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Article Cite This: Chem. Mater. 2017, 29, 10045−10052

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Selenophene-Incorporated Quaterchalcogenophene-Based Donor− Acceptor Copolymers To Achieve Efficient Solar Cells with Jsc Exceeding 20 mA/cm2 Fong-Yi Cao,† Cheng-Chun Tseng,† Fang-Yu Lin,† Yuzhong Chen,‡ He Yan,‡ and Yen-Ju Cheng*,† †

Department of Applied Chemistry, National Chiao Tung University, 1001 University Road, Hsinchu, 30010 Taiwan Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration & Reconstruction, Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong

‡

S Supporting Information *

ABSTRACT: Three selenophene-incorporated quaterchalcogenophenebased donor−acceptor copolymers PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se are designed and synthesized. To systematically fine-tune the molecular properties and investigate the chalcogen effect, PFBT2Th2Se and PFBT2Se2Th hybridize two thiophenes and two selenophenes as the donor with different isomeric main-chain placement while thiophene-free PFBT4Se uses quaterselenophene as the donor. On account of the selenophene’s advantageous features such as higher quinoidal population and higher molecular polarizability, the three polymers show good light-harvesting ability, strong intermolecular interactions, high crystallinity, and high charge mobilities. Bulk-heterojunction solar cells incorporating these selenophenecontaining polymers have exhibited promising photovoltaic performance with impressive current densities over 20 mA/cm2. The device with the PFBT2Se2Th:PC71BM blend showed a PCE of 9.02% with a Jsc of 21.02 mA/cm2. In addition, the device using quaterselenophene-based PFBT4Se:PC71BM blend exhibited a PCE of 8.92% with a superior Jsc of 22.63 mA/cm2 which represents one of the highest current densities from polymer:fullerene-based solar cells reported in the literature.

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INTRODUCTION Bulk-heterojunction (BHJ) polymer solar cells (PSCs) are considered important for the generation of renewable energy.1−13 The development of superior p-type conjugated polymers plays a major role in advancing this area. Donor− acceptor (D-A) conjugated copolymers have been widely adopted as p-type materials due to their tunable molecular properties and good solution processability.14−17 On the basis of the structure of the building blocks, current state-of-the-art p-type polymers can be generally classified into two major types. One is the alternating copolymers containing benzo[1,2b:4,5-b′]dithiophene (BDT) and thienothiophene (TT) units.18−34 In this category, the PTB7-based polymers featuring amorphous character with good solubility have achieved more than 10% efficiencies by low-temperature solution processing.35,36 Another emerging D−A copolymer is based on an electron-rich quaterthiophene (Th4) motif as a donor (D) with an electron-deficient difluorobenzothiadiazole (FBT) unit as an acceptor (A). Hsu et al. demonstrated a PTh4FBT polymer prepared by the polymerization of 5,6-difluoro-4,7-bis(5(trimethylstannyl)thiophen-2-yl)benzothiadiazole with 5,5′-dibromo-4,4′-dihexadecyl-2,2′-bithiophene monomer.37 This polymer showed broad absorption, high crystallinity, and strong aggregation which are beneficial characteristics for generating © 2017 American Chemical Society

and transporting charge efficiently. On the basis of the design concept of this polymer, research efforts have been directed toward side-chain engineering or combining different donor and acceptor building blocks to seek optimization of the polymer properties.38−50 Yan et al. further modified the structure of PTh4FBT by changing the two alkyl side chains from the two outer thiophene units to the two inner thiophene units that are adjacent to the central FBT unit, leading to a polymer known as PffBT4T-2OD. By carefully controlling the temperature-dependent aggregation behavior of PffBT4T-2OD to achieve appropriate polymer domains, the PffBT4T-2ODbased devices using thicker active layers achieved superior efficiencies of over 10%.42,44,47 Similarly, Takimiya and coworkers reported another quaterthiophene-based polymer PNTz4T by replacing the FBT unit with the naphthobisthiadiazole (NT) acceptor.48 The device using highly crystalline PNTz4T also exhibited an efficiency over 10%.49 Besides utilizing the quaterthiophene as the electron-rich unit, Huang et al. used another thiophene-based building block, 2,5-bis(3alkylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT), to copolyReceived: August 31, 2017 Revised: November 13, 2017 Published: November 14, 2017 10045

DOI: 10.1021/acs.chemmater.7b03688 Chem. Mater. 2017, 29, 10045−10052

Article

Chemistry of Materials

Figure 1. Chemical structures of PFBT2Th2Se, PFBT2Se2Th, PFBT4Se, and PffBT4T-2OD.

Scheme 1. Synthetic Route to PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se

would be a rational and promising strategy to further develop new highly crystalline polymers. However, compared to the thiophene-based counterpart, the preparation of alkylated selenophene building blocks is synthetically more challenging. To this end, we first design and synthesize two new FBT-based D−A copolymers PFBT2Se2Th and PFBT2Th2Se which hybridize two thiophenes and two selenophenes as electronrich donor moieties. The repeating unit in PFBT2Se2Th contains two unsubstituted thiophenes and two 2-octyldodecylselenophenyl groups attached to a FBT unit (Figure 1). By switching the position of thiophene and selenophene moieties in PFBT2Se2Th, another isomeric polymer PFBT2Th2Se was also designed and synthesized, which allows us to modulate the molecular properties and simultaneously investigate the structural isomeric effect. Moreover, for the first time, allselenphene-based PFBT4Se using a quaterselenophene unit as the donor was also prepared (Figure 1). Organic solar cells using these selenophene-incorporated polymers have exhibited superior photovoltaic efficiencies with extraordinary current densities over 20 mA/cm2. The PFBT2Th2Se:PC71BM-based

merize with the NT acceptor to make a new D−A polymer exhibiting high efficiencies.43 Belonging to the chalcogenophene family, selenophene generally has chemical and physical properties similar to that of thiophene. Nevertheless, selenophene also has distinct features that are promising for creating new organic semiconductors.51−59 It has been documented that polyselenophene is prone to have quinoidal population higher than that of polythiophene, resulting in more planar backbone and longer effective conjugation length.60 Moreover, selenium with higher polarizability tends to induce intermolecular selenium− selenium (Se−Se) interactions or selenium−aromatic interactions in selenophene-containing molecules.60 These beneficial properties endow selenophene-based materials with narrower optical bandgap, enhanced light-harvesting ability, and higher crystallinity, leading to improved charge carrier mobilities in field-effect transistors or higher photocurrents in OPV devices in comparison with corresponding thiophene-based counterparts. 61−65 Substitution of thiophene moieties in the quaterthiophene−FBT−base D−A copolymer by selenophenes 10046


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Figure 2. Normalized UV−visible absorption spectra of PffBT4T-2OD, PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se in (a) o-DCB and (b) the solid state.

Table 1. Intrinsic Properties of PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se λmax (nm)

a

polymer

Mn (kDa)

PDI

Tm (°C)

Td (°C)

o-DCBa

film

λonset (nm)b

Egopt (eV)c

EHOMO (eV)

ELUMO (eV)

Egele (eV)

PFBT2Th2Se PFBT2Se2Th PFBT4Se

34.0 15.2 50.7

2.2 2.2 3.4

295 287 314

412 428 413

662 658 671

722 729 746

781 800 822

1.59 1.55 1.51

−5.39 −5.39 −5.33

−3.56 −3.62 −3.65

1.83 1.77 1.68

o-DCB = o-dichorobenzene. bCalculated in the solid state. cEgopt = 1240/λonset.

Figure 3. Normalized UV−visible absorption spectra of (a) PFBT2Th2Se, (b) PFBT2Se2Th, and (c) PFBT4Se in gradually elevated temperature from 30 °C to 100 °C. (d) Plot of IT/I30 versus solution temperature.

device exhibited a power conversion efficiency (PCE) of 8.68% with an open-circuit voltage (Voc) of 0.68 V, a fill factor (FF) of 69.1%, and short-circuit current density (Jsc) of 18.5 mA/cm2. The device with the PFBT2Se2Th:PC71BM blend showed a highest PCE of 9.02% and a Jsc of 21.02 mA/cm2. More importantly, the device using the PFBT4Se:PC71BM blend performed a PCE of 8.92% with a Jsc of 22.63 mA/cm2 which

represents one of the highest current densities ever reported for the polymer−fullerene-based solar cells in the literature.

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RESULTS AND DISCUSSION Syntheses of the three polymers are shown in Scheme 1. 4,7Bis(5-bromo-4-(2-octyldodecyl)selenophen-2-yl)-5,6-difluorobenzothiadiazole (FBT2Se) was prepared by Stille coupling of 10047


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whereas PFBT2Se2Th has the lower LUMO energy and thus the smaller bandgap. The quarterselenophene-based PFBT4Se has the highest-lying HOMO level and the narrowest bandgap. The trend of the electrochemical bandgap is consistent with that of the optical bandgap measured by the UV−visible absorption. To further understand the backbone conformation, quantum-chemical calculations were performed with the Gaussian09 suite15 employing the ωB97X-D density functional in combination with the 6-31G(d,p) basis set as shown in Figure 4. The dimer molecules denoted as 2(FBT2Th2Se), 2-

1 with 2 followed by the NBS bromination. FBT2Se was copolymerized with 5,5′-bis(trimethylstannyl)-2,2′-bithiophene (2Th) or 5,5′-bis(trimethylstannyl)-2,2′-biselenophene (2Se) to obtain PFBT2Se2Th and PFBT4Se, respectively. In a similar manner, 4,7-bis(5-bromo-4-(2-octyldodecyl)thiophenyl-2-yl)5,6-difluorobenzothiadiazole (FBT2Th) was copolymerized with 5,5′-bis(trimethylstannyl)-2,2′-biselenophene (2Se) to form PFBT2Th2Se. The synthesis of the β-alkylated selenophene, 2-trimethylstannyl-4-(2-octyldodecyl) selenophene (1), is depicted in Supporting Information. From the DSC measurements, PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se showed clear melting points at 295 °C, 287 °C, and 314 °C upon heating and crystallization points at 272 °C, 270 °C, and 290 °C upon cooling, respectively, indicating that the three selenophene-incorporated polymers are highly crystalline. In the UV−visible absorption spectra as shown in Figure 2, PFBT2Th2Se exhibited a broad band covering the visible region with the λmax at 722 nm in the solid state, while the isomeric PFBT2Se2Th with the selenophenes attached to the FBT unit showed a more red-shifted λmax at 729 nm. The closer proximity between the more electron-donating selenophene rings and the FBT acceptor in PFBT2Se2Th might induce stronger intramolecular charge transfer, leading to the more red-shifted λmax in comparsion with PFBT2Th2Se. Furthermore, the quarterselenophene-based PFBT4Se displayed the broadest absorption band with the most red-shifted λmax at 746 nm. The absorption of the quarterthiophene-based PffBT4T-2OD included in Figure 2 showed the most blueshifted curve. The optical bandgaps of PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se, estimated to be 1.59, 1.55, and 1.51 eV, respectively, are gradually decreased with increasing content of selenophene. The detailed optical properties are summarized in Table 1. The temperature-dependent UV−visible absorption spectra were measured to investigate the aggregation behavior of selenophene-based polymers in o-DCB as shown in Figure 3. When the solution temperature increases to 100 °C, λmax of PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se was hypsochromically shifted by 80, 75, and 105 nm, respectively, with concomitant disappearance of the vibronic peaks at the longer wavelengths, indicating that the three polymers strongly aggregate at room temperature in solution. To quantify the degree of aggregation, we made a plot of IT/I30 as a function of solution temperature where IT is absorbance intensity at the wavelength of 720 nm for PFBT2Th2Se, 720 nm for PFBT2Se2Th, and 745 nm for PFBT4Se at different temperature of 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, and 100 °C and I30 is the intensity at 30 °C. The isomeric effect plays an important in dictating intermolecular interactions. This plot shows that the IT/I30 value of PFBT2Th2Se is higher than that of the other two polymers from 70 °C to 30 °C, suggesting that the aggregation strength of PFBT2Th2Se with the thiophene rings attached to the FBT acceptor is stronger than that of PFBT2Se2Th. Electrochemical properties of the polymers were evaluated by cyclic voltammetry (CV) and are shown in Table 1 and Figure S1. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se were estimated to be −5.39/− 3.56, −5.39/−3.62, and −5.33/−3.65 eV with the corresponding electrochemical bandgap of 1.83, 1.77, and 1.68 eV, respectively. Both PFBT2Th2Se and PFBT2Se2Th consisting of thiophene/selenophene units have similar HOMO energy,

Figure 4. DFT-optimized conformation of (a) 2(FBT2Th2Se), (b) 2(FBT2Se2Th), and (c) 2(FBT4Se).

(PFBT2Se2Th), and 2(DFBT4Se) (2-octyldodecyl is simplified to methyl group) are used for simulation of the polymers. The optimal geometries of 2(FBT2Th2Se), 2(FBT2Se2Th), and 2(FBT4Se) structures are displayed in Figure 4. The dihedral angle between the 3-methylthiophene and the neighboring unsubstituted selenophene in 2(FBT2Th2Se) is 42.15°. Similarly, the dihedral angle between the 3-methylselenophene and the neighboring unsubstituted thiophene in 2(FBT2Se2Th) or unsubstituted selenophene in 2(FBT4Se) is 46.79° and 45.60°, respectively. These results reveal that 2(FBT2Th2Se) has a backbone more planar than that of 2(FBT2Se2Th). Compared to the bithiophene-based PFBT2Se2Th counterpart, PFBT2Th2Se with two unsubstituted selenophenes connected together might be more susceptible to forming a quinoidal structure, resulting in a more planar backbone conformation. Nevertheless, the backbone of 2(FBT4Se) is slightly more twisted than that of 2(FBT2Th2Se). The much larger selenium atoms might come into play to cause certain intramolecular steric hindrance. The inverted bulk heterojunction devices based on the ITO/ ZnO/polymer:PC71BM:MoO3/Ag configuration were fabricated. The characteristics of devices were measured under AM 1.5G illumination at 100 mW/cm2 and are summarized in Table 2. The J−V curve and external quantum efficiency (EQE) spectra are depicted in Figure 5. The devices using PFBT2Th2Se:PC71BM (1:2 in wt %) with 5 vol % diphenyl ether (DPE) as an additive showed a PCE of 8.68% with a Voc of 0.68 V, an FF of 69.1%, and a high Jsc of 18.46 mA/cm2. It has been known that strong aggregation of the polymers can enhance domain purity and further improve charge transport in the active layer. Under similar conditions, the device using the PFBT2Se2Th:PC71BM blend (1:2 in wt %) presented a PCE of 10048


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Table 2. Characteristics of ITO/ZnO/Polymer:PC71BM/MoO3/Ag Devices with PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se as Donor Materials

a

polymer

polymer:PC71BM in wt %a

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

PFBT2Th2Se PFBT2Se2Th PFBT4Se

1:2 1:2 1:2

0.68 0.66 0.62

18.46 21.02 22.63

69.1 65.0 63.6

8.68 (8.55) 9.02 (8.89) 8.92 (8.67)

With 5 vol % diphenyl ether as the additive. The values in parentheses are the averaged PCE of 15 devices.

Figure 5. (a) J−V curves and (b) EQE spectra of the devices using PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se as donor materials under AM 1.5G illumination at 100 mW/cm2.

Figure 6. Two-dimensional GIWAXD images of the (a) PFBT2Th2Se:PC71BM, (b) PFBT2Se2Th:PC71BM, and (c) PFBT4Se:PC71BM thin films.

leads to the lowest Voc value compared to the other two bithiophene−selenophene hybridized polymers. Although PFBT2Th2Se and PFBT2Se2Th with the same HOMO (−5.39 eV) energy level have different Voc values, the difference in Voc is only marginal (0.68 V for PFBT2Th2Se and 0.66 V for PFBT2Se2Th). The small difference might be attributed to morphology variations. Two-dimensional grazing-incidence wide-angle X-ray diffraction (GIWAXD) measurements were employed to study the molecular orientations for polymer:PC71BM (1:2 in wt %) films prepared identically to the device fabrication. As shown in Figure 6, PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se thin films exhibited strong (101) diffraction in the out-of-plane direction at qz = 1.75 Å−1, qz = 1.71 Å−1, and qz = 1.71 Å−1 corresponding to the periodic π−π stacking of the polymers. The results suggest that all the polymer crystallites adopt face-on orientations which are beneficial for charge carrier transport in the vertical direction in the active layer. Furthermore, PFBT2Th2Se shows a π−π stacking distance (3.59 Å) shorter than that of PFBT2Se2Th (3.68 Å) and PFBT4Se (3.68 Å) with FBT2Se units, indicating that the biselenophene-based

9.02% with a Voc of 0.66 V, an FF of 65.0%, and an impressive Jsc of 21.02 mA/cm2. Furthermore, the device incorporating PFBT4Se displayed a PCE of 8.92% with a Voc of 0.62 V, an FF of 63.6%, and a superior Jsc of 22.63 mA/cm2. The PFBT4Sebased device exhibited a panchromatic external quantum efficiency (EQE) plateau between 300 and 850 nm in the UV−vis−NIR region (Figure 6b). The Jsc values calculated from integration of the EQE spectra agree well with the Jsc obtained from J−V measurements. Due to the enhancement of light-harvesting ability of the selenophene-incorporated polymers, the devices using the selenophene-containing polymers showed Jsc values higher (18.46 mA/cm2 for PFBT2Th2Se; 21.02 mA/cm2 for PFBT2Se2Th and 22.63 mA/cm2 for PFBT4Se) than that of the device using thiophene-based PffBT4T-2OD (18.4 mA/cm2) reported in the literature.44 To the best of our knowledge, this Jsc value represents the highest current density among D−A copolymer:PC71BM-based solar cells reported in the literature. The increasing Jsc is mainly attributed to the improvement of light-harvesting ability of the selenophene-incorporated polymers. However, the all-selenophene PFBT4Se with the highest-lying HOMO energy level 10049


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(DSC) was conducted on a TA Q200 instrument under nitrogen atmosphere at a heating/cooling rate of 10 °C/min. Thermogravimetric analysis (TGA) was recorded on a PerkinElmer Pyris under nitrogen atmosphere at a heating rate of 10 °C/min. Electrochemical cyclic voltammetry was conducted on a CH instruments electrochemical analyzer. A carbon glass electrode was used as the working electrode and a Ag/AgCl electrode as the reference electrode, while 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile was the electrolyte. CV curves were calibrated using ferrocence as the standard, whose HOMO is set at −4.8 eV with respect to zero vacuum level. The HOMO energy levels were obtained from the equation HOMO = −(Eoxonset − E(ferrocene)onset + 4.8) eV. The LUMO levels were obtained from the equation LUMO = −(Eredonset − E(ferrocene)onset + 4.8) eV. Surface topography was investigated using Veeco diInnova AFM and standard tips (type Tap 300; L, 135 μm; FREQ, 300 MHz; k, 40 N/m). GIXS experiments were conducted at National Synchrotron Radiation Research Center (NSRRC) on beamline BL23A in Taiwan. The samples were irradiated with an X-ray energy of 10.09 keV (λ = 1.23 Å) at a fixed incident angle of 0.08° through a coupled double crystal Si(111)/multilayer (Mo/B4C) monochromator, and the GIXS patterns were recorded on a 2D image detector (Pilatus 1M-F area detector). The polymer films for GIXS measurement were prepared under conditions identical to those used for the OPV devices. Device Fabrication. The solar cell devices were fabricated under the following procedures: The ITO-coated glass substrate was pretreated by ultrasonic cleaning in detergent, DI-water, acetone, and isopropyl alcohol for 10 min, respectively, and subsequently treated with UV−ozone for 45 min. The ZnO layer was prepared according to the literature.66 Furthermore, the blending solutions with ratio 1:2 (in wt %) as well as 5 vol % diphenyl ether (DPE) as an additive were spin-coated on top of the ITO/ZnO substrate to form active layers with a thickness of ca. 250 nm determined by atomic force microscopy. After the thermal annealing at 100 °C in the glovebox, the MoO3 layer (7 nm) and silver anode were deposited by thermal evaporation at a pressure below 10−6 Torr. The devices without encapsulation were characterized under ambient conditions. Current− voltage characteristics were measured using a Keithley 2400 SMU under irradiation from an AM 1.5G San-Yi solar simulator with JIS AAA spectrum. The characteristics of the solar cells were optimized by testing approximately 25 cells. IPCE spectra were measured using a lock-in amplifier with a current preamplifier under short-circuit conditions with illumination by monochromatic light from a 250 W quartz-halogen lamp (Osram) passing through a monochromator (Spectral Products CM110). Furthermore, the hole-only devices were fabricated with a structure of ITO/PEDOT:PSS (40 nm)/active layer/ MoO3/Ag and measured in dark.

PFBT2Th2Se has the strongest intermolecular interaction which is consistent with the results of the temperaturedependent absorption as well as the theoretical calculations. We also fabricated the hole-only devices based on the ITO/ PEDOT:PSS/polymer:PC71BM/MoO3/Ag configuration to evaluate the hole mobility of the polymer:PC71BM (1:2 in wt %) blend by using the space-charge limit current (SCLC) model (Figure S2). The hole mobilities were estimated to be 2.71 × 10−3, 5.66 × 10−3, and 3.75 × 10−3 cm2/(V s) for PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se, respectively. The high hole mobility is also crucial for making a thicker active layer over 250 nm. The high hole mobilities can be attributed to the highly crystalline nanostructure with face-on orientations which are favorable for vertical charge transport. Although PFBT2Se2Th shows the highest hole mobility of 5.66 × 10−3 cm2/(V s) and PCE of 9.02% compared to the other polymers, PFBT2SeTh exhibits an FF (65.0%) lower than that of PFBT2Th2Se (69.1%). PFBT2SeTh shows a molecular weight (15.2 kDa) much lower than that of PFBT2Th2Se (34.0 kDa). The lower molecular weight of PFBT2SeTh could affect the charge transport in the bulk, resulting in a lower FF. To gain a deeper insight into the nanoscale morphology of the photoactive layers, atomic force microscopy measurement was employed. As shown in Figure S3, the PFBT2Th2Se:PC71BM blend showed a larger surface roughness and phase separation owing to the strongest polymer aggregation. On the other hand, the PFBT2Se2Th:PC71BM and PFBT4Se:PC71BM blend showed more homogeneous phase separation with obvious nanofiber structures which can facilitate carrier transport as well as reduce charge recombination.

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CONCLUSION In summary, we developed three new selenophene-incorporated difluorobenzothiadiazole-based D−A copolymers PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se. PFBT2Th2Se and PFBT2Se2Th hybridize two thiophenes and two selenophenes as the donor with different isomeric main-chain placement while the all-selenophene-based PFBT4Se utilizes quaterselenophene as the donor. By changing the selenophene/ thiophene arrangement and the number of selenophenes in the polymer to systematically fine-tune and investigate the isomeric properties, the three polymers show good light-harvesting ability, strong intermolecular aggregation, high crystallinity, and high charge mobilities. Bulk-heterojunction solar cells incorporating these selenophene-containing polymers have exhibited promising photovoltaic performance with superior current densities exceeding 20 mA/cm2. The device with the PFBT2Se2Th:PC71BM blend showed the highest PCE of 9.02% with an impressive Jsc of 21.02 mA/cm2. In addition, the device using quaterselenophene-based PFBT4Se:PC71BM blend exhibited a PCE of 8.92% with a superior Jsc of 22.63 mA/cm2 which represents one of the highest current densities from PSCs reported in the literature. This research demonstrates that synthesis of functionalized selenophene derivatives is promising for creating new selenophene-based materials with better light-harvesting ability and stronger molecular packing to achieve superior photocurrent.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03688. Electrochemical properties, SCLC measurements, atomic force microscopy measurements, thermal properties, synthetic procedures, NMR spectra, and computation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

EXPERIMENTAL DETAILS

He Yan: 0000-0003-1780-8308 Yen-Ju Cheng: 0000-0003-0780-4557

General Measurement and Characterization. 1H and 13C NMR spectra were measured using a Varian 400 MHz instrument spectrometer. UV−visible spectra were collected using a UV-Hitachi U-4100 UV−vis spectrophotometer. Differential scanning calorimetery

Notes

The authors declare no competing financial interest. 10050


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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology and the Ministry of Education, and Center for Interdisciplinary Science (CIS) of the National Chiao Tung University, Taiwan, for financial support. We thank the National Center of Highperformance Computing (NCHC) in Taiwan for computer time and facilities. We also thank the National Synchrotron Radiation Research Center (NSRRC) in Taiwan, and Dr. U-Ser Jeng and Dr. Chun-Jen Su in at BL23A1 station for help with the GIWAXS measurements. We thank Hong Kong Innovation and Technology Commission for the support through the projects ITC-CNERC14SC01 and ITS/083/15.

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