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Gerardo Hernandez-Sosa,* Serpil Tekoglu, Sebastian Stolz, Ralph Eckstein, Claudia Teusch, Jannik Trapp, Uli Lemmer, Manuel Hamburger, and Norman Mechau Printing functional inks by means of roll-to-roll compatible techniques are expressed as an almost universal motivation in publications dealing with solution-processed organic electronics. The advances in this field during the past two decades have yielded a number of high-performance material systems for a wide variety of applications such as photovoltaics, lighting, and integrated circuits or sensors, to name a few.[1] However, in the race to improve device performance, material processability by high-throughput techniques and process scalability for largearea production have commonly taken a secondary place in the research program. Conventional printing techniques have proven to be very reliable in the graphical industry, and are expected to allow a continuous, high-volume large-area fabrication of optoelectronic devices.[2,3] Nevertheless, these production techniques possess inherent restrictions for the processability of a material, for example, a specific viscosity window, film-thickness range, lateral resolution, etc.[3,4] Depending on the desired application, a suitable functional ink formulation should be developed for the specific printing or coating method. However, the formulated ink should achieve large-area pinhole-free films that are homogeneous on the nanometer scale and present minimal loss of device and material performance compared with labscale devices. In this work, we study the material properties, device performance, ink formulation, and the gravure-printing processing of polymer-based light-emitting electrochemical cells (LECs). Dr. G. Hernandez-Sosa, S. Tekoglu, S. Stolz, R. Eckstein, J. Trapp, Prof. U. Lemmer, Dr. N. Mechau Light Technology Institute Karlsruhe Institute of Technology Engesserstr. 13, 76131 Karlsruhe, Germany E-mail: [email protected] Dr. G. Hernandez-Sosa, S. Tekoglu, S. Stolz, R. Eckstein, Dr. M. Hamburger, Dr. N. Mechau InnovationLab, Speyererstr., 4, 69115 Heidelberg, Germany Prof. U. Lemmer Institute of Microstructure Technology Karlsruhe Institute of Technology Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany C. Teusch, Dr. M. Hamburger Organisch-Chemisches Institut Ruprecht-Karls-Universität Heidelberg Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
DOI: 10.1002/adma.201305541
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The Compromises of Printing Organic Electronics: A Case Study of Gravure-Printed Light-Emitting Electrochemical Cells
LECs represent a technology with distinct processing advantages over organic light-emitting diodes (OLEDs). Commonly, the fabrication of high-efficiency OLEDs requires multilayered devices with very controlled thicknesses.[5] Conversely, LECs are single-active-layer devices with a high tolerance to thickness variations, which eliminates the challenges associated with printing or coating multilayered devices with complex architectures.[6] The single active layer is composed of a semiconductor and an electrolyte blend, sandwiched between two electrodes regardless of their work function (i.e. no low-work-function metal is needed as a cathode). When a voltage is applied to the device, the mobile ions contained in the layer migrate to the oppositely charged electrode to electrostatically compensate injected carriers, giving rise to electrochemically doped regions in the semiconductor. These doped regions facilitate the injection and transport of charge carriers, which will recombine radiatively in an intermediate intrinsic region.[7] State-of-the-art devices have yielded lifetimes over several 1000 h at 10 cd A−1 and over 4000 h at 600 cd m−2 for polymer- and ionic transition metal complex (iMTC)-based LECs, respectively.[8,9] The printed polymer LECs in this work are based on poly(methyl methacrylate) (PMMA) and tetrabutylammonium tetrafluoroborate (TBABF4) as the polymer solid electrolyte (PSE), and a poly(phenylvinylene) derivative commonly known as “Super Yellow” (SY) as the emitting semiconductor material. The PSE was characterized by cyclic voltammetry measurements to investigate its suitability in LECs. Moreover, the rheological properties of the ink formulation were optimized by adjusting the solid concentration, the ratio between components, and the molecular weight (Mw) of PMMA. Our results highlight the unavoidable compromise existing between device performance and printability, when moving from lab-scale processing to the fabrication of devices by high-throughput techniques. The printed devices reached a luminance of ca. 1000 cd m−2 at 10 V, a continuous operation above 100 cd m−2 over several hours, and a shelf lifetime over six months, making them suitable for the production of smart packaging. Determination of the electrochemical stability of PSEs is crucial in order to hinder the undesired electrochemical degradation of the device.[10,11] The operation of an LEC requires the redox potentials of the emitting material to be within the electrochemical stability window of the PSE. In other words, the electrochemical doping of the semiconductor should take place at a potential where the irreversible oxidation or reduction of the PSE has not occurred. Figure 1a presents the cyclic voltammogram of the PSE (PMMA + TBABF4) in CH3CN. The
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Figure 1. a) Cyclic voltammetry measurements of the PSE composed of PMMA and TBABF4. The dashed lines represent the n and p doping potentials of SY according to ref.[12] b) J–V–L characteristics of two LECs prepared with different ratios of SY:PMMA:TBABF4 measured at a sweep speed of 0.1 V s−1. c) Lifetime characteristics of the same devices operated in galvanostatic mode at three different current densities.
PSE exhibits a strong and irreversible reduction peak with an onset at ca. −2 V, while no oxidation peak is visible. The dashed lines in Figure 1a represent the oxidation (p-type doping) and reduction (n-type doping) potentials of SY versus Fc/Fc+, +0.4 V and −2.2 V, respectively. The SY redox potentials were obtained by cyclic voltammetry (shown in Figure S1 in the Supporting Information) and correspond to the typical values observed
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in the literature.[12] Accordingly, the oxidation potential of SY (for p-type doping) is within the electrochemical stability window of PMMA + TBABF4. However, the reduction potential of SY is positioned close to the stability limit of the electrolyte, a common problem also found in previous reports for polyethylene oxide-based PSEs, which could be a source of undesired electrochemical reactions, which limit the operating lifetime of the device.[8,11,13,14] Figure 1b shows the current–voltage–luminance (J–V–L) characteristics of two spin coated LEC formulations with different SY:PMMA:TBABF4 fractions: 1:1.25:0.25 and 1:0.125:0.025. It can be observed that a higher ratio of the PSE material reduces the turn-on voltage (Von) of the device (i.e., the voltage at which L > 1 cd m−2), and allows a higher luminance at lower voltages. Von was found to be ca. 8.5 V and ca. 15 V for the highest and lowest amounts of PSE respectively. This effect is related to the effective ionic conductivity of the blend and relative amount of PSE.[13,15,16] A larger ratio of PSE facilitates ionic transport and as a consequence the more rapid formation of the p and n doping fronts.[13,16] This effect can also be observed in the lifetime characteristics presented in Figure 1c. The measurements were obtained in galvanostatic mode at three different current densities. It can be observed that both the composition and the operation conditions affect the lifetime and maximum luminance achieved by the devices. A large amount of PSE will allow faster turn on times but shorten the lifetime, as has been reported elsewhere.[10,14] On the other hand, a higher current density accelerates the undesired side reactions shortening the operation lifetime.[17] Detailed studies of the effect of the operation conditions on the device lifetime are already present in literature, where pulsed current operation was used to delay the deterioration of iMTC based LECs.[17,18] The optoelectronic characteristics of these two devices are compared in Table 1. It can be observed that the turn-on time (τon – the time needed to reach 100 cd m−2) is reduced by ca. 360%, ca. 40%, and ca. 20%, for current densities of 10 mA cm−2, 20 mA cm−2, and 33 mA cm−2, respectively, when using the formulation with more PSE. However, lifetimes (τ50) of up to 12 h, 8 h, and 3.5 h for the 1:0.125:0.025 LEC formulation were achieved when operating at the same respective currents. These results suggest that a formulation with a reduced amount of PSE would be preferred when looking towards printed devices with a maximized lifetime. Gravure printing is a promising option for the highthroughput fabrication of optoelectronic devices. It is suitable for a wide variety of solvents and it allows for high printing speeds (1–10 m s−1). Moreover, its lateral resolution on the Table 1. Performance characteristics of the LECs prepared with different SY:PMMA: TBABF4 ratios. SY:PMMA: TBABF4 ratio
Vona) [V]
Lmaxa) @ 10 mA cm−2 @ 20 mA cm−2 @ 33 mA cm−2 [cd m−2]
τonb) [min]
τ50 [h]
τonb) [min]
τ50 [h]
τonb) [min]
τ50 [h]
1:1.25:0.25
8.5
1270
12
0.35
3
0.43
0.7
0.89
1:0.125:0.025
15
105
330
12.6
7.2
8.1
3.3
3.6
a)
−1 b)
−2
Sweep Speed 0.1 Vs ; Time to reach 100 cd m .
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micrometer scale makes it particularly suitable for the production of organic field-effect transistors, OLEDs, or solar cells.[3,19] A schematic of the gravure-printing process is depicted in Figure 2a. In the first stage (a), the cells of the engraved gravure cylinder are loaded with the functional ink which usually has a viscosity between 10 and 100 mPa s.[3,4] Subsequently, a blade removes the excess ink (b). In the third step (c), the ink
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Figure 2. a) Scheme of a gravure-printing process. b) Shear-rate viscosity dependency for four different LEC ink formulations containing different SY:PMMA:TBABF4 ratios at a concentration of 10 g L−1. c) Contrastenhanced photographs of gravure-printed films using different LEC ink formulations. The inset shows FFT images of the corresponding image.
is transferred from the gravure cell to the substrate (typically half of the cell volume), which requires good wetting properties from the fluid to the substrate. In the final stage (d), two competing processes, film drying and surface demodulation, take place. The transferred wet film usually presents an inhomogeneous thickness arising from hydrodynamic instabilities during the ink-transfer process. These hydrodynamic instabilities produce a pattern in the printing direction, commonly called viscous fingering, where the wavelength depends on the printing parameters as well as the fluid properties.[20–22] Its leveling time depends directly on the viscosity and inversely on the surface tension and wet-film thickness.[23] Films suitable for device fabrication will be produced only if the drying time is longer than the leveling time.[21,22] All the different stages of the gravure-printing process induce different amounts of shear forces on the fluid. For example, ink transfer and film drying take place at high and low shear rates respectively. Therefore, the viscosity of the fluid at high shear rates should be just low enough to reduce hydrodynamic instabilities, while assuring good pattern reproducibility. At low shear rates, the viscosity should not exhibit a high increase in order to favor film leveling of the fingering patterns.[20–23] In other words, a small shear thinning of the viscosity (i.e., Newtonian-like fluid behavior) favors a better printing outcome.[21,24] The shear-dependent viscosity of four LEC ink formulations with different SY:PMMA:TBABF4 ratios, taking the ratios presented in Figure 1b as upper and lower limits, are shown in Figure 2b. It can be observed that the behavior of the viscosity is affected by the ratio of the components within the blend. The printing outcomes of the different ink formulations are shown in Figure 2c. The inset of each figure corresponds to the fast Fourier transformation (FFT) of the corresponding image and should serve as a qualitative measure of the homogeneity of the film. A modulation in the microscope image would yield a pattern in the FFT image, whereas an ideal homogeneous layer would be represented as a single dot in the center of the diagram. It is observed that the best printing results are obtained from the formulations with a high PMMA content, where the viscous fingering pattern is minimized. This is a consequence of the more Newtonian behavior of these formulations, as can be seen in Figure 2b. SY has been reported to exhibit a strong non-Newtonian behavior; therefore, SY-rich formulations would tend to present moreviscous fingering when gravure printed.[21] Consequently, the optimal printing formulation would require a higher amount of PMMA than that observed for the best device performance (i.e., 1:0.125:0.025). It has been reported that the Mw of the polymer contained in the PSE affects its ionic mobility, the film morphology, and consequently the performance of the LECs.[15] Moreover, the viscosity of a polymer is proportional to its Mw and therefore changing the Mw would be a viable route to modify the rheology, the printing outcome, and the performance of the devices. Figure 3a shows the shear-rate-dependent viscosity of the LEC formulations (1:1.125:0.25) using four different Mw PMMA values with a total solid concentration of 10 g L−1 and 15 g L−1. It can be observed that the main effect of the Mw of PMMA on the viscosity of the ink formulations takes place at low shear rates. Additionally, a lower solid concentration
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yields lower viscosity values and a more Newtonian behavior of the viscosity. The results show that for this specific material combination, the magnitude and shear-rate dependency of the viscosity are mainly ruled by the properties of SY. Nevertheless, it would be expected that the Mw of the PSE would play a more important role in LECs based on oligomer or small-molecule emitters. The microscopy images and the corresponding FFT images of the printing fields are shown in Figure 3b. It can be observed that all the samples prepared at 10 g L−1 give acceptable results due to the lower and more Newtonian viscosity. For the 15 g L−1 formulations, the lower molecular weight (i.e., lower viscosity) LEC formulations
tend to yield more-homogeneous layers. Solid concentrations lower than 10 g L−1 will also result in homogeneous layers, as is presented in Figure S2 in the Supporting Information for a PMMA Mw of 350 kg mol−1. However, lower solid concentrations would results in thinner layers, which would in turn affect device performance, or, in the worst case, device production yield. Generally, this can be compensated to a certain extent by the gravure-printing plate settings (i.e., gravure-cell volume, tonal value, etc.); however, it would need to be investigated on a case to case basis depending on the functional ink properties. Figure 4 presents the J–V–L characteristics of a gravureprinted device, as prepared and after six months of storage in a N2-filled glovebox. The device was printed on an indium tin oxide (ITO)-covered poly(ethylene terphthalate) (PET) substrate using a PMMA with Mw of 350 kg mol−1 and an SY:PMMA:TBABF4 ratio of 1:1.25:0.25, the most-suitable formulation among those tested in this work. The maximum luminances recorded at 10 V for the as-prepared and stored device, respectively, were ca. 2000 cd m−2 and ca. 750 cd m−2, whereas Von of ca. 6.5 V and ca. 7.2 V were observed. The current–voltage characteristics also display a deterioration after the device has been stored, as the maximum current achieved at 10 V reduced by ca. 50%. A photograph of a device under operation is presented in the inset of Figure 4, showing the mechanical flexibility and homogeneity of the light emission. In summary, we have presented the fabrication of polymerbased LECs by the gravure-printing technique under ambient conditions. The PSE was characterized by cyclic voltammetry
Figure 3. a) Shear-rate viscosity dependency of LEC ink formulations using four different Mw and two different solid concentrations. SY:PMMA:TBABF4 ratios were set constant (1:1.25:0.25). b) Contrast-enhanced photographs of gravure-printed films using different LEC ink formulations. The insets show the corresponding FFT images.
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corresponding weight ratio of SY, PMMA, and TBABF4, and stirred overnight. The viscosity of the solutions was measured at 23 °C using a fluid volume of 1 mL on the cone-plate geometry using a Haake MARS rheometer. Gravure Printing: Printing samples were prepared on ITO-covered PET substrates using an RK gravure-printing proofer with a field size of 2.25 cm2 at a speed of ca. 1 m s−1. The cell volume per area of the gravure plate was 14 mL m−2 with 54 lines per cm. Thickness measurements were performed using a Veeco profilometer. Printed devices were fabricated on PET substrates with ITO pre-patterned electrodes from the LEC formulation of 10 g L−1 and a Mw of 350 kg mol−1. Device Characterization: Reference spin-coated samples were fabricated using the 350 kg mol−1 PMMA as a base for the PSE. Both the spin-coated and printed LECs were fabricated with Ag top contacts (100 nm) using a shadow mask to define the active area of the device (0.24 cm2). The electrical and optical characterization of the devices were performed using a calibrated BOTEST characterization system at a measuring speed of 0.1 V s−1. Figure 4. Current–voltage–luminance characteristics of a gravure-printed LEC. Inset: Photograph of a flexible LEC in operation.
to investigate its electrochemical-stability window and ensure its compatibility with the semiconductor polymer. The rheological properties of the blend, which assure a suitable printed-layer quality for the fabrication of devices, were identified by altering the material ratio, the PMMA Mw, and the solid concentration. The proposed approach shows that a polymer-based LEC system offers the possibility of developing ink formulations for a given printing technique without changing its chemical composition. The printed devices achieved maximum luminance values of ca. 2000 cd m−2 at 10 V, a Von of 6.5 V, and a shelf lifetime of over six months making them promising for large-area low-end applications like packaging or advertisement. As in the present case, adjustment of all the different parameters to obtain a functional ink for organic optoelectronic devices will inevitably result in a compromise between performance and printability. However, this drawback can be minimized if the different variables of the printing process (e.g., solvent type, viscosity, substrate wetting, etc.) are taken into account during the early stages of the materialdevelopment process.
Experimental Section Cyclic Voltammetry (CV): A three-electrode set-up consisting of a Pt working electrode, a Pt/Ti counter electrode, and a Ag wire pseudoreference electrode placed in a glass vessel was employed for the CV measurements. 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) (>99%, Sigma–Aldrich) + 2 M poly(methyl methacrylate) (PMMA) (Mw = 350 kg mol−1, Sigma–Aldrich) in CH3CN was used as electrolyte system. The molarity of the PMMA corresponded to the concentration of the monomeric repeat units. The voltage sweeps were driven and the current measured using a VERSASTAT 3 potentiostat (Princeton Applied Research). After measurement, a calibration scan was performed by adding a small amount of ferrocene (Di(cyclopentadienyl)iron) (>98%, Sigma–Aldrich) to the sample as an internal reference redox system. All the CV measurements were performed under an Ar atmosphere. Ink Formulation and Viscosity Measurements: The LEC formulations for the printing investigations were prepared as follows: SY (PDY132 LIVILUX, Merck), PMMA (Sigma–Aldrich, Mw = 350 kg mol−1) and TBABF4 (Sigma–Aldrich) were dissolved separately in anisole for several hours at concentrations of 5 g L−1, 10 g L−1, 12.5 g L−1, and 15 g L−1. Subsequently, the solutions were mixed at the
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors acknowledge financial support via the Gutenberg Plus project (FKZ: 13N11903) and MORPHEUS (FKZ: 13N11707) of the Leading-Edge Cluster Forum Organic Electronics managed by InnovationLab GmbH within the High-Tech Strategy for Germany of the Federal Ministry of Education and Research. Received: November 7, 2013 Revised: December 31, 2013 Published online: February 26, 2014 [1] a) T. Ameri, N. Li, C. J. Brabec, Energy Environ. Sci. 2013, 6, 2390; b) L. Duan, L. Hou, T.-W. Lee, J. Qiao, D. Zhang, G. Dong, L. Wanga, Y. Qiu, J. Mater. Chem. 2010, 20, 6392; c) K.-J. Baeg, M. Caironi, Y.-Y. Noh, Adv. Mater. 2012, 25, 4210; d) L. Torsi, M. Magliulo, K. Manoli, G. Palazzo, Chem. Soc. Rev. 2013, 42, 8612. [2] J. R. Sheats, J. Mater. Res. 2004, 19, 1974. [3] R. R. Søndergaard, M. Hösel, F. C. Krebs, J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 16. [4] H. Kipphan, Handbook of Print Media: Technologies and Production Methods, Springer, Berlin, Germany, 2001. [5] B. Geffroy, P. le Roy, C. Prat, Polym. Int. 2006, 55, 572. [6] a) Q. B. Pei, G. Yu, C. Zhang, Y. Yang, A. J. Heeger, Science 1995, 269, 1086; b) A. Sandström, H. F. Dam, F. C. Krebs, L. Edman, Nat. Commun. 2012, 3, 1002; c) L. Edman, Electrochim. Acta 2005, 50, 3878. [7] a) Q. Pei, Y. Yang, G. Yu, C. Zhang, A. J. Heeger, J. Am. Chem. Soc. 1996, 118, 3922; b) S. B. Meier, St. van Reenen, B. Lefevre, D. Hartmann, H. J. Bolink, A. Winnacker, W. Sarfert, M. Kemerink, Adv. Funct. Mater. 2013, 23, 3531; c) S. Van Reenen, P. Matyba, A. Dzwilewski, R. Janssen, L. Edman, M. Kemerink, J. Am. Chem. Soc. 2010, 132, 13776. [8] S. Tang, L. Edman, J. Phys. Chem. Lett. 2010, 1, 2727. [9] D. Tordera, S. Meier, M. Lenes, R. D. Costa, E. Ortí, W. Sarfert, H. J. Bolink, Adv. Mater. 2012, 24, 897. [10] J. H. Shin, P. Matyba, N. D. Robinson, L. Edman, Electrochim. Acta 2007, 52, 6456. [11] P. Matyba, M. R. Andersson, L. Edman, Org. Electron. 2008, 9, 699.
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[12] A. Sandström, P. Matyba, L. Edman, Appl. Phys. Lett. 2010, 96, 053303. [13] J. Fang, P. Matyba, N. D. Robinson, L. Edman, J. Am. Chem. Soc. 2008, 130, 4562. [14] R. D. Costa, A. Pertegás, E. Ortí,H. J. Bolink, Chem. Mater. 2010, 22, 1288. [15] G. Hernandez-Sosa, R. Eckstein, S. Tekoglu, T. Becker, F. Mathies, U. Lemmer, N. Mechau, Org. Electron. 2013, 14, 2223. [16] S. van Reenen, P. Matyba, A. Dzwilewski, R. A. J. Janssen, L. Edman, M. Kemerink, Adv. Funct. Mater. 2011, 21,1795. [17] D. Tordera, J. Frey, D. Vonlanthen, E. Constable, A. Pertegás, E. Ortí, H. J. Bolink, E. Baranoff, M. K. Nazeeruddin, Adv. Energy Mater. 2013, 3, 1338. [18] N. M. Shavaleev, R. Scopelliti, M. Grätzel, M. K. Nazeeruddin, A. Pertegás, C. Roldán-Carmona, D. Tordera, H. J. Bolink, J. Mater. Chem. C 2013, 1, 2241. [19] a) H. Kang, R. Kitsomboonloha, J. Jang, V. Subramanian. Adv. Mater. 2012, 22, 3065; b) S. Tekoglu, G. Hernandez-Sosa, E. Kluge, U. Lemmer, N. MechauOrg. Electron. 2013, 14, 3493; c) P. Kopola,
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Gerardo Hernandez-Sosa,* Serpil Tekoglu, Sebastian Stolz, Ralph Eckstein, Claudia Teusch, Jannik Trapp, Uli Lemmer, Manuel Hamburger, and Norman Mechau Printing functional inks by means of roll-to-roll compatible techniques are expressed as an almost universal motivation in publications dealing with solution-processed organic electronics. The advances in this field during the past two decades have yielded a number of high-performance material systems for a wide variety of applications such as photovoltaics, lighting, and integrated circuits or sensors, to name a few.[1] However, in the race to improve device performance, material processability by high-throughput techniques and process scalability for largearea production have commonly taken a secondary place in the research program. Conventional printing techniques have proven to be very reliable in the graphical industry, and are expected to allow a continuous, high-volume large-area fabrication of optoelectronic devices.[2,3] Nevertheless, these production techniques possess inherent restrictions for the processability of a material, for example, a specific viscosity window, film-thickness range, lateral resolution, etc.[3,4] Depending on the desired application, a suitable functional ink formulation should be developed for the specific printing or coating method. However, the formulated ink should achieve large-area pinhole-free films that are homogeneous on the nanometer scale and present minimal loss of device and material performance compared with labscale devices. In this work, we study the material properties, device performance, ink formulation, and the gravure-printing processing of polymer-based light-emitting electrochemical cells (LECs). Dr. G. Hernandez-Sosa, S. Tekoglu, S. Stolz, R. Eckstein, J. Trapp, Prof. U. Lemmer, Dr. N. Mechau Light Technology Institute Karlsruhe Institute of Technology Engesserstr. 13, 76131 Karlsruhe, Germany E-mail: [email protected] Dr. G. Hernandez-Sosa, S. Tekoglu, S. Stolz, R. Eckstein, Dr. M. Hamburger, Dr. N. Mechau InnovationLab, Speyererstr., 4, 69115 Heidelberg, Germany Prof. U. Lemmer Institute of Microstructure Technology Karlsruhe Institute of Technology Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany C. Teusch, Dr. M. Hamburger Organisch-Chemisches Institut Ruprecht-Karls-Universität Heidelberg Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
DOI: 10.1002/adma.201305541
Adv. Mater. 2014, 26, 3235–3240
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The Compromises of Printing Organic Electronics: A Case Study of Gravure-Printed Light-Emitting Electrochemical Cells
LECs represent a technology with distinct processing advantages over organic light-emitting diodes (OLEDs). Commonly, the fabrication of high-efficiency OLEDs requires multilayered devices with very controlled thicknesses.[5] Conversely, LECs are single-active-layer devices with a high tolerance to thickness variations, which eliminates the challenges associated with printing or coating multilayered devices with complex architectures.[6] The single active layer is composed of a semiconductor and an electrolyte blend, sandwiched between two electrodes regardless of their work function (i.e. no low-work-function metal is needed as a cathode). When a voltage is applied to the device, the mobile ions contained in the layer migrate to the oppositely charged electrode to electrostatically compensate injected carriers, giving rise to electrochemically doped regions in the semiconductor. These doped regions facilitate the injection and transport of charge carriers, which will recombine radiatively in an intermediate intrinsic region.[7] State-of-the-art devices have yielded lifetimes over several 1000 h at 10 cd A−1 and over 4000 h at 600 cd m−2 for polymer- and ionic transition metal complex (iMTC)-based LECs, respectively.[8,9] The printed polymer LECs in this work are based on poly(methyl methacrylate) (PMMA) and tetrabutylammonium tetrafluoroborate (TBABF4) as the polymer solid electrolyte (PSE), and a poly(phenylvinylene) derivative commonly known as “Super Yellow” (SY) as the emitting semiconductor material. The PSE was characterized by cyclic voltammetry measurements to investigate its suitability in LECs. Moreover, the rheological properties of the ink formulation were optimized by adjusting the solid concentration, the ratio between components, and the molecular weight (Mw) of PMMA. Our results highlight the unavoidable compromise existing between device performance and printability, when moving from lab-scale processing to the fabrication of devices by high-throughput techniques. The printed devices reached a luminance of ca. 1000 cd m−2 at 10 V, a continuous operation above 100 cd m−2 over several hours, and a shelf lifetime over six months, making them suitable for the production of smart packaging. Determination of the electrochemical stability of PSEs is crucial in order to hinder the undesired electrochemical degradation of the device.[10,11] The operation of an LEC requires the redox potentials of the emitting material to be within the electrochemical stability window of the PSE. In other words, the electrochemical doping of the semiconductor should take place at a potential where the irreversible oxidation or reduction of the PSE has not occurred. Figure 1a presents the cyclic voltammogram of the PSE (PMMA + TBABF4) in CH3CN. The
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Figure 1. a) Cyclic voltammetry measurements of the PSE composed of PMMA and TBABF4. The dashed lines represent the n and p doping potentials of SY according to ref.[12] b) J–V–L characteristics of two LECs prepared with different ratios of SY:PMMA:TBABF4 measured at a sweep speed of 0.1 V s−1. c) Lifetime characteristics of the same devices operated in galvanostatic mode at three different current densities.
PSE exhibits a strong and irreversible reduction peak with an onset at ca. −2 V, while no oxidation peak is visible. The dashed lines in Figure 1a represent the oxidation (p-type doping) and reduction (n-type doping) potentials of SY versus Fc/Fc+, +0.4 V and −2.2 V, respectively. The SY redox potentials were obtained by cyclic voltammetry (shown in Figure S1 in the Supporting Information) and correspond to the typical values observed
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in the literature.[12] Accordingly, the oxidation potential of SY (for p-type doping) is within the electrochemical stability window of PMMA + TBABF4. However, the reduction potential of SY is positioned close to the stability limit of the electrolyte, a common problem also found in previous reports for polyethylene oxide-based PSEs, which could be a source of undesired electrochemical reactions, which limit the operating lifetime of the device.[8,11,13,14] Figure 1b shows the current–voltage–luminance (J–V–L) characteristics of two spin coated LEC formulations with different SY:PMMA:TBABF4 fractions: 1:1.25:0.25 and 1:0.125:0.025. It can be observed that a higher ratio of the PSE material reduces the turn-on voltage (Von) of the device (i.e., the voltage at which L > 1 cd m−2), and allows a higher luminance at lower voltages. Von was found to be ca. 8.5 V and ca. 15 V for the highest and lowest amounts of PSE respectively. This effect is related to the effective ionic conductivity of the blend and relative amount of PSE.[13,15,16] A larger ratio of PSE facilitates ionic transport and as a consequence the more rapid formation of the p and n doping fronts.[13,16] This effect can also be observed in the lifetime characteristics presented in Figure 1c. The measurements were obtained in galvanostatic mode at three different current densities. It can be observed that both the composition and the operation conditions affect the lifetime and maximum luminance achieved by the devices. A large amount of PSE will allow faster turn on times but shorten the lifetime, as has been reported elsewhere.[10,14] On the other hand, a higher current density accelerates the undesired side reactions shortening the operation lifetime.[17] Detailed studies of the effect of the operation conditions on the device lifetime are already present in literature, where pulsed current operation was used to delay the deterioration of iMTC based LECs.[17,18] The optoelectronic characteristics of these two devices are compared in Table 1. It can be observed that the turn-on time (τon – the time needed to reach 100 cd m−2) is reduced by ca. 360%, ca. 40%, and ca. 20%, for current densities of 10 mA cm−2, 20 mA cm−2, and 33 mA cm−2, respectively, when using the formulation with more PSE. However, lifetimes (τ50) of up to 12 h, 8 h, and 3.5 h for the 1:0.125:0.025 LEC formulation were achieved when operating at the same respective currents. These results suggest that a formulation with a reduced amount of PSE would be preferred when looking towards printed devices with a maximized lifetime. Gravure printing is a promising option for the highthroughput fabrication of optoelectronic devices. It is suitable for a wide variety of solvents and it allows for high printing speeds (1–10 m s−1). Moreover, its lateral resolution on the Table 1. Performance characteristics of the LECs prepared with different SY:PMMA: TBABF4 ratios. SY:PMMA: TBABF4 ratio
Vona) [V]
Lmaxa) @ 10 mA cm−2 @ 20 mA cm−2 @ 33 mA cm−2 [cd m−2]
τonb) [min]
τ50 [h]
τonb) [min]
τ50 [h]
τonb) [min]
τ50 [h]
1:1.25:0.25
8.5
1270
12
0.35
3
0.43
0.7
0.89
1:0.125:0.025
15
105
330
12.6
7.2
8.1
3.3
3.6
a)
−1 b)
−2
Sweep Speed 0.1 Vs ; Time to reach 100 cd m .
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micrometer scale makes it particularly suitable for the production of organic field-effect transistors, OLEDs, or solar cells.[3,19] A schematic of the gravure-printing process is depicted in Figure 2a. In the first stage (a), the cells of the engraved gravure cylinder are loaded with the functional ink which usually has a viscosity between 10 and 100 mPa s.[3,4] Subsequently, a blade removes the excess ink (b). In the third step (c), the ink
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Figure 2. a) Scheme of a gravure-printing process. b) Shear-rate viscosity dependency for four different LEC ink formulations containing different SY:PMMA:TBABF4 ratios at a concentration of 10 g L−1. c) Contrastenhanced photographs of gravure-printed films using different LEC ink formulations. The inset shows FFT images of the corresponding image.
is transferred from the gravure cell to the substrate (typically half of the cell volume), which requires good wetting properties from the fluid to the substrate. In the final stage (d), two competing processes, film drying and surface demodulation, take place. The transferred wet film usually presents an inhomogeneous thickness arising from hydrodynamic instabilities during the ink-transfer process. These hydrodynamic instabilities produce a pattern in the printing direction, commonly called viscous fingering, where the wavelength depends on the printing parameters as well as the fluid properties.[20–22] Its leveling time depends directly on the viscosity and inversely on the surface tension and wet-film thickness.[23] Films suitable for device fabrication will be produced only if the drying time is longer than the leveling time.[21,22] All the different stages of the gravure-printing process induce different amounts of shear forces on the fluid. For example, ink transfer and film drying take place at high and low shear rates respectively. Therefore, the viscosity of the fluid at high shear rates should be just low enough to reduce hydrodynamic instabilities, while assuring good pattern reproducibility. At low shear rates, the viscosity should not exhibit a high increase in order to favor film leveling of the fingering patterns.[20–23] In other words, a small shear thinning of the viscosity (i.e., Newtonian-like fluid behavior) favors a better printing outcome.[21,24] The shear-dependent viscosity of four LEC ink formulations with different SY:PMMA:TBABF4 ratios, taking the ratios presented in Figure 1b as upper and lower limits, are shown in Figure 2b. It can be observed that the behavior of the viscosity is affected by the ratio of the components within the blend. The printing outcomes of the different ink formulations are shown in Figure 2c. The inset of each figure corresponds to the fast Fourier transformation (FFT) of the corresponding image and should serve as a qualitative measure of the homogeneity of the film. A modulation in the microscope image would yield a pattern in the FFT image, whereas an ideal homogeneous layer would be represented as a single dot in the center of the diagram. It is observed that the best printing results are obtained from the formulations with a high PMMA content, where the viscous fingering pattern is minimized. This is a consequence of the more Newtonian behavior of these formulations, as can be seen in Figure 2b. SY has been reported to exhibit a strong non-Newtonian behavior; therefore, SY-rich formulations would tend to present moreviscous fingering when gravure printed.[21] Consequently, the optimal printing formulation would require a higher amount of PMMA than that observed for the best device performance (i.e., 1:0.125:0.025). It has been reported that the Mw of the polymer contained in the PSE affects its ionic mobility, the film morphology, and consequently the performance of the LECs.[15] Moreover, the viscosity of a polymer is proportional to its Mw and therefore changing the Mw would be a viable route to modify the rheology, the printing outcome, and the performance of the devices. Figure 3a shows the shear-rate-dependent viscosity of the LEC formulations (1:1.125:0.25) using four different Mw PMMA values with a total solid concentration of 10 g L−1 and 15 g L−1. It can be observed that the main effect of the Mw of PMMA on the viscosity of the ink formulations takes place at low shear rates. Additionally, a lower solid concentration
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yields lower viscosity values and a more Newtonian behavior of the viscosity. The results show that for this specific material combination, the magnitude and shear-rate dependency of the viscosity are mainly ruled by the properties of SY. Nevertheless, it would be expected that the Mw of the PSE would play a more important role in LECs based on oligomer or small-molecule emitters. The microscopy images and the corresponding FFT images of the printing fields are shown in Figure 3b. It can be observed that all the samples prepared at 10 g L−1 give acceptable results due to the lower and more Newtonian viscosity. For the 15 g L−1 formulations, the lower molecular weight (i.e., lower viscosity) LEC formulations
tend to yield more-homogeneous layers. Solid concentrations lower than 10 g L−1 will also result in homogeneous layers, as is presented in Figure S2 in the Supporting Information for a PMMA Mw of 350 kg mol−1. However, lower solid concentrations would results in thinner layers, which would in turn affect device performance, or, in the worst case, device production yield. Generally, this can be compensated to a certain extent by the gravure-printing plate settings (i.e., gravure-cell volume, tonal value, etc.); however, it would need to be investigated on a case to case basis depending on the functional ink properties. Figure 4 presents the J–V–L characteristics of a gravureprinted device, as prepared and after six months of storage in a N2-filled glovebox. The device was printed on an indium tin oxide (ITO)-covered poly(ethylene terphthalate) (PET) substrate using a PMMA with Mw of 350 kg mol−1 and an SY:PMMA:TBABF4 ratio of 1:1.25:0.25, the most-suitable formulation among those tested in this work. The maximum luminances recorded at 10 V for the as-prepared and stored device, respectively, were ca. 2000 cd m−2 and ca. 750 cd m−2, whereas Von of ca. 6.5 V and ca. 7.2 V were observed. The current–voltage characteristics also display a deterioration after the device has been stored, as the maximum current achieved at 10 V reduced by ca. 50%. A photograph of a device under operation is presented in the inset of Figure 4, showing the mechanical flexibility and homogeneity of the light emission. In summary, we have presented the fabrication of polymerbased LECs by the gravure-printing technique under ambient conditions. The PSE was characterized by cyclic voltammetry
Figure 3. a) Shear-rate viscosity dependency of LEC ink formulations using four different Mw and two different solid concentrations. SY:PMMA:TBABF4 ratios were set constant (1:1.25:0.25). b) Contrast-enhanced photographs of gravure-printed films using different LEC ink formulations. The insets show the corresponding FFT images.
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corresponding weight ratio of SY, PMMA, and TBABF4, and stirred overnight. The viscosity of the solutions was measured at 23 °C using a fluid volume of 1 mL on the cone-plate geometry using a Haake MARS rheometer. Gravure Printing: Printing samples were prepared on ITO-covered PET substrates using an RK gravure-printing proofer with a field size of 2.25 cm2 at a speed of ca. 1 m s−1. The cell volume per area of the gravure plate was 14 mL m−2 with 54 lines per cm. Thickness measurements were performed using a Veeco profilometer. Printed devices were fabricated on PET substrates with ITO pre-patterned electrodes from the LEC formulation of 10 g L−1 and a Mw of 350 kg mol−1. Device Characterization: Reference spin-coated samples were fabricated using the 350 kg mol−1 PMMA as a base for the PSE. Both the spin-coated and printed LECs were fabricated with Ag top contacts (100 nm) using a shadow mask to define the active area of the device (0.24 cm2). The electrical and optical characterization of the devices were performed using a calibrated BOTEST characterization system at a measuring speed of 0.1 V s−1. Figure 4. Current–voltage–luminance characteristics of a gravure-printed LEC. Inset: Photograph of a flexible LEC in operation.
to investigate its electrochemical-stability window and ensure its compatibility with the semiconductor polymer. The rheological properties of the blend, which assure a suitable printed-layer quality for the fabrication of devices, were identified by altering the material ratio, the PMMA Mw, and the solid concentration. The proposed approach shows that a polymer-based LEC system offers the possibility of developing ink formulations for a given printing technique without changing its chemical composition. The printed devices achieved maximum luminance values of ca. 2000 cd m−2 at 10 V, a Von of 6.5 V, and a shelf lifetime of over six months making them promising for large-area low-end applications like packaging or advertisement. As in the present case, adjustment of all the different parameters to obtain a functional ink for organic optoelectronic devices will inevitably result in a compromise between performance and printability. However, this drawback can be minimized if the different variables of the printing process (e.g., solvent type, viscosity, substrate wetting, etc.) are taken into account during the early stages of the materialdevelopment process.
Experimental Section Cyclic Voltammetry (CV): A three-electrode set-up consisting of a Pt working electrode, a Pt/Ti counter electrode, and a Ag wire pseudoreference electrode placed in a glass vessel was employed for the CV measurements. 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) (>99%, Sigma–Aldrich) + 2 M poly(methyl methacrylate) (PMMA) (Mw = 350 kg mol−1, Sigma–Aldrich) in CH3CN was used as electrolyte system. The molarity of the PMMA corresponded to the concentration of the monomeric repeat units. The voltage sweeps were driven and the current measured using a VERSASTAT 3 potentiostat (Princeton Applied Research). After measurement, a calibration scan was performed by adding a small amount of ferrocene (Di(cyclopentadienyl)iron) (>98%, Sigma–Aldrich) to the sample as an internal reference redox system. All the CV measurements were performed under an Ar atmosphere. Ink Formulation and Viscosity Measurements: The LEC formulations for the printing investigations were prepared as follows: SY (PDY132 LIVILUX, Merck), PMMA (Sigma–Aldrich, Mw = 350 kg mol−1) and TBABF4 (Sigma–Aldrich) were dissolved separately in anisole for several hours at concentrations of 5 g L−1, 10 g L−1, 12.5 g L−1, and 15 g L−1. Subsequently, the solutions were mixed at the
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors acknowledge financial support via the Gutenberg Plus project (FKZ: 13N11903) and MORPHEUS (FKZ: 13N11707) of the Leading-Edge Cluster Forum Organic Electronics managed by InnovationLab GmbH within the High-Tech Strategy for Germany of the Federal Ministry of Education and Research. Received: November 7, 2013 Revised: December 31, 2013 Published online: February 26, 2014 [1] a) T. Ameri, N. Li, C. J. Brabec, Energy Environ. Sci. 2013, 6, 2390; b) L. Duan, L. Hou, T.-W. Lee, J. Qiao, D. Zhang, G. Dong, L. Wanga, Y. Qiu, J. Mater. Chem. 2010, 20, 6392; c) K.-J. Baeg, M. Caironi, Y.-Y. Noh, Adv. Mater. 2012, 25, 4210; d) L. Torsi, M. Magliulo, K. Manoli, G. Palazzo, Chem. Soc. Rev. 2013, 42, 8612. [2] J. R. Sheats, J. Mater. Res. 2004, 19, 1974. [3] R. R. Søndergaard, M. Hösel, F. C. Krebs, J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 16. [4] H. Kipphan, Handbook of Print Media: Technologies and Production Methods, Springer, Berlin, Germany, 2001. [5] B. Geffroy, P. le Roy, C. Prat, Polym. Int. 2006, 55, 572. [6] a) Q. B. Pei, G. Yu, C. Zhang, Y. Yang, A. J. Heeger, Science 1995, 269, 1086; b) A. Sandström, H. F. Dam, F. C. Krebs, L. Edman, Nat. Commun. 2012, 3, 1002; c) L. Edman, Electrochim. Acta 2005, 50, 3878. [7] a) Q. Pei, Y. Yang, G. Yu, C. Zhang, A. J. Heeger, J. Am. Chem. Soc. 1996, 118, 3922; b) S. B. Meier, St. van Reenen, B. Lefevre, D. Hartmann, H. J. Bolink, A. Winnacker, W. Sarfert, M. Kemerink, Adv. Funct. Mater. 2013, 23, 3531; c) S. Van Reenen, P. Matyba, A. Dzwilewski, R. Janssen, L. Edman, M. Kemerink, J. Am. Chem. Soc. 2010, 132, 13776. [8] S. Tang, L. Edman, J. Phys. Chem. Lett. 2010, 1, 2727. [9] D. Tordera, S. Meier, M. Lenes, R. D. Costa, E. Ortí, W. Sarfert, H. J. Bolink, Adv. Mater. 2012, 24, 897. [10] J. H. Shin, P. Matyba, N. D. Robinson, L. Edman, Electrochim. Acta 2007, 52, 6456. [11] P. Matyba, M. R. Andersson, L. Edman, Org. Electron. 2008, 9, 699.
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