Rational design of diketopyrrolopyrrole-based oligomers for high performance small molecular photovoltaic materials via an extended framework and multiple fluorine substitution

Two narrow-bandgap extended π-conjugated D₂-A_w-D₁-A_s-D₁-A_w-D₂ type small molecules (DPPBIT and DPPBIT4F) based on diketopyrrolopyrrole derivatives as the stronger acceptor core coupled with indacenodithiophene, benzothiadiazole or difluorobenzothiadiazole are designed and synthesized for application as donor materials in solution-processed small-molecule organic solar cells. The impacts of installation of four fluorine atoms to the two weaker acceptor units (benzothiadiazole) on the photophysical properties, the HOMO/LUMO energy level, charge carrier mobilities and the morphologies of blend films, and their photovoltaic properties are investigated. DPPBIT and DPPBIT4F exhibit similar broad and intense optical absorption covering the range from 300 to 900 nm and relatively low-lying HOMO energy levels. These two materials display significantly different photovoltaic performances. Compared with the best PCE of 2.7% for the blend films of DPPBIT and PC₇₁BM, the BHJ-OSC devices based on DPPBIT4F and PC₇₁BM exhibit the best PCE of 5.4% and very-high FF of 0.69 upon CH₂Cl₂ vapor annealing for 30 s, which is one of the best reported photovoltaic performances based on DPP-core small molecules in single-junction BHJ solar cells. Exposure to CH₂Cl₂ vapor allows for a re-organization of the blend films, which increases the intensity and vibrational feature of absorption and dramatically improves the balance of charge carrier mobility and PCE.