Effect of end-groups on the photovoltaic property of diphenyl substituted diketopyrrolopyrrole derivatives

In this work, a diphenyl substituted diketopyrrolopyrrole (DPP) and its two derivatives end-capped with fluorine and n-butyl respectively, namely PDPPP, FPDPPPF, and RPDPPPR, are designed and synthesized. The resulting molecules exhibit similar energy structures, i.e. both relatively narrow optical band gaps (1.75–1.79 eV) and deep highest occupied molecular orbital (HOMO) energy levels (−5.18 to −5.25 eV), implying that all of them are potentially good electron donors for organic solar cells (OSCs). However, three molecules show different photovoltaic performances when they are blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) to fabricate OSCs: the RPDPPPR-based device gives the highest power conversion efficiency (PCE) of 1.59%, whereas the PCEs of PDPPP and FPDPPPF-based OSCs are 0.46% and 0.55%, respectively. Through atomic force microscopy (AFM), X-ray diffraction (XRD) and space charge limited current (SCLC) characterizations, the prominent role of end-groups in the photovoltaic properties of DPP derivatives is disclosed: terminal alkyl chains in RPDPPPR can promote molecular crystallization and lead to the formation of finer phase-separation domains in the blended film, which are in favor of charge generation and transportation in the photovoltaic devices. Thus, RPDPPPR provides the best photovoltaic property among three DPP molecules.