Compared with conventional liquid crystal displays, organic light emitting devices (OLEDs) are lightweight, fast to respond, have wide viewing angles, and can be flexible and foldable. OLEDs are widely used in smartphones. However, because the excited-state energy levels of organic molecules are strongly affected by vibronic coupling, OLED displays typically have limited color purity, which restricts their use in high-end display applications. To address this issue, researchers have made significant progress in material design and structural engineering to improve color purity in OLEDs.
Scope and approach
This article first introduces rules for quantifying emission color, then summarizes recent design strategies and results for high color-purity emitters based on fluorescent, phosphorescent, and thermally activated delayed fluorescence (TADF) materials. It also reviews microcavity effects and device-level approaches to improve emission purity. Finally, the prospects and remaining challenges for high color-purity OLEDs in displays are discussed.
Color coordinates and color purity
To quantify emission color, W. David Wright and colleagues derived color matching functions from human color perception experiments.
Using matrix transformation and convolution, three tristimulus values can be calculated, which yield the chromaticity coordinates (CIEx, CIEy, 1-CIEx-CIEy). The pair (CIEx, CIEy) is commonly used to represent a light source's chromaticity, as illustrated in Figure 1(c).
Color purity is defined as the ratio of the distances along the line connecting the test device's chromaticity to the equal-energy point and to the dominant wavelength point. It expresses the vividness or saturation of a color. For displays, higher RGB color purity is generally preferred.
Figure 1: (a) color matching functions; (b) standard color matching functions; (c) chromaticity coordinates.
High color-purity organic emitters
Vibronic coupling is one of the main factors that reduce color purity in organic emitters, as shown schematically in Figure 2(a) and 2(b). Figure 2(c) summarizes mechanisms that affect OLED color purity: vibronic coupling promotes radiative transitions but also causes strong perturbation of emission energy by molecular vibrations, broadening the emission spectrum and reducing color purity.
This article analyzes effective methods to improve emission color purity for fluorescent, phosphorescent, and TADF materials, and summarizes recent material design strategies and results. For fluorescent materials, key strategies include designing twisted and rigid structures to suppress structural relaxation and molecular aggregation. For phosphorescent materials, increasing the ligand-centered (LC) contribution in MLCT/LC-mixed triplet states is important. For TADF materials, designing structures that exhibit multi-resonance effects has proven effective.
High color-purity microcavity devices
In OLED devices, an optical microcavity formed by two conductive planar mirrors can be considered a Fabry-Pérot resonator. If the cavity resonance frequency matches the OLED emission frequency, the intensity at the electroluminescence peak is enhanced while off-resonant wavelengths are suppressed, resulting in spectral narrowing and improved color purity. This phenomenon is known as the microcavity effect. Recent device structures and optimization strategies that exploit the microcavity effect to achieve high color purity are reviewed below.
Figure 2: Schematic of lambda/2, lambda, and 3lambda/2 resonance modes in microcavity OLEDs.
Conclusion and outlook
This article analyzed the main challenges to achieving high color-purity OLED devices and summarized recent advances at both the material and device levels. On the material side, molecular design can suppress vibronic coupling that broadens emission spectra. On the device side, microcavity effects can modify spectra to improve color purity.
Although many high color-purity OLED devices suitable for displays have been reported, challenges remain. Further study is needed on electron transitions among vibrational levels and on developing organic emitters that combine narrow full width at half maximum (FWHM) with high quantum efficiency. Continued investigation of emission mechanisms and device structure optimization will drive improvements in OLED resolution and color purity. Narrowband-emission OLEDs are expected to play an important role in next-generation high-resolution, wide-gamut displays.
