We report the synthesis of a family of multifluorine substituted oligomers and the corresponding polymer that have the same backbones but different conjugation lengths and amounts of fluorine atoms on the backbone. The physical properties and photovoltaic performances of these materials were systematically investigated using optical absorption, charge mobility, atomic force microscopy, transmission electron microscopy, grazing incidence X-ray diffraction, resonant soft X-ray scattering methods, and photovoltaic devices. The power conversion efficiencies (PCEs) based on oligomers were much higher than that in the polymer. Moreover, the devices based on BIT6F and BIT10F, which have an axisymmetric electron-deficient difluorobenzothiadiazole as the central unit, gave slightly higher PCEs than those with centrosymmetric electron-rich indacenodithiophene (IDT) as the central unit (BIT4F or BIT8F). Using proper solvent vapor annealing (SVA), particularly using thermal annealing (TA) followed by SVA, the device performance could be significantly improved. Notably, the best PCE of 9.1% with a very high FF of 0.76 was achieved using the medium-sized oligomer BIT6F with the optimized film morphology. This efficiency is the highest value reported for organic solar cells from small-molecules without rhodanine terminal group. More excitingly, devices from the shortest oligomer BIT4F showed an impressively high FF of 0.77 (the highest FF value reported for solution-processed small-molecule organic solar cells). These results indicate that photovoltaic performances of oligomers can be modulated through successive change in chain-length and fluorine atoms, alternating spatial symmetric core, and combined post-treatments.

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A series of tetrafluorine‐substituted small molecules with a D1‐A‐D2‐A‐D1 linear framework based on indacenodithiophene and difluorobenzothiadiazole is designed and synthesized for application as donor materials in solution‐processed small‐molecule organic solar cells. The impacts of thiophene π‐bridge and multiple fluorinated modules on the photophysical properties, the energy levels of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), charge carrier mobility, the morphologies of blend films, and their photovoltaic properties as electron donor material in the photoactive layer are investigated. By incorporating multiple fluorine substituents of benzothiadiazole and inserting two thiophene spacers, the fill factor (FF), open‐circuit voltage, and short‐circuit current density are dramatically improved in comparison with fluorinated‐free materials. With the solvent vapor annealing treatment, further enhancement in charge carrier mobility and power conversion efficiency (PCE) are achieved. Finally, a high PCE of 8.1% with very‐high FF of 0.76 for BIT‐4F‐T/PC71BM is achieved without additional additive, which is among one of the highest reported for small‐molecules‐based solar cells with PCE over 8%. The results reported here clearly indicate that high PCE in solar cells based small molecules can be significantly increased through careful engineering of the molecular structure and optimization on the morphology of blend films by solvent vapor annealing.

The synthesis of a series of tetrafl uorine-substituted, wide-bandgap, small molecules consisting of various π-conjugated spacers (furan, thiophene, selenophene) between indacenodithiophene as the electron-donating core and the electron-defi cient difl uorobenzothiadiazole unit is reported and the effect of the π-conjugated spacers on the photovoltaic properties is investigated. The alteration of the π-conjugated spacer enables fi ne-tuning of the photophysical properties and energy levels of the small molecules, and allows the adjustment of the charge-transport properties, the morphology of the photoactive fi lms, as well as their photovoltaic properties. Moreover, most of these devices exhibit superior device performances after CH 2 Cl 2 solvent annealing than without annealing, with a high fi ll factor (0.70-0.75 for all cases). Notably, the devices based on the new molecule BIT4FTh (with thiophene as the spacer) show an outstanding PCE of 8.7% (with an impressive FF of 0.75), considering its wide-bandgap (1.81 eV), which is among the highest effi ciencies reported so far for small-molecules-based solar cells. The morphologies of the photoactive layers with/without CH 2 Cl 2 solvent annealing are characterized by atomic force microscopy, transmission electron microscopy and two-dimensional grazing incidence X-ray diffraction analysis. The results reported here clearly indicate that highly effi cient small-molecules-based solar cells can be achieved through rational design of their molecular structure and optimization of the phaseseparated morphology via an adapted solvent-vapor annealing process.