Impact of Acceptor Fluorination on the Performance of All-Polymer

Subscriber access provided by READING UNIV

Article

Impact of Acceptor Fluorination on the Performance of All-Polymer Solar Cells Kedar D. Deshmukh, Rukiya Matsidik, Shyamal Prasad, Naresh Chandrasekaran, Adam Welford, Luke A. Connal, Amelia C. Y. Liu, Eliot Gann, Lars Thomsen, Dinesh Kabra, Justin M Hodgkiss, Michael Sommer, and Christopher R. McNeill ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14582 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Impact of Acceptor Fluorination on the Performance of All-Polymer Solar Cells Kedar D. Deshmukh,a Rukiya Matsidik,b,c Shyamal K. K. Prasad,d Naresh Chandrasekaran,a,e,f Adam Welford,a Luke A. Connal,g Amelia C. Y. Liu,h,i Eliot Gann,a,j,k Lars Thomsen,j Dinesh Kabra,e Justin M. Hodgkiss,d Michael Sommer,b,c,l and Christopher R. McNeilla* a

Department of Materials Science and Engineering, Wellington Road, Clayton, Victoria, 3800, Australia.

b

Institut für Makromolekulare Chemie, University of Freiburg, Stefan-Meier-Str. 31, 79104 Freiburg, Germany

c

Freiburger Materialforschungszentrum FMF, University of Freiburg, Stefan-Meier-Str. 21, 79104, Freiburg,

Germany d

MacDiarmid Institute for Adv. Mater. and Nanotechnology, and School of Chemical and Physical Sciences,

Victoria University of Wellington, Wellington 6140, New Zealand e

Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India

f

IITB-Monash Research Academy, IIT Bombay, Mumbai 400076, India

g

Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia

h

Monash Centre for Electron Microscopy, Monash University, Clayton, Victoria 3800, Australia

i

School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia

j

Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia

k

Present address: Materials Science and Engineering Division National Institute of Standards and

Technology, Gaithersburg, Maryland 20899, United States l

Present address: TU Chemnitz, Polymerchemie, Straße der Nationen 62, 09111 Chemnitz, Germany

* Corresponding author. Email: [email protected]

KEYWORDS: All-polymer solar cells, planarization, fluorination, morphology, photo-physics, GIWAXS, R-SoXS.

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Here we systematically study the effect of fluorination on the performance of all-polymer solar cells employing a naphthalene diimide (NDI) based polymer acceptor with thiophene-flanked phenyl co-monomer. Fluorination of the phenyl co-monomer with either 2 or 4 fluorine units is used to create a series of acceptor polymers with either no fluorination (PNDITPhT) bi-fluorination (PNDITF2T) or tetra-fluorination (PNDITF4T). In blends with the donor polymer PTB7-Th, fluorination results in an increase in power conversion efficiency from 3.1 % to 4.6 % despite a decrease in open circuit voltage from 0.86 V (unfluorinated) to 0.78 V (tetra-fluorinated). Countering this decrease in open-circuit voltage is an increase in short circuit current from 7.7 mA/cm2 to 11.7 mA/cm2 as well as an increase in fill factor from 0.45 to 0.53. The origin of the improvement in performance with fluorination is explored using a combination of morphological, photophysical and charge transport studies. Interestingly, fluorination is found not to affect the ultrafast charge generation kinetics but instead is found to improve charge collection yield subsequent to charge generation, linked to improved electron mobility and improved phase separation. Fluorination also leads to improved light absorption, with the blue-shifted absorption profile of the fluorinated polymers complementing the absorption profile of the low-band gap PTB7-Th.

Introduction Recent advances in polymer solar cells have pushed the efficiency of single-junction polymer solar cells to over 13%.1 All-polymer solar cells, which employ an electron accepting polymer have also shown rapid progress in the past couple of years with efficiencies doubling to over 9%.2 While all-polymer solar cells have historically lagged their fullerene and small molecule counterparts, they have certain advantages in including better absorption, mechanical flexibility and control over viscosity in printing applications.3 There has been significant process recently in understanding the morphology of the all-polymer solar cell with studies attributing the improvement in performance to improved control of intermixing of donor and acceptor phases, increased face-on stacking of the backbone and optimum domain sizes.4-9 The recent increase in the efficiency of all-polymer solar


Page 2 of 44

Page 3 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cells can be attributed mainly to the successful pairing of low-band gap donor polymers and in particular NDI-based acceptor polymers such as P(NDIT2OD-T2) that were initially developed for application in n-channel transistors.10-13 An increasing number of studies have reported new polymer acceptors based on modifications to the chemical structure of NDI (or perylene diimide) polymers resulting in varying degrees of success.14-21 In a recent study, Li et al. for example reported the synthesis of a derivative of P(NDI2OD-T2) with a certain fraction of repeat units having only a single thiophene (T) unit, realized by synthesizing a series of random co-polymers PNDI-Tx (where x stands for the percentage of single thiophenes relative to total donor units).20 The best efficiency of 7.8% with a fill-factor of 71% was obtained for the PTB7-Th:PNDI-T10 blend (with x = 10%) optimised with solvent vapour annealing. The authors attributed the high PCE to the balanced charge mobilities that effectively suppress bimolecular recombination. In another study, Yan et al. synthesized a perylene diimide based polymer with vinyl linkers (PDI-V) and paired it with PTB7-Th to yield an efficiency of 7.6% without any solvent additives.21 The high efficiency in this case was attributed to the planar polymer backbone and the high electron mobility. Halogenation – and in particular fluorination – of the donor polymer backbone has been successfully employed to enhance the device performance of polymer solar cells and to improve the performance of polymer-based organic field-effect transistors. Fluorination of the polymer backbone has been shown to improve charge carrier mobility by promoting backbone planarity, optimising molecular energy levels for charge generation, and helping to improve morphology.22-31 Fluorination of polymer donors with electron withdrawing fluorine atoms also improves cell VOC by lowering the HOMO energy level of the donor.32, 33 Albrecht et al. for example reported an improvement of all the J-V parameters upon fluorination of the donor polymer PCPDTBT.34 Cells based on fluorinated PCPDTBT were also found to show a weaker field dependence of photocurrent generation which was attributed to suppressed geminate recombination due to an optimal phase separated morphology. Furthermore, reduced non-geminate recombination was observed and attributed to improved charge transport. Son et al. have reported a study on increasing the level of fluorination of the PTBF



polymer family.35 They found that monofluorination of the thienothiophene led to the best results while perfluorination of the polymer backbone led to poor compatibility with the fullerene acceptor used leading to poorer performance. In another study, Stuart et al. observed an improvement in the VOC and FF and a very slight increase in the JSC through the fluorination of the polymer PBnDT-DTBT.29 The authors attributed this improvement to reduced bimolecular recombination with GIWAXS and RSoXS analysis pointing to improved crystalline ordering and the creation of pure domains on the size scale of 100 nm, aiding the charge transport. On the other hand, fluorination in some studies has not been an effective strategy.22, 36 In contrast to the numerous studies on fluorinating donor polymers, much less effort has been directed into fluorination of acceptor polymers.5, 37-41 Jung et al. for example were the first to show the effect of fluorination of the thiophene groups in the polymer P(NDI2OD-T2) along with modification of the side-chain length. The authors reported a significant increase in fill-factor and modest gains in short circuit current density and the open circuit voltage.5, 37 Fluorination was found to increase the bandgap of P(NDI2OD-T2) and surprisingly led to a very slight improvement in VOC from 0.79 V to 0.81 V. The increase in FF from 0.56 to 0.63 was accompanied by balanced charge mobilities in the blend. Simultaneous modifications to the side chain lead to an increase in the JSC in addition to improvement in VOC and FF. The authors concluded that fluorination results in improved ordering and more face-on oriented crystallites, suppressing bimolecular recombination. Uddin et al. reported the same polymer acceptor PNDI2OD-T2F with a different polymer donor to achieve an efficiency of 6% due to significant improvement in the short circuit current and fill-factor.5 When the fluorinated PNDI2OD-T2F was blended with PBDTTTPD, the efficiency increased from 2.2% to 6%. Transient Absorption Spectroscopy pointed to longer lived polarons and faster hole transfer due to improved HOMO/HOMO energy offsets. In addition, fluorination was shown to improve the morphology via better intermixing as evident from the lack of distinguishing size scale feature in RSoXS data. In the articles reporting the non-NDI based fluorinated acceptors, two new acceptor strategies have been discussed. Long et al. reported a new double B-N bridged bipyridine based


Page 4 of 44

Page 5 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


acceptor, which was paired with PTB7-Th to achieve an efficiency of 6.2% with a high VOC of 1.07 V due to higher energy CT states.38 A more recent article by Liu et al., employed a fluorinated thieno pyrrole dione (TPD) based acceptor with PTB7-Th as donor to fabricate solar cells with over 4% efficiency.39 Their DFT analysis revealed that fluorinating the backbone lead to planarization, with further modifications to the side chains resulting in balanced charge mobilities due to the induced morphological changes. In this study, we systematically investigate the effect of fluorinating the NDI-based acceptor polymer PNDITPhT (see Figure 1). Bi-fluorination or tetra-fluorination of PNDITPhT is achieved by substituting either two or all four H atoms of the phenyl ring to produce either PNDITF2T or PNDITF4T.42, 43 The performance of these polymers as acceptors in all-polymer solar cells is assessed by blending with the donor polymer PTB7-Th. With fluorination a systematic decrease in the LUMO energy results, accompanied by a decrease in cell VOC from 0.86 V (no fluorination) to 0.76 V (tetrafluorination). However overall solar cell performance is found to increase as a function of the degree of fluorination with the PTB7-Th:PNDITF4T blend showing the best performance with a 4.8 % power conversion efficiency. A detailed exploration of the reasons for the improvement in the performance is provided by employing various morphological characterization techniques combined with photophysics and device physics, examining the photocurrent generation process from charge generation to charge transport and collection. We note that similar polymer have recently been reported by other groups.40,

41

However by performing in-depth microstructural, photophysical and charge

transport studies we are able to more definitively comment about the origin of the improvement in efficiency with fluorination.

Results and discussion Materials Figure 1 presents the chemical structures of the three acceptor polymers used in this study. All polymers were synthesized via direct arylation polymerization (DAP) according to previously



reported methods,42, 43 with synthetic details provided in the experimental section. PNDITPhT is similar to the well-studied P(NDI2OD-T2) differing in the insertion of a phenyl ring between the two thiophene rings. PNDITF2T has two fluorine atoms substituted on the phenyl ring while PNDITF4T has all four hydrogen atoms substituted for fluorine atoms. Molecular weight analysis provided by high-temperature gel permeation chromatography (HT-GPC), table 1, indicates that the number average molecular weight, Mn, for each polymer is in the range of 36 to 53 kDa, with values from 1H NMR analysis in the range of 24 to 30 kDa. 1H NMR analysis also shows that all polymers are predominantly terminated by the thiophene-based monomer at either side, which is explained by the mechanism of DAP which involves an intramolecular oxidative addition step of the Pd catalyst into the second C-Br function of the monomer NDIBr2.44 The dispersity of the samples varies somewhat from 3.16 for PNDITPhT to 1.56 for PNDITF4T. Interestingly, higher dispersity values are associated with lower values of Mn meaning that the samples have similar values of weight-average molecular weight, which is arguably more important in determining phase separation (the longer chains in the sample will interact with each other sooner during solvent evaporation meaning that they drive phase separation more than shorter chains). Thus in general the acceptor polymers have similar molecular weight which is important when comparing the performance of acceptor polymers.45 All three polymers showed high thermal stability having onset temperatures of around 440 °C (table 1). Melting and crystallization temperatures (Tm, Tc) and corresponding enthalpies (∆Hm, ∆Hc), taken from the second heating and first cooling cycles, respectively, gradually increased with increasing number of fluorine atoms in the polymer backbone (Table 1). Figure 1 (d) presents the energy levels of these three acceptor polymers displayed alongside those of the donor polymer PTB7-Th. Consistent with previous studies,37, 38, 41 fluorination of the acceptor polymer has a stronger effect on the energy of the HOMO than on the energy of the LUMO. In particular the HOMO is shifted from -5.7 eV to -5.9 eV to -6.1 eV going from PNDITPhT to PNDITF2T to PNDITF4T (a total change of 0.4 eV) while the LUMO is only shifted from -3.76 eV to 3.81 eV to -3.87 eV (a total change of only ~ 0.1 eV). The new acceptor polymers – both neat and in


Page 6 of 44

Page 7 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


blend – show increased amount of aggregation with increasing amount of fluorination. The F4 neat polymer and blends show gelation in solution occurring due to non-covalent interactions i.e. aggregation of the fluorinated chains.46 At room temperature the solution attains an extremely high viscosity due to the stronger intermolecular interactions. As the solution temperature is increased the aggregation is revered resulting in a homogenous solution from which thin films are spin coated.47 To ensure smooth films, the substrate has to be heated to 70 °C. Optical properties Figure 2 presents the ultraviolet to visible to near-infrared absorption spectra of neat films and blends normalized to film thickness (see also intensity normalized plots in figure S4). The three acceptor polymers all have a π to π* absorption band located at ~ 370 nm along with a chargetransfer absorption band located in the visible to near-infrared consistent with other donor-acceptor copolymers. As anticipated in the above discussion of the energy levels of these materials, there is a strong blue-shift in the lowest energy charge transfer band with fluorination. The dominant absorption peak in the charge-transfer absorption band shifts from around 680 nm for PNDITPhT to 630 nm for PNDITF2T and further to 590 nm for the PNDITF4T. Also shown for reference in Figure 2 (a) is the absorption spectrum of neat PTB7-Th, which – by virtue of being a low band-gap polymer – absorbs in a broad range of wavelength from 790 nm to about 550 nm, and has its lowest energy vibronic peak at 715 nm.48 Figure 2 (b) shows the absorption spectra of polymer blends based of PTB7-Th as donor blended with either PNDITPhT or PNDITF2T or PNDITF4T as acceptor. The blend absorption spectra are dominated by the features of PTB7-Th as it has a stronger absorption intensity than the three acceptor polymers. Differences in the absorption spectra of the three blends are evident due to the variation in the absorption profile of the acceptor. As the absorption of the acceptor polymer blueshifts with fluorination, the absorption intensity in the near-infrared end of the spectrum decreases, with an increase in absorption intensity in the visible observed for the PTB7-Th:PNDITF4T blend.



Thus the absorption profile of PNDITF4T can be regarded as being complementary to that of PTB7-Th, helping to fill out the absorption spectrum in the visible region (see also figure S4). Photovoltaic properties and charge mobility All-polymer solar cells were fabricated with an inverted architecture with indium tin oxide as the electron collecting electrode combined with ZnO and PEIE interlayers on top. MoOx and Ag were used to form the top, hole collecting contact. Details of device fabrication are provided in the experimental section. For all devices, the active layer was spin-coated from a chlorobenzene solution with a donor:acceptor ratio of 1:1 by weight and with the thickness of the active layer optimized for the best performance (with thickness varying between 95 nm and 105 nm, see table 2 for details). Figure 3 presents the light current-voltage (J-V), external quantum efficiency (EQE) and dark J-V data for devices fabricated out of the three blends. For each device the J-V curves were obtained under 1 Sun (AM1.5G) conditions with the device characteristics summarized in table 2. For the sake of convenience, the blends will be referred to as Ph (PTB7-Th:PNDITPhT), F2 (PTB7-Th:PNDITF2T) and F4 (PTB7-Th:PNDITF4T). Strikingly, despite a decrease in open-circuit voltage, fluorination of the acceptor polymer leads to an increase in power conversion efficiency (PCE) from 3.1 % for Ph to 3.7 % for F2 to 4.5 % for F4. The observation that the F4 polymer yielded the highest efficiency is in contrast to the results of Li et al. who observed a sharp drop in efficiency going from bi-fluorination to tetra-fluorination, possibly due to differences in molecular weight, or strong solution aggregation of the tetrafluorinated polymer during processing.41 VOC decreases from 0.86 V for the Ph blend to 0.80 V for the F2 blend and finally to 0.78 V for the F4 blend. This decrease in VOC of 0.08 V going from Ph to F4 is roughly consistent with the corresponding change in LUMO energy of 0.11 eV. The observed changes in VOC with fluorination does not perfectly correspond with the measured changes in LUMO energy, with VOC changing by only 0.02 V going from the F2 blend to the F4 blend with the LUMO energy changing by 0.06 eV. However VOC is affected by many factors including charge density and


Page 8 of 44

Page 9 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


recombination rates.49 Indeed, examining the dark J-V curves (figure 3 (c)) the turn-on voltage for injection (which will also be largely affected by changes in the LUMO values) changes from 0.68 V for the Ph blend to 0.61 V for the F2 blend and 0.49 V for the F4 blend. Nevertheless, the decrease in VOC with fluorination is modest, with the overall improvement in device performance derived from substantial improvements in FF and JSC in contrast to the commonly observed improvement in open circuit voltage dominating the improvement in cell performance when fluorinating the donor polymer.28, 50 FF is found to increase from 0.45 for Ph to 0.52 for F4, with JSC increasing from 7.7 mA/cm2 for Ph to 11.7 mA/cm2 for F4. Peak EQE also systematically increases with fluorination from 40 % for Ph to 47 % for F2 to 55 % for F4. The shape of the EQE spectrum also broadens with fluorination, see figure S4 (c), attributed to the complementary absorption profile of PNDITF4T. Comparing the shape of the EQE spectra with the UV-Vis data (Figure 2) it is evident that improvement in light absorption with fluorination cannot fully explain the observed increase in JSC even though the F4 blend certainly benefits from the complementary absorption profile of PNDITF4T. Indeed, despite the F4 blend having a similar absorption strength at ~ 700 nm, the F4-based device has a significantly higher EQE at this wavelength. The F4 device also has a slightly smaller active layer film thickness than the other blends meaning that the increase in EQE in the F4 device cannot be due to an increase in light absorption alone. Rather, fluorination must be either improving the charge generation efficiency, the charge separation efficiency or the charge collection efficiency (or a combination of the above). The study of Li et al.41 – that also investigated the effect of fluorination in this series of acceptor polymers – found that the bifluorinated polymer optimised solar cell performance, with a PCE of 2.5% achieved for the corresponding F2 blend. Here however we find that the tetrafluorinated F4 maximises performance. The performance metrics observed here (VOC, JSC, FF) in general are also higher than those reported by Li et al., with a maximum PCE of 4.5 % achieved for the tetrafluorinated blend. The reason for the lower performance metrics of Li et al. could be due to their use of a ‘standard’ device architecture with an inverted device architecture found to give better



cell performance in our experiments. Differences in the molecular weight of PTB7-Th used could also be responsible, however Li et al. do not report this detail. Indeed, the observation by Li et al. that solar cell performance decreased going from F2 to F4 is likely to be related to the lower molecular weight of their F4 polymer (Mn ~ 39 kg/mol ) compared to their F2 polymer (Mn ~ 51 kg/mol). To understand if the enhanced JSC and FF could be affected by changes in charge mobility, we have calculated the electron and hole mobilities in the three blends using Space Charge Limited Current (SCLC) measurements.51 The space-charge limited current was verified using three different thickness for each blend and ensuring that mobilities were consistent for the blends (see Figure S5 in the Supplementary Information). The values for hole and electron mobility are provided in table 3. The hole mobility of the blends does not vary much with values of µhole = 4.9 × 10-4 cm2/V.s, 3.7 × 104

cm2/V.s and 7.6 × 10-4 cm2/V.s recorded for the Ph, F2 and F4 blends respectively. However the

electron mobility shows an increase with the level of fluorination. In particular the electron mobility is observed to increase from 8.4 × 10-5 cm2/V.s for the Ph blend to 1.1 × 10-4 cm2/V.s for the F2 blend and to 4.3 × 10-4 cm2/Vs for the F4 blend, representing a five-fold increase in electron mobility going from the Ph blend to the F4 blend. This increase in mobility with fluorination is attributed to improved planarization of the backbone of the fluorinated polymers as observed for other systems.39, 52

Morphology To examine the potential influence of differences in morphology on cell performance, thinfilm microstructure has been assessed using a combination of Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectroscopy, Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) and Resonant Soft X-ray Scattering (R-SoXS). Together these techniques enable insight into film morphology, domain size, film crystallinity and surface composition. The morphology data is then combined with


Page 10 of 44

Page 11 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


photoluminescence quenching measurements to provide a comprehensive understanding of the blend microstructure. Figure 4 presents the AFM and TEM images of the blends The AFM images of the neat polymers are provided in Figure S6. Neat films of PNDITPhT, PNDITF2T (and to a lesser extent PNDITF4T) exhibit a fibrillar nature with evidence of liquid crystalline ordering. These surface features are distinct to that exhibited by PTB7-Th which does not show any evidence of long-range correlated ordering. The neat PNDITF4T film shows a higher surface roughness which may be taken as evidence of increased aggregation. The surfaces of the blends exhibit less of a fibrillar nature pointing to the likelihood of a PTB7-Th rich surface and/or reduced ordering when in blend. The film surface roughness increases for the F4 blend (2.1 nm) as compared to 0.81 nm for Ph and 0.82 nm for F2 suggesting a coarser underlying morphology. TEM images for all the blends reveal a relatively intermixed morphology with a lack of overly coarse phase separation reported in earlier all-polymer systems.10 Fluorination appears to lead to clearer contrast with the F4 blend exhibiting distinct dark features on the scale of 50-100 nm suggesting a more phase-separated morphology. Hints of domain structure on a similar length scale are seen in the Ph and F2 blends although the contrast in the TEM images is not as sharp suggesting that these domains are less pure than in the F4 blend. Photoluminescence quenching and R-SoXS measurements presented below provide further evidence for domain coarsening with fluorination. We also note some small black dots in the TEM images which we attribute to acceptor aggregates / crystallites.10 Surface-sensitive NEXAFS spectroscopy was performed to probe the chemical composition and the polymer chain backbone tilt of the top layer and buried interfaces, which are in contact with the charge transport interlayers. The buried interface was exposed by floating off the film in deionised water and picking up top side down on a doped Si wafer. The chemical composition of the surface was determined by fitting the blend spectrum with the individual spectra of the donor and



acceptor polymers. The average tilt angle of the C 1s → π* transition dipole moment was also determined by measuring the angular dependence of the NEXAFS spectra. For further details the reader is referred to previous work.53 Table 4 provides a summary of the NEXAFS parameters obtained from fitting to the data, with the NEXAFS scans presented in the supporting information. All the blends in discussion are rich in the donor polymer at the top surface, as corroborated by AFM scans. The F4 blend in particular has a surface composition of 91% PTB7-Th that should benefit from an inverted architecture where holes are collected at the top electrode. The buried interface exhibits a mild enrichment of the acceptor polymer for the Ph and F2 blends (60 and 70 wt.% respectively) with the F4 blend showing a slight enrichment in PTB7-Th (40 wt.%). All blends show similar average tilt angles of the conjugated backbones at both top and buried interfaces. In particular, the polymer chains at the top and bottom interfaces do not show a strong preference for edge-on or face-on orientation. Thin film crystallinity is next assessed with GIWAXS. Figure 5 presents the two-dimensional GIWAXS images of the neat polymers with 1-D scattering profiles along the in-plane (IP) and out-ofplane (OOP) directions shown in Figure S9. The neat Ph polymer shows a semicrystalline microstructure similar to P(NDI2OD-T2) with lamellar, backbone and π-π stacking peaks evident. Neat F2 films show similar scattering features but with narrower lamellar stacking and π-stacking peaks indicating slightly larger crystallites. Details of the analysis of the neat polymers are provided in Table S1 in the supplementary information. With further fluorination to F4 the scattering features become less sharp indicating a decrease in crystalline order along the lamellar and π-stacking directions. Fluorination is found to produce smaller lamellar stacking distances, with the d-spacing of the (100) peak decreasing from 2.33 nm (neat Ph) to 2.29 nm (neat F2) to 2.24 nm (neat F4). Fluorination also produces a slight decrease in the π-π stacking distance decreasing from 0.37 nm for neat Ph to 0.35 nm for neat F2 and F4. In comparison neat PTB7-Th shows broader scattering peaks with a (100) d-spacing of 2.4 nm and π-π stacking distance of 0.39 nm. Note that the GIWAXS results presented are of as-cast (not annealed) thin-films which are far from equilibrium. Hence while


Page 12 of 44

Page 13 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


enthalpy values from DSC suggest an increase in the degree of crystallinity with increasing fluorination, this is not reflected in the GIWAXS data of as-cast thin films where the degree of crystallinity is limited by the kinetics of molecular packing during solution deposition. The GIWAXS patterns of the blends, Figure 5 (b, d, f), show scattering features consistent with features seen in the neat patterns of the donor and acceptor polymers, indicating that the polymers crystallise separately. Due to the similar lamellar and π-stacking d-spacings of the donor and acceptor polymers it is not easy to separately extract information about the d-spacing and coherence lengths of the donor and acceptor polymers in the blend. Table 5 presents the results of fitting the composite (100) and π-stacking peaks to single Gaussian peaks. The d-spacing of the composite lamellar peak in the blends is closer to that of the spacing of the neat fluorinated polymers than PTB7-Th suggesting that the scattering patterns are dominated by the more crystalline acceptor polymers. The lamellar d-spacing of the Ph blend is slightly lower than that of the F2 blend at 2.27 nm compared to 2.29 nm. This spacing decreases in the F4 blend to 2.24 nm consistent with that of neat F4. The coherence lengths of the blends are in general lower than that of the neat acceptor films which could either be due to smaller crystallites in the blend or a broadening of the peaks due to scattering from less-ordered PTB7-Th. The d-spacings and coherence lengths corresponding to the π-π stacking peak are similar for the three blends. The Hermann’s S parameter has also been calculated based on the χ-dependence of the (100) peak intensity using the following equation:54 S=

χ χ χχ 3 χ χχ

− 1 (1)

S runs from 1 for perfectly edge-on crystallites to -0.5 for perfectly face-on crystallites. The S values corresponding to GIWAXS images taken at the critical angle are summarised in Table S1 (neat films) and Table 5 (blends). S values have also been calculated as a function of X-ray angle of incidence (α) and are plotted in Figure S10 (neat films) and Figure 6 (c) (blends). For both neat films and blends,



the F2 samples show the most pronounced face-on orientation with bulk S values of -0.18 (neat F2) and -0.25 (F2 blend). The more pronounced face-on orientation of the blends in general compared to neat acceptor films can be attributed to the greater propensity of PTB7-Th to pack face-on with an S value of -0.29 for neat PTB7-Th donor. Interestingly neat films of F4 exhibit a pronounced angle dependence of S (Figure S10) with S varying from S = 0.4 for low incident angles to S = -0.15 for high incident angles. Such an observation is consistent with a layer of edge-on oriented chains at the top surface on top of a face-on bulk. To get a better picture of the blend film morphology, Resonant Soft X-ray Scattering was employed to give information regarding domain spacing and purity.4 The data shown in Figure 7 was collected at an energy of 286.8 eV, where there is a maximum scattering contrast between the donor and acceptor phases. Data is also shown that was acquired at the non-resonant energy of 270 eV where scattering contrast is largely due to electron density differences. The corresponding size scale (top axis) is calculated as lc= 2π/q, where q is the q value on the bottom axis. All samples exhibit relatively broad scattering profiles indicating a large range of sizes, ranging from 10 nm through to 200 nm. There are key differences in the scattering profiles with increasing degree of fluorination which are characterised by (i) an increase in scattering intensity at low-q, and (ii) an overall increase in scattering intensity. These changes can be interpreted as reflecting (i) a coarsening of morphology with increasing fluorination leading to larger features that produce more scattering a lower q values, and (ii) an increase in phase purity. These observations are consistent with the TEM results – where an increase in contrast is seen in the F4 blend – and with previous RSoXS studies of other systems that have shown that fluorination leads to larger and purer domains.29 Finally, photoluminescence (PL) quenching measurements have been used to provide further information about domain size and purity. PL was measured of neat polymers films and thin film blends with excitation wavelengths of 532, 625 and 715 nm. By the virtue of their light absorption (Figure 2) it can be seen that wavelengths 532 nm and 625 nm, excite both the donor and


Page 14 of 44

Page 15 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


acceptor phases and the wavelength 715 nm mainly excites the donor phase in the blend. For the F4 blend the complementary absorption profiles mean that exciting at 715 nm exclusively probes the donor phase while exciting at 532 nm largely excites the acceptor phase. Figure 8 presents the calculated PL quenching values. In general, quenching values of greater than 90% are observed indicating efficient exciton dissociation. The Ph blend exhibits relatively high quenching values (~ 94 %) for all excitation wavelengths with the PL quenching values reducing slightly to about 93 % for the F2 blend. The data for the F4 blend shows interesting differences where a much lower quenching fraction of 90 % is observed for 715 nm excitation compared to 95 % for 532 nm excitation. This observation suggests purer donor phases than acceptor phases; whether this situation only exists for the F4 blend compared to the other blends is hard to untangle with the absorption bands of these acceptor polymers overlapping with that of PTB7-Th. Looking at the 715 nm quenching data, the systematic decrease in quenching efficiency when probing the donor phase is consistent with the increase in domain size and decrease in domain purity observed by R-SoXS and TEM. Charge generation Figure 9 presents the results of transient absorption (TA) spectroscopy experiments that resolve the charge generation and recombination kinetics in each of the blends after excitation at 532 nm. Our experimental setup allows us to collect both short (1 ns) time delays via two different pump-probe configurations. Figure 9 (a) shows a series of TA spectra for PTB7-Th:PNDITF4T, which are nearly identical to the blends with the Ph and F2 acceptor polymers (Supporting Information Figure S11). The TA spectra are dominated by signatures of the donor polymer, and they reveal conversion from excitons to charge pairs; excitons have a photoinduced absorption peak around 0.8 eV, charges have a photoinduced absorption peak around 1.1 eV, along with a sub-gap peak around 1.5 eV that is likely due to electroabsorption. Both types of excitation also exhibit ground-state bleaching from around 1.6 – 2.3 eV.



Charge generation and recombination dynamics were evaluated by bi-linearly decomposing each of the TA surfaces into pairs of spectra and kinetics. This linear algebra approach can produce infinite pairs of spectra and kinetic solutions related via vector rotation. However, by using the known exciton and charge spectra as spectral masks, we ensured that the spectra and kinetics of species correspond to real species, and we used the ground-state bleaching intensity as an internal reference to normalise the populations. Figure 9 (b) shows the charge dynamics for each of the three blends, from the subpicosecond to microsecond timescale. In all cases, approximately half of the charge population appears promptly after photoexcitation, with the remaining charges formed following exciton diffusion on the picosecond timescale before recombination sets in from around 100 ps. The only small difference between these kinetics profiles is that the yield of prompt charge generation is slightly lower with higher fluorine content in the acceptor polymer, which is an expected result of improved phase separation. The charge recombination kinetics for each of the blends are indistinguishable at this fluence. Thus, we conclude that the charge generation and recombination dynamics vary little between the three blends and do not account for the observed differences in device efficiencies. Charge collection and recombination To study the kinetics of charge collection subsequent to charge separation, transient photocurrent measurements have been performed, see Figure 10. Figure 10 presents the photocurrent response of devices to a 200 µs square pulse of 635 nm light. Figure parts (a, c, e) present the response as a function of applied bias, while parts (b, d, f) present the response as a function of pulse intensity at short-circuit. The F4 blend shows a much ‘squarer’ response indicating a steady response after turn-on, while the Ph blend show a decrease in photocurrent due to recombination on the time frame of tens to hundreds of nanoseconds. The magnitude of the photocurrent decrease after turn on increases with light intensity and is also more prominent closer


Page 16 of 44

Page 17 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to open-circuit conditions, see Figure S12 for normalized traces. Previous modeling of the transient response of polymer solar cells has linked such light-intensity dependent recombination on this time scale to recombination due to the slow extraction of charges from the device.55, 56 The improved charge collection efficiencies with increasing fluorination can be linked to the improved electron mobility with fluorination, and potentially to the improved morphology.

Discussion Our results indicate that fluorination can be a viable strategy for improving the performance of all-polymer solar cells. In spite of a reduction in VOC, significant gains in fill-factor and short-circuit current push the efficiency to 4.6% for the F4 blend. With fluorination of the PNDITPhT polymer, a drop in VOC is observed, going from 0.86 V to 0.82 V for the F2 and to 0.77 V for the F4 blends. The drop in VOC can be associated with a change in the LUMO level, since the VOC is strongly influenced by the difference in the LUMO of the acceptor and the HOMO of the donor polymer.57, 58 This decrease in VOC is compensated for by significant increases both JSC (increasing from 7.7 mA/cm2 to 11.7 mA/cm2) and FF (increasing from 0.45 to 0.53). The improvement in JSC can be partially explained by the improved in light absorption with fluorination, with the blue-shifted absorption of the fluorinated polymers providing a more complementary absorption profile to that of PTB7-Th. However improvement in light absorption alone cannot explain the increase in JSC. Interestingly, from the transient absorption spectroscopy data, fluorination does not seem to influence charge generation dynamics on the ultrafast time frame. In principle one may have expected that the increased LUMO/LUMO and HOMO/HOMO separation that accompanies fluorination may have aided charge separation due to the formation of a more energetic CT,59 however on the timescales probed by transient absorption spectroscopy little changes were seen. Therefore any changes in the energetic landscape at the donor/acceptor interface are likely to be minimal or have little effect on charge generation. From the mobility data and transient photocurrent data, the improvement in JSC and fill-factor can be attributed instead to differences in the efficiency of charge collection



Page 18 of 44

subsequent to charge separation. Fluorination leads to an increase in electron mobility in the blend from 8.4 × 10-5 cm2/Vs to 4.3 × 10-4 cm2/Vs, with the transient response of the fluorinated cells showing suppressed recombination features. The improved charge collection efficiency may also be related to an improved morphology with the fluorinated blends having a coarser phase-separated morphology as indicated by TEM, R-SoXS measurements and PL quenching measurements. Interestingly, fluorination may actually cause an increase in the miscibility of PTB7-Th in the acceptor, with the PL quenching efficiency of the acceptor phase actually increasing with fluorination, despite the coarsening of the morphology observed by TEM. This increased mixing in the acceptor phase does not seem to affect charge collection, possibly aided by an increased mobility of the acceptor polymer. Indeed, recent studies on organic field-effect transistor (OFET) materials have found that a highly crystalline microstructure is not necessary for high mobilities.60, 61 Instead, low degrees of energetic disorder promoted by a high degree of backbone planarity

appear to be more

important.60 Thus for the fluorinated polymers studied here, the higher mobilities achieved by fluorination can lead to overall improvement in solar cell performance even though this leads to a decrease in crystalline order. It is also interesting to note that the crystallinity of the blends does not seem to play a significant role. In the F4 blend – which shows the best performance –reduced crystalline ordering compared to F2 blend is observed. The crystallites in the F4 blend also show a reduced face-on orientation compared to the F2 blends indicating a limited influence of microstructural order and orientation on overall cell performance.

Conclusions We have systematically studied the effect of acceptor fluorination on the performance of allpolymer solar cells. A systematic increase in power conversion efficiency was observed with increasing degree of fluorination driven by improvements in FF and JSC., even though fluorination led to a decrease in VOC from 0.86 V to 0.77 V. The increases in FF and JSC were attributed to improvements in light absorption and charge collection, with the fluorinated polymers having complementary absorption profiles to the low band-gap donor and higher electron mobilities. The


Page 19 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


improved charge collection yields were also associated with an improved morphology, with fluorination leading to coarser domains. Interestingly the charge generation dynamics on short time scales were not affected by fluorination, with thin-film crystallinity also not playing a dominant role.

Experimental Details Materials: The thiophene-flanked phenyl monomers bearing F atom with varying number from 0 to 4 were synthesized via direct arylation using previously reported protocol.62 All polymers were synthesized using DAP according to a previously published report.43 A typical procedure exemplified by PNDITF2T is as following: To a high-pressure SEC vial equipped with a magnetic stir bar, 197.51 mg (0.18 mmol) NDIBr2, 50.10 mg (0.18 mmol) TF2T, 74.63 mg (0.54 mmol) K2CO3, and 18.38 mg (0.18 mmol) pivalic acid were carefully added. Under N2 atmosphere 0.6 ml degassed 1chloronaphtahlene (CN) was added. After stirring the mixture at room temperature for 5 min, 1.65 mg (1.8 ×10-3 mmol) Pd2dba3 was added and the reaction vial was sealed and placed into a preheated oil bath at 90 °C. After 24 h, the reaction mixture was cooled to room temperature, further diluted with chloroform and precipitated into methanol (200 ml). The precipitated polymer was filtered off and purified by Soxhlet extraction using acetone, ethyl acetate, and iso-hexanes and finally collected with chloroform. The chloroform polymer solution was filtered through a short silica plug. The chloroform was removed and the polymer was dried under high vacuum overnight to give 214 mg polymer in 98 % yield. PNDITPhT was prepared using the same procedure as PNDITF2T in 98 % yield. For the synthesis of PNDITF4T mesitylene was used as reaction solvent at 120 °C and the polymerization was carried out for 5.5 h. Here, Soxhlet extraction was done using acetone, ethylethetate, iso-hexanes, and chloroform. Finally the polymer was collected with chlorobenzene. After solvent evaporation and drying, the polymer was collected in 96 % yield. The donor polymer PTB7-Th was purchased from 1-Materials Inc. (batch no. YY8162) with a molar mass of Mn = 25 kDa and Đ= 3.6.



Material characterisation: Molecular weights of polymer were characterized by high-temperature gel permeation chromatography (HT-GPC) performed in 1,2,4-trichlorobenzene (TCB). An Agilent PL220 instrument was used equipped with differential refractive index (DRI), viscometry (VS) and dual angle light scatter (LS 90 + 15) detectors. The system uses 2 x PLgel Mixed D columns (300 x 7.5 mm) and a PLgel 5 µm guard column. The mobile phase is TCB with 250 ppm BHT (butylated hydroxytoluene) additive. Samples were run at 1 mL/min at 160°C. Polystyrene standards (Agilent EasyVials) were used to create a third order calibration. Analyte samples were filtered through a stainless steel frit with 10 μm pore size before injection. Respectively, experimental molar mass (Mn, SEC) and dispersity (Đ) values of synthesized polymers were determined by conventional calibration using Agilent GPC/SEC software. UV-Vis absorption spectra were collected on an Agilent 8453 UV-Visible spectrophotometer over the range of 220-1100 nm. Photoluminescence spectroscopy was performed on a Horiba Jobin Yvon Fluorolog 3 spectrofluorometer with an excitation wavelength of 620 nm. TEM measurements were performed at the Monash Centre for Electron Microscopy with a JEOL JEM 210Ph FEGTEM operated at 200 kV in bright-field mode using a small (20 mm) objective aperture to enhance massthickness contrast with a constant defocus of -10 µm to enhance phase contrast from the phase boundaries. AFM measurements were performed at the Melbourne Centre for Nanofabrication on a Veeco Nanoscope V microscope using ScanAsyst mode. Device fabrication: Devices were fabricated with an ITO/ZnO/PEIE/BHJ blend/MoOx/Ag geometry. The ITO/glass substrate was first cleaned in acetone for 10 min, followed by cleaning with IPA in an ultrasonic bath. This was followed by oxygen plasma cleaning for 10 min. Approximately 10 nm of zinc oxide was deposited by spin-coating a precursor solution of zinc acetate dihydrate (0.1 M in 2-methoxy methanol with 1:1 zinc:amine ratio) and curing for 30 min at 190 ˚C. Polyethleneamine (PEIE) was then spin coated on the cured ZnO film from a 0.4 wt.% solution in 2-methoxyethanol and cured at


Page 20 of 44

Page 21 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 ˚C for 15 min. A double layer of ZnO + PEIE was found to be more effective than just either of the interlayers. After transferring substrates to a nitrogen glovebox the active layer was then spin coated from chlorobenzene as the solvent with a solvent concentration of 12-14 g/L. The active layer thickness was optimized for the best efficiency and varied from 115 ± 6 nm for Ph, 90 ± 5 nm for F2 and 90 ± 5 nm for F4. 15 nm of molybdenum oxide was deposited on the active layer in vacuo followed by 100 nm of Ag. The area of each device was 4.5 mm2 and no masks were used during the measurements. The devices were encapsulated using glass and epoxy adhesive before removal from the glove box for testing. Device characterization: Device performance under simulated sunlight was performed using a Photo Emission Tech. model SS50AAA solar simulator with the current–voltage curves measured with a Keithley 2635 source meter. The intensity of the solar simulator was set using a calibrated silicon reference cell with a KG3 glass filter (PV Measurements). The output of the solar simulator was characterized with a spectroradiometer (PV Measurements) with a spectral mismatch of 1.01 to 1.05 determined for all the cells. External quantum efficiency (EQE) was measured as a function of wavelength by dispersing light from a tungsten filament (Newport 250 W QTH) through a monochromators (Oriel Cornerstone 130) with a spot size smaller than the device active area (~1 mm2). Light intensity of less than 1 mW/cm2 was used with short-circuit current recorded using a Keithley 2635 source measure unit. The system was calibrated by placing a calibrated photodiode (Thorlabs FDS-100CAL) in the device under test position and referencing the intensity measured to that of another silicon photodiode. Space-charge-limited current (SCLC) measurements: Contacts for hole-only and electron-only devices were carefully selected to ensure efficient injection of the desired carrier and effective blocking of the other carrier. For each material three thicknesses were spin coated for each blend to ensure that the current density scaled with the film thickness and that the current is not injection limited. For the hole-only devices, a configuration of ITO/MoOx/Active layer/Au was used. The MoOx layer was 15 nm thick and the Au layer was 30 nm



Page 22 of 44

with both deposited via thermal evaporation in vacuo. For electron only devices a configuration of ITO/PEDOTPSS/Al/Active layer/Ca/Al was adopted. For both the hole-only and electron-only devices the film thickness was 150 ± 6 nm for Ph, 125 ± 7 for F2 and 135 ± 5 nm for F4 blends. The bottom Al layer was 35 nm thick. The Ca layer 20 nm thick along with 80 nm Al was thermally evaporated on top. Space-charge limited current measurements were made on encapsulated devices with currentvoltage curves acquired using a Keithley 2635 Source Meter. All measurements, unless otherwise stated, were performed in forward bias, where the ITO side of the device was biased positively. Measurements were made in the absence of illumination by placing the devices in a dark chamber. The data was analyzed in OriginPro 8 and Matlab by fitting of current voltage curves to the Murgatroyd equation. 51 Near Edge X-ray Absorption Fine Structure Spectroscopy: NEXAFS Measurements were performed at the Soft X-ray spectroscopy beamline at the Australian Synchrotron.63 Data was acquired using total electron yield (TEY) mode where the current flowing to neutralize the sample following photoemission is used to measure the strength of X-ray absorption. Data were normalized to the incident photon flux using the ‘‘stable monitor method’’ with further details regarding normalization and analysis provided elsewhere.64 For determination of surface composition, blend spectra acquired at 55° were fitted to neat reference spectra using single-value decomposition. Further details of the fitting process can be found elsewhere.65 Average tilt angles were determined by measuring angle-dependent NEXAFS spectra and fitting to the expression: 64 1 1 I = [1 + 3 cos θ − 1 3 cos γ − 1 ] 3 2 (2) Where; I is the area of the π* manifold, θ is the X-ray incidence angle and γ is the average tilt angle of the carbon 1s to π* transition dipole moment. Data were analyzed using QANT65 implemented in IgorPro. Grazing Incidence Wide Angle X-ray Scattering:


Page 23 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


GIWAXS measurements were performed at the SAXS/WAXS beamline at the Australian Synchrotron.66 9 keV photons were used with data taken with a grazing angle of 0.15° that was above the critical angle of the polymer films but below the critical angle of the substrate. Data was acquired with a Dectris - Pilatus 1M detector. Three 1 s exposures were taken with different lateral positions of the detector with these images stitched to remove the gaps between the detector elements. Sample-to-detector distance was calibrated with a silver behenate sample. Data were analyzed using a modified version of the Nika software packing in IgorPro 6.38.67 Resonant Soft X-ray Scattering R-SoXS measurements were performed at beamline 11.0.1.2 at the Advanced Light Source, Berkeley, California.68 Samples were floated off onto silicon nitride membranes with a photon energy of 286.8 eV used that maximised materials contrast with X-ray fluorescence still minimal at this energy. Two-dimensional transmission X-ray scattering patterns were recorded on a Princeton PI-MTE in-vacuum CCD detector. Two different sample-to-detector distances of 300 mm and 150 mm were used an combined in software to achieve the reported q-range. The two-dimensional scattering patterns were azimuthally integrated to on-dimensional scattering profiles used a customized version of the Nika X-ray scattering package, implemented in IgorPro.67 Transient absorption spectroscopy TA spectroscopy was carried out using the system described in the literature based on a 3KHz amplified Ti:Sapphire laser (Spectra-Physics Spitfire), with different configurations for fast (1 ns) timescales.69, 70 For fast timescales, narrow band ~100-fs excitation (pump) pulses were generated using a TOPAS-C (light conversion) and chopped at half of the 3-kHz laser rep-rate. Probe pulses spanning the visible to near-IR were generated by focusing the 800 nm fundamental laser pulses onto a 3 mm YAG crystal, and the differential transmission (ΔT/T) spectra were obtained by comparing probe shots with and without excitation. Each time delay was averaged via 2-4 scans to obtain a wavelength dependent TA surface. Excitation fluences were kept to below a few μJ/cm2 to suppress bimolecular processes. Slower timescale measurements employed the same probing



scheme, but with the excitation source replaced with a Q-switched Nd:YVO4 laser (AOT lasers), whose ~700 ps pulses at 532 nm were synchronized to the femtosecond laser via an electronic delay generator. Films for TA spectroscopy were kept under dynamic vacuum during measurements. TA surfaces were decomposed into spectral and kinetic components using matrix operations informed by measured spectra. Singular value decomposition was first used to identify the number of spectral components required. These solutions were then rotated using least-squares fitting to give the best fit to spectral masks that correspond to each species. The mask for PTB7-Th exciton was the TA signal averaged from 1 – 100 ps after excitation of a neat PTB7-Th film, while the charge spectra corresponded to the signal remaining after PTB7-Th exciton has decayed in the blended films. Transient photocurrent measurements. Transient photocurrent measurements were made by recording the short-circuit current response of devices to a 200 μs square pulse from an LED on an Agilent Technologies InfiniiVision DSO-X 3032A digital oscilloscope (DOSC). Illumination was provided by a Lumex SSL-LX3044SIC red LED (636 nm wavelength) driven by an Agilent 3522A function generator. Cells were connected in series with the 50 Ω terminated input of the oscilloscope. Light intensity was varied using a set of neutral density filters. For voltage-dependent measurements, the cells were connected in series with the second channel of the function generator operated in DC mode.

Supporting Information. NMR characterisation; normalised absorption and quantum efficiency spectroscopy plots; SCLC data; AFM of neat films; NEXAFS spectroscopy data and fits; 1D GIWAXS data plots of neat films; plot of S vs. angle of incidence for neat films; additional TA data; normalized transient photocurrent plots.

Acknowledgements Part of this research was undertaken on the soft X-ray and SAXS/WAXS beamlines at the Australian Synchrotron, part of ANSTO, and at beamline 11.0.1.2 of the Advanced Light Source at


Page 24 of 44

Page 25 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Lawrence Berkeley National Laboratory. C.R.M., E.G. and K.D. acknowledge travel funding provided by the International Synchrotron Access Program (ISAP) managed by the Australian Synchrotron and funded by the Australian Government. This work was also performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). The authors acknowledge use of facilities within the Monash Centre for Electron Microscopy. CRM acknowledges support from the Australian Research Council (FT100100275). JMH and SKKP acknowledge support from a Rutherford Discovery Fellowship to JMH. The Advanced Light Source was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. RM and MS acknowledge funding from the DFG (IRTG SOMAS 1642).



References 1. Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J., Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148-7151. 2. Fan, B.; Ying, L.; Wang, Z.; He, B.; Jiang, X.-F.; Huang, F.; Cao, Y., Optimisation of processing solvent and molecular weight for the production of green-solvent-processed all-polymer solar cells with a power conversion efficiency over 9%. Energy & Environ. Sci. 2017, 10, 1243-1251. 3. Cheng, P.; Bai, H.; Zawacka, N. K.; Andersen, T. R.; Liu, W.; Bundgaard, E.; Jørgensen, M.; Chen, H.; Krebs, F. C.; Zhan, X., Roll-Coated Fabrication of Fullerene-Free Organic Solar Cells with Improved Stability. Adv. Sci. 2015, 2, 1500096. 4. Swaraj, S.; Wang, C.; Yan, H.; Watts, B.; Luning, J.; McNeill, C. R.; Ade, H., Nanomorphology of bulk heterojunction photovoltaic thin films probed with resonant soft X-ray scattering. Nano Lett. 2010, 10, 2863-2869. 5. Uddin, M. A.; Kim, Y.; Younts, R.; Lee, W.; Gautam, B.; Choi, J.; Wang, C.; Gundogdu, K.; Kim, B. J.; Woo, H. Y., Controlling Energy Levels and Blend Morphology for All-Polymer Solar Cells via Fluorination of a Naphthalene Diimide-Based Copolymer Acceptor. Macromolecules 2016, 49, 63746383. 6. Lee, C.; Li, Y.; Lee, W.; Lee, Y.; Choi, J.; Kim, T.; Wang, C.; Gomez, E. D.; Woo, H. Y.; Kim, B. J., Correlation between Phase-Separated Domain Sizes of Active Layer and Photovoltaic Performances in All-Polymer Solar Cells. Macromolecules 2016, 49, 5051-5058. 7. McNeill, C. R., Morphology of all-polymer solar cells. Energy & Environ. Sci. 2012, 5, 56535667. 8. Zhou, N.; Lin, H.; Lou, S. J.; Yu, X.; Guo, P.; Manley, E. F.; Loser, S.; Hartnett, P.; Huang, H.; Wasielewski, M. R.; Chen, L. X.; Chang, R. P. H.; Facchetti, A.; Marks, T. J., Morphology-Performance Relationships in High-Efficiency All-Polymer Solar Cells. Adv. Energy Mater. 2014, 4, 1300785. 9. Deshmukh, K. D.; Prasad, S. K. K.; Chandrasekaran, N.; Liu, A. C. Y.; Gann, E.; Thomsen, L.; Kabra, D.; Hodgkiss, J. M.; McNeill, C. R., Critical Role of Pendant Group Substitution on the Performance of Efficient All-Polymer Solar Cells. Chem. Mater. 2017, 29, 804-816. 10. Deshmukh, K. D.; Qin, T.; Gallaher, J. K.; Liu, A. C. Y.; Gann, E.; O'Donnell, K.; Thomsen, L.; Hodgkiss, J. M.; Watkins, S. E.; McNeill, C. R., Performance, morphology and photophysics of high open-circuit voltage, low band gap all-polymer solar cells. Energy & Environ. Sci. 2015, 8, 332-342. 11. Gao, L.; Zhang, Z.-G.; Xue, L.; Min, J.; Zhang, J.; Wei, Z.; Li, Y., All-Polymer Solar Cells Based on Absorption-Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%. Adv. Mater. 2016, 28, 1884-1890. 12. Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler, M.; Facchetti, A., A high-mobility electron-transporting polymer for printed transistors. Nature 2009, 457, 679-686. 13. Mori, D.; Benten, H.; Okada, I.; Ohkita, H.; Ito, S., Highly efficient charge-carrier generation and collection in polymer/polymer blend solar cells with a power conversion efficiency of 5.7%. Energy & Environ. Sci. 2014, 7, 2939-2943. 14. Zhou, E.; Cong, J.; Wei, Q.; Tajima, K.; Yang, C.; Hashimoto, K., All-polymer solar cells from perylene diimide based copolymers: Material design and phase separation control. Angew. Chem. Int. Ed. 2011, 50, 2799-2803. 15. Earmme, T.; Hwang, Y. J.; Murari, N. M.; Subramaniyan, S.; Jenekhe, S. A., All-polymer solar cells with 3.3% efficiency based on naphthalene diimide-selenophene copolymer acceptor. .J Am. Chem. Soc. 2013, 135, 14960-14963. 16. Zhang, Y.; Wan, Q.; Guo, X.; Li, W.; Guo, B.; Zhang, M.; Li, Y., Synthesis and photovoltaic properties of an n-type two-dimension-conjugated polymer based on perylene diimide and benzodithiophene with thiophene conjugated side chains. J. Mater. Chem. A 2015, 3, 18442-18449. 17. Hwang, Y. J.; Earmme, T.; Courtright, B. A. E.; Eberle, F. N.; Jenekhe, S. A., n-Type Semiconducting Naphthalene Diimide-Perylene Diimide Copolymers: Controlling Crystallinity, Blend


Page 26 of 44

Page 27 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Morphology, and Compatibility Toward High-Performance All-Polymer Solar Cells. J. Am. Chem. Soc. 2015, 137, 4424-4434. 18. Lee, C.; Kang, H.; Lee, W.; Kim, T.; Kim, K. H.; Woo, H. Y.; Wang, C.; Kim, B. J., Highperformance all-polymer solar cells via side-chain engineering of the polymer acceptor: the importance of the polymer packing structure and the nanoscale blend morphology. Adv. Mater. 2015, 27, 2466-2471. 19. Hwang, Y.-J.; Courtright, B. A. E.; Ferreira, A. S.; Tolbert, S. H.; Jenekhe, S. A., 7.7% Efficient All-Polymer Solar Cells. Adv. Mater. 2015, 27, :4578-4584. 20. Li, Z.; Xu, X.; Zhang, W.; Meng, X.; Ma, W.; Yartsev, A.; Inganäs, O.; Andersson, M. R.; Janssen, R. A. J.; Wang, E., High Performance All-Polymer Solar Cells by Synergistic Effects of Fine-Tuned Crystallinity and Solvent Annealing. J. Am. Chem. Soc 2016, 138, 10935-10944. 21. Guo, Y.; Li, Y.; Awartani, O.; Zhao, J.; Han, H.; Ade, H.; Zhao, D.; Yan, H., A Vinylene-Bridged Perylenediimide-Based Polymeric Acceptor Enabling Efficient All-Polymer Solar Cells Processed under Ambient Conditions. Adv. Mater. 2016, 28, 8483-8489. 22. Wang, C.; Mueller, C. J.; Gann, E.; Liu, A. C. Y.; Thelakkat, M.; McNeill, C. R., Influence of fluorination on the microstructure and performance of diketopyrrolopyrrole-based polymer solar cells. Journal of Polymer Science Part B: Polymer Physics 2016, 55(1) 49-59 (2017). 23. Jo, J. W.; Jung, J. W.; Jung, E. H.; Ahn, H.; Shin, T. J.; Jo, W. H., Fluorination on both D and A units in D-A type conjugated copolymers based on difluorobithiophene and benzothiadiazole for highly efficient polymer solar cells. Energy & Environ. Sci.2015, 8, 2427-2434. 24. Wang, N.; Chen, Z.; Wei, W.; Jiang, Z., Fluorinated Benzothiadiazole-Based Conjugated Polymers for High-Performance Polymer Solar Cells without Any Processing Additives or Posttreatments. J. Am. Chem. Soc 2013, 135, 17060-17068. 25. Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W., Fluorine Substituted Conjugated Polymer of Medium Band Gap Yields 7% Efficiency in Polymer−Fullerene Solar Cells. J. Am. Chem. Soc 2011, 133, 4625-4631. 26. Zhou, C.; Zhang, G.; Zhong, C.; Jia, X.; Luo, P.; Xu, R.; Gao, K.; Jiang, X.; Liu, F.; Russell, T. P.; Huang, F.; Cao, Y., Toward High Efficiency Polymer Solar Cells: Influence of Local Chemical Environment and Morphology. Advanced Energy Materials 2016, 1601081-n/a. 27. Zhang, M.; Guo, X.; Zhang, S.; Hou, J., Synergistic Effect of Fluorination on Molecular Energy Level Modulation in Highly Efficient Photovoltaic Polymers. Adv. Mater. 2014, 26, 1118-1123. 28. Zhang, Y.; Chien, S.-C.; Chen, K.-S.; Yip, H.-L.; Sun, Y.; Davies, J. A.; Chen, F.-C.; Jen, A. K. Y., Increased open circuit voltage in fluorinated benzothiadiazole-based alternating conjugated polymers. Chem. Comm. 2011, 47, 11026-11028. 29. Stuart, A. C.; Tumbleston, J. R.; Zhou, H.; Li, W.; Liu, S.; Ade, H.; You, W., Fluorine Substituents Reduce Charge Recombination and Drive Structure and Morphology Development in Polymer Solar Cells. J. Am. Chem. Soc 2013, 135, 1806-1815. 30. Li, Z.; Lin, H.; Jiang, K.; Carpenter, J.; Li, Y.; Liu, Y.; Hu, H.; Zhao, J.; Ma, W.; Ade, H.; Yan, H., Dramatic performance enhancement for large bandgap thick-film polymer solar cells introduced by a difluorinated donor unit. Nano Energy 2015, 15, 607-615. 31. Chen, S.; An, Y.; Dutta, G. K.; Kim, Y.; Zhang, Z.-G.; Li, Y.; Yang, C., A Synergetic Effect of Molecular Weight and Fluorine in All-Polymer Solar Cells with Enhanced Performance. Adv. Funct. Mater. 2017, 27, 1603564. 32. Liu, P.; Zhang, K.; Liu, F.; Jin, Y.; Liu, S.; Russell, T. P.; Yip, H.-L.; Huang, F.; Cao, Y., Effect of Fluorine Content in Thienothiophene-Benzodithiophene Copolymers on the Morphology and Performance of Polymer Solar Cells. Chem. Mater.2014, 26, 3009-3017. 33. Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L., Highly Efficient Solar Cell Polymers Developed via Fine-Tuning of Structural and Electronic Properties. J. Am. Chem. Soc 2009, 131, 77927799. 34. Albrecht, S.; Janietz, S.; Schindler, W.; Frisch, J.; Kurpiers, J.; Kniepert, J.; Inal, S.; Pingel, P.; Fostiropoulos, K.; Koch, N.; Neher, D., Fluorinated Copolymer PCPDTBT with Enhanced Open-Circuit



Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells. J. Am. Chem. Soc 2012, 134, 14932-14944. 35. Son, H. J.; Wang, W.; Xu, T.; Liang, Y.; Wu, Y.; Li, G.; Yu, L., Synthesis of Fluorinated Polythienothiophene-co-benzodithiophenes and Effect of Fluorination on the Photovoltaic Properties. J. Am. Chem. Soc 2011, 133, 1885-1894. 36. Homyak, P.; Liu, Y.; Ferdous, S.; Liu, F.; Russell, T. P.; Coughlin, E. B., Effect of Pendant Functionality in Thieno[3,4-b]thiophene-alt-benzodithiophene Polymers for OPVs. Chem. Mater.2015, 27, 443-449. 37. Jung, J. W.; Jo, J. W.; Chueh, C. C.; Liu, F.; Jo, W. H.; Russell, T. P.; Jen, A. K., FluoroSubstituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71. Adv. Mater. 2015, 27, 3310-3317. 38. Long, X.; Ding, Z.; Dou, C.; Zhang, J.; Liu, J.; Wang, L., Polymer Acceptor Based on Double B←N Bridged Bipyridine (BNBP) Unit for High-Efficiency All-Polymer Solar Cells. Adv. Mater. 2016, 28, 6504-6508. 39. Liu, S.; Kan, Z.; Thomas, S.; Cruciani, F.; Brédas, J.-L.; Beaujuge, P. M., Thieno[3,4-c]pyrrole4,6-dione-3,4-difluorothiophene Polymer Acceptors for Efficient All-Polymer Bulk Heterojunction Solar Cells. Angew. Chem. Int. Ed. 2016, 55, 12996-13000. 40. Deng, P.; Ho, C. H. Y.; Lu, Y.; Li, H.-W.; Tsang, S.-W.; So, S. K.; Ong, B. S., Naphthalene diimidedifluorobenzene-based polymer acceptors for all-polymer solar cells. Chem. Comm. 2017, 53, 32493252. 41. Li, X.; Liu, X.; Sun, P.; Feng, Y.; Shan, H.; Wu, X.; Xu, J.; Huang, C.; Chen, Z.-K.; Xu, Z.-X., Effect of fluorination on n-type conjugated polymers for all-polymer solar cells. RSC Adv. 2017, 7, 1707617084. 42. Luzio, A.; Fazzi, D.; Nübling, F.; Matsidik, R.; Straub, A.; Komber, H.; Giussani, E.; Watkins, S. E.; Barbatti, M.; Thiel, W.; Gann, E.; Thomsen, L.; McNeill, C. R.; Caironi, M.; Sommer, M., Structure– Function Relationships of High-Electron Mobility Naphthalene Diimide Copolymers Prepared Via Direct Arylation. Chem. Mater.2014, 26, 6233-6240. 43. Matsidik, R.; Komber, H.; Sommer, M., Rational Use of Aromatic Solvents for Direct Arylation Polycondensation: C–H Reactivity versus Solvent Quality. ACS Macro Lett. 2015, 4, 1346-1350. 44. Nübling, F.; Komber, H.; Sommer, M., All-Conjugated, All-Crystalline Donor–Acceptor Block Copolymers P3HT-b-PNDIT2 via Direct Arylation Polycondensation. Macromolecules 2017, 50, 19091918. 45. Zhou, N.; Dudnik, A. S.; Li, T. I. N. G.; Manley, E. F.; Aldrich, T. J.; Guo, P.; Liao, H.-C.; Chen, Z.; Chen, L. X.; Chang, R. P. H.; Facchetti, A.; Olvera de la Cruz, M.; Marks, T. J., All-Polymer Solar Cell Performance Optimized via Systematic Molecular Weight Tuning of Both Donor and Acceptor Polymers. J. Am. Chem. Soc. 2016, 138, 1240-1251. 46. Zhu, D.; Shang, J.; Ye, X.; Shen, J., Aggregation and Gelation of Aromatic Polyamides with Parallel and Anti-parallel Alignment of Molecular Dipole Along the Backbone. Sci. Rep. 2016, 6, 39124. 47. Coniglio, A.; Arcangelis, L. D.; Gado, E. D.; Fierro, A.; Sator, N., Percolation, gelation and dynamical behaviour in colloids. J. Phys: Condens. Matter 2004, 16, S4831. 48. Cui, C.; Wong, W.-Y.; Li, Y., Improvement of open-circuit voltage and photovoltaic properties of 2D-conjugated polymers by alkylthio substitution. Energy & Environ. Sci.2014, 7, 2276-2284. 49. Maurano, A.; Hamilton, R.; Shuttle, C. G.; Ballantyne, A. M.; Nelson, J.; O’Regan, B.; Zhang, W.; McCulloch, I.; Azimi, H.; Morana, M.; Brabec, C. J.; Durrant, J. R., Recombination Dynamics as a Key Determinant of Open Circuit Voltage in Organic Bulk Heterojunction Solar Cells: A Comparison of Four Different Donor Polymers. Adv. Mater. 2010, 22, 4987-4992. 50. Peng, Q.; Liu, X.; Su, D.; Fu, G.; Xu, J.; Dai, L., Novel Benzo[1,2-b:4,5-b′]dithiophene– Benzothiadiazole Derivatives with Variable Side Chains for High-Performance Solar Cells. Adv. Mater. 2011, 23, 4554-4558.


Page 28 of 44

Page 29 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


51. Murgatroyd, P. N., Theory of space-charge-limited current enhanced by Frenkel effect. J. Phys. D: Appl. Phys. 1970, 3, 151-156. 52. Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y., High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends. Nat. Mater. 2005, 4, 864-868. 53. Schuettfort, T.; Thomsen, L.; McNeill, C. R., Observation of a Distinct Surface Molecular Orientation in Films of a High Mobility Conjugated Polymer. J. Am. Chem. Soc 2013, 135, 1092-1101. 54. Strobl, G., The Physics of Polymers. 3rd edition ed.; Springer: Berlin, 2007. 55. Hwang, I.; McNeill, C. R.; Greenham, N. C., Drift-diffusion modeling of photocurrent transients in bulk heterojunction solar cells. J. Appl. Phys. 2009, 106, 094506. 56. Li, Z.; Lakhwani, G.; Greenham, N. C.; McNeill, C. R., Voltage-dependent photocurrent transients of PTB7:PC70BM solar cells: Experiment and numerical simulation. J. Appl. Phys. 2013, 114, 034502. 57. Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.; Fromherz, T.; Rispens, M. T.; Sanchez, L.; Hummelen, J. C., Origin of the Open Circuit Voltage of Plastic Solar Cells. Adv. Funct. Mater. 2001, 11, 374-380. 58. Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J. V., On the origin of the opencircuit voltage of polymer-fullerene solar cells. Nat. Mater. 2009, 8, 904-909. 59. Pal, S. K.; Kesti, T.; Maiti, M.; Zhang, F.; Inganäs, O.; Hellström, S.; Andersson, M. R.; Oswald, F.; Langa, F.; Österman, T.; Pascher, T.; Yartsev, A.; Sundström, V., Geminate Charge Recombination in Polymer/Fullerene Bulk Heterojunction Films and Implications for Solar Cell Function. J. Am. Chem. Soc 2010, 132, 12440-12451. 60. Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Olivier, Y.; Cornil, J.; Beljonne, D.; Sirringhaus, H., Approaching disorder-free transport in high-mobility conjugated polymers. Nature 2014, 515, 384-388. 61. Chen, H.; Hurhangee, M.; Nikolka, M.; Zhang, W.; Kirkus, M.; Neophytou, M.; Cryer, S. J.; Harkin, D.; Hayoz, P.; Abdi-Jalebi, M.; McNeill, C. R.; Sirringhaus, H.; McCulloch, I., Dithiopheneindenofluorene (TIF) Semiconducting Polymers with Very High Mobility in Field-Effect Transistors. Adv. Mater. 2017, 29, 1702523. 62. Matsidik, R.; Martin, J.; Schmidt, S.; Obermayer, J.; Lombeck, F.; Nübling, F.; Komber, H.; Fazzi, D.; Sommer, M., C–H Arylation of Unsubstituted Furan and Thiophene with Acceptor Bromides: Access to Donor–Acceptor–Donor-Type Building Blocks for Organic Electronics. J. Org. Chem. 2015, 80, 980-987. 63. Cowie, B. C. C.; Tadich, A.; Thomsen, L., The Current Performance of the Wide Range (90– 2500 eV) Soft X-ray Beamline at the Australian Synchrotron. AIP Conf. Proc. 2010, 1234, 307-310. 64. Nahid, M. M.; Gann, E.; Thomsen, L.; McNeill, C. R., NEXAFS spectroscopy of conjugated polymers. Eur. Polym. J. 2016, 81, 532-554. 65. Gann, E.; McNeill, C. R.; Tadich, A.; Cowie, B. C. C.; Thomsen, L., Quick AS NEXAFS Tool (QANT): A program for NEXAFS loading and analysis developed at the Australian Synchrotron. J. Synchrotron Rad. 2016, 23, 374-380. 66. Kirby, N. M.; Mudie, S. T.; Hawley, A. M.; Cookson, D. J.; Mertens, H. D. T.; Cowieson, N.; Samardzic-Boban, V., A low-background-intensity focusing small-angle X-ray scattering undulator beamline. J. Appl. Cryst. 2013, 46, 1670-1680. 67. Ilavsky, I., Nika - software for 2D data reduction. J. Appl. Cryst. 2012, 45, 324-328. 68. Gann, E.; Young, A.; Collins, B. A.; Yan, H.; Nasiatka, J.; Padmore, H. A.; Ade, H.; Hexemer, A.; Wang, C., Soft X-ray Scattering Facility at the Advanced Light Source with Real-time Data Processing and Analysis. Rev. Sci. Instrum. 2012, 83, 045110. 69. Barker, A. J.; Chen, K.; Hodgkiss, J. M., Distance Distributions of Photogenerated Charge Pairs in Organic Photovoltaic Cells. J. Am. Chem. Soc. 2014, 136, 12018-12026.



70. Gallaher, J. K.; Prasad, S. K. K.; Uddin, M. A.; Kim, T.; Kim, J. Y.; Woo, H. Y.; Hodgkiss, J. M., Spectroscopically tracking charge separation in polymer : fullerene blends with a three-phase morphology. Energy & Environ. Sci. 2015, 8, 2713-2724.


Page 30 of 44

Page 31 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Chemical structures of the acceptor polymers PNDITPhT, (a), PNDITF2T, (b), and PNDITF4T (c). Molecular energy levels of the polymer donor and the three acceptors, (d).



Figure 2. UV-Vis- near IR absorption spectra normalized to thickness of neat films of PNDITPhT, PNDITF2T, PNDITF4T, and PTB7-Th, (a), and blends of the three acceptor polymers with PTB7-Th, (b).


Page 32 of 44

Page 33 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Device characteristics of optimized cells for each blend: Light J-V curves, (a), External Quantum Efficiency (EQE) spectra, (b), and Dark J-V curves, (c).



Figure 4. Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) images of PTB7-Th:PNDITPhT (a,b) PTB7-Th:PNDITF2T(c,d) and PTB7-Th:PNDITF4T (e,f).


Page 34 of 44

Page 35 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Two-dimensional GIWAXS images of neat polymers PNDITPhT, (a), PNDITF2T, (c) and PNDITF4T, (e). 1:1 wt. ratio blends with PTB7-Th with Ph, (b), F2, (d), and F4, (f).



Figure 6. One-dimensional scattering profiles taken from the 2D GIWAXs images of the blends, Inplane (IP) profiles, (a), and out of plane (OOP) line profiles, (b). Plot of Hermann’s S-parameter vs. the incident angle, (c).


Page 36 of 44

Page 37 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. R-SoXS scattering profiles of the blends. Scattering profiles are shown for X-ray energies of 286.8 eV (resonant) and 270 eV (non-resonant).



Figure 8. Photoluminescence quenching fraction of the blends as a function for excitation at 532 nm, 625 nm and 715 nm wavelength.


Page 38 of 44

Page 39 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. (a) Series of transient absorption spectra for a PTB7-Th:PNDITF4T blend after excitation at 532 nm (1.2 µJ/cm2). (b) Charge dynamics of PTB7-Th:PNDITPhT, PTB7-Th:PNDITF2T, and PTB7Th:PNDITF4T after excitation at 532 nm (1 – 2 µJ/cm2). The charge populations were obtained after bilinearly decomposing the TA surfaces and applying spectral masks to define exciton and charge populations.



Figure 10. Transient photocurrent results taken as a function of applied bias (a, c, e) and light intensity at short-circuit (b, d, e). (a, b) show results for the PBT7-Th:Ph blend; (c, d) show results for the PBT7-Th:F2 blend; (e, f) show results for the PTB7-Th:F4 blend.


Page 40 of 44

Page 41 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mn/Ða

Mn,NMR

LUMOb

HOMOc

Egd

Tm/ TC

( kDa)

(kDa)

(eV)

(eV )

(ev)

(°C)

(J/g)

PNDITPhT

36/3.16

23.6

-3.76

-5.7

1.56

277.3/258.9

9.40/10.0

440

PNDITF2T

38/2.16

24.6

-3.81

-5.9

1.65

278.8/277.9

12.0/11.9

440

PNDITF4T

53/1.56

29.7

-3.87

-6.1

1.80

300.7/280.1

18.5/18.1

440

∆Hm / ∆HC

Ton

Polymer e

e

f

(°C)

Table 1. Key material properties of the acceptor polymer. aDetermined by High Temperature GPC. b

Measured by Cyclic voltammetry (CV). cMeasured from Photo-Electron Spectroscopy in Air (PESA).

d

Band-gap calculated from the onset of the absorption in the UV-VIS spectra. eDetermined from

Differential Scanning Calorimetry (DSC) measurements (10 K/min, second cycle). fDetermined from Thermogravimetric Analysis (TGA) .

VOC

Max. JSC

(V)

(mA/cm )

Blend

Efficiency

Max. EQE

(%)

(%)

Fill factor 2

Theo. JSC from EQE 2

(mA/cm ) PTB7-Th:PNDITPhT

0.86

7.7

0.45 ± 0.02

3.1 ± 0.02

40

7.3

PTB7-Th: PNDITF2T

0.80

9.8

0.50 ± 0.01

3.8 ± 0.03

47

9.2

PTB7-Th:PNDITF4T

0.78

11.7

0.52 ± 0.02

4.6 ± 0.02

55

11.3

Table 2. Summary of photovoltaic properties of inverted all-polymer solar cells.



Page 42 of 44

Diode Hole mobility

Diode Electron mobility

(cm2/V.s)a

(cm2/V.s)b

PTB7-Th:PNDITPhT

4.9 × 10-4

8.4 × 10-5

PTB7-Th: PNDITF2T

3.7 × 10

PTB7-Th:PNDITF4T

7.6 × 10-4

Blend

-4

1.1 × 10

-4

4.3 × 10-4

Table 3. aHole mobility determined from hole-only SCLC devices. bElectron mobility determined from electron-only SCLC devices.

Blend

Wt.% of donor

Wt.% of acceptor

Avg. Tilt angle

Avg. Tilt angle

(top)

(bottom)

(top)

(bottom)

PTB7-Th:PNDITPhT

77 ± 3

60 ± 1

52.2 ± 0.3 °

47.2 ± 0 .1 °

PTB7-Th:PNDITF2T

65 ± 1

70 ± 2

52.6 ± 0.1 °

43.5 ± 0.5 °

PTB7-Th: PNDITF4T

91 ± 1

42 ± 2

51.5 ± 0.2 °

48.0 ± 0.3 °

Table 4. Summary of top and bottom surface chemical composition and average back-bone tilt angles determined by NEXAFS spectroscopy of the three blends.


Page 43 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Blend

In-plane (100) peak

Out-of-plane (010) peak

S

parame

d-spacing (nm)

Coherence Length

d-spacing

Coherence Length

(nm)

(nm)

(nm)

(nm)

PTB7-Th:Ph

2.27 ± 0.01

8.0 ± 0.08

0.37 ± 0.01

1.74 ± 0.01

-0.19

PTB7-Th:F2

2.29 ± 0.01

11.85 ± 0.19

0.37 ± 0.01

1.85 ± 0.01

-0.25

PTB7-Th:F4

2.24 ± 0.01

7.15 ± 0.05

0.37 ± 0.01

1.96 ± 0.01

-0.22

Table 5. Summary of key parameters extracted from the GIWAXS data via fitting of the in-plane and out-of-plane sector profiles and polar plots for the polymers blends



TOC Graphic


Page 44 of 44

Impact of Acceptor Fluorination on the Performance of All-Polymer

Recommend Documents