Bias, Length, or Coupling: What Controls the Quantum Efficiency of Molecular Electroluminescence?

Using currents to control light emission of individual molecules has multiple motivations, including organic light-emitting diode design and the development of display technologies, optical interconnects in nanoscale circuits, chemical reaction mapping and detection with molecular resolution, optoelectronic logic gates, or single-photon sources for quantum control. However, experiments struggle to discern the specific roles of intrinsic and extrinsic factors on the emitted power and the quantum efficiency. Among such factors, the applied potential, the electrode coupling, and the molecular length are critical. Through carefully validated time-dependent quantum electrodynamics modeling, we decipher how these parameters can be chosen to achieve an exponential enhancement in the electroluminescence efficiency of individual polymers, with polyphenylenevinylene and two other aromatic oligomers as case studies. In particular, we show that the applied bias is the primary factor determining emission power, while the quantum efficiency is mostly controlled by the polymer length.