Multilayer Polymer Light‐Emitting Diodes: White‐Light Emission with High Efficiency

Abstract

Organic and polymer light-emitting diodes (OLEDs/ PLEDs) that emit white light are of interest and potential importance for use in active-matrix displays (with color filters) and because they might eventually be used for solid-state lighting. [1,2] In such applications, the fabrication of large-area devices and the use of low-cost manufacturing technology will be the major issues.[2] The fabrication of PLEDs by processing the active materials from solution (e.g., by using printing technology) promises to be less expensive than that of OLEDs, where deposition of the active layers requires the use of vacuum technology. [2,3] Several approaches have been used to generate white light from OLEDs and PLEDs. [2,4±9] In each case, however, the efficiency was modest and the lifetime was limited by that of the blue emitters. [3,7±12] White light is characterized by three quantities: The CIE (Commission Internationale d'Eclairage) coordinates, the color temperature (CT), and the color rendering index (CRI). [13] ªPureº white light has CIE coordinates of (0.333, 0.333), and is obtained by balancing the emission of the colors employed. For illumination applications, the CT needs to be equivalent to that of a blackbody source at temperatures between 3000 and 7500 K. The CRI is a numerical measure of how ªtrueº colors look when viewed with the light source. CRI values range from 0 to 100, with 100 representing true color reproduction.We show that high-performance, multilayer, white-lightemitting PLEDs can be fabricated by using a blend of luminescent semiconducting polymers and organometallic complexes as the emission layer, and water-soluble (or ethanol-soluble) polymers and/or small molecules as the holeinjection/transport layer (HIL/HTL) and the electron injection/transport layer (EIL/ETL). Each layer is spin-cast sequentially from solution. These multilayer PLEDs emit illumination-quality white light with high brightness, high luminous efficiency, suitable CT, stable CIE coordinates, and high CRI.The device fabrication is described in the Experimental section. The device architecture and the molecular structure of the polymers and small molecules used for fabrication of the devices are shown in Scheme 1. The energy levels of the top of the p-band (highest occupied molecular orbital, HOMO) and the bottom of the p*-band (lowest occupied molecular orbital, LUMO) of PFO-ETM, PVK-SO 3 Li, and t-Bu-PBD-SO 3 Na, and the work functions of Ba and PEDOT:PSS are given in Scheme 2a. The corresponding energy levels of PFO-ETM, Ir(HFP) 3 , and fluorenone are shown in Scheme 2b.The selection of materials for the HIL/HTL and EIL/ETL is critical for achieving the band alignment that favors charge injection from the electrodes, for achieving good transport of the respective carriers through the HTL and ETL to the active emission layer, and for hole-blocking (electron-blocking) at the ETL (HTL). PVK-SO 3 Li was selected for the HIL/HTL because its HOMO energy level at ±5.75 eV is well-aligned with the HOMO energy level of PFO-ETM at ±5.80...

Solution-processed multilayer small-molecule light-emitting devices with high-efficiency white-light emission

“…Finally, we fabricated solution-processed multilayer white OLEDs by incorporating green-emitting Ir(ppy) 3 (0.2 wt%) and red-emitting tris(2-phenyl-1-quinoline)iridium(III) (Ir(phq) 3 ) (0.7 wt%) into the blue EML. Although the electroluminescence (EL) spectra of white OLEDs typically depend on current density (in other words, luminance) 4,6,19,[34][35][36][37] , the resulting solution-processed white OLED surprisingly showed no perceived change in the EL spectra under varying current density, as shown in Fig. 6a.…”

“…To achieve the multilayer structures by solution processing, research efforts have focused on p-conjugated polymers that afford a robust hydrophobic layer, on which a hydrophilic layer can be deposited from orthogonal solvents, such as water or water/alcohol mixture [6][7][8][9][10] . In situ cross-linking reactions have also been explored to afford covalently bound structures that are highly resistant to processing solvents [11][12][13][14][15][16][17][18][19] .…”

Conjugated Zwitterionic Polyelectrolyte as the Charge Injection Layer for High-Performance Polymer Light-Emitting Diodes

“…[6][7][8][9][10][11][12] A range of CPEs with different counterions (anion/cation) and backbones 13,14 have been synthesized in attempts to understand the role of the ions in the charge injection process. 15,16 The promising electronic properties of CPEs in combination with their solubility in polar solvents offer a unique route to the realization of high-performance polymer light-emitting diodes (PLEDs) without the use of reactive Ca or Ba cathodes.…”

Solution processed multilayer polymer light-emitting diodes based on different molecular weight host

“…Furthermore, these layers may also act to block the counter charge carrier confining electron-hole pairs in the active region and increasing the probability of generating emissive excitons that are essential for high quantum efficiency OLEDs. In an effort to emulate this, multilayer solution processable PLEDs have been suggested by many based on water-alcohol-soluble polymers salts [3][4][5] or inorganic ionic materials. 6 Other methods like thermally or UV cross-linked hole transporting layer, [7][8][9] insoluble interlayer of various conjugated polymers forms upon annealing, 2,10 and intermediate liquid buffer layer 11 have also been used to spin-coat multilayer devices.…”