A heptacyclic carbon–oxygen-bridged ladder-type building block for A–D–A acceptors

Abstract

A heptacyclic carbon–oxygen-bridged ladder-type unit, CO_i7, was designed. Two corresponding nonfullerene acceptors, CO_i7IC and CO_i7DFIC, were developed. The single crystal structure of CO_i7IC indicates an S-shaped backbone and a packing via π–π interaction of the end groups. PTB7-Th:CO_i7DFIC solar cells gave a power conversion efficiency of 8.32%, with a decent fill factor of 71.9%.

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Owing to their strong light-harvesting capability, tunable energy levels and good morphological stability, nonfullerene acceptors (NFAs) show greater potential than fullerene acceptors in organic solar cells.¹ The power conversion efficiency (PCE) for NFA-based solar cells has exceeded 14%.² Acceptor–donor–acceptor (A–D–A) small molecule acceptors could be the most successful class of NFAs. They generally consist of a carbon-bridged (C-bridged) ladder-type electron-donating core unit, such as indacenodithiophene (IDT), and two strong electron-withdrawing end units, such as 1,1-dicyanomethylene-3-indanone (IC).³ The C-bridged core units are crucial for making high-performance A–D–A NFAs: (1) the extended conjugation systems facilitate π electron delocalization, reducing the bandgaps and enhancing the light absorption of the acceptors; (2) the rigid molecular structures favour improvement in the electron mobility;⁴ and (3) the out-of-plane side chains avoid overaggregation of the molecules, helping to realize an optimal morphology.⁵ However, the medium electron-donating capability of C-bridged units is unfavourable for making NFAs with stronger light-harvesting capability.⁶ In this regard, our group started using carbon–oxygen-bridged (CO-bridged) ladder-type core units for constructing NFAs with strong light absorption. The rationale is that electron-rich oxygen atoms can greatly improve the electron-donating capability of the unit via the conjugation effect. We first reported the synthesis of pentacyclic, hexacyclic and octacyclic CO-bridged units, and the corresponding A–D–A NFAs.⁷ CO-Bridged NFAs exhibited strong light-harvesting capability and high performance in solar cells. A PCE of 14.62% was achieved.⁸ Here, we report a heptacyclic CO-bridged ladder-type unit, CO_i7 (“i” indicates that C–O bonds point to the core of the molecule (inward), and “7” stands for seven fused rings), and its use in preparing two A–D–A acceptors, CO_i7IC and CO_i7DFIC (Fig. 1). The single crystal structure of CO_i7IC indicates an S-shaped backbone of the molecule and a packing via π–π interaction of the end groups. Solar cells based on CO_i7DFIC and a copolymer donor, PTB7-Th, gave a PCE of 8.32%, with a decent fill factor of 71.9%.

Fig. 1 The structures of CO_i7IC, CO_i7DFIC and PTB7-Th.

The synthesis of CO_i7IC and CO_i7DFIC is similar to that of previously reported CO-bridged NFAs (Scheme S1, ESI†).⁷ The details are given in the ESI.† CO_i7IC and CO_i7DFIC were characterized by nuclear magnetic resonance (NMR) and mass spectroscopy (see the ESI†). The structure of CO_i7IC was also confirmed by the single crystal XRD analysis (Fig. 2). The top view of CO_i7IC indicates the Z configuration of C=C double bonds connecting CO_i7 and IC units, and the locked conformation between CO_i7 and IC, in which the carbonyl group is close to the sulfur atom of thiophene. Such a molecular geometry is consistent with previous theoretical calculations,⁹ and S⋯O interaction plays an important role. The side view of the molecule indicates an S-shaped backbone of CO_i7IC, which is quite different from those of C-bridged A–D–A NFAs.^9–11 The S-shaped geometry could result from the two non-coplanar CO-bridges, which might induce a twist of the backbone plane. Being viewed along the a-axis, the material presents a one-dimensional stacking via terminal π–π interaction.¹¹ The π–π interaction distance is around 3.5 Å.

Fig. 2 The single crystal structure of CO_i7IC. (a) Top view; (b) side view; and (c) packing structure viewed along the a-axis (side chains are omitted for clarity). Note: grey, carbon; red, oxygen; blue, nitrogen; orange, sulphur.

The absorption spectra for CO_i7IC and CO_i7DFIC in chloroform and as films are shown in Fig. S11 (ESI†). In solution, CO_i7IC and CO_i7DFIC show a strong intramolecular charge transfer (ICT) band at 500–750 nm, presenting an absorption maximum at 676 nm and 689 nm, respectively. For films, the absorption maximum shifts to 698 nm and 717 nm, respectively, suggesting enhanced intermolecular interaction in the solid state. The absorption onsets for CO_i7IC and CO_i7DFIC films are 775 nm and 802 nm, respectively, corresponding to optical bandgaps (E^opt_g) of 1.60 eV and 1.55 eV, respectively. The fluorine atoms enhance the electron-withdrawing capability of the end units and strengthen the ICT, thus leading to a smaller E^opt_g and stronger light-harvesting capability for CO_i7DFIC.¹² The energy levels estimated from cyclic voltammetry measurements are shown in Fig. S13 (ESI†).¹³ The highest occupied molecular orbital (HOMO) levels for CO_i7IC and CO_i7DFIC are −5.66 eV and −5.78 eV, respectively, and the lowest unoccupied molecular orbital (LUMO) levels are −3.92 eV and −4.06 eV, respectively. The donor PTB7-Th exhibits a HOMO at −5.38 eV and a LUMO at −3.11 eV.

Bulk heterojunction solar cells with an ITO/ZnO/PTB7-Th:NFA/MoO₃/Ag structure were made to evaluate the photovoltaic performance of CO_i7IC and CO_i7DFIC.¹⁴J–V curves and external quantum efficiency (EQE) spectra are shown in Fig. 3, and the performance data are listed in Table 1. The best PTB7-Th:CO_i7IC cells gave a PCE of 6.36%, with an open-circuit voltage (V_oc) of 0.82 V, a short-circuit current density (J_sc) of 13.12 mA cm⁻² and a fill factor (FF) of 58.9%. These cells have a D/A ratio of 1 : 1.4 (w/w), an active layer thickness of 105 nm and 0.2 vol% 1,8-diiodooctane (DIO) as the additive (Tables S1–S3, ESI†). The best PTB7-Th:CO_i7DFIC cells gave a PCE of 8.32%, with a V_oc of 0.67 V, a J_sc of 17.36 mA cm⁻² and a FF of 71.9%. These cells have a D/A ratio of 1 : 1.4 (w/w), an active layer thickness of 83 nm and 0.4 vol% DIO as the additive (Tables S4–S6, ESI†). CO_i7DFIC cells gave a lower V_oc than CO_i7IC cells because of the deeper LUMO of CO_i7DFIC, since V_oc is proportional to (LUMO_acceptor – HOMO_donor).¹⁵ In contrast, CO_i7DFIC cells gave much higher J_sc and FF than CO_i7IC cells. From CO_i7IC cells to CO_i7DFIC cells, the EQE spectra broaden and intensify (Fig. 3b). The broadening of the EQE spectra is consistent with the enhanced light-harvesting capability of CO_i7DFIC, while the enhanced EQE might be due to improved charge generation and transport in CO_i7DFIC cells. We investigated exciton dissociation probabilities (P_diss) (Fig. S14, ESI†). P_diss for CO_i7IC and CO_i7DFIC cells are 90.7% and 92.4%, respectively, suggesting more efficient charge generation in CO_i7DFIC cells.² The 71.9% FF for CO_i7DFIC cells is among the highest values reported for PTB7-Th:NFA solar cells to date.^9,16 A high FF suggests balanced charge transport and suppressed charge recombination in the active layer. We measured hole and electron mobilities (μ_h and μ_e) using the space charge limited current (SCLC) method (Fig. S15 and S16 and Table S7, ESI†).¹⁷ Compared with the PTB7-Th:CO_i7IC blend film, the PTB7-Th:CO_i7DFIC blend film shows higher μ_h and μ_e of 4.73 × 10⁻⁴ cm² V⁻¹ s⁻¹ and 6.95 × 10⁻⁵ cm² V⁻¹ s⁻¹, respectively, and more balanced charge carrier transport (μ_h/μ_e is closer to 1). We studied bimolecular recombination by plotting J_sc against light intensity (P_light) (Fig. S17, ESI†).¹⁸ The data were fitted to a power law: J_sc ∝ P^α_light. The α values for CO_i7IC and CO_i7DFIC cells are 0.933 and 0.969, respectively, suggesting less charge recombination in the latter. Enhanced mobilities, balanced charge transport and reduced charge recombination lead to a high FF for CO_i7DFIC cells. The film morphology was studied using an atomic force microscope (AFM) (Fig. S18, ESI†). Compared with the PTB7-Th:CO_i7IC blend film, the PTB7-Th:CO_i7DFIC blend film presents very clear nanofiber structures with ∼16 nm diameter. Such an optimal nanoscale phase separation favors charge generation and transport, also accounting for the higher J_sc and FF for PTB7-Th:CO_i7DFIC cells. DIO additive can promote phase separation during the film-forming process due to its low dissolving capability to NFAs.^18a The XRD profiles for the pure and blend films are shown in Fig. S19 (ESI†). The CO_i7IC and CO_i7DFIC films gave a broad diffraction peak centered at 2θ of ∼24°, corresponding to a π–π stacking d-spacing of ∼3.7 Å. Sharp diffraction peaks at 2θ of 9.04°, 10.80°, 22.96° and 26.34° were also observed for the CO_i7DFIC film, suggesting a higher crystallinity of CO_i7DFIC. The PTB7-Th:CO_i7IC and PTB7-Th:CO_i7DFIC blend films show similar XRD profiles, with a broad diffraction peak centered at 2θ of ∼24°.

Fig. 3 J–V curves (a) and EQE spectra (b) for the solar cells.

Table 1 Performance data for the solar cells

D:A	V oc [V]	J sc [mA cm^−2]	FF [%]	PCE [%]
a The data in the parentheses are integrated current densities from EQE spectra. b The data in the parentheses are averages for 10 cells.
PTB7-Th:COi7IC	0.82	13.12 (12.60)^a	58.9	6.36 (6.25)^b
PTB7-Th:COi7DFIC	0.67	17.36 (16.71)^a	71.9	8.32 (8.00)^b

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Conclusions

In summary, a heptacyclic carbon–oxygen-bridged ladder-type unit, CO_i7, was designed. Two corresponding A–D–A acceptors, CO_i7IC and CO_i7DFIC, were prepared. The single crystal structure of CO_i7IC indicates an S-shaped backbone and a one-dimensional stacking via terminal π–π interaction. A PCE of 8.32% and a decent FF of 71.9% obtained from PTB7-Th:CO_i7DFIC solar cells demonstrate the potential of the CO_i7 building block for developing efficient nonfullerene acceptors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We greatly acknowledge the National Natural Science Foundation of China (U1401244, 21572041, 51503050, 51773045, 21772030 and 21704021), the National Key Research and Development Program of China (2017YFA0206600) and the Youth Association for Promoting Innovation (CAS) for financial support.

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Footnotes

† Electronic supplementary information (ESI) available: Material preparation and characterization, solar cell fabrication and measurements. CCDC 1837067. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8qm00285a

‡ K. Jin and C. Deng contributed equally to this work.

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