Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Organic solar cell optimization requires careful balancing of current-voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor-donor-acceptor (A-D-A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward nearIR region, down to approximate to 860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the V-oc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (approximate to 400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T: NCBDT devices demonstrate an impressive power conversion efficiency over 12%-among the best for solution-processed organic solar cells.