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Field induced electro-optical characterization in poly-3-hexylthiophene MIS capacitor
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Thin Solid Films 516 (2008) 2568 – 2572 www.elsevier.com/locate/tsf
Field induced electro-optical characterization in poly-3-hexylthiophene MIS capacitor Eiji Itoh ⁎, Hiroshi Nagai, Keiichi Miyairi Department of Electrical and Electronic Engineering, Shinshu University, 4-17-1, Wakasato, Nagano, 380-8553, Japan Available online 29 April 2007
Abstract We investigated the electro-optical properties of the transmitted light in MIS capacitor consisting of P3HT/polyimide double-layered device by optical charge modulation spectroscopy (CMS) technique. A pronounced charge-induced subgap transition, the direct evidence for the polaronic nature of the transition from delocalized polaron, was observed in accumulation in the wave length between 650 nm and 1100 nm, and it almost disappeared in fully depletion of positive gate voltage. The charge induced optical transition measured at 766 nm increases linearly with the amplitude of AC modulation in accumulation, whereas it was suppressed when the positive large gate bias was applied to the film. On the other hand, the CMS curves observed between 460 nm and 650 nm changed a little with gate voltage and it was considered as the electro-absorption effect related to the interfacial traps at P3HT/polyimide interface. © 2007 Elsevier B.V. All rights reserved. Keywords: Polythiophene; MIS capacitor; Charge modulation spectroscopy; Polyimide
1. Introduction Thin-film, polymer-based organic field-effect transistor (pOFET) is considered as very attractive device recently because all the layer s of a p-OFET can be deposited and patterned at low temperature by a combination of low-cost solution-processing and direct-write printing, which is suitable for realization of low-cost, large-area electronic devices on flexible substrates such as polyimide, polyethylenetelephthalate (PET), and polyethylenenaphthalate (PEN) sheets. And therefore they are expected to be used for the active-matrix of flexible-electronic paper-like display, simple-low-cost radiofrequency identification (RFID) tags compatible with existing communication standards, etc. [1–8]. One promising material that has undergone intensive study for p-OFET applications is regioregular poly (3-hexylthiophene) (RR-P3HT) because reported value of mobility in P3HT is comparable to a-Si [1–4]. When electronic charges are injected into an organic semiconductor, such as P3HT, sub-band gap states are formed to accommodate this charge, so-called polarons are formed at the P3HT/insulator ⁎ Corresponding author. E-mail address: [email protected] (E. Itoh). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.102
interface. Subsequently, the additional optical transitions are created in the optical absorption spectra [1–4,8]. It is well known that a large number of majority charges should be accumulated to obtain the field effect of p-OFET at the semiconductor/gate insulator interface. It is therefore considered that a detectable change in the optical absorption spectra should be observed by changing the gate voltage. The nature of the charge carriers in P3HT has been investigated experimentally by Sirringhaus et al. using optical charge modulation spectroscopy (CMS) technique [8–11]. The influence of substantial CMS changes due to the subgap transition were then considered as delocalization of the polarons in the microcrystalline P3HT over several adjacent polythiophene chains [9,11]. Since the CMS change enhanced by the use of highly ordered RR-P3HT, with a high hole mobility due to the efficient delocalization of polarons, CMS change under the gate biasing seems to be the useful technique to investigate the charging phenomena in P3HT as well as the conventional capacitance–voltage (C–V) and FET characteristics. In this study, we have investigated the electro-optical properties of the transmitted light by CMS technique in an MIS capacitor consisting of ITO/polyimide/P3HT/Au doublelayered structure. The capacitance–voltage characteristics were
E. Itoh et al. / Thin Solid Films 516 (2008) 2568–2572
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Fig. 1. The configuration of samples and chemical structures of molecules used in this study.
also measured in order to obtain the relationship between the optical transition of P3HT and the behaviors of charges accumulated at P3HT/polyimide interface. 2. Experiments Fig. 1 shows the configuration of samples and chemical structures of molecules used in this study. A 2-mm-wide ITO transparent electrode coated on glass slide was used as gate electrode. Partially fluorinated polyimide with a thickness of 150 nm was then deposited as a gate insulator by spin-coating of precursor material followed by the thermal imidization at 350 °C for 2 h. The sample was then soaked in ethanol for a few minutes to dedope the unexpected ionic contaminant. In this study, regioregular P3HT with regioregularity of higher than 98% was used as a p-type semiconductor kindly supplied from Merck Ltd. A 30-nm-thick P3HT was spin-coated from 0.5 wt% chloroform solution on polyimide. Finally, a 2-mm-wide semitransparent Au top electrode was evaporated. The electrode area of the sample was 4 mm2. Sample A was then transferred into the grove box filled with Ar atmosphere and it was heat
treated at 100 °C on a hot plate for 30 min in order to remove the adsorbate such as oxygen and water molecules. The sample A was then encapsulated by glass slide which is surrounded by UV cured resin. On the other hand, sample B was not encapsulated. Instead of encapsulation, it was fixed in a vacuum chamber and then heated at 100 °C for 1 h. Fig. 2 shows the experimental set up of the electro-optical characterization of MIS capacitors. The charge modulation spectroscopy (CMS) of P3HT MIS diode was measured by applying the DC coupled AC voltage (Vex = VDC + ΔV). A monochromated light by using monochromator or optical bandpass filter was exposed to the MIS diode, and the transmitted light was collected by a Si photodiode. The signal from the photodiode which is connected to the I–V converter is then measured by a lock-in amplifier with reference to the AC voltage. The DC offset voltage VDC was applied between − 30 and + 30 V, whereas the amplitude and the frequency of AC voltage was kept at 1 V and 200 Hz, respectively. The CMS characteristic was measured as a function of wavelength of incident light and the DC offset voltage. The result was then
Fig. 2. The experimental set up of the electro-optical characterization, charge modulation spectroscopy, in this study.
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Fig. 3. The C–V curves of ITO/polyimide (ca. 150 nm)/P3HT (ca. 30 nm)/Au MIS diode as a function of gate voltage. Sample A was encapsulated in Ar with glass slide after heat treatment in Ar atmosphere, and then measured in a ambient atmosphere. Sample B was measured in a vacuum after the heat treatment without encapsulation.
compared with the capacitance–voltage (C–V) characteristic of P3HT MIS diode. Here, the C–V curves of P3HT MIS diode was measured by precise LCR meter (Agilent 4284) as a function of gate DC bias VDC which is applied to ITO bottom electrode, whereas the amplitude of AC voltage was kept at 1 V [12,13]. 3. Results and discussions Fig. 3 shows the C–V curves of ITO/polyimide (150– 200 nm)/P3HT (ca. 30 nm)/Au MIS diode as a function of gate voltage VG measured in Ar (sample A) and in a vacuum (sample B). When a large negative gate voltage is applied to the ITO electrode with respect to the top Au electrode, holes are accumulated at insulator/P3HT interface. The capacitance measured at − 20 V corresponds to the capacitance of polyimide insulator. As increasing the gate voltage toward positively, accumulated holes almost disappear at particular voltage. We defined this voltage as the threshold voltage VT1. P3HT is depleted of free charge carriers (hole) and depletion layer is formed at the interface for VG N VT1. The capacitance decreases with the increment of VG and then becomes a constant value at VT2. The P3HT layer might be fully depleted at VT2. It should be noted here that VT1 and VT2 for sample A are estimated as +2 V and +15.5 V, whereas those for sample B are estimated as − 3.5 V and + 4.1 V. That is, both VT1 and VT2 of sample A, which is encapsulated in Ar atmosphere, are larger than those of sample B, and VT2–VT1 is also larger for sample A. It is known that p-type organic semiconductor with low ionization energy (typically less than 5.0 eV), such as P3HT, tend to give large positive shift of VT1 and C–V curve changes slowly with the increment of VG upon exposure to air probably due to the oxygen doping [14,15]. And heat treatment in a
vacuum is often required to cancel the formation of charge– transfer complex between P3HT and oxygen [12,13,15]. Therefore, we considered that this large positive shift of VT1 and VT2 in sample A is caused by the doping of oxygen molecules penetrated through UV cured resin. Unexpected contaminant, which is still remain in the film after the dedoping process, may also influence the C–V curves, and optimum treatment procedure should be established as soon as possible. Fig. 4 shows the CMS characteristics, that is, the relationship between the change in the optical transmission and the wavelength of the incident light at various DC voltages in sample A. Here, sample A was illuminated by monochromatic light through optical bandpass filters for 470, 500, 550, 600, 694, 766, 830, 905 and 1069 nm. The bandwidth of each bandpass filter is ca. 10 nm. It is interestingly noted here that the optical transmission decreases above 600 nm whereas it increases below 600 nm. The decrease in ΔT/T at the wavelengths between 694 nm and 1069 nm means the formation of new optical absorption since no absorption was observed in intrinsic P3HT film. It should be noted here that the magnitude of ΔT/T gradually decreased with the increment of VG above VT1 (∼ 0 V) and CMS change above 694 nm almost disappears at the voltage above VT2 (∼+ 20 V). This chargeinduced subgap transition is considered as the direct evidence for the polaronic nature of the charge carriers. Sirringhaus's group reported that there is a broad change in CMS curves at the energy levels between 1.3 and 1.8 eV and it is attributed to the charge induced transition from radical cations of thiophene molecules. They reported that the pronounced charge-induced transition observed at around 1.75 eV (∼ 700 nm) was considered as the transition from delocalized polaron over the two conjugated chains because it was pronounced in highly ordered high mobility RR P3HT [9–11]. It should be noted here that ΔT/T observed at the voltage between VT1 and VT2 gives an important information about the charging behavior in the MIS
Fig. 4. The relationship between the change in the optical transmission ΔT/T and the wavelength of the incident light (CMS curves) at various DC offset voltages in sample A.
E. Itoh et al. / Thin Solid Films 516 (2008) 2568–2572
Fig. 5. The relationship between ΔT/T and the wavelength of the incident light (CMS curves) at various DC offset voltages of − 10 V, 0 and +10 V in sample B.
devices. The resistivity of depleted P3HT is not enough high and some carriers can move across the depleted region of P3HT film between Au and P3HT/polyimide interface at the low frequency such as 200 Hz. Both the condition of CMS measurement such as frequency of AC modulation and the heat treatment process should be optimized. Similar CMS curves were again observed in sample B as shown in Fig. 5. It should be noted here that ΔT/T between 460 nm and 650 nm seems to be independent of VG, whereas ΔT/T between 650 nm and 1100 nm depends on VG and it almost disappears above VT2 of sample B. The quick change in CMS curves between 0 V and + 10 V corresponds to C–V curve of sample B in Fig. 3. When the sample was biased into a fully depleted state or relatively under high electric field, the experiment is basically an electroabsorption experiment. We therefore considered that ΔT/T below 650 nm is probably due to the electroabsorption of P3HT, and the CMS change observed in both accumulation and depletion seems to be the evidence of existence of deep trap level or high resistance region in P3HT. The hole mobility of RR-P3HT FET prepared from chloroform solution on polyimide gate insulator used in this study was only 9 × 10− 5 cm2/Vs. It is noted here that RRP3HT device prepared from xylene on the other type of polyimide gives the higher mobility (N 2 × 10− 3 cm2/Vs), and ΔT/T disappears for VDC ≫ VT2. The details of the results of RR-P3HT device from xylene is under investigation. In our previous study, the impedance spectroscopy of MIS capacitor consisting of ITO/polyimide/P3HT/Au structure gives us the information of interfacial trap with a high density of ∼ 1012 cm− 2 eV− 1 at P3HT/polyimide interface [12,13]. We assumed that these interfacial traps formed at P3HT/polyimide interface might be the origin of this high resistance region. The electronic charges penetrating thorough P3HT would be trapped by interfacial states in both accumulation and depletion. In addition, high electric field formed due to the space charges at the P3HT/polyimide interface may cause the CMS change below 650 nm. On the other hand, the magnitudes of ΔT/T
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observed in Figs. 4 and 5 seem to be much smaller than the reported value (2–3 × 10− 2%) of highly ordered RR-P3HT MIS diode by Sirringhaus's group even though the thickness of the gate insulator and the amplitude of AC voltage was comparable [9–11]. Since ΔT/T observed in highly ordered RR-P3HT MIS diode becomes larger than disordered one [9–11], we attributed the small value of ΔT/T to the disordering of RR-P3HT at P3HT/polyimide interface and very low mobility of our P3HT FET. Finally, we investigated the relationship between the amplitude of AC voltage and the magnitude of ΔT/T measured at 766 nm in sample A as shown in Fig. 6. The magnitude of ΔT/T increases almost linearly with the amplitude of AC when the offset voltage was kept at 0 V, whereas the slope of the curve decreases with the increment of DC offset voltage VDC and the non-linear relation is observed at VDC = + 20 V. As discussed in Fig. 3, holes are accumulated at the voltage below +2 V in sample A. The magnitude of ΔT/T should be proportional to either the number of accumulated holes at P3HT/polyimide interface or the electric field in P3HT layer (probably at the interface region). Especially, ΔT/T measured at 766 nm may strongly depend on the number of polaronic charges formed at the P3HT/polyimide interface. That is, the polaronic charges linearly increase with the increment of the magnitude of gate voltage during the half cycle of AC voltage, especially when the total value of VG becomes lower than VT1. The magnitude of ΔT/T therefore increases with the amplitude of AC voltage. On the other hand, ΔT/T measured at VDC = + 20 V should be small when the sample is fully depleted. The small change of ΔT/T for |VAC| b 6–7 V is probably attributed to the migration of charge carriers through oxygen doped P3HT. The slope of ΔT/T gradually increases with the amplitude of AC voltage when the half cycle of the AC voltage overlaps with the voltage region between VT1 and VT2. And the same slope with VDC = 0 V would be obtained when the AC voltage overlaps with the voltage region below VT1. The experimental results in Fig. 6
Fig. 6. The relationship between the amplitude of AC voltage and the magnitude of CMS change measured at 766 nm in sample A at the gate voltage (DC offset) of 0, +10 and +20 V.
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correspond well to the aforementioned discussions. The combination of CMS technique and C–V measurement thus seems to be the useful method to understand the carrier dynamics in MIS device.
as well as FET measurement is useful method to understand the interfacial charging phenomena in polymer-based MIS devices.
4. Conclusion
This work was supported by the CLUSTER of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
In this paper, we investigated the electro-optical properties of the transmitted light in MIS capacitor consisting of P3HT/ polyimide double-layered device by optical charge modulation spectroscopy (CMS) technique. The relationship between the DC gate voltage and the capacitance was also measured in order to obtain the relationship between the charging phenomena and CMS results. Capacitance of MIS diode under negative gate biasing saturates at the capacitance of polyimide gate insulator, whereas it decreases with VG above threshold voltage. The threshold voltage shifts toward positively in the sample encapsulated in Ar atmosphere, and it was attributed to the oxygen doping penetrate through the UV cured resin. The pronounced charge-induced, subgap transition observed in CMS characteristics was considered as the direct evidence for the polaronic nature of the accumulated charges for VG b 0 V. The CMS curves observed between 650 nm and 1000 nm strongly depended on VG, whereas CMS curves observed between 460 nm and 650 nm changed a little with VG and it was considered as the electro-absorption effect in dielectrics-like P3HT probably reflected the very low hole mobility and interfacial traps in our RR-P3HT FET device. We therefore considered that the combination of CMS and C–V measurement
Acknowledgements
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[8] [9]
[10] [11] [12] [13] [14] [15]
E. Ebisawa, T. Kurosawa, S. Nara, J. Appl. Phys. 54 (1983) 3255. J.H. Burroughes, C.A. Jones, R.H. Friend, Nature 335 (1988) 137. Z. Bao, A. Dodabalapur, A.J. Lovinger, Appl. Phys. Lett. 69 (1996) 4108. H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. G. Wang, J. Swensen, D. Moses, A.J. Heeger, J. Appl. Phys. 93 (2003) 6137. S.R. Forrest, Nature 428 (2004) 911. H. Sirringhaus, L. Buergi, T. Kawase, R.H. Friend, in: C.R. Kagan, P. Andry (Eds.), Thin Film Transistors, Marcel Dekker, New York, 2003, p. 427. H. Sirringhaus, Adv. Mater. 17 (2005) 2411. H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W. Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P. Herwigs, D.M. de Leeuws, Nature 401 (1999) 685. B.J. Brown, H. Sirringhaus, R.H. Friend, Synthetic Metals 101 (1999) 557. D. Beljonne, J. Cornil, H. Sirringhaus, P.J. Brown, M. Shkunov, R.H. Friend, J.L. Bredas, Adv. Func. Mater. 11 (2001) 229. I. Torres, D.M. Taylor, E. Itoh, Appl. Phys. Lett. 85 (2004) 314. E. Itoh, I. Torres, D.M. Taylor, Jpn. J. Appl. Phys. 44 (2005). M.S.A. Abdou, F.P. Orfino, Y. Son, S. Holdcroft, J. Am. Chem. Soc. 119 (1997) 4518. E.J. Meijer, C. Deteheverry, P.J. Baesjou, E. van Veenendaal, D.M. de Leeuw, T.M. Klapwijk, J. Appl. Phys. 93 (2003) 4831.
Thin Solid Films 516 (2008) 2568 – 2572 www.elsevier.com/locate/tsf
Field induced electro-optical characterization in poly-3-hexylthiophene MIS capacitor Eiji Itoh ⁎, Hiroshi Nagai, Keiichi Miyairi Department of Electrical and Electronic Engineering, Shinshu University, 4-17-1, Wakasato, Nagano, 380-8553, Japan Available online 29 April 2007
Abstract We investigated the electro-optical properties of the transmitted light in MIS capacitor consisting of P3HT/polyimide double-layered device by optical charge modulation spectroscopy (CMS) technique. A pronounced charge-induced subgap transition, the direct evidence for the polaronic nature of the transition from delocalized polaron, was observed in accumulation in the wave length between 650 nm and 1100 nm, and it almost disappeared in fully depletion of positive gate voltage. The charge induced optical transition measured at 766 nm increases linearly with the amplitude of AC modulation in accumulation, whereas it was suppressed when the positive large gate bias was applied to the film. On the other hand, the CMS curves observed between 460 nm and 650 nm changed a little with gate voltage and it was considered as the electro-absorption effect related to the interfacial traps at P3HT/polyimide interface. © 2007 Elsevier B.V. All rights reserved. Keywords: Polythiophene; MIS capacitor; Charge modulation spectroscopy; Polyimide
1. Introduction Thin-film, polymer-based organic field-effect transistor (pOFET) is considered as very attractive device recently because all the layer s of a p-OFET can be deposited and patterned at low temperature by a combination of low-cost solution-processing and direct-write printing, which is suitable for realization of low-cost, large-area electronic devices on flexible substrates such as polyimide, polyethylenetelephthalate (PET), and polyethylenenaphthalate (PEN) sheets. And therefore they are expected to be used for the active-matrix of flexible-electronic paper-like display, simple-low-cost radiofrequency identification (RFID) tags compatible with existing communication standards, etc. [1–8]. One promising material that has undergone intensive study for p-OFET applications is regioregular poly (3-hexylthiophene) (RR-P3HT) because reported value of mobility in P3HT is comparable to a-Si [1–4]. When electronic charges are injected into an organic semiconductor, such as P3HT, sub-band gap states are formed to accommodate this charge, so-called polarons are formed at the P3HT/insulator ⁎ Corresponding author. E-mail address: [email protected] (E. Itoh). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.102
interface. Subsequently, the additional optical transitions are created in the optical absorption spectra [1–4,8]. It is well known that a large number of majority charges should be accumulated to obtain the field effect of p-OFET at the semiconductor/gate insulator interface. It is therefore considered that a detectable change in the optical absorption spectra should be observed by changing the gate voltage. The nature of the charge carriers in P3HT has been investigated experimentally by Sirringhaus et al. using optical charge modulation spectroscopy (CMS) technique [8–11]. The influence of substantial CMS changes due to the subgap transition were then considered as delocalization of the polarons in the microcrystalline P3HT over several adjacent polythiophene chains [9,11]. Since the CMS change enhanced by the use of highly ordered RR-P3HT, with a high hole mobility due to the efficient delocalization of polarons, CMS change under the gate biasing seems to be the useful technique to investigate the charging phenomena in P3HT as well as the conventional capacitance–voltage (C–V) and FET characteristics. In this study, we have investigated the electro-optical properties of the transmitted light by CMS technique in an MIS capacitor consisting of ITO/polyimide/P3HT/Au doublelayered structure. The capacitance–voltage characteristics were
E. Itoh et al. / Thin Solid Films 516 (2008) 2568–2572
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Fig. 1. The configuration of samples and chemical structures of molecules used in this study.
also measured in order to obtain the relationship between the optical transition of P3HT and the behaviors of charges accumulated at P3HT/polyimide interface. 2. Experiments Fig. 1 shows the configuration of samples and chemical structures of molecules used in this study. A 2-mm-wide ITO transparent electrode coated on glass slide was used as gate electrode. Partially fluorinated polyimide with a thickness of 150 nm was then deposited as a gate insulator by spin-coating of precursor material followed by the thermal imidization at 350 °C for 2 h. The sample was then soaked in ethanol for a few minutes to dedope the unexpected ionic contaminant. In this study, regioregular P3HT with regioregularity of higher than 98% was used as a p-type semiconductor kindly supplied from Merck Ltd. A 30-nm-thick P3HT was spin-coated from 0.5 wt% chloroform solution on polyimide. Finally, a 2-mm-wide semitransparent Au top electrode was evaporated. The electrode area of the sample was 4 mm2. Sample A was then transferred into the grove box filled with Ar atmosphere and it was heat
treated at 100 °C on a hot plate for 30 min in order to remove the adsorbate such as oxygen and water molecules. The sample A was then encapsulated by glass slide which is surrounded by UV cured resin. On the other hand, sample B was not encapsulated. Instead of encapsulation, it was fixed in a vacuum chamber and then heated at 100 °C for 1 h. Fig. 2 shows the experimental set up of the electro-optical characterization of MIS capacitors. The charge modulation spectroscopy (CMS) of P3HT MIS diode was measured by applying the DC coupled AC voltage (Vex = VDC + ΔV). A monochromated light by using monochromator or optical bandpass filter was exposed to the MIS diode, and the transmitted light was collected by a Si photodiode. The signal from the photodiode which is connected to the I–V converter is then measured by a lock-in amplifier with reference to the AC voltage. The DC offset voltage VDC was applied between − 30 and + 30 V, whereas the amplitude and the frequency of AC voltage was kept at 1 V and 200 Hz, respectively. The CMS characteristic was measured as a function of wavelength of incident light and the DC offset voltage. The result was then
Fig. 2. The experimental set up of the electro-optical characterization, charge modulation spectroscopy, in this study.
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Fig. 3. The C–V curves of ITO/polyimide (ca. 150 nm)/P3HT (ca. 30 nm)/Au MIS diode as a function of gate voltage. Sample A was encapsulated in Ar with glass slide after heat treatment in Ar atmosphere, and then measured in a ambient atmosphere. Sample B was measured in a vacuum after the heat treatment without encapsulation.
compared with the capacitance–voltage (C–V) characteristic of P3HT MIS diode. Here, the C–V curves of P3HT MIS diode was measured by precise LCR meter (Agilent 4284) as a function of gate DC bias VDC which is applied to ITO bottom electrode, whereas the amplitude of AC voltage was kept at 1 V [12,13]. 3. Results and discussions Fig. 3 shows the C–V curves of ITO/polyimide (150– 200 nm)/P3HT (ca. 30 nm)/Au MIS diode as a function of gate voltage VG measured in Ar (sample A) and in a vacuum (sample B). When a large negative gate voltage is applied to the ITO electrode with respect to the top Au electrode, holes are accumulated at insulator/P3HT interface. The capacitance measured at − 20 V corresponds to the capacitance of polyimide insulator. As increasing the gate voltage toward positively, accumulated holes almost disappear at particular voltage. We defined this voltage as the threshold voltage VT1. P3HT is depleted of free charge carriers (hole) and depletion layer is formed at the interface for VG N VT1. The capacitance decreases with the increment of VG and then becomes a constant value at VT2. The P3HT layer might be fully depleted at VT2. It should be noted here that VT1 and VT2 for sample A are estimated as +2 V and +15.5 V, whereas those for sample B are estimated as − 3.5 V and + 4.1 V. That is, both VT1 and VT2 of sample A, which is encapsulated in Ar atmosphere, are larger than those of sample B, and VT2–VT1 is also larger for sample A. It is known that p-type organic semiconductor with low ionization energy (typically less than 5.0 eV), such as P3HT, tend to give large positive shift of VT1 and C–V curve changes slowly with the increment of VG upon exposure to air probably due to the oxygen doping [14,15]. And heat treatment in a
vacuum is often required to cancel the formation of charge– transfer complex between P3HT and oxygen [12,13,15]. Therefore, we considered that this large positive shift of VT1 and VT2 in sample A is caused by the doping of oxygen molecules penetrated through UV cured resin. Unexpected contaminant, which is still remain in the film after the dedoping process, may also influence the C–V curves, and optimum treatment procedure should be established as soon as possible. Fig. 4 shows the CMS characteristics, that is, the relationship between the change in the optical transmission and the wavelength of the incident light at various DC voltages in sample A. Here, sample A was illuminated by monochromatic light through optical bandpass filters for 470, 500, 550, 600, 694, 766, 830, 905 and 1069 nm. The bandwidth of each bandpass filter is ca. 10 nm. It is interestingly noted here that the optical transmission decreases above 600 nm whereas it increases below 600 nm. The decrease in ΔT/T at the wavelengths between 694 nm and 1069 nm means the formation of new optical absorption since no absorption was observed in intrinsic P3HT film. It should be noted here that the magnitude of ΔT/T gradually decreased with the increment of VG above VT1 (∼ 0 V) and CMS change above 694 nm almost disappears at the voltage above VT2 (∼+ 20 V). This chargeinduced subgap transition is considered as the direct evidence for the polaronic nature of the charge carriers. Sirringhaus's group reported that there is a broad change in CMS curves at the energy levels between 1.3 and 1.8 eV and it is attributed to the charge induced transition from radical cations of thiophene molecules. They reported that the pronounced charge-induced transition observed at around 1.75 eV (∼ 700 nm) was considered as the transition from delocalized polaron over the two conjugated chains because it was pronounced in highly ordered high mobility RR P3HT [9–11]. It should be noted here that ΔT/T observed at the voltage between VT1 and VT2 gives an important information about the charging behavior in the MIS
Fig. 4. The relationship between the change in the optical transmission ΔT/T and the wavelength of the incident light (CMS curves) at various DC offset voltages in sample A.
E. Itoh et al. / Thin Solid Films 516 (2008) 2568–2572
Fig. 5. The relationship between ΔT/T and the wavelength of the incident light (CMS curves) at various DC offset voltages of − 10 V, 0 and +10 V in sample B.
devices. The resistivity of depleted P3HT is not enough high and some carriers can move across the depleted region of P3HT film between Au and P3HT/polyimide interface at the low frequency such as 200 Hz. Both the condition of CMS measurement such as frequency of AC modulation and the heat treatment process should be optimized. Similar CMS curves were again observed in sample B as shown in Fig. 5. It should be noted here that ΔT/T between 460 nm and 650 nm seems to be independent of VG, whereas ΔT/T between 650 nm and 1100 nm depends on VG and it almost disappears above VT2 of sample B. The quick change in CMS curves between 0 V and + 10 V corresponds to C–V curve of sample B in Fig. 3. When the sample was biased into a fully depleted state or relatively under high electric field, the experiment is basically an electroabsorption experiment. We therefore considered that ΔT/T below 650 nm is probably due to the electroabsorption of P3HT, and the CMS change observed in both accumulation and depletion seems to be the evidence of existence of deep trap level or high resistance region in P3HT. The hole mobility of RR-P3HT FET prepared from chloroform solution on polyimide gate insulator used in this study was only 9 × 10− 5 cm2/Vs. It is noted here that RRP3HT device prepared from xylene on the other type of polyimide gives the higher mobility (N 2 × 10− 3 cm2/Vs), and ΔT/T disappears for VDC ≫ VT2. The details of the results of RR-P3HT device from xylene is under investigation. In our previous study, the impedance spectroscopy of MIS capacitor consisting of ITO/polyimide/P3HT/Au structure gives us the information of interfacial trap with a high density of ∼ 1012 cm− 2 eV− 1 at P3HT/polyimide interface [12,13]. We assumed that these interfacial traps formed at P3HT/polyimide interface might be the origin of this high resistance region. The electronic charges penetrating thorough P3HT would be trapped by interfacial states in both accumulation and depletion. In addition, high electric field formed due to the space charges at the P3HT/polyimide interface may cause the CMS change below 650 nm. On the other hand, the magnitudes of ΔT/T
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observed in Figs. 4 and 5 seem to be much smaller than the reported value (2–3 × 10− 2%) of highly ordered RR-P3HT MIS diode by Sirringhaus's group even though the thickness of the gate insulator and the amplitude of AC voltage was comparable [9–11]. Since ΔT/T observed in highly ordered RR-P3HT MIS diode becomes larger than disordered one [9–11], we attributed the small value of ΔT/T to the disordering of RR-P3HT at P3HT/polyimide interface and very low mobility of our P3HT FET. Finally, we investigated the relationship between the amplitude of AC voltage and the magnitude of ΔT/T measured at 766 nm in sample A as shown in Fig. 6. The magnitude of ΔT/T increases almost linearly with the amplitude of AC when the offset voltage was kept at 0 V, whereas the slope of the curve decreases with the increment of DC offset voltage VDC and the non-linear relation is observed at VDC = + 20 V. As discussed in Fig. 3, holes are accumulated at the voltage below +2 V in sample A. The magnitude of ΔT/T should be proportional to either the number of accumulated holes at P3HT/polyimide interface or the electric field in P3HT layer (probably at the interface region). Especially, ΔT/T measured at 766 nm may strongly depend on the number of polaronic charges formed at the P3HT/polyimide interface. That is, the polaronic charges linearly increase with the increment of the magnitude of gate voltage during the half cycle of AC voltage, especially when the total value of VG becomes lower than VT1. The magnitude of ΔT/T therefore increases with the amplitude of AC voltage. On the other hand, ΔT/T measured at VDC = + 20 V should be small when the sample is fully depleted. The small change of ΔT/T for |VAC| b 6–7 V is probably attributed to the migration of charge carriers through oxygen doped P3HT. The slope of ΔT/T gradually increases with the amplitude of AC voltage when the half cycle of the AC voltage overlaps with the voltage region between VT1 and VT2. And the same slope with VDC = 0 V would be obtained when the AC voltage overlaps with the voltage region below VT1. The experimental results in Fig. 6
Fig. 6. The relationship between the amplitude of AC voltage and the magnitude of CMS change measured at 766 nm in sample A at the gate voltage (DC offset) of 0, +10 and +20 V.
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correspond well to the aforementioned discussions. The combination of CMS technique and C–V measurement thus seems to be the useful method to understand the carrier dynamics in MIS device.
as well as FET measurement is useful method to understand the interfacial charging phenomena in polymer-based MIS devices.
4. Conclusion
This work was supported by the CLUSTER of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
In this paper, we investigated the electro-optical properties of the transmitted light in MIS capacitor consisting of P3HT/ polyimide double-layered device by optical charge modulation spectroscopy (CMS) technique. The relationship between the DC gate voltage and the capacitance was also measured in order to obtain the relationship between the charging phenomena and CMS results. Capacitance of MIS diode under negative gate biasing saturates at the capacitance of polyimide gate insulator, whereas it decreases with VG above threshold voltage. The threshold voltage shifts toward positively in the sample encapsulated in Ar atmosphere, and it was attributed to the oxygen doping penetrate through the UV cured resin. The pronounced charge-induced, subgap transition observed in CMS characteristics was considered as the direct evidence for the polaronic nature of the accumulated charges for VG b 0 V. The CMS curves observed between 650 nm and 1000 nm strongly depended on VG, whereas CMS curves observed between 460 nm and 650 nm changed a little with VG and it was considered as the electro-absorption effect in dielectrics-like P3HT probably reflected the very low hole mobility and interfacial traps in our RR-P3HT FET device. We therefore considered that the combination of CMS and C–V measurement
Acknowledgements
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