Solution-Processed Diketopyrrolopyrrole-Containing Small-Molecule Organic Solar Cells with 7.0% Efficiency: In-Depth Investigation on the Effects of Structure Modification and Solvent Vapor Annealing

A family of narrow band gap extended conjugated D₂-A₂-D₁-A₁-D₁-A₂-D₂ type small molecules based on diketopyrrolopyrrole derivatives as the stronger acceptor core (A₁) coupled with indacenodithiophene (IDT; D₁) and difluorobenzothiadiazole (A₂) are synthesized, and their properties as donor materials in solution-processed small-molecule organic solar cells are investigated. The impacts of replacing the thiophene ring by a more electron-deficient thiazole ring and inserting thiophene spacer between electron-donating (D₁) and electron-accepting (A₁ and A₂) aromatic moieties on bulk properties, such as the photophysical properties, the HOMO/LUMO energy level, charge carrier mobilities, and the morphologies of blend films, as well as optimization on device performance via solvent vapor annealing are investigated. NDPPFBT shows outstanding efficiencies up to 7.00% after THF vapor annealing for 60 s because of a very high fill factor (FF) of 0.73 and high V_oc of 0.89 V. The reported efficiency is among one of the highest values for small-molecules-based organic solar cells from an electron-accepting unit as core and appears as the first diketopyrrolopyrrole-based small-molecule bulk-heterojunction organic solar cells with PCE over 7% with high FF and V_oc.