DEGRADATION OF CZ SILICON SOLAR CELLS BY SUB-BANDGAP ILLUMINATION

摘要

Time-dependent measurements of the performance degradation of Czochralski-silicon solar cells reveal that the degradation curves are composed of two parts: a fast initial decay, characterised by a time constant of seconds, followed by an asymptotic degradation, proceeding within hours. These degradation processes are related to the formation of two different boron-oxygen-related recombination centres which are both composed of one boron and two oxygen atoms. Despite this similarity the formation processes of the two centres are different. In this work, we provide experimental evidence that the fast degradation process proceeds after a change of the charge state of a latent centre. By applying defined voltages to the p-n junction of high-efficiency silicon solar cells, we show that the electron quasi-Fermi level needs to be shifted across the energy level of E_v+(635±20) meV in order to activate the fast-forming boron-oxygen centre. This finding is supported by an optical approach. Using infrared sub-bandgap illumination with different cut-off wavelengths, we demonstrate that the fast-forming centre can also be activated by optical injection of electrons into the latent centre.