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1 January 2011 Thin-film transistor as a probe to study carrier transport in amorphous organic semiconductors
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We describe how to use the thin-film transistor (TFT) technique to quantify carrier transport of amorphous organic semiconductors relevant to organic electronic devices. We have chosen several amorphous materials, including arylamine compounds, 4,4′-N,N′-dicarbazole-biphenyl (CBP), and a phosphorescent dye molecule [Ir(ppy)3] for investigations. Generally, the field effect (FE) mobility was found to be about one order of magnitude smaller than that obtained from an independent time-of-flight (TOF) technique. For N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) and N,N′-Bis(3-methylphenyl)-N,N′-bis(phenyl)-9,9-spirobifluorene (spiro-TPD), the FE mobilities were found to be 1.7×10−5 and 1.3 ×10−5cm2/Vs, respectively. Temperature-dependent measurements were carried out to study the FE mobility. It was found that the energetic disorder increased in the neighborhood of a gate dielectric layer. This factor is one of the origins causing the discrepancy between TFT and TOF mobilities. We also examined how the hole transport of CBP is affected by Ir(ppy)3 when it is doped into CBP.



Organic electronics has grown tremendously over the past 20 years due to its broad applications in photonic devices. Examples are organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and organic photovoltaic cells.1, 2 , 3 Among these devices, phosphorescent organic light-emitting diodes (PHOLEDs) have captured considerable attention due to their extraordinarily high power efficiencies. OLEDs exceeding 100 lm/W have been reported and are seriously considered to be the next-generation ultrathin solid state lighting devices.4 Irrespective of the precise nature of these devices, charge-carrier transport remains one of the most critical issues in understanding the device operations. In fact, the ability of quantifying the carrier transport of an organic semiconductor is essential for device development and optimization.

There are abundant techniques for studying carrier transport.5 In this contribution, we demonstrate how to employ thin-film transistor (TFT) technique to study the hole transporting properties of amorphous organic semiconductors. Generally, TFT technique is not viewed as the most appropriate tool to study intrinsic carrier transport of an organic semiconductor due to the presence of a gate dielectric layer. The gate dielectric may conceivably affect charge transport in two ways. (i) The orientations of the organic molecules could be different in the neighborhood of a gate dielectric layer. Thus, charge transport could be very different from the case in which the gate dielectric layer is absent. (ii) The gate dielectric may modify the energy landscape of charges hopping in its neighborhood. In the case of an amorphous organic semiconductor, the first limitation is removed because the orientations of the molecules are random. Yet, the second limitation remains. In this contribution, we show how to delineate factor (ii) from TFT data using data obtained independently from a time-of-flight (TOF) technique. We examine the feasibility of the TFT technique to study the hole transport properties of two arylamine compounds, N-diphenyl-N,N-bis(3-methylphenyl)-(1,1-biphenyl)-4,4-diamine (TPD) and N,N-Bis(3-methylphenyl)-N,N-bis(phenyl)-9,9-spirobifluorene (spiro-TPD).6, 7 Extensions of the TFT technique to study the charge conducting properties of tris(2-phenylpyridine) iridum [Ir(ppy)3] and 4,4-N,N-dicarbazole-biphenyl (CBP), will be presented. The chemical structures of all molecules studied are shown in Fig. 1. From x-ray-diffraction and ellipsometery experiments, neat films of TPD, spiro-TPD, and CBP were known to form smooth amorphous films.8, 9 The structure of Ir(ppy)3 film is not known, but it has a molecular structure similar to another metal chelate tris(hydroxyquinoline) aluminum (Alq3), which is also known to form an amorphous film.8, 9 In phosphorescent OLEDs, CBP is generally used as a host material for Ir(ppy)3.10 When Ir(ppy)3 is doped into CBP, CBP molecules facilitate energy transfer to Ir(ppy)3. But a sound understanding of the carrier transport property in the doped system is still unknown. Thus, we have also taken the transport measurements on Ir(ppy)3 doped into CBP. Figure 2 shows the work functions and energy levels of all materials used in the experiments. Typically, we use gold (Au) as the source and drain electrodes in a p-type OTFT. The work function of Au is ∼5.1 eV. In a Ir(ppy)3-based OTFT, hole injection is effective as the highest occupied molecular orbital (HOMO) of Ir(ppy)3 is only about 5.2 eV.10 But, in the case of CBP, it has a relatively large HOMO value of ∼5.7 eV.11 To facilitate hole injection, a thin layer of molybdenum oxide (MoOx), which has a relatively high work function of between 5.6 and 6.7 eV, was inserted between the organic layer and gold (source/drain) electrode.12, 13

Fig. 1

Chemical structures of (a) TPD, (b) spiro-TPD, (c) CBP, and (d) Ir(ppy)3.


Fig. 2

Energy levels of corresponding materials.



Experimental Details

CBP was purchased from E-ray while TPD, spiro-TPD, and Ir(ppy)3 were purchased from Luminescence Technology Corporation. A heavily doped silicon (p-Si) wafer, etched with a 300 nm thin layer of SiO2, was used as the substrate. The dielectric capacitance per unit area Ci of the Si wafer substrate is 11 nFcm−2 as measured by an impedance analyzer. Before the deposition of organic film, the substrate was cleaned by ethanol and acetone in an ultrasonic bath, followed by UV-ozone treatment.14 An organic layer was deposited on the pretreated substrate by thermal evaporation at high vacuum. The coating rate and thickness of the organic film were 0.5 Å/s and 100 nm, respectively. The thickness was monitored by a quartz-crystal sensor. In the case of Ir(ppy)3-doped CBP films, the coating rates for both organics were equal to 1 Å/s. The concentrations of Ir(ppy)3 in CBP were 0, 0.5, 10, 20, 50, and 100%. Subsequently, 20 nm of MoOx capped with a 100 nm of gold (Au) were thermally deposited via a shadow mask to define the source and drain electrodes.15, 16 After the fabrication, the sample was loaded into a vacuum cryostat. The sample temperature was regulated between 237 and 377 K for the TFT measurement. From the TFT data, field-effect (FE) mobilities from both the linear and saturation regions were evaluated using the standard equations,17

[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} {\rm Linear\hspace*{3pt} region\hspace*{-2pt}:}\hspace*{24pt} I_{{\rm DS,Lin}} = (W/L)\mu _{{\rm FE, Lin}} C_i \left[ {(V_{\rm G} - V_{\rm T})V_{{\rm DS}} - (V_{{\rm DS}}^2 /2)} \right]. \end{equation}\end{document} Linear region :I DS , Lin =(W/L)μ FE , Lin Ci(VGVT)V DS (V DS 2/2).
[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} \hspace*{-84pt}{\rm Saturation\hspace*{3pt} region\hspace*{-2pt}:}\hspace*{12pt}I_{{\rm DS,Sat}} = (W/2L)\mu _{{\rm FE,Sat}} C_i (V_{\rm G} - V_{\rm T})^2. \end{equation}\end{document} Saturation region :I DS , Sat =(W/2L)μ FE , Sat Ci(VGVT)2.

In Eqs. 1, 2, W and L are, respectively, the channel width and length of the OTFT; Ci is the capacitance of the gate dielectric per unit area; VDS and VG are, respectively, the voltage applied to the drain and gate electrode; VT is the threshold voltage and μFE is the FE mobility. Under the linear region, the FE mobility μFE,Lin could be extracted from the slope of IDS versus VG plot. Similarly, at the saturation region, the FE mobility μFE,Sat could be extracted from the slope of (IDS)1/2 versus VG plot.


Results and Discussion


Transport Measurement of Arylamine Compounds

Figure 3 shows the output characteristics (IDS versus VDS) of TPD and spiro-TPD based OTFTs at 290 K. The gate voltage was varied in steps of –10 V starting from 0 to –80 V. Both devices exhibited p-type TFT characteristics with well-defined linear and saturation regions. Plotting (IDS)1/2 versus VG, the FE mobility μFE,Sat at saturation region (80–100 V) was extracted from slope of the fitting line. μFE,Sat of the TPD and spiro-TPD OTFTs were 1.7×10−5 and 1.3×10−5cm2/Vs, respectively,. Correspondingly, we can also extract VT from the plot of (IDS)1/2 versus VG. The threshold voltages are quite small for both TPD (–0.1 V) and spiro-TPD (2.2 V).

Fig. 3

Output characteristics of top contact (a) TPD and (b) spiro-TPD based OTFTs at 290 K. The gate voltages varied in steps of –10 V between 0 and –80 V.


Temperature-dependent measurements were carried out for both TPD and spiro-TPD OTFTs between 237 and 359 K. We then analyzed the data by the Gaussian disorder model (GDM).18 In GDM, each molecule in the amorphous thin film is treated as a transport site for hopping conduction. Under the external field, charge carriers transport within the sites by hopping. The carrier mobility is field- and temperature-dependent and can be described by

[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} \mu (F,T) = \mu _\infty \exp [ {- ({2\sigma /3kT})^2} ]\exp \{{{\rm C}F^{1/2} [ {({\sigma /kT})^2 - \Sigma ^2} ]} \}, \end{equation}\end{document} μ(F,T)=μexp[(2σ/3kT)2]exp{CF1/2[(σ/kT)2Σ2]},
where F, T, and k are the applied electric field, the absolute temperature, and the Boltzmann constant, respectively; μ is the high-temperature limit of mobility, and C is a constant. The energetic disorder σ can be interpreted as the width of Gaussian distribution of the density of states (DOS) of energy for the transporting sites; the positional disorder Σ describes the geometric randomness of the material. At low field (F→0), the second exponential term in Eq. 3 approaches 1, and thus the temperature-dependence mobility (zero-field mobility μ0) becomes
[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} \mu (0,T) = \mu _0 = \mu _\infty \exp [ {- \left({2\sigma /3kT} \right)^2} ]. \end{equation}\end{document} μ(0,T)=μ0=μexp[2σ/3kT2].

The zero-field mobility (μ0) in each temperature was approximately equal to the FE linear mobility (μFE,Lin) at low field. μFE,Lin can be evaluated from the expression of linear current IDS,Lin [Eq. 1]. Plotting IDS,Lin versus VGFE,Lin at low field (VDS = –10 V) was extracted from slope of the fitting line. At room temperature (290 K), the low-field μFE,Lin for TPD and spiro-TPD were 9.7×10−6 and 7.0×10−6cm2/Vs, respectively. From Eq. 4, we plotted a graph ln μFE,Lin versus (1/T)2. The energetic disorder σ and the high-temperature limit mobility μ were respectively extracted from the slope and y intercept of the fitted line as shown in Fig. 4. The extracted σ for TPD and spiro-TPD are both ∼85 meV. We have compared our results to data from an independent TOF technique.6, 7 Table 1 shows the transport parameters (μ, σ, and μ) of TPD and spiro-TPD in OTFT and TOF techniques.

Table 1

Transport parameters of TPD and spiro-TPD extracted from TFT and TOF.

MaterialTechniqueHole mobility at 290 K (cm2V−1s−1)VT (V)σ (meV)μ∞ (cm2V−1s−1)
TPDTFT1.7×10−5 (saturation) 9.7×10−6 (linear)−0.1851.7×10−3
Spiro-TPDTFT1.3×10−5 (saturation) 7.0×10−6 (linear)2.2861.3×10−3

Fig. 4

Field-effect linear mobilities (at VDS = –10 V) from TFT and zero-field mobilities (τ1/2) from TOF versus (1000/T)2 for (a) TPD and (b) spiro-TPD. The open and closed symbols are the TOF and TFT data, respectively. The energetic disorders σ and the high-temperature limit mobilities μ were respectively extracted from the slope and y intercept of the fitted lines.


Two points should be noted from Table 1: (i) For both TPD and spiro-TPD, the TFT mobilities extracted in the linear regions are lower than those from the saturation regions. The origin of the discrepancies are most likely related to constact resistances at the S and D electrodes. For the saturation regions, the contact effects should be negligible as VDS are large (80–100 V). In contrast, contacts between MoOx and organics only give rise to a voltage drop of a fraction of electron volts. Thus, for the purpose of mobility evaluation, μFE,Sat is a more reliable parameter. (ii) μFE,Sat is one order of magnitude smaller lower than TOF results The reduction in mobility in TFT may arise from scattering of charge carriers by local surface roughness at the gate dielectric. In addition, the increase of the energetic disorder σ [Eq. 3] is expected to reduce the mobility. According to Borsenberger amd Bässler, the total width of the hopping DOS is the convolution of the intrinsic DOS and extrinsic energetic fluctuations arising from, e.g., polarization in a neighboring medium.19 If the individual contributions are independent, then the square of total width of the DOS, [TeX:] \documentclass[12pt]{minimal}\begin{document}$\sigma _{{\rm total}}^2$\end{document} σ total 2 is equal to the sum of the square of the individual contribution. In our experiment, [TeX:] \documentclass[12pt]{minimal}\begin{document}$\sigma _{{\rm total}}^2$\end{document} σ total 2 is just [TeX:] \documentclass[12pt]{minimal}\begin{document}$\sigma _{{\rm TFT}}^2$\end{document} σ TFT 2 . There are two contributions to [TeX:] \documentclass[12pt]{minimal}\begin{document}$\sigma _{{\rm TFT}}^2$\end{document} σ TFT 2 : one is the intrinsic DOS [TeX:] \documentclass[12pt]{minimal}\begin{document}$\sigma _{{\rm TFT}}^2$\end{document} σ TFT 2 as obtained from TOF and the additional contribution is due to the random dipoles from the gate dielectric [TeX:] \documentclass[12pt]{minimal}\begin{document}$\sigma _{{\rm dielectric}}^2$\end{document} σ dielectric 2 . Combining together these two factors gives rise to

[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{eqnarray} \sigma _{{\rm TFT}}^2 = \sigma _{{\rm TOF}}^2 + \sigma _{{\rm dielectric}}^2. \nonumber \\[-24pt] \end{eqnarray}\end{document} σ TFT 2=σ TOF 2+σ dielectric 2.

From Eq. 5, the value of σdielectric in both TPD and spiro-TPD was ∼40 meV, which is in excellent agreement with other arylamine compounds (NPB and 2TNATA) reported previously.16 Figure 5 depicts in a schematic diagram why TFT and TOF mobilities should differ.

Fig. 5

Schematic diagram showing the surface roughness and polar insulating surface of the gate dielectric. The former effectively increases the charge hopping distance while the latter broadens the energetic disorder.



Transport Measurement of Host and Guest Materials in PHOLEDs

From Sec. 3.1, we establish that it is feasible to use TFT to measure carrier mobilities of amorphous organic materials. Because both TPDs and spiro-TPD contain the same arylamine functional group, it will be interesting to determine if TFT can be extended to other materials that contain different functional groups. To this end, we extended our study to PHOLED materials Ir(ppy)3 and CBP that contain phenylpyridine and carbazol groups, respectively. Figure 6 shows the output characteristics of a pristine Ir(ppy)3 film at room temperature (292 K). The current–voltage curves show well-defined p-type characteristics. From Eqs. 1, 2, the hole mobility of Ir(ppy)3 can be extracted.20 The results are μLin = 9.5×10−6 cm2V−1s−1 and μSat = 1.7×10−5 cm2V−1s−1. The mobility values are in the range of a typical amorphous organic hole transporter in OLED applications. For example, NPB, is a commonly used hole-transporting material for OLEDs. The FE mobility is 2.4×10−5 cm2V−1s−1 as measured by the TFT technique in the saturation region.21 Judging from the hole mobility values, Ir(ppy)3 can be viewed as a good hole transporter.

Fig. 6

Output characteristics of top contact (a) Ir(ppy)3- and (b) CBP-based OTFTs at 290 K. The gate voltages varied in steps of − 10 V between 0 and –80 V.


Figure 6 shows the output characteristics of a pristine CBP film at room temperature (292 K). In spite of a relative large HOMO level (∼5.8 eV) of CBP, well-behaved output characteristics can still be observed. The result indicates that MoOx can indeed possess a high work function to facilitate hole injections into CBP. This finding is actually consistent with a previous publication.22 Applying the same approach as Ir(ppy)3, the hole mobilities of CBP are μLin = 1.2×10−5 cm2V−1s−1 and μSat = 1.5×10−5 cm2V−1s−1 in the linear and saturation, respectively.

We have established that Ir(ppy)3 is actually a good hole transporter. Its transport mechanism was further investigated by temperature-dependent measurements. The results were analyzed by the GDM, which is outlined in Sec. 3.1. Figure 7 shows the semi-log plots of linear FE mobilities versus (1000/T)2 of Ir(ppy)3 and CBP OTFTs, respectively. The extracted σ for Ir(ppy)3 in an OTFT is 88 meV. It is comparable to the data from other amorphous organic hole transporters measured by the TFT technique, such as phenylamine compounds (e.g., NPB).15, 16 , 23 In those cases, the range of σ is within 80–90 meV. The extracted σ and μ of pristine CBP are 95 meV and 7.3×10−3 cm2V−1s−1, respectively. Figure 8 is a summary of the transport parameters of the four materials in this work.

Fig. 7

FE linear mobilities (at VDS = –10 V) from TFT versus (1000/T)2 for (a) Ir(ppy)3 and (b) CBP. The energetic disorders σ and high-temperature limit mobilities μ were respectively extracted from the slope and y intercept of the fitted lines.


Fig. 8

Transport parameters of TPD, spiro-TPD, Ir(ppy)3, and CBP extracted from TFT and TOF (CBP's TOF data from literature; see Refs. 24, 25).


Ir(ppy)3 is often a dopant in CBP rather than a host.26, 27 , 28 Therefore, it is more meaningful to study the hole transport properties in a Ir(ppy)3-doped CBP film. The inset in Fig. 9 shows the TFT output characteristics of a 20% Ir(ppy)3-doped CBP film. The output current IDS is dramatically decreased compared to a 100% Ir(ppy)3 film. The FE mobility is μSat = 1.7×10−7 cm2V−1s−1, evaluated in the saturation region. The value is two orders of magnitude smaller than the pristine Ir(ppy)3 film. Figure 9 shows the hole mobility μSat of Ir(ppy)3-doped CBP films at different doping concentrations. In the system of Ir(ppy)3-doped CBP, we realize a large HOMO difference between CBP and Ir(ppy)3 (∼0.5 eV). Thus, Ir(ppy)3 molecules can be regarded as deep traps. In principle, holes can be injected from the source electrode, through the MoOx interlayer, into both CBP and Ir(ppy)3 molecules. Injection of holes into Ir(ppy)3, however, is energetically more favorable. At a low doping level (e.g., <10%), once holes are injected, hopping among the dopant molecules is not preferable due to a longer hopping distance. Injected holes can only hop among the host molecules but may be trapped by dopant molecules. Thus, it is expected that hole mobility decreases with the doping concentration. In contrast, at a sufficient high doping concentration (e.g., >10%), holes are likely to hop among the dopant molecules rather than the host because of reduced hopping distances. Thus, it is expected that hole mobility reaches at a optimized value and gradually increases with the doping concentration. Actually, at 10% of doping concentration, the TFT signals were barely detectable at room temperature (290 K) and the mobility value was evaluated from a higher temperature (394 K).

Fig. 9

Saturation mobility μSat of CBP:Ir(ppy)3 OTFT film versus doping concentration. Inset: TFT output characteristics of a 20% Ir(ppy)3-doped CBP film.




In this contribution, we employ TFT technique to study carrier transport in amorphous organic electronic materials. Compared to other techniques (e.g., TOF, dark-injection space-charge-limited current, and JV characteristic), TFT only consumes small quantities of materials. Although we used a film thickness of 100 nm in our experiments, the film thickness, in principle, can be reduced to <10 nm because only several monolayers of molecules are needed to form a complete conducting channel. We studied hole transport in the spiro-TPD- and TPD-based TFTs. The data are comparable to an independent TOF technique. Thus, TFT can be viewed as a powerful platform for mobility determination when the material under investigation is in limited quantity. Besides arylamine compounds, we have extended our studies to the phosphorescent organic light emitter Ir(ppy)3, host material CBP, and Ir(ppy)3-doped CBP.


Support of this research by the Research Committee of Hong Kong Baptist University and the Research Grant Council of Hong Kong under Grant No. FRG2/09–10/077 and HKBU211209E are gratefully acknowledged.



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© 2011 Society of Photo-Optical Instrumentation Engineers (SPIE) 1947-7988/2011/1(1)/011011/9/$25.00
Shu-Kong So, Wing H. Choi, and Chi H. Cheung "Thin-film transistor as a probe to study carrier transport in amorphous organic semiconductors," Journal of Photonics for Energy 1(1), 011011 (1 January 2011).
Published: 1 January 2011

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