Baijun Chen

Electron drift mobility and electroluminescent efficiency of tris(8-hydroxyquinolinolato) aluminum

The electron drift mobility in films of tris(8-hydroxyquinolinolato) aluminum (Alq) deposited at different rates (0.2, 0.4, and 0.7 nm/s) on silicon has been determined by the time-of-flight technique. It has been found that the drift mobility of electrons in Alq increased by about two orders of magnitude as the deposition rate decreased from 0.7 to 0.2 nm/s. Further, the electron drift mobility in all Alq samples increased linearly with the square root of the applied electric field. Electroluminescent devices with a structure of indium tin oxide/α-naphthylphenylbiphenyl amine (NPB, 90 nm)/Alq (90 nm)/Mg:Ag were fabricated at different Alq deposition rates. The device efficiency was found to increase with increasing electron mobility in Alq. As the electron is the minority carrier in the present device, an increase in electron mobility in Alq would thus lead to an increase in device efficiency.

Improved Time-of-Flight Technique for Measuring Carrier Mobility in Thin Films of Organic Electroluminescent Materials

Using an improved time-of-flight (TOF) technique, the drift mobilities of electrons and holes in organic films prepared on silicon or indium-tin-oxide (ITO)-coated glass substrates have been determined. For the samples on silicon, the silicon was also used as a carrier-generating layer. This substantially increased the number of charge carriers generated and thus resulted in a higher intensity electrical signal. Consequently, the thickness of the organic layers can be reduced to less than 1/10 of the typical values (several microns) required in the conventional TOF measurement. The typical thickness of the organic layer in the present work is 400 nm. For organic materials with a high optical absorption coefficient, samples for the TOF measurement can be prepared by directly depositing these materials onto ITO glass substrates with a thickness of about 1000 nm. For both types of substrate, the thickness of the organic layer is much closer to the typical value used in organic electroluminescent devices. The signal, and thus the accuracy, in the present measurement were much improved over those of the conventional TOF measurement. The logarithm of the drift mobility changed linearly with the square root of the applied electric field.

Measurement of electron/hole mobility in organic/polymeric thin films using modified time-of-flight apparatus

Abstract We report, using modified time-of-flight (TOF) apparatus, the measurement of the drift mobility of electrons/holes in thin films of vapordeposited tris(8-hydroxyquinolinolato)aluminum (Alq 3 ) and spin-cast poly( N -vinylcarbazole) (PVK) based on silicon. Drift mobilities of both carriers are strongly electric field and temperature dependent. At room temperature and an electric field of 2 × 10 5 V cm −1 , the effective mobilities of electron and hole are 1 × 10 −5 and 7.14 × 10 −6 cm 2 V −1 s −1 , respectively, in a 200 nm thick samples corresponding to the two materials.

Exciton confinement in organic multiple quantum well structures

High-quality organic multiple quantum well (MQW) structures based on tris(8-hydroxyquinoline) aluminium and 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) have been prepared by use of a multisource-type high-vacuum organic molecular beam deposition system. Small-angle x-ray diffraction measurements indicate that the MQW structures have a very uniform layered structure throughout the entire stack. Both optical absorption and photoluminescence (PL) measurements reveal the evidence for exciton confinement in the /PBD MQW structures. The thickness-dependent blue shift of the PL peak observed upon decreasing the well width is attributed to the change of exciton energy as a result of quantum confinement. Theoretical calculations have been performed and the results agree with the experimental results. Light-emitting diodes (LEDs) based on the /PBD MQW structure with greater efficiency, a narrower electroluminescence spectrum and less of a blue shift than the single heterostructure LEDs have also been fabricated.

Carrier transport and high-efficiency electroluminescence properties of copolymer thin films

A novel copolymer with moieties capable of charge transport and electroluminescence was synthesized. The drift mobilities of electron and hole in the spin-coated films of the copolymer on silicon substrate have been determined by the time-of-flight technique. The shape of the photo-current curves obtained for the transport of both electron and hole is typical for dispersive transport in organic polymers. Under an electrical field strength of 5:0 £ 10 5 V/cm, the drift mobility of electron and hole is 4:78 £ 10 26 and 3:46 £ 10 26 cm 2 /V s, respectively. A high-efficiency electroluminescent device with the bi-layer structure of ITO/copolymer/2-(4-biphenylyl)-5-(4-tert-butylphenl)-1,3,4,oxadiazole (PBD) /LiF/Al was fabricated. The device emitted a bright blue-green light peaking at wavelength of 496 nm, originating from copolymer with a maximum current efficiency of 10 cd/A and a maximum luminescence efficiency of 2.9 lm/W at the DC drive voltage of 12 V. q 2000 Elsevier Science S.A. All rights reserved.

Fabrication and characterization of high quality organic multiple quantum well structures

Abstract High quality organic multiple quantum well (MQW) structures consisting of alternating layers of tris (8-hydroxyquinoline) aluminum (Alq 3 ) and 2-(4-biphenylyl)-5(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) have been prepared by a multisource-type high-vacuum organic molecular beam deposition (OMBD) system which we have designed specially to improve the large-area uniformity and abrupt interfaces. Small angle X-ray diffraction, optical absorption and photoluminescence measurement results indicate that the MQW structures have a very uniform layered structure throughout the entire stack, and interfacial roughness is smaller than 1.0 nm. Exciton energy shift to a high energy region with decreasing Alq 3 layer thickness has been observed.

Performance optimization of organic electroluminescent devices

Performance of organic electroluminescent (EL) devices has been found to depend critically on various processing parameters including the purity and deposition rate of organic materials and substrate temperature. The effects of these processing parameters were systematically investigated by using the time-of-flight measurement of carrier mobility, photoluminescence, Raman as well as photoemission spectroscopies. It was observed that carrier mobility in organic EL materials could be improved substantially by either increasing the material purity or decreasing the deposition rate. Concomitantly, the increase in carrier mobility also led to considerable enhancement in the efficiency of EL devices. By depositing organic EL materials at elevated substrate temperatures, significant improvement in EL efficiency was also obtained. The EL devices thus fabricated consisted of a crystalline hole-transporting layer (HTL). Such devices not only showed improved efficiency but also enhanced stability. The improvement in stability is attributed to the fact that the HTL was already crystalline in the fabricated device so that subsequent operation and storage of the device would not lead to further crystallization. Thus, the widely accepted degradation mechanism via operation-induced crystallization and interfacial diffusion appears not important in the devices fabricated with a crystalline HTL.

HIGH EFFICIENT GREEN EMISSION FROM ORGANIC MULTI-QUANTUM WELLS STRUCTURE

Organic green light emitting devices (LEDs) with multi-quantum wells (MQWs) structure were fabricated. Aromatic diamine was used as hole-transporting layer and potential barrier layer; tris (8-hydroxyquinoline) aluminum acted as electron transporting layer and MQWs green emitting layer. The influence of the barrier layer thickness and quantum well number to the device performance was also investigated. The barrier thickness must be thin (such as 4 nm) enough to tunnel through and distribute charge carriers uniformly in different wells (mainly electrons). The organic MQWs LEDs showed enhanced electroluminescent efficiencies. Maximum luminous efficiency and external quantum efficiency were 1.24 lm/W and 1.04%, respectively.

Influence of Minority Carrier Mobility on Organic Electroluminescent Device Characteristics

The drift mobility of electron in thin films of tris(8-hydroxyquinolinolato) aluminum (Alq) deposited at different rates (0.2 nm/s, 0.4 nm/s and 0.7 nm/s) on silicon has been determined by the time-of-flight (TOF) technique. It has been found that the drift mobility of electron in Alq increased for about two orders of magnitude as the deposition rate decreased from 0.7 to 0.2 nm/s. Further, the electron drift mobility in all Alq samples increased linearly with the square root of the applied electric field. Electroluminescent devices with a structure of ITO / NPB(90 nm) / Alq(90 nm) / Mg:Ag were fabricated at different Alq deposition rates. The device efficiency was found to increase with increasing electron mobility in Alq. As electron is the minority carrier of the present device, an increase in electron mobility in Alq would thus lead to an increase in device efficiency.

Organic electroluminescent devices and their application

Several organic electroluminescent devices have been fabricated by multi-source high vacuum deposition system. For high brightness organic electroluminescent device, the maximum brightness is over 40000 cd/m2. For quantum well structures, quantum size effect has been investigated and the high light emission efficiencies of the devices have been obtained. White-light emission from organic multi-quantum well structures is proposed at first. Brightness of the white-light MQW devices reaches 4000 cd/m2 at 17 V.