Monitoring a-Si:H solar cells degradation by two sides blue lightcollection

In view of the persistent lack of correlation between single layer material properties and solar cell performance, one is at present interested in experimental techniques which allow the characterisation of material properties within a full p-i-n cell. In this paper, the authors introduce a novel method to monitor i-layer properties during degradation of a-Si:H p-i-n solar cell. Two blue, strongly absorbed light beams shine from both p- and n- side on the cell, giving rise to an approximately constant density of charge trapped in the deep states over the i-layer. The sign and magnitude of the constant space charge can be systematically varied by the ratio of the two light beams. The deformation in the shape of the built-in electric field within the cell, as induced by this charge, dramatically affects the collection of photogenerated carriers in a very distinctive way. A simple analytical model, based on the `standard' DOS model for a-Si:H and involving a reduced set of parameters is presented; it allows the determination of the density of the deep defects by fitting experimental collection data. A more detailed modelling of the experiment, using a finite element a-Si:H device simulator, is also presented.