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Electrochemical route to fabricate porous organic polymers film and its application for polymer solar cells
Dyes and Pigments 142 (2017) 132e138
Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Electrochemical route to fabricate porous organic polymers film and its application for polymer solar cells Cong Liu a, *, Haiyuan Luo a, Guang Shi a, Li Nian b, **, Zhenguo Chi c, Yuguang Ma d a
School of Chemistry and Environment, South China Normal University, Guangzhou, Guangdong 510006, China South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong 510006, China c Materials Science Institute, School of Chemistry and Chemical Engineering, SunYat-sen University, Guangzhou, Guangdong 510275, China d Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong 510640, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2017 Received in revised form 9 March 2017 Accepted 9 March 2017 Available online 14 March 2017
A facile and controllable electrochemical polymerization of porous organic polymers (POP) film using carbazolyl triphenylethylene derivative (TPCz) as building block for the applications in polymer solar cells(PSCs) was developed. The POP film is an interlocked network film of poly-TPCz (designated as PTPCz) which has smooth surface morphology and porous nanostructures. The film can increase the work function (WF) of the anode of PSCs, the contact with the active layer, and enhance hole-extraction process. By using PEDOT:PSS/PTPCz instead of PEDOT:PSS as the anode interlayer, the optimized PSCs shows a 13% enhancement in power conversion efficiency (PCE) which is 8.54%. These results indicate the potential of electrochemical synthesized POP film as efficient interlayer for PSCs. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Polymer solar cells Anode interlayer Electrochemical polymerization Porous organic polymer
1. Introduction Porous organic polymers (POP), with properties of large surface areas, high chemical stabilities, and low skeletal density, have attracted great attention in recent years due to their potential applications in catalysis, gas storage, and gas separations [1e8]. By using rigid or contorted building blocks, POP with high surface area has been fabricated from chemical polymerization. For example, Chen et al. reported a carbazole-based porous organic polymer via straightforward carbazole-based oxidative coupling polymerization, which showed a high Brunauer-Emmett-Teller (BET) surface area (up to 2220 m2 g1) and exhibited a dominant pore diameter centred at about 0.62 nm [9]. Chen et al. synthesized tetraphenylethylene-based porous organic polymers through a Suzuki coupling polycondensation or oxidative coupling polymerization, for which the BET specific surface area values could be tuned between 472 and 810 m2 g1 and the adsorption capacity for hydrogen is up to 1.07 wt% at 1.13 bar and 77 K [10]. Due to their cross-linked structure, POP are usually insoluble and hard to be processed, which may limit their application in many filed, including potential application in polymer solar cells * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Liu), [email protected] (L. Nian). http://dx.doi.org/10.1016/j.dyepig.2017.03.017 0143-7208/© 2017 Elsevier Ltd. All rights reserved.
(PSCs). To solve this problem, electropolymerization is a new effective choice to synthesize POP film, in which thin films of POPs can be deposited on the electrode with controllable thickness readily [11,12]. Gu et al. synthesized highly luminescent POP films to detect explosives by electropolymerization method, in which multiple peripheral units of carbazole were designed for site-specific electropolymerization of three dimensional porous skeletons [13]. Recently, POP films were also emerged to be used as interlayer materials in PSCs to improve the device performance [14]. However, there are still few reports on the application of POP films in high performance PSCs. The properties of POPs are affected by multiple factors such as building blocks, pore structure, material morphology, and so on. So far, it is still a challenge to develop POP films with potential use and good performance for PSCs. In this work, a propeller-like building block, an aggregationinduced emission (AIE) molecule based on carbazolyl triphenylethylene derivative (TPCz), is selected as a single component for the electrochemical synthesis of POP film for potential application as anode interlayer in PSCs. The chemical structure of electrochemical deposited precursor TPCz was showed in Fig. 1a. The POP film is an interlocked network film of poly-TPCz (designated as PTPCz) synthesized by electropolymerization on the electrode in which the TPCz units are directly linked by the crossing of carbazole. The twist and rigid structure of TPCz ensures the growth of the high surface
C. Liu et al. / Dyes and Pigments 142 (2017) 132e138
Fig. 1. a) The chemical structure of TPCz. b) The first cycle of CV curve for TPCz (1 mg/ mL) in a mixed solvent of acetonitrile and CH2Cl2 (V/V ¼ 1/1) with 0.1MTBAPF6 as supporting electrolytes and PEDOT:PSS-modified ITO as the working electrode, potential from 0 to 1.5 V and the scan rate was 0.2 V s1 c) Recorded for 10 scan cycles of CV curves at a scan rate of 0.2 V s1 with scan range of 0 Ve0.97 V.
area porous network structure. The peripheral carbazole groups in TPCz enable efficient electrochemical coupling with structurally well-defined coupling products (dimeric carbazyl) [15,16]. The PTPCz film shows 3D nanostructures with inherently structural pores (z1 nm) and smooth surface morphology benefiting the
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Fig. 2. a) FT-IR spectra of TPCz powder and POP film. b) The UVevis absorption spectra of TPCz spin-coated film and POP film. c) The PL spectra of the TPCz spin-coated film and POP film.
contact with active layer of PSCs. In addition, the PTPCz film increases the work function (WF) of the anode, which contributes to the hole-extraction from the active layer. By using PEDOT:PSS/ PTPCz instead of PEDOT:PSS as the anode interlayer, the power
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Fig. 3. Schematic representation of synthesis of PTPCz and model structure of POP film simulated by Density Functional Theory (DFT) calculations.
conversion efficiency (PCE) of the PSCs with PTB7:PC71BM as the active layer can be raised from 7.58% to 8.54%. 2. Experimental 2.1. Materials and characterization TPCz was prepared according to literature methods [17]. Tetrabutylammoniumhexafluorophosphate (TBAPF6, 98%) was used as the supporting electrolyte; TBAPF6 was purchased from Sigma Aldrich, USA, and dried for 24 h under vacuum before use. All solvents were dried and distilled before use. The ITO-coated glass substrates (ITO) were cleaned by detergent, deionized water, acetone and iso-propanol under sonication for 15min, respectively. A Bruker Vector 33 Fourier transform infrared spectroscopy (FTIR) spectrometer (Germany) was used for FT-IR analysis. Fluorescence spectra were obtained on a Hitachi FL-2500 spectrometer.
Fig. 4. a) HR-TEM images of the POP film. (The film was scrapped off the electrode, suspended in ethanol, and then transferred onto the copper mesh for the TEM measurements). b) AFM image of the POP film on PEDOT:PSS/ITO substrate in tapping mode (dynamic force mode).
Fig. 5. The thickness of the POP film under different CV cycles and potentials at the scan rate of 0.2 V s1 (black line: 0e0.94 V; red line: 0e0.97 V). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
C. Liu et al. / Dyes and Pigments 142 (2017) 132e138
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Fig. 6. a) KPFM image of PEDOT:PSSfilm. b) KPFM image of PEDOT:PSS/PTPCz film. c) The first cycle of CVofPTPCzfilmmeasured in acetonitrile containing 0.1 M Bu4NPF6 as electrolyte (scan rate of 0.05 V s1).
UVeVis absorption spectra were recorded on a Shimadzu UV-1700 spectrophotometer. The atomic force microscopy (AFM) and kelvin probe force microscopy (KPFM) image were recorded using a Seiko SPA 400 with an SPI 3800 probe station. The thickness of the film was measured on an Ambios Tech. XP-2profilometer. The highresolution transmission electron microscopy (HR-TEM) experiment was performed using a JEOL model JEM-3010 with an acceleration voltage of 300 kV. The POP film was prepared by the cyclic voltammetry (CV) mode in which the thickness of POP film can be facilely controlled by scan cycles and potential. The CV experiments were performed using a standard one-compartment, three-electrode electrochemical cell attached to a CHI-660C Electrochemical Workstation (Shanghai, Chenhua). An Ag/Agþ non-aqueous electrode was used as the reference electrode, ITO (~2 cm2) spin-coating a thin layer (40 nm) of poly(ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) was used as the working electrode and titanium metal was used as the counter electrode (area: ~3 cm2). TPCz (1 mg/mL)
and TBAPF6 (0.1 M) was dissolved in a mixed solvent of acetonitrile and CH2Cl2 (V/V ¼ 1/1), which was carefully purified and purged by dry nitrogen prior to the electrochemical measurements, and the resulting solution was used as the electrolyte solution. After the electrochemical polymerization, the resulting POP film was washed with a mixture of acetonitrile and CH2Cl2 (V/V ¼ 1/1) to remove any unreactive precursors and supporting electrolytes, and then dried in a vacuum oven. 2.2. The fabrication and characterization of PSCs PTPCz was deposited on ITO/PEDOT:PSS via electrochemical deposition method mentioned above. The ITO/PEDOT:PSS/PTPCz substrates were then transferred into a nitrogen-filled glove box. PTB7 was blended with PC71BM and dissolved in chlorobenzene (CB) with the addition of a small amount of diiodooctane (DIO) (CB: DIO ¼ 97:3, v/v). The blended ratios of polymer: PC71BM was 1:1.5 by weight. The solutions were then spin-coated onto the
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ITO/PEDOT:PSS/PTPCz and ITO/PEDOT:PSS substrates. The thicknesses of the active layers were about 100 nm. Subsequently, the film was transferred into a thermal evaporator. Then, 20 nm of Ca and 100 nm of Al were deposited through a shadow mask (defined active area of 0.16 cm2) onto the photoactive layer by thermoevaporation in a vacuum chamber with base pressure of 2 106 mbar. All device fabrication processes were carried out in a N2-filled glove box (Braun GmbH). PCEs were measured under an AM1.5G solar simulator (Japan, SAN-EI, XES-40S1). The power of the sun simulation was calibrated before the testing using a standard silicon solar cell, giving a value of 1000 W/m2 in the test. The current density-voltage (J-V) characteristics were recorded with a Keithley 2400 source meter.
The microporous structure of POP film was observed directly by HR-TEM. As shown in Fig. 4a, the POP film has uniform micropores with diameter of less than 1 nm, which is similar to some of the previous reported microporous organic polymers [9,13]. The diameter of the micropores was fitted with the result simulated by DFT calculations (Fig. 3). TPCz is a propeller-like molecule containing four peripheral rigid carbazole moieties for desite-specific electropolymerization which help the form of three dimensional porous polymer with inherently porosity and high physicochemical stability [17]. As anode interlayers in PSCs, the permanent
3. Results and discussion The electropolymerizable precursor TPCz was characterized first by CV. Fig. 1b shows the CV of TPCz recorded for the first scan cycle from 0 V to 1.5 V, and the onset oxidative potential of TPCz appeared at about 0.93 V, which is attributed to the oxidation of carbazole [16]. The carbazole groups are oxidized rapidly when the potential increased above 0.93 V and the peak potential shown at 1.10 V. The obvious reductive peak observed at 0.73 V due to the reduction of the dimeric carbazole cation [18,19]. The peaks appeared at potentials of 1.38 and 0.93 V are attributed to the oxidation and reduction of triphenylethylene focal core, respectively. So the potential range used in the electropolymerization of TPCz is from 0 V to 0.97 V, at which range the polymerization occurred only at the carbazole groups. For a consecutive scan of 10 cycles (Fig. 1c), a new oxidation peak appears at 0.82 V from the second scan which is assigned to the oxidation of the formed dimeric carbazoles. The peak current rise gradually with the increase of scan cycles indicated the growth of the PTPCz film on the electrode. The structures of TPCz power and the POP film were analyzed by FT-IR spectroscopy (Fig. 2a). The peak at 727 cm1 in POP film (and 725 cm1 in TPCz powder sample) and the peak at 747 cm1 in POP film (and 749 cm1 in TPCz powder sample) are attributed to the disubstituted benzene ring of unsubstitued carbazole ring [19]. The new peak at 802 cm1 in POP film is attributed to the trisubstituted benzene ring, indicating the formation of dimeric carbazole [20,21]. Compared with TPCz powder, the intensity of the peaks of the unsubstitued carbazole ring (727 cm1 and 747 cm1) are much lower for POP film, indicating that most of the carbazole in EP film formed into dimeric carbazole [22]. The UVevis spectra of the TPCz spin-coated film and the POP film are shown in Fig. 2b. For the spincoated film of TPCz, the absorption peak at 297 nm is attributed to the p-p* transition of the carbazole group. While in POP film, the absorption peak of carbazole group is about 10 nm red-shifted to 307 nm, which is attributed to the expanded p-electron delocalization caused by the formation of dimeric carbazole [22]. The PL spectra of the TPCz spin-coated film and the POP film are presented in Fig. 2c. The POP film exhibits strong photoluminescence properties and the maximum wavelength (lmax) at about 502 nm, which shows a 27 nm red-shift comparing with that of the TPCz spincoated film. As there are four carbazole units in a TPCz molecule, a cross-linked film is deposited by the electrochemical coupling reaction of carbazole groups which may increase the planarity of the molecules and cause a significant increase in the conjugated extent of the molecules, resulting in the red-shift of the PL spectra in POP film. These results indicate that POP films are achieved by cross-linking between the carbazole units. The electrochemical polymerization mechanism of TPCz is shown in Fig. 3. This POP film is chemically stable and cannot dissolve in common solvents, such as CH2Cl2, HCl and NaOH, due to its cross-linking structures.
Fig. 7. The J-V characteristics of the PSCs. a) under AM 1.5G irradiation at 1000 W/m2 and b) in the dark. c) EQE curves. The device configurations are ITO/PEDOT:PSS/PTPCz/ PTB7:PC71BM/Ca/Al.
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Table 1 Devices performance of PSCs with different anode interlayer under 1000 Wm2 AM illumination. The Statistic data was achieved from 10 independent devices. Interlayer
Voc(V)
Jsc (mA/cm2)
FF(%)
PCE (%)a
Rs (U cm2)
Rsh (kU cm2)
PEDOT:PSS PEDOT:PSS/PTPCz
0.73 ± 0.01 0.74 ± 0.01
14.91 ± 0.22 15.98 ± 0.25
68.25 ± 0.42 70.62 ± 0.51
7.38 ± 0.15(7.58) 8.32 ± 0.16 (8.54)
6.63 3.78
0.33 1.24
a
The best PCEs in the brackets.
structural pores of POP film will benefit the contact with active layer and hole-transport. The morphology of the POP film on electrode was directly measured by contact mode of AFM. The POP film exhibited a very smooth and uniform surface morphology with root-meansquare roughness (RMS) of only 1.1 nm (Fig. 4b), which satisfied the requirement of high performance interlayer for PSCs. The PTPCz film grew on the electrode with a size of 1.5 cm 1 cm. The thickness of POP film can be easily controlled by adjusting the scan cycles and the oxidative potentials. As shown in Fig. 5, the thickness increase linearly with the scan cycles and the average thickness increase from one scanning cycle is about 2.5 nm under the oxidative potential of 0.97 V. While at the low oxidative potential of 0.94 V, the average thickness increase from one scanning cycle is about 1.6 nm. So the preparation of a POP film with a thickness of tens of nm requires only a small amount of the monomer and can be completed within a few minutes. Because the POP film is too thin, collected enough samples for BrunauereEmmetteTeller (BET) measurements by electropolymerized is too difficult. Since the WF of the interlayer has great influence on devices performance, the WF of PEDOT:PSS and PEDOT:PSS/PTPCz film were determined by KPFM. The contact potential difference (CPD) originates from the WF difference between the tip and sample surfaces [23]. The WF of the sample surface is obtained by measuring the CPD. As shown in Fig. 6a and b, the PEDOT:PSS/PTPCz film exhibits a WF of 5.23 eV, while in the same experiment, the WF for PEDOT:PSS is 5.14 eV. The increased WF of the PEDOT:PSS/PTPCz film will minimize the hole-collection barrier in PSCs, suggesting it may be more competent as an anode interlayer in PSCs than PEDOT:PSS film. For an anode interlayer material, the WF should be close to the highest occupied molecular orbital (HOMO) value of the donor material in the active layer to facilitate hole-collection, while its lowest unoccupied molecular orbital (LUMO) level should be high enough to block electrons [14]. The first cycle of CV of PTPCz film shown in Fig. 6c and the LUMO energy level of PTPCz film located at 2.34 eV, which can provide good electron-blocking capability as the anode interlayer in the PSCs. The PEDOT:PSS/PTPCz film was used as anode interlayer for the fabrication of PSCs with device configuration of ITO/PEDOT:PSS/ PTPCz/PTB7:PC71BM/Ca/Al, where poly(thieno[3,4-b]-thiophene/ benzodithiophene) (PTB7) and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) act as electron donor and acceptor, respectively. As a reference, devices were also fabricated with PEDOT:PSS as the anode interlayer. The current density-voltage (J-V) characteristics of the PSCs under AM 1.5G irradiation at1000 W/m2 and in the dark are shown in Fig. 7a and b respectively. The extracted device performance metrics, including Rs and shunt resistance (Rsh), are summarized in Table 1. Devices with PEDOT:PSS/PTPCz anode interlayer exhibited a maximal power conversion efficiency (PCEmax) of 8.54% with an open-circuit voltage (Voc) of 0.74 V, a short-circuit current density (Jsc) of 16.23 mA cm2, and a fill factor (FF) of 71.11%, which is significantly higher than the value obtained from the reference device using PEDOT:PSS as the anode interlayer which showed a PCEmax of 7.58% (Voc of 0.73 V, Jsc of 15.12 mA cm2 and FF of 68.67%). A significant positive effect of the PEDOT:PSS/PTPCz based device is that all of the photovoltaic parameters are enhanced. The Rsh variation of the devices (0.33 kU cm2 for PEDOT:PSS based device to 1.24 kU cm2 for PEDOT:PSS/PTPCzbased device) may be the major
reason for the slightly increased Voc. As for the Rsh improvement, it was attributed to the reduced leakage current as shown in Fig. 7b. The EQE spectra of PSCs in Fig. 7c supports the increase in Jsc and the calculated Jsc obtained by integration of the EQE curves (16.19 mA cm2 and 15.07 mA cm2 for PEDOT:PSS/PTPCz and PEDOT:PSS based devices respectively) showed less than 2% mismatch compared with Jsc values obtained from the J-Vcurves. Both of the inherently structural pores for PTPCz film and the increased WF by PTPCz modifying minimized the contact resistance between interlayer and active layer (Table 1, Rs) and enhanced the holetransportation in PSC, resulting in high FF. 4. Conclusions In conclusion, we have demonstrated an electrochemical synthesized POP film based on propeller-like building block and its application as anode interlayer for high performance PSCs. The twist and rigid structure of the TPCz precursor ensured the growth of 3D nanostructures porous network structure for PTPCz film. In addition, the PTPCz film increases the WF of the anode. All of them contribute to the better contact with the active layer and the enhanced hole-extraction process. By using PEDOT:PSS/PTPCz instead of PEDOT:PSS as the anode interlayer, the PCE of the PSCs with PTB7:PC71BM as the active layer increases from 7.58% to 8.54%. These results indicated that electrochemical synthesized POP film had great potential applications in efficient interlayer for PSCs. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant numbers: 51203054), China Postdoctoral Science Fundation (Grant numbers: 2014M552192) and China Postdoctoral Science Fundation (Grant numbers: 2015T80906). L.Nian was financially supported by Joint International Research Laboratory of Optical Information and Scientific Research Start Project Funding of South China Normal University. References [1] Yuan S, Dorney B, White D, Kirklin S, Zapol P, Yu L, et al. Microporous polyphenylenes with tunable pore size for hydrogen storage. Chem Commun 2010;46:4547e9. [2] Mackintosh HJ, Budd PM, McKeown NB. Catalysis by microporous phthalocyanine and porphyrin network polymers. J Mater Chem 2008;18:573e8. [3] Lee JY, Wood CD, Bradshaw D, Rosseinsky MJ, Cooper AI. Hydrogen adsorption in microporous hypercrosslinked polymers. Chem Commun 2006;25:2670e2. [4] Wood CD, Tan B, Trewin A, Niu HJ, Bradshaw D, Rosseinsky MJ, et al. Hydrogen storage in microporous hypercrosslinked organic polymer networks. Chem Mater 2007;19:2034e48. [5] McKeown NB, Gahnem B, Msayib KJ, Budd PM, Tattershall CE, Mahmood K, et al. Towards polymer-based hydrogen storage materials: engineering ultramicroporous cavities within polymers of intrinsic microporosity. Angew Chem Int Ed 2006;45:1804e7. [6] Wood CD, Tan B, Trewin A, Su F, Rosseinsky MJ, Bradshaw D, et al. Microporous organic polymers for methane storage. Adv Mater 2008;20:1916e21. [7] McKeown NB, Budd PM, Msayib KJ, Ghanem BS, Kingston HJ, Tattershall CE, et al. Polymers of intrinsic microporosity (PIMs): bridging the void between microporous and polymeric materials. Chem Eur J 2005;11:2610e20. [8] Park HB, Jung CH, Lee YM, Hill AJ, Pas SJ, Mudie ST, et al. Polymers with cavities tuned for fast selective transport of small molecules and ions. Science 2007;318:254e8.
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[9] Chen Q, Luo M, Hanmmershøj P, Zhou D, Han Y, Laursen BW, et al. Microporous polycarbazole with high specific surface area for gas storage and separation. J Am Chem Soc 2012;134:6084e7. [10] Chen Q, Wang JX, Yang F, Zhou D, Bian N, Zhang XJ, et al. Tetraphenylethylenebased fluorescent porous organic polymers: preparation, gas sorption properties and photoluminescence properties. J Mater Chem 2011;21:13554e60. [11] Gu C, Chen YC, Zhang ZB, Xue SF, Sun SH, Zhang K, et al. Electrochemical route to fabricate film-like conjugated microporous polymers and application for organic electronics. Adv Mater 2013;25:3443e8. [12] Gu C, Huang N, Gao J, Xu F, Xu Y, Jiang DL. Controlled synthesis of conjugated microporous polymer films: versatile platforms for highly sensitive and labelfree chemo- and biosensing. Angew Chem Int Ed 2014;53:4850e85. [13] Gu C, Huang N, Wu Y, Xu H, Jiang DL. Design of highly photofunctional porous polymer films with controlled thickness and prominent microporosity. Angew Chem 2015;127:11702e6. [14] Gu C, Chen YC, Zhang ZB, Xue SF, Sun SH, Zhong CM, et al. Achieving high efficiency of PTB7-based polymer solar cells via integrated optimization of both anode and cathode interlayers. Adv Energy Mater 2014;4:1301771e5. [15] Roncali J. Electrogenerated functional conjugated polymers as advanced electrode materials. J Mater Chem 1999;9:1875e93. [16] Heinze J, Frontana-Uribe BA, Ludwigs S. Electrochemistry of conducting polymers-persistent models and new concepts. Chem Rev 2010;110:4724e71.
[17] Yang ZY, Chi ZG, Xu BJ, Li HY, Zhang XQ, Li XF, et al. High-Tg carbazole derivatives as a new class of aggregation-induced emission enhancement materials. J Mater Chem 2010;20:7352e9. [18] Desbene-Monvernay A, Lacaze PC, Dubois JE. Polaromicrotribometric (PMT) and IR, ESCA, EPR spectroscopic study of colored radical films formed by the electrochemical oxidation of carbazoles. J Electroanal Chem 1981;129: 229e41. [19] Desbene-Monvernay A, Lacaze PC, Delamar M. A quantitative study of crosslinking in electrodeposited carbazole films using tetrabutylammonium tribromide bromination and XPS analysis. J Electroanal Chem 1992;334: 241e6. [20] Saçak M, Akbulut U, Cheng C, Batchelder DN. Raman and i.r. spectroscopy of electrochemically obtained conducting and non-conducting poly(N-vinylcarbazole). Polymer 1994;35:2495e500. lu B. Oxidative polymerization of N[21] Saraç SA, Sezer E, Ustamehmetog substituted carbazoles. Polym Advan Technol 1997;8:556e62. [22] Gu C, Chen YC, Zhang ZB, Xue SF, Sun SH, Zhang K, et al. Electrochemical route to fabricate film-like conjugated microporous polymers and application for organic electronics. Adv Mater 2013;25:3443e8. [23] Kitamura S, Suzuki K, Iwatsuki M. High resolution imaging of contact potential difference using a novel ultrahigh vacuum non-contact atomic force microscope technique. Appl Surf Sci 1999;140:265e70.
Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Electrochemical route to fabricate porous organic polymers film and its application for polymer solar cells Cong Liu a, *, Haiyuan Luo a, Guang Shi a, Li Nian b, **, Zhenguo Chi c, Yuguang Ma d a
School of Chemistry and Environment, South China Normal University, Guangzhou, Guangdong 510006, China South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong 510006, China c Materials Science Institute, School of Chemistry and Chemical Engineering, SunYat-sen University, Guangzhou, Guangdong 510275, China d Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong 510640, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2017 Received in revised form 9 March 2017 Accepted 9 March 2017 Available online 14 March 2017
A facile and controllable electrochemical polymerization of porous organic polymers (POP) film using carbazolyl triphenylethylene derivative (TPCz) as building block for the applications in polymer solar cells(PSCs) was developed. The POP film is an interlocked network film of poly-TPCz (designated as PTPCz) which has smooth surface morphology and porous nanostructures. The film can increase the work function (WF) of the anode of PSCs, the contact with the active layer, and enhance hole-extraction process. By using PEDOT:PSS/PTPCz instead of PEDOT:PSS as the anode interlayer, the optimized PSCs shows a 13% enhancement in power conversion efficiency (PCE) which is 8.54%. These results indicate the potential of electrochemical synthesized POP film as efficient interlayer for PSCs. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Polymer solar cells Anode interlayer Electrochemical polymerization Porous organic polymer
1. Introduction Porous organic polymers (POP), with properties of large surface areas, high chemical stabilities, and low skeletal density, have attracted great attention in recent years due to their potential applications in catalysis, gas storage, and gas separations [1e8]. By using rigid or contorted building blocks, POP with high surface area has been fabricated from chemical polymerization. For example, Chen et al. reported a carbazole-based porous organic polymer via straightforward carbazole-based oxidative coupling polymerization, which showed a high Brunauer-Emmett-Teller (BET) surface area (up to 2220 m2 g1) and exhibited a dominant pore diameter centred at about 0.62 nm [9]. Chen et al. synthesized tetraphenylethylene-based porous organic polymers through a Suzuki coupling polycondensation or oxidative coupling polymerization, for which the BET specific surface area values could be tuned between 472 and 810 m2 g1 and the adsorption capacity for hydrogen is up to 1.07 wt% at 1.13 bar and 77 K [10]. Due to their cross-linked structure, POP are usually insoluble and hard to be processed, which may limit their application in many filed, including potential application in polymer solar cells * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Liu), [email protected] (L. Nian). http://dx.doi.org/10.1016/j.dyepig.2017.03.017 0143-7208/© 2017 Elsevier Ltd. All rights reserved.
(PSCs). To solve this problem, electropolymerization is a new effective choice to synthesize POP film, in which thin films of POPs can be deposited on the electrode with controllable thickness readily [11,12]. Gu et al. synthesized highly luminescent POP films to detect explosives by electropolymerization method, in which multiple peripheral units of carbazole were designed for site-specific electropolymerization of three dimensional porous skeletons [13]. Recently, POP films were also emerged to be used as interlayer materials in PSCs to improve the device performance [14]. However, there are still few reports on the application of POP films in high performance PSCs. The properties of POPs are affected by multiple factors such as building blocks, pore structure, material morphology, and so on. So far, it is still a challenge to develop POP films with potential use and good performance for PSCs. In this work, a propeller-like building block, an aggregationinduced emission (AIE) molecule based on carbazolyl triphenylethylene derivative (TPCz), is selected as a single component for the electrochemical synthesis of POP film for potential application as anode interlayer in PSCs. The chemical structure of electrochemical deposited precursor TPCz was showed in Fig. 1a. The POP film is an interlocked network film of poly-TPCz (designated as PTPCz) synthesized by electropolymerization on the electrode in which the TPCz units are directly linked by the crossing of carbazole. The twist and rigid structure of TPCz ensures the growth of the high surface
C. Liu et al. / Dyes and Pigments 142 (2017) 132e138
Fig. 1. a) The chemical structure of TPCz. b) The first cycle of CV curve for TPCz (1 mg/ mL) in a mixed solvent of acetonitrile and CH2Cl2 (V/V ¼ 1/1) with 0.1MTBAPF6 as supporting electrolytes and PEDOT:PSS-modified ITO as the working electrode, potential from 0 to 1.5 V and the scan rate was 0.2 V s1 c) Recorded for 10 scan cycles of CV curves at a scan rate of 0.2 V s1 with scan range of 0 Ve0.97 V.
area porous network structure. The peripheral carbazole groups in TPCz enable efficient electrochemical coupling with structurally well-defined coupling products (dimeric carbazyl) [15,16]. The PTPCz film shows 3D nanostructures with inherently structural pores (z1 nm) and smooth surface morphology benefiting the
133
Fig. 2. a) FT-IR spectra of TPCz powder and POP film. b) The UVevis absorption spectra of TPCz spin-coated film and POP film. c) The PL spectra of the TPCz spin-coated film and POP film.
contact with active layer of PSCs. In addition, the PTPCz film increases the work function (WF) of the anode, which contributes to the hole-extraction from the active layer. By using PEDOT:PSS/ PTPCz instead of PEDOT:PSS as the anode interlayer, the power
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Fig. 3. Schematic representation of synthesis of PTPCz and model structure of POP film simulated by Density Functional Theory (DFT) calculations.
conversion efficiency (PCE) of the PSCs with PTB7:PC71BM as the active layer can be raised from 7.58% to 8.54%. 2. Experimental 2.1. Materials and characterization TPCz was prepared according to literature methods [17]. Tetrabutylammoniumhexafluorophosphate (TBAPF6, 98%) was used as the supporting electrolyte; TBAPF6 was purchased from Sigma Aldrich, USA, and dried for 24 h under vacuum before use. All solvents were dried and distilled before use. The ITO-coated glass substrates (ITO) were cleaned by detergent, deionized water, acetone and iso-propanol under sonication for 15min, respectively. A Bruker Vector 33 Fourier transform infrared spectroscopy (FTIR) spectrometer (Germany) was used for FT-IR analysis. Fluorescence spectra were obtained on a Hitachi FL-2500 spectrometer.
Fig. 4. a) HR-TEM images of the POP film. (The film was scrapped off the electrode, suspended in ethanol, and then transferred onto the copper mesh for the TEM measurements). b) AFM image of the POP film on PEDOT:PSS/ITO substrate in tapping mode (dynamic force mode).
Fig. 5. The thickness of the POP film under different CV cycles and potentials at the scan rate of 0.2 V s1 (black line: 0e0.94 V; red line: 0e0.97 V). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. a) KPFM image of PEDOT:PSSfilm. b) KPFM image of PEDOT:PSS/PTPCz film. c) The first cycle of CVofPTPCzfilmmeasured in acetonitrile containing 0.1 M Bu4NPF6 as electrolyte (scan rate of 0.05 V s1).
UVeVis absorption spectra were recorded on a Shimadzu UV-1700 spectrophotometer. The atomic force microscopy (AFM) and kelvin probe force microscopy (KPFM) image were recorded using a Seiko SPA 400 with an SPI 3800 probe station. The thickness of the film was measured on an Ambios Tech. XP-2profilometer. The highresolution transmission electron microscopy (HR-TEM) experiment was performed using a JEOL model JEM-3010 with an acceleration voltage of 300 kV. The POP film was prepared by the cyclic voltammetry (CV) mode in which the thickness of POP film can be facilely controlled by scan cycles and potential. The CV experiments were performed using a standard one-compartment, three-electrode electrochemical cell attached to a CHI-660C Electrochemical Workstation (Shanghai, Chenhua). An Ag/Agþ non-aqueous electrode was used as the reference electrode, ITO (~2 cm2) spin-coating a thin layer (40 nm) of poly(ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) was used as the working electrode and titanium metal was used as the counter electrode (area: ~3 cm2). TPCz (1 mg/mL)
and TBAPF6 (0.1 M) was dissolved in a mixed solvent of acetonitrile and CH2Cl2 (V/V ¼ 1/1), which was carefully purified and purged by dry nitrogen prior to the electrochemical measurements, and the resulting solution was used as the electrolyte solution. After the electrochemical polymerization, the resulting POP film was washed with a mixture of acetonitrile and CH2Cl2 (V/V ¼ 1/1) to remove any unreactive precursors and supporting electrolytes, and then dried in a vacuum oven. 2.2. The fabrication and characterization of PSCs PTPCz was deposited on ITO/PEDOT:PSS via electrochemical deposition method mentioned above. The ITO/PEDOT:PSS/PTPCz substrates were then transferred into a nitrogen-filled glove box. PTB7 was blended with PC71BM and dissolved in chlorobenzene (CB) with the addition of a small amount of diiodooctane (DIO) (CB: DIO ¼ 97:3, v/v). The blended ratios of polymer: PC71BM was 1:1.5 by weight. The solutions were then spin-coated onto the
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ITO/PEDOT:PSS/PTPCz and ITO/PEDOT:PSS substrates. The thicknesses of the active layers were about 100 nm. Subsequently, the film was transferred into a thermal evaporator. Then, 20 nm of Ca and 100 nm of Al were deposited through a shadow mask (defined active area of 0.16 cm2) onto the photoactive layer by thermoevaporation in a vacuum chamber with base pressure of 2 106 mbar. All device fabrication processes were carried out in a N2-filled glove box (Braun GmbH). PCEs were measured under an AM1.5G solar simulator (Japan, SAN-EI, XES-40S1). The power of the sun simulation was calibrated before the testing using a standard silicon solar cell, giving a value of 1000 W/m2 in the test. The current density-voltage (J-V) characteristics were recorded with a Keithley 2400 source meter.
The microporous structure of POP film was observed directly by HR-TEM. As shown in Fig. 4a, the POP film has uniform micropores with diameter of less than 1 nm, which is similar to some of the previous reported microporous organic polymers [9,13]. The diameter of the micropores was fitted with the result simulated by DFT calculations (Fig. 3). TPCz is a propeller-like molecule containing four peripheral rigid carbazole moieties for desite-specific electropolymerization which help the form of three dimensional porous polymer with inherently porosity and high physicochemical stability [17]. As anode interlayers in PSCs, the permanent
3. Results and discussion The electropolymerizable precursor TPCz was characterized first by CV. Fig. 1b shows the CV of TPCz recorded for the first scan cycle from 0 V to 1.5 V, and the onset oxidative potential of TPCz appeared at about 0.93 V, which is attributed to the oxidation of carbazole [16]. The carbazole groups are oxidized rapidly when the potential increased above 0.93 V and the peak potential shown at 1.10 V. The obvious reductive peak observed at 0.73 V due to the reduction of the dimeric carbazole cation [18,19]. The peaks appeared at potentials of 1.38 and 0.93 V are attributed to the oxidation and reduction of triphenylethylene focal core, respectively. So the potential range used in the electropolymerization of TPCz is from 0 V to 0.97 V, at which range the polymerization occurred only at the carbazole groups. For a consecutive scan of 10 cycles (Fig. 1c), a new oxidation peak appears at 0.82 V from the second scan which is assigned to the oxidation of the formed dimeric carbazoles. The peak current rise gradually with the increase of scan cycles indicated the growth of the PTPCz film on the electrode. The structures of TPCz power and the POP film were analyzed by FT-IR spectroscopy (Fig. 2a). The peak at 727 cm1 in POP film (and 725 cm1 in TPCz powder sample) and the peak at 747 cm1 in POP film (and 749 cm1 in TPCz powder sample) are attributed to the disubstituted benzene ring of unsubstitued carbazole ring [19]. The new peak at 802 cm1 in POP film is attributed to the trisubstituted benzene ring, indicating the formation of dimeric carbazole [20,21]. Compared with TPCz powder, the intensity of the peaks of the unsubstitued carbazole ring (727 cm1 and 747 cm1) are much lower for POP film, indicating that most of the carbazole in EP film formed into dimeric carbazole [22]. The UVevis spectra of the TPCz spin-coated film and the POP film are shown in Fig. 2b. For the spincoated film of TPCz, the absorption peak at 297 nm is attributed to the p-p* transition of the carbazole group. While in POP film, the absorption peak of carbazole group is about 10 nm red-shifted to 307 nm, which is attributed to the expanded p-electron delocalization caused by the formation of dimeric carbazole [22]. The PL spectra of the TPCz spin-coated film and the POP film are presented in Fig. 2c. The POP film exhibits strong photoluminescence properties and the maximum wavelength (lmax) at about 502 nm, which shows a 27 nm red-shift comparing with that of the TPCz spincoated film. As there are four carbazole units in a TPCz molecule, a cross-linked film is deposited by the electrochemical coupling reaction of carbazole groups which may increase the planarity of the molecules and cause a significant increase in the conjugated extent of the molecules, resulting in the red-shift of the PL spectra in POP film. These results indicate that POP films are achieved by cross-linking between the carbazole units. The electrochemical polymerization mechanism of TPCz is shown in Fig. 3. This POP film is chemically stable and cannot dissolve in common solvents, such as CH2Cl2, HCl and NaOH, due to its cross-linking structures.
Fig. 7. The J-V characteristics of the PSCs. a) under AM 1.5G irradiation at 1000 W/m2 and b) in the dark. c) EQE curves. The device configurations are ITO/PEDOT:PSS/PTPCz/ PTB7:PC71BM/Ca/Al.
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Table 1 Devices performance of PSCs with different anode interlayer under 1000 Wm2 AM illumination. The Statistic data was achieved from 10 independent devices. Interlayer
Voc(V)
Jsc (mA/cm2)
FF(%)
PCE (%)a
Rs (U cm2)
Rsh (kU cm2)
PEDOT:PSS PEDOT:PSS/PTPCz
0.73 ± 0.01 0.74 ± 0.01
14.91 ± 0.22 15.98 ± 0.25
68.25 ± 0.42 70.62 ± 0.51
7.38 ± 0.15(7.58) 8.32 ± 0.16 (8.54)
6.63 3.78
0.33 1.24
a
The best PCEs in the brackets.
structural pores of POP film will benefit the contact with active layer and hole-transport. The morphology of the POP film on electrode was directly measured by contact mode of AFM. The POP film exhibited a very smooth and uniform surface morphology with root-meansquare roughness (RMS) of only 1.1 nm (Fig. 4b), which satisfied the requirement of high performance interlayer for PSCs. The PTPCz film grew on the electrode with a size of 1.5 cm 1 cm. The thickness of POP film can be easily controlled by adjusting the scan cycles and the oxidative potentials. As shown in Fig. 5, the thickness increase linearly with the scan cycles and the average thickness increase from one scanning cycle is about 2.5 nm under the oxidative potential of 0.97 V. While at the low oxidative potential of 0.94 V, the average thickness increase from one scanning cycle is about 1.6 nm. So the preparation of a POP film with a thickness of tens of nm requires only a small amount of the monomer and can be completed within a few minutes. Because the POP film is too thin, collected enough samples for BrunauereEmmetteTeller (BET) measurements by electropolymerized is too difficult. Since the WF of the interlayer has great influence on devices performance, the WF of PEDOT:PSS and PEDOT:PSS/PTPCz film were determined by KPFM. The contact potential difference (CPD) originates from the WF difference between the tip and sample surfaces [23]. The WF of the sample surface is obtained by measuring the CPD. As shown in Fig. 6a and b, the PEDOT:PSS/PTPCz film exhibits a WF of 5.23 eV, while in the same experiment, the WF for PEDOT:PSS is 5.14 eV. The increased WF of the PEDOT:PSS/PTPCz film will minimize the hole-collection barrier in PSCs, suggesting it may be more competent as an anode interlayer in PSCs than PEDOT:PSS film. For an anode interlayer material, the WF should be close to the highest occupied molecular orbital (HOMO) value of the donor material in the active layer to facilitate hole-collection, while its lowest unoccupied molecular orbital (LUMO) level should be high enough to block electrons [14]. The first cycle of CV of PTPCz film shown in Fig. 6c and the LUMO energy level of PTPCz film located at 2.34 eV, which can provide good electron-blocking capability as the anode interlayer in the PSCs. The PEDOT:PSS/PTPCz film was used as anode interlayer for the fabrication of PSCs with device configuration of ITO/PEDOT:PSS/ PTPCz/PTB7:PC71BM/Ca/Al, where poly(thieno[3,4-b]-thiophene/ benzodithiophene) (PTB7) and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) act as electron donor and acceptor, respectively. As a reference, devices were also fabricated with PEDOT:PSS as the anode interlayer. The current density-voltage (J-V) characteristics of the PSCs under AM 1.5G irradiation at1000 W/m2 and in the dark are shown in Fig. 7a and b respectively. The extracted device performance metrics, including Rs and shunt resistance (Rsh), are summarized in Table 1. Devices with PEDOT:PSS/PTPCz anode interlayer exhibited a maximal power conversion efficiency (PCEmax) of 8.54% with an open-circuit voltage (Voc) of 0.74 V, a short-circuit current density (Jsc) of 16.23 mA cm2, and a fill factor (FF) of 71.11%, which is significantly higher than the value obtained from the reference device using PEDOT:PSS as the anode interlayer which showed a PCEmax of 7.58% (Voc of 0.73 V, Jsc of 15.12 mA cm2 and FF of 68.67%). A significant positive effect of the PEDOT:PSS/PTPCz based device is that all of the photovoltaic parameters are enhanced. The Rsh variation of the devices (0.33 kU cm2 for PEDOT:PSS based device to 1.24 kU cm2 for PEDOT:PSS/PTPCzbased device) may be the major
reason for the slightly increased Voc. As for the Rsh improvement, it was attributed to the reduced leakage current as shown in Fig. 7b. The EQE spectra of PSCs in Fig. 7c supports the increase in Jsc and the calculated Jsc obtained by integration of the EQE curves (16.19 mA cm2 and 15.07 mA cm2 for PEDOT:PSS/PTPCz and PEDOT:PSS based devices respectively) showed less than 2% mismatch compared with Jsc values obtained from the J-Vcurves. Both of the inherently structural pores for PTPCz film and the increased WF by PTPCz modifying minimized the contact resistance between interlayer and active layer (Table 1, Rs) and enhanced the holetransportation in PSC, resulting in high FF. 4. Conclusions In conclusion, we have demonstrated an electrochemical synthesized POP film based on propeller-like building block and its application as anode interlayer for high performance PSCs. The twist and rigid structure of the TPCz precursor ensured the growth of 3D nanostructures porous network structure for PTPCz film. In addition, the PTPCz film increases the WF of the anode. All of them contribute to the better contact with the active layer and the enhanced hole-extraction process. By using PEDOT:PSS/PTPCz instead of PEDOT:PSS as the anode interlayer, the PCE of the PSCs with PTB7:PC71BM as the active layer increases from 7.58% to 8.54%. These results indicated that electrochemical synthesized POP film had great potential applications in efficient interlayer for PSCs. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant numbers: 51203054), China Postdoctoral Science Fundation (Grant numbers: 2014M552192) and China Postdoctoral Science Fundation (Grant numbers: 2015T80906). L.Nian was financially supported by Joint International Research Laboratory of Optical Information and Scientific Research Start Project Funding of South China Normal University. References [1] Yuan S, Dorney B, White D, Kirklin S, Zapol P, Yu L, et al. Microporous polyphenylenes with tunable pore size for hydrogen storage. Chem Commun 2010;46:4547e9. [2] Mackintosh HJ, Budd PM, McKeown NB. Catalysis by microporous phthalocyanine and porphyrin network polymers. J Mater Chem 2008;18:573e8. [3] Lee JY, Wood CD, Bradshaw D, Rosseinsky MJ, Cooper AI. Hydrogen adsorption in microporous hypercrosslinked polymers. Chem Commun 2006;25:2670e2. [4] Wood CD, Tan B, Trewin A, Niu HJ, Bradshaw D, Rosseinsky MJ, et al. Hydrogen storage in microporous hypercrosslinked organic polymer networks. Chem Mater 2007;19:2034e48. [5] McKeown NB, Gahnem B, Msayib KJ, Budd PM, Tattershall CE, Mahmood K, et al. Towards polymer-based hydrogen storage materials: engineering ultramicroporous cavities within polymers of intrinsic microporosity. Angew Chem Int Ed 2006;45:1804e7. [6] Wood CD, Tan B, Trewin A, Su F, Rosseinsky MJ, Bradshaw D, et al. Microporous organic polymers for methane storage. Adv Mater 2008;20:1916e21. [7] McKeown NB, Budd PM, Msayib KJ, Ghanem BS, Kingston HJ, Tattershall CE, et al. Polymers of intrinsic microporosity (PIMs): bridging the void between microporous and polymeric materials. Chem Eur J 2005;11:2610e20. [8] Park HB, Jung CH, Lee YM, Hill AJ, Pas SJ, Mudie ST, et al. Polymers with cavities tuned for fast selective transport of small molecules and ions. Science 2007;318:254e8.
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