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Electrochemical Tuning of Morphological and Optoelectronic Characteristics of DonorAcceptor Spiro-Fluorene Polymer Film. Application in the Building of an Electroluminescent Device Daniel Heredia,† Luciana Fernandez,† Luis Otero,† Musubu Ichikawa,*,‡ Chi-Yen Lin,§ Yuan-Li Liao,§ Shao-An Wang,§ Ken-Tsung Wong,*,§ and Fernando Fungo*,† †
Departamento de Química, Universidad Nacional de Río Cuarto, Río Cuarto, Codigo Postal X5804BYA, Río Cuarto, Argentina Department of Functional Polymer Science, Shinshu University, Ueda, Nagano 386-8567, Japan § Department of Chemistry, National Taiwan University, Taipei 106, Taiwan ‡
ABSTRACT: The electrochemical polymerization conditions for the synthesis of spirobifluorene-based polymer (EF-CN1) films with adequate properties to be applied in an organic light emitting diode (OLED) were studied. We demonstrate that, after optimization in the parameters of electrochemical polymerization, we were able to obtain a conducting polymeric film with very smooth surface, absence of pinholes, and high optoelectronic activity, which was compatible as an efficient hole transporting layer in OLED. The electrochemical deposited polymers, and polymer films obtained through drop casting from solution of chemically synthesized polymer (C-CN1), were characterized and the results compared. The films were studied by cyclic voltammetry, UVvis, fluorescence spectra, scanning electron mictroscopy, and atomic force microscopy. The built OLEDs achieve current-luminous efficiency and external quantum efficiency of 2.6 cd/A and 0.50%, respectively.
1. INTRODUCTION Conducting polymers incorporated into organic electronic and optoelectronic devices, such as organic solar cells15 (OSC), organic field effect transistors (OFET),69 electrochromic cells (EC),1012 and organic light-emitting diodes (OLED),1316 have captured the attention of many researches. One of the major advantages of using organic polymers in organic electronics and optoelectronics is the potential of replacing rigid and brittle Si and glass substrates, rendering the development of large area flexible devices (FD) possible. It is reasonable to expect that the use of conducting polymers will introduce a significant advance in the construction and application of devices for energy generation, image display, lighting systems and others. Thus, the improvement and application of FD are directly associated to the development of new suitable polymeric materials and deposition processes.11,13,14 Up to now, two major approaches are used to deposit organic material thin films in optoelectronic devices: thermal evaporation and solution-processing. Although thermal evaporation with the use of fine mask can produce well-patterned films, the process is slow and demands expensive vacuum equipments. In addition, thermal evaporation requires materials with high sublimation capability and excellent thermal stability, which are properties not easily being obtained in polymers. On the other hand, low-cost solution processes, such as spin-coating and deep-coating, widely used in the production of nonpatterned films, require material with intrinsic high solubility and usually produce a large amount of waste. Moreover, films acquired from r 2011 American Chemical Society
solution processes are often with low homogeneity, inadequate for the application on the delicate device fabrication. An alternative technique to produce promising conducting polymer films is electropolymerization, with the adoption of electroactive monomers. The polymerization through the electrochemical deposition techniques allows us to synthesize the polymeric film in one step with fine control over the thickness; moreover, a mold in the working electrode allows an excellent and simple pattern of the film. A number of reports revealed that the polymeric films obtained electrochemically over conductive solid substrates are highly stable, showing compact layers, with excellent sticking to the electrode, high charge transport capability, and considerable fluorescence properties in some special cases.1721 All of these properties are key parameters in the manufacture of organic FD. However, a few number of studies show real application of the electrochemical film deposition in optoelectronic devices like OLED and OSCs.22,23 Despite of the fine control on the film thickness, the electrochemical parameter control in the polymer formation can significantly affect the optoelectronic properties and film morphology. It is been known that the morphology and topology of the films can influence the performance of electronic devices significantly.22,2429 Therefore, understanding how electrochemical Received: May 30, 2011 Revised: September 20, 2011 Published: September 26, 2011 21907
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The Journal of Physical Chemistry C parameters affect these mentioned properties is essential to our research to use electropolymerization deposition methods. An illustrative example has already been reported by Ma and co-workers about how the electropolymerization parameters affect the efficiency of optoelectronic devices. They showed an elaborated strategy to fabricate high-quality electropolymeric films with high fluorescence and smooth surface morphology, which is useful for the construction of luminescent devices with efficiency comparable to those made by traditional methods. Fluorene-based molecular systems attracted considerable attention because they are functional materials for organic electronics.30 For example, they are promising candidates for blue OLEDs,18,2036 liquid crystals,37 and OFETs30,38 Remarkably, polyfluorenes and oligofluorenes end-capped with hole-transporting moieties improve the performance of OLEDs.3943 Among the fluorene-based derivatives, molecules based on spirobifluorene building blocks have shown particularly impressive performance for example in OSC,4446 and the spirobifluorene-based polymer has also been reported to exhibit extraordinary thermal and chromatic stability.45 In this paper, we are going to explore the electrochemical polymerization for the synthesis of a spirobifluorene-based polymer (2,20 -dicyano-7,70 -bis(400 -N,N-diphenylaminostyryl)9,90 -spirobifluorene, CN1, Figure 1). We are going to demonstrate that, after well-manipulation and optimization in the condition of electrochemical polymerization, we are able to obtain conducting polymeric films with a very smooth surface, absence of pinholes, and high optoelectronic activity, which was highly compatible as an efficient hole transporting layer (HTL) in OLED.
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All of the chemicals were provided by Sigma-Aldrich in spectroscoic or electrochemical grade. Anhydrous toluene (TOL), acetonitrile (ACN), 1,2-dichloroethane (DCE), tetra-n-butylammonium perchlorate (TBAP), tris(8-quinolinato)aluminum (Alq3), and tetra-n-butylammonium hexafluorphosphate (TBAHFP) were used as received. The synthesis and characterization of CN1 monomer have been reported previously.47 The CN1 polymer was obtained both by electro-polymerization over Pt or indium tin oxide (ITO) electrodes (Delta Technologies) and by chemical oxidative polymerization in solution. Film thickness was measured using a Tencor profilometer model Alpha-step 500. The electroluminescent (EL) properties were measured using an EL measurement system (Precise Gauge, EL1003) with a Keithley 2400 source meter. 2.2. Polymer Films Electrodeposition and Electrochemical Study. CN1 polymer films were electrodeposited and studied by cyclic voltammetry (Autolab Electrochemical Instruments potentiostat-galvanostat) under different experimental conditions. The CN1 monomer concentration ranged from 0.1 to 1.0 mM in different acetonitrile/toluene (ACN/Tol) solvent composition, containing 0.2 M (TBAHFP) or tetra-n-butylammonium perchlorate (TBAP) as supporting electrolytes. Polymer films were wrought under different applied potential programs, as described below. A platinum coil was uses as counter electrode and a silver wire as a quasireference electrode (QRE). When the cyclic voltammogram (CV) experiments were complete, ferrocene was added to the cell as an internal standard, and the QRE was calibrated with Fc/Fc+ = 0.40 V vs SCE. In order to estimate the frontier energy orbitals of the electrodeposited film, the
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Figure 1. Chemical structure of 2,20 -dicyano-7,70 -bis(400 -N,N-diphenylaminostyryl)-9,90 -spirobifluorene (CN1).
potentials relative to the conventional scale (NHE) were converted to the absolute energy scale taking the energy of the electrons in NHE as 4.5 eV.48 The LUMO level in the polymer was estimated by subtracting the excitation energy from the redox potentials of the film.49 2.3. Polymer Chemical Synthesis. A total of 5 mg of CN1 monomer was dissolved in 25 mL of anhydrous chloroform. An equimolar amount of FeCl3 in a mixture of MeCN/chloroform (1/2)50 was added dropwise under stirring. The mixture was stirred for 24 h at room temperature and then poured into methanol. A yellow precipitate was formed. The insoluble product was separated by centrifugation (5 min, 10000 rpm/min). Then, the polymer was washed with ACN and centrifuged again. This procedure was repeated three times in order to remove the free monomer and the rest of FeCl3. The product obtained was finally dried under vacuum, affording a yellow solid with a yield of 75%. 2.4. Film Surface Analysis. The morphology of CN1 polymer films was performed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The samples were analyzed on a Carl Zeiss EVO MA 10 with electron beam energy of 20 KeV. Topology images were made with a commercial atomic force microscope (Agilent 5500). Microfabricated Si3N4 cantilevers (Mikromasch USA) with a nominal radius of 20 nm were used for topographic imaging. The applied force was 1 nN and the scan rate was 1 s/line. rms values were calculated from the images obtained (256 256 pixel). 2.5. Absorption an Emission Spectroscopy. UV visible absorption measurements were performed in solution and as solid films using a Shimadzu UV 2401PC spectrophotometer at room temperature. A Spex Fluoromax apparatus was employed for the fluorescence measurements. 2.6. OLED Devices Build Procedure. Double-layer OLED devices were built using the following configuration: ITO/polymer film (Variable thickness)/Alq3 (50 nm)/LiF (0.5 nm):Al (200 nm). Indium tin oxide (ITO, two mm wide bands) on a glass substrate with a resistance of 14 Ω was cleaned with detergents and deionized water, ultrasonicated in isopropanol, dried, and treated with oxygen plasma for 5 min. The CN1 Polymer film was electrodeposited as hole-transporting layer (HTL). Then, Alq3 was vacuum deposited onto the surface of the film. An ultrathin LiF layer (0.5 nm) on the top of Alq3 was followed by the deposition of the Al cathode (200 nm). The characterization of the devices performances was carried out at room temperature under ambient atmosphere.
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Polymeric Films. ITO is one of the most frequently used electrode materials in the development of optoelectronic devices.51 Thus, the deposition of optical and/or electro-active organic materials over ITO surface 21908
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Figure 3. Tridimensional idealized structure obtained by molecular mechanics calculations (MM+ at HyperChem software) of three CN1 units in the polymer structure.
Figure 2. (a) Successive voltammograms of CN1 electrodeposition on the ITO electrode. 0.5 mM CN1 solution in ACN-0.1 M TBAP, scan rate 0.1 V/s. (b) Cyclic voltammogram of film electropolymerized on ITO. ACN-0.1 M TBAP, scan rate 0.1 V/s.
is of high interest. Repetitive CV electropolymerization using a 0.5 mM CN1 solution in ACN containing 0.1 M TBAP between 0 and 1.25 V vertex potentials on ITO electrode, produces stable and conductive polymer films, as previously reported.47,52 The successive potential scans show a gradual increase of current signal, which is in agreement with the formation of an electroactive conducting film on the electrode surface. The CN1 radical cation derived from triphenylamine (TPA) moieties undergoes dimerization to produce tetraphenylbenzidine (TPB) producing a polymer due to the spiro CN1 molecular structure characteristic.47 Figure 2a shows 20 successive voltammograms of CN1 electrodeposition on the ITO electrode. We refer to this electrochemically deposited film (EF) obtained under the above-described conditions as EF-CN1-A. The EF-CN1-A deposited on ITO has electrochemical characteristics similar to those observed in other electrode surfaces, such as Pt47 and Au52 (See Figure 2b). We attributed the electrochemical and optical characteristics of the polymer to the particular molecular structure of the monomer, where two fluorene rings holding TPA substituents are perpendicularly bonded through a sp3 hybridized carbon atom. This configuration precludes π-orbital interactions between the two branches,53 allowing independent dimerization of both TPA. Figure 3 shows a tridimensional idealized structure obtained by molecular mechanics calculations (MM+) of the polymer. According to this spatial geometry, fluorene cores are in a perpendicular fashion that avoids aggregation.30 Also, this structure could minimize self-quenching of the highly fluorescent fluorene centers in the solid state. In the present case the CN1 molecular structure remains in the polymer chains; thus, we can presume that the EF-CN1-A polymer would have good fluorescence properties. However, when we analyzed the optical and morphological characteristics of the EF-CN1-A polymer, we observed a lack of fluorescence, and surface inhomogeneities, together with cracks or pinholes. It is already known that the morphology of the films is quite essential in the construction of optoelectronic devices. The presence of pinhole and/or surface roughness would produce an
Figure 4. SEM photograph of EF-CN1-A obtained at different magnifications (a) 500, and (b) 2500.
electrical shortcut or different current path that significantly influences the stability, optoelectronics properties, and performance of the device. The surface morphology of EF-CN1-A was analyzed by scanning electron microscopy (SEM) and the obtained image is shown in Figure 4. The image in Figure 4a shows the presence of an inhomogeneous and granulate surface in the film. At a higher magnification micrograph (Figure 4b), the grains show irregular shapes and sizes, and the pinholes are also observed. These results imply that the film obtained under the described condition is not good enough to be used in the construction of optoelectronic devices. In order to investigate if the lack of fluorescence and the morphological characteristics of the film are due to the nature of the polymer, or the result from the electrodeposition process, we conducted the polymerization by chemical (C) oxidation in homogeneous phase of the CN1 monomer. The obtained product (called, CCN1) is stable and soluble in chloroform. The absorption spectrum of the polymer CCN1 in CHCl3 solution is shown in Figure 5, together with the characteristic spectra of the CN1 monomer and electrochemical deposited films (EF-CN1). The absorption spectrum of CN1 monomer (Figure 5a) shows a ππ transition at λ = 303 nm and a charge transfer (CT) absorption band with maximum at λ = 397 nm.47 On the other hand, CCN1 shows the same ππ* transition maximum as the CN1 monomer, but the charge transfer band is broader and the absorption maximum moves 9 nm to longer wavelength. This can be explained having into account the CN1 polymer formation mechanism and its molecular structure.47 The TPB centers present in the polymer chain have an oxidation potential lower than the CN1 phenyl amines moieties, due to the extension of conjugation,47 which is consistent with the red-shift observed on the absorption spectra. On the other hand, it has also been observed that, in the CCN1 spectrum, the extinction 21909
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Figure 5. Absorption spectra of (a) CN1 monomer and (b) CCN1 polymer in CHCl3 solution. (c) An ITO electrode with chemical synthesized polymer film. (d and e) Electrodeposited polymer film obtained under different electrochemical conditions (see text).
Figure 6. Absorption spectra of CF-CN1, deposited on a ITO electrode, at different applied potentials. ACN, 0.1 M TBAHFP.
coefficient relationship between the ππ* and charge transfer transition bands is larger than the CN1 monomer, indicating that the polymer structure affects the optical properties. Therefore, in order to study the optoelectronic properties of CCN1 in solid state, we modified ITO electrodes with polymers films using drop coating techniques, which called CF-CN1. These films shown red shift of the CT maximum and broadening of the CT band regarding to the observed in CCN1 chloroform solution, which reveals that the optical properties of the polymer are affected in solid state films prepared by drop-coating techniques. However, we must note that when the film is deposited through electrochemical techniques the obtained light absorption spectra (EF-CN1-A, Figure 5c) are quite different. The extinction coefficient relationship between the ππ* and charge transfer transition bands increases, and the CT maximum moves to lower
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Figure 7. Emission spectra of CN1 monomer and CCN1 polymer in CHCl3 solution (a and b). Emission spectra of polymer films obtained under different condition (ce; see text).
wavelengths and appears as a shoulder of the ππ*main transition. These facts could be attributed to the presence of different chemical structures between the polymers formation by both, chemical and electrochemical procedures, or an effect of the film deposition methods. Thus, we conducted spectroelectrochemical analysis of CF-CN1 in order to verify its molecular structure. The redox and electrochromic characteristics of the CF-CN1 do not differ from those films obtained by electrodeposition.47 The behavior of CF-CN1 agrees with the presence of TPB in its molecular structure. Upon film oxidation, the spectra in Figure 6 show the formation of the TPB radical cation (absorption maximum 488 nm) and dication (absorption maximum at 728 nm), demonstrating that TPB residues are also present in the structure of the material synthesized in the homogeneous phase47,54 and that the chemical structures of the material obtained from both procedures are the same. Thus, the difference observed in the absorption spectra could be due to the nature of the electrochemical deposition method that involves at the same time the generation of the oxidized state, polymer synthesis, and film formation in high double-layer potential drop.55 It is plausible that the films obtained in this condition are different than those obtained by other techniques, such as dip coating, drop coating, or spin coating. On the other hand, the polymers obtained by oxidation in homogeneous phase present capability of fluorescence emission, both in solution (CCN1) and as solid film (CF-CN1). Figure 7 depicts the steady state emission spectra of the chemical sensitized polymer, together with the characteristic spectra of CN1 monomer. The CN1 emission spectrum shows broad emission bands with a maximum at 502 nm in CHCl3 that undergo a strong solvent-polarity dependence, which is evidence of the photoinduced electron-transfer process, resulting in a large change in the dipole moment of the excited state.47,52 CCN1 in solution presents a similar emission band, but with a lower efficiency (50% regarding to the monomer). On the other hand, Figure 7 shows that CF-CN1 in the solid state (drop casting) retains emission capability. However, the absorption maximum has a 5 nm blueshift to shorter wavelength, with regard to the observation in the 21910
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Figure 8. SEM photograph of EF-CN1-C obtained at different magnifications (a) 500, and (b) 2500.
CCN1 solution, indicating that the solid state may slightly affect the polymer fluorescence properties; yet, the capability of fluorescence emission is not altered too much. Thus, it can be inferred that the absence of light emission in the electrochemically deposited film (EF-CN1-A) could be due to effects originated from the electrodeposition process. As already remarked, electrochemical deposition involves in one step the generation of charged polymer precursors and film deposition in high electric field, which is different of other physical deposition methods. Also, it is probable that in the electrochemically deposited film remain trapped oxidized centers and electrolyte salt. These charges can quench the exited states efficiently, and diminish the fluorescence drastically.18,2022,31 In order to minimize the presence of charged states trapped in the electrochemically deposited films, the experimental procedure was modified. In a first stage the electrolyte composition was maintained, and the applied potential program was systematically modified. The film growing voltamperogram was initiated at negative potential (0.4 V), and was added a sixty second waiting time between each cycle of the polymerization process at 0.4 V. After this procedure, we found that the film obtained (EF-CN1-B) was luminescent (Figure 7d). The spectrum shows that the position and shape of the emission band is identical to that of the polymer film obtained by drop coating deposition technique. However, the morphological characteristics of the EFCN1-B polymer resemble those observed by SEM images showed in Figure 4. Surface inhomogeneities remain and limit the applicability of the material in the construction of optoelectronics devices. With the objective of improving the surface quality of the film, further study on the effects of electrolyte composition over film characteristics was investigated. Taking into account of the polymer solubility, we modified the electrolyte medium by addition of low polarity solvent (toluene). We speculated that the increase of polymer solubility in the electrodeelectrolyte interface could avoid abrupt precipitation and grain inhomogeneities formation. Also, as Ma and co-workers21 clearly showed, the performances of optoelectronic devices constructed by polymer electrooxidation methods are affected by the size of the anion in the electrolyte. Thus, we replace ClO4 by PF6, which has a larger size and presents higher solubility in low polarity medium, which could also increase the solubility of polymer precursors, driving to the formation of more homogenously deposited film. Finally, the scan rate was increased with the purpose of to generate thin material deposited in each cycle, and to avoid the formation of trapped charges. The electrodeposited material (called EF-CN1-C) formed under these conditions holds redox characteristics and optical properties similar to the films obtained before (EF-CN1-B, see Figures 2, 5, and 7), but a blue shift in emission maximum of 5 nm is observed with
Figure 9. AFM photograph of EF-CN1-C film (left) and the naked ITO electrode (right). The applied force was 1 nN and the scan rate was 1 s/line.
Figure 10. ITO/polymer EF-CN1/Alq3/LiF-Al OLED energy diagram.
regard to the other films. As it was already mentioned, the CN1 fluorescence phenomenon is originated from a photoinduced charge separated state, which is highly dependent on the surrounding environment polarity. Therefore, as the electrochemical deposition program affects films morphology, it is also possible that the environment conditions (as charge excess distribution and polarity) are also influenced by the films growing conditions, and their effects are manifest in the florescence spectra. On the other hand, EF-CN1-C shows remarkable changes in its surface morphology, and Figure 8 shows SEM images of a film obtained from 0.5 mM CN1 solution in ACN/ TOL (1:2) containing 0.1 M TBAHFP at 0.5 V/s. The SEM images of EF-CN1-C indicate that the film surface is homogeneous, without cracks or pinholes. At a higher magnification micrograph (Figure 8b), it can be seen that the film presents a total covering of the ITO, without leaving islands, and the surface morphology is totally soft. The quality of the electrodeposited film in the ACN-TOL mixture is similar to that observed in those obtained by drop casting. Microscopic morphology and roughness of the films also were examined by atomic force microscopy, as is shown in Figure 9. AFM 3D micrographs in Figure 9 provide surface inspection (1 1 μm) of the microstructural arrays, topological structure, porosity, and film quality of the deposited layers. The results showed a small particles agglomerate, with a homogeneous 21911
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Figure 11. (a) OLED current density and luminance as function of the applied voltage. (b) Luminescence as function of the applied voltage for different polymer EF-CN1-C film thickness: 37.2 nm (black), 38.5 nm (red), 5 cycles-95.5 nm (blue), and 10 cycles 202.2 nm (green). The inset shows the EFCN1-C film thickness as function of the number of growing potential cycles.
distribution at micrometric length scale and low roughness (rms 37 Å). On the other hand, it is interesting to note that the EF-CN1-C electrodeposited film has a surface roughness lower than that of the ITO surface (rms 70 Å). Therefore, we can conclude that, through the manipulation of electrochemical conditions, it is possible to tune the key parameters in the devices construction: optoelectrical and surface morphology properties of electrodeposited films, at the same time. 3.2. Fabrication and Characterization of OLED. The capability as hole transporter layer of the EF-CN1 films was evaluated in the follow OLED conformation ITO/polymer EFCN1/Alq3/LiF-Al. Figure 10 shows the energy diagram obtained from the electrochemical data. The scheme also shows the energy level of ITO and LiF-Al contacts and Alq3 emitter. When the OLEDs were constructed from rough and inhomogeneous electrodeposited films (EF-CN1-A and EF-CN1-B), the devices were not electroluminescent, and even some of them showed an electrical short cut. On the other hand, when the OLEDs were built with the smooth EF-CN1-C film electrodeposited from the ACN-TOL electrolyte, the devices were electroluminescent and reproducible. The results are depicted in Figure 11a, where it is observed that the jV curve shows three different shapes at different voltage intervals. This behavior is mainly observed in devices built with thin EF-CN1-C films (less than ∼50 nm); meanwhile most of the OLEDs holding thick EFCN-C film present a jV response where is not possible to identify isolated processes in the current curve. In Figure 11a, at applied voltage below ∼7.5 V, an exponential growing of the current is observed, without light emission. When the applied potential reaches the emission onset voltage, a change in the current slope is observed. Finally, close to the OLED breakpoint (up of 17.5 V) a third zone is identified. The origin of this fact is not fully understood, but the electric behavior could be related to the CN1 polymer characteristics. CN1 is not a conjugated polymer, is a redox polymer type, which was deposited in undoped state. As was described above, the CN1 film electrochemical behavior showed two defined redox processes (related to the formation of the TPB radical cation and dication). Thus, the two first processes observed in Figure 11a could involve different oxidized states of the film. However, this is a speculation, and superficial defects in the thinnest films and deleterious process associated at different jV curves cannot be rejected.
The turn on potential and light emitting intensity strongly depends on the film thickness (Figure 11b), a parameter easily controlled by electrochemical deposition method. It is important to note that the OLED performance is improved as the film thickness decreases, and it reaches a maximum at 38.5 nm (formed after two potential cycles, (see inset in Figure 11b), reflecting a relationship with the charge mobility in the film. The luminescence reaches a maximum at hole transporter layer thickness of 38.5 nm with a turn on potential of ∼7.5 V. This value is slightly high for thin devices; however, it is similar to that reported in other OLED built with similar thickness for related hole transport material.56 It is interesting to note the different luminescence behavior observed between a device built from one and two deposition cycles, taking into account that there are very small differences in the polymer layer thickness (red and black curves Figure 11b). From a practical point of view, both devices have the same EF-CN1-C film thickness. Therefore, the difference observed in the devices performance cannot be explicated in base of film thickness. We also must take into account the electrodeposition processes. The thickness of the CN1 electrodeposited films were controlled by the variation of the number of oxidation potential cycles applied to the electrode, but there is very small or not thickness difference between one or two cycles (see inset figure 11 b). As was depicted in manuscript, the CN1 electrodeposition mechanism involves the oxidation of triphenylamine (TPA) moieties and TPA radical cations dimerization. In the first CV this process occurs over naked ITO, and the electrocatalytic phenomenon is different from the subsequent deposition cycles, where the heterogeneous charge transfer process occurs in the polymer surface. Moreover, in the second (and also subsequent cycles) the film is oxidized (and then reduced) generating active species (TPA radical cations) that increase the polymer chain (and also cross-linkage between chains) that affects the internal morphology and electronic properties of the film. Thus, it is not unexpected to obtain remarkable different properties between films formed by only one deposition cycle and the others ones. Moreover, we have been shown that the work function of conducting surfaces can be engineered by electrodeposition of spirobifluorene compounds with donoracceptor pairs. The results obtained showed that gold electrode work function can be changed over more than 0.4 eV by 21912
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The Journal of Physical Chemistry C electrodeposition of one or two CN1 polymer deposition cycles, in similar condition than the used in Figure 11, meanwhile subsequent deposition cycles (and concomitant thickness increases) do not produce large changes in the work function. Although this effect was observed in gold electrode, similar effects can be present when the polymer is grow over ITO surface. On the other hand, we acquire a maximum current-luminous efficiency of 2.6 cd/A, which is good enough when we take into account that Alq3 based devices usually show the currentluminous efficiency around 5 cd/A.57,58 The corresponding external quantum efficiency (EQE) of electron-photon conversion is around 0.5%. Note that the theoretical limit of EQE for fluorescent OLEDs is 5%.59,60
4. CONCLUSION The morphological and optoelectronic properties of electrodeposited polymer films are strongly affected by the electrodeposition processes. They can be tuned through the smart control of the electrochemical variables and conditions. We showed the potential and versatility of the electrochemical synthesis in the generation of a hole transporter material used in the construction of an OLED device ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
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’ ACKNOWLEDGMENT We thank Japan Society for the Promotion of Science (JSPS), Consejo Nacional de Investigaciones Científicas y Tecnicas (CONICET), Universidad Nacional de Río IV (UNRC), and the National Science Council of Taiwan for financial support. L.F., L.O., and F.F. are scientific members of CONICET. ’ REFERENCES (1) Tang, W.; Hai, J.; Dai, Y.; Huang, Z.; Lu, B.; Yuan, F.; Tang, J.; Zhang, F. Sol. Energy Mater. Sol. Cells 2010, 94, 1963–1979. (2) G€unes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324–1338. (3) Cheng, Y-J; Yang, S-H; Hsu, C.-S. Chem. Rev. 2009, 109, 5868–5923. (4) Spanggaard, H.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2004, 83, 125–146. (5) Chavhan, S. D.; Abellon, R. D.; Van Breemen, A. J. J. M.; Koetse, M. M.; Sweelssen, J.; Savenije, T. J. J. Phys. Chem. C 2010, 114, 19496–19502. (6) Sekitani, T.; Someya, T. Adv. Mater. 2010, 222, 2228–2246. (7) Wu, W.; Liu, Y.; Zhu, D. Chem. Soc. Rev. 2010, 39, 1489–1502. (8) Yamashita, Y. Chem. Lett. 2009, 38, 870–875. (9) Rao, M.; Ortiz, R. P.; Facchetti, A.; Marks, T. J.; Narayan, K.S. J. Phys. Chem. C 2010, 114, 20609–20613. (10) Argun, A. A.; Aubert, P.-H.; Thompson, B. C.; Schwendeman, I.; Gaupp, C. L.; Hwang, J.; Pinto, N. J.; Tanner, D. B.; MacDiarmid, A. G.; Reynolds, J. R. Chem. Mater. 2004, 16, 4401–4412. (11) Yang, C.-H.; Huang, L.-R.; Chih, Y.-K.; Chung, S.-L. J. Phys. Chem. C 2007, 111, 3786–3794. (12) Beaujuge, P. M.; Reynolds, J. R. Chem. Rev. 2010, 110, 268–320. (13) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Chem. Rev. 2009, 109, 897–1091.
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