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J. Phys. Chem. C 2008, 112, 5174-5184
Blue-Green, Orange, and White Organic Light-Emitting Diodes Based on Exciplex Electroluminescence of an Oligoquinoline Acceptor and Different Hole-Transport Materials Abhishek P. Kulkarni and Samson A. Jenekhe* Department of Chemical Engineering and Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1750 ReceiVed: August 12, 2007; In Final Form: January 10, 2008
Exciplex electroluminescence colors ranging from blue-green to yellow-orange to white were observed in blends and bilayers of a blue light-emitting oligoquinoline acceptor, 6,6′-bis(2-4-diphenylquinoline) (B1PPQ), and different arylamine donors with varying ionization potential (IP) values. The blend composition is shown to be an effective tool to tune the exciplex electroluminescence colors for a given donor/acceptor (D/A) pair. In the D/A blends, exciplex emission was observed under both optical and electrical excitation. However, in the D/A bilayers, exciplex emission was observed only under electrical excitation, highlighting the critical role of the electron-hole capture at the D/A interface in the exciplex formation. Yellow (CIE ) 0.41, 0.52) to orange (CIE ) 0.57, 0.42) exciplex electroluminescence was achieved from binary blends of B1PPQ and an arylamine donor with an IP of 5.0 eV. Bilayers of B1PPQ with the same donor gave white electroluminescence colors (CIE ) 0.35, 0.32) because of the contribution of both bulk (blue) and exciplex (orange) emissions. Blue-green electroluminescence colors (CIE ) 0.16, 0.34) were observed in both blends and bilayers of B1PPQ and a donor with an IP of 5.3 eV. In the D/A bilayer diodes, the competing bulk and exciplex emissions were highly sensitive to the acceptor film thickness and the applied voltage. These results reveal the complex charge-transfer excited-state dynamics at D/A interfaces frequently encountered in multicomponent organic light-emitting diodes.
Introduction Organic light-emitting diodes (OLEDs) based on conjugated small molecules and polymer semiconductors are being developed for applications in full-color displays and solid-state lighting.1,2 Major challenges remain, including the need to significantly improve the emission color coordinates, performance, and durability of red, green, blue (RGB) and white OLEDs. In a typical OLED consisting of multilayers or blends of hole-transport materials (electron donors) and electrontransport materials (electron acceptors), the electroluminescence (EL) arises from the radiative decay of excitons in the bulk of the emissive material. In some cases, EL originates from an intermolecular charge-transfer excited-state complex (exciplex) formed at the interface between the donor (D) and acceptor (A) and is often red-shifted compared to the bulk emission of the individual components.3-12 The emission wavelength of the exciplex depends critically on the difference between the ionization potential (IP) of the donor and the electron affinity (EA) of the acceptor,3,13 thereby providing a simple approach to tune the EL colors by judiciously choosing the donor and acceptor with appropriate energy levels. Although OLED colors including red, green, blue (RGB)4-11 and white12 based on exciplex EL have been achieved, the charge-transfer excitedstate dynamics at the D/A interfaces, particularly under electrical excitation in the diode, is not fully understood. Hence, in addition to allowing manipulation of OLED color, studies of exciplex formation at the D/A organic interface are also important from a fundamental scientific viewpoint. In addition to the appropriate energy levels of the D and A, strong intermolecular π-orbital overlap between the D and A is * Corresponding author. E-mail:
[email protected].
essential to exciplex formation, which in turn depends on the spatial orientation of the molecules at the D/A interface.3,13,14 Thus, in a binary D/A blend, one would expect the blend composition to affect the exciplex stabilization energy and thereby the exciplex emission color and efficiency. In a bilayer D/A diode, the competition between bulk and exciplex (interface) EL emission would depend critically on the relative film thicknesses of the D and A layers and the applied voltage (electric field). However, only a few studies of the variation in exciplex EL efficiency as a function of the D/A blend composition7c,11a or the relative film thicknesses of the D/A films11c have been performed. Systematic investigations of such structure-property relationships are needed to develop a complete understanding of the excited-state processes at D/A interfaces commonly encountered in OLEDs. We herein report studies of the photophysics and electroluminescence of exciplex-forming blends and bilayers consisting of a blue-emitting oligoquinoline acceptor15 and different donor molecules with different IP values. We recently developed a family of new blue-emitting n-type oligomers having a common 6,6′-bis(4-phenylquinoline) core15 exemplified by 6,6′-bis(2,4diphenylquinoline) (B1PPQ) in Chart 1. Blue OLEDs (CIE ) 0.15, 0.10) with a high 2.95% external quantum efficiency (EQE) at 1215 cd/m2 were obtained from bilayer devices of donor poly(N-vinylcarbazole) (PVK) and B1PPQ (EA ) 2.7 eV, IP ) 5.7 eV).15b Because of the high and similar IP value of PVK (5.8 eV), exciplex formation was not possible at the PVK/B1PPQ interface, and hence blue EL was achieved from the bulk of the B1PPQ thin film.15 However, polyquinolines and oligoquinolines are known to be good electron-acceptor (ntype) materials,16 quite capable of forming exciplexes with suitable donors. The donor molecules we have selected for study
10.1021/jp076480z CCC: $40.75 © 2008 American Chemical Society Published on Web 03/11/2008
OLEDs Based on Exciplex Electroluminescence CHART 1: Molecular Structures of the Electron Donor Molecules (MTDATA, TAPC, TTA) and the Electron Acceptor Molecule (B1PPQ)
include the commonly used triarylamine-based hole-transport materials17-19 in OLEDs such as 4,4′,4′′-tris(3-methylphenylamino)triphenylamine (MTDATA, IP ) 5.0 eV),18 1,1-bis(di4-tolylaminophenyl)cyclohexane (TAPC, IP ) 5.3 eV),19 and tris(p-tolyl)amine (TTA, IP ) 5.3 eV),17 whose molecular structures are also shown in Chart 1. The variation in the exciplex emission color and efficiency was studied as a function of both blend composition and the relative film thickness in bilayer devices. In the case of blends, a series of five blends containing 10, 20, 35, 50, and 80 mol % of the acceptor B1PPQ were made with each of the three different donor molecules. In the bilayers, three different film thicknesses of B1PPQ (7, 28, and 43 nm) were used for a fixed thickness of the donor layer. Exciplex EL colors ranging from blue-green to yellow-orange and white were observed from the blend and bilayer OLEDs. Experimental Section Materials. The oligoquinoline B1PPQ was synthesized and fully characterized in our laboratory as previously reported.15 TTA, TAPC, and MTDATA (sublimed grade purity) were purchased from Sensient Imaging Technologies. Preparation of Blends and Thin Films. Binary blends of B1PPQ with TTA, TAPC, and MTDATA were made by mixing appropriate volumes of ∼0.7 wt % solutions of the donor (TTA, TAPC, or MTDATA) and acceptor (B1PPQ) molecules in chloroform. Compositions of blends in this report refer to molar percentage (mol %) of B1PPQ. A series of five blend compositions containing 10, 20, 35, 50, and 80 mol % B1PPQ were prepared in each case. Thin films for optical absorption and PL measurements were obtained by spin-coating the blend solutions on glass slides at 1800 rpm. All the films were dried at 60 °C typically overnight in vacuum to remove any residual solvent. Blend films (∼35 nm thick) were homogeneous and showed good optical transparency, except for some TTA-based films which tended to crystallize. In such cases, 20 wt % polystyrene (PS) was added to the blends (total 80 wt %) to prevent this crystallization and improve the film quality obtained by spin-coating. Optical Absorption and Photoluminescence Spectroscopy. Optical absorption spectra were recorded using a Perkin-Elmer model Lambda 900 UV/vis/near-IR spectrophotometer. Steadystate PL emission and PL excitation (PLE) spectra were acquired on a Photon Technology International (PTI) Inc. model QM2001-4 spectrofluorimeter. Time-Resolved Photoluminescence Decay Dynamics. Fluorescence decays were measured on a PTI model QM-2001-4
J. Phys. Chem. C, Vol. 112, No. 13, 2008 5175 spectrofluorimeter equipped with Strobe Lifetime upgrade. The instrument utilizes a nanosecond flash lamp as an excitation source and a stroboscopic detection system. All measurements were done at room temperature. The decay curves were analyzed using a multiexponential fitting software provided by the manufacturer. Reduced χ2 values, Durbin-Watson parameters, and weighted residuals were used as the goodness-of-fit criteria. Fabrication and Characterization of OLEDs. All the OLEDs were fabricated as sandwich structures between a lithium fluoride/aluminum (LiF/Al) cathode and an indiumtin oxide (ITO) anode. A 50-nm-thick poly(ethylene dioxythiophene)/poly(styrene sulfonate) blend (PEDOT; Φ ) 5.2 eV) hole-injection layer was used in some cases. The blend devices had the architecture ITO/(donor/B1PPQ)/TPBI (30 nm)/ LiF/Al, where the thin film of 1,3,5-tris(N-phenylbenzimidizol2-yl)benzene (TPBI; IP ) 6.2-6.7 eV, EA ) 2.7 eV)1b acted mainly as the hole-blocking layer. The bilayer devices were of the type ITO/PEDOT/(PS:donor ) 25:75)/B1PPQ/LiF/Al. ITOcoated glass substrates (Delta Technologies Ltd.) were cleaned sequentially in ultrasonic baths of detergent, 2-propanol, deionized water, and acetone, and then dried at 60 °C in a vacuum overnight. For the blend diodes, thin films (35-40 nm) of the binary blends of B1PPQ and the donors were obtained directly on top of ITO by spin-coating from their chloroform solution. In the case of TTA and TAPC blends, 20 wt % polystyrene was added as a supporting matrix to facilitate the spin-coating of uniform films. The TPBI thin film (30 nm) was then obtained by evaporation from resistively heated quartz crucibles at a rate of ∼0.1-0.4 nm/s in a vacuum evaporator (Edwards Auto 306) at base pressures of
