Development of tri-chromatic WOLED device

Chapter 4 Development of Fluorescence Deep Blue System and Tri-

4.6 Development of tri-chromatic WOLED device

In previous experiments, we already developed an efficient and stable fluorescent deep blue system. By taking the advantage of the efficient and saturate blue color, we fabricated tri-chromatic white OLED device composed of red, green and blue colors to improve the low NTSC ratio and CRI value of conventional di-chromatic white OLED devices, In our tri-chromatic white

OLED device, we used

4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyra n (DCJTB) doped into co-host system of Rb/Alq3 as red emission layer [44],

Alq3 doped 10-(2-benzothiazolyl)-1,1,7,7-

tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]-pyrano[6,7,8-ij]quinolizin-1 1-one (C545T) as green emission layer, in particular, the sky blue emission system (MADN doped DSA-Ph) is replaced by our new deep blue emission system developed in previous section. We used 2BpSA-BiPh as deep blue emitter and

-MADN as blue host material. The chemical structures of red,

green, and deep blue emitters are shown in Table 2-1. The deep blue device with structure of ITO/CFx/NPB (50 nm)/-MADN: 5% 2BpSA-BiPh (40 nm)/Alq3

(10 nm)/LiF (1 nm)/Al (200 nm) shows the EL efficiency of 4.6 cd/A and 2.3 lm/W with an emission peak at 456 nm and CIEx,y of (0.14, 0.15) at 6.3 V and 20 mA/cm². Figure 4-12 depicts the EL spectrum of this deep blue device.

Figure 4-12 EL spectrum of 2BpSA-BiPh doped deep blue device at 20 mA/cm².

Firstly, we fabricated a white device with structure of ITO/CFx/NPB (100 nm)/-MADN: 5% 2BpSA-BiPh (25 nm)/Alq3: 1% C545T (10 nm)/60% Alq3: 40% Rb: 1% DCJTB (5 nm)/BPhen (25 nm)/LiF (1 nm)/Al (200 nm). The sequence of three emission layers is blue, green, and red (B/G/R). The device can achieve an EL efficiency of 5.1 cd/A and 2.1 lm/W at 7.6 V and 20 mA/cm². The EL spectrum covers a wide range of visible region as shown in Figure 4-13(a), however, the blue emission intensity is much weaker than the red emission, leading to undesirable white emission with CIEx,y

coordinates of (0.44,

0.41). Furthermore, a significant EL color shift of this B/G/R device is observed with respect to various drive currents as the CIEx,y

coordinates is shifted from

(0.521, 0.410) at 2 mA/cm² to (0.384, 0.404) at 300 mA/cm² with ΔCIE_x,y=

±(0.137, 0.006) as shown in Figure 4-13(b). The red emission dominates the EL spectrum at low current density of 2 mA/cm², and the intensity of blue and green emission intenisty increase gradually with the increasing current density. We inferred the unstable EL color is due to the RZ shifts toward the blue emitting

layer under high current stress.

Figure 4-13 (a) EL spectrum of B/G/R device at different current density. (b) CIEx,y coordinates vs current density characteristics of B/G/R device.

This phenomenon can be rationalized by the injected holes are often the dominate carriers in most OLED devices, because of the easier hole injection from ITO anode and higher hole mobility of hole transport materials as compared to the electron injection from metal cathode and electron mobility of electron transport materials. Therefore, the carrier RZ is expected to be close to the red emission layer which is nearby the cathode side. In addition, it is known that Rb with LUMO/HOMO of 3.2/5.4 eV and DCJTB with LUMO/HOMO of 2.7/4.9 eV can be carrier traps for holes and electrons as illustrated in Figure 4-14, especially at low electric field [45], which easily cause the problematic white emission color change with various drive currents in white OLED device with multi-emission-layer structure.

(a) (b)

Figure 4-14 Energy diagram of B/G/R device.

In order to alleviate the unstable color issue associated with the undesirable carrier-trapping characteristic of red emission layer, we switched the sequence of three emission layers to red, blue, and green, (R/B/G) and also inserted an additional thin NPB layer (3.5 nm) between red and blue emission layers as an electron-blocking layer to enhance the carrier recombination probability in blue and green emission layer. The device structure is ITO/CFx/NPB (80 nm)/60%

Alq3: 40% Rb: 1% DCJTB (10 nm)/NPB (3.5 nm)/-MADN: 5%

2BpSA-BiPh (20 nm)/Alq3: 1% C545T (10 nm)/BPhen (25 nm)/LiF (1 nm)/Al (200 nm). Figure 4-15(a) plots the EL spectrum of this R/B/G device, it shows a more balanced white emission with of CIEx,y

of (0.36, 0.36) at 20 mA/cm

² and the relative intensity of blue light has been increased as compared with the previous device. This phenomenon exhibits the recombination zone indeed shift toward the blue emission layer by switching the sequence of emission layers and improve white CIE_x,y

coordinates. Moreover, albeit the red emission still

dominates the EL spectrum at low current density due to carrier-trapping effect, the EL color shift with respect to varying drive currents has also been improved

to ΔCIEx,y = ±(0.033, 0.097) from 2 mA/cm² to 300 mA/cm² as shown in Figure 4-15(b). However, the reduced relative intensity of yellow emission leads to a lower EL efficiency of 4.4 cd/A at 20 mA/cm² as compared to the B/G/R device.

Figure 4-15 (a) EL spectrum of R/B/G device at different current densities. (b) CIEx,y coordinates vs current density characteristics of R/B/G device.

To further enhance the power efficiency of our tri-chromatic white OLED, we introduce p-i-n structure into device architecture, in which 50% v/v WO3

doped NPB [46] and 2% Cs2CO3 doped BPhen [47] were used as the p-doped transport layer and n-doped transport layer, respectively. We also optimized the device structure by tuning the thickness of p-doped and n-doped layers, the optimized device structure is ITO/p-HTL (20 nm)/NPB (10 nm)/60% Alq3: 40%

Rb: 1% DCJTB (10 nm)/NPB (4 nm)/-MADN: 5% 2BpSA-BiPh (20 nm)/Alq3: 1% C545T (10 nm)/BPhen (25 nm)/n-ETL (20 nm)/Al (200 nm).

As shown in Figure 4-16, this p-i-n R/B/G white device shows a broad EL spectrum in visible region with three main peaks, clearly indicating the emissions of 2BpSA-BiPh, C545T, and DCJTB at 456, 476, 520, and 620 nm, respectively. Detailed EL performances of these devices measured at 20 mA/cm²

(a) (b)

are summarized in Table 4-4. The p-i-n R/B/G white device shows a much lower drive voltage and a dramatic gain in power efficiency as compared with other

tri-chromatic white devices. The p-i-n R/B/G white device can achieve 8 cd/A

and 4.5 lm/W at 5.5 V and 20 mA/cm²with a white CIEx,y of (0.34, 0.35).

Figure 4-16 EL spectra of tri-chromatic white devices at 20 mA/cm². Table 4-4 EL performances of tri-chromatic WOLED devices at 20 mA/cm².

Based on these results, we conclude that: (1) due to the smallest bandgap and carrier-trapping characteristic of red emitter, the position and thickness of red emission layer is important; (2) the efficient exciton confinement is one of the most important factors in controlling the RZ shift under various drive currents; (3) the device structure can be optimized by tuning the thickness of

Device Voltage (V)

Current Eff.

(cd/A)

Power Eff.

(lm/W)

E. Q. E.

(%)

CIEx,y

B/G/R 7.6 5.1 2.1 2.3 (0.44, 0.41)

R/B/G 5.8 4.4 2.4 2.4 (0.36, 0.36)

p-i-n R/B/G 5.6 8.0 4.5 4.3 (0.34, 0.35)

p-doped and n-doped layers without affecting J-V characteristic because of their

high conductivity.

We also tested the operational device lifetime of devices I and II under a constant current density of 20 mA/cm² in a dry box as plotted in Figure 4-17.

After driving of 200 hr, the luminance decay of R/B/G device and p-i-n R/B/G device are 15% and 22%, respectively. Assuming the scalable law of Coulombic degradation [45] for driving at L0 of 100 cd/m², the half-decay lifetime (t1/2) of R/B/G device and p-i-n R/B/G device are projected to be around 11518 hrs and 16000 hrs, respectively.

Figure 4-17 Device operational lifetime of B/G/R and p-i-n R/G/B devices.

4.7 Summary

In this chapter, we developed -MADN as an effective wide bandgap host material for doped deep blue OLED device. We found -MADN can be more efficient in Förster energy-transfer to the deep blue dopant (SA-BiPh). It can also make the injected carriers for recombination more balanced in the emitting layer due to its low-lying HOMO level, resulting in significant improvement in

EL performance giving rise to blue OLED with EL efficiencies of 3.3 cd/A and 1.3 lm/W and a deep blue CIEx,y color coordinates of (0.15, 0.13).

In addition, we also demonstrated one tri-chromatic WOLED device composed of our new deep blue system, green, and red emitters. Detailed EL performances of di- and tri-chromatic WOLED devices are summarized in Table 4-5. The lower current efficiency of tri-chromatic WOLED device is attributed to the less intensity around the emission wavelength of 555 nm, which is the most sensitive light for human eyes. However, the tri-chromatic WOLED device achieved a higher E. Q. E. of 4.3% and radiance of 0.79 W/Srm² as compared to those of conventional WOLED and p-i-n di-chromatic WOLED device (0.67 W/Srm² and 0.68 W/Srm², respectively).

Table 4-5 EL performances of WOLED devices at 20 mA/cm².

As shown in Figure 4-18, it is evident that the weak intensity at wavelength

< 450 nm and > 620 nm of conventional WOLED has been enhanced and a wider EL spectrum can be obtained by introducing deep blue system and red co-host system (MADN:SA-BiPh and Alq3:Rb:DCJTB) into the WOLED device.

Thereby, the CRI value of tri-chromatic WOLED can be enhanced to 87.

Furthermore, after attaching color filters, the color gamut of p-i-n R/B/G device

Device Voltage

can reach 73.2% of NTSC standard and CIEx,y coordinates of RGB and white points are (0.66, 0.33), (0.27, 0.62), (0.13, 0.13), and (0.36, 0.36), respectively,.

These results indicate the tri-chromatic WOLEDs with high power efficiency, high CRI, and high color gamut indeed have the potential to be used for full-color display and light source applications.

Figure 4-18 (a) The transmittance of RGB color filters and EL spectra of

di-chromatic and tri-chromatic WOLEDs. (b) Spectra of RGB colors after

attaching color filters. (c) The color gamut of di-chromatic and tri-chromatic WOLEDs.

(a) (b)

(c)

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100

Chapter 5 Carrier Transport Properties of Anthracene-Based Materials 5.1 Bipolar nature of anthracene-based materials

The charge carrier (hole and electron) transport property is a key factor in determining the performance of OLEDs [1,2]. Two widely used theories for describing the charge mobilities in organic materials are coherent band theory [3,4] and incoherent hopping model [5,6], respectively. In the former, the charges transfer through valence or conduction bands is formed by the overlapping molecular orbital with strong coupling between neighboring molecules. In contrast, the latter is a dominant mechanism at room temperature because the dynamic structure disorder invalidates the band model due to a strong coupling of the lattice phonons with charge motion. Accordingly, the hopping model is suitable for most organic materials, and the theoretical prediction of mobility is in good agreement with experimental observations [7].

Generally, organic π-conjugated materials are assumed to transport charge at room temperature via a thermally activated hopping-type mechanism. The hole and electron transfer process between adjacent spatially separated segments can be summarized as follows:

A⁺ + A → A + A⁺, A^- + A → A + A-

(5.1) where A represents the neutral species undergoing charge transfer, and the A⁺/A -species contains the hole/electron. Assuming the temperature is sufficiently high to reasonably treat vibrational modes classically, the rate (KET) of intermolecular charge hopping can be described by the Marcus theory [8] in following equation:

101 temperature, t is the electronic coupling matrix element between the two species, dictated largely by orbital overlap. Obviously,

 and t are the two most

important parameters and have a dominant impact on the charge-transfer rate.

An evaluation of t would require the relative positions of the molecules in the solid state as it is related to the energetic splitting of the frontier orbitals of the interacting molecules, and it can be obtained by two approaches: one is to resort to Koopmans’ theorem [9,10]; the other is to directly calculate the coupling matrix element of the frontier orbitals [11,12]. Due to organic materials are arranged randomly in the manner of an amorphous film in OLED devices, the range of intermolecular charge transfer in the solid state is limited. The mobility of charges has been demonstrated to be largely related to the reorganization energy (



) for OLED materials and in general, it has good agreement with the experimental observations [6,7,13,14].

The reorganization energy (



) reflects the changes in the geometry of the two molecules when going from the initial to the final state. This term originates from the fact that the geometry of a charged



-conjugated molecule differs significantly from that of the corresponding neutral molecule, owing to a marked redistribution of the



-electron bond densities. The reorganization energy for hole transport (



+) can be estimated as the sum of two terms [15], as illustrated in Figure 5-1: (1) the relaxation energy (



1) between the energy of the charged molecule in its fully relaxed cation geometry and that in the geometry characteristic of the ground state, and (2) the relaxation energy (



2) between the

102

energy of the neutral molecule in its equilibrium geometry and that in the geometry characteristic of the charged system. In general, these two contributions are nearly equal to one another. Similarly, the reorganization energy for electron transport (



-) equals the sum of the two relaxation energies



3 (neutral to anion) and



4 (anion back to neutral).

Figure 5-1 Calculation model of the reorganization energy [15].

Cornil et al. [14] calculated the reorganization energies of oligoacenes containing from 3 to 5 rings for holes and electrons and compared to the results extracted from gas-phase UV photoelectron spectroscopy spectra for holes [16]

as shown in Figure 5-2. There is a very good quantitative agreement between theory and experiment for holes; importantly, the

 values have globally the

same order of magnitude for electrons and holes. In general,



varies in the range 0.1–0.5 eV; the smaller values are obtained, as expected, when the extent of geometric deformations is minimal when going from the neutral to the charged state. This is the case, for instance, for oligoacenes [16] or phthalocyanines [17]. Table 5-1 summarizes the reorganization energies of

103

benzene, naphthalene and anthracene. As expected, as the size of the molecule increases, there is in general a decrease in the reorganization energy due to the more rigid molecular geometry (Figure 5-2 also shows the same trend). It can also be observed that the molecular reorganization energy values do not provide markedly different contributions to hole and electron transport in organic conjugated materials, indicating these materials possess bipolar charge transport nature.

Figure 5-2 Evolution of the internal reorganization energy in oligoacenes as a function of the inverse number of rings, as calculated at the density functional theory level for holes (●) and electrons (■) [14] , and estimated as twice the relaxation energy extracted from gas-phase UV photoelectron spectroscopy measurements (○) for holes [16].

104

Table 5-1 Theoretical evaluation of the reorganization energies (



, eV) for hole and electron transport for anthracene-based materials [15,18].

In 2008, Yang et al. calculated the reorganization energies of aryl-substituted anthracene derivatives (MAT and TAT) and triphenylamine-substituted anthracene derivatives [18] (MAN and TAN, values are shown in Table 5-1 and structures are shown in Figure 5-3). These reorganization energies are all two or three times higher than that of anthracene for either



+ or



-. This increasing reorganization energy indicates lower charge-carrier mobility than that of unsubstituted anthracene. We can also see that there is a large difference of reorganization energies between these anthracene-based materials, indicating the various substituents on 9-/10- positions of anthracene moiety would really affect the conformation of molecular packing and further change the charge-carrier transport abilities of each molecule. Furthermore, all these aryl-substituted and triphenylamine-substituted anthracene derivatives are found to have a more

Molecule 1 2 + 3 4 

105

similar reorganization energy for hole and electron transport, which indicates that these anthracene-based materials could be expected to possess the bipolar nature of a balanceable hole and electron transport property. Raghunath et al. did the similar calculations on the reorganization energy of isomeric ADN-type derivatives [15] (values are also summarized in Table 5-1 and structures can be found in Figure 4-1). From Table 5-1, DPA, -ADN, -ADN, and -TBADN all have similar reorganization energy for hole and electron transport property like those aryl-substituted and triphenylamine-substituted anthracene derivatives discussed above. It can be also observed that the difference between



+ and



- of

-TMADN and -TMADN are slightly larger than those of other compounds

listed in the table, suggesting that the intense steric effect of four methyl substituents to the molecular structure of ADN would strongly affect the optimized geometry either ground state or charged state, thereby, further influence the reorganization energy. The results also indicate that the charge-carrier transport property of ADN-type materials could be tuned by modification of molecular structure, even for the design of an electron transport material. On the other hand, there are some reports about ambipolar materials [19,20,21], which have been proven to have similar reorganization energies of



+ or



-. Therefore, we can conclude that most anthracene-based materials have the potential to be bipolar compound, due to its similar reorganization energies for hole and electron transport (



+ and



-) from computational results.

106

Figure 5-3 Chemical structures of aryl-substituted anthracene derivatives and triphenylamine-substituted anthracene derivatives.

5.2 Mobility measurements of anthracene-based materials in literatures

In 2006, So et al. examined the influence of various number of t-butyl substituents of ADN-type host materials on their charge-carrier transport properties by time-of-flight (TOF) measurements [22,23]. The general sample structure was [ITO/ADN compound (5~7

m)/Al (15 nm)]. The

anthracene-based compounds were: ADN, TBADN, DTBADN, and TTBADN, respectively (structures are shown in Figure 4-1). The transition time (



) of these compounds are determined from Figure 5-5(a) and all carrier mobilities of ADN compounds at 290 K are depicted in Figure 5-5(b), which all obey the Poole-Frenkel behavior [24,25],

 ( E )  

₀

exp(  E )

(5.3) where



is the Poole-Frenkel factor, and



0 is the zero field mobility.

MAT MAN

TAT TAN

107

Figure 5-4 (a) TOF transient signals of ADN, TBADN and TTBADN at 290K under applied field strengths of 0.56, 0.58, and 1.45 MV/cm, respectively [22].

(b) Field dependent electron and hole mobilities of ADN, TBADN, DTBADN and TTBADN at 290 K [23].

Interestingly, from the results, all ADN-type compounds are ambipolar and have similar Poole–Frenkel slope



(similar results were obtained in the cases of MADN and

,-MADN [26]). Their hole and electron mobilities as reported

have values in the range (1–5) × 10^-7 cm²/Vs at E = (5–8) × 10⁵V/cm.

Furthermore, a systematic reduction in both the hole and electron mobilities can be observed as the number of t-butyl group increases. From the frontier orbitals of ADN and TTBADN as shown in Figure 5-6, it depicts that the HOMO and LUMO in both ADN compounds are localized on the anthracene moiety and do not involve the naphthyl and t-butyl groups. In other words, the naphthyl and

t-butyl groups effectively act mostly as inert spacers for charge-carrier transport.

As holes hop among HOMOs while electrons hop in LUMOs in adjacent molecules, it is expected that carrier transports only occur on the anthracene moieties and have similar spatial extents in all cases. As a result, all these

(a) (b)

108

ADN-type compounds behave similarly and have almost identical PF slope.

Furthermore, the presence of the two or four t-butyl group, increases the average intermolecular separation over which charge hopping from one site (anthracene moeity) to another localized on another molecule would be more difficult.

Therefore, electron and hole mobilities are effectively reduced with bulky substituents. Similar observations have also been reports previously in hole transport of rubrene and tetra (t-butyl) rubrene [27].

Figure 5-5 The frontier orbitals of (a) ADN and (b) TTBADN.

In 2006, Wu et al. also reported ADN’s mobility data by using terfluorene [structure can be found in Figure 5-6(a)] as the charge-generation material (CGL) for TOF measurement [28]. The large bandgap terfluorene material was found to possess mobility over 10^-3 cm²/Vs for both carriers. Its large absorption

在文檔中以蒽為主體之非晶態材料雙極性載子傳輸特性與摻雜電性研究及其在有機電激發光元件之應用 (頁 112-0)