Synthesis and characterization of cyano-substituted pyridine derivatives for applications as exciton blockers in photovoltaic devices

A new class of cyano-substituted pyridine derivatives have been synthesized and characterized. The relationship between their chemical structures, energy gap, energy levels, electron mobility and device performance was studied systematically by applying the new materials as EBL in standard CuPc/C₆₀/EBL organic photovoltaic (OPV) devices. Their properties fulfill all the requirements for a good EBL material, such as a wide energy gap (E_g, 3.12–3.50 eV), a deep highest occupied molecular orbital (HOMO, 6.71–7.80 eV) energy level, a high electron mobility (in the order of 10⁻⁵ cm² V⁻¹ s⁻¹) and a high glass transition temperature (T_g, 139–189 °C), thus most of them show performances surpassing that of BCP. In particular, one device shows an efficiency of 2.12% and a half-efficiency lifetime of 340 h, which are, respectively, 44% and more than 5 times better than those (1.47%, 60 h) of the BCP-based device. Furthermore, we found that the lowest unoccupied molecular orbital (LUMO) energy level of the EBL can also influence the leakage current and thus the efficiency of the OPV devices. It is expected that these studies would provide useful guidance for designing high-performance EBL materials.