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Technical issues in graphene anode organic light emitting diodes
DIAMAT-06394; No of Pages 6 Diamond & Related Materials xxx (2015) xxx–xxx
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Technical issues in graphene anode organic light emitting diodes Jaehyun Moon a, Jin-Wook Shin a, Hyunsu Cho a, Jun-Han Han a, Nam Sung Cho a, Jong Tae Lim a, Seung Koo Park a, Hong Kyw Choi b, Sung-Yool Choi c, Ji-Hoon Kim d, Min-Jae Maeng d, Jaewon Seo d, Yongsup Park d, Jeong-Ik Lee a,⁎ a
Soft I/O Interface Research Section, Electronics and Telecommunications Research Inst. (ETRI), Daejeon, Republic of Korea University of Science and Technology, Daejeon, Republic of Korea Dept. of Electrical Engineering, Korean Advanced Inst. of Science and Technology, Daejeon, Republic of Korea d Dept. of Physics, Kyung Hee University, Seoul, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 28 November 2014 Received in revised form 25 March 2015 Accepted 25 March 2015 Available online xxxx Keywords: Graphene Organic light emitting diode Optical properties Interfacial properties Patterning
a b s t r a c t Optical, interfacial and patterning issues of anode graphene films in organic light emitting diode (OLED) applications were investigated. In the optical part, the microcavities of graphene and indium tin oxide (ITO) anode OLEDs were contrasted. With the use of graphene one may avoid spectral and organic stack design problems related to microcavity problems. However, due to the weak microcavity, emission enhancement using interference designs is practically impossible. By inserting an electron acceptor insert at the graphene/hole transport layer (HTL) interface, it was possible to enhance the current density by factor of three. Based on in situ ultraviolet photoelectron spectroscopy (UPS) results, the insert was interpreted as being a charge generation layer. Graphene patterning using laser or plasma methods turned out to be problematic. None of those methods could offer acceptable dimension accuracy and preserved graphene quality. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene is a two-dimensional film, in which carbon atoms are arranged in a hexagonal array. The thickness of one layer graphene is only 0.345 nm. Since the publication of Geim and Novoselov's article in year 2004, various outstanding properties and perspectives of graphene have reported in the literature [1–4]. As an ordinary course, graphene and multilayered graphene have been experimented in a variety of applications. As being thin, electrically conductive, optically transparent and mechanically flexible, graphene has gained extensive attention as a possible replacement for indium tin oxide (ITO), which has been so far dominantly used as the material for anode or transparent electrode in OLEDs [5–7]. Potentially, due to its thinness, electrical conductivity and mechanical compliance, graphene films can be used in various flexible electronics. In this article, we deal with the technical issues of graphene as an anode for OLEDs. Because graphene and ITO are different in many aspects, one needs to pay close attention to the differences to make most of the graphene anode. In this article, we focus on three aspects, which we consider as of high importance in realizing efficient graphene anode equipped OLEDs. Three topics are optical, interfacial and patterning considerations. In the optical part, we discuss microcavity and contrast its role to that of ITO anode case [8,9]. In the interfacial consideration, we compare the energy alignments of ITO and graphene with respect to ⁎ Corresponding author. E-mail address: [email protected] (J.-I. Lee).
adjacent organic layer, hole transport layer (HTL), and discuss a strategy which is useful in improving the hole transport [10,11]. In the patterning part, we investigate the patterning issue of graphene films [12]. So far, the patterning issue has been treated as a cursory issue. However, to realize graphene film as a component in integrated electronics devices, the patterning is an actual hurdle which has to be overcome. 2. Actual examples Before investigating the aforementioned technical issues, it is useful to compare the actual performances of OLEDs which have ITO or graphene anodes, and figure out the technical issues. We have used a multi-layered graphene (MLG) grown by a chemical vapor deposition (CVD) method [13]. We will refer an OLED with ITO anode as ITOOLED and an OLED with MLG anode as MLG-OLED. Fig. 1 shows various characteristics of phosphorescent green OLEDs. The green emitter was tris(2-phenylpyridinato-C2,N)iridium(III) (Ir(ppy)3), which was doped into a host of 2,6-bis[3-(carbazol-9-yl)phenyl]pyridine (DCzPPy). Both the current density (J) and luminance (L) levels of ITO-OLED were observed to be higher than those of MLG-OLED (Fig. 1(a)). Correspondingly the ITO-OLED showed higher external quantum efficiencies (EQEs) than that of MLG-OLED (Fig. 1(b)). The EQE of MLG-OLED is approximately 90% of ITO-OLED at a 1000 cd/m2. The results of Fig. 1(a) and (b) can be attributed to various factors. The hole injection of MLG is not effective as ITO. The hole injection is closely related to the energy alignment of the work function of the anode and the highest occupied
http://dx.doi.org/10.1016/j.diamond.2015.03.020 0925-9635/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: J. Moon, et al., Technical issues in graphene anode organic light emitting diodes, Diamond Relat. Mater. (2015), http:// dx.doi.org/10.1016/j.diamond.2015.03.020
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Fig. 1. (a) The JVL characteristics of ITO anode and MLG anode OLEDs. (b) The EQEs of ITO anode and MLG anode OLEDs. (c) The EL spectra of ITO anode OLED as a function of viewing angle. (d) The EL spectra of MLG anode OLED as a function of viewing angle.
molecular orbital (HOMO) level of the adjacent organics. Also, the sheet resistance can be a factor. Typical OLED ITO anode has a sheet resistance (Rs) lower than 50 Ω/sq, while typical MLG has approximately 250 Ω/sq [14]. The high Rs of MLG causes the increase of the operating voltage of the OLED. Potentially, the high Rs can be problematic in achieving luminance uniformity over large area OLED lighting panels. The optical
properties of MLG can be a factor. The extinction coefficient (k) of graphene is reported to be high as 1.3 [15–17]. Such high k will cause significant absorption loss of the generated light. Fig. 1(c) and (d) compare the normalized electroluminescence (EL) spectra of ITO and MLG OLEDs as a function of viewing angle. In the case of ITO-OLED, shoulder development in the EL spectrum is apparent as the viewing angle changes.
Fig. 2. (a) Schematics of simulation cell and optical components. (b) Simulated EQEs of ITO anode and MLG anode OLEDs as a function of HTL thickness. Inset is the actual OLED structure used in simulations.
Please cite this article as: J. Moon, et al., Technical issues in graphene anode organic light emitting diodes, Diamond Relat. Mater. (2015), http:// dx.doi.org/10.1016/j.diamond.2015.03.020
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However, in the case of MLG-OLED, the EL spectrum is a very weak function of viewing angle. The results strongly indicate that the internal optics of ITO and MLG OLEDs are very different. To sum up, compared to ITO OLED, MLG OLED turned out to have differences in electrical performance and EL spectral characteristics, which have to be considered in making MLG-OLEDs. 3. Optical consideration In a simplified picture, conventional bottom emissive type OLEDs can be structurally described as a vertical stack of a metallic cathode, organic layers and a transparent anode. Fig. 2(a) schematically illustrates the simulation cell used in the simulations. Also optical components present in the OLED are depicted. HTL, ETL and EL refer to electron transport layer (ETL), hole transport layer (HTL) and emissive layer (EL), respectively. We have performed optical simulations, which is based on multiple interference and dipole oscillation theories. The light source or the dipole was positioned at the EL side of the ETL/EL interface. The theoretical background of the simulations can be found elsewhere [18]. Optical constants (n, k) of organic materials were measured using an ellipsometer (M-2000d, J.A. Woollam Co.). The optical constants of graphene were obtained from published literature [15,16]. In the simulation courses, we have varied the thickness of the HTL, while fixing the ETL thickness as 60 nm. The thicknesses of metallic Al cathode and EL were 100 nm and 20 nm, respectively. In bottom emission type OLEDs with ITO as the anode, microcavity exists between the metallic cathode and the ITO anode [19]. The light generated in the organic layer not only travels downward, which is designated as DEO, but also upward (UEO) toward the highly reflective metallic cathode. We refer the downward and upward traveling optical components as DEO and U EO, respectively. The optical components due to the reflection at the metallic cathode surface and anode/glass interface are referred as UER and DER, respectively. In order to maximize the light out-coupling of OLED, the thicknesses of organics are adjusted to an optical condition which corresponds to a constructive interference. Fig. 2(b) shows the simulated efficiencies (ηs) as a function of the HTL thickness. Here, the efficiency (η) refers to the external quantum efficiency (EQE, %). In the ITO anode case, η is observed to oscillate in a sinusoidal fashion, which marks the presence of microcavity. The difference in the η varies from 16% to 26%, depending on the choice of HTL thickness. Thus, from the perspective of η, the choice of organic thickness bears huge importance in ITO anode equipped OLEDs. However, compared to the ITO OLED case, the presence of microcavity is rather weak in graphene anode equipped OLEDs. The influence of microcavity in graphene anode OLED diminishes as the graphene thickness increases. It is important to notice that the overall η of graphene anode OLED is always lower than that of ITO anode OLED. The lower efficiency of graphene anode OLED seems to have its origin in, at least, two reasons. First, although the refractive index of graphene is reported as being 2.4, due to its fine thickness, graphene cannot effectively contribute in forming a reflective surface at the organic/graphene interface. Second, absorption can play a role in the loss of generated light. When light passes through a medium, absorption takes place. The light intensity (I) decays as I ~ exp(−2πk / λO)IO. Here λO and IO refer to wavelength and the incident light intensity, respectively. Thus light passing through a medium with bigger k is expected to experience larger decay in I. The reported k value of graphene is 1.3, while the k of ITO is 0.05 [20]. Because the presence of microcavity is weak in graphene anode OLEDs, the choice of organic thickness can be alleviated but the enhancement of the light out-coupling by means of a microcavity method is fairly limited. Also the high k value of graphene reduces the out coupling to a nonnegligible degree. OLED with thinner graphene anode shows higher EQE in the whole range considered. This is understood as the effect of higher transmittance which is equivalent to lower absorption. Comments regarding the choice of graphene film thickness are worthwhile here. As graphene film becomes thicker the sheet resistance decreases,
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leading to lower operation voltage and better luminance uniformity. However, as the absorption obstacle persists, thicker graphene cannot always be favored over thinner graphene. The choice of graphene film thickness depends on the application area of OLED. In OLED display applications the emission area of a pixel is about 50 μm × 50 μm. In such case the non-uniformity due to sheet resistance is not present. Thus thinner graphene films may be favored. In contrast, in OLED lighting applications, in which large area is favored, thicker graphene films may be used. Practically, to resolve the trade-off relation between transmittance and sheet resistance in large area OLED, auxiliary metal lines may be used. 4. Interfacial considerations In OLEDs, holes and electrons recombine in the emission layer to produce photons. Thus enhancing the current of electrons and holes toward the emission zone is of prime importance to ensure high performance OLEDs. In order to facilitate the current, a hole injection layer (HIL) is frequently inserted between the anode and HTL. In this work, we have used 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HatCN) as our HIL [21]. In order to explore the effect of Hat-CN on the electrical properties, we have fabricated hole only devices (HODs) with MLG anode. Our HODs have a stack structure of MLG/HTL (TAPC, 100 nm)/cathode (Al, 100 nm) and MLG/HIL(Hat-CN, 10 nm)/HTL (TAPC, 100 nm)/cathode (Al, 100 nm). TAPC is an acronym of 4,4′cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine], and is commonly used as a hole transport layer (HTL) in OLED applications. The MLG were obtained by a chemical vapor deposition method. Ni and CH4 were used as the catalytic agent and carbon source gas, respectively. Technical details can be found elsewhere [6]. Fig. 3(a) compares the current density (J) and applied voltage (V) characteristics of three HODs. As can be readily noticed, the use of Hat-CN as the HIL improves the J level significantly. To be specific the J increases three folds at an applied voltage of 4 V. The results of Fig. 3(a) strongly imply that the use Hat-CN HIL can significantly contribute in enhancing the efficiencies of MLG anode OLEDs. In other to elucidate the reason of improvement in J level by inserting a Hat-CN layer, we have performed in situ surface analyses using a He 1 ultraviolet photoelectron spectroscopy (UPS) method and constructed energy level diagrams. Fig. 3(b) and (c) show the energy level diagrams for MLG/TAPC and MLG/Hat-CN/TAPC interfaces, respectively. The work functions (ϕs ~ 5.1 eV) of MLG turned out to be not sensitive to the adjacent organic layer. The hole injection barriers (ϕhs) were estimated using the energy difference between the Fermi level (Ef) and the onset of HOMO peak. Details of the UPS methods in studying organic interfaces can be found elsewhere [10]. The ϕhs of MLG/TAPC and MLG/Hat-CN/TAPC were 0.65 eV and 3.03 eV, respectively. While a ϕh of 0.65 eV will not preclude hole injection toward TAPC, ϕh of 3.03 eV will effectively block hole injection at the MLG/ Hat-CN interface. Thus, enhancement in the J cannot be due to the lowered energy barrier between MLG and Hat-CN. Hat-CN is a high mobility strong n-type organic semiconductor, which can extract electrons from adjacent organics [10,22]. Thus the role of Hat-CN is not an energy barrier modifier but a charge generation layer (CGL) [23,24]. Under applied voltage, the electrons in the HOMO level of TAPC are extracted toward the Hat-CN LUMO level, which effectively contributes in hole generation in the TAPC layer. In this picture, the energy barrier between MLG and Hat-CN is irrelevant to the charge transport but the energy difference between the LUMO level of Hat-CN and the HOMO level of TAPC is. Fig. 3(d) illustrates the scenario of Hat-CN as a CGL under applied voltage. Because UPS measurements cannot locate the energy level of unoccupied energy levels, the LUMO level of Hat-CN cannot be positioned exactly. However, because the clear enhancement in J has been observed by using a Hat-CN HIL, the LUMO level of Hat-CN and HOMO level of TAPC must be very close, which implies the possibility of Fermi pinning of Hat-CN LUMO level [24]. In this work, we have
Please cite this article as: J. Moon, et al., Technical issues in graphene anode organic light emitting diodes, Diamond Relat. Mater. (2015), http:// dx.doi.org/10.1016/j.diamond.2015.03.020
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Fig. 3. (a) The JV characteristics of ITO anode and MLG anode HODs. (b) The energy level diagram of MLG/TAPC interface. (c) The energy level diagram of MLG/Hat-Cn/TAPC interface. (d) The charge transport taking place between the LUMO of Hat-Cn and HOMO of TAPC (see text for details).
Fig. 4. (a) SEM image of IR laser patterned MLG film. (b) Periphery of IR laser patterned MLG film. (c) The Raman spectra of pristine and plasma patterned MLG. Inset shows the configuration of plasma patterning and possible plasma leak paths. (d) Periphery of plasma patterned MLG film.
Please cite this article as: J. Moon, et al., Technical issues in graphene anode organic light emitting diodes, Diamond Relat. Mater. (2015), http:// dx.doi.org/10.1016/j.diamond.2015.03.020
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introduced Hat-CN to modify the MLG/HTL interface. The HOMO position of TAPC at the interface indicates that the electrons from the TAPC HOMO can be easily excited to the Hat-CN LUMO. This process is a good reason for using Hat-CN as a layer between TAPC and an anode where its actual role is generating electron–hole pairs at the interface with TAPC and transporting them toward the anode. Certainly other HIL candidates exist and wait to be studied. 5. Patterning consideration In this section, we discuss an issue of practical importance. In graphene film patterning, two issues are important. First, to be used as an anode, graphene films must be patterned into accurate dimensions. Second, the patterning method should not deteriorate the graphene. Graphene films can be patterned using laser. Laser sources equipped with moving stage facilities may enable direct patterning of graphene into various patterns. In this course laser ablates the graphene film to form the desired pattern. Fig. 4(a) and (b) show SEM images of our laser patterned MLG films, which have been transferred on a glass substrate. In laser patterning, we have used infrared (IR) laser with a wavelength of λ = 1064 nm. As can be readily noticed, the use laser for patterning purpose is problematic. First, defining the dimension accurately turns out to be a difficult task. The dotted lines in Fig. 4(a) represent the edges, on which horizontal MLG films were supposed to terminate. However, significant narrowing (~30%) of the MLG film can be observed. Second, in the laser exposed regions, MLG films wrap on the glass surface to form MLG bundles (Fig. 4(b)). The bundles are thought to being formed at the expense of neighborhood MLG films. Bundle forming at least is one reason of the MLG narrowing. The observations of Fig. 4(a) and (b) are, presumably, due to the high temperature in local region. Compared to MLG, glass is almost transparent to IR [25]. Also the heat conductivity of glass is lower. Thus the laser induced heat will be directed toward MLG. Graphene films can be patterned using oxygen plasma and a shadow mask. In this process, the exposed region is etched off by plasma, while shadowed region remains, forming patterns. Due to its simplicity, this method has been widely used in laboratory. Fig. 4(c) compares the Raman profiles collected from pristine MLG and patterned MLG. The plasma power and oxygen flow were 50 W and 30 sccm, respectively, and plasma was applied for 30 s. The Raman profile of patterned MLG clearly shows the presence of the D peak (~1350 cm−1). The presence of D peak signifies the deterioration of graphene. The D peak appears when the honeycomb bonding of graphene is broken to result in structural damage [2]. As illustrated in the inset of Fig. 4(c), plasma can leak into the physical gap between the MLG surface and the lower part of the shadow mask and induce structural damage. The plasma induced damage will change the various properties of MLG such as work function and sheet resistance, and also limit the hole injection toward the HTL. The edge region of plasma patterned MLG bears irregularity (Fig. 4(d)). In order to obtain well-defined patterns, photolithography methods can be suggested. However, in this method, the problem of surface residues may take place. Removal of residues still remains as a challenging task in graphene applications [26–29]. So far no definite method for patterning graphene films exists and needs to be established. 6. Summary In this work, we have investigated the technical problems of using MLG as an anode in OLED applications. Based on OLED results, we have pointed out the differences between ITO and MLG OLEDs. In MLG-OLED the presence of microcavity is weak. This offers advantages of viewing angle independent EL spectrum and freedom in choosing the thickness of organics. However, enhancing the intensity of emission by means of a microcavity method cannot be applied in MLG-OLEDs. The J–V characteristics of MLG-OLED can be significantly enhanced by inserting a charge generation layer between MLG and HTL. Hole only devices and in-situ UPS approaches can be useful in selecting the charge
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generation layer. Concerning pattering of MLG, IR laser and O2 plasmashadow mask methods are not appropriate. The former did not offer defined dimension of patterns and the later induced surface damage of MLG. On fundamental level, the graphene process itself is limiting the applications of graphene. In most cases, graphene has to be grown, delaminated and transferred to the substrate of interest. During the delamination and transfer processes, graphene films can be easily damaged and contaminated. While MLG has been shown as a working transparent anode in OLEDs, there exist many hurdles to overcome to make graphene applicable in OLEDs. Especially, from engineering perspective, developing a reliable patterning process is important. Prime Novelty Statement In this article, we have presented the characteristics of graphene anode OLEDs and provided in depth investigations on important technical issues, which are very important in using graphene as a transparent electrode in OLEDs. We believe that our investigations and results are of high importance in realizing graphene OLEDs. Acknowledgments This work was financially supported by the Development Program of MOTIE/KEIT (10044412, Development of basic and applied technologies for OLEDs with Graphene), Korea. References [1] K. Novoselov, A. Geim, S. Morozov, D. Jiang, Y. Zhang, S. Dubonos, I. Grigorieva, A. Firsov, Electric field in atomically thin carbon films, Science 306 (2004) 666–669. [2] A. Ferrari, J. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. Novoselov, S. Roth, A. Geim, Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006) 187401–187404. [3] P. Avouris, Z. Chen, V. Perebeinos, Carbon-based electronics, Nat. Nanotechnol. 2 (2007) 605–615. [4] D. Rogers, A perfect fit, Plast. Electron. 5 (2014) 35–37. [5] F. Bonaccorso, Z. Sun, T. Hasan, A. Ferrari, Graphene photonics and optoelectronics, Nat. Photonics 4 (2010) 611–622. [6] J. Hwang, H.K. Choi, J. Moon, T. Kim, J.-W. Shin, C.W.J. Joo, J.-H. Han, D.-H. Cho, J.W. Huh, J.-I. Lee, H.Y. Chu, Multilayered graphene anode for blue phosphorescent organic light emitting diodes, Appl. Phys. Lett. 100 (2012) 133304–133307. [7] J. Moon, J. Hwang, H.K. Choi, T.Y. Kim, S.-Y. Choi, C.W. Joo, J.-H. Han, J.-W. Shin, B.J. Lee, D.-H. Cho, J.W. Huh, S.K. Park, N.S. Cho, H.Y. Chu, J.-I. Lee, Large area organic light emitting diodes with multilayered graphene anodes, Proc. SPIE 8476 (2012) U1–U5. [8] T. Tsutsui, N. Takada, S. Saito, F. Ogino, Sharply directed emission in organic electroluminescent diodes with an optical-microcavity structure, Appl. Phys. Lett. 65 (1994) 1868–1870. [9] J.W. Huh, J.-W. Shin, D.-H. Cho, J. Moon, C.W. Joo, S.K. Park, J. Hwang, N.S. Cho, J. Lee, J.-H. Han, H.Y. Chu, J.-I. Lee, A randomly nano-structured scattering layer for transparent organic light emitting diodes, Nanoscale 6 (2014) 10727–10733. [10] Y.-K. Kim, J. Kim, Y. Park, Energy level alignment at a charge generation interface between 4, 4′-bis(N-phenyl-1-naphthylamino)biphenyl and 1,4,5,8,9,11hexaazatriphenylene-hexacarbonitrile, Appl. Phys. Lett. 94 (2009) 63305–63307. [11] C. Small, S.-W. Tsang, J. Kido, S. So, F. So, Origin of enhanced hole injection in inverted organic devices with electron accepting interlayer, Adv. Funct. Mater. 22 (2012) 3261–3266. [12] J.-Y. Hong, J. Jang, Micropatterning of graphene sheets: recent advances in techniques and applications, J. Mater. Chem. 22 (2012) 8179–8191. [13] A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M. Dresselhaus, K. Jing, Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition, Nano Lett. 9 (2009) 30–35. [14] S. Bae, H. Kim, Y. Lee, X. Xu, J.S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. Kim, Y. Song, Y.-J. Kim, K.S. Kim, B. Özyilmaz, J.-H. Ahn, B.H. Hong, S. Iijima, Roll-to-roll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574–578. [15] J. Wu, M. Agrawal, H. Becerril, Z. Bao, Z. Liu, Y. Chen, P. Peumans, Organic lightemitting diodes on solution-processed graphene transparent electrodes, ACS Nano 4 (2010) 43–48. [16] R. Nair, P. Blake, A. Grigorenko, K. Novoselov, T. Booth, T. Stauber, N. Peres, A. Geim, Structure constant defines visual transparency of graphene, Science 210 (2008) 1308. [17] S.-Y. Kim, J.-J. Kim, Outcoupling efficiency of organic light emitting diodes employing graphene as the anode, Org. Electron. 13 (2012) 1081–1085. [18] M. Furno, R. Meerheim, S. Hofmann, B. Lüssem, K. Leo, Efficiency and rate of spontaneous emission in organic electroluminescent devices, Phys. Rev. B 85 (2012) (115205-11526).
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Technical issues in graphene anode organic light emitting diodes Jaehyun Moon a, Jin-Wook Shin a, Hyunsu Cho a, Jun-Han Han a, Nam Sung Cho a, Jong Tae Lim a, Seung Koo Park a, Hong Kyw Choi b, Sung-Yool Choi c, Ji-Hoon Kim d, Min-Jae Maeng d, Jaewon Seo d, Yongsup Park d, Jeong-Ik Lee a,⁎ a
Soft I/O Interface Research Section, Electronics and Telecommunications Research Inst. (ETRI), Daejeon, Republic of Korea University of Science and Technology, Daejeon, Republic of Korea Dept. of Electrical Engineering, Korean Advanced Inst. of Science and Technology, Daejeon, Republic of Korea d Dept. of Physics, Kyung Hee University, Seoul, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 28 November 2014 Received in revised form 25 March 2015 Accepted 25 March 2015 Available online xxxx Keywords: Graphene Organic light emitting diode Optical properties Interfacial properties Patterning
a b s t r a c t Optical, interfacial and patterning issues of anode graphene films in organic light emitting diode (OLED) applications were investigated. In the optical part, the microcavities of graphene and indium tin oxide (ITO) anode OLEDs were contrasted. With the use of graphene one may avoid spectral and organic stack design problems related to microcavity problems. However, due to the weak microcavity, emission enhancement using interference designs is practically impossible. By inserting an electron acceptor insert at the graphene/hole transport layer (HTL) interface, it was possible to enhance the current density by factor of three. Based on in situ ultraviolet photoelectron spectroscopy (UPS) results, the insert was interpreted as being a charge generation layer. Graphene patterning using laser or plasma methods turned out to be problematic. None of those methods could offer acceptable dimension accuracy and preserved graphene quality. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene is a two-dimensional film, in which carbon atoms are arranged in a hexagonal array. The thickness of one layer graphene is only 0.345 nm. Since the publication of Geim and Novoselov's article in year 2004, various outstanding properties and perspectives of graphene have reported in the literature [1–4]. As an ordinary course, graphene and multilayered graphene have been experimented in a variety of applications. As being thin, electrically conductive, optically transparent and mechanically flexible, graphene has gained extensive attention as a possible replacement for indium tin oxide (ITO), which has been so far dominantly used as the material for anode or transparent electrode in OLEDs [5–7]. Potentially, due to its thinness, electrical conductivity and mechanical compliance, graphene films can be used in various flexible electronics. In this article, we deal with the technical issues of graphene as an anode for OLEDs. Because graphene and ITO are different in many aspects, one needs to pay close attention to the differences to make most of the graphene anode. In this article, we focus on three aspects, which we consider as of high importance in realizing efficient graphene anode equipped OLEDs. Three topics are optical, interfacial and patterning considerations. In the optical part, we discuss microcavity and contrast its role to that of ITO anode case [8,9]. In the interfacial consideration, we compare the energy alignments of ITO and graphene with respect to ⁎ Corresponding author. E-mail address: [email protected] (J.-I. Lee).
adjacent organic layer, hole transport layer (HTL), and discuss a strategy which is useful in improving the hole transport [10,11]. In the patterning part, we investigate the patterning issue of graphene films [12]. So far, the patterning issue has been treated as a cursory issue. However, to realize graphene film as a component in integrated electronics devices, the patterning is an actual hurdle which has to be overcome. 2. Actual examples Before investigating the aforementioned technical issues, it is useful to compare the actual performances of OLEDs which have ITO or graphene anodes, and figure out the technical issues. We have used a multi-layered graphene (MLG) grown by a chemical vapor deposition (CVD) method [13]. We will refer an OLED with ITO anode as ITOOLED and an OLED with MLG anode as MLG-OLED. Fig. 1 shows various characteristics of phosphorescent green OLEDs. The green emitter was tris(2-phenylpyridinato-C2,N)iridium(III) (Ir(ppy)3), which was doped into a host of 2,6-bis[3-(carbazol-9-yl)phenyl]pyridine (DCzPPy). Both the current density (J) and luminance (L) levels of ITO-OLED were observed to be higher than those of MLG-OLED (Fig. 1(a)). Correspondingly the ITO-OLED showed higher external quantum efficiencies (EQEs) than that of MLG-OLED (Fig. 1(b)). The EQE of MLG-OLED is approximately 90% of ITO-OLED at a 1000 cd/m2. The results of Fig. 1(a) and (b) can be attributed to various factors. The hole injection of MLG is not effective as ITO. The hole injection is closely related to the energy alignment of the work function of the anode and the highest occupied
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Please cite this article as: J. Moon, et al., Technical issues in graphene anode organic light emitting diodes, Diamond Relat. Mater. (2015), http:// dx.doi.org/10.1016/j.diamond.2015.03.020
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Fig. 1. (a) The JVL characteristics of ITO anode and MLG anode OLEDs. (b) The EQEs of ITO anode and MLG anode OLEDs. (c) The EL spectra of ITO anode OLED as a function of viewing angle. (d) The EL spectra of MLG anode OLED as a function of viewing angle.
molecular orbital (HOMO) level of the adjacent organics. Also, the sheet resistance can be a factor. Typical OLED ITO anode has a sheet resistance (Rs) lower than 50 Ω/sq, while typical MLG has approximately 250 Ω/sq [14]. The high Rs of MLG causes the increase of the operating voltage of the OLED. Potentially, the high Rs can be problematic in achieving luminance uniformity over large area OLED lighting panels. The optical
properties of MLG can be a factor. The extinction coefficient (k) of graphene is reported to be high as 1.3 [15–17]. Such high k will cause significant absorption loss of the generated light. Fig. 1(c) and (d) compare the normalized electroluminescence (EL) spectra of ITO and MLG OLEDs as a function of viewing angle. In the case of ITO-OLED, shoulder development in the EL spectrum is apparent as the viewing angle changes.
Fig. 2. (a) Schematics of simulation cell and optical components. (b) Simulated EQEs of ITO anode and MLG anode OLEDs as a function of HTL thickness. Inset is the actual OLED structure used in simulations.
Please cite this article as: J. Moon, et al., Technical issues in graphene anode organic light emitting diodes, Diamond Relat. Mater. (2015), http:// dx.doi.org/10.1016/j.diamond.2015.03.020
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However, in the case of MLG-OLED, the EL spectrum is a very weak function of viewing angle. The results strongly indicate that the internal optics of ITO and MLG OLEDs are very different. To sum up, compared to ITO OLED, MLG OLED turned out to have differences in electrical performance and EL spectral characteristics, which have to be considered in making MLG-OLEDs. 3. Optical consideration In a simplified picture, conventional bottom emissive type OLEDs can be structurally described as a vertical stack of a metallic cathode, organic layers and a transparent anode. Fig. 2(a) schematically illustrates the simulation cell used in the simulations. Also optical components present in the OLED are depicted. HTL, ETL and EL refer to electron transport layer (ETL), hole transport layer (HTL) and emissive layer (EL), respectively. We have performed optical simulations, which is based on multiple interference and dipole oscillation theories. The light source or the dipole was positioned at the EL side of the ETL/EL interface. The theoretical background of the simulations can be found elsewhere [18]. Optical constants (n, k) of organic materials were measured using an ellipsometer (M-2000d, J.A. Woollam Co.). The optical constants of graphene were obtained from published literature [15,16]. In the simulation courses, we have varied the thickness of the HTL, while fixing the ETL thickness as 60 nm. The thicknesses of metallic Al cathode and EL were 100 nm and 20 nm, respectively. In bottom emission type OLEDs with ITO as the anode, microcavity exists between the metallic cathode and the ITO anode [19]. The light generated in the organic layer not only travels downward, which is designated as DEO, but also upward (UEO) toward the highly reflective metallic cathode. We refer the downward and upward traveling optical components as DEO and U EO, respectively. The optical components due to the reflection at the metallic cathode surface and anode/glass interface are referred as UER and DER, respectively. In order to maximize the light out-coupling of OLED, the thicknesses of organics are adjusted to an optical condition which corresponds to a constructive interference. Fig. 2(b) shows the simulated efficiencies (ηs) as a function of the HTL thickness. Here, the efficiency (η) refers to the external quantum efficiency (EQE, %). In the ITO anode case, η is observed to oscillate in a sinusoidal fashion, which marks the presence of microcavity. The difference in the η varies from 16% to 26%, depending on the choice of HTL thickness. Thus, from the perspective of η, the choice of organic thickness bears huge importance in ITO anode equipped OLEDs. However, compared to the ITO OLED case, the presence of microcavity is rather weak in graphene anode equipped OLEDs. The influence of microcavity in graphene anode OLED diminishes as the graphene thickness increases. It is important to notice that the overall η of graphene anode OLED is always lower than that of ITO anode OLED. The lower efficiency of graphene anode OLED seems to have its origin in, at least, two reasons. First, although the refractive index of graphene is reported as being 2.4, due to its fine thickness, graphene cannot effectively contribute in forming a reflective surface at the organic/graphene interface. Second, absorption can play a role in the loss of generated light. When light passes through a medium, absorption takes place. The light intensity (I) decays as I ~ exp(−2πk / λO)IO. Here λO and IO refer to wavelength and the incident light intensity, respectively. Thus light passing through a medium with bigger k is expected to experience larger decay in I. The reported k value of graphene is 1.3, while the k of ITO is 0.05 [20]. Because the presence of microcavity is weak in graphene anode OLEDs, the choice of organic thickness can be alleviated but the enhancement of the light out-coupling by means of a microcavity method is fairly limited. Also the high k value of graphene reduces the out coupling to a nonnegligible degree. OLED with thinner graphene anode shows higher EQE in the whole range considered. This is understood as the effect of higher transmittance which is equivalent to lower absorption. Comments regarding the choice of graphene film thickness are worthwhile here. As graphene film becomes thicker the sheet resistance decreases,
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leading to lower operation voltage and better luminance uniformity. However, as the absorption obstacle persists, thicker graphene cannot always be favored over thinner graphene. The choice of graphene film thickness depends on the application area of OLED. In OLED display applications the emission area of a pixel is about 50 μm × 50 μm. In such case the non-uniformity due to sheet resistance is not present. Thus thinner graphene films may be favored. In contrast, in OLED lighting applications, in which large area is favored, thicker graphene films may be used. Practically, to resolve the trade-off relation between transmittance and sheet resistance in large area OLED, auxiliary metal lines may be used. 4. Interfacial considerations In OLEDs, holes and electrons recombine in the emission layer to produce photons. Thus enhancing the current of electrons and holes toward the emission zone is of prime importance to ensure high performance OLEDs. In order to facilitate the current, a hole injection layer (HIL) is frequently inserted between the anode and HTL. In this work, we have used 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HatCN) as our HIL [21]. In order to explore the effect of Hat-CN on the electrical properties, we have fabricated hole only devices (HODs) with MLG anode. Our HODs have a stack structure of MLG/HTL (TAPC, 100 nm)/cathode (Al, 100 nm) and MLG/HIL(Hat-CN, 10 nm)/HTL (TAPC, 100 nm)/cathode (Al, 100 nm). TAPC is an acronym of 4,4′cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine], and is commonly used as a hole transport layer (HTL) in OLED applications. The MLG were obtained by a chemical vapor deposition method. Ni and CH4 were used as the catalytic agent and carbon source gas, respectively. Technical details can be found elsewhere [6]. Fig. 3(a) compares the current density (J) and applied voltage (V) characteristics of three HODs. As can be readily noticed, the use of Hat-CN as the HIL improves the J level significantly. To be specific the J increases three folds at an applied voltage of 4 V. The results of Fig. 3(a) strongly imply that the use Hat-CN HIL can significantly contribute in enhancing the efficiencies of MLG anode OLEDs. In other to elucidate the reason of improvement in J level by inserting a Hat-CN layer, we have performed in situ surface analyses using a He 1 ultraviolet photoelectron spectroscopy (UPS) method and constructed energy level diagrams. Fig. 3(b) and (c) show the energy level diagrams for MLG/TAPC and MLG/Hat-CN/TAPC interfaces, respectively. The work functions (ϕs ~ 5.1 eV) of MLG turned out to be not sensitive to the adjacent organic layer. The hole injection barriers (ϕhs) were estimated using the energy difference between the Fermi level (Ef) and the onset of HOMO peak. Details of the UPS methods in studying organic interfaces can be found elsewhere [10]. The ϕhs of MLG/TAPC and MLG/Hat-CN/TAPC were 0.65 eV and 3.03 eV, respectively. While a ϕh of 0.65 eV will not preclude hole injection toward TAPC, ϕh of 3.03 eV will effectively block hole injection at the MLG/ Hat-CN interface. Thus, enhancement in the J cannot be due to the lowered energy barrier between MLG and Hat-CN. Hat-CN is a high mobility strong n-type organic semiconductor, which can extract electrons from adjacent organics [10,22]. Thus the role of Hat-CN is not an energy barrier modifier but a charge generation layer (CGL) [23,24]. Under applied voltage, the electrons in the HOMO level of TAPC are extracted toward the Hat-CN LUMO level, which effectively contributes in hole generation in the TAPC layer. In this picture, the energy barrier between MLG and Hat-CN is irrelevant to the charge transport but the energy difference between the LUMO level of Hat-CN and the HOMO level of TAPC is. Fig. 3(d) illustrates the scenario of Hat-CN as a CGL under applied voltage. Because UPS measurements cannot locate the energy level of unoccupied energy levels, the LUMO level of Hat-CN cannot be positioned exactly. However, because the clear enhancement in J has been observed by using a Hat-CN HIL, the LUMO level of Hat-CN and HOMO level of TAPC must be very close, which implies the possibility of Fermi pinning of Hat-CN LUMO level [24]. In this work, we have
Please cite this article as: J. Moon, et al., Technical issues in graphene anode organic light emitting diodes, Diamond Relat. Mater. (2015), http:// dx.doi.org/10.1016/j.diamond.2015.03.020
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Fig. 3. (a) The JV characteristics of ITO anode and MLG anode HODs. (b) The energy level diagram of MLG/TAPC interface. (c) The energy level diagram of MLG/Hat-Cn/TAPC interface. (d) The charge transport taking place between the LUMO of Hat-Cn and HOMO of TAPC (see text for details).
Fig. 4. (a) SEM image of IR laser patterned MLG film. (b) Periphery of IR laser patterned MLG film. (c) The Raman spectra of pristine and plasma patterned MLG. Inset shows the configuration of plasma patterning and possible plasma leak paths. (d) Periphery of plasma patterned MLG film.
Please cite this article as: J. Moon, et al., Technical issues in graphene anode organic light emitting diodes, Diamond Relat. Mater. (2015), http:// dx.doi.org/10.1016/j.diamond.2015.03.020
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introduced Hat-CN to modify the MLG/HTL interface. The HOMO position of TAPC at the interface indicates that the electrons from the TAPC HOMO can be easily excited to the Hat-CN LUMO. This process is a good reason for using Hat-CN as a layer between TAPC and an anode where its actual role is generating electron–hole pairs at the interface with TAPC and transporting them toward the anode. Certainly other HIL candidates exist and wait to be studied. 5. Patterning consideration In this section, we discuss an issue of practical importance. In graphene film patterning, two issues are important. First, to be used as an anode, graphene films must be patterned into accurate dimensions. Second, the patterning method should not deteriorate the graphene. Graphene films can be patterned using laser. Laser sources equipped with moving stage facilities may enable direct patterning of graphene into various patterns. In this course laser ablates the graphene film to form the desired pattern. Fig. 4(a) and (b) show SEM images of our laser patterned MLG films, which have been transferred on a glass substrate. In laser patterning, we have used infrared (IR) laser with a wavelength of λ = 1064 nm. As can be readily noticed, the use laser for patterning purpose is problematic. First, defining the dimension accurately turns out to be a difficult task. The dotted lines in Fig. 4(a) represent the edges, on which horizontal MLG films were supposed to terminate. However, significant narrowing (~30%) of the MLG film can be observed. Second, in the laser exposed regions, MLG films wrap on the glass surface to form MLG bundles (Fig. 4(b)). The bundles are thought to being formed at the expense of neighborhood MLG films. Bundle forming at least is one reason of the MLG narrowing. The observations of Fig. 4(a) and (b) are, presumably, due to the high temperature in local region. Compared to MLG, glass is almost transparent to IR [25]. Also the heat conductivity of glass is lower. Thus the laser induced heat will be directed toward MLG. Graphene films can be patterned using oxygen plasma and a shadow mask. In this process, the exposed region is etched off by plasma, while shadowed region remains, forming patterns. Due to its simplicity, this method has been widely used in laboratory. Fig. 4(c) compares the Raman profiles collected from pristine MLG and patterned MLG. The plasma power and oxygen flow were 50 W and 30 sccm, respectively, and plasma was applied for 30 s. The Raman profile of patterned MLG clearly shows the presence of the D peak (~1350 cm−1). The presence of D peak signifies the deterioration of graphene. The D peak appears when the honeycomb bonding of graphene is broken to result in structural damage [2]. As illustrated in the inset of Fig. 4(c), plasma can leak into the physical gap between the MLG surface and the lower part of the shadow mask and induce structural damage. The plasma induced damage will change the various properties of MLG such as work function and sheet resistance, and also limit the hole injection toward the HTL. The edge region of plasma patterned MLG bears irregularity (Fig. 4(d)). In order to obtain well-defined patterns, photolithography methods can be suggested. However, in this method, the problem of surface residues may take place. Removal of residues still remains as a challenging task in graphene applications [26–29]. So far no definite method for patterning graphene films exists and needs to be established. 6. Summary In this work, we have investigated the technical problems of using MLG as an anode in OLED applications. Based on OLED results, we have pointed out the differences between ITO and MLG OLEDs. In MLG-OLED the presence of microcavity is weak. This offers advantages of viewing angle independent EL spectrum and freedom in choosing the thickness of organics. However, enhancing the intensity of emission by means of a microcavity method cannot be applied in MLG-OLEDs. The J–V characteristics of MLG-OLED can be significantly enhanced by inserting a charge generation layer between MLG and HTL. Hole only devices and in-situ UPS approaches can be useful in selecting the charge
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generation layer. Concerning pattering of MLG, IR laser and O2 plasmashadow mask methods are not appropriate. The former did not offer defined dimension of patterns and the later induced surface damage of MLG. On fundamental level, the graphene process itself is limiting the applications of graphene. In most cases, graphene has to be grown, delaminated and transferred to the substrate of interest. During the delamination and transfer processes, graphene films can be easily damaged and contaminated. While MLG has been shown as a working transparent anode in OLEDs, there exist many hurdles to overcome to make graphene applicable in OLEDs. Especially, from engineering perspective, developing a reliable patterning process is important. Prime Novelty Statement In this article, we have presented the characteristics of graphene anode OLEDs and provided in depth investigations on important technical issues, which are very important in using graphene as a transparent electrode in OLEDs. We believe that our investigations and results are of high importance in realizing graphene OLEDs. Acknowledgments This work was financially supported by the Development Program of MOTIE/KEIT (10044412, Development of basic and applied technologies for OLEDs with Graphene), Korea. References [1] K. Novoselov, A. Geim, S. Morozov, D. Jiang, Y. Zhang, S. Dubonos, I. Grigorieva, A. Firsov, Electric field in atomically thin carbon films, Science 306 (2004) 666–669. [2] A. Ferrari, J. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. Novoselov, S. Roth, A. Geim, Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006) 187401–187404. [3] P. Avouris, Z. Chen, V. Perebeinos, Carbon-based electronics, Nat. Nanotechnol. 2 (2007) 605–615. [4] D. Rogers, A perfect fit, Plast. Electron. 5 (2014) 35–37. [5] F. Bonaccorso, Z. Sun, T. Hasan, A. Ferrari, Graphene photonics and optoelectronics, Nat. Photonics 4 (2010) 611–622. [6] J. Hwang, H.K. Choi, J. Moon, T. Kim, J.-W. Shin, C.W.J. Joo, J.-H. Han, D.-H. Cho, J.W. Huh, J.-I. Lee, H.Y. Chu, Multilayered graphene anode for blue phosphorescent organic light emitting diodes, Appl. Phys. Lett. 100 (2012) 133304–133307. [7] J. Moon, J. Hwang, H.K. Choi, T.Y. Kim, S.-Y. Choi, C.W. Joo, J.-H. Han, J.-W. Shin, B.J. Lee, D.-H. Cho, J.W. Huh, S.K. Park, N.S. Cho, H.Y. Chu, J.-I. Lee, Large area organic light emitting diodes with multilayered graphene anodes, Proc. SPIE 8476 (2012) U1–U5. [8] T. Tsutsui, N. Takada, S. Saito, F. Ogino, Sharply directed emission in organic electroluminescent diodes with an optical-microcavity structure, Appl. Phys. Lett. 65 (1994) 1868–1870. [9] J.W. Huh, J.-W. Shin, D.-H. Cho, J. Moon, C.W. Joo, S.K. Park, J. Hwang, N.S. Cho, J. Lee, J.-H. Han, H.Y. Chu, J.-I. Lee, A randomly nano-structured scattering layer for transparent organic light emitting diodes, Nanoscale 6 (2014) 10727–10733. [10] Y.-K. Kim, J. Kim, Y. Park, Energy level alignment at a charge generation interface between 4, 4′-bis(N-phenyl-1-naphthylamino)biphenyl and 1,4,5,8,9,11hexaazatriphenylene-hexacarbonitrile, Appl. Phys. Lett. 94 (2009) 63305–63307. [11] C. Small, S.-W. Tsang, J. Kido, S. So, F. So, Origin of enhanced hole injection in inverted organic devices with electron accepting interlayer, Adv. Funct. Mater. 22 (2012) 3261–3266. [12] J.-Y. Hong, J. Jang, Micropatterning of graphene sheets: recent advances in techniques and applications, J. Mater. Chem. 22 (2012) 8179–8191. [13] A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M. Dresselhaus, K. Jing, Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition, Nano Lett. 9 (2009) 30–35. [14] S. Bae, H. Kim, Y. Lee, X. Xu, J.S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. Kim, Y. Song, Y.-J. Kim, K.S. Kim, B. Özyilmaz, J.-H. Ahn, B.H. Hong, S. Iijima, Roll-to-roll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574–578. [15] J. Wu, M. Agrawal, H. Becerril, Z. Bao, Z. Liu, Y. Chen, P. Peumans, Organic lightemitting diodes on solution-processed graphene transparent electrodes, ACS Nano 4 (2010) 43–48. [16] R. Nair, P. Blake, A. Grigorenko, K. Novoselov, T. Booth, T. Stauber, N. Peres, A. Geim, Structure constant defines visual transparency of graphene, Science 210 (2008) 1308. [17] S.-Y. Kim, J.-J. Kim, Outcoupling efficiency of organic light emitting diodes employing graphene as the anode, Org. Electron. 13 (2012) 1081–1085. [18] M. Furno, R. Meerheim, S. Hofmann, B. Lüssem, K. Leo, Efficiency and rate of spontaneous emission in organic electroluminescent devices, Phys. Rev. B 85 (2012) (115205-11526).
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Please cite this article as: J. Moon, et al., Technical issues in graphene anode organic light emitting diodes, Diamond Relat. Mater. (2015), http:// dx.doi.org/10.1016/j.diamond.2015.03.020