Vertical phase separation and light-soaking effect improvements by photoactive layer spin coating initiation time control in air-processed inverted organic solar cells

Abstract

Inverted type organic solar cells have been regarded more promising over normal type, owing to a favorable vertical phase separation and air-processability. Many techniques, such as thermal or solvent annealing, slow growth method, melting bi-layers etc. have been employed to improve the morphology of photoactive layer, which is one of the advantages of inverted type OSCs. As a result, the device performance is greatly enhanced which is also reflected in the current-density versus voltage curve. However, some of the methods presented above are difficult and costly to perform. In addition, inverted OSCs still employ certain metal oxide electron-transporting layers, which impose UV activation time due to light-soaking effect. Here we show a facile method that can control the vertical phase separation as well as minimize the light-soaking effect. By leaving the photoactive solution with a low volatile solvent in air before spin coating, conductive polymers (donor) and fullerene derivatives (acceptor) were separated vertically in a way that their charge extraction is enhanced. Besides, oxygen and impurities at the contact on ETL were removed by leaving the solution for a certain period. Mechanisms of these improvements were evidenced by various analytical techniques and empirical tests using different materials in different environments. We anticipate this finding can expedite the OSC research and its usage in all air-processed inverted OSCs.