Nonplanar Perylene Diimide-Based Small Molecule and Its Polymer as Electron Acceptors

Perylene diimide (PDI)-based small molecular and polymeric electron acceptors were designed, simulated, and synthesized. The corresponding organic photovoltaics (OPVs) were fabricated. The relationship between the molecular structure and optoelectronic properties was investigated subsequently. The designed nonplanar (∼90°) spirodithiophenefluorene (SDTF) center controls the molecular crystallization of PDI moieties that provide a good electron transport path. Bromination of the fluorene group in the SDTF center broadens the light absorption spectra and reduces the lowest unoccupied molecular orbital energy level. Moreover, the strongly electronegative Br atoms increased the molecular interaction and the π-π* stacking, thus improving the charge transport of the electron acceptor in the active layer with a polymer donor (PTB7-Th). This donor/acceptor active layer exhibited a strong photoresponse with a maximum value of 57% recorded at a 435 nm wavelength and complete photoluminescence quenching and achieved balanced hole/electron mobility (μ_h/μ_e= 3.71) and favorable morphology to obtain good OPV performance. Polymerizing the brominated monomer, the PDI-based polymeric electron acceptor was synthesized with a lower band gap of nearly −4.0 eV. The film of the polymer acceptor paired with the polymer donor achieved a higher open-circuit voltage (V_oc) of 0.94 V and a broader photoresponse with a maximum value of 51% recorded at 610 nm. The bulk heterojunction all-polymer solar cell achieved the power conversion efficiency of 3.96%.