Limits to Silicon Solar Cell Efficiency due to Free Carrier Absorption
At wavelengths above 1μm (near the bandgap of silicon), light has a much lower chance of being absorbed by solar cells which reduces their efficiency. To combat this. different light trapping techniques have been developed to improve solar cell efficiency in this range. These techniques refract light as it enters a solar cell, keeping it in the cell longer and increasing the chance of absorption. Light trapping is proven to be very useful in improving solar cell performance, however, at higher illumination free carrier absorption (FCA) could reduce its effectiveness. FCA occurs when already-excited carriers absorb more light but then quickly lose the additional energy through thermalization (heat). As illumination increases the number of light-generated carriers increases, creating more FCA. Increased FCA should decrease the average distance light travels in a cell as more light is absorbed by the free carriers. Therefore, it is hypothesized that light trapping effectiveness decreases in the near-bandgap range as illumination increases.
The Z-factor describes the average distance light travels in a cell and is the most common means of quantify light trapping. A solar simulator was constructed to measure the quantum efficiency (QE) of different cells under varying illumination levels, from which a cell’s Z-factor can be determined. It is expected that changes observed in a cell’s Z-factor will give insight as to how effective a cell’s respective light trapping method is at higher light levels.
Determining how Z changes under different lighting conditions will be used to quantify how light-induced FCA affects light trapping with increased illumination. A better understanding of how light trapping works at higher light levels will allow for further optimization, particularly for concentrator solar cells. Concentrator photovoltaics have the potential for much higher efficiencies and could help lead to increased renewable energy systems.