Chapter 6 Admittance Spectroscopy Measurements for Conductive-Doped
6.3 p-type doped system composed of WO 3 -doped MADN
6.3.1 Admittance measurements of WO 3 -doped MADN layer
In our experiments, a series of hole-only devices were fabricated for studying the effect of hole injection and electrical characteristics. The hole-only device structure was ITO/p-doped HTL (60 nm)/Alq3 (60 nm)/ Al (150 nm), in which the doping concentration of WO3 in MADN are 0%, 10%, 20% and 33%, respectively.
Figure 6-14 plots the I-V characteristics of the hole-only devices. It is shown that the WO3-doped hole-only devices all greatly outperform the undoped device, indicating that doping WO3 into MADN promotes the injection of hole from ITO anode. At a small applied bias, the device with 33% WO3 has the best I-V characteristics than most of the other WO3-doped devices.
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Figure 6-14 I-V characteristics of WO3-doped hole-only devices.
To understand the electrical doping phenomena, the hole-only devices are further investigated by AS as shown in Figure 6-15. As we discussed in the PAK2 case (Section 6.2.1), the low-frequency peaks (G/F peaks at 250 Hz for undoped device, and 20 kHz for 10% WO3 device) are assigned to be associated with a single RC time constant of the p-doped MADN layer and high-frequency peaks (G/F peaks at 0.34-0.58 MHz for all devices) are associated with the single RC time constant of parasitic series resistance [27]. It can also be observed in Figure 6-15, the G/F peak of p-doped MADN layer gradually shifts toward the high frequency region and finally mixes with the series resistance peak as WO3 percentage is increased from 0% to 20%. This result indicates that doping WO3 into MADN can greatly reduce the resistance of p-doped MADN layer according to the relationship of 2f = (RC)-1.
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Figure 6-15 G/F-F spectra at zero bias and room temperature of hole-only devices with various WO3 doping concentration.
Figure 6-16 plots the temperature-dependent G/F-F spectra measured at zero bias of these hole-only devices, it is found that the high-frequency peak is temperature-independent and the low-frequency peak is temperature-dependent.
Furthermore, the Ea of p-doped MADN layer can be derived from the slope of relationship between of ln(F) and 1000/T as plotted in Figure 6-17(a). In this case, Ea represents the energy separation between the edge of the HOMO level and Fermi level of MADN as shown in Figure 6-17(b).
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Figure 6-16 Temperature-dependent G/F-F spectra at zero bias of hole-only devices with various doping concentration.
Figure 6-17 (a) Characteristic of ln(F) vs 1000/T of WO3-doped devices derived from the low-frequency peaks in Figure 6-16. (b) Schematic energy diagram of ITO/p-doped MADN interface.
(a) (b)
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As shown, when the doping concentration of WO3 is increased from 0% to 10% and then to 20%, the Ea of MADN could be greatly reduced from 0.655 eV to 0.177 eV and then to 0.09 eV. However, as the doping concentration further increases to 33%, the p-doped MADN peak still cannot be resolved from the series resistance peak even at a temperature of 78 K. As a result, the Ea cannot be obtained exactly, but it could be estimated to be less than 0.09 eV. Owing to that the Ea represents the energy separation between the edge of HOMO level and Fermi level, the decrease of Ea indicates that the WO3 incorporation reduces this separation, which lowers the hole injection energy barrier between MADN and ITO, thus improves the hole injection from ITO to MADN.
The improvement can be demonstrated in Figure 6-14, where the room-temperature I-V characteristics of the hole-only devices in logarithm scale are plotted. As is shown, the current is gradually improved from the undoped device to the device doped with 33% WO3 at a small applied bias which agrees well with the results concluded from Figure 6-17(a). Because the current condition of the hole-only device at the small applied bias is mainly dominated by hole injection from ITO to HTL layer, the enhanced current condition at the small applied bias can be ascribed to the reduced width of energy barrier between MADN and ITO as well. At the high applied bias, however, the device doped with 10% WO3 outperforms the other devices, suggesting that the hole injection barrier becomes negligible at the high applied bias. Consequently under high bias, it is likely that the current conduction is dominated by the carrier transport in the bulk of hole-only device instead. In the heavy doping consideration, the WO3 could be diffused into the Alq3 layer resulting in creating trap center near the interface between MADN and Alq3 which limits the carrier
157
transport via the interface between MADN and Alq3. As a result, the device doped with 33% WO3 shows a worse current condition than that of the devices doped with 10% and 20% at high applied bias. Following this study, we conclude that the p-doped MADN layer is expected to improve hole injection and reduce ohmic loss in OLED devices.
6.3.2 Device performance of using p-doped layer composed of MADN and WO
3In our experiments, we fabricated the OLED devices with the p-doped HTL composed of MADN doped with WO3 with structure of ITO/p-type HTL (50 nm)/MADN (10 nm)/Alq3 (75 nm)/LiF (1 nm)/Al (150 nm), in which the doping concentration of WO3 are 0%, 10%, 20%, and 33%, respectively.
Figure 6-18 depicts the L-J-V characteristics of these WO3-doped devices. It is notable that higher current density and luminance can be achieved at lower applied bias by using WO3 as p-dopant. The overall EL performances of these devices are summarized in Table 6-3. Device with 10% WO3 doping achieved a power efficiency of 2.4 lm/W at 20 mA/cm2, which is 40% higher than that of undoped device. The results show excellent agreement with the conclusions obtained from the study of hole-only device and clearly demonstrate that the MADN doped with WO3 could be adopted for an efficient p-HTL in OLEDs device.
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Figure 6-18 L-J-V characteristics of the WO3-doped devices.
Table 6-3 EL performances of OLED devices at 20 mA/cm2.
In summary, we demonstrated that the MADN doped with WO3 decreases the resistance of MADN and the hole injection energy barrier between MADN and ITO resulting in improving the ohmic loss and hole injection. In Alq3 based OLEDs, using MADN doped with WO3 as p-HTL can achieve a current efficiency of 4.0 cd/A and a power efficiency of 2.4 lm/W.
6.4 n-type doped system composed of CsF-doped MADN
In this section, we further report the development of the n-doped transport layers using MADN as a host, in which cesium fluoride (CsF) is used as
n-dopant, repectively. The carrier injection property and the effect of CsF
WO3 conc.
(%)
Voltage (V)
Current Eff.
(cd/A)
Power Eff.
(lm/W)
E. Q. E.
(%) CIEx,y
0 6.6 3.5 1.7 1.4 (0.35, 0.56)
10 5.2 4.0 2.4 2.1 (0.35, 0.55)
20 5.3 3.5 2.1 3.5 (0.30, 0.65)
33 5.4 3.9 2.3 5.8 (0.30, 0.64)
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incorporation into MADN are investigated by measuring I-V curves and temperature-dependent AS. The AS results show that the incorporation of CsF into MADN can greatly reduce the resistance and activation energy of MADN layer resulting in improving electron injection. We also demonstrate the application of the n-doped organic layer of MADN doped CsF in OLED devices.
6.4.1 Admittance measurements of CsF-doped MADN layer
In our experiments, a series of electron-only devices were fabricated for studying the effect of electron injection and electrical characteristics. The electron-only device structure was ITO/Alq3 (80 nm)/n-doped ETL (10 nm)/Al (150 nm), in which the doping concentration of CsF in MADN are 0%, 10%, 20% and 30%, respectively.
Figure 6-19 plots the I-V characteristics of these electron-only devices.
Lower operational voltage and higher current density dependency were observed in CsF doped electron-only device as compared to that of undoped device, which indicate that the electron injection from Al cathode can also be improved by doping CsFinto MADN.
Figure 6-19 I-V characteristics of CsF-doped electron-only devices.
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To understand the electrical doping phenomena, the electron-only devices are further investigated by AS as shown in Figure 6-20. As we discussed in the PAK2 case (Section 6.2.1), the low-frequency peaks (G/F peaks at 200-4000 Hz) are assigned to be associated with a single RC time constant of the n-doped MADN layer and high-frequency peaks (G/F peaks at 0.13-0.15 MHz) are associated with a single RC time constant of parasitic series resistance [27]. It is also found that the high-frequency peak is temperature-independent and the low-frequency peak is temperature-dependent. Furthermore, the Ea of n-doped MADN layer can be derived from the slope of relationship between of ln(F) and 1000/T as plotted in Figure 6-21(a), when the doping percentage of CsF was increased from 10%, to 20%, and 30%, the activation energy (Ea) of MADN could be greatly reduced from 0.157 eV to 0.145 eV and then to 0.099 eV. In this case, Ea represents the energy separation between the edge of the LUMO level and Fermi level of MADN as shown in Figure 6-21(b), the decrease of Ea
indicates that the Cs2CO3 incorporation reduces this separation, which lowers the electron injection energy barrier between MADN and Al, thus improves the electron injection from Al cathode to MADN.
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Figure 6-20 Temperature-dependent G/F-F spectra at zero bias of electron-only devices with various doping concentration.
Figure 6-21 (a) Characteristic of ln(F) vs 1000/T of CsF-doped devices derived from the low-frequency peaks in Figure 6-20. (b) Schematic energy diagram of Al/n-doped MADN interface.
(a) (b)
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6.4.2 Device performances of using n-doped layer composed of CsF-doped MADN
As illustrated in Figure 6-22, three following devices are fabricated to test the efficacy of the n-doped ETL composed of MADN doped with CsF. First one is the standard device with conventional structure of ITO ITO/CFx/NPB (60 nm)/Alq3 (75 nm)/LiF (1 nm)/Al (150 nm), second and third are i-i-n devices with structures of ITO/CFx/HTL (60 nm)/Alq3 (65 nm)/MADN: 30% CsF (10 nm)/Al (150 nm), in which NPB and MADN were used as HTL, respectively.
Figure 6-22 Device architecture of standard device and i-i-n devices.
The EL performances are summarized in Table 6-4, it is apparent that i-i-n devices with MADN doped CsF layer can achieve higher efficiency and low drive voltage at 20 mA/cm2. Significantly, inducing the n-doped layer of MADN doped CsF can improve the electron injection from Al cathode, which fully agrees the results of I-V characteristic and AS measurement. These results indicate that MADN can be used as an efficient host for n-type dopant CsF and reduce the drive voltage in OLED devices. Furthermore, the performance of
163
i-i-n device with MADN as hole transport layer (HTL) can boost up to 6.1 cd/A as shown in Figure 6-23, the enhancement of device performance agrees the result of the utilization of MADN as HTL, which we will discuss in Section 7.3.
Table 6-4 EL performances of OLED devices at 20 mA/cm2.
Figure 6-23 Current efficiency vs current density characteristics of OLED devices.
6.5 Summary
In this chapter, we study the electrical characteristics and doping effect of
p-doped (WO
3-doped MADN) and n-doped (PAK2-doped BPhen and CsF-doped MADN) organic layers by measuring I-V curves and temperature-dependent AS. The results indicate that incorporations of p-typeDevice (HTL)
Voltage (V)
Current Eff.
(cd/A)
Power Eff.
(lm/W)
E. Q. E.
(%) CIEx,y
Standard 6.3 3.6 1.8 1.1 (0.36, 0.55)
i-i-n (NPB) 5.2 3.3 2.0 1.1 (0.36, 0.55)
i-i-n (MADN) 5.5 6.1 3.5 1.9 (0.38, 0.55)
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(WO3) and n-type (PAK2 and CsF) dopants can reduce the activation energy, which represents the energy separation of between the Fermi level and the HOMO level for p-type case, the LUMO level for n-type cases, respectively, and can further improve the carrier injection from the electrodes.
On the other hand, the results of PAK2 case also show that potassium metal carboxylates are also useful for n-type doping with an easy and stable evaporation process as compared to inorganic salts. However, the mechanism of thermal deposition of these kind of materials is still not well understood, even we investigated this phenomenon by QCM.
Moreover, the results of MADN cases imply that MADN can be a host for both p-type (WO3) and n-type (CsF) dopants and these p-doped and n-doped MADN layers can effectively applied to OLED devices, which is an useful information for simplifying the OLED device architecture by multifunctionally utilizing MADN, and the further experiments are described in next chapter.
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Chapter 7
Organic Light-Emitting Diodes based on one Multifunctional Bipolar Material
7.1 Introduction
Since Tang and Van Slyke developed the multi-layer OLEDs [1], tremendous efforts have been directed toward improving the device performance and operational lifetime. It was recognized that E. Q. E. of OLEDs depends heavily on the efficiency of carrier injection, transport and recombination as well as the balance of the holes and electrons [2], however, the mechanism underlying these key performance parameters are not well understood.
The basic OLED device has a bilayer organic thin-film structure such as ITO/NPB/Alq3/LiF/Al, where ITO is the anode, LiF/Al is the cathode, NPB and Alq3 are the HTL and ETL, respectively. During operation, the injected holes and electrons recombine at or near the HTL/ETL interface, producing EL.
However, this typical NPB/Alq3 structure in general does not necessarily provide the configuration to achieve a balanced carrier injection/transportation that leads to recombination. One of the reasons is that the excess holes would accumulate at HTL/ETL interface and generate NPB+ and Alq3
+ radical cations.
It has been suggested that exciton quenching at the NPB/Alq3 interface due to the accumulation of NPB+ radical cations at the interface impacts significantly on the current efficiency [3,4]. Moreover, it has also been reported that Alq3 cationic species would easily be produced when hole carriers exist excessively, resulting in deterioration of device lifetime [5,6,7].
It has been shown that much improved OLED performance can be realized
168
using a HIL/HTL structure, where HIL is the “hole-injection” layer inserted between the anode and the HTL. For example, with CuPc [8] as the HIL as in CuPc/NPB/Alq3 where Alq3 also functions as the emissive layer, long-lived OLEDs have been obtained. Another common HIL material is 4,4’,4”-tris[N-(3-methylphenyl)-N-phenylamino] triphenylamine (MTDATA) [9], with which enhanced current efficiency and operational stability have been demonstrated. High-efficiency OLEDs have also been reported in various HIL/HTL configurations [10,11,12,13,14]. Furthermore, low-voltage and high-efficiency OLEDs can be realized with a p-doped HTL [15,16,17], in which the layer thickness can be readily adjusted for optimal light extraction. It has been suggested that the enhanced performance in HIL/HTL devices is due to a sequence of cascaded hole-injection barriers present in the HIL/HTL/ETL structure, which produces a “balanced” electron-hole recombination at the HTL/ETL interface [18,19].
Consequently, finding a way of reducing the number of holes or increasing the number of electrons reaching the emission layer is considered one of the most direct and economic solutions to improve device efficiency. Recently, it was found that the hole mobility can be efficiently controlled by incorporating the composite hole transporting layer (c-HTL) of NPB: copper phthalocyanin (CuPc) (1:1) to balance the charge carriers [20] from which the device efficiency can be significantly enhanced. However, the fabrication process is complicated, which requires precise control and in particular, using the environmentally unfriendly CuPc.
In addition to the well-known property of large bandgap and high fluorescence quantum yield, anthracene-based derivatives have been shown to
169
possess bipolar character and moderate-to-high carrier mobilities. In this chapter, we summarize examples of anthracene-based compounds that have successfully been used as charge transport materials both for hole and electron transport layers, respectively. We also demonstrate the bipolar nature of MADN by studying the electrical characteristics of carrier-only devices and further simplify the OLED device structure by taking the advantage of this bipolar nature.
7.2 Review of anthracene-based materials used in HTL of OLED devices
As early in 2002, Shi and Tang at Kodak had claimed in a US patent in which anthracene-based derivatives were found to be useful as hole transport layer in producing efficient OLED devices [21]. Representative structures are shown in Figure 7-1. In the past, although arylamines have been used extensively as hole transport materials in OLED devices, they do have a number of deficiencies. First, as a class of organic materials, they are relatively strong electron donors, meaning that they can be readily oxidized and may be unstable in ambient environments. Second, when placed adjacent to an ETL as HTL in a OLED device, the arylamines may interact with the strongly accepting electron transport layer to produce non-emissive charge-transfer species which will result in a loss of EL. Third, because of the low ionization potential of the arylamines HTL and the hole-blocking ETL will cause the holes to localize in the arylamines to form the non-emissive amine radical cations which will also lead to quenching. To alleviate the aforementioned shortcomings in HTL, anthracene-based derivatives have been used recently for improving the OLED device performance.
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Figure 7-1 Chemical structures of anthracene-based HTLs.
In 2002, Tao et al. [22] synthesized a series of diaminoanthracene derivatives and used them as hole transport material in a green emitter. The structure of 9,10-bis(2-naphthylphenylamino)anthracene (-NPA) is depicted in Figure 7-1 as well. The
-NPA-based device, with structure of ITO/-NPA (40
nm)/Alq3 (50 nm)/Mg:Ag alloy (10:1, 55 nm), achieved an EL efficiency of 7.7 cd/A and 5.4 lm/W with an EL peaking at 530 nm and CIEx,y of (0.31, 0.63). In particular, the device has an extremely low turn-on voltage of 2.6 V without anyp-dopant, suggesting that diaminoanthracene derivatives are excellent for hole
transport layer with HOMO of 5.54 eV. After that, there was another patent by Yu et al. [23] claiming that the introduction of diarylamino groups on 2-/6- positions of anthracene moiety could improve hole transport property considerably and is useful as hole transport layer in OLEDs.Later, triphenylamine end-capped anthracene derivatives have also been characterized for efficient blue emitter as well as HTL [24] whose typical structure of 9-Phenyl-10-(4-triphenylamine)anthrancene (PhAA) is also shown in Fig. 7-1. These type of materials show high Tg of 104-162 °C with good
171
thermal stability, and strong blue emission of quantum yields of 0.44-0.48.
Based on these compounds, efficient deep-blue emissions have been achieved by using a simplified two-layer device architecture of ITO/PhAA (50 nm)/TPBI (30 nm)/LiF/MgAg with maximum efficiency of 3.0 cd/A (2.4 lm/W) and CIEx,y of (0.14, 0.14).
7.3 Using MADN as HTL
In our previous work, MADN has been found to be an efficient blue host material which forms stable thin-film morphology upon thermal evaporation and has a wide energy bandgap [25]. Moreover, the hole mobility of MADN has been measured to be (3-9) × 10-3 cm2/Vs by TOF technique and discussed in
Section 5.3.2, which is even higher than that of common-used hole transport
material NPB (5.1 × 10-4 cm2/Vs) [26], indicating that MADN can be used as a hole-transport material in OLED devices.In this section, we study the electrical characteristics of I-V dependence and AS of devices in which the conventional HTL, NPB, is replaced with MADN and find that it could simultaneously improve the carrier recombination in the device and significantly enhance the device efficiency and operational lifetime as well.
7.3.1 Admittance measurements of MADN/Alq
3bilayer structure
For studying the transport phenomenon and electrical characteristics, two additional hole-only devices were also fabricated. The structure of hole-only devices were ITO/CFx/MADN (30 nm)/Alq3 (60 nm)/Al (150 nm) and ITO/CFx/NPB (30 nm)/Alq3 (60 nm)/Al (150 nm), respectively. Figure 7-2 shows the I-V characteristics of the hole-only devices. Higher operational voltage and smaller current density dependency were observed in device B with172
MADN as compared to that of the device A. For instance, the turn-on voltage of hole-only devices A and B are 2.4 V and 3.0 V, respectively. We attribute the high drive voltage of MADN device to the large energy gap between MADN and ITO anode as compared to those of NPB hole-only device. (The HOMO
MADN as compared to that of the device A. For instance, the turn-on voltage of hole-only devices A and B are 2.4 V and 3.0 V, respectively. We attribute the high drive voltage of MADN device to the large energy gap between MADN and ITO anode as compared to those of NPB hole-only device. (The HOMO