Chapter 5 Carrier Transport Properties of Anthracne-Based Materials
5.4 Using buckminsterfullerene (C60) as charge generation layer
5.4.1 TOF results of MADN with C60 as CGL
The sample structure of MADN is ITO/C60 (0.5
m)/MADN (5.5 m)/Al
(120 nm). The thicknesses of organic materials under testing were still set substantially larger than that of C60 so that the total transit time is larger than the time resolution of the electronic system and the transit time across the C60 layer is negligible in comparison with the total transit time.Figure 5-16(a) is the representative TOF transient signals of holes for MADN measured at an applied field of 2.12 × 105 V/cm. The double logarithmic representation of photocurrents is shown in the inset of Figure 5-16(a). The photocurrent transients for holes of MADN exhibit non-dispersive behaviors as characterized by a plateau and a well-defined turning point at t =
tr. Figure 5-16(b) depicts the dependence of the hole mobilities on the electric field of MADN. MADN exhibits a hole mobility of (2.7-3.8) × 10-3 cm2/Vs at an electric field from 6.43 × 104 V/cm to 2.61 × 105 V/cm.Figure 5-16 (a) Representative TOF transient for holes of MADN at E = 2.12 × 105 V/cm. (b) Hole mobilities vs E1/2 of MADN.
(a) (b)
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5.4.2 TOF results of TBADN with C60 as CGL
The sample structure of TBADN is ITO/C60 (0.25
m)/TBADN (3.4
m)/ITO (120 nm). Figure 5-17(a) is the representative TOF transient signals of
holes for TBADN measured at an applied field of 2 × 105 V/cm. The double logarithmic representation of photocurrents is shown in the inset of Figure 5-17(a). The photocurrent transients for holes of TBADN exhibit non-dispersive behaviors as characterized by a plateau and a well-defined turning point at t =
tr. Figure 5-17(b) shows the dependence of the hole mobilities on the electric field of TBADN. TBADN exhibits a hole mobility of (1.5-2.9) × 10-3 cm2/Vs at an electric field from 7.21 × 104 V/cm to 3.39 × 105 V/cm.Figure 5-17 (a) Representative TOF transient for holes of TBADN at E = 2 × 105 V/cm. (b) Hole mobilities vs E1/2 of TBADN.
5.4.3 Discussions
As shown in Figure 5-18, MADN and TBADN exhibit non-dispersive hole-transport behaviors, and the field dependence of hole mobilities follows the Poole-Frenkel relationship (Equation 5.3): mobility (
) is proportional to the(a) (b)
125
square root of the applied electric field (E1/2). The fitted value of the zero-field mobility (
0), and the Poole-Frenkel factor (
) of MADN and TBADN with C60 as CGL are summarized in Table 5-4. It is apparent that the CGL-TOF results almost identical to the results of typical TOF. Moreover, by incorporating C60 as the CGL, a more practical thickness of 3~5 m for MADN and TBADN were used in the TOF sample configuration, indicating C60 is useful as the general CGL for hole TOF measurements of ADN-type materials.Figure 5-18 Hole mobilities vs E1/2 of MADN and TBADN.
Table 5-4 The fitted values of zero-field hole mobility and Poole-Frenkel factor of MADN and TBADN.
sample structure
zero-field mobility (0, cm2/Vs)
Poole-Frenkel factor [, (cm/V)1/2]
MADN 1.55 × 10-3 1.85 × 10-3
C60/MADN 2.02 × 10-3 1.22 × 10-3
TBADN 1.06 × 10-3 1.81 × 10-3
C60/TBADN 9.41 × 10-4 1.98 × 10-3
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However, the hole transients of ADN are still highly dispersive due to the same crystallization problem even using this CGL technique, and no transit time and electron mobility can be extracted. On the other hand, it is difficult for the photo-generated electrons to inject into ADN-type materials from C60 due to the large energy barrier between the LUMOs of C60 and ADN-type materials (LUMOs of C60 and ADN-type materials are 3.6 eV and 2.5 eV, respectively), suggesting that C60 is not suitable to be a CGL for electron TOF measurements of ADN-type materials.
5.5 Summary
In this chapter, we measured the hole mobilities of MADN and TBADN by TOF technique, and we found that ADN-type materials have high hole mobilities in the ranges of 2 × 10-3 to 9 × 10-3 cm2/Vs, and have almost identical and small Poole-Frenkel factors. Furthermore, the various substituents on anthracene moiety would affect the conformation of molecular packing and further change the charge-carrier transport abilities of each molecule. For instance, the bulky t-butyl substitution is an effective means of tuning hole mobility in ADN-type compounds, which can increase the intermolecular distance and further suppress hole mobility. In addition, we also have shown that C60 is useful as the CGL of TOF measurements for hole mobilities of ADN-type materials. Such a scheme shall have the advantages of simplifying the instrument and reducing material consumption in the TOF mobility measurements.
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Chapter 6
Admittance Spectroscopy Measurements for Conductive-Doped Organic Layers
6.1 Introduction
As an alternative to inorganic semiconductors, organic materials have recently gained much attention [1]. Originally, much of the research has concentrated on single crystals, which can show mobilites of a few cm2/Vs at room temperature and even much higher values at low temperature, as shown in the pioneering work of Karl [2]. However, for practical applications as thin films, organic semiconductors with disordered structures, such as evaporated small-molecule materials of polymers processed from solution are prevailing. In photoconductors for copiers and laser printers, organic semiconductors are already broadly applied.
Organic semiconductors have unique physical properties, which offer many advantages to inorganic semiconductors: (1) The extremely high absorption coefficients in the visible range of some dyes offer the possibility to prepare very thin photodetectors and photovoltaic cells [3]. Due to the small thickness of the layers, the requirements on chemical and structural perfection are reduced since the excitation energy does not have to travel long ways. (2) Many fluorescent dyes emit strongly red shifted to their absorption. Thus, there are almost no reabsorption losses in OLEDs [4], which, together with the low index of refraction, circumvents the key problems of inorganic LED. (3) Since organic semiconductors consist of molecular structures with saturated electron systems, the number of intrinsic defects in disordered systems is much lower than in
131
inorganic amorphous semiconductors, where a large number of dangling bonds exist. (4) There is a nearly unlimited number of chemical compounds available, and it is possible to tailor materials.
It is worthwhile to remind that the breakthrough of the classical silicon technology came in the very moment the conduction type was no longer determined by impurities but could be controlled by doping. Unlike inorganic semiconductors, up to now, organic dyes are usually prepared in a nominally undoped form. However, controlled and stable doping is a prerequisite for the realization and the efficiency of many organic-based devices. If we succeed in shifting the Fermi level towards the transport states, this could reduce ohmic losses, ease carrier injection from contacts and increase the built-in potential of Schottky- or p-n junctions.
In this chapter, we study the electrical characteristics and doping effect of p- and n-doped organic layers by measuring current-voltage (I-V) curves and temperature-dependent AS. The fundamental theory of AS are describe in
Section 2.10. Then, we also demonstrate the application of these
conductivity-doped organic layers in OLED devices.6.2 n-type doped system composed of PAK2-doped BPhen
Since the demonstration of the first double-layer OLED device in 1987 [4], a great deal of effort has been made to improve their performance. Improving the charge injection is one important way to enhance device performance.
Efficient electron injection in OLED devices is more difficult to achieve without the use of low work-function metals. Using low work-function metals or alloys as cathode [4,5,6,7,8], remarkable improvements in device performance have been achieved. Unfortunately, the low work-function metals are readily oxidized
132
and limit the lifetime of the device. Despite its relatively high work function, an Al cathode is widely used due to its high resistance to corrosion, stability, and processibility.
Later, a number of groups have reported that electron injection can be significantly improved by inserting a thin layer of large band gap inorganic and organic salts or insulators at the organic/metal interface [7,9,10,11,12,13,14,15,16,17,18]. The improvements on the electron injection have been attributed to tunneling injection of electrons, as sufficient potential difference may be maintained between Al and Alq3 in the presence of a thin insulating layer [9,19,20,21]. In these cases, a thin LiF is the most commonly used insulator to be inserted between Al and Alq3 [10]. Another explanation for the mechanism of electron injection into the LiF/Al composite cathode is a chemical reaction between Al and LiF [22]. Another class of electron injection materials is organometallic complexes with reactive metals such as alkali metal carboxylates [23,24] and 2-(hydroxyl)quinoline lithium (Liq) [25]. However, the effectiveness of these electron injection materials is very sensitive to the choice of metal and only Al has been found to provide good device performance.
An alternative approach for efficient carrier injection is to introduce a p-i-n structure into an OLED device as described in Section 1.4. In 2004, Canon Inc.
reported that cesium carbonate (Cs2CO3) [26], which can facilitate electron injection from a wide range of metal electrodes. However, fabrication can be performed only at the high deposition temperature of Cs2CO3 (around 500 °C under a vacuum). Therefore, finding a satisfactory n-type dopant with a low deposition temperature is important in developing OLEDs with low power consumption.
133
In this section, we developed an n-doped ETL that consists of dipotassium phthalate [C6H4(COOK)2, abbreviation as PAK2 in this thesis] incorporated into BPhen, PAK2 can grow at a stable rate around 330 0C at a base vacuum of 10-6 torr, significantly facilitating the fabrication process. The electrical characteristics of this n-doped layer are also investigated by measuring current-voltage (I-V) curves and temperature-dependent AS and we also study the mechanism of the thermal evaporation process of PAK2 by quartz crystal microbalance (QCM) method and scanning auger spectroscopy.
6.2.1 Admittance measurements of PAK2-doped BPhen layer
A series of electron-only devices were also fabricated to study the electron injection and electrical characteristics of using PAK2 as n-type dopant. The structure of electron-only devices was ITO/Alq3 (60 nm)/n-doped ETL (30 nm)/Al (150 nm), in which the n-doped ETL is composed of BPhen doped PAK2, and doping concentration of PAK2 were 0%, 5%, 10% and 20%, respectively.
Figure 6-1 plots the I-V characteristics of electron-only devices and reveals that the PAK2-doped devices all greatly outperform the undoped device, indicating that doping PAK2 into BPhen promotes the injection of electron from the Al cathode. The 5% PAK2-doped device B shows the best I-V characteristics amongst in all electron-only devices, even at small applied bias, probably due to the different extent of electron injection with various PAK2 doping concentration. The electrical properties of this n-doped layer were investigated by temperature-dependent AS with an equivalent circuit model to elucidate this phenomenon.
134
Figure 6-1 I-V characteristics of PAK2-doped electron-only devices.
Figure 6-2(a) shows the capacitance-frequency (C-F) and conductance/frequency-frequency (G/F-F) spectra of 5% PAK2-doped electon-only device measured at 1.8 V and room temperature. The spectra show two capacitance drops and G/F peaks at inflexion frequencies and around 22 kHz and 0.22 MHz, suggesting the presence of two geometric resistance-capacitance (RC) time constant effects. Based on these spectra, an equivalent circuit model as shown in Figure 6-2(b) is developed, where CAlq3,
R
Alq3, and CBPhen, RBPhen represent the geometric capacitance and resistance of the Alq3 and BPhen layers, respectively, and Rs represent the series resistance which can be ascribed to parasitic effects due to lead/contact resistances [27].135
Figure 6-2 (a) G/F-F spectrum and capacitance-frequency C-F spectrum of 5%
PAK2-doped device measured at 2 V and room temperature. (b) Schematic representation of equivalent circuit model.
In these electron-only devices, RAlq3 can be treated as an open circuit because it is much larger than RBPhen and Rs, and RBPhen can be also reasonably assumed to be larger than Rs. Based on this equivalent circuit, the total equivalent capacitance is related to equation 2.11, which is given by
2
From the equivalent capacitance equations 6.1 and 6.2, it can be found that
C() equals to C
Alq3 when the frequency ( is low enough. As increases, C(
) becomes a value of series combination of CAlq3 and CBPhen. When
further increases, C(
) drops to zero due to series resistance Rs. As shown in Figure 6-2(a), the capacitance of 4.7 nF at 100 Hz is comparable to the value of 4.64 nF(a)
(b)
136 active area and Alq3 layer thickness), respectively.
As frequency increases, the carriers charging the BPhen layer cannot follow ac probing frequency and the capacitance drops at the inflexion frequency of ~2 kHz which equals to the inverse of the RC time constant of the BPhen layer by the relationship of
where the value of CBPhen is 9.29 nF calculated from the same equation as CAlq3. When frequency is increased beyond this inflexion frequency, the capacitance reaches a plateau with a value of 3.2 nF, which is also comparable to the calculated value of series combination of CAlq3 and CBPhen (CAlq3+BPhen) as shown below: small value of series resistance Rs. The C-F spectrum shows excellent agreement with the results of equivalent circuit model. It is notable that the two inflexion frequencies can be more clearly observed in conductance/frequency-frequency (G/F-F) spectra.
Furthermore, we measured the conductance/frequency-frequency (G/F-F)
137
spectra of these electron-only devices at various biases as plotted in Figure 6-3, in which the distinct G/F peaks are proportional to the dielectric loss. The loss peak can be described by the classical Debye frequency response which is given by bias-independent peak at high frequency region at 585 kHz, even at high applied bias, this peak is assigned to the resistance-capacitance (RC) time constant of parasitic series resistance as we discussed in previous paragraph. We attribute this result to the energy barrier between the work function of Al (4.2 eV) and the lowest unoccupied molecular orbital (LUMO) of BPhen (2.9 eV) in undoped device is too high to be measured by admittance spectroscopy.
On the other hand, the rest spectra of PAK2-doped devices all show two distinct G/F peaks: a bias-independent peak at high-frequency region of 0.16-0.22 MHz, which is assigned as the RC time constant of parasitic series resistance; a bias-dependent peak at low frequency region of ~1 kHz, which is associated with the RC time constant of PAK2-doped layer. Moreover, Figure 6-3 also reveals that the signal of PAK2-doped layer cannot be clearly observed at a bias of under 1 V, because a high energy barrier between Al and BPhen, limiting the AS measurements. We suggest that the energy barrier becomes negligible as the bias is increased over 1 V.
138
Figure 6-3 G/F-F spectra of PAK2-doped electron-only devices at various biases.
Furthermore, the electrical properties of PAK2-doped layer can be characterized from the temperature-dependent AS measurements. Figure 6-4 displays temperature-dependent G/F-F spectra of 5% PAK2-doped device measured at 1 V, 1.2 V, 1.6 V, and 1.8 V, respectively. It is evident that the signal of parasitic series resistance at high frequency region is temperature-independent and would not shift at different temperature. On the other hand, the signal of PAK2-doped layer at low frequency region is temperature-dependent, it would shift toward higher frequency region at high temperature.
139
Figure 6-4 Temperature-dependent G/F-F spectra of 5% PAK2-doped devices measured at various biases.
The BPhen peak evidently depends significantly on temperature and the series-resistance peaks are all independent of temperature. Furthermore, the activation energy (Ea
) can be obtained from these temperature-dependent peaks
by a simple geometric equation derived from equation 2.12.)
0
exp(
T k F E
F
B
a
(6.7)where F0 is the pre-exponential factor, Ea
is the activation energy which
represents the energy separation between the edge of the Fermi level and the LUMO level of BPhen in this model, kB is Boltzmann’s constant and T is the temperature. Therefore, the Ea can be derived from the slope of relationship between of ln(F) and 1000/T as plotted in Figure 6-5(a).140
Figure 6-5 (a) Characteristics of ln(F) vs 1000/T of 5% PAK2-doped device at various biases derived from the low-frequency peaks in Figure 6-4. (b) Relationship between Ea and applied bias of PAK2-doped devices.
Figure 6-5(b) plots the relationship between Ea and applied bias of PAK2-doped devices. The calculated Ea values of the PAK2-doped devices are around 0.5~0.6 eV which is much smaller than the Ea (half band-gap, 1.7 eV) of pristine BPhen (The Fermi level of ideally pure organic semiconductors should be close to the middle of the gap) [29]. Based on these AS results, the incorporation of PAK2 into BPhen increases the Fermi level of BPhen from deep to shallow, further reducing the interface energy barrier and increasing the efficiency of electron injection from the Al cathode. Moreover, the 5%
PAK2-doped device has the smallest Ea value, which fully agrees with the result of I-V measurement, indicating that increasing PAK2 concentration from 5% to 20% would not further improve the performance of electron injection, which might be attributable to some other effects of carrier quenching and defect generations.
(a) (b)
141
6.2.2 Elucidation of the mechanism of thermal-evaporated PAK2 layer
In previous section, we studied the electrical characteristics of BPhen doped PAK2 layer from I-V curves and AS measurements and demonstrate the incorporation of PAK2 can reduce the electron injection barrier from Al cathode.
Similar result was reported for sodium acetate (CH3COONa) used as an efficient EIL in OLED devices [24]. Furthermore, higher sodium (Na) content in overlaid Al cathodes was observed for Alq3/CH3COONa/Al than Alq3/NaF/Al by x-ray photoelectron spectroscopy of detached Al cathodes [23]. It was also found by XPS that Na metal with a small amount of CH3COONa was deposited during vapor deposition of CH3COONa [30]. However, it is still a difficult task to evaluate the possibility of alkali metals formation by thermal decomposition.
For this purpose, the QCM is a simple and extremely sensitive method to measure the negative frequency shift caused by increasing mass during deposition of alkali metal salts. By using QCM, Ganzorig and Fujihira demonstrated the thermal decomposition of the Na salts of acetate and benzoate during vacuum vapor deposition [31]; Qiu also investigated the electron injection mechanism of evaporated Cs2CO3 [32]. To estimate the possible evaporation behavior of PAK2, we use QCM method to measure the negative frequency shift caused by the vacuum thermal evaporation of PAK2. In order to reduce the possible experimental errors, two thermally stable materials (NPB and MADN) were selected for comparison, and their slopes of the linear relationship (frequency shift vs mass loss) are compared to that of PAK2.
The frequency shift of the QCM (∆f) can be converted to the mass loss load on the surface of the quartz crystal wafer using the standard Sauerbrey formula
142 change in the mass and adsorbed onto the crystal, and A (cm2) is the deposition area on the quartz crystal.
If the source material is assumed to be thermally stable during the evaporation, the mass adsorbed onto the crystal surface (∆m) would be in direct ratio the mass loss in the boat (∆M): Then, according to equation 6.9, the following equation can be obtained:
A
where K is a positive constant that is independent of the materials used. It can be seen that ∆f is in direct ratio to ∆M. On the contrary, if the source material decomposes and only a fraction (d%) is deposited, ∆m would be in direct ratio
where K is a positive constant that is independent of the materials used. It can be seen that ∆f is in direct ratio to ∆M. On the contrary, if the source material decomposes and only a fraction (d%) is deposited, ∆m would be in direct ratio