Structural control of bulk heterojunction films based on oligothiophene with sterically-bulky groups

Organic photovoltaic solar cells based on oligothiophene and fullerene have been investigated intensively. Until now, the morphologies of bulk heterojunction (BHJ) structures based on oligothiophene and fullerene have been difficult to control because the oligothiophene molecules have lacked substituent groups and therefore have interacted strongly with one another to the point of aggregating into microfibril structures. We succeeded in fabricating homogeneous oligothiophene:C₆₀ BHJ films by adding sterically-bulky groups to oligothiophene in order to control the extent of aggregation. We found that the aggregation onset and maximum fill factor occurred at higher oligothiophene concentrations in BHJ films with C ₆₀ for the substituted molecules-the ratio that provides as much oligothiophene as possible for good photovoltaic performance but not so much as to incur aggregation-fill factor was improved. Here in, we report the morphologies and photovoltaic performances of single-component oligothiophene films and BHJ films based on oligothiophenes.