Xiaochen Liu

Efficient orange-red phosphorescent organic light-emitting diodes using an in situ synthesized copper(I) complex as the emitter

Inexpensive and eco-friendly luminescent Cu(I) complexes are ideal phosphorescent emitters for high efficiency organic light-emitting diodes (OLEDs). A series of pyrazinyl carbazole (CPz) compounds were designed and synthesized to obtain efficient luminescent Cu(I) complexes through reaction with CuI using a vacuum codeposition method. Based on photophysical studies of the CPz compounds and their codeposited CuI–CPz films, the compound 9-(3-(6-(carbazol-9-yl)pyrazin-2-yl)phenyl)-carbazole (CPzPC) with CuI was chosen as the emissive layer for OLED fabrication, where the emitter was identified as Cu2(μ-I)2(CPzPC)4 on the basis of X-ray absorption spectroscopy. After optimizing the device architecture and material selection including the CuI doping concentration and hole transporting layer, an efficient orange-red emitting OLED with a maximum emission band, an external quantum efficiency (EQE), and a luminance of 590 nm, 6.6% and 8619 cd m−2 (10 V), respectively, was achieved.

Red emissive organic light-emitting diodes based on codeposited inexpensive Cu-I complexes

Inexpensive materials made of abundant natural resources such as CuI complexes are essential to sustain the development of organic light emitting diode (OLED) technology for mass market applications such as solid-state illumination. CuI complexes, however, mostly are neither soluble nor stable toward sublimation, which is a road block for the development of efficient CuI complex based OLEDs using traditional methods of synthesis, sublimation and vacuum evaporation. In this work, two isoquinolyl carbazole (CIQ) compounds 9-(8-(carbazol-9-yl)isoquinolin-5-yl)-carbazole (DCIQ) and 9-(4-(5-(4-(carbazol-9-yl)phenyl)isoquinolin-8-yl)phenyl)-carbazole (DCDPIQ) were synthesized to codeposition with copper iodide (CuI) to form red emissive dimeric CuI complex doped film in situ, which could be utilized directly as the emissive layer (EML) in OLEDs. After a systematic study of the two compounds and their codeposited CuI : CIQ films, as well as optimizing the CuI doping concentration, it is found that red OLEDs can be achieved, showing a maximum emission band, an external quantum efficiency (EQE), a luminance of 643 nm, 3.5%, 3290 cd m−2 for DCIQ, and 635 nm, 3.6%, 853 cd m−2 for DCDPIQ, respectively.

Structural and photophysical study of copper iodide complex with P^N or P^N^P ligand

Two P^N-type ligands, 8-(diphenylphosphino)quinoline (L1) and 2-[2-(diphenylphosphino)ethyl]pyridine (L2), and two P^N^P-type ligands, 2,6-bis((diphenylphosphino)methyl)pyridine (L3) and 2,6-bis((di-tert-butylphosphino)methyl)pyridine (L4), were synthesized to coordinate with copper iodide (CuI). As a result, CuI complexes with rich structures, such as discrete complexes with formulae of [Cu2I2(L1)2] (5), [Cu3I3(L2)2] (6), and [CuI(L3)] (7), and polymeric complexes with repeating units of [Cu2I2(L4)] (8), and [Cu3I3(L4)] (9) were synthesized and characterized by single crystal X-ray diffraction. Besides the intriguing structures, these complexes showed rich photoluminescent properties, with emission colour that varied from blue to red and a photoluminescence quantum yield (PLQY) from 1.6 to 29.9% in the solid state. Molecular orbital calculation and experimental study showed that the emissions involve halide to ligand charge transfer (XLCT), metal to ligand charge transfer (MLCT), and/or cluster-centered (CC) excited states.

Carbazolylphosphines and carbazolylphosphine oxides: facilely synthesized host materials with tunable mobilities and high triplet energy levels for blue phosphorescent OLEDs

A high triplet energy level (ET) and balanced carrier mobility are critical factors for host materials in efficient blue phosphorescent organic light-emitting diodes (PHOLEDs). Herein, we report a facile synthesis of four compounds, dicarbazolylphenylphosphine (DCPP), dicarbazolylphenylphosphine oxide (DCPPO), tricarbazolylphosphine (TCP), and tricarbazolylphosphine oxide (TCPO), and their application as host materials in a classic blue phosphorescent emitter bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic). The four compounds show a high ET of up to 3.0 eV and tunable mobilities. We fabricated four OLEDs with a device structure of ITO/MoO3 (1 nm)/N,N′-dicarbazolyl-3,5-benzene (mCP):MoO3 (20 wt%, 10 nm)/mCP (30 nm)/Host:FIrpic (7 wt%, 20 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB, 40 nm)/LiF (1 nm)/Al (100 nm), all of which display maximum external quantum efficiencies (EQEs) exceeding 20%. The DCPPO-based device reaches the highest EQE of 27.5%, the maximum luminance of 14 070 cd m−2, and the lowest efficiency roll-off of 22.2% from 1 to 10 mA cm−2, which is among the best-performing FIrpic-based PHOLEDs without using any light extraction method.

A series of dinuclear cuprous iodide complexes chelated with 1,2-bis(diphenylphosphino)benzene derivatives: structural, photophysical and thermal properties

Three new bisphosphine ligands, 4-phenyl-1,2-bis(diphenylphosphino)benzene (Ph-dppb), 4-pyrrolyl-1,2-bis(diphenylphosphino)benzene (Pr-dppb), and 4,5-dimethoxyl-1,2-bis(diphenylphosphino)benzene (OMe-dppb), were synthesized to coordinate with cuprous iodide (CuI), and compared with 1,2-bis(diphenylphosphino)benzene (dppb). Single crystal X-ray diffraction and elemental analysis experiments indicate that three binuclear CuI complexes with formulae of [Cu(μ2-I)Ph-dppb]2, [Cu(μ2-I)Pr-dppb]2, and [Cu(μ2-I)OMe-dppb]2 were obtained, which are the same as the reference complex [Cu(μ2-I)dppb]2. The three complexes showed a maximum emission band (λmax) in the range of 484–535 nm which varies in the electron effect of the substituted groups (i.e. methoxy, pyrrolyl, phenyl) on [Cu(μ2-I)dppb]2, and in high photoluminescence quantum yield (PLQY) from 53 to 90% in the solid state. Furthermore, the complexes [Cu(μ2-I)Ph-dppb]2 and [Cu(μ2-I)Pr-dppb]2 exhibited a thermal decomposition temperature exceeding 380 °C. The high PLQY and excellent thermal stability of these CuI complexes imply their great potential application as efficient emitters in organic light-emitting diodes (OLEDs).

Review on photoluminescence and electroluminescence study of copper(I) complexes

Copper(I) complexes have attracted considerable interest because of their rich photophysical properties and potential applications as inexpensive, abundant materials in organic light-emitting diode (OLED). Herein we review photoluminescence and electroluminescence study on several common copper(I) complexes [Cu(NN)2]+, [Cu(NN)(PP)]+ and (CuX) m L n (where NN indicates a chelating bisimine ligand, typically a substituted 1,10-phenanthroline; PP denotes a bisphosphine ligand; X means halide, such as I, Br, Cl and L equals a ligand that contains N or P coordination site). The relationship between structure and photoluminescence properties of copper(I) complexes and recent progress on copper(I) complexes based OLEDs are discussed.