Contact resistance

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**Understanding Contact Resistance and Current Crowding in Solar Cells**

Contact resistance is a significant issue in silicon solar cells, occurring at the interface between the cell and the metal contact. To minimize losses, the top N+ layer must be heavily doped. However, excessive doping can lead to problems like the formation of a “dead layer,” reducing the cell’s efficiency in capturing light-generated carriers.

**Impact of Contact Resistance on Power Loss**

Current crowding is a phenomenon where the conductivity of the finger metal is much higher than that of the semiconductor beneath it. This leads to non-uniform current distribution towards the contacts, with the highest concentration at the edge of the finger contact. The transfer width, denoted as W T, characterizes the degree of current crowding.

**Calculating Power Loss due to Contact Resistance**

The power loss due to contact resistance can be determined by considering the contact resistance at the finger, which depends on parameters like transfer width, sheet resistance, and finger length. The power loss at each side of the finger is calculated as I²R C, where I is the current and R C is the contact resistance.

**Fractional Power Loss and Series Resistance**

The fractional power loss due to contact resistance is the ratio of power dissipated in the contact resistance to the power generated over the region. This fraction helps assess the efficiency of the solar cell in converting sunlight into electricity. Additionally, the contribution to series resistance in ohm cm² can be evaluated to understand the overall resistance in the solar cell.

**Challenges in Commercial Solar Cells**

In commercial screen-printed solar cells, contact resistance can vary across the wafer due to complexities in the firing process of silver paste. Small differences in surface topology and local heating can lead to significant variations in the quality of the silver-silicon bond, impacting the overall performance of the solar cell.

Understanding and addressing contact resistance and current crowding are crucial for optimizing the efficiency and performance of solar cells, ultimately enhancing their ability to harness solar energy effectively.