The effect of diketopyrrolopyrrole (DPP) group inclusion in p-cyanophenyl end-capped oligothiophene used as a dopant in P3HT:PCBM BHJ solar cells

In this work, two p -cyanophenyl end-capped oligothiophenes, DPP-(2TPhCN) 2 and di- ( p -CNPh)4T , were compared as dopants in the P3HT:PC 60 BM bulk heterojunction (BHJ) layer of inverted organic solar cells. Inclusion of DPP-(2TPhCN) 2 significantly increased the average efficiency of the solar cells, while the increase using di- ( p -CNPh)4T doping in the cell efficiency was minor. In the BHJ photoactive layer, the dopant molecules are close to and interact with P3HT and PC 60 BM molecules. Intra- and intermolecular interactions of the dopant molecules with P3HT and PC 60 BM were studied in chloroform solutions. Energy or electron transfer from the dopant molecules to PC 60 BM takes place as the fluorescence emission intensity and lifetime of the dopant molecules decreased in the presence of PC 60 BM. In the case of doping with DPP-(2TPhCN) 2 , doped cells had higher absorbance than the non-doped reference cell and doping broadens the cell absorption to the near IR-region. Thus, the dopant molecules act as additional light absorbers in the photoactive layer and transfer energy or electrons to PC 60 BM, which increases the short circuit current and power conversion efficiency of the cell. Also, the emission of the cells doped with DPP-(2TPhCN) 2 decreased when compared to that of the reference cell. In this case, P3HT can give electrons or energy to dopant molecules and the cell current and efficiency are further increased.