Flexographic printing of polycarbazole-based inverted solar cells

The mass production of organic photovoltaics requires high throughput processes capable of producing a functional active layer which is homogenous, pinhole free and of a specified thickness. Common methods to achieve this are slot-die coating and gravure printing. We have demonstrated the fabrication of poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole (PCDTBT) based photovoltaic cells using the flexographic printing technique, which enables high throughput patterned active layers to be deposited on flexible substrates at lower cost. This was achieved by optimizing the flexographic plate pattern, print speed, the solvent and the drying process. By incorporating halftone patterning, a common process in the graphics printing industry, and optimizing the printing speed, the homogeneity of the active layer print was significantly improved. Further studies of suitable solvents and drying conditions led to reduced pinhole formation and improved uniformity. The functionality of the flexographically printed active layer was demonstrated by fabrication of 1 cm² photovoltaic cells which showed an efficiency of up to 3.5%, which is comparable to alternative deposition techniques. These results demonstrate the suitability of flexography as a fabrication technique for bulk heterojunction organic photovoltaics.