ELSEVIER Synthetic Metals 85 (1997) 1249-1250 Light-emitting diodes of poly(3-hexylthiophene) Langmuir-Blodgett films T. &terg?+rd, A. J. Pal, J. Paloheimo, H. Stubb ifbo Akademi University, Department of Physics, Porthmsgatan 3, FIN-20500 ifbo, Finland Abstract Poly(3-hexylthiophene) Langmuir-Blodgett films have been used as emitting layers in light-emitting diodes. The effect of the film thickness and additional carrier-transport layers on current-voltage characteristics and quantum efficiency were studied, and the electroluminescence spectra measured. Hole transporting poly(9-vinylcarbazole) was used both as a separate layer in a heterostructure device and in a blend with emitting material. The electron-transport material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-l,3,4-oxadiazole was used in a blend with the emitting material. The connections between the diode structure and the electro-optical properties as well as operation mechanisms are discussed. Keywords: Conjugated polymers; Electroluminescence; Langmuir-Blodgett techniques; Light-emitting diodes; Poly(3-hexylthiophene) 1. Introduction Light-emitting diodes (LEDs) based on organic materials such as polymers, oligomers and dye compounds have received a lot of attention during the last few years [l]. One of the goals is to be able to fabricate LEDs with specific properties. In order to do so one has to have a good knowledge about the physics of the device. The use of Langmuir-Blodgett (LB) films in LEDs [2,3] makes it possible to investigate the effects of the device structure, due to the precise thickness of the films and well-defined boundaries between different layers. In this work we study the effect of the hole-transport material poly(9-vinylcarbazole) (PVK) [4] and the electron-transport material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-l,3,4-oxadiazole (PBD) [5] on the properties of LB LEDs where poly(3- hexylthiophene) (PHT) is the emitting material. The differences between using PVK and PHT in a blend together [ 63 or having them separated in different layers in a multilayer device are investigated. The electron-transport material PBD is only used in a blend with PHT. 2. Experimental The details of the LB film fabrication can be found elsewhere [7, 81. The thickness of the LEDs is between 7 and 13 LB monolayers, where one monolayer is = 3 nm thick. In all cases, the films consisted of 40 mol.% of arachidic acid mixed with the active materials. The LB films were deposited on Indium Tin Oxide (ITO) coated glass at a surface pressure of 20 mN/m. The semitransparent IT0 (sheet resistance less than lOR/O ) worked as the anode and vacuum evaporated thin films of aluminum were used as cathode. The structures of the LEDs are given in Table 1. The emitted light was detected with a calibrated silicon photodiode and a Keithley 617 electrometer. The I-V curves of the LEDs were measured using a Keithley 230 voltage source and Keithley 2001 multimeter. 0379-6779/97/%17.00 0 1997 Elsevier Science S.A. All rights reserved PII 80379-6779(96)04342-l The photoluminescence (PL) and electroluminescence (EL) spectra were typically recorded with a Perkin-Elmer LS-50 luminescence spectrometer. Table 1. Structure of the LEDs. Structure Materials (1) homostructure/l3 monolayers ITO-PHT-Al (2) homostructure03 monolayers ITO-PHT/PVK-Al (3) heterostructure/5+6 monolayers ITO-PVK-PHT-Al (4) homostructure/ll monolayers ITO-PHT/PBD-AI 3. Results Figure 1 shows the luminance vs. current density curves for the four different structures. The luminance of structure (1) was found to be independent of the device thickness, in agreement with Ref. [3]. The other structures showed a more complicated thickness dependence of the luminance. It can be seen from Fig. 1 that the luminance decreases when adding the hole-transport material and increases when adding the electron-transport material. The results can be explained by assuming that in structure (1) the injection of electrons is weaker than injection of holes, and that the balance needed for high quantum efficiency can be approached by using the electron-transport layers while the use of hole-transport material has the reverse effect. One can further see that the decrease of luminance is larger for the multilayer device ITO-PVK-PHT-Al than for the device based on a blend, ITO-PHT/PVK-Al, suggesting that a heterostructure device which has the emitting and hole-transporting materials separated allows a more effective hole injection. Figure 2 gives the PL and EL spectra for PHT. The spectra for the other structures were also measured and found to be similar to the one shown. The EL spectrum has its maximum at about 2.0 eV and is slightly blue shifted compared to the PL spectra of PHT LB films. The shift can be explained by thermal heating of the sample.