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Highly Efficient Photovoltaic Polymers Based on Benzodithiophene and Quinoxaline with Deeper HOMO Levels Delong Liu,†,‡ Wenchao Zhao,† Shaoqing Zhang,† Long Ye,†,‡ Zhong Zheng,†,‡ Yong Cui,† Yu Chen,†,‡ and Jianhui Hou*,†,‡ †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: We present the synthesis and photovoltaic application of four conjugated polymers composed of benzo[1,2-b:4,5-b′]dithiophene (BDT)-based and 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (DTQx)-based units. Fluorination of the DTQx units and the conjugated side groups of the BDT unit shows synergistic effect on molecular energy level modulation of the polymers, and as a result, the polymer PBQ-4 exhibits the deepest HOMO and LUMO levels in these four polymers. The characterizations of the photovoltaic properties of the polymer solar cells (PSCs) based on these four polymers reveal that the fluorination has little influence on short-circuit current density (JSC) and fill factor (FF) but is very helpful to enhance open-circuit voltage (VOC) of the devices. Benefiting from the synergistic effect of the fluorination, the device based on PBQ-4 shows a high VOC of 0.90 V, which is 0.26 V higher than the polymer without fluorine and ca. 0.10 V higher than the other two polymers with less fluorine. As a result, a power conversion efficiency (PCE) of 8.55% was recorded in the PBQ-4 based device, which is much higher than those of the other three polymers and also the highest one for the BDT-Qx-based polymers.

1. INTRODUCTION Polymer solar cells (PSCs) with bulk heterojunction structure (BHJ) have shown great potentials in making large area and flexible solar cell panels through roll-to-roll solution coating process.1−5 In this type of solar cell, conjugated polymers and fullerene derivatives have been widely used as the electron donor and acceptor materials, respectively. As is well-known, open-circuit voltage (VOC) of the BHJ PSCs is directly proportional to the gap between the highest occupied molecular orbital (HOMO) level of donor and the lowest unoccupied molecular orbital (LUMO) level of acceptor in their active layers.6,7 Therefore, new conjugated polymers with deeper HOMO levels are favorable to get higher VOC and thus to promote power conversion efficiency (PCE). In fact, molecular energy level modulation has become one of the most important topics for molecular design of conjugated polymers in PSCs. In the past decade, various methods have been successfully developed and adopted to modulate molecular energy levels of conjugated polymers, and these methods can be classified into two types. First, molecular energy levels of conjugated polymers can be tuned by changing their conjugated backbones. For example, benzo[1,2-b:4,5-b′]dithiophene (BDT) is an important and broadly used conjugated building block in photovoltaic polymers; when 4,8-bis(alkyloxy)-BDT units were copolymerized with different conjugated building blocks like 2,1,3benzothiadiazole (BT), thiophene, thieno[3,4-b]pyrazine © XXXX American Chemical Society

(TPZ), and so on, the HOMO and LUMO levels of the BDT-based polymers can be varied from −4.56 to −5.16 eV and from −2.66 to −3.46 eV, respectively, while the BDTpolymers with varied conjugated backbones often showed different optical band gaps.8−20 Second, molecular energy levels of conjugated polymers can be modulated by changing the substituents on their backbones. For instance, for the conjugated polymers based on 4,8-bis(2-ethylhexyloxy)-BDT and thieno[3,4-b]thiophene (TT), when the substituents on their TT units were changed from carboxylate to ketone group, the HOMO level of the polymer can be reduced from −5.01 to −5.12 eV due to the enhanced electron-withdrawing effect of the functional group.21,22 Moreover, according to the reported works, it can be concluded that although both types of methods mentioned above can be utilized for molecular energy level modulation, the first one has a few disadvantages. When the backbone of a conjugated polymer is changed, its band gap and absorption spectrum can be changed; meanwhile, the target polymer often shows different morphological properties compared to the starting polymer, and hence its photovoltaic properties will be changed distinctly. Therefore, from the point of view of molecular energy level fine-tune, i.e., modulating molecular energy level without changing other properties of Received: April 20, 2015 Revised: July 12, 2015

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DOI: 10.1021/acs.macromol.5b00829 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Molecular Structures and Synthesis Method of the Polymers

the effects of fluorination position on photovoltaic properties, and the investigation of PBQ-4 shows the synergistic effect of the fluorination. The polymers were characterized by UV−vis absorption spectroscopy, electrochemical cyclic voltammetry (CV), ultraviolet photoelectron spectroscopy (UPS), and so on, and also the photovoltaic and morphological properties of the blends based on these polymers and PC71BM were carefully investigated. The results clearly demonstrate that the fluorination on BDT-T and DTQx building blocks has a synergistic effect on molecular energy level modulation, and as a result, the polymer PBQ-4 showed the deepest HOMO level in these polymers and the device based on PBQ-4:PC71BM exhibited a high VOC of 0.90 V and a high PCE of 8.55%.

conjugated polymers, the second method mentioned above shows great superiority and thus has attracted particular attention in recent years. The introduction of fluorine onto backbone of conjugated polymer has been proven to be an effective method to reduce HOMO and LUMO levels. For example, when fluorine was introduced onto TT unit in the polymer PBDTTT-C, the HOMO and LUMO levels of the target polymer (PBDTTTCF) could be tuned from −5.12 to −5.22 eV and −3.35 to −3.45 eV so VOC of the corresponding PSC device was improved from 0.70 to 0.76 V.21 As reported by You and coworkers in 2011, for the polymer based on BnDT and HTAZ, when fluorine was introduced onto the HTAZ unit, HOMO and LUMO levels of the polymer were reduced from −5.29 to −5.36 eV and −2.87 to −3.05 eV, respectively, and hence VOC of the corresponding devices can be improved from 0.70 to 0.79 V.23 More recently, more and more polymers with deeper HOMO and LUMO levels were designed and synthesized by the fluorination of their backbones, and consequently, enhanced VOC and thus higher PCE in PSC devices were realized.24−28 On the other hand, since fluorine is a very small functional group, the introduction of fluorine has little influence on steric hindrance between the conjugated building blocks.29,30 Therefore, developing new methods to utilize the fluorination more effectively is of great importance to design conjugated polymer for the applications in PSCs. In this contribution, following our recent work that the fluorination effect on molecular energy level modulation,31 four conjugated polymers based on thiophene-substituted BDT (BDT-T) and 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (DTQx) building blocks were synthesized, characterized, and applied in PSCs. As shown in Scheme 1, from the polymer PBQ-1 to PBQ-2 or to PBQ-3, fluorine atoms were introduced onto the two conjugated side groups of BDT-T or to the DTQx unit. Then, fluorine atoms were introduced onto both BDT-T and DTQx blocks to derive the polymer PBQ-4. The comparisons among PBQ-1, PBQ-2, and PBQ-3 demonstrate

2. RESULTS AND DISCUSSION 2.1. Synthesis of the Polymers. As shown in Scheme 1, the polymers were prepared by Stille coupling polycondensation reaction between the bis(trimethyltin)-substituted BDT-T monomer (BDT-T or BDT-T-2F) and the dibromo-DTQx monomer (DTQx or DTQx-2F). These four polymers were synthesized with the same conditions, i.e., using toluene as solvent and Pd(PPh3)4 as catalyst; the polymerizations were carried out under refluxing temperature. The molecular weight of the polymers was estimated by gel permeation chromatography (GPC) using chloroform as the eluent at 40 °C and monodispersed polystyrene as the standard. For PBQ-1, PBQ2, PBQ-3, and PBQ-4, the number-average molecular weight (Mn) is 68.5, 10.2, 18.3, and 22.7 kDa, and the polydispersity index (PDI) is 3.07, 2.27, 3.58, and 2.40, respectively. The reason why the Mn of PBQ-1 is higher than for the other three materials probably is that the reactivity of the tin monomers is different. 32,33 According to the results obtained from thermogravimetric analysis (TGA), the decomposition onset temperatures (Td) of these polymers are all above 330 °C (see Figure S1 in Supporting Information). Moreover, the crystallinity of the polymers was investigated by the X-ray diffraction (XRD) method, and no reflection peak can be B


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Figure 1. Normalized UV−vis absorption spectra of the polymers in chloroform solutions (a) and thin films (b).

Table 1. Optical Parameters and Physical Propriety of the Polymers polymer

soln λmax (nm)

soln λedge (nm)

film λmax (nm)

film λedge (nm)

Egopt (eV)

εsoln (104) (M−1 cm−1)

εfilm (104) (cm−1)

Mn (kDa)

PDI

Tda (°C)

PBQ-1 PBQ-2 PBQ-3 PBQ-4

598 593 613 618

725 725 725 710

636 619 600 583

755 745 737 716

1.64 1.66 1.68 1.73

4.54 4.77 5.15 6.17

5.45 7.30 7.31 8.59

68.5 10.2 18.3 22.7

3.07 2.27 3.58 2.40

349 357 354 365

a

Decomposition temperature at 5% weight loss.

Figure 2. Theoretical calculations of the dimers of the four polymers by density functional theory (DFT) at the B3LYP/6-31G(d,p) level.

Table 2. Electrochemical Parameters of Polymers

a

polymer

HOMOa (eV)

LUMOa (eV)

HOMOb (eV)

LUMOb (eV)

HOMOc (eV)

LUMOc (eV)


−5.05 −5.19 −5.19 −5.35

−3.29 −3.46 −3.43 −3.55

−4.66 −4.86 −4.92 −5.05

−2.95 −3.15 −3.21 −3.3

−4.74 −4.86 −4.77 −4.9

−2.42 −2.51 −2.51 −2.61

Energy levels were determined by CV. bEnergy levels were determined by UPS. cEnergy levels were determined by theoretical calculation.

extinction coefficient (εfilm = 8.59 × 104 cm−1), while the polymer PBQ-1 showed the lowest extinction coefficient (εfilm = 5.45 × 104 cm−1); for PBQ-2 and PBQ-3, a similar εfilm of ca. 7.30 × 104 cm−1 was recorded. The improvement of extinction coefficient is probably due to the auxochromic effect of fluorine atom. The results obtained from the absorption measurements show that the band gap of the polymer will be slightly enlarged by the fluorination on both BDT-T and DTQx units, and the extinction coefficient of the polymer film can be slightly enhanced by introducing the fluorine atoms. 2.3. Molecular Energy Levels. We used three methods to evaluate the HOMO and LUMO levels of the four polymers, including quantum chemistry calculation, CV, and UPS methods. For quantum chemistry calculation, the density functional theory (DFT) method using Gaussian 09 package at the B3LYP/6-31G(d,p) level was adopted. In order to simplify the calculation, all the alkyls were replaced by methyl groups, and the dimers were used for the calculations (see Figure 2). Electrochemical CV measurements were carried out

distinguished from the XRD analysis for all of these four polymers (see Figure S2), indicating that they are all amorphous. 2.2. Optical Absorption Properties. UV−vis absorption spectra of the polymers in dilute chloroform solution and thin film are shown in Figures 1a and 1b. In order to make clear comparisons, the detailed results obtained from the UV−vis absorption measurements are collected in Table 1. In solution state, PBQ-1, PBQ-2, and PBQ-3 display very similar absorption spectra with absorption edges at ca. 725 nm, while the absorption edge of PBQ-4 locates at 710 nm. In solid thin films, the absorption edges of PBQ-1, PBQ-2, PBQ-3, and PBQ-4 are 755, 745, 737, and 716 nm, corresponding to optical band gaps (Egopt) of 1.64, 1.66, 1.68, and 1.73 eV, respectively. The extinction coefficients of the polymers in solution (εsoln) and thin film (εfilm) were measured and are listed in Table 1. The εsoln of PBQ-1, PBQ-2, PBQ-3, and PBQ-4 are 4.54 × 104, 4.77 × 104, 5.15 × 104, and 6.17 × 104 M−1 cm−1, respectively. As regards polymers thin films, PBQ-4 showed the highest C


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Figure 3. CV curves (a) of the polymers on glassy carbon electrodes measured in 0.1 mol/L Bu4NPF6 acetonitrile solutions at a scan rate of 20 mV/ s. UPS spectra of the onset (b) and the secondary edge region (c) of the polymers.

Figure 4. J−V curves of PSCs based on PBQ-1:PC71BM with different D/A ratios (a) and varied amount of DIO (b).

which corresponds to a HOMO level of −5.19 eV and is 0.14 eV lower compared to that of PBQ-1; for PBQ-4, the HOMO level is at −5.35 eV, which is 0.30 eV lower than that of PBQ-1. For the UPS measurements, as shown in Figure 3b,c, the HOMO levels of PBQ-1, PBQ-2, PBQ-3, and PBQ-4 are at −4.66, −4.86, −4.92, and −5.05 eV, respectively. Overall, although the HOMO levels for these polymers obtained from these three methods are different, these values demonstrate the same trend; i.e., the HOMO level of PBQ-1 can be reduced by introducing fluorine atoms onto the BDT-T or DTQx units, and also the fluorination on these two building blocks has a synergistic effect on reducing the HOMO level. 2.4. Photovoltaic Properties. PSC devices with an architecture of indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/BHJ blend/Ca/Al were fabricated and characterized to investigate photovoltaic properties of these four polymers. Herein, PC71BM was selected as the electron acceptor material, and o-dichlorobenzene (o-DCB) was used as the processing solvent for fabricating the active layers. First of all, three different compositions of the polymer:PC71BM blends (1:1, 1:1.5, and 1:2; polymer:PC71BM, w/w) were scanned to get the optimal D/A ratio. Taking the blends of PBQ-1:PC71BM as an example, as shown in Figure 4a, when

by the well-known method; i.e., glassy carbon electrode, saturated calomel electrode, and Pt wire were used as the working electrode, reference electrode, and counter electrode, respectively, and 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6) solution in acetonitrile (CH3CN) was used as the electrolyte.34−39 For the UPS measurements, the samples were prepared and characterized referring to a recent work.40 UPS was identified as an effective means to measure the work function. The HOMO and LUMO levels obtained from these three methods are listed in Table 2. As shown in Figure 2, the HOMO surfaces of these four polymers are well delocalized along their backbones, while their LUMO surfaces are mainly localized onto the DTQx units. Since these four polymers show very similar HOMO and LUMO surfaces in shape, it can be concluded that the introduction of fluorine onto BDT-T or/and DTQx units has little influence on delocalization properties of the π-electrons. According to the theoretical calculations, PBQ-2 and PBQ-3 show lower HOMO and LUMO levels compared to PBQ-1, while PBQ-4 shows the lowest HOMO and LUMO levels in these four polymers. In CV and UPS measurements, a similar trend as obtained in theoretical calculations can be confirmed. For example, as shown in Figure 3a, the onset potentials of the p-doping processes of PBQ-2 and PBQ-3 are at ca. 0.79 V, D


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Table 3. Device Performance of the PSCs Based on PBQ-1:PC71BM with Different D/A Ratios and Different Additive Amounts PCE (%) processing solvent DCB

DCB DCB DCB DCB a

+ + + +

1% 3% 5% 8%

DIO DIO DIO DIO

D/A (w/w)

VOCa (V)

JSCa (mA/cm2)

FFa (%)

av

max

thicknessb (nm)

1:1 1:1.5 1:2 1:1.5 1:1.5 1:1.5 1:1.5

0.75 0.73 0.71 0.69 0.65 0.63 0.63

12.07 12.14 9.63 8.35 11.02 12.39 12.55

55.57 58.65 63.17 56.92 65.30 69.81 66.37

5.03 5.20 4.32 3.26 4.67 5.40 5.25

5.14 5.24 4.53 3.28 4.79 5.63 5.44

80 82 85 80 85 80 82

Average data obtained from six devices. bThe thickness of active layer in optimal device.

Figure 5. J−V curves (a) and EQE curves (b) of PSCs based on polymer:PC71BM (1:1.5, w/w) using 5% DIO as additive under standard solar illumination conditions.

Table 4. Optimized Photovoltaic Performances of Devices Based on Polymer:PC71BM (1:1.5, w/w) Using 5% DIO as Additives under Standard Solar Illumination Conditions PCE (%)

a

polymer

a

a

VOC (V)

JSC (mA/cm )


0.63 0.75 0.79 0.90

12.39 12.96 13.46 13.52

2

JSCb

a

2

(mA/cm ) 11.94 11.78 12.92 12.60

FF (%)

av

max

EQE

thicknessc (nm)

69.81 63.14 67.58 70.02

5.40 6.13 7.19 8.52

5.63 6.25 7.39 8.55

5.35 5.67 7.00 7.97

80 80 82 85

Average data obtained from six devices. bCalculated from the EQE spectra shown in Figure 6b. cThe thickness of active layer in optimal device.

(0.64 V); the device of PBQ-4 showed a VOC of 0.90 V, which is the highest one in these four devices. The results clearly confirm that the introduction of fluorine onto BDT-T and DTQx units has a synergistic effect on enhancing output voltages in device. Moreover, in comparison to the device based on PBQ-1, the device based on PBQ-4 showed a slightly higher JSC (13.58 mA/cm2) and similar FF (0.70). As a result, a PCE of 8.55% was obtained for the device of PBQ-4, which is the highest one in these four devices and also the highest one for the PSCs based on BDT-Qx-based conjugated polymers. The external quantum efficiencies (EQE) of the PSCs fabricated under the optimal condition were measured, and the plots are shown in Figure 5b. According to the results of EQE measurements, the PBQ-3-based device showed broader response range but lower EQE than the PBQ-4-based device, so the integral current densities obtained from the EQE measurements are quite similar, which is coincident with the results observed in the J−V measurements. The devices based on PBQ-4 showed slightly narrower photovoltaic response in long wavelength range but comparatively higher quantum efficiency than the devices based on PBQ-2 and PBQ-3, which is coincident with the trends obtained in UV−vis absorption spectra; i.e., PBQ-4 possess broader optical band gaps but higher extinction coefficient than the other polymers.

the D/A ratios were changed from 1:1 to 1:1.5 and then to 1:2, VOC of the corresponding devices reduced from 0.75 to 0.74 V and then to 0.72 V. The trend was coincident with the reported works, which may be caused by the increased permittivity or the lowering of effective LUMO level.41−43 Overall, the device with a D/A ratio of 1:1.5 showed an optimal PCE of 5.24%. Second, a trace amount of 1,8-diiodooctane (DIO) was used as the additive. As shown in Figure 4b and Table 3, 1%, 3%, 5%, and 8% (DIO/o-DCB, v/v) of DIO were scanned, and it was found that after the addition of DIO, VOC of the device reduced from 0.74 to 0.64 V, while the fill factor (FF) improved 0.58 to 0.70; as a result, the device processed with 5% DIO showed a PCE of 5.63%. The same methods were used to get the optimal D/A ratios and DIO amount for the other three polymers. Interestingly, for the devices based on the other three polymers, the optimal D/A ratio is 1:1.5 and the optimal volume ratio of DIO is 5%. Then, the PSC devices of these four polymers were compared under the same and optimal conditions. In Figure 5a, the current density−voltage (J−V) curves of the four types of devices are shown, and the corresponding photovoltaic parameters are listed in Table 4. The devices of PBQ-2 and PBQ-3 showed VOC of 0.76 and 0.80 V, respectively, and both these two values are higher than that of the device of PBQ-1 E


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Figure 6. Taping mode AFM topography, phase, and TEM images of (a−c) PBQ-4:PC71BM films (1:1.5 processed by o-DCB) and (d−f) PBQ4:PC71BM films (1:1.5 processed by o-DCB with 5% DIO as additive).

2.5. Morphological Properties of the Polymer:PC71BM Blends. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to investigate the morphological properties of the blend films processed by using pure DCB and the mixture of DCB and 5% DIO.44,45 Since the optimal device fabrication conditions for these four blend films are the same, the blend of PBQ-4:PC71BM is taken as the example, and the AFM and TEM images of the blend films based on other three polymers are provided in Figure S3. As shown in Figures 6a and 6b, when pure DCB was used as the processing solvent, the blend film of PBQ-4:PC71BM shows large size aggregations in AFM measurement, and also the large size aggregations (>100 nm) can be observed in TEM image (see Figure 6c). However, when 5% DIO was used as the additive, the morphological properties of the blend film changed significantly. As shown in Figure 6e, the size of the aggregations in the blend film is reduced, and also this phenomenon can be confirmed in the TEM image shown in Figure 6f. Meanwhile, after the use of DIO, the surface of the blend film became smoother, i.e., the mean square surface roughness (Rq) of the blend film reduced from 1.85 nm (see Figure 6a) to 1.57 nm (see Figure 6d). According to the morphological properties of the PBQ-4:PC71BM blend films, it can be concluded that the use of DIO prevents the formation of large size aggregation in the blend film so the blend film processed by the mixture of DCB and DIO shows enhanced photovoltaic properties.

the conjugated side groups of the BDT units shows a synergistic effect on molecular energy level modulation of the polymer, and as a result, the polymer PBQ-4 exhibits the lowest HOMO and LUMO levels in these four polymers. Characterizations of the PSCs based on these four polymers reveal that the fluorination has little influence on JSC and FF but is very helpful to enhance VOC. Benefiting from the synergistic effect of the fluorination, the device based on PBQ-4 showed a high VOC of 0.90 V, which is 0.26 V higher than the polymer without fluorine and ca. 0.1 V higher than the other two polymers with less fluorine. As a result, a PCE of 8.55% was recorded in the PBQ-4-based device, which is the highest one for the polymers based on BDT and Qx building blocks.

3. CONCLUSION In this work, four conjugated polymers based on BDT and DTQx building blocks were synthesized and applied in PSCs. The results clearly show that the fluorination on the BDT or Qx units of PBQ-1 has little influence on thermal stabilities and optical absorption properties of the polymer, but the molecular energy levels of PBQ-1 can be effectively reduced. More importantly, we found that the fluorination of the Qx units and

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ASSOCIATED CONTENT

* Supporting Information S

1 H NMR and/or 13C NMR spectra for monomers and polymers, TGA plots, X-ray diffraction patterns, tapping mode AFM topography, phase and TEM images, and details of the photovoltaic data for polymers. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00829.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86-10-82615900 (J.H.). The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from National Basic Research Program 973 (2014CB643501), NSFC (Nos. 51173189, 21325419, and 91333204), and the Chinese Academy of Science (No. XDB12030200). F


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