Investigations of the Conjugated Polymers Based on Dithienogermole

Aug 7, 2014 - Dithienogermole (DTG) Units for Photovoltaic Applications ... the comparisons among these four polymers provide fundamental information ...
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Investigations of the Conjugated Polymers Based on Dithienogermole (DTG) Units for Photovoltaic Applications Qi Wang,†,‡ Shaoqing Zhang,†,‡ Long Ye,‡ Yong Cui,†,‡ Huili Fan,*,† and Jianhui Hou*,†,‡ †

School of Chemistry and Biology Engineering, University of Science and Technology Beijing, Beijing 100083, China State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

‡

S Supporting Information *

ABSTRACT: Conjugated polymers based on the dithienogermole (DTG) units showed promising properties for the applications in polymer solar cells (PSCs), so that the investigations of the natures and photovoltaic properties of the DTG-based polymers with varied backbone structures would be of great interest. In this work, four DTG-based polymers named as PDTG-BDD, PDTG-IID, PDTG-BT, and PDTG-DPP were synthesized and characterized. The results indicate that the DTG-based polymers show varied absorption bands and molecular energy levels. In X-ray diffraction measurements, these polymers show different laminar packing and π−π stacking distances in solid films. The PSC devices based on the four DTGbased polymers were fabricated, and their photovoltaic properties were characterized. The results show that different device fabrication conditions are needed to get optimal photovoltaic performance of these four polymers. The device of PDTG-BDD shows a PCE of 6.3% with a high Voc of 0.935 V, a FF of 65.0%, and a Jsc of 10.3 mA/cm2, which is the highest one in these four polymers; the devices of PDTG-DPP showed a miserably low Jsc of 3.19 mA/cm2 due to the unfavorable morphologies of the polymer:PC71BM blend. Overall, the comparisons among these four polymers provide fundamental information for understanding the correlations among molecular structures and photovoltaic properties of the DTG-based polymers, and how to control or modulate the bandgaps, molecular energy levels, and morphologies of the DTG-polymers will be the key to fully explore their potentials as photovoltaic materials.

1. INTRODUCTION Polymer solar cells (PSCs) have attracted tremendous interest due to their potentials in providing clean energy. As known, the power conversion efficiency (PCE), one of the key parameters for PSCs, can be affected by many factors including the natures of the materials used in the devices and the architectures of the devices. The application of novel donor polymers provides an efficient approach to enhance the photovoltaic performance of PSCs.1−6 In recent years, the donor−acceptor (D−A) copolymers which consist of electron-donating and -withdrawing building blocks have been widely used as the electron donors in highly efficient PSCs due to their superiorities in realizing tunable properties like absorption spectra, molecular energy levels, mobilities, etc. Therefore, many new building blocks with electron-rich properties, like dithienosilole (DTS)7−12 and benzodithiophene (BDT),13−18 were designed and applied in constructing photovoltaic polymers, and these building blocks have played critical roles in promoting the development of the PSC field. In 2011, the dithienogermole (DTG) unit was synthesized and successfully applied in designing highly efficient photovoltaic polymers. The D−A copolymers containing DTG commonly show strong π−π stacking interaction and thus highly ordered crystalline structures in solid state. By using the copolymer of DTG and N-octylthienopyrrolodione (TPD) © 2014 American Chemical Society

(named as PDTG-TPD) as the electron donor material in PSCs, an excellent PCE up to 8.5% has been achieved, which was one of the highest efficiencies then.19 However, although the modulations of band gaps and energy levels of the D−A copolymers based on other types of electron donating building blocks have been well reported,20−24 these strategies have been seldom investigated in the DTG-based polymers.25−34 Considering that the investigation of the natures and photovoltaic properties of the DTG-based polymers with varied backbone structures would be of great interest, four building blocks with electron withdrawing effect including benzodithiophenedione (BDD),35,36 isoindigo (IID),37,38 2,1,3benzothiadiazole (BT), 39−41 and diketopyrrolopyrrole (DPP)42−45 were copolymerized with the DTG units, and four DTG-based polymers named as PDTG-BDD, PDTG-IID, PDTG-BT, and PDTG-DPP as shown in Scheme 1 were synthesized and characterized in this work. The results indicate that the DTG-based polymers show varied absorption bands and molecular energy levels and also different crystalline structures in solid films; thus, these four polymers show varied photovoltaic behaviors in PSC devices. The comparisons Received: April 21, 2014 Revised: July 21, 2014 Published: August 7, 2014 5558

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Scheme 1. Molecular Structures and Synthesis of Four DTG-Based Polymers

devices with a structure of ITO/PEDOT: PSS/polymers:PC71BM/ Ca/Al were fabricated. The details of the preparation conditions of the active layers are provided in the Photovoltaic Properties section. Synthesis of the Polymers. Polymerization of PDTG-BDD and PDTG-DPP. The bis(trimethyltin) monomer DTG (0.242 g, 0.3 mmol) and the dibromide monomer BDD (0.181 g, 0.3 mmol) or DPP (0.272 g, 0.3 mmol) were dissolved in 7 mL of toluene in a 50 mL two-neck round-bottom flask with a reflux condenser. After flushing by argon for 5 min, 15 mg of Pd2(dba)3 and 45 mg of P(otol)3 were added to the flask. After being purged by argon for 20 min, the reactant was heated to 110 °C and stirred for 16 h under argon and cooled. The polymer was precipitated from 100 mL of methanol. After being filtered through a Buchner funnel, 0.220 g of the crude product was obtained and then purified by column chromatography using silica gel and chloroform as the stationary phase and the eluent, respectively. After being concentrated by rotary evaporation, the polymers were precipitated from methanol. Finally, the product polymers were dried under vacuum for 24 h. PDTG-BDD: Yield: 56% (0.156 g). Elements analysis: calculated for C50H70GeO2S4: C, 66.43; H, 7.81; found: C, 66.25; H, 7.80. PDTG-DPP: Yield: 54% (0.193 g). Elemental analysis: calculated for C70H106GeN2O2S4: C, 69.57; H, 8.84; N, 2.32; found: C, 69.15; H, 8.81; N, 2.47. Polymerization of PDTG-IID. The synthetic processes of PDTGIID were almost the same as PDTG-BDD, except the amounts of the monomers and the reaction time of the polymerization; i.e., DTG (0.256 g, 0.319 mmol) and IID (0.205 g, 0.319 mmol) were used, and the time of the polymerization was 15 min only. PDTG-IID: Yield: 59% (0.183 g). Elemental analysis: calculated for C56H76GeN2O2S2: C, 71.10; H, 8.10; N, 2.96; found: C, 70.15; H, 7.87; N, 2.94. Polymerization of PDTG-BT. The DTG monomer (0.483 g, 0.6 mmol) and the acceptor monomer BT (0.177 g, 0.6 mmol) were used for polymerization. After flushing by argon for 5 min, 30 mg of Pd(PPh3)4 was added to the flask. The reactant was purged by argon for another 20 min and then heated to 110 °C. Then, the reactant was stirred at 110 °C for 20 h under the protection of argon. The polymer was purified by the same procedure as mentioned above. PDTG-BT: Yield: 57% (0.214 g). Elemental analysis: calculated for C30H38GeN2S3: C, 60.51; H, 6.43; N, 4.70; found: C, 60.18; H, 6.33; N, 4.45.

among these four polymers provide fundamental information for understanding the correlations among molecular structures and photovoltaic properties of the DTG-based polymers.

2. EXPERIMENTAL SECTION Materials. The BDD monomer (see Scheme 1) was prepared according to our previous reported method.35 4,4′-Bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]germole (DTG) was prepared according to the reported method,25 and (E)-1,1′-bis(2-ethylhexyl)isoindigo (IID), 2,1,3-benzothiadiazole (BT), and 2,5-bis(2-hexyldecyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) were obtained from Solarmer Materials Inc. Pd(PPh3)4 was commercially available from Frontier Scientific Inc. Pd2(dba)3 and P(o-tol)3 were purchased from Tokyo Chemical Industry Inc. All of the commercial available chemicals were used without any further purification. Instruments and Measurements. The elemental analysis data were obtained by using a FLASH EA1112 elemental analyzer. Molecular weight and polydispersity (PDI) of these four DTG-based polymers were estimated by using the gel permeation chromatography (GPC) method with chloroform as eluent and monodispersed polystyrenes as standard at 45 °C. Thermogravimetric analysis (TGA) measurements were performed on a TGA-2050 (TA Instruments Inc). UV−vis absorption spectroscopy measurements were performed using a Hitachi U-3100 UV−vis spectrophotometer. The CV measurements were carried out on a CHI650D electrochemical workstation by using Pt disk (working electrode), Pt wire (counter electrode), and Ag/Ag+ electrode (reference electrode), and the ferrocene/ferrocenium redox couple (Fc/Fc+) was used as the internal calibration. The external quantum efficiency (EQE) curves were recorded with a QE-R3011 (Enli Technology Co., Ltd.). Atom force microscopy (AFM) images were measured by a Nanoscope III A (Vecco) in the tapping mode. Transmission electron microscopy (TEM) was obtained on a JEM-2010 instrument at accelerating voltage of 200 kV. The space-charge-limited current (SCLC) method46 was used for the hole mobility measurements with the device structure of ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (35 nm)/polymer:PC71BM/Au(30 nm). The XRD samples of the polymer films were prepared by casting the solutions of the polymers in chloroform in a concentration of 30 mg/mL; AFM and TEM samples of blend films were prepared the same as the PSC devices just without metal electrodes on the active layers. Fabrication of the PSC Devices. PSC devices were fabricated and measured as our reported method as follows and tested using the same solar simulator and silicon reference cell.47 After cleaning the indium tin oxide (ITO) glass substrates (sheet resistance of 15 Ω/ square) by deionized water, acetone, and isopropanol, successively, the substrates were treated with UV-ozone for 15 min. Then, a 35 nm thick PEDOT:PSS (Heraeus Materials, 4083) layer was spin-cast onto the ITO substrate and then annealed at 150 °C for 15 min under the ambient atmosphere. The active layers were prepared onto the PEDOT:PSS layers by spin-coating, and then 20 nm Ca and 100 nm Al were successively deposited onto the active layer. Finally, the PSC

3. RESULTS AND DISCUSSION Synthesis and Basic Properties of the Polymers. The synthetic routes of PDTG-BDD, PDTG-IID, PDTG-BT, and PDTG-DPP are shown in Scheme 1. The Stille coupling reactions were employed for preparing these four polymers, and the yields of the polymerizations were around 60%. Since the monomers have different relativities, varied polymerization conditions were employed. For example, considering that the BDD monomer has similar molecular structure as TPD, for PDTG-BDD, Pd2(dba)3-P(o-tol)3 was used as the catalyst and the polymerization was carried out in toluene;25 the reported 5559

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Table 1. Molecular Weight, Thermal Stability, and XRD Data of the Polymers polymer

Mwa

Mna

PDIa

Td (°C)

2θ (deg)

d100b (Å)

2θ (deg)

d010b (Å)

PDTG-BDD PDTG-IID PDTG-BT PDTG-DPP

61.1K 515K 15.4K 10.3K

24.2K 133K 2.14K 6.85K

2.52 3.88 7.23 1.50

407 409 411 419

5.36 8.75 5.25 5.12

16.47 10.09 16.81 17.24

21.83 23.75 25.18 24.10

4.07 3.74 3.53 3.69

a

Mw, Mn, and PDI of the polymers were measured by GPC using monodispersed polystyrene as standard and chloroform as eluent. bd100 and d010 are calculated from d sin θ = nλ.

Figure 1. (a) TGA plots of four DTG-based polymers with a heating rate of 10 °C/min under the inert atmosphere. (b) DSC thermograms of four DTG-based polymers under the inert atmosphere.

Figure 2. (a) X-ray diffraction patterns of the polymers films casted from chloroform onto Si substrates. (b) Cyclic voltammograms of the polymers’ films in 0.1 mol/L Bu4NPF6 in acetonitrile solution at a scan rate of 50 mV/s. (c) Normalized absorption spectra of the solutions of the polymers in chloroform. (d) Normalized absorption spectra of the solid films of the polymers.

synthetic methods for the polymers named as P3TI38 and PSBTBT7 were referred for the polymerizations of PDTG-IID and PDTG-BT, respectively. It should be noted that during the polymerization reaction of PDTG-IID the polymer can be precipitated from the toluene after being refluxed for 15 min, meaning that the coupling reaction between DTG and IID units is very quick. The molecular weights and PDI values of these four DTG-based polymers are listed in Table 1. The

number-average molecular weights (Mn) of PDTG-BDD, PDTG-IID, PDTG-BT, and PDTG-DPP are 24.2K (PDI = 2.52), 133K (PDI = 3.88), 2.14K (PDI = 7.23), and 6.85K (PDI = 1.50), respectively. PDTG-BDD, PDTG-IID, and PDTG-DPP show moderate PDI values (1.50−3.88), while PDTG-BT has a very low value of Mn (2.14K) and a high PDI of 7.23, which may be due to its poor solubility. PDTG-BT can only be dissolved in chloroform and hot o-DCB, while the 5560

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Table 2. Optical Properties and Electrochemical Properties of Four DTG-Based Polymers

a

polymer

λmax,sol (nm)

λmax,film (nm)

λedge,film (nm)

εfilm (10−2/nm)

Egopt a (eV)

Eonsetox (eV)

HOMO (eV)

LUMOb (eV)

PDTG-BDD PDTG-IID PDTG-BT PDTG-DPP

595 750 688 810

602 714 701 802

751 817 877 952

0.87 0.94 0.93 0.90

1.65 1.52 1.41 1.30

0.61 0.58 0.19 0.24

−5.41 −5.38 −4.99 −5.04

−3.76 −3.86 −3.58 −3.74

μHole (cm2 V−1 s−1) 1.17 2.88 5.25 2.35

× × × ×

10−4 10−4 10−4 10−3

Calculated from Egopt = 1240/λedge,film. bCalculated from LUMO = HOMO + Egopt.

can be tuned effectively from −5.41 to −4.99 eV, through copolymerizing DTG units with four different electron withdrawing units. Since open circuit voltage (Voc) of PSC is directly proportional to the energy levels offset between the HOMO of the donor and the LUMO of the acceptor, the PSCs of PDTG-BDD or PDTG-IID would possess higher Voc than the PSCs of PDTG-BT or PDTG-DPP. Additionally, the HOMO level and Egopt of PDTG-BT are slightly different with previously reported.26,27 Compared to their Si-bridged analogous, the DTS-based polymers, the DTG-based polymers usually have higher HOMO levels; e.g., the HOMO of PDTG-IID and PDTGBT are −5.38 and −4.99 eV, respectively, which are all slightly higher than PDTS-IID (HOMO = −5.55 eV)49 and PDTS-BT (HOMO = −5.05 eV).7 As a result, Voc of PDTS-IID and PDTS-BT are higher than the DTG-based polymers, respectively.51,52 For the comparison between PDTS-BT and PDTG-BT, the results were not in exact accordance with previous report,53 which may be probably due to the effects of the alkyl chains. As listed in Table S5, density functional theory (DFT) calculations by the B3LYP/6-31G* basis were used to demonstrate the electronic structures (HOMO and LUMO surfaces) of the four DTG-based polymers, with a molecular main chain length n = 2. The molecular energy levels obtained from the CV measurements are coincident with the theoretical results obtained from the quantum chemical calculations. As shown in Figure S7, the HOMO surfaces of the four DTGbased polymers are distributed along the conjugated backbone evenly. For PDTG-IID and PDTG-BT, LUMO surfaces are mainly located at the electron deficient units (IID and BT, respectively). Optical Properties. The absorption spectra of these four DTG-based polymers in chloroform solutions and as solid films are shown in Figures 2c and 2d, respectively. The four polymers show much different absorption spectra. As listed in Table 2, the peaks of PDTG-BDD, PDTG-IID, PDTG-BT, and PDTGDPP in solution are located at 595, 750, 688, and 810 nm, respectively. From solution state to solid film, the absorption peaks of PDTG-BDD and PDTG-BT are red-shifted, while those of PDTG-IID and PDTG-DPP are blue-shifted. These four polymers show much different absorption edges at long wavelength direction, and the optical band gaps are 1.65 eV for PDTG-BDD, 1.52 eV for PDTG-IID, 1.41 eV for PDTG-BT, and 1.30 eV for PDTG-DPP. Generally speaking, PDTG-BT and PDTG-DPP exhibit broader absorption spectra in long wavelength region than PDTG-BDD and PDTG-IID, probably meaning that higher photocurrent densities may be obtained in the PSCs based on PDTG-BT and PDTG-DPP than the other two polymers. Moreover, as listed in Table 2, the four DTGbased polymers exhibit relatively high and much similar absorption coefficients in film, which are all around 0.9 × 10−2/nm, implying they would have no shortage in absorption coefficients. PDTG-IID and PDTG-BT show narrower optical

other three polymers show good solubilities in chloroform, chlorobenzene, and o-DCB The TGA plots of these four DTG-based polymers are shown in Figure 1a, and their decomposition temperatures (Td) are collected in Table 1. Clearly, all these four polymers exhibit excellent thermal stability with Td values over 400 °C, which are adequate for fabrication process of PSC devices. For polymer PDTG-DPP, the thermal stability is even better than the Sibridged analogue, PDTS-DPP,48 and the Td = 409 °C of PDTG-IID is the same as the Si-bridged analogue, PDTS-IID.49 The differential scanning calorimetry (DSC) measurements were used to probe the crystallization process of the polymers in the solid state. As shown in Figure 1b, there are no apparent exothermic and endothermic peaks in DSC analysis for all the four polymers. X-ray Diffraction Analysis. The formation of ordered structure within thin polymer film can be investigated by the Xray diffraction (XRD) measurement. The XRD patterns of the thin films of these four polymers casted from chloroform are shown in Figure 2a. The results indicate that all these four polymers show two pronounced reflections. The peaks at low 2θ zone and high 2θ zone are attributed to the ordered laminar packing and interchain π−π stacking, respectively. According to the Bragg equation and the reflection peaks, the d-spacing of the laminar packing and the distance of the π−π stacking of the polymers can be calculated. As listed in Table 1, the d-spacings of the laminar packing for PDTG-BDD, PDTG-IID, PDTGBT, and PDTG-DPP are 16.47, 10.09, 16.81, and 17.24 Å, respectively. The XRD data of DTG-BT were consistent with the reported work.50 Clearly, PDTG-DPP shows the longer dspacing for laminar packing than the other three polymers, which should be ascribed to the long alkyl side chains on the DPP units. Moreover, these polymers show different π−π stacking distances, which are 4.07, 3.74, 3.53, and 3.69 Å for PDTG-BDD, PDTG-IID, PDTG-BT, and PDTG-DPP, respectively. Obviously, PDTG-BT has smaller π−π stacking distances than the other three polymers. The d-spacing of the laminar packing and the distance of the π−π stacking of the polymer PDTG-BT are similar to its Si-bridged analogue, PDTS-BT.51 Electrochemical Properties. Electrochemical cyclic voltammetry (CV) was performed to evaluate molecular energy levels of the polymers. Figure 2b shows the CV plots of the four DTG-based polymers’ films. As listed in Table 2, the oxidation onset potentials of PDTG-BDD, PDTG-IID, PDTG-BT, and PDTG-DPP are 0.61, 0.58, 0.19, and 0.24 V, corresponding to the highest occupied molecular orbital (HOMO) levels of −5.41, −5.38, −4.99, and −5.04 eV, respectively. It should be noted that since it is very hard to get a sharp n-doping signal for these four polymers, CV curves of n-doping processes are not provided here. The lowest unoccupied molecular orbital (LUMO) levels of the four polymers are estimated using “LUMO = HOMO + Egopt”, where Egopt is the optical band gap of the polymer. It is obvious that the molecular energy levels 5561

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Table 3. Photovoltaic Properties of PSCs Devices of Four DTG-Based Polymer/PC71BM Blends polymer

D/A ratio

DIO additive

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

thickness (nm)

PDTG-BDD

1:2 1:3 1:4 1:3 2:1 1:1 1:2 1:1 2:1 1:1 1:2 1:1 1:1 1:2 1:3 1:2

w/o w/o w/o 7% w/o w/o w/o 3% w/o w/o w/o 3% w/o w/o w/o 3%

0.894 0.906 0.885 0.935 0.827 0.798 0.798 0.835 0.621 0.613 0.566 0.561 0.538 0.547 0.544 0.515

7.66 8.66 8.63 10.3 2.70 3.82 2.84 10.7 6.06 11.2 11.1 12.0 1.53 1.86 1.71 3.19

43.0 49.3 48.1 65.0 26.7 40.8 44.0 45.5 36.0 47.5 46.6 53.2 58.5 60.0 62.3 63.6

2.95 3.86 3.68 6.28 0.60 1.24 1.00 4.07 1.36 3.25 2.93 3.59 0.48 0.61 0.58 1.05

65 70 69 71 66 75 80 71 60 62 66 63 67 70 74 67

PDTG-IID

PDTG-BT

PDTG-DPP

band gaps, comparing to their Si-bridged analogous;51,52 however, the optical band gaps of PDTG-DPP and PDTSDPP are very similar.48 Photovoltaic Properties, External Quantum Efficiency, and Mobility. The bulk-heterojunction PSC devices with a structure of ITO/PEDOT:PSS/polymers:PC71BM/Ca/Al were fabricated to investigate photovoltaic properties of these four DTG-based polymers. The active layers were prepared in the solutions of o-DCB with the concentration of 10 mg/mL (polymer/o-DCB). Different spin speeds were used to control the thickness of the active layers of the PSC devices. First, a series of D/A (polymer/PC71BM, w/w) ratios were scanned from 1:0.5 to 1:4 to get the optimal D/A ratios for each polymer. The current density−voltage (J−V) curves are provided in the Supporting Information, and the detailed data are collected in Table 3. It shows that the optimal D/A ratios for the PDTG-IID:PC71BM and PDTG-BT:PC71BM blends are 1:1, and the optimal D/A ratios of for PDTG-DPP:PC71BM and PDTG-BDD:PC71BM are 1:2 and 1:3, respectively. From the device based on PDTG-BDD:PC71BM with the optimal D/ A ratio (polymer:PC71BM = 1:3), a PCE of 3.86% with Voc = 0.906 V, short circuit current density (Jsc) = 8.66 mA/cm2, and fill factor (FF) = 49.3% can be recorded. The PCEs of the devices of PDTG-IID, PDTG-BT, and PDTG-DPP with optimal D/A ratios are 1.24, 3.25, and 0.61%, respectively. Furthermore, 1,8-diiodooctane (DIO) was used as the solvent additive to further enhance photovoltaic performance of the devices. Herein, varied amounts of DIO, from 1% to 10% (v/v, DIO/o-DCB), were fully scanned, and the active layers were treated with an inert solvent according to our recent report.54 The J−V curves and the photovoltaic parameters of the PSC devices with varied amounts of DIO are provided in the Supporting Information, and meanwhile, the photovoltaic results obtained with the addition of optimum amount of DIO are shown in Table 3 and Figure 3. When 7% DIO was added, the Jsc of the PDTG-BDD devices slightly increased from 8.66 to 10.3 mA/cm2, while the FF was significantly improved from 49.3% to 65.0%, so that the PCEs of the devices increased from 3.86% to 6.28%. For the PDTG-IID/PC71BM system, when 3% DIO was added, the Jsc values was improved tremendously from 3.82 to 10.7 mA/cm2, so the PCE was improved from 1.24% to 4.07%. The performance of devices based on PDTG-IID/PC71BM are higher than PDTS-IID/

Figure 3. J−V curves of the devices of four DTG-based polymers fabricated under the optimal conditions.

PC71BM system.49,52 The improvement of the efficiency comes from the increase of Jsc of the devices, which probably attributes to higher hole mobility of the DTG-based polymers and more favorable phase separation morphology of blend films.53 The photovoltaic properties of the PDTG-BT-based device is not very sensitive to the use of DIO; i.e., for the PDTG-BT-based devices with and without the use of DIO, similar PCE values were recorded. The devices based on PDTG-BT in this work show higher Voc and FF than those of the reported works, which might be due to the difference of the molecular weight; i.e., the Mn of PDTG-BT in the reported work is 31 K (PDI = 3.2), while in this work, the Mn is 2.14K only (PDI = 7.23).50 Unfortunately, although the polymer PDTG-DPP shows the broadest absorption spectrum in these four polymers and the corresponding devices show relatively good FF values (FF = 60%), the PSC devices based on this polymer exhibit much lower Jsc values than the devices based on the other three polymers. External quantum efficiency (EQE) curves of the devices based on the four DTG-based polymers prepared through the optimal fabrication processes and the absorption spectra of the active layers of the devices are shown in Figures 4a and 4b. As shown, both the devices based on PDTG-IID and PDTG-BT possess a broad response range covering from 330 to 850 nm, and EQE peak values of the PSCs are 54% and 48%, 5562

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Figure 4. (1) EQE curves and (2) absorption spectra of the active layers of four DTG-based polymers.

Figure 5. AFM height (a−d) and phase (e−h) images and TEM images (i−l) of blend films of polymers/PC71BM processed under optimal conditions.

PDTG-BDD, PDTG-IID, PDTG-BT, and PDTG-DPP, respectively. From PDTG-BDD, PDTG-IID, PDTG-BT, to PDTG-DPP, the polymer:PCBM films show gradually increasing surface roughness. The polymer PDTG-DPP possesses good solubility, excellent thermostability, broad absorption, and relatively better hole mobility; however, the domain sizes of PCBM in blend film are too large (diameters between 200 and 500 nm) to get efficient exciton dissociation. This explains why the EQE response values and the Jsc of devices based on PDTG-DPP are miserably low. Transmission electron microscopy (TEM) was used to further investigate the bulk morphologies of the blend films to confirm the findings in AFM measurements. TEM images of the blend films of PDTG-BDD/PC71BM, PDTG-IID/PC71BM, PDTG-BT/PC71BM, and PDTG-DPP/PC71BM processed with optimal amount of DIO are displayed in Figure 5i−l. The aggregations of PCBM can be easily observed in the TEM images. Fox example, some little dark spots can be found in the blend film of PDTG-BT:PC71BM in Figure 5k; the image of the blend film of PDTG-DPP:PC71BM in Figure 5l shows dark

respectively. The PSC device of PDTG-BDD shows a response range covering from 330 to 750 nm, with EQE peak value of 67%, which is the highest value among the four types of devices. Although PDTG-DPP owns a more broad response range from 330 to 900 nm, but the EQE peak value is 11% only. Moreover, the hole mobilities of PDTG-BDD, PDTG-IID, PDTG-BT, and PDTG-DPP are 1.17 × 10−4, 2.88 × 10−4, 5.25 × 10−4, and 2.35 × 10−3 cm2 V−1 s−1, respectively. There is no big difference among hole mobilities of PDTG-BDD, PDTG-IID, and PDTGBT, and the hole mobility of PDTG-DPP is the highest one among the four polymers, indicating that PDTG-DPP would have no shortage in charge transport, so that we suspected that the blend of PDTG-DPP:PC71BM may have some serious problems in morphologies. Morphology Study. Nanoscale surface morphologies of the blend films of the blends processed with the use of the optimum amounts of DIO were investigated by the atomic force microscope (AFM). As shown in Figure 5a−h, the rootmean-square roughness (Rq) values obtained from the height image was 0.82, 1.61, 3.41, and 5.62 nm for the blends of 5563

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ball-like aggregations with diameters between 200 and 500 nm, which is coincident with the AFM results (Figure 5h).

4. CONCLUSION In this work, four DTG-based D−A copolymers were designed and synthesized. By copolymerizing with four different acceptor units, bandgaps and molecular energy levels of the DTG-based polymers can be tuned effectively. These four polymers show similar thermal properties in the TGA and DSC measurements, while they have different crystalline structures in solid state; i.e., varied laminar packing and π−π stacking distances can be observed in the XRD characterizations. The PSC devices based on the four DTG-based polymers were fabricated, and their photovoltaic properties were characterized. The results show that different device fabrication conditions are needed to get optimal photovoltaic performance of these four polymers. The device of PDTG-BDD shows a PCE of 6.3% with a high Voc of 0.935 V, a FF of 65.0%, and a Jsc of 10.3 mA/cm2, which is the highest one in these four polymers; the devices of PDTG-DPP showed a miserably low Jsc of 3.19 mA/cm2 due to the unfavorable morphologies of the polymer:PC71BM blend. These results suggest that DTG is a promising building block for constructing high performance photovoltaic polymers, and how to control or modulate the bandgaps, molecular energy levels, and morphologies of the polymers will be the key to fully explore their potentials as photovoltaic materials.

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ASSOCIATED CONTENT

* Supporting Information S

FT-IR and 1H NMR spectra and the theoretical simulation results for all of the polymers, J−V curves and the detailed photovoltaic data for the devices based on these four DTGbased polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +86-10-82615900 (J.H.). *E-mail [email protected]; Tel +86-10-62332681 (H.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from International S&T Cooperation Program of China (2011DFG63460), Ministry of Science and Technology of China, (No. 2014CB643501), and NSFC (Nos. 91333204, 51373181, 21325419, 51261160496).

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