Efficient oligothiophene:fullerene bulk heterojunction organic photovoltaic cells

Novel small-molecule bulk heterojunction photovoltaic (PV) cells consisting of oligothiophen (alpha-sexithiophen: 6T) and fullerene (C60) have been developed. Oligothiophen is well known as a good hole-transport material, and by changing the number of thiophen rings and making chemical modifications or substitutions, its characteristics relevant to PV applications (such as carrier mobility, energy level, packing, and ordered structure) can be controlled. Thus far it has been difficult to fabricate films of oligothiophene--fullerene blends with suitable morphology by using the common co-evaporation method, because oligothiophene crystallizes easily during film deposition. The present study found that the morphology of 6T:C60 blends strongly depending on the composition of 6T:C60. Suitable morphology was obtained only for films deposited with the co-evaporation of excess C60. It is likely that excess C60 prevents the crystallization of 6T. By successfully controlling the film morphology, we were able to demonstrate good PV performance in oligothiophene:fullerene bulk heterojunction PV cells for the first time. Moreover, it was found that PV performance could be further improved by inserting a C60 layer between the blend layer and exciton blocking layer.