The limited relevance of SWE dangling bonds to degradation in high-quality a-Si:H solar cells

Abstract

Contributions of different light-induced defect states to degradation of solar cells have been established for high-quality p-i-n solar cells with i layers of protocrystalline a-Si:H deposited at very low rates, whose nanostructure is dominated by hydrogen-passivated divacancies. Nature of the different light-induced gap states and their respective roles as electron and hole recombination centers were characterized in the thin films from their photocurrents, and in corresponding solar cells from their Shockley-Reed-Hall carrier recombination currents. The results were directly related to three light-induced states, with 'A' and 'B' within 0.2 eV and 'C' 0.4 eV below midgap, identified from subgap absorption. The A and B states are efficient electron, while the C states are very efficient hole recombination centers. Under 1-sun illumination, the former dominate the electron lifetimes, while the latter are key to solar cell operation as is confirmed by the direct correlation of their creation with the degradation of VOC and 1-sun fill factor (FF). It is also shown that the apparent correlation found earlier between the cell FF and electron lifetimes is due to the same long-term degradation kinetics of the light-induced changes in the B t and C states. © 2011-2012 IEEE.