Elsevier

Thin Solid Films

Volume 518, Issue 2, 30 November 2009, Pages 703-706
Thin Solid Films

Vapor deposition polymerization of vinyl compounds and fabrication of OLED having double emissive layers

https://doi.org/10.1016/j.tsf.2009.07.073Get rights and content

Abstract

Physical vapor deposition polymerization was achieved by evaporating vinyl monomers followed by heat treatment. This technique was utilized for preparing a hole-transport layer (HTL) and emissive layers (EMLs) of organic light emitting diodes (OLEDs). Adivinyl derivative of tetraphenyldiaminobiphenyl, DvTPD, was used both for the HTL and for the host material of red phosphorescent EML. A vinyl derivative of bis(N-carbazolyl)benzene, vmCP, was used for the host material of blue phosphorescent EML. The ELMs were doped with red or blue-emitting phosphorescent iridium complexes modified with styril units. After characterizing the devices that have single EML of red or blue emission, an OLED having multilayered ELM was prepared by stacking these layers to achieve quasi-white emission. It was found that the deposition polymerization was effective to improve the emission efficiency. The improvement appeared to be related to the proper carrier balance in the EML.

Introduction

Since the pioneering work by Tang et al. [1], most of the organic light emitting diodes (OLEDs) have been developed by constructing heterojunctions of hole and electron transport layers, although single-layer structure has been frequently adopted for polymer OLEDs [2]. Starting from the early devices having a structure of simple single hetero junction [1], [3], device efficiency has been improved by introducing such concepts as double heterojunction [4], carrier blocking layer [5], etc. making the device structure progressively complicated. The heterojunction structure is convenient for optimizing materials by separating the functions to different materials, and is also effective to confine carriers and excitons by manipulating the band structure. The phosphorescent device, which stands out as an important landmark in achieving high efficiency, consisted of as many as eight different layers including the electrodes [6]. Multilayeredstructure is especially important in developing white light emitting OLEDs by stacking emissive layers (EMLs) of different colors [8]. Such multilayered structures can be constructed by physical vapor deposition (PVD) of small molecules without much difficulty. On the other hand, the small molecules are limited in such properties as thermal stability, mechanical robustness, and flexibility. Although polymer materials are superior in these characteristics, the conventional wet coating processes are not feasible for realizing the complicated multilayered structures that are commonly adopted for the vapor-deposited OLEDs.

With a purpose to compromise the structure controllability of PVD and material stability of polymers, the authors have developed several strategies to prepare polymeric thin films by vapor deposition polymerization [7]. The vapor deposition polymerization is a technique that evaporates small molecules and polymerizes them on the substrate surface. It has the common characteristic of PVD, and is capable of depositing nanometer-scale and high-purity polymer thin films and multilayered structures without using solvents. Several schemes for deposition polymerization have been proposed according to the polymer materials. We have found in particular that vinyl polymer thin films can be easily prepared by ultraviolet [9], electron [10], or thermally assisted deposition polymerization methods [11], and can be applied for constructing organic electronic devices including OLEDs. In this paper, this scheme was employed to prepare OLEDs that consist of more than one polymer layers by taking advantage of its capability to form polymer multilayers. Above all, two polymer EMLs of different colors were stacked to obtain a broad emission spectrum. The OLED characteristics were investigated in connection with the effect of polymerization.

Section snippets

Experimental

In this work, polymeric thin films were obtained by physical vapor deposition of vinyl monomers followed by thermal annealing in the vacuum deposition chamber. Fig. 1 shows the OLED structures (a, b) and the newly developed vinyl compounds (c) used in this work. Three types of OLEDs were prepared in this study; those having single EML of red or blue emission in structure Fig. 1(a), and one having double EMLs of red and blue emissions in structure Fig. 1(b). N,N′-diphenyl-N,N′-bis(4-vinylphenyl)

Red-emitting device

Red-emitting OLEDs of the structure in Fig. 1(a) were prepared using an EML of DvTPD doped with Ir(piq)2acac-vb. Devices of an identical structure were prepared with and without the post-deposition annealing process to compare the effect of polymerizing the HTL and EML layers. Fig. 2 shows the current density–luminance (a) and luminance–external quantum efficiency (b) characteristics of these devices. At a driving current of 2.5 mA/cm2, the device prepared without polymerization showed operation

Conclusions

Polymer thin films were prepared by PVD of vinyl compounds followed by annealing in the vacuum deposition system. This technique was applied for preparing HTL and EMLs to construct OLEDs. DvTPD was used both for the HTL and for the host material of red-emitting EML doped with Ir(piq)2acac-vb. A blue-emitting EML was prepared by combining vmCP host material with FIrpic-vp dopant. Taking advantage of the capability of stacking polymer layers by PVD, an OLED having double EMLs was prepared to

Acknowledgement

This research was supported by a Grant-in-Aid for Scientific Research No. 20360347 from the Japan Society of the Promotion of Science.

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