Toward more efficient liquid crystal displays

Liquid crystal displays (LCDs) are popular as computer screens, televisions, and mobile phone displays. They consist primarily of two linear polarizers with some liquid crystal materials between them. Liquid crystal materials act as voltage-controlled light valves that control the amount of light emitted from a pixel of an LCD. However, LCDs are not self-emissive thus must be backlit. Since the polarized backlight source is essential for LCDs, if we could use linearly polarized organic LEDs (OLEDs) as LCD backlights directly, we could remove polarizers to avoid absorptive loss and so improve the power efficiency of LCDs significantly.

Previous investigation of polarized OLEDs has mainly focused on fluorescent OLEDs. These are inefficient, however, and interest has largely shifted toward phosphorescent OLEDs (PPOLEDs),¹ which should be able to attain much higher efficiencies.^{2, 3} We have formed a mesogenic (i.e., liquid crystal) host-guest emitting system, which successfully realized PPOLEDs.⁴ We used a mesogenic phosphorescent platinum complex, N200, combined with a mesogenic oligofluorene host, F(MB)5—see Figure 1(a)—followed by adequate molecular alignment techniques.

Figure 1. Molecular structures of mesogenic phosphorescent platinum complex (N200) and mesogenic oligofluorene host: F(MB)5 (a). Polarized photoluminescence (PL) spectra of F(MB)5:N200 mixture films using poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as the alignment layer (b), where ∥ and ⊥ indicate polarization directions parallel with and perpendicular to the rubbing direction. a.u.: Arbitrary units.

We prepared mixture films using N200 and F(MB)5 coated onto a rubbed layer of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate), known as PEDOT:PSS. The PEDOT:PSS layer here serves as an alignment layer for the mixture films. We aligned the molecules in the host-guest system with a thermal annealing process.

We took photoluminesence (PL) spectra to analyze the annealed F(MB)5:N200 (25%) mixture film coated on the rubbed PEDOT:PSS layer. We measured polarization parallel and perpendicular to the rubbing direction: see Figure 1(b). We assigned the red emission in the mixture film to metal-metal-to-ligand charge transfer (MMLCT) of the N200 molecules that associated with the intermolecular interaction between the Pt(II) metal cores.⁵ Interestingly, N200 MMLCT emission along the rubbing direction is weaker than that perpendicular to the rubbing direction, while the F(MB)5 emission along the rubbing direction is stronger than that perpendicular to the rubbing direction. From the polarized PL spectra of the F(MB)5:N200 mixture film, we deduced that the axis of the columnar stack of N200 molecules in aggregates is more or less perpendicular to the rubbing direction and that the oligofluorene backbone is roughly aligned with the rubbing direction (see Figure 2).

We fabricated a PPOLED using indium tin oxide (ITO), the 25wt% film mixture of F(MB)5 and N200, here named F25N, 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBi), lithium fluoride (LiF), and aluminum (Al). The layer structure (in sequence) was ITO/rubbed PEDOT:PSS(30nm)/F25N(55nm)/ TPBi(45nm)/LiF(0.5nm)/Al(150nm).

Figure 2. Schematic representation of the morphologies and molecular alignment in our novel aligned mixture (host-guest) films containing 25wt% F(MB)5 and N200. MMLCT: Metal-metal-to-ligand charge transfer. Pt: Platinum.

Figure 3. Polarized electroluminescent (EL) spectra (a), current-voltage-light characteristics (b), and external quantum efficiency and current efficiency versus current density (c) of the our polarized phosphorescent organic LED device.

We measured the electroluminescence (EL) characteristics of the polarized OLEDs (see Figure 3). Our device shows purely N200 MMLCT emission, indicating carrier trapping and direct recombination of electron-hole pairs on N200 guest molecules. Similar to N200 MMLCT emission in PL of the aligned mixture films, EL perpendicular to the rubbing direction is stronger than that along it. The dichroic ratio, R_EL=EL_⊥/EL_∥ ≈2 (where ∥ and ⊥ indicate polarization directions parallel with and perpendicular to the rubbing direction). The red-emitting polarized OLED exhibits a turn-on voltage of 3–4V, a maximum brightness exceeding 2000cd/m², and a maximum current efficiency of 2.4cd/A. The EL efficiency of the polarized OLED is low compared to state-of-the-art PPOLEDs nowadays, but represents the very first workable polarized PPOLEDs.

In summary, we have successfully realized workable polarized PPOLEDs by combining a recently developed mesogenic phosphorescent metal—that is, Pt(II)—complex and a mesogenic oligofluorene host to form the corresponding mesogenic host-guest emitting system. In the host-guest film, the Pt(II) complex tends to aggregate and self-assemble into the columnar stacking arrangement, exhibiting MMLCT emission of N200. With the rubbed conducting polymer PEDOT:PSS as the alignment layer, polarized red-emitting OLEDs adopting the aligned host-guest system were successfully implemented, showing an EL dichroic ratio of ≈2, a maximal brightness exceeding 2000cd/m², and a current efficiency of up to 2.4cd/A. The particular alignment mechanism of guest aggregates and the unique MMLCT emission mechanism results in the polarized OLED exhibiting stronger EL perpendicular to the rubbing direction than along it. Next, we plan to investigate both the morphology effect on the host-guest system and other mesogenic phosphorescent metal complexes to further improve the efficiency of polarized PPOLEDs.

The authors gratefully acknowledge financial support from the National Science Council of Taiwan.