Fluorination Effects on Indacenodithienothiophene Acceptor Packing

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Fluorination Effects on Indacenodithienothiophene Acceptor Packing and Electronic Structure, End-Group Redistribution, and Solar Cell Photovoltaic Response Thomas J Aldrich, Micaela Matta, Weigang Zhu, Steven M. Swick, Charlotte L. Stern, George C. Schatz, Antonio Facchetti, Ferdinand S. Melkonyan, and Tobin J. Marks J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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Journal of the American Chemical Society

Fluorination Effects on Indacenodithienothiophene Acceptor Packing and Electronic Structure, End-Group Redistribution, and Solar Cell Photovoltaic Response Thomas J. Aldrich,† Micaela Matta,*,† Weigang Zhu,† Steven M. Swick,† Charlotte L. Stern,† George C. Schatz,*,† Antonio Facchetti,*,†,§ Ferdinand S. Melkonyan,*,† Tobin J. Marks*,†,‡ Department of Chemistry and the Materials Research Center and ‡ Department of Materials Science and Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States § Flexterra Corporation, 8025 Lamon Avenue, Skokie, Illinois 60077, United States †

ABSTRACT: Indacenodithienothiophene (IDTT)-based postfullerene electron acceptors, such as ITIC (2,2′-[[6,6,12,12-tetrakis(4hexylphenyl)-6,12-dihydrodithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene-2,8-diyl]-bis[methylidyne(3-oxo-1H-indene2,1(3H)-diylidene)]]bis[propanedinitrile]), have become synonymous with high power conversion efficiencies (PCEs) in bulk heterojunction (BHJ) polymer solar cells (PSCs). Here we systematically investigate the influence of end-group fluorination density and positioning on the physicochemical properties, single-crystal packing, end-group redistribution propensity, and BHJ photovoltaic performance of a series of ITIC variants, ITIC-nF (n = 0, 2, 3, 4, and 6). Increasing n from 0 → 6 contracts the optical bandgap, but only marginally lowers the LUMO for n > 4. This yields enhanced photovoltaic short-circuit current density and good open-circuit voltage, so that ITIC-6F achieves the highest PCE of the series, approaching 12% in blends with the PBDB-TF donor polymer. Single-crystal diffraction reveals that the ITIC-nF molecules cofacially interleave with ITIC-6F having the shortest π–π distance, 3.28Ǻ. This feature together with ZINDO-level computed intermolecular electronic coupling integrals as high as 57 meV, and B3LYP/DZP-level reorganization energies as low as 147 meV, rival or surpass the corresponding values for fullerenes, ITIC0F, and ITIC-4F, and track a positive correlation between the ITIC-nF space-charge limited electron mobility and n. Finally, a heretofore unrecognized solution-phase redistribution process between the 2-(3-oxo-indan-1-ylidene)-malononitrile-derived endgroups (EGs) of IDTT-based NFAs, i.e., EG1-IDTT-EG1 + EG2-IDTT-EG2 2 EG1-IDTT-EG2, with implications for the entire ITIC PSC field, is identified and mechanistically characterized, and the effects on PSC performance are assessed.

INTRODUCTION π-Conjugated organic semiconductors are essential components in polymer solar cells (PSCs) containing interpenetrating bulk heterojunction (BHJ) blend photoactive layers of solution-processable p-type (donor) polymers and ntype (acceptor) molecules.1–5 While the dominant acceptor materials for much of the past decade have been based on fullerenes and rylene diimides,3,6–10 the recent advent of indacenodithienothiophene (IDTT)-based nonfullerene acceptors (NFAs) has enabled monumental increases in PSC power conversion efficiency (PCE),11–15 now demonstrated to exceed 17% in tandem PSCs.16 These remarkable performance increases are partially attributed to advantageous IDTT-based NFA properties, which include: strong oscillator strengths, low optical bandgaps, high electron mobilities, low internal reorganization energies, rapid charge transfer with donor materials, and synthetic tunability.17–23 Since the first report of the IDTT-based NFA ITIC-0F (Figure 1a),17 diverse structural variants have shown promise in high-efficiency PSCs.11,12,14,24 Among the most noteworthy to date is tetrafluorinated ITIC-4F (Figure 1a), which when paired with certain wide-bandgap donor polymers, has broad applicability in high-efficiency binary25–30 and ternary singlejunction31 as well as tandem PSCs.32 IDTT NFA end-group fluorination downshifts the frontier molecular orbitals (FMOs), thereby lowering the open-circuit voltage (VOC) values in BHJ PSCs relative to those with ITIC-0F. However,

Figure 1. (a) Chemical structures of the symmetrical and unsymmetrical ITIC-nF NFAs (n = 0, 2, 3, 4, 6) and the donor polymer PBDB-TF. (b) Thin film UV-vis spectra. (c) CVderived frontier MO energetics.

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Scheme 1. Synthetic Routes to the Trifluorinated End-Group and the ITIC-nF Electron Acceptors

fluorination also contracts the NFA optical bandgap, and the subsequent enhancements in short-circuit current density (JSC) enabled by the broader BHJ optical cross-section more than compensate for the aforementioned reductions in VOC, affording overall higher PCEs.26,29,33,34 Nevertheless, minimizing the tradeoff between JSC and VOC has been identified as a critical strategy for optimizing NFA-based PSC performance.35 However, despite the potential of fluorinated IDTT acceptors in high-performance PSCs, systematic studies of fluorination density and positioning effects on acceptor properties and photovoltaic performance have not been thoroughly carried out, especially from a combined experiment - theory standpoint. In this study, we compare and contrast the chemical properties, electronic structures, crystal packings, chemical reactivities, and BHJ photovoltaic performance characteristics of an IDTT-based NFA series, ITIC-nF (Figure 1a), where the end-group fluorination density and positioning are systematically varied. X-ray data on other IDTT-related acceptors anticipate close intermolecular π–π end-group interactions,18,36–38 and therefore we hypothesized that unsymmetrical EG-IDTT-EG’ structures with one fluorinated and one non-fluorinated end-group (ITIC-2F and ITIC-3F, Figure 1a) might enhance molecular ordering and electron transport in informative ways relative to symmetrical EGIDTT-EG NFAs.39,40 It will be seen that comparison of the ITIC-0F, ITIC-4F, and ITIC-6F low-temperature crystal structures offers insight into their solid state intermolecular interactions as does the strength and directionality of ZINDOlevel computed intermolecular electronic coupling integrals. While ITIC-nF acceptors pack cofacially, ITIC-6F interleaves with the shortest π–π stacking distances and greatest computed electronic coupling between neighboring molecules. Additionally, heavy fluorination is shown to reduce the computed internal reorganization energies to as low as 147 meV, lower than those in fullerenes. We also report a previously unrecognized NFA end-group redistribution process occurring in the blend solutions prior to BHJ film deposition. Namely, in solutions containing one unsymmetrical NFA (EG-IDTT-EG’) or a mixture of two

symmetrical NFAs (EG-IDTT-EG + EG’-IDTT-EG’), endgroup redistribution results in the formation of all possible symmetrical and unsymmetrical NFA combinations, i.e., EGIDTT-EG, EG-IDTT-EG’, and EG’-IDTT-EG’. We also present a materials processing methodology which minimizes redistribution. Importantly, note that ITIC-0F, ITIC-2F, and ITIC-4F readily participate in end-group redistribution, whereas ITIC-3F and ITIC-6F are less reactive. 13C isotopic labeling studies yield results consistent with a [2+2] cycloaddition/cycloreversion mechanism for this process. Finally, the ITIC-nF acceptors are investigated here in BHJ PSCs with the wide-bandgap donor polymer poly{[4,8bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5b']dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c']dithiophene1,3-diyl]]} (PBDB-TF, Figure 1a),18,41,42 previously reported in high-efficiency binary PSCs with ITIC-0F and ITIC4F.25,28,30,31,42 PBDB-TF:ITIC-nF PSC chemical structuremorphology-performance relationships are investigated using UV-vis spectroscopy, DSC, single-crystal diffraction, cyclic voltammetry, AFM, grazing incidence wide-angle X-ray scattering (GIWAXS), as well as photovoltaic and spacecharge limited current (SCLC) mobility measurements. The results underscore the beneficial effects of IDTT NFA fluorination on PSC performance metrics, with hexafluorinated ITIC-6F achieving the highest PCEs, approaching 12%, due to enhanced JSCs and comparable VOCs relative to ITIC-4F-based PSCs. Lastly, the effects of endgroup redistribution on the performance and BHJ morphology of binary blend PSCs containing unsymmetrical ITIC-nF acceptors as well as ternary blend PSCs containing two different symmetrical ITIC-nF acceptors are scrutinized. RESULTS AND DISCUSSION We begin with the synthesis of the NFA series, ITIC-nF (n = 0, 2, 3, 4, 6), and systematically investigate the effects of end-group fluorination on their optical absorption, electrochemical, and thermal properties as well as on their electronic structures, including B3LYP-computed internal reorganization energies. Single-crystal X-ray diffraction


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elucidates the ITIC-nF geometry, solid state packing, and ZINDO-calculated intermolecular electronic coupling. ITICnF solution-phase redistribution is assessed in photovoltaic blend solutions by NMR spectroscopy and the reaction mechanism is investigated by 13C isotopic labeling experiments. The photovoltaic properties of the ITIC-nF series are characterized in BHJ PSCs with the donor polymer, PBDB-TF. The effects of redistribution on ITIC-nF PSC performance are evaluated by comparing with BHJ blends prepared following protocols that suppress it. The PSC BHJ blend morphologies are characterized by AFM and GIWAXS, and their charge transport characteristics are determined by SCLC mobility measurements. Taken together, these results demonstrate the photovoltaic performance benefits achieved via structurally advantageous NFA fluorination. ITIC-nF Acceptor Synthesis. The synthesis and characterization of the ITIC-nF acceptors are summarized in Scheme 1 and in the Supporting Information (Schemes S1 and S2). First, the trifluorinated end-group 2-(4,5,6-trifluoro-3oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (4) is synthesized by the condensation of malononitrile with dione 3. Because two resulting isomers are possible, the structure of 4 is confirmed by ITIC-6F single-crystal X-ray diffraction data (vide infra), which is itself synthesized via the Knoevenagel condensation reaction between 4 and 6,6,12,12-tetrakis(4hexylphenyl)-6,12-dihydrodithieno[2,3-d:2',3'-d']-sindaceno[1,2-b:5,6-b']dithiophene-2,8-dicarboxaldehyde (5, Scheme 1). A two-step synthetic sequence was initially attempted for unsymmetrical ITIC-2F. First, reaction of 5 with 1 equiv of fluorine-free 2-(3-oxo-indan-1-ylidene)-malononitrile (6) affords the mono-condensed product 8. However, the subsequent reaction of 8 with 1.5 equiv of 2-(5,6-difluoro-3oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (7) yields three major products identified to be ITIC-0F, ITIC-2F, and ITIC-4F, indicating that both condensation and retrocondensation of end-groups with IDTT are facile under these reaction conditions.31,43 Nevertheless, ITIC-2F is obtained, albeit in 15% yield due to purification challenges. We believe that the reversibility of end-group condensation with IDTT will ultimately aid in the concise synthesis of chemically diverse NFA libraries. As a proof of concept, the one-pot reaction of 5, 6, and 7 easily generates three different NFAs: ITIC-0F, ITIC-2F, and ITIC-4F (Scheme S2). Furthermore, this principle is utilized in the synthesis of unsymmetrical ITIC-3F from the reaction of ITIC-6F with 1.3 equiv of 6 (Scheme 1). ITIC-nF Acceptor Characterization. ITIC-nF optical absorption properties are summarized in Table 1. In chlorobenzene (PhCl) solutions, ITIC-nF fluorination generally red-shifts the absorption profile and λmax (Figure S1), however ITIC-3F is slightly red-shifted versus ITIC-4F, in agreement with the computationally predicted trend (Table S7). The solution extinction coefficients also exhibit an increasing trend with ITIC-nF fluorination, consistent with previous reports.26,33 The ITIC-nF thin film optical absorption spectra are shown in Figure 1b. Again, increased fluorination generally red-shifts the λmax and absorption onset, resulting in a lower optical bandgap (Egopt), however ITIC-3F (1.54 eV) exhibits a slightly lower Egopt than ITIC-4F (1.55 eV) and ITIC-6F exhibits the lowest (1.51 eV). The ITIC-nF frontier molecular orbital (FMO) energies estimated from the CV oxidation and reduction potentials are

shown in Figures 1c and S2. All of the ITIC-nF exhibit reasonable energy level alignment relative to the PBDB-TF donor polymer FMOs.11 The ITIC-nF LUMO energies monotonically decrease with increasing n, consistent with electronTable 1. ITIC-nF Acceptor Optical Absorption Properties and Calculated Internal Reorganization Energies Acceptor

Solutiona

Filmb

λmax (nm)

ε × 10–5 (M–1cm–1)

λmax (nm)

Egopt (eV)

λint (meV)c

ITIC-0F

668

1.62

708

1.59

155d

ITIC-2F

676

1.74

722

1.56

158

ITIC-4F

682

1.95

728

1.55

158

ITIC-3F

685

1.70

730

1.54

152

ITIC-6F

695

2.00

745

1.51

147

a

Solution UV-vis spectra in PhCl (0.010 mg mL–1). b UV-vis spectra of PhCl-cast films. c Calculated internal reorganization energies at the B3LYP/DZP level of theory. d See ref. 18.

withdrawing fluorine effects.34,44 Note however that the difference in LUMO energy between ITIC-4F and ITIC-6F (~10 meV) is dramatically smaller than that between ITIC-0F and ITIC-2F (70 meV), which suggests that fluorination of the end-group fragment at the 4-position has less electronic influence than does fluorination at the 5- or 6-positions (see also Figures S5–S9).34 The ITIC-nF acceptor thermal properties were next investigated by DSC (Figure S3). None of the present acceptors exhibit a thermal transition below 200 °C. Rather, ITIC-2F, ITIC-3F, and ITIC-6F exhibit cold crystallization transitions in the first DSC heating cycles at 202 °C, 213 °C, and 243 °C, respectively (Table S3).42 In contrast, no thermal transitions are observed in the respective second DSC heating/cooling cycles of these molecules. Interestingly, the B3LYP/DZP-level internal reorganization energies, λint, of ITIC-2F and ITIC-4F are slightly greater than that of ITIC-0F, while those of ITIC-3F and ITIC-6F are lower (Table 1). Note that the λint of ITIC-6F (147 meV) is also lower than those for fullerenes, PC61BM and PC71BM.45 Reorganization energy is a measure of the energetic costs attributed to structural relaxation following ionization (electron addition), and the lower values for ITIC3F and ITIC-6F may favor greater intermolecular electron carrier mobility.18,46 Solid State Packing and Intermolecular Electronic Coupling. Solid state ordering plays a critical role in defining both the strength and directionality of electronic coupling interactions in organic semiconductors.47–50 Despite the complex and often predominantly amorphous nature of BHJ photoactive layers, the analysis of molecular single-crystal packing provides complementary insight into structure/selfassembly relationships that can assist in both the rational design of new materials and in the understanding of their charge transport properties in BHJ blends.18,51 Currently however, there is a surprising paucity of single-crystal structural data for IDTT-derived acceptors, including ITIC-4F, considering their broad functionality and fundamental importance. Consequently, single-crystals of ITIC-0F, ITIC2F, ITIC-4F, ITIC-6F, and a cocrystal of ITIC-0F with ITIC4F were grown via slow vapor diffusion, and diffraction data were then acquired (see SI for complete detail).52



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The crystal structures of ITIC-0F, ITIC-4F, and ITIC-6F are shown in Figure 2. While all three molecules crystallize in triclinic unit cells, striking differences in intermolecular packing and conjugated IDTT-core planarity are evident. Notably, all three NFAs exhibit intramolecular S···O=C interactions with distances closer than the sum of the S and O van der

Figure 2. Single-crystal structures of (a) ITIC-0F, (b) ITIC-4F, and (c) ITIC-6F. (Left) π-face-on perspective of the single molecules highlighting the intramolecular S···O distance with the last digit uncertainty in parentheses. (Right) Crystal packing diagrams illustrating the relationships between an arbitrary central molecule (colored) and four near neighbors. Calculated electronic coupling values, |J|, are relative to central molecule. Alkyl chains are truncated to methyl groups and hydrogen atoms are omitted for ease of viewing.

Waals radii (3.25 Å), potentially suggesting a conformational lock.53,54 While the central IDTT units are generally planar, torsions between the IDTT cores and the end-groups are observed. Both ITIC-0F and ITIC-6F exhibit relatively small torsions of 4.1(5)° and 8.0(8)°, respectively, while that of ITIC-4F is larger, 16.1(7)°. Furthermore, dramatic differences in intermolecular packing are evident. Specifically, ITIC-0F and ITIC-6F pack face-to-face in “brick-like” planes separated by their alkyl substituents, while ITIC-4F packs in both a faceto-face and an edge-to-face fashion (Figure 2). Note that ITIC-2F as well as the ITIC-0F:ITIC-4F cocrystal adopt a packing structure similar to that of ITIC-4F, however due to a high degree of disorder about the crystallographic inversion center and the resulting absence of any preferential difluorinated/non-fluorinated end-group ordering, additional analysis of these crystals was not performed (See SI and Figures S13 and S14).55 In ITIC-0F and ITIC-6F, substantial π–π interactions between the electron deficient end-groups with interplanar spacings of ~3.4 Å for ITIC-0F, and 3.95 Å and 3.28 Å for ITIC-6F are observed. Note that the close 3.28 Å ITIC-6F π–π distance is one of the shortest yet observed for photovoltaically efficient IDTT NFAs,18,36–38,56,57 and is similar to those observed in crystalline fullerene acceptors, such as PC61BM and PC71BM.58,59 Additionally, remarkably close 3.16 Å CN···π distances are observed between neighboring ITIC-6F molecules (Figure 2c). These interactions may reflect favorable lone-pair···π electrostatic effects, which can be

observed with highly electron-deficient π-systems including perfluoroarenes.60–62 In contrast, the closest intermolecular distances for ITIC-4F are 3.21 Å and 3.35 Å, resulting from S···π (lone-pair···π) and π–π interactions, respectively (Figure 2b).63,64 We next assess the impact of single-crystal packing on intermolecular electronic coupling. First, an arbitrary ‘central’ molecule within the crystal lattice was selected as a reference. Next, the transfer integrals between the reference and its nearest neighbors were computed using a ZINDO Hamiltonian (see SI for details and Tables S4–S6).18,65 The crystal structure of ITIC-0F features four neighbors having non-negligible transfer integrals. The largest |J| coupling value of 11.4 meV results from the closer intermolecular π–π contacts (Figure 2a). Tetrafluorinated ITIC-4F also exhibits four nearneighbors, but the largest coupling arises from edge-to-face interactions with |J| = 17.1 meV (Figure 2b). Interestingly, a previous analysis of another ITIC-0F polymorph exhibiting edge-to-face packing found a similar result, with a maximum |J| value (16 meV) also arising from edge-to-face interactions.18,56 In marked contrast, ITIC-6F exhibits dramatically larger electronic couplings to its nearest neighbors with |J| values of 45.2 and 56.8 meV (Figure 2c). Note that the trend in computed couplings parallels that found in the experimental electron mobilities (vide infra), and that the electronic coupling in ITIC-6F is the largest yet reported for an IDTT-based NFA and even exceeds those reported for


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crystalline PC61BM.66 Interestingly, stronger coupling is not found between the closely spaced π–π pairs, but rather for the pairs sharing the close CN···π interactions, in accord with the importance of nitrile interactions to both the intermolecular coupling and electron mobility of some n-type organic semiconductors.67 Lastly, the electronic coupling directionality was assessed by summing all |J| contributions along each crystallographic axis (see SI, Figure S4). While coupling in ITIC-0F has the lowest magnitude, it is the most isotropic and may enable more robust charge transport in tortuous BHJ blends.68 In contrast, ITIC-4F and ITIC-6F exhibit large |J| anisotropy along their a and b crystallographic axes, respectively. ITIC-nF Solution-Phase Redistribution and Chemical Instability. Due to the reversible end-group reactivity

Figure 3. Analysis of ITIC-nF end-group redistribution in photovoltaic active layer solutions prepared according to conventional scrambled (SCR) conditions. (a) Chemical structures of ITIC-2F and ITIC-4F. (b) 19F{1H} NMR spectra of the purified ITIC-2F and ITIC-4F NFAs compared to those of ITIC-2F SCR and ITIC-0F:ITIC-4F SCR active layer solutions. (c) HRMS analysis of the ITIC-2F SCR active layer solution.

observed during the syntheses of ITIC-2F and ITIC-3F, the chemical stability of ITIC-nF molecules within photovoltaic blend solutions was additionally evaluated. Specifically, active layer blend solutions of PBDB-TF and ITIC-2F as well as of PBDB-TF and ITIC-3F were first prepared following conventional procedures, i.e., by dissolving PBDB-TF and NFA (1:1 mass ratio) in anhydrous PhCl containing 0.5 vol% of the commonly-utilized PSC processing additive 1,8diiodooctane (DIO) at 50 °C for 16 h inside a glovebox (see SI for complete details).25,28,42 Note that the unsymmetrical ITICnF acceptors were deliberately selected because end-group redistribution in these molecules will form chemically distinct products. As will be seen below, these conventional active layer preparation conditions result in ITIC-nF end-group redistribution and consequently blends prepared by these methods will be referred to as “scrambled” and will be denoted by SCR.

The extent of end-group redistribution in these mixtures was assessed by solution 19F{1H} NMR spectroscopy because 1H NMR is uninformative (Figure S18). Surprisingly, signals attributed to both ITIC-2F and ITIC-4F are present in the 19F{1H} NMR spectrum of the conventionally-prepared PBDB-TF:ITIC-2F active layer solution (ITIC-2F SCR spectrum, Figure 3b). The peak ratio of ITIC-2F:ITIC-4F is 10:6, implying a 10:3 molar ratio due to ITIC-4F symmetry. The formation of ITIC-4F in this blend indicates the existence of a solution-phase redistribution process between ITIC-nF end-groups, i.e., 2 ITIC-2F ITIC-4F + ITIC-0F. Note that the reaction stoichiometry requires the formation of ITIC-0F and ITIC-4F in equal quantities. Although ITIC-0F cannot be directly quantified by 1H NMR spectroscopy due to peak overlap (Figure S18), its presence in the final active layer mixture is supported by HRMS analysis, where molecular ions consistent with those expected for ITIC-0F, ITIC-2F, and ITIC-4F are observed (Figure 3c). Thus, redistribution in the PBDB-TF:ITIC-2F active layer blend solution generates a final acceptor composition of ITIC-2F:ITIC-0F:ITIC-4F in a 10:3:3 molar ratio. Consequently, this blend is determined to be 60% scrambled. Analogously, scrambling in the conventionally-prepared PBDB-TF:ITIC-3F active layer blend solution is found to generate ITIC-0F and ITIC-6F (Figure S17). However, scrambling in the ITIC-3F blend solution is less extensive, only 5% (Table S11). This suggests slower redistribution reaction kinetics for the trifluorinated end-group relative to both the di- and non-fluorinated ones. Scrambling of ITIC-2F was also investigated in PhCl solutions containing neither PBDB-TF nor DIO, and while it is found to be somewhat slower, it nevertheless occurs (Figure S20). The reversibility of the end-group redistribution reaction was additionally evaluated in active layer blend solutions containing two different symmetrical ITIC-nF acceptors in a 1:1 molar ratio. These blend solutions were prepared following the same conventional procedures as noted above. Based on the combined NMR spectroscopic and HRMS data on these blends, we conclude that the end-group redistribution reaction is reversible. Specifically, after 16 h at 50 °C, the PBDB-TF:ITIC-0F:ITIC-4F blend solution is found to additionally contain the unsymmetrical acceptor ITIC-2F (ITIC-0F:ITIC-4F SCR spectrum, Figure 3b). Likewise the PBDB-TF:ITIC-0F:ITIC-6F blend solution is found to additionally contain ITIC-3F (Figure S17). The extent of endgroup scrambling in the ITIC-0F:ITIC-4F and ITIC-0F:ITIC6F blend solutions is determined by 19F{1H} NMR spectroscopy to be 34% and 4%, respectively (Table S11). Again, these results indicate a reduced propensity of the trifluorinated end-group to participate in the redistribution process. These results indicate that under standard active layer solution preparation conditions, the end-groups of IDTT-based NFAs undergo reversible redistribution. In solutions containing NFAs with chemically different end-groups, as is the case with unsymmetrical ITIC-nF molecules or with two different symmetrical ITIC-nF molecules, redistribution results in the generation of new and chemically distinct acceptors which were not initially present in the solution. Due to the unexpected nature of this chemical instability and the absence of any obvious catalyst, those factors giving rise to this reactivity were further investigated. ITIC-nF End-Group Redistribution Reaction Mechanism. The mechanism of the end-group redistribution



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occurring under conventional “scrambled” active layer solution preparation conditions was next examined. While transition-metal-catalyzed C=C/C=C cross metathesis reactions have been extensively characterized,69,70 relatively less is known about uncatalyzed olefin exchange reactions, such as the present redistribution process, including their mechanisms. Previously, photocatalyzed71–73 as well as thermally induced associative [2+2] cycloaddition/cycloreversion (CA/CR) mechanisms via cyclobutane intermediates have been proposed for C=C/C=C redistributions.74–76 Similarly, C=N/C=C redistribution between imines and Knoevenagel compounds has recently been shown to proceed through a four-membered ring intermediate via a CA/CR mechanism.77 However, a H2Ocatalyzed dissociative retro-condensation/condensation (RC/C) mechanism via aldehyde intermediates has also been proposed for C=C/C=C redistribution in other reports.78 Thus, we next outline a series of 13C isotopic labeling experiments

Figure 4. Mechanistic investigation of the ITIC-nF end-group redistribution reaction. (a) Reaction between unlabeled ITIC-0F (1.0 equiv) and 13CHO-IDTT(13C)-2F (1.0 equiv) in PhCl:DIO (99.5:0.5 v:v) with PBDB-TF at 50 °C for 24 h. Expected products of the RC/C and CA/CR mechanisms are shown. Selected regions of the (b) 19F{1H}, (c) 13C, and (d) 1H NMR spectra of the product mixture.

performed on model ITIC-nF molecules that enable the CA/CR and RC/C redistribution mechanisms to be rigorously distinguished. Note that ITIC-nF scrambling occurs under light-free conditions, precluding a photocatalyzed pathway (Figure S16). Because no intermediates indicative of either the CA/CR or RC/C mechanisms [i.e., cyclobutanes or aldehydes, respectively] are observed in the 1H NMR spectra of the photovoltaic blends prepared following conventional procedures (Figure S18), the C=C/C=C end-group redistribution mechanism of the ITIC-nF molecules is probed here by isotopic labeling (Scheme S5). First, the redistribution reaction between ITIC-0F and the 13C-labeled difluorinated mono-condensed 13CHO-IDTT(13C)-2F was investigated under the conventionally-prepared active layer blend solution conditions (Figure 4a). Note that the 13CHO-IDTT(13C)-2F aldehyde group (13CHO) reactivity is predicted to depend

dramatically on the operative reaction mechanism. Specifically, in a CA/CR process, the 13CHO moieties (v, Figure 4a) should be unreactive.79 In marked contrast, in a RC/C process, the 13CHO moieties (ii, Figure 4a) are expected to be reactive, and an equivalent of 12CHO moieties will be formed for every 13CHO which reacts with a nucleophilic endgroup (i or iii, Figure 4a). Thus, the CA/CR and RC/C mechanisms should be distinguishable by observing whether or not the 13C-enrichment of the aldehyde group is preserved during the reaction. After reaction of ITIC-0F with 13CHO-IDTT(13C)-2F for 24 h at 50 °C, the 19F{1H} NMR spectrum reveals ~20% endgroup redistribution conversion with the major fluorinated product identified as ITIC-2F (Figures 4b and S21).80 The 13C NMR analysis also indicates ~20% conversion with the major 13C-labeled product identified as 13CHO-IDTT(13C)-0F (Figures 4c and S22). Note that the 13C signal from the


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aforementioned generated ITIC-2F is largely absent indicating that it is unlabeled (Figure S22). Next, the 1H NMR low field region was examined to assess the isotopic distribution of the aldehyde-containing products. An overlapped set of doublets assignable to 13C-bound aldehydic H atoms on both 13CHOIDTT(13C)-0F and 13CHO-IDTT(13C)-2F is observed, however virtually no singlet aldehyde signal from 12CHO groups is present (Figure 4d and S23). This indicates that the 13CHOIDTT(13C)-2F aldehyde is essentially unreactive under the conventional photovoltaic active layer solution conditions. Analogous results are obtained when either unlabeled ITIC-4F or ITIC-6F is reacted with 13CHO-IDTT(13C)-0F (Section 11 in the SI). Collectively, these findings indicate that the endgroup redistribution process does not proceed via a dissociative H2O-catalyzed RC/C mechanism. Moreover, no evidence of the RC/C mechanism can be observed even under forcing conditions, e.g. with 1 vol% H2O in the PhCl solvent (Figure S23).81 Instead, these results are consistent with an associative CA/CR end-group redistribution mechanism occurring through a four-membered cyclobutane intermediate (v, Figure 4a). As this mechanism does not produce ‘free’ nucleophilic end-groups (i and iii, Figure 4a) through a retrocondensation process, we also conclude that end-group redistribution in the active layer solutions proceeds by a distinctly different mechanism than during ITIC-nF synthesis (vide supra). ITIC-nF Binary Blend Solar Cell Performance. Next, PSCs with the inverted cell architecture, ITO/ZnO/active layer/MoO3/Ag, containing a binary BHJ active layer blend of PBDB-TF and ITIC-nF were fabricated and evaluated to assess the photovoltaic (PV) response of the ITIC-nF acceptors. First, an “unscrambled” material processing methodology was developed to reduce ITIC-nF end-group redistribution observed in the active layer solutions prepared following conventional procedures (vide supra). Specifically, PBDB-TF was first pre-dissolved separately in a mixture of PhCl:DIO (99.5:0.5 v:v)25,28 at 50 °C for 16 h, cooled to 25 °C, and then added to the ITIC-nF acceptor to achieve a 1:1 mass ratio in

Figure 5. Comparison of the PBDB-TF:ITIC-nF binary and ternary PSC photovoltaic performances. (a) J–V response of the binary PSCs and (d) corresponding EQE spectra. (b) Comparison of the J–V response for the scrambled (SCR) and unscrambled binary PSCs and (e)



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corresponding EQE spectra. (c) Comparison of the J–V response for the scrambled (SCR) and unscrambled ternary PSCs and (f) corresponding EQE spectra.

Table 2. Photovoltaic Metrics of the PBDB-TF:ITIC-nF-Based PSCs Acceptor(s)

Conditiona

Scramb. %b

VOC (V)c

JSC (mA cm–2)c

FF (%)c

PCE (%)c

µh × 104 (cm2V–1s–1)

µe × 104 (cm2V–1s–1)

ITIC-0F

-

-

1.00 (1.00)

14.8 (15.1)

56.5 (57.6)

8.33 (8.72)

2.0 ± 0.6

2.7 ± 1.0

-

3

0.91 (0.92)

16.7 (17.3)

65.6 (65.7)

10.03 (10.38)

1.3 ± 1.0

2.0 ± 0.3

SCR

60

0.91 (0.91)

16.3 (16.9)

64.8 (65.5)

9.67 (10.07)

0.9 ± 0.2

5±2

-

-

0.83 (0.83)

17.6 (18.3)

67.7 (68.4)

9.91 (10.39)

2.0 ± 1.1

5±2

-

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