Oligothiophene - Fullerene Bulk Heterojunction Organic Photovoltaic Cells

The purpose of this study is to investigate the device structure for higher efficiency in the case of employing oligothiophene (6T) as a donor material and fullerene (C60) as an acceptor material in the small molecular bulk heterojunction organic PV cells. 6T is well-known as a high mobility material in the study of organic transistor, so we expected the improvement of hole transport properties. Firstly we examined the 6T/C60 heterojunction characteristics by making bilayer pn junction type PV cells. As a result, it showed excellent photovoltaic characteristics: PCE of 0.75%, Jsc of 3.9mA/cm², Voc of 0.35V, FF of 0.55 at AM1.5G 100mW/cm². Next we investigated the relation between device structure and photovoltaic characteristics in 6T/C60 bulk heterojunction PV cells. And it was found that all characteristics extremely depended on the composition of the 6T/C60 blend layer. In the case of equal volume ratio, (this is optimum ratio in the metal phthalocyanine and fullerene bulk heterojunction PV cells) PV characteristics has decreased rather compare to bilayer type. But as the ratio of C60 increased, the characteristics were extremely improved. As a result of optimization of device structure, we achieved high efficiency: PCE of 2.0%, Jsc of 5.6mA/cm², Voc of 0.70, FF of 0.51. In particular, Voc of this PV cells is highest value as the small molecular bulk heterojunction cells. Blend morphology is remarkably different between C60 excess blend and equal blend. C60 excess cells has smooth surface, in contrast equal blend cells surface is rough. It is likely that phase separation occurs in equal blend cells and affects carrier transport network. It is concluded that 6T/C60 bulk heterojunction PV cells with C60 excess blend layer is promising cells for higher efficiency