Solution-Processable Small Molecules for High-Performance Organic

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Solution-Processable Small Molecules for High-Performance Organic Solar Cells with Rigidly Fluorinated 2,2’-Bithiophene Central Cores Zhenguo Wang, Zuojia Li, Jiang Liu, Jun Mei, Kai Li, Ying Li, and Qiang Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01784 • Publication Date (Web): 21 Apr 2016 Downloaded from http://pubs.acs.org on April 24, 2016

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ACS Applied Materials & Interfaces

Solution-Processable Small Molecules for High-Performance Organic Solar Cells with Rigidly Fluorinated 2,2’-Bithiophene Central Cores

Zhenguo Wang, † Zuojia Li, † Jiang Liu,*‡ Jun Mei,‡ Kai Li, † Ying Li, † Qiang Peng*†

†

Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of

Chemistry, and State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China ‡

Chengdu Green Energy and Green Manufacturing Technology R&D Center, Chengdu 610207, P. R. China.

*To whom correspondence should be addressed: Tel: +86-28-86510868; fax: +86-28-86510868; e-mail: Jiang Liu: [email protected] Qiang Peng: [email protected]

ACS Paragon Plus Environment


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Abstract Small molecules containing an oligothiophene backbone are simple but effective donor materials for organic solar cells (OSCs). In this work, we incorporated rigid 2,2’-bithiophene (BT) or fluorinated 2,2’-bithiophene (FBT) as the central unit, and synthesized two novel small molecules (TTH-D3TRh and TTF-D3TRh) with an oligothiophene backbone and 3-ethylrhodanine end groups. Both molecules exhibit good thermal stability as well as strong and broad absorption. The fluorination of BT central unit made TTF-D3TRh possess a relatively lower-lying HOMO energy level, better molecular stacking and higher mobility in comparison with TTH-D3TRh. Conventional OSCs were fabricated to evaluate the photovoltaic property of these two molecules. Without extra post-treatments, the conventional devices based on TTH-D3TRh and TTF-D3TRh showed high PCEs of 5.00% and 5.80%, respectively. TTF-D3TRh device exhibited a higher performance, which can be attributed to the improved Voc of 0.92 V, Jsc of 10.04 mA cm-2, and FF of 62.8%. Using an inverted device structure, the OSCs based on TTH-D3TRh and TTF-D3TRh showed largely elevated PCEs of 5.89% and 7.14%, respectively. The results indicated that these structurally simple TTH-D3TRh and TTF-D3TR are potential donor candidates for achieving highly efficient OSCs. The strategy of fluorination and rigidity designation is an effective approach to develop oligothiophene-based small molecular donors for highly efficient solar cell applications. Keywords: organic solar cells; fluorination; rigidity; oligothiophene; high performance


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1. Introduction Solution-processed organic solar cells (OSCs) have attracted wide interests as a promising utilization approach of renewable energy resource due to their low-cost, flexibility, and applicability in large-area devices in recent years.1-4 Based on the concept of bulk-heterojunction (BHJ), the power conversion efficiencies (PCEs) of OSCs based on polymers and small molecules had rapidly increased over 10% in the past few years.3,5-8 Although the great progress has been made, it is still a challenge to improve the PCEs for commercial applications in the future. Compared with the polymeric OSCs (P-OSCs), small molecular OSCs (SM-OSCs) process many advantages, such as the defined molecular structures with no batch to batch variations, the variable structural modification for fine tuning of their HOMO and LUMO energy levels, and the simply accessible synthesis procedures.9,10 Thus, SM-OSCs have attracted more and more attention recently. The related research work was focused on developing novel small molecular donor materials,11-14

using new device

structures,15-17 optimizing fabrication processing techniques,18-20 and incorporating effective interlayers.21 In SM-OSCs, donor material innovation is considered as the key point to obtain high-performance devices. Oligothiophene is a simple but effective system in organic photovoltaic small molecules, and has been widely employed in SM-OSCs because of their high polarizability, good carrier mobility, and tunable optical and electrochemical properties.3,22-24 In the past few years, Chen et al. reported a series of small molecules containing an oligothiophene backbone with different end groups, such as



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dicyanovinyl,24

alkyl

cyanoacetate,25

3-ethylrhodanine,26

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and

2-(1,1-dicyanomethylene)rhodanine,3 which exhibited with maximum PCEs from 3.7% to 10.08%. The significant progress in this molecular system could be comparable to those obtained in OSCs based on conjugated polymers, implying its promising future in practical applications. The previous work also demonstrated that the central building block played a crual effect on the enhancement of photovoltaic properties of oligothiophene-based molecules.3,22-34 Thus, some rigid aromatic units were then incorporated as the central core to elevate the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and overall device performance, such as the thieno[3,2-b]thiophene

(TT),27

cyclopenta[2,1-b;3,4-b’]-dithiophene

(DTC),28

dithieno[3,2-b:2’,3’-d]silole (DTS),28 dithieno[3,2-b;6,7-b]carbazole (DTCz),29 and benzo[1,2-b:4,5-b’]dithiophene (BDT)30-34. However, the synthesis precedures of these aromatic units are relatively complex, which will increase the cost of the whole device fabrication. It is still a challenge to develop effectively photovoltaic oligothiophenes with simple structures for high-performance SM-OSCs. In this work, two new small molecules based on the simple structure of oligothiophene with 3-ethylrhodanine end groups, TTH-D3TRh and TTF-D3TRh (Figure 1), were designed and synthesized for highly efficient SM-OSCs. Bare 2,2’-bithiophene (BT) or fluorinated 2,2’-bithiophene (FBT) were employed here as the central cores, which showed a rigid skeleton compared with thiophene or alkyl-substituted thiophene units. The rigid nature would induce better molecular packing with improved backbone aggregations. On the other side, fluorine atoms were


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introduced on donor and/or acceptor skeletons because they had some advantages, which had been verified in polymeric donor materials in P-OSCs.35-41 For example, the fluorination would induce a more rigid and planar conjugated structure via the supramolecular forces without increasing deleterious steric effects. It would also induce a lower-lying HOMO energy level and higher carrier mobility to elevate the Voc, Jsc and FF of the related solar cell devices. As expected, these two molecules indeed showed strong solar absorptions, low-lying HOMO levels, high crystallinity, and good film forming property from solution process, especially for TTF-D3TRh with a fluorinated 2,2’-bithiophene central core. Using the simple spin-coating fabrication method, the PCEs of 5.00% and 5.80% were achieved easily for TTH-D3TRh- and TTF-D3TRh-based conventional devices. The TTF-D3TRh device exhibited a higher efficiency, which was attributed to the improved Voc of 0.92 V, Jsc of 10.04 mA cm-2, and FF of 62.8% induced by the fluorination on the 2,2’-bithiophene central core. After employing an inverted device structure, TTH-D3TRh- and TTF-D3TRh-based OSCs showed a largely elevated PCEs of 5.89% and 7.14%, respectively.

2. Results and discussion 2.1 Molecular design, synthesis and characterization The synthetic routes of TTH-D3TRh and TTF-D3TRh are depicted in Scheme 1. The two important intermediate compounds of 6 and 7 were synthesized from compound 3 and compound 4 or 5 by the Stille coupling reaction. TTH-D3TRh and TTF-D3TRh were finally synthesized from the corresponding aldehyde compounds 6 or 7 with



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3-octyl-rodanine via Knoevenagel condensation reactions. The detailed synthesis procedures and characterization data are provided in the experimental section. TTH-D3TRh and TTF-D3TRh exhibited an excellent film forming property and good solubility in general organic solvents, such as tetrahydrofura (THF), chloroform (CF), chlorobenzene (CB), and dichlorobenzene (DCB). The thermal stability of TTH-D3TRh and TTF-D3TRh were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) thermograms. As shown in Figure 2a, the degradation temperatures (Td, 5% weight loss) of TTH-D3TRh and TTF-D3TRh were measured to be about 387 oC and 382 oC in nitrogen atmosphere, respectively. The results showed that the fluorination would not improve the thermal stability of the BT-based molecules. Such good thermal properties for TTH-D3TRh and TTF-D3TRh are enough for avoiding the degradation of the active layer and device deformation in SM-OSCs. From the Figure 2b, TTH-D3TRh exhibited a melting temperature (Tm) and crystallization temperature (Tc) of 148 oC and 104 oC. After introduction of fluorine atoms on BT skeleton, TTF-D3TRh exhibited a higher Tm of 165 oC and a higher Tc of 133 oC. Clear Tms on the heating process and Tcs on the cooling process indicated that TTH-D3TRh and TTF-D3TRh had a tendency to crystallize. Obviously, the fluorination would increase this tendency, which might be helpful for the molecular stacking in the solid state, generating the relatively improved carrier mobility.42 2.2 Optical absorption and electrochemical property Solution and solid film absorption spectra of TTH-D3TRh and TTF-D3TRh are


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measured and provided in Figure 3. The detailed absorption data are also provided in Table 1. As shown in Figure 3a, both molecules showed the similar and strong absorption in the wavelength region of 300-700 nm. The fluorinated TTF-D3TRh exhibited a little blue-shifted absorption which was also observed in the previously reported literature.43,44 The reason is that the incorporated fluorine atoms possess the strong electron-withdrawing property, which limited the π-delocalization of the molecular system.43,44 However, TTF-D3TRh exhibited a larger extinction coefficient (ε) of 1.0×105 M-1 cm-1 than that (ε = 9.8×104 M-1 cm-1) of TTH-D3TRh. In the solid state, both molecules exhibit an apparent red-shifted and broadened absorption profile compared with their solution counterparts (Figure 3b). It is interesting that TTH-D3TRh and TTF-D3TRh showed clear vibronic shoulder peaks at about 646 nm, which was attributed to an enhanced π-π packing between the conjugated backbones. The stronger shoulder absorption was also observed for TTF-D3TRh, which would increase the whole absorption of TTF-D3TRh compared with TTH-D3TRh. The introduction of fluorine atoms would be responsible for this change because it would enhance the intermolecular interactions and the closed π-π stacking property via the supramolecular forces induced by F-H, F-S interactions and so on. The fluorination would also increase the molecular planarity (vide infra) of TTF-D3TRh, which led to the improved absorption from its solution to solid state.43,44 By determination of the absorption edges of the thin films, the optical band gaps (EgOPT) were determined to be 1.74 and 1.73 eV for TTH-D3TRh and TTF-D3TRh, respectively. The energy levels of the donor materials are important for the selection of the



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suitful acceptors in SM-OSCs. Thus, cyclic voltammetry (CV) measurements were carried out in an anhydrous CH3CN solution of tetrabutylammonium perchlorate (n-Bu4NClO4) (0.1 M) by using a three-electrode cell. A platinum plate covered with a thin film of the resulting molecule was used as the working electrodes. A platinum wire and Ag/AgCl (0.1 M) were employed as the counter and reference electrodes, respectively. The reference electrode (Ag/AgCl) was calibrated to be 4.40 eV by the Fc/Fc+ system according to the previous way.45-48 As shown in Figure 4a, the HOMO levels were measured to be 5.12 and 5.28 eV for TTH-D3TRh and TTF-D3TRh, respectively. The introduced fluorine atoms can reduce the HOMO level of TTF-D3TRh effectively, which is expected to get a higher Voc from its solar cell devices fabricated later. From the onset potentials of the reduction process, LUMO levels were also calculated to be about -2.68 and -2.80 eV for TTH-D3TRh and TTF-D3TRh, respectively. Thus, the band gaps (EgCV) of TTH-D3TRh and TTF-D3TRh were determined to be about 2.50 and 2.48 eV. The changing trends agree well with those obtained by the optical way. As shown in Figure 4b, the LUMO gaps of 1.20-1.48 eV between the small molecules and PC71BM would provide an enough driving force to promote the efficient exciton dissociation at the donor and acceptor interface, which would thus improve the electron transfer for overcoming the related binding energy of the intrachain exciton.49 The corresponding electrochemical data of TTH-D3TRh and TTF-D3TRh are also summarized in Table 1. 2.3 Theoretical calculations In order to further understand the fluorination effect on the molecular configurations


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and electronic properties of TTH-D3TRh and TTF-D3TRh, density functional theory (DFT) simulations were carried out to evaluate the stationary points as stable states for optimizing the molecular conformations and single-point energies by using the method of B3LYP/6-31G(d).50 The wave functions (HOMO and LUMO) of TTH-D3TRh and TTF-D3TRh are shown in Figure 5. The results showed that the electron density of LUMO was most localized on the sides of the end groups of rhodanine. However, the wave function of HOMO exhibited the main delocalization along the electron-rich skeleton of the oligothiophene backbone. In comparison with the HOMOs, HOMO-1 orbitals varied from the middle section to the ending sections, which indicated that internal charge transfer would be occurred in this molecular system. For the fluorinated TTF-D3TRh, the tangency became more evident. A similar change was also found from the LUMO to LUMO+1. The whole DFT results implied that the effective charge-transfer process would be possible between the oligothiophene backbone and rhodanine groups. The HOMO and LUMO levels of TTH-D3TRh and TTF-D3TRh were determined to be -5.00/-2.75 eV and -5.04/-2.80 eV, respectively. The band gaps were then calculated to be 2.25 and 2.16 eV. The change tangency is consistent with those obtained from UV-vis and CV measurements. Furthermore, the dihedral angles of the two thiophene rings of the central BT (TTH-D3TRh) or fluorinated BT (TTF-D3TRh) units were calculated to be 11.27o and 0.52o, respectively. The results indicated that the incorporation of fluorine atoms would effectively increase the planarity of TTF-D3TRh, which could be helpful for the enhancement of the molecular packing and thereby the elevation of the carrier



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mobility. 2.4 Crystallinity and hole mobility The molecular packing properties of TTH-D3TRh and TTF-D3TRh in the solid state, were evaluated by XRD measurements. Figure 6 shows the XRD patterns of the prepared small molecule films spin-coated from their choroform (CF) solutions. As show in Figure 6a, TTH-D3TRh and TTF-D3TRh showed the first peaks at 2θ=5.15° and 2θ=5.25°, which were attributed to the (100) diffraction from a lamellar packing structure.18,30,51 The d100-spacings (d1s) were calculated to be about 17.14 Å and 16.81 Å, respectively. TTF-D3TRh showed a relatively stronger and sharper peak, implying its higher crystallinity than TTH-D3TRh. The smaller d1 value of TTF-D3TRh may be attributed to the introduction of fluorine atoms, which promotes the lamellar stacking of the whole molecule. Another weak and broad (010) peaks appeared at 2θ=23.40° and 2θ=24.60° for TTH-D3TRh and TTF-D3TRh, were originated from the π-π stacking nature. The corresponding π-π stacking distances (dπ) between the coplanar small molecular skeletons were then calculated to be 3.80 Å and 3.61 Å for TTH-D3TRh

and

TTF-D3TRh,

respectively.

Compared

with

TTH-D3TRh,

TTF-D3TRh showed a shorter dπ value, which indicated that the introduced fluorine atoms effectively enhanced the π-π stacking property of TTF-D3TRh. Obviously, the fluorination made the oligothiophene backbone of TTF-D3TRh more planar (as discussed in theoretical calculations section). On the other side, the generating F-H, F-S and other weak supramolecular interactions induced from the introduced fluorine atoms also prompted the oligothiophene backbones to approach much closer. Thus,


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the stronger π-π stacking property of TTF-D3TRh would be expected to achieve the higher carrier mobility. In order to make a deeper insight of the crystallinity of the active blends based on these

two

small

molecules,

thin

films

of

TTH-D3TRh:PC71BM

and

TTF-D3TRh:PC71BM were also investigated by XRD measurements. As shown in Figure 6, TTH-D3TRh:PC71BM and TTF-D3TRh:PC71BM films exhibited a reduced (100) diffraction intensity with the angles at 2θ = 4.84° and 2θ = 4.90°, corresponding to the d1 values of 18.23 Å and 18.01 Å, respectively. The increased d1 values were mainly due to the introduction of PC71BM, which limited the aggregation of the molecular donors. Furthermore, TTF-D3TRh:PC71BM blend film still had an obvious (200) diffraction peak at 2θ = 9.83°, as could be found in the pristine TTF-D3TRh film, indicating its well-ordered molecular stacking even with the incorporation of PC71BM. For the π-π stacking property, TTF-D3TRh:PC71BM also showed the stronger packing structure than that of TTH-D3TRh:PC71BM, which was similar to their pristine films. The results indicated the TTF-D3TRh:PC71BM blend would be expected to achieve the relatively higher carrier mobility in the solar cell devices. The hole mobility of the blend films commonly plays a positive role on the Jsc and FF of the related SM-OSCs. Higher hole mobility is considered as the good issue for improved carrier transport without a large photocurrent loss derived from recombination of charges.52,53 Here, the space charge limited current (SCLC) method was employed to evaluate the vertical carrier mobility. The hole-only devices were then fabricated and investigated with a configuration of indium tin oxide



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(ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT):polystyrene sulfonate (PSS)/small molecule:PC71BM/Au. The hole mobility could be determined by the Mott-Gurney equation.52,53 From Figure 6b, the hole mobilities of TTH-D3TRh and TTF-D3TRh devices are calculated to be 5.21×10-4 cm2 V-1 s-1 and 7.24×10-4 cm2 V-1 s-1, respectively. As expected, TTF-D3TRh device exhibits the higher mobility, which can be attributed to its better crystallinity as well as molecular stacking nature of TTF-D3TRh as mentioned above. Obviously, the improved hole mobility would be helpful for the elevation of Jsc and FF parameters of the related SM-OSCs. 2.5 Photovoltaic properties To evaluate the photovoltaic performance of TTH-D3TRh and TTF-D3TRh, the solar cell

devices

were

first

fabricated

with

a

conventional

structure

of

ITO/PEDOT:PSS/small molecule:PC71BM/Ca/Al, and tested under the illumination of AM 1.5G, 100 mW cm-2. The fabrication conditions were optimized by varying the blend weight ratio, active layer thicknesses, and processing additives. The active layers were finally spin-coated from the chloroform solutions of the small molecule and PC71BM with an optimal weight ratio of 1:0.8 (w/w). 1% 1,8-diiodooctane (DIO) was added into the solution as the processing additive to control the surface morphology of the active layer. The J-V curves and external quantum efficiency (EQE) curves of the best TTH-D3TRh and TTF-D3TRh devices are provided in Figure 7a and 7b. The corresponding photovoltaic data are also summarized in table 2. As shown in Figure 7a, TTH-D3TRh device exhibited a PCE of 5.00%, with a Voc of 0.86 V, a Jsc of 9.35 mA cm-2 and an FF of 62.4%. When using TTF-D3TRh as the


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donor material, the device showed a higher PCE of 5.80%, with a Voc of 0.92 V, a Jsc of 10.04 mA cm-2 and an FF of 62.8%. Clearly, the better efficiency of TTF-D3TRh device was attributed to the improved Voc and Jsc, while the FF was almost kept the same value. Since the Voc is related to the energy offset between the HOMO level of the donor and the LUMO level of the acceptor,54,55 the obtained higher Voc of 0.92 V should originate from the lower-lying HOMO levels of TTF-D3TRh as determined in the CV experiments. The increased Jsc could be confirmed by EQE evaluations. As shown in Figure 7b, the EQE response of TTH-D3TRh and TTF-D3TRh devices exhibited a broad profile ranging from 300 to 750 nm, which was contributed to the absorptions of both the small molecular donor and the PC71BM acceptor. The maximum EQE values reached about 53% and 57% for the TTH-D3TRh and TTF-D3TRh devices, respectively. Apparently, the EQE results agreed well with the measured Jsc values from J-V evaluations. To obtain deeper insight into the effect of fluorination on the photovoltaic property of the designed small molecules, the dark J-V characteristics curves and the photocurrent density versus effective voltage (Jph-Veff) curves of the SM-OSCs were also investigated. The related curves are plotted in Figure 8a and 8b. Compared to the TTH-D3TRh device (Figure 8a), the TTF-D3TRh device exhibited a higher rectification ratio and a smaller leakage current, which implied the suppressed charge recombination and effective charge extraction. Figure 8b indicated that Jphs of both devices had a relatively linear dependence on the voltage at a low value of Veff, which reached saturation when the effective voltage Veffs increased. Thus, TTF-D3TRh



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device exhibited a higher Jph under the same Veff, suggesting a less bimolecular recombination, and much better interfacial contact than those of TTH-D3TRh device.13,56 The morphology of the active layers with fine nanophase separation is positive for highly efficient SM-OSCs. The surface morphologies of the TTH-D3TRh:PC71BM and TTF-D3TRh:PC71BM films were studied by atomic force microscopy (AFM) measurement. As shown in Figure 9, the TTF-D3TRh:PC71BM blend exhibited a more homogenous morphology with the relatively smaller domain size than that observed from the TTH-D3TRh:PC71BM film. The average root mean square (RMS) roughness

values of

TTF-D3TRh:PC71BM and

TTH-D3TRh:PC71BM were

determined to be 8.57 nm and 11.32 nm. The larger aggregation feature also indicated that the worse miscibility of TTH-D3TRh and PC71BM, which would obstruct the carrier transport and thus induce lower Jsc and FF values. Apparently, the fluorination of the central BT unit would be responsible for the elevations of Voc, Jsc, FF and the overall device performance. In comparison with the conventional devices, inverted OSCs should exhibit a higher efficiency and longer-term ambient stability. The reason is that a stable metal electrode with high work-function is employed in the inverted devices and the stable anode buffer layers is used to instead the acidic PEDOT:PSS layer.57,58 Therefore, the inverted

OSCs

were

fabricated

and

characterized

with

a

structure

of

ITO/ZnO/PNFBr/small molecule:PC71BM/MoO3/Ag. Here, ZnO and MoO3 were used as the electron-transport and hole-transport layers, respectively. To balance the


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energy barriers and improve the interfacial contact, a thin layer of PNFBr was spin-coated on the surface of ZnO layer.57,58 As shown in Figure 7a, both inverted devices showed a significantly elevated PCE values of 5.89% and 7.14%. The raised device performances could be due to the increase of Jsc and FF simultaneously, although the Voc was kept the same level in compared with their conventional devices. The results indicated that the Jsc values were elevated from 9.35 and 10.04 mA cm-2 to 10.26 and 11.03 mA cm-2 for TTH-D3TRh and TTF-D3TRh, respectively, which was also confirmed by the enhanced EQE response (Figure 7b). The integrated Jsc values from the EQE curves were in accordance with those obtained from J-V evaluations within 5% error. At last, the highest PCE more than 7.00% was obtained for the TTF-D3TRh device, which implied its potential application in OSCs as a small molecular donor. The significantly increased FFs should result from the enhanced interfacial contact between the active layer and ITO electrode. This would boost an efficient electron collection and suppress the opposite charge recombination.59,60

Conclusions In summary, we designed and synthesized two photovoltaic small molecular donors, TTH-D3TRh and TTF-D3TRh, with a simple oligothiophene skeleton, rigid 2,2’-bithiophene (BT) central core and 3-ethylrhodanine end groups. TTH-D3TRh and TTF-D3TRh exhibit strong and broad absorptions. The fluorination of BT central unit made TTF-D3TRh possess a lower-lying HOMO level, better molecular stacking and higher mobility in comparison with TTH-D3TRh. Using the simple spin-coating fabrication approach, the high PCEs of 5.00% and 5.80% were achieved for



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TTH-D3TRh- and TTF-D3TRh-based SM-OSCs without extra post-treatments. As expected, TTF-D3TRh device showed the significant elevations of Voc and Jsc, which could be attributed to above fluorination effect. After employing an inverted device structure, the OSCs exhibited largely elevated PCEs of 5.89% and 7.14% for TTH-D3TRh and TTF-D3TRh, respectively. The results indicated that these simple BT-based small molecules still are promising donor candidates for obtaining highly efficient SM-OSCs, and the improved photovoltaic property will be significantly elevated through just a little variation of the molecular configuration with a rigid fluorinated BT central unit.

3. Experimental section 3.1 Materials All the chemicals were purchased from Adamas, Alfa Asear, Aladdin, and Aldrich Chemical Co., which were used in this work without purification. All solvents for reactions were distilled first before usage. Potassium 4-oxo-2-thioxothiazolidin-3-ide (1),

3-octyl-rodanine

[2,2':5',2''-terthiophene]-5-carbaldehyde 2,2’-bithiophene

(4),

(2), (3),

5''-bromo-3,3''-dioctyl5,5’-bis(tributylstannyl)-

5,5’-bis(tributylstannyl)-3,3’-difluoro-2,2’-bithiophene

(5)

were synthesized according to the reported literatures.15,61-64 Compound 6. Compound 3 (0.51 g, 0.88 mmol) and compound 4 (0.30 g, 0.40 mmol) were first dissolved in dry toluene (10 mL). After the solution was degassed with argon gas for several minutes, Pd(PPh3)4 (50 mg, 0.26 mmol) was added immediately. The reaction mixture was stirred at 100 °C for 24 h under argon protection, and then


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poured into water (50 mL). After extracting with CH2Cl2, the combined organic layer was dried over anhydrous MgSO4. The solvent was removed by evaporation to afford the crude product. Finally, the pure compound 6 was obtained by column chromatography separation (silica gel; dichloromethane:petroleum ether = 1:1) as a red solid (0.31g, 67.39%). 1H NMR (400 MHz, CDCl3, δ/ppm): 9.83 (s, 2H, CHO), 7.60 (s, 2H, ArH), 7.25 (d, 2H, J=4.44, ArH), 7.12 (d, 2H, J=3.60, ArH), 7.08 (s, 4H, ArH), 7.03 (s, 2H, ArH), 2.80 (m, 8H, CH2), 1.69 (br, 8H, CH2), 1.33 ( m, 54H, CH2), 0.87 (t, 24H, J=6.56, CH3)

13

C NMR (100 MHz, CDCl3, δ/ppm): 182.60, 155.70,

153.05, 141.20, 141.00, 140.42, 140.22, 139.14, 137.77, 135.25, 134.86, 133.89, 130.58, 128.64, 127.82, 126.19, 118.65, 118.14, 117.79, 99.95, 40.93, 32.54, 31.91, 30.63, 30.32, 29.66, 29.50, 29.31, 28.92, 25.96, 23.05, 22.66. Anal. calcd (%) for C66H82O2S8: C, 68.11; H, 7.10; S, 22.04; found: C, 68.21; H, 7.15; S, 22.13. MS (MALDI-TOF-MS) m/z: calcd for C66H82O2S8 [M]+, 1162.4080; found,1162.4070 Compound 7. Compound 7 was synthesized from compound 3 and compound 5 according to a similar procedure described for compound 6 with a yield of 64.70%. 1H NMR (400 MHz, CDCl3, δ/ppm): 9.86 (s, 2H, CHO),7.61 (s, 2H, ArH), 7.25 (s, 2H, ArH), 7.13 (s, 2H, ArH), 7.08 (br, 4H, ArH), 2.84 (m, 8H, CH2), 1.72 (br, 8H, CH2), 1.42 (m, 54H, CH2), 0.91 (m, 24H, CH3).

13

C NMR (100 MHz, CDCl3, δ/ppm):

182.47, 154.49, 151.87 141.16, 140.85, 140.39, 140.28, 139.02, 137.62, 134.84, 134.53, 130.06, 127.80, 126.83, 126.26, 126.08, 113.11, 77.35, 77.03, 76.71, 31.90, 31.89, 30.46, 30.30, 29.65, 29.55, 29.46, 29.44, 29.30, 29.28, 22.69, 14.12. Anal. calcd (%) for C66H80F2O2S8: C, 66.07; H, 6.72; S, 21.38; found: C, 66.11; H, 6.74, S,



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

21.56. MS (MALDI-FTICR) m/z: calcd for C66H80F2O2S8 [M]+, 1198.3890; found, 1198.3870. TTH-D3TRh. Compound 6 (0.31 g, 0.27 mmol), three drops of piperidine and compound 2 (0.65 g, 2.7 mmol) were first dissolved in dry CHCl3 (35 mL) under argon protection. The reaction mixture was refluxed for 24h and cooled to room temperature subsequently. After extracting with CHCl3 for several times, the combined organic layer was dried over anhydrous MgSO4. The crude product was obtained after evaporation of the solvent. TTH-D3TRh was finally purified by column chromatography separation (silica gel; dichloromethane:petroleum ether = 1:1) as a black solid (0.32 g, 74.0%). 1H NMR (400MHz, CDCl3, δ/ppm): 7.75 (s, 2H, ArH), 7.21 (s, 2H, ArH), 7.10 (d, 2H, J=4.44, ArH), 7.05 (d, 2H, J=4.76, ArH), 4.09 (t, 4H, J=7.86, CH2), 2.79 (s, 4H, CH2), 1.69 (m, 8H, CH2), 1.35 (br, 10H, CH2), 0.88 (br, 62H, CH2), 0.81 (m, 24H, CH3). 13C NMR (100 MHz, CDCl3, δ/ppm): 192.42, 167.64, 141.04, 139.55, 137.63, 137.38, 136.02, 135.89, 135.18, 134.60, 129.23, 127.27, 126.78, 126.07, 124.89, 124.46, 124.37, 120.47, 77.36, 77.04, 76.72, 31.93, 31.80, 30.48, 30.31, 29.68, 29.63, 29.52, 29.47, 29.33, 29.15, 27.02, 26.83, 22.72, 22.68, 14.17, 14.12. Anal. calcd for C88H116N2O2S12: C, 65.30; H, 7.22; N, 1.73; S, 23.77; found: C, 65.40; H, 7.32; N, 1.71; S, 23.65. MS (MALDI-FTICR) m/z: calcd for C88H116N2O2S12 [M]+, 1616.5685 ; found 1616.5680. TTF-D3TRh. TTF-D3TRh was synthesized as a black solid from compound 7 and compound 2 according to a similar procedure described for TTH-D3TRh with a yield of 77.0%. 1H NMR (400MHz, CDCl3, δ/ppm): 7.65 (s, 2H, ArH), 7.41 (s, 2H, ArH),


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7.14 (d, 4H, J=0.0078, ArH), 7.00 (d, 4H, J=0.05, ArH), 4.15 (m, 4H, CH2), 2.70 (m, 8H, CH2), 1.70 (m, 12H, CH2), 1.33 (br, 60H, CH2), 0.88 (br, 18H, CH3).

13

C NMR

(100 MHz, CDCl3, δ/ppm): 191.67, 167.25, 155.50, 154.35, 152.97, 151.36, 140.30, 138.22, 136.84, 134.54, 131.08, 128.32, 126.47, 125.32, 122.56, 120.49, 117.72, 112.65, 44.76, 40.90, 39.81, 38.86, 32.52, 32.03, 31.77, 30.25, 29.92, 29.71, 29.58, 29.43, 29.15, 28.91, 27.01, 26.83, 25.90, 23.12, 22.70, 14.33, 14.17, 12.32, 10.95. Anal. calcd (%) for C88H114F2N2O2S12: C, 57.22; H, 6.22; N, 1.52; S, 31.25; found: C, 57.32; H, 6.35; N, 1.42; S, 31.34. MS (MALDI-FTICR) m/z: calcd for C88H114F2N2O2S12 [M]+, 1652.5697; found 1652.5497 3.2 Measurements and Characterization The molecular structures of the intermediate compounds and target compounds were confirmed by NMR spectra on a Bruker ARX 400 spectrometer and elemental analysis on a Carlo Erba 116 elemental analyzer. The thermal, optical, electrochemical and crystallinity properties of the prepared donor materials were measured on a TA Instrument Model SDT Q600 simultaneous TGA/DSC analyzer, Cary 300 spectrophotometer, CHI660 potentiostat/galvanostat electrochemical workstation, Bruker Inova atomic microscope and Philips X-ray diffractometer, respectively. The test conditions and procedures are referred to our previously reported literatures.47,48,50, 58

3.3 Device fabrication and evaluations The detailed device fabrication procedures and characterization methods of conventional SM-OSCs, inverted SM-OSCs and hole-only devices in this work were



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according to our previously reported literatures.47,48,50, 58

Acknowledgments This work was financially supported by the NSFC (51573107, 21432005), the Youth Science and Technology Foundation of Sichuan Province (2013JQ0032), the Foundation

of

State

Key

Laboratory

of

Polymer

Materials

Engineering

(sklpme2014-3-05, sklpme2015-2-01), the Synergistic Innovation Joint Foundation of CAEP-SCU (XTCX2014008), and the Fundamental Research Funds for the Central Universities (2012SCU04B01, YJ2011025).

References (1) Service, R. F. Outlook Brightens for Plastic Solar Cells. Science 2011, 332, 293. (2) He, Z. C.; Zhong, C. M.; Huang, X.; Wong, W. Y.; Wu, H. B.; Chen, L. W.; Su, S. J.; Cao, Y. Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells. Adv. Mater. 2011, 23, 4636-4643. (3) Kan, B.; Li, M. M.; Zhang, Q.; Liu, F.; Wan, X. J.; Wang, Y. C.; Ni, W.; Long, G. K.; Yang, X.; Feng, H. R.; Zuo, Y.; Zhang, M. T.; Huang, F.; Cao, Y.; Russell, T. P.; Chen, Y. S. A Series of Simple Oligomer-like Small Molecules Based on Oligothiophenes for Solution-Processed Solar Cells with High Efficiency. J. Am. Chem. Soc. 2015, 137, 3886-3893. (4) Vohra, V.; Kawashima, K.; Kakara, T.; Koganezawa, T.; Osaka, I.; Takimiya, K.; Murata, H. Efficient Inverted Polymer Solar Cells Employing Favourable Molecular Orientation. Nat. Photonics 2015, 9, 403-408. (5) Chen, J. D.; Cui, C. H.; Li, Y. Q.; Zhou, L.; Ou, Q. D.; Li, C.; Li, Y. F.; Tang, J. X.


Page 21 of 45

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


Single-Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Adv. Mater. 2015, 27, 1035-1041. (6) Liu, C.; Yi, C.; Wang, K.; Yang, Y. L.; Bhatta, R. S.; Tsige, M.; Xiao, S. Y.; Gong, X. Single-Junction Polymer Solar Cells with Over 10% Efficiency by a Novel Two-Dimensional Donor-Acceptor Conjugated Copolymer. ACS Appl. Mater. Interfaces 2015, 7, 4928-4935. (7) Yu, W.; Huang, L.; Yang, D.; Fu, P.; Zhou, L.; Zhang, J.; Li, C. Efficiency Exceeding 10% for Inverted Polymer Solar Cells with a ZnO/Ionic Liquid Combined Cathode Interfacial Layer. J. Mater. Chem. A 2015, 3, 10660-10665. (8) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293/1-8. (9) Lin, Y. Z.; Li, Y. F.; Zhan, X. W. Small Molecule Semiconductors for High-efficiency Organic Photovoltaics. Chem. Soc. Rev. 2012, 41, 4245-4272. (10) Chen, Y. S.; Wan, X. J.; Long, G. K. High Performance Photovoltaic Applications Using Solution-Processed Small Molecules. Acc. Chem. Res. 2013, 46, 2645. (11) Liu, Y. S.; Wan, X. J.; Wang, F.; Zhou, J. Y.; Long, G. K.; Tian, J. G.; Chen, Y. S. High-Performance Solar Cells Using a Solution-Processed Small Molecule Containing Benzodithiophene Unit. Adv. Mater. 2011, 23, 5387-5391. (12) Zhou, J. Y.; Wan, X. J.; Liu, Y. S.; Zuo, Y.; Li, Z.; He, G. R.; Long, G. K.; Ni, W.; Li,

C.

X.;

Su,

X.

C.;

Chen,

Y.

S.

Small

Molecules

Based

on

Benzo[1,2-b:4,5-b’]dithiophene Unit for High-Performance Solution-Processed



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

Organic Solar Cells. J. Am. Chem. Soc. 2012, 134, 16345-16351. (13) Zhang, Q.; Kan, B.; Liu, F.; Long, G. K.; Wan, X. J.; Chen, X. Q.; Zuo, Y.; Ni, W.; Zhang, H. J.; Li, M. M.; Hu, Z. C.; Huang, F.; Cao, Y.; Liang, Z. Q.; Zhang, M. T.; Russell, T. P.; Chen, Y. S. Small-Molecule Solar Cells with Efficiency Over 9%. Nat. Photonics 2014, 9, 35-41. (14) Cheng, M.; Yang, X. C.; Chen, C.; Tan, Q.; Sun, L. C. Molecular Engineering of Small Molecules Donor Materials Based on Phenoxazine Core Unit for Solution-Processed Organic Solar Cells. J. Mater. Chem. A 2014, 2, 10465-10469. (15) Liu, Y. S.; Chen, C. C.; Hong, Z. R.; Gao, J.; Yang, Y.; Zhou, H. P.; Dou, L. T.; Li, G.; Yang, Y. Solution-Processed Small-Molecule Solar Cells: Breaking the 10% Power Conversion Efficiency. Sci. Rep. 2013, 3, 3356/1-8. (16) Riede, M.; Uhrich, C.; Widmer, J.; Timmreck, R.; Wynands, D.; Schwartz, G.; Gnehr, W. M.; Hildebrandt, D.; Weiss, A.; Hwang, J.; Sundarraj, S.; Erk, P.; Pfeiffer, M.; Leo, K. Efficient Organic Tandem Solar Cells based on Small Molecules. Adv. Funct. Mater. 2011, 21, 3019-3028. (17) Lee, S. H.; Seo, J. W.; Lee, J. Y. Stable Inverted Small Molecular Organic Solar Cells Using a p-Doped Optical Spacer. Nanoscale 2015, 7, 157-165. (18) Lin, Y. Z.; Ma, L. C.; Li, Y. F.; Liu, Y. Q.; Zhu, D. B.; Zhan, X. W. Small-Molecule Solar Cells with Fill Factors up to 0.75 via a Layer-by-Layer Solution Process. Adv. Energy Mater. 2014, 4, 1300626/1-6. (19) Montcada, N. F.; Pelado, B.; Viterisi, A.; Albero, J.; Coro, J.; d. l. Cruz, P.; Langa, F.; Palomares, E. High Open Circuit Voltage in Efficient Thiophene-Based


Page 22 of 45

Page 23 of 45

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


Small Molecule Solution Processed Organic Solar Cells. Org. Electron. 2013, 14, 2826-2832. (20) Mukherjee, S.; Proctor, C. M.; Tumbleston, J. R.; Bazan, G. C.; Nguyen, T. Q.; Ade, H. Importance of Domain Purity and Molecular Packing in Efficient Solution-Processed Small-Molecule Solar Cells. Adv. Mater. 2015, 27, 1105-1111. (21) Min, J.; Zhang, H.; Stubhan, T.; Luponosov, Y. N.; Kraft, M.; Ponomarenko, S. A.; Ameri, T.; Scherf, U.; Brabec, C. J. A Combination of Al-Doped ZnO and a Conjugated Polyelectrolyte Interlayer for Small Molecule Solution-Processed Solar Cells with an Inverted Structure. J. Mater. Chem. A 2013, 1, 11306-11311. (22) Mishra, A.; Bäuerle, P. Small Molecule Organic Semiconductors on the Move: Promises for Future Solar Energy Technology. Angew. Chem. Int. Ed. 2012, 51, 2020-2067. (23) Long, G. K.; Wan, X. J.; Kan, B.; Liu, Y. S.; He, G. R.; Li, Z.; Zhang, Y. W.; Zhang, Y.; Zhang, Q.; Zhang, M. M.; Chen, Y. S. Investigation of Quinquethiophene Derivatives with Different End Groups for High Open Circuit Voltage Solar Cells. Adv. Energy Mater. 2013, 3, 639-646. (24) Liu, Y. S.; Wan, X. J.; Yin, B.; Zhou, J. Y.; Long, G. K.; Yin, S. G.; Chen, Y. S. Efficient

Solution

Processed

Bulk-Heterojunction

Solar

Cells

Based

a

Donor-Acceptor Oligothiophene. J. Mater. Chem. 2010, 20, 2464-2468. (25) Liu, Y. S.; Wan, X. J.; Wang, F.; Zhou, J. Y.; Long, G. K.; Tian, J. G.; You, J. B.; Yang, Y.; Chen, Y. S. Spin-Coated Small Molecules for High Performance Solar Cells. Adv. Energy Mater. 2011, 1, 771-775.



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

Page 24 of 45

(26) Li, Z.; He, G. R.; Wan, X. J.; Liu, Y. S.; Zhou, J. Y.; Long, G. K.; Zuo, Y.; Zhang, M. M.; Chen, Y. S. Solution Processable Rhodanine-Based Small Molecule Organic Photovoltaic Cells with a Power Conversion Efficiency of 6.1%. Adv. Energy. Mater. 2012, 2, 74-77. (27) Zhang, Q.; Wang, Y. C.; Kan, B.; Wan, X. J.; Liu, F.; Ni, W.; Feng, H. R.; Russellbc, T. P.; Chen, Y. S. A Solution-Processed High Performance Organic Solar Cell Using a Small Molecule with the Thieno[3,2-b]thiophene Central Unit. Chem. Commun. 2015, 51, 15268-15271. (28) Ni, W.; Li, M. M.; Liu, F.; Wan, X. J.; Feng, H. R.; Kan, B.; Zhang, Q.; Zhang, H. T.; Chen, Y. S. Dithienosilole-Based Small-Molecule Organic Solar Cells with an Efficiency over 8%: Investigation of the Relationship between the Molecular Structure and Photovoltaic Performance. Chem. Mater. 2015, 27, 6077-6084. (29) Feng, H. R.; Li, M. M.; Ni, W.; Liu, F.; Wan, X. J.; Kan, B.; Wang, Y. C.; Zhang, Y. M.; Zhang, Q.; Zuo, Y.; Yang, X.; Chen, Y. S. Investigation of the Effect of Large Aromatic Fusion in the Small Molecule Backbone on the Solar Cell Device Fill Factor. J. Mater. Chem. A 2015, 3, 16679-16687. (30) Liu, Y. S.; Wan, X. J.; Wang, F.; Zhou, J. Y.; Long, G. K.; Tian, J. G.; Chen, Y. S. High-Performance Solar Cells using a Solution-Processed Small Molecule Containing Benzodithiophene Unit. Adv. Mater. 2011, 23, 5387-5391. (31) Zhou, J. Y.; Wan, X. J.; Liu, Y. S.; Zuo, Y.; Li, Z.; He, G. R.; Long, G. K.; Ni, W.; Li,

C.

X.;

Su,

X.

C.;

Chen,

Y.

S.

Small

Molecules

Based

on

Benzo[1,2-b:4,5-b’]dithiophene Unit for High-Performance Solution-Processed


Page 25 of 45

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


Organic Solar Cells. J. Am. Chem. Soc. 2012, 134, 16345-16351. (32) Zhou, J. Y.; Zuo, Y.; Wan, X. J.; Long, G. K.; Zhang, Q.; Ni, W.; Liu, Y. S.; Li, Z.; He, G. R.; Li, C. X.; Kan, B.; Li, M. M.; Chen, Y. S. Solution-Processed and High-Performance

Organic

Solar

Cells

Using

Small

Molecules

with

a

Benzodithiophene Unit. J. Am. Chem. Soc. 2013, 135, 8484-8487. (33) Li, M. M.; Liu, F.; Wan, X. J.; Ni, W.; Kan, B.; Feng, H. R.; Zhang, Q.; Yang, X.; Wang, Y. C.; Zhang, Y. M.; Shen, Y.; Russell, T. P.; Chen, Y. S. Subtle Balance Between Length Scale of Phase Separation and Domain Purification in Small-Molecule Bulk-Heterojunction Blends under Solvent Vapor Treatment. Adv. Mater. 2015, 27, 6296-6302. (34) Cui, C. H.; Guo, X.; Min, J.; Guo, B.; Cheng, X.; Zhang, M. J.; Brabec, C. J.; Li, Y. F. High-Performance Organic Solar Cells Based on a Small Molecule with Alkylthio-Thienyl-Conjugated Side Chains without Extra Treatments. Adv. Mater. 2015, 27, 7469-7475. (35) Price, S. C.; Stuart, A. C.; Yang, L. Q.; Zhou, H. X.; You, W. Fluorine Substituted Conjugated

Polymer

of

Medium

Band

Gap

Yields

7%

Efficiency

in

Polymer-Fullerene Solar Cells. J. Am. Chem. Soc. 2011, 133, 4625-4631. (36) Peng, Q.; Liu, X. J.; Su, D.; Fu, G. W.; Xu, J.; Dai, L. M. Novel Benzo[1,2-b:4,5-b’]dithiophene-Benzothiadiazole Derivatives with Variable Side Chains for High-Performance Solar Cells. Adv. Mater. 2011, 23, 4554-4558. (37) Li, K.; Li, Z. J.; Feng, K.; Xu, X. P.; Wang, L. Y.; Peng, Q. Development of Large Band-Gap Conjugated Copolymers for Efficient Regular Single and Tandem



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

Organic Solar Cells. J. Am. Chem. Soc. 2013, 135, 13549-13557. (38) Tumbleston, J. R.; Stuart, A. C.; Gann, E.; You, W.; Ade, H. Fluorinated Polymer Yields High Organic Solar Cell Performance for a Wide Range of Morphologies. Adv. Funct. Mater. 2013, 23, 3463-3470. (39) Fei, Z. P.; Shahid, M.; Yaacobi-Gross, N.; Rossbauer, S.; Zhong, H. L.; Watkins, S. E.; Anthopoulos, T. D.; Heeney, M. Thiophene fluorination to enhance photovoltaic performance in low band gap donor-acceptor polymers. Chem. Commun. 2012, 48, 11130-11132. (40) Li, Z. K.; Lin, H. R.; Jiang, K.; Carpenter, J.; Li, Y. K.; Liu, Y. H.; Hu, H. W.; Zhao, J. B.; Ma, W.; Ade, H.; Yan, H. Dramatic performance enhancement for large bandgap thick-film polymer solar cells introduced by a difluorinated donor unit. Nano Energy 2015, 15, 607-615. (41) Yang, Y. C.; Wu, R. M.; Wang, X.; Xu, X. P.; Li, Z. J.; Li, K.; Peng, Q. Isoindigo Fluorination to Enhance Photovoltaic Performance of Donor-Acceptor Conjugated Copolymers. Chem. Commun. 2014, 50, 439-441. (42) Fei, Z. P.; Boufflet, P.; Wood, S.; Wade, J.; Moriarty, J.; Gann, E.; Ratcliff, E. L.; McNeill, C. R.; Sirringhaus, H.; Kim, J. S.; Heeney, M. Influence of Backbone Fluorination in Regioregular Poly(3-alkyl-4-fluoro)thiophenes. J. Am. Chem. Soc. 2015, 137, 6866-6879. (43) Jo, J. W.; Bae, S.; Liu, F.; Russell, T. P.; Jo, W. H. Comparison of Two D-A Type Polymers with Each Being Fluorinated on D and A Unit for High Performance Solar Cells. Adv. Funct. Mater. 2015, 25, 120-125.


Page 26 of 45

Page 27 of 45

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


(44) Schroeder, B. C.; Huang, Z. G.; Ashraf, R. S.; Smith, J.; D’Angelo, P.; Watkins, S. E.; Anthopoulos, T. D.; Durrant, J. R.; McCulloch, I. Silaindacenodithiophene-Based Low Band Gap Polymers-The Effect of Fluorine Substitution on Device Performances and Film Morphologies. Adv. Funct. Mater. 2012, 22, 1663-1670. (45) Li, Y. F.; Cao, Y.; Gao, J.; Wang, D. L.; Yu, G.; Heeger, A. J. Electrochemical Properties of Luminescent Polymers and Polymer Light-Emitting Electrochemical Cells. Synth. Met. 1999, 99, 243-248. (46) Peng, Q.; Lu, Z. Y.; Huang, Y.;

Xie, M. G.;

Han, S. H.; Peng, J. B.; Cao, Y.

Synthesis and Characterization of New Red-Emitting Polyfluorene Derivatives Containing

Electron-Deficient

2-Pyran-4-ylidene-Malononitrile

Moieties.

Macromolecules 2004, 37, 260-266. (47) Xu, X. P.; Wu, Y. L.; Fang, J. F.; Li, Z. J.; Wang, Z. G.; Li, Y.; Peng, Q. Side-Chain

Engineering

of

Benzodithiophene-Fluorinated

Quinoxaline

Low-Band-Gap Co-polymers for High-Performance Polymer Solar Cells. Chem. Eur. J. 2014, 20, 13259-13271. (48) Xu, X. P.; Feng, K.; Li, K.; Peng, Q. Synthesis and Photovoltaic Properties of Two-Dimensional Benzodithiophene-Thiophene Copolymers with Pendent Rational Naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole Side Chains. J. Mater. Chem. A 2015, 3, 23149-23161. (49) Brabec, C. J.; Winder, C.; Sariciftci, N. S.; Hummelen, J. C.; Dhanabalan, A.; van Hal, P. A.; Janssen, R. A. J. A Low-Bandgap Semiconducting Polymer for Photovoltaic Devices and Infrared Emitting Diodes. Adv. Funct. Mater. 2002, 12,



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

Page 28 of 45

709-712. (50) Zeng, Z.; Li, Y.; Deng, J. F.; Huang, Q.; Peng, Q. Synthesis and Photovoltaic Performance of Low Band Gap Copolymers Based on Diketopyrrolopyrrole and Tetrathienoacene with Different Conjugated Bridges. J. Mater. Chem. A 2014, 2, 653-662. (51) Prosa, T. J.; Winokur, M. J. X-ray Structural Studies of Poly(3-alkylthiophenes): An Example of an Inverse Comb. Macromolecules 1992, 25, 4364-4372. (52) Guo, X.; Zhou, N.; Lou, S. J.; Hennek, J. W.; Ponce, O. R.; Butler, M. R.; Boudreault, P. L.; Strzalka, J.; Morin, P. O.; Leclerc, M.; Lopez, N. J. T.; Ratner, M. A.;

Chen,

L.

X.;

Chang,

R.

P.;

Facchetti,

A.;

Marks,

T.

J.

Bithiopheneimide-Dithienosilole/Dithienogermole Copolymers for Efficient Solar Cells: Information from Structure-Property-Device Performance Correlations and Comparison to Thieno[3,4-c]pyrrole-4,6-dione Analogues. J. Am. Chem. Soc. 2012, 134, 18427-18439. (53) Peng, Q.; Lim, S. L.; Wong, I. H.; Xu, J.; Chen, Z. K. Synthesis and Photovoltaic Properties of Two-Dimensional Low-Bandgap Copolymers Based on New Benzothiadiazole Derivatives with Different Conjugated Arylvinylene Side Chains. Chem. Eur. J. 2012, 18, 12140-12151. (54) Peng, Q.; Park, K.; Lin, T.; Durstock, M.; Dai, L. M. Donor-π-Acceptor Conjugated Copolymers for Photovoltaic Applications: Tuning the Open-Circuit Voltage by Adjusting the Donor/Acceptor Ratio. J. Phys. Chem. B 2008, 112, 2801-2808.


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(55) Warnan, J.; Cabanetos, C.; Bude, R.; Labban, A. E.; Li, L.; Beaujuge, P. M. Electron-Deficient N-Alkyloyl Derivatives of Thieno[3,4-c]pyrrole-4,6-dione Yield Efficient Polymer Solar Cells with Open-Circuit Voltages > 1 V. Chem. Mater. 2014, 26, 2829-2835. (56) Proctor, C. M.; Kim, C.; Neher, D.; Nguyen, T. Q. Nongeminate Recombination and Charge Transport Limitations in Diketopyrrolopyrrole-Based Solution-Processed Small Molecule Solar Cells. Adv. Funct. Mater. 2013, 23, 3584-3594. (57) Peng, Q.; Huang, Q.; Hou, X. B.; Chang, P. P.; Xu, J.; Deng, S. J. Enhanced Solar Cell Performance by Replacing Benzodithiophene with Naphthodithiophene in Diketopyrrolopyrrole-Based Copolymers. Chem. Commun. 2012, 48, 11452-11454. (58) Feng, K.; Xu, X. P.; Li, Z. J.; Li, Y.; Li, K.; Yu, T.; Peng, Q. Low Band Gap Benzothiophene-Thienothiophene Copolymers with Conjugated Alkylthiothieyl and Alkoxycarbonyl Cyanovinyl Side Chains for Photovoltaic Applications. Chem. Commun. 2015, 51, 6290-6292. (59) Dong, Y.; Hu, X. W.; Duan, C. H.; Liu, P.; Liu, S. J.; Lan, L. Y.; Chen, D. C.; Ying, L.; Su, S. J.; Gong, X.; Huang, F.; Cao, Y. A Series of New Medium-Bandgap Conjugated Polymers Based on Naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) for High-Performance Polymer Solar Cells. Adv. Mater. 2013, 25, 3683-3688. (60) He, Z. C.; Zhong, C. M.; Su, S. J.; Xu, M.; Wu, H. B.; Cao, Y. Enhanced Power-Conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure. Nat. Photonics 2012, 6, 591-595. (61) Zhou, J. Y.; Wan, X. J.; Liu, Y. S.; Long, G. K.; Wang, F.; Li, Z.; Zuo, Y.; Li, C.



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X.; Chen, Y. S. A Planar Small Molecule with Dithienosilole Core for High Efficiency Solution-Processed Organic Photovoltaic Cells. Chem. Mater. 2011, 23, 4666-4668. (62) Zhang, M. J.; Guo, X.; Zhang, S. Q.; Hou, J. H. Synergistic Effect of Fluorination on Molecular Energy Level Modulation in Highly Efficient Photovoltaic Polymers. Adv. Mater. 2014, 26, 1118-1123. (63) Jo, J. W.; Jung, J. W.; Wang, H. W.; Kim, P.; Russell, T. P.; Jo, W. H. Fluorination of Polythiophene Derivatives for High Performance Organic Photovoltaics. Chem. Mater. 2014, 26, 4214-4220. (64) Wei, Y.; Yang, Y.; Yeh, J. M. Synthesis and Electronic Properties of Aldehyde End-Capped Thiophene Oligomers and Other α, ω-Substituted Sexithiophenes. Chem. Mater. 1996, 8, 2659-2666.


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Legends for Schemes, Figures and Tables Figure 1. Molecular structures of TTH-D3TRh and TTF-D3TRh. Scheme 1. Synthetic routes to TTH-D3TRh and TTF-D3TRh. Figure 2. (a) Thermogravimetric analysis (TGA) and (b) differential scanning calorimetry (DSC) curves of the TTH-D3TRh and TTF-D3TRh. Figure 3. (a) UV-vis absorption spectra of the TTH-D3TRh and TTF-D3TRh in chloroform solutions and (b) in thin solid films. Figure 4. (a) Cyclic voltammograms (CV) of TTH-D3TRh and TTF-D3TRh at a scan rate of 50 mV s-1. (b) Schematic energy diagram of the materials used in the SM-OSCs. Figure 5. Optimized molecular structures, molecular orbital surfaces of the LUMO+1, LUMO, HOMO, and HOMO-1 for the TTH-D3TRh and TTF-D3TRh, obtained by Gaussian 09 at the B3LYP/6-31G(d) level. Figure 6. (a) XRD patterns of the small molecule films on silicon wafers. (b) J-V characteristics of the hole-only devices based on small molecule:PC71BM measured at ambient temperature. Figure 7. (a) J-V curves of the best devices under a simulated AM 1.5 G irradiation (100 mW cm-2). (b) External quantum efficiency (EQE) curves of the best devices. Figure 8. (a) Dark J-V characteristics for TTH-D3TRh and TTF-D3TRh (b) Photocurrent

density

versus

effective

voltage

(Jph-Veff)

characteristics

for

TTH-D3TRh and TTF-D3TRh under constant incident light intensity (AM 1.5G, 100 mW cm-2).



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

Figure 9. AFM height (top) and phase (below) images of TTH-D3TRh:PC71BM (a, c) and TTF-D3TRh:PC71BM (b,d) blend films. Image size: 5×5 µm. Table 1. Optical and electrochemical data of the molecules Table 2. Photovoltaic data of the optimized devices based on TTH-D3TRh and TTF-D3TRh.


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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


S C8H17 N

C8H17C8H17

S

H O

S S

S

S

S

O

S

S

S

C8H17C8H17

H

S

TTH-D3TRh H

N C8H17

S

F TTF-D3TRh

S C8H17 N O

C8H17C8H17

S

F S

S F

O

S

S

S

S S

S C8H17C8H17

S

N C8H17 S

Figure 1. Molecular structures of TTH-D3TRh and TTF-D3TRh.



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

Scheme 1. Synthetic routes to TTH-D3TRh and TTF-D3TRh.


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(a)

(b) 2.0 100

TTH-D3TRh TTF-D3TRh

60

40

TTH-D3TRh TTF-D3TRh

1.5

Heat flow (mW/mg)

80

Weight (%)

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


1.0 0.5 0.0 -0.5 -1.0

20 100

200

300

400 o

Temperature ( C)

500

600

100

150

200 o

Temperature ( C)

Figure 2. (a) Thermogravimetric analysis (TGA) and (b) differential scanning calorimetry (DSC) curves of the TTH-D3TRh and TTF-D3TRh.



(a)

Absorption coefficient (10 cm )

(b) 1.0

4

-1

TTH-D3TRh TTF-D3TRh

-1

-1

Absorption coeffecient (10 M cm )

0.8

5

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

Page 36 of 45

0.6 0.4 0.2 0.0 300

400

500

600

700

800

5

TTH-D3TRh TTF-D3TRh

4 3 2 1 0 300

400

500

Wavelength (nm)

600

700

800

900

Wavelength (nm)

Figure 3. (a) UV-vis absorption spectra of the TTH-D3TRh and TTF-D3TRh in chloroform solutions and (b) in thin solid films.


(a)

(b) -2.5

-2.68 -2.80

0.04

-0.04

-4.0 -4.5 -5.0 -5.5

ITO -4.7

-5.25 -5.12 -5.28

-6.0 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

TTF-D3TRh

-3.5

1.5

-4.0 Al -4.28 PC71BM

-0.02

-2.87

PEDOT:PSS

Energy Level (eV)

0.00

-0.06 -2.5

Ca

-3.0

TTH-D3TRh TTH-D3TRh

0.02

Current (mA)

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


TTH-D3TRh

Page 37 of 45

-6.0

Voltage (V)

Figure 4. (a) Cyclic voltammograms (CV) of TTH-D3TRh and TTF-D3TRh at a scan rate of 50 mV s-1. (b) Schematic energy diagram of the materials used in the SM-OSCs.



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

Figure 5. Optimized molecular structures, molecular orbital surfaces of the LUMO+1, LUMO, HOMO, and HOMO-1 for the TTH-D3TRh and TTF-D3TRh, obtained by Gaussian 09 at the B3LYP/6-31G(d) level.


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(a) 2000

(b)

TTH-D3TRH TTF-D3TRH TTH-D3TRH:PC71BM

100

TTF-D3TRH:PC71BM

J (A m )

-2 1/2

1500

1000

10 TTH-D3TRh TTF-D3TRh

1/2

Intensity (a .u.)

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


1

500 4

5

6

10

15

20

25

30

35

0

0.1 5

10

15

20

2θ (deg)

25

30

35

40

0

1

2

3

4

5

6

Vappl-Vbi-Va (V)

Figure 6. (a) XRD patterns of the small molecule films on silicon wafers. (b) J-V characteristics of the hole-only devices based on small molecule:PC71BM measured at ambient temperature.



(a)

2

(b)

60

0 -2 -4

TTH-D3TRh (Conventional device) TTH-D3TRh (Inverted device) TTF-D3TRh (Conventional device) TTF-D3TRh (Inverted device)

50 40

EQE (%)

-2

Current density (mA cm )

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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-6 -8

20 10

-10 -12 -0.2

TTH-D3TRh (Conventional device) TTH-D3TRh (Inverted device) TTF-D3TRh (Conventional device) TTF-D3TRh (Inverted device)

30

0.0

0.2

0.4

Voltage (V)

0.6

0.8

1.0

0 300

400

500

600

700

800

Wavelength (nm)

Figure 7. (a) J-V curves of the best devices under a simulated AM 1.5 G irradiation (100 mW cm-2). (b) External quantum efficiency (EQE) curves of the best devices.


Page 41 of 45

(a) 1000

(b)

100

10

TTH-D3TRh TTF-D3TRh

10 1

TTH-D3TRh TTF-D3TRh

-2

Jph (mA cm )

-2

Current density (mA cm )

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


0.1 0.01

1

1E-3 1E-4 -1

0

1

0.01

Voltage (V)

0.1

1

Veff (V)

Figure 8. (a) Dark J-V characteristics for TTH-D3TRh and TTF-D3TRh (b) Photocurrent density versus effective voltage (Jph-Veff) characteristics for TTH-D3TRh and TTF-D3TRh under constant incident light intensity (AM 1.5G, 100 mW cm-2).



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

Figure 9. AFM height (top) and phase (below) images of TTH-D3TRh:PC71BM (a, c) and TTF-D3TRh:PC71BM (b,d) blend films. Image size: 5×5 µm.


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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


Table 1. Optical and electrochemical data of the molecules

Abs. (nm)

Abs. (nm)

EgOPT[a]

HOMO

LUMO

EgCV[b]

λSol max

film λ max

(eV)

(eV)

(eV)

(eV)

TTH-D3TRh

509

591,646

1.74

-5.18

-2.68

2.50

TTF-D3TRh

501

585,646

1.73

-5.28

-2.80

2.48

molecules

[a] Optical band gap ( EgOPT) was estimated from the wavelength of the optical absorption edge of the molecule film. [b] Electrochemical band gap ( EgCV) was calculated from the LUMO and HOMO energy levels.



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

Page 44 of 45

Table 2. Photovoltaic data of the optimized devices based on TTH-D3TRh and TTF-D3TRh. Voc

Jsc

FF

PCEmax/ PCEave

Rs

Rsh

(V)

(mA cm-2)

(%)

(%)

(Ω cm2)

(kΩ cm2)

TTH-D3TRh[a]

0.86

9.35

62.4

5.00(4.90)

13.56

0.46

TTH-D3TRh[b]

0.86

10.26

66.8

5.89(5.62)

12.84

0.59

TTF-D3TRh[a]

0.92

10.04

62.8

5.80(5.58)

13.42

0.63

TTF-D3TRh[b]

0.93

11.03

69.6

7.14(6.97)

12.03

0.72

molecules

[a] Data obtained from the conventional device. [b] Data obtained from the inverted device.


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Table of Contents (TOC)


Solution-Processable Small Molecules for High-Performance Organic

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