Novel Donor–Acceptor Random Copolymers Containing

Feb 28, 2014 - Photovoltaic devices based on these random copolymers were ..... Organic Solar Cells: Material Selection and Device Architecture Acc. C...
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Novel Donor−Acceptor Random Copolymers Containing Phenanthrocarbazole and Diketopyrrolopyrrole for Organic Photovoltaics and the Significant Molecular Geometry Effect on Their Performance Zhiqiang Deng,† Lie Chen,*,†,‡ Feiyan Wu,†,‡ and Yiwang Chen†,‡ †

Institute of Polymers/Department of Chemistry and ‡Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China S Supporting Information *

ABSTRACT: A series of donor−acceptor (D−A) random copolymers PPC-T-DPP and PPC-TT-DPP based on electron-rich 6H-phenanthro[1;10,9,8-cdefg]carbazole (PC) and electron-deficient 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) building blocks with two different linking units, thiophene (T) and thieno[3,2-b]thiophene (TT), were successfully synthesized. All of the copolymers show excellent light absorption, and their photoelectric properties can be easily adjusted by tailoring the molar ratio of PC to DPP. Among the polymer solar cells (PSCs) based on the copolymers with T linking unit, the PPC-T-DPP_1/2 (the molar ratio of PC:DPP = 1:2):PC61BM ([6,6]-phenyl-C61butyric acid methyl ester) based device achieves the best efficiency of 2.0%. After replacing T with TT, a slightly enhanced power conversion efficiency (PCE) of 2.2% is obtained from the PPC-TT-DPP_1/2:PC61BM-based device due to the better charge mobility of TT. However, with the addition of 1-chloronaphthalene (CN) to a volume ratio of 10%, the PCE of the device with PPC-T-DPP_1/2:PC61BM is improved to 2.8%, even higher than the CN-treated PPC-TT-DPP_1/2:PC61BM device (2.4%). The density functional theory (DFT) calculation results reveal that the incorporation of the axisymmetrical PC molecule with centrosymmetrical DPP unit together induces a large dihedral angle and big backbone torsion, and the more planar TT linkage seems to deteriorate molecular packing compared to the T unit, consequently leading to the worse performance.

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strong π-stacking structures in their solid-state films. Incorporation of π-extended fuse rings should be one of the most effective strategies to ensure strong intermolecular interactions between the neighboring polymer main chains.6−12 Among various donor blocks with fused aromatic rings, 6Hphenanthro[1,10,9,8-cdefg]carbazole (PC) and its derivatives have been demonstrated to meet well the requirements for organic optoelectronic applications,13−15 for their unusual and remarkable properties, such as perfect planar structure, strong electron-donating ability, and facile synthesis. More importantly, PC-based copolymers show high hole mobility up to 0.3 cm2 V−1 S−1.15,16 In addition, the copolymers containing electron-deficient diketopyrrolopyrrole (DPP) have been found to exhibit high hole mobility close to 2 cm2 V−1 S−1, due to the planarity of the DPP skeleton.17−21 Single-junction cells with DPP-based copolymers have afforded a power conversion efficiency (PCE) of 7.0%.22 Considering that the fused aromatic PC and DPP block may strongly influence the molecular π-stacking and the chargetransport properties of copolymers,23,24 we take advantage of

INTRODUCTION Polymer solar cells (PSCs) have generated notable scientific interest due to the potential advantages of PSCs over inorganicbased solar cells, including low cost, light weight, and fast/ cheap roll to roll production.1−4 Typically, p-type semiconductor (electron donor, such as conjugated polymers) and n-type semiconductor (electron acceptor, such as [6,6]-phenylC61-butyric acid methyl ester (PC61BM)) are blended in a bulk heterojunction (BHJ) configuration as the core components for PSCs.5 To achieve high efficiency of PSCs, the most critical challenge at the molecular level is to develop p-type conjugated polymers that simultaneously possess (i) a narrow band gap, for matching the high photon flux region of the solar spectrum to ensure enough light harvesting, which depends on the absorption range and extinction coefficient, and (ii) high hole mobility for efficient charge transport. To broaden the response wavelength range of conjugated polymer, one attractive approach is to construct donor−acceptor (D−A) conjugated polymers by covalently linking acceptor segments to the conjugated polymer. However, the charge mobility of these D− A copolymers is quite low to achieve the desired power conversion efficiency (PCE) for application. An important design principle of D−A conjugated polymers with high charge-transporting characteristic is to obtain the © 2014 American Chemical Society

Received: November 17, 2013 Revised: February 23, 2014 Published: February 28, 2014 6038

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Scheme 1. Synthetic Routes of Copolymers with Different Feed Molar Ratios

Table 1. Molecular Weight and Thermal Properties of Copolymers molar ratio (PC/DPP) copolymer

Mw (kg/mol)

Mn (kg/mol)

PDI

Td (°C)

PPC-T-DPP_2/1 PPC-T-DPP_1/1 PPC-T-DPP_1/2 PPC-TT-DPP_1/2

43.6 45.2 50.7 49.5

14.0 17.6 21.5 17.8

3.12 2.57 2.36 2.78

403 402 400 411

a

feed ratio

actual ratiob

2.00 1.00 0.50 0.50

1.82 0.88 0.65 0.71

a

The decomposition temperature corresponding to a 5% weight loss. bCalculated by comparing the respective integration of the H atom (N−C−H) of PC and DPP.

reported literature procedure.16 The synthetic routes and structural characterizations were provided in the Supporting Information (SI) (Scheme SI 1 and Figures SI 1−4). As shown in Scheme 1, using tris(dibenzylideneacetone)dipalladium (Pd2(dba)3) catalyst and tri-o-tolylphosphine ligand (P(oTolyl)3), the copolymers PPC-T-DPP_2/1, PPC-T-DPP_1/1, PPC-T-DPP_1/2, and PPC-TT-DPP_1/2 were synthesized by Stille coupling polymerization with the feed molar ratio of PC to DPP being 2:1, 1:1, 1:2 (T as the linking unit), and 1:2 (TT as the linking unit). To ensure enough solubility and selforganization, long branched 1-octylnonane and 2-octyldodecyl side chains were substituted on the nitrogen atom of the PC and DPP unit, respectively. All copolymers are highly soluble (15−20 mg/mL) in common organic solvent such as chloroform, tetrahydrofuran, chlorobenzene, and 1,2-dichlorobenzene. The structures of the copolymers have been confirmed by 1H NMR spectra, as revealed in Figures SI 5−8 (Supporting Information). The actual compositions of the copolymers calculated by comparing the respective integration of the H atom (N−C−H) of PC and DPP are consistent with feed ratios of the comonomers. The molecular weight (Mn and

the PC donor block and DPP acceptor unit to create novel condensed aromatic D−A random conjugated copolymers. To precisely adjust the conjugated backbone packing and the photoelectric properties of copolymers, thiophene (T) and thieno[3,2-b]thiophene (TT) were chosen as the bridge to afford a series of copolymers, poly{6H-phenanthro[1,10,9,8cdefg]carbazole−thiophene−3,6-di(thiophen-2-yl)pyrrolo[3,4c]pyrrole-1,4(2H,5H)-dione} and poly{6H-phenanthro[1,10,9,8-cdefg]carbazole−thieno[3,2-b]thiophene−3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione}, namely, PPC-T-DPP and PPC-TT-DPP, respectively, by tailoring the feed ratios of comonomers (donor, acceptor, and linking unit). 25 The photophysics properties and morphology of these copolymers have been studied, and the effect of molecular geometry on the molecular packing and their corresponding photovoltaic properties are also given special attention.

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RESULTS AND DISCUSSION The monomer N-(1-octylnonane)-4,8-dibromo-phenanthro[1,10,9,8-cdefg]carbazole (PC) was prepared following the 6039

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Mw) and polydispersity index (PDI) of the copolymers were obtained by performing gel permeation chromatography (GPC) relative to polystyrene standards in tetrahydrofuran at 30 °C, and the related data are shown in Table 1. Thermal Properties. Thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) of copolymers have been carried out to determine the thermal properties of the copolymers. From Figure SI 9 (SI) and Table 1 we can see that all copolymers exhibit sufficient thermal stability for PSC applications with 5% weight loss temperatures between 400 and 420 °C, and the feed molar ratio has little effect on thermal stabilities of copolymers. Besides, DSC curves of these random copolymers reveal no clear liquid-crystalline phase transition from 40 to 350 °C (Figure SI 10, SI). However, there is a melting transition in PPC-T-DPP_1/2 at ∼250 °C caused by the melting of the copolymer backbone. Optical Properties. The solution and thin-film optical absorption spectra of the studied copolymers are shown in Figure 1, and the correlated optical parameters are summarized

in Table 2. Thanks to the strong push−pull interaction between the PC donor and DPP acceptor unit, all of the studied copolymers display broad absorption bands with two distinct peaks at 400−550 nm and 600−850 nm, corresponding to the localized π−π* transition (LT) from the PC segment and intramolecular charge transfer (ICT) between the DPP acceptor and PC donor unit, respectively. In Figure 1a, PPCT-DPP_2/1, PPC-T-DPP_1/1, and PPC-T-DPP_1/2 in diluted chloroform have obvious absorption variation. As the ratio of PC donor unit in the copolymers decreases, the absorbance intensity in the LT region decreases, while the intensity of the ICT band shows the opposite tendency, due to the increased content of the DPP segment. When the copolymers were fabricated into films, a distinct bathochromic shift (ca. 5−28 nm) is observed in Figure 1b, indicating the stronger intermolecular interactions in the solid state. After replacing the thiophene (T) bridge with thieno[3,2-b]thiophene (TT), the profiles of solution absorption spectra remain almost unchanged (Figure 1c). However, it is unexpected that the absorption spectrum of the PPC-TTDPP_2/1 film is blue-shifted compared to that of PPC-TDPP_2/1, which may give the sign that TT with larger conjugation area did not improve the coplanarity of the copolymer backbone. The optical band gaps (Egopt) of copolymers are deduced from the absorption onsets of around 850 nm, giving the band gap of copolymers with the following order: PPC-T-DPP_2/1 (1.39 eV) < PPC-TT-DPP_2/1 (1.42 eV) < PPC-T-DPP_1/1 (1.46 eV) < PPC-T-DPP_1/2 (1.57 eV). The optical band gap matches well with the solar flux, implying that these copolymers have great potential for effective photon harvesting. Electrochemical Properties. Cyclic voltammetry (CV) was used to investigate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The CV measurements were conducted in a 0.1 M Bu4PF6/CH3CN solution. The platinum disc working electrode was coated with the copolymer thin film by using a polymer solution in chlorobenzene. As shown in Figure SI 11 (SI), the copolymers show relatively stable and reversible pdoping and n-doping processes. Generally, a promising D−A copolymer for highly efficient PSCs should possess a narrow band gap of ∼1.5 eV with suitable HOMO and LUMO levels. Estimated from the onset oxidation and reduction potentials in the cyclic voltammogram, these copolymers have low-lying HOMO levels from −5.38 to −5.29 eV, which would offer a relatively high open circuit voltage (Voc) and suitable LUMO levels from −3.66 to −3.70 eV for efficient charge transfer from copolymer to PCBM (Table 3). The deepest HOMO level of −5.38 eV was obtained from PPC-T-DPP_2/1, and increasing the DPP content makes the HOMO level increase to −5.34 eV for PPC-T-DPP_1/1 and −5.31 eV for PPC-T-DPP_1/2. In contrast, the LUMO level decreases along with the enhanced content of the DPP unit. As a result, the electrochemical band gaps of these copolymers move to a narrower region with the rise of DPP content, and the lowest value reaches to 1.61 eV for PPC-T-DPP_1/2. Additionally, the HOMO and LUMO levels of PPC-TT-DPP_1/2 are −5.29 and −3.67 eV, respectively, exhibiting a slightly broader band gap than PPC-T-DPP_1/2 due to the increased LUMO level. These results are well consistent with the UV observation. Energy level diagrams of copolymer are revealed in Figure 2. Density functional theory (DFT) computation is also performed to give a deep insight into the electronic structures of copolymers PPC-T-DPP and

Figure 1. Absorption spectra of PPC-T-DPP_2/1, PPC-T-DPP_1/1, and PPC-T-DPP_1/2 in dilute chloroform solution (a) and as thin film (b) and absorption spectra of PPC-T-DPP_1/2 and PPC-TTDPP_1/2 in dilute chloroform solution and as thin film (c). 6040

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Table 2. Summary of Optical Properties of Copolymers thin film

chloroform solution polymer

λmax (nm)

λonset (nm)

Egopt(eV)

λmax (nm)

λonset (nm)

Egopt(eV)

PPC-T-DPP_2/1 PPC-T-DPP_1/1 PPC-T-DPP_1/2 PPC-TT-DPP_1/2

654,456 660,461 706,452 704,472

796 843 854 862

1.56 1.47 1.45 1.44

654,456 694,476 734,456 712,486

802 850 890 872

1.57 1.46 1.39 1.42

AM 1.5 G radiation (100 mW/cm2). Details of device fabrication and characterization are summarized in the Supporting Information, and all device parameters are presented in Table 4. Among the PPC-T-DPP-based devices, the best power conversion efficiency (PCE) of 2.0% is obtained from the device based on PPC-T-DPP_1/2: PC61BM (1:2, w:w) with a short-circuit current (Jsc) of 5.82 mA·cm−2, an open-circuit voltage (Voc) of 0.714 V, and a fill factor (FF) of 47.3%. Replacing the linking bridge thiophene with thieno[3,2b]thiophene, the PPC-TT-DPP_1/2-based device obtains a better PCE of 2.2%, combining a higher Jsc of 6.97 mA cm −2, a Voc of 0.732 V, and a FF of 43.8%. The PPC-TT-DPP_1/2 possesses the worse light absorption but achieves the better Jsc with respect to the PPC-T-DPP_1/2 counterpart, probably ascribed to the higher charge mobility of the planar TT moiety (discuss later). All of the copolymers show relatively high Voc with respect to the most DPP-based copolymer with the Voc of ∼0.6 V, owing to the big fused ring structure. However, compared with the state-of-the-art D−A copolymers boosting the high PCE up to 7−9%, the poor efficiency of these copolymers results from low Jsc and FF, probably correlated with the unfavorable morphology and crystallinity caused by the strong twist between the big DPP and PC group. Different from most PSCs for which the device efficiency can be improved greatly upon heating, thermal annealing exerts little favor on the performance of these copolymers. Since the smallmolecule additive 1-chloronaphthalene (CN) can strongly affect the morphology of mixtures of low band gap copolymers with PC61BM,28 devices based on the addition of CN to the active layer were fabricated, and the corresponding current density−voltage (J−V) curves of devices are shown in Figure 3. Solar cells made from PPC-T-DPP_1/2:PC61BM (1:2, w/w) with the addition of CN to an optimal ratio of 10% (v/v) achieve an obviously increased PCE of 2.8% with a Jsc of 6.46 mA·cm−2, Voc of 0.763 V, and FF of 57.8%, which is 40% enhancement over the nonadditive device. However, the additive-treated device based on PPC-TT-DPP_1/2 only obtains a slightly improved PCE of 2.4%. The inferior PCE of PPC-TT-DPP_1/2 than PPC-T-DPP_1/2 originates from

Table 3. Summary of Electrochemical Properties of Copolymers polymer

Eoxonset (V)

Eredonset (V)

HOMO (eV)

LUMO (eV)

Egel (eV)

PPC-T-DPP_2/1 PPC-T-DPP_1/1 PPC-T-DPP_1/2 PPC-TT-DPP_1/2

0.98 0.94 0.91 0.90

−0.74 −0.71 −0.70 −0.72

−5.38 −5.34 −5.31 −5.29

−3.66 −3.69 −3.70 −3.67

1.72 1.65 1.61 1.62

Figure 2. Energy level diagrams of copolymer.

PPC-TT-DPP, and the molecular simulation is carried out with one repeating unit (n = 1) for representation by the DFT B3LYP/6-31G(**) level with the Gaussian 09 package.26 For more convenient simulation, the alkyl groups are replaced by methyl groups, where the electronic properties and equilibrium geometries will not be influenced significantly.27 As shown in Figure SI 12 (SI), the theoretic calculations predict that the HOMO coefficients are fully distributed over the whole molecules, while the LUMOs are principally localized on the DPP segment. The full localization of HOMO coefficients along the molecular axis is beneficial for the efficient hole transport through π-orbital interactions. Photovoltaic Characteristics. Photovoltaic devices based on these random copolymers were fabricated with typical device structure based on glass/ITO/PEDOT:PSS/Polymer:PCBM/LiF/Al configuration and tested under simulated

Table 4. Photovoltaic Performance of Copolymers in Standard BHJ Devices with PC61BM polymer PPC-T-DPP_1/1 PPC-T-DPP_2/1 PPC-T-DPP_1/2 PPC-T-DPP_1/2 PPC-T-DPP_1/2 PPC-T-DPP_1/2 PPC-TT-DPP_1/2 PPC-TT-DPP_1/2 PPC-TT-DPP_1/2 PPC-TT-DPP_1/2

polymer/PC61BM 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2

with 5% with 10% CN with 20% CN with 5% CN with 10% CN with 20% CN

Voc (V)

Jsc (mA cm−2)

FF (%)

PCEmax (%)

0.748 0.823 0.714 0.731 0.763 0.725 0.732 0.734 0.748 0.726

3.02 4.07 5.82 5.98 6.46 1.75 6.97 7.33 7.78 2.62

28.9 35.3 47.3 50.2 57.8 27.3 43.8 45.0 40.5 26.9

0.7 1.2 2.0 2.2 2.8 0.3 2.2 2.4 2.4 0.5

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to further investigate their structural organization in the solar cells, as shown in Figure SI 14b (SI). After the PC61BM was blended, despite some sharp peaks caused by PC61BM crystallites,29 the peak at low angle (2θ = 4°) remains in the PPC-T-DPP_1/2:PC61BM blend but is difficult to detect in PPC-TT-DPP_1/2:PC61BM film. It can be clearly manifested that replacing T with TT bridge between PC and DPP block did not optimize the morphology of the blend film but disturbed the molecular packing in the active layer instead. Film Topography. Atomic force microscopy (AFM) was used to investigate the surface morphology of the active layers. Figure 4a,b,c,d shows the typical height images of poly-

Figure 3. Characteristic J−V curves of the BHJ solar cell fabricated from PPC-T-DPP_1/2 and PPC-TT-DPP_1/2 and from their respective blend with or without 1-chloronaphthalene (CN).

the lower FF, suggesting the less ordered stacking of PPC-TTDPP_1/2:PC61BM blends. Hole-only devices were fabricated to measure the hole mobility using the space charge limited current (SCLC) method with a device configuration of glass/ITO/PEDOT:PSS/Polymer:PC61BM/MoO3/Al. The detailed information and mobility curves can be found in the SI (Figure SI 13). The calculated hole mobility values of copolymer are 2.11 × 10−4 cm2 V−1 S−1 for PPC-T-DPP_2/1, 1.75 × 10−4 cm2 V−1 S−1 for PPC-T-DPP_1/1, and 3.08 × 10−4 cm2 V−1 S−1 for PPC-T-DPP_1/2. Higher hole mobility of PPC-T-DPP_1/2 can well explain the higher PCE in solar cells. Additionally, the hole mobility of 3.79 × 10−4 cm2 V−1 S−1 for PPC-TT-DPP_1/ 2 is slightly higher than 3.08 × 10−4 cm2 V−1 S−1 for PPC-TDPP_1/2, leading to a better Jsc in the photovoltaic device. Similarly, CN also was added to the devices which help hole mobility up to 3.94 × 10−4 and 4.51 × 10−4 cm2 V−1 S−1 for PPC-T-DPP_1/2 and PPC-TT-DPP_1/2, respectively. These results of SCLC measurement are far worse than the expectation, hinting the inferior charge-transportation channel may be due to the disorder stacking of copolymer:PC61BM blends. To get a comprehensive view of the reason for the poor performance, X-ray diffraction (XRD) and atomic force microscopy (AFM) were applied to determine the thin-film microstructures and morphologies of these copolymers. Figure SI 14a (SI) shows the X-ray patterns of the random copolymer thin films. For PPC-T-DPP random copolymers with a thiophene bridge, the sharp diffraction peak at low angle 2θ = 4° corresponds to the lamellar structure with d-spacing of 22.10 Å, and the weak peak at high angle 2θ = 21° (3.86 Å) is related to the π−π stacking distance. Incorporation of the TT bridge between PC and DPP block moves the peak associated to the lamellar structure to the higher angle (2θ = 4.68°, 18.87 Å) with stronger reflection intensity, meaning a more compact interlayer stacking. Nevertheless, the half peak width of this peak in PPC-TT-DPP_1/2 is much larger than the one in PPCT-DPP_1/2, inferring the imperfect crystals of PPC-TTDPP_1/2. Such imperfect crystals may come from the worse coplanarity of the backbones, leading to the inferior optical absorption compared with PPC-T-DPP_1/2, which is in good agreement with the blue-shift UV band of PPC-TT-DPP_1/2 film. The blend films of PPC-T-DPP_1/2:PC61BM and PPC-TTDPP_1/2:PC61BM were also detected by XRD measurements

Figure 4. Topography images by AFM of blend films of PPC-TDPP_2/1:PC61BM (a), PPC-T-DPP_1/1:PC61BM (b), PPC-TDPP_1/2:PC61BM (c), and PPC-TT-DPP_1/2:PC61BM (d).

mer:PC61BM blend films, and the corresponding phase images can be found in the Supporting Information (Figure SI 15a,b,c,d). All blend films exhibit small root-mean-square (RMS) roughness of 0.36 nm for PPC-T-DPP_2/1, 0.34 nm for PPC-T-DPP_1/1, 0.30 nm for PPC-T-DPP_1/2, and 0.84 nm for PPC-TT-DPP_1/2. Among these blend films, the PPCT-DPP_1/2:PC61BM film shows the most elongated domains, which is favorable for efficient charge separation, transfer, and transportation. It is worthy to note that although the device based on PPC-T-DPP_1/2:PC61BM obtains a slightly lower PCE than the one made from PPC-TT-DPP_1/2:PC61BM with the TT bridge, the morphology of the PPC-T-DPP_1/ 2:PC61BM blend film is smoother with larger crystal grain and better film interconnectivity than PPC-TT-DPP_1/2:PC61BM film. Therefore, it can be inferred that the rigid TT as a linking unit instead of a T bridge between such big donor and acceptor building blocks will twist the molecular planarity, leading to a disordered molecular packing, as revealed by XRD analysis. Addition of the CN into the blends can further amplify the influence of molecular planarity on the molecular packing. When slow crystallization in the CN additive treats solvent, the PPC-T-DPP_1/2:PC61BM blend can develop much more ordering packing than PPC-TT-DPP_1/2:PC61BM, affording better performance. In addition, for the PPC-T-DPP_1/ 2:PC61BM blend, adding the volume ratio of CN to 10% promotes a more homogeneous morphology with RMS roughness of 0.68 nm compared to the nonadditive film and the film with 5% volume ratio additive, as shown in Figure 5. Further increasing the ratio of CN to 20%, severe phase separation is formed with RMS roughness of ∼3.32 nm, leading to a reduced performance. Similarly, the same phenomenon is also observed in the PPC-TT-DPP_1/2:PC61BM blend film, and the lowest RMS roughness of 1.36 nm is found in the film with 10% volume ratio additive. 6042

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Figure 5. Topography in 3D images by AFM of blend films of PPC-T-DPP_1/2:PC61BM with 5% CN (a), 10% CN (b), 20% CN (c), and PPC-TTDPP_1/2:PC61BM with 5% CN (d), 10% CN (e), and 20% CN (f).

Figure 6. Optimized geometry of PC-T/TT-DPP modeled oligomers by DFT/B3LYP/6-31G**.

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absorption. The absorption spectrum and energy level are welltuned through the variation of PC to DPP composition in the copolymers. Among the PPC-T-DPP based devices, the best PCE of 2.0% was obtained from PPC-T-DPP_1/2 (PC:DPP = 1:2), with a short-circuit current (Jsc) of 5.82 mA·cm−2, an open-circuit voltage (Voc) of 0.714 V, and a fill factor (FF) of 47.3%. After changing T to TT, the slightly better PCE is achieved with a higher Jsc of 6.97 mA cm−2 due to the better charge mobility of the planar TT moiety (confirmed by SCLC measurement). The DFT calculation results reveal a large dihedral angle between the PC and DPP molecule and big backbone torsion in copolymer backbones, and the XRD patterns show the weak crystalline feature in this kind of random copolymers. These defects of molecular geometry strongly influence their charge-transport properties and lead to the poor PCE in the resulting solar cells. Additionally, the bigger backbone torsion in PPC-TT-DPP_1/2 results in the inferior spectral absorption (blue-shift UV band) and lower PCE in the CN-treated device compared with PPC-T-DPP_1/ 2. Therefore, we can conclude that simply increasing the fused rings to form a condensed aromatic structure may afford a negative function on molecular arrangement, and a comprehensive understanding on the whole molecular geometry seems more important for PSC molecular design.

The PC unit is a highly extended fused ring, and the DPP unit is also a favorable building block with strong planarity; thus, it is surprising that PPC-T-DPP and PPC-TT-DPP show such unsatisfactory performance. From the AFM observation, the morphologies of PPC-T-DPP_1/2:PC61BM and PPC-TTDPP_1/2:PC61BM are not so bad to afford such low PCE of devices. It is speculated that the origin behind the poor efficiency should be correlated to the unfavorable packing in the organic phase of the bulk heterojunction, influenced by molecular geometry of these copolymers. Therefore, molecular simulation is executed on these random copolymers with the different arrangements of PC-T-DPP, PC-T-PC-T-DPP, PC-TDPP-T-DPP, PC-TT-DPP, PC-TT-PC-TT-DPP, and PC-TTDPP-TT-DPP. As shown in Figure 6a, for PC-T-DPP, the energy-minimized dihedral angle between the planes of PC and T is 53° (outward direction) and −15° (inward direction) between the same T and DPP, so the interfacial angle between PC and DPP is ∼70°. When the molecule arranges in the mode of PC-T-PC-T-DPP (Figure 6b), the dihedral angle between the two adjacent PC blocks increases greatly to 100°, and the one between PC and DPP is unchanged. However, for the mode of PC-T-DPP-T-DPP (Figure 6c), insertion of one more DPP unit enables the slight decrease of the dihedral angle between PC and DPP, explaining why the copolymer with the more molar weight of DPP (PPC-T-DPP_1/2) obtained the better performance. When changing T to TT (Figure 6d,e,f), the adjacent blocks maintain the large dihedral angle (about 60° between PC and DPP), although the more planar TT unit decreases this angle by ∼10°. Since the PC molecule can be regarded as fusing three hexatomic rings along one side of the vertical orientation of the carbazole core, this largely uniaxially extending π-conjugation will cause a large dihedral angle with the neighboring unit. On the other hand, incorporation of the axisymmetrical PC molecule with centrosymmetrical DPP together results in the nonlinear-shaped backbone, even with the help of the T or TT bridge. Obviously, such a large dihedral angle and big backbone torsion lead to the disordered molecular packing in the thin film, consequently resulting in the unacceptable performance. Therefore, although extending π-conjugation structure is a favorable means for molecular design, simply increasing more fused rings to form large a πconjugation structure would be counterproductive, and other factors should be involved, like the molecular symmetry and linkage. In addition, in the cases of PC-T-DPP_1/2 and PC-TTDPP_1/2, the dominant arrangements are PC-T-DPP-T-DPP and PC-TT-DPP-TT-DPP, respectively. It can be found that the dihedral angles between the linking unit (T/TT) and DPP unit get bigger by replacing T with TT (13° vs 16°, 6.3° vs 9°, 12° vs 17°) (Figure 6c and f). This means that the T bridge can facilitate more ordered molecular packing than TT, especially promoted by slow crystallization in the CN additive-treated solvent. The results again demonstrate that the most important design principle of D−A conjugated polymers for high performance PSCs should be consideration of the whole molecular geometry, rather than only pursuing π-extended fused rings.

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ASSOCIATED CONTENT

S Supporting Information *

The synthetic routes and structural characterizations, TGA, DSC, CV, and SCLC curves of copolymer, image of calculated HOMO and LUMO levels, XRD patterns, and additional AFM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 791 83969562. Fax: +86 791 83969561. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51263016 and 51003045).

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REFERENCES

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CONCLUSIONS In conclusion, a series of random copolymers based on the multifused aromatic ring donor PC with strong acceptor DPP were designed and prepared to create a novel condensed aromatic D−A random conjugated copolymer with broad 6044

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