Effect of light irradiation on the characteristics of organic field-effect transistors

Transcription

1 JOURNAL OF APPLIED PHYSICS 100, Effect of light irradiation on the characteristics of organic field-effect transistors Yong-Young Noh, a Jieun Ghim, Seok-Ju Kang, Kang-Jun Baeg, and Dong-Yu Kim b Center for Frontier Materials, Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-Dong, Buk-Gu, Gwangju , Republic of Korea Kiyoshi Yase Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Higashi, Tsukuba, Ibaraki , Japan Received 21 July 2006; accepted 28 August 2006; published online 2 November 2006 The effect of light irradiation on the characteristics of organic field-effect transistors containing sexithiophene 6-T and pentacene was examined. Organic phototransistors OPTs in which 6-T and pentacene were incorporated were fabricated. Their response behaviors were investigated under conditions of irradiation by either modulated or continuous ultraviolet light with various intensities. Both devices showed two distinguishable responses, i.e., fast and slow responses from photoconductive and photovoltaic effects, respectively. The fast response is mainly the result of the generation of mobile carriers by the absorption of a photon energy higher than the band gap energy of the semiconductor and, subsequently, an increase in conductance via a greater flow of photogenerated mobile carriers into the channel layer. On the other hand, the slow response, which was confirmed by a light induced shift in the threshold voltage V th or the switch-on voltage V O, is the result of a slow release of accumulated and trapped electrons in the semiconductor-gate dielectric interface. The V O is defined as the flatband voltage of devices. Below the V O, the channel current with the gate voltage is off current, and the channel current increases with the gate voltage above the V O. The speed of release of the accumulated charge was dependent on the type of semiconductor used. Pentacene OPTs showed a particularly long retention time. Even after storage for ten days, the shifted V O or V th for the pentacene OPTs by light irradiation was not restored to the original value of the fresh devices. We conclude that this long sustained V th shift renders them attractive for use in light-addressable nonvolatile memory devices American Institute of Physics. DOI: / I. INTRODUCTION Over the past ten years, remarkable advances have been made in the development of organic field-effect transistors OFETs based on organic semiconductors. 1 The performance of OFETs has been greatly improved by the surface treatment of gate dielectrics, 2 8 the development of semiconducting materials or gate insulators, 9 15 and the development of processing techniques Transistors based on pentacene, a prototype material for OFET performance, routinely show field-effect mobility values FET 1cm 2 /V s, a current ratio of the on to the off state I on /I off 10 8, and a threshold voltage V th 0 V. Based on those achievements, applications of OFETs, for example, in chemical or biological sensors, are currently under investigation Organic phototransistors OPTs are one of the viable candidates in which OFETs could be used OPTs are a type of optical transducer because such devices simultaneously require photoconducting properties and high transistor performance. 31,32 PTs combine light detection a Present address: OE group, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK; electronic mail: yyn21@cam.ac.uk b Electronic mail: kimdy@gist.ac.kr and signal amplification properties in a single device without the noise increment associated with avalanche photodiodes To date, a few reports on OPTs have appeared in which a conjugated organic polymer or oligomer is used However, the performance such as responsivity, ratio of photocurrent to dark current I ph /I dark, and charge carrier mobility of these devices were relatively lower than those of their inorganic counterparts. OPTs therefore need to be further improved for future applications. In our previous report, we demonstrated a substantially improved OPT, prepared using various vacuum evaporated oligomers such as 2,5-bis-biphenyl-4-yl-thieno 3, 2,- b thiophene BPTT and copper phthalocyanine CuPC, and concluded that the reason for the enhanced performance of OPTs is mainly based on the selection of the wavelength of the incident light. 29,30 In this paper, we present some recent findings on the response behaviors of OPTs under modulated light and discuss a light induced memory effect produced by accumulated and trapped charges. High performance OPTs using sexithiophene 6-T and pentacene were fabricated and their response behaviors were investigated under conditions of irradiation using various intensities of modulated or continuous ultraviolet UV light. Both devices showed two different distinguishable responses, i.e., a fast and a slow response /2006/100 9 /094501/6/$ , American Institute of Physics

2 Noh et al. J. Appl. Phys. 100, on irradiation. The fast response can be attributed to a modulation in drain current I D by irradiation with an energy higher than the band-gap energy of the semiconductor. The slow response was associated with the slow recovery of a switch-on voltage V O or V th, which had been shifted via light irradiation or light induced hysteresis behavior in the transfer curves of the OPTs. The reason for these response behaviors on light irradiation are discussed herein. II. EXPERIMENT 6-T and pentacene were obtained from Sigma Aldrich and were purified by vacuum gradient sublimation before evaporation T and pentacene thin films nm thick were deposited on the SiO nm, 10 nf/cm 2 surface of a heavily doped silicon wafer, used as the gate electrode, at a deposition rate of nm/s and a substrate temperature T S of 120 and 30 C, respectively. A more detailed description of the evaporation process can be found in our previous report After evaporation, the OPTs were completed by evaporating a 50 nm thick gold layer 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 L and a width W 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 a wavelength of nm peak wavelength=365 nm using a Hamamatsu LC5 instrument 38 or visible light irradiation using a xenon lamp with various color filters, which permits the wavelength of incident light to be selected in air. The variation in light intensity was obtained using neutral density filters with various transmittances. A Hamamatsu LC5 is a strong UV spotlight source that uses a mercury-xenon lamp. The intensity of the UV light can be maintained constant with a built-in sensor and feedback control. The light was illuminated from the top open side of the devices, i.e., the opposite side of the bottom gate electrode. The light intensity was measured using an Ophir BC-20 photodetector with a calibrated 2A-SH head. The illuminated area was slightly larger than the measured transistor channels. The irradiated light intensity of the active device was determined by dividing the measured light intensity by the dimensions of the transistor channel area. The temperature of the device was monitored using a conventional thermocouple to avoid measurement error, which might be induced by the heating of the devices during the illumination. All measurements were performed at room temperature. III. RESULT AND DISCUSSION OFETs of 6-T and pentacene with a top-contact configuration were fabricated, and various device properties including FET, I on /I off, and V th were measured. Figures 1 a and 1 b show the typical I D versus gate voltage V G at fixed source-drain voltage V D = 50 V and I D characteristics with various V G for the p-channel of 6-T deposited OFETs at a T S of 120 C, respectively. The films showed only sharp diffraction peaks in the x-ray diffractograms corresponding to FIG. 1. a Transfer characteristics of 6-T FET T S =120 C measured at V D = 50 V and b output characteristics with V G in the range from 0 to 50 V in steps of 10 V. the 00l order of the molecular long axis of the thin film phase and a layered morphology with an edge-on orientation at the T S used, consistent with prior reports. 39,40 All topcontact devices in which 6-T or pentacene was used as the active layer showed well-defined linear and saturation characteristics, as shown in Fig. 1 b. The FET and V th of the OFETs were obtained from Eq. 1 for the saturation regimes as proposed by Horowitz et al. 41 I sat D = WC i FET V G V th 2, 2L where L is the channel length, W the channel width, and C i the capacitance per unit area of the gate dielectric layer C i =10 nf/cm 2 for 300 nm thick SiO 2. The calculated FET, I on /I off, and V th obtained by plotting I D and I D 1/2 vs V G,as shown in Fig. 1 a, are 0.09 cm 2 V 1 s 1, , and 4.5 V, respectively. These parameters for the pentacene OFETs, fabricated at a T s of 30 C, were also calculated using the same procedures. The obtained FET, I on /I off, and V th for the pentacene devices 100 nm thick are 0.7 cm 2 V 1 s 1,2 10 6, and 15 V, respectively. For measurement of OPT characteristics, the output and transfer characteristics of the 6-T and pentacene devices were measured under exposure to UV or visible light with top illumination; i.e., the light impinged on the opposite side of high doped n-type silicon wafer used as a bottom gate electrode. 1

3 Noh et al. J. Appl. Phys. 100, FIG. 2. a Output characteristics of 6-T OPTs measured in the dark dashed line or under modulated UV illumination of 1.5 mw/cm 2 solid line at V G =0 V. The UV light was switched on and off manually during the measurement. b Transfer characteristics of 6-T OPTs V D = 50 V measured in the dark filled squares, under UV illumination filled circles, immediately after the light was turned off filled triangles, and again after storage in the dark for five days opened squares. Figures 2 a and 2 b show the output and transfer characteristics, respectively, for the 6-T device in the dark and under UV irradiation at 1.5 mw/cm 2. The 6-T OPTs, measured under UV irradiation, showed a large increase in I D with almost a saturation shape even at V G =0 V. When measured in the dark, the devices showed a maximum I D in the nanoampere region because the device was in the turn-off state in the V G =0 A number of excitons, subsequently electrons and holes, 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 increases in I D. This indicates that light can play a role as an additional terminal that optically controls device operation along with the conventional three terminals; source, drain, and gate electrodes. 4 Two important parameters that indicate the performance of a photodetector are the responsivity R in A/W and the ratio of photo-on and dark-off I D at V G =V O I ph /I dark. R is defined as 26,42 R = I ph = I D,illum I D,dark, 2 P inc EA where I ph is the generated drain current by light irradiation, P inc the optical power incident on the channel of the device, I D,illum the drain current under illumination, I D,dark the drain current in the dark, E the irradiation of the incident light power/area, and A the effective device area. The 6-T and pentacene OPTs 100 nm thick show high R of and A/W and high I ph /I dark of 1300 and V G =0 V, respectively, under 365 nm UV light with an intensity of 1.5 mw/cm 2. The I ph /I dark was obtained at V G =V O in order to exclude the effect of electrically induced hole carriers by the gate electrode. This excellent I ph /I dark for pentacene OPTs is 100 times greater than previously reported values for OPTs and even for amorphous silicon ,43 Regarding the performance of the devices, the thickness of the semiconducting layer was dependent on the wavelength of the incident light. However, this subject is not discussed because it is beyond the scope of this paper. The one interesting finding is that the increase in I D was not only induced via more photogenerated mobile carriers, i.e., holes. The intensity of I D was changed with a rather fast speed following the modulation of light when the light was switched on and off during the measurement of the output curve, as shown in Fig. 2 a. This behavior clearly results from the contribution of photogenerated excitons and, subsequently, free electrons and holes in the semiconductor. However, I D was not restored to the original off value of the fresh devices measured in the dark when the light was turned off. This suggests that some other behavior as well as the contribution of I D via photogenerated holes occurred at the same time during the light irradiation of the OPTs. Another behavior was confirmed through the measurement of transfer characteristics for devices, which were measured in the dark after the light was turned off, as shown in Fig. 2 b. Figure 2 b shows the transfer characteristics of 6-T OPTs V D = 50 V measured in the dark or under UV irradiation. In addition, the same device was measured again just after the light was turned off and after five days of storage under ambient conditions in the dark. It should be noted that the transfer curves for the 6-T OPTs showed a V th shift with illumination. This photoinduced V th shift can be attributed to a photovoltaic effect by the accumulation of less mobile carriers electrons for this case in inorganic and organic PTs. 29,44 When absorption occurs in the p-type 6-T channel, photogenerated holes easily flow to the drain electrode, whereas photogenerated electrons accumulate under the Au source electrode or at the interface between a semiconductor and a gate dielectric. These accumulated electrons effectively lower the potential barrier between the source and the semiconductor channel, leading to an effective shift in V th and V O. 45 In particular, we presume that a large number of photogenerated electrons were accumulated or trapped at the semiconductor-dielectric interface by hydroxyl groups formed on the SiO 2 dielectric. 46 Figure 2 b also shows that the accumulated electrons are released at a very slow speed. When the device was measured after the UV light was turned off, V O did not recover to the original value of the fresh devices. Moreover, the shifted V O of the device by light irradiation was not restored to the original value of fresh devices, even after storage under dark ambient conditions for one day, and finally recovered five days later with a small decrease in the maximum I D at V G = 50 V. This indicates that the accumulated charges in the semiconductor and the interface between the semiconductor and the gate dielectric are released at a very slow speed. Figure 3 a shows the output characteristics of the pen-

4 Noh et al. J. Appl. Phys. 100, FIG. 4. Transfer characteristics of pentacene OPTs measured under UV irradiation of 1.5 mw/cm 2 at V D = 50 V. The device was scanned from +20 to 50 V and subsequently from 50 to +20 V. FIG. 3. a Output characteristics of pentacene OPTs measured in the dark at V G =0 V dashed line and 10 V dotted line or under modulated UV illumination of 1.5 mw/cm 2 at V G = 10 V solid line. The UV light was switched on and off manually during the measurement. b Transfer characteristics of pentacene OPTs V D = 50 V measured in the dark filled squares, under UV illumination filled circles, and then after storage in the dark for five days filled triangles and ten days opened squares. tacene device measured in the dark and under UV irradiation at 1.5 mw/cm 2. The irradiated devices showed a highly enhanced I D approximately eight times compared with that of the dark device when measured under UV light at V G = 10 V. However, the increase in I D by light irradiation was not only induced via photo-generated holes, similar to 6-T OPTs. The modulated I D value with a rather fast speed following the modulation of light was around 1 A, which corresponds to no more than 1/7 of the total increase in I D by light irradiation in the saturation region. This suggests that a large fraction of the increased I D in the pentacene OPTs can be attributed to a shift in V th induced by the accumulation of one type of carrier electron from the photogenerated exciton in the semiconductor-dielectric interface, thus showing a slow response to modulated light. Figure 3 b shows the transfer characteristics of pentacene OPTs 50 nm thick, V D = 50 V measured in the dark or under 400 nm UV irradiation and of the same device measured in the dark after five and ten days of storage under dark ambient conditions. The shifted V O or V th of the device by light irradiation did not recover its original value for fresh devices, and only 50% of the maximum shift value was sustained even after storage for ten days. It is important to note that the speed of release of the accumulated charge is dependent on the type of semiconductor. Pentacene OPTs did not show fully restored transfer curves even after one month of storage. This long retention time in the V th shift suggests the possibility of applications for light-addressable memory devices. Figure 4 shows the light induced hysteresis behavior of the transfer curves of pentacene OPTs on sweeping the V G from 20 to 50 V at V D = 50 V. The pentacene OPTs did not show hysteresis behavior when measured in the dark. Such a clockwise hysteresis is the result of a distortion in the applied V G by accumulated electrons. The channel resistance modulation memory ratio at V G =0 is around 1000 and the memory window, defined as the difference between the threshold voltage on the upsweep V th =9.9 V and downsweep V th = 6.8 V, is 16.7 V. This memory window value for the pentacene OPTs is comparable to that of an organic nonvolatile memory field-effect transistor with ferroelectric polymer gate dielectrics. 47 The retention time of the pentacene OPTs is not ideal for commercialization, but might be improved via alternative methods, i.e., the extremely slow release of trapped carriers such as device operation at a low temperature or the insertion of a barrier layer with a larger band-gap semiconductor to block a detrapping of trapped electrons. 27 The subsistence of two characteristics in organic OPTs, which consist of a fast responsive photoconductive behavior by photogenerated mobile carriers holes and a slow responsive photovoltaic behavior by accumulated and trapped photogenerated carriers electrons, was confirmed by a comparative fitting of the measured data with theoretical values. Figures 5 a and 5 b show the photocurrent of 6-T OPTs as a function of incident light power under turn-on V G = 50 V and turn-off I D at V O states at V D = 50 V. It is known that two different effects, i.e., photoconductive and photovoltaic effects, are operative in an active semiconductor layer as the result of the photoirradiation of inorganic phototransistors When the device is in the turn-on state V G V th, the photovoltaic effect is significant because the photovoltage is induced by the large number of accumulated trapped electrons under the source, whereas, when the device is in the turn-off state V G V th, the I D shows a relatively small increase with optical power due to a photoconductive

5 Noh et al. J. Appl. Phys. 100, IV. CONCLUSION The electrical characteristics of 6-T and pentacene based OPTs were examined under UV light illumination. The major outcome of the light irradiation of OFETs is a considerable increase in the off-state I D by photons absorbed in the semiconductors. A number of excitons are generated by the absorbed photons, which are of higher energy than the band gap of the semiconductors and are split into electrons and holes by a source-drain field. Subsequently, one type of carrier more mobile carriers easily flows into the channel layer to drain electrode, whereas less mobile carriers accumulate and are trapped in the semiconductor-dielectric interface. These two carriers induce two different characteristics, i.e., photoconductive and photovoltaic effects, in the OPTs. The photoconductive effect by more mobile carriers showed a rather fast response to modulated illuminated light, whereas photovoltaic effects showed a very slow response via a slow release of accumulated electrons. The speed of release of the accumulated or trapped charges on the trap sites is dependent on the type of semiconductor. The pentacene OPTs showed a particularly long retention time several weeks. This long sustained V th shift by light irradiation suggests the possibility of applications in light-addressable field-effect transistor memory devices. FIG. 5. Photocurrent of 6-T OPTs as a function of incident light power under a turn-on V G = 50 V and b turn-off I D at V O states at V D = 50 V. Symbols denote measured data points and solid lines indicate the results fitted using Eqs. 3 and 4. effect. The photocurrent caused by the photovoltaic effect can be expressed as 44,45 I ph,pv = G M V th = AkT q ln 1+ q P opt I pd hc, where is the quantum efficiency, P opt the incident optical power, I pd the dark current for electrons, hc/ the photon energy, G M the transconductance, V th 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 as 42 I ph,pc = q p pe WD = BP opt, 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. 5, I ph are plotted as a function of incident optical powers at the device turn-on state V G = 50 V and V D = 50 V and turnoff state V G was selected at the minimum I D ; V D = 50 V. The symbols denote the measured data points and the solid lines indicate fitted results using Eqs. 3 and 4. The wellfitted line indicates that the 6-T 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, such characteristics are consistent with CuPC and BPTT OPTs, as described in our previous report. 29, ACKNOWLEDGMENTS The authors wish to thank Dr. Yuji Yoshida AIST for fruitful discussions related to FET measurements. This work was financially supported by the Korea Science and Engineering Foundation KOSEF via the National Research Laboratory NRL program, Heeger Center for Advanced Materials, 21st Century Frontier R&D Program of Ministry of Science and Technology MOST, and Program for Integrated Molecular System. One of the authors Y.Y.N. was supported by the Korea Science and Engineering Foundation KOSEF for visiting research at AIST. 1 C. D. Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater. Weinheim, Ger. 14, F.-J. Meyer zu Heringdorf, M. C. Reuter, and R. M. Tromp, Nature London 412, I. Kymissis, C. D. Dimitrakopoulos, and S. Purushothaman, IEEE Trans. Electron Devices 48, A. Salleo, M. L. Chabinyc, M. S. Yang, and R. A. Street, Appl. Phys. Lett. 81, S. Kobayashi et al., Nat. Mater. 3, M. Shtein, J. Mapel, J. B. Benziger, and S. R. Forrest, Appl. Phys. Lett. 81, H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, and E. P. Woo, Science 290, T. W. Kelly, L. D. Boardman, T. D. Dunbar, D. V. Muyres, M. J. Pellerite, and T. P. Smith, J. Phys. Chem. B 107, C. J. Drury, C. M. J. Mutsaers, C. M. Hart, M. Matters, and D. M. de Leeuw, Appl. Phys. Lett. 73, H. E. Katz, X. M. Hong, A. Dodabalapur, and R. Sarpeshkar, J. Appl. Phys. 91, J. Veres, S. D. Ogier, S. W. Leeming, D. C. Cupertino, and S. M. Khaffaf, Adv. Funct. Mater. 13, I. M. Ruthnberg, O. A. Scherman, R. H. Grubbs, W. Jiang, E. Garfunkel, and Z. Bao, J. Am. Chem. Soc. 126, M. Halik et al., Nature London 431, H. Klauk, M. Halik, U. Zschieschang, G. Schimid, W. Radlik, and W. Weber, J. Appl. Phys. 92, L.-L. Chua, P. K. H. Ho, H. Sirringhaus, and R. H. Friend, Appl. Phys.

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