The power conversion efficiencies of organic bulk-heterojunction solar cells delicately depend on the morphology of the two semiconductors which are blended in the light-harvesting layer. When deposited from non-halogenated solvents, solvent additives are often needed to craft well-defined bi-continous networks of polymers and fullerenes, but despite their widespread use, the working principle of solvent additives is still under debate. In this work, a series of structurally similar, substituted benzaldehydes is investigated towards their use for suppressing liquid-liquid demixing and hence phase separation of polymer:fullerene blends during thin-film deposition. Besides the commonly accepted good fullerene solubility, the solvent additives must exhibit lowest polymer solubility for best solar cell performance. This study adds an important criterion to the mandatory properties of solvent additives that are suitable to produce highly efficient organic solar cells.
We demonstrate a new organic solar cell fabrication and characterization technique that allows for a quick
screening of new materials and material combinations (i.e. blends) as active layers for solution processed
organic solar cells with respect to the optimization of the active layer thicknesses, thereby saving precious
material resources. Therefore we bar coat wedge-shaped layers by “horizontal dipping”. The photocurrent
under short circuit conditions, the external quantum efficiency and the absorption of those wedge-shaped
layers were then recorded spatially resolved. From the results the device photocurrent vs. the layer thickness
can be extracted allowing for conclusions about the optimum absorber layer thickness.