Investigation of degradation in solar cells from different mc-Si materials

The impact of light induced degradation on small lab-type 2x2 cm2 solar cells made from standard p-type mc-Si material is compared to the degradation of lowly compensated UMG material on the one hand and intentionally Fe and Fe/Cu contaminated material (several ppma of Fe or Fe and Cu are introduced into the melt of electronic grade feedstock before ingot casting) on the other hand. The material is taken from three different ingot heights (bottom, middle and top), respectively. All cells are annealed at 200 °C in the dark for several minutes before they are exposed to one sun illumination and the development of the electrical parameters over time is determined. The reference and UMG material behave as expected and show a degradation which is correlated to the boron and interstitial oxygen concentration over the ingot height and therefore can be attributed to B-O related defects. The intentionally Fe and Cu contaminated samples in contrast show a degradation behavior that is more probably correlated to the distribution of the Fe and Cu contamination, as the absolute degradation towards the top of the ingot is higher for highly contaminated material (20 ppma Fe) than in the reference material and the observed time constants for degradation are different. Fe/Cu contaminated material exhibits too low cell performance in the top and bottom part of the ingot to show any significant degradation effects. The lowly contaminated material (2 ppma Fe) behaves like the reference material as long as the wafer material does not contain additional contaminants originating from the crucible walls. Material from near the crucible wall by contrast shows a stronger light induced degradation.