Alkoxythiophene and alkylthiothiophene π-bridges enhance the performance of A–D–A electron acceptors

Abstract

Three nonfullerene acceptors CO5DFIC, CO5DFIC-OT and CO5DFIC-ST were developed. CO5DFIC-OT and CO5DFIC-ST have alkoxythiophene and alkylthiothiophene π-bridges, respectively, and they show higher LUMO levels and enhanced light-harvesting capability compared to CO5DFIC without π-bridges. CO5DFIC-OT and CO5DFIC-ST solar cells gave higher open-circuit voltage, short-circuit current density and power conversion efficiency than CO5DFIC cells.

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Recently, considerable progress has been made in organic solar cells due to the rapid development of efficient nonfullerene acceptors (NFAs).¹ Compared with fullerene acceptors, NFAs have advantages like tunable energy levels, strong light-harvesting capability and good morphological stability.^1a NFA solar cells offer high power conversion efficiencies (PCEs) (>14% for single-junction and >17% for tandem).² Acceptor–donor–acceptor (A–D–A) small molecule acceptors are promising NFAs.³ A–D–A NFAs usually consist of a ladder-type electron-donating core unit and two electron-accepting end units. π-Bridges can be added between the core and end units to tune optoelectronic properties. A–D–A NFAs with π-bridges like thiophene,⁴ thieno[3,4-b]thiophene,⁵ benzo[c][1,2,5]thiadiazole⁶ and benzo[d][1,2,3]triazole⁷ have been reported. Alkoxythiophene (OT) and alkylthiothiophene (ST) are electron-rich π-bridges widely used for improving the performance of donor materials.⁸ S⋯O and S⋯S noncovalent interactions can lock the conformation, enhance molecular packing and facilitate charge transport for the donors.⁸ However, their application in A–D–A NFAs is less explored.⁹ In this work, we developed three A–D–A acceptors CO5DFIC, CO5DFIC-OT and CO5DFIC-ST, and studied the effect of OT and ST π-bridges on material properties and photovoltaic performance (Fig. 1). Both OT and ST can lift the lowest unoccupied molecular orbital (LUMO) levels and improve the light-harvesting capability of A–D–A NFAs, thus enhancing the open-circuit voltage (V_oc), short-circuit current density (J_sc) and PCE for solar cells. When blended with donor PTB7-Th, CO5DFIC-ST with ST π-bridges afforded a PCE of 9.73%, which is much higher than that of CO5DFIC (5.58%).

Fig. 1 Structures of CO5DFIC, CO5DFIC-OT and CO5DFIC-ST.

The synthetic routes to CO5DFIC, CO5DFIC-OT and CO5DFIC-ST are shown in Scheme S1 (ESI†). Core unit CO5 was prepared according to our previous report.^1c OT and ST π-bridges were introduced via Stille coupling. The difluoro-substituted 1,1-dicyanomethylene-3-indanone (DFIC) end units were introduced via Knoevenagel condensation. The details are given in the ESI.† Intermediates and the final NFAs were characterized by nuclear magnetic resonance (NMR) and mass spectroscopy (see the ESI†). These molecules show good solubility in common solvents such as chloroform, toluene and chlorobenzene.

The absorption spectra of CO5DFIC, CO5DFIC-OT and CO5DFIC-ST in chloroform and as films are shown in Fig. S15 (ESI†) and Fig. 2a, respectively. In solution, they show a strong intramolecular charge transfer (ICT) absorption band, with a peak at 684 nm, 811 nm and 753 nm, respectively (Table 1). For films, the spectra show bathochromic shifts. From solution to films, the redshifts for CO5DFIC, CO5DFIC-OT and CO5DFIC-ST are 35 nm, 52 nm and 67 nm, respectively. The larger redshifts for CO5DFIC-OT and CO5DFIC-ST than that of CO5DFIC suggest that OT and ST π-bridges favor molecular packing in the solid state. The absorption onsets for CO5DFIC, CO5DFIC-OT and CO5DFIC-ST films are 794 nm, 972 nm and 928 nm, respectively, corresponding to optical bandgaps (E^opt_g) of 1.56 eV, 1.28 eV and 1.34 eV, respectively. Smaller E^opt_g values of CO5DFIC-OT and CO5DFIC-ST suggest that the electron-rich OT and ST π-bridges strengthen the ICT, leading to a bandgap shrink. Meanwhile, the ICT bands of CO5DFIC-OT and CO5DFIC-ST are broader than that of CO5DFIC. The FWHM are 148 nm, 206 nm and 209 nm for CO5DFIC, CO5DFIC-OT and CO5DFIC-ST, respectively. The smaller E^opt_g and broadened ICT bands demonstrate the improved light-harvesting capability of CO5DFIC-OT and CO5DFIC-ST, which could bring high J_sc for solar cells. Compared with the spectrum of CO5DFIC, the spectra of CO5DFIC-OT and CO5DFIC-ST are more complementary with that of donor PTB7-Th (absorption band at 500–800 nm).

Fig. 2 (a) Absorption spectra of PTB7-Th, CO5DFIC, CO5DFIC-OT and CO5DFIC-ST films; and (b) their energy level diagrams.

Table 1 Optical and electrochemical data for CO5DFIC, CO5DFIC-OT and CO5DFIC-ST

Acceptors	λ sol [nm]	λ film [nm]	λ on [nm]	E ^optg ^a [eV]	E ^onox [V]	E ^onred [V]	HOMO^b [eV]	LUMO^c [eV]	E ^ecg ^d [eV]
a E ^optg = 1240/λon. b HOMO = −(E^onox + 4.8). c LUMO = −(E^onred + 4.8). d E ^ecg = LUMO−HOMO.
CO5DFIC	684	719	794	1.56	1.16	−0.51	−5.96	−4.29	1.67
CO5DFIC-OT	811	863	972	1.28	0.71	−0.85	−5.51	−3.95	1.56
CO5DFIC-ST	753	820	928	1.34	0.75	−0.88	−5.55	−3.92	1.63

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The energy levels of CO5DFIC, CO5DFIC-OT and CO5DFIC-ST were evaluated by cyclic voltammetry (CV) (Fig. S16, ESI†).¹⁰ The energy level diagram is given in Fig. 2b. The highest occupied molecular orbital (HOMO) levels of CO5DFIC, CO5DFIC-OT and CO5DFIC-ST are −5.96 eV, −5.51 eV and −5.55 eV, respectively, and the LUMO levels are −4.29 eV, −3.95 eV and −3.92 eV, respectively. The electrochemical bandgaps (E^ec_g) of CO5DFIC, CO5DFIC-OT and CO5DFIC-ST are 1.67 eV, 1.56 eV and 1.63 eV, respectively, which are close to E^opt_g. The E^ec_g values are 0.1–0.3 eV higher than E^opt_g, which might result from the interface barrier between the film and the electrode surface during CV measurements.¹¹ OT and ST π-bridges lift both HOMO and LUMO levels of the materials. The higher LUMO levels of CO5DFIC-OT and CO5DFIC-ST are favorable for obtaining higher V_oc in solar cells. Donor PTB7-Th has a HOMO at −5.39 eV and a LUMO at −3.12 eV.^1e

Inverted solar cells with a structure of ITO/ZnO/PTB7-Th:NFA/MoO₃/Ag were made to evaluate the performance of CO5DFIC, CO5DFIC-OT and CO5DFIC-ST.¹²J–V curves and external quantum efficiency (EQE) spectra are shown in Fig. 3a and b, respectively, and the performance data are listed in Table 2. The best PTB7-Th:CO5DFIC cells gave a PCE of 5.58%, with a V_oc of 0.58 V, a J_sc of 14.19 mA cm⁻² and a fill factor (FF) of 67.4%. These cells have a D/A ratio of 1 : 1 (w/w), an active layer thickness of 90 nm and 3 vol% 1-chloronaphthalene (CN) as the additive (Tables S1–S3, ESI†). The best PTB7-Th:CO5DFIC-OT cells gave a PCE of 7.66%, with a V_oc of 0.71 V, a J_sc of 17.58 mA cm⁻² and a FF of 61.0%. These cells have a D/A ratio of 1 : 1.5 (w/w), an active layer thickness of 85 nm and 3 vol% CN as the additive (Tables S4–S6, ESI†). The best PTB7-Th:CO5DFIC-ST cells gave a PCE of 9.73%, with a V_oc of 0.74 V, a J_sc of 20.71 mA cm⁻² and a FF of 63.7%. These cells have a D/A ratio of 1 : 2.5 (w/w), an active layer thickness of 102 nm and 3 vol% CN as the additive (Tables S7–S9, ESI†). Compared with CO5DFIC cells, CO5DFIC-OT and CO5DFIC-ST cells show higher performance due to higher V_oc and J_sc. The higher V_oc values for CO5DFIC-OT and CO5DFIC-ST cells are due to the higher LUMO levels of CO5DFIC-OT and CO5DFIC-ST, and the higher J_sc values originate from the stronger light-harvesting capability of CO5DFIC-OT and CO5DFIC-ST. CO5DFIC-OT and CO5DFIC-ST cells gave much broader EQE spectra than CO5DFIC cells (Fig. 3b). The integrated current densities obtained from EQE spectra for CO5DFIC, CO5DFIC-OT and CO5DFIC-ST cells are 13.78 mA cm⁻², 16.46 mA cm⁻² and 19.60 mA cm⁻², respectively, which are consistent with the J_sc values obtained from J–V measurements. The maximum EQE (EQE_max) for CO5DFIC, CO5DFIC-OT and CO5DFIC-ST cells are 65%, 55% and 71%, respectively, suggesting that charge generation is more efficient in CO5DFIC-ST cells but less efficient in CO5DFIC-OT cells. Indeed, the exciton dissociation probabilities (P_diss) for CO5DFIC, CO5DFIC-OT and CO5DFIC-ST cells are 91.5%, 87.5% and 95.3%, respectively, consistent with EQE_max (Fig. S17, ESI†).¹³ In contrast to V_oc and J_sc, the FF decreased in CO5DFIC-OT and CO5DFIC-ST cells. To understand this, we measured hole and electron mobilities (μ_h and μ_e) of PTB7-Th:NFA blend films by using the space charge limited current (SCLC) method (Fig. S18 and S19, Table S10, ESI†).¹⁴ Compared with the PTB7-Th:CO5DFIC film, PTB7-Th:CO5DFIC-OT and PTB7-Th:CO5DFIC-ST films show higher μ_h but lower μ_e. The μ_h/μ_e values of PTB7-Th:CO5DFIC, PTB7-Th:CO5DFIC-OT and PTB7-Th:CO5DFIC-ST films are 1.8, 9.1 and 6.7, respectively. The unbalanced charge transport in CO5DFIC-OT and CO5DFIC-ST cells accounts for the lower FF. We also studied bimolecular recombination by plotting J_sc against light intensity (P_light) (Fig. S20, ESI†). The data were fitted to a power law: J_sc ∝ P^α_light. The α values of CO5DFIC, CO5DFIC-OT and CO5DFIC-ST cells are 99.6%, 95.9% and 98.6%, respectively, suggesting more bimolecular recombination in CO5DFIC-OT and CO5DFIC-ST cells,¹⁵ which also leads to a lower FF. The morphology of the active layers was studied by using an atomic force microscope (AFM) (Fig. S21, ESI†). The PTB7-Th:CO5DFIC film presents a coarser surface than PTB7-Th:CO5DFIC-OT and PTB7-Th:CO5DFIC-ST films. The root-mean-square (RMS) roughnesses are 3.67 nm, 1.86 nm and 2.69 nm, respectively. All blend films present clear nanostructures, suggesting good nanoscale phase separation.

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

Table 2 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:CO5DFIC	0.58	14.19 (13.78)^a	67.4	5.58 (5.37)^b
PTB7-Th:CO5DFIC-OT	0.71	17.58 (16.46)^a	61.0	7.66 (7.37)^b
PTB7-Th:CO5DFIC-ST	0.74	20.71 (19.60)^a	63.7	9.73 (9.37)^b

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Conclusions

The electron-rich alkoxythiophene (OT) and alkylthiothiophene (ST) π-bridges were used to develop efficient A–D–A NFAs. The two acceptors, CO5DFIC-OT and CO5DFIC-ST, present higher LUMO levels and enhanced light-harvesting capability compared to CO5DFIC without π-bridges. CO5DFIC-OT and CO5DFIC-ST solar cells gave higher V_oc, J_sc and PCEs than CO5DFIC cells. A 9.73% efficiency was obtained from PTB7-Th:CO5DFIC-ST cells. The results suggest that OT and ST π-bridges are useful in developing high-performance A–D–A acceptors. Further studies on design and synthesis are ongoing with the goal to improve the photocurrent.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2017YFA0206600), the National Natural Science Foundation of China (U1401244, 21572041, 51503050, 51773045, 21772030, 21704021, 51473139 and 20974091) and the Youth Association for Promoting Innovation (CAS). X. Wang also acknowledges the Innovation Platform Open Foundation of University of Hunan Province (14K092) and the Hunan 2011 Collaborative Innovation Center of Chemical Engineering & Technology for financial support.

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Footnotes

† Electronic supplementary information (ESI) available: Preparation and characterization of materials, fabrication and measurements of solar cells. See DOI: 10.1039/c8qm00647d

‡ These authors contributed equally to this work.

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