The efficiency of polymer based bulk heterojunction (BHJ) solar cells mainly depends on the film morphology of the absorption layer and the interface properties between the stacked layers. A comparative study using atomic force microscopy (AFM) and optical in situ thin film drying measurements is performed. The strong impact of distinct drying scenarios on the polymer:fullerene BHJ layer morphology is investigated by AFM. The AFM images show a systematic dependency of structure sizes at the surface on drying kinetics. In addition thin film optical measurements for the determination of thin film drying kinetics and parameters are performed using a dedicated experimental setup. The data are used as the input for a quantitative simulation of the drying process. The film thickness decreases linearly during drying while the solvent mass fraction decreases moderately over a wide range until it drops rapidly. Subsequently the remaining solvent fraction evaporates considerably slower. Our work gives a fundamental understanding of the film formation kinetics and prerequisites for the systematic optimization of the film morphology in solution processed organic photovoltaic devices.

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