5,6-Difluorobenzothiazole-Based Conjugated Polymers with Large

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5,6-Difluorobenzothiazole Based Conjugated Polymers with Large Band Gaps and Deep HOMO Levels Jiangman Sun, Ping Cai, Feilong Pan, Lianjie Zhang, Zhulin Liu, Zhitian Liu, Yong Cao, and Junwu Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03643 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 14, 2018

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ACS Applied Materials & Interfaces

5,65,6-Difluorobenzothiazole Based Conjugated Polymers with Large Band Gaps and Deep HOMO Levels Jiangman Sun,† Ping Cai,† Feilong Pan,† Lianjie Zhang,*† Zhulin Liu,† Zhitian Liu,‡ Yong Cao,† and Junwu Chen*† †Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, P. R. China. ‡ School of Materials Science & Engineering, Wuhan Institute of Technology, Wuhan 430205, P. R. China. ABSTRACT: A 5,6-difluorobenzothiazole (ffBTz) based dibromo monomer was successfully synthesized, from which new fluorinated conjugated polymers PF-ffBTz and PFN-ffBTz were prepared via copolymerizations with two fluorene based diboronic ester monomers. Twisted fluorene-ffBTz backbones enable PF-ffBTz and PFN-ffBTz with large band gaps up to 3.10 eV and deep-lying HOMO levels down to −6.2 eV. The chemical structures of PF-ffBTz and PFN-ffBTz impart some new functionalities of fluorinated conjugated polymers. PF-ffBTz can show deep blue electroluminescent (EL) emission, with high external quantum efficiency (EQE) of 3.71%. PFN-ffBTz, with amino-functionalized side chains on the fluorene unit, can serve as an efficient cathode interlayer in inverted polymer solar cells (PSCs), showing better photovoltaic performances if compared with a ZnO interlayer. In addition, it is found that using an optical filter to cutoff short wavelength section (≤ 380 nm) of incident light can significantly elevate photo stability of PSCs under continuous illumination. KEYWORDS: 5,6-difluorobenzothiazole; fluorinated conjugated polymer; deep blue electroluminescent polymer; cathode interlayer polymer; photo stability of polymer solar cells 1. INTRODUCTION Designs of new conjugated polymers have drawn broad attention during the past two decades, with aiming to develop synthetic methodology and expand optoelectronic device applications, such as polymer light-emitting diodes (PLEDs),1−3 polymer solar cells (PSCs),3−5 and organic fieldeffect transistors (OFETs).6 Alcohol/water soluble conjugated polymers are attractive materials that can modify electrode interfacial properties of PLEDs and PSCs.7−9 The band gap (Eg) and energy levels of a conjugated polymer can determine its optoelectronic properties greatly, especially towards the PLED and PSC applications.1−5 Blue PLED is still a big challenge among RGB emissions, and highly efficient deep blue PLEDs are normally based on random conjugated polymers.10−13 Fluorinated aromatic cycles are attractive building blocks for constructing conjugated polymers (see examples listed in Tables S1 and S2 in Supporting Information).14−17 The Eg values of the fluorinated conjugated polymers are typically between 1.44 and 2.18 eV. Among fluorinated aromatic cycles, 5-fluoro-2,1,3benzothiadiazole (fBT),18 5,6-difluoro-2,1,3benzothiadiazole (ffBT or FBT),18−21 5,6-difluoro-2,1,3benzoselenadiazole (ffBSe),22 and 5,6difluorobenzotriazole (ffBTA)23−25 (Chart 1), belong to a group of fluorinated benzo-five-membered heterocycles that have been extensively investigated in constructing novel fluorinated conjugated polymers mainly for the PSC application (Table S1). These fluorinated conjugated pol-

Chart 1. The reported fluorinated benzo-five-membered heterocycles for fluorinated conjugated polymers.

ymers possess HOMO levels between −5.15 and −5.72 eV, which are active layer materials in PSCs and OFETs. Synthesis of a new building block plays an important role in developing new conjugated polymers with novel optoelectronic properties. In this work, a 5,6difluorobenzothiazole (ffBTz) based dibromo monomer 4 was designed to construct a new category of fluorinated conjugated polymers PF-ffBTz and PFN-ffBTz, via copolymerizations with two fluorene based diboronic ester monomers (Figure 1). To the best of our knowledge, no ffBTzbased fluorinated conjugated polymer was reported before. Interestingly, PF-ffBTz and PFN-ffBTz containing the asymmetric ffBTz moiety possess large Eg values of 3.10 and 3.03 eV, respectively, obviously larger than the reported fluorinated conjugated polymers. Moreover, HOMO levels of PF-ffBTz and PFN-ffBTz are −6.20 and −6.00 eV, respectively, significantly deeper than the reported conjugated polymers based on fluorinated benzo-five-

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Figure 1. (A) Synthetic route for 4,7-dibromo-5,6-difluoro-2-heptylbenzo[d]thiazole (4). Reagents: (a) octanoyl chloride, pyridine, 120 °C, 2h; (b) P4S10, toluene, gentle reflux, 3h; (c) NaH, toluene; (d) LDA, trimethyl chlorosilane, THF, −78 °C, 3h; (e) Br2, chloroform and acetic acid, 16h. (B) Suzuki coupling reactions for polymers PF-ffBTz and PFN-ffBTz. membered heterocycles. Normally, a deeper HOMO level can impart an organic semiconductor with better antioxidation ability. In PLEDs, alternating copolymer PF-ffBTz shows an efficient deep blue electroluminescent (EL) emission, with a maximum external quantum efficiency (EQE) of 3.71%. The CIE x and y coordinates of (0.163, 0.056) of the EL spectrum is very close to CIE 1931 standard of (0.14, 0.08) for primary blue of The National Television System Committee (NTSC). PFN-ffBTz, with aminofunctionalized side chains on the fluorene unit, is a hydrophilic conjugated polymer. When serving as a cathode interlayer on ITO cathode in inverted PSCs, PFN-ffBTz can tune work function of ITO and achieve much better layerlayer contact at the cathode side for improving electron collection.26,27 With PTB7:PC71BM blend film15,28 as the active layer, PSCs can display excellent PCE of 8.74%. Moreover, it is found that using an optical filter to cutoff short wavelength section (≤ 380 nm) of incident light is benefit for well retaining of initial efficiency of PSCs under continuous illumination, supplying a new strategy to elevate photo stability of PSCs. 2. RESULTS AND DISCUSSION The synthetic route for 4,7-dibromo-5,6-difluoro-2heptylbenzo[d]thiazole (4), a ffBTz based dibromo monomer, is shown in Figure 1A. With 2,4,5-trifluoroaniline as the starting material, its amidation with octanoyl chloride in anhy-

drous pyridine at 120 °C for 2 h readily afforded compound 1 in a good yield of 83%. A further transformation was carried out by a treatment with phosphorus pentasulfide in anhydrous toluene, giving compound 2 in 71% yield. 5,6-Difluoro2-heptylbenzo[d]thiazole (3) was obtained in a good yield of 84% by cyclization of 2 with sodium hydride. The subsequent lithiation and bromination effectively converted 3 to the targeted dibromo ffBTz based monomer 4 in a yield of 75%. The two fluorene based diboronic ester monomers were synthesized according to previous reports.29,30 Polymers PFffBTz and PFN-ffBTz were prepared by Suzuki coupling reactions of the diboronic ester monomers and 4, which were obtained in 91% and 51% yields, respectively. The low yield for PFN-ffBTz should be ascribed to its hydrophilicity from the amino-functionalized side chains, and only the high molecular portion of the polymer could be collected during sedimentation in methanol. The molecular weights of PF-ffBTz and PFNffBTz are listed in Table 1. The Mw values for PF-ffBTz and PFN-ffBTz are 196.0 and 34.8 kg/mol, respectively, indicating a much higher propagation activity for PF-ffBTz during the polymerization. The two polymers possess comparable Mw/Mn around 1.68. Thermal stability of the polymers was investigated with thermogravimetric analysis (TGA). The decomposition temperatures (Td) of PF-ffBTz and PFN-ffBTz for 5% weight loss are 414 and 383 °C, respectively, all showing fairly good thermal stability.


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Table 1. Molecular weights, absorption, optical band gaps (Eg), and electrochemical properties of PF-ffBTz and PFN-ffBTz M wa

Mw/Mna

λabs-solnb λabs-film

Eg c

Eox

HOMOCVd

HOMOUPS LUMOe

(nm)

(nm)

(eV)

(V)

(eV)

(eV)

(eV)

Copolymer (kg/mol) PF-ffBTz

196.0

1.67

358

359

3.10

1.40

−6.20

−6.29

−3.19

PFN-ffBTz

34.8

1.69

358

364

3.03

1.20

−6.00

−6.08

−3.05

Estimated by GPC in THF on the basis of a polystyrene calibration. b THF solution (1×10−5 M). c Estimated with the absorption edges of thin solid films. d Calculated according to HOMO = −e(Eox + 4.8). eCalculated from the HOMOUPS level and optical bandgap.

a

The solution (1×10−5 M in THF) and film absorption spectra of PF-ffBTz and PFN-ffBTz are shown in Figure 2A. The solutions of PF-ffBTz and PFN-ffBTz exhibit almost identical absorption peaks (λabs-soln) at 358 nm. In literature, polyfluorenes (PFs) typically display λabs-soln values at around 385 nm.10 The obviously blue-shifted absorptions of PF-ffBTz and PFN-ffBTz relative to those of PFs imply that PF-ffBTz and PFN-ffBTz may possess much twisted backbones.10,26,31 To address the possibility, we calculated the optimal geometry of 4,7-bis(9,9-dimethylfluorene-2-yl)-5,6-difluoro-2methylbenzo[d]thiazole (F-ffBTz-F), a model molecule for the ffBTz based polymers (Figure 2B). The quantum chemistry simulation was optimized by TD-B3LYP/6-31G(d). The calculation indicates the twisting angles between fluorene and ffBTz are of 42.37° and 48.99°, reflecting the non-coplanar backbones of the polymers. As a comparison, terfluorene shows obviously smaller twisting angels of ~37° (Figure S1). The frontier molecular orbitals of F-ffBTz-F are illustrated in Figure S2. The electrons in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) can be delocalized on the both fluorene and ffBTz units, suggesting that PF-ffBTz and PFN-ffBTz possess very weak D-A effect on the polymer backbone. The result is also consistent with the only one peak existed in the solution absorption. The films of PF-ffBTz and PFN-ffBTz show absorption peaks (λabs-film) at 359 and 364 nm, respectively, very close to corresponding λabs-soln values. The behavior demonstrates that the two polymers exhibit very weak backbone aggregation and can also support their twisted backbone structures.32 The absorption edges for the films of PF-ffBTz and PFN-ffBTz are 400 and 409 nm, respectively and the calculated optical band gaps (Eg) are thus 3.10 and 3.03 eV, respectively. The Eg value of PF-ffBTz is larger than 2.92 eV for PF and that of PFN-ffBTz is also larger than 2.91 eV for PFN.30 Thus the conjugation length of the backbone of PF-ffBTz or PFN-ffBTz is probably shorter than that of a polyfluorene. It should be noted that PF-ffBTz and PFN-ffBTz possess the largest the Eg values among the reported fluorinated conjugated polymers.

The HOMO levels of PF-ffBTz and PFN-ffBTz were characterized by cyclic voltammetry (CV) measurement and ultraviolet photoelectron spectroscopy (UPS). Their HOMOCV levels were obtained from the onset of oxidation potential (Eox) during CV measurement with an Ag/Ag+ electrode as the reference electrode and ferrocene as the internal standard (Figure S3). The Eox values for PF-ffBTz and PFN-ffBTz are of 1.40 and 1.20 V, respectively, giving HOMOCV levels of –6.20 and –6.00 eV, respectively (Table 1). The UPS spectra are shown in Figure S4. The calculated

HOMOUPS levels for PF-ffBTz and PFN-ffBTz are of –6.29 and –6.08 eV, respectively, further confirming the deeplying HOMO levels of the two polymers. Thus the HOMO levels of PF-ffBTz and PFN-ffBTz are significantly deeperlying to those of PF and PFN at around –5.7 eV.29 The deeper-lying HOMO levels of PF-ffBTz and PFN-ffBTz should be ascribed to the ffBTz unit induced more twisting of the polymer backbones. Similar effect had been observed before for some conjugated polymers.26,33 It should be noted that the HOMO levels of PF-ffBTz and PFN-ffBTz are also deeper than those of the reported conjugated polymers



based on fluorinated benzo-five-membered heterocycles. In addition, relative to PF-ffBTz, smaller Eg and upper-lying HOMO level can be found for PFN-ffBTz, indicating the polar amino-functionalized side chains of PFN-ffBTz can result in stronger backbone aggregation in the solid state, giving a decreased backbone twisting.26 The LUMO levels of –3.19 and –3.05 eV for PF-ffBTz and PFN-ffBTz respectively are derived from their corresponding HOMOUPS levels and Eg values (Table1). The hole and electron mobilities of the two polymers were obtained via space charge limited current (SCLC) method, as measured with hole- and electrononly devices,34 respectively. PF-ffBTz and PFN-ffBTz show hole mobilities of 5.8×10−9 and 3.0×10−11 cm2/(V s), respectively, while their electron mobilities are of 3.1×10−8 and 3.0×10−7 cm2/(V s), respectively. The not high mobilities of the two polymers should be related to their twisted main chains.

The photoluminescence (PL) spectra of solution and film of PF-ffBTz are shown in Figure 3A. The PL spectrum of the PF-ffBTz solution demonstrates a deep blue emission peak at 404 nm, with a full width at half-maximum (PLfwhm) of 72 nm. The PF-ffBTz film exhibits a red-shifted emission peak at 419 nm, also showing an obviously decreased PLfwhm (56 nm). The electroluminescence (EL) spectrum of PF-ffBTz based PLED (ITO/PEDOT:PSS (40 nm)/PVK (40 nm)/PF-ffBTz (80nm)/CsF (1.5 nm)/Al) is also shown in Figure 3A. The EL spectrum displays a deep blue emission with a peak at 408 nm and a further decreased ELfwhm of 46 nm. The CIE x and y coordinates of (0.163, 0.056) of the EL spectrum is very close to CIE 1931 standard of (0.14, 0.08) for primary blue of The National Television System Committee (NTSC). The PF-ffBTz based PLED shows a turn-on voltage of 4.25 V, a maximum luminance of 1497 cd/m2, and a notable maximum external quantum efficiency (EQE) of 3.71% (Figure 3B). The EQE values of the device can be higher than 3% for a luminance range from 9.7 to 615 cd/m2. The device performance is much better than the reported 0.8% EQE of PF in its blue EL emission.35 Thus PF-ffBTz is a promising alternating conjugated polymer that can show highly efficient deep blue EL emission. In previous reports, highly efficient deep blue PLEDs are normally based on random conjugated polymers with small portions of the key chromophores.10−13 The alternating copolymerization for PF-ffBTz can supply a welldefined backbone structure. Unfortunately, based on this device configuration, PF-ffBTz and PF displayed very limited life-time (Figure S5), indicating that achieving a long life-time deep blue PLED is extremely challenging. Considering the large Eg nature of the fluorene-ffBTz backbone, we synthesized PFN-ffBTz with aminofunctionalized side chains towards a new interlayer polymer with a large Eg. When an interlayer polymer is utilized as a cathode interlayer in an inverted PSC, its large Eg can show some possible advantages, such as more incident light passing through and potentially lower excitation extent under illumination. To evaluate the cathode interlayer property of PFN-ffBTz, we fabricated inverted PSCs with a device configuration of ITO/PFN-ffBTz (~6 nm)/active layer (100 nm)/MoO3 (10 nm)/Al (100 nm). The thin cathode interlayer of PFN-ffBTz on the ITO cathode was casted from its methanol:acetic acid (99.5:0.5, v/v) mixture with a concentration of 0.5 mg/mL and then the interlayer was

dried at 120 °C for 10 min. The thickness of the cathode interlayer was calibrated by relative absorbance at 364 nm, with a quartz substrate. Chlorobenzene is the solvent for deposition of PTB7:PC71BM (1:1.5) active layer. It was found that spin-coating chlorobenzene on a PFN-ffBTz interlayer (with a quartz substrate) showed almost no decrease of the absorbance. Thus the thin film of PFN-ffBTz on ITO is robust to endure solvent erosion during further spin-coating of the active layer. As measured by a Kelvin probe, the work function of ITO was shifted from the initial value of –4.7 eV to –4.2 eV by PFN-ffBTz. The decreased work function of ITO caused by the interlayer polymer matches well with the LUMO level of PC71BM and supplies good capability for electron extraction.8,26 MoO3 is a widely utilized hole extraction layer in combination with Al as the top anode in inverted PSCs.26 Inverted PSCs based on bare ITO, ITO/PF-ffBTz, and ITO/ZnO cathodes were also fabricated for a comparison study. In order to give certain protection of the PSCs, the area of the active layer of each device was covered by a glass plate with UV-glue for the adhesion in a N2 glovebox, with UV-curing for 3 min. The measurements of the photovoltaic performances were carried out under illumination of AM1.5G simulated solar light at 100 mW/cm2 in air under ambient condition.


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Figure 4. (A) J-V curves of inverted PSCs based on different interlayer on ITO cathode. (B) Photo stability of the inverted PSCs with a ZnO or a PFN-ffBTz interlayer on ITO cathode, as measured under continuous illumination of simulated white light (AM1.5G) at 100 mW/cm2 in air. Photo stability of the inverted PSCs were compared without or with protection by a 380 nm optical filter (a UV filter to cut off lights with wavelengths ≤ 380 nm). Figure 4A shows J-V characteristics of the inverted PSCs. Their photovoltaic parameters are summarized in Table 2. For the PSC without an interlayer on ITO, the device displays an open-circuit voltages (Voc) of 0.4 V, a short-circuit current (Jsc) of 14.7 mA/cm2, and a fill factor (FF) of 49.9%. The PCE of the PSC is 2.93%. Inserting PFN-ffBTz as the cathode interlayer can largely improve the photovoltaic performance, and the inverted solar cell shows Voc, Jsc, and FF of 0.76 V, 15.6 mA/cm2, and 73.5%, respectively. The calculated PCE for the device is 8.74%, a high efficiency for the PTB7:PC71BM active layer.15,28 The averaged PCEs for the bare ITO and ITO/PFN-ffBTz cathodes are 2.82% and 8.67%, showing an increase of 207% by the cathode modification. Using PF-ffBTz as interlayer shows a PCE of 3.25%, close to that of the bare ITO cathode. The results indicate the amino-functionalized side chains of PFN-ffBTz play a key role in the cathode modification. Inverted PSC based on ZnO interlayer exhibits Voc, Jsc, and FF of 0.76 V, 16.1 mA/cm2, and 68.6%, respectively, giving a PCE of 8.38%. Relative to ZnO, the PFN-ffBTz interlayer can show a higher efficiency due to its obviously larger FF of 73.5%. Series resistances (Rs) of the PSC devices are also shown in Table 2, among which the device based on the PFN-ffBTz interlayer can display a lowest Rs of 3.6 Ω cm2. Generally, inserting the PFN-ffBTz interlayer between ITO and active layer can significantly improve layer-layer contacts that are favorable for electron collection by the ITO cathode. Similar behavior was also observed before for cathode modification with an interlayer polymer in conventional PSCs.36,37 Table 2. Photovoltaic performances of PTB7:PC71BM based inverted polymer solar cells with or without PFN-ffBTz as interlayer on ITO cathode Cathode interlayer No

Voc (V) 0.40

Jsc (mA/cm2) 14.7

FF (%) 49.9

Rsa PCE (%) (Ω cm2) 2.93 9.0

PFN-ffBTz

0.76

15.6

73.5

8.71

3.6

PF-ffBTz

0.42

15.7

49.4

3.25

7.6

ZnO

0.76

16.1

68.6

8.38

4.0

a Series resistance deduced from the inverse slope near Voc in the J–V curve.

Towards industrial application, seeking effective strategies to elevate photo stability of PSCs under illumination is very important.38 Although UV irradiation is only a small portion of sun light, its nature of high energy light can usually cause great deteriorations of active layer and organic interlayers in PSCs.39 The photo stability of the inverted PSCs with a ZnO or a PFN-ffBTz cathode interlayer was measured under continuous illumination of simulated white light (AM1.5G) at 100 mW/cm2 in air under ambient condition. We compared protection effect by a 380 nm optical filter (a UV filter to cut off short wavelength portion (≤ 380 nm) of incident light) (Figure 4B). Without protection by the optical filter, the ZnO- and PFN-ffBTz-based PSC devices show fast decreasing of PCE, only retaining 50% and 75% of initial efficiency after illumination for 120 min, respectively. With protection by the optical filter, the twotype PSC devices can show significantly enhanced photo stability. The ZnO- and PFN-ffBTz-based PSC devices can retain 88% and 96% of initial efficiency after illumination for 120 min, respectively. The results demonstrate that the short wavelength portion (≤ 380 nm) of incident light is the major cause for the instability of PSC device under the continuous illumination. The results also suggest that using a UV protection film at the incident light side would be a useful strategy to prolong the working lifetime of PSCs. 3. CONCLUSION In summary, PF-ffBTz and PFN-ffBTz as a new category of fluorinated conjugated polymers were successfully synthesized. The two polymers comprise a largely twisted fluorene-ffBTz connection as evidenced by almost no shift of UV absorption spectra from solution to film and quantum chemistry simulation of the model molecule, achieving the largest band gap (3.1 eV) among fluorinated conjugated polymers and the deepest-lying HOMO level (–6.20 eV) among conjugated polymers based on fluorinated benzo-



five-membered heterocycles. PF-ffBTz shows an efficient deep blue EL emission, with an EL peak at 408 nm, a narrow ELfwhm of 46 nm, CIE x and y coordinates of (0.163, 0.056) matching NTSC primary blue standard, and a maximum EQE of 3.71%. PFN-ffBTz with amino-functionalized side chains on the fluorene unit can be utilized as an efficient cathode interlayer on ITO cathode in inverted PSCs. With PTB7:PC71BM blend film as the active layer, the PFNffBTz based PSC device displays excellent PCE of 8.74%, higher than the 8.38% for a ZnO interlayer. Moreover, our results also indicate that using an optical filter to cut off short wavelength section (≤ 380 nm) of incident light can be an effective strategy to elevate photo stability of PSCs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami. xxxxxxx. Synthesis details and structural characterizations of compounds and polymers, device fabrication and characterization, cyclic curves of polymers, UPS spectra, and Eg values and energy levels of representative fluorinated conjugated polymers.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. W. Chen). *E-mail: [email protected] (L. Zhang). ORCID Lianjie Zhang: 0000-0003-3555-4372 Junwu Chen: 0000-0003-0190-782X Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank the financial support of National Natural Science Foundation of China (U1401244, 21225418, 51521002, 91633301), the National Basic Research Program of China (973 program 2013CB834705, 2014CB643505), Natural Science Foundation of Guangdong Province (2016A030312002), and GDUPS (2013).

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5,6-Difluorobenzothiazole-Based Conjugated Polymers with Large

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