Tunneling of free charge carriers through really thin barrier layers at the highly doped semiconductors is critical for ohmic contacts in solar cells with very high solar energy conversion efficiencies.
For proper design of efficient Solar Cells Contact resistance losses must be properly studied to optimize the produced Solar cells and ensure maximum power is obtained from the system. Contact resistance losses happen at the interface between the silicon solar cell and the metal contact. To keep the losses from top contact low, the top N+ layer must be as heavily doped as possible. Yet, a high level of doping creates other problems.
If a high amount of phosphorus is diffused into silicon, the additional phosphorus will lie at the surface of the cell, thus creating a later named dead layer, where there is little possibility of collection for light generated carriers. A lot of the commercial cells have a poor response because of this “dead layer”. Thus, the region directly under the contacts should be heavily doped, while the doping level for the emitter is controlled by the trade offs between maintaining a high emitter diffusion length and obtaining a low saturation current in the emitter.
Contact Resistance in commercial solar cells
In the typical commercial screen printed solar cells, the contact resistance changes across the wafer. The silver paste firing physics are quite complex, so small variance in the surface topology and the local heating cause large variance in the silver-silicon bond quality.
There are many ways to measure the contact resistance of a solar cell. A common technique is to bias the cell at the maximum power point and measure voltage drops along the cell after that. This method can be automated to produce a map for the solar device to show the regions of really large contact resistance.