Enhanced light emission from top-emitting organic light-emitting diodes by optimizing surface plasmon polariton losses

Abstract: We demonstrate enhanced light extraction for monochrome top-emitting organic light-emitting diodes (OLEDs). The enhancement by a factor of 1.2 compared to a reference sample is caused by the use of a hole transport layer (HTL) material possessing a low refractive index (1.52). The low refractive index reduces the in-plane wave vector of the surface plasmon polariton (SPP) excited at the interface between the bottom opaque metallic electrode (anode) and the HTL. The shift of the SPP dispersion relation decreases the power dissipated into lost evanescent excitations and thus increases the outcoupling efficiency, although the SPP remains constant in intensity. The proposed method is suitable for emitter materials owning isotropic orientation of the transition dipole moments as well as anisotropic, preferentially horizontal orientation, resulting in comparable enhancement factors. Furthermore, for sufficiently low refractive indices of the HTL material, the SPP can be modeled as a propagating plane wave within other organic materials in the optical microcavity. Thus, by applying further extraction methods, such as micro lenses or Bragg gratings, it would become feasible to obtain even higher enhancements of the light extraction.

• The paper shows that incorporating a low-index HTL mitigates SPP losses, leading to an estimated 20% improvement in OLED outcoupling efficiency.
• The paper combines theoretical models and experimental validations to demonstrate SPP resonance shifts in both isotropic and anisotropic emitter configurations.
• The paper suggests that integrating low-index HTL with additional outcoupling techniques can further enhance OLED performance in practical applications.

Enhanced Light Extraction in Top-Emitting OLEDs via Surface Plasmon Polariton Engineering

Summary

The paper "Enhanced light emission from top-emitting organic light-emitting diodes by optimizing surface plasmon polariton losses" (1503.01309) details a comprehensive theoretical and experimental investigation into the enhancement of external quantum efficiency (EQE) in top-emitting organic light-emitting diodes (OLEDs) through refractive index engineering at the metal/organic interface. Specifically, the study demonstrates that incorporating a low-refractive-index hole transport layer (HTL), such as PEDOT:PSS, adjacent to the opaque metallic anode substantially mitigates losses to surface plasmon polariton (SPP) modes, thereby increasing outcoupling efficiency and overall EQE in both isotropic and anisotropic emitter configurations.

Background and Motivation

OLED efficiency is fundamentally limited by optical losses—chiefly waveguide (WG) modes, absorption, and, for top-emitting architectures, pronounced SPP losses at the metal/organic interface. While established means targeting electrical and internal quantum efficiencies ($\gamma$ and $\eta_\text{IQE}$) approach unity, the dominant bottleneck is the outcoupling efficiency ($\eta_\text{out}$). Traditional device stacks are highly susceptible to SPP-induced energy dissipation, and attempts to circumvent SPP losses via optical microcavity redesign introduce alternative loss mechanisms (e.g., higher-order WG modes).

The manipulation of emitter dipole alignment—a shift toward preferential horizontal orientation—has been shown to further suppress SPP losses, although an irreducible SPP component remains. The present study extends these approaches by directly altering the SPP dispersion through refractive index minimization at the key metal/dielectric boundary, thus decoupling the emitter layer from SPP channels and indicating a path to exceed previous outcoupling limits for planar OLEDs.

Theoretical Analysis

The authors formalize the effect of the refractive index at the HTL/metal boundary on the SPP dispersion relation. For a simplified single-interface, reducing $n_\text{HTL}$ shifts the SPP resonance to a lower in-plane wavevector ($u$), as calculated from classical and transfer-matrix models. This shift decreases the phase space where power can be lost into evanescent SPP modes. In fully stratified OLED stacks, this effect persists but with modulated magnitude depending on the adjacent layer thickness and the spectral overlap between emitter and plasmonic resonance.

For a realistic outcoupling spectrum, the device can be decomposed into contributions from outcoupled modes ($u < u_\text{TIR}$), trapped WG modes ($u_\text{TIR} < u < \nu_\text{active}$), and SPP/evanescent losses ($u > \nu_\text{active}$). The primary technical advance is the demonstration that the SPP resonance can be shifted below $\nu_\text{active}$, effectively converting it from an evanescent field into a propagating mode accessible to macro- or micro-structured extraction schemes.

The mechanism was modeled for both HTL and ETL index modulation, confirming that the proximity of the low-index layer to the metallic electrode is critical; similar changes in the ETL have a two-fold lower efficiency impact.

Experimental Implementation

Devices incorporating both isotropic (Ir(ppy)$_3$) and anisotropic (Ir(ppy)$_2$(acac)) green phosphorescent emitters were fabricated (A: high-$n$ HTL, isotropic; B: low-$n$ HTL, isotropic; C: high-$n$ HTL, anisotropic; D: low-$n$ HTL, anisotropic).

In these devices, the conventional high-index HTL (Spiro-TTB:F6-TCNNQ, $n=1.77$) was replaced with PEDOT:PSS ($n=1.52$), yielding a refractive index contrast of 14%. Structural and electrical consistency was maintained throughout, with the PEDOT:PSS deposited by spin-coating onto the bottom Al anode.

Results

Numerical and experimental enhancement: The optimized device simulations predicted a $\sim20\%$ increase in outcoupling efficiency due to SPP engineering, regardless of dipole orientation. Specifically, isotropic devices showed a simulated and measured EQE boost by factors of 1.23 and 1.19, respectively, while anisotropic devices demonstrated factors of 1.19 and 1.18. The enhancement is robust against dipole anisotropy: even near-complete horizontal dipole alignment preserves significant gains.

Optical analysis: Power dissipation analysis reveals that the majority of the efficiency increase is attributable to the SPP resonance shift and corresponding reduction of dissipative evanescent coupling. Additional benefits arise from the minor enhancement of WG mode emission, but these are second-order contributions.

SPP accessibility: The displacement of the SPP resonance into the propagating region renders previously lossy plasmonic channels available for extraction via additional optical outcoupling methods—e.g., micro-lens arrays or Bragg gratings. This is expected to provide multiplicative gains when combined with established outcoupling techniques.

Practical considerations: Introduction of PEDOT:PSS increases leakage current due to potential impurity incorporation from the spin-coating process; however, peak EQE and optical performance remain consistent with theoretical expectations.

Implications and Future Developments

Practical:

• Device architectures optimized for low-index HTL integration can immediately benefit premium display and lighting technologies via improved luminous efficacy.
• The approach is compatible with both isotropic and anisotropic emitters, broadening the spectrum of applicable emitter chemistries.
• Realignment of SPP resonances suggests a route towards even higher efficiencies when paired with advanced patterned extraction layers.

Theoretical:

• The results delineate a hard pathway for circumventing the traditional $\sim30\%$ outcoupling efficiency barrier in planar OLEDs.
• Extension to white OLEDs and broadband emitters is plausible, as the SPP shift occurs across the visible spectrum.
• The findings potentially impact broader plasmonic-photonic hybrid device studies by providing systematic means to control SPP distribution via photonic design.

Future speculation: The integration of low-index HTL materials is likely to become a reference standard for high-efficiency OLED stack design. As new transparent conducting polymers and process technologies mature, refractive index engineering at metal interfaces could be exploited further to create multi-photon outcoupling platforms, especially when coupled with machine learning-driven design and optimization of photonic architectures.

Conclusion

This work establishes that surface plasmon polariton losses, one of the principal constraints on planar top-emitting OLED performance, can be effectively controlled by introducing a low-index HTL adjacent to the opaque metallic anode. The demonstrated increases in outcoupling efficiency—approximately 20% under realistic conditions and invariant across emitter dipole anisotropy—set a new benchmark for planar OLED design. Beyond immediate practical application, this strategy is compatible with concurrent outcoupling methods, offers gains across the visible spectrum, and advances theoretical boundaries for the optical performance of organic devices (1503.01309).