Macromolecular Rapid Communications
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Dithienobenzothiadiazole-Based Conjugated Polymer: Processing Solvent-Relied Interchain Aggregation and Device Performances in FieldEffect Transistors and Polymer Solar Cells Jun Huang, Yongxiang Zhu, Junwu Chen,* Lianjie Zhang, Junbiao Peng, Yong Cao
DTfBT-Th3, a new conjugated polymer based on dithienobenzothiadiazole and terthiophene, possesses a bandgap of ≈1.86 eV and a HOMO level of −5.27 eV. Due to strong interchain aggregation, DTfBT-Th3 can not be well dissolved in chlorobenzene (CB) and o-dichlorobenzene (DCB) at room temperature (RT), but the polymer can be processed from hot CB and DCB solutions of ≈100 °C. In CB, with a lower solvation ability, a certain polymer chain aggregation can be preserved, even in hot solution. DTfBT-Th3 displays a field-effect hole mobility of 0.55 cm2 V−1 s−1 when fabricated from hot CB solution, which is higher than that of the device processed from hot DCB (0.16 cm2 V−1 s−1). In DTfBT-Th3-based polymer solar cells, a good power conversion efficiency from 5.37% to 6.67% can be achieved with 150−300 nm thick active layers casted from hot CB solution, while the highest efficiency for hot DCB-processed solar cells is only 5.07%. The results demonstrate that using a solvent with a lower solvation ability, as a “wet control” process, is beneficial to preserve strong interchain aggregation of a conjugated polymer during solution processing, showing great potential to improve its performances in optoelectronic devices.
1. Introduction Polymer solar cells (PSCs) have attracted considerable attention over the past several years due to their unique advantages of low cost, light weight, and great potential for the realization of flexible and large-area devices.[1,2] The bulk heterojunction (BHJ) PSC, a promising device structure for a high power conversion efficiency (PCE), Dr. J. Huang, Y. Zhu, Prof. J. Chen, Dr. L. Zhang, Prof. J. Peng, Prof. Y. Cao Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, China E-mail: [email protected]
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involves the use of a phase-separated blend of an electrondonating conjugated polymer and an electron-accepting fullerene derivative as the active layer.[3–6] Early polymer donors, such as dialkoxy-substituted poly(para-phenylene vinylene)s (PPVs), poly(3-hexylthiophene) (P3HT), low band-gap polyfluorenes and polycarbazoles, etc., are composed of simple aromatic building blocks.[4–7] During the past several years, increasing efforts have been paid to introduce bigger fused rings to construct various low band-gap D-A conjugated polymers as polymer donors for BHJ PSCs,[8–24] among which PTB7 derived from benzodithiphene (BDT) and thienothiophene (TT) is the typical example.[24] The introduction of bigger fused rings can show great potential to tune band-gaps and energy levels of the resulting conjugated polymers,[25] from which
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DOI: 10.1002/marc.201400461
Dithienobenzothiadiazole-Based Conjugated Polymer: Processing Solvent-Relied Interchain Aggregation and Device. . .
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largely elevated power conversion efficiencies (PCEs) of to 6.67% were achieved for active layers deposited with >9% have been realized.[26] hot CB solutions, where varied thicknesses of the active layers were investigated toward future printable PSCs. Dithieno-based fused rings, have been utilized to construct D-A conjugated polymers for BHJ PSCs.[27] Integration of a dithiophene and an aromatic ring can generate larger π-electron delocalization and establish stronger π-π 2. Experimental Section stacking ability for efficient charge transport. So far, D-A The synthesis and characterization of 2, 3, and DTfBT-Th3 are conjugated polymers comprising dithienocyclopentadiene described in Supporting Information. OFET and PSC fabrications (CPDT),[27] dithienosilole (DTS),[28] dithienogermole (DTG),[14] and characterizations are also described there. dithienopyrrole (DTP),[29] dithienopyran,[18,30] dithienocarbazole (DTC),[31] dithienobenzodithiophene (DBD),[9] and 2,2'-bithiophene-3,3'-dicarboximide (BTI)[13] have been 3. Results and Discussion reported, and some of them have shown excellent PCEs over 8% in BHJ PSCs.[13,18,31] The results indicate that dithieno3.1. Synthesis of DTfBT-Th3 based fused rings are very important heterocycles for the constructions of highly efficient polymer donors. Dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2-c][1,2,5]thiadiaThe synthetic route for polymer DTfBT-Th3 is shown zole (DTfBT) is a four-ring-fused structure of dithiophene in Scheme 1. 5,8-Dibromodithieno[3′,2′:3,4;2″,3″:5,6] and 2,1,3-benzothiadiazole (BT),[7] which was synthesized benzo[1,2-c][1,2,5]-thiadiazole (compound 1), the starting material, was synthesized according to a report by several years ago.[32] However, there are very few reports Arroyave et al.[32] Through a Stille coupling reaction with on DTfBT-based conjugated polymers. Recently, Mei et al. reported alternating copolymers derived from BDT and tributyl(4-(2-octyldodecyl)thiophen-2-yl)stannane, comDTfBT, with PCEs up to 4.5%.[33] pound 2 was obtained as a red liquid in a yield of 70%. The selection of bulky alkyl chains on the thiophenes is We are interested in using DTfBT as a building block for to supply solubility of the target polymer. Then comnew conjugated polymers. The big coplanar skeleton of pound 2 was brominated with N-bromosuccinimide DTfBT would be useful to achieve high mobility of a con(NBS) to afford the dibromo monomer 3 with a good jugated polymer, which is an important factor for applicayield of 82%. The DTfBT-Th3 was obtained by conventions in organic field-effect transistors (OFETs) and PSCs. In this work, a new DTfBT-based monomer containing tional Stille polymerization reaction of monomer 3 and flanking thiophenes was synthesized for the preparation 2,5-bis(trimethylstannyl)thiophene in CB solution, with of a conjugated polymer DTfBT-Th3 comprising DTfBT and Pd2(dba)3 as the catalyst and P(o-tol)3 as the ligand at terthiophene (Th3) (Scheme 1). The chemical structure of 110 °C for 3 d. The crude polymer was precipitated in methanol. The solid was placed in a Soxhlet thimble, the polymer represents that the backbone of a polythioand extracted consecutively with acetone, ethyl acetate, phene is partially fused by BT units (see TOC). DTfBTCH2Cl2, and CB. The CB fraction was precipitated in methTh3 possesses a band-gap of ≈1.86 eV and a HOMO level of −5.27 eV. Chlorobenzene (CB) and o-dichlorobenzene anol again and dark polymer with a yield of 70% was (DCB) are two common solvents for the processing of obtained after dried in a vacuum oven. DTfBT-Th3 showed conjugated polymers for OFETs and PSCs. Unexpectedly, very limited solubility in common solvents at room temDTfBT-Th3 showed obviously different absorption behavperature (RT), including the extracting solvent CB. Fortunately, the polymer could be dissolved in hot CB and DCB. iors in CB and DCB, resulting from different aggregations of polymer chains. It was found that CB with lower solvation ability than DCB to the polymer could be benefit to preserve strong interchain aggregation. Fortunately, high quality films of DTfBT-Th3 could be deposited with hot CB or DCB solutions, which is important in fabrications of high performance devices. OFET fabricated with hot CB solution exhibited a higher hole mobility (μh) of 0.55 cm2 V−1 s−1. This is the first report on OFET mobility for DTfBT-based conjugated polymers. Using DTfBT-Th3 as Scheme 1. Synthetic route for DTfBT-Th3. the polymer donor in BHJ PSCs, PCEs up
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High temperature GPC indicates that DTfBT-Th3 possesses a number-averaged molecular weight of 34 Kg/mol with a polydispersity (PDI) of 2.04. A polymer chain of DTfBTTh3 approximately comprises 32 repeating units. 3.2. Thermal and Electrochemical Properties The thermal stability of DTfBT-Th3 was investigated by thermogravimetric analysis (TGA). DTfBT-Th3 showed no decomposition at 390 °C and 5% weight loss at 432 °C (Figure S1a, Supporting Information). We also carried out differential scanning calorimetry (DSC) analysis, however, no obviously thermal transition was found in temperature range from 30 to 250 °C. The HOMO level of DTfBT-Th3 was determined from the onset of oxidation potential (Eox) during cyclic voltammetry (CV) measurement with an Ag/Ag+ electrode as the reference electrode and an energy level of ferrocene of −4.80 eV as the internal standard (Figure S1b, Supporting Information). The Eox of the polymer is 0.47 V and the calculated HOMO level is −5.27 eV, deeper than that of P3HT. The lowest unoccupied molecular orbital (LUMO) level of DTfBT-Th3 is obtained from the onset of reduction potential (Ered). The Ered of DTfBT-Th3 is −1.50 V, corresponding LUMO level is −3.30 eV. The deduced electrochemcial band-gap of the polymer is 1.97 eV. 3.3. Absorption Spectra Due to the poor solubility of DTfBT-Th3 in both CB and DCB at RT, we tried to elevate temperature to enhance dissolving of the polymer. Delightedly, under heating by boiling water, the solubility of DTfBT-Th3 in CB or DCB could be obviously modified. Unexpectedly, the two dilute solutions (1 × 10−5 M) showed large difference when gradually cooling to RT (28 °C, see Figure S2, Supporting Information). The color of the RT CB solution was bluish violet while dark purple was found for the DCB solution. The color difference at RT was further confirmed by their UV-vis spectra (Figure 1a). The RT CB solution exhibited a main absorption peak at 622 nm and a shoulder peak at 572 nm. The relative intensity ratio of the shoulder peak to the main peak is 0.84. But only one absorption peak at 584 nm was found for the RT DCB solution. Thin solid films (60 nm) of DTfBTTh3 on quartz substrates were deposited with both CB and DCB solutions of ≈100 °C and their absorption spectra also showed big difference (Figure 1b). The absorption spectrum of the film casted with the hot CB solution exhibited two peaks very similar to those of the RT solution as shown in Figure 1a, with negligible peak shifts. The relative intensity ratio of the shoulder peak to the main peak decreases to 0.76. The absorption spectrum of the film casted with the warm DCB solution showed two poor-resolved peaks at 559 and 598 nm with comparable intensity. The film also
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possessed weak absorption at 300 nm, which was very faint for the film deposited with the CB solution. Usually, a strong absorption peak of a film at longer wavelength is closely associated with aggregation.[34] If compare the absorption spectra of the RT CB solution and the film casted with CB solution, the decreased ratio of the shoulder peak to the main peak for the film, in other words, the relatively enhanced main peak, indicates the main peak should be related to aggregation.[34,35] The very small difference between the two absorption spectra also demonstrates that polymer chains of DTfBT-Th3 in the RT CB solution already establish very strong aggregation. As a comparison, the evolution of the absorption peaks of the RT DCB solution to its film (Figures 1a and 1b) represents relatively weak aggregation ability for DTfBT-Th3 chains. The results reveal that processing solvents have great effect on the aggregation behavior of the polymer chains. The above absorption behavior of DTfBT-Th3 is somewhat unusual. We further investigated aggregation behavior of DTfBTTh3 in solutions at different temperatures (Figures 1c and 1d) because the aggregation of a polymer in solution would become weaker at a higher temperature.[34–36] During a heating process from 28 to 90 °C, the two absorption peaks of DTfBT-Th3 in the CB solution decreased continuously and became almost comparable at 90 °C, without obvious shifts. The existing of such an aggregation-related peak at 90 °C further confirms that polymer chains of DTfBT-Th3 possess very strong interchain aggregation ability in the CB solution. However, the evolution process of the absorption spectra of the DCB solution upon the heating was quite different (Figure 1d). The absorption peak showed continuous blue-shifting, from 584 nm at RT to 524 nm at 90 °C, demonstrating the conjugation length of the DTfBT-Th3 backbone decreased upon the heating. This could be attributed to more twisting or decreased coplanarity of the building blocks in the repeating units at a higher temperature.[34–36] It can be concluded that DCB should have higher solvation ability to DTfBT-Th3 if compared with CB. Based on the absorption spectra of the RT solutions (Figure 1a), DTfBT-Th3 in the CB and DCB solutions showed large molar extinction coefficients (εmax) of 6.4 × 104 and 7.2 × 104 mol−1 L cm−1, respectively. The optical band gap (Eg) of DTfBT-Th3 was estimated with the absorption edge of polymer film (Figure 1b). The Eg of the film casted with the CB solution was 1.85 eV, slightly smaller than the 1.87 eV of the DCB-processed film. 3.4. Atomic Force Microscopy The qualities of DTfBT-Th3-based films were analyzed with atomic force microscopy (AFM). Neat films of DTfBT-Th3 and blend films of DTfBT-Th3 and [6,6]-phenyl-C71-butyric
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Dithienobenzothiadiazole-Based Conjugated Polymer: Processing Solvent-Relied Interchain Aggregation and Device. . .
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Figure 1. UV-vis spectra of (a) chlorobenzene (CB) and o-dichlorobenzene (DCB) solutions (1 × 10 –5 M) at room temperature, (b) films on quartz substrates casted from hot CB and DCB solutions, (c) CB solution at different temperature, and (d) DCB solution at different temperature of DTfBT-Th3.
acid methyl ester (PC71BM) with a ratio of 1:2 were deposited for the measurements, towards the OFET and PSC applications, respectively. The AFM images of the neat films (≈60 nm) on wafer, casted with CB and DCB solutions at ≈100 °C, are shown in Figures S3a,b (Supporting Information), respectively. Their root-mean square (RMS) surface roughness values are 1.868 and 1.676 nm, respectively. The results indicate that the two films are of good quality and the aggregates of DTfBT-Th3 polymer chains existed in the hot CB solution may not affect the film formation too much, and large aggregates deteriorating film quality could not be found. Usually, a low quality film is encountered when using a poor solvent.[37] For the blend films (≈150 nm) casted with CB and DCB solutions on ITO substrate (Figures S3c,d, Supporting Information), their RMS values are 1.460 and 2.505 nm, respectively. 3.5. Organic Field-Effect Transistors We fabricated DTfBT-Th3-based OFETs with a simple bottom-gate, top-contact configuration.[38] The polymer films (≈60 nm) were spin-coated with CB and DCB
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solutions at ≈100 °C on octyltrichlorosilane (OTS) treated SiO2 gate insulators. Then gold films (≈50 nm) were deposited under vacuum as the source and drain electrodes. The OFETs were annealed at 100 °C in N2 glovebox for 10 min. The characterizations of the unencapsulated OFETs were performed under an ambient environment with relative humidity of ≈75%. The output and transfer characteristics are shown in Figure 2. From the slope of the curve of (IDS)1/2 vs VG (Figure 2b), the calculated μh for the OFET fabricated with the hot CB solution was 0.55 cm2 V−1 s−1), with on/off current ratio (Ion/Ioff ) of 2 × 104 and threshold voltage (Vth) of 3 V. The OFET fabricated with the hot DCB solution showed μh of 0.16 cm2 V−1 s−1, Ion/Ioff of 5 × 104, and Vth of 0 V. The results demonstrate the performance of a DTfBT-Th3-based OFET exists certain reliance on the processing solvent. This work supplies the first OFET performance for DTfBT-based conjugated polymers and the results well demonstrate that DTfBT, a big coplanar heterocycle, is a notable building block to construct high mobility conjugated polymers. In previous reports, some conjugated polymers could display very high mobilities normally at
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annealing temperatures of 150 °C or above, and lower annealing temperatures of ≈100 °C could only achieve relatively lower mobilities.[38] It should be noted that P3HT could display μh from ≈0.01 to ≈0.3 cm2 V−1 s−1.[39,40] Using CB as the processing solvent, the established strong aggregation of DTfBT-Th3 chains as well as its high quality film would play very positive contributions to the carrier transport of the polymer, where interchain hoppings may be greatly accelerated.[34] The good hole mobilities of 0.55 and 0.16 cm2 V−1 s−1 of the DTfBT-Th3-based OFETs fabricated with hot CB and DCB solutions, respectively, suggest the polymer films should possess certain aggregation orders. X-ray diffractions (XRD) of the films were performed and the diffraction patterns were shown in Figure S4, Supporting Information. The CB- and DCB-casted DTfBT-Th3 films showed almost identical out-of-plane diffraction peaks at 2θ of 3.84° and 3.80°, respectively, which corresponded to d-spacing values of 23.15 and 23.29 Å for the interdigitation of the alkyl side chains, respectively. Unfortunately, no 010 in-plain diffractions could be observed, thus no d-spacing data for π-π stacking could be given based on the measurement.
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3.6. Polymer Solar Cells Single junction BHJ PSCs with power conversion efficiencies (PCEs) over 9% have been realized. However, such high PCEs were typically achieved with thin active layers of ≈100 nm.[26] High efficiency PSCs with thicker active layers would be not only more compatible to solution printing technology, a probable commercialization processing for PSCs, but also more effective in the utilization of incident light.[1] For large area high-speed printing, maintaining high PCEs during large thickness variations of the active layer should be required.[36,41–44] In principle, high hole mobility of a polymer donor can accelerate hole transport in the active layer of a PSC and decrease recombination of holes and electrons, from which high PCE can be realized with a thick active layer. We are interested in the utilization of a conjugated polymer with strong interchain aggregation and high hole mobility for thick-film PSCs.[34] The good hole mobility, applicable band-gap, and suitable energy level alignment of the DTfBT-Th3 suggest us to fabricate DTfBT-Th3based PSCs with thicker active layers. DTfBT-Th3-based
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PSCs were fabricated in a device configuration of ITO/ PEDOT:PSS (40 nm)/DTfBT-Th3:PC71BM = 1:2/Ca (5 nm)/Al (100 nm), where PC71BM was utilized as the acceptor. The active layers with the different thickness were deposited via spin-coating at different spin-speeds from both hot CB and DCB solutions (≈100 °C), so as to reveal whether photovoltaic performance exists processing solvent reliance in the DTfBT-Th3-based PSCs. The measurements of the PSCs were carried out under illumination of AM1.5G simulated solar light at 100 mW cm−2. The J-V characteristics for the PSCs are shown in Figures 3a and 3b. Their photovoltaic parameters are summarized in Table 1. For active layers casted with the hot CB solutions, the PSCs with the different active layer thickness (80−300 nm) showed almost comparable open-circuit voltage (Voc) (0.78−0.80 V). For a thin active layer of 80 nm, the PSC showed a low short-circuit current (Jsc) of 9.10 mA cm−2 and a good fill factor (FF) of 69.0%, corresponding to the lowest PCE of 4.96%. A thicker active layer of 110 nm increased Jsc to 9.90 mA cm−2 and FF to 70.9%, yielding a higher PCE of 5.62%. Further increasing thickness to 150 nm could maintain a high FF of 70.9% and again enhance Jsc to 11.9 mA cm−2, providing the best PCE of 6.67% among all PSCs. An averaged PCE of 6.57% was achieved from 8 devices with thickness ≈150 nm. A higher active layer thickness of 185 nm further increased Jsc to 12.7 mA cm−2 but encountered a decreased FF of 66.1%, affording a lower PCE of 6.38%. The PSC with 300 nm thick active layer showed decreased Jsc and FF, showing a PCE of 5.37%. Changing the processing solvent to DCB, the PSCs with the different active layer thickness (120−420 nm) showed comparable Voc from 0.72 to 0.73 V. In comparison to above CB-processed PSCs, the relatively lower Voc might be related to the larger surface roughness of the DCB-casted active layer as observed by AFM.[37] A thin active layer of 120 nm displayed a Jsc of 9.20 mA cm−2 and a FF of 68.9%, giving a PCE of 4.60%. A thicker active layer of 150 nm showed a higher Jsc of 10.6 mA cm−2 and a slightly lower FF of 66.7%, 3
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resulting in a PCE of 5.07%. An averaged PCE of 5.00% was obtained from 6 devices with thickness ≈150 nm. Further increasing thickness of the active layer to 240 and 420 nm did not increase Jsc. The lower FF values of 64.2% and 56.3% resulted in lower PCEs of 4.95% and 4.22%, respectively. In general, for the two types of PSCs processed with different solvents, increased series resistances (Rs) and decreased FF (Table 1) could be found along with the increase of active layer thickness. Figure 3c shows the external quantum efficiency (EQE) of the two types of PSCs with 150 nm thick active layer as the example. The PSC fabricated with the hot CB solution exhibited a maximum EQE of 63.5% at 480 nm. In wavelength range from 430 to 630 nm, the device possessed EQE more than 57%. When using DCB as the processing solvent, the PSC displayed a maximum EQE of 61.5% at 475 nm. Generally, the spectral response of the CB-fabricated PSC was obviously higher than that of the DCB-fabricated device, well corresponding to the higher Jsc of the CBfabricated PSC. In combination with the higher Voc and FF, much better PCE was achieved for the CB-fabricated PSC. The processing solvent-relied OFET and PSC performances of DTfBT-Th3 demonstrate the advantage of a solvent with less solvation ability to preserve strong polymer chain aggregation during solution processing and consequently result in positive contributions. Judicious controls of solution conditions for fabrications of PSCs had been noted before for polymer donors with strong interchain aggregations, and better photovoltaic performances were achieved when active layer was processed with a blend solvent to dissolve polymer[37] or with a solvent to dissolve polymer at RT.[35] Conjugated polymers with strong interchain aggregations or co-facial stackings are of importance for high performance OFETs. Indeed, selection of a more suitable solvent to deposit a polymer film as a “wet control” process for enhancing aggregation was also introduced before,[45] although thermal annealing, a “dry control” process for enhancing aggregation, was more widely utilized in the fabrication of an OFET.[38,40]
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Figure 3. J-V characteristics of PSCs with different active layer thickness casted from (a) chlorobenzene (CB) or (b) o-dichlorobenzene (DCB). (c) EQE curves of PSCs with a 150 nm thick active layer casted from CB or DCB.
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Table 1. Photovoltaic performances of DTfBT-Th3 based PSCs using chlorobenzene (CB) or dichlorobenzene (DCB) as processing solvent for casting active layers at different thickness.
solvent CB
DCB
thickness [nm]
Voc [V]
Jsc [mA cm−2]
FF [%]
PCEa) [%]
Rsb) [Ω cm−2]
80
0.79
9.10
69.0
4.96 (4.85)
39
110
0.80
9.90
70.9
5.62 (5.58)
40
150
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11.9
70.9
6.67 (6.57)
44
185
0.78
12.4
66.1
6.38 (6.28)
44
300
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11.3
60.6
5.37 (5.33)
59
120
0.73
9.20
68.4
4.60 (4.54)
47
150
0.72
10.6
66.7
5.07 (5.00)
52
240
0.73
10.6
64.2
4.95 (4.81)
54
420
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10.3
56.3
4.22 (4.12)
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a)
Data in the parentheses are the averaged values based on over five devices. b)Series resistance.
The processing solvent-relied interchain aggregation and device performances of DTfBT-Th3 should be related to the length of the alkyl side chains of DTfBT-Th3. In this work, two 2-octyldodecyl groups were selected and DTfBT-Th3 only showed limited solubility in CB or DCB at room temperature. It would be possible to realize a DTfBTTh3 derivative with better solubility at room temperature if introducing longer alkyl side chains. Possibly higher molecular weight and more desirable aggregation ability of such a DTfBT-Th3 derivative would play important roles in improving device performances.
4. Conclusions In summary, a new DTfBT-based monomer containing flanking thiophenes was synthesized for the preparation of a conjugated polymer DTfBT-Th3 comprising DTfBT and terthiophene. Due to strong interchain aggregation, DTfBT-Th3 could not be well dissolved in CB and DCB at room temperature, but DTfBT-Th3 could be processed with hot CB and DCB solutions of ≈100 °C. Based on UV absorption spectra, it was CB with lower solvation ability, which could maintain certain polymer chain aggregation in hot solution, with a similar absorption style to those of RT CB solution and CB-processed film. The processing solventrelied interchain aggregation obviously resulted in different device performances. DTfBT-Th3 displayed an OFET hole mobility of 0.55 cm2 V−1 s−1 when fabricated with the hot CB solution, higher than the 0.16 cm2 V−1 s−1 of the hot DCB solution-processed OFET. Moreover, a higher PCE of 6.67% was achieved from solar cells with active layers deposited by hot CB solution. In general, the results suggest that DTfBT, a big coplanar heterocycle, is a notable building block to construct high mobility conjugated polymers for efficient polymer solar cells. Moreover, this work also demonstrates that using a solvent with a lower
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solvation ability, as a “wet control” process, would be beneficial to preserve strong interchain aggregation of a conjugated polymer during solution processing, showing great potential to improve its performances in optoelectronic devices.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: This work is supported by the National Basic Research Program of China (973 program 2013CB834705 and 2014CB643505), National Natural Science Foundation of China (21225418 and 51173048), China Postdoctoral Science Foundation (2014M552194), and GDUPS (2013). Received: August 17, 2014; Revised: September 8, 2014; Published online: October 6, 2014; DOI: 10.1002/marc.201400461 Keywords: D–A conjugated copolymers; dithienobenzothiadiazoles; salvation ability; organic field-effect transistors; polymer solar cells [1] G. Dennler, M. C. Scharber, C. J. Brabec, Adv. Mater. 2009, 21, 1323. [2] F. C. Krebs, Sol. Energy Mater. Sol. Cells 2009, 93, 394. [3] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789. [4] Y.-J. Cheng, S.-H. Yang, C.-S. Hsu, Chem. Rev. 2009, 109, 5868. [5] J. Chen, Y. Cao, Acc. Chem. Res. 2009, 42, 1709. [6] G. Li, R. Zhu, Y. Yang, Nat. Photonics 2012, 6, 153. [7] N. Blouin, A. Michaud, M. Leclerc, Adv. Mater. 2007, 19, 2295. [8] H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Nat. Photon. 2009, 3, 649. [9] H. J. Son, L. Lu, W. Chen, T. Xu, T. Zheng, B. Carsten, J. Strzalka, S. B. Darling, L. X. Chen, L. Yu, Adv. Mater. 2013, 25, 838. [10] S. Shi, X. Xie, P. Jiang, S. Chen, L. Wang, M. Wang, H. Wang, X. Li, G. Yu, Y. Li, Macromolecules 2013, 46, 3358.
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Communication
Dithienobenzothiadiazole-Based Conjugated Polymer: Processing Solvent-Relied Interchain Aggregation and Device Performances in FieldEffect Transistors and Polymer Solar Cells Jun Huang, Yongxiang Zhu, Junwu Chen,* Lianjie Zhang, Junbiao Peng, Yong Cao
DTfBT-Th3, a new conjugated polymer based on dithienobenzothiadiazole and terthiophene, possesses a bandgap of ≈1.86 eV and a HOMO level of −5.27 eV. Due to strong interchain aggregation, DTfBT-Th3 can not be well dissolved in chlorobenzene (CB) and o-dichlorobenzene (DCB) at room temperature (RT), but the polymer can be processed from hot CB and DCB solutions of ≈100 °C. In CB, with a lower solvation ability, a certain polymer chain aggregation can be preserved, even in hot solution. DTfBT-Th3 displays a field-effect hole mobility of 0.55 cm2 V−1 s−1 when fabricated from hot CB solution, which is higher than that of the device processed from hot DCB (0.16 cm2 V−1 s−1). In DTfBT-Th3-based polymer solar cells, a good power conversion efficiency from 5.37% to 6.67% can be achieved with 150−300 nm thick active layers casted from hot CB solution, while the highest efficiency for hot DCB-processed solar cells is only 5.07%. The results demonstrate that using a solvent with a lower solvation ability, as a “wet control” process, is beneficial to preserve strong interchain aggregation of a conjugated polymer during solution processing, showing great potential to improve its performances in optoelectronic devices.
1. Introduction Polymer solar cells (PSCs) have attracted considerable attention over the past several years due to their unique advantages of low cost, light weight, and great potential for the realization of flexible and large-area devices.[1,2] The bulk heterojunction (BHJ) PSC, a promising device structure for a high power conversion efficiency (PCE), Dr. J. Huang, Y. Zhu, Prof. J. Chen, Dr. L. Zhang, Prof. J. Peng, Prof. Y. Cao Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, China E-mail: [email protected]
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involves the use of a phase-separated blend of an electrondonating conjugated polymer and an electron-accepting fullerene derivative as the active layer.[3–6] Early polymer donors, such as dialkoxy-substituted poly(para-phenylene vinylene)s (PPVs), poly(3-hexylthiophene) (P3HT), low band-gap polyfluorenes and polycarbazoles, etc., are composed of simple aromatic building blocks.[4–7] During the past several years, increasing efforts have been paid to introduce bigger fused rings to construct various low band-gap D-A conjugated polymers as polymer donors for BHJ PSCs,[8–24] among which PTB7 derived from benzodithiphene (BDT) and thienothiophene (TT) is the typical example.[24] The introduction of bigger fused rings can show great potential to tune band-gaps and energy levels of the resulting conjugated polymers,[25] from which
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largely elevated power conversion efficiencies (PCEs) of to 6.67% were achieved for active layers deposited with >9% have been realized.[26] hot CB solutions, where varied thicknesses of the active layers were investigated toward future printable PSCs. Dithieno-based fused rings, have been utilized to construct D-A conjugated polymers for BHJ PSCs.[27] Integration of a dithiophene and an aromatic ring can generate larger π-electron delocalization and establish stronger π-π 2. Experimental Section stacking ability for efficient charge transport. So far, D-A The synthesis and characterization of 2, 3, and DTfBT-Th3 are conjugated polymers comprising dithienocyclopentadiene described in Supporting Information. OFET and PSC fabrications (CPDT),[27] dithienosilole (DTS),[28] dithienogermole (DTG),[14] and characterizations are also described there. dithienopyrrole (DTP),[29] dithienopyran,[18,30] dithienocarbazole (DTC),[31] dithienobenzodithiophene (DBD),[9] and 2,2'-bithiophene-3,3'-dicarboximide (BTI)[13] have been 3. Results and Discussion reported, and some of them have shown excellent PCEs over 8% in BHJ PSCs.[13,18,31] The results indicate that dithieno3.1. Synthesis of DTfBT-Th3 based fused rings are very important heterocycles for the constructions of highly efficient polymer donors. Dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2-c][1,2,5]thiadiaThe synthetic route for polymer DTfBT-Th3 is shown zole (DTfBT) is a four-ring-fused structure of dithiophene in Scheme 1. 5,8-Dibromodithieno[3′,2′:3,4;2″,3″:5,6] and 2,1,3-benzothiadiazole (BT),[7] which was synthesized benzo[1,2-c][1,2,5]-thiadiazole (compound 1), the starting material, was synthesized according to a report by several years ago.[32] However, there are very few reports Arroyave et al.[32] Through a Stille coupling reaction with on DTfBT-based conjugated polymers. Recently, Mei et al. reported alternating copolymers derived from BDT and tributyl(4-(2-octyldodecyl)thiophen-2-yl)stannane, comDTfBT, with PCEs up to 4.5%.[33] pound 2 was obtained as a red liquid in a yield of 70%. The selection of bulky alkyl chains on the thiophenes is We are interested in using DTfBT as a building block for to supply solubility of the target polymer. Then comnew conjugated polymers. The big coplanar skeleton of pound 2 was brominated with N-bromosuccinimide DTfBT would be useful to achieve high mobility of a con(NBS) to afford the dibromo monomer 3 with a good jugated polymer, which is an important factor for applicayield of 82%. The DTfBT-Th3 was obtained by conventions in organic field-effect transistors (OFETs) and PSCs. In this work, a new DTfBT-based monomer containing tional Stille polymerization reaction of monomer 3 and flanking thiophenes was synthesized for the preparation 2,5-bis(trimethylstannyl)thiophene in CB solution, with of a conjugated polymer DTfBT-Th3 comprising DTfBT and Pd2(dba)3 as the catalyst and P(o-tol)3 as the ligand at terthiophene (Th3) (Scheme 1). The chemical structure of 110 °C for 3 d. The crude polymer was precipitated in methanol. The solid was placed in a Soxhlet thimble, the polymer represents that the backbone of a polythioand extracted consecutively with acetone, ethyl acetate, phene is partially fused by BT units (see TOC). DTfBTCH2Cl2, and CB. The CB fraction was precipitated in methTh3 possesses a band-gap of ≈1.86 eV and a HOMO level of −5.27 eV. Chlorobenzene (CB) and o-dichlorobenzene anol again and dark polymer with a yield of 70% was (DCB) are two common solvents for the processing of obtained after dried in a vacuum oven. DTfBT-Th3 showed conjugated polymers for OFETs and PSCs. Unexpectedly, very limited solubility in common solvents at room temDTfBT-Th3 showed obviously different absorption behavperature (RT), including the extracting solvent CB. Fortunately, the polymer could be dissolved in hot CB and DCB. iors in CB and DCB, resulting from different aggregations of polymer chains. It was found that CB with lower solvation ability than DCB to the polymer could be benefit to preserve strong interchain aggregation. Fortunately, high quality films of DTfBT-Th3 could be deposited with hot CB or DCB solutions, which is important in fabrications of high performance devices. OFET fabricated with hot CB solution exhibited a higher hole mobility (μh) of 0.55 cm2 V−1 s−1. This is the first report on OFET mobility for DTfBT-based conjugated polymers. Using DTfBT-Th3 as Scheme 1. Synthetic route for DTfBT-Th3. the polymer donor in BHJ PSCs, PCEs up
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High temperature GPC indicates that DTfBT-Th3 possesses a number-averaged molecular weight of 34 Kg/mol with a polydispersity (PDI) of 2.04. A polymer chain of DTfBTTh3 approximately comprises 32 repeating units. 3.2. Thermal and Electrochemical Properties The thermal stability of DTfBT-Th3 was investigated by thermogravimetric analysis (TGA). DTfBT-Th3 showed no decomposition at 390 °C and 5% weight loss at 432 °C (Figure S1a, Supporting Information). We also carried out differential scanning calorimetry (DSC) analysis, however, no obviously thermal transition was found in temperature range from 30 to 250 °C. The HOMO level of DTfBT-Th3 was determined from the onset of oxidation potential (Eox) during cyclic voltammetry (CV) measurement with an Ag/Ag+ electrode as the reference electrode and an energy level of ferrocene of −4.80 eV as the internal standard (Figure S1b, Supporting Information). The Eox of the polymer is 0.47 V and the calculated HOMO level is −5.27 eV, deeper than that of P3HT. The lowest unoccupied molecular orbital (LUMO) level of DTfBT-Th3 is obtained from the onset of reduction potential (Ered). The Ered of DTfBT-Th3 is −1.50 V, corresponding LUMO level is −3.30 eV. The deduced electrochemcial band-gap of the polymer is 1.97 eV. 3.3. Absorption Spectra Due to the poor solubility of DTfBT-Th3 in both CB and DCB at RT, we tried to elevate temperature to enhance dissolving of the polymer. Delightedly, under heating by boiling water, the solubility of DTfBT-Th3 in CB or DCB could be obviously modified. Unexpectedly, the two dilute solutions (1 × 10−5 M) showed large difference when gradually cooling to RT (28 °C, see Figure S2, Supporting Information). The color of the RT CB solution was bluish violet while dark purple was found for the DCB solution. The color difference at RT was further confirmed by their UV-vis spectra (Figure 1a). The RT CB solution exhibited a main absorption peak at 622 nm and a shoulder peak at 572 nm. The relative intensity ratio of the shoulder peak to the main peak is 0.84. But only one absorption peak at 584 nm was found for the RT DCB solution. Thin solid films (60 nm) of DTfBTTh3 on quartz substrates were deposited with both CB and DCB solutions of ≈100 °C and their absorption spectra also showed big difference (Figure 1b). The absorption spectrum of the film casted with the hot CB solution exhibited two peaks very similar to those of the RT solution as shown in Figure 1a, with negligible peak shifts. The relative intensity ratio of the shoulder peak to the main peak decreases to 0.76. The absorption spectrum of the film casted with the warm DCB solution showed two poor-resolved peaks at 559 and 598 nm with comparable intensity. The film also
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possessed weak absorption at 300 nm, which was very faint for the film deposited with the CB solution. Usually, a strong absorption peak of a film at longer wavelength is closely associated with aggregation.[34] If compare the absorption spectra of the RT CB solution and the film casted with CB solution, the decreased ratio of the shoulder peak to the main peak for the film, in other words, the relatively enhanced main peak, indicates the main peak should be related to aggregation.[34,35] The very small difference between the two absorption spectra also demonstrates that polymer chains of DTfBT-Th3 in the RT CB solution already establish very strong aggregation. As a comparison, the evolution of the absorption peaks of the RT DCB solution to its film (Figures 1a and 1b) represents relatively weak aggregation ability for DTfBT-Th3 chains. The results reveal that processing solvents have great effect on the aggregation behavior of the polymer chains. The above absorption behavior of DTfBT-Th3 is somewhat unusual. We further investigated aggregation behavior of DTfBTTh3 in solutions at different temperatures (Figures 1c and 1d) because the aggregation of a polymer in solution would become weaker at a higher temperature.[34–36] During a heating process from 28 to 90 °C, the two absorption peaks of DTfBT-Th3 in the CB solution decreased continuously and became almost comparable at 90 °C, without obvious shifts. The existing of such an aggregation-related peak at 90 °C further confirms that polymer chains of DTfBT-Th3 possess very strong interchain aggregation ability in the CB solution. However, the evolution process of the absorption spectra of the DCB solution upon the heating was quite different (Figure 1d). The absorption peak showed continuous blue-shifting, from 584 nm at RT to 524 nm at 90 °C, demonstrating the conjugation length of the DTfBT-Th3 backbone decreased upon the heating. This could be attributed to more twisting or decreased coplanarity of the building blocks in the repeating units at a higher temperature.[34–36] It can be concluded that DCB should have higher solvation ability to DTfBT-Th3 if compared with CB. Based on the absorption spectra of the RT solutions (Figure 1a), DTfBT-Th3 in the CB and DCB solutions showed large molar extinction coefficients (εmax) of 6.4 × 104 and 7.2 × 104 mol−1 L cm−1, respectively. The optical band gap (Eg) of DTfBT-Th3 was estimated with the absorption edge of polymer film (Figure 1b). The Eg of the film casted with the CB solution was 1.85 eV, slightly smaller than the 1.87 eV of the DCB-processed film. 3.4. Atomic Force Microscopy The qualities of DTfBT-Th3-based films were analyzed with atomic force microscopy (AFM). Neat films of DTfBT-Th3 and blend films of DTfBT-Th3 and [6,6]-phenyl-C71-butyric
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acid methyl ester (PC71BM) with a ratio of 1:2 were deposited for the measurements, towards the OFET and PSC applications, respectively. The AFM images of the neat films (≈60 nm) on wafer, casted with CB and DCB solutions at ≈100 °C, are shown in Figures S3a,b (Supporting Information), respectively. Their root-mean square (RMS) surface roughness values are 1.868 and 1.676 nm, respectively. The results indicate that the two films are of good quality and the aggregates of DTfBT-Th3 polymer chains existed in the hot CB solution may not affect the film formation too much, and large aggregates deteriorating film quality could not be found. Usually, a low quality film is encountered when using a poor solvent.[37] For the blend films (≈150 nm) casted with CB and DCB solutions on ITO substrate (Figures S3c,d, Supporting Information), their RMS values are 1.460 and 2.505 nm, respectively. 3.5. Organic Field-Effect Transistors We fabricated DTfBT-Th3-based OFETs with a simple bottom-gate, top-contact configuration.[38] The polymer films (≈60 nm) were spin-coated with CB and DCB
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solutions at ≈100 °C on octyltrichlorosilane (OTS) treated SiO2 gate insulators. Then gold films (≈50 nm) were deposited under vacuum as the source and drain electrodes. The OFETs were annealed at 100 °C in N2 glovebox for 10 min. The characterizations of the unencapsulated OFETs were performed under an ambient environment with relative humidity of ≈75%. The output and transfer characteristics are shown in Figure 2. From the slope of the curve of (IDS)1/2 vs VG (Figure 2b), the calculated μh for the OFET fabricated with the hot CB solution was 0.55 cm2 V−1 s−1), with on/off current ratio (Ion/Ioff ) of 2 × 104 and threshold voltage (Vth) of 3 V. The OFET fabricated with the hot DCB solution showed μh of 0.16 cm2 V−1 s−1, Ion/Ioff of 5 × 104, and Vth of 0 V. The results demonstrate the performance of a DTfBT-Th3-based OFET exists certain reliance on the processing solvent. This work supplies the first OFET performance for DTfBT-based conjugated polymers and the results well demonstrate that DTfBT, a big coplanar heterocycle, is a notable building block to construct high mobility conjugated polymers. In previous reports, some conjugated polymers could display very high mobilities normally at
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Figure 2. I–V characteristics of the DTfBT-Th3 based unencapsulated OFET devices annealed at 100 °C in N2 for 10 min, as measured in ambient conditions with a relative humidity of ≈75%: (a) output and (b) transfer characteristics of an OFET casted from chlorobenzene (CB), (c) output and (d) transfer characteristics of an OFET casted from o-dichlorobenzene (DCB).
annealing temperatures of 150 °C or above, and lower annealing temperatures of ≈100 °C could only achieve relatively lower mobilities.[38] It should be noted that P3HT could display μh from ≈0.01 to ≈0.3 cm2 V−1 s−1.[39,40] Using CB as the processing solvent, the established strong aggregation of DTfBT-Th3 chains as well as its high quality film would play very positive contributions to the carrier transport of the polymer, where interchain hoppings may be greatly accelerated.[34] The good hole mobilities of 0.55 and 0.16 cm2 V−1 s−1 of the DTfBT-Th3-based OFETs fabricated with hot CB and DCB solutions, respectively, suggest the polymer films should possess certain aggregation orders. X-ray diffractions (XRD) of the films were performed and the diffraction patterns were shown in Figure S4, Supporting Information. The CB- and DCB-casted DTfBT-Th3 films showed almost identical out-of-plane diffraction peaks at 2θ of 3.84° and 3.80°, respectively, which corresponded to d-spacing values of 23.15 and 23.29 Å for the interdigitation of the alkyl side chains, respectively. Unfortunately, no 010 in-plain diffractions could be observed, thus no d-spacing data for π-π stacking could be given based on the measurement.
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3.6. Polymer Solar Cells Single junction BHJ PSCs with power conversion efficiencies (PCEs) over 9% have been realized. However, such high PCEs were typically achieved with thin active layers of ≈100 nm.[26] High efficiency PSCs with thicker active layers would be not only more compatible to solution printing technology, a probable commercialization processing for PSCs, but also more effective in the utilization of incident light.[1] For large area high-speed printing, maintaining high PCEs during large thickness variations of the active layer should be required.[36,41–44] In principle, high hole mobility of a polymer donor can accelerate hole transport in the active layer of a PSC and decrease recombination of holes and electrons, from which high PCE can be realized with a thick active layer. We are interested in the utilization of a conjugated polymer with strong interchain aggregation and high hole mobility for thick-film PSCs.[34] The good hole mobility, applicable band-gap, and suitable energy level alignment of the DTfBT-Th3 suggest us to fabricate DTfBT-Th3based PSCs with thicker active layers. DTfBT-Th3-based
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PSCs were fabricated in a device configuration of ITO/ PEDOT:PSS (40 nm)/DTfBT-Th3:PC71BM = 1:2/Ca (5 nm)/Al (100 nm), where PC71BM was utilized as the acceptor. The active layers with the different thickness were deposited via spin-coating at different spin-speeds from both hot CB and DCB solutions (≈100 °C), so as to reveal whether photovoltaic performance exists processing solvent reliance in the DTfBT-Th3-based PSCs. The measurements of the PSCs were carried out under illumination of AM1.5G simulated solar light at 100 mW cm−2. The J-V characteristics for the PSCs are shown in Figures 3a and 3b. Their photovoltaic parameters are summarized in Table 1. For active layers casted with the hot CB solutions, the PSCs with the different active layer thickness (80−300 nm) showed almost comparable open-circuit voltage (Voc) (0.78−0.80 V). For a thin active layer of 80 nm, the PSC showed a low short-circuit current (Jsc) of 9.10 mA cm−2 and a good fill factor (FF) of 69.0%, corresponding to the lowest PCE of 4.96%. A thicker active layer of 110 nm increased Jsc to 9.90 mA cm−2 and FF to 70.9%, yielding a higher PCE of 5.62%. Further increasing thickness to 150 nm could maintain a high FF of 70.9% and again enhance Jsc to 11.9 mA cm−2, providing the best PCE of 6.67% among all PSCs. An averaged PCE of 6.57% was achieved from 8 devices with thickness ≈150 nm. A higher active layer thickness of 185 nm further increased Jsc to 12.7 mA cm−2 but encountered a decreased FF of 66.1%, affording a lower PCE of 6.38%. The PSC with 300 nm thick active layer showed decreased Jsc and FF, showing a PCE of 5.37%. Changing the processing solvent to DCB, the PSCs with the different active layer thickness (120−420 nm) showed comparable Voc from 0.72 to 0.73 V. In comparison to above CB-processed PSCs, the relatively lower Voc might be related to the larger surface roughness of the DCB-casted active layer as observed by AFM.[37] A thin active layer of 120 nm displayed a Jsc of 9.20 mA cm−2 and a FF of 68.9%, giving a PCE of 4.60%. A thicker active layer of 150 nm showed a higher Jsc of 10.6 mA cm−2 and a slightly lower FF of 66.7%, 3
0 80 nm 110 nm 150 nm 185 nm 300 nm
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resulting in a PCE of 5.07%. An averaged PCE of 5.00% was obtained from 6 devices with thickness ≈150 nm. Further increasing thickness of the active layer to 240 and 420 nm did not increase Jsc. The lower FF values of 64.2% and 56.3% resulted in lower PCEs of 4.95% and 4.22%, respectively. In general, for the two types of PSCs processed with different solvents, increased series resistances (Rs) and decreased FF (Table 1) could be found along with the increase of active layer thickness. Figure 3c shows the external quantum efficiency (EQE) of the two types of PSCs with 150 nm thick active layer as the example. The PSC fabricated with the hot CB solution exhibited a maximum EQE of 63.5% at 480 nm. In wavelength range from 430 to 630 nm, the device possessed EQE more than 57%. When using DCB as the processing solvent, the PSC displayed a maximum EQE of 61.5% at 475 nm. Generally, the spectral response of the CB-fabricated PSC was obviously higher than that of the DCB-fabricated device, well corresponding to the higher Jsc of the CBfabricated PSC. In combination with the higher Voc and FF, much better PCE was achieved for the CB-fabricated PSC. The processing solvent-relied OFET and PSC performances of DTfBT-Th3 demonstrate the advantage of a solvent with less solvation ability to preserve strong polymer chain aggregation during solution processing and consequently result in positive contributions. Judicious controls of solution conditions for fabrications of PSCs had been noted before for polymer donors with strong interchain aggregations, and better photovoltaic performances were achieved when active layer was processed with a blend solvent to dissolve polymer[37] or with a solvent to dissolve polymer at RT.[35] Conjugated polymers with strong interchain aggregations or co-facial stackings are of importance for high performance OFETs. Indeed, selection of a more suitable solvent to deposit a polymer film as a “wet control” process for enhancing aggregation was also introduced before,[45] although thermal annealing, a “dry control” process for enhancing aggregation, was more widely utilized in the fabrication of an OFET.[38,40]
0 0.0
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Figure 3. J-V characteristics of PSCs with different active layer thickness casted from (a) chlorobenzene (CB) or (b) o-dichlorobenzene (DCB). (c) EQE curves of PSCs with a 150 nm thick active layer casted from CB or DCB.
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J. Huang et al.
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Table 1. Photovoltaic performances of DTfBT-Th3 based PSCs using chlorobenzene (CB) or dichlorobenzene (DCB) as processing solvent for casting active layers at different thickness.
solvent CB
DCB
thickness [nm]
Voc [V]
Jsc [mA cm−2]
FF [%]
PCEa) [%]
Rsb) [Ω cm−2]
80
0.79
9.10
69.0
4.96 (4.85)
39
110
0.80
9.90
70.9
5.62 (5.58)
40
150
0.79
11.9
70.9
6.67 (6.57)
44
185
0.78
12.4
66.1
6.38 (6.28)
44
300
0.78
11.3
60.6
5.37 (5.33)
59
120
0.73
9.20
68.4
4.60 (4.54)
47
150
0.72
10.6
66.7
5.07 (5.00)
52
240
0.73
10.6
64.2
4.95 (4.81)
54
420
0.73
10.3
56.3
4.22 (4.12)
83
a)
Data in the parentheses are the averaged values based on over five devices. b)Series resistance.
The processing solvent-relied interchain aggregation and device performances of DTfBT-Th3 should be related to the length of the alkyl side chains of DTfBT-Th3. In this work, two 2-octyldodecyl groups were selected and DTfBT-Th3 only showed limited solubility in CB or DCB at room temperature. It would be possible to realize a DTfBTTh3 derivative with better solubility at room temperature if introducing longer alkyl side chains. Possibly higher molecular weight and more desirable aggregation ability of such a DTfBT-Th3 derivative would play important roles in improving device performances.
4. Conclusions In summary, a new DTfBT-based monomer containing flanking thiophenes was synthesized for the preparation of a conjugated polymer DTfBT-Th3 comprising DTfBT and terthiophene. Due to strong interchain aggregation, DTfBT-Th3 could not be well dissolved in CB and DCB at room temperature, but DTfBT-Th3 could be processed with hot CB and DCB solutions of ≈100 °C. Based on UV absorption spectra, it was CB with lower solvation ability, which could maintain certain polymer chain aggregation in hot solution, with a similar absorption style to those of RT CB solution and CB-processed film. The processing solventrelied interchain aggregation obviously resulted in different device performances. DTfBT-Th3 displayed an OFET hole mobility of 0.55 cm2 V−1 s−1 when fabricated with the hot CB solution, higher than the 0.16 cm2 V−1 s−1 of the hot DCB solution-processed OFET. Moreover, a higher PCE of 6.67% was achieved from solar cells with active layers deposited by hot CB solution. In general, the results suggest that DTfBT, a big coplanar heterocycle, is a notable building block to construct high mobility conjugated polymers for efficient polymer solar cells. Moreover, this work also demonstrates that using a solvent with a lower
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solvation ability, as a “wet control” process, would be beneficial to preserve strong interchain aggregation of a conjugated polymer during solution processing, showing great potential to improve its performances in optoelectronic devices.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: This work is supported by the National Basic Research Program of China (973 program 2013CB834705 and 2014CB643505), National Natural Science Foundation of China (21225418 and 51173048), China Postdoctoral Science Foundation (2014M552194), and GDUPS (2013). Received: August 17, 2014; Revised: September 8, 2014; Published online: October 6, 2014; DOI: 10.1002/marc.201400461 Keywords: D–A conjugated copolymers; dithienobenzothiadiazoles; salvation ability; organic field-effect transistors; polymer solar cells [1] G. Dennler, M. C. Scharber, C. J. Brabec, Adv. Mater. 2009, 21, 1323. [2] F. C. Krebs, Sol. Energy Mater. Sol. Cells 2009, 93, 394. [3] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789. [4] Y.-J. Cheng, S.-H. Yang, C.-S. Hsu, Chem. Rev. 2009, 109, 5868. [5] J. Chen, Y. Cao, Acc. Chem. Res. 2009, 42, 1709. [6] G. Li, R. Zhu, Y. Yang, Nat. Photonics 2012, 6, 153. [7] N. Blouin, A. Michaud, M. Leclerc, Adv. Mater. 2007, 19, 2295. [8] H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Nat. Photon. 2009, 3, 649. [9] H. J. Son, L. Lu, W. Chen, T. Xu, T. Zheng, B. Carsten, J. Strzalka, S. B. Darling, L. X. Chen, L. Yu, Adv. Mater. 2013, 25, 838. [10] S. Shi, X. Xie, P. Jiang, S. Chen, L. Wang, M. Wang, H. Wang, X. Li, G. Yu, Y. Li, Macromolecules 2013, 46, 3358.
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