Novel π-Conjugated Polymer Based on an Extended Thienoquinoid

Communication Cite This: Chem. Mater. 2018, 30, 319−323

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Novel π‑Conjugated Polymer Based on an Extended Thienoquinoid Yunhao Cai,† Xiaonan Xue,† Guangchao Han,‡ Zhaozhao Bi,§ Bingbing Fan,† Tao Liu,† Dongjun Xie,∥ Lijun Huo,*,† Wei Ma,§ Yuanping Yi,‡ Chuluo Yang,∥ and Yanming Sun*,† †

Heeger Beijing Research and Development Center, School of Chemistry and Environment, Beihang University, Beijing 100191, China ‡ Beijing National Laboratory for Molecular Science, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China ∥ Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, China S Supporting Information *

ABSTRACT: With a fused terthiophene, dithieno[3,4-b:2′,3′-d]thiophene (DTT) is used as a thienoquinoidal building block for conjugated polymers. However, due to its highly extended heteroarene structure, chemical synthesis of the functional DTT unit is a challenge. All known DTT-based polymer films were prepared by an electrochemical polymerization approach. In this study, we first synthesized the functional DTT monomer and DTT-based conjugated polymer, PDBT-DTT, via a typical Stille coupling reaction. With another fused thiophene ring, DTT shows increased aromatic (weaker quinoid) character compared to the thieno-[3,4-b]thiophene (TT) unit. Moreover, PDBT-DTT showed much higher photovoltaic performance as well as better thermal and photostability than its TT-based analogue (PDBT-TT). The results demonstrate that DTT is a potential quinoidal building block for constructing high-performance conjugated polymers.

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rganic semiconductors have garnered considerable interest due to their numerous applications in organic electronics such as organic field-effect transistors (OFETs), organic lightemitting diodes (OLEDs), organic solar cells (OSCs), etc.1−5 Significant progress has been made in this respect over the past decade with the development of π-conjugated polymers.6−11 In most cases, high-performance conjugated polymers are designed and synthesized with a “donor−acceptor” (D−A) approach, in which electron-rich and electron-deficient components are alternated in the backbone of polymers.12,13 Quinoidal structures incorporating expanded para-quinodimethane units with electron-withdrawing groups have been widely used as the acceptor units for designing conjugated polymers.14−16 The quinoidal molecules can possess unique properties including low band gap along with high absorption coefficients, deep highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, near-infrared fluorescence, and singlet-fission property as well as prominent charge transport ability because of their favorable planarity originating from double bonds and extended π-conjugation.17−21 Several representative conjugated polymers with quinoidal structures are shown in Figure 1.22−26 Among these quinoidal units, the thieno-[3,4-b]thiophene (TT) unit with functional groups has recently emerged as an extremely attractive electronwithdrawing building block for conjugated polymers.27−29 Yu et al. first applied the TT unit into organic photovoltaic materials and then developed a series of polythienothiophene-cobenzodithiophene copolymers.30−32 From then on, intensive efforts have been dedicated toward the design of TT-based copolymers with state-of-the-art power conversion efficiencies (PCEs) over 10%.33,34 © 2018 American Chemical Society

Figure 1. Representative π-conjugated polymers with quinoidal structures.

It should be noted that in comparison with the aromatic form, the quinoid form is energetically less stable since obtaining a quinoidal structure needs to destroy the degree of aromaticity and a loss in the stabilization energy.35,36 To achieve a chemically stable quinoidal structure, an alternative building block with enhanced aromaticity is highly desirable. Dithieno[3,4-b:2′,3′d]thiophene (DTT) (see Figure 1 for its chemical structure) is an appealing quinoidal unit where an aromatic thienothiophene moiety is fused to each thiophene ring. Despite apparent advantages of the DTT unit, such as highly efficient π-electron delocalization, low electronic π → π* band gap, and specific optical properties, its synthesis remains a challenge due to its highly extended heteroarene structure.37−39 To the best of our knowledge, until now, there is no report for functional/soluble Received: November 2, 2017 Revised: January 6, 2018 Published: January 8, 2018 319

DOI: 10.1021/acs.chemmater.7b04592 Chem. Mater. 2018, 30, 319−323

Communication

Chemistry of Materials

chain to achieve compound 1. Compound 1 was then converted into the bischloromethyl product of compound 2 with titanium(IV) chloride as catalyst under violent stirring conditions. Subsequently, the ring-closing reaction of compound 2 was carried out in a methanol solution of equimolar sodium sulfide to obtain compound 3. Then compound 3 was hydrolyzed in an aqueous solution of sodium hydroxide to get compound 4, and the formyl group was removed by catalytic copper power in a quinoline solution at reflux temperature with a yield of 60%. Compound 6 was synthesized by lithium diisopropylamide and N-formylpiperidine. The key intermediate of 7 was obtained by a condensation reaction between compound 6 and 2-ethylhexyl thioglycolate in a water-free system. Compound 9 was synthesized by 3-chloroperbenzoic acid as an oxidant, and then the target monomer of DTT was finished by a simple bromide process. In the following polymerization process, a typical Stille coupling reaction was carried out by copolymerizing DTT and DBT to obtain PDBTDTT (Scheme 1). PDBT-DTT has good thermal stability with a decomposition temperature higher than 405 °C under inert atmosphere (Figure S1). DFT calculations were performed to determine the geometric and electronic structure of PDBT-DTT using the tunedωB97XD functional and the 6-31G** basis set (Figures S2 and S3).44 It has been proved that more thiophene units in β-fused thiophene could facilitate the localization of quinoidal properties on the para-quinodimethane unit.45 As illustrated in Figure 2, the

DTT derivatives. The well-studied low band gap poly(dithieno[3,4-b:2′,3′-d]thiophene) (pDTT) polymer can be only obtained by an electrochemical polymerization method (Figure 1).40−43 In this contribution, we first report an efficient synthetic route for a new functional and energy level tunable DTT derivative (DTT-ester), in which the most reactive site is substituted by a soluble ester group (Scheme 1). The electron deficient ester Scheme 1. Synthetic Route of the Functional DTT-E Unit, PDBT-DTT, and PDBT-TTa

a

(i) DCC, DMAP, ethanol; (ii) chloromethyl ethyl ether, tin(IV) chloride, 50 °C, overnight; (iii) Na2S, methanol; (iv) NaOH, THF, 60 °C, overnight; (v) Cu, quinoline; vi) LDA, 0 °C, N-formylpiperidine, then room temperature 12 h; vii) 2-ethylhexyl thioglycolate, K2CO3, DMF; viii) McPBA, ethyl acetate, −78 °C; ix) acetic anhydride, reflux, 30 min; x) NBS, DMF; xi) Pd(ph3P)4, toluene, reflux, overnight.

moiety can decrease the HOMO level and band gap of DTT, which can lead to better oxidative stability (Figure 1b). By incorporating dithienobenzodithiophenes (DBT) as the electron donor and DTT-ester as the acceptor, a novel conjugated copolymer, poly{dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene-co-dithienothiophene}(PDBT-DTT), was synthesized. Its chemical structure is shown in Scheme 1. Compared to its TT-based analogue (PDBT-TT, see Scheme S2 for its structure), PDBT-DTT has a deeper ionization potential (IP). Moreover, the DTT unit in PDBT-DTT showed greater bond length variation than the TT unit in PDBT-TT, and with obviously longer C−C and shorter CC lengths, suggesting that PDBT-DTT exhibits more aromatic (weaker quinoid) character than PDBT-TT. To evaluate the application in organic electronics, organic solar cells (OSCs) based on PDBT-DTT have been fabricated. As a result, PDBT-DTT showed much higher photovoltaic performance than that of PDBT-TT, regardless of the acceptor materials used. More importantly, we noticed that PDBT-DTT devices exhibit better thermal and photostability than PDBT-TT devices. The results suggested that DTT is a highly promising quinoidal unit for constructing high-performance conjugated polymers for organic electronics. We tried several synthetic routes for the DTT-ester unit (Scheme S1). Finally, we succeeded in obtaining the target compound by a fine molecular design. The synthetic route was described as follows: first, an esterification of 3-bromothiophene2-carboxylic acid was done by introducing a soluble ethyl side

Figure 2. Plots of C−C and CC bond lengths in DBT-DTT and DBT-TT. The red and blue arrows highlight the different units possessing contrasting bond length alternation characters.

DTT unit in PDBT-DTT shows greater bond length variations than the TT unit in PDBT-TT, with obviously longer C−C and shorter CC lengths, especially for the C13−C14 length. This result discloses that DTT exhibits slightly weaker quinoidal (stronger aromatic) character than TT.46,47 Based on the DFT results, the PDBT-DTT molecule has a nearly planar backbone (Figure S4), which is favorable for charge transport.48,49 Figure 3a shows the UV−vis absorption spectra of PDBTDTT. In a dilute chloroform solution, PDBT-DTT displays a

Figure 3. (a) UV−vis absorption spectra of PDBT-DTT (in solution and film). (b) Cyclic voltammetry measurement of PDBT-DTT (in solution). 320

DOI: 10.1021/acs.chemmater.7b04592 Chem. Mater. 2018, 30, 319−323

Communication

Chemistry of Materials maximum absorption peak at 578 nm. Compared to the peak in the solution, PDBT-DTT film shows ca. 36 nm red-shift in absorption (614 nm) with the onset value of 693 nm, corresponding to an optical band gap (Egopt) of 1.79 eV. In terms of PDBT-TT, its absorption spectra in solution and film are nearly identical with Egopt of 1.63 eV. The maximum absorption coefficient of PDBT-DTT film was measured to be 50474 cm−1 and is slightly higher than that of PDBT-TT, indicating its good light-absorbing capability (Figure S4). Electrochemical cyclic voltammetry (CV) was carried out to investigate the energy level of the two polymers (Figures 3b and S4). The HOMO and the LUMO energy levels of PDBT-DTT and PDBT-TT are −5.42, −5.36 and −3.32, −3.36 eV, respectively (Figure 3b). Moreover, ultraviolet photoelectron spectroscopy (UPS) was performed to measure the IP of neat polymer films (Figure S5). The IP and electron affinity (EA) of PDBT-DTT and PDBT-TT are calculated to be 4.91, 4.66 and 3.12, 3.03 eV, respectively. The results agree well with the trend of open-circuit voltage (Voc) observed in OSCs and also are consistent with the DFT results. In order to evaluate the application of PDBT-DTT in organic electronics, organic solar cells (OSCs) were fabricated with PC71BM as the acceptor initially. Chloroform was used as the host solvent, and 1,8-diiodooctane (DIO) was used as the solvent additive to optimize the active layer morphology.50 The detailed data for device optimizations are summarized in Figures S6 and S7 and Tables S1−S8. Under the optimal condition, the PDBTDTT:PC71BM device showed a power conversion efficiency (PCE) of 7.01% with a short-circuit current (Jsc) of 13.60 mA cm−2, a Voc of 0.84 V, and a fill factor (FF) of 61%. Furthermore, the application of PDBT-DTT in nonfullerene OSCs has been also tested. 3,9-Bis(2-methylene-(3-(1,1-dicyanomethylene)cyclopentane-1,3-dione-[c]thiophen))-5,5,11,11-tetrakis(4hexylphenyl)dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITCPTC) with complementary absorption with PDBT-DTT was employed as the acceptor (Scheme S3).51 As a result, a high PCE of 8.74%, with a Jsc of 15.60 mA cm−2, a Voc of 0.79 V, and a FF of 67% was achieved, demonstrating that PDBTDTT is a very promising donor candidate for incorporation in various kinds of organic photovoltaic devices. In contrast, OSCs based on PDBT-TT displayed a much lower photovoltaic performance. The hole and electron mobilities in the PDBTDTT:ITCPTC blend were measured as 1.58 × 10−3 and 2.50 × 10−3 cm2/(V s), respectively (Figures S8 and S9 and Table S9). The high carrier mobility and balanced carrier transport are one of the main reasons of high PCEs achieved in PDBT-DTT devices. The device stability was also tested. As shown in Figure 4c and 4d, PDBT-DTT devices exhibited increased photostability and thermal stability than PDBT-TT cells, which are crucially important for industry production of OSCs. The corresponding incident photon conversion efficiency (IPCE) spectra of OSCs are shown in Figure 4b. Broad IPCE spectra from 300 to 750 nm were observed in PC71BM-based solar cells, and spectra from 300 to 800 nm were observed for nonfullerenebased devices. Especially for the PDBT-DTT:ITCPTC device, IPCE exceeds 65% from 565 to 730 nm, with a peak of 70% at 710 nm, indicative of efficient photon harvesting and charge collection. The PDBT-DTT:ITCPTC device yielded a higher photocurrent. The calculated Jsc from the IPCE spectra is 15.36 mA cm−2, consistent with the value (15.60 mA cm−2) obtained from J−V measurements (see Table 1). The surface morphologies of the blend films were studied by using atomic force microscopy (AFM) and transmission electron

Figure 4. (a, b) J−V characteristics and the corresponding IPCE spectra of optimal PDBT-DTT and PDBT-TT-based OSCs under the illumination of AM1.5G, 100 mW cm−2. (c, d) Thermal and light stability test of PDBT-DTT:ITCPTC and PDBT-TT:ITCPTC devices.

microscopy (TEM) (Figures S10−S12). It can be seen from AFM and TEM images that without DIO additive, the surfaces of PDBT-DTT:PC71BM and PDBT-DTT:ITCPTC films are smooth with root-mean-square (RMS) roughness of 1.15 and 1.42 nm, respectively. With the DIO additive, both of the blend films showed a better phase separation, and the RMS values increased to 2.45 and 1.78 nm, which is probably another reason for high PCEs achieved in PDBT-DTT:ITCPTC devices. Grazing-incidence wide-angle X-ray scattering (GIWAXS) was performed to investigate the molecular packing and orientation of the films. Figure 5 shows the 2D GIWAXS patterns and corresponding in-plane and out-of-plane linecuts. For the neat films, PDBT-DTT exhibited a preferential face-on orientation, as indicated by a conspicuous (100) lamellar diffraction peak at qxy ≈ 0.32 Å−1 (d ≈ 19.6 Å) and a strong (010) π−π stacking peak at qz ≈ 1.70 Å−1 (d ≈ 3.70 Å). The faceon orientation is proved to be beneficial to vertical charge transport. Correspondingly, PDBT-DTT neat films show a high hole mobility of ∼3.0 × 10−3 cm2/(V s) (Table S9). When blending with PC71BM, PDBT-DTT showed a mixed face-on and edge-on orientation, as evidenced by the increased lamellar scattering and decreased π−π stacking in an out-of-plane direction. Although the crystalline nature of PDBT-DTT was hampered by ITCPTC in the blend film, the lamellar scattering of PDBT-DTT in the in-plane direction is still obvious. Together with the distinct π−π stacking peak in the out-of-plane direction, PDBT-TT still showed a preferred face-on molecular orientation. As a result, the PDBT-DTT:ITCPTC blend exhibited slightly higher hole and electron mobilities than those of PDBTDTT:PC71BM films. In conclusion, we first synthesized the functional and soluble DTT unit and DTT-containing conjugated polymer, PDBTDTT, which has a main absorption ranging from 400 to 700 nm, a deep IP value, and a high hole mobility of ∼3.0 × 10−3 cm/(V s). With one more thiophene ring fused with the TT unit, DTT shows increased aromatic (weaker quinoid) character than the TT unit. In a preliminary evaluation of the potential of the PDBT-DTT for organic electronics, OSCs based on PDBTDTT and different types of acceptors were fabricated. A higher photovoltaic performance and better device stability than devices based on its TT analogue were achieved. These promising results 321

DOI: 10.1021/acs.chemmater.7b04592 Chem. Mater. 2018, 30, 319−323

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Chemistry of Materials

Table 1. Photovoltaic Performance of OSCs Based on PDBT-TT and PDBT-DTT under the Illumination of AM1.5G, 100 mW cm−2

a

active layer

Voc

Jsc [mA/cm2]

FF [%]

PCEa [%]

PCEmax [%]

PDBTDTT:PC71BM PDBT-DTT:ITCPTC PDBT-TT:PC71BM PDBT-TT:ITCPTC

0.84 ± 0.01 0.79 ± 0.01 0.51 ± 0.01 0.50 ± 0.01

13.5 ± 0.1 15.6 ± 0.1 10.5 ± 0.2 11.2 ± 0.2

60.7 ± 0.7 68.2 ± 0.8 56.3 ± 0.9 37.6 ± 0.5

6.9 ± 0.1 8.6 ± 0.2 3.1 ± 0.1 2.2 ± 0.1

7.01 8.74 3.09 2.17

The average PCE value was calculated from 10 devices for each condition.

make DTT and its derivatives attractive building blocks for developing high-performance π-conjugated polymers.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04592. Experimental details, synthesis and characterizations, device fabrication and characterizations, DFT calculation, UV absorption spectra, cyclic voltammogram measurement, UPS measurements, AFM and TEM images, SCLC measurements, and device performance (PDF)

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Figure 5. 2D GIWAXS patterns of (a) neat PDBT-DTT film, (b) neat ITCPTC film, (c) PDBT-DTT:PC71BM film, and (d) PDBTDTT:ITCPTC film. (e) Out-of-plane and in-plane linecuts of PDBTDTT, ITCPTC, and the blend films.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wei Ma: 0000-0002-7239-2010 Yuanping Yi: 0000-0002-0052-9364 Chuluo Yang: 0000-0001-9337-3460 Yanming Sun: 0000-0001-7839-3199 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) (nos. 21734001, 51473009, 21674007) and the International Science & Technology Cooperation Program of China (no. 2014DFA52820). 322

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DOI: 10.1021/acs.chemmater.7b04592 Chem. Mater. 2018, 30, 319−323