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

Abstract

In this work, two p-cyanophenyl end-capped oligothiophenes, DPP-(2TPhCN)₂ and di-(p-CNPh)4T, were compared as dopants in the P3HT:PC₆₀BM 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 PC₆₀BM molecules. Intra- and intermolecular interactions of the dopant molecules with P3HT and PC₆₀BM were studied in chloroform solutions. Energy or electron transfer from the dopant molecules to PC₆₀BM takes place as the fluorescence emission intensity and lifetime of the dopant molecules decreased in the presence of PC₆₀BM. 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 PC₆₀BM, 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.