Quasi-surface emission in vertical organic lightemitting transistors with network electrode Chang-Min Keum,1 In-Ho Lee,1 Sin-Hyung Lee,1 Gyu Jeong Lee,1 Min-Hoi Kim,2 and Sin-Doo Lee1,* 1
Department of Electrical and Computer Engineering, Seoul National University, Kwanak P.O. Box 34, Seoul 151600, South Korea 2 School of Global Convergence Studies, Hanbat National University, 125 Dongseodaero, Yuseong-gu, Daejeon 305719, South Korea * [email protected]
Abstract: We demonstrate a vertical-type organic light-emitting transistor (VOLET) with a network electrode of closed topology for quasi-surface emission. In our VOLET, the spatial distribution of the surface emission depends primarily on the relative scale of the aperture in the network electrode to the characteristic length for the charge carrier recombination. Due to the closed topology in the network of the source electrode, the charge transport and the resultant carrier recombination are substantially extended from individual network boundaries toward the corresponding aperture centers in the source electrode. The luminance was found to be well-controlled by the gate voltage through an organic semiconducting layer over the network source electrode. ©2014 Optical Society of America OCIS codes: (230.0230) Optical devices; (230.0250) Optoelectronics; (230.3670) Lightemitting diodes; (250.0250) Optoelectronics; (160.4890) Organic materials; (160.6000) Semiconductor materials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Y. L. Guo, G. Yu, and Y. Q. Liu, “Functional organic field-effect transistors,” Adv. Mater. 22(40), 4427–4447 (2010). M. O’Neill and S. M. Kelly, “Ordered materials for organic electronics and photonics,” Adv. Mater. 23(5), 566– 584 (2011). M. Muccini, “A bright future for organic field-effect transistors,” Nat. Mater. 5(8), 605–613 (2006). F. Cicoira and C. Santato, “Organic light emitting field effect transistors: advances and perspectives,” Adv. Funct. Mater. 17(17), 3421–3434 (2007). M. Muccini, W. Koopman, and S. Toffanin, “The photonic perspective of organic light-emitting transistors,” Laser Photonics Rev. 6(2), 258–275 (2012). R. Capelli, S. Toffanin, G. Generali, H. Usta, A. Facchetti, and M. Muccini, “Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes,” Nat. Mater. 9(6), 496–503 (2010). B. Liu, M. A. McCarthy, Y. Yoon, D. Y. Kim, Z. Wu, F. So, P. H. Holloway, J. R. Reynolds, J. Guo, and A. G. Rinzler, “Carbon-nanotube-enabled vertical field effect and light-emitting transistors,” Adv. Mater. 20(19), 3605–3609 (2008). M. A. McCarthy, B. Liu, E. P. Donoghue, I. Kravchenko, D. Y. Kim, F. So, and A. G. Rinzler, “Low-voltage, low-power, organic light-emitting transistors for active matrix displays,” Science 332(6029), 570–573 (2011). A. Hepp, H. Heil, W. Weise, M. Ahles, R. Schmechel, and H. von Seggern, “Light-emitting field-effect transistor based on a tetracene thin film,” Phys. Rev. Lett. 91(15), 157406 (2003). J. Zaumseil, C. L. Donley, J. S. Kim, R. H. Friend, and H. Sirringhaus, “Efficient top-gate, ambipolar, lightemitting field-effect transistors based on a green-light-emitting polyfluorene,” Adv. Mater. 18(20), 2708–2712 (2006). J. Zaumseil, R. H. Friend, and H. Sirringhaus, “Spatial control of the recombination zone in an ambipolar lightemitting organic transistor,” Nat. Mater. 5(1), 69–74 (2006). E. B. Namdas, P. Ledochowitsch, J. D. Yuen, D. Moses, and A. J. Heeger, “High performance light emitting transistors,” Appl. Phys. Lett. 92(18), 183304 (2008). J. Zaumseil, C. R. McNeill, M. Bird, D. L. Smith, P. Paul Ruden, M. Roberts, M. J. McKiernan, R. H. Friend, and H. Sirringhaus, “Quantum efficiency of ambipolar light-emitting polymer field-effect transistors,” J. Appl. Phys. 103(6), 064517 (2008).
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14750
14. B. B. Y. Hsu, C. H. Duan, E. B. Namdas, A. Gutacker, J. D. Yuen, F. Huang, Y. Cao, G. C. Bazan, I. D. W. Samuel, and A. J. Heeger, “Control of Efficiency, Brightness, and Recombination Zone in Light-Emitting Field Effect Transistors,” Adv. Mater. 24(9), 1171–1175 (2012). 15. T. Oyamada, H. Sasabe, Y. Oku, N. Shimoji, and C. Adachi, “Estimation of carrier recombination and electroluminescence emission regions in organic light-emitting field-effect transistors using local doping method,” Appl. Phys. Lett. 88(9), 093514 (2006). 16. K. Yamane, H. Yanagi, A. Sawamoto, and S. Hotta, “Ambipolar organic light emitting field effect transistors with modified asymmetric electrodes,” Appl. Phys. Lett. 90(16), 162108 (2007). 17. M. Schidleja, C. Melzer, and H. von Seggern, “Electroluminescence from a pentacene based ambipolar organic field-effect transistor,” Appl. Phys. Lett. 94(12), 123307 (2009). 18. N. Suganuma, N. Shimoji, Y. Oku, S. Okuyama, and K. Matsushige, “Organic light-emitting transistors with split-gate structure and PN-hetero-boundary carrier recombination sites,” Org. Electron. 9(5), 834–838 (2008). 19. V. Maiorano, A. Bramanti, S. Carallo, R. Cingolani, and G. Gigli, “Organic light emitting field effect transistors based on an ambipolar p-i-n layered structure,” Appl. Phys. Lett. 96(13), 133305 (2010). 20. K. Nakamura, T. Hata, A. Yoshizawa, K. Obata, H. Endo, and K. Kudo, “Metal-insulator-semiconductor-type organic light-emitting transistor on plastic substrate,” Appl. Phys. Lett. 89(10), 103525 (2006). 21. K. Nakamura, T. Hata, A. Yoshizawa, K. Obata, H. Endo, and K. Kudo, “Improvement of Metal–Insulator– Semiconductor-Type Organic Light-Emitting Transistors,” Jpn. J. Appl. Phys. 47(3), 1889–1893 (2008). 22. M. A. McCarthy, B. Liu, and A. G. Rinzler, “High current, low voltage carbon nanotube enabled vertical organic field effect transistors,” Nano Lett. 10(9), 3467–3472 (2010). 23. A. J. Ben-Sasson and N. Tessler, “Unraveling the physics of vertical organic field effect transistors through nanoscale engineering of a self-assembled transparent electrode,” Nano Lett. 12(9), 4729–4733 (2012). 24. A. J. Ben-Sasson, Z. H. Chen, A. Facchetti, and N. Tessler, “Solution-processed ambipolar vertical organic field effect transistor,” Appl. Phys. Lett. 100(26), 263306 (2012). 25. H. Kleemann, A. A. Günther, K. Leo, and B. Lüssem, “High-performance vertical organic transistors,” Small 9(21), 3670–3677 (2013). 26. A. J. Ben-Sasson and N. Tessler, “Patterned electrode vertical field effect transistor: Theory and experiment,” J. Appl. Phys. 110(4), 044501 (2011). 27. N. J. Watkins, L. Yan, and Y. Gao, “Electronic structure symmetry of interfaces between pentacene and metals,” Appl. Phys. Lett. 80(23), 4384–4386 (2002). 28. F. Amy, C. Chan, and A. Kahn, “Polarization at the gold/pentacene interface,” Org. Electron. 6(2), 85–91 (2005).
1. Introduction Recently, rapid progress in organic electronics and photonics [1, 2] has led to the advancement of organic light-emitting transistors (OLETs) where the functionality of electrical switching and the capability of light generation are integrated in a single building block [3–5]. In addition to the intrinsic switching function, the OLET exhibits, in principle, higher quantum efficiency and a larger aperture ratio than an organic light-emitting diode [3, 6]. However, except for a few cases of carbon nanotube (CNT)-based vertical OLETs [7, 8], most of the OLETs have been constructed in lateral source-drain geometries and have shown line- or band-type emission characteristics [9–14] since the effective recombination zone for holes and electrons is limited within a hundred nanometer range between the source electrode and the drain electrode [3]. Thus, toward improving the device performance and extending the light emission area, a number of approaches such as multilayer structures [6, 15], modified electrodes [16–18], and p-i-n [19] or metal-insulator-semiconductor types [20, 21] have been suggested to effectively control the charge injection, the charge transport, and the charge carrier recombination throughout the entire device. Despite good material properties, the CNT-based vertical field-effect transistors [7, 8, 22] may suffer from the uniformity as well as the long-term stability resulting from the random distribution as well as the poor connectivity of the CNTs used for the source electrode. Particularly, the interface between the CNT source electrode in contact with an adjacent layer, which is either an active layer of an organic semiconductor or an additional charge transport layer, becomes inevitably irregular and exhibits a wide range of the landscape of charge traps. Therefore, in a vertical architecture, the alternative is to introduce a source electrode with periodic vacancies instead of the CNTs. For example, the use of perforated electrodes [23, 24] and interdigitated electrodes [25] was found to provide a useful way of producing short channel length devices.
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14751
In this case, the electric field should be precisely controlled through the vacancies of the source electrode from the gate electrode. In this work, we demonstrated the first vertical OLET (VOLET) showing the quasisurface emission with the help of a network source electrode of closed topology having periodic vacancies (or apertures). An optical methodology of characterizing the apparent carrier recombination zone for light emission on a macroscopic level was also developed. The cross-sectional view of a unit VOLET cell is schematically illustrated in Fig. 1(a). Compared to a standard vertical-type transistor [23–25], our OLET has an emission layer interfaced with both the organic semiconductor layer and a source insulator placed on the top of the network source electrode. As shown in Fig. 1(a), the charge transport occurs in two steps; the first step is represented by the flow of the lateral charge (qL) in the plane of the organic semiconductor and the second by the flow of the vertical charge (qV) out of the plane of the organic semiconductor into the emission layer through the vacancy (or aperture) of the source electrode. Here, qV can be described in terms of the space-charge limited current [26] and is essentially the charge which participates in the recombination process for light emission. Since the charge recombination and the resultant light emission are primarily governed by the pathway as well as the magnitude of qL from each side of the aperture, the geometrical dimension of the aperture and the thickness of each layer play critical roles on the light emission characteristics including the emission zone and the spatial distribution around the aperture boundary.
Fig. 1. (a) Cross-sectional view of our VOLET with a network source electrode having periodic apertures. The lateral charge transport (qL) and the vertical charge transport (qV) are depicted as solid and dashed arrows, respectively. (b) Illustration of our VOLET architecture with a network source electrode and the stacked layers of functional materials.
In our VOLET, the charge injection from the network source electrode and the apparent luminance through the aperture were found to be well-controlled by the gate voltage in a quasi-surface manner.
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14752
2. Experimental A prerequisite for the VOLET in Fig. 1(b) is to construct a network source electrode which should be precisely patterned together with the source insulator and chemically compatible with the gate insulator and the organic semiconductor (pentacene) in direct contact. Moreover, the electrical properties of the source electrode at the interface with the organic semiconductor should be properly tailored. For the fabrication of our OLET, a glass substrate with a pre-patterned gate electrode, made of 70 nm-thick indium-tin-oxide (ITO), was cleaned in a bath for ultra-sonication using acetone, isopropyl alcohol, methanol, and deionized water in sequence. A gate insulator of silicon dioxide (SiO2) of 300 nm thick was prepared over the gate electrode by plasma-enhanced chemical vapor deposition and subsequently poly(4vinylphenol) (PVP) mixed with methylated poly(melamine-co-formaldehyde) in propylene glycol methyl ether acetate in 2 wt.% was spin-coated at the rate of 3000 rpm for 30 sec. The PVP layer was then soft-baked at 100°C for 10 min to remove the residual solvent and was thermally cross-linked at 200°C for 30 min. In constructing the network source electrode, gold (Au) of 15 nm thick was deposited by thermal evaporation under the pressure of about 10−6 Torr, followed by the preparation of the source insulator from a solution of the PVP in 10 wt.%. For the source insulator, the PVP solution was spin-coated and baked under the same conditions for the gate insulator described above. The PVP source insulator was patterned using a photoresist (PR) (AZ 5214, AZ electronic materials) through conventional photolithographic and dry-etching processes. After the removal of the PR, the network source electrode having periodic vacancies (or apertures) was produced by wet-etching in an etchant (TFA, Transene) for Au. Note that the patterned PVP source insulator was served as a mask. A 100 nm-thick layer of pentacene, being a p-type organic semiconductor, was then deposited over the network source electrode by thermal evaporation at the deposition rate of 0.1 nm/s under about 10−7 Torr. A 50nm-thick hole transport layer of N,N'-di(1-naphthyl)-N,N'diphenylbenzidine (α-NPD), an emission layer of tri-(8-hydroxyquinoline)aluminum (Alq3) of 50 nm thick, and the drain electrode of LiF/Al (0.5 nm/60 nm thick) were deposited on the pentacene layer in sequence. The light emission characteristics of our OLET having the network source electrode were observed with an optical microscope (Eclipse E600, Nikon) and analyzed using an image processing software (Image J, National Institute of Health). The spatial distribution of the light emission depending on the physical dimension of each aperture in the network source electrode was resolved in terms of the light intensity across the aperture. The luminance of our OLET as a function of the gate voltage was measured using a spectrometer (CS-2000, Konica Minolta) in conjunction with a source meter (2636A, Keithley). 3. Results and discussion 3.1 Quasi-surface light emission in VOLET Figure 2 shows the microscopic images of three network source electrodes, having different sizes of the apertures, where the cross-linked PVP layers were prepared. One side of each square aperture (Wa) and the separation (We) between two adjacent apertures were set to be equal. The apertures of 20, 10, and 5 μm in size were shown in Figs. 2(a), 2(b), and 2(c), respectively. For small apertures, their shapes were much rounded and became even close to circles due to the patterning resolution in our study.
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14753
Fig. 2. Microscopic images of three network source electrodes of Au, having different sizes of the apertures, covered with the cross-linked PVP layers. The size of each aperture and the separation between two adjacent apertures (or the dimension of each electrode region) were denoted by Wa and We, respectively. The value of Wa = We was varied to be (a) 20 μm, (b) 10 μm, and (c) 5 μm.
For given organic semiconductor (pentacene) and emission material (Alq3), the aperture size is a critical factor for achieving the light emission covering the whole aperture area. More specifically, the aperture size in relative to the characteristic length for the charge carrier recombination (or the length scale of the recombination zone) plays an essential role on whether the apparent emission occurs within the entire region of the aperture or not. In short, simply taking the characteristic length for the charge recombination as Wa/2, the emission area becomes Wa2, meaning that the quasi-surface emission over the whole aperture is expected. We first examine the light emission characteristics for three types of the VOLETs with three different aperture sizes of 20, 10, and 5 μm. Figures 3(a)-3(c) show the microscopic images for the light emission from the three VOLETs, taken with an optical microscope, at the drain voltage of –10 V and the gate voltage of –10 V under zero source voltage. For the aperture of 20 μm, the light emission occurs only in the vicinity of the edges as shown in Fig. 3(a).
Fig. 3. Microscopic images for the light emission through the apertures of the network source electrode in three VOLETs with different values of (a) Wa = 20 μm, (b) Wa = 10 μm, and (c) Wa = 5 μm at the drain voltage of –10 V and the gate voltage of –10 V under zero source voltage. (d) Intensity profiles along the yellow line across a single aperture for each OLET given in (a)-(c). Green, blue, and red colors represent Wa = 20, 10, and 5 μm, respectively. In each case, the FWHM (depicted by the gray arrow) was estimated from two intensity curves separated from one another by extrapolation.
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14754
As clearly seen in Figs. 3(b) and 3(c), the light emission becomes to cover nearly the entire area of the aperture with decreasing the aperture size. In characterizing the spatial distribution of the light emission, the intensity profiles along the yellow line across a single aperture were measured and analyzed using two emission curves as shown in Fig. 3(d). For the case of 5 μm, the apparent intensity across the aperture remains fairly constant owing to the overlap of two curves in the middle of the aperture. In contrast, for the case of 20 μm, two intensity curves were far separated from one another. This indicates that the closed topology (totally different from the interdigitated geometry) of the source electrode plays a critical role on extending the carrier recombination zone so as to achieve the quasi-surface emission in the OLET with the network source electrode. From two intensity curves separated from one another by extrapolation as shown in Fig. 3(d), the full width at half maximum (FWHM) was estimated to be about 3.9 ± 0.5 μm which is somewhat smaller than the aperture size of 5 μm. The FWHM is indirectly related to qL in terms of the carrier mobility of the organic semiconductor, the charge injection barrier at the interface with the source electrode, and the geometrical factors such as the device architecture. 3.2 Control of luminance in VOLET by gate voltage We now describe the gate voltage-dependent luminance of our VOLET with the aperture size of Wa = 5 μm in the network source electrode. Figures 4(a)-4(c) represent the light emission features through 6 × 6 apertures in the source electrode at the gate voltage of –50 V, 0 V, and + 50 V for fixed drain voltage of –10 V, respectively. From Fig. 4(b), it was found that our VOLET was in “the normally ON state” at the zero gate voltage, meaning that the charges were partially injected by the electric field between the drain electrode and the source electrode. This is because the potential barrier (about 0.5 eV), representing the difference in the energy level between the highest occupied molecular orbital of pentacene and the work function of Au, is not high enough to block the charge injection [23, 27, 28]. This potential barrier can be reduced by the application of a negative bias at the gate electrode so that more charges can be injected and recombined to enhance the light emission as clearly seen in Fig. 4(a).
Fig. 4. Light emission features through 6 × 6 apertures of Wa = 5 μm in the source electrode at the gate voltage of (a) –50 V, (b) 0 V, and (c) + 50 V at the drain voltage of –10 V. (d) Luminance as a function of the gate voltage. The inset shows a photograph together with a zoom-in microscopic image for the light emission from the OLET (2.5 mm × 2 mm in size) with 50,000 apertures in the source electrode at the gate voltage of –50 V and the drain voltage of –10 V.
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14755
Under a positive bias at the gate electrode, the charge injection becomes suppressed due to the increase of the effective potential barrier and thus the VOLET is switched off as shown in Fig. 4(c). Although the light emission of our VOLET was not uniform in aperture by aperture, it is evident that the experimental results fully support the validity of our device concept. Figure 4(d) shows the luminance as a function of the gate voltage at the drain voltage of –10 V. The inset shows a photograph together with a zoom-in microscopic image for the light emission from a prototype OLET panel (2.5 mm × 2 mm in size) with 50,000 apertures in the source electrode at the gate voltage of –50 V for fixed drain voltage of –10 V. The luminance was measured to be about 170 cd/m2 and the contrast ratio was about 40. These values are still far behind those for practical applications such as emissive displays. However, the optoelectronic performance might be much improved through the optimization of the materials as well as the geometric parameters in analogy to the CNT-based transistors and OLETs [7, 22]. It should be emphasized that our VOLET provides indeed the light emission in a quasi-surface manner and the gate voltage-dependent luminance as observed from a prototype panel of 2.5 mm × 2 mm in size. 4. Conclusion We constructed for the first time a new type of the VOLET showing the quasi-surface emission with the help of a network source electrode of closed topology. In contrast to the interdigitated geometry, the closed topology was found to allow the charge transport and the carrier recombination process in 2-dimension, extended from a bilateral direction, so that the quasi-surface emission of light was produced. Our results indicate that the light emission area depends on the relative size of the aperture in the network electrode to the characteristic length scale for the charge carrier recombination. It was found that the luminance was wellcontrolled by the gate voltage. Although the improvement of the device performance through tailoring the materials and engineering the interfaces among different layers remains to be made, our approach to the surface emission-type VOLET serves as a new platform for developing advanced optoelectronic devices as well as next generation displays. Acknowledgments This work was supported by the National Research Foundation of Korea under the Ministry of Education, Science and Technology of Korea through the Grant 2011-0028422.
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14756
Department of Electrical and Computer Engineering, Seoul National University, Kwanak P.O. Box 34, Seoul 151600, South Korea 2 School of Global Convergence Studies, Hanbat National University, 125 Dongseodaero, Yuseong-gu, Daejeon 305719, South Korea * [email protected]
Abstract: We demonstrate a vertical-type organic light-emitting transistor (VOLET) with a network electrode of closed topology for quasi-surface emission. In our VOLET, the spatial distribution of the surface emission depends primarily on the relative scale of the aperture in the network electrode to the characteristic length for the charge carrier recombination. Due to the closed topology in the network of the source electrode, the charge transport and the resultant carrier recombination are substantially extended from individual network boundaries toward the corresponding aperture centers in the source electrode. The luminance was found to be well-controlled by the gate voltage through an organic semiconducting layer over the network source electrode. ©2014 Optical Society of America OCIS codes: (230.0230) Optical devices; (230.0250) Optoelectronics; (230.3670) Lightemitting diodes; (250.0250) Optoelectronics; (160.4890) Organic materials; (160.6000) Semiconductor materials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Y. L. Guo, G. Yu, and Y. Q. Liu, “Functional organic field-effect transistors,” Adv. Mater. 22(40), 4427–4447 (2010). M. O’Neill and S. M. Kelly, “Ordered materials for organic electronics and photonics,” Adv. Mater. 23(5), 566– 584 (2011). M. Muccini, “A bright future for organic field-effect transistors,” Nat. Mater. 5(8), 605–613 (2006). F. Cicoira and C. Santato, “Organic light emitting field effect transistors: advances and perspectives,” Adv. Funct. Mater. 17(17), 3421–3434 (2007). M. Muccini, W. Koopman, and S. Toffanin, “The photonic perspective of organic light-emitting transistors,” Laser Photonics Rev. 6(2), 258–275 (2012). R. Capelli, S. Toffanin, G. Generali, H. Usta, A. Facchetti, and M. Muccini, “Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes,” Nat. Mater. 9(6), 496–503 (2010). B. Liu, M. A. McCarthy, Y. Yoon, D. Y. Kim, Z. Wu, F. So, P. H. Holloway, J. R. Reynolds, J. Guo, and A. G. Rinzler, “Carbon-nanotube-enabled vertical field effect and light-emitting transistors,” Adv. Mater. 20(19), 3605–3609 (2008). M. A. McCarthy, B. Liu, E. P. Donoghue, I. Kravchenko, D. Y. Kim, F. So, and A. G. Rinzler, “Low-voltage, low-power, organic light-emitting transistors for active matrix displays,” Science 332(6029), 570–573 (2011). A. Hepp, H. Heil, W. Weise, M. Ahles, R. Schmechel, and H. von Seggern, “Light-emitting field-effect transistor based on a tetracene thin film,” Phys. Rev. Lett. 91(15), 157406 (2003). J. Zaumseil, C. L. Donley, J. S. Kim, R. H. Friend, and H. Sirringhaus, “Efficient top-gate, ambipolar, lightemitting field-effect transistors based on a green-light-emitting polyfluorene,” Adv. Mater. 18(20), 2708–2712 (2006). J. Zaumseil, R. H. Friend, and H. Sirringhaus, “Spatial control of the recombination zone in an ambipolar lightemitting organic transistor,” Nat. Mater. 5(1), 69–74 (2006). E. B. Namdas, P. Ledochowitsch, J. D. Yuen, D. Moses, and A. J. Heeger, “High performance light emitting transistors,” Appl. Phys. Lett. 92(18), 183304 (2008). J. Zaumseil, C. R. McNeill, M. Bird, D. L. Smith, P. Paul Ruden, M. Roberts, M. J. McKiernan, R. H. Friend, and H. Sirringhaus, “Quantum efficiency of ambipolar light-emitting polymer field-effect transistors,” J. Appl. Phys. 103(6), 064517 (2008).
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14750
14. B. B. Y. Hsu, C. H. Duan, E. B. Namdas, A. Gutacker, J. D. Yuen, F. Huang, Y. Cao, G. C. Bazan, I. D. W. Samuel, and A. J. Heeger, “Control of Efficiency, Brightness, and Recombination Zone in Light-Emitting Field Effect Transistors,” Adv. Mater. 24(9), 1171–1175 (2012). 15. T. Oyamada, H. Sasabe, Y. Oku, N. Shimoji, and C. Adachi, “Estimation of carrier recombination and electroluminescence emission regions in organic light-emitting field-effect transistors using local doping method,” Appl. Phys. Lett. 88(9), 093514 (2006). 16. K. Yamane, H. Yanagi, A. Sawamoto, and S. Hotta, “Ambipolar organic light emitting field effect transistors with modified asymmetric electrodes,” Appl. Phys. Lett. 90(16), 162108 (2007). 17. M. Schidleja, C. Melzer, and H. von Seggern, “Electroluminescence from a pentacene based ambipolar organic field-effect transistor,” Appl. Phys. Lett. 94(12), 123307 (2009). 18. N. Suganuma, N. Shimoji, Y. Oku, S. Okuyama, and K. Matsushige, “Organic light-emitting transistors with split-gate structure and PN-hetero-boundary carrier recombination sites,” Org. Electron. 9(5), 834–838 (2008). 19. V. Maiorano, A. Bramanti, S. Carallo, R. Cingolani, and G. Gigli, “Organic light emitting field effect transistors based on an ambipolar p-i-n layered structure,” Appl. Phys. Lett. 96(13), 133305 (2010). 20. K. Nakamura, T. Hata, A. Yoshizawa, K. Obata, H. Endo, and K. Kudo, “Metal-insulator-semiconductor-type organic light-emitting transistor on plastic substrate,” Appl. Phys. Lett. 89(10), 103525 (2006). 21. K. Nakamura, T. Hata, A. Yoshizawa, K. Obata, H. Endo, and K. Kudo, “Improvement of Metal–Insulator– Semiconductor-Type Organic Light-Emitting Transistors,” Jpn. J. Appl. Phys. 47(3), 1889–1893 (2008). 22. M. A. McCarthy, B. Liu, and A. G. Rinzler, “High current, low voltage carbon nanotube enabled vertical organic field effect transistors,” Nano Lett. 10(9), 3467–3472 (2010). 23. A. J. Ben-Sasson and N. Tessler, “Unraveling the physics of vertical organic field effect transistors through nanoscale engineering of a self-assembled transparent electrode,” Nano Lett. 12(9), 4729–4733 (2012). 24. A. J. Ben-Sasson, Z. H. Chen, A. Facchetti, and N. Tessler, “Solution-processed ambipolar vertical organic field effect transistor,” Appl. Phys. Lett. 100(26), 263306 (2012). 25. H. Kleemann, A. A. Günther, K. Leo, and B. Lüssem, “High-performance vertical organic transistors,” Small 9(21), 3670–3677 (2013). 26. A. J. Ben-Sasson and N. Tessler, “Patterned electrode vertical field effect transistor: Theory and experiment,” J. Appl. Phys. 110(4), 044501 (2011). 27. N. J. Watkins, L. Yan, and Y. Gao, “Electronic structure symmetry of interfaces between pentacene and metals,” Appl. Phys. Lett. 80(23), 4384–4386 (2002). 28. F. Amy, C. Chan, and A. Kahn, “Polarization at the gold/pentacene interface,” Org. Electron. 6(2), 85–91 (2005).
1. Introduction Recently, rapid progress in organic electronics and photonics [1, 2] has led to the advancement of organic light-emitting transistors (OLETs) where the functionality of electrical switching and the capability of light generation are integrated in a single building block [3–5]. In addition to the intrinsic switching function, the OLET exhibits, in principle, higher quantum efficiency and a larger aperture ratio than an organic light-emitting diode [3, 6]. However, except for a few cases of carbon nanotube (CNT)-based vertical OLETs [7, 8], most of the OLETs have been constructed in lateral source-drain geometries and have shown line- or band-type emission characteristics [9–14] since the effective recombination zone for holes and electrons is limited within a hundred nanometer range between the source electrode and the drain electrode [3]. Thus, toward improving the device performance and extending the light emission area, a number of approaches such as multilayer structures [6, 15], modified electrodes [16–18], and p-i-n [19] or metal-insulator-semiconductor types [20, 21] have been suggested to effectively control the charge injection, the charge transport, and the charge carrier recombination throughout the entire device. Despite good material properties, the CNT-based vertical field-effect transistors [7, 8, 22] may suffer from the uniformity as well as the long-term stability resulting from the random distribution as well as the poor connectivity of the CNTs used for the source electrode. Particularly, the interface between the CNT source electrode in contact with an adjacent layer, which is either an active layer of an organic semiconductor or an additional charge transport layer, becomes inevitably irregular and exhibits a wide range of the landscape of charge traps. Therefore, in a vertical architecture, the alternative is to introduce a source electrode with periodic vacancies instead of the CNTs. For example, the use of perforated electrodes [23, 24] and interdigitated electrodes [25] was found to provide a useful way of producing short channel length devices.
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14751
In this case, the electric field should be precisely controlled through the vacancies of the source electrode from the gate electrode. In this work, we demonstrated the first vertical OLET (VOLET) showing the quasisurface emission with the help of a network source electrode of closed topology having periodic vacancies (or apertures). An optical methodology of characterizing the apparent carrier recombination zone for light emission on a macroscopic level was also developed. The cross-sectional view of a unit VOLET cell is schematically illustrated in Fig. 1(a). Compared to a standard vertical-type transistor [23–25], our OLET has an emission layer interfaced with both the organic semiconductor layer and a source insulator placed on the top of the network source electrode. As shown in Fig. 1(a), the charge transport occurs in two steps; the first step is represented by the flow of the lateral charge (qL) in the plane of the organic semiconductor and the second by the flow of the vertical charge (qV) out of the plane of the organic semiconductor into the emission layer through the vacancy (or aperture) of the source electrode. Here, qV can be described in terms of the space-charge limited current [26] and is essentially the charge which participates in the recombination process for light emission. Since the charge recombination and the resultant light emission are primarily governed by the pathway as well as the magnitude of qL from each side of the aperture, the geometrical dimension of the aperture and the thickness of each layer play critical roles on the light emission characteristics including the emission zone and the spatial distribution around the aperture boundary.
Fig. 1. (a) Cross-sectional view of our VOLET with a network source electrode having periodic apertures. The lateral charge transport (qL) and the vertical charge transport (qV) are depicted as solid and dashed arrows, respectively. (b) Illustration of our VOLET architecture with a network source electrode and the stacked layers of functional materials.
In our VOLET, the charge injection from the network source electrode and the apparent luminance through the aperture were found to be well-controlled by the gate voltage in a quasi-surface manner.
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14752
2. Experimental A prerequisite for the VOLET in Fig. 1(b) is to construct a network source electrode which should be precisely patterned together with the source insulator and chemically compatible with the gate insulator and the organic semiconductor (pentacene) in direct contact. Moreover, the electrical properties of the source electrode at the interface with the organic semiconductor should be properly tailored. For the fabrication of our OLET, a glass substrate with a pre-patterned gate electrode, made of 70 nm-thick indium-tin-oxide (ITO), was cleaned in a bath for ultra-sonication using acetone, isopropyl alcohol, methanol, and deionized water in sequence. A gate insulator of silicon dioxide (SiO2) of 300 nm thick was prepared over the gate electrode by plasma-enhanced chemical vapor deposition and subsequently poly(4vinylphenol) (PVP) mixed with methylated poly(melamine-co-formaldehyde) in propylene glycol methyl ether acetate in 2 wt.% was spin-coated at the rate of 3000 rpm for 30 sec. The PVP layer was then soft-baked at 100°C for 10 min to remove the residual solvent and was thermally cross-linked at 200°C for 30 min. In constructing the network source electrode, gold (Au) of 15 nm thick was deposited by thermal evaporation under the pressure of about 10−6 Torr, followed by the preparation of the source insulator from a solution of the PVP in 10 wt.%. For the source insulator, the PVP solution was spin-coated and baked under the same conditions for the gate insulator described above. The PVP source insulator was patterned using a photoresist (PR) (AZ 5214, AZ electronic materials) through conventional photolithographic and dry-etching processes. After the removal of the PR, the network source electrode having periodic vacancies (or apertures) was produced by wet-etching in an etchant (TFA, Transene) for Au. Note that the patterned PVP source insulator was served as a mask. A 100 nm-thick layer of pentacene, being a p-type organic semiconductor, was then deposited over the network source electrode by thermal evaporation at the deposition rate of 0.1 nm/s under about 10−7 Torr. A 50nm-thick hole transport layer of N,N'-di(1-naphthyl)-N,N'diphenylbenzidine (α-NPD), an emission layer of tri-(8-hydroxyquinoline)aluminum (Alq3) of 50 nm thick, and the drain electrode of LiF/Al (0.5 nm/60 nm thick) were deposited on the pentacene layer in sequence. The light emission characteristics of our OLET having the network source electrode were observed with an optical microscope (Eclipse E600, Nikon) and analyzed using an image processing software (Image J, National Institute of Health). The spatial distribution of the light emission depending on the physical dimension of each aperture in the network source electrode was resolved in terms of the light intensity across the aperture. The luminance of our OLET as a function of the gate voltage was measured using a spectrometer (CS-2000, Konica Minolta) in conjunction with a source meter (2636A, Keithley). 3. Results and discussion 3.1 Quasi-surface light emission in VOLET Figure 2 shows the microscopic images of three network source electrodes, having different sizes of the apertures, where the cross-linked PVP layers were prepared. One side of each square aperture (Wa) and the separation (We) between two adjacent apertures were set to be equal. The apertures of 20, 10, and 5 μm in size were shown in Figs. 2(a), 2(b), and 2(c), respectively. For small apertures, their shapes were much rounded and became even close to circles due to the patterning resolution in our study.
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Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14753
Fig. 2. Microscopic images of three network source electrodes of Au, having different sizes of the apertures, covered with the cross-linked PVP layers. The size of each aperture and the separation between two adjacent apertures (or the dimension of each electrode region) were denoted by Wa and We, respectively. The value of Wa = We was varied to be (a) 20 μm, (b) 10 μm, and (c) 5 μm.
For given organic semiconductor (pentacene) and emission material (Alq3), the aperture size is a critical factor for achieving the light emission covering the whole aperture area. More specifically, the aperture size in relative to the characteristic length for the charge carrier recombination (or the length scale of the recombination zone) plays an essential role on whether the apparent emission occurs within the entire region of the aperture or not. In short, simply taking the characteristic length for the charge recombination as Wa/2, the emission area becomes Wa2, meaning that the quasi-surface emission over the whole aperture is expected. We first examine the light emission characteristics for three types of the VOLETs with three different aperture sizes of 20, 10, and 5 μm. Figures 3(a)-3(c) show the microscopic images for the light emission from the three VOLETs, taken with an optical microscope, at the drain voltage of –10 V and the gate voltage of –10 V under zero source voltage. For the aperture of 20 μm, the light emission occurs only in the vicinity of the edges as shown in Fig. 3(a).
Fig. 3. Microscopic images for the light emission through the apertures of the network source electrode in three VOLETs with different values of (a) Wa = 20 μm, (b) Wa = 10 μm, and (c) Wa = 5 μm at the drain voltage of –10 V and the gate voltage of –10 V under zero source voltage. (d) Intensity profiles along the yellow line across a single aperture for each OLET given in (a)-(c). Green, blue, and red colors represent Wa = 20, 10, and 5 μm, respectively. In each case, the FWHM (depicted by the gray arrow) was estimated from two intensity curves separated from one another by extrapolation.
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14754
As clearly seen in Figs. 3(b) and 3(c), the light emission becomes to cover nearly the entire area of the aperture with decreasing the aperture size. In characterizing the spatial distribution of the light emission, the intensity profiles along the yellow line across a single aperture were measured and analyzed using two emission curves as shown in Fig. 3(d). For the case of 5 μm, the apparent intensity across the aperture remains fairly constant owing to the overlap of two curves in the middle of the aperture. In contrast, for the case of 20 μm, two intensity curves were far separated from one another. This indicates that the closed topology (totally different from the interdigitated geometry) of the source electrode plays a critical role on extending the carrier recombination zone so as to achieve the quasi-surface emission in the OLET with the network source electrode. From two intensity curves separated from one another by extrapolation as shown in Fig. 3(d), the full width at half maximum (FWHM) was estimated to be about 3.9 ± 0.5 μm which is somewhat smaller than the aperture size of 5 μm. The FWHM is indirectly related to qL in terms of the carrier mobility of the organic semiconductor, the charge injection barrier at the interface with the source electrode, and the geometrical factors such as the device architecture. 3.2 Control of luminance in VOLET by gate voltage We now describe the gate voltage-dependent luminance of our VOLET with the aperture size of Wa = 5 μm in the network source electrode. Figures 4(a)-4(c) represent the light emission features through 6 × 6 apertures in the source electrode at the gate voltage of –50 V, 0 V, and + 50 V for fixed drain voltage of –10 V, respectively. From Fig. 4(b), it was found that our VOLET was in “the normally ON state” at the zero gate voltage, meaning that the charges were partially injected by the electric field between the drain electrode and the source electrode. This is because the potential barrier (about 0.5 eV), representing the difference in the energy level between the highest occupied molecular orbital of pentacene and the work function of Au, is not high enough to block the charge injection [23, 27, 28]. This potential barrier can be reduced by the application of a negative bias at the gate electrode so that more charges can be injected and recombined to enhance the light emission as clearly seen in Fig. 4(a).
Fig. 4. Light emission features through 6 × 6 apertures of Wa = 5 μm in the source electrode at the gate voltage of (a) –50 V, (b) 0 V, and (c) + 50 V at the drain voltage of –10 V. (d) Luminance as a function of the gate voltage. The inset shows a photograph together with a zoom-in microscopic image for the light emission from the OLET (2.5 mm × 2 mm in size) with 50,000 apertures in the source electrode at the gate voltage of –50 V and the drain voltage of –10 V.
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14755
Under a positive bias at the gate electrode, the charge injection becomes suppressed due to the increase of the effective potential barrier and thus the VOLET is switched off as shown in Fig. 4(c). Although the light emission of our VOLET was not uniform in aperture by aperture, it is evident that the experimental results fully support the validity of our device concept. Figure 4(d) shows the luminance as a function of the gate voltage at the drain voltage of –10 V. The inset shows a photograph together with a zoom-in microscopic image for the light emission from a prototype OLET panel (2.5 mm × 2 mm in size) with 50,000 apertures in the source electrode at the gate voltage of –50 V for fixed drain voltage of –10 V. The luminance was measured to be about 170 cd/m2 and the contrast ratio was about 40. These values are still far behind those for practical applications such as emissive displays. However, the optoelectronic performance might be much improved through the optimization of the materials as well as the geometric parameters in analogy to the CNT-based transistors and OLETs [7, 22]. It should be emphasized that our VOLET provides indeed the light emission in a quasi-surface manner and the gate voltage-dependent luminance as observed from a prototype panel of 2.5 mm × 2 mm in size. 4. Conclusion We constructed for the first time a new type of the VOLET showing the quasi-surface emission with the help of a network source electrode of closed topology. In contrast to the interdigitated geometry, the closed topology was found to allow the charge transport and the carrier recombination process in 2-dimension, extended from a bilateral direction, so that the quasi-surface emission of light was produced. Our results indicate that the light emission area depends on the relative size of the aperture in the network electrode to the characteristic length scale for the charge carrier recombination. It was found that the luminance was wellcontrolled by the gate voltage. Although the improvement of the device performance through tailoring the materials and engineering the interfaces among different layers remains to be made, our approach to the surface emission-type VOLET serves as a new platform for developing advanced optoelectronic devices as well as next generation displays. Acknowledgments This work was supported by the National Research Foundation of Korea under the Ministry of Education, Science and Technology of Korea through the Grant 2011-0028422.
#209871 - $15.00 USD (C) 2014 OSA
Received 9 Apr 2014; accepted 1 Jun 2014; published 9 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.014750 | OPTICS EXPRESS 14756
