Electrical and Photo-Induced Degradation of ZnO Layers in Organic Photovoltaics

We present the case of degradation of organic solar cells by sunlight concentrated to a moderate level (similar to 4 suns). This concentration level is not enough for sufficient acceleration of the photobleaching or trap-generation in the photoactive layer and therefore such short treatment (100 minutes) does not affect the short-circuit current of the device. However, a significant degradation of V-OC and FF has been recorded by measurements of the cell current-voltage curves with a variation of light intensity, for the devices before and after the treatment. The same degradation was found to occur after short application of forward voltage biases in the dark. This kind of degradation is found to be repairable, and could even be prevented by simple electrical treatment (short pulses of the reverse bias). Moreover, even the fresh cells can be improved by the same process. Generation and degeneration of shunts in ZnO hole-blocking layer as underlying physical mechanisms for the cell degradation and restoration, respectively, can explain the results.