Influence of Solvent and Solvent Additive on the Morphology of PTB7

Article pubs.acs.org/JPCB

Influence of Solvent and Solvent Additive on the Morphology of PTB7 Films Probed via X‑ray Scattering Shuai Guo,† Eva M. Herzig,†,‡ Anna Naumann,† Gregory Tainter,† Jan Perlich,§ and Peter Müller-Buschbaum*,† †

Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien, James-Franck-Str. 1, 85748 Garching, Germany ‡ Technische Universität München, Munich School of Engineering, James-Franck-Str. 1, 85748 Garching, Germany § Deutsches Elektronen-Synchrotron at DESY, Notkestr. 85, 22603 Hamburg, Germany ABSTRACT: Films of the semiconducting polymer poly[[4,8-bis[(2ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] with 40% fluorinated monomers, denoted PTB7-F40, are spin coated out of different solvents onto PEDOT:PSS films. The influence of the used solvents chlorobenzene, 1,2-dichlorobenzene, and 1,2,4-trichlorobenzene as well as the influence of the additive 1,8-diiodooctane (DIO) is probed with grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS). As seen with GISAXS, without DIO, the films are homogeneous and show roughness correlation with the PEDOT:PSS film surface. With DIO, an inner film structure with a size of 50−75 nm is found and the roughness correlations weaken. In addition, as seen in GIWAXS, the crystalline part of the films is influenced by the used solvent if DIO is added.

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INTRODUCTION Semiconducting polymers have applications in many different emerging areas such as organic electronics and organic photovoltaics.1−7 Tremendous work has been focused on organic photovoltaics, both in academic and in industrial areas, due to its mechanical flexibility, light weight, and potential high throughput production. Recently, conjugated polymers with low band gap have been used in these organic solar cells. Devices based on the copolymer series using thieno[3,4b]thiophene-alt-benzodithiophene as the electron donor material, blended with fullerenes in a bulk heterojunction (BHJ) morphology, have demonstrated very promising power conversion efficiencies (PCE) of 8−10%,8−12 which almost reach the threshold for commercial applications in photovoltaics.13 With the aim to further increase the PCE of organic solar cells, additional treatments such as morphology control, interface engineering, plasmonic effect, and many more approaches have been examined.14−17 Among all of them, morphology control is the most efficient and popular method to improve the device performance. It can be realized for instance by the introduction of solvent additives, carbon nanotubes, or solvent treatments.18−21 Moreover, it is known that the selection of the solvent used for the deposition of the active layer also has a crucial impact on morphology and, therefore, the device performance of organic solar cells.22,23 Despite the massive interest in these complex films or functional stacks, a basic understanding about the behavior of the novel conjugated polymers, which are relevant for highly © 2013 American Chemical Society

efficient organic solar cells or other organic electronic applications, is important as well. Typically, all related investigations directly focus on BHJ films or on full devices using polymer and fullerene components. In such complex samples, a full decoupling of the underlying mechanisms is difficult. Moreover, fundamental knowledge about the phase separation, molecular orientation, and crystallinity in pure polymer films is still very limited. Since these polymers are the main component of organic solar cells, such fundamental insights can help to understand the more complex scenarios in the devices, which, e.g., make use of the BHJ geometry. Zhou et al. reported PBnDT-DTffBT (poly[benzo[1,2-b:4,5b′]-dithiophene-alt-5,6-difluoro-4,7-dithien-2-yl-2,1,3-benzothiadiazole]) as the first successful application of a fluorinated conjugated donor−acceptor polymer in organic photovoltaics, which has reached beyond 7% efficiency.24 Since then, fluorine doping has been widely used to synthesize more efficient polymers. In particular, PTB7 (polymer poly[[4,8-bis[(2ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) based organic solar cells showed very good efficiencies, if the additive 1,8-diiodooctane (DIO) was used.25−27 However, all the previous conclusions, such as DIO only selectively dissolves the PCBM phase and does not influence the PTB7 phase, have Received: October 10, 2013 Revised: December 10, 2013 Published: December 16, 2013 344

dx.doi.org/10.1021/jp410075a | J. Phys. Chem. B 2014, 118, 344−350

The Journal of Physical Chemistry B

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Figure 1. (a) 2D GISAXS data of PTB7-F40 films prepared without (top row) and with (bottom row) solvent additive. The used solvents CB (chlorobenzene), DB (1,2-dichlorobenzene), and TB (1,2,4-trichlorobenzene) are indicated. Each data set is shown as a function of the in-plane component of the scattering vector qxy and the component along the surface normal qz. The intensity is shown in logarithmic scale. (b) The corresponding vertical cuts of the 2D intensity as a function of the detector angle αi + αf. The bottom three curves are PTB7-F40 films made of CB, DB, and TB without additive DIO, and the top three are with additive. The gray box indicates the position of the specular beamstop.

often been made based on the investigation of the blended polymer:fullerene system.14,28,29 Thus, the potential influence of solvent and solvent additive on an individual component such as PTB7, which is actually essential for accurately understanding more complex mixtures, has been overlooked. As shown in the present work, the selection of solvent and solvent additive already plays a great role in the domain formation of the pure polymer films. To entirely demonstrate these effects, we investigate to what extent the PTB7 phase is affected by the solvent additive DIO. Instead of focusing on the full and complex BHJ blend system, pure polymer PTB7 thin films are investigated. Thin films are made out of three different solvents, with and without the addition of DIO. The molecular structure of the investigated polymer PTB7-F40 is shown in Scheme 1. Compared to the most investigated PTB7 (refers to PTB7-F100), the frame of PTB7-F40 is similar except that the ratio between the fluorinated thieno[3,4-b]thiophene submonomer and the non-fluorinated one is statistically 0.4 (see Scheme 1). Although different degrees of fluorination of PTB7, e.g., PTB7-F20, PTB7-F60, and PTB7-F80, have been investigated already,25−27 a deep understanding on how the solvent additive DIO influences the pure polymer PTB7-F40 is still unknown. To detect the crystalline structure and to probe the morphology of the films, the advanced scattering techniques grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS) are used. Structural changes introduced by the selected solvent and the additive are reported.

Scheme 1. Molecular Structure of Probed Material PTB7F40

ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] with 40% fluorinated monomers, denoted PTB7-F40 (1-Material), and the solvents chlorobenzene (Carl Roth), 1,2-dichlorobenzene, and 1,2,4-trichlorobenzene (Sigma-Aldrich) as well as the additive 1,8-diiodooctane denoted DIO (Sigma-Aldrich) were used as supplied. PTB7-F40 was dissolved in each solvent at a fixed concentration of 10 mg/mL. DIO was used as a solvent additive with 3 vol % and added to the polymer solution. All samples were spin coated on PEDOT:PSS covered silicon (Si) substrates from Si-Mat, which were precleaned in an acid bath.30 The PEDOT:PSS (purchased from Sigma-Aldrich) layers were prepared (spin coated for 60 s at 3000 rpm) and annealed afterwards for 10 min at 150 °C. To obtain a similar

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EXPERIMENTAL SECTION Samples. The polymer poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2345



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film thickness from different solvents, spin coating of PTB7F40 was done immediately after the PEDOT:PSS annealing for 30 s at 2000 rpm at room temperature for chlorobenzene and at 900 and 400 rpm at 100 °C for 1,2-dichlorobenzene and 1,2,4trichlorobenzene, respectively. The use of silicon instead of ITO substrates, which are common in organic solar cells, was to reduce the background scattering. The characteristic lateral structures of the polymer are comparable for both types of substrates, PEDOT:PSS on Si or on ITO/glass.31 GISAXS. The GISAXS measurements were performed at the synchrotron beamline BW4, HASYLAB (DESY) in Hamburg, Germany (λ = 0.138 nm).32 An X-ray beam with a size of 23 × 36 μm2 was selected. The scattered intensity was collected with a MarCCD detector. Two moveable beamstops were used to protect the detector from the high intensity of the direct and reflected beams. To allow excellent sampling statistics at large qy values, a rod-shaped beamstop at qy = 0 blocking relatively high intensity replaces the point-like beamstop in the case of a long data acquisition time. An incident angle of 0.30° was selected, well above the critical angle of the investigated materials PTB7-F40 (0.16°), hence probing the inner film structures. The sample−detector distance (SDD) was 2.084 m. To gain quantitative information about structure information, horizontal and vertical line cuts of the 2D GISAXS data were performed. The vertical cut at the position qy = 0 gives structure information along the surface normal of the substrate. The horizontal cut at the critical angle of each material contains the information about the lateral structures, such as domain sizes, size distributions, spatial correlations, etc. GIWAXS. The GIWAXS measurements were also performed at beamline BW4.33 To record crystalline structures, the SDD was reduced to 0.104 m. Vertical and horizontal sector integrals were performed on the 2D GIWAXS data to obtain detailed information on the crystalline parts of the films.

respective cut, the intensity modulation gradually disappears. The off-centered vertical cuts are slices of the 2D GISAXS data, which are done with a fixed width Δqy at different positions qy. The information about the smallest replicated in-plane structure is extracted by identifying the qy value at which the off-centered vertical cut does not show any intensity modulation anymore.35 The extracted values for the smallest replicated in-plane structure are 470 nm in the case of CB and 555 nm in the case of DB and TB. Thus, the solvent used has no strong influence on the roughness spectrum of the PTB7F40 film surface. The addition of the additive DIO changes the 2D GISAXS patterns. No pronounced periodic intensity oscillations in the qz direction are visible anymore. Consequently, these films exhibit no strong thickness correlations, which means that the PTB7-F40 film surface and the PEDOT:PSS film surface become statistically more independent. The added DIO acts as a plasticizer. Due to its relatively high boiling point, it remains inside the films after spin coating.21 Thus, the surface structure of the spin coated films is not frozen-in directly after spin coating but can change subsequent to the spin coating, agreeing with the investigation by Ye et al.38 As the thickness correlation is an energetically unfavorable state for the polymer film surface,33 the film smoothens to reduce its free energy and thereby loses part of its correlated roughness. The smallest replicated in-plane structures increase (CB + DIO 758 nm, DB + DIO 1300 nm, TB + DIO 825 nm). As a consequence, the intensity undulation arising from correlated roughness changes after adding the additive DIO and becomes much less pronounced, as seen from Figure 1b. Thus, the film surfaces of PTB7-F40 films prepared with or without additive have very different roughness spectra. Further insights about the inner film structure are gained from horizontal line cuts of the 2D GISAXS data. These cuts are done at the critical angle of PTB7-F40, and are fitted with a 1D paracrystal model within the frame of the distorted wave Born approximation (DWBA).39,40 In this model, contributions from two different structure and form factors, as well as the resolution function, are considered. As observed from Figure 2, all the neat PTB7-F40 films do not show prominent scattering features in the horizontal line cuts, due to the absence of periodic lateral inner film structures in the nanometer regime. Thus, the films are rather homogeneous in the direction parallel to the substrate, as is expected for homopolymer films. It is concluded that, without the additive DIO, no polymer aggregation is observed. In comparison, the appearance of a shoulder in the intensity of the horizontal line cuts is seen for all the films prepared with DIO additive. It indicates the formation of lateral structures inside the films caused by the addition of DIO. From the fits of the data, the corresponding structure sizes are extracted. By the addition of DIO, structures of 50, 70, and 75 nm are formed for CB, DB, and TB, respectively. Obviously, the low volatility of DIO causes structure formation in the ternary system (polymer, solvent, additive). Considering the different volatilities of the host solvents that it decreases from CB (131 °C) to DB (181 °C) and TB (215 °C), the differences in the structure sizes can be explained. It was also reported by Lou et al. that the radius of PTB7 aggregates increased very slightly from 34 to 37 Å with the addition of DIO, as probed by SAXS measurments.41 To identify the molecular orientation, the polymer backbone spacing, and the crystal size in the PTB7-F40 films, GIWAXS measurements are performed.3,42,43 The 2D GIWAXS data are

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RESULTS AND DISCUSSION GISAXS measurements are performed to probe the structural information of the thin films with a high statistical relevance and thereby gain insights on the morphology of the films. Figure 1a shows the 2D GISAXS scattering patterns for PTB7F40 films made from chlorobenzene (CB), 1,2-dichlorobenzene (DB), and 1,2,4-trichlorobenzene (TB), without and with the additive DIO. For the three GISAXS patterns in the top row of Figure 1a, the most prominent feature observed is an oscillation of the scattered intensity in the qz direction. In vertical line cuts extracted from the 2D GISAXS data at qy = 0, these intensity oscillations are more visible (see Figure 1b). The pure PTB7F40 films show this feature in the GISAXS data irrespective of the solvent used. The intensity oscillations are caused by socalled correlated roughness, indicating a locally constant film thickness of two adjacent thin films. This phenomenon was observed for several other homopolymer films.34−37 Moreover, it has also been observed in more complex samples such as bilayer films or full functional stacks used for organic solar cells.31,35 In the present investigation, the observed periodic intensity oscillation arises from the thickness correlation between the PEDOT:PSS film surface and that of the PTB7F40 film. This means that the PTB7-F40 film surface follows the roughness of the underlying PEDOT:PSS film surface and acts as a bandpass filter, which transmits a part of the roughness spectrum offered by the PEDOT:PSS film surface. Therefore, in off-centered vertical cuts toward larger qy values of the 346



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This is in agreement with a previous investigation on neat PTB7 polymer made of DB by Hammond et al.; nevertheless, their work mainly focused on the molecular order in polymer/ fullerene bulk film.44 In order to have a quantitative evaluation, vertical and horizontal sector integrals of the 2D GIWAXS data are performed (see Figure 4). In the present work, as the polymer PTB7-F40 in the prepared films has a very low crystallinity, we follow the approach by Hammond et al.44 We assume that a missing crystal population extracted from the inaccessible region in Figure 3 is unlikely. Therefore, the data analysis is proceeded on the original 2D detector images for particularly this system, as seen from many others’ work.17,20,44 From the horizontal cuts (Figure 4a), no obvious changes are observed, either in the case of the different host solvents or caused by the additive DIO. In contrast, in the vertical cuts, moderate changes are observed (Figure 4b). Thus, the used solvents and the usage of DIO as additive affect the crystallinity of the films to a certain extent. By fitting all intensity peaks in the cuts (both directions) with Gaussian functions and applying the Scherrer equation, changes in the polymorph and in the crystal size are extracted. In the horizontal cuts, seen in Figure 4a, the peak at lower q values of 3.3 nm−1 is the (100) Bragg peak. It corresponds to the polymer PTB7 backbone spacing in the alkyl side chain direction, which is around 1.9 nm. The weak peak at q ∼ 16 nm−1 originates from the π−π stacking distance in the (010) direction. In the vertical cuts shown in Figure 4b, peaks occur at similar q positions. In contrast to the horizontal cuts, in the vertical direction, much less pronounced reflection at low q

Figure 2. Horizontal line cuts of the 2D GISAXS data performed at the critical angle of PTB7-F40. The bottom three curves are extracted from measurements on neat PTB7-F40 films without DIO additive and the top three curves from films with added DIO. The used solvents are CB (black), DB (red), and TB (blue), and the fits are shown with solid lines (green). For clarity, the curves are shifted along the intensity axis.

shown in Figure 3. Pronounced scattering intensity is observed for all samples at the same high q values in these GIWAXS data, indicating the π−π stacking distance of the polymer backbone.

Figure 3. 2D GIWAXS data of PTB7-F40 film made of CB, DB, and TB after background correction shown from left to right as indicated. In the upper row, samples are prepared without solvent additive, in the bottom row with the addition of DIO. 347



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volatility of the solvent. It is likely that such domains will be present in BHJ films with PC71BM as well, which helps to explain the best device performance from this solvent in the corresponding PTB7:PC71BM solar cells. The polymer aggregation observed after addition of DIO completes the common interpretation that DIO only selectively dissolves the PCBM phase and does not influence the PTB7 phase.14,46 The crystallinity of the PTB7 films is not influenced by the solvent used as long as no DIO is added. In the case that DIO is added, the overall structural order in the solid state film gets solvent dependent, and it may further assist in the charge transport in a corresponding solar cell device. Since BHJ solar cells based on PTB7 show high efficiencies only in the case that DIO is added, this is an intriguing observation.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 (0) 89/289-12451. Fax: +49 (0) 89/289-12473. E-mail: [email protected].

Figure 4. (a) Horizontal and (b) vertical sector integrals of the 2D GIWAXS data of PTB7-F40 films prepared from CB (black), DB (red), and TB (blue) without (solid lines) or with (dash lines) DIO. All of the curves are shifted along the y axis for clarity of presentation.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the GreenTech Initiative (Interface Science for Photovoltaics - ISPV) of the EuroTech Universities together with TUM.solar in the frame of the Bavarian Collaborative Research Project “Solar Technologies go Hybrid” (SolTec). G.T. thanks the Erasmus Mundus scholarship “MaMaSelf” program for funding. Portions of this research were carried out at the synchrotron light sources DORISIII at DESY. DESY is a member of the Helmholtz Association (HGF).

region is observed for all samples, caused most likely by the beamstop instead of the (100) reflection, as shown in Figure 3. The very strong (010) Bragg peaks at 16.5 nm−1 in the vertical direction associated with the π−π stacking alignment, indicating the prevailing crystallite orientation for PTB7-F40 films is the face-on orientation, which confirms others’ investigation on the PTB7 system.26,41,44 Films spin coated out of CB have a less pronounced (010) Bragg peak and thus less overall ordered crystallites. Upon addition of DIO, the number of ordered crystallites is even further reduced for the films made from CB. The film spin coated from DB has the strongest developed structural order, but it is reduced as well by the addition of DIO. Only films made from TB exhibits an improved crystallinity if DIO is added. In contrast, the crystal sizes extracted from the Scherrer equation show nearly no influence from either the different solvents or the additive DIO used in the film preparation. This observation is in agreement with previous work done by Lou et al., which showed that the addition of DIO has a negligible influence on the PTB7 crystal size.41 However, in the case of other semiconducting polymers, such as PCPDTBT, it was reported that additive processed films are substantially better ordered than that of the neat films made of solvent CB. Such a difference in the influence of solvent additive on the crystalline part of the films could be ascribed to the big difference in the molecular structures.45

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CONCLUSION With GISAXS and GIWAXS, the structure and morphology of PTB7-F40 films are probed and the influence of the solvent used for spin coating the polymer is determined. Irrespective of the solvent used (CB, DB, and TB), the films are homogeneous and have smooth surfaces with a roughness correlation with the underlying PEDOT:PSS film surface. Thus, not only on average but also locally the film thickness is constant. Upon the addition of the commonly used additive DIO, the roughness correlations change and are significantly less pronounced. Moreover, the films exhibit an inner structure size, which depends on the solvent used. For films made from CB, the smallest polymer domains are obtained, due to the high 348



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