Influence of Fluorination and Molecular Weight on the Morphology and

Article pubs.acs.org/JPCC

Influence of Fluorination and Molecular Weight on the Morphology and Performance of PTB7:PC71BM Solar Cells Xiaoxi He,† Subhrangsu Mukherjee,‡ Scott Watkins,§ Ming Chen,§ Tianshi Qin,§ Lars Thomsen,∥ Harald Ade,‡ and Christopher R. McNeill*,⊥ †

Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, United States § CSIRO Materials Science and Engineering, Private Bag 10, Clayton South MDC, Victoria 3168, Australia ∥ Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3169, Australia ⊥ Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia ‡

S Supporting Information *

ABSTRACT: The device performance and microstructure of a series of PTB7-based polymers with varied molecular weight and degree of fluorination are investigated. Although the energy level of the highest occupied molecular orbital is found to increase with degree of fluorination, a strong relative molecular weight dependence of device performance dominates any underlying fluorination-dependent trend on overall performance. Microstructural investigation using a combination of X-ray techniques reveals a striking effect of polymer molecular characteristics on film morphology, with the size of PC71BM domains systematically decreasing with increasing polymer molecular weight. Furthermore, the relative purity of the mixed PTB7:PC71BM domain is found to systematically decrease with increasing molecular weight. When domain sizes with and without the use of the solvent additive diiodooctane (DIO) are compared, the effectiveness of DIO in reducing PC71BM domain sizes is also found to be strongly dependent on the molecular weight of the polymer. It is found that molecular weights of at least 150 kg mol−1 are required for DIO to be effective in reducing the PC71BM domain size in order to produce high short-circuit current densities. Finally, it is shown that relatively high device efficiencies can be achieved with low degrees of fluorination; an efficiency of 4.6% is achieved for a degree of fluorination of only 5.3%.

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PTB7:PC71BM system4 (along with several other highperformance systems7−9), small amounts of so-called solvent additives are added to the host solvent to achieve an optimal microstructure. Specifically, it has been shown that adding ∼3 vol % of the additive 1,8-diiodooctane (DIO) to solutions leads to a significant boost in device performance attributed to a reduction of the size of PC71BM domains from ∼200 nm to ∼20 nm.4 The effectiveness of DIO has been linked to its ability to selectively dissolve PC71BM aggregates in solution.10 Domains larger than ∼20 nm are generally not beneficial to device performance as the limited exciton diffusion length of organic semiconductors (∼10 nm) requires domains of order 10−20 nm for efficient exciton harvesting.5 Domains that are too small can lead to reduced performance because of increased recombination.11 Fundamental to understanding the high performance of PTB7-based solar cells is the influence of the chemical and

INTRODUCTION Steady progress is continuing in the development of polymer solar cells with efficiencies of over 9% reported for singlejunction devices and over 10% for tandem cells.1−3 Cells based on the thienothiophene-benzodithiophene copolymer PTB74 (see Figure 1 for chemical structure) exhibit the highest literature-reported single-junction efficiencies1 in blends with the fullerene derivative PC71BM. Blending of semiconducting polymers with high electron affinity fullerene derivatives is necessary to drive exciton dissociation, with bound excitons being the primary product of photogeneration.5 The polymer acts as the electron donor (or hole acceptor) while the fullerene derivative acts as the electron acceptor (or hole donor). The active layer of polymer−fullerene solar cells is prepared by dissolving polymer and fullerene in a common solvent and casting a blended thin film either by spin-coating (typical for laboratory experiments) or any number of printing techniques (more appropriate for a manufacturing setting). The microstructure of the resultant layer is sensitive to the solution deposition process; fine-tuning of the active layer morphology is required to optimize device performance.6 For the © 2014 American Chemical Society

Received: February 4, 2014 Revised: April 7, 2014 Published: April 11, 2014 9918

dx.doi.org/10.1021/jp501222w | J. Phys. Chem. C 2014, 118, 9918−9929

The Journal of Physical Chemistry C

Article

solvent additive DIO, we find that the effectiveness of DIO in reducing PC71BM domains is influenced by the molecular weight of the polymer. This observation indicates that the effectiveness of DIO is not exclusively related to its interaction with PC71BM aggregates in the solution and that morphology evolution in devices is more complicated than previously thought.

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EXPERIMENTAL DETAILS The PTB7 derivatives were provided by 1-Material Inc. and PC71BM was obtained from Nano-C. All materials were used as received. The relative molecular weights were independently determined with an Agilent Technologies 1200 series gel permeation chromatograph in chlorobenzene at 80 °C using two PL mixed B columns in series and calibrated against narrow polydispersity polystyrene standards. The HOMO energy levels of the derivatives were measured with a Riken Keiki AC-2 photoelectron spectrometer in air. UV−vis optical absorption measurements were performed with a Hewlett-Packard G1103A UV−vis spectrophotometer. PTB7-Fx and PC71BM were codissolved with a weight ratio of 1:1.5 (total concentration, 25 g L−1) in chlorobenzene at 70 °C. 1,8-diiodooctane (DIO) was added as an additive with varied volume fraction of 1%, 2%, 3%, 4%, and 5%. The spin coating speed was adjusted to optimize film thickness. The optimal volume fraction of DIO for all polymers was independently determined and found to be 3% in all cases. Films measured by scanning transmission X-ray microscopy (STXM), grazing incidence wide-angle X-ray scattering (GIWAXS), near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy, and resonant soft X-ray scattering (RSoXS) were spin-coated onto NaPSS-coated glass substrates (STXM, R-SoXS) or silicon wafers (NEXAFS, GIWAXS). Solar cell devices were fabricated on ITO-coated glass covered with a ∼40 nm layer of poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid (PEDOT:PSS). NaPSS and PEDOT:PSS were annealed at 150 °C for 10 and 30 min, respectively, under flowing nitrogen prior to transfer to a nitrogen glovebox for further processing steps. PTB7-Fx:PC71BM films were cast from chlorobenzene solution with 3 vol % DIO to give films ∼100 nm thick for solar cells. (Film thickness was separately varied for each system, and a film thickness of 100 nm found to be optimum.) Aluminum electrodes of ∼100 nm were evaporated on top of the active layer. The devices were completed with epoxy and an encapsulation glass cover. Pure films were spincoated from chlorobenzene solutions (14 g L−1 for PTB7-Fx and 50 g L−1 for PC71BM) for characterization using X-ray photoelectron spectroscopy (XPS), in-air UV photoelectron spectroscopy, NEXAFS spectroscopy, and UV−vis spectroscopy. The steady-state device characteristics were examined by measuring both the external quantum efficiency (EQE) spectra and the device current−voltage characteristics under a solar simulator. EQE spectra were measured using a 100 W tungsten halogen lamp dispersed through a monochromator with a Keithley 237 source measure unit (SMU) used to measure the short-circuit current at various wavelengths. Incident light intensity was continuously monitored during measurement by a reference photodiode calibrated by a Hamamatsu S8746-01 calibrated photodiode. Current−voltage characteristics were measured under ∼100 mW cm−2 AM1.5G conditions using the Keithley 237 SMU with illumination provided by an ABET

Figure 1. Chemical structures of PTB7, PC71BM, and PTB7-Fx.

physical properties of PTB7-based polymers on device performance. In particular, the role of the fluorine atom grafted onto the thieno[3,4-b]-thiophene unit of PTB7 is a subject of interest. Several groups have reported high-efficiency fluorinesubstituted conjugated polymers.12−16 Price et al. suggested that the substituted fluorine atoms have a minimal effect on the optical and electrochemical properties of the polymer, but rather have a profound effect on the hole mobility of the polymer and thus the photovoltaic performance.12 Price et al. also found that fluorination of the polymer backbone lowered both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymer, hence increasing the energy difference between the HOMO of the polymer and the LUMO of the fullerene acceptor. Monofluorinated thienothiophene has also been found to give better solar cell performance compared with perfluorination of the polymer backbone.15 In addition, changes in crystallinity, internal polarization, and photochemical stability have been attributed to fluorination.15 The enhancement of the open-circuit voltage of solar cells caused by fluorination of copolymers is also reported in the work done by Albrecht et al.16 Furthermore, Stuart et al. found that fluorination of polymers similar to PTB7 reduces charge recombination and affects the structure and morphology of polymer solar cells.17 By studying fluorination and side-chain length, Yang et al. have shown that judicious side-chain choice can further improve the performance of fluorinated polymers.18 Recently, by studying PTB7-like polymers where the fraction of thieno[3,4-b] thiophene units that possess a fluorine atom was systematically varied from 0% to 100%, Wang et al. have claimed an optimum degree of fluorination in the range of 20− 40%.19 Here, we study the same series of PTB7-like polymers where the fraction of thieno[3,4-b] thiophene units that are monofluorinated is varied, with a view to understanding the influence of fluorination not only on device performance but also upon film microstructure. However, by accounting for differences in the molecular weight of the polymers studied, we observe a striking molecular weight dependence of device performance and morphology evolution that dwarfs any fluorination-dependent trend. Moreover, by comparing the morphology of films prepared with and without the use of the 9919



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traditional 1D I(q) function as I(q)·q2 resembles the power spectral density of the 2D data. Two detector distances were used to achieve the q-range of the data presented here.

class AAA solar simulator calibrated to a silicon reference cell, correcting for spectral mismatch. Grazing-incidence wide-angle scattering (GIWAXS) measurements were performed at the SAXS/WAXS beamline at the Australian Synchrotron. Twelve kiloelectronvolt photons were used with 2D scattering patterns recorded on an MAR-165 CCD detector. The sample-to-detector distance was calibrated using a silver behenate standard. A grazing angle of ∼0.10° was used that is above the critical angle of the polymer films but below the critical angle of the substrate. Data acquisition times of 60 s were used with no evidence found for beam damage when comparing data taken at shorter and longer acquisition times. GIWAXS data were analyzed using the software SAXS15ID version 3299. X-ray diffraction data are expressed as a function of the scattering vector, q, that has a magnitude of (4π/λ)sin θ, where θ is half the scattering angle and λ is the wavelength of the incident radiation. Surface-sensitive XPS and near-edge X-ray fine-structure (NEXAFS) spectroscopy at the carbon edge were recorded at the Soft X-ray Spectroscopy Beamline at the Australian Synchrotron.20 Nearly perfectly linearly polarized photons (P ≈ 1) from an undulator X-ray source with high spectral resolution of E/ΔE ≤ 10 000 were focused into an ultrahigh vacuum chamber on ∼0.4 × 1 mm2 sample area. For XPS measurement an X-ray energy of 1486.7 eV was used with photoelectrons detected using a SPECS Phoibos 150 Hemispherical Analyzer with a pass energy of 40 eV. The stability of the energy of the X-ray was monitored using a bulk gold reference. XPS spectra were analyzed in the following manner. A Shirley background was applied to remove the electronscattering background and maintain the intrinsic line shape from the raw data. The relevant peaks (C 1s, F 1s) were then fitted to a Gaussian, and the area under the peak was divided with the appropriate sensitivity factors. Normalized to the intensity of the C 1s peak, the area under the F 1s peak for each sample was compared to that of the F100 sample to determine the F-content relative to the case of 100% fluorination. For NEXAFS measurement X-ray absorption was measured via Auger electron yield (AEY) using a SPECS Phoibos 150 Hemispherical Analyzer set to a kinetic energy of 230 eV. The recorded signal was normalized to the incident photon flux using the “stable monitor method”, in which the sample signal is compared consecutively to a clean reference sample and the time variations in flux measured via a gold mesh.21 The normalized spectra were scaled by subtracting a background which scales according to the atomic scattering factors of the material prior to the onset of the first feature setting (that is, the absorption measured at 280 eV) and then normalizing to the value at 320 eV. STXM measurements were performed at the PolLux beamline at the Swiss Light Source.22−24 Samples were prepared by floating off onto TEM grids. An X-ray energy of 283.0 eV was used that was found to maximize materials contrast based on the difference spectrum of the two materials. Full details can be found in previous publications.25,26 R-SoXS characterization was conducted at Beamline 11.0.1.2 of the Advanced Light Source (ALS) in the soft X-ray energy regime (∼283 eV) following previously established methods and protocols.18,27,28 A section of film was floated onto 1.5 × 1.5 mm2 silicon nitride window. The 1D averaged intensity is multiplied by q2, which then corresponds to an azimuthal integration of the 2D data. This reflects the fractional distribution of domain spacing more directly than the

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RESULTS AND DISCUSSION The chemical structures of the PTB7-like polymers used in this study are shown in Figure 1. In a method that is the same as that reported in the work of Wang et al.,19 the fluorine atom of the thieno[3,4-b] thiophene unit has been randomly substituted by hydrogen with variation from 0% to 100% in increments of 20%. When there are no hydrogen atoms in the thieno[3,4-b] thiophene unit, the material is marked as PTB7 or PTB7-F100. PTB7-F00 indicates that all the fluorine atoms were substituted by hydrogen atoms. The properties of the PTB7-Fx series (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) including batch number, weightaverage molecular weight (Mw), number-average molecular weight (MN), and polydispersity index (PDI) are listed in Table 1, with the relative molecular weights independently Table 1. Properties of the PTB7-Fx Batches Studied polymer

batch no.

MW (kg mol−1)

MN (kg mol−1)

PDI

PTB7-F00 PTB7-F20 PTB7-F40 PTB7-F60 PTB7-F80 PTB7-F100

SX6−073 YY2−158 YY2−138 SX6−077 YY2−150 YY2−253

49 265 305 354 142 73

22.8 78.6 77.8 105 28.3 32.1

2.15 3.37 3.92 3.36 5.01 2.27

determined with high-temperature gel permeation chromatography. It is recognized that GPC measurements compared with polystyrene standards actually measure molecular volume. However, the difference in molecular volume caused by the substitution of hydrogen with fluorine atoms in these highly substituted polymers was assumed to be small, thus making the relative molecular weight measurements an appropriate proxy for absolute molecular weight. Figure 2a presents the optical absorption properties of neat PTB7-Fx thin films. Neat PTB7-Fx thin films in general exhibit a dominant absorption peak around 675 nm with a shoulder around 620 nm. Similar to P3HT and many other semiconducting polymers,29 the presence of this resolvable vibronic progression in the solid-state absorption spectra indicates the presence of polymer aggregation, which in the case of PTB7 is J-like. Of the batches investigated, PTB7-F100 has the largest peak at 675 nm relative to the 620 nm shoulder, followed by PTB7-F20. However, in general, all the neat PTB7-Fx thin films show similar absorption spectra; no systematic change in optical properties with fluorination is observed. (Note that the reduced vibronic structure in PTB7-F00 may be related to the low molecular weight of this polymer and hence reduced aggregation.) When blended with PC71BM, the absorption of the polymer is complemented by the fullerene to provide strong absorption over almost the entire visible range (Figure 2b). The absorption spectra of blend films were normalized to the 475 nm PC71BM peak. The PTB7-F00 blend shows weak absorption from 600 to 750 nm compared with the other batches, suggesting reduced aggregation that may be due to the significantly lower molecular weight of this batch compared to the other batches. This is consistent with the low ordering as observed with WAXS detailed below. When the ratio of the 675 nm peak height to the 620 nm peak height is examined, the F20 and F40 batches appear to have the highest degree of 9920



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Figure 2. Optical absorption spectra of neat films of PTB7-Fx (a) and films of PTB7-Fx blended with PC71BM (b). The data in panel a has been normalized to the peak at 620 nm, whereas the data in panel b has been normalized to the PC71BM peak at 475 nm.

Figure 3. (a) Current−voltage characteristics of polymer solar cells under AM1.5 condition with a light intensity of 100 mW/cm2. (b) External quantum efficiency of PTB7-Fx:PC71BM solar cells fabricated with 3 vol % DIO.

aggregation, followed by F100, F80, F60, and F00. Interestingly the variation of aggregation signature is not monotonic with fluorine content, suggesting other parameters are at work. Figure 3a,b presents the characteristics of optimized devices based on blends with a weight ratio of 1:1.5 PTB7-Fx:PC71BM. Each polymer device was optimized with respect to active layer thickness and DIO content. The optimized performance metrics reported below were found to be repeatable over multiple device fabrications. The device performance parameters are summarized in Table 2. Most devices show good quantum efficiencies in the range between 400 and 700 nm. In the PTB7-F00 device, the EQE is only about 40% with a maximum EQE of 45.3% at 405 nm. The maximum EQE in the PTB7-F100 device is higher than that of PTB7-F00, with a typical EQE of 50% and a peak EQE of 58.4% at 395 nm, but lower than previous values reported for PTB7-F100.1,4 PTB7F20, F40, F60, and F80 devices exhibit even higher EQEs, which is reflected by the high short-circuit currents JSC, above 16 mA cm−2. The device based on PTB7-F80:PC71BM has the best performance, with a VOC of 0.69 V, a JSC of 16.1 mA cm−2, a FF of 51.0%, and a corresponding PCE of 5.7%. The PTB7F00 device in contrast has the lowest PCE, with almost half the JSC compared with that of the other PTB7 derivatives. Specifically, the PTB7-F00 device shows a JSC of only 8.3 mA cm−2, a VOC of 0.61 V, and a FF of 41.3%, with a corresponding PCE of 2.1%. This low JSC is consistent with the low EQE of F00 over the whole measured wavelength range. Similar to the optical properties, there is only a weak dependence of device parameters to fluorine content (Figure S1 of Supporting Information), indicating that other material parameters appear to be more important than the degree of fluorination. Significantly, these results demonstrate that a high photovoltaic performance, close to 5%, can be achieved with an

Table 2. Summary of Photovoltaic Parameters of PTB7Fx:PC71BM Solar Cells polymer

VOC (V)

JSC (mA cm−2)

FF (%)

PCE (%)


0.61 0.62 0.62 0.59 0.69 0.66

−8.3 −16.2 −16.2 −16 −16.1 −12.2

41.3 46.2 54.0 41.1 51.0 50.4

2.1 4.6 5.5 3.9 5.7 4.1

apparent fluorine content as low as 20% (actually shown below to be only 5.3% by independent measurement of F-content). It is noted that the fill factors (and hence overall device efficiencies) are lower than those reported for record PTB7based cells.1,30 Improved fill factors can be expected by further optimizing contacts and interfacial layers.30 As discussed below, the variation of short-circuit current density with material parameters will be of primary interest. Because the EQEs and short-circuit currents reported here are consistent with other high-efficiency PTB7-based cells, our results are relevant to state-of-the-art devices. Because of the weak link between the specified degree of fluorination and optical and device parameters, the actual fluorine content of the PTB7-Fx polymers was checked with Xray photoelectron spectroscopy (XPS); the results are summarized in Table 3 (see Figure S2 of Supporting Information for full XPS data). Surprisingly, it is clear that the actual fluorine contents as measured by XPS are in some cases far from the expected fluorine content and are typically lower than expected. Note that the highest molecular weight polymer shows the largest discrepancy. However, although the actual fluorine content is not systematically spread, there is still 9921



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their polymers was not separately measured and the molecular weights of their polymers were not reported. The surface topography of PTB7-Fx:PC71BM blend films as measured by atomic force microscopy (AFM) are shown in Figure 4a−f. For the PTB7-F00:PC71BM film, domains of ∼200

Table 3. Measured Fluorine Content of PTB7-Fx Polymers polymer

real F%


0 5.3 19.2 13.6 79.9 100

a sufficient absolute spread of fluorination to enable investigation of the influence of fluorine content on the properties of both films and solar cells. Notably, there is a large discrepancy between the specified and actual fluorination values in the case of PTB7-F20, F40, and F60. However, even when accounting for the real fluorine content values, a strong correlation between the degree of fluorination and device properties is still lacking, as shown in Figure S3 of Supporting Information. Nevertheless, the previous statement still holds that devices with low fluorine content can achieve good performance; the PTB7-F20 batch with only ∼5% fluorination shows a power conversion efficiency of nearly 5%. For convenience, the samples will still be termed “PTB7-Fx”. The HOMO levels of the polymers, determined by AC-2 measurement, are displayed in Table 4. For comparison, the Table 4. HOMO Energy Level Values of PTB7-Fx Measured by Riken Keiki AC-2 Photoelectron Spectroscopy; Difference between the HOMO Values of PTB7 Derivatives and LUMO Values of PC71BM Are Also Calculated polymer

EHOMO (eV)

EHOMO − ELUMO (eV)


−4.86 −4.94 −4.93 −5.01 −4.99 −5.34

−0.56 −0.64 −0.63 −0.71 −0.69 −1.04

Figure 4. AFM images of PTB7-Fx/PC71BM blends prepared with diiodooctane (DIO): (a) PTB7-F00:PC 71 BM, (b) PTB7F20:PC71BM, (c) PTB7-F40:PC71BM, (d) PTB7-F60:/PC71BM, (e) PTB7-F80:PC71BM, and (f) PTB7-F100:PC71BM. Note that part (a) has a height range (0 to 50 nm) different from that of the other images (0 to 10 nm).

HOMO value of P3HT is 4.65 eV under identical conditions;31 the LUMO level of PC71BM is ∼4.3 eV.32 Accordingly, the differences between the HOMO values of PTB7 derivatives and LUMO values of PC71BM are also listed in Table 4. In general, the HOMO level is found to increase with fluorination; the HOMO level is 5.34 eV for PTB7-F100 and 4.86 eV for PTB7F00. Note that while there are changes in the measured HOMO values we should not expect to see changes in the optical band gap as the LUMO is also lowered by fluorination.12,33 We also note that our value of 5.34 eV for the HOMO of PTB7-F100 is different from the value of 5.15 previously reported by Liang et al.4 This discrepancy is explained by the different measurement techniques used; the HOMO of the PTB7-F100 film was directly measured by in-air photoelectron spectroscopy compared to the solution-based cyclic voltammetry measurement of Liang et al. This lowering of the HOMO with fluorination, all else being equal, should therefore lead to a higher VOC as confirmed by Figure S3 of Supporting Information where VOC is plotted against measured fluorine content. While Figure S3 does support an improvement in VOC resulting from fluorination, there is still scatter in the data due to other factors. Interestingly, Wang et al.,19 who studied similar polymers, did not see systematic variation in VOC with fluorination although the actual fluorine content of

nm are clearly seen, in contrast to the surface of the other blend films which are largely smooth and featureless. The 200 nm domains are likely to be PCBM-rich clusters like those observed in PTB7-F100 films processed without the solvent additive DIO.4 As the molecular ordering of the active layer of polymer solar cells has been linked to device performance in multiple material systems,33−36 grazing incidence wide-angle X-ray scattering has been used to probe this aspect, with results presented in Figures 5a−f and 6a−f. For the neat PTB7-Fx films, a distinct in-plane peak is observed at qxy = 0.32 ± 0.01 Å−1 (see Figure S4 of Supporting Information for line profiles) corresponding to the (100) reflection with a lamellae spacing of 19.6 ± 0.6 Å. The (100) peak position of the PTB7-F60 sample is slightly different, at qxy = 0.30 ± 0.01 Å−1, suggesting a slightly larger periodic lamellae spacing of 20.9 ± 0.7 Å compared with that of other PTB7 derivatives. The observation that the (100) peak is found largely along qxy rather than qz implies that the polymer lamellae are predominantly stacking in a “face-on” polymer backbone orientation in which the π-conjugated polymer 9922



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Figure 6. 2D GIWAXS scattering patterns of blends of PTB7-Fx with PC71BM. Because of the missing wedge in GIWAXS, the qz direction provides only approximate qz values.

Figure 5. 2D GIWAXS scattering patterns of neat PTB7-Fx. Because of the missing wedge in GIWAXS, the qz direction provides only approximate qz values.

Table 5. Summary of Peak Positions and Spacings of PTB7Fx Crystallites in Neat Films

backbone planes lie parallel to the substrate surface. (010) peaks are found predominantly about the qz axis corresponding to a π−π stacking direction perpendicular to the substrate, consistent with a face-on orientation of polymer lamellae, which is different from most other bulk heterojunction organic photovoltaic materials, such as P3HT,37 but consistent with previous reports on PTB series polymers.38,39 The (010) peaks of PTB7-F20, F40, F60, and F80 are located at qz = 1.60 ± 0.01 Å−1. However, for PTB7-F00 and PTB7-F100, the (010) peaks are found at 1.56 ± 0.01 Å−1 and 1.62 ± 0.01 Å−1, respectively. The corresponding π−π stacking distances are 3.93 Å for PTB7-F20, 40, 60, and 80; 4.03 Å for PTB7-F00; and 3.88 Å for PTB7-F100. Table 5 summarizes all the peak positions and spacings. Although it is difficult to reliably compare the magnitude of the scattering intensities from sample to sample, there are significant differences suggesting variations in the relative degree of crystallinity. The F60 sample appears to be the most crystalline; the F00 sample is practically amorphous. Again, no systematic fluorination-dependent trends are found in the GIWAXS data. Furthermore, the broad scattering peaks indicate that the crystallites are relatively small (

Influence of Fluorination and Molecular Weight on the Morphology and

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