Summary

In this introduction, we have provided an overview on the state-of-the art of the characteristic of LECs, which have achieved remarkable performance cover all the visible spectral region, from blue to red. However, for future application in solid-state lighting or display technique, the performance of LECs should be further improved. Thus, in this thesis, we would develop the novel high-gap materials to be used for LECs and study the effect of carrier balance in LECs.

Figure 1-1 Schematic representations of a typical multilayer OLED and a LEC based on ionic transition metal complexes.

Figure 1-2 Materials used in the first polymer-based LECs.

Figure 1-3 Materials used in the first ionic transition metal complex (iTMC) -based LECs.

Figure 1-4 The Principle of Light-Emitting Electrochemical Cells

Figure 1-5 Ionic iridium complexes A - H.

A B

C D

E, F G, H

Figure 1-6 Ionic iridium complexes I - M.

I J K

M L

Figure 1-7 The Ionic liquids of [BMIM⁺][PF₆^–], [EMIM⁺][PF₆^–] and [HMIM⁺][PF₆^–].

Chapter 2 An Ionic Terfluorene Derivative for Saturated Deep-Blue Solid State Light-Emitting Electrochemical Cells

2.1 Introduction

LECs possess several advantages over conventional OLEDs. In LECs, electrochemically doped regions induced by spatially separated ions under a bias form ohmic contacts with electrodes, resulting in balanced carrier injection and low operating voltages and, consequently, high power efficiencies.[1,59] As such, LECs generally require only a single emissive layer, which can be processed readily from solution, and, conveniently, they can feature air-stable electrodes, whereas OLEDs typically require more sophisticated multilayer structures and low-work-function cathodes.[60-61]

Recently, cationic Ir complexes have been explored widely for their use in LECs because of their high luminescence efficiencies, tunable light emission colors, and high compatibility with ionic electrolytes. By tailoring the structures of their chelating ligands, emissions from cationic Ir complexes can cover such a large color range to achieve full-color displays and white light emission.[8,22,30]

To date, however, the development of efficient saturated blue-emitting ionic Ir complexes has lagged behind those of other colors. The complexes that have

the bluish-green region. Recently, He et al. reported a blue-emitting cationic Ir complex exhibiting EL centered at 460 nm.[17] Bolink et al. found that the origin of the large spectral shift in the EL, ranging from 476 to 560 nm, of the blue-emitting cationic Ir complex [Ir(ppy-F₂)₂Me₄phen]PF₆ was related to the concentration of the ionic transition metal complex in the thin film.[63] The difficulty in effecting color-shifting toward the deep-blue region with Ir-based cationic complexes is mainly due to the intrinsically narrow energy gaps in such cationic complexes relative to those of neutral ones [e.g., Ir(dfppy)₂(pic), where dfppy is 2-(2,4-difluorophenyl)pyridine and pic is picolinic acid], thereby limiting the possibility of spectral tuning through molecular design.[64-65]

Moreover, thermal population to accessible ligand field states (a possible nonradiative decay pathway) leads to low emission efficiencies, which further restricts the development of blue-emitting cationic Ir complexes.[66] Therefore, alternative approaches for the development of saturated blue-emitting materials for the use in LECs remain in high demand to complete the emitting color gap of cationic Ir complexes.

Polyfluorenes (PFs) are used widely as efficient blue emitters in polymer light-emitting diodes (PLEDs) because they generally possess high PL quantum efficiencies and high thermal stabilities.[67-69] The rigidity of the

coplanar fluorene structure in the conjugated backbone and the flexibility of functionalization on the C9 position have crucial effects on the characteristics and processability of the resulting polymers, as well as modulating their intermolecular interactions when in the form of thin films. LECs based on PF/

PEO mixtures were reported by Yang et al. as early as 1997. Blue-green LECs incorporating an emissive layer blend comprising a PF featuring polyether-type side groups and lithium triflate has achieved an EQE of 4%.[70] In this case, however, the emission resulted mainly from aggregation of PF, shifting the emission wavelength to the green region.[71] Although much effort has been exerted to improve the compatibility of the polymer and the ionic electrolytes [e.g., introducing oligo(ethylene glycol) units at the C9 positions of the PF or directly end-capping C9-substituted alkyl chains with ionic species],[75] no LECs based on PFs have yet avoided the phenomena of green emissions generated from either aggregation or keto defects.[69]

In this study, to avoid the intrinsic tendency of aggregation that is widely observed for PF derivatives, we selected members of the terfluorene family—low-molecular-weight analogues of PFs—to realize saturated blue-emitting LECs. Terfluorene derivatives possess emission wavelengths in the deep-blue region with ultra-high luminance quantum yields (close to

unity).[76-77] They also exhibit bipolar carrier transport capability (μ_h, μ_e > 10^–4 cm² V^–1 s^–1), which is beneficial to device performance.[78-79] Herein, we report the use of a terfluorene-based ionic compound (1) to achieve saturated deep-blue EL from two LEC devices: device I and device II provided Commission Internationale de l’Eclairage (CIE)[80] coordinates (x, y) of (0.151, 0.122) and (0.159, 0.115), respectively, extremely close to the blue coordinates (0.14, 0.08) in the NTSC color gamut. We prepared the ionic terfluorene derivative 1 through simple attachment of 1-methylimidazolium moieties to the terminal positions of the alkyl substituents of the central fluorene moiety, rendering a hydrophobic terfluorene core bearing movable anions, enabling the formation of homogeneous films through spin-coating and, consequently, efficient blue-emitting LECs.