Near-IR absorption and photocurrent generation using a first-of-its-kind boron difluoride formazanate non-fullerene acceptor

Abstract

Herein, we report the synthesis and characterization of the first non-fullerene acceptor (NFA) containing a boron difluoride formazanate (BF₂fz) core end-capped with N-annulated perylene diimides (PDIs). Electronic coupling between the BF₂fz core and the PDI endcaps enabled tuning of the lowest unoccupied molecular orbital, leading to near-panchromatic optical absorption. Post-deposition solvent vapor annealing of the new NFA resulted in a significant red-shift in the optical spectra, which stretched into the near-IR. Proof-of-concept organic photovoltaic (OPV) devices were constructed to demonstrate the potential of this new material as an NFA. SVA treatment of the active layer resulted in a 2-fold increase in power conversion efficiency (PCE), due mainly to increases in the BF₂PDI₂ generated photocurrent that extended into the near-IR.

---

Introduction

For applications in organic photovoltaics (OPVs), π-conjugated materials are perfectly suited for use as non-fullerene acceptors (NFAs) because both physical and optoelectronic properties may be optimized to match the electron-donor material with which they are paired in the active layer.^1,2 One class of non-fullerene acceptors (NFAs) recognized for high redox and thermal stability, as well as high molar absorptivity, are perylene diimides (PDIs).^3,4 Despite such favorable qualities, the efficiency of charge-extraction for PDI-based NFA materials is often hindered by the formation of large π–π stacking domains throughout the BHJ.¹ To promote more favorable phase separation in the BHJ, PDI self-assembly may be disrupted through either direct dimerization of PDIs^5–8 or insertion of a core between the PDI-moieties.^9–20

The Welch research group has previously explored the optoelectronic influence of thienoisoindigo, thienyl- and pyridyl-diketopyrrolopyrrole cores that were acetylene-linked to N-annulated PDI (Fig. 1).²¹ Strong electronic communication between the cores and PDI endcaps adjusted the highest occupied molecular orbital (HOMO) energy level of these molecules, affording near-panchromatic absorptions ideally suited for OPV. In search of a core that could achieve similar absorptivity, while instead influencing the lowest unoccupied molecular (LUMO) energy level, we targeted an acetylene-terminated boron difluoride formazanate (BF₂fz), a chromophore popularized by the Gilroy research group.^22–24 The properties of BF₂fz materials are highly sensitive to structural variation at the N-aryl substituents,^25–27 meaning the optoelectronic properties can be tuned by the choice of endcap material. In this contribution, we report the synthesis, characterization, and NFA capability of a new π-conjugated material comprised of a BF₂fz core acetylene-linked to N-annulated PDI endcaps (BF₂PDI₂).

Fig. 1 Depicting previously investigated HOMO-modifying and newly synthesized LUMO-modifying N-annulated PDI-core-PDI materials. R = hexyl (HOMO modifying cores) or 2-ethylhexyl (LUMO modifying core).

Characterization

The BF₂fz core²² and N-annulated PDI (Br-PDIN-EH)⁸ end caps were connected via a Sonogashira cross-coupling reaction to generate BF₂PDI₂ (see ESI,† for full experimental details). The identity of BF₂PDI₂ was confirmed by ¹H, ¹¹B, ¹³C, and ¹⁹F NMR spectroscopy, as well as MALDI TOF mass spectrometry and CHN elemental analysis (Fig. S1–S7, ESI†). The ¹¹B and ¹⁹F NMR spectra exhibited diagnostic 1 : 2 : 1 triplet and 1 : 1 : 1 : 1 quartet coupling patterns, respectively.²⁵ Moreover, the ¹H NMR spectrum showed no evidence of a formazan related N-H peak (at ∼15 ppm),²² strongly suggesting the BF₂fz core remained intact throughout synthesis and purification.

The thermal properties of BF₂PDI₂ were measured using a combination of differential scanning calorimetry and thermal gravimetric analysis (Fig. S8 and S9, ESI†). No obvious glass transitions or melting points were observed between 100–300 °C, while the onset of molecular decomposition was not observed until >300 °C. The high thermal stability observed in BF₂PDI₂ may be attributed to significant π-conjugation between the BF₂fz core and the N-annulated PDI endcaps. This notion was supported by the density functional theory (DFT) optimized structure of BF₂PDI₂ at the B3LYP-6-31G(d,p) level of theory (Fig. S14, ESI†). The PDI units were nearly co-planar with each other, with only a small degree of bending caused by the BF₂fz core. This DFT optimized ‘dragonfly’ structure corresponded well with previously determined X-ray crystallography structures for compounds using the same BF₂fz core.^23,24

Next, the electronic properties of BF₂PDI₂ were probed using cyclic voltammetry (CV), with additional information provided by DFT calculations. The CV of BF₂PDI₂ was comprised of three irreversible oxidation events, as well as four fully reversible reduction events (Fig. 2). While the number oxidation events could be determined by differential pulse voltammetry (Fig. S13, ESI†), the precise electrochemical origin may not be readily assigned. On the other hand, the first and fourth reductions (E_1/2 = −0.85 and −1.96 V vs. Fc^+/0) may be attributed to BF₂fz core, while the second and third reduction events (E_1/2 = −1.32 and −1.60 V vs. Fc^+/0) may be attributed to N-annulated PDI as the current passed during these redox processes was 2× larger. By DFT calculations, the first oxidation event appeared to be delocalized across the entire molecule, while the first reduction event was largely centered on the BF₂fz core (Fig. S15, ESI†). The predicted HOMO–LUMO energy levels correlate well with the observed CV data. Together, these data strongly suggest that the BF₂fz core has a higher electron affinity than the N-annulated PDI endcaps, which served to lower the LUMO energy level of BF₂PDI₂, when compared to related compounds with different cores.²¹

Fig. 2 Cyclic voltammogram of BF₂PDI₂, measured at 100 mV s⁻¹ in CH₂Cl₂ under argon with 0.1 M TBAPF₆ supporting electrolyte (WE = glassy carbon, CE = Pt-wire, pseudo-RE = Ag/AgCl). Inset are all measured oxidation (Ox) and reduction (R) events, referenced to Fc^+/0.

The optical properties of BF₂PDI₂ were measured by UV-visible-nearIR absorption spectroscopy (UV-vis-nIR). The UV-vis-nIR profile of BF₂PDI₂ in solution exhibited a broad absorbance between 450–750 nm, with an absorption maximum (λ_max) at 543 nm attributable to the PDI endcaps and a low energy shoulder just beyond 600 nm attributed to the BF₂fz core (Fig. 3). By overlaying the optical spectra of BF₂PDI₂ with its constituent fragments (BF₂Ace₂ and PDIN-EH, Fig. S11, ESI†), it was revealed that the low energy shoulder attributed to BF₂fz core was red-shifted in BF₂PDI₂, emphasizing the extension of π-conjugation induced by the PDI endcaps. When spin-cast into a thin-film, the optical profile of BF₂PDI₂ underwent several notable changes. Beyond the slightly red-shifted onset of absorption (λ_onset), the broadening of λ_max was also accompanied by an appreciable increase in the intensity of the low-energy shoulder, consistent with enhanced electron delocalization along the π-conjugated backbone.²¹

Fig. 3 UV-vis-nIR absorption profile of BF₂PDI₂ in CHCl₃ solution (red), thin-film spin-cast from o-dichlorobenzene (green), and the same thin-film solvent vapor annealed from CHCl₃ (blue).

In the past, we have shown that post-deposition solvent vapor annealing (SVA) can induce structural order within the solid-state morphology of similar PDI–core–PDI materials.^11,28–31 Exposure of the BF₂PDI₂ thin-film to CHCl₃ solvent vapors caused the once broad solid-state optical profile to resolve into two distinct absorption regions. The PDI-based λ_max region was split into two peaks (λ = 508 and 560 nm), while the low-energy shoulder developed fine structure (λ_max = 790 nm) that extended well into the near-IR (λ_onset = 850 nm). This behaviour is strongly indicative of molecular reorganization and/or aggregation of BF₂PDI₂ into ordered domains with strong intermolecular electronic-coupling in the solid-state.^32–34

OPV devices

The NFA capabilities of BF₂PDI₂ were assessed by constructing some proof-of-concept OPV devices with the following inverted architecture: ITO/ZnO/ternary active layer/MoOx/Ag (full experimental details in ESI†). A ternary active layer comprised of electron-donor polymer PPDT2FBT (FBT), PC₆₁BM, and BF₂PDI₂ was employed for OPV devices. FBT was selected because of its complementary absorption with the solvent vapor annealed film of BF₂PDI₂ (Fig. S16, ESI†). PC₆₁BM was added to this ternary blend to assist with charge mobility.³⁵ Using a FBT : BF₂PDI₂ : PC₆₁BM blend ratio of (1 : 1 : 0.5) at 10 mg mL⁻¹ total concentration, the best OPV devices achieved an open circuit voltage (V_oc) of 0.69 V, a short circuit current (J_sc) of 1.1 mA cm⁻², and a fill factor (FF) of 41%, leading to a power conversion efficiency (PCE) of 0.3% (Table S1, ESI†).

Post-deposition treatment of these films with CHCl₃ SVA resulted in the desired optical profile shift of BF₂PDI₂, giving the OPV devices a near-panchromatic absorbance (Fig. 4A). Device performance was maximized after 5 min of CHCl₃ SVA, where both the J_sc and FF increased to 1.9 mA cm⁻² and 49%, respectively, leading to a 2-fold increase in PCE to 0.6% (Fig. 4B). Longer SVA treatments led to OPV performances similar to as-cast devices (Fig. S17 and Table S2, ESI†). This phenomena has previously been observed with our N-annulated PDI–core–PDI materials and was attributed to over-annealing of the active layer.^11,28

Fig. 4 Optical absorption profile (A), J–V curves (B), an external quantum efficiency plots (C) comparing the effects of solvent vapor annealing with CHCl₃ on ternary blend OPV devices with a FBT : BF₂PDI₂ : PC₆₁BM (1 : 1 : 0.5) active layer at 10 mg mL⁻¹ total concentration.

To better understand the observed enhancement in OPV performance for FBT : BF₂PDI₂ : PC₆₁BM (1 : 1 : 0.5) ternary blends that were SVA using CHCl₃ for 5 min, photoluminescence (PL) and external quantum efficiency (EQE) experiments were performed. PL measurements of the ternary blend showed efficient quenching of the FBT polymer before and after SVA treatment (Fig. S18, ESI†). Analysis of the EQE spectra, the measured photocurrent clearly displays contributions from all components of the ternary blend (Fig. 4C). Following SVA treatment, photocurrent generation was extended well into the near-IR (λ_max = 790 nm), clearly emphasizing the increased contribution from the new NFA material, BF₂PDI₂.

Surface morphology differences between the as-cast and SVA treated devices were also analyzed by atomic force microscopy (AFM). AFM height images showed significant differences between the as-cast and SVA treated devices (Fig. 5), as the root-mean square surface roughness of the films increased from 1.9 nm to 4.8 nm. X-ray diffraction (XRD) measurements of ternary blend films before and after CHCl₃ SVA failed to show any distinguishing features that could be attributed to BF₂PDI₂ (Fig. S19, ESI†).³⁶ Together, these data suggest that SVA enhanced OPV performance by inducing BF₂PDI₂ aggregation into phase separated domains, rather than crystallization.

Fig. 5 Atomic force microscopy height images of ternary blend OPV devices with an FBT : BF₂PDI₂ : PC₆₁BM (1 : 1 : 0.5) active layer at 10 mg mL⁻¹ total concentration.

Conclusions

In conclusion, we have synthesized and fully characterized a first-of-its-kind NFA containing a BF₂fz core. The use of acetylene-linkers enabled through-conjugation between the BF₂fz core and the PDI endcaps, leading to an altered LUMO energy level that resulted in near-panchromatic absorption for BF₂PDI₂. The new compound is a rare example of an acceptor–acceptor–acceptor type π-conjugated architecture. Treatment of solid-state BF₂PDI₂ with CHCl₃ SVA resulted in a significant bathochromic shift of the optical spectra, leading to near-IR absorption. The overall performance in a series of proof-of-concept OPV devices, where BF₂PDI₂ was used as an NFA in a ternary blend system, was improved 2-fold by SVA treatment of the active layer. The increase in PCE could be mainly attributed to favorable aggregation of BF₂PDI₂ in the active layer which increased the generated photocurrent. This notion was supported by EQE and AFM measurements. This work demonstrates the utility of the BF₂fz organic dye to construct narrow bandgap π-conjugated materials for use in organic electronics and provides the groundwork for further development.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

GCW acknowledges the Canada Research Chairs Program, CFI JELF (34102), NSERC DG (2019-04392), and the University of Calgary. JBG acknowledges CFI JELF (33977), NSERC DG (2018-04240), and the University of Western Ontario. JSD acknowledges NSERC CGS-D Scholarship. JK acknowledges Alberta Graduate Excellence Scholarship (AGES) program. Compute Canada and Westgrid are acknowledged for computational resources. This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund (CFREF).

References

1. S. Rajaram, R. Shivanna, S. K. Kandappa and K. S. Narayan, Nonplanar Perylene Diimides as Potential Alternatives to Fullerenes in Organic Solar Cells, J. Phys. Chem. Lett., 2012, 3, 2405–2408.
2. C. B. Nielsen, S. Holliday, H.-Y. Chen, S. J. Cryer and I. McCulloch, Non-Fullerene Electron Acceptors for Use in Organic Solar Cells, Acc. Chem. Res., 2015, 48, 2803–2812.
3. Z. Liu, Y. Wu, Q. Zhang and X. Gao, Non-Fullerene Small Molecule Acceptors Based on Perylene Diimides, J. Mater. Chem. A, 2016, 4, 17604–17622.
4. W. Chen and Q. Zhang, Recent Progress in Non-Fullerene Small Molecule Acceptors in Organic Solar Cells (OSCs), J. Mater. Chem. C, 2017, 5, 1275–1302.
5. D. Sun, D. Meng, Y. Cai, B. Fan, Y. Li, W. Jiang, L. Huo, Y. Sun and Z. Wang, Non-Fullerene-Acceptor-Based Bulk-Heterojunction Organic Solar Cells with Efficiency over 7%, J. Am. Chem. Soc., 2015, 137, 11156–11162.
6. D. Meng, D. Sun, C. Zhong, T. Liu, B. Fan, L. Huo, Y. Li, W. Jiang, H. Choi, T. Kim, J. Y. Kim, Y. Sun, Z. Wang and A. J. Heeger, High-Performance Solution-Processed Non-Fullerene Organic Solar Cells Based on Selenophene-Containing Perylene Bisimide Acceptor, J. Am. Chem. Soc., 2016, 138, 375–380.
7. A. D. Hendsbee, J.-P. Sun, W. K. Law, H. Yan, I. G. Hill, D. M. Spasyuk and G. C. Welch, Synthesis, Self-Assembly, and Solar Cell Performance of N-Annulated Perylene Diimide Non-Fullerene Acceptors, Chem. Mater., 2016, 28, 7098–7109.
8. S. V. Dayneko, A. D. Hendsbee and G. C. Welch, Combining Facile Synthetic Methods with Greener Processing for Efficient Polymer-Perylene Diimide Based Organic Solar CellsSmall, Methods, 2018, 2, 1800081.
9. Y. Lin, Y. Wang, J. Wang, J. Hou, Y. Li, D. Zhu and X. Zhan, A Star-Shaped Perylene Diimide Electron Acceptor for High-Performance Organic Solar Cells, Adv. Mater., 2014, 26, 5137–5142.
10. Y. Duan, X. Xu, H. Yan, W. Wu, Z. Li and Q. Peng, Pronounced Effects of a Triazine Core on Photovoltaic Performance–Efficient Organic Solar Cells Enabled by a PDI Trimer-Based Small Molecular Acceptor, Adv. Mater., 2017, 29, 1605115.
11. S. M. McAfee, S. V. Dayneko, P. Josse, P. Blanchard, C. Cabanetos and G. C. Welch, Simply Complex: The Efficient Synthesis of an Intricate Molecular Acceptor for High-Performance Air-Processed and Air-Tested Fullerene-Free Organic Solar Cells, Chem. Mater., 2017, 29, 1309–1314.
12. J. Liu, S. Xie, S. Feng, M. Li, L. Wu, X. Xu, X. Chen, C. Li and Z. Bo, A Propeller-Shaped Perylene Diimide Hexamer as a Nonfullerene Acceptor for Organic Solar Cells, J. Mater. Chem. C, 2018, 6, 9336–9340.
13. H. Lin, S. Chen, H. Hu, L. Zhang, T. Ma, J. Y. L. Lai, Z. Li, A. Qin, X. Huang, B. Tang and H. Yan, Reduced Intramolecular Twisting Improves the Performance of 3D Molecular Acceptors in Non-Fullerene Organic Solar Cells, Adv. Mater., 2016, 28, 8546–8551.
14. Q. Wu, D. Zhao, A. M. Schneider, W. Chen and L. Yu, Covalently Bound Clusters of Alpha-Substituted PDI—Rival Electron Acceptors to Fullerene for Organic Solar Cells, J. Am. Chem. Soc., 2016, 138, 7248–7251.
15. J. Zhang, Y. Li, J. Huang, H. Hu, G. Zhang, T. Ma, P. C. Y. Chow, H. Ade, D. Pan and H. Yan, Ring-Fusion of Perylene Diimide Acceptor Enabling Efficient Nonfullerene Organic Solar Cells with a Small Voltage Loss, J. Am. Chem. Soc., 2017, 139, 16092–16095.
16. Q. Zhang, X. Xu, S. Chen, G. B. Bodedla, M. Sun, Q. Hu, Q. Peng, B. Huang, H. Ke, F. Liu, T. P. Russell and X. Zhu, Phenylene-Bridged Perylenediimide-Porphyrin Acceptors for Non-Fullerene Organic Solar Cells, Sustain, Energy Fuels, 2018, 2, 2616–2624.
17. T. A. Welsh, A. Laventure, T. Baumgartner and G. C. Welch, Dithienophosphole-Based Molecular Electron Acceptors Constructed Using Direct (Hetero)Arylation Cross-Coupling Methods, J. Mater. Chem. C, 2018, 6, 2148–2154.
18. T. A. Welsh, A. Laventure, A. F. Alahmadi, G. Zhang, T. Baumgartner, Y. Zou, F. Jäkle and G. C. Welch, Borane Incorporation in a Non-Fullerene Acceptor To Tune Steric and Electronic Properties and Improve Organic Solar Cell Performance, ACS Appl. Energy Mater., 2019, 2, 1229–1240.
19. Z. Luo, T. Liu, Z. Chen, Y. Xiao, G. Zhang, L. Huo, C. Zhong, X. Lu, H. Yan, Y. Sun and C. Yang, Isomerization of Perylene Diimide Based Acceptors Enabling High-Performance Nonfullerene Organic Solar Cells with Excellent Fill Factor, Adv. Sci., 2019, 6, 1802065.
20. J. D. B. Koenig, A. Laventure and G. C. Welch, Harnessing Direct (Hetero)Arylation in Pursuit of a Saddle-Shaped Perylene Diimide Tetramer, ACS Appl. Energy Mater., 2019, 2, 8939–8945.
21. J. R. Cann, C. Cabanetos and G. C. Welch, Spectroscopic Engineering toward Near-Infrared Absorption of Materials Containing Perylene Diimide, ChemPlusChem, 2017, 82, 1359–1364.
22. S. M. Barbon and J. B. Gilroy, Boron Difluoride Formazanate Copolymers with 9,9-Di-n-Hexylfluorene Prepared by Copper-Catalyzed Alkyne–Azide Cycloaddition Chemistry, Polym. Chem., 2016, 7, 3589–3598.
23. J. S. Dhindsa, R. R. Maar, S. M. Barbon, M. O. Avilés, Z. K. Powell, F. Lagugné-Labarthet and J. B. Gilroy, A π-Conjugated Inorganic Polymer Constructed from Boron Difluoride Formazanates and Platinum(II) Diynes, Chem. Commun., 2018, 54, 6899–6902.
24. J. S. Dhindsa, A. Melenbacher, S. M. Barbon, M. J. Stillman and J. B. Gilroy, Altering the Optoelectronic Properties of Boron Difluoride Formazanate Dyes via Conjugation with Platinum(II)-Acetylides, Dalton Trans., 2020 10.1039/C9DT03417J.
25. R. R. Maar, S. M. Barbon, N. Sharma, H. Groom, L. G. Luyt and J. B. Gilroy, Evaluation of Anisole-Substituted Boron Difluoride Formazanate Complexes for Fluorescence Cell Imaging, Chem. – Eur. J., 2015, 21, 15589–15599.
26. R. R. Maar, R. Zhang, D. G. Stephens, Z. Ding and J. B. Gilroy, Near-Infrared Photoluminescence and Electrochemiluminescence from a Remarkably Simple Boron Difluoride Formazanate Dye, Angew. Chem., Int. Ed., 2019, 58, 1052–1056.
27. J. B. Gilroy and E. Otten, Formazanate Coordination Compounds: Synthesis, Reactivity, and Applications, Chem. Soc. Rev., 2020, 49, 85–113.
28. S. M. McAfee, A.-J. Payne, S. V. Dayneko, G. P. Kini, C. E. Song, J.-C. Lee and G. C. Welch, A Non-Fullerene Acceptor with a Diagnostic Morphological Handle for Streamlined Screening of Donor Materials in Organic Solar Cells, J. Mater. Chem. A, 2017, 5, 16907–16913.
29. S. M. McAfee, A.-J. Payne, A. D. Hendsbee, S. Xu, Y. Zou and G. C. Welch, Toward a Universally Compatible Non-Fullerene Acceptor: Multi-Gram Synthesis, Solvent Vapor Annealing Optimization, and BDT-Based Polymer Screening, Sol. RRL, 2018, 2, 1800143.
30. D. Zomerman, J. Kong, S. M. McAfee, G. C. Welch and T. L. Kelly, Control and Characterization of Organic Solar Cell Morphology Through Variable-Pressure Solvent Vapor Annealing, ACS Appl. Energy Mater., 2018, 1, 5663–5674.
31. A.-J. Payne, N. A. Rice, S. M. McAfee, S. Li, P. Josse, C. Cabanetos, C. Risko, B. H. Lessard and G. C. Welch, Donor or Acceptor? How Selection of the Rylene Imide End Cap Impacts the Polarity of π-Conjugated Molecules for Organic Electronics, ACS Appl. Energy Mater., 2018, 1, 4906–4916.
32. A.-J. Payne, S. Li, S. V. Dayneko, C. Risko and G. C. Welch, An Unsymmetrical Non-Fullerene Acceptor: Synthesis via Direct Heteroarylation, Self-Assembly, and Utility as a Low Energy Absorber in Organic Photovoltaic Cells, Chem. Commun., 2017, 53, 10168–10171.
33. M. E. Farahat, C.-S. Tsao, Y.-C. Huang, S. Hsiung Chang, W. Budiawan, C.-G. Wu and C.-W. Chu, Toward Environmentally Compatible Molecular Solar Cells Processed from Halogen-Free Solvents, J. Mater. Chem. A, 2016, 4, 7341–7351.
34. G. L. Schulz and S. Ludwigs, Controlled Crystallization of Conjugated Polymer Films from Solution and Solvent Vapor for Polymer Electronics, Adv. Funct. Mater., 2017, 27, 1603083.
35. S. B. Srivastava, S. K. Srivastava and S. P. Singh, Molecular-Shape-Induced Efficiency Enhancement in PC61BM and PC71BM Based Ternary Blend Organic Solar Cells, J. Phys. Chem. C, 2017, 121, 17104–17111.
36. F. Tintori, A. Laventure and G. C. Welch, Additive Induced Crystallization of a Twisted Perylene Diimide Dimer within a Polymer Matrix, Soft Matter, 2019, 15, 5138–5146.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0qm00109k

---