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Development of intrinsically fullerene-compatible polymers: Strategy for developing high performance organic solar cells using a non-halogenated solvent
Dyes and Pigments 132 (2016) 103e109
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Development of intrinsically fullerene-compatible polymers: Strategy for developing high performance organic solar cells using a nonhalogenated solvent Jea Woong Jo a, Yujeong Kim a, Min Jae Ko a, b, Hae Jung Son a, * a b
Photoelectronic Hybrid Research Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 February 2016 Received in revised form 28 March 2016 Accepted 2 April 2016 Available online 28 April 2016
Because large amounts of solvents are needed for large-scale fabrication of organic solar cells, it is important to develop solar cell processes using environmentally friendly solvents. In the research, we introduce a new approach to control the morphology of polymer:PC71BM blend films by an additive-free and non-halogenated solvent system. Incorporation of a fullerene-compatible o-dichlorobenzene group as the side chain of polythienothiophene-co-benzodithiophene derivatives importantly enhances the compatibility of the polymer with PC71BM and improves the nano-scale morphology of the blend film processed from o-xylene. We studied the effect of introducing the o-dichlorobenzene group onto the polymer on the morphology, charge mobility, and solar cell performances as comparing the cases of the polymer before modification. The developed polymer:PC71BM-based solar cells prepared using o-xylene show a remarkably improved performance with a PCE of 6.07%. © 2016 Published by Elsevier Ltd.
Keywords: Organic photovoltaic Conjugated polymer Compatibility Non-halogenated solvent Morphology
1. Introduction Organic photovoltaic (OPV) technology has developed remarkably since the use of polymer bulk heterojunctions (BHJs) as active layers. The power conversion efficiencies (PCEs) of polymer BHJ OPVs have risen above 10% owing to the development of high performance photo-active [1] and interfacial materials [2], and the optimization of device-processing methods [3]. However, to achieve a fully mature technology from the current level of research and development into cost-effective products for commercialization, many research challenges need to be overcome [4], such as ensuring the long-term stability of the OPV devices and developing the solution-processing techniques and module architectures for large-scale device fabrication. It is necessary to replace the toxic halogenated organic solvents that are currently used to prepare the BHJ active layers to non-halogenated solvents that are environmentally friendly. Moreover, it is important to prepare the BHJ active films without using solvent additives such as halogenated 1,8-diiodooctane and 1-chloronaphthalene [5]. The morphology of the BHJ active layer is strongly dependent on
* Corresponding author. E-mail address: [email protected] (H.J. Son). http://dx.doi.org/10.1016/j.dyepig.2016.04.008 0143-7208/© 2016 Published by Elsevier Ltd.
the processing solvent, and most polymer BHJ films prepared by using a non-halogenated solvent have an inappropriate morphology with large phase separations between the electrondonating conjugated polymer and the electron-accepting fullerene derivative. Several attempts to find fabrication methods that use solvents such as toluene, o-xylene, and 1,2,3trimethylbenzene and provide the BHJ films with appropriate morphologies have achieved solar cell devices with PCEs above 6% [6]. However, the developed methods are appropriate only for a few polymers and thus their ranges of applications are limited. One main reason for the poor morphology of the BHJ films is a low miscibility of the pullepush low bandgap polymers with phenyl-C61 (or C71)-butyric acid methyl esters (PCBMs). The pullepush conjugated polymers usually show good p-p interactions between the polymer chains due to their rigid backbones, however, which sometimes result in unfavorable properties of the polymers such as low solubilities in non-halogenated solvents and low miscibilities with PCBMs [6a,b,i]. Therefore, it is important to develop new electron-donating polymers that show high solubilities and are miscible with fullerene derivatives regardless of the processing solvent and solvent additive. In our previous report, we demonstrated that the incorporation of the o-dichlorobenzyl group, which is fullerene-compatible due to a similar Hansen solubility
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yield remarkably enhanced performances with the PCEs up to 6.07%. 2. Results and discussion
Scheme 1. Chemical structures of conjugated polymers.
parameter to those of fullerene derivatives, at the end of the conjugated polymer's side chain produces increased miscibility between the polymer and PCBM [7]. Inspired by this result, conjugated polymer donors based on polythienothiophene-cobenzodithiophene were prepared herein by introducing the fullerene-compatible o-dichlorobenzene-terminated side chains onto pullepush type conjugated polymers (Scheme 1). This approach is expected to make the polymers more miscible with PCBMs in non-halogenated solvents, which can induce proper phase separations in the polymer:PCBM blend films after spincoating. We investigated the effects of the side chain modification on the morphology and photovoltaic properties of the BHJ films processed by the representative non-halogen solvent, o-xylene. When compared with the polymer analogues without the side chain modification, it was found that the synthesized polymers exhibit similar optical and electrochemical characteristics. However, the polymer films blended with PC71BM by using o-xylene without an additive exhibit a much improved morphology with significantly reduced phase separations. As a result, the solar cell devices introducing the resulting BHJ active layers were found to
In our study, we use polythienothiophene-co-benzodithiophene as the basic polymer backbone because it is a good low bandgap polymer for the BHJ solar cells. Therefore, PTB7-DCB and PTB7-ThDCB polymers bearing an o-dichlorobenzyl group at the ends of their alkyl side chains were synthesized via Stille coupling reactions in a toluene/DMF cosolvent with Pd(PPh3)4 as the catalyst. The detailed synthesis and purification procedures are described in Scheme S2. The number average molecular weights (Mn) and polydispersities (PDIs) of the resulting polymers were measured with gel permeation chromatography (GPC) by using chlorobenzene as the eluent. As listed in Table 1, all polymers have similar molecular weights in the range of 42e51 kg/mol and PDI values of 2.09e2.64 regardless of the side chain modification, which suggests that the effects of their molecular weight on the morphology of their polymer:PCBM blend films and thus on their photovoltaic properties are not significant. The optical properties of the synthesized polymers were explored with UVeVis absorption spectroscopy, as shown in Fig. 1a and b. The absorption spectra of PTB7-DCB and PTB7-Th-DCB in an o-xylene solution are very similar to those of the corresponding polymers without the o-dichlorobenzyl group over the whole wavelength range, with absorption maximum peaks at 676 nm and 705 nm for PTB7-DCB and PTB7-Th-DCB, respectively. However, the spectra of the PTB7-DCB and PTB7-Th-DCB films contain slight redshifts in their absorption onsets compared with those of the PTB7 and PTB7-Th films, although the overall shapes of the absorption spectra are similar between the polymers with and without the odichlorobenzyl group. The optical bandgaps (Eg) of the polymers were estimated from the absorption onsets in the film spectra to be 1.61, 1.60, 1.56, and 1.55 eV for PTB7, PTB7-DCB, PTB7-Th, and PTB7Th-DCB, respectively. The electrochemical properties of the polymers were investigated with cyclic voltammetry and their individual cyclic voltammograms and calculated energy levels are shown in Fig. 1c and Table 1. The highest occupied molecular orbital (HOMO) energy levels of PTB7-Th (5.31 eV) and PTB7-Th-DCB (5.27 eV) are deeper than those of PTB7 (5.22 eV) and PTB7-DCB (5.19 eV), which is probably due to the weaker electron-donating ability of the thienyl side chain [8]. Notably, when the o-dichlorobenzyl group is incorporated into the polymer, PTB7 and PTB7-Th consistently show slightly increased HOMO energy levels, which is probably due to the molecular conformational change of the polymer chain. The influence of the o-dichlorobenzyl group on the miscibility between the polymer and PC71BM was studied by photoluminescence (PL) measurements. Fig. S1a and b show the PL spectra of the PC71BM films including various amounts of PTB7 or PTB7-DCB, which were recorded with excitations at 380 nm [7], and Fig. S1c exhibits a comparison of the normalized PL intensity
Table 1 Characteristics of polymers. Polymer
Mn (kg/mol)
PDI
lmax solution (nm)
lmax film (nm)
Ega (eV)
HOMO (eV)
LUMOb (eV)
PTB7 PTB7-DCB PTB7-Th PTB7-Th-DCB
45 44 51 42
2.09 2.64 2.38 2.16
680 676 703 705
680 686 704 713
1.61 1.60 1.56 1.55
5.22 5.19 5.31 5.27
3.61 3.59 3.75 3.72
a b
Determined from the onset of UVeVis absorption spectra. Eg þ HOMO.
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and PTB7-DCB. Both the PL intensities of the PC71BM films were gradually decreased with increases of the polymer's contents in the blend film. However, the degree of the PL quenching increases more rapidly for the PC71BM film with PTB7-DCB compared with the films with PTB7. Because the PL from the excitation at 380 nm is PC71BM emission dominant [9a], the degree of PL quenching is able to show relative non-radiative quenching of excitons generated within the PC71BM domains by the polymer, which is dependent on the distance from the exciton to the polymer chain present near the polymer:PC71BM interface [9bed]. Therefore, from the relative PL intensity, we can compare the relative total polymer/PC71BM interface areas of the blend films. As a result, the introduction of odichlorobenzyl group at the terminal of the side chain may lead to the enhanced miscibility of polymer with PC71BM and thus more efficient PL quenching in the blend films. OPV devices were prepared with the conventional device configuration glass/ITO/PEDOT:PSS/polymer:PC71BM/TiO2/Al [10]. The polymer:PC71BM active layers were prepared in o-xylene without any additive and the effects of introducing the o-dichlorobenzyl groups into the polymers on the photovoltaic properties were studied. The blend ratios of PTB7:PC71BM, PTB7-DCB:PC71BM, PTB7-Th:PC71BM, and PTB7-Th-DCB:PC71BM were varied and it was found that the best device performances were obtained at the ratios 1:1.3, 1:1.1, 1:0.9, and 1:1.1, respectively. The JeV characteristics of the devices under AM 1.5G illumination are shown in Fig. 2a and
Fig. 1. UVeVis absorption spectra of polymers in (a) o-xylene solutions and (b) films. (c) Cyclic voltammograms of polymers.
change at the maximum emission wavelength of 710 nm according to the polymer amount between the PC71BM films including PTB7
Fig. 2. (a) JeV curves and (b) EQE spectra of polymer:PC71BM solar cells prepared with o-xylene solvent.
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DCB-based device has the highest JSC of all the OPV devices. The JSC,EQE values obtained by integrating the EQE spectra are close to the JSC values determined from the JeV curves (Table 2), which suggests that the JeV measurements have a high reliability. These results demonstrate that the introduction of the fullerenecompatible o-dichlorobenzyl group is a promising strategy for the fabrication of high performance OPV devices that are processed with a non-halogenated solvent. Additionally, we also prepared OPVs using a 1,8-diiodooctane (DIO) additive. Fig. S2 shows JeV characteristics of devices under AM 1.5G illumination and corresponding photovoltaic parameters are summarized in Table S1. Compared with devices prepared without DIO, solar cells processed with the DIO additive showed enhanced PCEs, which may be attributed to the improved morphology of polymer:PC71BM blend; the PTB7 and PTB7-Th devices show PCEs of 6.03 and 6.46%, respectively. However, higher PCEs are observed with the polymers modified with the o-
the relevant photovoltaic parameters are summarized in Table 2. The PTB7-DCB device exhibits a PCE of 4.22%, which is higher than that of PTB7 (PCE ¼ 2.46%), because of its enhanced short-circuit current (JSC) and fill factor (FF); the JSC is increased by the addition of DCB from 7.50 mA/cm2 to 11.66 mA/cm2 and the FF is increased from 42% to 49%. The improvement in the solar cell performance that results from attaching the o-dichlorobenzyl group is also evident in the PTB7-Th-based solar cell devices. The PTB7-Th-DCB device exhibits higher JSC (15.11 mA/cm2) and FF (49%) values than the corresponding 13.70 mA/cm2 and 37% of the PTB7-Th device and, as a result, the PCE (6.07%) of the PTB7-Th-DCB device is higher than that of the PTB7-Th device (PCE ¼ 4.11%). Fig. 2b shows the external quantum efficiency (EQE) spectra of the OPV devices measured under the condition of monochromatic light. PTB7-Th-DCB exhibits much larger EQE values than the other polymers, as high as ~60% in the wavelength range of 420e720 nm. This result is consistent with the observation that the PTB7-Th-
Table 2 Photovoltaic properties of OPV devicesa fabricated with o-xylene solvent under standard AM 1.5G illumination. Polymer
Polymer: PC71BM
Thickness (nm)
VOC (V)
JSC (JSC,EQEb) (mA/cm2)
FF (%)
PCEmax (avec) (%)
PTB7 PTB7-DCB PTB7-Th PTB7-Th-DCB
1:1.3 1:1.1 1:0.9 1:1.1
90 100 90 100
0.78 0.77 0.81 0.82
7.50 11.66 13.70 15.11
42 47 37 49
2.46 4.22 4.11 6.07
a b c
(7.56) (11.41) (13.52) (15.06)
(2.34) (3.92) (3.91) (5.60)
Conventional device configuration: ITO/PEDOT:PSS/polymer:PC71BM/TiO2/Al. Integrated from EQE data. Average PCE calculated from 8 devices.
Fig. 3. TEM images of (a) PTB7:PC71BM, (b) PTB7-DCB:PC71BM, (c) PTB7-Th-DCB:PC71BM, and (d) PTB7-Th-DCB:PC71BM blend films prepared with o-xylene solvent. The scale bar denotes 500 nm.
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Table 3 Charge carrier mobilities of polymer:PC71BM blend films prepared with o-xylene solvent. Polymer
Hole mobilities, mh (cm2/V$s)
PTB7 PTB7-DCB PTB7-Th PTB7-Th-DCB
6.75 2.18 5.94 1.48
105 104 105 104
dichlorobenzyl group (i.e., PTB7-DCB and PTB7-Th-DCB). PTB7-DCB and PTB7-Th-DCB devices show PCEs of 6.53 and 6.99%, respectively. Therefore, it is suggested that the introduction of the odichlorobenzyl group is also effective to improve the photovoltaic performances in solar cells using a non-halogen solvent with additives. The morphologies of the photoactive layers in the OPV devices were investigated with transmission electron microscopy (TEM). As shown in Fig. 3a, there is a large phase separation between the polymer and PC71BM in the PTB7:PC71BM blend film, which can disrupt efficient exciton dissociations due to the limited diffusion length (~10 nm) of the excitons. In contrast, much smaller dark spots attributed to aggregated PC71BM are evident in the PTB7DCB:PC71BM blend film shown in Fig. 3b; this dramatically reduced phase separation is attributed to the enhanced miscibility between the conjugated polymer and PC71BM that arises from the side chain modification. Similarly, the morphology of the PTB7Th:PC71BM blend film is improved by the introduction of the odichlorobenzyl group, as shown in Fig. 3c and d, although the effect is not as high as in the case of PTB7; the dark spots in the TEM image are much smaller and the film has more homogeneous and fine features. The enhanced miscibility of polymers after side chain modification can be also confirmed by the AFM images of polymer:PC71BM blend films (Fig. S3), where polymers with the odichlorobenzyl group (i.e., PTB7-DCB and PTB7-Th-DcB) showed smaller domain sizes compared with their analogues (i.e., PTB7 and PTB7-Th). Another important feature of the o-xylene-processed polymer:PC71BM films is that the polymers with a thienyl side chain exhibit better miscibility with PCBM than those with the alkoxy side chain. The synergetic effects of the introduction of the o-dichlorobenzyl group and thienyl side chains result in the optimal morphology of the PTB7-Th-DCB:PC71BM film with a nanoscale bicontinuous network when it is spin-coated from o-
Electron mobilities, me (cm2/V$s) 3.47 7.14 3.75 6.35
106 105 106 105
me/mh 19.5 3.1 15.8 2.3
xylene without any additive. The improved morphology is expected to enhance the JSC and FF values of the associated OPV device and thus produce the highest efficiency of the polymers. We compared the charge carrier mobilities of the polymer:PC71BM films. The hole (mh) and electron (me) mobilities were calculated from the dark JeV curves of the hole-only and electrononly devices by using the space-charge limited current (SCLC) model, as shown in Fig. S4 and Table 3. We found that the hole mobilities of the polymers incorporating the o-dichlorobenzyl group are 2e3 times higher than those of the corresponding polymers without the modification: 2.18 104 cm2/V s for PTB7DCB (cf. 6.75 105 cm2/V s for PTB7) and 1.48 104 cm2/V s for PTB7-Th-DCB (cf. 5.94 105 cm2/V s for PTB7-Th). The blend films of the polymers with the o-dichlorobenzyl group also show much higher electron mobilities: 7.14 105 cm2/V s for PTB7-DCB (cf. 3.47 106 cm2/V s for PTB7) and 6.35 105 cm2/V s for PTB7-ThDCB (cf. 3.75 106 cm2/V s for PTB7-Th). Furthermore, the hole and electron mobilities of the PTB7-DCB and PTB7-Th-DCB blend films show a better balance with a me/mh ratio of 2.3e3.1 compared to those of PTB7 and PTB7-Th (me/mh ratio ¼ 15.8e19.5). The improved charge balance in the solar cell device may importantly contribute to reducing charge recombination losses and thus, enhancing the FF values of PTB7-DCB and PTB7-Th-DCB [11]. It has been reported that two components with similar surface energies have better miscibility in blend films [7,12]. To compare the surface energies of PC71BM and the polymers, the contact angles of water and diiodomethane (DIM) were measured (Fig. S5), and then the surface energies of the materials were calculated with the OwensWendt geometric mean equation (Table S2). As shown in Fig. 4, PTB7-Th-DCB has a surface energy of 43.5 mJ/m2, which is the nearest of the polymers to that of PC71BM (49.7 mJ/m2); this high similarity of the surface energies is closely correlated with the fine-featured morphology of the PTB7-Th-DCB:PC71BM blend film. 3. Conclusions
Fig. 4. Comparison of surface energy values of PC71BM and polymers.
In this paper, we introduce a new approach to the synthesis of donor polymers for high-performance OPV devices in which the photoactive layer is processed by using a non-halogenated solvent such as o-xylene. The incorporation of the o-dichlorobenzylterminated side chain into PTB7 and PTB7-Th is demonstrated to improve the morphology of the photoactive layers, although they are spin-coated by a non-halogenated solvent without any additive, which is most likely due to the enhanced miscibility of the polymers with PC71BM. In particular, the PTB7-Th-DCB:PC71BM film shows a much better morphology, i.e. it contains a nanoscale phase separation, than those of the other polymers, which leads to efficient exciton dissociations and charge transport in its OPV device and thereby a promising PCE of 6.07%. We recommend several important points for achieving high performance OPV devices with environmentally benign processes. First, it is important that the conjugated polymer has a chemical structure with a good compatibility with fullerene derivatives, which is advantageous for the polymer:fullerene blend film to form the appropriate morphology despite the use of a non-halogenated solvent. Second, the incorporation of a fullerene-compatible side chain such as the
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o-dichlorobenzyl group is a facile method that can widen the range of solvents that are used to process the associated OPV devices. In particular, this approach will be applicable to the many high performance polymers that necessitate a use of chlorinated solvents and additives to achieve the good BHJ film morphologies. [5]
Acknowledgments This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System and Basic Science Research Program (2015R1A1A1A05001115) funded by the National Research Foundation under the Ministry of Science, Korea Institute of Science and Technology (KIST, 2E25392); this work was also supported by the Nano$Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2012M3A7B4049989). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2016.04.008.
[6]
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Development of intrinsically fullerene-compatible polymers: Strategy for developing high performance organic solar cells using a nonhalogenated solvent Jea Woong Jo a, Yujeong Kim a, Min Jae Ko a, b, Hae Jung Son a, * a b
Photoelectronic Hybrid Research Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 February 2016 Received in revised form 28 March 2016 Accepted 2 April 2016 Available online 28 April 2016
Because large amounts of solvents are needed for large-scale fabrication of organic solar cells, it is important to develop solar cell processes using environmentally friendly solvents. In the research, we introduce a new approach to control the morphology of polymer:PC71BM blend films by an additive-free and non-halogenated solvent system. Incorporation of a fullerene-compatible o-dichlorobenzene group as the side chain of polythienothiophene-co-benzodithiophene derivatives importantly enhances the compatibility of the polymer with PC71BM and improves the nano-scale morphology of the blend film processed from o-xylene. We studied the effect of introducing the o-dichlorobenzene group onto the polymer on the morphology, charge mobility, and solar cell performances as comparing the cases of the polymer before modification. The developed polymer:PC71BM-based solar cells prepared using o-xylene show a remarkably improved performance with a PCE of 6.07%. © 2016 Published by Elsevier Ltd.
Keywords: Organic photovoltaic Conjugated polymer Compatibility Non-halogenated solvent Morphology
1. Introduction Organic photovoltaic (OPV) technology has developed remarkably since the use of polymer bulk heterojunctions (BHJs) as active layers. The power conversion efficiencies (PCEs) of polymer BHJ OPVs have risen above 10% owing to the development of high performance photo-active [1] and interfacial materials [2], and the optimization of device-processing methods [3]. However, to achieve a fully mature technology from the current level of research and development into cost-effective products for commercialization, many research challenges need to be overcome [4], such as ensuring the long-term stability of the OPV devices and developing the solution-processing techniques and module architectures for large-scale device fabrication. It is necessary to replace the toxic halogenated organic solvents that are currently used to prepare the BHJ active layers to non-halogenated solvents that are environmentally friendly. Moreover, it is important to prepare the BHJ active films without using solvent additives such as halogenated 1,8-diiodooctane and 1-chloronaphthalene [5]. The morphology of the BHJ active layer is strongly dependent on
* Corresponding author. E-mail address: [email protected] (H.J. Son). http://dx.doi.org/10.1016/j.dyepig.2016.04.008 0143-7208/© 2016 Published by Elsevier Ltd.
the processing solvent, and most polymer BHJ films prepared by using a non-halogenated solvent have an inappropriate morphology with large phase separations between the electrondonating conjugated polymer and the electron-accepting fullerene derivative. Several attempts to find fabrication methods that use solvents such as toluene, o-xylene, and 1,2,3trimethylbenzene and provide the BHJ films with appropriate morphologies have achieved solar cell devices with PCEs above 6% [6]. However, the developed methods are appropriate only for a few polymers and thus their ranges of applications are limited. One main reason for the poor morphology of the BHJ films is a low miscibility of the pullepush low bandgap polymers with phenyl-C61 (or C71)-butyric acid methyl esters (PCBMs). The pullepush conjugated polymers usually show good p-p interactions between the polymer chains due to their rigid backbones, however, which sometimes result in unfavorable properties of the polymers such as low solubilities in non-halogenated solvents and low miscibilities with PCBMs [6a,b,i]. Therefore, it is important to develop new electron-donating polymers that show high solubilities and are miscible with fullerene derivatives regardless of the processing solvent and solvent additive. In our previous report, we demonstrated that the incorporation of the o-dichlorobenzyl group, which is fullerene-compatible due to a similar Hansen solubility
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yield remarkably enhanced performances with the PCEs up to 6.07%. 2. Results and discussion
Scheme 1. Chemical structures of conjugated polymers.
parameter to those of fullerene derivatives, at the end of the conjugated polymer's side chain produces increased miscibility between the polymer and PCBM [7]. Inspired by this result, conjugated polymer donors based on polythienothiophene-cobenzodithiophene were prepared herein by introducing the fullerene-compatible o-dichlorobenzene-terminated side chains onto pullepush type conjugated polymers (Scheme 1). This approach is expected to make the polymers more miscible with PCBMs in non-halogenated solvents, which can induce proper phase separations in the polymer:PCBM blend films after spincoating. We investigated the effects of the side chain modification on the morphology and photovoltaic properties of the BHJ films processed by the representative non-halogen solvent, o-xylene. When compared with the polymer analogues without the side chain modification, it was found that the synthesized polymers exhibit similar optical and electrochemical characteristics. However, the polymer films blended with PC71BM by using o-xylene without an additive exhibit a much improved morphology with significantly reduced phase separations. As a result, the solar cell devices introducing the resulting BHJ active layers were found to
In our study, we use polythienothiophene-co-benzodithiophene as the basic polymer backbone because it is a good low bandgap polymer for the BHJ solar cells. Therefore, PTB7-DCB and PTB7-ThDCB polymers bearing an o-dichlorobenzyl group at the ends of their alkyl side chains were synthesized via Stille coupling reactions in a toluene/DMF cosolvent with Pd(PPh3)4 as the catalyst. The detailed synthesis and purification procedures are described in Scheme S2. The number average molecular weights (Mn) and polydispersities (PDIs) of the resulting polymers were measured with gel permeation chromatography (GPC) by using chlorobenzene as the eluent. As listed in Table 1, all polymers have similar molecular weights in the range of 42e51 kg/mol and PDI values of 2.09e2.64 regardless of the side chain modification, which suggests that the effects of their molecular weight on the morphology of their polymer:PCBM blend films and thus on their photovoltaic properties are not significant. The optical properties of the synthesized polymers were explored with UVeVis absorption spectroscopy, as shown in Fig. 1a and b. The absorption spectra of PTB7-DCB and PTB7-Th-DCB in an o-xylene solution are very similar to those of the corresponding polymers without the o-dichlorobenzyl group over the whole wavelength range, with absorption maximum peaks at 676 nm and 705 nm for PTB7-DCB and PTB7-Th-DCB, respectively. However, the spectra of the PTB7-DCB and PTB7-Th-DCB films contain slight redshifts in their absorption onsets compared with those of the PTB7 and PTB7-Th films, although the overall shapes of the absorption spectra are similar between the polymers with and without the odichlorobenzyl group. The optical bandgaps (Eg) of the polymers were estimated from the absorption onsets in the film spectra to be 1.61, 1.60, 1.56, and 1.55 eV for PTB7, PTB7-DCB, PTB7-Th, and PTB7Th-DCB, respectively. The electrochemical properties of the polymers were investigated with cyclic voltammetry and their individual cyclic voltammograms and calculated energy levels are shown in Fig. 1c and Table 1. The highest occupied molecular orbital (HOMO) energy levels of PTB7-Th (5.31 eV) and PTB7-Th-DCB (5.27 eV) are deeper than those of PTB7 (5.22 eV) and PTB7-DCB (5.19 eV), which is probably due to the weaker electron-donating ability of the thienyl side chain [8]. Notably, when the o-dichlorobenzyl group is incorporated into the polymer, PTB7 and PTB7-Th consistently show slightly increased HOMO energy levels, which is probably due to the molecular conformational change of the polymer chain. The influence of the o-dichlorobenzyl group on the miscibility between the polymer and PC71BM was studied by photoluminescence (PL) measurements. Fig. S1a and b show the PL spectra of the PC71BM films including various amounts of PTB7 or PTB7-DCB, which were recorded with excitations at 380 nm [7], and Fig. S1c exhibits a comparison of the normalized PL intensity
Table 1 Characteristics of polymers. Polymer
Mn (kg/mol)
PDI
lmax solution (nm)
lmax film (nm)
Ega (eV)
HOMO (eV)
LUMOb (eV)
PTB7 PTB7-DCB PTB7-Th PTB7-Th-DCB
45 44 51 42
2.09 2.64 2.38 2.16
680 676 703 705
680 686 704 713
1.61 1.60 1.56 1.55
5.22 5.19 5.31 5.27
3.61 3.59 3.75 3.72
a b
Determined from the onset of UVeVis absorption spectra. Eg þ HOMO.
J.W. Jo et al. / Dyes and Pigments 132 (2016) 103e109
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and PTB7-DCB. Both the PL intensities of the PC71BM films were gradually decreased with increases of the polymer's contents in the blend film. However, the degree of the PL quenching increases more rapidly for the PC71BM film with PTB7-DCB compared with the films with PTB7. Because the PL from the excitation at 380 nm is PC71BM emission dominant [9a], the degree of PL quenching is able to show relative non-radiative quenching of excitons generated within the PC71BM domains by the polymer, which is dependent on the distance from the exciton to the polymer chain present near the polymer:PC71BM interface [9bed]. Therefore, from the relative PL intensity, we can compare the relative total polymer/PC71BM interface areas of the blend films. As a result, the introduction of odichlorobenzyl group at the terminal of the side chain may lead to the enhanced miscibility of polymer with PC71BM and thus more efficient PL quenching in the blend films. OPV devices were prepared with the conventional device configuration glass/ITO/PEDOT:PSS/polymer:PC71BM/TiO2/Al [10]. The polymer:PC71BM active layers were prepared in o-xylene without any additive and the effects of introducing the o-dichlorobenzyl groups into the polymers on the photovoltaic properties were studied. The blend ratios of PTB7:PC71BM, PTB7-DCB:PC71BM, PTB7-Th:PC71BM, and PTB7-Th-DCB:PC71BM were varied and it was found that the best device performances were obtained at the ratios 1:1.3, 1:1.1, 1:0.9, and 1:1.1, respectively. The JeV characteristics of the devices under AM 1.5G illumination are shown in Fig. 2a and
Fig. 1. UVeVis absorption spectra of polymers in (a) o-xylene solutions and (b) films. (c) Cyclic voltammograms of polymers.
change at the maximum emission wavelength of 710 nm according to the polymer amount between the PC71BM films including PTB7
Fig. 2. (a) JeV curves and (b) EQE spectra of polymer:PC71BM solar cells prepared with o-xylene solvent.
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DCB-based device has the highest JSC of all the OPV devices. The JSC,EQE values obtained by integrating the EQE spectra are close to the JSC values determined from the JeV curves (Table 2), which suggests that the JeV measurements have a high reliability. These results demonstrate that the introduction of the fullerenecompatible o-dichlorobenzyl group is a promising strategy for the fabrication of high performance OPV devices that are processed with a non-halogenated solvent. Additionally, we also prepared OPVs using a 1,8-diiodooctane (DIO) additive. Fig. S2 shows JeV characteristics of devices under AM 1.5G illumination and corresponding photovoltaic parameters are summarized in Table S1. Compared with devices prepared without DIO, solar cells processed with the DIO additive showed enhanced PCEs, which may be attributed to the improved morphology of polymer:PC71BM blend; the PTB7 and PTB7-Th devices show PCEs of 6.03 and 6.46%, respectively. However, higher PCEs are observed with the polymers modified with the o-
the relevant photovoltaic parameters are summarized in Table 2. The PTB7-DCB device exhibits a PCE of 4.22%, which is higher than that of PTB7 (PCE ¼ 2.46%), because of its enhanced short-circuit current (JSC) and fill factor (FF); the JSC is increased by the addition of DCB from 7.50 mA/cm2 to 11.66 mA/cm2 and the FF is increased from 42% to 49%. The improvement in the solar cell performance that results from attaching the o-dichlorobenzyl group is also evident in the PTB7-Th-based solar cell devices. The PTB7-Th-DCB device exhibits higher JSC (15.11 mA/cm2) and FF (49%) values than the corresponding 13.70 mA/cm2 and 37% of the PTB7-Th device and, as a result, the PCE (6.07%) of the PTB7-Th-DCB device is higher than that of the PTB7-Th device (PCE ¼ 4.11%). Fig. 2b shows the external quantum efficiency (EQE) spectra of the OPV devices measured under the condition of monochromatic light. PTB7-Th-DCB exhibits much larger EQE values than the other polymers, as high as ~60% in the wavelength range of 420e720 nm. This result is consistent with the observation that the PTB7-Th-
Table 2 Photovoltaic properties of OPV devicesa fabricated with o-xylene solvent under standard AM 1.5G illumination. Polymer
Polymer: PC71BM
Thickness (nm)
VOC (V)
JSC (JSC,EQEb) (mA/cm2)
FF (%)
PCEmax (avec) (%)
PTB7 PTB7-DCB PTB7-Th PTB7-Th-DCB
1:1.3 1:1.1 1:0.9 1:1.1
90 100 90 100
0.78 0.77 0.81 0.82
7.50 11.66 13.70 15.11
42 47 37 49
2.46 4.22 4.11 6.07
a b c
(7.56) (11.41) (13.52) (15.06)
(2.34) (3.92) (3.91) (5.60)
Conventional device configuration: ITO/PEDOT:PSS/polymer:PC71BM/TiO2/Al. Integrated from EQE data. Average PCE calculated from 8 devices.
Fig. 3. TEM images of (a) PTB7:PC71BM, (b) PTB7-DCB:PC71BM, (c) PTB7-Th-DCB:PC71BM, and (d) PTB7-Th-DCB:PC71BM blend films prepared with o-xylene solvent. The scale bar denotes 500 nm.
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Table 3 Charge carrier mobilities of polymer:PC71BM blend films prepared with o-xylene solvent. Polymer
Hole mobilities, mh (cm2/V$s)
PTB7 PTB7-DCB PTB7-Th PTB7-Th-DCB
6.75 2.18 5.94 1.48
105 104 105 104
dichlorobenzyl group (i.e., PTB7-DCB and PTB7-Th-DCB). PTB7-DCB and PTB7-Th-DCB devices show PCEs of 6.53 and 6.99%, respectively. Therefore, it is suggested that the introduction of the odichlorobenzyl group is also effective to improve the photovoltaic performances in solar cells using a non-halogen solvent with additives. The morphologies of the photoactive layers in the OPV devices were investigated with transmission electron microscopy (TEM). As shown in Fig. 3a, there is a large phase separation between the polymer and PC71BM in the PTB7:PC71BM blend film, which can disrupt efficient exciton dissociations due to the limited diffusion length (~10 nm) of the excitons. In contrast, much smaller dark spots attributed to aggregated PC71BM are evident in the PTB7DCB:PC71BM blend film shown in Fig. 3b; this dramatically reduced phase separation is attributed to the enhanced miscibility between the conjugated polymer and PC71BM that arises from the side chain modification. Similarly, the morphology of the PTB7Th:PC71BM blend film is improved by the introduction of the odichlorobenzyl group, as shown in Fig. 3c and d, although the effect is not as high as in the case of PTB7; the dark spots in the TEM image are much smaller and the film has more homogeneous and fine features. The enhanced miscibility of polymers after side chain modification can be also confirmed by the AFM images of polymer:PC71BM blend films (Fig. S3), where polymers with the odichlorobenzyl group (i.e., PTB7-DCB and PTB7-Th-DcB) showed smaller domain sizes compared with their analogues (i.e., PTB7 and PTB7-Th). Another important feature of the o-xylene-processed polymer:PC71BM films is that the polymers with a thienyl side chain exhibit better miscibility with PCBM than those with the alkoxy side chain. The synergetic effects of the introduction of the o-dichlorobenzyl group and thienyl side chains result in the optimal morphology of the PTB7-Th-DCB:PC71BM film with a nanoscale bicontinuous network when it is spin-coated from o-
Electron mobilities, me (cm2/V$s) 3.47 7.14 3.75 6.35
106 105 106 105
me/mh 19.5 3.1 15.8 2.3
xylene without any additive. The improved morphology is expected to enhance the JSC and FF values of the associated OPV device and thus produce the highest efficiency of the polymers. We compared the charge carrier mobilities of the polymer:PC71BM films. The hole (mh) and electron (me) mobilities were calculated from the dark JeV curves of the hole-only and electrononly devices by using the space-charge limited current (SCLC) model, as shown in Fig. S4 and Table 3. We found that the hole mobilities of the polymers incorporating the o-dichlorobenzyl group are 2e3 times higher than those of the corresponding polymers without the modification: 2.18 104 cm2/V s for PTB7DCB (cf. 6.75 105 cm2/V s for PTB7) and 1.48 104 cm2/V s for PTB7-Th-DCB (cf. 5.94 105 cm2/V s for PTB7-Th). The blend films of the polymers with the o-dichlorobenzyl group also show much higher electron mobilities: 7.14 105 cm2/V s for PTB7-DCB (cf. 3.47 106 cm2/V s for PTB7) and 6.35 105 cm2/V s for PTB7-ThDCB (cf. 3.75 106 cm2/V s for PTB7-Th). Furthermore, the hole and electron mobilities of the PTB7-DCB and PTB7-Th-DCB blend films show a better balance with a me/mh ratio of 2.3e3.1 compared to those of PTB7 and PTB7-Th (me/mh ratio ¼ 15.8e19.5). The improved charge balance in the solar cell device may importantly contribute to reducing charge recombination losses and thus, enhancing the FF values of PTB7-DCB and PTB7-Th-DCB [11]. It has been reported that two components with similar surface energies have better miscibility in blend films [7,12]. To compare the surface energies of PC71BM and the polymers, the contact angles of water and diiodomethane (DIM) were measured (Fig. S5), and then the surface energies of the materials were calculated with the OwensWendt geometric mean equation (Table S2). As shown in Fig. 4, PTB7-Th-DCB has a surface energy of 43.5 mJ/m2, which is the nearest of the polymers to that of PC71BM (49.7 mJ/m2); this high similarity of the surface energies is closely correlated with the fine-featured morphology of the PTB7-Th-DCB:PC71BM blend film. 3. Conclusions
Fig. 4. Comparison of surface energy values of PC71BM and polymers.
In this paper, we introduce a new approach to the synthesis of donor polymers for high-performance OPV devices in which the photoactive layer is processed by using a non-halogenated solvent such as o-xylene. The incorporation of the o-dichlorobenzylterminated side chain into PTB7 and PTB7-Th is demonstrated to improve the morphology of the photoactive layers, although they are spin-coated by a non-halogenated solvent without any additive, which is most likely due to the enhanced miscibility of the polymers with PC71BM. In particular, the PTB7-Th-DCB:PC71BM film shows a much better morphology, i.e. it contains a nanoscale phase separation, than those of the other polymers, which leads to efficient exciton dissociations and charge transport in its OPV device and thereby a promising PCE of 6.07%. We recommend several important points for achieving high performance OPV devices with environmentally benign processes. First, it is important that the conjugated polymer has a chemical structure with a good compatibility with fullerene derivatives, which is advantageous for the polymer:fullerene blend film to form the appropriate morphology despite the use of a non-halogenated solvent. Second, the incorporation of a fullerene-compatible side chain such as the
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o-dichlorobenzyl group is a facile method that can widen the range of solvents that are used to process the associated OPV devices. In particular, this approach will be applicable to the many high performance polymers that necessitate a use of chlorinated solvents and additives to achieve the good BHJ film morphologies. [5]
Acknowledgments This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System and Basic Science Research Program (2015R1A1A1A05001115) funded by the National Research Foundation under the Ministry of Science, Korea Institute of Science and Technology (KIST, 2E25392); this work was also supported by the Nano$Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2012M3A7B4049989). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2016.04.008.
[6]
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