JOURNAL OF APPLIED PHYSICS 98, 074505 共2005兲
Highly sensitive thin-film organic phototransistors: Effect of wavelength of light source on device performance Yong-Young Noha兲 and Dong-Yu Kimb兲 Center for Frontier Materials, Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-Dong, Buk-Gu, Gwangju 500-712, Republic of Korea
Kiyoshi Yasec兲 Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
共Received 11 April 2005; accepted 16 August 2005; published online 7 October 2005兲 Organic phototransistors 共OPTs兲 were fabricated from pentacene and copper phthalocyanine 共CuPC兲 based on the geometry of organic field-effect transistors 共OFETs兲; and the effect of the wavelength of the incident light source on their performance was examined. High performance OFETs with pentacene and CuPC were fabricated and the characteristics of the OPTs were examined under UV and visible-light irradiations with top illumination. The CuPC and pentacene OPTs show a high responsivities of 0.5–2 and 10– 50 A / W and maximum IPh / IDark of 3000 and 1.3⫻ 105, respectively, under 365 nm UV light. However, under visible light, at a wavelength of 650 nm, the pentacene OPTs had 100 times less responsivity, 0.15– 0.45 A / W, and a IPh / IDark of 1000, even though an absorption coefficient three times larger was observed at this wavelength than at 365 nm. A strong correlation was found between the performance of the OPTs and the incident photon to current conversion efficiency spectra of an organic semiconductor. The strong dependence on the wavelength of incident light of the performance of the prepared OPTs can be explained by an internal filter effect in which light with a large absorption coefficient is filtered at the top surface and through the bulk of the film when light is directed onto the opposite side of the OFET gate electrode. Thus, light cannot efficiently contribute to the generation of charge carriers in the channel regions that were formed in the first two molecular layers adjacent to the dielectric interface. Consequently, the most efficient OPTs were produced when the following conditions of incident light were satisfied: The photon energies 共or frequencies兲 should be 共i兲 larger than the band gap and 共ii兲 have a relatively small absorption coefficient, since the light can penetrate down to the channel layer more efficiently when it is near the dielectric interface without any loss in absorption through the film. © 2005 American Institute of Physics. 关DOI: 10.1063/1.2061892兴 I. INTRODUCTION
Because of their unique advantages, electronic and optoelectronic devices based on conjugated organic oligomers or polymers have been the subject of active investigation, which include ease of processing and the possibility of producing flexible devices via low-temperature fabrication. Organic light-emitting diodes 共OLEDs兲, a major example of such devices, have encouraged many researchers, since they have been commercialized, and show excellent performance. Organic field-effect transistors 共OFETs兲, potential components of flexible devices, are also expected to be the next candidates for commercialization even though their intrinsic limitations on charge-carrier mobility hinder a wide range of applications.1–4 Photodetectors or photovoltaic cells, in which organic semiconductors are used, are another interesting devices due to their fascinating photodetection or photoconducting characteristics in the UV or visible region.5–7 Therefore, a numa兲
Present address: Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK. b兲 Electronic mail:
[email protected] c兲 Electronic mail:
[email protected] 0021-8979/2005/98共7兲/074505/7/$22.50
ber of groups have reported on the potential production of organic semiconductors for use in photodetecting or photosensing devices.8–14 Among these, photo-field-effect transistors, phototransistors 共PTs兲, are viable candidates in the area of organic semiconductors for use in optical transducers because they combine light detection and signal amplification properties in a single device without the noise increment associated with avalanche photodiodes.15,16 PTs employ the principle of photodiodes, but the amplifying action of the transistors makes these devices more sensitive.17 In particular, PTs with a field-effect transistor geometry have an advantage compared with bipolar junction transistors, since they can be integrated into a large area circuit with a high density. To date, only a few reports on organic phototransistors 共OPTs兲 with a FET geometry, in which a conjugated organic polymer or oligomer thin films with polycrystalline or amorphous morphologies are used, have appeared.18–22 However, the performance of such devices with respect to responsivity, the ratio of photocurrent to dark current 共IPh / IDark兲, and charge carrier mobility were relatively lower than those of their inorganic counterparts and, furthermore, the performances of their FETs were relatively poor compared with the
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rent conversion efficiency 共IPCE兲 spectra of the organic semiconductor used. The reasons for these results are discussed in this paper. II. EXPERIMENT
FIG. 1. Cross-sectional view of the OPTs and the structure of the CuPC and pentacene used in this study.
well-known pentacene or oligothiophene devices. In our previous report, we demonstrated the preparation of greatly improved OPTs, in which a biphenyl end-capped fused bithiophene 共BPTT兲 oligomer was used in their production.23 The BPTT OPTs showed 82 A / W and 2.0⫻ 105 as the maximum responsivity and an IPh / IDark under UV light irradiation. In addition, the mechanism for photocurrent amplification in the BPTT OPTs were found to be the photovoltaic 共turn on兲 and photocurrent effects 共turn off兲, as evidenced by a comparison of experimental results and the fitting results from theoretical equations. In this paper, we present the results of an investigation of the effect of the wavelength of the incident light source on the performance of OPTs with a field-effect transistor structure based on copper phthalocyanine 共CuPC兲 and pentacene 关Fig. 1兴, in an extension of our previous studies. Highperformance OFETs were fabricated using CuPC and pentacene and the characteristics of their OPTs were investigated under irradiation by various wavelengths of both UV and visible lights. Herein, we report that the CuPC and pentacene OPTs show remarkably high responsivities of 0.5–2 and 10– 50 A / W and maximum IPh / IDark of 3000 and 1.3⫻ 105, respectively, under 365 nm UV light. However, under visible light at a wavelength of 650 nm, the pentacene OPTs showed lower responsivities of 0.15– 0.45 A / W and an IPh / IDark of 1000. Consequently, a strong correlation was found between the performance of our OPTs and the incident photon to cur-
The chemical structures of CuPC and pentacene are shown in Fig. 1. CuPC and pentacene were obtained from the TCI Co., Ltd. The semiconducting oligomers were purified by vacuum gradient sublimation before evaporation.24 A CuPC and pentacene thin film 共⬃100 nm thick兲 was deposited on the SiO2 共300 nm and 10 nF/ cm2兲 surface of a heavily doped silicon wafer, used as the gate electrode, at deposition rates of 0.01– 0.04 nm/ s and substrate temperatures of 150 and 30 ° C, respectively. A more detailed description of the evaporation process can be found in our previous report.25–27 After evaporation, the OPTs were completed by evaporating a 50-nm-thick layer of gold through a shadow mask to form source and drain electrodes on the semiconducting thin films in the form of a top-contact geometry. This device had a channel length and a width of 20 m and 5 mm, respectively. The characteristics of the OPTs were examined with a Keithley 4200 semiconductor characterization system in the dark or under UV at wavelengths of 300– 400 nm 共peak wavelength= 365 nm兲 using a Hamamatsu LC5 instrument or visible light irradiation using a xenon lamp with various color filters, which permit the wavelength of the incident light to be selected, in air and the variation in light intensity was achieved using neutral density filters with various transmittances. The light was illuminated from the top 共open兲 side of the devices 共as shown in Fig. 1兲. The light intensity was measured using an Ophir 共BC-20兲 photodetector with a calibrated 2A-SH head. The temperature of the device was monitored using a conventional thermocouple to avoid measurement errors, which might be induced by the heating of the devices during the illumination. All measurements were performed at room temperature. The absorption spectra of the CuPC and pentacene films were obtained with a Shimadzu UV-3101PC. CuPC and pentacene thin films 共150 nm兲 were deposited onto indium tin oxide 共ITO兲-coated glass substrates for the IPCE measurements. The samples were completed by evaporating a 50-nm-thick layer of Al through a shadow mask as a top electrode. The action spectra of the monochromatic IPCEs for the samples were measured with a CEP-99W system 共Bunkoh-keiki Co., Ltd.兲. III. RESULT AND DISCUSSION
The OFETs of pentacene and CuPC with a top-contact configuration were fabricated to measure various device properties, such as field-effect mobility 共FET兲, the current ratio of the on and off states 共IOn / IOff兲, and threshold voltage 共VTh兲. Figures 2共a兲 and 2共b兲 show typical drain current 共ID兲 versus gate voltage 共VG兲 values at a fixed source-drain voltage 共VD = −50 V兲 and ID characteristics with various VG for the p-channel FETs of pentacene deposited at a substrate temperature 共Ts兲 of 30 ° C, respectively. The pentacene films showed sharp diffraction peaks in x-ray diffractograms, corresponding to the 共00l兲 order of the molecular long axis of
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FIG. 2. 共a兲 Transfer characteristics of the pentacene FET measured at VD = −50 V and 共b兲 output characteristics with VG which varies from 0 to − 50 V in steps of −10 V.
the thin-film phase and a layered morphology with an edge-on orientation at the Ts used, which are consistent with previously reported data.28,29 All top-contact devices in which pentacene is used as the active layer showed welldefined linear and saturation characteristics 关as shown in Fig. 2共b兲兴. The FET and VTh of the OFETs were obtained from Eq. 共1兲 for the saturation regimes as proposed by Horowitz,1 sat ID = 兵W/2L其FETCi共VG − VTh兲2 ,
共1兲
where L is the channel length, W the channel width, and Ci the capacitance per unit area of the gate dielectric layer 共Ci = 10 nF/ cm2 for 300-nm-thick SiO2兲. The calculated FET, IOn / IOff, and VTh by plotting ID and ID1/2 vs VG 关as shown in Fig. 2共a兲兴 are 0.49 cm2 V−1 s−1, 1.5⫻ 106, and −18 V, respectively. The parameters for the CuPC OFETs, fabricated at a substrate temperature 共Ts兲 of 150 ° C, were also calculated using the same procedures. The obtained FET, IOn / IOff, and VTh for the CuPC devices were 0.02 cm2 V−1 s−1, 1 ⫻ 105, and −15 V, respectively. For the measurement of OPT characteristics, the output and transfer characteristics of the pentacene and CuPC devices were measured under exposure to UV or visible light with top illumination 共as shown in Fig. 1兲. Figures 3共a兲 and 3共b兲 show the output characteristics of a pentacene and CuPC device in the dark and under UV irradiation at an intensity of 1.55 mW/ cm2. Both the pentacene and CuPC OFETs, measured under UV irradiation, showed a
FIG. 3. Output characteristic of 共a兲 pentacene and 共b兲 CuPC OPTs measured in the dark 共solid line: VG = 0 V; dashed line: VG = −10 V兲 or under UV irradiation 共dotted line: VG = 0 V; dash-dotted line: VG = −10 V兲 with 1.55 mW/ cm2. 共c兲 Transfer characteristics of the pentacene OPTs measured in the dark 共closed squares兲 or under UV with various light intensities at VD = −50 V.
large increase in ID with well-formed saturation. When measured in the dark, the devices showed a maximum ID in the submicroampere region because the device was in a turn-off state for VG = 0. The light-induced ID was immediately restored to the original value as soon as the UV light was switched off during the measurements. A number of charge carriers are generated when light with a photon energy equal to or higher than the band-gap energy of an organic semiconductor is absorbed, thus leading to an increase in ID. This indicates that light can play a role as an additional terminal
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that optically controls device operation, along with the conventional third terminals, the source, drain, and gate electrodes.15,16 Figure 3共c兲 shows the transfer characteristics for pentacene FETs in the dark or under UV irradiation at different powers at VD = −50 V. The IPh / IDark was obtained at VG = 4 V 共switch on voltage VO兲 in order to exclude the effect of electrically induced hole carriers by the gate electrode. The maximum IPh / IDark was 1.3⫻ 105 and the value for all the devices measured were typically greater than 104 under illumination by UV light at 5.0 mW/ cm2. This excellent IPh / IDark is similar to that of a BPTT device23 of 2.0⫻ 105 and 100 times greater than previously reported values for OPTs,18–22 even for amorphous silicon.30 Moreover, those performances were reproduced with the same devices even after storage under ambient conditions for more than 1 month. The transfer curves for the pentacene OPTs showed a VTh shift with increasing incident optical power. This photoinduced VTh shift 共VTh,Ph兲 can be attributed to the wellknown photovoltaic effect resulting from the accumulation of less mobile carriers in inorganic and organic PTs.23,31 When absorption occurs in a p-type pentacene channel, the photogenerated holes easily flow to the drain electrode whereas photogenerated electrons accumulate under the Au source electrode. These accumulated electrons effectively lower the potential barrier between the source and the pentacene channel because of the existence of an energy barrier of ⬃1 eV between the gold and pentacene, thus leading to a thickness-dependent variation in VTh for the case of topcontact geometry.32,33 The lowered injection barrier induced by light irradiation results in an effective decrease in VTh and a significant increase in ID.22,31 In particular, we presume that a large number of photogenerated electrons were accumulated or trapped at the semiconductor-dielectric interface by hydroxyl groups that have been formed on the used SiO2 dielectric.34 The measured VTh,Ph for the pentacene device was as high as 30 V under UV light with 5.0 mW/ cm2 and a similar value was also observed for the BPTT devices.23 The VTh,Ph could be analyzed as the shielding effect of VG, which was also used to explain the well-known bias-stress effect in OFETs, by trapped electrons in organic semiconductors.35–37 In fact, the pentacene OFETs on SiO2 dielectric showed a threshold voltage shift as large as 30 V by a bias-stress effect.35 These VTh,Ph persist for several minutes and the values are then restored to the original value. The time required for restoring can be shortened by light irradiation or other sources with comparable energy through the detrapping of trapped electrons or recombination between trapped electrons and holes.21,37 Figure 4 shows the transfer characteristics of CuPC OFETs in the dark or under UV irradiation with different powers at VD = −50 V. The maximum IPh / IDark was 3000 and that for all measured devices were typically greater than 1000 under UV light at an intensity of 1.55 mW/ cm2. The transfer curves for the CuPC OPTs also showed a VTh,Ph value of around 15 V with increasing incident optical power as was observed in the pentacene and BPTT OPTs, even
J. Appl. Phys. 98, 074505 共2005兲
FIG. 4. Transfer characteristics of the CuPC OPTs measured in the dark 共closed squares兲 or under UV with various light intensities at VD = −50 V.
though the value is smaller. The measured maximum responsivity in CuPC OPTs is 2 A / W and typically 0.5– 2 A / W under 365 nm light. Figure 5共a兲 shows the transfer characteristics of pentacene PTs in the dark or under irradiation with light of various monochromatic wavelengths from the near UV to the visible range. In dark conditions, the device showed a zero VO while the device showed a gate voltage shift to a positive voltage
FIG. 5. 共a兲 Transfer characteristics of the pentacene OPTs measured in the dark 共closed squares兲 or under a xenon lamp at an intensity of 5 mW/ cm2 with color filters of 650, 600, 550, 500, 450 and 400 nm to obtain monochromatic light at VD = −10 V. 共b兲 The responsivity of pentacene PTs as a function of incident optical power under 365 nm UV light or 650 nm visible light 共inset兲 at VG = 0 V and VD = −50 V.
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FIG. 6. 共a兲 IPCE 共%兲 for ITO/pentacene 共140 nm兲 / Al cell and 共b兲 the UVvis absorption spectrum of a pentacene film 共140 nm兲.
FIG. 7. 共a兲 IPCE 共%兲 for ITO/CiPC 共150 nm兲 / Al cell and 共b兲 the UV-vis absorption spectrum of the CuPC film 共150 nm兲.
under light irradiation by the above mentioned VTh,Ph. The VTh,Ph also showed a wavelength dependence. A larger VTh,Ph was obtained when light having a higher energy was used and the maximum VTh,Ph 共⬃10 V兲 was observed for a wavelength of 400 nm. This indicates that the number of trapped electrons, which are generated by photoirradiation, is the largest at this wavelength. Thus, a better responsivity would be expected under shorter wavelength light for the case of pentacene OPTs. The responsivity of pentacene OPTs was measured under exposure to 650 and 365 nm light. Figure 5共b兲 and the inset show the responsivity of pentacene OPTs versus the incident light power measured under 365 and 650 nm irradiations, respectively. The measured maximum responsivity for the pentacene OPTs was 47 A / W and typically 10– 40 A / W under the 365 nm light. Whereas, the measured responsivity in the same device decreased by around 100 times under the 650 nm light with the same range of light intensity 关as shown in the inset of Fig. 5共b兲兴. The measured maximum responsivity is 0.45 A / W and typically 0.15– 0.45 A / W under the 650 nm light. These results are consistent with our expectations and are also strongly correlated with the IPCE curve for the pentacene thin films. Figures 6共a兲 and 6共b兲 show the IPCE and absorption spectra of a 140-nm-thick pentacene film sandwiched between the ITO and Al electrodes, respectively. The most interesting result is the dissimilarity of the IPCE curve to the absorption spectrum. Such behavior can be accounted for by the well-known antibatic characteristics by the internal filter effect.38–43 The relationship between photocurrent and absorption spectra can be classified as one of two types for organic semiconductors: symbatic and antibatic. If the maximum photocurrent is obtained for the most strongly absorbed light, the photocurrent response is said to be symbatic with the absorption spectrum. If the maximum of the photocurrent is obtained for the weakest absorbed light, the photocurrent response is said to be antibatic. Both types of photocurrent action spectra were observed depending on the applied bias and the electrode through which it is illuminated. In our case, the sample was illuminated through the ITO electrode in the the measurement of the IPCE spectra. The wavelength range
of light with a high absorption coefficient would be largely absorbed near the ITO. The generated hole can be collected by the ITO electrode while electrons generated by the absorbed light are not collected by the Al electrode through pentacene due to the unipolar transport properties of the charge carriers of organic semiconductors. Therefore, the light absorbed in a narrow zone close to the Al electrode can only contribute to the efficient generation of current, since the less mobile carrier, electrons, that are generated in this region can be directly collected by the Al electrode and the hole can also be simultaneously collected by the ITO electrode via pentacene. This is the so called internal filter effect, which means that light having a large absorption coefficient is filtered in semiconductors by absorption and does not contribute to the generation of current. Therefore, the shape of the IPCE spectrum is the reverse of the absorption spectrum 共as shown in Fig. 6兲 and the same behavior is also observed in the IPCE and the absorption spectrum of CuPC 共as shown in Fig. 7兲. In the case of pentacene, illumination by a short wavelength of less than 550 nm leads to efficient photocurrent generation while efficient photocurrent generation is not observed upon exposure to light with a wavelength of 650 nm, the band-gap energy of pentacene, even though the absorption coefficient is very large at this wavelength, as shown in the absorption curve for pentacene. This IPCE curve shows a strong correlation with the dependence of responsivity and VTh,Ph on the wavelength of the incident light of pentacene OPTs, as described above 共Fig. 5兲. On the other hand, the absorbance at 650 nm for the pentacene film is three times larger than at 365 nm. This, therefore, indicates that the performance of the OPTs is strongly dependent on the IPCE spectrum and not absorption. This correlation between the performance of OPTs and IPCE might be explained by the location of charge generation in the OFETs by photoirradiation. It was recently found, both theoretically and experimentally, that an effective charge flow layer at an insulator-semiconductor interface is formed in the first two molecular layers next to the dielectric interface, which dominates charge transport in OFETs.44,45 Consequently, it might be concluded that the most efficient OPTs would be obtained under conditions which satisfy the
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OPTs also follow the photovoltaic effect in the turn-on state and the photoconductive effect in the turn-off state, analogous to their inorganic counterparts. Moreover, those characteristics are consistent with CuPC and BPTT OPTs as described in our previous report.23 Therefore, these characteristics suggest the existence of a universal mechanism in OPTs with a limitation in device geometry 共top contact兲 and the crystallinity of semiconducting layer 共polycrystalline兲. IV. CONCLUSIONS
FIG. 8. Photocurrent of the pentacene PTs as a function of incident light power under the turn-on 共VG = −50 V, closed squares兲 and turn-off 共minimum ID, open squares兲 states at VD = −50 V. The symbols denote the measured data points and the solid lines indicate the fitted results using Eqs. 共2兲 and 共3兲.
two conditions of incident light: The incident photon energies should be 共i兲 larger than the band gap and 共ii兲 have a relatively small absorption coefficient, since light can penetrate more efficiently down to the channel layer near the dielectric interface without any large loss in absorption by the film. These photons can lead to an efficient increase in ID. Figure 8 shows the photocurrent of pentacene PTs as a function of incident light power under the turn-on 共VG = −50 V兲 and turn-off 共minimum ID兲 states at VD = −50 V. It is known that two different effects, i.e., photoconductive and photovoltaic effects, occur in the active semiconductor layer as the result of photoirradiation for inorganic phototransistors.15–17 When the device is in the turn-on state 共VG ⬎ VTh兲, the photovoltaic effect is significant because a photovoltage is induced by the large number of accumulated trapped electrons under the source. Whereas, when the device is in the turn-off state 共VG ⬍ VTh兲, the ID shows a relatively small increase with optical power due to a photoconductive effect. The photocurrent caused by the photovoltaic effect can be expressed as31 IPh,Pv = G M ⌬VTh =
冉
冊
qPopt AkT ln 1 + , q IPdhc
共2兲
where is the quantum efficiency, Popt the incident optical power, IPd the dark current for electrons, hc / the photon energy, G M the transconductance, ⌬VTh the threshold voltage shift, and A the fitting parameter. The photocurrent induced by a photoconductive effect in the device turn-off state can be described as46 IPh,Pc = 共q p pE兲WD = BPopt ,
共3兲
where p is the hole mobility, p the hole concentration, E the electrical field in the channel, W the gate width, D the depth of absorption region, and B the fitting parameter. In Fig. 8, IPh are plotted as a function of incident optical powers at the device turn-on state 共VG = −50 V and VD = −50 V兲 and the turn-off state 共VG was selected at the minimum ID, VD = −50 V兲. The symbols denote the measured data points and the solid lines indicate the fitted results using Eqs. 共2兲 and 共3兲. The well-fitted line indicates that pentacene
In conclusion, highly photosensitive organic thin-film phototransistors with an OFET configuration were fabricated based on pentacene and CuPC, and the effect on the wavelength of the light source of the device performance was examined. The pentacene and CuPC OPTs were optically switched even under irradiation by a low power of light 共⬃ several microwatts兲 and showed excellent photosensitivity and high IPh / IDark when light above the band-gap energy of the semiconductor is used. The performances of CuPC and pentacene OPTs were strongly correlated with their IPCE spectra. The pentacene OPTs showed a 100 times higher responsivity and IPh / IDark under illumination by 365 nm light compared with 650 nm light, even though the absorbance at 650 nm for the pentacene film was three times larger than at 365 nm. This shows that the selection of the wavelength is a very important requirement for obtaining high performance OPTs. A comparison of the measured results and theory confirms that the OPTs followed photovoltaic 共turn-on state兲 and photoconductive 共turn-off state兲 behaviors. The major advantage of using organic or polymer semiconductors for PTs is the possibility of preparing flexible devices. It seems likely that OPTs could be fabricated on various flexible substrates such as polymers or clothing for uses as visible-light detectors and UV intensity indicators in the presence of solar light. The promising opportunity for near-IR applications can be further opened by the tuning of the wavelength for sensing via organic semiconductors with a smaller band gap. ACKNOWLEDGMENTS
The authors wish to thank Dr. Yuji Yoshida 共AIST兲 for fruitful discussions related to FET measurements. This work was financially supported by the Korea Science and Engineering Foundation 共KOSEF兲 via National Research Laboratory 共NRL兲 program, Heeger Center for Advanced Materials, 21th Century Frontier R&D Program of Ministry of Science and Technology 共MOST兲, and Program for Integrated Molecular System, respectively. One of the authors 共Y.Y.N.兲 was supported by the Korea Science and Engineering Foundation 共KOSEF兲 for visiting research at AIST. G. Horowitz, Adv. Mater. 共Weinheim, Ger.兲 10, 365 共1998兲. H. E. Katz and Z. Bao, J. Phys. Chem. B 104, 671 共2000兲. C. D. Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater. 共Weinheim, Ger.兲 14, 99 共2002兲. 4 J. M. Shaw and P. F. Seidler, IBM J. Res. Dev. 45, 3 共2001兲. 5 P. Peumans, A. Yakimov, and S. R. Forrest, J. Appl. Phys. 93, 3693 共2003兲. 6 C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen, Adv. Funct. Mater. 11, 15 共2001兲. 1 2 3
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