Strategies for improving the efficiency of CZ-silicon solar cells
Recent investigations have firmly established that the metastable defect which is responsible for the light-induced degradation in Czochralski silicon (Cz-Si) is correlated with oxygen and boron. Thus, an attractive way to improve the material quality is to substitute boron by gallium as the p-type dopant. We have verified that no degradation and excellent carrier lifetimes close to the theoretical limit are observed over a wide doping concentration range. Stable efficiencies higher than 20 % can be achieved with an RP-PERC cell structure on gallium-doped Cz-Si with resistivities from 0.12 omega cm to 1.5 omega cm. A maximum efficiency of 22.2 % was obtained at 0.3 omega cm despite the Ga-doped Cz-Si having a significant concentration of interstitial oxygen. Standard Cz-Si (boron-doped, oxygen-contaminated) can be improved by an optimized high-temperature step without external gettering. Choosing the optimal process parameters, it is possible to increase the stable lifetime significantly in both conventional tube furnace and rapid thermal processing (RTP) system. Using the RTP system, the stable lifetime can be improved by a factor of two within 120 s.