Molecular Reorientation and Structural Changes in Cosolvent-Treated

Article pubs.acs.org/JPCC

Molecular Reorientation and Structural Changes in CosolventTreated Highly Conductive PEDOT:PSS Electrodes for Flexible Indium Tin Oxide-Free Organic Electronics Claudia M. Palumbiny,† Christoph Heller,† Christoph J. Schaffer,† Volker Körstgens,† Gonzalo Santoro,‡ Stephan V. Roth,‡ and Peter Müller-Buschbaum*,† †

Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, James-Franck-Strasse 1, 85748 Garching, Germany ‡ Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany S Supporting Information *

ABSTRACT: Cosolvent addition of glycerol (G) and the use of the cosolvent ethylene glycol (EG) increase the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films to values on the order of indium tin oxide conductivity. The underlying morphological changes are probed via scanning electron microscopy as well as advanced scattering techniques microfocused grazing incidence small- and wide-angle Xray scattering. The enhancement in conductivity is ascribed to fundamental morphological changes and molecular reorientation within crystalline domains. Thereby, the conductivity enhancement is directly correlated to domain ruptures toward smaller and more densely packed PEDOT domains together with an enhanced crystallinity, the removal of PSS molecules, and moreover a reorientation of the conjugated PEDOT molecules. The latter is reported and quantified here for PEDOT:PSS films for the first time.

1. INTRODUCTION The transparency and the potential to manufacture photovoltaic (PV) devices based on organic materials (OPV) on flexible substrates allow numerous smart design opportunities as well as roll-to-roll mass production.1 However, challenges such as the limited resource of the rare metal indium and the resulting high cost of the commonly used indium tin oxide (ITO) are still present. ITO is primarily used as the transparent electrode material in organic electronic devices. In addition, the brittleness of ITO limits the roll-to-roll (R2R) processing of organic solar cells with printing technologies. This limitation is essentially due to the fact that the efficiency of OPV devices was shown to decrease rapidly with the number of bending cycles.2 Flexibility,3 however, is one of the main advantages of OPV devices over conventional PV devices. To substitute ITO as the transparent conducting electrode, several attempts have been made using organic materials such as carbon nanotubes,4 graphene,5 or conducting polymers.2,6−10 In this context, the polymer blend poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has attracted a great deal of attention, particularly in combination with cosolvent modification,2,7,8,11−14 and is used in R2R processes as the standard electrode.15 Further, in dye-sensitized solar cells, PEDOT:PSS modified with cosolvents has begun to be used as an electrode replacement.16 Even in silicon research, PEDOT:PSS has begun to find its application by combining silicon and polymers in photovoltaic and/or artificial photosynthetic systems.17 The promising properties of PEDOT:PSS, such as its solution © 2014 American Chemical Society

processability, a high optical transparency, its easy and cheap commercial availability, its hole conductivity, and its mechanical flexibility, make PEDOT:PSS desirable for application in optoelectronic devices and use as a widely used electron blocking layer. Besides, for its application in photovoltaic devices, PEDOT:PSS already plays a crucial role in other devices such as electronic papers, organic light-emitting diodes (OLED), or organic field effect transistors (OFET). Because the material is already integrated in many devices, there are a variety of applications that will profit from PEDOT:PSS as an ITO substitute. Because of easy processing, for most applications the nontreated pristine water-soluble complex PEDOT:PSS (e.g., Heraeus Clevios PH1000 Ossila M122) is used, reaching conductivity values of 0.01−10 S/cm.18 However, as a replacement for ITO as a stand-alone electrode, PEDOT:PSS is still lacking in conductivity by several orders of magnitude. It is known that adding non-acidic organic cosolvents with high boiling points and strong polarities, e.g., glycerol (G) or ethylene glycol (EG) as used in this work, increases the conductivity to 1000 S/cm,19,20 as declared by the supplier Heraeus Clevios/Ossila. Thereby, the cosolvent is believed to act as a structuring agent for the resulting film morphology. Received: February 12, 2014 Revised: May 8, 2014 Published: June 1, 2014 13598

dx.doi.org/10.1021/jp501540y | J. Phys. Chem. C 2014, 118, 13598−13606

The Journal of Physical Chemistry C

Article

its core−shell complex on the right. The scope of this work is to correlate the conductivities directly to the inner film morphology together with the molecular orientation within the crystals. Such an improved understanding of the underlying structure is necessary for controlling the performance of practical devices. To address the correlation between inner film structure and conductivity, we investigated the changes in the inner film structure of PEDOT:PSS (Heraeus Clevios PH1000, Ossila M122, 1:2.5) films that arise with a recently developed posttreatment method using EG as a cosolvent. Thereby, the conductivity is increased, reaching values on the order of ITO as reported by Kim et al.19 Additionally, a comparison is drawn with a so-called cosolvent additive for PEDOT:PSS using G. Cosolvent addition of G is known to enhance the conductivity of the resulting layer also.8,40 Furthermore, G acts as a plasticizer for PEDOT:PSS, allowing the application of structuring methods for enhanced light absorption and efficiency enhancement in OPV devices as shown before in contiguous studies.41 Note that addition of cosolvents is also mentioned in literature as doping9 or secondary doping.42,43 It allows for rearrangement of the molecules within the thin film as discussed in the results part discussing inner film morphology. Six different concentrations of G, ranging from 0 to 50 mg/mL, were used. For each G additive concentration, the comparison is drawn to additional cosolvent post-treatment using EG. The routine for G additive (termed method 1) and EG post-treatment (termed method 2) is shown in Figure 1b and discussed in detail in the Experimental Section. At this point, it should be mentioned that even after the samples have been annealed as described in the Experimental Section, the high-boiling point solvents remain in the film at a sufficiently low concentration so as not to degrade the conductivity. Instead, the conductivity is increased by more than 3 orders in magnitude as discussed in the Results, clearly showing that residuals of the cosolvent do not affect the conductivity of thin PEDOT:PSS films. The surface morphology was probed via scanning electron microscopy (SEM). For detailed and quantitative investigation of the inner film morphology, crystallinity, and molecular reorientation, the solvent-treated films were probed using powerful techniques such as microfocused grazing incidence small- and wide-angle X-ray scattering (μ-GISAXS and μGIWAXS, respectively). The measurements were performed on beamline P03/MiNaXS of the PETRA III storage ring at DESY (Hamburg, Germany).44 In contrast to the surface sensitive information obtained from SEM, the microfocused X-ray scattering methods μ-GISAXS and μ-GIWAXS provide statistically relevant results averaged over the illuminated film volume corresponding to the size of the top electrode and thereby of the typical active area of an OPV device.45,46 From the μGISAXS measurements, the size, separation, and distribution of the PEDOT-rich domains present underneath the film surface are extracted. Changes in crystallinity and molecular reorientation of the conjugated PEDOT molecules with respect to the substrate surface are observed via μ-GIWAXS. The changes in surface and inner film morphology, the crystallinity, and the molecular orientation are thereby related to the changes in conductivity. For a complete and thorough investigation of the films, their conductivity, thickness, and optical properties are studied using four-point probe, X-ray reflectometry (XRR)/profilometry, and transmission spectroscopy (UV−vis) measurements, respectively.

Several attempts to explain the increase in conductivity for cosolvent-treated PEDOT:PSS films have been made, e.g., by measuring an enhanced overall crystallinity with wide-angle Xray scattering (WAXS).21 Removal of the insulating PSS was not directly observed during the process but was indicated indirectly by Fourier transform infrared spectroscopy (FTIR),22 X-ray photoelectron spectroscopy (XPS),14,20,22−25 UV−vis,25 and cyclic voltammetry (CV),18,24 while a vertical phase separation showing a capping layer of PSS for pristine PEDOT:PSS was observed with XPS and atomic force microscopy (AFM).14,18,20,22,24−29 An enhancement of the doping level of the conjugated polymer was also found,29 yet the precise morphology inside the film, as well as both the crystallinity and moreover the molecular orientation, has a very strong influence on the charge transport through the conjugated polymer film.30,31 In recent years, the in-plane morphology32,33 and vertical phase separation of blend systems have attracted an increasing amount of interest31−34 as well as the investigation of molecular structures in conjugated polymers.34,35 In this work, we address the inner film structure and the molecular orientation providing new insight into the underlying effects with cosolvent treatment leading to the high conductivity of PEDOT:PSS. PEDOT:PSS consists of two ionic bound polymers as indicated in the chemical structure shown in Figure 1a. The

Figure 1. (a) Structural formula of PEDOT:PSS (left). Model of a core−shell PEDOT:PSS domain in solution (right). (b) Solvent additive and post-treatment routine of the PEDOT:PSS film.

conjugated PEDOT polymer displays good conductivity that arises from the alternating double bonds along the backbone. The PSS that is needed for the solubility of PEDOT, however, is nonconductive. Because of the presence of PSS in the shell, the complex is soluble in water and thin films can be prepared by spin-casting the solution. Further, PEDOT has a high affinity for PSS that, because of its polyanionic nature, acts as a counterion for PEDOT.36 In aqueous solution, the PEDOT:PSS complex tends to form PEDOT-rich cores surrounded by a PSS-rich shell.21,26,30,37−39 The structural formula of PEDOT:PSS is sketched in Figure 1a together with 13599



Article

damage of the films, measuring times of 0.1−5 s were chosen and summed up to 61 s images for improving statistics. For the μ-GIWAXS measurements, an inclination angle of 0.12−0.15° and a sample detector distance of 0.114 m were chosen. Images were summed up to a 260 s counting time. Beam damage was excluded for both GISAXS and GIWAXS by comparing images and cuts sequentially. 2.7. GISAXS Fitting. For fitting the GISAXS data, the distorted wave Born approximation (DWBA) and the local monodisperse approximation were used (Figure S3 of the Supporting Information).

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. For the fabrication of the PEDOT:PSS thin films, microscope glass slides (Carl Roth) were cleaned in an acidic [1:1.56:3.67 solution of distilled H2O (DI-H2O), 30% H2O2, and 96% H2SO4] hot bath at 80 °C for 15 min.47 After being acidically cleaned, the glass slides were rinsed with DI-H2O for 2 min each and blow dried with nitrogen (N2). On top of the cleaned substrate, the PEDOT:PSS (PH1000, Ossila M122, 1:2.5) film was spincast from an aqueous solution (1500 rpm, ACL 9, 60 s) after sonication for 10 min and filtering [polyvinylidene fluoride (PVDF) filter with a 0.45 μm pore size]. After each spincoating step, an annealing procedure in air for 10 min at 140 °C was performed. For the doped films, the filtered PEDOT:PSS was added to the cosolvent glycerol (Carl-Roth 3783.1) at the required concentration (0−50 mg/mL) and stirred with magnetic stirrers for 20 min prior to spin-coating. For posttreatment of the films, the cosolvent ethylene glycol (SigmaAldrich catalog no. 324558) was used. The film was bathed for 3 min in the EG cosolvent before spinning off the remaining solvent (1500 rpm, ACL 9, 30 s) and subsequent annealing for 10 min at 140 °C. See Figure 1b for a graphic representation of the routine. 2.2. X-ray Reflectivity/Profilometry. For thin films, determination of the correct film thickness is essential. The film thickness was measured by X-ray reflectometry (XRR) (D8 Advance, Bruker) and for comparison to literature with a profilometer (Dektak 150 Surface Profiler, Veeco). XRR measurements (copper Kα, λ = 0.154 nm, 0−10°, steps of 0.02°, automatic absorber) are nondestructive and very precise. The precise determination of film thicknesses is important because small deviations in thickness lead to a large difference in conductivity. Profilometry measurements perpendicular to a scratched line of the film are fast but imprecise for soft matter because of the weight of 1 mg of the measuring prod (12.5 μm diameter) on the sample. 2.3. UV−Vis Spectroscopy. The transmission (UV−vis spectrometer, Lambda35, PerkinElmer) for all films was measured from 190 to 1100 nm (λ) with a scanning speed of 120 nm/min. 2.4. Conductivity. Conductivity measurements were performed with a four-point probe (Cascade Microtech, C4S54/5) consisting of four equally spaced tungsten carbide test prods (1 mm distance and 135 μm radius, 40−70 g weight). The current is sent through the two outer prods while the potential difference between the inner two prods is measured. 2.5. Scanning Electron Microscopy. The SEM measurements (N-vision 40 SEM, Zeiss) were performed using secondary electrons, an in-lens detector for material sensitive measurements, an electron energy of 1.5−2.0 kV, and a working distance of 1.0−1.5 mm. The contrast and brightness were further adjusted using Wayne Rasband’s ImageJ version 1.44p.48 2.6. μ-GISAXS and μ-GIWAXS. The μ-GISAXS measurements were performed at the P03/MiNaXS beamline of the PETRA III storage ring at DESY.44 A 2.3 m long sample− detector distance (evacuated flight tube) was chosen to access the desired q range. The synchrotron radiation (photon wavelength of 0.0957 nm) impinged the film under an inclination angle of 0.35°. Data were collected with a Pilatus 300k detector (Dectris, Baden, Switzerland; 487 pixels × 619 pixels, pixel size of 172 μm × 172 μm). To exclude beam

3. RESULTS AND DISCUSSION G additive (method 1) and EG post-treatment (method 2) of PEDOT:PSS films are compared by investigating the influence on basic properties such as conductivities, film thicknesses, and absorption coefficients as well as determining the effect on morphology in terms of the surface, inner film morphology, and molecular orientation. 3.1. Basic Properties. 3.1.1. Film Thickness. The determination of thickness is based on X-ray reflectivity (XRR) measurements. For comparison, measurements with a profilometer are conducted as well, as such data are frequently found in the literature. Note that it is important to determine the exact film thickness for thin films to obtain accurate conductivity values. For this reason, the more precise thicknesses obtained from XRR measurements should be used when calculating conductivities. Thicknesses of PEDOT:PSS films with and without EG post-treatment for all G additive concentrations are shown in Figure 2. The

Figure 2. Film thicknesses for six different G additive concentrations of PEDOT:PSS films with and without additional EG post-treatment. The thicknesses are measured via profilometry (left) as well as with the more precise X-ray reflectometry (XRR, right). The dotted line represents the theoretical thickness for G-doped PEDOT:PSS films after total evaporation of the G.

theoretical thickness is shown for comparison (dotted line), representing the thicknesses for total evaporation of the G dopant from the film after annealing. For EG-post-treated samples, the film thickness decreases by approximately 25 nm in a manner independent of the initial G additive concentration. This is mainly due to selective removal of excess PSS from the system (see UV−vis experiments). 3.1.2. Conductivity. As discussed above, to use PEDOT:PSS as an electrode, its conductivity needs to be enhanced to the 13600



Article

level of that of ITO, which is on the order of 103 S/cm. We show by four-point probe measurements that EG posttreatment (method 2) increases the lateral conductivity dramatically, reaching values of 1000 S/cm, independent of the initial G additive concentration (see Figure 3). The

Figure 4. Absorption coefficients of a pure PSS film and PEDOT:PSS films with six different G additive concentrations together with EGpost-treated films for all six initial G additive concentrations. The data clearly show that EG post-treatment leads to optically denser films.

(SEM), Figure 5b, is shown for the qy range of interest between 0.1 and 0.8 nm−1 in Figure 5a for G concentrations of 0 and 50 mg/mL with and without EG post-treatment. An enhanced

Figure 3. Conductivity measurement of PEDOT:PSS films for six different G additive concentrations and additional EG post-treatment for each G additive concentration.

conductivity is also increased steadily with an increased G additive concentration (method 1). The conductivity is influenced by the removal of the PSS capping layer as well as the morphological and molecular changes discussed in the following. For the application of highly conductive PEDOT:PSS directly as an electrode in ITO-free OPV or OLED devices, the lateral conductivity becomes more important than the vertical conductivity. The charges are injected from the active layer directly to the PEDOT electrode, where they need to be transported in the lateral direction to the external contacts. At this point, it should be mentioned that the change in work function with cosolvents has to be kept in mind for application.9,13,49 3.1.3. Absorption Coefficient. To address the optical properties of PEDOT:PSS, the absorption coefficients of PEDOT:PSS with and without EG cosolvent post-treatment are calculated from the UV−vis spectroscopy and XRR/ profilometry thickness for all G additive concentrations (Figure 4). For comparison, the absorption coefficients are shown together with the absorption coefficient of pure PSS. It is obvious that the EG-post-treated PEDOT:PSS (method 2) films are optically more dense than films that are only G-doped. However, the transparency of the films remains. Especially the EG post-treatment leads to the removal of PSS molecules. Thereby, the PEDOT molecules gain space and collapse (see Inner Film Morphology). This explains the decrease in film thickness as well as the increase in optical density for higher wavelengths, because at these wavelengths PSS hardly contributes to the absorption. The removal of PSS has also been confirmed by XPS measurements, e.g., by Alemu et al.22 and Kim et al.23 3.2. Morphology. 3.2.1. Surface Morphology. For investigation of the surface morphology, the films are probed with SEM and the power spectral density (PSD) is extracted from the micrographs by Fourier transformation and radial integration. The lateral scattering obtained from the surface

Figure 5. (a) PSD functions extracted from the (b) SEM images of films without and with G additive as well as additional EG posttreatment for each film. 13601



Article

changes in the lateral scattering and show how the radius and the distance of the small domains decrease with EG posttreatment. For the larger domains, the radius decreases with an increase in G additive concentration and decreases further by a constant factor for additional EG post-treatment. For the larger domains, the difference in distance between G additive and additional EG post-treatment decreases with an increase in G additive concentration. In total, a clear tendency toward smaller domains and domain rupture at high additive concentrations as well as post-treatment is observed. For the latter, the domains are additionally more densely packed. Note that a different film treatment can lead to different observations caused by different film formation times.50 Here the films are directly transferred onto a heating plate (T = 140 °C) after each spin-casting step (Figure 1b). Thereby, aggregation of the domains within the films after solvent treatment is suppressed. 3.2.3. Surface versus Inner Film Morphology. To draw a direct comparison between the surface and the inner film structures, the horizontal line cuts (μ-GISAXS) (Figure 6) are compared to the PSD function (Figure 5a) extracted from the 2D SEM images (Figure 5b) by Fourier transformation and radial integration. The enhanced intensity for the three cosolvent-treated films (G-doped, EG-post-treated, and both G-doped and EG-post-treated) compared to that of the pristine PEDOT:PSS film is similar in the PSD and μ-GISAXS data. We conclude that the structural changes observed in the PEDOT:PSS inner film are similar on the surface. With the statistical information about the inner film and the similar changes locally on the surface, it can be confirmed that the local picture of the topography also represents the inner film morphology of the whole film and, in turn, that the topography is similar on the whole sample and not only locally. This similarity of the surface and inner film structure is not always given and needs to be confirmed for other systems when using these techniques. Further, from the μ-GISAXS fits, detailed information about the shape, size, and distribution of domains present in the inner film volume can be extracted. 3.2.4. Molecular Orientation. Besides the morphological changes on a larger scale, also the orientation of the PEDOT polymer is expected to have a great impact on the lateral conductivity of the film.51 To investigate this molecular order in detail, μ-GIWAXS measurements are performed. From the scattering signal corresponding to the π−π stacking distance for the PEDOT polymer,21,51 the orientation of the PEDOT stacking planes is extracted by integration along the azimuthal scattering from the π−π stacking (Figure 7a). The edge-on molecular orientation, with the polymer chain and the π−π stacking in the plane, is distinguished from the face-on orientation, with the π−π stacking vertical to the plane, by the angle in the data plot (2D raw scattering data are displayed in Figure S2 of the Supporting Information). π−π scattering between 0° and 45° as well as between 135° and 180° results from edge-on oriented crystals, whereas π−π scattering between 45° and 135° results from face-on oriented crystals, findings similar to those of other investigations of crystalline structure of conjugated polymers with GIWAXS.52 The data show that the pristine PEDOT:PSS film (G additive concentration of 0 mg/mL, no EG post-treatment) exhibits edge-on as well as face-on orientation. However, from the pristine film, an overall low intensity is probed, and hence, the pristine film has a crystallinity that is low compared to those of the other films.

scattering intensity is observed for the G-doped (method 1), EG-post-treated (method 2), and both G-doped and EG-posttreated (method 1 and method 2) samples. The enhanced intensity indicates a structural change at the surface in the presence of cosolvents. However, SEM provides only information about the surface while the inner film structures are not accessible with this technique. 3.2.2. Inner Film Morphology. To investigate the inner film morphology, μ-GISAXS is used. The two-dimensional (2D) μGISAXS signal is measured for the non-doped and the highly G-doped films, without (method 1) and with (method 2) EG post-treatment. From the 2D scattering images, a clear change in the scattering pattern from the pristine film to the G-doped, EG-post-treated, and both G-doped and EG-post-treated films is observed (2D raw scattering data are displayed in Figure S1 of the Supporting Information). The lateral scattering along qy at the material sensitive Yoneda region29 is enhanced for all modifications of the pristine film (Figure 6). For more detailed

Figure 6. Horizontal line cuts from the 2D μ-GISAXS data for films of six different G additive concentrations with and without additional EG post-treatment together with their fits (red lines). The data are shifted along the y-axis for the sake of clarity.

information, horizontal line cuts of the 2D μ-GISAXS data at the Yoneda region are performed. Figure 6 shows the cuts together with their fits. It is evident that the difference in scattering intensity between non-post-treated and EG-posttreated films (Figure 6) decreases with increasing G additive concentration, following the trend of conductivity (Figure 3). By fitting the horizontal line cuts of the μ-GISAXS measurements, we extract the domain radii, distances, distributions, and shapes of domains from the fitting model used. In Figure 8, the radii and distances, as extracted from the form and structure factors, respectively, are plotted. The dashed lines, together with the gray arrows, act as a guide to the eye for the observed 13602



Article

stacking of the molecules, and the lowest conductivity is along the side chains.51 Along the backbone, the charge is transported by delocalized electrons, whereas charge transport along the π−π stacking is described by hopping transport. For EG-post-treated films, the PEDOT molecules in the crystals show an enhanced edge-on orientation. In edge-on oriented crystals, the backbone as well as the π−π stacking is oriented parallel to the substrate surface. The lateral conductivity within one crystal and most likely also between two edge-on oriented crystals is therefore improved by enhanced edge-on orientation. This enhancement of the edge-on orientation gives rise to the assumption that the increase in lateral conductivity is not only due to the formation of smaller and densely packed PEDOT domains (μ-GISAXS) and the known removal of PSS22,23 (see UV−vis) but also due to the reorientation of the molecules within the crystals (μGIWAXS). This reorientation of PEDOT molecules from faceon to edge-on due to EG post-treatment is observed here for the first time, to the best of our knowledge. Addition of G to PEDOT:PSS leads to an enhanced face-on orientation of the crystals with an increased G additive concentration. Because the conductivity of the films with high G additive concentrations is improved similar to the case with EG post-treatment (Figure 3), at first glance this observation conflicts with the expectation and observation for EG posttreatment. However, the overall crystallinity increases strongly with an increase in G additive concentration. This leads to less amorphous regions between crystals, and therefore, better charge transport between the crystals is the dominating effect for the increase in conductivity in the case of the addition of G. On the basis of the results obtained by μ-GIWAXS data analysis, we conclude that the increase in conductivity by G additive and EG post-treatment results from two different effects. For the G additive, the conductivity is enhanced by a strongly increased overall crystallinity with face-on oriented crystals. In the case of EG-post-treated films, the crystals have an enhanced edge-on orientation compared to that of the Gdoped films. The pathways for charge transport are hence more direct along the lateral plane for EG post-treatment than for G additive. Therefore, the increased conductivity of EG-posttreated films is explained by the improved edge-on orientation of PEDOT:PSS and more direct percolation paths for the charges. The enhancement of the conductivity for G-doped films is therefore dominated by the structural changes and the enhanced overall crystallinity together with the known removal of PSS. Further enhancement of the conductivity for additional EG post-treatment, however, is influenced additionally by the edge-on improvement within the crystals. 3.2.5. Model for Structural Changes and Molecular Orientation. Via combination of the information obtained by the different methods discussed above, a model for the morphological changes with G additive and EG post-treatment is developed. In the proposed model, the sizes and distances of the PEDOT domains are obtained from the μ-GISAXS fitting within the assumed model as shown in Figure 8 for all films. The scattering intensities due to the form factors are compared to each other to estimate the ratio of the amount of small to large scattering domains. Note that one large domain leads to more intense scattering compared to one small domain for an equal exposed volume. To take this into account, the following estimation is made on the basis of the general relation between the scattering intensity (I) and the mass (m) of spherical objects.53

Figure 7. (a) Integration over the semicircle in the 2D μ-GIWAXS data along the π−π stacking for films of four G additive concentrations with and without additional EG post-treatment. (b) Ratio of edge-on to face-on orientation of PEDOT molecules extracted from the μGIWAXS π−π scattering. Dotted lines act as guides to the eye.

G additive (method 1) leads to a reorientation of the molecules toward more face-on oriented crystals. The trend toward face-on orientation is enhanced strongly for increased G additive concentrations. Further, the overall intensity and hence the overall crystallinity are enhanced at higher G additive concentrations. EG post-treatment (method 2) in contrast leads to a reorientation of the PEDOT crystallites toward enhanced edgeon orientation independent of the initial G additive concentration. The scattering intensity and thereby the overall crystallinity for EG-post-treated films are independent of the initial G additive concentration before the EG post-treatment. For quantitative evaluation of the molecular orientation, the edge-on to face-on ratio is calculated. To obtain the edge-on to face-on ratio, first the areas under the partial curves corresponding to the molecular orientations are calculated. Second, the specific areas are divided by each other. Here the edge-on to face-on orientation is determined by dividing the area corresponding to the edge-on oriented crystals by the area corresponding to the face-on oriented crystals (Figure 7b). For comparing the molecular orientation to the conductivity, it is important to note that the highest hole conductivity in the conjugated PEDOT polymer crystal is along the backbone chain, the second highest conductivity is along the π−π 13603



Article

The model in Figure 9 shows the structural and molecular changes with cosolvent treatment. The inner film structure changes toward smaller and denser packed domains. This reduces the insulating PSS spacing between the conducting PEDOT domains. Further, the reorientation of the molecules within the domains has an impact on the conductivity because the conductivity within a crystal of conjugated molecules is anisotropic. For the EG post-treatment and G additive, the molecular reorientation is different and the conductivity increase is explained by different dominating effects as discussed for molecular reorientation. The change in inner film morphology, crystallinity, and molecular orientation together with the removal of PSS explains the increase in the conductivity of PEDOT:PSS films.

4. CONCLUSION We conclude that the enhancement of conductivity of cosolvent-treated films is ascribed to partial removal of the non-conductive PSS molecules, morphological changes toward smaller and more densely packed PEDOT-rich domains, with improved interconductivity. For the G additive, an enhanced crystallinity with face-on oriented molecules within the crystals is observed. Moreover, a reorientation of the conjugated molecules toward improved edge-on orientation is found for EG post-treatment. Together, these molecular and structural changes lead to the highly improved lateral conductivity that is important for the application of PEDOT:PSS as an electrode. In particular, the reorientation of the PEDOT molecules with EG post-treatment, which is observed here for the first time, requires consideration. It is remarkable that the molecular orientation for G additive and EG post-treatment is drastically different, however, leading to comparable enhanced conductivities. This indicates that the structural changes on the mesoscale together with the removal of PSS and the enhanced crystallinity might have a stronger influence than the molecular orientation within the crystalline domains. Via correlation of the inner film morphology and the molecular orientation within the crystalline domains to the film conductivity, these results assist in the directed modifications of PEDOT:PSS thin films with optimized properties for practical devices.

Figure 8. Distances and radii from the structure (a and b) and form (c and d) factors of the μ-GISAXS analysis for films with six different G additive concentrations with and without additional EG posttreatment. Dotted lines act as guides to the eye.

I(FF) ∝ nm2 = n(ρV )2 ∝ (4ρπR3)2 ∝ (R3)2 ∝ R6

R1 = xR 2 ; I1(R 2) = I2(R 2) × x 6

I1(FF1) x6

: I2(FF2) ↔ domain ratio

where n is the particle concentration, ρ the scattering length density, V the volume of a particle, R the radius of the scattering objects extracted from the fits, and x the scaling factor for the domain ratio. From this, a model that contains the relative distances and radii from the fit and the estimated number ratio of scattering objects per volume is developed. In Figure 9, a graphical representation, derived from the morphological changes (μ-GISAXS) (Figure 8), is depicted together with the molecular reorientation of the molecules within the PEDOT crystals (μ-GIWAXS) (Figure 7b). All structural size parameters (Figure 8) are depicted in the correct ratio using the model described above.

■

ASSOCIATED CONTENT

S Supporting Information *

2D GISAXS and GIWAXS scattering data, a direct comparison of SEM and GISAXS, XRR measurements, and the GISAXS fitting model used. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +49/(0)89/28912451. Fax: +49/(0)89/289-12473. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been supported by the GreenTech InitiativeInterface Science for Photovoltaics (ISPV) of the EuroTech Universities together with the International Graduate School of Science and Engineering (IGSSE), TUM. C.M.P. thanks the International Doctorate Program in NanoBioTechnology (IDK-NBT)-Elite Network of Bavaria for a doctoral fellowship

Figure 9. Illustration of the morphological changes together with the respective and preferred molecular orientation of pristine PEDOT, Gdoped PEDOT, EG-post-treated PEDOT, and PEDOT subjected to G additive and EG post-treatment. 13604



Article

and Work Function for High-Performance Indium Tin Oxide-Free Optoelectronics. ACS Appl. Mater. Interfaces 2012, 4, 2551−2560. (15) Krebs, F. C. All Solution Roll-to-Roll Processed Polymer Solar Cells Free From Indium-Tin-Oxide and Vacuum Coating Steps. Org. Electron. 2009, 10, 761−768. (16) Yue, G.; Wu, J.; Xiao, Y.; Lin, J.; Huang, M.; Lan, Z. Application of Poly(3,4-ethylenedioxythiophene):Polystyrenesulfonate/Polypyrrole Counter Electrode for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2012, 116, 18057−18063. (17) Walter, M. G.; Liu, X.; O’Leary, L. E.; Brunschwig, B. S.; Lewis, N. S. Electrical Junction Behavior of Poly(3,4-ethylenedioxythiophene) (PEDOT) Contacts to H-Terminated and CH3-Terminated p-, n-, and n+-Si(111) Surfaces. J. Phys. Chem. C 2013, 117, 14485− 14492. (18) Xia, Y.; Ouyang, J. Significant Different Conductivities of the Two Grades of Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate), Clevios P and Clevios PH1000, Arising from Different Molecular Weights. ACS Appl. Mater. Interfaces 2012, 4, 4131−4140. (19) Kim, Y. H.; Sachse, C.; Machala, M. L.; May, C.; MüllerMeskamp, L.; Leo, K. Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post-Treatment for ITO-Free Organic Solar Cells. Adv. Funct. Mater. 2011, 21, 1076−1081. (20) Crispin, X.; Jakobsson, F. L. E.; Crispin, A.; Grim, P. C. M.; Andersson, P.; Volodin, A.; van Haesendonck, C.; Van der Auweraer, M.; Salaneck, W. R.; Berggren, M. The Origin of the High Conductivity of Poly(3,4-ethylenedioxythiophene)−Poly(styrenesulfonate) (PEDOT−PSS) Plastic Electrodes. Chem. Mater. 2006, 18, 4354−4360. (21) Takano, T.; Masunaga, H.; Fujiwara, A.; Okuzaki, H.; Sasaki, T. PEDOT Nanocrystal in Highly Conductive PEDOT:PSS Polymer Films. Macromolecules 2012, 45, 3859−3865. (22) Alemu, D.; Wei, H.-Y.; Ho, K.-C.; Chu, C.-W. Highly Conductive PEDOT:PSS Electrode by Simple Film Treatment with Methanol for ITO-free Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 9662−9671. (23) Kim, J. Y.; Jung, J. H.; Lee, D. E.; Joo, J. Enhancement of Electrical Conductivity of Poly(3,4-ethylenedioxythiophene)/Poly(4styrenesulfonate) by a Change of Solvents. Synth. Met. 2002, 126, 311−316. (24) Jönsson, S. K. M.; Birgerson, J.; Crispin, X.; Greczynski, G.; Osikowicz, W.; Denier van der Gon, A. W.; Salaneck, W. R.; Fahlman, M. The Effects of Solvents on the Morphology and Sheet Resistance in Poly(3,4-ethylenedioxythiophene)−Polystyrenesulfonic Acid (PEDOT−PSS) Films. Synth. Met. 2003, 139, 1−10. (25) Pietsch, M.; Bashouti, M. Y.; Christiansen, S. The Role of Hole Transport in Hybrid Inorganic/Organic Silicon/Poly(3,4-ethylenedioxy-thiophene):Poly(styrenesulfonate) Heterojunction Solar Cells. J. Phys. Chem. C 2013, 117, 9049−9055. (26) Xia, Y.; Ouyang, J. PEDOT:PSS films with Significantly Enhanced Conductivities Induced by Preferential Solvation with Cosolvents and their Application in Polymer Photovoltaic Cells. J. Mater. Chem. 2011, 21, 4927−4936. (27) Nardes, A. M.; Kemerink, M.; Janssen, R. A. J.; Bastiaansen, J. A. M.; Kiggen, N. M. M.; Langeveld, B. M. W.; van Breemen, A. J. J. M.; de Kok, M. M. Microscopic Understanding of the Anisotropic Conductivity of PEDOT:PSS Thin Films. Adv. Mater. 2007, 19, 1196−1200. (28) Timpanaro, S.; Kemerink, M.; Touwslager, F. J.; De Kok, M. M.; Schrader, S. Morphology and Conductivity of PEDOT/PSS Films Studied by Scanning−Tunneling Microscopy. Chem. Phys. Lett. 2004, 394, 339−343. (29) Reyes-Reyes, M.; Cruz-Cruz, I.; López-Sandoval, R. Enhancement of the Electrical Conductivity in PEDOT:PSS Films by the Addition of Dimethyl Sulfate. J. Phys. Chem. C 2010, 114, 20220− 20224. (30) Chen, W.; Nikiforov, M. P.; Darling, S. B. Morphology Characterization in Organic and Hybrid Solar Cells. Energy Environ. Sci. 2012, 5, 8045−8074.

and the Center for NanoScience (CeNS) for support. C.H. thanks the Stiftung Industrieforschung for financial support. C.J.S. thanks the Bavarian State Ministry of Sciences, Research and Arts for funding through the International Graduate School Materials Science of Complex Interfaces (CompInt). We thank K. Kyriakos for a discussion of light scattering, P. Weiser from the Walter Schottky Institute (WSI) and the Center for Nanotechnology and Nanomaterial (ZNN) of TUM and K. Sarkar for support with SEM, M. A. Niedermeier, C. Jendrzejewski, and E. M. Herzig for help during the synchrotron experiments, J. Schlipf and K. Sarkar for proofreading, and D. Magerl for implementing the fitting model and discussions. GISAXS and GIWAXS measurements were conducted at synchrotron light source PETRA III at DESY, a member of the Helmholtz Association (HGF).

■

REFERENCES

(1) Søndergaard, R.; Hösel, M.; Angmo, D.; Larsen-Olsen, T. T.; Krebs, F. C. Roll-to-Roll Fabrication of Polymer Solar Cells. Mater. Today 2012, 15, 36−49. (2) Na, S.-I.; Kim, S.-S.; Jo, J.; Kim, D.-Y. Efficient and Flexible ITOFree Organic Solar Cells Using Highly Conductive Polymer Anodes. Adv. Mater. 2008, 20, 4061−4067. (3) Kaltenbrunner, M.; White, M. S.; Glowacki, E. D.; Sekitani, T.; Someya, T.; Sariciftci, N. S.; Bauer, S. Ultrathin and Lightweight Organic Solar Cells with High Flexibility. Nat. Commun. 2012, 3, 770. (4) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Transparent, Conductive Carbon Nanotube Films. Science 2004, 305, 1273−1276. (5) Wu, J.; Becerril, H. A.; Bao, Z.; Liu, Z.; Chen, Y.; Peumans, P. Organic Solar Cells with Solution-Processed Graphene Transparent Electrodes. Appl. Phys. Lett. 2008, 92, 263302−263302-3. (6) Emmott, C. J. M.; Urbina, A.; Nelson, J. Environmental and Economic Assessment of ITO-free Electrodes for Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2012, 97, 14−21. (7) Fehse, K.; Walzer, K.; Leo, K.; Lövenich, W.; Elschner, A. Highly Conductive Polymer Anodes as Replacements for Inorganic Materials in High-Efficiency Organic Light-Emitting Diodes. Adv. Mater. 2007, 19, 441−444. (8) Zhang, F.; Johansson, M.; Andersson, M. R.; Hummelen, J. C.; Inganäs, O. Polymer Photovoltaic Cells with Conducting Polymer Anodes. Adv. Mater. 2002, 14, 662−665. (9) Huang, J.; Miller, P. F.; Wilson, J. S.; de Mello, A. J.; de Mello, J. C.; Bradley, D. D. C. Investigation of the Effects of Doping and PostDeposition Treatments on the Conductivity, Morphology, and Work Function of Poly(3,4-ethylenedioxythiophene)/Poly(styrene sulfonate) Films. Adv. Funct. Mater. 2005, 15, 290−296. (10) Lim, T. H.; Oh, K. W.; Kim, S. H. Self-assembly Supramolecules to Enhance Electrical Conductivity of Polyaniline for a Flexible Organic Solar Cells Aanode. Sol. Energy Mater. Sol. Cells 2012, 101, 232−240. (11) Younchan, Y.; Jongwoon, P.; Park, B. Solution-Processed Flexible ITO-Free Organic Light-Emitting Diodes Using Patterned Polymeric Anodes. J. Disp. Technol. 2010, 6, 252−256. (12) Vosgueritchian, M.; Lipomi, D. J.; Bao, Z. Highly Conductive and Transparent PEDOT:PSS Films with a Fluorosurfactant for Stretchable and Flexible Transparent Electrodes. Adv. Funct. Mater. 2012, 22, 421−428. (13) Na, S.-I.; Wang, G.; Kim, S.-S.; Kim, T.-W.; Oh, S.-H.; Yu, B.-K.; Lee, T.; Kim, D.-Y. Evolution of Nanomorphology and Anisotropic Conductivity in Solvent-modified PEDOT:PSS Films for Polymeric Anodes of Polymer Solar Cells. J. Mater. Chem. 2009, 19, 9045−9053. (14) Yeo, J.-S.; Yun, J.-M.; Kim, D.-Y.; Park, S.; Kim, S.-S.; Yoon, M.H.; Kim, T.-W.; Na, S.-I. Significant Vertical Phase Separation in Solvent-Vapor-Annealed Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) Composite Films Leading to Better Conductivity 13605



Article

(31) Ruderer, M. A.; Müller-Buschbaum, P. Morphology of Polymerbased Bulk Heterojunction Films for Organic Photovoltaics. Soft Matter 2011, 7, 5482−5493. (32) Moons, E. Conjugated Polymer Blends: Linking Film Morphology to Performance of Light Emitting Diodes and Photodiodes. J. Phys.: Condens. Matter 2002, 14, 12235−12260. (33) Anselmo, A. S.; Lindgren, L.; Rysz, J.; Bernasik, A.; Budkowski, A.; Andersson, M. R.; Svensson, K.; van Stam, J.; Moons, E. Tuning the Vertical Phase Separation in Polyfluorene:Fullerene Blend Films by Polymer Functionalization. Chem. Mater. 2011, 23, 2295−2302. (34) Müller-Buschbaum, P. The Active Layer Morphology of Organic Solar Cells Probed with Grazing Incidence Scattering Techniques. Adv. Mater. 2014, DOI: 10.1002/adma.201304187. (35) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, M.; et al. A Strong Regioregularity Effect in Self-Organizing Conjugated Polymer Films and High-Efficiency Polythiophene:Fullerene Solar Cells. Nat. Mater. 2006, 5, 197−203. (36) Spanninga, S. A.; Martin, D. C.; Chen, Z. X-ray Photoelectron Spectroscopy Study of Counterion Incorporation in Poly(3,4-ethylenedioxythiophene). J. Phys. Chem. C 2009, 113, 5585−5592. (37) Lang, U.; Müller, E.; Naujoks, N.; Dual, J. Microscopical Investigations of PEDOT:PSS Thin Films. Adv. Funct. Mater. 2009, 19, 1215−1220. (38) Crispin, X.; Marciniak, S.; Osikowicz, W.; Zotti, G.; van der Gon, A. W. D.; Louwet, F.; Fahlman, M.; Groenendaal, L.; De Schryver, F.; Salaneck, W. R. Conductivity, Morphology, Interfacial Chemistry, and Stability of Poly(3,4-ethylene dioxythiophene)− Poly(styrene sulfonate): A Photoelectron Spectroscopy Study. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2561−2583. (39) Yoneda, Y. Anomalous Surface Reflection of X Rays. Phys. Rev. 1963, 131, 2010−2013. (40) Lai, S. L.; Chan, M. Y.; Fung, M. K.; Lee, C. S.; Lee, S. T. Concentration Effect of Glycerol on the Conductivity of PEDOT Film and the Device Performance. Mat. Sci. .Eng., B 2003, 104, 26−30. (41) Meier, R.; Birkenstock, C.; Palumbiny, C. M.; MüllerBuschbaum, P. Efficiency-improved Organic Solar Cells Based on Plasticizer Assisted Soft Embossed PEDOT:PSS Layers. Phys. Chem. Chem. Phys. 2012, 14, 15088−15098. (42) Martin, B. D.; Nikolov, N.; Pollack, S. K.; Saprigin, A.; Shashidhar, R.; Zhang, F.; Heiney, P. A. Hydroxylated Secondary Dopants for Surface Resistance Enhancement in Transparent Poly(3,4ethylenedioxythiophene)−Poly(styrenesulfonate) Thin Films. Synth. Met. 2004, 142, 187−193. (43) Elschner, A.; Kirchmeyer, S.; Lovenich, W.; Merker, U.; Reuter, K. PEDOT: Principles and Application of an Intrinsically Conductive Polymer; CRC Press: Boca Raton, FL, 2010. (44) Buffet, A.; Rothkirch, A.; Döhrmann, R.; Körstgens, V.; Abul Kashem, M. M.; Perlich, J.; Herzog, G.; Schwartzkopf, M.; Gehrke, R.; Müller-Buschbaum, P.; et al. P03, the Microfocus and Nanofocus X-ray Scattering (MiNaXS) Beamline of the PETRA III Storage Ring: The Microfocus Endstation. J. Synchrotron Radiat. 2012, 19, 647−653. (45) Müller-Buschbaum, P. Grazing Incidence Small-Angle X-ray Scattering: An Advanced Scattering Technique for the Investigation of Nanostructured Polymer Films. Anal. Bioanal. Chem. 2003, 376, 3−10. (46) Perlich, J.; Rubeck, J.; Botta, S.; Gehrke, R.; Roth, S. V.; Ruderer, M. A.; Prams, S. M.; Rawolle, M.; Zhong, Q.; Körstgens, V.; et al. Grazing Incidence Wide Angle X-ray Scattering at the Wiggler Beamline BW4 of HASYLAB. Rev. Sci. Instrum. 2010, 81, 105105− 105105-7. (47) Müller-Buschbaum, P. Influence of Surface Cleaning on Dewetting of Thin Polystyrene Films. Eur. Phys. J. E: Soft Matter Biol. Phys. 2003, 12, 443−448. (48) Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W. NIH Image to ImageJ: 25 Years of Image Analysis. Nat. Methods 2012, 9, 671−675. (49) Nardes, A. M.; Kemerink, M.; de Kok, M. M.; Vinken, E.; Maturova, K.; Janssen, R. A. J. Conductivity, Work Function, and Environmental Stability of PEDOT:PSS Thin Films Treated with Sorbitol. Org. Electron. 2008, 9, 727−734.

(50) Snaith, H. J.; Kenrick, H.; Chiesa, M.; Friend, R. H. Morphological and Electronic Consequences of Modifications to the Polymer Anode ‘PEDOT:PSS’. Polymer 2005, 46, 2573−2578. (51) Aasmundtveit, K. E.; Samuelsen, E. J.; Pettersson, L. A. A.; Inganäs, O.; Johansson, T.; Feidenhans’l, R. Structure of Thin Films of Poly(3,4-ethylenedioxythiophene). Synth. Met. 1999, 101, 561−564. (52) Ruderer, M. A.; Prams, S. M.; Rawolle, M.; Zhong, Q.; Perlich, J.; Roth, S. V.; Müller-Buschbaum, P. Influence of Annealing and Blending of Photoactive Polymers on Their Crystalline Structure. J. Phys. Chem. B 2010, 114, 15451−15458. (53) Pedersen, J. S. iNANOSchool Home Page of Aarhus University, Aarhus, Denmark. http://inanoschool.au.dk/fileadmin/inano/ iNANOSchool/iNANOschool_2006/N9/SAXS%20proteins.pdf (accessed November 19, 2013).

13606


Molecular Reorientation and Structural Changes in Cosolvent-Treated

Recommend Documents