The effect of a concentration graded cathode for organic solar cells

We present the effects of a concentration graded Li:Al cathode when it is made by one-step evaporation method using single alloy sources on the performance of organic solar cells. The concentration profile of the Li:Al cathode and related interface energy levels were investigated by means of secondary ion mass spectroscopy and ultraviolet photoelectron spectroscopy, in comparison with those of a common Al cathode. The results indicate that interfacial lithium accumulation introduces a cascade decrease of the work function (WF) of the cathode. The WF graded cathode applied to bulk heterojunction solar cells resulted in increased short circuit current and power conversion efficiency. Furthermore, the Li:Al cathode avoids the formation of interface Al–C complex, which may cause disruption of electron transport.