The influence of substrate heating on morphology and layer growth in C60:ZnPc bulk heterojunction solar cells

The change of morphology in mixed layers due to different substrate temperature Tsub of organic solar cells containing C-60 and zinc phthalocyanine (ZnPc) is studied. Heating the substrate during deposition of the bulk heterojunction C-60:ZnPc leads to a significant improvement of solar cell performance, mainly due to an increase in photocurrent and fill factor (FF). This is attributed to improved charge carrier percolation pathways within the C-60:ZnPc blend. Using atomic force microscopy, scanning electron microscopy, transmission electron microscopy, organic field effect transistor, X-ray diffraction, and absorption measurements, we observe aggregation, cluster-like, and polycrystalline growth of the heated bulk layer. This provides better transport percolation paths by inducing a phase separation of the molecules. Heated blend layer with thickness of 60 nm shows high performance without loss in FF. When heating the substrate to the optimum temperature of 110 degrees C, a power conversion efficiency of 3.0% is achieved, compared to 1.4% for an identical device prepared on a substrate held at room temperature. (C) 2010 Elsevier B.V. All rights reserved.