Revealing the electronic structure of organic emitting semiconductors at the single-molecule level

Revealing the electronic structure of organic emitting molecules is instructive for tuning the electron-hole balance, one of the key factors in regulating the organic light emitting diode (OLED) performance. Herein, we introduced single molecule conductance measurement (SMCM) technology to probe the conductance of three-model emitting molecules on the Au surface, finding that their hole transporting ability across the metal-molecule interface can be suppressed after electron-withdrawing arms are connected to the center component. This observation would benefit the electron-hole balance of the film in large scale OLED devices whose holes are excessively relative to electrons. I-V modeling reveals that the conductance decrease between molecules is owing to the reduced metal-molecule coupling rather than the impaired energy level alignment. The electronic structure variation between molecules could also be revealed by photophysical measurement, electrochemical analysis, and density functional theory (DFT) simulations, which give supportive evidence of the SMCM result.