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Estimation of the Charge Injection Barrier at a Metal/ Organic Semiconductor Interface Based on Accumulated Charge Measurement: The Effect of Offset Bias Voltages Hiroyuki Tajima, Kesuke Yoshida, Seiichi Sato, Tomofumi Kadoya, and Jun-ichi Yamada J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04456 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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The Journal of Physical Chemistry

Estimation of the Charge Injection Barrier at a Metal/Organic Semiconductor Interface Based on Accumulated Charge Measurement: The Effect of Offset Bias Voltages Hiroyuki Tajima,†* Kesuke Yoshida,† Seiichi Sato,† Tomofumi Kadoya,† Jun-ichi Yamada†

†

Graduate School of Material Science, University of Hyogo, 3-2-1 Kohto, Kamigori-gun, Hyogo 678-1297, Japan

ABSTRACT The effect of the offset bias voltage on the threshold voltage of the hole injection into the organic-semiconductor (OS) layer was examined in detail, in the accumulated charge measurement (ACM) for the n-type Si/SiO2/OS/Ag (OS = zinc phthalocyanine [ZnPc] or metal-free phthalocyanine [H2Pc]) capacitor. The threshold highly depends on the offset bias voltages, when the OS layer is in the hole-depletion regime. On the other hand, the threshold was nearly constant when the OS layer operated in the hole-accumulation regime. The hole injection barrier of the Ag/OS interface was obtained by the threshold in the accumulation regime. The obtained values were 0.41 eV and 0.05 eV for H2Pc/Ag and ZnPc/Ag interfaces, respectively. The study revealed that accurate estimation of the injection barrier is possible by examining the offset voltage dependence in the ACM.

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1. INTRODUCTION While charge injection from electrodes is crucial in electronic devices, our understanding of injection barrier for metal/organic semiconductor interfaces is still insufficient.1 In the case of inorganic semiconductors, various techniques to determine injection barrier are described in the standard textbook.2 In these techniques, an ohmic contact at one of the two electrodes is prepared, and the injection barrier formed at the other electrode is investigated. However, in the case of organic semiconductors, it is not easy to prepare a good quality ohmic contact, and the application of standard techniques is limited. In this context, the barrier heights of metal/organic semiconductors were mainly studied using spectroscopic techniques, such as ultraviolet photoelectron spectroscopy (UPS),3-8 photoelectron yield spectroscopy (PYS),9,10 and inverse photoelectron spectroscopy (IPES).11,12,7 UPS and PYS provide the energy of the highest occupied molecular orbital (HOMO) relative to the vacuum potential (ionization energy [IE]). On the other hand, IPES gives the energy of the lowest unoccupied molecular orbital (LUMO) relative to the vacuum potential (the electron affinity [EA]). Assuming the Schottky-Mott rule and taking the vacuum level shift (∆VL) into account, the charge injection barrier is given by Wm+∆VL-EA (in the case of n-type organic semiconductors) or by IE-Wm+∆VL (in the case of p-type organic semiconductors), where Wm is the work function of the metal electrode.6 Recently, we reported a direct method to determine the charge-injection barrier, namely the accumulated charge measurement (ACM), which is based on displacement current measurement (DCM).13-20 The injection barrier evaluated by this technique includes non-ideal factors such as impurity effects.21,22 The ACM was first applied to metal-free phthalocyanine (H2Pc)23 and later to pentacene.24 The sample for the ACM


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has a structure consisting of a back electrode (M1), an insulator (I), an organic semiconductor (OS), and a top electrode (M2), as shown in Fig. 1a. This setup is the same as the one used in the DCM technique. DCM is a powerful method for investigating the effect of carrier behavior on charge injection, discharging, accumulation, and trapping. This technique has been applied to organic light emitting diodes, organic transistors, and organic solar cells. In conventional DCM, displacement current is analyzed as a function of the applied voltage. Therefore, the technique cannot determine the barrier-height voltage, because the applied voltage consists of the voltage drop at the insulator layer and the internal voltage at the semiconductor layer. In our previous study on the ACM, we have reported the method to determine the accumulated charge accurately, and to obtain the plot of the degree of charge injection as a function of internal voltage. We could then evaluate the barrier-height voltage at the metal/organic interface directly from the threshold of the internal voltage in the plot. In our studies, Ag, MoO3/Ag, and Au were used for the M2 electrodes, and n-type Si (n-Si) was used for the M1 electrode for most measurements. One of the criticisms against these results is that the work function difference between the M1 and M2 electrodes is not explicitly considered, because of which the flat band condition at the zero bias condition is not satisfied. As this condition is an important assumption in the analysis, it may cause a serious error in the estimation of the injection barrier if it is not fulfilled. In order to overcome this drawback, we examined the effect of M1 electrodes on the estimation of the hole injection barrier, by comparing the results for indium-tin oxide (ITO)/SiO2/pentacene/Ag and n-Si/SiO2/pentacene/Ag capacitors.24 We found that the threshold of the internal voltage, which provides an estimation of the



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injection barrier, was 0.5 eV for the former, and 0.4 eV for the latter. Based on this result, we concluded that the back electrode affects the estimation of the injection barrier in the ACM technique, but the effect is within the error range of 0.1 eV. However, the work function difference between n-Si and ITO is approximately 1 eV; we have not yet examined other cases. Moreover, charges in the SiO2 layer induce an effect similar to that of the work function difference.2 In order to overcome this problem and to improve the reliability of the ACM as a tool to evaluate the injection barrier in organic/metal interface, it is essential to clarify how the work function difference affects the estimation of the injection barrier in the ACM. In the sample shown in Fig. 1a, the purpose of the offset bias voltage, added to the applied voltage, is equivalent to that of the work function difference between the M1 and M2 electrodes, as M2 is separated from M1 by an insulator layer. We then performed the ACM for various offset voltages. We determined the threshold voltage of the hole injection as a function of the offset voltage. We showed that the injection barrier can be precisely determined by examining the offset-voltage dependence in the ACM.

2. EXPERIMENTAL SECTION 2.1 The principle of the measurement Figure 1 shows the principle of the ACM. In this scheme, we consider the case when the Fermi energy of the M1 electrode is higher than that of the M2 electrode (i.e. the work function of M1 is smaller than that of M2), and a constant offset voltage of Voff is applied. In the present ACM, we measure the change of the accumulated charge Qacc, as a function of the change of voltage Va. To evaluate Qacc, the simplest way is to integrate the displacement current to the bias-voltage waveform as shown in Fig. 1b. In


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this waveform, the initial applied voltage of Va+Voff and the final applied voltage of Voff should be maintained for a long time, and the integration should be performed from A to B. Here A is the start of the voltage sweep and B is the end of the application of voltage. Long time maintenance of the constant voltage of Voff is necessary for the integration, in order to evaluate Qacc precisely. However, the time-consuming integration also deteriorates the accuracy of Qacc, because of the leakage current of the current-voltage amplifier, etc. Therefore, in the actual procedure, we evaluated Qacc by using the voltage oscillation technique described in Ref. 21. This technique gives accurate estimate of Qacc, without the necessity of long time integration. Figure 1c shows the diagram at Va = 0 V. Due to Voff and the work function difference between the M1 and M2 electrodes, accumulated charge of ±Q' appears on both sides of the insulating layer even when Va is zero. When Va is applied, the extra charges of ±Qacc appear on both sides, as shown in Fig. 1d. Figure 1e shows the comparison of the diagram shown in Fig. 1c and Fig. 1d. The voltage change Va is a sum of an extra voltage drop within the insulator layer VI and that of within the OS layer VOS, i.e. Va = VI + VOS . Using the Gauss' law, the former drop is given by VI = Qacc/CI, where CI is the capacitance of the insulator layer. Then, VOS is given by

VOS = Va - Qacc/CI .

(1)

Note, that this equation is correct at any time even when band bending due to the charge injection occurs, however Figs. 1c-e show the simple case where charge injection does not occurs. As can be seen from Fig. 1e, VOS corresponds to the relative change of



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potential in the OS layer at the boundary, in contact with the I layer. We can define the degree of charge injection ∆Q as

∆Q = Qacc - CVa,

(2)

where C is a series combination of capacitances of the I and OS layers, given by C = CICOS/(CI + COS), where COS is the capacitance of the OS layer. ∆Q is zero when no charge is injected into the layer of the organic semiconductor. It increased with the increase of the injected charges, and finally reached ∆Q = Qacc - CVa = CIVa - CVa = VaCI2/(CI+COS). Thus, this parameter reflects the magnitude of charge injection into the OS layer. In the plot of VOS vs. ∆Q, there is a threshold of VOS above which ∆Q drastically increases. In this paper, we define this threshold as Vth. In previous papers,23,24 we neglected the potential difference between the M1 and M2 electrodes, and assumed the existence of the flat band condition in the OS layer at Va = 0 V. In this case, the charge injection barrier between the M2 and OS layers should coincide with Vth. Thus, we can directly evaluate the injection barrier from the plot of VOS versus ∆Q. Actually, there is a potential difference between the M1 and M2 electrodes. This difference influences the estimation of the injection barriers. We examined this effect in detail, by performing ACM for various values of Voff.

2. 2 Experimental implementation A highly doped n-Si wafer (ρ < 0.01 Ω.cm) with a thermally grown SiO2 layer of 100 nm thickness was treated with hexamethyldisilazane (HMDS), and was used as a


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substrate. The n-Si substrate worked as the back electrode M1, and the layer of SiO2 was used as the insulating layer I. The OS layer was a 120 nm thick film of either H2Pc or zinc phthalocyanine (ZnPc). The top electrode M2 was made of Ag with a thickness of 100 nm. The capacitance of the insulating layer CI was independently measured to be 0.307±0.003 nF/mm2 using n-Si/SiO2(100 nm)/Al capacitor as reference. The layers of the OS and Ag were formed by vacuum deposition at room temperature. The area of the top electrode (Ag) was approximately 7 mm2, which is smaller than the area of the OS and the back electrode (n-Si). The areas of the top electrode in all the samples were precisely measured using a digital microscope. The displacement current was measured by using a digital oscilloscope (DL850, Yokokawa), a function generator (WF1946A), and a homemade current amplifier with a gain of 6800 V/A. The accumulated charge Qacc was obtained by integrating the displacement current. The measurements were performed at room temperature, in a nitrogen atmosphere.

3. RESULTS 3. 1 Measurements for the n-Si/SiO2(100 nm)/H2Pc(120 nm)/Ag(100 nm) capacitor Figure 2a shows the accumulated charge Qacc as a function of Va, for the n-Si/SiO2/H2Pc/Ag capacitor. A constant slope of 0.31 nF/mm2 for Va > 2 V, and 0.24 nF/mm for Va < 0 V was obtained. The slope of 0.31 nF/mm2 is consistent with CI, the capacitance of the SiO2 layer, which was measured independently. This indicates that the charge is injected at the I/OS interface at Va > 2 V. The small slope in the negative side was consistent with the large barrier height for Va < 0 V. We measured the series capacitance C for the layer of SiO2 and H2Pc to be 0.24 nF/mm2 from this data.



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Figure 2b shows the plot of ∆Q versus VOS obtained using Eqs. (1) and (2). The plot indicates as VOS reaches a threshold, above which ∆Q steeply increases. As mentioned in section 2.1, this threshold is denoted by Vth. Vth depends on the offset voltage, and is 0.86 V at Voff = -3 V, 0.46 V at Voff = 0 V, and 0.41 V at Voff = 1 V. For Voff ≥ 2V, the threshold does not appear, but the plots of ∆Q versus VOS are almost overlap with the plot of Voff = 1 V. The value of Vth at Voff = 0 V is consistent with that of reported previously.23

3. 2 Measurements for the n-Si/SiO2(100 nm)/ZnPc(120 nm)/Ag(100 nm) capacitor Figure 3a shows Qacc as a function of Va for the n-Si/SiO2/ZnPc/Ag capacitor. A constant slope of 0.31 nm/mm2 is observed for Va > 2 V. This slope does not depend on Voff, and is consistent with CI, the capacitance of the insulating layer, measured independently. This indicates that the charge is injected at the I/OS interface at Va > 2 V. The small slope in the negative side was consistent with the large barrier height for Va < 0 V. We evaluated the series capacitor C for the layer of SiO2 and ZnPc to be 0.23 nF/mm2 from this data. The effect of Voff is larger in this capacitor compared with the n-Si/SiO2/H2Pc/Ag capacitor. This effect explicitly appears at Va < 0 V. This indicates that a hole is injected even at Va = 0 V, because of the low injection barrier of the ZnPc/Ag contact. Figure 3b shows the plot of VOS versus ∆Q obtained from Qacc and Va using Eqs. (1) and (2). ∆Q abruptly increases above a threshold voltage Vth. Vth at Voff = 0 V is 0.107 V. Vth depends on Voff, and is 0.42 V at Voff = -3 V, and 0.054 V at Voff = 1 V. As Voff increases above 2 V, Vth is further decreased. In the ∆Q-VOS plot of n-Si/SiO2/ZnPc/Ag capacitor, the curve explicitly turns


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backward. This is due to the effect of charge spreading, the phenomenon when charges are injected into an area greater than the area of the M2 electrode. In this case, the effective capacitance is more than CI, and the value of VOS calculated from Eq. (1) decreases. Similar phenomena are observed in ACM studies on pentacene24 and in DCM studies.13-15 This observed charge spreading effect is stronger in ZnPc than in H2Pc. This is due to the lower hole injection barrier, as a consequence, greater hole density in ZnPc than those in H2Pc.

4. DISCUSSION Figure 4 shows the plot of Vth as a function of Voff for both H2Pc and ZnPc. The two curves exhibit very similar characteristics against Voff. The change of Vth is very large at Voff ≤ 0 V, and becomes small at Voff ≥ 1 V. In this section, we discuss this result in detail, on the basis of a schematic model illustrated in Fig. 5. Fig. 5a illustrates the schematic energy diagram of the actual sample. M1, I, M2 in Fig. 1a represents n-Si, SiO2, Ag, respectively. The work function of n-Si is smaller than that of Ag. VFB denote the potential difference between n-Si and Ag, where a flat-band state in the organic semiconductor layer is realized. Unless we consider the pinning of charges at the interface, VFB is approximately the work function difference between n-Si and Ag, and a rough estimate of VFB is given by the Schottky-barrier height between n-Si and Ag. Thus, we deduced this parameter, which is approximately 0.8 eV according to the literature.2 First, we consider the case of Voff = VFB, as shown in Fig. 5b. In this case, the flat-band state in the organic semiconductor is realized for Va = 0 V. The Ag electrode and organic semiconductor at the boundary with the insulating layer are on the same



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potential level at VOS = Vth. Thus, the potential change Vth is the same as the charge injection barrier φB. This is the case, what we have assumed in previous reports.23, 24 Next, we consider the case of Voff < VFB (Fig. 5c). When VOS = Vth, the potential of Ag electrode is the same as that of the organic semiconductor at the boundary, contacting the insulating layer. This is the same as in the case of Fig. 5a. The difference is the state after the charge extraction (i.e. Va = 0 V). In this case, all the holes at the I/OS interface are extracted and a depletion state is realized. Then potential shift denoted by δ, appears in the OS layer. Consequently, Vth is the sum of δ and φB. This is the reason of the very large shift of Vth observed for Voff ≤ 0 V. Finally, we discuss the case of Voff > VFB, as shown in Fig. 5d. In this case, a large number of excess holes are injected. Thus, injected holes are not completely extracted even at Va = 0 V. The excess holes are accumulated at the I/OS interface. Because of this accumulation, the voltage drop in the OS layer is relatively small, and of the order of kT / q ( = 25.9 meV at 300 K).2 Neglecting this small voltage drop, we can derive the relation Vth = φB. This relation is the same as the one derived for the case of Voff = VFB. On the basis of the above-mentioned discussion, it is expected that Vth tends to saturate above Voff = VFB. This is consistent with the observed result that Vth is nearly saturated at Voff above 1 V. Figure 5e shows the schematic plot of ∆Q as a function of VOS, expected due to the above-mentioned consideration. In this figure, the backward turning of the curve due to the charge spreading effect is neglected for simplicity. Since Vth is almost saturated at Voff > 1 V, as can be seen in Fig. 4, we assume that VFB = 1 V. This assumption of VFB = 1 V, is consistent with the estimation of VFB = 0.8 V, deduced from the Schottky-barrier height between n-Si and Ag. Thus, the values


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of the hole injection barrier are estimated to be 0.054 V for ZnPc and 0.41 V for H2Pc, which are the values of Vth at Voff = 1 V. In a previous paper,23 we determined the hole injection barrier of H2Pc in the measurement at Voff = 0 V. Thus, the obtained value of the hole injection barrier was overestimated by 0.05 eV. In the case of pentacene/Ag interface, we measured the hole injection barrier to be 0.5 eV, when we use n-Si as a back electrode, and to be 0.4 eV when we use ITO as a back electrode.24 The work function of ITO is almost the same as, or slightly larger than that of Ag, while the work function of n-Si is less than that of Ag. Thus, a hole accumulation regime is expected for the ITO back electrode, while a hole depletion regime is expected for the n-Si back electrode. Thus, we consider that the latter estimate of 0.4 eV is more accurate than the former. According to a data book based on PYS in the atmosphere, IE of ZnPc and H2Pc are 4.8 eV and 5.1 eV, respectively, and Wm of Ag is 4.8 eV.9 Using these parameters and neglecting ∆VL, we calculate the hole injection barriers for ZnPc/Ag and H2Pc/Ag interfaces to be 0 eV and 0.3 eV. These values are consistent with the estimates of the injection barriers obtained in this study. On the other hand, according to the UPS studies under ultra-high vacuum, IE of ZnPc and H2Pc are 4.8 eV8 and 4.96 eV7, respectively, and Wm of Ag is 4.3 eV.6 Using these parameters and neglecting ∆VL, we estimate the hole injection barriers for ZnPc/Ag and H2Pc/Ag interfaces to be 0.5 eV and 0.7 eV, respectively. If we take ∆VL into account, the above-mentioned values of the injection barrier become larger. Thus, the estimation based on UPS studies under ultra-high vacuum is not consistent with the results obtained in this study. Ishii et al reported a very large shift of the UPS spectra in the ZnTPP/M (M = Mg, Al, Ag) interface, after the sample is exposed to an O2 atmosphere.3 This significant shift is not



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observed in the ZnTPP/Au interface. Based on this result, they concluded that the oxidation of the metal substrate is the origin of this shift. The 0.2 eV difference of IE between ZnPc and H2Pc is relatively consistent with the 0.36 eV difference in the injection barriers between Ag/ZnPc and Ag/H2Pc. Therefore, if we take this oxidation effect for Ag electrode into consideration, the inconsistency in the estimated values may be solved. At the end of this section, we briefly discuss the errors in the estimated values. There are several sources of errors in the estimation of injection barriers using ACM. One source of error is associated with the estimation of VFB. In this paper, it is assumed that VFB = 1 V, based on the saturating behavior of Vth and the Schottky-barrier height between n-Si and Ag. However, VFB may deviate from the estimated value when the charged traps are taken into consideration. In the case of ZnPc/Ag, Vth = 0.054 V at Voff = 1 V and Vth = 0.025 V at Voff = 2 V. Thus, if it is assumed that VFB = 2 V instead of VFB = 1 V, then the estimated value of φB changes from 0.054 V to 0.025 V. Then, the error due to uncertainty in VFB was calculated to be 0.03 V. In the case of H2Pc/Ag, the value of Vth cannot be obtained from the data shown in Fig. 2. However, the curve of Voff = 2 V in Fig. 2 almost overlaps with the curve of Voff = 1 V. Thus, the value of Vth at Voff = 2 V and Voff = 1 V can be considered to be almost the same. An important point is that the error associated with the flat-band condition depends on the choice of electrodes, and causes a systematic deviation. When n-Si was used as a back electrode and Ag as a top electrode, the ACM, without applying an offset voltage, overestimates the injection barrier by ~0.05 eV, as can be seen in Fig. 4. Other influential sources of errors are the error associated with the estimation of the top-electrode area and the error associated with the estimation of Qacc. Although


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accurate estimation of these errors is very difficult, the total error of φB originated from these errors can be considered to be ±0.05 eV, judging from the reproducibility in a series of measurements.

5. CONCLUSION In conclusion, we have developed a modified procedure to estimate the carrier injection barrier at the metal/OS interface on the basis of ACM. By examining the Voff dependence, we succeeded in compensating the work function difference between the top and back electrodes in the ACM. We applied this technique to ZnPc/Ag and H2Pc/Ag junctions and obtained the hole injection barriers to be 0.05 eV and 0.41 eV, respectively. When we used n-Si as a back electrode and Ag as a top electrode, the ACM, without applying an offset voltage overestimates the injection barrier by ~0.05 eV. In order to determine the injection barrier accurately, ACM should be performed for OS operating in the charge accumulation regime.

AUTHOR INFORMATION Corresponding Author *(H. T.) [email protected]

ACKNOWLEDGEMENTS This work was supported by Grants in Aid for Scientific Research (15K13628 and 17K18020) and by Hyogo Science and Technology Association.



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Brütting, W.; Ishii, H. Charge Accumulation at Organic Semiconductor Interfaces due to a Permanent Dipole Moment and its Orientational Order in Bilayer Devices. J. Appl. Phys. 2012, 111, 114508. (19) 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. (20) Egusa, S.; Miura, A.; Gemma, N.; Azuma, M. Carrier Injection Characteristics of Organic Electroluminescent Device. Jpn. J. Appl. Phys. 1994, 33, 2741−2745. (21) Jung, S.; Kim, C-H.; Bonnassieux, Y.; Horowitz, G. Injection Barrier at Metal/ Organic Semiconductor Junctions with a Gaussian Density-of-States. J. Phys. D: Appl. Phys. 2015, 48, 395103. (22) Horowitz, G. Validity of the Concept of Band Edge in Organic Semiconductors. J. Appl. Phys. 2015, 118, 115502 . (23) 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. (24) Kadoya, T.; Otsuka, M.; Ogino, A.; Sato, S.; Yokomatsu, T.; Maenaka, K.; Yamada, J.; Tajima, H. Estimation of Charge-Injection Barriers at the Metal/Pentacene Interface through Accumulated Charge Measurement. J. Phys. Chem. C 2017, 121, 2882-2888.


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Figure. 1. (a) Schematic illustration of the sample and the setup of measurement. V(t) and I(t) denote the applied voltage and the displacement current I(t), respectively. The accumulated charge Qacc was evaluated by the integration of the displacement current. (b) The equivalent bias-voltage waveform for the displacement current measurement. In the actual measurements, we used voltage oscillating technique described in Ref. 21. (c), (d) Schematic potential diagram when Va = 0 V and Va≠0, respectively. Note that the diagrams assume work function of M1 electrode is smaller than that of M2 electrode. (e) Effect of the voltage Va on the potential diagram. Note that VOS represents the potential change of the OS layer at the interface with I layer. The abbreviations in the illustration is as follows: (M1, M2) back and top electrodes; (I) the layer of insulator; (OS) the layer of organic semiconductor; (Voff) offset bias voltage; (Va) applied bias voltage added to Voff; (VI, VOS) voltage drops caused by Va in the layers of I and OS, respectively; (Q') accumulated charge caused by Voff and the work-function difference between the M1 and M2 electrodes; (Qacc) accumulated charge caused by Va; (CI) the capacitance of insulator layer; (COS) the capacitance of the organic semiconductor layer; (C) the series capacitance for the layer of insulator and organic semiconductor.



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Figure 2. (a) Accumulated charge Qacc as a function of the applied voltage Va for n-Si/SiO2(100 nm)/H2Pc(120 nm)/Ag(100 nm) capacitor. Note that 0.31 nF/mm2 slope value above a Va of approximately 3 V is consistent with the value of CI = 0.31 nF/ mm2. This implies that a complete charge injection into H2Pc film is obtained in this voltage region. (b) Degree of charge injection ∆Q as a function of voltage drop within the OS layer VOS. The threshold voltages of charge injection Vth were evaluated based on this plot.


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Figure. 3 (a) Accumulated charge Qacc as a function of the applied voltage Va for n-Si/SiO2(100 nm)/ZnPc(120 nm)/Ag(100 nm) capacitor. Note that 0.31 nF/mm2 slope value above a Va of approximately 3 V is consistent with the value of CI = 0.31 nF/ mm2. This implies that a complete charge injection into H2Pc film is obtained in this voltage region. (b) Degree of charge injection ∆Q as a function of voltage drop within the OS layer VOS. The threshold voltages of charge injection Vth were evaluated based on this plot. The backward warping of the curves is due to the charge spreading effect. (See the text for details.)



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Figure. 4 Threshold voltages of charge injection as a function of offset voltage (Voff). Flat-band condition is approximately realized for Voff = 1 V. The hole injection barriers were evaluated by the value of Vth at Voff = 1 V. The region of accumulation (depletion) indicates that hole is accumulated (depleted) at Va = 0 V.


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Figure. 5 (a) Sample structure used in measurement. VFB denote the potential difference between n-Si and Ag, where flat-band state in the organic semiconductor layer is realized. (b-d) The energy diagrams for the OS layer and Ag electrode at VOS = Vth (right panels) and at VOS = 0 (left panels) in the flat-band condition (Voff = VFB; b), the charge-depletion regime (Voff < VFB; c), and the charge-accumulation regime (Voff > VFB; d). In the charge-depletion regime, potential gradient (illustrated by δ in the figure) emerge at Va = VOS = 0 V, and Vth does not coincide with φB. In the charge-accumulation regime, potential gradient is negligible at Va = VOS = 0 V and Vth almost coincides with φB. (e) Schematic figure of ∆Q as a function of VOS. In the flat-band condition and in the accumulated-charge regime, ∆Q steeply increases at VOS = φB.



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Organic

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