Anomalously bright single-molecule upconversion electroluminescence

Abstract

Efficient upconversion electroluminescence is highly desirable for a broad range of optoelectronic applications, yet to date, it has been reported only for ensemble systems, while the upconversion electroluminescence efficiency remains very low for single-molecule emitters. Here we report on the observation of anomalously bright single-molecule upconversion electroluminescence, with emission efficiencies improved by more than one order of magnitude over previous studies, and even stronger than normal-bias electroluminescence. Intuitively, the improvement is achieved via engineering the energy-level alignments at the molecule-substrate interface so as to activate an efficient spin-triplet mediated upconversion electroluminescence mechanism that only involves pure carrier injection steps. We further validate the intuitive picture with the construction of delicate electroluminescence diagrams for the excitation of single-molecule electroluminescence, allowing to readily identify the prerequisite conditions for producing efficient upconversion electroluminescence. These findings provide deep insights into the microscopic mechanism of single-molecule upconversion electroluminescence and organic electroluminescence in general.

Fig. 1. Efficient UCEL from a single H₂Pc molecule.

a Schematic of STM-induced single-molecule electroluminescence. Insets show the structure and STM image (−0.7 V, 2 pA) of H₂Pc. b STML spectra acquired from H₂Pc/3ML-NaCl/Au(111) at different bias (V_b): 1.5, 1.7, 2.0, –1.7 and –2.0 V. c Bias-dependent intensity integrated over the Q_x peak. Inset: Differential conductance (dI/dV) curve of H₂Pc, with the two peaks assigned to the HOMO and LUMO states. d Dependence of molecular emission intensities I_ph on tunneling currents I_e at V_b = 1.7 and 2.0 V.

Fig. 2. Schematic diagrams of contrasting single-molecule UCEL mechanisms.

In the upconversion region, the first tunneling electron excites the neutral molecule from the S₀ ground state to the T₁ triplet state by either IES (a) or via a transient anionic state through two sequential carrier injection steps (b). Then, the second tunneling electron can promote the molecule from the T₁ state to the S₁ state via a transient cationic state through another two sequential carrier injection steps, as shown identically on the right panels in a and b. Here we use solid black arrows for carrier injections (CI), dashed green arrows for inelastic electron scattering (IES), red arrows for photon emission, vertical dashed lines to illustrate level shifting due to charging/discharging, and pink wavy lines to connect transitions that occur simultaneously. The same annotations are adopted for other similar figures throughout the whole manuscript. See Supplementary Notes 3 & 4 for more details.

Fig. 3. Simulated EL diagrams for single-molecule electroluminescence.

a Energy-level diagram for a single-molecule junction. $E_{{\mathrm{T}}_1}$ ($E_{{\mathrm{S}}_1}$) is the energy of the lowest excited spin-triplet (spin-singlet) states, ${\varphi }_{\mathrm{e}}$ (${\varphi }_{\mathrm{h}}$) is the electron (hole) injection barrier defined as the energy difference between the molecular LUMO (HOMO) and the Fermi level of the substrate. The molecular levels are assumed to be pinned to the substrate. b Numerically simulated exciton excitation efficiency ${\eta }_{\mathrm{ex}}$ as a function of V_b and ${\varphi }_{\mathrm{e}}$ based on the quantum master equation model. In each region, only the dominant mechanism is highlighted. The simulation parameters are given in Supplementary Note 5. c Schematics for different one-electron excitation mechanisms.

Fig. 4. Tuning driving voltages for anomalously bright UCEL.

Differential conductance (dI/dV) (a) and bias-dependent electroluminescence intensities integrated over the S₁ peak (b) for H₂Pc (upper panel) and platinum phthalocyanine (PtPc, lower panel) molecules with different NaCl thicknesses on Au(111). c Corresponding energy-level diagrams.