Difluorobenzothiadiazole-Based Small-Molecule Organic Solar Cells with 8.7% Efficiency by Tuning of π-Conjugated Spacers and Solvent Vapor Annealing

The synthesis of a series of tetrafluorine-substituted, wide-bandgap, small molecules consisting of various pi-conjugated spacers (furan, thiophene, selenophene) between indacenodithiophene as the electron-donating core and the electron-deficient difluorobenzothiadiazole unit is reported and the effect of the pi-conjugated spacers on the photovoltaic properties is investigated. The alteration of the pi-conjugated spacer enables fine-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 films, as well as their photovoltaic properties. Moreover, most of these devices exhibit superior device performances after CH2Cl2 solvent annealing than without annealing, with a high fill 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 efficiencies reported so far for small-molecules-based solar cells. The morphologies of the photoactive layers with/without CH2Cl2 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 efficient small-molecules-based solar cells can be achieved through rational design of their molecular structure and optimization of the phase-separated morphology via an adapted solvent-vapor annealing process.