Photon-degradation effects in terrestrial silicon solar cells

Abstract

An unexplained instability in terrestrial solar-cell output under various use conditions has been observed. The effect is similar to but not identical with previously observed photon-induced degradation phenomenon in radiation-damaged space solar cells. Reduction in cell output is observed in n+/p cells upon exposure to illumination or upon the application of a sufficiently high forward bias. Conversely, an enhancement in output is observed when p+/n cells are illuminated. Investigations performed on n+/p cells indicates that a recombination center located at Ec−0.37 eV in the forbidden band is responsible for the loss in output. The center is electrically inactive in its ground state but can be activated either by raising the minority-carrier quasi-Fermi level sufficiently close to the latent-center energy level in the band gap, or by direct excitation of electrons from the valence band to the latent-center level. The center has been identified as a complex of a lattice defect and a silver atom or cluster of atoms. The effect can be prevented either by preventing the introduction of silver or by eliminating lattice damage through sufficient surface material removal prior to diffusion while at the same time restricting diffusion temperatures to 875 °C or less. The effect in n-base cells is not fully understood at the present. It appears, however, that in either type of cell it is the p material that is responsible for the observed instabilities.