Improvement in interlayer structure of p–i–n-type organic solar cells with the use of fullerene-linked tetrabenzoporphyrin as additive†
The additive effect on small-molecule-based p–i–n-type devices has been little investigated so far. We focus on the improvement of the miscibility of tetrabenzoporphyrin (BP) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) blend film by addition of fullerene-linked tetrabenzoporphyrin (BP–C60) as an additive to the interlayer (i-layer). BP is one of the most promising p-type organic semiconductors, and BP films can be prepared readily by heating as-cast films of the precursor (a bicyclo[2.2.2]octadiene-fused porphyrin; CP), that results in changes from amorphous CP films to polycrystalline BP films. Because of the high crystallinity of BP, large BP grains on the scale of tens to hundreds of nanometers are generated in blend films of BP and PC61BM during film fabrication. We found that the addition of BP–C60 as an additive (3, 5, 7, and 10 wt%) to the i-layer composed of BP and PC61BM improves the miscibility of BP and PC61BM. The power conversion efficiency of p–i–n-type organic solar cells consisting of a blend film of BP and PC61BM (i-layer) sandwiched by BP (p-layer) and PC61BM (n-layer) improved by up to 50% as compared to that of a control device after the addition of BP–C60 to the i-layer. The film morphology was investigated using atomic force microscopy, fluorescence microspectroscopy, two-dimensional grazing-incident wide-angle X-ray diffraction measurements, and scanning electron microscopy. Interacting with both BP and PC61BM, the addition of BP–C60 led to changes in the grain size as well as an increase in the size of the BP/PC61BM interface and hence effective charge separation in the p–i–n device. This morphological improvement is attributable to the ability of BP–C60, which exhibits the characteristics of both BP and C60, to promote the compatibility of BP and PC61BM. This study is a significant step towards the development of high-performance p–i–n-type solar cells and should pave the way for the fabrication of high-performance bulk-heterojunction layers in solution-processed organic photovoltaic devices.