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Estimation of Charge-Injection Barriers at the Metal/Pentacene Interface Through Accumulated Charge Measurement Tomofumi Kadoya, Masato Otsuka, Akinari Ogino, Seiichi Sato, Tokuji Yokomatsu, Kazusuke Maenaka, Jun-ichi Yamada, and Hiroyuki Tajima J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12215 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

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The Journal of Physical Chemistry

Estimation of Charge-Injection Barriers at the Metal/Pentacene Interface Through Accumulated Charge Measurement Tomofumi Kadoya,†, Masato Otsuka,† Akinari Ogino,† Seiichi Sato,† Tokuji Yokomatsu,§ Kazusuke Maenaka,§ Jun-ichi Yamada,† and Hiroyuki Tajima†, †Graduate

School of Material Science, University of Hyogo, 3-2-1 Koto, Kamigori-cho,

Ako-gun, Hyogo 678-1297, Japan §

Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo,

671-2280 Japan

ABSTRACT The charge-injection barrier from metal electrodes to thin-film pentacene is investigated using accumulated charge measurements. When a gold electrode is deposited on a pentacene film, the interface forms a Schottky contact with a hole-injection barrier of 0.2 eV. However, interfacial carrier motion is reversible between charge injection and discharge. The result suggests that the reported electrical hysteresis in typical pentacene transistors is caused by carrier traps that are localized primarily in the SiO2/pentacene interface. The Ag/pentacene junction has a large barrier height of 0.5 eV. The barrier height is significantly reduced and an Ohmic contact is realized by using molybdenum oxide (MoO3) as a buffer layer.

1 ACS Paragon Plus Environment


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1. INTRODUCTION Charge injection from a metal electrode to an organic semiconductor is still a central issue in organic electronics.1 In organic devices, a large charge-injection barrier gives rise to large contact resistance, which reduces the device performance.2 It has been reported that several remedies, such as insertion of a buffer layer37 and chemical doping,811 can improve the charge-injection efficiency. The energy levels of organic semiconductors and metals have been investigated mainly using spectroscopic techniques such as photoemission yield spectroscopy, inverse photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy (UPS).1220 These methods provide deep insights into the electric states at the interface between the metal electrode and organic semiconductor. Since interfacial dipoles arise at the interface, an additional interfacial potential (vacuumlevel shift VL) appears, usually in an unfavorable direction, to increase the Schottky barrier.13,14 In this context, assuming the Schottky-Mott rule, the barrier height is generally estimated to be IE  (Wm  VL) for p-type semiconductors and Wm  VL  EA for n-type semiconductors, where IE is the ionization energy, EA is the electron affinity, and Wm is the work function of the metal electrode. Optical measurement is a general method of investigating the metal/organic interface. However, optical methods have severe limitations in terms of sample thickness and substrate materials. Therefore, optical methods usually cannot be used to study the sample directly with the actual device structure. Recently, we reported a new method to determine the charge-injection barrier, namely accumulated charge measurement (ACM).21 This technique is based on displacement current measurement (DCM). DCM methods have several advantages over conventional spectroscopic methods. DCM is capable of performing measurement of the actual device


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structure. It is therefore noteworthy that we can obtain practical information about the interface, including all factors such as the interfacial dipole, carrier traps, and impurity effects. In addition, DCM makes it possible to observe the carrier motion during charge injection, discharge, accumulation, and trapping. This technique has widely been applied to organic thin-film devices, not only in two-terminal capacitor with sandwiched structures22-27 but also in three-terminal transistor structures.28-33 In two-terminal sandwiched DCM techniques, displacement current is analyzed as a function of the applied voltage V. This standard DCM cannot determine the barrier-height voltage VB directly from V, because V includes the voltage drop at the insulator layer and an internal voltage at the organic-semiconductor layer. In our previous study, we have reported the analyzing method that VB is directly estimated from V.21 Thus, the estimation of injection barrier is enabled by using the simple electrical measurements. This is the most remarkable point in ACM. Using ACM, we can obtain precise information of the accumulated charge at the metal/organic interface. Pentacene is one of the representative p-channel materials. A gold (Au) electrode is commonly used for pentacene-based organic devices. When pentacene is deposited on the Au electrode, charge transfer occurs from Au to pentacene. After charge transfer, only a few layers of pentacene have a positive charge, and the interfacial dipole produces an injection barrier that is larger than the native energy-level difference. It has also been reported that the strength of the interfacial dipole is somewhat decreased in the form of the Au on pentacene structure.34 This is one of the reasons for the superior performance of top-contact organic devices compared to bottom-contact ones, in particular organic transistors.35 Thin films of pentacene have also been investigated by DCM.28-33,36,37 Frisbie et al have reported that the barrier height at the metal/pentacene interface from



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Arrhenius

plots,

where

nonsymmetric

device

structure

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consisting

of

metal/pentacene/indium-tin-oxide (ITO) is used.38 This measurement is based on the assumption that the pentacene/ITO interface forms an ideal Ohmic contact. In fact, the formation of high quality Ohmic contact is difficult in organic thin-film devices, even deep-work-function metals are used. Therefore, the charge-injection barrier in the actual pentacene device is still unclear. In the present paper, we report the barrier height from several electrodes to pentacene and investigate carrier motion at the metal/pentacene interface.

2. EXPERIMENTAL SECTION 2.1 Principle of ACM (modified DCM) Figure 1 shows a schematic of ACM. Because we have already reported a detailed description of the method,21 we only provide a brief outline here. The sample can be regarded as two capacitors (based on silicon dioxide and pentacene), connected in series (Figure 1a). The series capacitance is given by C = CSiO2Cp/(CSiO2 + Cp), where CSiO2 and Cp are the capacitances of silicon dioxide and pentacene, respectively. When the applied voltage V is zero, the vacuum potential is flat and no charge carriers are accumulated (Figure 1b). When finite V is applied, charge accumulation occurs. Qacc is the total accumulated charge including all the charges that appear in the SiO2 layer, at the SiO2/pentacene boundary, and at the pentacene/electrode interface (Figure 1c). After V is increased above the injection barrier, charge carriers are injected and accumulated in the pentacene film. Q represents the degree of charge injection in pentacene (Figure 1d). For a precise estimate of Qacc, four kinds of voltage sweep are used, as described in Figure 2a. PZ is an abbreviation of positive to zero and NZ is negative to zero. In the PZ (NZ)


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mode, the voltage sweep starts at a positive (negative) offset to allow charge to accumulate in the pentacene. Then, the voltage decreases (increases) to zero (region 1), and then increases (decreases) to maximum V again (region 2). The voltage sweep from zero to V corresponds to charge injection. The voltage sweep from V to zero is discharge. This voltage sweep is repeated twice (in region 3 and 4). In these voltage sweeps, the conversion of charge injection and discharge is immediately performed so that we can observe carrier motion that is mostly derived from the metal/semiconductor interface. In the same way, the ZP (ZN) mode starts from a zero offset. The positive voltage sides (PZ and ZP modes) correspond to hole accumulation. The negative voltage sides (NZ and ZN modes) correspond to electron accumulation. Figure 2b shows the typical time dependence of the displacement current for the Ag/SiO2 capacitor at V = 0.5 V. The displacement current is normalized by the electrode-surface area. The waveform corresponds to the ZP voltage sweep. The displacement current is step-like, as expected for an ideal capacitor. According to the data, the capacitance density of the insulating SiO2 layer, CSiO2, is estimated to be 0.30 nF mm2. Qacc for holes is defined as the accumulated charge at t = 0 in the PZ mode (Figure 2a). Similarly, Qacc for electrons corresponds to the charge at t = 0 in the NZ mode. However, estimation of Qacc at t = 0 is difficult. In the PZ and NZ modes, the voltage sweeps start from a constant value V, which occurs over a long time to allow charge to accumulate. Then, discharge begins. In the typical I-V curve using DCM, a rapid increase is observed in the displacement current for charge injection, whereas discharge current gradually decreases.29-31,36,37 In charge injection, the transient process of charge-sheet formation occurs and the injected charge is immediately captured in trapping sites. On the other hand, charge-sheet annihilation during discharge is comparatively slow because of the



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slow release of trapped carriers.37 To precisely estimate Qacc at t = 0 in the PZ and NZ modes, data regarding the ZP and ZN modes are necessary. Note that the integration of the displacement current occurs over a number of repetitions, for which charge injection and discharge is periodically performed. As described in Figure 2a, the repetition is performed basically twice. In this context, we have reported the following equations21: Qacc (hole) = Q (PZ(1)) + Q (PZ(2)) + Q (PZ(3))  [Q (ZP(1)) + Q (ZP(2)) + Q (ZP(3)) + Q (ZP(4))] and Qacc (electron) = Q (NZ(1)) + Q (NZ(2)) + Q (NZ(3))  [Q (ZN(1)) + Q (ZN(2)) + Q (ZN(3)) + Q (ZN(4))] where, Q (1, 2, 3, and 4) is the integration of the displacement current in each region. Q (1, 2, 3, and 4) is defined as 𝑄(m) =

∫ 𝐼𝑑𝑖𝑠 (𝑡)d𝑡, (𝑚 = 1,2,3,4; 𝑋 = 𝑃𝑍, 𝑁𝑍, 𝑍𝑃, 𝑍𝑁) 𝑋(𝑚)

where Idis(t) is the time-dependent displacement current. Using these equations, precise estimation of Qacc is possible. Using Qacc, the voltage drop across the SiO2 layer is estimated by use of VSiO2 = Qacc/CSiO2. Subtracting the voltage drop at the silicon dioxide VSiO2 from V, we can estimate the internal voltage drop Vi at the pentacene layer. In a similar manner, Q is evaluated by subtracting CV (C is the series capacitance: C = CSiO2Cp/(CSiO2 + Cp)) from Qacc. Then, VB is determined by plotting Q as a function of Vi. These equations are correct at all times, even when charge injection is incomplete and the charges are trapped in the pentacene film.


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2.2 Experiment After being washed, highly doped n-type silicon wafers with thermally grown silicon dioxide layers of 100 nm in thickness were treated with hexamethyldisilazane. Pentacene was purchased from Tokyo Kasei and was used without further purification. Pentacene was vacuum evaporated to form an approximately 100-nm-thick film under 103 Pa of pressure. Molybdenum oxide (MoO3) as the hole-transport layer was similarly evaporated at 10 nm in thickness. Finally, the metal electrode was evaporated to approximately 50 nm in thickness using a mask on the pentacene. The surface area of the top electrode was smaller than the combined area of pentacene and the backside electrode. The spread of charge carriers in the pentacene film was possible.36,37 The spread effect is discussed based on the Q vs. V plots, which are presented in the section 3.1. The resulting substrates were set in the sample holder. Gold wires (10 m,  diameter) were attached to the metal electrode and the doped-Si using silver paste. The displacement current was measured using a digital oscilloscope, a function generator, and a homemade current amplifier. All measurements were performed at 0.1-V intervals, at room temperature in a N2 atmosphere. The voltage scan rate was 1 V ms1 for all measurements. All measurements were repeated 16 times and the signal was averaged to improve the signal-to-noize ratio. The current was measured at a low frequency (0.5 to 1 Hz) to ensure that there was sufficient time for charge injection.

3. RESULTS AND DISCUSSION 3.1 The Au/pentacene interface Figure 3 shows the time dependence of the displacement current at the Au/pentacene junction. When V =  0.5 V, charge injection is insufficient and the deviation is small



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(Figure 3a). On the other hand, the displacement current at V = 5 V exhibits a significant deviation in the PZ and ZP modes (Figure 3b). In contrast, the curves of the ZN and NZ modes are completely step-like. This phenomenon indicates that holes have already been injected into the pentacene. In the PZ curve, three processes are observed in region 1, where V is decreased from 5 to 0 V. The first process is a large step of the current from 0 to 0.3 A mm2; as shown in Figure 2b, this response is due to the series capacitor. The second process is from t = 0 to 3 ms; although the voltage begins to decrease, the current remains saturated. This phenomenon indicates that V  2 V is sufficient voltage to allow full hole injection into pentacene, because the voltage scan speed is 1 V ms1. The third process is from t = 3 to 5 ms; the current gradually decreases corresponding to the discharge process. In region 2, in which V is increased from 0 to 5 V, the three processes are also confirmed. The first process is a step in the current from 0.2 to 0.2 A mm2. Because the sign of dV/dt is changed, the displacement current flows in reverse direction. This process is related to the response of the conventional DCM method. 29-31,36,37 The second process is from approximately t = 5 to 7 ms. Because the curve shows a linear increase, smooth hole injection occurs. The third process is from t = 7 to 10 ms; the current is nearly saturated and the curve has a plateau at approximately 0.3 A mm2. This phenomenon means that holes are fully injected again and a charge sheet is formed over the pentacene/SiO2 interface. The same behavior is confirmed in regions 3 and 4. Note that the displacement current shows symmetrical behavior between the PZ and ZP voltage sweeps. The results indicate that carrier motion at the Au/pentacene interface is essentially reversible and that no hysteresis occurs between charge injection and discharge. In thin-film pentacene transistors with a Au electrode, a large current hysteresis in forward and reverse voltage sweeps is typically observed.39 However, note that the


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present method investigates carrier motion at the metal/pentacene interface, and does not include information regarding the SiO2/pentacene interface. These results suggest that most of the trapping sites, which are responsible for the transistor hysteresis, are localized in the SiO2/pentacene interface, and that the effect of the Au/pentacene interface is small. On the other hand, deviation in the current for the ZN and NZ modes has not been observed, even at high voltages. The present observation means that hole injection, rather than electron injection, is dominant. This result is consistent with the previously reported energy diagrams; pentacene has generally been used as a hole transport material. Qacc plotted as a function of V is depicted in Figure 4a. Since electron injection does not occur, the slopes of the straight parts in the electron side correspond to a seriescapacitance density of 0.20 nF mm2, which is smaller than that of CSiO2 (0.30 nF mm2). In that case, the pentacene works as an insulator. This result is satisfied using the approximation shown in Figure 1c. In contrast, the gradients of the curves gradually increases with increasing V for hole transport. Because of hole injection, the seriescapacitance density increases reaching 0.30 nF mm2 near V = 4 V. Results confirm that the series-capacitance density exceeded 0.30 nF mm2 above V = 5 V. This phenomenon is attributed to the carrier spread effect.36,37 Figure 4b shows a schematic of the accumulated charges in pentacene. Initially, charge injection is insufficient, such that series-capacitance density is less than 0.30 nF mm2. When the series-capacitance density reaches CSiO2 (0.30 nF mm2), hole injection into the pentacene is complete and a charge sheet is formed. Therefore, the charge-spread effect is prominent at the high voltages, where the series-capacitance density is larger than 0.30 nF mm2. Figure 5 shows Q plotted as a function of Vi after CV and VSiO2 are subtracted from Qacc and V using C = 0.20 nF mm2 and CSiO2 = 0.30 nF mm2. Q at the hole side rapidly



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changes above Vi = 0.2 V. When an ideal Ohmic contact is realized, the charge-injection barrier is expected to be zero. Therefore, the Au/pentacene interface is found to be a Schottky contact and the interface has a hole-injection barrier of 0.2 eV. This value includes all factors that militate the barrier height. Above Vi = 0.2 V, the curve is slightly warped backwards; which is ascribed to the carrier spread corresponding to V  6 V in Figure 3b. Although Au has a comparatively deep work function of 5.1 eV,40 the Au/pentacene interface has an interfacial dipole with a strength that is found to be dependent on pentacene thickness.34 Because no electron is injected, Q at the electron side does not exhibit significant change.

3.2 Ag/pentacene interface and the effect of the MoO3 buffer layer Figure 6 shows the results for the Ag and Ag/MoO3 electrodes. At the Ag/pentacene junction, deviation from the step-like waveform is clearly observed in the PZ mode; however, the deviation is small in the ZP mode (Figure 6a). In addition, the deviation of the PZ curve in region 1 is different from that of the Au electrode. Although V starts to decrease, the current increases from t = 0 to 2 ms, and then, gradually decreases from t = 2 to 5 ms, meaning that the response of the displacement current is delayed. These observations suggest that charge injection and discharge occur at this voltage; however, the mobility which is perpendicular to the substrate is low. Therefore, the deviation of the PZ curve in region 2, which corresponds to t = 8 to 10 ms, is smaller than that of region 1. At the electron side, electron injection does not occur, such that the series-capacitance density is a constant (C = 0.17 nF mm2, Figure 6b). In hole transport, the seriescapacitance density starts to change above V = 1.5 V. Full charge injection is observed at approximately V = 4 V. As depicted in Figure 6c, VB is determined to be 0.5 eV at the


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Ag/pentacene interface. The barrier height is larger than that of the Au electrode; this correlation is good agreement with that of a previous report.38 When using MoO3, the work function is expected to shift down to approximately 5.3 eV.39 The deep-workfunction electrode has been widely used to enhance hole injection in organic electronics.41 In the Ag/MoO3/pentacene junction, a large displacement current has been observed in the ZP and PZ modes. The curves are very similar to that of the Au/pentacene junction (Figure 4b); therefore, the same interpretation of the transport process is supported. However, the waveform is not a completely step-like, even in the ZN and NZ modes, which might be due to the displacement current derived from the carrier motion in the MoO3 layer. Although the origin of this current is unclear, the same behavior has been observed in our previous report.21 Results confirm that hole injection occurs at V  0 V (Figure 6d). The series-capacitance density rapidly reaches 0.30 nF mm2 because of very smooth hole injection (Figure 6e). The hole-injection barrier is less than 0.1 eV, which is estimated from the immediate increase of Qacc during hole transport (Figure 6f). The barrier height is significantly reduced, as compared to the Ag/pentacene interface. The resulting junction shows that VB  0.1 V, such that the resulting interface is a nearly Ohmic contact. In pentacene-based organic transistors, several papers report that an Ohmic contact is achieved using MoO3, though the interface has a small activation energy less than 0.1 eV.39,42 The thickness of MoO3 has been reported to significantly influence the interfacial characteristics; only very thin MoO3 provides Ohmic hole injection.43 In our experiment, an Ohmic contact is observed using approximately 10 nm of MoO3.

3.3 Effect of built-in potential To investigate the built-in potential derived from the backside electrode, we measured



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the Ag/pentacene junction using the ITO-based substrate, where the 100 nm SiO2 layers were fabricated on the ITO surface using chemical vapor deposition. Figure 7 shows the Q vs. Vi plots. The barrier height is found to be 0.4 eV for the ITO/SiO2 substrate. This value is relatively good agreement with the value of the doped-Si/SiO2 substrate. It is likely that the sample dependence causes this range of difference (approximately 0.1 V). Because the obtained results are within the error range of 0.1 V, the effects of the built-in potential is small.

4. CONCLUSION In conclusion, carrier injection from metal electrodes to pentacene is investigated using ACM. The Au/pentacene interface has a small hole-injection barrier of 0.2 eV. Although we consider a Schottky interface, carrier motion at the interface is reversible between carrier injection and discharge. This result indicates that the electrical hysteresis in the typical pentacene transistors is attributed to carrier traps, which mostly exist in the SiO2/pentacene interface. The Ag/pentacene contact has a hole-injection barrier of 0.5 eV. By inserting MoO3 into the Ag/pentacene interface, the barrier height is significantly reduced and an Ohmic contact is realized. The present technique is applicable to other materials including polymers, n-type and ambipolar semiconductors. Further measurements are currently being performed by our group. ACM offers another method for studying the metal/organic interface and it will find a wide range of applications in organic electronics.


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Supporting Information Available The data of the displacement current for the Ag, Ag/MoO3 electrode, and the data using the ITO-based substrate. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *(T. K. and H. T.) E-mail: [email protected], [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS The authors are grateful to the University of Hyogo, MEMS Device Development Support Center. This work was partly supported by Grants in Aid for Scientific Research (15K13682). T. K. and M. O. made equal contributions to this work. The Authors acknowledge Mr. Jordan Ropez for a critical reading of the manuscript.



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(21) Tajima. H.; Miyao. F.; Mizukoshi. M.; Sato. S. Determination of Charge Injection Barrier Using the Displacement Current Measurement Technique. Org. Electron. 2016, 34, 193199 (2016). (22) Egusa. S.; Gemma. N.; Miura. A.; Mizushima, K.; Azuma, M. Carrier Injection Characteristic of the Metal/Organic Junctions of Organic Thin-Film Devices. J. Appl. Phys. 1992, 71, 20422044. (23) Egusa, S.; Miura, A.; Gemma, N.; Azuma, M. Carrier Injection Characteristics of Organic Electroluminescent Device. Jpn. J. Appl. Phys. 1994, 33 Part 1, 27412745 (1994). (24) Usui, H.; Watanabe, M.; Arai, C.; Hibi, K.; Tanaka, K. Vapor Deposition Polymerization of a Polyimide Containing Perylene Units Characterized by Displacement Current Method. Jpn. J. Appl. Phys. 2005, 44 Part 1, 28102814. (25) Abiko, N.; Sugi, K.; Suenaga, T.; Kimura, Y.; Ishii, H.; Niwano, M. Carrier Injection Characteristics of Metal/Tris-(8-hydroquinoline) Aluminum Interface with Long Chain Alkane Insertion Layer. Jpn. J. Appl. Phys. 2006, 45 Part 1, 442446. (26) Ohta, H.; Morishita, K.; Furukawa, S. Evaluation of Charge Transfer Characteristics of Metal-Free Phthalocyanine Thin Films by Displacement Current Measurement. Thin Solid Films 2008, 516, 26002606. (27) Noguchi, Y.; Sata, N.; Tanaka, Y.; Nakayama, Y.; Ishii, H. Threshold Voltage Shift and Formation of Charge Traps Induced by Light Irradiation During the Fabrication of Organic Light-Emitting Diodes. Appl. Phys. Lett. 2008, 92, 203306203308. (28) Suzuki, S.; Yasutake, Y.; Majima, Y. Frequency Dependences of Displacement Current and Channel Current in Pentacene Thin-Film Transistors. Jpn. J. Appl. Phys. 2008, 47, 31673169.


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(29) Majima, Y.; Kawakami, D.; Suzuki, S.; Yasutake, Y. Simultaneous Measurements of Drain-to-Source Current and Carrier Injection Properties of Top-Contact Pentacene ThinFilm Transistors. Jpn. J. Appl. Phys. 2007, 46, 390393. (30) Ogawa, S.; Kimura, Y.; Ishii, H.; Niwano, M. Carrier Injection Characteristics in Organic Field Effect Transistors Studied by Displacement Current Measurement. Jpn. J. Appl. Phys. 2003, 42, L1275 L1278. (31) Ogawa, S.; Naijo, T.; Kimura, Y.; Ishii, H.; Niwano, M. Photoinduced Doping Effect of Pentacene Field Effect Transistor in Oxygen Atmosphere Studied by Displacement Current Measurement. Appl. Phys. Lett. 2005, 86, 252104252106. (32) Suzuki, S.; Yasutake, Y.; Majima, Y. Interface Trap Level in Top-Contact Pentacene Thin-Film Transistors Evaluated by Displacement Current Measurement. Org. Electron. 2010, 11, 594598. (33) Yoshita, S.; Tamura, R.; Taguchi, D.; Weis, M.; Lim, E.; Manaka, T.; Iwamoto, M. Displacement Current Analysis of Carrier Behavior in Pentacene Field Effect Transistor with Poly(Vinylidene Fluoride and Tetrafluoroethylene) Gate Insulator. J. Appl. Phys. 2009, 106, 024505024508. (34) Watkins, N. J.; Yan, L.; Gao, Y. Electronic Structure Symmetry of Interfaces Between Pentacene and Metals. Appl. Phys. Lett. 2002, 80, 43844386. (35)

Shibata,

K.;

Wada,

H.;

Ishikawa,

K.;

Takezoe,

H.;

Mori,

T.

(Tetrathiafulvalene)(Tetracyanoquinodimethane) as a Low-Contact-Resistance Electrode for Organic Transistors. Appl. Phys. Lett. 2007, 90, 193509193511. (36) Tanaka, Y.; Noguchi, Y.; Kraus, M.; Brütting, W.; Ishii, H. Displacement Current Measurement of A Pentacene Metal-Insulator-Semiconductor Device to Investigate Both Quasi-Static and Dynamic Carrier Behavior Using A Combined Waveform. Org. Electron.



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FIGURE CAPTIONS

Figure 1. (Color online) (a) Schematic of the device structure. Energy-level diagrams in cases in which (b) the applied bias voltage is zero, (c) charges are accumulated at the metal/pentacene interface, and (d) charge injection into pentacene occurs. Q indicates the degree of charge injection. In these figures, a Schottky contact is assumed at the metal/pentacene interface. Note the small built-in electric field in the pentacene layer is neglected in (b). This effect is discussed in the section 3.3.



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Figure 2. (Color online) (a) Bias voltage sweep patterns used in ACM: (PZ) V()  0  V()  0  V(), (ZP) 0  V()  0  V()  0, (NZ) V()  0  V()  0  V(), (ZP) 0  V()  0  V()  0. (b) Time dependence of the displacement current for the Ag/SiO2 capacitor at V = 0.5 V. The figure corresponds to CSiO2 = 0.30 nF mm2.


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Figure 3. (Color online) Time dependence of the displacement current measurement for the Au/pentacene interface. (a) |V| = 0.5 V and (b) |V| = 5 V.



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Figure 4. (Color online) (a) Q as a function of V for the PZ, ZP, NZ, and ZN voltage sweep modes. The dotted lines correspond to series-capacitance densities of 0.20 and 0.30 nF/mm2, respectively. (b) Schematics of the accumulation of charges as a function of C.


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Figure 5. (Color online) Q plotted as a function of Vi.



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Figure 6. (Color online) Time dependence of the displacement current, Qacc vs. V plots, and Q vs. Vi plots for (a), (b), (c) the Ag electrode and (d), (e), (f) the Ag/MoO3 (10 nm) electrode. The dotted lines represent the series-capacitance density.


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Figure 7. (Color online) Q vs. Vi plots of the Ag/pentacene junction. The red curve corresponds to doped-Si/SiO2; the blue curve denotes the ITO/SiO2 substrate.



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Table of Contents Graphic


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(b) vacuum level

(a)

(c)

VSiO2 Vi

V metal pentacene SiO2 doped-Si

(d) VSiO2

V

SiO2

Vi pentacene

A

n-Si

metal

n-Si

n-Si

 

Qacc

metal

 

Qacc C C =CSiO2Cp / (CSiO2  Cp)

ACS Paragon Plus Environment

  

Qacc

Q metal

     

Qacc Q = Qacc  CV Vi = V  Qacc/CSiO2


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(a)

1

2

3

ZP NZ ZN PZ

4

(b)

Electron

Hole

(c)

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Electron

Hole

0.17 nF mm−2

|V| = 5V

0.30 nF mm−2

VB = 0.5 V

(d)

1 ZP NZ ZN PZ

2

3

4

|V| = 5V

(e)

Electron

0.16 nF mm−2

Hole

(f)

Electron

Hole

0.30 nF mm−2

VB < 0.１ V


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0.4

A

doped-Si

0.3 

metal pentacene SiO2

Q (nC mm )

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0.2 0.1

Electron

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Hole

Hole-injection barrier of 0.2 eV

0.0 -0.1 -0.2 -2.0

VB = 0.2 V -1.0

0.0 Vi (V)


1.0

Pentacene

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