Bias, length, or coupling? What controls the quantum efficiency of electroluminescent single-polymers
Since the first evidence of luminescence of organic polymers in STM junctions, efforts have been invested in elucidating how to leverage the voltage, anchoring chemistry, and molecular structure to optimize emission power and efficiency. Understanding the fundamentals underlying current-driven molecular emission is important not only for OLED engineering, but also to control luminescence at the atomic scale toward the mastering of single or localized photon sources. However, the difficulty in isolating the separate roles of the variables at play in molecular junction experiments, has precluded a general comprehension of their distinctive effects on the emitted power and the quantum yield. In the present report, we use time-dependent electronic structure simulations based on quantum electrodynamics to disentangle the incidence of bias, electronic coupling and molecular length on device performance, with polyphenylene-vinylene as a case study. A careful validation demonstrates that our approach can achieve quantitative agreement with available experimental data. Through its application we identify the applied bias as the main factor determining emission power. The quantum efficiency, however, is influenced only minimally by bias and electronic coupling, and is instead dominated by polymer length, on which it depends exponentially. Thus, using longer polymer chains emerges as the primary strategy for achieving higher efficiencies. Our results thereby provide key prescriptions for designing single-molecule electroluminescent platforms.