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Perylene diimide-benzodithiophene D-A copolymers as acceptor in all-polymer solar cells
Organic Electronics 41 (2017) 49e55
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Perylene diimide-benzodithiophene D-A copolymers as acceptor in all-polymer solar cells Youdi Zhang a, Xia Guo a, **, Wenyan Su a, Bing Guo a, Zhuo Xu a, Maojie Zhang a, ***, Yongfang Li a, b, * a
State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2016 Received in revised form 24 November 2016 Accepted 26 November 2016
Two n-type conjugated D-A copolymers with perylene diimide (PDI) as acceptor unit and benzodithiophene (BDT) as donor unit, P(PDI-BDT-Ph) and P(PDI-BDT-Th), were synthesized and applied as electron acceptor in all-polymer solar cells (all-PSCs). P(PDI-BDT-Ph) and P(PDI-BDT-Th) films exhibit similar absorption spectra in the visible region with optical bandgap (Eg) of 1.65 eV and 1.55 eV respectively, and the identical LUMO level of 3.89 eV. The all-PSCs based on P(PDI-BDT-Ph) as acceptor and PTB7-Th as donor demonstrated a power conversion efficiency (PCE) of 4.31% with a short-circuit current density (Jsc) of 11.94 mA cm2, an open-circuit voltage (Voc) of 0.81 V, and a fill factor (FF) of 44.49%. By contrast, the corresponding all-PSCs with P(PDI-BDT-Th) as acceptor showed a relative lower PCE of 3.58% with a Jsc of 11.36 mA cm2, Voc of 0.79 V, and FF of 40.00%. © 2016 Elsevier B.V. All rights reserved.
Keywords: Perylene diimide n-type conjugated polymers Polymer acceptors All-polymer solar cells
1. Introduction All-polymer solar cells (all-PSCs) with n-type conjugated polymer as acceptor have developed rapidly in recent years due to the potential advantages of easy-tuning absorption spectra and molecular energy levels as well as better flexibility of the polymer acceptor over the traditional fullerene derivative acceptor PCBM [1e16]. However, the power conversion efficiencies (PCEs) of the all-PSCs are still lag behind those of the polymer:fullerene PSCs. Thus, it is crucial to design and synthesize more suitable n-type conjugated polymer acceptors for improving the photovoltaic performance of the all-PSCs [17e19]. Perylene diimides (PDIs)-based D-A copolymers possess higher electron mobility and suitable LUMO energy levels for the application as acceptor in the all-PSCs. In 2007, Zhan et al. synthesized a
* Corresponding author. Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Guo), [email protected] (M. Zhang), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.orgel.2016.11.038 1566-1199/© 2016 Elsevier B.V. All rights reserved.
PDI-based D-A copolymer with PDI as acceptor unit and dithieno [3,2-b:2,3-d]thiophene as donor unit, and the all-PSCs with the polymer as acceptor and a two-dimension (2D)-conjugated polythiophene derivative as donor displayed a PCE of ca. 1% [20]. Then the PCE was further improved to ca. 1.5% by the molecular structural modification of the polymer donor and polymer acceptor [21]. Subsequently, a series of n-type PDI-based conjugated copolymers were designed and synthesized by Zhou et al. and the PCE of the allPSCs with the PDI-based polymer as acceptor reached over 2% by using a 2D-conjugated polythiophene derivative as donor [3]. Bao et al. achieved a higher PCE of 4.4% in 2014 for the all-PSCs with the PDI-based polymer as acceptor and the isoindigo-containing polymer as donor [22]. In the past few years, 2D-conjugated polymers based on benzodithiophene (BDT) unit with conjugated side chains play an important role in the application as polymer donor in high performance PSCs [23e32]. Recently, we applied the 2D-conjugation concept to n-type conjugated polymers and synthesized a 2Dconjugated D-A copolymer P(PDI-BDT-T) with alkylthienyl substituted benzodithiophene (BDT) as donor unit and PDI as acceptor unit, and the all-PSCs with P(PDI-BDT-T) as polymer acceptor demonstrated a PCE of 4.71% [33]. In order to extend the family of the n-type 2D-conjugated polymers, herein we
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synthesized two new n-type 2D-conjugated D-A alternative copolymers P(PDI-BDT-Th) and P(PDI-BDT-Ph) with alkoxy-thienyl or alkoxy-phenyl substituted BDT as donor unit and PDI as acceptor unit (Scheme 1), and compared their physicochemical and photovoltaic properties in the all-PSCs with the polymers as acceptor and a narrow bandgap polymer PTB7-Th as donor. 2. Experimental section 2.1. General procedure for the polymerization Monomers 2Br-PDI [34e38], BDT-Th-Sn [39,40] and BDT-Ph-Sn [41,42] were prepared by using experimental procedures reported in literatures. 2Br-PDI (0.30 mmol), BDT-Th-Sn or BDT-Ph-Sn (0.30 mmol) were dissolved into 10 mL of toluene in a flask under nitrogen. The flask equipped with a return pipe was then degassed and filled with argon for three times. Pd(PPh3)4 (18 mg) was added into the flask. Then, the reaction mixture was treated for 18e24 h at 110 C. After the reaction mixture was cooled to room temperature, the polymer was precipitated in 150 mL of methanol. Subsequently, the crude product was collected by filtration, dried in vacuum drying oven, and then purified by Soxhlet extraction with methanol, acetone, and hexane. P(PDI-BDT-Th) (364 mg, 77%): Mn ¼ 25.1 KDa, PDI ¼ 1.95, Anal. Calcd for C96H126N2O6S4 (%): C 75.25, H 8.29, N 1.83; found (%): C 74.92, H 8.07, N 1.64; 1H NMR (400 MHz, CDCl3) d 8.82 (d, 1H), 8.55e8.26 (m, 2H), 8.18e7.87 (m, 1H), 7.20e6.97 (m, 1H), 6.35e6.10 (m, 1H), 4.39e3.76 (m, 4H), 1.95 (d, 1H), 1.70 (d, 1H), 1.27 (m, 39H), 1.00e0.72 (m, 13H). P(PDI-BDT-Ph) (374 mg, 80%): Mn ¼ 27.8 KDa, PDI ¼ 1.67, Anal. Calcd for C102H134N2O6S2 (%): C 79.13, H 8.72, N 1.81; found (%): C 79.25, H 8.57, N 1.75; 1H NMR (400 MHz, CDCl3) d 8.76 (d, 1H), 8.52e8.19 (m, 2H), 7.64 (s, 3H), 7.00 (s, 3H), 3.99 (d, 4H), 2.00 (s, 1H), 1.68 (d, 1H), 1.28 (m, 40H), 0.91e0.76 (m, 12H). 2.2. Instruments and measurements UVevis absorption spectra were taken on an Agilent Technologies Cary Series UVeViseNIR Spectrophotometer in chloroform solution and in thin film. The electrochemical cyclic voltammetry (CV) was performed on a Zahner Ennium Electrochemical
Workstation with glassy carbon disk, Pt wire, and Ag/Agþ electrode as working electrode, counter electrode, and reference electrode respectively, in a 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile electrolyte solution. Atomic force microscopy (AFM) measurements were carried out on a Dimension 3100 (Veeco) Atomic Force Microscope in the tapping mode. Transmission electron microscopy (TEM) was performed using a JEOL 2200FS instrument at 160 kV accelerating voltage. X-ray diffraction (XRD) measurement was performed using a Bruker D8 Advance Instrument at 40 kV voltage and 200 mA current with Cu Ka radiation. 2.3. Charge carrier mobility measurement The charge carrier mobility was measured by the space charge limited current (SCLC) method with the hole-only device of ITO/ PEDOT:PSS/Active layer/MoO3/Al for the hole mobility measurement or with the electron-only device of ITO/ZnO/Active layer/Ca/ Al for the electron mobility measurement. After measurements of the J-V curves of the devices, the hole mobilities and the electron mobilities were calculated according to the following equation,
8 V2 J ¼ εr ε0 m 3 9 L where εr is the dielectric constant of the polymer, ε0 is the permittivity of the vacuum, m0 is the zero-field mobility, J is the current density, L is the thickness of the blend films, V ¼ Vappl e Vbi, Vappl is the applied potential, and Vbi is the built-in potential from the difference in the work function of the anode and the cathode (in the hole-only and electron-only device,Vbi value is 0.2 V and 0 V, respectively). 2.4. Device fabrication and characterization The ITO-coated glass substrate was cleaned in an ultrasonic bath with deionized water, acetone, and isopropanol, each process was approximately 15 min, and then dried under a stream of dry nitrogen. Subsequently, the ITO-coated glass substrate was treated by UV-ozone for 15 min. PEDOT:PSS (Clevios P VP AI 4083, H.C. Starck) solution was then spin-coated onto the pre-cleaned ITO coated glass substrates, and thermal-treated at 150 C for 15 min to get the
Scheme 1. Molecular structures of polymer donor PTBT-Th, and polymer acceptors P(PDI-BDT-Th) and P(PDI-BDT-Ph).
Y. Zhang et al. / Organic Electronics 41 (2017) 49e55
PEDOT:PSS film with thickness of ~30 nm. The blend solutions of PTB7-Th and P(PDI-BDT-Th) or P(PDIBDT-Ph) were prepared by simultaneously dissolving PTB7-Th and P(PDI-BDT-Th) or P(PDI-BDT-Ph) with different D/A weight ratio, in dichlorobenzene without or with solvent additives (1chloronaphthalene), and spin-coated on the ITO/PEDOT: PSS electrode at 3000 rpm for 60 s, resulting in the active layer with a film thickness of ~90 nm. Then 20 nm thick Ca layer covered with 100 nm Al electrode were thermally deposited under a pressure of 2 104 Pa, through a shadow mask on top of the active layer. The active area of the pixels, as defined by the overlap of anode and cathode area, was 0.038 cm2. The thickness was measured by Ambios Technology D-100 surface profilometer. The current density-voltage (J-V) curves were measured in N2 atmosphere using AM1.5G solar simulator with an irradiation light intensity of 100 mWcm2. The external quantum efficiency (EQE) of the devices was measured by Solar Cell Spectral Response Measurement System QE-R3011 (Enli Technology CO., Ltd.). The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. 3. Results and discussion 3.1. Synthesis As shown in Scheme 2, P(PDI-BDT-Th) and P(PDI-BDT-Ph) were synthesized by the same method as our previously reported [33], with a yield of 77% and 80%, respectively. The polymers could well dissolve in common organic solvents, e.g. chloroform, toluene, chlorobenzene and o-dichlorobenzene (o-DCB). The values of the number average molecular weight (Mn) for P(PDI-BDT-Th) and P(PDI-BDT-Ph) are 25.1 KDa and 27.8 KDa, with their polydispersity indices of 1.95 and 1.67, respectively, which were estimated by high temperature gel permeation chromatography (GPC) at 160 C using 1,2,4-trichlorobenzene as the solvent and polystyrene with narrow molecular weight distribution as a standard. 3.2. Thermal properties Thermogravimetric analysis (TGA) measurements were carried out to evaluate the thermal stability of the polymers P(PDI-BDTTh) and P(PDI-BDT-Ph), and the TGA plots of the two polymers are shown in Fig. 1a. P(PDI-BDT-Th) shows an excellent thermal stability with high decomposition temperature (Td) at 5% weight loss of ca. 346 C, which is similar with that of P(PDI-BDT-T) (360 C) [33]. Importantly, Td of P(PDI-BDT-Ph) is at ca. 466 C, indicating that attaching phenyl to the side chains is beneficial for improving the thermal stability in comparison with thienyl side chain.
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From the DSC thermograms of polymers P(PDI-BDT-Th) and P(PDI-BDT-Ph) at a heating rate of 10 C min1 (Fig. 1b), no obvious exotherm or endotherm peaks are observed from 50 to 250 C, which reveals that the polymers are amorphous materials. 3.3. Optical and electrochemical properties Fig. 2a shows the absorption spectra of P(PDI-BDT-Th) and P(PDI-BDT-Ph) in chloroform solution and thin films on quartz substrates. And the corresponding optical data, including the absorption peaks (labs), absorption edge wavelength (lonset) and their optical band gaps are listed in Table 1. The absorption spectra of the two polymers in thin films are identical in shape to their solution spectra, and show broad absorption covering the entire visible light range from 300 nm to 800 nm. P(PDI-BDT-Th) and P(PDI-BDT-Ph) display three main absorption peaks, including a strong p-p* transition peak of donor unit and a p-p* transition peak of acceptor unit in the wavelength range of 300e420 nm [43], a PDI segment absorption peak at 450e600 nm [44] and an intramolecular charge-transfer transition peak which is the broad tail/shoulder extending in the wavelength range of 600e800 nm. And absorption spectra of P(PDI-BDT-Th) and P(PDI-BDT-Ph) is similar in shape to that of P(PDI-BDT-T) at 300e800 nm. The absorption edges of the two polymer films are at 800 nm for P(PDI-BDT-Th) and 752 nm for P(PDI-BDT-Ph) corresponding to the optical band gaps of 1.55 and 1.65 eV, respectively. The electronic energy levels of the conjugated polymers P(PDIBDT-Th) and P(PDI-BDT-Ph) thin films on working electrode were estimated by electrochemical cyclicvoltammetry (CV) [45,46]. Fig. 2b displays cyclic voltammograms of the polymer films, from which the onset reduction and onset oxidation potentials (Ered and Eox) can be observed to be 0.82/0.79 V vs. Ag/Agþ for P(PDI-BDTTh) and 0.82/0.96 V vs. Ag/Agþ for P(PDI-BDT-Ph), respectively. Thus, the LUMO and HOMO energy levels of the two polymers can be estimated from the Ered and Eox values according to the equations: [47] HOMO ¼ -e (Eox þ 4.71) (eV); LUMO ¼ -e (Ered þ 4.71) (eV). The estimated LUMO and HOMO levels are 3.89/5.50 eV for P(PDI-BDT-Th) and 3.89/5.67 eV for P(PDI-BDT-Ph), as shown in Table 1. The LUMO level (3.89 eV) of the two n-type conjugated polymers is identical to that of the previously reported electronic acceptor P(PDI-BDT-T) and close to that of PCBM, indicating that the two polymers could be used as acceptor in PSCs instead of PCBM from the viewpoint of electronic energy levels. The HOMO level (5.67 eV) of P(PDI-BDT-Ph) is ca. 0.17 eV lower than that (5.50 eV) of P(PDI-BDT-Th), which could be due to the weaker electron-donating ability of alkoxy-phenyl group in P(PDI-BDT-Ph) than the alkoxy-thienyl group in P(PDI-BDT-Th) [48]. It should be mentioned that the HOMO energy level (5.50 eV) of P(PDI-BDT-
Scheme 2. Synthesis procedures of the n-type conjugated polymers P(PDI-BDT-Th) and P(PDI-BDT-Ph).
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Fig. 1. (a) TGA and (b) DSC plots of P(PDI-BDT-Th) (curve with square) and P(PDI-BDT-Ph) (curve with circle) with a heating rate of 10 C min1 under the inert atmosphere.
Fig. 2. (a) UVevis absorption spectra of P(PDI-BDT-Th) and P(PDI-BDT-Ph) in chloroform solution and in solid films; (b) Cyclic voltammograms of P(PDI-BDT-Th) and P(PDI-BDTPh) films on a platinum electrode in 0.1 mol L1 Bu4NPF6 acetonitrile solutions at a scan rate of 50 mV s1.
Table 1 Optical and electrochemical properties of polymers P(PDI-BDT-Th) and P(PDI-BDT-Ph). Polymers
P(PDI-BDT-Th) P(PDI-BDT-Ph) a b
labs (nm) Solution
Film
330/387/554 309/374/546/665
330/396/554 309/377/546/674
lonseta (nm)
Egb (eV)
LUMO (eV)
HOMO (eV)
800 752
1.55 1.65
3.89 3.89
5.50 5.67
Measured in thin films. Optical band gap.
Th) is ca. 0.20 eV up-shifted than that (5.70 eV) of P(PDI-BDT-T) [33] due to the stronger electron-donating ability of alkoxy substituent on P(PDI-BDT-Th) than alkyl substituent on P(PDI-BDT-T). In addition, in comparison with the LUMO (3.61 eV) and HOMO (5.25 eV) of PTB7-Th [33], the two n-type polymers of P(PDI-BDTTh) and P(PDI-BDT-Ph) are suitable polymer acceptors with PTB7Th as polymer donor in the energy level point of view. 3.4. Photovoltaic performance of All-PSCs To evaluate the photovoltaic properties of the two n-type copolymers, all-PSCs were fabricated with PTB7-Th as donor and P(PDI-BDT-Th) or P(PDI-BDT-Ph) as acceptor in the conventional device structure of ITO/PEDOT:PSS/PTB7-Th:P(PDI-BDT-Th) or P(PDI-BDT-Ph)/Ca/Al, where ITO is indium tin oxide, PEDOT is poly(3,4-ethylenedioxythiophene), and PSS is poly(styrene sulfonate). As we all know, the weight ratios of donor and acceptor play important role in the photovoltaic properties of all-PSCs. So photovoltaic performance of the all-PSCs was optimized by using three different weight ratios of 2:1, 1.5:1 and 1:1 for PTB7Th:P(PDI-BDT-Th) and 1.5:1, 1:1 and 1:1.5 for PTB7-Th:P(PDI-
BDT-Ph) blends. In addition, solvent additive of 1chloronaphthalene (CN) was also used to optimize the photovoltaic performance [49]. Table S1 in SI list the photovoltaic parameters of the all-PSCs under different fabrication conditions. Obviously, the optimal D/A weight ratio and additive treatment condition were found to be 1.5:1 (w/w) with 1 vol % CN additive for PTB7-Th:P(PDI-BDT-Th) and 1:1 (w/w) with 2 vol % CN for PTB7Th:P(PDI-BDT-Ph). Fig. 3a and Fig. S1a in SI show the current density-voltage (JeV) curves of the all-PSCs with the optimized donor/acceptor weight ratios, and Table 2 lists the corresponding photovoltaic parameters. Under the optimized condition, the maximum PCE of the all-PSCs based on PTB7-Th:P(PDI-BDT-Ph) reached 4.31% with a Jsc of 11.94 mA cm2, a Voc of 0.81 V and FF of 44.49%, while the best PCE of the all-PSCs based on PTB7-Th:P(PDIBDT-Th) was 3.58% with a Jsc of 11.36 mA cm2, a Voc of 0.79 V and FF of 40.00%. Nevertheless, the PCE of the device based on PTB7Th:P(PDI-BDT-Th) or PTB7-Th:P(PDI-BDT-Ph) is lower than that of the anteriorly reported device based on PTB7-Th:P(PDI-BDT-T) as active layer (PCE ¼ 4.71%), Similar Voc values of ca. 0.80 V for the two devices are reasonable because of the identical LUMO energy level of the two polymer acceptors. The higher photovoltaic
Y. Zhang et al. / Organic Electronics 41 (2017) 49e55
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Fig. 3. (a) J-V curves and (b) EQE curves of the optimized all-PSCs based on PTB7-Th:P(PDI-BDT-Ph) (1:1, w/w) without and with CN additive treatment.
Table 2 Photovoltaic performance data of the all-PSCs based on PTB7-Th:P(PDI-BDT-Th) and PTB7-Th:P(PDI-BDT-Ph) with optimal D/A weight ratios, without and with solvent additive treatment of various CN content under the illumination of AM 1.5G, 100 mW cm2. D/A (w/w)
Additives
PTB7-Th:P(PDI-BDT-Th) 1.5:1 w/o 1.5:1 1% CN PTB7-Th:P(PDI-BDT-Ph) 1:1 w/o 1:1 2% CN
Voc (V)
Jsc (mA cm2)
FF (%)
PCEmax./PCEaave(%)
Thickness (nm)
0.76 ± 0.01 0.79 ± 0.00
5.07 ± 0.09 11.28 ± 0.31
49.01 ± 0.15 39.97 ± 0.05
1.91/1.85 3.58/3.49
87 80
0.80 ± 0.00 0.81 ± 0.00
6.75 ± 0.09 11.83 ± 0.11
45.33 ± 0.14 44.45 ± 0.08
2.45/2.41 4.31/4.25
103 90
performance of P(PDI-BDT-Ph) than P(PDI-BDT-Th) could be ascribed to the enhanced charge mobilities of P(PDI-BDT-Ph) which will be discussed below. The external quantum efficiency (EQE) plots of the all-PSCs based on PTB7-Th:P(PDI-BDT-Th) and PTB7-Th:P(PDI-BDT-Ph) are shown in Fig. 3b and Fig. S1b (see SI). The EQE plots exhibit a broad response ranges from 300 nm to ca. 800 nm in the entire visible light region, in which the short wavelength sunlight in the range of 300e550 nm is mainly attributable to the contribution of polymer acceptors and the photoresponse in the long-wavelength range of 600e800 nm is mainly corresponding to the absorption of the polymer donor. For the all-PSCs based on PTB7-Th:P(PDIBDT-Th), with 1 vol% CN additive, the maximum EQE value at ca. 700 nm is increased significantly from 26% for the device without additive treatment to 52% for the device with 1 vol% CN additive treatment. While for the all-PSCs based on PTB7-Th:P(PDI-BDTPh), the maximum EQE value at ca. 700 is enhanced from 36% for the device without additive to 56% with 2 vol% CN solvent additive. These results indicate that the EQE values agree well with the Jsc values obtained from J-V measurements. 3.5. Charge carrier mobilities The influence of the internal electron and hole transport on the photovoltaic performance of the all-PSCs is evaluated by measuring charge carrier mobilities with the space-charge-limited current (SCLC) method based on the Mott-Gurney equation [50]. The electron mobilities were measured by electron-only device of ITO/ ZnO/sample film/Ca/Al and the hole mobilities were measured by hole-only devices of ITO/PEDOT:PSS/sample film/MoO3/Al. Table 3 summarized the hole and electron mobilities of pure P(PDI-BDTTh) film, pure P(PDI-BDT-Ph) film, PTB7-Th:P(PDI-BDT-Th) and PTB7-Th:P(PDI-BDT-Ph) blend films without and with the solvent additive treatments. Fig. 4 and Fig. S2 in SI show the currentvoltage plots for the mobility measurements of the two blend films. The electron mobilities of pure P(PDI-BDT-Th) and P(PDIBDT-Ph) are found to be 4.27 105 cm2 V1 s1 and
Table 3 Hole and electron mobility values of the pristine P(PDI-BDT-Ph) and PTB7Th:P(PDI-BDT-Ph) blends without and with CN additive treatment by the SCLC method. Materials P(PDI-BDT-Ph) P(PDI-BDT-Th) PTB7-Th:P(PDI-BDT-Ph) PTB7-Th:P(PDI-BDT-Th)
Additive e e w/o 2% CN w/o 1% CN
me (cm2/V s) 1.00 4.27 1.16 2.77 8.02 9.62
4
10 105 104 104 105 105
mh (cm2/V s) e e 6.74 2.89 1.57 8.03
103 103 103 103
1.0 104 cm2 V1 s1, respectively. The higher electron mobility of P(PDI-BDT-Ph) could be attributed to the more extended pconjugation of BDT-Ph unit with phenylene conjugated side chain in comparision with the BDT-Th unit [26,51]. However, the electron mobilities of the two new polymer acceptors are lower than that of the previously reported polymer acceptor P(PDI-BDT-T) (3.11 103 cm2 V1 s1) [33], due probably to the bigger alkoxy substituents of P(PDI-BDT-Th) and P(PDI-BDT-Ph) which weakens their interchain interaction. The electron and hole mobilities of the PTB7-Th:P(PDI-BDT-Th) blend film without additive treatment were 8.02 105 and 1.57 103 cm2 V1 s1 respectively, which were increased to 9.62 105 and 8.03 103 cm2 V1 s1 respectively for the blend film with 1 vol% CN additive treatment. While for the PTB7-Th:P(PDI-BDT-Ph) blend film, the electron mobility was increased from 1.16 104 cm2 V1 s1 for the film without additive treatment to 2.77 104 cm2 V1 s1 for the film with 2 vol% CN additive treatment; but the hole mobility was decreased from 6.74 103 cm2 V1 s1 for the film without additive treatment to 2.89 103 cm2V1 s1 for the film with 2 vol% CN additive treatment. Obviously, the higher electron mobility of P(PDI-BDT-Ph) film and the more balanced electron and hole transport in the blend films of PTB7-Th:P(PDI-BDT-Ph) with the CN additive treatment should be responsible for the good photovoltaic performance of the corresponding all-PSCs.
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Fig. 4. Current-voltage plots for the measurements of (a) electron mobilities and (b) hole mobilities of the devices based on PTB7-Th:P(PDI-BDT-Ph) (1:1, w/w) without and with CN additive treatments.
3.6. Morphologies It is well-known that morphology control is critical to the photovoltaic performance of the all-PSCs. In order to investigate the morphology changes of PTB7-Th:P(PDI-BDT-Th) and PTB7Th:P(PDI-BDT-Ph) blends without and with the solvent additive treatments, atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements were carried out, and the morphology images are shown in Fig. 5aeh. The surface roughnesses of the PTB7-Th:P(PDI-BDT-Th) blend film without and with 1 vol% CN additive were 0.94 and 1.11 nm, respectively. While the surface roughnesses of the PTB7-Th:P(PDI-BDT-Ph) blend film without and with 2 vol% CN additive treatment were found to be 1.32 and 1.37 nm, respectively. Obviously, with the additive treatment, the surface roughnesses of both all-polymer blend films were slightly increased. The slightly larger roughness of the P(PDI-BDT-
Ph)-based blend film with solvent additive treatment could be caused by the aggregation of the acceptor, which benefits the higher electron mobility of the blend as mentioned above. In comparison of the TEM images of the polymer blend films with additive treatment (Fig. 5j, l) with that of the blend films without additive treatment (Fig. 5i,k), it can be seen that the morphology and the donor/acceptor interpenetrating network were improved obviously for the films with additive treatment. Especially, With the 2 vol% CN additive treatment, PTB7-Th:P(PDIBDT-Ph) blend film showed a uniform nanoscale fiber morphology with size of ca. 10 nm within the range of exciton diffusion length (Fig. 5l). The good morphology of the PTB7-Th:P(PDI-BDT-Ph) blend film with 2 vol% CN solvent additive treatment is beneficial to exciton dissociation and charge transport, thus leading to an improvement of the PCE with a high Jsc [51e53].
Fig. 5. AFM and TEM Morphology images of PTB7-Th:P(PDI-BDT-Th) and PTB7-Th:P(PDI-BDT-Ph) blends, AFM height images: PTB7-Th:P(PDI-BDT-Th) without (a) and with 1% CN additive (b), PTB7-Th:P(PDI-BDT-Ph) without (c) and with 2% CN additive (d); AFM phase images: PTB7-Th:P(PDI-BDT-Th) without (e) and with 1% CN additive (f), PTB7-Th:P(PDIBDT-Ph) without (g) and with 2% CN additive (h); TEM images: PTB7-Th:P(PDI-BDT-Th) without (i) and with 1% CN additive (j), PTB7-Th:P(PDI-BDT-Ph) without (k) and with 2% CN additive (l), respectively.
Y. Zhang et al. / Organic Electronics 41 (2017) 49e55
4. Conclusions Two PDI-BDT-based n-type D-A copolymers P(PDI-BDT-Th) and P(PDI-BDT-Ph) were synthesized and used as acceptor in all-PSCs, for investigating the effect of aromatic conjugated side chains (alkoxy-phenyl groups or alkoxy-thenyl groups) on BDT unit on the photovoltaic performance of the polymer acceptors. The two n-type polymers show good thermal stability with the decomposition temperatures (Td) at 5% weight loss of 346 C for P(PDI-BDT-Th) and 466 C for P(PDI-BDT-Ph), and identical LUMO level of 3.89 eV. The all-PSCs based on P(PDI-BDT-Ph) as acceptor and PTB7-Th as donor with 2 vol% CN additive treatment exhibited a optimum PCE of 4.31%, which is higher than that (3.58%) of the optimized P(PDI-BDT-Th)-based devices. The higher electron mobility of P(PDI-BDT-Ph) polymer acceptor and the good nanofiber morphologies of the active layer blend film partially explain the better photovoltaic performance of the P(PDI-BDT-Ph)-based all-PSCs. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 91333204, 51203168, 51422306, 51503135 and 51573120), the Priority Academic Program Development of Jiangsu Higher Education Institutions, Jiangsu Provincial Natural Science Foundation (Grant No. BK20150332), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 15KJB430027) and the Ministry of Science and Technology of China (973 project, No. 2014CB643501), China Postdoctoral Science Foundation Funded Project (Project No. 2016M591902). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2016.11.038. References [1] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498. [2] M.M. Alam, S.A. Jenekhe, Chem. Mater. 16 (2004) 4647e4656. [3] E. Zhou, J. Cong, Q. Wei, K. Tajima, C. Yang, K. Hashimoto, Angew. Chem. Int. Ed. 50 (2011) 2799e2803. [4] C.R. McNeill, Energy. Environ. Sci. 5 (2012) 5653e5667. [5] T. Earmme, Y.-J. Hwang, N.M. Murari, S. Subramaniyan, S.A. Jenekhe, J. Am. Chem. Soc. 135 (2013) 14960e14963. [6] T. Earmme, Y.J. Hwang, S. Subramaniyan, S.A. Jenekhe, Adv. Mater. 26 (2014) 6080e6085. [7] H. Li, T. Earmme, G. Ren, A. Saeki, S. Yoshikawa, N.M. Murari, S. Subramaniyan, M.J. Crane, S. Seki, S.A. Jenekhe, J. Am. Chem. Soc. 136 (2014) 14589e14597. [8] X. Guo, A. Facchetti, T.J. Marks, Chem. Rev. 114 (2014) 8943e9021. [9] C. Mu, P. Liu, W. Ma, K. Jiang, J. Zhao, K. Zhang, Z. Chen, Z. Wei, Y. Yi, J. Wang, Adv. Mater. 26 (2014) 7224e7230. [10] D. Mori, H. Benten, I. Okada, H. Ohkita, S. Ito, Adv. Energy Mater. 4 (2014) 1301006. [11] J.W. Jung, J.W. Jo, C.C. Chueh, F. Liu, W.H. Jo, T.P. Russell, A.K.Y. Jen, Adv. Mater. 27 (2015) 3310e3317. [12] K.D. Deshmukh, T. Qin, J.K. Gallaher, A.C.Y. Liu, E. Gann, K. O'Donnell, L. Thomsen, J.M. Hodgkiss, S.E. Watkins, C.R. McNeill, Energy. Environ. Sci. 8 (2015) 332e342.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Perylene diimide-benzodithiophene D-A copolymers as acceptor in all-polymer solar cells Youdi Zhang a, Xia Guo a, **, Wenyan Su a, Bing Guo a, Zhuo Xu a, Maojie Zhang a, ***, Yongfang Li a, b, * a
State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2016 Received in revised form 24 November 2016 Accepted 26 November 2016
Two n-type conjugated D-A copolymers with perylene diimide (PDI) as acceptor unit and benzodithiophene (BDT) as donor unit, P(PDI-BDT-Ph) and P(PDI-BDT-Th), were synthesized and applied as electron acceptor in all-polymer solar cells (all-PSCs). P(PDI-BDT-Ph) and P(PDI-BDT-Th) films exhibit similar absorption spectra in the visible region with optical bandgap (Eg) of 1.65 eV and 1.55 eV respectively, and the identical LUMO level of 3.89 eV. The all-PSCs based on P(PDI-BDT-Ph) as acceptor and PTB7-Th as donor demonstrated a power conversion efficiency (PCE) of 4.31% with a short-circuit current density (Jsc) of 11.94 mA cm2, an open-circuit voltage (Voc) of 0.81 V, and a fill factor (FF) of 44.49%. By contrast, the corresponding all-PSCs with P(PDI-BDT-Th) as acceptor showed a relative lower PCE of 3.58% with a Jsc of 11.36 mA cm2, Voc of 0.79 V, and FF of 40.00%. © 2016 Elsevier B.V. All rights reserved.
Keywords: Perylene diimide n-type conjugated polymers Polymer acceptors All-polymer solar cells
1. Introduction All-polymer solar cells (all-PSCs) with n-type conjugated polymer as acceptor have developed rapidly in recent years due to the potential advantages of easy-tuning absorption spectra and molecular energy levels as well as better flexibility of the polymer acceptor over the traditional fullerene derivative acceptor PCBM [1e16]. However, the power conversion efficiencies (PCEs) of the all-PSCs are still lag behind those of the polymer:fullerene PSCs. Thus, it is crucial to design and synthesize more suitable n-type conjugated polymer acceptors for improving the photovoltaic performance of the all-PSCs [17e19]. Perylene diimides (PDIs)-based D-A copolymers possess higher electron mobility and suitable LUMO energy levels for the application as acceptor in the all-PSCs. In 2007, Zhan et al. synthesized a
* Corresponding author. Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Guo), [email protected] (M. Zhang), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.orgel.2016.11.038 1566-1199/© 2016 Elsevier B.V. All rights reserved.
PDI-based D-A copolymer with PDI as acceptor unit and dithieno [3,2-b:2,3-d]thiophene as donor unit, and the all-PSCs with the polymer as acceptor and a two-dimension (2D)-conjugated polythiophene derivative as donor displayed a PCE of ca. 1% [20]. Then the PCE was further improved to ca. 1.5% by the molecular structural modification of the polymer donor and polymer acceptor [21]. Subsequently, a series of n-type PDI-based conjugated copolymers were designed and synthesized by Zhou et al. and the PCE of the allPSCs with the PDI-based polymer as acceptor reached over 2% by using a 2D-conjugated polythiophene derivative as donor [3]. Bao et al. achieved a higher PCE of 4.4% in 2014 for the all-PSCs with the PDI-based polymer as acceptor and the isoindigo-containing polymer as donor [22]. In the past few years, 2D-conjugated polymers based on benzodithiophene (BDT) unit with conjugated side chains play an important role in the application as polymer donor in high performance PSCs [23e32]. Recently, we applied the 2D-conjugation concept to n-type conjugated polymers and synthesized a 2Dconjugated D-A copolymer P(PDI-BDT-T) with alkylthienyl substituted benzodithiophene (BDT) as donor unit and PDI as acceptor unit, and the all-PSCs with P(PDI-BDT-T) as polymer acceptor demonstrated a PCE of 4.71% [33]. In order to extend the family of the n-type 2D-conjugated polymers, herein we
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synthesized two new n-type 2D-conjugated D-A alternative copolymers P(PDI-BDT-Th) and P(PDI-BDT-Ph) with alkoxy-thienyl or alkoxy-phenyl substituted BDT as donor unit and PDI as acceptor unit (Scheme 1), and compared their physicochemical and photovoltaic properties in the all-PSCs with the polymers as acceptor and a narrow bandgap polymer PTB7-Th as donor. 2. Experimental section 2.1. General procedure for the polymerization Monomers 2Br-PDI [34e38], BDT-Th-Sn [39,40] and BDT-Ph-Sn [41,42] were prepared by using experimental procedures reported in literatures. 2Br-PDI (0.30 mmol), BDT-Th-Sn or BDT-Ph-Sn (0.30 mmol) were dissolved into 10 mL of toluene in a flask under nitrogen. The flask equipped with a return pipe was then degassed and filled with argon for three times. Pd(PPh3)4 (18 mg) was added into the flask. Then, the reaction mixture was treated for 18e24 h at 110 C. After the reaction mixture was cooled to room temperature, the polymer was precipitated in 150 mL of methanol. Subsequently, the crude product was collected by filtration, dried in vacuum drying oven, and then purified by Soxhlet extraction with methanol, acetone, and hexane. P(PDI-BDT-Th) (364 mg, 77%): Mn ¼ 25.1 KDa, PDI ¼ 1.95, Anal. Calcd for C96H126N2O6S4 (%): C 75.25, H 8.29, N 1.83; found (%): C 74.92, H 8.07, N 1.64; 1H NMR (400 MHz, CDCl3) d 8.82 (d, 1H), 8.55e8.26 (m, 2H), 8.18e7.87 (m, 1H), 7.20e6.97 (m, 1H), 6.35e6.10 (m, 1H), 4.39e3.76 (m, 4H), 1.95 (d, 1H), 1.70 (d, 1H), 1.27 (m, 39H), 1.00e0.72 (m, 13H). P(PDI-BDT-Ph) (374 mg, 80%): Mn ¼ 27.8 KDa, PDI ¼ 1.67, Anal. Calcd for C102H134N2O6S2 (%): C 79.13, H 8.72, N 1.81; found (%): C 79.25, H 8.57, N 1.75; 1H NMR (400 MHz, CDCl3) d 8.76 (d, 1H), 8.52e8.19 (m, 2H), 7.64 (s, 3H), 7.00 (s, 3H), 3.99 (d, 4H), 2.00 (s, 1H), 1.68 (d, 1H), 1.28 (m, 40H), 0.91e0.76 (m, 12H). 2.2. Instruments and measurements UVevis absorption spectra were taken on an Agilent Technologies Cary Series UVeViseNIR Spectrophotometer in chloroform solution and in thin film. The electrochemical cyclic voltammetry (CV) was performed on a Zahner Ennium Electrochemical
Workstation with glassy carbon disk, Pt wire, and Ag/Agþ electrode as working electrode, counter electrode, and reference electrode respectively, in a 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile electrolyte solution. Atomic force microscopy (AFM) measurements were carried out on a Dimension 3100 (Veeco) Atomic Force Microscope in the tapping mode. Transmission electron microscopy (TEM) was performed using a JEOL 2200FS instrument at 160 kV accelerating voltage. X-ray diffraction (XRD) measurement was performed using a Bruker D8 Advance Instrument at 40 kV voltage and 200 mA current with Cu Ka radiation. 2.3. Charge carrier mobility measurement The charge carrier mobility was measured by the space charge limited current (SCLC) method with the hole-only device of ITO/ PEDOT:PSS/Active layer/MoO3/Al for the hole mobility measurement or with the electron-only device of ITO/ZnO/Active layer/Ca/ Al for the electron mobility measurement. After measurements of the J-V curves of the devices, the hole mobilities and the electron mobilities were calculated according to the following equation,
8 V2 J ¼ εr ε0 m 3 9 L where εr is the dielectric constant of the polymer, ε0 is the permittivity of the vacuum, m0 is the zero-field mobility, J is the current density, L is the thickness of the blend films, V ¼ Vappl e Vbi, Vappl is the applied potential, and Vbi is the built-in potential from the difference in the work function of the anode and the cathode (in the hole-only and electron-only device,Vbi value is 0.2 V and 0 V, respectively). 2.4. Device fabrication and characterization The ITO-coated glass substrate was cleaned in an ultrasonic bath with deionized water, acetone, and isopropanol, each process was approximately 15 min, and then dried under a stream of dry nitrogen. Subsequently, the ITO-coated glass substrate was treated by UV-ozone for 15 min. PEDOT:PSS (Clevios P VP AI 4083, H.C. Starck) solution was then spin-coated onto the pre-cleaned ITO coated glass substrates, and thermal-treated at 150 C for 15 min to get the
Scheme 1. Molecular structures of polymer donor PTBT-Th, and polymer acceptors P(PDI-BDT-Th) and P(PDI-BDT-Ph).
Y. Zhang et al. / Organic Electronics 41 (2017) 49e55
PEDOT:PSS film with thickness of ~30 nm. The blend solutions of PTB7-Th and P(PDI-BDT-Th) or P(PDIBDT-Ph) were prepared by simultaneously dissolving PTB7-Th and P(PDI-BDT-Th) or P(PDI-BDT-Ph) with different D/A weight ratio, in dichlorobenzene without or with solvent additives (1chloronaphthalene), and spin-coated on the ITO/PEDOT: PSS electrode at 3000 rpm for 60 s, resulting in the active layer with a film thickness of ~90 nm. Then 20 nm thick Ca layer covered with 100 nm Al electrode were thermally deposited under a pressure of 2 104 Pa, through a shadow mask on top of the active layer. The active area of the pixels, as defined by the overlap of anode and cathode area, was 0.038 cm2. The thickness was measured by Ambios Technology D-100 surface profilometer. The current density-voltage (J-V) curves were measured in N2 atmosphere using AM1.5G solar simulator with an irradiation light intensity of 100 mWcm2. The external quantum efficiency (EQE) of the devices was measured by Solar Cell Spectral Response Measurement System QE-R3011 (Enli Technology CO., Ltd.). The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. 3. Results and discussion 3.1. Synthesis As shown in Scheme 2, P(PDI-BDT-Th) and P(PDI-BDT-Ph) were synthesized by the same method as our previously reported [33], with a yield of 77% and 80%, respectively. The polymers could well dissolve in common organic solvents, e.g. chloroform, toluene, chlorobenzene and o-dichlorobenzene (o-DCB). The values of the number average molecular weight (Mn) for P(PDI-BDT-Th) and P(PDI-BDT-Ph) are 25.1 KDa and 27.8 KDa, with their polydispersity indices of 1.95 and 1.67, respectively, which were estimated by high temperature gel permeation chromatography (GPC) at 160 C using 1,2,4-trichlorobenzene as the solvent and polystyrene with narrow molecular weight distribution as a standard. 3.2. Thermal properties Thermogravimetric analysis (TGA) measurements were carried out to evaluate the thermal stability of the polymers P(PDI-BDTTh) and P(PDI-BDT-Ph), and the TGA plots of the two polymers are shown in Fig. 1a. P(PDI-BDT-Th) shows an excellent thermal stability with high decomposition temperature (Td) at 5% weight loss of ca. 346 C, which is similar with that of P(PDI-BDT-T) (360 C) [33]. Importantly, Td of P(PDI-BDT-Ph) is at ca. 466 C, indicating that attaching phenyl to the side chains is beneficial for improving the thermal stability in comparison with thienyl side chain.
51
From the DSC thermograms of polymers P(PDI-BDT-Th) and P(PDI-BDT-Ph) at a heating rate of 10 C min1 (Fig. 1b), no obvious exotherm or endotherm peaks are observed from 50 to 250 C, which reveals that the polymers are amorphous materials. 3.3. Optical and electrochemical properties Fig. 2a shows the absorption spectra of P(PDI-BDT-Th) and P(PDI-BDT-Ph) in chloroform solution and thin films on quartz substrates. And the corresponding optical data, including the absorption peaks (labs), absorption edge wavelength (lonset) and their optical band gaps are listed in Table 1. The absorption spectra of the two polymers in thin films are identical in shape to their solution spectra, and show broad absorption covering the entire visible light range from 300 nm to 800 nm. P(PDI-BDT-Th) and P(PDI-BDT-Ph) display three main absorption peaks, including a strong p-p* transition peak of donor unit and a p-p* transition peak of acceptor unit in the wavelength range of 300e420 nm [43], a PDI segment absorption peak at 450e600 nm [44] and an intramolecular charge-transfer transition peak which is the broad tail/shoulder extending in the wavelength range of 600e800 nm. And absorption spectra of P(PDI-BDT-Th) and P(PDI-BDT-Ph) is similar in shape to that of P(PDI-BDT-T) at 300e800 nm. The absorption edges of the two polymer films are at 800 nm for P(PDI-BDT-Th) and 752 nm for P(PDI-BDT-Ph) corresponding to the optical band gaps of 1.55 and 1.65 eV, respectively. The electronic energy levels of the conjugated polymers P(PDIBDT-Th) and P(PDI-BDT-Ph) thin films on working electrode were estimated by electrochemical cyclicvoltammetry (CV) [45,46]. Fig. 2b displays cyclic voltammograms of the polymer films, from which the onset reduction and onset oxidation potentials (Ered and Eox) can be observed to be 0.82/0.79 V vs. Ag/Agþ for P(PDI-BDTTh) and 0.82/0.96 V vs. Ag/Agþ for P(PDI-BDT-Ph), respectively. Thus, the LUMO and HOMO energy levels of the two polymers can be estimated from the Ered and Eox values according to the equations: [47] HOMO ¼ -e (Eox þ 4.71) (eV); LUMO ¼ -e (Ered þ 4.71) (eV). The estimated LUMO and HOMO levels are 3.89/5.50 eV for P(PDI-BDT-Th) and 3.89/5.67 eV for P(PDI-BDT-Ph), as shown in Table 1. The LUMO level (3.89 eV) of the two n-type conjugated polymers is identical to that of the previously reported electronic acceptor P(PDI-BDT-T) and close to that of PCBM, indicating that the two polymers could be used as acceptor in PSCs instead of PCBM from the viewpoint of electronic energy levels. The HOMO level (5.67 eV) of P(PDI-BDT-Ph) is ca. 0.17 eV lower than that (5.50 eV) of P(PDI-BDT-Th), which could be due to the weaker electron-donating ability of alkoxy-phenyl group in P(PDI-BDT-Ph) than the alkoxy-thienyl group in P(PDI-BDT-Th) [48]. It should be mentioned that the HOMO energy level (5.50 eV) of P(PDI-BDT-
Scheme 2. Synthesis procedures of the n-type conjugated polymers P(PDI-BDT-Th) and P(PDI-BDT-Ph).
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Y. Zhang et al. / Organic Electronics 41 (2017) 49e55
Fig. 1. (a) TGA and (b) DSC plots of P(PDI-BDT-Th) (curve with square) and P(PDI-BDT-Ph) (curve with circle) with a heating rate of 10 C min1 under the inert atmosphere.
Fig. 2. (a) UVevis absorption spectra of P(PDI-BDT-Th) and P(PDI-BDT-Ph) in chloroform solution and in solid films; (b) Cyclic voltammograms of P(PDI-BDT-Th) and P(PDI-BDTPh) films on a platinum electrode in 0.1 mol L1 Bu4NPF6 acetonitrile solutions at a scan rate of 50 mV s1.
Table 1 Optical and electrochemical properties of polymers P(PDI-BDT-Th) and P(PDI-BDT-Ph). Polymers
P(PDI-BDT-Th) P(PDI-BDT-Ph) a b
labs (nm) Solution
Film
330/387/554 309/374/546/665
330/396/554 309/377/546/674
lonseta (nm)
Egb (eV)
LUMO (eV)
HOMO (eV)
800 752
1.55 1.65
3.89 3.89
5.50 5.67
Measured in thin films. Optical band gap.
Th) is ca. 0.20 eV up-shifted than that (5.70 eV) of P(PDI-BDT-T) [33] due to the stronger electron-donating ability of alkoxy substituent on P(PDI-BDT-Th) than alkyl substituent on P(PDI-BDT-T). In addition, in comparison with the LUMO (3.61 eV) and HOMO (5.25 eV) of PTB7-Th [33], the two n-type polymers of P(PDI-BDTTh) and P(PDI-BDT-Ph) are suitable polymer acceptors with PTB7Th as polymer donor in the energy level point of view. 3.4. Photovoltaic performance of All-PSCs To evaluate the photovoltaic properties of the two n-type copolymers, all-PSCs were fabricated with PTB7-Th as donor and P(PDI-BDT-Th) or P(PDI-BDT-Ph) as acceptor in the conventional device structure of ITO/PEDOT:PSS/PTB7-Th:P(PDI-BDT-Th) or P(PDI-BDT-Ph)/Ca/Al, where ITO is indium tin oxide, PEDOT is poly(3,4-ethylenedioxythiophene), and PSS is poly(styrene sulfonate). As we all know, the weight ratios of donor and acceptor play important role in the photovoltaic properties of all-PSCs. So photovoltaic performance of the all-PSCs was optimized by using three different weight ratios of 2:1, 1.5:1 and 1:1 for PTB7Th:P(PDI-BDT-Th) and 1.5:1, 1:1 and 1:1.5 for PTB7-Th:P(PDI-
BDT-Ph) blends. In addition, solvent additive of 1chloronaphthalene (CN) was also used to optimize the photovoltaic performance [49]. Table S1 in SI list the photovoltaic parameters of the all-PSCs under different fabrication conditions. Obviously, the optimal D/A weight ratio and additive treatment condition were found to be 1.5:1 (w/w) with 1 vol % CN additive for PTB7-Th:P(PDI-BDT-Th) and 1:1 (w/w) with 2 vol % CN for PTB7Th:P(PDI-BDT-Ph). Fig. 3a and Fig. S1a in SI show the current density-voltage (JeV) curves of the all-PSCs with the optimized donor/acceptor weight ratios, and Table 2 lists the corresponding photovoltaic parameters. Under the optimized condition, the maximum PCE of the all-PSCs based on PTB7-Th:P(PDI-BDT-Ph) reached 4.31% with a Jsc of 11.94 mA cm2, a Voc of 0.81 V and FF of 44.49%, while the best PCE of the all-PSCs based on PTB7-Th:P(PDIBDT-Th) was 3.58% with a Jsc of 11.36 mA cm2, a Voc of 0.79 V and FF of 40.00%. Nevertheless, the PCE of the device based on PTB7Th:P(PDI-BDT-Th) or PTB7-Th:P(PDI-BDT-Ph) is lower than that of the anteriorly reported device based on PTB7-Th:P(PDI-BDT-T) as active layer (PCE ¼ 4.71%), Similar Voc values of ca. 0.80 V for the two devices are reasonable because of the identical LUMO energy level of the two polymer acceptors. The higher photovoltaic
Y. Zhang et al. / Organic Electronics 41 (2017) 49e55
53
Fig. 3. (a) J-V curves and (b) EQE curves of the optimized all-PSCs based on PTB7-Th:P(PDI-BDT-Ph) (1:1, w/w) without and with CN additive treatment.
Table 2 Photovoltaic performance data of the all-PSCs based on PTB7-Th:P(PDI-BDT-Th) and PTB7-Th:P(PDI-BDT-Ph) with optimal D/A weight ratios, without and with solvent additive treatment of various CN content under the illumination of AM 1.5G, 100 mW cm2. D/A (w/w)
Additives
PTB7-Th:P(PDI-BDT-Th) 1.5:1 w/o 1.5:1 1% CN PTB7-Th:P(PDI-BDT-Ph) 1:1 w/o 1:1 2% CN
Voc (V)
Jsc (mA cm2)
FF (%)
PCEmax./PCEaave(%)
Thickness (nm)
0.76 ± 0.01 0.79 ± 0.00
5.07 ± 0.09 11.28 ± 0.31
49.01 ± 0.15 39.97 ± 0.05
1.91/1.85 3.58/3.49
87 80
0.80 ± 0.00 0.81 ± 0.00
6.75 ± 0.09 11.83 ± 0.11
45.33 ± 0.14 44.45 ± 0.08
2.45/2.41 4.31/4.25
103 90
performance of P(PDI-BDT-Ph) than P(PDI-BDT-Th) could be ascribed to the enhanced charge mobilities of P(PDI-BDT-Ph) which will be discussed below. The external quantum efficiency (EQE) plots of the all-PSCs based on PTB7-Th:P(PDI-BDT-Th) and PTB7-Th:P(PDI-BDT-Ph) are shown in Fig. 3b and Fig. S1b (see SI). The EQE plots exhibit a broad response ranges from 300 nm to ca. 800 nm in the entire visible light region, in which the short wavelength sunlight in the range of 300e550 nm is mainly attributable to the contribution of polymer acceptors and the photoresponse in the long-wavelength range of 600e800 nm is mainly corresponding to the absorption of the polymer donor. For the all-PSCs based on PTB7-Th:P(PDIBDT-Th), with 1 vol% CN additive, the maximum EQE value at ca. 700 nm is increased significantly from 26% for the device without additive treatment to 52% for the device with 1 vol% CN additive treatment. While for the all-PSCs based on PTB7-Th:P(PDI-BDTPh), the maximum EQE value at ca. 700 is enhanced from 36% for the device without additive to 56% with 2 vol% CN solvent additive. These results indicate that the EQE values agree well with the Jsc values obtained from J-V measurements. 3.5. Charge carrier mobilities The influence of the internal electron and hole transport on the photovoltaic performance of the all-PSCs is evaluated by measuring charge carrier mobilities with the space-charge-limited current (SCLC) method based on the Mott-Gurney equation [50]. The electron mobilities were measured by electron-only device of ITO/ ZnO/sample film/Ca/Al and the hole mobilities were measured by hole-only devices of ITO/PEDOT:PSS/sample film/MoO3/Al. Table 3 summarized the hole and electron mobilities of pure P(PDI-BDTTh) film, pure P(PDI-BDT-Ph) film, PTB7-Th:P(PDI-BDT-Th) and PTB7-Th:P(PDI-BDT-Ph) blend films without and with the solvent additive treatments. Fig. 4 and Fig. S2 in SI show the currentvoltage plots for the mobility measurements of the two blend films. The electron mobilities of pure P(PDI-BDT-Th) and P(PDIBDT-Ph) are found to be 4.27 105 cm2 V1 s1 and
Table 3 Hole and electron mobility values of the pristine P(PDI-BDT-Ph) and PTB7Th:P(PDI-BDT-Ph) blends without and with CN additive treatment by the SCLC method. Materials P(PDI-BDT-Ph) P(PDI-BDT-Th) PTB7-Th:P(PDI-BDT-Ph) PTB7-Th:P(PDI-BDT-Th)
Additive e e w/o 2% CN w/o 1% CN
me (cm2/V s) 1.00 4.27 1.16 2.77 8.02 9.62
4
10 105 104 104 105 105
mh (cm2/V s) e e 6.74 2.89 1.57 8.03
103 103 103 103
1.0 104 cm2 V1 s1, respectively. The higher electron mobility of P(PDI-BDT-Ph) could be attributed to the more extended pconjugation of BDT-Ph unit with phenylene conjugated side chain in comparision with the BDT-Th unit [26,51]. However, the electron mobilities of the two new polymer acceptors are lower than that of the previously reported polymer acceptor P(PDI-BDT-T) (3.11 103 cm2 V1 s1) [33], due probably to the bigger alkoxy substituents of P(PDI-BDT-Th) and P(PDI-BDT-Ph) which weakens their interchain interaction. The electron and hole mobilities of the PTB7-Th:P(PDI-BDT-Th) blend film without additive treatment were 8.02 105 and 1.57 103 cm2 V1 s1 respectively, which were increased to 9.62 105 and 8.03 103 cm2 V1 s1 respectively for the blend film with 1 vol% CN additive treatment. While for the PTB7-Th:P(PDI-BDT-Ph) blend film, the electron mobility was increased from 1.16 104 cm2 V1 s1 for the film without additive treatment to 2.77 104 cm2 V1 s1 for the film with 2 vol% CN additive treatment; but the hole mobility was decreased from 6.74 103 cm2 V1 s1 for the film without additive treatment to 2.89 103 cm2V1 s1 for the film with 2 vol% CN additive treatment. Obviously, the higher electron mobility of P(PDI-BDT-Ph) film and the more balanced electron and hole transport in the blend films of PTB7-Th:P(PDI-BDT-Ph) with the CN additive treatment should be responsible for the good photovoltaic performance of the corresponding all-PSCs.
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Y. Zhang et al. / Organic Electronics 41 (2017) 49e55
Fig. 4. Current-voltage plots for the measurements of (a) electron mobilities and (b) hole mobilities of the devices based on PTB7-Th:P(PDI-BDT-Ph) (1:1, w/w) without and with CN additive treatments.
3.6. Morphologies It is well-known that morphology control is critical to the photovoltaic performance of the all-PSCs. In order to investigate the morphology changes of PTB7-Th:P(PDI-BDT-Th) and PTB7Th:P(PDI-BDT-Ph) blends without and with the solvent additive treatments, atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements were carried out, and the morphology images are shown in Fig. 5aeh. The surface roughnesses of the PTB7-Th:P(PDI-BDT-Th) blend film without and with 1 vol% CN additive were 0.94 and 1.11 nm, respectively. While the surface roughnesses of the PTB7-Th:P(PDI-BDT-Ph) blend film without and with 2 vol% CN additive treatment were found to be 1.32 and 1.37 nm, respectively. Obviously, with the additive treatment, the surface roughnesses of both all-polymer blend films were slightly increased. The slightly larger roughness of the P(PDI-BDT-
Ph)-based blend film with solvent additive treatment could be caused by the aggregation of the acceptor, which benefits the higher electron mobility of the blend as mentioned above. In comparison of the TEM images of the polymer blend films with additive treatment (Fig. 5j, l) with that of the blend films without additive treatment (Fig. 5i,k), it can be seen that the morphology and the donor/acceptor interpenetrating network were improved obviously for the films with additive treatment. Especially, With the 2 vol% CN additive treatment, PTB7-Th:P(PDIBDT-Ph) blend film showed a uniform nanoscale fiber morphology with size of ca. 10 nm within the range of exciton diffusion length (Fig. 5l). The good morphology of the PTB7-Th:P(PDI-BDT-Ph) blend film with 2 vol% CN solvent additive treatment is beneficial to exciton dissociation and charge transport, thus leading to an improvement of the PCE with a high Jsc [51e53].
Fig. 5. AFM and TEM Morphology images of PTB7-Th:P(PDI-BDT-Th) and PTB7-Th:P(PDI-BDT-Ph) blends, AFM height images: PTB7-Th:P(PDI-BDT-Th) without (a) and with 1% CN additive (b), PTB7-Th:P(PDI-BDT-Ph) without (c) and with 2% CN additive (d); AFM phase images: PTB7-Th:P(PDI-BDT-Th) without (e) and with 1% CN additive (f), PTB7-Th:P(PDIBDT-Ph) without (g) and with 2% CN additive (h); TEM images: PTB7-Th:P(PDI-BDT-Th) without (i) and with 1% CN additive (j), PTB7-Th:P(PDI-BDT-Ph) without (k) and with 2% CN additive (l), respectively.
Y. Zhang et al. / Organic Electronics 41 (2017) 49e55
4. Conclusions Two PDI-BDT-based n-type D-A copolymers P(PDI-BDT-Th) and P(PDI-BDT-Ph) were synthesized and used as acceptor in all-PSCs, for investigating the effect of aromatic conjugated side chains (alkoxy-phenyl groups or alkoxy-thenyl groups) on BDT unit on the photovoltaic performance of the polymer acceptors. The two n-type polymers show good thermal stability with the decomposition temperatures (Td) at 5% weight loss of 346 C for P(PDI-BDT-Th) and 466 C for P(PDI-BDT-Ph), and identical LUMO level of 3.89 eV. The all-PSCs based on P(PDI-BDT-Ph) as acceptor and PTB7-Th as donor with 2 vol% CN additive treatment exhibited a optimum PCE of 4.31%, which is higher than that (3.58%) of the optimized P(PDI-BDT-Th)-based devices. The higher electron mobility of P(PDI-BDT-Ph) polymer acceptor and the good nanofiber morphologies of the active layer blend film partially explain the better photovoltaic performance of the P(PDI-BDT-Ph)-based all-PSCs. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 91333204, 51203168, 51422306, 51503135 and 51573120), the Priority Academic Program Development of Jiangsu Higher Education Institutions, Jiangsu Provincial Natural Science Foundation (Grant No. BK20150332), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 15KJB430027) and the Ministry of Science and Technology of China (973 project, No. 2014CB643501), China Postdoctoral Science Foundation Funded Project (Project No. 2016M591902). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2016.11.038. References [1] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498. [2] M.M. Alam, S.A. Jenekhe, Chem. Mater. 16 (2004) 4647e4656. [3] E. Zhou, J. Cong, Q. Wei, K. Tajima, C. Yang, K. Hashimoto, Angew. Chem. Int. Ed. 50 (2011) 2799e2803. [4] C.R. McNeill, Energy. Environ. Sci. 5 (2012) 5653e5667. [5] T. Earmme, Y.-J. Hwang, N.M. Murari, S. Subramaniyan, S.A. Jenekhe, J. Am. Chem. Soc. 135 (2013) 14960e14963. [6] T. Earmme, Y.J. Hwang, S. Subramaniyan, S.A. Jenekhe, Adv. Mater. 26 (2014) 6080e6085. [7] H. Li, T. Earmme, G. Ren, A. Saeki, S. Yoshikawa, N.M. Murari, S. Subramaniyan, M.J. Crane, S. Seki, S.A. Jenekhe, J. Am. Chem. Soc. 136 (2014) 14589e14597. [8] X. Guo, A. Facchetti, T.J. Marks, Chem. Rev. 114 (2014) 8943e9021. [9] C. Mu, P. Liu, W. Ma, K. Jiang, J. Zhao, K. Zhang, Z. Chen, Z. Wei, Y. Yi, J. Wang, Adv. Mater. 26 (2014) 7224e7230. [10] D. Mori, H. Benten, I. Okada, H. Ohkita, S. Ito, Adv. Energy Mater. 4 (2014) 1301006. [11] J.W. Jung, J.W. Jo, C.C. Chueh, F. Liu, W.H. Jo, T.P. Russell, A.K.Y. Jen, Adv. Mater. 27 (2015) 3310e3317. [12] K.D. Deshmukh, T. Qin, J.K. Gallaher, A.C.Y. Liu, E. Gann, K. O'Donnell, L. Thomsen, J.M. Hodgkiss, S.E. Watkins, C.R. McNeill, Energy. Environ. Sci. 8 (2015) 332e342.
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