Selenophene-Incorporated Quaterchalcogenophene-Based Donor

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Selenophene-Incorporating 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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03688 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Chemistry of Materials

Selenophene-Incorporating Quaterchalcogenophene-Based Donor-Acceptor Copolymers to Achieve Efficient Solar Cells with Jsc Exceeding 20 mA/cm2 Fong-Yi Cao,1 Cheng-Chun Tseng,1 Fang-Yu Lin,1 Yuzhong Chen,2 He Yan2 and Yen-Ju Cheng*1 1

Department of Applied Chemistry, National Chiao Tung University, 1001 University Road,

Hsinchu, 30010 Taiwan 2

Department of Chemistry and Energy Institute, The Hong Kong University of Science and

Technology, Clear Water Bay, Kowloon, Hong Kong. E-mail: [email protected]

ABSTRACT: Three selenophene-incorporating quaterchalcogenophene-based 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 the different isomeric main-chain placement while the 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 selenophene-containing 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 performed a PCE of 8.92 % with a superior Jsc of 22.63 mA/cm2 which represents one of the highest current densities from the polymer:fullerene-based solar cells reported in the literature. 1

ACS Paragon Plus Environment


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INTRODUCTION Bulk-heterojunction (BHJ) polymer solar cells (PSCs) are considered as an important technique to generate 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 processibility.14-17 Judging from the structure of the building blocks, the current state-of-the-art p-type polymers can be generally classified into two major types One is the alternating copolymers containing benzo[1,2-b:4,5-b′]dithiophene (BDT) and thienothiophene (TT) units.18-34 In this category, the PTB7-based polymers featuring amorphous characters with good solubility have achieved more than 10% efficiencies by low-temperature solution processing.35,36 Another emerging D-A copolymers are 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

5,6-difluoro-4,7-bis(5-(trimethylstannyl)thiophen-2-yl)benzothiadiazole 5,5'-dibromo-4,4'-dihexadecyl-2,2'-bithiophene

monomer.37

This

polymer

of with

showed

broad

absorption, high crystallinity and strong aggregation which are beneficial characteristics for generating and transporting charge efficiently. On the basis of the design concept of this polymer, research efforts have been directed towards side-chain engineering or combining different donor and acceptor building blocks to seek for 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-2OD-based 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 2


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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(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT), to copolymerize with the NT acceptor to make a new D-A polymer exhibiting high efficiencies.43 Belonging to the chalcogenophene family, selenophene generally has similar chemical and physical properties with 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 higher quinoidal population than 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 the selenophene-containing molecules.60 These beneficial properties endow selenophene-based materials with narrower optical bandgap, enhanced light-harvesting ability and higher crystallinity, leading to the improved charge carrier mobilities in field-effect transistors or higher photocurrents in OPV devices in comparison with the corresponding thiophene-based counterparts.61-65 Substitution of thiophene moieties in the quaterthiophene-FBT-base D-A copolymer by selenophenes would be a rational and promising strategy to further develop new highly crystalline polymer. 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 the electron-rich 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, all-selenphene-based PFBT4Se using a quaterselenophene unit as the 3



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donor was also prepared (Figure 1). Organic solar cells using these selenophene-incorporating polymers have exhibited superior photovoltaic efficiencies with extraordinary current densities over 20 mA/cm2. The PFBT2Th2Se:PC71BM-based 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.

RESULTS AND DISCUSSION

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

The

synthesis

of

the

three

polymers

are

shown

in

Scheme

1.

4,7-Bis(5-bromo-4-(2-octyldodecyl)selenophen-2-yl)-5,6-difluorobenzothiadiazole (FBT2Se) was prepared by Stille coupling of 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 4


similar

manner,

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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 the supporting information.

Scheme 1. Synthetic route of PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se.

From the DSC measurements, the PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se showed clear melting points at 295, 287, 314 oC upon heating and crystallization points at 272, 270, 290 oC upon cooling, respectively, indicating that the three selenophene-incorporating 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 solid state, while the isomeric PFBT2Se2Th with the selenophenes attached to the FBT unit showd 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 5



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red-shifted λmax in comparsion with PFBT2Th2Se. Furthermore, the quarterselenophene-based PFBT4Se displayed a broadest absorption band with the most red-shifted λmax at 746 nm. The absorption of the quarterthiophene-based PffBT4T-2OD included in the Figure 2 showed the most blue-shifted curve. The optical bandgaps of PFBT2Th2Se, PFBT2Se2Th and PFBT4Se, estimated to be 1.59 eV, 1.55 eV and 1.51 eV, respectively, are gradually decreased with increasing the content of selenophene. The detailed optical properties are summarized in Table 1. Table 1. Summary of the intrinsic properties of the PFBT2Th2Se, PFBT2Se2Th and PFBT4Se. Polymers

Mn (kDa)

PDI

λmax (nm)

Tm

Td

(°C)

(°C)

o-DCB

λonset

Egopt

EHOMO

ELUMO

Egele

Film

(nm)a

(eV)b

(eV)

(eV)

(eV)

PFBT2Th2Se

34.0

2.2

295

412

662

722

781

1.59

-5.39

−3.56

1.83

PFBT2Se2Th

15.2

2.2

287

428

658

729

800

1.55

-5.39

−3.62

1.77

PFBT4Se

50.7

3.4

314

413

671

746

822

1.51

-5.33

−3.65

1.68

o-DCB = ortho-dichorobenzene, acalculated in the solid state, bEgopt = 1240/λonset.

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

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, the λmax of PFBT2Th2Se, PFBT2Se2Th and PFBT4Se was hypsochromically shifted by 80 nm, 75 nm and 105 nm, respectively, with the concomitant 6


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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, 40, 50, 60, 70, 80, 90, 100 oC and I30 is the intensity at 30 oC. The isomeric effect plays an important in dictating intermolecular interactions. This plot shows that the IT/I30 value of PFBT2Th2Se is higher than the other two polymers from 70 to 30 oC, suggesting that the aggregation strength of PFBT2Th2Se with the thiophene rings attached to the FBT acceptor is stronger than that of PFBT2Se2Th.

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. Electrochemical properties of the polymers were evaluated by cyclic voltammetry (CV) and are 7



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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/−3.65 and −5.33/−3.65 eV with the corresponding electrochemical bandgap of 1.83 eV, 1.77 eV and 1.68 eV, respectively. Both PFBT2Th2Se and PFBT2Se2Th hybridizing thiophene/selenophene units have the similar HOMO energy, 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(PFBT2Se2Th) and 2(DFBT4Se) (the 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 more planar backbone than 2(FBT2Se2Th). Compared to the bithiophene-based PFBT2Se2Th counterpart, the PFBT2Th2Se with two unsubstituted selenophenes connected together might be more susceptible to forming the quinoidal structure, resulting in more planar backbone conformation. Nevertheless, the backbone of 2(FBT4Se) is slightly more twisting than that of 2(FBT2Th2Se). The much larger selenium atoms might come into play to cause certain intramolecular steric hindrance.

8


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

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 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 the strong aggregation of the polymers can enhance domain purity and further improve the charge transport in the active layer. Under the similar conditions, the device using the PFBT2Se2Th:PC71BM blend (1:2 in wt %) presented a PCE of 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, FF of 63.6 %, and a superior Jsc of 22.63 mA/cm2. The PFBT4Se-based device exhibited a panchromatic external quantum efficiency (EQE) plateau between 300 and 850 9



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nm in UV-Vis-NIR region (Figure 6b). The Jsc values calculated from integration of the EQE spectra agree well with the Jsc obtained from the J−V measurements. Due to the enhancement of light-harvesting ability of the selenophene-incorporating polymers, the devices using the selenophene-containing polymers

showed higher Jsc values (18.46 mA/cm2 for PFBT2Th2Se;

21.02 mA/cm2 for PFBT2Se2Th and 22.63 mA/cm2 for PFBT4Se) than the device using the 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 the D-A

copolymer:PC71BM-based solar cells reported in the literature. The increasing Jsc is mainly attributed to the improvement of the light-harvesting ability of the selenophene-incorporating polymers. However, the all-selenophene PFBT4Se with the highest-lying HOMO energy level 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 the morphology variations. Table 2. Characteristics of ITO/ZnO/polymer:PC71BM:MoO3/Ag devices with PFBT2Th2Se, PFBT2Se2Th and PFBT4Se as the donor materials.

a)

Polymer

Polymer:PC71BM in wt%

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

PFBT2Th2Se

1:2

0.68

18.46

69.1

8.68 (8.55)

PFBT2Se2Th

1:2

0.66

21.02

65.0

9.02 (8.89)

PFBT4Se

1:2

0.62

22.63

63.6

8.92 (8.67)

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

10


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

The two-dimensional grazing-incidence wide-angle X-ray diffraction (GIWAXD) measurements were employed to study the molecular orientations for the polymer:PC71BM (1:2 in wt%) films prepared identially 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, the PFBT2Th2Se shows the shorter π–π stacking distance of 3.59 Å than that of PFBT2Se2Th (3.68 Å) and PFBT4Se (3.68 Å) with the FBT2Se units, indicating that the biselenophene-based PFBT2Th2Se has the strongest intermolecular interaction which is consistent with the results of the temperature-dependent absorption as well as the theoretical calculations.

11



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Figure

6.

Two-dimensional

GIWAXD

images

of

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the

a)

PFBT2Th2Se:PC71BM,

b)

PFBT2Se2Th:PC71BM and c) PFBT4Se:PC71BM thin films.

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 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/Vs for PFBT2Th2Se, PFBT2Se2Th and PFBT4Se, respectively. The high hole-mobility is also crucial for making thicker active layer over 250 nm. The high hole mobilities can be attributed to the highly crystalline nanostructure with the face-on orientations which are favorable for vertical charge transport. Although PFBT2Se2Th shows a highest hole mobility of 5.66 × 10-3 cm2/Vs and PCE of 9.02% compared to the other polymers, PFBT2SeTh exhibits a lower FF of 65.0% than PFBT2Th2Se (69.1%). PFBT2SeTh shows much lower molecular weight of 15.2 kDa than that of the 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 the phase separation owing to the strongest polymer aggregation. On the other hand, PFBT2Se2Th:PC71BM and PFBT4Se:PC71BM blend showed more homogeneous phase separation with obvious nano-fiber structures which can facilitate the carrier transport as well as reduce the 12


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charge recombination. CONCLUSION In

summary,

we

developed

three

new

selenophene-incorporating

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 selenophene 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 a highest PCE of 9.02 % with an impressive Jsc of 21.02 mA/cm2. In addition, the device using quaterselenophene-based PFBT4Se:PC71BM blend performed 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 demonstrate 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.

EXPERIMENTAL DETAILS General Measurement and Characterization. 1

H and

13

C NMR spectra were measured using Varian 400 MHz instrument spectrometer .

UV-Visible spectra was collected using the UV-HITACHI U-4100 UV−vis spectrophotometer. Differential scanning calorimetery (DSC) was conducted on a TA Q200 Instrument under nitrogen atmosphere at a heating/cooling rate of 10 °C/min. Thermogravimetric analysis (TGA) was 13



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recorded on a Perkin-Elmer 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 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 identical conditions used for the OPV devices.

Device Fabrication. The solar cell devices were fabricated under the following procedures: The ITO-coated glass substrate was pre-treated by ultrasonic cleaning in 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 ZnO/ITO substrate to form the active layers with the thickness of ca. 250 nm determined by atomic force microscopy. After the thermal annealing at 100 °C in the glove box, 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 in the ambient condition. Current-voltage characteristics 14


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were measured using a Keithley 2400 SMU under the irradiation of 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 the structure of ITO/PEDOT:PSS (40 nm)/active layer/MoO3/Ag and measured in dark.

ASSOCIATED CONTENT Supporting information: Electrochemical properties, SCLC measurements, atomic force microscopy measurements, thermal properties, synthetic procedures, NMR spectra and computation.

ACKNOWLEDGEMENTS 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 High-performance 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 the help with the GIWAXS measurements.

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831. (66) Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifter, J. A. J. Heeger, Inverted Polymer Solar Cells Integrated with a Low-Temperature-Annealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679-1683.

Table of content Selenophene-Incorporating Quaterchalcogenophene-Based Donor-Acceptor Copolymers to Achieve Efficient Solar Cells with Jsc Exceeding 20 mA/cm2

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Selenophene-Incorporated Quaterchalcogenophene-Based Donor

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