Donor End-Capped Hexafluorinated Oligomers for Organic Solar Cells

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Donor End-Capped Hexafluorinated Oligomers for Organic Solar Cells with 9.3% efficiency by Engineering the Position of #-Bridge and Sequence of Two-Step Annealing Jin-Liang Wang, Kai-Kai Liu, Sha Liu, Fei Xiao, Zheng-Feng Chang, YuQing Zheng, Jin-Hu Dou, Ru-Bo Zhang, Hong-Bin Wu, Jian Pei, and Yong Cao Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 16 Jan 2017 Downloaded from http://pubs.acs.org on January 16, 2017

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Chemistry of Materials

Donor End-Capped Hexafluorinated Oligomers for Organic Solar Cells with 9.3% efficiency by Engineering the Position of π-Bridge and Sequence of Two-Step Annealing Jin-Liang Wang,* ,†,§ Kai-Kai Liu, †,§ Sha Liu, ‡,§ Fei Xiao, † Zheng-Feng Chang, † Yu-Qing Zheng, # Jin-Hu Dou, # Ru-Bo Zhang,† Hong-Bin Wu,*, ‡ Jian Pei,* , #and Yong Cao‡ †

Key Laboratory of Cluster Science of Ministry of Education, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, China.

‡

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, China. #

College of Chemistry and Molecular Engineering, Peking University, Beijing, China.

ABSTRACT: A pair of isomeric hexafluorinated oligomers (Th6FSe and Se6FTh), which are with the same aromatic compositions (difluorobenzothiadiazoles central core, IDT units, and donor end-capped groups), but differ in the π-bridge position (selenophene and thiophene), were designed and successfully synthesized. The potential of the resulted oligomers as donor materials for BHJ-OSCs was systematically investigated through optical absorption, AFM, TEM, GIXD, charge mobility measurement, and photovoltaic device fabrication. It was found that the π-bridge sequences in the resulted oligomers play a subtle but key role in device performances. Moreover, as a result of increase of crystalline content and desired phase separation after rapid SVA or combined TA and SVA treatment, the device performance of the resultant devices undergo significant enhancement. Notably, the Se6FTh devices showed a best PCE of ca. 9.3% with SVA+TA treatment, which is the highest PCE of BHJ-OSCs based on donor end-capped oligomers. These primary study demonstrated that the sequence of π-bridge and annealing treatments play critical roles for improving ordered and crystalline morphology and enhanced PCE, and hence can provide an useful strategy toward highly efficient oligomers for BHJ-OSCs.

INTRODUTION Organic solar cells with bulk-heterojunction structure have been considered as a low cost and useful techniques for utilizing solar energy owing to their advantages of light-weight 1-7 and flexibility. Recently, much contributions were focused on developing narrow bandgap polymeric donor materials and organic solar cells with power conversion efficiency 8-22 (PCE) around 12% have been reported. On the other hand, when compared to polymer counterpart, small molecules/oligomers exhibited more well-defined structures, which can provide better reproducibility in the device performance and clear relationships of structure-device perfor23-27 mance. Recently, significant progresses have been made in small-molecule-based organic solar cells and their PCEs is 28-45 now comparable to these of polymers solar cells. Despite these advances, two main strategies are currently being used in the design of high efficient small molecular donor. The first method relies on oligothiophene/benzodithiophene (BDT) or other alternating donor units capped with relatively electron-withdrawing units, such as rhodamine and analogues, which was called as “A-D-A” 31-45 sequence system with PCEs in range of 6%-10%. The second method relies on an electron-rich central unit symmetrically substituted with electron-deficient unit, and subse-

quent end-capping of donor terminal units, which was called as D-A-D or D-A-D-A-D sequenced small molecules. It is critical to note that most of D-A-D or D-A-D-A-D sequenced small molecules/oligomers with electron-rich units as terminal groups hardly achieve high PCE (>9%) due to their lim46-50 ited building blocks and their poor film morphology. Thus combination the molecular structure and film morphology optimization of small molecules/oligomers remains the key factor for high level device performance in donor end-capped small molecules/oligomers based devices.

Chart 1: Chemical structures of two isomers with different π-bridge position (selenophene and thiophene).

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Scheme 1. Synthesis of two isomeric oligomers Th6FSe and Se6FTh with different π-bridge position.

As minor changes of heteroatom substitution in the chemical structure of may induce large variations in the resultant film morphology, thus it can be a promising strategy 51-52 to enhance PCEs. As compared with the S atom, Se atom is much larger in size and less electronegative, thus selenophene ring-containing materials tend to be more extended in absorption spectrum and showed higher charge mobility in 53-56 comparison with those of thiophene-based materials. However, owing to their higher HOMO energy levels and lower Voc, several reported selenophene containing materials showed lower PCE than that of thiophene containing materi57-58 als. Therefore, it is necessary to understand how to optimize the structure of selenophene-based materials to achieve higher Jsc and Voc simultaneously. Besides, the impact of the moieties sequence of isomers on their optoelectronic properties has been advocated but rarely studied. Meanwhile, owing to intramolecular hydrogen bonds, fluorine substitution in conjugated materials can lead to more rigid and planar backbone, which usually resulted in improved PCEs when com59-65 pared to the devices from those non-fluorine analogue. Consequently, developing highly efficient (PCE>9%) donor end-capped and fluorinated small molecule/oligomers with rational design in moiety sequence is important and challenging in advancing donor materials for organic solar cells. Here, we reported the design and synthesis of a pair of isomeric hexafluorinated oligomers, named as Th6FSe and Se6FTh, which are with the same aromatic compositions (electron-deficient difluorobenzothiadiazoles central core, electron-rich IDT units, and donor end-capped groups) but differ in the positions of π-bridge (selenophene and thio-

phene) (see Chart 1), respectively. Special attention is paid to the influence of the sequence of the π-bridge on the band gap and the HOMO/LUMO energy levels in the solid state, charge transport properties and the morphologies of blend films, and the resultant photovoltaic properties. As compared to that of the device from Th6FSe, higher PCEs were observed in the devices from Se6FTh. As a result of optimized film morphology and charge transport by solvent vapor annealing (SVA) or combined thermal annealing (TA) and SVA treatment, the best PCE of 9.26% were achieved for these devices using Se6FTh:PC71BM blends after SVA and followed with TA treatment. To the best of our knowledge, this is the highest PCE for solution-processed organic solar cells from donor unit end-capped oligomers, and is among one of the best oligomeric donor materials for organic solar cells. Surprisingly, among various post annealing treatments, the device based on Th6FSe:PC71BM blends gave the highest PCE of 7.17% after TA and followed with SVA treatment. These results provide insights into understanding how OSC performance is influenced by alternating the position of π-bridge and the sequence of TA and SVA treatment in a pair of isomeric oligomers, and hence provides a guideline for further designing highly efficient oligomers for organic solar cells.

RESULT AND DISCUSSION The synthetic routes to two isomeric oligomers are presented in Scheme 1. Treatment of indacenodithiophene (IDT) with N-bromosuccinimide (NBS) (1.0 eq.) in mixture


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solvents of THF and DMF gave monobromide 1. 3 was prepared by a Stille coupling reaction between 1 and monotin 49 reagent 2 in 61% isolated yield. Then 3 can be lithiated by LDA followed by quenching with trimethyltin chloride to afford monotin reagent 4 with IDT unit. Selenophene functionalized benzothiadiazole 5 was obtained by a Stille coupling reaction between 4,7-dibromo-5,6-difluorobenzo[1,2,5]thiadiazole and 2-tributyltinselenophene in 82% isolated yield, which was then converted to corresponding dibromide 6 using NBS. Finally, Th6FSe was obtained as a dark solid through the Stille coupling reactions between monotin 4 and dibromide 6 in good isolated yield. For the isomer Se6FTh case, 8 was obtained by a Stille coupling reac66 tion between 7 and 2-tributyltinselenophene in 91% isolated yield. Treatment of 8 with LDA followed by trimethyltin chloride and recrystallized from methanol/chloroform to afford the desired product 9 as red solid. 10 was obtained by a Stille coupling reaction between 1 and monotin reagent 9 in 53% isolated yield. Then 10 can be lithiated by LDA followed by quenching with trimethyltin chloride to afford monotin reagent 11. Se6FTh was synthesized by Stille coupling of 11 67 and 12 in 67% isolated yield. All compounds were purified by silica gel column chromatography, and their structures 1 13 and purity were fully characterized by H NMR, C NMR, ESI/MALDI-TOF MS, and elemental analysis. Under N2 atmosphere, the onset temperature with 5% weight-loss by o thermogravimetric analysis (TGA) was over 400 C for Th6FSe and Se6FTh (Figure S1), which demonstrated that good thermal stability of them.

Normalized Absorbance Intensity

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0 900

Figure 1: The absorption spectra of these two isomers in -6 chloroform solutions (1×10 M) and in the thin films. The absorption spectra of two isomeric oligomers both in diluted chloroform solutions and in thin films obtained by spin-coating were recorded in Figure 1. Two isomers both showed two intensive absorption bands with considerably high molar extinction coefficient in solutions and thin films due to π-π* transition of conjugated backbone and the intramolecular charge transfer (ICT) for donor and acceptor units. In solution, the oligomer Th6FSe showed an absorption peak at 433 nm and another more obvious absorption peak at 596 nm. Although the spectral profile of Se6FTh is nearly identical to that of the isomer Th6FSe in solution, the

ICT absorption band of Se6FTh appeared at 603 nm, which is about 7 nm red-shifted compared to that of Th6FSe. In addition to the ICT band, the absorption spectrum of Se6FTh displayed less intensity and slight blue-shift in the range of 300-500 nm in comparison with that of Th6FSe. These observation suggested that the donor and acceptor moieties in Se6FTh had stronger D-A coupling interaction than that of Th6FSe. In contrast, the absorption spectra of these πconjugated oligomers in thin films were obviously red-shifted with shoulder features at ICT band in comparison with those of diluted solutions. Compared with its absorption maximum in solutions, Th6FSe showed red-shift of ca.17 nm with a weaker shoulder at longer wavelengths in the thin films. However, the thin film of Se6FTh displayed the larger redshift of 33 nm on the maximum absorption peak (λmax) and exhibited obvious 0-0 and 0-1 vibrational peak relative to those of Th6FSe, which may relate to an enhanced interchromophore interaction and a higher π-electron delocalization through the whole molecular backbone upon Se6FTh. Optical bandgaps were estimated from the absorption onsets of the thin films as 1.68 eV for Th6FSe and 1.75 eV for Se6FTh, respectively, which indicated that bandgaps were finely modulated via delicate manipulation the π-bridge positions. Obviously, all the above mentioned spectral differences in solutions and thin films are originated from the structural variations between Th6FSe and Se6FTh, in which two thiophene π-bridges and selenophene π-bridges were exchanged their positions. To estimate energy levels of these oligomers and understand the relationship between the chemical structure and the redox properties of the desired materials, the electrochemical curves and energy levels of these materials were studied and calculated by cyclic voltammetry (CV). As shown in Figure S2 and Table 1, CV curves of thin films based on two isomers both showed one quasi-reversible oxidation or reduction waves. The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) energy levels calculated from the onset oxidation curves and the onset reduction curves, and were -5.24 eV/3.19 eV for Th6FSe and -5.28 eV/-3.22 eV for Se6FTh, respectively. As shown in Table 1, for both HOMO and LUMO energy levels, Th6FSe is slightly higher than Se6FTh. The slightly deep-lying HOMO energy level of Se6FTh should provide higher Voc in organic solar cells in comparison with that of Th6FSe. To evaluate the potential of these two isomers as promising donor materials for organic solar cells and shed light on intrinsic differences, organic solar cells using these oligomers as donors were fabricated, with a device structure of ITO/PEDOT:PSS/each isomer:PC71BM/PFN/Al. By varying the D/A ratio in a broad range, an optimal weight ratio between oligomer/PC71BM was obtained as 1:3 for both isomers (see Table S1-S2 and Figure S3). The current density-voltage characteristics (J-V) of best devices are displayed in Figure 2. Interestingly, Se6FTh-based device without any post treatment showed a better performance (a PCE of 5.64%) than the Th6FSe devices (a PCE of 4.31%) (see Table 2). The observed higher Voc in the Se6FTh device is consistent with its slightly lower-lying HOMO energy levels. Indeed, the dependence of the photovoltaic performance on the isomeric structures



indicated that optimization of sequence of the π-bridge are necessary for the donor molecular design. Recently, several reports showed that rapid SVA and TA were effective post treatments to optimize the morphology in the active layer, which is critical to achieve improved device 68-73 performance in BHJ-OSCs. More recently, Chen and coworkers used two step annealing (TA-SVA) treatments with chloroform as vapor solvent to optimize the morphology of bulk films and dramatically improved the PCE and stability 74-75 in BHJ-OSCs. For our cases, upon CH2Cl2 vapor treatment, both Se6FTh/PC71BM and Th6FSe/PC71BM devices also showed improved PCEs in comparison with those of as-cast devices. It is worthy to note that proper time treatment (30 s) can provide the highest PCE for both cases (see Table S3-S4). For example, upon CH2Cl2 vapor treatment for 30 s, the device from Se6FTh/PC71BM showed an improved PCE of 8.09% -2 (with a Jsc of 13.05 mA cm , a Voc of 0.87 V, and FF of 0.71), while the Th6FSe device also exhibited a much improved PCE of 6.96%. The improvement is mainly owing to an increase in Jsc and FF, while Voc suffered from an obvious reduction after SVA. Nevertheless, as shown in Table 2, TA alone can avoid such reduction in Voc. Therefore, we applied a two-

step treatment consisting of TA and a consequent SVA treatment (referred to as "TA+SVA") or a two-step treatment consisting of SVA and a consequent TA treatment (referred to as "SVA+TA ") to further enhance the device performances in comparison with SVA treatment. Moreover, systematically studies on difference of TA+SVA treatment and SVA+TA treatment to optimize the morphology of active layer have received little attention. The PCE of Th6FSe device increase slightly to 7.17% after the TA+SVA treatment, as a result of slightly improved FF and Voc. However, after SVA+TA treatment, the PCE decreased to 6.61%, mainly due to an obvious reduction in Jsc. Surprisingly, the PCE of the Se6FTh device increased to 8.44% after TA+SVA treatment and further dramatically improved to 9.26% (with a high Jsc of 14.74 mA, a Voc of 0.87 V, and a high FF of 0.72) after SVA+TA treatment. Moreover, the average PCE by accounting about twenty individual devices for the blend of Se6FTh/PC71BM was 8.95%. The obtained best PCE in the Se6FTh device here is among the highest for OSCs from organic oligomeric donor materials. It is more important to note that such outstanding PCE is the highest values of BHJ-OSCs based on donor end-capped 46-50 oligomers.

Table 1. Photophysical and electrochemical properties of two isomers. Compd

λmaxabs. (sol)(nm) 433, 596 431, 603

Th6FSe Se6FTh

λmaxabs (film) (nm) 440,613 437, 595, 636

Eox(onset) a (V) 0.44 0.48

Ered(onset) a (V) -1.61 -1.58

EHOMO (eV) -5.24 -5.28

ELUMO (eV) -3.19 -3.22

b

Eg(cv) (eV) 2.05 2.06

Eg(opt) (eV) 1.68 1.75

+

a

b

potentials are measured relative to a Fc/Fc redox couple as an external reference. estimated from the onset of thin-film absorption. (a)

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40 30

10 0.5 As-cast

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Figure 2: J-V curves (a-b), plots of PCE (c) and FF (d) dependence on the device treatment conditions, and corresponding EQE curves (e-f) based on the best result of these oligomers and PC71BM before/after post annealing treatments.


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The different responses in device performance upon postcurves showed a maximal value of 77% at 440 nm and > 60% treatment for two isomers is not very clear at this stage. This over a broad wavelength range between 400 nm and 650 nm. should be related to chemical structure, molecular packing, These results indicated a highly efficient photo-electron conand morphology and carrier transporting factors. Recent reversion was archived in the devices after post annealing ports provide evidence that the increase of end-group ground treatment. All the integrated current density from the EQE dipole moment has a positive impact on molecular packing, curves showed 2-5% mismatch with the Jsc value from J-V 48 the FF, and mobility of devices. Due to the symmetrical curve (see table 2). On the other side, it should be noted that structure and the same central core of our isomers, we think the change in optical absorption cannot be mainly responsithat difference of molecular packing interaction of isomers ble, although subtle changes in spectral absorption after post would be mainly contributed from the ground dipole moannealing treatments were clearly observed (see Figure S6). ment of the rest half of the molecules. The rest half of the These subtle changes could originate from molecular reormolecular ground-state dipole moments was calculated using ganization and/or improved nanostructural order (the effect DFT B3LYP/6-31G(d,p) method and yielded magnitudes of will be discussed below). 0.30 D and 0.50 D for Th6FSe and Se6FTh, respectively Meanwhile, given the important role of film morphology of (Figure S5). These empirical results were consistent with the the active layer in determining the device performance of higher FF and PCE of Se6FTh, which suggested that exOSCs, the surface morphology of the photoactive layers was change of the positions of π-bridge play an important role for firstly studied by tapping mode AFM. Figure 3 and S7 highly efficient donor materials. showed the height and phase images for the as-cast, SVA, Figure 2 and S4 gave the variation of these cell parameters TA+SVA, SVA+TA films. For the as-cast film, a smooth surbefore/after post annealing treatments and exchanging the face with a small root-mean-square (RMS) roughness was positions of π-bridge. Interesting, the TA+SVA treatment was clearly seen. Upon SVA, TA+SVA, and SVA+TA treatment, the best treatment for the improvement of FF. It was found the RMS of Se6FTh:PC71BM film obviously increased from that Th6FSe-based devices gave the largest enhancement of 0.7 nm (as-cast) to 1.1 nm, 1.2 and 1.3 nm, respectively, which PCE under TA+SVA treatment and Se6FTh-based devices is consistent with other group’s report on the changes of the 68-75 gave the largest enhancement of PCE under SVA+TA treatblends with/without any post-treatments. In contrast, the ment in comparison with the initial performance based on RMS of Th6FSe:PC71BM films increased slightly after the the as-cast film. The Jsc showed a similar changing trend to treatments. The results indicated that exchange the position the PCE values. The improvement in Jsc can be further eviof the π-bridging building block could affect the response of denced by their photo response, which was also known as the blend surface morphology during post annealing. EQE curve (see Figure 2 e & f). Taking the Se6FTh//PC71BM device as an example, upon SVA+TA treatment, the EQE Table 2. A summary of the device performances and charge carrier mobilities from these oligomers and PC71BM. oligomer

Jsc 2 (mA/cm )

Calculated Jsc 2 (mA/cm )

Voc (V)

e

FF

PCE (best) (%)

μe 2 -1 -1 (cm V s )

μh 2 -1 -1 (cm V s )

-4 -5 9.74 4.17±0.28(4.31) Th6FSe 10.10±0.11 0.86±0.01 0.49±0.01 1.4×10 5.0×10 a -4 -4 11.82 4.83±0.11(6.96) Th6FSe 12.34±0.33 0.83±0.01 0.67±0.01 3.2×10 3.2×10 b 4.27±0.33(4.56) 9.72±0.19 0.87±0.01 0.50±0.03 Th6FSe c -4 -4 11.59 6.84±0.22(7.17) Th6FSe 11.78±0.20 0.87±0.01 0.67±0.01 4.4×10 3.3×10 d -4 -4 11.05 6.44±0.10(6.61) Th6FSe 11.23±0.18 0.85±0.00 0.67±0.01 3.2×10 2.7×10 -4 -4 11.12 5.40±0.17(5.64) Se6FTh 11.18±0.15 0.90±0.00 0.54±0.11 1.9×10 1.4×10 a -4 -4 12.75±0.17 12.59 0.87±0.00 0.71±0.01 7.87±0.13(8.09) Se6FTh 3.5×10 7.3×10 b 11.54±0.17 0.90±0.01 0.53±0.01 5.55±0.12(5.72) Se6FTh c -4 -4 13.56±0.12 13.18 0.86±0.00 0.71±0.01 8.13±0.07(8.44) Se6FTh 2.9×10 4.3×10 d -4 -4 14.30±0.25 13.96 0.87±0.01 0.72±0.004 8.95±0.17(9.26) Se6FTh 3.2×10 7.4×10 a b c d e with SVA; withTA; with TA+SVA; with SVA+TA; The average values are calculated from >15 devices with standard deviation



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2

Figure 3. Tapping mode AFM height (a) and phase (b) images 5×5 μm of for blend film with Se6FTh/PC71BM blend film before/after post different annealing treatments. SVA+TA treatment, an enhanced π-π stacking feature Besides the AFM, high-resolution TEM was used to further ((010) diffraction) in the out-of-plane direction was ocinvestigate the interior morphology of these films becurred, which indicated that there exists a predominant fore/after post annealing treatments. As shown in Figure 4, “face-on” crystalline orientation in these blend films. For both of the as-cast film exhibited a homogeneous morpholoexample, the (010) diffraction out-plane direction after -1 gy, which is consistent with the AFM images. When these SVA+TA treatment located at 1.89 Å corresponding to a π-π stacking distances of 3.32 Å, implying the existence films were treated with SVA, the length scale of phase separation obviously increased. Upon TA + SVA treatment, a furof very strong intermolecular interactions. Meanwhile, nanocrystalline size along π-π staking direction was esther slightly increased length scale of phase separation (ca. 15 nm) and better bicontinuous morphology were clearly seen timated through the Scherrer equation. The crystal size in (010) direction with SVA+TA treatment (ca. 33 Å) is for both of Th6FSe and Se6FTh blends. Meanwhile, the slight larger than that of as-cast film (ca. 31 Å). MoreoSe6FTh blend films with SVA+TA treatment showed imver, the alkyl-to-alkyl spacing in molecular crystals with proved dispersed fibrous structures of 15-20 nm width as compared with the film with TA+SVA treatment. Such SVA+TA treatment is slight smaller than that of as-cast film, indicating the tighter alkyl chain stacking with changes of morphology in Se6FTh blend film originated from SVA+TA treatment. These results indicated the SVA+TA better aggregation and reorganization of Se6FTh. This observation thus can partially explain why the Se6FTh device with treatment can effectively promote nucleation and growth of molecular crystals. Moreover, the GIXD patSVA +TA treatment showed the best results among post annealing treatments. In contrast, a smaller scale phase separaterns of the Th6FSe:PC71BM films with TA+SVA treattion with smaller domain sizes was observed for Th6FSe ment also showed an enhanced diffraction in the out-ofsamples in comparison with TA+SVA treatment. Thus effiplane direction when compared to that of the as-cast ciency of excitons splitting with SVA +TA treatment would film (see Figure S8). Although the crystal size of Th6FSe:PC71BM films in (010) direction with TA+SVA be low and charge transport would be also less efficient in treatment (ca. 26 Å) is obviously larger than that of ascomparison with that of Th6FSe samples with TA+SVA cast film (ca. 21 Å), we noted the crystal sizes are smaller treatment. than those of Se6FTh/PC71BM blend films. Overall, it is We further applied Grazing incidence X-ray diffractions (GIXD) technique to investigate the effect of the indicated that the combination of TA and SVA treatpost annealing treatment on the crystallinity and organiments represent an effective approach to control and zation of these thin films. As shown in Figure 5, the asoptimize crystalline morphology in our isomeric hexcast Se6FTh/PC71BM thin film showed a very weak (010) afluorinated oligomer/ PC71BM blend films, particular to -1 diffraction peaked at 1.88 Å in out-of-plane direction, Se6FTh/PC71BM blend films. which corresponding to a π-π stacking distances of 3.34 Å. After post annealing treatments, in particular with


6

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Figure 4. TEM images of blend films based on (a)Th6FSe/PC71BM or (b)Se6FTh/PC71BM before/after post annealing treatments.

Figure 5. GIXD patterns of blend films based on Se6FTh/PC71BM before/after post annealing treatments. To investigate the effect of the different position of πbridges units and post-annealing treatments on charge transporting properties in these blend films, space charge limited current (SCLC) measurements was carried out to determine the charge mobilities. Both of electron and hole mobilities of the blends were deduced by analysing the J-V curves of single carrier devices (see Table 2 and Figure S9-10). The Se6FTh:PC71BM film showed substantially higher hole mobility than that of Th6FSe:PC71BM film, which can be assigned a relatively higher degree of molecular ordering in the solid state, thus exhibiting a better intramolecular charge transport. Moreover, all of hole mobilities increased about 2-5 times with these post-annealing treatments. For example, the Se6FTh:PC71BM film with the SVA +TA treat-4 2 -1 ment exhibited a higher hole mobilities (7.4×10 cm V s 1 ) than those using either single SVA treatment or the TA +SVA treatment. In addition, electron mobility (μe) of the blends with the treatments showed slight increase in comparison with those of as-cast films. Therefore, less charge recombination loss after post annealing treatment was achieved owing to higher hole mobility and the more balanced between the hole and electron mobility, which led to most of the photo-generated charge carriers can be readily collected at the electrode with post-annealing treatments, revealing by their high FFs 76-78 (over 0.7).

CONCLUSION In summary, we have synthesized a pair of isomeric hexafluorinated oligomers with electron-rich terminal units (Th6FSe and Se6FTh) and studied their applications in BHJ-OSCs. Both oligomers have the same building blocks, but differ in the position of π-bridge. As a result of increase of crystalline content and desired phase separation morphology after rapid SVA or combined TA and SVA treatment, the device performance of the resultant devices from these two isomers undergo significant enhancement. Of which, the Se6FTh based devices showed higher PCE than the Th6FSe device. Such differences were further correlated with nanoscale morphology of the blend films, which was systematically investigated using optical absorption, AFM, TEM, GIXD and charge mobility. Notably, relative larger bandgap oligomer Se6FTh exhibited an outstanding PCE of 9.3%, which is the highest PCE for solution-processed BHJOSCs based on donor end-capped oligomers. These primary study demonstrated that the position of π-bridge and sequence of annealing treatments play critical roles for improving ordered and crystalline morphology and

enhancing PCE, and hence can provide an useful strategy toward more efficient oligomers for BHJ-OSCs.

EXPERIMENTAL Materials and Characterization: All air and water sensitive reactions were performed under nitrogen atmosphere. Tetrahydrofuran (THF) and Toluene was dried over Na and freshly distilled prior to use. The other materials were of the common commercial level and used as 1 13 received. H and C NMR spectra were recorded on a Bruker ARX-400 or ARX-500 spectrometer. All chemical 1 shifts were reported in parts per million (ppm). H NMR 13 chemical shifts were referenced to TMS (0 ppm), and C NMR chemical shifts were referenced to CDCl3. MALDITOF-MS was recorded on a Bruker mass spectrometer. Thermal gravity analyses (TGA) were carried out on a TA Instrument Q600 analyser. Elemental analyses were performed using a German elemental analyser. Absorption spectra were recorded on Hitachi UH5300 UV-vis spectrometer. Cyclic voltammetry (CV) was performed on CHI workstation. Glassy carbon electrode was used as a working electrode and a platinum wire as a counter electrode, these films were drop-cast on a glass carbon working electrode from THF solutions. Measurements were carried out at a scan rate of 50 mV/s in CH3CN containing 0.1 M n-Bu4NPF6 as the supporting electrolyte. All potentials were recorded versus Ag/AgNO3 (0.01 M in anhydrous acetonitrile) reference electrode and calibrat+ ed with the redox couple of Fc/Fc under the same experimental conditions. The film morphology was studied by atomic force microscopy (AFM, Veeco MultiMode V) operating in tapping mode. The TEM sample structure was ITO/PEDOT:PSS/Blend/Methanol. A 40-nm-thick PEDOT:PSS anode buffer layer was spin-cast on the ITO o anode substrate, then dried in air at 150 C for 20 minutes. Prior to spin coating, the chlorobenzene soluo tion was heated up at 70 C. These oligomers:PC71BM active layer were prepared by spin-coating their mixture of chlorobenzene solution with various weight ratios. The active layers were placed in a glass petri dish containing 1 mL CH2Cl2 for 30 s for solvent vapor annealing o or thermal annealing at 70 C for 10 min and then were placed in a glass petri dish containing 1 mL CH2Cl2 for 30 s for solvent vapor annealing or were placed in a glass petri dish containing 1 mL CH2Cl2 for 30 s for solvent o vapor annealing and then thermal annealing at 70 C for 10 min. Subsequently, the films were placed into a plass petri dish full of deionized water until the blend film separated from the ITO substrate. Then, the separated blend film was then placed onto the copper mesh for


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TEM test. The JEOL 2100F was used for high-resolution TEM test. The X-ray diffraction data were obtained at beamline BL14B1 of the Shanghai Synchrotron Radiation Facility (SSRF). BHJ-OSC fabrication. Device preparation and characterization were carried out in clean room conditions with protection against dust and moisture. Solar simulator illumination intensity was determined by a singlecrystal silicon reference cell (Hamamatsu S1133, with KG5 visible color filter) calibrated by the National Renewable Energy Laboratory (NREL). The device structure was ITO/PEDOT:PSS/oligomer:PC71BM/PFN/Al, a 40-nmthick PEDOT:PSS anode buffer layer was spin-cast on the o ITO substrate, then dried in a vacuum oven at 100 C overnight. Prior to spin coating, the chlorobenzene soluo tion was heated up at 70 C. These oligomers:PC71BM active layer were prepared by spin-coating their mixture of chlorobenzene solution with various weight ratios. The active layers were placed in a glass petri dish containing 1 mL CH2Cl2 for 30 s for solvent vapor annealing o or thermal annealing at 70 C for 10 min and then were placed in a glass petri dish containing 1 mL CH2Cl2 for 30 s for solvent vapor annealing or were placed in a glass petri dish containing 1 mL CH2Cl2 for 30 s for solvent o vapor annealing and then thermal annealing at 70 C for 10 min. A 5 nm PFN layer was then spin-coated from methanol solution in presence of a trace amount of acetic acid onto the active layer. Subsequently, the films were transferred into a vacuum evaporator and 80 nm of Al were deposited as cathode. The active area of OPV 2 2 devices is 0.16 cm (~2×8 mm , as well defined by a shadow mask). The film thickness of photoactive layer was monitored by a surface profiler (Alfa Step-500, Tencor). The values of power conversion efficiency were determined from J-V characteristics measured by a Keithley 2400 source-measurement unit under AM 1.5G spectrum from a solar simulator (Oriel model 91192). 3: In a 100 mL two-neck round-bottom flask, 2 (0.36 g, 0.55 mmol), 1 (0.43 g, 0.5 mmol), Pd2(dba)3 (23 mg, 0.025 mmol), tri(o-tolyl)phosphine (31 mg, 0.1 mmol) was added. The flask was evacuated and back-filled with N2 three times, and then degassed toluene (60 mL) was injected into the mixture. The resulting solution was stirred at room temperature for 12 h under the N2 atmosphere. The solvents were then removed under reduced pressure. The dark residue was purified by silica gel chromatography, eluting with PE-CH2Cl2 (30:1) to give purple oil 1 (0.37 g, 61%). H NMR (CDCl3, 400 MHz, ppm): δ 8.208.23 (m, 1H, Th-H), 8.18-8.19 (d, J = 3.6 Hz, 1H, Th-H), 7.31-7.32 (m, 3H, Ph-H, Th-H), 7.26-7.27 (d, J = 3.6 Hz, 1H, Th-H), 7.24-7.25 (d, J = 4.8 Hz, 1H, Th-H), 7.21-7.22 (d, J = 3.6 Hz, 1H, Th-H), 7.13-7.14 (d, J = 3.6 Hz, 1H, ThH), 6.98-7.99 (m, 1H, Th-H), 6.73-6.74 (d, J = 3.6 Hz, 1H, Th-H), 2.81-2.85 (t, J = 7.6 Hz, 2H, CH2), 1.98-1.99 (m, 8H, CH2), 1.70-1.73 (m, 2H, CH2), 1.33-1.43 (m, 6H, CH2, CH), 13 0.54-0.95 (m, 63H, CH2, CH3). C NMR (CDCl3, 100 MHz, ppm): δ (155.9, 155.8, 155.7), (155.2, 155.1, 155.0), 153.4, 152.8, (150.94, 150.74, 150.35, 150.15, JCF = 259, 20 Hz), (150.85, 150.65, 148.27, 148.07, JCF = 259, 20 Hz), (148.5, 148.4, JCF = 8 Hz), 146.3, (142.30, 142.26), 142.1, (141.3, 141.2, JCF = 7 Hz), 137.9, 136.5, 135.5, 134.4, (131.9, 131.8, 131.6, 131.5), (130.1, 129.9), 126.2, 125.0, 124.0, 123.1, 122.9, 122.7, 119.9, (114.3, 144.2, JCF = 9 Hz), (111.0, 110.9, JCF = 11 Hz),

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54.2, 53.8, 44.4, (44.0, 43.9), (35.2, 35.1), 34.5, (34.1, 34.0), (31.8, 31.6), 30.4, (29.1, 28.99, 29.95), 28.8, 28.4, (27.8, 27.6, 27.5, 27.4, 27.3, 27.22, 27.16), (23.1, 22.8), (14.4, 14.3), (11.1, 11.0, 10.8), 10.6. HR-ESI-MS (m/z): calcd for + + C72H92F2N2S6: 1214.5547 (M ), Found: 1214.5570 (M ). 4: To a solution of 3 (0.30 g, 0.25 mmol) in anhydrous THF (80 mL) was added a solution of lithium diisopropylamide in THF (5.0 mL, 0.50 mmol) dropwise in N2 atmosphere at -78 °C. The mixture was stirred at −78 °C for 1 h and Me3SnCl (0.10 g, 0.50 mmol) in anhydrous THF (10 mL) was added. The mixture solution was warmed up to room temperature and stirred for 10 h. The mixture solution was quenched with water, and extracted with chloroform. The organic extracts were washed with brine and dried over anhydrous Na2SO4. After removal of the solvent under the reduced pressure, the resulting crude product was obtained as purple oil (0.31 g, 92%) and 1 used in the next step without any further purification. H NMR (CDCl3, 400 MHz, ppm): δ 8.23-8.24 (m, 1H, Th-H), 8.20-8.21 (d, J = 3.6 Hz, 1H, Th-H), 7.30-7.32 (m, 3H, PhH, Th-H), 7.23-7.26 (m, 2H, Th-H), 7.15-7.16 (d, J = 3.6 Hz, 1H, Th-H), 7.02 (m, 1H, Th-H), 6.74-6.75 (d, J = 3.6 Hz, 1H, Th-H), 2.81-2.85 (t, J = 7.6 Hz, 2H, CH2), 1.96-1.98 (m, 8H, CH2), 1.69-1.73 (m, 2H, CH2), 1.33-1.41 (m, 6H, CH2, CH), 0.54-0.91 (m, 63H, CH2, CH3). 0.39 (s, 9H, 13 Sn(CH3)3). C NMR (CDCl3, 100 MHz, ppm): δ 157.3, 155.8, 153.7, 153.4, 152.8, (151.2, 151.0, 148.6, 148.4, JCF = 259, 20 Hz), (148.9, 148.8, JCF = 9 Hz), 147.9, 146.6, 142.5, 141.5, 139.4, 137.5, 136.5, 135.2, 134.4, 132.1, 132.0, 131.9, 131.8, 130.6, 130.1, 125.2, 124.3, (123.3, 123.2, JCF = 14 Hz), 120.0, (114.5, 114.3, JCF = 14 Hz), 111.3, 54.2, 53.2, 44.2, 44.0, 43.8, 35.1, 34.4, 34.3, 34.1, 34.0, (31.8, 31.7), 30.5, (29.02, 28.95, 28.8), 28.4, (27.5, 27.4, 27.2), 23.1, 22.8, 14.3, (11.0, 10.8, 10.7, 10.6), -8.1. HR-ESI-MS (m/z): calcd for C75H100F2N2S6Sn: + + 1378.5205 (M ), Found: 1378.5189 (M ). Th6FSe: In a 100 mL two-neck round-bottom flask, 6 (0.059 g, 0.10 mmol), 4 (0.34 g, 0.25 mmol), Pd2(dba)3 (4.5 mg, 0.0050 mmol), tri(o-tolyl)phosphine (6.5 mg, 0.020 mmol) was added. The flask was evacuated and back-filled with N2 three times, and then degassed toluene (70 mL) was injected into the mixture. The resulting solution was stirred at room temperature for 12 h under the N2 atmosphere. The solvents were then removed under reduced pressure. The dark residue was purified by silica gel chromatography, eluting with PE-CH2Cl2 1 (3:1) to give dark solid (0.19 g, 68%). H NMR (CDCl3, 400 MHz, ppm): δ 8.41-8.43 (m, 2H, Se-H), 8.24-8.26 (m, 2H, Th-H), 8.22-8.23 (d, J = 3.6 Hz, 2H, Th-H), 7.51 (m, 2H, Se-H), 7.33-7.34 (m, 6H, Th-H, Ph-H), 7.25-7.27 (m, 4H, Th-H), 7.22-7.23 (m, 2H, Th-H), 7.16-7.17 (d, J = 3.6 Hz, 2H, Th-H), 6.74-6.75 (d, J = 3.6 Hz, 2H, Th-H), 2.82-2.85 (t, J = 7.6 Hz, 4H, CH2), 2.01 (m, 16H, CH2), 1.70-1.74 (m, 4H, CH2), 1.32-1.43 (m, 12H, CH2, CH), 0.56-0.99 (m, 13 126H, CH2, CH3). C NMR (CDCl3, 125 MHz, ppm): δ 156.3, 153.3, (151.1, 150.9, 149.0, 148.8, JCF = 260, 19 Hz), (150.9, 150.7, 148.9, 148.7, JCF = 260, 19 Hz), (149.01, 148.96, JCF = 8 Hz), 148.4, 146.8, 142.7, 142.4, 141.9, 140.7, 138.2, 136.1, 134.5, 134.3, 134.1, 132.2, 132.13, 132.06, 132.0, 130.3, 130.2, 125.6, 125.3, 124.4, 123.5, 120.9, 120.1, 114.4, 113.2, 111.5, 54.4, 44.5, 44.4, 44.0, 35.4, 34.7, 34.6, 34.3, 34.1, (31.82, 31.76), 30.5, 29.1, (29.03, 28.97), 28.5, 27.8, 27.6, 27.5, 27.4, 23.2, 23.1, 22.8, (14.42, 14.34, 14.28), 11.0, 10.9, 10.7. MALDI-TOF MS (m/z): calcd for C158H186F6N6S13Se2: + + 2857.9 (M , 100%), Found: 2858.6 (M ). Elemental


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Analysis: calcd for C158H186F6N6S13Se2: C, 66.40; H, 6.56; N, 2.94. Found: C, 66.18; H, 6.66; N, 2.87. 10: In a 100 mL two-neck round-bottom flask, 9 (0.43 g, 0.6 mmol), 1 (0.43 g, 0.5 mmol), Pd2(dba)3 (23 mg, 0.025 mmol), tri(o-tolyl)phosphine (31 mg, 0.1 mmol) was added. The flask was evacuated and back-filled with N2 three times, and then degassed toluene (90 mL) was injected into the mixture. The resulting solution was stirred at room temperature for 12 h under the N2 atmosphere. The solvents were then removed under reduced pressure. The dark residue was purified by silica gel chromatography, eluting with PE-CH2Cl2 (30:1) to give red oil (0.33 1 g, 53%). H NMR (CDCl3, 400 MHz, ppm): δ 8.37-8.40 (m, 1H, Se-H), 8.20-8.21 (d, J = 3.6 Hz, 1H, Th-H), 7.48 (m, 1H, Se-H), 7.31 (m, 2H, Ph-H), 7.23-7.24 (d, J = 3.6 Hz, 2H, Th-H), 7.19-7.21 (m, 1H, Th-H), 7.15-7.16 (d, J = 3.6 Hz, 1H, Th-H), 6.97-7.98 (m, 1H, Th-H), 6.74-6.75 (d, J = 3.6 Hz, 1H, Th-H), 2.81-2.85 (t, J = 7.6 Hz, 2H, CH2), 1.98 (m, 8H, CH2), 1.70-1.73 (m, 2H, CH2), 1.33-1.41 (m, 6H, 13 CH2, CH), 0.52-0.95 (m, 63H, CH2, CH3). C NMR (CDCl3, 100 MHz, ppm): δ (155.9, 155.8, 155.7), (155.2, 155.1, 155.0), 153.4, 152.8, 151.0, (150.8, 150.6, 148.2, 148.0, JCF = 255, 20 Hz), (148.8, 148.7, JCF = 10 Hz), (148.3, 148.2, JCF = 10 Hz), 146.4, 142.6, 142.1, 141.3, 140.3, 136.5, 135.5, 134.4, 133.8, 131.6, 130.0, 126.2, 125.3, 125.1, 124.0, 123.0, 122.7, 120.5, (114.3, 114.2, JCF = 14 Hz), 112.8, (110.9,110.8, JCF = 10 Hz), 54.2, 53.8, 44.4, 43.8, (35.14, 35.09), 34.5, (34.1, 34.0), (31.8, 31.7), 30.4, 29.9, 29.1, 28.4, (27.5, 27.4, 27.3), (23.07, 23.04, 22.8), (14.42, 14.34, 14.3), (11.1, 11.0, 10.8), (10.6, 10.5). HR-ESI-MS (m/z): calcd for C72H92F2N2S5Se: + + 1262.5000 (M ), Found: 1262.5023 (M ). 11: To a solution of 10 (0.35 g, 0.28 mmol) in anhydrous THF (65 mL) was added a solution of lithium diisopropylamide in THF (5 mL, 0.5 mmol) dropwise in N2 atmosphere at -78 °C. The mixture was stirred at −78 °C for 1 h and Me3SnCl (0.11 g, 0.56 mmol) in anhydrous THF (10 mL) was added. The mixture solution was warmed up to room temperature and stirred for 10 h. The mixture solution was quenched with water, and extracted with chloroform. The organic extracts were washed with brine and dried over anhydrous Na2SO4. After removal of the solvent under the reduced pressure, the resulting crude product was obtained as dark oil (0.37 g, 93%) and used 1 in the next step without any further purification. H NMR (CDCl3, 400 MHz, ppm): δ 8.36-8.38 (m, 1H, Se-H), 8.19-8.20 (d, J = 3.6 Hz, 1H, Th-H), 7.46-7.47 (d, J = 3.6 Hz, 1H, Se-H), 7.30 (m, 2H, Ph-H), 7.22-7.23 (d, J = 3.6 Hz, 1H, Th-H), 7.19-7.20 (m, 1H, Th-H) 7.14-7.15 (d, J = 3.6 Hz, 1H, Th-H), 7.02 (m, 1H, Th-H), 6.73-6.74 (d, J = 3.6 Hz, 1H, Th-H), 2.81-2.85 (t, J = 7.6 Hz, 2H, CH2), 1.96 (m, 8H, CH2), 1.70-1.73 (m, 2H, CH2), 1.33-1.41 (m, 6H, CH2, CH), 0.55-0.95 (m, 63H, CH2, CH3). 0.39 (s, 9H, 13 Sn(CH3)3). C NMR (CDCl3, 100 MHz, ppm): δ (157.4, 157.3), 155.8, 153.7, 152.8, (151.2, 151.0, 148.6, 148.5, JCF = 255, 18 Hz), (150.9, 150.7, 148.3, 148.1, JCF = 255, 20 Hz), (149.0, 148.9, JCF = 9 Hz), 146.6, 142.8, (141.5,141.4, JCF = 6 Hz), 140.0, 139.4, 136.5, 135.2, 134.4, (134.1, 133.0, JCF = 9 Hz), 133.7, (131.8, 131.7, JCF = 10 Hz), 130.6, 130.1, 125.2, 124.2, 123.3, (120.6, 120.5, JCF = 9 Hz), 114.5, 114.3, 113.0, (110.0, 110.9, JCF = 12 Hz), 54.2, 53.2, 44.2, 43.9, 43.8, 35.1, 34.4, (31.8, 31.7), 30.5, 29.9, (29.03, 28.96, 28.82), 28.4, (27.6, 27.5), 23.1, 22.8, 14.4, (10.8, 10.6), -8.1. HR-ESI-MS (m/z): + calcd for C75H100F2N2S5SeSn: 1262.5000 ([M-SnMe3] ), + Found: 1262.5135 ([M-SnMe3] )

Se6FTh: In a 100 mL two-neck round-bottom flask, 12 (0.050 g, 0.10 mmol), 11 (0.36 g, 0.25 mmol), Pd2(dba)3 (4.5 mg, 0.0050 mmol), tri(o-tolyl)phosphine (6.5 mg, 0.020 mmol) was added. The flask was evacuated and back-filled with N2 three times, and then degassed toluene (55 mL) was injected into the mixture. The resulting solution was stirred at room temperature for 12 h under the N2 atmosphere. The solvents were then removed under reduced pressure. The dark residue was purified by silica gel chromatography, eluting with PE-CH2Cl2 1 (3:1) to give dark solid (0.19 g, 67%). H NMR (CDCl3, 400 MHz, ppm): δ 8.40-8.41 (m, 2H, Se-H), 8.26-8.27 (m, 2H, Th-H), 8.22-8.23 (d, J = 3.6 Hz, 2H, Th-H), 7.50 (m, 2H, Se-H), 7.33-7.35 (m, 6H, Th-H, Ph-H), 7.25-7.27 (m, 4H, Th-H), 7.21 (m, 2H, Th-H), 7.16-7.17 (d, J = 3.6 Hz, 2H, Th-H), 6.74-6.75 (d, J = 3.6 Hz, 2H, Th-H), 2.81-2.85 (t, J = 7.6 Hz, 4H, CH2), 2.01 (m, 16H, CH2), 1.70-1.72 (m, 4H, CH2), 1.33-1.41 (m, 12H, CH2, CH), 0.55-0.97 (m, 126H, 13 CH2, CH3). C NMR (CDCl3, 125 MHz, ppm): δ 156.3, 153.3, (151.2, 151.0, 148.7, 148.4, JCF = 259, 21 Hz), (149.1, 148.9, JCF = 21 Hz), 146.9, 142.5, 141.8, 140.6, 138.1, 136.1, 134.6, 134.4, 134.3, 134.0, 132.3, 132.1, 125.6, 125.3, 124.5, 123.6, 120.6, 120.2, 114.4, (113.5, 113.4, JCF = 16 Hz), (111.6, 111.5, JCF = 16 Hz), 54.4, 44.5, 44.1, 35.4, 34.7, 34.6, 34.3, 34.2, 31.8, 30.5, 29.0, 28.6, 27.7, 27.6, 27.4, 23.1, 22.8, 14.4, 14.3, 11.0, 10.6. MALDI-TOF MS (m/z): calcd for + + C158H186F6N6S13Se2: 2857.9 (M , 100%), Found: 2857.6 (M , 100%). Elemental Analysis: calcd for C158H186F6N6S13Se2: C, 66.40; H, 6.56; N, 2.94. Found: C, 66.62; H, 7.51; N, 2.88.

ASSOCIATED CONTENT Supporting Information. The synthesis and characterization of these precursors, additional experimental results, and 1 13 the copies of H NMR, C NMR, and MALDI TOF MS are described in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected]; [email protected].

Author Contributions The manuscript was written through contributions of all au[+] thors. J.-L. Wang, K.-K. Liu, S. Liu contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the grants from the National Natural Science Foundation of China (21472012, 21672023, 91333206); the Thousand Youth Talents Plan of China; Beijing Natural Science Foundation (2152027). The authors are grateful for having been able to use the beamline BL14B1 (Shanghai Synchrotron Radiation Facility) for 2DGIXD experiments.

REFERENCES


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TOC: Th6FSe Se6FTh

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Th6FSe Se6FTh 0.7

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PCE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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13

Donor End-Capped Hexafluorinated Oligomers for Organic Solar Cells

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