Ring Fusion of Thiophene–Vinylene–Thiophene (TVT) Benefits Both

Jun 8, 2018 - Ring fusion can downshift the HOMO energy level and increase the absorption coefficient of the corresponding polymer. Combining with ...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Ring Fusion of Thiophene−Vinylene−Thiophene (TVT) Benefits Both Fullerene and Non-Fullerene Polymer Solar Cells Xiaochen Wang,† Ailing Tang,† Fan Chen,†,‡ and Erjun Zhou*,† †

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Conjugated polymers based on thiophene−vinylene− thiophene (TVT) and ethenylene fused TVT (ETVT) combined with alkylated dithienylbenzothiadiazole (DTBT) were designed and synthesized to investigate the effect of ring fusion on the properties of TVT based photovoltaic polymers. It is found that ring fusion of the TVT segment significantly affects molecular architecture and optoelectronic properties of the polymer. Ring fusion can downshift the HOMO energy level and increase the absorption coefficient of the corresponding polymer. Combining with reduced energy loss, PETVTTBT shows superior photovoltaic performance to PTVTTBT, in both fullerene and non-fullerene polymer solar cells. Particularly, in ITIC based polymer solar cells, simultaneous enhancement in the JSC, VOC, and FF is demonstrated after ring fusion. As a result, PCE of PETVTTBT based solar cells increases drastically by 120% over that of PTVTTBT (3.69%), reaching 8.18%.

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polymers have been synthesized and applied in field effect transistors (FETs).28−34 Many noticeable, record high charge carrier mobilities were obtained from TVT based polymer semiconductors. Nowadays, charge carrier mobility of TVT based polymer has reached 11 cm2 V−1 s−1.35 In contrast, there are far fewer reports on TVT based photovoltaic polymers, and the PCEs are usually lower than 6% mainly due to the high energy loss. Therefore, it is necessary to design and synthesize TVT derivatives and analogues to reduce the energy loss of corresponding polymers and retain the advantages of extended conjugation and high charge carrier mobility. As we known, almost all the high-performance photovoltaic polymers contain the thiophene unit but scarcely contain the vinylene segment. Hence, derivation and modification of vinylene segment should be the critical issue to improve photovoltaic performance of TV based polymers. In this study, vinylene was bridged to adjacent thiophene with CC subunits in the TVT unit to form ethenylene fused TVT (ETVT). This building block ETVT can be also considered as the benzene embedded TVT derivatives, as shown in Figure 1. Compared with that of vinylene, strong electronic localization of double bonds in benzene is favorable to downshift the highest occupied molecular orbital (HOMO) of the polymers and thus increase the open-circuit voltage (Voc) of corresponding solar cells. Meanwhile, from vinylthiophene to benzothiophene, the change of chemical structure will cause the

INTRODUCTION During the recent decades, photovoltaic polymers have been the focus of both scientific research and industrial application, owing to the adjustable chemical structures and properties, excellent film forming property, and potential to fabricate various lightweight and flexible polymer solar cells (PSCs).1−3 To improve the photovoltaic performance of semiconducting polymers, significant accomplishments have been achieved in the molecular engineering, and most of these efforts have been focused on developing new building blocks or fine-tuning the structures of building blocks.4−6 Among the various conjugated electron-donating building blocks, the thienylenevinylene (TV) unit has attracted much interest in the design of new conjugated copolymers for the application in optoelectronic devices.7−18 The presence of a vinylene spacer between two thiophene units endows TV with extended conjugation and coplanarity.19−21 As a result of the enhanced intra- and interchain π electrons interaction, the polymers exhibit reduced energy band gap and higher charge carrier mobility, which are favorable to increase both short-circuit current density (Jsc) and fill factor (FF).22−26 However, the combination of vinylene and thiophene always leads to increased energy loss (voltage loss) and consequently decreased power conversion efficiency (PCE) of the polymer solar cells. As discussed in our previous report, poly(thienylenevinylene) derivatives (PTVs) showed significantly higher voltage loss in solar cells than the average value of photovoltaic polymers.27 This phenomenon also existed in the (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) based polymers, which is a kind of important donor unit in semiconducting polymers. Plenty of TVT based conjugated © XXXX American Chemical Society

Received: April 16, 2018 Revised: May 31, 2018

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DOI: 10.1021/acs.macromol.8b00805 Macromolecules XXXX, XXX, XXX−XXX

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EXPERIMENTAL SECTION

Materials. 5,6-Difluoro-4,7-bis(5-bromo-4-(2-octyldodecyl)-2thienyl)-2,1,3-benzothiadiazole (DTBT), 1,2-bis(5-trimethylstannanyl-2-thienyl)ethene (TVT-Sn), and trimethyl(5-(2(trimethylstannyl)benzo[b]thiophen-6-yl)thiophen-2-yl)stannane (ETVT-Sn) were synthesized according to literature procedures.38−40 N-Bromosuccinimide (NBS) was purified by recrystallization from water before use. Tetrahydrofuran (THF) was dried over Na/ benzophenone ketyl and freshly distilled prior to use. Other reagents and solvents were commercial grade and used as received without further purification. All reactions were performed under a nitrogen atmosphere. Measurements and Characterization. Nuclear magnetic resonance spectra (NMR) were recorded in deuterated solvents using a Bruker Avance III HD 400 spectrometer at room temperature. Chemical shifts of NMR were reported in ppm. The molecular weights of the polymers were measured by the gel permeation chromatography (GPC) method. The number-average molecular weights (Mn), weightaverage molecular weights (Mw), and polydispersity index (PDI, Mw/ Mn) were measured on a PL-220 (Polymer Laboratories) chromatograph connected to a differential refractometer with polystyrenes as reference standard and 1,2-dichlorobenzene (DCB) as an eluent (at 150 °C). Elemental analyses were performed on a Flash EA 1112 analyzer. UV−vis absorption spectra were recorded on a Shimadzu UV-3600 UV−vis−NIR spectrophotometer or a Hitachi UH5300 spectrophotometer. Absorption spectra measurements of the polymer solutions were carried out in DCB at different temperature. Absorption of the polymer films was measured on the quartz plates with the polymer films spin-coated from the polymer solutions in DCB and dried in air. The electrochemical cyclic voltammetry was conducted on Shanghai Chenhua CHI620 electrochemical workstation with a platinum plate, platinum wire, and Ag/AgCl electrode as the working electrode, counter electrode, and reference electrode, respectively, in a 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6)−acetonitrile solution. Polymer thin films were formed by drop-casting of polymer solutions in DCB on the working electrode and then dried in air. Atomic force microscopy (AFM) images of the thin films were obtained on a Multimode 8 scanning probe microscope operating in tapping mode. Fabrication of Photovoltaic Devices. PSCs were fabricated with ITO glass as a positive electrode, Ca/Al as a negative electrode, and the blend film of the active layer between them as a photosensitive layer. The ITO glass was precleaned and modified by a thin layer of PEDOT:PSS, which was spin-cast from a PEDOT:PSS aqueous solution on the ITO substrate, and the thickness of the PEDOT:PSS

Figure 1. Chemical structures and frontier molecular orbital (MO) diagrams of TVT and ETVT.

rearrangement of electronic distribution of the molecular orbitals and consequently the variation of energy loss of corresponding solar cells. Therefore, it is worthy to make further studies to gain much relevant data of inherent law about the relationship of energy loss and chemical structure. In this study, we transform the vinylene in TVT unit to conjugated double bonds in benzene in ETVT unit and systematically investigate the effect on the photovoltaic property of the resulting polymers. To construct D−A type conjugated polymers, alkylated dithienylbenzothiadiazole (DTBT) was selected as an acceptor building block. It has been proved that DTBT can endow polymers with suitable absorption and molecular energy levels, as well as high charge carrier mobility, which guarantee the sunlight harvesting, charge separation, and transportation in the polymer solar cells.36,37 The long branched 2-octyldodecyl groups were introduced to the thiophene segments to improve the solubility of polymers for the purification, characterization, and application in optoelectronic devices.

Scheme 1. Chemical Structures and Synthetic Route of ETVT and the Polymers

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Figure 2. Absorption spectra of PTVTTBT (a) and PETVTTBT (b) solutions (0.01 mg mL−1 in DCB) at temperatures as indicated.

Table 1. Optical Properties of the Polymersa polymer PTVTTBT PETVTTBT a

λabs in DCB (nm)

ε in DCB (M−1 cm−1)

λonset in DCB (nm)

λabs in films (nm)

λonset in films (nm)

Egopt (eV)

326, 441, 587, 703 326, 429, 590, 646

3.6 × 10 3.8 × 104

768 702

474, 643, 703 437, 605, 653

771 716

1.61 1.73

4

Optical properties in DCB were recorded at 20 °C.

layer was about 35 nm. The photosensitive layer was prepared by spincoating a blend solution of polymer and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) or ITIC in chlorobenzene on the ITO/ PEDOT:PSS electrode. Then, the cathode was deposited on the active layer by vacuum evaporation under ∼3 × 10−4 Pa. The thickness of the photosensitive layer was ca. 100 nm, measured on an Ambios Tech XP-2 profilometer. The effective area of one cell was ca. 4 mm2. The current−voltage (I versus V) measurement of the devices was conducted on a computer-controlled Keithley 236 source measure unit. A xenon lamp with an AM 1.5 filter was used as the white-light source, and the optical power at the sample was 100 mW/cm2. Synthesis of the Polymers. The synthesis of the polymers was carried out using palladium-catalyzed Stille coupling between monomer DTBT (1 equiv) and TVT-Sn or ETVT-Sn (1.02 equiv), as shown in Scheme 1. Monomers, Pd2(dba)3 (2.5% in mol), (o-tol)3P (20% in mol), and toluene were added in a reaction vial with a magnetic stirring bar. After being purged by three freeze−pump−thaw cycles, the mixture was heated at 100 °C for 12 h. Then, bromobenzene (5 equiv) was added as end-cappers. After stirring for another 6 h, the reaction solution was cooled to room temperature and then added dropwise to 120 mL of ethanol. The black precipitate was filtered into a Soxhlet funnel and extracted by methanol, hexane, chloroform, and chlorobenzene successively. The polymers in hot chlorobenzene solution were further purified by preparative gel permeation chromatography. The products fraction were concentrated and precipitated in ethanol to recover the polymers. After filtration by 0.45 μm nylon filters, the products were dried under vacuum overnight to yield the titled polymer as black solids. PTVTTBT. Yield: 52%; Mn = 25.7 kDa, Mw = 29.3 kDa, PDI = 1.14. Anal. Calcd for (C64H90F2N2S5)n: C, 70.80; H, 8.36; N, 2.58. Found: C, 70.67; H, 8.49; N, 2.53. PETVTTBT. Yield: 39%; Mn = 18.5 kDa, Mw = 22.0 kDa, PDI = 1.19. Anal. Calcd for (C66H90F2N2S5)n: C, 71.43; H, 8.17; N, 2.52. Found: C, 71.37; H, 8.28; N, 2.55.

PTVTTBT was also prepared for the purpose of compare. The polymers have moderate molecular weight; numberaverage molecular weight (Mn)/weight-average molecular weight (Mw) of PTVTTBT and PETVTTBT are 25.7/29.3 and 18.5/22.0 kDa, respectively. Especially to deserve to be mentioned, both polymers show extremely low polydispersity index (PDI, Mw/Mn) of 1.14 and 1.19, after the standard purification procedure. The polymers can be dissolved in hot chlorobenzene, dichlorobenzene, or trichlorobenzene and processed to form smooth and pinhole-free films upon spin coating. Thermal stability of the polymers was evaluated by thermogravimetric analysis (TGA) under a nitrogen atmosphere at a heating rate of 10 °C min−1. Compared to that of PTVTTBT, onset decomposition temperature corresponding to 5% weight loss of PETVTTBT is raised by 25 °C, from 423 to 448 °C, while the residual weight ratio of the latter increased by nearly 70%, from 25.2% to 42.5%, as shown in Figure S1. Obviously, structural rigidification by ring fusion of TVT units is beneficial to improve the thermostability of the conjugated polymers. The excellent thermal stability of the polymers is perfectly adequate for their applications in PSCs, OFETs, and other optoelectronic devices. Optical Properties. Figure 2 shows the absorption spectra of PTVTTBT and PETVTTBT in DCB solutions at different temperatures. Table 1 summarizes the optical data at 20 °C, including the absorption peak wavelengths (λabs), absorption edge wavelengths (λonset), optical band gap (Egopt), and molar absorption coefficient (ε) of the polymers. The absorption spectrum of PTVTTBT recorded from dilute DCB solution at 20 °C features three absorption bands: the first one located at 326 nm, the second one at 441 nm, which can be assigned to intrinsic absorptions of the benzothiadiazole unit and localized π−π* transitions,41 and the third broader band in the long wavelength region, beyond 500 nm, corresponding to intramolecular charge transfer (ICT) between the donor and acceptor units. Besides the three absorption peaks, there are two aggregation peaks at 493 and 705 nm, indicating that strong π−π stacking between the polymer backbones has formed aggregation or ordered packing at 20 °C even in the diluted solution. With the increase of temperature, absorption spectra of PTVTTBT solutions appear regular changes. From 20 to 80 °C, absorption wavelength of the first band keeps

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RESULTS AND DISCUSSION Synthesis and Characterization. The chemical structure and synthesis route of ETVT and the polymers are outlined in Scheme 1. The building block ETVT was synthesized by crosscoupling reaction between 6-bromobenzothiophene and tributyl(thiophen-2-yl)stannane in almost quantitative yield. Then stannylation reaction was performed to introduce two trialkyltin substituents on ETVT to afford ETVT-Sn in 88% yield for further reaction. The ETVT based polymer PETVTTBT was synthesized by palladium-catalyzed Stille coupling polymerization. The TVT-containing analogue C

DOI: 10.1021/acs.macromol.8b00805 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. (a) Chemical structures of PC71BM and ITIC. (b) Film absorption spectra. (c) Energy level diagram of PTVTTBT, PETVTTBT, PC71BM, and ITIC.

obvious decrease in intensity, while the second absorption bands assumes only a slightly reduce in intensity. In another word, along with the increase of solution temperature, the intensity of both the second and third absorption bands of PETVTTBT solution display the tendency declining at the beginning, rising up later and descending at last. Meanwhile, the intensity of aggregation absorption peaks of PETVTTBT solution gradually decreases and nearly disappears completely above 60 °C, with steady wavelength at 451 and 646 nm. Absorption spectra of as spin-coated films of PTVTTBT and PETVTTBT are shown in Figure 3b. Optical data of the films are summarized in Table 1. The enhanced intermolecular π−π interaction in the solid state obviously influences the absorption of the polymers. In the polymer films, intrinsic absorptions of the building blocks degenerate significantly to near disappearance compared with that in solution. At normal atmospheric temperature, absorption of the polymers changed from complex multipeak feature in solution into bimodal band absorption spectra in film with bathochromic shift. In the solid state, the intensities of aggregation absorption increase greatly, exceeding the main absorption peaks of intramolecular long-range interaction, hence changing the shape of the absorption spectra curves. Because the strong intermolecular interaction already exists in the polymer solutions, the red-shift of absorption edge wavelengths is weakened obviously for both polymers, from solution to film. In comparison with solution, PTVTTBT film displays exactly the same aggregation absorption peak, with a slight red-shift of 3 nm in edge wavelengths. By contrast, PETVTTBT manifests a definite red-shift of 7 nm in aggregation absorption and 14 nm in absorption edge in film, relative to its solution, suggesting that the ring fusion of TVT segment alleviates excessive molecular aggregation in solution. Electrochemical Properties. Cyclic voltammetry (CV) was employed to study the electrochemical properties of these conjugated polymers. From the onset oxidation potential (Eox) and reduction potentials (Ered) in the cyclic voltammogram, ionization potential (IP) and electron affinity (EA) could be readily estimated, which correspond to the highest occupied

constant, corroborating that the absorption at 326 nm originates in intrinsic absorptions of building block (benzothiadiazole); second and third absorption bands shift hypsochromically from 441 to 420 nm and 587 to 555 nm, respectively, implying that enhanced thermal motion of solvent molecule and the polymer chain at higher temperature affect both the short- and long-range intermolecule interactions of building blocks in PTVTTBT. Meanwhile, temperature increase has complex effects on absorption peak intensities of the polymer solution. From 20 to 60 °C, all the intensities of the three absorption bands improve gradually. However, along with further rise in the solution temperature, from 60 to 80 °C, the intensities of the absorption peaks are diverse in various breeds. The intensity of the first absorption band has the trend of strengthening continuously; the intensity of the second band is almost keeping invariable; the intensity of the third band presents a trend of getting lower. In this process, the intensity of aggregation peak at 705 nm gradually declines and nearly disappears completely above 65 °C, with steady wavelength. At the same time, the wavelength and intensity of the aggregation peak at 493 nm seem to remain unchanged and overlapped by the stepwise blue-shifted third absorption band. The PETVTTBT solution also demonstrates three absorption bands at 20 °C. In consequence of its rise in temperature, the intensity of the first absorption band of PETVTTBT solution increases regularly, with the constant peak wavelength at 326 nm, corresponding to the intrinsic absorption of benzothiadiazole, as discussed above. Similar to that of PTVTTBT, the peaks of second and third absorption bands of PETVTTBT solution showed a blue-shift from 429 to 390 nm and 590 to 526 nm, respectively, with the increase of temperature from 20 to 80 °C. Compared to that of PTVTTBT, the absorption spectra of PETVTTBT are more susceptible to temperature. From 20 to 35 °C, absorption intensity of both the second and third absorption bands of PETVTTBT solution is steadily dwindling; from 35 to 60 °C, the intensities of the two peaks exhibit a gradual improvement; from 60 to 80 °C, the third absorption bands shows a relatively D

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Figure 4. Simulated molecular geometries (top) and frontier molecular orbitals (bottom) diagram obtained by DFT calculations for simplified molecules of PTVTTBT and PETVTTBT (n = 2). Color code: gray (C), white (H), blue (N), yellow (S), and turquoise (F).

Figure 5. Potential energy surface scan of TVT and ETVT units.

polymer PETVTTBT, though downshifts the HOMO energy level by 0.1 eV. Therefore, compared to that of PTVTTBT, the band gap (EgCV) of PETVTTBT is increased significantly, which is supported by their absorption spectra (Figure 2). The decrease in the HOMO level for PETVTTBT could contribute to the increase of the open-circuit voltage (Voc) for both fullerene and non-fullerene solar cells. The electrochemical band gaps of PTVTTBT and PETVTTBT are 1.71 and 1.81

molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the polymers, respectively. Cyclic voltammograms of the polymer films are shown in Figure S2, and the energy levels of the polymers are calculated and summarized in Figure 3c. The HOMO and LUMO energy levels of PTVTTBT are −5.15 and −3.44 eV, calculated from its Eox and Ered (0.78 and −0.93 V vs Ag/AgCl), respectively. Interestingly, the ring fusion of TVT units has a negligible effect on the LUMO energy level of the resulting E

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Figure 6. (a, b) J−V characteristics and (c, d) EQE spectra of the optimized PSCs under illumination of AM 1.5G at 100 mW cm−2.

eV, respectively, in good agreement with their optical band gaps. Theoretical Calculations. To evaluate the influence of ring fusion of TVT unit on molecular architecture and consequently on the optoelectronic properties of the corresponding polymers, density functional theory (DFT) calculations were performed to verify stationary points as stable states for the optimized conformations and single point energies at the B3LYP/6-31G (d) level of theory in vacuum using the Gaussian 09 program package (see ref S1 in the Supporting Information). The final energies were calculated as the sum of single point and zero point energies. In particular, the HOMO and LUMO level positions and related electron distributions were calculated. Moreover, all the alkyl chains were replaced by methyl groups in the calculation to avoid excessive computation demand. Optimized geometries of the polymers in the ground state are depicted in Figure 4. The polymer PTVTTBT showed excellent planar conformation, with all the dihedral angles between adjacent aromatic rings less than 0.5°. After ring fusing, dihedral angle between thiophene and neighbor unit increased to 24.4° in the ETVT segment in PETVTTBT compared with that of almost 0° in the TVT segment in PTVTTBT. At the same time, the dihedral angles between ETVT unit and neighboring two thiophenes are 19.3° and 18.6° in PETVTTBT, respectively. To further understand such a huge difference for dihedral angles and obtain more insight into the effect of ring fusion on TVT unit, relaxed potential surface energy scans were carried out for TVT and ETVT at 10° intervals. The dihedral angle was constrained (0−360°, altogether 37 conformational isomers), and all other degrees of freedom were allowed to relax to their energy minima at the B3LYP/6-31G(d) level. As shown in Figure 5, great differences are observed for the energy curves against torsion degree between TVT and ETVT. There are two potential rotatable bonds in TVT and consequently three rotamers, i.e., trans,trans-TVT, cis,transTVT, and cis,cis-TVT. Taking trans,trans-TVT as a reference,

the calculated relative conformational energies are 4.96 and 9.64 kJ mol−1 for cis,trans-TVT and cis,cis-TVT, respectively. By contrast, there is a single rotatable bond in ETVT. Because of the nonplanar structure, two pairs of enantiomers exist in the ETVT unit, denoted by cis-ETVT and trans-ETVT. Figure 5c shows the general trend of conformational energy against torsion degree of ETVT. However, the curve is not smooth. So, more careful calculation was performed at 0.5° intervals to get the exact optimized geometry of ETVT subunit. Because of the same energy in enantiomers, the conformational energy of one of each enantiomer was investigated. The calculation was performed around the angles corresponding to energy minima, in the range of 25°−35° and 145°−155° for cis-ETVT and trans-ETVT, respectively. As seen in Figure 5d, in optimized geometry, dihedral angles between thiophene and benzothiophen ring are 28.5° and 152° for cis-ETVT and trans-ETVT, respectively. The energy difference between cis and trans conformation is only 0.14 kJ mol−1 for ETVT. The wave functions of the frontier molecular orbital are depicted in Figure 4. As can be observed, the HOMO are delocalized along the whole π-conjugated backbone while the LUMO are mostly concentrated on the benzothiadiazole based acceptor groups for both of the polymers. These images provide further evidence of the formation of well-defined D−A structure and the intramolecular charge transfer (ICT) behavior of the material (i.e., the HOMO to LUMO transition is a donor to acceptor intramolecular charge transfer).42 The calculated HOMO/LUMO energy levels of the polymers are −2.88/− 4.69 and −2.85/−4.92 eV, for PTVTTBT and PETVTTBT, respectively. Although discrepancies exist between calculation and experimental results, the trends of variation in the energy levels are identical. The ring fusion of TVT units lowers the HOMO energy level of the resulting polymer, while it does not obviously impact the LUMO energy level. As a result, the band gap of PETVTTBT is increased significantly compared with that of PTVTTBT. These results are in concert with the optical and electrochemical data. F

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Macromolecules Table 2. Device Parameters of the Optimized PSCs under the Illumination of AM 1.5G at 100 mW cm−2 D/A (w:w) PTVTTBT/PC71BM (1:1) PETVTTBT/PC71BM (1:1) PTVTTBT/ITIC (1.5:1) PETVTTBT/ITIC (1:1) a

Voc (V) 0.66 0.76 0.78 0.91

Jsc (mA cm−2)

PCE (%)a

FF (%)

13.42 13.07 9.01 15.02

68.94 68.07 52.46 59.84

6.10 6.76 3.69 8.18

[6.04 [6.63 [3.58 [8.09

± ± ± ±

0.08] 0.14] 0.12] 0.11]

μh (cm2 V−1 s−1) 7.03 8.11 7.48 6.51

× × × ×

−3

10 10−3 10−3 10−3

μe (cm2 V−1 s−1) 4.61 2.87 6.92 1.34

× × × ×

10−3 10−3 10−4 10−4

Average PCEs with standard deviations in square brackets are based on 12 devices.

Figure 7. Plots of ln(JL3/V2) versus (V/L)0.5 for the measurement of hole (a) and electron (b) mobility in the active layers by the SCLC method.

Photovoltaic Properties. To investigate the influence of ring fusion of TVT units on the photovoltaic properties of the conjugated polymers, bulk heterojunction PSC devices based on PTVTTBT and PETVTTBT were fabricated. PC71BM and ITIC (Figure 3a) were used as the representative electron acceptors to evaluate the photovoltaic performance of the polymers in both fullerene and non-fullerene solar cell systems. The current density−potential (J−V) curves and external quantum efficiency (EQE) spectra are shown in Figure 6, and the corresponding device parameters are summarized in Table 2 and Table S1. Using PC71BM as the acceptor, as-cast solar cells based on PTVTTBT showed moderate PCE of 6.10%, with Voc of 0.66 V, Jsc of 13.42 mA cm−2, and FF of 68.94%. By contrast, the devices based on PETVTTBT exhibited obvious increased Voc of 0.76 V and slightly decreased Jsc of 13.07 mA cm−2 and FF of 68.07%. The increased value of Voc of corresponding PSCs is in accordance with the decrease of the HOMO energy levels of the polymer PETVTTBT. Because of the relatively low absorption beyond 600 nm of PC71BM, photoresponse of the solar cells in long wavelength range depends on the optical property of the polymers. As shown in Figure 6c, the flat EQE curves indicate the balanced contribution from polymer and PC71BM in both of PTVTTBT and PETVTTBT based devices. The photoresponse of solar cells based on the polymers is consistent well with their absorption characteristics. The PTVTTBT based device shows broad EQE up to about 800 nm, with the intensity of above 50% in the range of 375−717 nm. The absorption coefficient hike led to higher EQE of PETVTTBT: PC71BM based solar cells in almost the whole range of 370−676 nm compared to that of PTVTTBT. However, the hypsochromic shift of PETVTTBT absorption edge brings about sharply falling in EQE beyond 676 nm. Their combined effects determine the slight reduction of Jsc by about 3%. Benefiting from the significant promotion of Voc, at last, a PCE of 6.76% was achieved for PETVTTBT: PC71BM based device, reaching an 11% improvement with respect to that of PTVTTBT.

Ring fusion of TVT influences more obviously on photovoltaic performance of the non-fullerene PSCs. Combining with the ITIC acceptor, the photovoltaic devices based on PTVTTBT yielded relatively low PCEs of 3.69%. After ring fusing, polymer PETVTTBT displayed a simultaneous enhancement of Voc (0.91 vs 0.78 V), Jsc (15.02 vs 9.01 mA cm−2), and FF (59.84% vs 52.46%) by 17%, 67%, and 14%, respectively, in the non-fullerene polymer solar cells. As a result, the drastic increase in the PCE is realized by 120% over the PTVTTBT based devices, reaching 8.18%. The superior photovoltaic performance of polymer PETVTTBT could be further confirmed by the EQE spectra of the devices. As seen in Figure 3b, ITIC makes up the absorption absence of PETVTTBT in the long wavelength range, particularly in the region beyond 675 nm. Combining its enhanced absorption coefficient, in ITIC based solar cells, PETVTTBT projects outdistanced photocurrent response in the entire spectrum range (Figure 6d), which further verifies the significantly higher Jsc than that of PTVTTBT. The calculated short-circuit currents from the integration of the EQE values are 9.00 and 14.58 mA cm−2 for PTVTTBT:ITIC and PETVTTBT:ITIC based solar cells, respectively, which agree well with corresponding J−V characteristics of the devices. To further study the impact of ring fusion of TVT segment on the charge carrier mobility in the active layers of the PSCs devices, the hole (μh) and electron mobilities (μe) in the photosensitive layers were measured by the space charge limited current (SCLC) method using devices with structure of ITO/PEDOT:PSS/polymer:acceptor/Au and ITO/TiOx/polymer:acceptor/Al. For unipolar transport in a trap-free semiconductor with an ohmic injecting contact, the SCLC can be approximated by the Mott−Gurney equation:43 J≅

⎛ 9 V ⎞ V2 εrε0μ0 exp⎜0.891γ ⎟ 8 L ⎠ L3 ⎝

(1)

where J is the current density, εr is the dielectric constant of the polymer, ε0 is the free-space permittivity (8.85 × 10−12 F/m), μ0 is the charge mobility at zero field, γ is a constant, L is the thickness of the blended film layer, V = Vappl − Vbi, Vappl is the G

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Figure 8. AFM height images (top) and phase images (5.0 μm × 5.0 μm) of the polymer:acceptor blend films.

applied potential, and Vbi is the built-in potential which results from the difference in the work function of the anode and the cathode (in this device structure, Vbi = 0.2 V). Figure 7 displays the ln(JL3/V2) versus (V/L)0.5 curve for the measurement of the μh and μe of the blends by the SCLC method. The calculated mobilities using eq 1 are summarized in Table 2. All the polymer:acceptor blends display relative high hole and electron mobilities at 10−4−10−3 cm2 V−1 s−1 order of magnitude. PETVTTBT shows comparable hole mobilities with PTVTTBT in both fullerene and non-fullerene solar cell systems, at 10−3 cm2 V−1 s−1 order of magnitude. The results prove that ring fusion of TVT segment does not affect charge carrier mobility of the resulting polymers in PSCs. In other words, ETVT based polymers keeps the advantage of high charge mobility of TVT derivatives in polymer solar cells. The μe in ITIC based solar cells is 1 order of magnitude lower than that of PC71BM based devices, which could be one of the most important reasons causing inferior FF in the ITIC based solar cells.26 This could be attributed to the lower electron mobility of ITIC than that of PC71BM.44 Atom force microscopy (AFM) was employed to analyze active layers of the polymer solar cells and to investigate the effect of ring fusion of TVT segment on the morphologies and microstructures of the polymer:acceptor blend films. As shown in Figure 8, all the blends display a very smooth surface. There is no significant difference between the PTVTTBT and PETVTTBT in blending films both with PC71BM and ITIC, in terms of their height and phase images. This result indicates that the ring fusion of TVT segment do not obviously affect the morphologies and microstructures of the polymer:acceptor blend films. PTVTTBT and PETVTTBT have almost the same average roughness (Ra) in the blend films based PC71BM and ITIC, with Ra of 2 and 3 nm, respectively. The increased Ra in ITIC based films should be ascribed to the strong π−π stacking of ITIC.45 As mentioned in the Introduction, one of the most recognized disadvantages of the TVT based photovoltaic polymers is the large energy loss. Figure 9 shows the plots of PCE against energy loss for the solar cells based on TVT containing polymers in this work and reported in the literature. The detailed data of the corresponding devices are summarized in Table S2. It can be seen that the energy loss in PSCs based on TVT derivatives are concentrated in the 0.8−1.2 eV regions. In PC71BM based solar cells, PTVTTBT and PETVTTBT

Figure 9. Plots of PCE against energy loss for TVT based polymer solar cell systems with different acceptors. Polymers in this work are shown with solid points. NFA represents non-fullerene acceptor (see Table S2 in the Supporting Information for the detailed data). The green dashed lines are the Eg − eVOC = 0.8 and 1.2 eV.

show comparable energy loss of 0.95 and 0.93 eV, which are among the typical values for the TVT based PSCs. For the nonfullerene PSCs with ITIC as acceptor, the VOC is 0.78 and 0.91 V, and the Eg referring to the onset of film absorption of ITIC (782 nm, Figure 3b) is 1.58 eV. Thus, the energy loss (Eloss) calculated from the definition of Eloss = Eg− eVOC is 0.80 and 0.67 eV, where Eg is the lower optical band gap of the donor and acceptor components. Compared to that of PTVTTBT, the reduction of Eloss in PETVTTBT based solar cells should be the result of electron rearrangement of CC subunits in ETVT after ring fusion. It should be noted that reason for the difference of energy loss is not clearly understood and maybe related to the loss in the formation of charge transfer (CT) states and the loss in the charges or CT states recombination.46 Further investigation is underway. It is clear that the energy loss of the PETVTTBT:ITIC system is among the lowest values reported so far for PSCs based on TVT derivatives (Figure 9 and Table S1). At the same time, the Eloss of 0.67 eV is smaller than that of most PSCs and approaching the empirically low threshold of 0.6 eV.47−49 More importantly, despite the low energy loss value, the PETVTTBT:ITIC system can still achieve a PCE as high as 8.18%, which is the champion efficiency of TVT based polymer solar cells. H

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Properties and Photovoltaic Performance. Prog. Polym. Sci. 2011, 36 (10), 1326−1414. (5) Jung, J. W.; Jo, J. W.; Jung, E. H.; Jo, W. H. Recent Progress in High Efficiency Polymer Solar Cells by Rational Design and Energy Level Tuning of Low Bandgap Copolymers with Various ElectronWithdrawing Units. Org. Electron. 2016, 31, 149−170. (6) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115 (23), 12666−12731. (7) Musser, A. J.; Al-Hashimi, M.; Maiuri, M.; Brida, D.; Heeney, M.; Cerullo, G.; Friend, R. H.; Clark, J. Activated Singlet Exciton Fission in a Semiconducting Polymer. J. Am. Chem. Soc. 2013, 135 (34), 12747− 12754. (8) Huang, H.; Chen, Z.; Ponce Ortiz, R.; Newman, C.; Usta, H.; Lou, S.; Youn, J.; Noh, Y. Y.; Baeg, K. J.; Chen, L. X.; Facchetti, A.; Marks, T. J. Combining Electron-neutral Building Blocks with Intramolecular “Conformational Locks” Affords Stable, High-mobility P- and N-Channel Polymer Semiconductors. J. Am. Chem. Soc. 2012, 134 (26), 10966−10973. (9) Cho, H.-H.; Kim, S.; Kim, T.; Sree, V. G.; Jin, S.-H.; Kim, F. S.; Kim, B. J. Design of Cyanovinylene-Containing Polymer Acceptors with Large Dipole Moment Change for Efficient Charge Generation in High-Performance All-Polymer Solar Cells. Adv. Energy Mater. 2018, 8 (3), 1701436. (10) Wang, Y.; Hasegawa, T.; Matsumoto, H.; Mori, T.; Michinobu, T. Rational Design of High-Mobility Semicrystalline Conjugated Polymers with Tunable Charge Polarity: Beyond BenzobisthiadiazoleBased Polymers. Adv. Funct. Mater. 2017, 27 (2), 1604608. (11) Choi, H. H.; Baek, J. Y.; Song, E.; Kang, B.; Cho, K.; Kwon, S. K.; Kim, Y. H. A Pseudo-Regular Alternating Conjugated Copolymer Using an Asymmetric Monomer: A High-Mobility Organic Transistor in Nonchlorinated Solvents. Adv. Mater. 2015, 27 (24), 3626−3631. (12) Huang, H.; Zhou, N.; Ortiz, R. P.; Chen, Z.; Loser, S.; Zhang, S.; Guo, X.; Casado, J.; López Navarrete, J. T.; Yu, X.; Facchetti, A.; Marks, T. J. Alkoxy-Functionalized Thienyl-Vinylene Polymers for Field-Effect Transistors and All-Polymer Solar Cells. Adv. Funct. Mater. 2014, 24 (19), 2782−2793. (13) Yun, H. J.; Kang, S. J.; Xu, Y.; Kim, S. O.; Kim, Y. H.; Noh, Y. Y.; Kwon, S. K. Dramatic Inversion of Charge Polarity in Diketopyrrolopyrrole-Based Organic Field-Effect Transistors via a Simple Nitrile Group Substitution. Adv. Mater. 2014, 26 (43), 7300−7730. (14) Khim, D.; Cheon, Y. R.; Xu, Y.; Park, W.-T.; Kwon, S.-K.; Noh, Y.-Y.; Kim, Y.-H. Facile Route to Control the Ambipolar Transport in Semiconducting Polymers. Chem. Mater. 2016, 28 (7), 2287−2294. (15) Kim, H. S.; Huseynova, G.; Noh, Y.-Y.; Hwang, D.-H. Modulation of Majority Charge Carrier from Hole to Electron by Incorporation of Cyano Groups in Diketopyrrolopyrrole-Based Polymers. Macromolecules 2017, 50 (19), 7550−7558. (16) Zhang, W.; Mao, Z.; Huang, J.; Gao, D.; Yu, G. HighPerformance Field-Effect Transistors Fabricated with Donor−Acceptor Copolymers Containing S···O Conformational Locks Supplied by Diethoxydithiophenethenes. Macromolecules 2016, 49 (17), 6401− 6410. (17) Speros, J. C.; Paulsen, B. D.; White, S. P.; Wu, Y.; Jackson, E. A.; Slowinski, B. S.; Frisbie, C. D.; Hillmyer, M. A. An ADMET Route to Low-Band-Gap Poly(3-hexadecylthienylene vinylene): A Systematic Study of Molecular Weight on Photovoltaic Performance. Macromolecules 2012, 45 (5), 2190−2199. (18) Dong, X.; Tian, H.; Xie, Z.; Geng, Y.; Wang, F. Donor− Acceptor Conjugated Polymers Based on Two-Dimensional Thiophene Derivatives for Bulk Heterojunction Solar Cells. Polym. Chem. 2017, 8 (2), 421−430. (19) Elandaloussi, E. H.; Frère, P.; Richomme, P.; Orduna, J.; Garin, J.; Roncali, J. Effect of Chain Extension on the Electrochemical and Electronic Properties of π-Conjugated Soluble Thienylenevinylene Oligomers. J. Am. Chem. Soc. 1997, 119 (44), 10774−10784. (20) Lu, C.; Wu, H. C.; Chiu, Y. C.; Lee, W. Y.; Chen, W. C. Biaxially Extended Quaterthiophene− and Octithiophene−Vinylene Conju-

CONCLUSION In summary, we designed and synthesized photovoltaic polymers based on TVT and ETVT donors and DTBT acceptor by the palladium-catalyzed Stille coupling reaction. The polymers possess good solubility and thermal stability. The ring fusion of TVT segment not only downshifted the HOMO energy level of the conjugated polymer but also increased absorption coefficient of PETVTTBT compared to that of nonfused polymer PTVTTBT. At the same time, PETVTTBT preserved high charge carrier mobility of TVT based polymers in solar cells. Integrating these advantage of ring fused TVT based polymers, PETVTTBT shows superior photovoltaic performance to PTVTTBT, in both fullerene and non-fullerene solar cells. Particularly, in ITIC based polymer solar cells, simultaneous enhancement in the JSC, VOC, and FF was demonstrated after ring fusion. As a result, PCE of PETVTTBT based solar cells increased drastically by 120% over that of PTVTTBT (3.69%), reaching 8.18%. In addition, energy loss of the PETVTTBT:ITIC system (0.67 eV) is among the lowest values reported so far for PSCs based on TVT derivatives, and smaller than that of most PSCs, approaching the empirically low threshold of 0.6 eV. These results indicate that ring fusion should be an effective method to reduce energy loss toward high performance PSCs based on TVT derivatives.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00805. Additional material characterization, device evaluation, and photovoltaic parameters of TVT derivative based polymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.Z.). ORCID

Xiaochen Wang: 0000-0001-8888-3503 Erjun Zhou: 0000-0003-1182-311X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of China (NSFC, No. 21504019, 51773046, 51673048, 51473040, and 21602040), the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDBSSW-SLH033), the National Key Research and Development Program of China (2017YFA0206600), and the National Natural Science Foundation of Beijing (No. 2162045).

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REFERENCES

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