Chapter 6 Admittance Spectroscopy Measurements for Conductive-Doped
6.2 n-type doped system composed of PAK2-doped BPhen
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.
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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)
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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.
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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.
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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)
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