Excess dark currents and transients in thin-film CdTe solar cells: implications for cell stability and encapsulation of scribe lines and cell ends in modules

We have isolated a non-linear, metastable, shunt-path failure mechanism located at the CdS/CdTe cell edge. In such cases, most performance loss, usually erratic, can be associated with the shunt path. We studied these shunt paths using dark current-transients and infrared (ir) imaging and find only one shunt path per cell and only at the cell corner wall, even in badly degraded cells. The effect on diminishing the cell\´s efficiency far exceeds what would be expected from the cell\´s linear shunt-resistance value. We propose that current transients and ir imaging be used as a "fingerprint" of the source and magnitude of excess currents to evaluate the contribution of scribe-line edges and cell ends in thin-film module performance and degradation due to environmental stress. Protection afforded by, or contamination due to, new or currently used encapsulants can then be evaluated.