Bias, length, or coupling? What controls the quantum efficiency of electroluminescent single-polymers

TL;DR

The paper addresses the fundamental question of what controls the efficiency and power of electroluminescence in molecular junctions. It develops a quantum-electrodynamics–based simulation framework (self-consistent tight-binding plus driven Liouville von Neumann open boundaries) to separate the effects of bias, length, and interfacial coupling, validating against experiments on OPV. The key findings are that emitted power is predominantly determined by the applied bias, while quantum efficiency increases exponentially with polymer length, with coupling strength playing nuanced roles; these results yield practical design guidance for optimizing single-molecule electroluminescent devices and inform macroscopic OLED materials.

Abstract

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.

Figures (10)

• Figure 1: (a) Chemical structure of the p-phenylenevinylene repeating unit. (b) Ball-and-stick models for the first three oligomers ($n=1, 2, 3$) of the oligo(p-phenylenevinylene) (OPV) series, shown forming a molecular junction between two electrodes (yellow spheres). $L$ is the overall length of the molecular wire.
• Figure 2: Current as a function of applied bias, for single oligophenylenvinylene molecules with varying lengths as indicated in the legend.
• Figure 3: Current as a function of oligomer length. (a) Effect of applied bias, with a metal-molecule coupling strength of 2 eV. (b) Effect of coupling strength, for an applied bias of 2 V. (c) Effect of coupling strength, for an applied bias of 3 V.
• Figure 4: Logarithm of the emitted power versus the logarithm of the current, for a metal-electrode coupling of 2 eV, and different oligomer lengths. These plots prove the empirical relation $P \propto I^n$ with $n$ equal to 1.5, 1.0 and 0.65 for oligomers containing 1, 2 and 3 units.
• Figure 5: Color map of the power emitted by the electroluminescence of oligophenylenvinylene, as a function of the applied bias and polymer length. The corresponding metal-molecule coupling parameter is 2 eV.