Sequential Processing for Organic Photovoltaics: Design Rules for Morphology Control by Tailored SemiOrthogonal Solvent Blends

obtaining such effi ciencies has proven to be a major challenge due to the high sensitivity of device performance on the nm-scale morphology of the polymer and fullerene components. [ 10–17 ] To achieve the ideal nm-scale bulk heterojunction (BHJ) morphology for photovoltaic applications, two solution-based processing methods can be used: blend casting (BC) [ 18,19 ] and sequential processing (SqP). [ 20,21 ] With BC, which is by far the most common way in which organic photovoltaics are processed, the active layer is formed by blending the donor and acceptor materials together in solution prior to deposition of the blended fi lm. In SqP, the photovoltaic active layer is formed by depositing the donor and acceptor materials sequentially in separate steps, where the top layer is deposited from a semi-orthogonal solvent with respect to the bottom. [ 22 ] The SqP method is newer and has received far less attention than the BC approach, but has shown great promise for the production of polymer-based solar cells. [ 23–26 ] For example, it has been demonstrated that SqP can produce fi lms with better macroscopic quality and scalability, [ 22,27 ] and yield devices with near-unity internal quantum effi ciencies that outperform their blend-cast counterparts. [ 28–30 ]