Elsevier

Thin Solid Films

Volume 696, 29 February 2020, 137781
Thin Solid Films

Balancing charge-transporting characteristics in bipolar host materials

https://doi.org/10.1016/j.tsf.2019.137781Get rights and content

Highlights

  • •

    Bipolar host materials with balanced charge transporting property were synthesized.

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    The optimum bipolar host showed well enhanced device performances.

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    Charge balance is a function of alkyl chain length and internal compactness.

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    Morphology of the device with bipolar host materials was maintained.

Abstract

Bipolar host materials with balanced charge transporting property were synthesized using a solvent-less green reaction method. The characteristics of these synthesized bipolar host materials were examined by thermal and spectroscopic analysis. The energy levels of these materials were estimated from cyclic voltammograms and absorption spectra. The optimized molecular geometries and spatial distributions were obtained from molecular simulations. Moreover, the current density vs. voltage features of single-carrier devices were investigated to assess the bipolar transport characteristics of the host materials. As the length of the alkyl chain increased, the electron-transporting capability and hole-transporting ability exhibited optimum values at the octyl chain attachment. The current efficiency, power efficiency and quantum efficiency values of white organic light-emitting diodes prepared by blending blue and yellow iridium phosphors with these bipolar hosts were high with decreasing alkyl chain length. The result was remarkably similar to the trend of current density characteristics of single-carrier devices. The power efficiency of the octyl chain attached bipolar host was approximately three times higher than that of typical blended hole and electron transporting materials. This enhanced efficiency was attributed to the well-balanced charge transfer by the bipolar host material inside an emissive layer. Moreover, the morphology of the device fabricated with a blend of charge transport materials changed due to deterioration, whereas that of the device fabricated with bipolar host materials did not change after 12 h of operation at 9 V.

Introduction

The use of white organic light emitting diodes (WOLEDs) as a lighting source has received growing attention as the next-generation lighting [1], [2], [3], [4], [5], [6], [7]. The production cost of WOLEDs should be reduced to improve the price competitiveness against a conventional lighting device. OLED devices prepared via a conventional vacuum process show improved efficiency; however, the equipment for this vacuum process is very expensive. One of the most probable methods to reduce the production cost is utilization of a solution process. The process of forming a single organic active layer by mixing electrons and hole transporting materials combined with active materials is used in the soluble OLED process worldwide. The lifetime and efficiency of the OLED device with a single active layer are drastically reduced if the organic active layer containing various materials is phase separated by deterioration, resulting from its continuous usage [8,9], or if inhomogeneities occur due to the relative miscibility difference [10,11]. Thus, utilization of a bipolar host material both reduces the numbers of mixed material and helps maintain the device stability [8]. A bipolar host containing both hole- and electron-transporting moieties facilitates the charge balance in the emitting layer, resulting in broad charge recombination zones [12], which reduces the triplet-triplet annihilation and leads to high efficiency and decreased efficiency roll-off [13,14].

Recently, OLEDs using phosphorescence have been employed worldwide as a lighting application because of their excellent efficiency. Bipolar hosts for solution-processable phosphorescence OLEDs require large energy gaps, high charge carrier mobility for both electrons and holes, high thermal stability and high solubility. The hosts with carbazole moieties have been extensively used in phosphorescence OLEDs because of their good hole mobility and high intrinsic triplet energy (~ 3.02 eV) [15]. OLED lighting usually employs white light, which is obtained by mixing blue and yellow emitters [16] or mixing blue, green and red emitters [17]. Thus, the bipolar host material must exhibit good transport properties for various band gaps [18], [19], [20].

In this study, solution-processable bipolar host materials were synthesized, and the charge-transporting properties for the hole and electron were controlled by attaching different sizes of alkyl groups, thus changing the molecular spatial geometries. WOLEDs were fabricated through a solution process. The device characteristics demonstrated that the host materials with balanced charge-transporting properties exhibits enhanced electroluminescence properties.

Section snippets

Synthesis of 9H-thioxanthene-9-one-S,S-dioxide (1)

Hydrogen peroxide (35% aqueous solution, 8 g, 235 mmol) was added to a solution of thioxanethene-9-one (25 g, 118 mmol) in acetic acid (400 mL) at room temperature. The resulting mixture was placed under reflux for 2 h and then cooled to room temperature to yield a precipitate, which was filtered and washed with n-hexane (400 mL) to produce yellow crystal. (Yield: 93%) 1H NMR (300 MHz, CDCl3, δ): 8.35(dd, J= = 7.5 Hz. J= = 1.5 Hz, 2H), 8.19 (dd, J= = 7.5 Hz. J= = 1.5 Hz, 2H), 7.88 (td, J

Results and discussion

The bipolar hosts composed of 9,9-Bis(9-alkylcarbazole)thioxanthene-S,S-dioxide were synthesized by a solvent-less green reaction method [21]. Here, the alkyls were ethylhexyl (BH1), octyl (BH2), dodecyl (BH3), and heptadecane (BH4) and were synthesized by multi-step reactions, as shown in Fig. 1. The thioxanthene-S,S-dioxide moiety possessed good electron-transporting ability, whereas the bis-alkylcarbazole moiety has good hole-transporting ability. The thermal properties of each compound were

Conclusions

The bipolar host materials demonstrated excellent thermal stability based on the TGA and DSC measurements. All four host materials showed quite similar thermal characteristics. The HOMO level of the host materials was obtained from the oxidation potential measured using CVs, whereas the LUMO level was calculated from the band gap estimated from the band edge of the absorption spectrum. The optimized molecular geometries and spatial distributions were obtained using the Gaussian 03 program. The

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Industrial Technology Innovation Program (Grant no. 10063277, Development of pattern deposition system based on roll to roll processing under low temperature and atmospheric pressure condition for smart thin film device fabrication) funded by Ministry of Trade, Industry & Energy.

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