Efficient Green Organic Light-emitting Diodes Research Articles

A series of new bipolar CBP derivatives with introduction of a electron-deficient moiety for efficient green organic light-emitting diodes

Abstract By introducing pyridine, benzimidazole and triazine groups, three novel CBP derivatives host materials, 9,9'-(2-(pyridin-3-yl)-[1,1′-biphenyl]-4,4′-diyl)bis(9H-carbazole) (CBPPY), 9,9'-(4''-(1- phenyl-1H-benzo[d]imidazol-2-yl)-[1,1':2′,1″-terphenyl]-4,4′-diyl)bis(9H-carbazole) (CBPBM) and 9,9'-(3''-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1':2′,1″-terphenyl]-4,4′-diyl)Bis (9H-carbazole) (CBPTZ) were designed and synthesized. These three compounds exhibited high thermal stabilities, appropriate HOMO-LUMO energy levels and improved electroluminescent (EL) performance compared with CBP compound in Ir(ppy)3 doped green organic light emitting diodes (OLED). In addition, devices based on CBPPY, CBPBM and CBPTZ showed lower driving voltages of 3.0, 2.6 and 2.5 V, and all devices achieved higher EL efficiencies. Especially, the CBPBM-based device reached the optimal external quantum efficiencies (EQE) of 18.9% with the maximum power efficiency (PE) of 56.7 l m/W, which suggested that CBPPY, CBPBM and CBPTZ were promising bipolar host materials for green OLEDs.

Highly efficient green organic light-emitting diodes containing luminescent tetrahedral copper(i) complexes

A series of highly emissive sublimable copper(I) complexes with tetrahedral geometries, Cu(dppb)(pz2Bph2) 1, Cu(dppb-F)(pz2Bph2) 2, and Cu(dppb-CF3)(pz2Bph2) 3 [dppb = 1,2-bis(diphenylphosphino)benzene, dppb-F = 1,2-bis[bis(3,5-difluorophenyl)phosphino]benzene, and dppb-CF3 = 1,2-bis[bis[3,5-bis(trifluoromethyl)phenyl]phosphino]benzene, pz2Bph2− = diphenyl-bis(pyrazol-1-yl)borate], were synthesized and investigated as luminescent guest molecules in prototype organic light-emitting diodes. Thermogravimetric analysis of 1–3 under vacuum revealed that introduction of F or CF3 substituents to the meta positions of the four peripheral phenyl groups in the dppb skeleton increased the ability of the copper(I) complexes to be sublimed. 1–3 exhibited strong green emission in amorphous films at 293 K with maximum emission wavelengths of 523–544 nm, quantum yields of 0.50–0.68, and decay times of 3.6–8.2 μs. Molecular orbital calculations indicated that the origin of green phosphorescence from 1–3 was a mixture of σ → π* and π → π* transitions. Conventional bottom-emitting devices with three-layer structures containing 1–3 produced bright green luminescence with maximum external quantum efficiencies of 11.9, 16.0, and 17.7% for 1, 2 and 3, respectively.

Integration of high brightness and low operating voltage green organic light-emitting diodes on complementary metal-oxide semiconductor backplane

We report high brightness and low operating voltage efficient green organic light-emitting diodes (OLEDs) based on silicon complementary metal-oxide semiconductor (CMOS) backplane which can be used in applications such as microdisplays. The small molecule top-emitting OLEDs are based on a fluorescent green emitter accompanied by blocking, doped charge transport layers, and an anode fabricated with standard CMOS processes of a 200 mm integrated circuit (IC) fab. The devices are designed to maximize the efficiency under low operative bias so as to fit the limited voltage budget of the IC. This was done by making optical simulations of the device structure, optimizing the organic layer thicknesses and charge injection in the n and p transport layers. The devices reach a current efficacy of 21.6 cd/A at a luminance of 20,000 cd/m2. The devices exhibit a voltage swing as low as 2.95 V for a contrast ratio of 1000. The optimized devices have a high lifetime of 6000 and 8800 h at 5000 cd/m2. Furthermore, aging inside the emission layer is investigated.

Structure-properties relationships in solution-processable single-material molecular emitters for efficient green organic light-emitting diodes

The electroluminescent properties of a series of solution-processable fluorescent molecular emitters have been systematically investigated. While the introduction of the electron-deficient benzothiadiazole unit in the structure confers efficient electron-injection on the emitter materials, they exhibit different hole-transport properties. The device characteristics of the OLEDs based on these various emitters are discussed on the basis of (i) the energy levels of their HOMO and LUMO and (ii) their hole-transport properties in relation with the charge-transport and blocking properties of the electron- and hole-transport layers.

Solution Processed Small Molecule-Based Green Phosphorescent Organic Light-Emitting Diodes Using Polymer Binder

The authors have demonstrated efficient green organic light-emitting diodes (OLEDs) by using polymer binder. We fabricated small molecular green OLEDs and mixed polymer as a binder such as polystyrene (PS) or poly(N-vinylcarbazole) (PVK). The 4,4'-N,N'-dicarbazole-biphenyl (CBP) is a small molecular material with excellent electrical properties however it become crystalline at high temperature. Polymer binder prevents crystallization of CBP and lead to high efficiency. Therefore, we added PS or PVK into CBP as a polymer binder. As a result, we obtained maximum luminous efficiency, power efficiency and quantum efficiency of 22.8 cd/A, 11.6 Im/W and 6.61% at 23% PS added device.

Highly Efficient Green Phosphorescent Organic Light-Emitting Diodes with High Electron Mobility

We demonstrated highly efficient green organic light-emitting diodes (OLEDs) by inserting a mixed hole blocking layer (HBL) between the emitting layer (EML) and electron transporting layer (ETL). We compared 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), N-arylbenzimidazoles trimer (TPBi), and 4,7-diphenyl-1,10-phenanthroline (Bphen) as the hole blocking layer. Thereafter, we mixed BCP with TPBi and Bphen. We obtained improved efficiency from the mixed HBL devices and the TPBi mixed with BCP device showed the best efficiency.

Highly efficient green organic light-emitting diodes from single exciplex emission

Spectral single and stable green exciplex emission was demonstrated from organic light-emitting diodes (OLEDs) with 4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine and 4,7-diphenyl-1,10-phenanthroline that function as electron donor (D) and acceptor (A), respectively. As 8-hydroxyquinoline aluminum (Alq3) was attached to the acceptor layer, electroluminescent (EL) properties of the two exciplex-type OLEDs with D/A-bilayer and D:A mixture layer configurations were markedly improved, i.e., a peak current efficiency of 7.6cd∕A at 2.38mA∕cm2 in three-layer device and a maximum luminance of 6620cd∕m2 at 8.7V in blend layer device were obtained, respectively, without changing the peak position (535nm) and the shape of EL spectrum. Discussion is given on the harvest of the pure green exciplex emission and enhancement of luminance which is obtained by inserting Alq3 layer.