Abnormal Temperature-Dependent Electron Transport in Hole

Article pubs.acs.org/JPCC

Abnormal Temperature-Dependent Electron Transport in Hole Transport Material N,N′‑Diphenyl‑N,N′‑bis(3- methylphenyl)-[1,1′biphenyl]-4,4′-diamine (TPD) Yun He, Yongmao Hu, Xiaoqing Chen, Huan Peng, Yintao You, Qi Zeng, Xindong Gao, and Xiaoyuan Hou* Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education) and State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China ABSTRACT: Charge transport is important both in the research and application of organic semiconductors. For hole transport materials, most works focus on the transport characteristics of holes while less attention is paid to that of electrons. In this study, electron currents in N,N′-diphenylN,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD) thin films were studied. It is found that temperature dependence of electron conduction of TPD varies with driving voltage: in low voltage range, the electron conduction increases with temperature, while in the high voltage range it decreases abnormally with increasing temperature. Neither trapped-charge-limited current with exponential distribution of traps nor trap free space-charge-limited current with electric-field-dependent mobility could be used to interpret all the observed phenomena. The inability of current models to explain the phenomena indicates that the electron transport mechanism of TPD is different from that of holes.

1. INTRODUCTION Organic optoelectronic devices such as organic light-emitting diodes (OLEDs) and organic solar cells have been extensively studied in past decades. In spite of achievements on device fabricating techniques, many works focused on the physical processes of the devices. One important aspect is charge conduction in organic materials and devices, usually investigated by the current−voltage (I−V) characteristics. Burrows et al. reported temperature- and thickness-dependent I−V characteristics of ITO/TPD/Alq/Mg:Ag devices.1 They observed a power law dependence of current I∝Vm+1 with m > 1 and concluded that trapped-charge-limited current (TCLC) with an exponential distribution of traps having a characteristic energy Et = 0.15 eV applied to the observation. This temperature-dependent TCLC was also reported by Stößel and Kumar et al.2,3 More interestingly, Brütting et al. found that the temperature-dependent current could be fitted either by TCLC with an exponential trap distribution and a fieldindependent mobility or trap free space-charge-limited current (TFSCLC) with a hopping type mobility of charge carriers in Al/Alq/Ca devices.4 These results showed that the conduction increased with temperature, and the experimental results could be well fitted by either TCLC or TFSCLC model. Some other groups reported similar results.5−7 However, Gao et al. found an abnormal phenomenon in that the electron conduction in hole transport (HT) material N,N′-bis(inaphthyl)-N,N′diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) decreased with increasing temperature, opposite to that of the holes.5 Whether the fact that electrons and holes behave differently for the conduction is an unique property of NPB only or is a general phenomenon for most organic small molecule HT materials has © 2013 American Chemical Society

not been studied so far, and it is worthy of further investigation. Moreover, in practical organic optoelectronic devices such as solar cells and light-emitting diodes,5,8 HT material does not serve as a transport layer for holes only; the electrons will also leak through. Hence, a better understanding of electron conduction behavior in HT materials will be helpful for carrier transport control and device performance improvement. In the present work, electron conduction characteristics of hole transport material N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′- diamine (TPD) is systemically studied. It is found that the temperature dependence of the electron conduction varies with the driving voltage. The electron conduction increases monotonically with temperature in the low voltage region, while in the high voltage region it shows an abnormal behavior: the electron conduction decreases nonmonotonically with increasing temperature. Both TCLC and TFSCLC models are used to interpret such abnormal temperature-dependent behaviors of electron conduction. However, neither of them is applicable to the whole range of j−V characteristics.

2. EXPERIMENTAL METHODS Indium tin oxide (ITO)-coated glass substrates with sheet resistance of 20 Ω per square were cleaned by successive ultrasonic cleaning in detergent and deionized water followed by final UV-ozone treatment. The commercial material TPD Received: February 24, 2013 Revised: April 16, 2013 Published: April 17, 2013 9143

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value, which decreases slightly with increasing temperature, the current density turns to increase rapidly with the driving voltage. For instance, the current density at 256 K increases from 7.1 × 10−9 mA/cm2 to 2.0 × 10−7 mA/cm2 in the range of 0−7 V, and changes five orders from 2.0 × 10−7 mA/cm2 to 2.3 × 10−2 mA/cm2 in the range of 7−35 V. On the other hand, each j−V curve below room temperature intersects with that at 296 K, and the intersections are in the voltage range of 27−30 V. Before intersections, in the voltage range of 0.5−7 V, the relationship of current and voltage agrees closely to Ohm’s law, and the corresponding resistance is about 1011 Ω; further investigation shows it is related to the finite insulation of the measurement system, and this part will be ignored in the following discussion. In the range of 10−27 V, current density increases with temperature, e.g., it increases from 1.4 × 10−6 mA/cm2 to 2.3 × 10−5 mA/cm2 at 14 V as temperature increases from 79 to 296 K. The variation of current density for different temperatures decreases with increasing voltage in this range and approaches zero in the range of 27−30 V, where the j−V curves intersect. After intersections, in the high voltage range of 30−35 V, the current density turns to change with temperature non-monotonically. With temperature increasing, it increases first in the range of 79−136 K and then decreases in the range of 136−296 K, as shown in Figure 1b more clearly with linear scales. The inset of Figure 1b shows the current variation with temperature at 23 and 35 V. At driving voltage of 23 V, the current density increases monotonically with temperature from 1.8 × 10−4 mA/cm2 to 8.1 × 10−4 mA/ cm2. This current variation is well-known and similar to that of most organic materials.1−4 However, the current variation with temperature at 35 V is quite different: it first increases from 0.038 mA/cm2 to 0.056 mA/cm2 as temperature increases from 79 to 136 K. Thereafter it continuously decreases to 0.018 mA/ cm2 as temperature increases to 296 K. This kind of temperature-dependent conduction in the high voltage range of 30−35 V is very abnormally different from previous observations that the conduction in most organic materials increases with temperature.1−4 Also, the conduction observed in this work is different from NPB, in which electron conduction decreases with increase of temperature monotonically.5 The j−V characteristics of the devices with other thickness of TPD (250 and 300 nm) were also investigated, and they showed the same voltage- and temperature-dependent behavior as described above. Figure 2 shows only the j−V of device with TPD thickness of 250 nm for simplicity. It is possible that the j−V characteristics may represent a progressive degradation of the sample during the experiments. To exclude this, the direction of the temperature and the voltage sweeps were reversed, and the temperature-dependent j−V characteristics did not change. According to a previous report, the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) level of TPD are 2.3 and 5.4 eV, respectively.9 The work function of Li is 2.8 eV.10 For devices in the present work, the energy difference between the work function of Li and the HOMO level of TPD is as high as 2.6 eV (holes can hardly be injected into TPD from Li), while the energy difference between the work function of Li and the LUMO level of TPD is only 0.5 eV. In such a case, the currents in devices are carried by electrons rather than holes. The electron current in the devices could be injection-limited current (ILC) or space-charge-limited current (SCLC). For the

with a purity of 99% was used without further purification. All the materials (TPD, lithium, aluminum) were deposited in one vacuum chamber at basic pressure of 1.0 × 10−5 Pa and the deposition rates were detected by a quartz-oscillator thickness monitor. After all the depositions finished, the devices with the structure ITO/Li/TPD/Li/Al were transferred first to a glovebox (O2 < 1 ppm and H2O < 1 ppm) from the deposition chamber, and then into a liquid nitrogen Dewar bottle. The whole transfer process was carried out without exposure to air. The device area was 16 mm2, and the current density−voltage (j−V) characteristics were recorded with a programmable voltage−current source (Keithley 236). During the measurement of j−V, the ITO side was positively biased with respect to the Al side. The device temperature could be continually adjusted from 78 to 300 K in the Dewar bottle. All the j−V characteristics given as figures in the present work were recorded through increasing voltage in a process of increasing temperature.

3. RESULTS AND DISCUSSION Figure 1a shows the j−V characteristics of the device with TPD thickness of 200 nm at different temperatures. For all the temperatures given, the current density increases slowly with voltage in low voltage range. When voltage exceeds a critical

Figure 1. j−V characteristics of the device with TPD thickness of 200 nm at different temperatures (a) double logarithmically scaled and (b) double linearly scaled. The inset shows the variation of current density versus temperature at 23 and 35 V. 9144

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Figure 3. j−V characteristics at different temperatures. Inset: temperature dependence of m below and above 25 V, obtained by fitting j−V characteristics to the form j∝Vm+1. Figure 2. j−V characteristics of the device with TPD thickness of 250 nm at different temperatures. Inset shows the current density versus electric field for three different thicknesses of TPD200 nm, 250 nm, and 300 nmat 296 K.

TCLC with exponential trap distribution, which is not shown here for simplicity. The derived characteristic energy of traps for holes is about 70 meV. In addition, current densities of electrons and holes are different by several orders of magnitude under equal electric fields, e.g., electron and hole current densities are 1.9 × 10−5 and 5 mA/cm2, respectively, under an electric field of 6.75 × 105 V/cm at 296 K. In the high-voltage range above 25 V, the dependence between m and 1/T is apparently not linear, as shown in the inset of Figure 3, which is not consistent with the TCLC model. Moreover, conduction decreases with increasing temperature are even contradicted with the characteristics of TCLC, as the higher the temperature, the more carriers would contribute to the current rather than being trapped. Besides TCLC, another widely used model to interpret j−V characteristics of organic materials is TFSCLC, with electric field-dependent mobility described in terms of hopping processes following a modified Poole-Frenkel law as6,12−14

former, the current under equal electric field should not vary with the thickness of TPD. The inset of Figure 2 shows the current density versus electric field of the devices with different thickness of TPD. The current densities under equal field are not the same, which means that the current is not injectionlimited. Moreover, the physical mechanism of ILC is mainly tunneling or thermal emission, in the case of the tunneling model the current is temperature independent, and for the thermal emission model the current should increase with temperature. Obviously, the current characteristic in the present work could not be explained by either of these two models. Many previous studies showed that j−V for most organic materials obeys the power law dependence of j∝Vm+1 (m > 1),1−4,7,11 where m is proportional to 1/T, which is in accordance with the characteristics of TCLC with the exponential distribution of traps. Nevertheless, most of the studies focused on properties of dominating carriers, i.e., holes in HT materials or electrons in electron transport materials. In the present work, it is found that the j−V characteristics of electrons in TPD also show power law voltage dependence. In the range of 7−35 V, the characteristics of j−V are very different from those reported previously.1−3 As shown in Figure 3 with a double logarithmic representation, the value of m of the curve at 296 K remains unchanged in the entire voltage range; however, for every other temperature, the values of m below and above about 25 V are different. The values of m at different temperatures are shown in the inset of Figure 3. The relationship between m and 1/T is approximately linear only in the low voltage range below 25 V, and the slope of the fitted curve (dashed line in the inset of Figure 3) is 424. The characteristics of j−V and the relation between m and 1/T below 25 V agree with the model of TCLC with exponential trap distribution.11 It is well known that for TCLC, if trap distribution is exponential, the j−V characteristics should show a power law of the form j∝Vm+1 with m = Et/kT, where Et is the characteristic energy of traps. According to the TCLC model, characteristic energy of traps for electrons in TPD is k·424 = 37 meV. In order to compare it with the characteristic energy of traps for holes in TPD, hole-only devices were also fabricated, and it is found that the j−V characteristics can be well fitted by

⎡ ⎛ 1 1 ⎞⎤ μ(E , T ) = μ00 exp⎢ −(Δ − βPF E )⎜ − ⎟⎥ ⎢⎣ kT0 ⎠⎥⎦ ⎝ kT

(1)

where μ00 is constant, Δ is the activation energy at zero electric field, T is the temperature, T0 is an empirical parameter, k is the Boltzmann constant, βPF is the Poole−Frenkel factor, and E is the electric field. Equation 1 was originally proposed by Gill as an empirical expression, with its validity confirmed for many organic materials.12,15−17 In the TFSCLC case, with Poole− Frenkel-type mobility in a more general expression, μ = μ0 exp(β√E), the j−V relationship is18 j=

9 V2 εε0μ0 3 exp(0.89β V /d ) 8 d

(2)

where μ0 = μ00 exp[−Δ(1/kT − 1/kT0)], β = βPF (1/kT − 1/ kT0), and V/d = E. The experimental results mentioned above could be fitted with the TFSCLC model based on field-dependent mobility. Figure 4 shows the fitted results of j−V curves at different temperatures with eq 2. In high voltage range of 20−35 V (see inset of Figure 4), the fitted curves agree well with the experimental data, and the fitted curves also intersect each other. Figure 5 shows parameters μ0 and β versus 1/T with μ0 in logarithmic scale obtained by fitting j−V to eq 2. According to the TFSCLC model and eq 1, the dependence lg(μ0) ∼ 1/T and β ∼ 1/T should both be linear for the whole temperature 9145

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the characteristic of temperature assistance hopping transport in disordered materials, and this behavior has been reported by many studies.14−17,19,20 However, at high electric field, the electron mobility in disordered material TPD abnormally shows the band-like transport behavior that is common in single crystals; this means that the temperature hinders the electron transport in TPD and cannot be explained by the transport mechanism applied for disordered materials. Organic semiconductor materials usually present large energy barriers against the injection of one type carriers, thus they usually play the role of HT or electron transport in working devices where structures are engineered to produce predominantly single carrier transport. In this sense, organic semiconductor materials are usually labeled as HT materials or electron transport materials, and many studies usually focused on the transport properties of the dominating carriers in a given material and paid less attention to the carriers of opposite polarity. In the present work, it is found that the temperature dependence of the conduction of electrons in the HT material TPD is quite different from that of holes, and cannot be interpreted satisfactorily and completely by the TFSCLC and TCLC models only, although each model applies to the conduction characteristics of carriers well for most organic materials.2,4,7,12 The inability of current models to explain the phenomena indicates that the electron transport mechanism of TPD is different from that of holes. The abnormal temperature dependence of conduction of electrons has also been found in another HT material, NPB.5 These might imply that the difference in temperature dependence of conduction of electrons and holes in the same material could be a general phenomenon for organic small-molecule HT materials. Whether the abnormal temperature dependence of electron conduction is unique to TPD and NPB or a general phenomenon for most organic small-molecule HT materials, as well as the mechanism beneath these apparently abnormal phenomena, needs to be further studied.

Figure 4. Measured (symbols) and fitted (solid lines) j−V characteristics at different temperatures. Inset shows the enlarged j− V characteristics in the range of 22.5−35 V.

Figure 5. Temperature dependence of parameters μ0 and β obtained by fitting j−V characteristics to eq 2.

range of 79−296 K. However, they are apparently not linear, as shown in Figure 5, indicating that the model does not apply to explain the full properties of electron conduction in TPD. The corresponding field-dependent electron mobility μ = μ0 exp (β√E) at different temperatures is also obtained and shown in Figure 6. In the low electric field region (1.2 MV/cm) it turns to decrease non monotonically with increasing temperature. The temperature dependence of mobility at low electric filed is just

4. CONCLUSIONS In conclusion, temperature dependence of electron conduction in TPD is voltage dependent. In the ranges of low and high voltages, opposite temperature dependences are observed respectively. Models TCLC with exponential trap distribution and TFSCLC with field-dependent mobility are used to try to interpret the experiment data, but neither one can explain the experimental results for the whole voltage range.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86 21 65103617. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China and the National Natural Science Foundation of China (NSFC).

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REFERENCES

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Figure 6. Field-dependent mobility obtained by fitting j−V characteristics to eq 2. Inset shows the mobility versus electric field at low electric field with mobility in logarithmical scale. 9146

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