Article pubs.acs.org/Macromolecules
Medium-Bandgap Conjugated Polymers Containing Fused Dithienobenzochalcogenadiazoles: Chalcogen Atom Effects on Organic Photovoltaics Jaewon Lee, Dong Hun Sin, J. Arul Clement, Chandramouli Kulshreshtha, Heung Gyu Kim, Eunjoo Song, Jisoo Shin, Hyeongjin Hwang, and Kilwon Cho* Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea S Supporting Information *
ABSTRACT: We designed, synthesized, and characterized a series of three medium-bandgap conjugated polymers (PBDTfDTBO, PBDTfDTBT, and PBDTfDTBS) consisting of fused dithienobenzochalcogenadiazole (fDTBX)-based weak electron-deficient and planar building blocks, which possess bandgaps of ∼2.01 eV. The fDTBX-based medium-bandgap polymers exhibit deep-lying HOMO levels (∼5.51 eV), which is beneficial for use in multijunction polymer solar cell applications. The resulting polymers with chalcogen atomic substitutions revealed that the difference in the electron negativity and atomic size of heavy atoms highly affects an intrinsic property, morphological feature, and photovoltaic property in polymer solar cells. The polymer solar cells based on sulfur-substituted medium-bandgap polymer showed power conversion efficiencies above 6% when blended with [6,6]-phenyl-C71-butyric acid methyl ester in a typical bulk-heterojunction single cell. These results suggest that the fDTBX-based medium-bandgap polymer is a promising alternative material for P3HT in tandem polymer solar cells for achieving high efficiency.
1. INTRODUCTION
Achieving high PCEs by simultaneously increasing the shortcircuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF) remains a challenge because JSC/VOC trade-off has been shown in the D−A type low-bandgap polymer-based PSCs.15 As a result, multijunction solar cells with multiple p−n bulk-heterojunction (BHJ) layers made of different semiconducting polymers for complementary absorption spectra have been considered as a promising way to overcome the aforementioned problem and further enhance the performance of PSCs.16,17 In this sense, medium-bandgap polymers with fascinating photovoltaic performances are highly desired as well as low-bandgap polymers.18 However, the exploration of medium-bandgap polymers with suitable photovoltaic properties for tandem cells has been largely overlooked compared to the extensive efforts into developing low-bandgap polymers.
Polymer solar cells (PSCs) have attracted considerable attention as a promising clean and renewable energy resource because of many competitive advantages for achieving lightweight, low-cost, large area, semitransparent, and flexible devices through roll to roll solution processing techniques.1−3 In the past decade, significant improvements in power conversion efficiencies (PCEs) of PSCs have been achieved due to the development of novel conjugated polymers for active layers,4−7 the incorporation of effective interlayers,8 understanding of polymeric solid state microstructures and photophysics,9−11 and the optimization of fabrication processing and device architecture techniques.12,13 Among these innovations, development of novel donor polymers constructed by the donor−acceptor (D−A) approach is highly effective to obtain high PCEs; the PCEs of single-junction PSCs with development of new D−A alternating copolymers have exceeded 10%.14 © XXXX American Chemical Society
Received: July 20, 2016 Revised: November 24, 2016
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DOI: 10.1021/acs.macromol.6b01569 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Scheme 1. Synthetic Procedure of (a) Electron-Donating Monomer 2Tin-BDT-T and (b) Electron-Withdrawing Monomers 2Br-DT-fDTBX (X = O, T, and S for Oxygen, Sulfur, and Selenium, Respectively)
More recently, Mei et al. reported a fused dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2-c][1,2,5]thiadiazole (fDTBT)based polymer and its significant bathochromic characteristic of ICT transition (>70 nm) compared to nonfused DTBTbased polymers.24 This ring fusion strategy demonstrates that an electron-accepting strength of fDTBT unit significantly decreases compared to the BT unit because of the fused two electron-rich thienyl rings. In addition, the fDTBT-based polymers exhibited high charge carrier mobilities probably due to the reduced conformational disorder resulting from its four-ring-fused structure.25−28 Despite these advantages, there are very few reports on fDTBT-based conjugated polymers in comparison with TAZ-based conjugated polymers for organic electronic applications. In this work, we designed and synthesized new acceptor units of fused dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2-c][1,2,5]chalcogenadiazole fDTBX (X = O, T, and S), namely fDTBO, fDTBT, and fDTBS, for constructing well-performing medium-bandgap polymers. The fused two electron-rich thienyl rings flanked with electron-deficient benzochalcogenadiazole (BO, BT, and BS) units can effectively decrease the electronwithdrawing property of the whole fDTBX moiety keeping high-lying LUMO energy level. To construct D−A alternating copolymers, a weak electron-rich unit of 2D conjugated benzo[1,2-b:4,5-b′]dithiophene with 2-alkylthienyl side groups (BDT-T) was employed as a comonomer with the fDTBX derivatives. This combination yields a synergetic effect of BDTT as a weak donor and fDTBX as weak acceptors, thereby contributing to produce medium-bandgap polymers with deeplying HOMO levels. Furthermore, by substituting chalcogen atoms from oxygen to sulfur to selenium within the fDTBX
Poly(3-hexylthiophene) (P3HT) has long been considered as a successful medium-bandgap polymer used in the bottom cell of tandem PSCs.19 However, P3HT-based solar cells usually suffered from low VOC values (≈0.6 V) when combined with fullerene derivatives due to its relatively high-lying highest occupied molecular orbital (HOMO) energy level.20 Modulating the electron-donating or -withdrawing strengths of donor or acceptor units in D−A alternating copolymers has been utilized as an effective way to obtain a low-lying HOMO energy level as well as medium-bandgap property.21 Moreover, D−A alternating copolymers have resulted in high charge mobilities compared to P3HT because of their strong intramolecular charge-transfer (ICT), namely the push−pull effect, along the conjugated backbones.22 Recently, Price et al. reported a medium-bandgap polymer with a promising photovoltaic property of 7.1% using fluorinated benzo[d][1,2,3]triazole (FTAZ) as the acceptor unit which possesses a weak electronwithdrawing ability and a deep-lying HOMO level.18The 2position nitrogen atom of TAZ is liable to donate its lone pair electron onto triazole ring, leading to a higher-lying lowest unoccupied molecular orbital (LUMO) energy level and thus wider Eg of resulting polymer when compared to benzothiadiazole (BT)-based polymers. Dong et al. reported on a series of medium-bandgap polymers (Eg ≈ 1.9 eV) based on naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) (TZNT) unit. One of them demonstrated a high PCE of up to 7.11% in an inverted BHJ solar cell structure.23 They designed and synthesized a new acceptor unit of TZNT inspired from a weak electron-withdrawing TAZ and employed a weak electron-rich unit of BDT as the comonomer to construct medium-bandgap polymers with deep-lying HOMO levels. B
DOI: 10.1021/acs.macromol.6b01569 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 2. Synthesis and Chemical Structure of Three fDTBX-Based Copolymers
unit, we finely tuned the material optoelectronic properties and investigated their influences into the PCEs of PSCs. The solar cell device fabricated with the PBDTfDTBT:[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) blend exhibited the highest photovoltaic performance among these three PBDTfDTBX derivatives with a high VOC of 0.86 V and a power conversion efficiency of 6.02%. This study exhibits the great potential of PBDTfDTBT in constructing low-cost, high-efficiency tandem solar cell applications.
Supporting Information. The chemical structures of the synthesized compounds and polymers were confirmed by elemental analysis and NMR spectroscopy. These polymers are readily soluble in chlorinated solvents, such as chloroform (CF), chlorobenzene (CB), and o-dichlorobenzene (DCB). The number-average molecular weights (Mn) of these three polymers from the CF fractions were 15.2, 14.3, and 15.3 kDa, with polydispersity index (PDI) of 1.44, 2.25, and 1.80, respectively (Table 1).
2. RESULTS AND DISCUSSION 2.1. Design, Synthesis, and Characterization. The detailed synthetic procedures of monomers are described in Scheme 1. The BDT-T unit was synthesized according to a modification of a previously reported synthetic pathway.29 The branched alkyl chain of 2-butyloctyl was employed to ensure the polymers to possess good processable solubility. Compound 3 has been synthesized following modified reported procedure.30 The diketone intermediate (2) was prepared from selective lithiation of 3-bromothiophene with n-butyllithium followed by quenching with oxalyl chloride, which yielded di-3thienylethanedione. Then it was subjected to oxidative intramolecular ring-closing reaction using iron trichloride.31 Formation of fDTBO can be achieved by carrying out reaction using a pressure vessel at 140 °C for 3 days in the presence of diketone and hydroxylamine.28,30 Condensation of diketone (3) with hydroxylamine in the presence of ethanol gave dioxime (4) which was directly reduced to diamine (5) by using Pd/C. Treatment of diamine with thionyl chloride in the presence of triethylamine in chloroform under reflux conditions afforded fDTBT.30 Compound fDTBS can be synthesized by reacting the diamine and selenium dioxide in ethanol.32 Bromination of fDTBO, fDTBT, and fDTBS was carried out using bromine in the presence of chloroform. Stille coupling reaction of compound 6 with (4-(2-ethylhexyl)thiophen-2yl)trimethylstannane afforded 7 in 65−75% yields. Bromination of compounds 7 using N-bromosuccinimide in the presence of THF to give final acceptor monomers in 80−85%. Finally, target polymers were prepared by performing the Stille cross-coupling reactions between the bis(trimethyltin) BDT monomer and the dibrominated DT-fDTBX monomers with Pd(PPh3)4 as a catalyst in anhydrous toluene and N,Ndimethylformamide (10:1 vol %), as shown in Scheme 2. The specific polymerization procedure is described in the
Table 1. Molecular Weights, Thermal Properties, and Solubility of PBDTfDTBX Polymers polymer
Mna (kDa)
Mwa (kDa)
PDIa
Tdb (°C)
solubilityc (g L−1)
PBDTfDTBO-L PBDTfDTBT-L PBDTfDTBS-L PBDTfDTBT-H
15.2 14.3 15.3 90.9
21.9 32.2 27.6 220.0
1.44 2.25 1.80 2.20
403 441 367 438
28.1 27.2 21.7 22.1
a
Determined by GPC using polystyrene standards and chlorobenzene (CB) as eluent. b5% weight loss temperatures measured by TGA under a nitrogen atmosphere. cThe concentration of the saturated solution in chlorobenzene (CB) at 25 °C according to the Beer− Lambert law.
2.2. Theoretical Calculations. To gain an in-depth understanding of the structural and electronic properties of the resulting polymers, density functional theory (DFT) calculations, using B3LYP/6-31G* model, were performed on the DT-fDTBX units (DT-fDTBO, DT-fDTBT, and DTfDTBS, Figure 1 and Figure S7) and dimeric model molecules (BDTfDTBO, BDTfDTBT, and BDTfDTBS, Figure 2 and Figure S8) with methyl-trimmed alkyl chains for computation simplicity.33,34 As shown in Figure 1a, the calculated HOMO and LUMO energy levels of DT-BT and DT-fDTBT indicate that the fused two electron-donating thienyl units flanked with electrondeficient BT ring can effectively decrease the electronwithdrawing property of the whole fDTBT moiety by possessing a higher-lying lowest unoccupied molecular orbital (LUMO) energy level. The N−X bonding distances in the present chalcogenadiazole substructures gradually increase from 1.37 Å (DT-fDTBO) to 1.65 Å (DT-fDTBT) to 1.80 Å (DTfDTBS) as the chalcogen atom becomes heavier from oxygen to sulfur to selenium (Figure 1b). The larger N−X distance C
DOI: 10.1021/acs.macromol.6b01569 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 1. (a) Energy level diagram of DT-BT and DT-fDTBT model molecules measured by DFT calculation (B3LYP functional/6-31G* basis set). (b) Illustration of dipole moment of DT-fDTBO, -T, and -S units. The arrow indicates the direction and magnitude of the dipole moment. The red, yellow, and orange symbols represent oxygen, sulfur, and selenium atoms, respectively.
Figure 2. Isodensity surface plot and energy level diagram of di-BDTfDTBX model molecules measured by DFT calculation (B3LYP functional/631G* basis set).
Figure 3. (a) Cyclic voltammograms and (b) TGA plots of PBDTfDTBX-L polymers. Normalized UV−vis absorption spectra of the polymers (c) in CB solution at a concentration of 0.025 g L−1 for 25 °C and (d) spin-coated thin films prepared by CB.
shoves the nitrogen atom out of the pentangular heterocyclic substructure in the chalcogenadiazole, leading to smaller bond
angles (θb) from 130.1° to 126.0° to 123.9° as it goes from DTfDTBO to DT-fDTBT to DT-fDTBS. This suggests that the D
DOI: 10.1021/acs.macromol.6b01569 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. Optical Properties and Molecular Energy Levels of PBDTfDTBX DFT
cyclic voltammetry
polymer
λmax solna (nm)
λmax filmb (nm)
λonset filmb (nm)
HOMO/LUMO (eV)
HOMO/LUMO (eV)
EgDFT (eV)
Egopt c (eV)
PBDTfDTBO PBDTfDTBT PBDTfDTBS
353, 492 351, 544 367, 515
363, 525 356, 543 371, 552
617 626 646
−4.94/−2.53 −4.84/−2.50 −4.80/−2.52
−5.51/−3.56 −5.44/−3.54 −5.40/−3.55
2.41 2.34 2.28
2.01 1.98 1.92
Measured in dilute chlorobenzene solution at a concentration of 0.05 g L−1. bSpin-cast from 10 mg mL−1 chlorobenzene solution. cEstimated from the onset of the UV−vis spectra measured from thin films.
a
bonding MO.40 The deep-lying HOMO levels ( PBDTfDTBO-L, i.e., in accordance to the degree of crystallite strength of the blend films. Although, PBDTfDTBS-L and PBDTfDTBO-L initial decays are quite similar; however, PBDTfDTBO-L decays slower until around the tenth microsecond. This confirms that the amount of recombination in PBDTfDTBO-L device has started quite early and even low concentration of charge carriers which already were dissociated have been still recombining until later times. A detail analysis of amount of recombination in these blends by transient absorption spectroscopy is ready for submission. Now considering the case of additive effect in the blends, the PBDTfDTBO-L, PBDTfDTBS-L, PBDTfDTBT-L, and PBDTfDTBT-H devices prepared with DIO decay faster than the devices prepared without DIO due to increase in charge carrier densities inside the devices.65,66 Their monoexponential carrier lifetimes were calculated as 2, 2.8, 2.5, and 16 μs, thus showing decay time in the following order: PBDTfDTBT-H > PBDTfDTBS-L > PBDTfDTBT-L > PBDTfDTBO-L. As indicated above, addition of DIO in solvent results in a dramatic change in the active layer morphology, which therefore formed interpenetrating network of polymer and PC71BM, and in fact enhances the carrier densities in the device. Because of the higher charge carrier densities, recombination kicks in the bulk and at open-circuit condition where no charge carriers are extracted, charges recombine faster with DIO than without DIO.67 Comparing PBDTfDTBS-L and PBDTfDTBT (-H or -L) blend films prepared with DIO, it is quite interesting to see that PBDTfDTBS-L decays slightly faster than the PBDTfDTBT-L blend although much sharper phase separation can be seen above in the PBDTfDTBT-L blend. However, we expect that larger size chalcogens, i.e., O < S < Se, interact more with PC71BM domains and thus induce the higher charge densities in the Se-containing blend film, which leads to greater amount of recombination and faster decay. This truly correlates with the highest VOC value (≈0.86 V) obtained with PBDTfDTBS-L device with DIO shown in Table 3. It is quite evident here that the PBDTfDTBO-L blend with DIO have slower lifetime than the PBDTfDTBS-L blend without DIO, but faster than the PBDTfDTBO-L blend without DIO. Its slowest decay even in DIO containing device confirms that there is not as much rise in the charge density in the blend due to poorest phase separation, as seen morphologically above. This also confirms that addition of DIO does not stop the recombination existing for longer times. Also, slight improvement of charge densities were still unable to improve the VOC of PBDTfDTBO-based device without DIO. Figure 9b shows TPC measurements of devices prepared with and without DIO under short-circuit conditions. The decay of photogenerated charges signifies the measurement of charge collection time. The devices processed with and without DIO shows the photocurrent decay in the order, PBDTfDTBT > PBDTfDTBS > PBDTfDTBO. This decay also corresponds to charge mobility of polymers; longer decay time signifies the higher mobility probably due to enhanced interconnectivity of polymer. Similar to previous reports,68 the addition of DIO in PBDTfDTBT, PBDTfDTBS, and PBDTfDTBO solar cell devices increases their charge collection times under shortcircuit condition because of improved percolation pathways resulting into better charge extraction. This also corresponds to increase in charge densities in DIO containing devices, as seen in Table 3. The higher JSC in DIO-processed devices enhances the polaron pair yield dissociation while reducing the geminate recombination. Figure 9c shows the total amount of extracted
3. CONCLUSIONS In summary, we successfully synthesized a series of polymers containing a BDTfDTBX (X = O, T, and S) backbone and investigated the effects of chalcogen atom substitution on the photovoltaic performance of PSCs. The substituted chalcogen atoms have a substantial effect on the optical, electrochemical, and photovoltaic properties of resultant polymers. We found that the S-containing fDTBT moiety in PBDTfDTBT induced a strong intermolecular packing of polymer and proper donor/ acceptor miscibility, leading to an improved nanoscale phaseseparated morphology in the polymer:PC71BM blend film. As a result, a power conversion efficiency of 6.02% with a high VOC value of 0.84 V was obtained for the PBDTfDTBT-H:PC71BMbased PSC. The results indicate that PBDTfDTBT is a promising polymer donor material, as a medium-bandgap polymer, for future application of large-area PSCs. 4. EXPERIMENTAL SECTION 4.1. Characterization of Compounds. 1H and 13C NMR spectra of intermediate monomers were recorded on a Bruker AVANCE 400 MHz NMR spectrometer in chloroform-d solution (CDCl3) with 0.003% TMS as internal reference at room temperature. 1H NMR spectra was recorded on a Bruker DRX 500 spectrometer operating at 500 MHz using 1,1,2,2-tetrachloroethane-d2 to avoid an overlap in the range of aromatic chemical shift. The number-average (Mn) and weight-average (Mw) molecular weights of the polymers were determined by gel-permeation chromatography (GPC, Shimadzu) with chlorobezene as the eluent at 80 °C, calibrated against narrow polydispersity polystyrene standard. TGA plots were measured with a TA Instruments, Inc. TGA 2050 under a nitrogen atmosphere at heating rate of 10 °C min−1. Differential scanning calorimetry (DSC) analysis was performed on a TA Instruments Q100 under flowing N2 at a heating and cooling rates of 5 °C min−1. UV−vis spectra were obtained with a Varian CARY-5000 UV−vis spectrophotometer. Atomic force microscopy (AFM) images were obtained with a MultiMode 8 scanning probe microscope (VEECO Instruments Inc.). Transmission electron microscopy (TEM) images were obtained using a Hitachi-7600 system by using an accelerating voltage of 100 kV. 4.2. PSC Fabrication and Measurement. The conventional structure of the PSC devices was prepared with a configuration of glass/ITO/MoO3/active layer (polymer:PC71BM)/LiF/Al. The ITOcoated glass substrates were percleaned by sequential sonications with detergent, deionized water, acetone, and isopropyl alcohol for 15 min each, followed by drying under a nitrogen stream. MoO3 was thermally evaporated with a layer thickness of 9 nm from MoO3 powder at high vacuum condition (3 × 10−6 Torr). Active layer solutions were prepared in CB (polymer/PC71BM blends) and kept on hot plate at 55 °C for 12 h in a N2 glovebox. DIO was added into the blend solutions and stirred for 1 h before photoactive layer deposition. Photoactive layers were spin-coated from the warm solution on the prepared glass/ITO/MoO3 substrate in a N2 glovebox with controlled thickness range of 80−250 nm and dried for 1 h under N2 environment. To deposit the electrodes, the prepared samples were transferred into a vacuum chamber (pressure
