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Dec 10, 2013 - Enhancing Photovoltaic Performance of Copolymers Containing Thiophene Unit with D–A Conjugated Side Chain by Rational Molecular ...
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Enhancing Photovoltaic Performance of Copolymers Containing Thiophene Unit with D−A Conjugated Side Chain by Rational Molecular Design Ping Shen,†,‡ Haijun Bin,†,‡ Lu Xiao,† and Yongfang Li†,§,* †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, China § College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China. S Supporting Information *

ABSTRACT: Rational molecular design of conjugated polymers and cautious optimization of morphologies of the active layer are critical for developing high performance polymer solar cells (PSCs). In this work, we designed and synthesized a new thiophene monomer TBTF attaching donor−acceptor (D−A) conjugated side chain with fluorinated 4,7-dithien-5-yl-2,1,3-benzodiathiazole (BTF) as acceptor unit, and synthesized two new two-dimension-conjugated (2D-conjugated) copolymers, P(BDT-TBTF) and P(BDTTBTF/DPP), for the application as donor materials in PSCs. P(BDT-TBTF) is a new side chain D−A copolymer of benzodithiophene (BDT) and TBTF units, and P(BDTTBTF/DPP) is a ternary D−A copolymer of BDT, TBTF and pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) units. The introduction of TBTF unit with D−A conjugated side chain and DPP unit forming the ternary copolymer provides the opportunity to tune the optoelectronic properties of the resulting polymers. As expected, the binary copolymer P(BDT-TBTF) shows an enhanced absorption coefficient and lower-lying HOMO energy level, and the ternary copolymer P(BDT-TBTF/DPP) possesses a small bandgap and quite broad absorption band matched well with solar spectrum. These features are beneficial to achieving reasonable high short-circuit current (Jsc) and high open-circuit voltage (Voc). Bulk-heterojunction PSCs based on P(BDT-TBTF) showed an initial power conversion efficiency (PCE) of 5.66% with a high Voc of 0.88 V and a Jsc of 11.23 mA cm−2, whereas P(BDT-TBTF/DDP) gave a PCE of 3.51% along with a higher Jsc of 13.15 mA cm−2. The Jsc and PCE of the devices were further improved by a simple methanol treatment, to 13.21 mA cm−2 and 6.21% for P(BDT-TBTF) and 14.56 mA cm−2 and 5% for P(BDT-TBTF/DPP), respectively. To the best of our knowledge, the PCE of 6.21% is the highest value reported for PSCs based on side chain D−A copolymers to date. This is a good example for a subtle tuning absorption properties, energy levels, charge transport and photovoltaic properties of the polymers by rational molecular design.

1. INTRODUCTION Polymer solar cells (PSCs) have attracted increasing attentions in the research community due to their potential advantages of low cost, lightweight, and large-scale roll-to-roll production.1−5 The most successful PSCs are the bulk heterojunction (BHJ) devices which are based on a blend active layer of electrondonating conjugated polymers and electron-accepting fullerene derivatives such as [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM). It is well recognized that an ideal polymer donor in PSCs should possess broad and strong absorption in the visible region, suitable electronic energy levels well matching with the fullerene acceptor, high charge transport ability, and appropriate compatibility with the fullerene acceptor to form © 2013 American Chemical Society

nanoscale bicontinuous interpenetrating network. Optimizing the properties of the polymer donor can offer high values of short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and result in high power conversion efficiency (PCE) of the PSCs. Two-dimension (2D)-conjugation and donor−acceptor (D− A) copolymerization have been proven to be two successful strategies in the molecular design of high performance conjugated polymer donor materials.3 The 2D-conjugated polymers (such as polythiophene derivatives with conjugated Received: September 11, 2013 Revised: December 1, 2013 Published: December 10, 2013 9575

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Figure 1. Chemical structures of PTG1 and the two new copolymers P(BDT-TBTF) and P(BDT-TBTF/DPP).

Scheme 1. Synthetic Routes of the Monomers and the Corresponding Copolymers

mobility and a moderate PCE of ∼5%.9a,b However the relatively narrow absorption band and weak absorbance limit further improvement of its photovoltaic properties. Recently, fluorine substitution on the acceptor unit has been successfully performed for improving the photovoltaic performance of the D−A copolymers.15−18 The fluorine substitution can further decrease HOMO energy level and enhance the interchain interaction of the D−A copolymers. However, most fluorinated acceptor units have been embedded in the main chains of the polymers, there are few attempt of introducing these units into side chains of conjugated polymers.10 In addition, pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) as a strong electron-withdrawing unit, is widely used to build high

side chains,6 copolymers based on benzodithiophene (BDT) with bithienyl conjugated substitutents,7 and copolymers with conjugated donor−acceptor (D−A) structured side chains8−10) show broad absorption, higher hole mobility and relatively lowlying HOMO energy levels, which are attractive for high performance conjugated polymer donor materials for PSCs. Low bandgap conjugated polymers with a D−A backbone have also attracted great attention for the high performance conjugated polymer donors.11−14 In a earlier research, Tan et al. reported a benzo[1,2-b:4,5-b′]dithiophene (BDT)-based 2D-conjugated D−A copolymer PTG1 (see Figure 1) with 4,7dithien-5-yl-2,1,3-benzodiathiazole as electron-withdrawing conjugated side chains. This polymer showed high hole 9576

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performance D−A copolymers.19 Moreover, DPP-containing units have been employed as the third copolymerization unit to build ternary copolymers and the optoelectronic properties of the copolymers were improved.20 Among the 2D-conjugated polymers, the copolymers based on thiophene unit with conjugated D−A side chain demonstrate the advantages of enhanced absorbance and lowlying HOMO energy level.9 On the basis of the aforementioned points, for further improving photovoltaic performance of the 2D-conjugated side chain D−A copolymers, we herein designed and synthesized a new 2D-conjugated side-chain D−A copolymer P(BDT-TBTF) and a random ternary DPPcontaining D−A copolymer P(BDT-TBTF/DPP) (as shown in Figure 1) based on BDT and thiophene unit with the fluorinated benzothiadiazole (BT)-containing conjugated side chain. It should be mentioned that the substituent on terminal thiophene of the conjugated side chain of P(BDT-TBTF) is different from the control polymer PTG1 besides the fluorine substitution. We used the branched 2-ethylhexyl group instead of hexyl in PTG1 to ensure good solubility of the resulting polymers P(BDT-TBTF), because the solubility usually reduces in the fluorinated polymers.10 In addition, the reactivity of the α position of the terminal thiophene is higher than that of the beta position. The alkyl chain (2-ethylhexyl) was attached on the α position of the terminal thiophene to improve the chemical stability of polymer in some extend. The 2D-conjugated side chain D−A copolymer P(BDT-TBTF) showed enhanced absorbance and lower HOMO energy level than the control polymer PTG19a,b (Figure 1) without the fluorine substitution on the BT unit of the conjugated side chain. And the ternary copolymer P(BDT-TBTF/DPP) displayed a broad absorption from 300 to 800 nm. The PSCs based on copolymer:PC70BM blends gave a high PCE of 5.66% for P(BDT-TBTF) and 3.51% for P(BDT-TBTF/DPP). Higher PCE values up to 6.21% and 4.99% were achieved by a simple methanol treatment before deposition of metal electrode for P(BDT-TBTF) and P(BDT-TBTF/DPP), respectively. To the best of our knowledge, the PCE of 6.21% is the highest value reported for PSCs based on side chain D−A copolymers to date.9c

with the aim of maintaining the length of the DTBT side chain because too long side chain (e.g., introducing other group at the end of DTBT) is deleterious to the hole mobility.9b Two branched 2-ethylhexyl replaces the hexyl (as shown in PTG1 in Figure 1) to ensure the good solubility of the resulting polymers. New compounds 2−4 and M1 were satisfactorily characterized by 1H NMR, 13C NMR and MS or MALDI-TOF. The copolymer P(BDT-TBTF) was directly synthesized by the Stille coupling reaction between the monomers M1 and M2. The DPP-containing ternary copolymer P(BDT-TBTF) was also obtained via the Stille coupling reaction of a distannane monomers M2 with the dibromide monomer M1 and M3 with a feed mole ratio of 1:0.7:0.3. The consideration for the selection of the mole ration of the three monomers is as follows: The electron-donating unit of M2 is equal to the sum of the two units of M1 with electron-withdrowing side chain and the electron-accepting M3. For the ratio of M1 to M3, making the strong electron-accepting unit M3 be less than that of the relatively weak electron-accepting unit M1. The copolymers have good solubility in common organic solvents such as chloroform, tetrahydrofuran, chlorobenzene, and odichlorobenzene (DCB). Table 1 summarized the polymerTable 1. Polymerization Results and Thermal Properties of Copolymers copolymer

yield (%)

Mn (kDa)a

Mw (kDa)a

PDI

Td (°C)b

P(BDT-TBTF) P(BDT-TBTF/DPP) PTG1c

75.6 87.3 78.0

11.1 11.9 20.2

29.8 22.4 52.5

2.68 1.88 2.6

339 334 368

a

Determined by GPC in THF based on polystyrene standards. Decomposition temperature, determined by TGA in nitrogen, based on 5% weight loss. cThe data from ref 9b.

b

ization results and thermal properties of the copolymers. The molecular weights and polydispersity indices (PDIs) of the polymers were determined by gel permeation chromatography (GPC) analysis with a polystyrene standard calibration. The two copolymers had approximately equal number-average molecular weights (Mn) of 11.5 kDa but different weightaverage molecular weights (Mw). The thermal stability of conjugated polymers is very important for their application in optoelectronic device. The thermal properties of the copolymers were obtained by thermogravimetric analysis (TGA). As shown in Figure 2 and

2. RESULTS AND DISCUSSION 2.1. Materials Synthesis and Chemical Characterization. Synthetic routes of the monomers and copolymers are shown in Scheme 1. The monomers M221 and M319a were synthesized by the reported methods. To eliminate the potential active site at one thiophene unit of fluorinated 4,7dithien-5-yl-2,1,3-benzodiathiazole (DTBT) side chain and optimize the molecular structure, we designed a new strategy to synthesize the fluorinated DTBT side chain. First, compound 3 was obtained via two steps: Stille coupling reaction between compound 1 and 5-(2-ethylhexyl)-2(tributylstannyl)thiophene to synthesize compound 2 which is further coupled with 4-(2-ethylhexyl)-2-(tributylstannyl)thiophene to get compound 3. Then, the aldehyde-functionalized intermediate 4 was prepared by Vilsmeier−Haack formylation of compound 3 under a mild condition. Finally, monomer M1 was synthesized from compounds 4 and (2,5dibromothiophen-3-yl-methyl) phosphonic acid diethyl ester, in a moderate yield (55.8%) according to the Wittig−Horner reaction, and purified using a silica gel column with an hexane/ dichloromethane eluent. Furthermore, we employed two (2ethylhexyl)thiophene to build fluorinated DTBT side chain

Figure 2. TGA plots of the polymers with a heating rate of 20 °C/min under N2 atmosphere. 9577

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Figure 3. UV−vis absorption spectra of P(BDT-TBTF), P(BDT-TBTF/DPP) and PTG1 in dilute chloroform solutions (10−5 mol L−1) (a) and P(BDT-TBTF) and P(BDT-TBTF/DPP) in the film states (b).

Table 2. Optical and Electrochemical Properties of the Two Copolymers copolymer

solution λs,max(nm)a

film λf,max(nm)b

λedge (nm)c

Egopt (eV)d

EHOMO (eV)e

ELUMO (eV)e

Egec (eV)f

P(BDT-TBTF) P(BDT-TBTF/DPP) PTG1g

384, 492 384, 486 674 504

392, 508 394, 494 660, 718 517

653 792 644

1.90 1.57 1.92

−5.36 −5.25 −5.44

−3.19 −3.27 −3.44

2.17 1.96 2.00

a Measured in dilute chloroform solution. bMeasured on quartz plate with polymers cast from chloroform solution. cAbsorption edge of the thin films. dEstimated from the onset wavelength of the absorption spectra: Egopt = 1240/λedge. eCalculated according to the equation: EHOMO/LUMO = −e(Eox/red + 4.41) (eV). fBandgap obtained from ELUMO − EHOMO. gThe data from ref 9b.

improved absorption coefficient should be attributed to the introduction of the strong dipole moment caused by fluorine in P(BDT-TBTF). In comparison with P(BDT-TBTF), the ternary copolymer P(BDT-TBTF/DPP) shows a significantly broad absorption covering a wavelength range of 300−800 nm with one more λs,max (at 674 nm) in long wavelength except for the two absorption peaks (at 384 and 486 nm) corresponding to P(BDT-TBTF). Apparently, the new absorption peaks at long wavelength region are attributed to the ICT interaction of the BDT and DPP units. In comparison with the absorption spectra of the polymer solution, the absorption peaks of P(BDT-TBTF) film (λf,max) is red-shifted (Figure 3b, Table 2). At the same time the absorption band is broadened remarkably, which can be elaborated readily by the full widths at half-maximum. As shown in Figure 3b, the full width at half-maximum of P(BDTTBTF) is about 290 nm, greater than that (about 200 nm) for its solution. This phenomenon results from the enhanced interchain interaction in the solid films, which can be attributed to the strong dipole moment caused by fluorine in P(BDTTBTF), as mentioned above. However, the film of the ternary copolymer P(BDT-TBTF/DPP) shows some different features. The two absorption peaks in short wavelength region are slightly red-shifted to 394 and 494 nm, while the two absorption peaks in long wavelength region are slightly blueshifted to 660 and 718 nm, respectively. The similar result was also observed in other DPP-based copolymers.22 The optical band gaps (Egopt) of P(BDT-TBTF) and P(BDT-TBTF/DPP) are 1.90 and 1.57 eV, respectively, calculated from the absorption edges (λedge) of solid state films (Table 2). P(BDT-TBTF/DPP) displays a lower bandgap than P(BDTTBTF) because of the introduction of DPP unit into polymer main chains. Furthermore, the bandgap of P(BDT-TBTF) is slightly lower than that of the control polymer PTG1 (1.92 eV), 9a,b implying that the introduction of fluorinated benzothiadiazole unit as the conjugated side chains benefits

Table 1, the degradation temperatures (Td) of the copolymers with 5% weight loss are similar at ca. 339 and 334 °C respectively, which is slightly lower than that of the analogue polymer PTG1 (∼360 °C).9a,b Even so, the thermal stability of the two polymers is good enough for the application in photovoltaic devices. 2.2. Optical Properties. The optical properties of the copolymers were investigated by UV−visible (UV−vis) and photoluminescence (PL) spectroscopy. Figure 3a shows the UV−vis absorption spectra of P(BDT-TBTF) and P(BDTTBTF/DPP) in dilute chloroform solution (10−5 mol L−1), along with the absorption spectrum of the control polymer PTG1 for comparison. The corresponding absorption data of the copolymers are summarized in Table 2. In solution, P(BDT-TBTF) displays two absorption peaks (λs,max). The first one located at 384 nm corresponds to the π−π* transition of the polymeric backbone in short wavelength region. The second one at 492 nm in long wavelength region is attributed to the ICT interaction between the donor unit in the main chain and the acceptor unit at the side chain. P(BDT-TBTF) exhibits a slight blue-shift of the maximum peak value for ICT (12 nm) as compared to the control copolymer PTG1 (504 nm, Table 2). This is probably because the bulky (2-ethyl)hexyl substituent in the fluorinated DTBT side chain disrupted the planar conformation of P(BDT-TBTF) to some extend. It should be noted that the molar extinction coefficient of P(BDT-TBTF) at peak wavelength (492 nm) is 5.59 × 104 M−1 cm−1, which is much higher than that of PTG1 (500 nm, 3.39 × 104 M−1 cm−1). This result demonstrates convincingly that P(BDT-TBTF) has a better ability of light-harvesting than that of PTG1, indicating the former could give a higher Jsc than the latter. It should be mentioned that the enhance absorbance of P(BDT-TBTF) solution is not from aggregation of the fluorinated polymer, because the absorption peak of P(BDTTBTF) film is obviously red-shifted (by 16 nm, see Table 2) compared to its solution and there is no absorption shoulder both in solution and solid film (see Figure 3). Therefore, the 9578

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devices, and they can be estimated from the onset oxidation and reduction potentials of cyclic voltammogram (CV).23 Figure S2 in the Supporting Information shows the CV curves of the two copolymer films on the Pt electrode in acetonitrile solution containing 0.1 M Bu4NPF6 as a supporting electrolyte. The onset potentials for oxidation (Eox) were observed to be 0.95 and 0.84 V vs. Ag/AgCl for P(BDT-TBTF) and P(BDTTBTF/DPP), respectively. On the other hand, the onset potentials for reduction (Ered) of them were found to be −1.22 and −1.14 V vs. Ag/AgCl, respectively. The energies of HOMO and LUMO (EHOMO/LUMO) was estimated from the Eox and Ered which was calibrated with the ferrocene/ferrocenium couple (Fc/Fc+ measured as 0.39 V vs Ag/AgCl). The Fc/Fc+ energy level used in EHOMO/LUMO calculations was assumed to be −4.80 eV.24 Thus, the HOMO and LUMO energy levels as well as the electrochemical energy gaps (Egec) of the copolymers were calculated according to the following equations:

to lower the bandgap of the resulting polymer in some extend. The absorption edge (792 nm) of the ternary copolymer P(BDT-TBTF/DPP) is red-shifted to the near-infrared region, implying that this polymer has great potential for effective photon-harvesting and could achieve a high short-circuit current in PSCs. The photoluminescence (PL) spectra of the copolymers (see Figure 4 and Figure S1 in the Supporting Information) in

E HOMO = −e(Eox + 4.41) (eV) E LUMO = −e(Ered + 4.41) (eV) Eg ec = e(Eox − Ered) (eV)

Figure 4. Normalized photoluminescence spectra of the copolymer P(BDT-TBTF) excited at different λs,max in dilute CHCl3.

The calculated HOMO and LUMO energy levels and Egec of the copolymers are also summarized in Table 2. The EHOMO levels of P(BDT-TBTF) and P(BDT-TBTF/ DPP) are calculated to be −5.36 and −5.25 eV, respectively. Regarding the threshold HOMO level for air stable conjugated polymers being estimated to be −5.2 eV,25 the deep HOMO levels of the two copolymers, especially for P(BDT-TBTF), should be beneficial to their chemical stability in ambient conditions. In addition, the deep EHOMO of the polymers is expected for higher open-circuit voltage (Voc) of the PSCs with the polymers as donor materials because the Voc is usually proportional to the difference between the HOMO level of the donor and the LUMO level of the acceptor.26 The EHOMO of P(BDT-TBTF) is lower than that of P(BDT-TBTF/DPP), indicating that it would achieve a higher Voc value. On the other hand, a polymer donor intended for use with a soluble fullerene acceptor (e.g., PC60BM or PC70BM) should have a LUMO offset of approximately 0.3−0.4 eV relative to the fullerene acceptor (ca. −3.91 eV for PC70BM27) for the effective charge transfer. The ELUMO of P(BDT-TBTF) and P(BDT-TBTF/ DPP) are −3.19 and −3.27 eV, respectively, which are

diluted CHCl3 solution are also measured. The copolymers are excited at the wavelength corresponding to the λs,max values. As shown in Figure 4, P(BDT-TBTF) exhibits almost the same PL spectra with a maximum emission peak at 648 nm when it is excited at 384 and 492 nm, respectively. As for the ternary copolymer, when it is excited with wavelengths of three λs,max (384, 486, and 674 nm), P(BDT-TBTF/DPP) displays almost the same PL spectra with a maximum emission peak at 704 nm (see Figure S1 in the Supporting Information). These results indicate that there is a thorough intramolecular energy transfer of the excitons from the conjugated side chains to the main chains. This phenomenon ensures that all photons absorbed by the copolymers are useful for the photovoltaic conversion.6b,9a−c 2.3. HOMO and LUMO Energy Levels Measured by Electrochemical Method. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the conjugated polymers are important parameters for the applications in optoelectronic

Table 3. Photovoltaic Properties for the PSCs Based on the Two Copolymers:PC70BM Devices with Different Weight Ratios under the Illumination of AM1.5G, 100 mW/cm2 active layer P(BDT-TBTF):PC70BM = 1:1 P(BDT-TBTF):PC70BM = 1:2 P(BDT-TBTF):PC70BM = 1:3 P(BDT-TBTF):PC70BM = 1:4 P(BDT-TBTF):PC70BM = 1:5 P(BDT-TBTF):PC70BM = 1:4b P(BDT-TBTF/DPP):PC70BM = P(BDT-TBTF/DPP):PC70BM = P(BDT-TBTF/DPP):PC70BM = P(BDT-TBTF/DPP):PC70BM =

1:1 1:2 1:3 1:2b

Voc (V)

Jsc (mA cm−2)

FF (%)

0.90 0.86 0.87 0.88 0.87 0.85 0.79 0.74 0.73 0.73

7.82 9.10 10.56 11.23 10.24 3.85 8.63 13.15 9.49 9.82

54.0 57.4 59.0 57.3 58.1 65.6 38.6 36.0 40.6 44.2

PCEmax (PCEaverage)a (%) 3.79 4.49 5.42 5.66 5.17 2.15 2.63 3.51 2.81 3.17

(3.66) (4.13) (5.20) (5.29) (4.96) (1.97) (2.43) (3.22) (2.69) (3.06)

PCEmax is the highest PCE of the devices and PCEaverage is the average value of five devices. bPSC devices were fabricated with 1% (v/v) DIO as an additive. a

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Figure 5. J−V curves of (a) the PSCs based on P(BDT-TBTF):PC70BM with different weight radios and (b) the PSCs with methanol treatment under the optimized wetting time.

weight ratio on PCEs depends mainly upon the Jsc of the devices. Additionally, it was observed that an increase in the PC70BM content resulted in a slight decrease in the Voc. This has previously been observed for other donor polymers,22b,29 The reduction in the Voc correlated with the decrease in the energy of the charge transfer (CT) states with increasing PC70BM concentrations.29c It is clear that the Voc of P(BDTTBTF) is higher than that of P(BDT-TBTF/DPP), which could be readily understood from the lower-lying HOMO energy level of the former (see Table 2). Next, in view of the favorable effect of a processing additive, such as 1,8-diiodooctane (DIO), upon the photovoltaic performance of the reference polymer PTG1,9a we also checked the effect of DIO additive on the two copolymers. Nevertheless, the results showed that the additive of DIO has no positive effect on the device performance of the two polymers. As shown in Table 3, when a little amount of DIO (1%, v/v) was added as the processing additive, the performance of devices based on P(BDT-TBTF) reduced sharply, with a poor PCE of 2.15%, mainly resulting from a sharp decrease of Jsc (from 11.23 to 3.85 mA cm−2) though the FF improved in some extend (from 57.3% to 65.6%). As for the ternary copolymer P(BDT-TBTF/DPP), the PCE dropped from 3.51% to 3.17% with the similar reason for a decreased Jsc from 13.15 to 9.82 mA cm−2. Recently, several groups have demonstrated that the device performance could be considerably enhanced by treatment with some polar solvents (such as methanol, ethanol and propanol) that are commonly used in the solution-processable cathode buffer layers.30 Here we tried to employ a polar solvent methanol to further optimize the photovoltaic performance of the PSCs based on the two copolymers. The process of the methanol treatment is as follows: (i) spin-coating the active layer and drying in inert atmosphere; (ii) adding methanol atop the active layer and waiting for a short time (10−20 s); (iii) spinning off methanol by spinning at high speed (such as 2500r); (iv) evaporating the metal cathode (Ca and Al) onto the active layer, finally. From Table 4, it can be seen that after methanol treatment the photovoltaic performance of the two polymers had been enhanced in some degree, especially for P(BDT-TBTF) where the PCEmax was further improved up to 6.21% with a largely increased Jsc from 11.23 to 13.21 mA cm−2. For P(BDT-TBTF/DPP), the performance improved more remarkably. The PCEmax increased up to 4.99%, along with an increase of Voc from 0.74 to 0.76 V, a Jsc from 13.15 to 14.56

apparently high enough for an effective charge transfer from the copolymers to the fullerene acceptor.28 2.4. Photovoltaic Properties. PSCs were fabricated from the two polymers as donor and PC70BM as acceptor with a traditional device structure of ITO/PEDOT:PSS/polymer:PC70BM/Ca/Al. The active layers were spin-coated from an o-dichlorobenzene (DCB) solution of the donor and acceptor. The corresponding open-circuit voltage (Voc), shortcircuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE) of the devices are summarized in Table 3. Figure 5 shows the current density−voltage (J−V) curves of the PSCs based on the blends of polymer:PC70BM under the illumination of AM1.5G, 100 mW cm−2 and Figure 6 shows the external quantum efficiency (EQE) curves of the PSCs.

Figure 6. EQE curves of the best PSCs based on P(BDTTBTF):PC70BM (1:4, w/w) and P(BDT-TBTF/DPP):PC70BM (1:2, w/w) with or without methanol treatment.

First, P(BDT-TBTF):PC70BM weight ratios ranging from 1:1 to 1:5 were tested in order to study the effect of donor and acceptor concentrations on photovoltaic performance. As shown in Figure 5a and Table 3, when the weight ratio of P(BDT-TBTF) and PC70BM changed from 1:1 to 1:4, the maximum PCE (PCEmax) of the device increased steadily from 3.79% to 5.66%. As the acceptor weight ratio is further increased to 1:5, the PCEmax decreased to 5.17%. Therefore, the optimal blend weight ratio for P(BDT-TBTF):PC70BM was determined to be around 1:4. The blend ratio of P(BDTTBTF/DPP):PC70BM was also optimized and the best PCE of 3.51% was achieved with the optimal blend weight ratio of 1:2 (see Table 3). For the two polymers, the effect of the blend 9580

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Table 4. Wetting Time-Dependent Photovoltaic Properties of the Devices Based on P(BDT-TBTF):PC70BM (1:4, w/w) and P(BDT-TBTF/DPP):PC70BM (1:2, w/w) with Methanol Treatment

a

polymer

wetting time (s)

Voc (V)

Jsc (mA cm−2)

FF (%)

P(BDT-TBTF) P(BDT-TBTF) P(BDT-TBTF) P(BDT-TBTF) P(BDT-TBTF/DPP) P(BDT-TBTF/DPP) P(BDT-TBTF/DPP) P(BDT-TBTF/DPP) PTG1a

0 10 20 30 0 10 20 30 20

0.88 0.87 0.88 0.84 0.75 0.76 0.75 0.74 0.83

12.49 13.07 13.21 12.40 14.18 14.56 14.04 13.40 10.16

54.6 53.5 53.4 55.5 41.1 45.1 46.0 43.9 50.2

PCEmax (PCEaverage) (%) 6.00 6.08 6.21 5.78 4.38 4.99 4.85 4.35 4.23

(5.91) (5.98) (6.04) (5.67) (4.33) (4.93) (4.78) (4.17) (4.05)

Device based on PTG1:PC70BM (1:4, w/w) with 1% (v/v) DIO as the additive.

Figure 7. Wetting time dependence of PSCs performance of P(BDT-TBTF) and P(BDT-TBTF/DPP): (a) Voc, (b) Jsc, (c) FF, and (d) PCE.

mA cm−2, and a FF from 36.0% to 45.1%. The external quantum efficiency (EQE) spectra were also measured (Figure 6) to verify the high Jsc, especially for treatments with methanol. Though the EQE response covers a relatively narrow spectrum (300−720 nm), a high EQE value greater than 50% from 410 to 590 nm was observed for P(BDT-TBTF)-based devices. Moreover, the highest EOE value surpass even 80% after treatments with methanol, which elucidates the high Jsc of P(BDT-TBTF)-based devices. The high EQE should attribute to its obviously better light-harvesting ability of P(BDT-TBTF) relative to PGT1 (see Figure 3a). As shown in Figure 6, the device based on the ternary copolymer P(BDT-TBTF/DPP) shows obviously broader EQE responses than P(BDT-TBTF), which is consistent with the higher Jsc values in the P(BDTTBTF/DPP) device. The Jsc values calculated by integrating the EQE data (Figure. 6) with the AM 1.5G spectrum were 11.91 and 13.35 mA cm−2 for P(BDT-TBTF) and P(BDTTBTF/DPP) devices, respectively. The EQE results agree with

the measured Jsc values, considering that