The carrier blocking property of polyterpenol thin films derived from non-synthetic precursor is studied using Electric Field Induced Optical Second Harmonic Generation (EFISHG) technique that can directly probe carrier motion in organic materials. A properly biased double-layer MIM device with a structure of indium zinc oxide (IZO)/polyterpenol/C₆₀/Al shows that by incorporating the polyterpenol thin film, the electron transport can be blocked while the hole transport is allowed. The inherent electron blocking hole transport property is verified using Al/C₆₀/Alq3/polyterpenol/IZO and Al/Alq3/polyterpenol/IZO structures. The rectifying property of polyterpenol is very promising and can be utilized in the fabrication of many organic devices

Bazaka, Kateryna

Iwamoto, Mitsumasa

Jacob, Mohan

Manaka, Takaaki

Taguchi, Dai

Jacob, Mohan V.

ResearchOnline at James Cook University

Electron-blocking hole-transport polyterpenol thin films

Mohan V. Jacob

Kateryna Bazaka

Dai Taguchi

Takaaki Manaka

Mitsumasa Iwamoto

Crossref

Chemical Physics Letters

Queensland University of Technology ePrints Archive

This is the author’s version of a work that was submitted/accepted for pub-lication in the following source:Jacob, Mohan V., Bazaka, Kateryna, Taguchi, Dai, Manaka, Takaaki, &Iwamoto, Mitsumasa(2012)Electron-blocking hole-transport polyterpenol thin films.Chemical Physics Letters, 528, pp. 26-28.This file was downloaded from: http://eprints.qut.edu.au/92070/c© Copyright 2012 ElsevierLicensed under the Creative Commons Attribution; Non-Commercial; No-Derivatives 4.0 International. DOI: 10.1016/j.cplett.2012.01.031Notice: Changes introduced as a result of publishing processes such ascopy-editing and formatting may not be reflected in this document. For adefinitive version of this work, please refer to the published source:http://doi.org/10.1016/j.cplett.2012.01.031   Electron-Blocking Hole-Transport Polyterpenol Thin Films Mohan V. Jacoba*, Kateryna Bazakaa, Dai Taguchib, Takaaki Manakab, Mitsumasa Iwamotob aElectronic Materials Research Lab, School of Engineering, James Cook University, Townsville QLD 4811, Australia bDepartment of Physical Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan  *email: Mohan.Jacob@jcu.edu.au Phone: +61-747814379 Fax: +61-7-47815177  Abstract: The carrier blocking property of polyterpenol thin films derived from non-synthetic precursor is studied using Electric Field Induced Optical Second Harmonic Generation (EFISHG) technique that can directly probe carrier motion in organic materials. A properly biased double-layer MIM device with a structure of indium zinc oxide (IZO)/polyterpenol/C60/Al shows that by incorporating the polyterpenol thin film, the electron transport can be blocked while the hole transport is allowed. The inherent electron blocking hole transport property is verified using Al/C60/Alq3/polyterpenol/IZO and Al/Alq3/polyterpenol/IZO structures. The rectifying property of polyterpenol is very promising and can be utilized in the fabrication of many organic devices. Keywords: EFISHG, polyterpenol, polymer thin film, organic semiconductors  Introduction Recent progress in the area of organic electronic circuits, especially displays, demand new and improved materials for increasing the device efficiency.  In electronics, the flexibility, light weight, and efficient integration of the organic materials based electronic devices offer unique attractions [1-2]. Organic electronics present exciting possibilities for the realization of large-area self-illuminating displays based on organic light emitting diodes (OLED) and organic field effect transistors (OFET) [3], organic solar cells [4] and photovoltaic devices, sensors [5], and plastic/organic electronics [6,7]. In OLEDs, holes and electrons are injected from opposite electrodes into an active molecular/macromolecular medium to produce light emission via exciton decay [8]. The efficiency of the OLEDs will be determined by the different layers used in the device; these include hole transport (HTL), emissive (EML), electron transport (ETL) and electron blocking (EBL) layers. Charge transport through the organic layers effect the efficiency of the devices. Therefore it is essential to incorporate additional layers to control both carriers. The HTL plays a significant role in the device performance by maximizing hole injection from the anode while blocking the efficiency-depleting electron overflow from the EML [8, 9].    There are number of hole/ electron transport/ blocking layers are available. HTLs such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOTPSS) [10],  polyaniline-camphorsulfonic acid    (PANI-CSA) [11], and polypyrrole-dodecylbenzene sulfonic acid (Ppy-BDSA) [12] have been reported in the literature.  The major detriments of many HTLs are only the annealing or ultraviolet photochemical processing requirement but  also factors such as low durability and volume shrinkage, or microcracking resulting in leakage current. In the mean time, only very few hole transport electron blocking layers available. Such layers can improve the efficiency of Organic Light Emitting Diodes (OLEDs). A balanced injection and accumulation of charge carriers are essential to improve the efficiency of OLEDs however due to the misalignment of their extremely low-lying highest occupied molecular orbits (HOMOs) against those of organic HTLs, unbalanced charge transport is prevalent. Sun et al [9] reported enhancement of efficieny of Quantum Dot LEDs with a high color purity by using multiple holeinjection (HIL)/HTL layers, composed of PEDOT, poly(N,N=-bis(4-butylphenyl)-N,N=-bis(phenyl)benzidine (poly-TPD) [or poly(N-vinylcarbozole) (PVK) blending with and without poly-TPD], and a salmon-based deoxyribonucleic acid (DNA) complexed with cetyltrimetylammonium (CTMA) to improve the hole transport. The DNA in conjunction with CTMA act as a hole transport electron blocking layer.   Majority of the currently utilized organic precursors used to fabricate charge transport controlling layers pose an environmental hazard, an impediment, which can be overcome using non-synthetic environmentally conscious alternatives. Polyterpenol is a polymer thin film of tepinen-4-ol, a monoterpene component of the essential oils of several plants, with demonstrated good electrical insulating performance. The developed material is characterized by transparency and stability over the environmental variations. Polyterpenol is used as an interfacial layer in the fabrication of a p-type OFET, substantially improving the mobility of the device [13]. The electrical insulating property of polyterpenol in terms of carrier blocking within double-layer MIM devices was further elucidated through time-resolved visualization of carrier motions using the Electric Field Induced Optical Second Harmonic Generation (EFISHG) technique.   Experiments The fabrication of IZO/polyterpenol/C60/Al double-layer diodes followed the previously outlined methodology [13, 14]. A thin layer of polyterpenol was deposited onto the IZO surface using RF plasma polymerization at a pressure of 100 mTorr, RF frequency of 13.56 MHz, and 100 W RF power. Terpinen-4-ol (Australian Botanical Products) vapors were released into the reactor at a constant rate. The achieved film thickness was 100 nm. Then, a layer of C60 followed by Al electrode were deposited onto the polyterpenol thin film using vacuum evaporation technique, under a pressure of 10-5 Torr and a deposition rate of approximately 1 nm/min.   The I-V characteristics of the IZO/polyterpenol/C60/Al diode were studied, with Al acting as the reference electrode. As shown in Figure1a), the characteristics were comparable to those of a rectifying    diode structure, and no hysteresis was observed. The magnitude of the current was similar to the pentacene-polyterpenol structure, which substantiated our previous findings regarding the insulating behavior of polyterpenol thin films. The results suggested that electron injection from Al electrode as well as hole injection from IZO electrode were probable, but electron injection from the IZO electrode and hole injection from the Al electrode were minor. Electron injection and transport through C60 and hole injection from IZO and transport through polyterpenol was therefore proposed as the mechanism behind the observed phenomenon. However, additional measurements on the structure had to be performed in order to eliminate other potential scenarios, such as electron injection from Al and successive transport through C60 and polyterpenol, or hole injection from IZO and successive transport through polyterpenol and C60 layers.   Time-resolved EFISHG can directly probe the carrier motion within the double-layer diodes, providing direct experimental evidence necessary to verify the carrier behavior within such a device.  The experimental setup used in this study can be found elsewhere [15]. In the experiment, external voltage (Vex) is applied to the double layer with capacitance Ci and conductance Gi and charges ±Qm are induced on the electrodes with a response time τRC = Rs C*, where C* is the total series capacitance of the diode. At time t >> τRC, charge carriers are injected into the device and transported along the direction of the electric field. Should the respective relaxation times between the blocking and the active layers differ, τ1 ≠ τ2, a charge Qs is accumulated at the boundary between these layers with a relaxation time τMW = (C1+C2)/ (G1+G2), τMW >> τRC,  due to the Maxwell-Wagner (MW) effect . The electric field distribution is directly correlated to the SHG signal changes [16, 17].   The EFISHG is distinct in that it is generated in proportion to the square of static electric field, a characteristic that sets it apart from other SHG sources. The expression Ish ∝ d2 P (2ω) 2 can be used to describe the intensity of the second harmonic signal, with the nonlinear polarization wave P (2ω) = 0 (2) : E(ω)E(ω) + 0 (3):E(0)E(ω)E(ω), the thickness of the centrosymmetric layer d, the permittivity of vacuum 0, the average electric field across the probed layer E(0), the electric field of the p-polarized light E(ω), and the nth-order nonlinear susceptibility tensor (n) (n = 2, 3, …), which is a material dependent parameter with ω. For C60, (2) =0, with only the SH generation due to the second term contribution expected to appear. The square-root of SH intensity (Ish)0.5 is used for probing the electric field E(0) which is the square-root of the absolute difference between the SH intensity at the baseline level and under applied external voltage. For C60, the wavelength of the probing light is set to 1000 nm, with the SH light intensity I (2ω) generated at 500 nm [18]. In this case, we used laser beam without focusing to probe average electric field in the device thickness direction originated from charge charging/discharging at the C60/polyterpenol interface.   Before carrying this EFISHG experiments, it is very important to confirm no SHG generation from terpenol, Al and IZO at a wavelength of 500nm, which is used for probing the SHG generation from the    C60 layer. Using Al/terpenol/IZO diodes,  we checked the absorption of the diode with polyterpenol thin film at 500 nm (see the inset of Fig. 1b). Figure 1b shows the intensity of the reflected second harmonic signal for the diode with polyterpenol thin film. As it is exhibited, at the selected wavelengths of the fundamental laser beam (1000 nm) and excited second harmonic electromagnetic signal (500 nm), the polyterpenol based structure has no significant contribution. That is, no SHG generation is expected from polyterpenol thin film layer.  Results and Discussions  The experimental results of time-resolved EFISHG of the double layer MIM device illustrating the charging and discharging mechanisms are presented in Figure 2(a). During charging, a step voltage was applied to the device, and it was removed in the discharging process. Figure 2(a) illustrates 3 conditions, namely 0 V, 3 V and 5 V on IZO with reference to Al electrode.  The changes in the intensity of the SH signal were interpreted in terms of the electric field distribution in C60 layer under the presence of injected charge carriers [15, 19]. The base line response was obtained for Vex = 0. For the forward bias conditions of 3 V or 5 V, electron injection into C60 layer from Al electrode to the polyterpenol layer and/or hole injection into polyterpenol from IZO was suggested [15]. After analyzing experimental data in the manner same as in our previous study, using a filtering method, it was shown that the decaying process during charging satisfied relaxation time dependence of applied voltage 1/V2. This suggested recombination of electrons and holes at the interface was probable, with the amount of positive charge less than that of negative charge [16]. The SH intensity increased from the baseline and a maximum SH intensity was obtained at 10-6 s, on replying electrode charging with a response time τRC. Smooth carrier injection followed, with electrons being transported and accumulated at the C60/polyterpenol interface. This was reflected by the rapid decrease in SH intensity to its initial baseline level, which it reached at approximately 10-3 s, corresponding to the MW relaxation time τMW. In this case, the electric field was relaxed since charges ±Qm accumulated on electrodes at  RC = 440 ns and the charges Qs accumulated at C60/polyterpenol.   In the case of reverse bias conditions, holes were candidates injected from Al electrode. As can be seen from Figure 2(b), the SH intensity for charging and discharging processes of the device under reverse bias conditions was a distinct feature in comparison with the forward bias. The charges ±Qm accumulated on electrodes at  RC=440 ns, but the Qs accumulated at C60/polyterpenol was very small. The SH peak was broader in the case of hole injection compared to electron injection, which was in agreement with our previous study [15, 19]. The injection commenced at approximately 1 sec. During the discharge phase of the experiment, first the charges on the electrodes disappeared at 1 sec, while the accumulated electrons stayed at the interface.  These results suggested that hole injection was blocked by the C60 layer, and electron injection was prohibited by the polyterpenol.      The inherent electron blocking hole transport property of the polyterpenol was further corroborated using Al/Alq3/polyterpenol/IZO and Al/C60/Alq3/polyterpenol/IZO structures, schematics for which are presented in Figures 3(a) and 3(b). In the case of Al/Alq3/polyterpenol/IZO device, holes from IZO side were transported through the polyterpenol and recombined at the Alq3 layer with the electrons transmitted from the Al electrode. As a result of this recombination, electromagnetic radiation was emitted, as illustrated in Figure 3(c). Fig 3(c) shows the intensity of the emitted radiation and the photoluminescence measured using a spectrometer (Andor Technology, SR163). A study was carried out using the structure shown in Fig 3(b), Al/C60/Alq3/polyterpenol/IZO. The n-type C60 promotes the transport of electrons injected from Al electrode and accumulate at the C60 Alq3 interface. As in the previous case holes are transported through the polyterpenol layer and a charge recombination happens at the Alq3 - C60 layer. The observed intensity and photoluminescence is measured and shown in Fig. 3(c). This study clearly elucidate the charge transport property of the polyterpenol thin film, which is a new environmentally friendly electron blocking layer that can be used in between the active organic layers and anode.  Conclusions The charge transport mechanism across plasma polymerized polyterpenol was studied by depositing the n-type organic semiconductor, C60 on the polyterpenol thin film and Al top electrodes. Using the EFISHG, charge accumulation under the steady state was demonstrated for electron injection from the Al/C60 (V > 0), whereas no charge accumulation was observed for hole injection (V < 0) from Al/C60. Resulting from these observations, electron blocking property of polyterpenol was proposed as a likely mechanism. Results from electroluminescence spectrum measurement on the Al/C60/Alq3/polyterpenol/IZO structure evidently confirmed the rectifying property of polyterpenol, with electron and hole recombination taking place in Alq3 and C60 layers, indicated by the EL wavelengths of the emitted light consistent with Alq3 and C60 photoluminescence spectra. This study substantiate that the environmentally friendly transparent polyterpenol thin film will be an excellent candidate to be used as a ‘hole transport electron blocking layer’ in organic electronic circuits and hence can be utilized in the fabrication of many organic devices, e.g., OLEDs. Acknowledgments  A part of this work is financially supported by a Grant-in-Aid for Scientific Research (S) (No.22226007) from Japanese Society for the Promotion of Science (JSPS) and GCOE.  K.B. thanks to Australian Postgraduate Award (APA) and Australian Institute of Nuclear Science and Engineering Postgraduate Award (AINSE PGRA).  References 1. S. R. Forrest, Nature 428 (2004) 911.    2. G. Gelinck, Nature 445 (2007) 268.  3. A. Facchetti, Mater. Today 10 (2007) 28.  4. U. Zhokhavets, T. Erb, G. Gobsch, M. Al-Ibrahim, O. Ambacher, Chem. Phys. Lett. 418 (2006) 347. 5. L. Kergoat, B. Piro, M. Berggren, M.-C. Pham, A. Yassar and M. C. Pham, Org. Electron. 13 (2012) 1.  6. H. Hu, X. Wang, J. Wang, L. Wan, F. Liu, H. Zheng, R.Chen, C. Xu, Chem. Phys. Lett. 484 (2010) 247. 7. W. Chen, D.-C. Qi, H. Huang, X. Gao, A. T. S. Wee, Adv. Funct. Mater. 21 (2011), 410-424. 8.  H. Yan, P.Lee, N. R.Armstrong, A.Graham, G. A. Evmenenko, P. Dutta, T. J. Marks, J. Am. Chem. Soc. 127 (2005) 3172. 9. Q. Sun, G. Subramanyam, L. Dai, M. Check, A. Campbell, R. Naik, J. Grote and Y.Wang, ACS Nano 3 (2009) 737. 10. L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J. R. Reynolds, J. R., Adv. Mater. (Weinheim, Ger.) 12 (2000) 481. 11. S. A. Carter, M. Angelopoulos, S. Karg, P. J. Brock, J. C.Scott, Appl. Phys. Lett. 70 (1997) 2067. 12. A. Elschner, F. Bruder, H. W. Heuer, F. Jonas, A. Karbach, S. Kirchmeyer, S. Thurm, R.  Wehrmann, Synth. Met. 111 (2000) 139. 13. M. V. Jacob, K. Bazaka, M. Weis, D. Taguchi, T. Manaka, M. Iwamoto,  Thin Solid Films 518 (2010), 6123.  14. K. Bazaka, M. V. Jacob, Mater. Lett. 63 (2009) 1594. 15. K. Bazaka, M. V. Jacob, D. Taguchi, T. Manaka, and M. Iwamoto, Chem. Phys. Lett. 503 (2011) 105. 16. D. Taguchi, L. Zhang, J. Li, M. Weis, T. Manaka, M. Iwamoto,  J. Phys. Chem. C 114 (2010) 15136. 17. E. Lim, Y. Shibata,T.  Manaka, M. Iwamoto, Thin Solid Films 518 (2009) 893. 18. L. Zhang, D. Taguchi, T. Manaka, M. Iwamoto, J. Appl. Phys. 110 (2011), 033715. 19. T. Manaka, E. Lim, R. Tamura, M. Iwamoto, Nat. Photonics 1 (2007) 581.                                    Figure 1 a) The I-V characteristics of the polyterpenol−C60 double layer device. Inset: The structure of the device.  b) The intensity of the reflected second harmonic signal of wavelength 500 nm.            Figure 2. SH intensity for charging and discharging processes of IZO/polyterpenol/C60/Al device under forward (a) and reverse (b) bias conditions, and the baseline for SH intensity at Vex = 0.            Figure 3. Experimental set-up and device structure (a) Al/C60/Alq3/polyterpenol/IZO, and (b) Al/Alq3/polyterpenol/IZO. (c) Observed electromagnetic radiation as a result of electron-hole recombination at Alq3 layer.          A polymer thin film is fabricated from non-synthetic terpenol.    EFISHG can directly probe carrier motion in organic materials.    The charge transport through the polyterpenol/C60  structure is investigated    Electron-Blocking Hole-Transport nature of polyterpenol thin film is confirmed.   Novel environmentally friendly polymer thin film for organic electronics.  

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Electron-blocking hole-transport polyterpenol thin films

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