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Perylene diimide–thienylenevinylene-based small molecule and polymer acceptors for solution-processed fullerene-free organic solar cells
Dyes and Pigments 114 (2015) 283e289
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Perylene diimideethienylenevinylene-based small molecule and polymer acceptors for solution-processed fullerene-free organic solar cells Shuixing Dai a, b, Yuze Lin c, d, Pei Cheng c, d, Yifan Wang c, d, Xingang Zhao c, Qidan Ling a, *, Xiaowei Zhan b, * a
Fujian Key Laboratory of Polymer Materials, College of Materials Science and Engineering, Fujian Normal University, Fuzhou 350007, China Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China Beijing National Laboratory for Molecular Sciences and Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China d University of Chinese Academy of Sciences, Beijing 100049, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2014 Received in revised form 25 November 2014 Accepted 28 November 2014 Available online 13 December 2014
A small molecule and a polymer based on perylene diimide and thienylenevinylene were designed and synthesized. Both small molecule and polymer exhibited excellent thermal stability with decomposition temperatures of >400 C and strong absorption in the visible region (300e800 nm). These two compounds showed highest occupied molecular orbital levels of 5.57 and 5.70 eV and lowest unoccupied molecular orbital levels of 3.72 and 3.67 eV, respectively. Solution processed fullerene-free polymer solar cells based on the small molecule acceptor and the polymer acceptor afforded power conversion efficiencies of up to 0.69% and 1.00%, respectively. Comparative studies of the absorption, energy levels, charge transport, morphology and photovoltaic properties of the small molecule and polymer were carried out. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Perylene diimide Thienylenevinylene Organic solar cell Solution process Nonfullerene acceptor Fullerene-free
1. Introduction Organic solar cells (OSCs) based on solution-processed bulk heterojunction (BHJ) devices have received intense attention in recent years because they present some advantages, such as large scale production using roll-to-roll process, flexibility, light weight and low cost [1e7]. To date, the best power conversion efficiencies (PCEs) reported in literature exceed 10%; the quick development of new donor materials plays an important role in the excellent performance of OSCs [8e10]. In a sharp contrast, fullerene and its derivatives are still the commonly used acceptors till now, including [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), PC71BM, indene C60 bis-adduct and indene C70 bis-adduct (ICBA) [11,12]. However, fullerenes still have some flaws, such as weak absorption in the visible region, difficult purification and limited energy level variation. Currently, non-
* Corresponding authors. E-mail addresses: [email protected] (Q. Ling), [email protected] (X. Zhan). http://dx.doi.org/10.1016/j.dyepig.2014.11.022 0143-7208/© 2014 Elsevier Ltd. All rights reserved.
fullerene acceptors have attracted increasing attention due to easy modification and purification, tunable energy level and broad absorption [13e16]. Perylene diimide (PDI) as a typical n-type organic semiconductor has good light-harvesting property, thermal and chemical stability, and strong electron-accepting ability. However, the PDI cores tend to aggregate and form large crystal domains in blended films because they have good planarity and strong intermolecular interaction. To solve this problem, some strategies have been used to modify PDI cores by side chain substitution on imide nitrogen atoms and/or substitution on bay-regions of PDIs [17e21]. In particular, PDI dimers, connecting two PDI units with a single bond or aromatic moiety at bay-positions or NeN bond at imide positions [22e27], and star-shaped PDIs [28] form twisted structures and suppress the self-aggregation and over-crystallization. Since the first PDI-based polymer acceptor for all-polymer solar cells was reported by Zhan et al., in 2007 [29], intensive effort has been devoted to searching for more suitable PDI polymer acceptors to improve PCEs [30e32]. Recently, PCEs of up to 4.4% were achieved for PDI polymer acceptors based OSCs [32].
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Fig. 1. Molecule structure of compounds 3 and 5.
In this contribution, we report the synthesis of a small molecule (compound 3) and a polymer (compound 5) based on perylene diimide and thienylenevinylene (Fig. 1). Both small molecule and polymer exhibited broad absorption (300e800 nm) and suitable energy levels matched with those of the low-bandgap polymer donor PBDTTT-C-T. We used compound 3 and compound 5 as an acceptor and PBDTTT-C-T as a donor to fabricate fullerene-free OSCs. We carried out comparative studies of the absorption, energy levels, charge transport, morphology and photovoltaic properties of the small molecule and polymer. 2. Experimental section 2.1. Measurements and characterization 1
H NMR spectra were recorded using a Bruker AVANCE 300 MHz or 400 MHz spectrometer. 13C NMR spectra were recorded using a Bruker AVANCE 600 MHz spectrometer. Mass spectra were recorded using Bruker Daltonics Biflex III MALDI-TOF Analyzer in the
MALDI mode. Elemental analysis was performed using a Flash EA1112 analyzer. UVevis absorption (solution in chloroform, thin film using quartz substrate) was measured using a Jasco V-570 spectrophotometer. Electrochemical measurements were conducted under nitrogen in a solution of tetra-n-butylammonium hexafluorophosphate (0.1 mol L1) in CH3CN employing a computer-controlled CHI660C electrochemical workstation, glassy carbon working electrode coated with compound 3 or compound 5 films, a platinum-wire auxiliary electrode, and an Ag/AgCl reference electrode. The potentials were referenced to a ferrocenium/ ferrocene (FeCpþ/0 2 ) couple using ferrocene as an external standard. Thermogravimetric analysis (TGA) measurements were performed using a Shimadzu thermogravimetric analyzer (Model DTG-60) under flowing nitrogen gas at a heating rate of 10 C min1. The nanoscale morphology of the blended films was observed using an atomic force microscope (AFM) (NanoMan VS, Veeco, USA) in the tapping mode. The gel permeation chromatography (GPC) measurements were performed using a Waters 515 chromatograph connected to a Waters 2414 refractive index detector, using THF as
Scheme 1. Synthetic routes to compounds 3 and 5.
S. Dai et al. / Dyes and Pigments 114 (2015) 283e289
285
Table 1 Thermal, optical and electrochemical data of compounds 3 and 5. Compound
3 5 a b
Fig. 2. TGA curves of compounds 3 and 5.
an eluent and polystyrene standards as calibrants. Three Waters Styragel columns (HT2, 3, 4) connected in series were used. Fourier transform infrared spectroscopy (FT-IR) was performed using a Bruker Tensor 27 with a thin KBr pellet. 2.2. Fabrication and characterization of the photovoltaic cells Photovoltaic cells were fabricated with the structure of ITO/ PEDOT:PSS/PBDTTT-C-T:acceptor/Ca/Al. The patterned indium tin oxide (ITO) glass (sheet resistance ¼ 15 U ,1) was pre-cleaned in an ultrasonic bath with acetone and isopropanol, and treated in an ultraviolet-ozone chamber (Jelight Company, USA) for 30 min. A thin layer (30 nm) of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT: PSS, Baytron PVP AI 4083, Germany) was spin-coated onto the ITO glass, and then baked at 150 C for 15 min. A blend solution of PDI acceptor: PBDTTT-C-T in chloroform (CF) or o-dichlorobenzene (o-DCB) was spin-coated on the PEDOT: PSS layer from. The thickness of the photosensitive layer was measured using AFM. The calcium layer (ca. 20 nm) and the aluminium layer (ca. 80 nm) were subsequently evaporated onto the surface of the photosensitive layer under vacuum (ca. 105 Pa) to form the negative electrode. The active area of the device was 4 mm2. An XES-70S1 (SAN-EI Electric Co., Ltd.) solar simulator (AAA grade, 70 70 mm2 photobeam size) coupled with AM 1.5 G solar
Td ( C)
426 447
opt a
lmax (nm)
Eg
Solution
Film
524 626
530 632
1.75 1.61
(eV)
HOMO (eV)
LUMO (eV)
Egec b (eV)
5.58 5.70
3.72 3.67
1.86 2.03
Estimated from the absorption edge in films. Obtained from electrochemistry.
spectrum filters was used as the light source, and the optical power at the sample was 100 mW cm2. A 2 2 cm2 monocrystalline silicon reference cell (SRC-1000-TC-QZ) was purchased from VLSI Standards Inc. The currentevoltage (IeV) measurement of the devices was conducted using a computer-controlled Agilent B2912A Precision Source/Measure Unit. The incident photon to converted current efficiency (IPCE) spectrum was measured using a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd.). The light intensity at each wavelength was calibrated using a standard single crystal Si photovoltaic cell. Hole-only or electrononly diodes were fabricated using the architectures: ITO/PEDOT: PSS/PBDTTT-C-T: acceptor/Au for holes and Al/PBDTTT-C-T: acceptor/Al for electrons. Mobilities were extracted by fitting the current densityevoltage curves using the MotteGurney relationship (space charge limited current). 2.3. Materials The materials and solvents were purchased commercially and used without any purification unless stated otherwise. Toluene and tetrahydrofuran (THF) were distilled from sodium benzophenone under nitrogen before use. Compound 1 was purchased from Suna Tech Inc. Compound 2 was synthesized according to the reported procedure [33,34], while compound 4 was synthesized according to the reported procedure [29]. 2.4. Synthesis 2.4.1. Compound 3 Compound 1 (0.3 mmol, 230 mg) and compound 2 (0.15 mmol, 78 mg) were dissolved in anhydrous toluene (20 cm3), then Pd(PPh3)4 (0.023 mmol, 27 mg) was added. The solution mixture was stirred at 110 C for 48 h under argon atmosphere. The mixture was cooled to room temperature, and then extracted with CHCl3.
Fig. 3. UVevis absorption spectra of compounds 3 and 5 in chloroform solution (a) and in thin film (b).
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Fig. 4. Cyclic voltammograms of compounds 3 and 5 (a) and energy alignment (b).
The organic phase was dried over anhydrous MgSO4. After removing the solvent, the residue was purified using column chromatography on silica gel employing CHCl3 as an eluent, affording a purple black solid (117 mg, 48.7%). MS (MALDI-TOF): m/ z 1561 [MþH]þ. FT-IR (KBr, cm1): 1705 (C]O, stretching band), 1656 (C]C, stretching band). 1H NMR (400 MHz, CDCl3): d 9.38 (d, J ¼ 8.4 Hz, 2H), 8.63-8.57 (m, 2H), 8.53-8.42 (m, 2H), 8.39-8.35 (m, 2H), 8.29-8.18 (m, 4H), 7.10-6.99 (m, 6H), 4.50 (t, J ¼ 5.2 Hz, 4H), 4.18-4.10 (m, 8H), 2.12-2.05 (m, 4H), 1.72-1.67 (m, 4H), 1.39-1.32 (m, 34H), 1.13 (t, J ¼ 7.2 Hz, 6H), 0.96-0.88 (m, 26H). Anal. Calc. for C98H104N4O10S2: C 75.35; H 6.71; N 3.59. Found: C 75.19; H 6.74; N 3.61.
with chloroform, and the extract was concentrated before it was precipitated with methanol again, affording a dark solid (110 mg, 38.5%). GPC: Mn ¼ 8962, Mw ¼ 15628, Mw/Mn ¼ 1.74. FT-IR (KBr, cm1): 1703 (C]O, stretching band), 1663 (C]C, stretching band). 1 H NMR (300 MHz, CDCl3): d 8.66 (br, 2H), 8.32 (br, 4H), 7.10-7.04 (br, 6H), 4.14 (br, 4H), 2.00 (br, 2H), 1.22 (br, 92H). 13C NMR (150 MHz, CDCl3): d 163.6, 145.3, 143.0, 142.1, 135.6, 134.2, 133.7, 133.0, 129.9, 129.2, 128.6, 128.0, 127.0, 125.3, 123.3, 122.4, 120.9, 53.6, 45.0, 36.9, 32.1, 31.9, 30.3, 29.9, 29.6, 26.7, 22.9, 14.3. Anal. Calc. for C82H112N2O4S2: C 78.54; H 9.00; N 2.23. Found: C 77.27; H 8.54; N 2.26. 3. Results and discussion
2.4.2. Compound 5 A mixture of compound 2 (83 mg, 0.16 mmol), compound 4 (195 mg, 0.16 mmol) and Pd(PPh3)4 (11 mg, 0.009 mmol) in anhydrous toluene was heated under reflux for 48 h under Ar. 2-(Tributylstannyl)thiophene (2.6 mm3, 0.008 mmol) was added, and the mixture was refluxed for another 12 h. Then bromothiophene (1.6 mm3, 0.016 mmol) was added, and the mixture was refluxed for additional 12 h. After allowed to cool to room temperature, the reaction mixture was poured into methanol (200 cm3). The isolated crude product was then subjected to soxhlet extraction with methanol, acetone, and hexane. Finally, the product was extracted
3.1. Synthesis and characterization Two compounds 3 and 5 were synthesized via Still coupling reactions between either bromo- (compound 1) or dibromosubstituted PDI (compound 4) and 1,2-dithienylethene ditin (compound 2) in one pot (Scheme 1). Compound 3 is soluble in chloroform (ca. 5 mg mm3 at 60 C), while compound 5 is readily soluble in common organic solvents such as dichlorobenzene and chloroform at room temperature. The thermal properties of compounds 3 and 5 were investigated using thermogravimetric
Fig. 5. JeV characteristics (a) and IPCE curves (b) of PBDTTT-C-T: compound 3 and PBDTTT-C-T: compound 5 (1:1, w/w) devices.
S. Dai et al. / Dyes and Pigments 114 (2015) 283e289 Table 2 Device data at different PBDTTT-C-T: 3 and PBDTTT-C-T: 5 weight ratios without or with DIO. A D/A (w/w) Solvent DIO (v/v, %) VOC (V) JSC (mA cm2) FF (%) PCE (%)a 3 3 3 5 5 5 5 5 5 a
1:1 1.5:1 1:1.5 1:1 1.5:1 1:1.5 1:1 1:1 1:1
CF CF CF DCB DCB DCB DCB DCB DCB
/ / / / / / 1 3 5
0.84 0.83 0.79 0.74 0.71 0.72 0.72 0.76 0.72
2.68 2.31 2.60 1.73 1.37 1.75 1.76 3.29 2.21
30.8 29.6 29.5 32.9 30.3 29.8 33.4 40.0 42.9
0.69 0.57 0.61 0.42 0.29 0.38 0.43 1.00 0.69
(0.67) (0.56) (0.60) (0.39) (0.27) (0.35) (0.41) (0.93) (0.64)
Average PCE in brackets.
PBDTTT-C-T: 3 PBDTTT-C-T: 5
DIO (v/v, %) / 3
carbon working electrode in a CH3CN solution containing 0.1 M [nBu4N]þ[PF6] using a potential scan rate of 100 mV s1. The two compounds show irreversible oxidation waves and quasi-reversible reduction waves (Fig. 4(a)). The HOMO and LUMO values were estimated from the oxidation and reduction potentials, respectively, assuming the absolute energy level of FeCpþ/0 to be 4.8 eV. 2 Compound 5 exhibits slightly down-shifted HOMO level (5.70 eV) than compound 3 (5.58 eV), while compound 5 exhibits slightly up-shifted LUMO level (3.67 eV) than compound 3 (3.72 eV) (Table 1). Fig. 4(b) shows the energy alignment for compounds 3, 5 and PBDTTT-C-T. 3.4. Organic solar cells
Table 3 Hole and electron mobilities of PBDTTT-C-T: 3 and PBDTTT-C-T: 5 (1:1, w/w). Blend
287
mh (cm2 V1 s1) 4
1.51 10 3.72 105
me (cm2 V1 s1) 5.02 105 6.91 105
analysis (TGA). Two compounds have excellent thermal stability with decomposition temperatures (Td, 5% weight loss) of 426 and 447 C (Fig. 2). 3.2. Absorption spectra The absorption spectra of compounds 3 and 5 in chloroform solution and in thin film are shown in Fig. 3. In solution, the small molecule shows two maximum absorption peaks at 394 and 524 nm and a shoulder peak at 570 nm due to the pep stacking and aggregation. The extinction coefficient of compound 3 is 5.3 104 M1 cm1. In solution, the polymer shows three absorption bands peaked at 392, 490 and 626 nm. Compared with the small molecule, the polymer exhibits a broader absorption. Relative to their solutions, the films of compounds 3 and 5 exhibit red-shifted absorption. The optical bandgaps of compounds 3 and 5 estimated from the absorption edge in the films are 1.75 and 1.61 eV, respectively (Table 1).
We used compounds 3 and 5 as the acceptor materials, and PBDTTT-C-T [35] as the donor material to fabricate BHJ solar cells. PBDTTT-C-T and compound 3 or compound 5 have complementary absorptions. The LUMO gap and HOMO gap between PBDTTT-C-T (HOMO ¼ 5.11 eV, LUMO ¼ 3.25 eV) and compound 3 or compound 5 are large enough to allow efficient exciton dissociation [36]. The difference between HOMO energy level of the donor and LUMO energy level of the acceptor is ca. 1.4 eV, which is beneficial to obtaining an open circuit voltage (VOC) in OSCs [37]. Fig. 5 shows the JeV and IPCE characteristics of PBDTTT-C-T: compound 3 and PBDTTT-C-T: compound 5-based devices under AM 1.5G illumination at an intensity of 100 mW cm2. The weight ratio of donor to acceptor (D/A) to some extent affects VOC, short circuit current density (JSC), fill factor (FF) and PCE of the compound 3- and compound 5-based devices (Table 2); the best PCE is 0.69% and 0.42%, respectively, when the D/A weight ratio is 1:1. The addition of 1,8-diiodooctane (DIO) additive leads to enhancement in performance of the compound 5-based devices; when the volume ratio of DIO is 3%, the best PCE of 1.00% is achieved. However, the addition of DIO exerts no obvious effect on the PCE of the compound 3-based devices. From IPCE curves (Fig. 5(b)), both the acceptor and the donor contribute to the generation of photocurrent of the devices. The two blended films exhibit broad photoresponse from 300 nm to 800 nm. 3.5. Measurements of hole and electron mobility
3.3. Electrochemistry The electrochemical properties of compounds 3 and 5 were investigated by cyclic voltammetry (CV) using films on a glassy-
The measurements of hole and electron mobilities of blended films were carried out using the method of space charge limited current (SCLC) [38]. Hole-only or electron-only diodes were
Fig. 6. J-V characteristics measured under dark for (a) hole-only and (b) electron-only devices based on PBDTTT-C-T: compound 3 and PBDTTT-C-T: compound 5 (1:1, w/w) blend films.
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Fig. 7. AFM height images (a and c) and phase images (b and d) of PBDTTT-C-T: compound 3 blend films without DIO (a and b) and with 3% DIO (c and d), and AFM height images (e and g) and phase images (f and h) of PBDTTT-C-T: compound 5 blend films without DIO (e and f) and with 3% DIO (g and h) spin-coated on ITO/PEDOT: PSS substrates. The scan size for all images is 1 mm 1 mm.
S. Dai et al. / Dyes and Pigments 114 (2015) 283e289
fabricated using the architectures of ITO/PEDOT:PSS/PBDTTT-C-T: acceptor/Au for holes and Al/PBDTTT-C-T: acceptor/Al for electrons. The hole and electron mobilities of PBDTTT-C-T: acceptor blend films are listed in Table 3. The hole mobilities of PBDTTT-C-T: compound 3 and PBDTTT-C-T: compound 5 are 1.51 104 and 3.72 105 cm2 V1 s1, respectively. The electron mobility of PBDTTT-C-T: compound 3 (5.02 105 cm2 V1 s1) is comparable to that of PBDTTT-C-T: compound 5 (6.91 105 cm2 V1 s1). The more balanced electron and hole transport of PBDTTT-C-T: compound 5 partially accounts for a higher efficiency relative to PBDTTT-C-T: compound 3 (Fig. 6). 3.6. Blended film morphology The morphology of the BHJ active layer was examined using atomic force microscope (AFM) in the tapping mode. Fig. 7 shows the AFM height images and phase images of PBDTTT-C-T: compound 3 and PBDTTT-C-T: compound 5 (1:1, w/w) blended films without or with 3% DIO. We found that the addition of 3% DIO led to larger phase separation and bigger root-mean-square (RMS) roughness for both compounds 3 and 5. When we add 3% DIO, PBDTTT-C-T: compound 3 blended film exhibits a typical cluster structure with many aggregated domains, and the RMS increases from 0.71 nm to 11.1 nm. When 3% DIO is added, PBDTTT-C-T: compound 5 blended film shows bicontinuous interdigitated network with an increased RMS roughness from 1.25 to 3.27 nm, leading to higher JSC and FF, and thus achieving better PCE. 4. Conclusions Small molecule and polymer acceptors based on perylene diimide and thienylenevinylene were synthesized. Both small molecule and polymer exhibit strong and broad absorption in the visible region. Compared with the small molecule 3, polymer 5 shows slightly down-shifted HOMO levels, up-shifted LUMO levels, and reduced optical band gaps. Solution processed organic solar cells based on PBDTTT-C-T/compound 3 and PBDTTT-C-T/compound 5 (1:1, w/w) exhibit PCEs up to 0.69% and 1.00%, respectively. The better PCE of compound 5-based devices is attributed to bicontinuous interdigitated network morphology and balanced hole and electron mobilities of the blended films. Acknowledgements This work was supported by the 973 Program (No. 2011CB808401), the NSFC (No. 91433114, 21025418, 51261130582), and the Chinese Academy of Sciences. References € sel M, Søndergaard RR, Jørgensen M. 25th anniversary [1] Krebs FC, Espinosa N, Ho article: rise to power e OPV-based solar parks. Adv Mater 2014;26:29e39. [2] Lin Y, Li Y, Zhan X. Small molecule semiconductors for high-efficiency organic photovoltaics. Chem Soc Rev 2012;41:4245e72. [3] Zhao X, Zhan X. Electron transporting semiconducting polymers in organic electronics. Chem Soc Rev 2011;40:3728e43. [4] Peet J, Heeger AJ, Bazan GC. “Plastic” solar cells: self-assembly of bulk heterojunction nanomaterials by spontaneous phase separation. Acc Chem Res 2009;42:1700e8. [5] Li G, Zhu R, Yang Y. Polymer solar cells. Nat Phot 2012;6:153e61. [6] Li YF. Molecular design of photovoltaic materials for polymer solar cells: toward suitable electronic energy levels and broad absorption. Acc Chem Res 2012;45:723e33. [7] Cheng Y-J, Yang S-H, Hsu C-S. Synthesis of conjugated polymers for organic solar cell applications. Chem Rev 2009;109:5868e923. [8] You J, Dou L, Yoshimura K, Kato T, Ohya K, Moriarty T, et al. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat Commun 2013;4:1446.
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Perylene diimideethienylenevinylene-based small molecule and polymer acceptors for solution-processed fullerene-free organic solar cells Shuixing Dai a, b, Yuze Lin c, d, Pei Cheng c, d, Yifan Wang c, d, Xingang Zhao c, Qidan Ling a, *, Xiaowei Zhan b, * a
Fujian Key Laboratory of Polymer Materials, College of Materials Science and Engineering, Fujian Normal University, Fuzhou 350007, China Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China Beijing National Laboratory for Molecular Sciences and Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China d University of Chinese Academy of Sciences, Beijing 100049, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2014 Received in revised form 25 November 2014 Accepted 28 November 2014 Available online 13 December 2014
A small molecule and a polymer based on perylene diimide and thienylenevinylene were designed and synthesized. Both small molecule and polymer exhibited excellent thermal stability with decomposition temperatures of >400 C and strong absorption in the visible region (300e800 nm). These two compounds showed highest occupied molecular orbital levels of 5.57 and 5.70 eV and lowest unoccupied molecular orbital levels of 3.72 and 3.67 eV, respectively. Solution processed fullerene-free polymer solar cells based on the small molecule acceptor and the polymer acceptor afforded power conversion efficiencies of up to 0.69% and 1.00%, respectively. Comparative studies of the absorption, energy levels, charge transport, morphology and photovoltaic properties of the small molecule and polymer were carried out. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Perylene diimide Thienylenevinylene Organic solar cell Solution process Nonfullerene acceptor Fullerene-free
1. Introduction Organic solar cells (OSCs) based on solution-processed bulk heterojunction (BHJ) devices have received intense attention in recent years because they present some advantages, such as large scale production using roll-to-roll process, flexibility, light weight and low cost [1e7]. To date, the best power conversion efficiencies (PCEs) reported in literature exceed 10%; the quick development of new donor materials plays an important role in the excellent performance of OSCs [8e10]. In a sharp contrast, fullerene and its derivatives are still the commonly used acceptors till now, including [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), PC71BM, indene C60 bis-adduct and indene C70 bis-adduct (ICBA) [11,12]. However, fullerenes still have some flaws, such as weak absorption in the visible region, difficult purification and limited energy level variation. Currently, non-
* Corresponding authors. E-mail addresses: [email protected] (Q. Ling), [email protected] (X. Zhan). http://dx.doi.org/10.1016/j.dyepig.2014.11.022 0143-7208/© 2014 Elsevier Ltd. All rights reserved.
fullerene acceptors have attracted increasing attention due to easy modification and purification, tunable energy level and broad absorption [13e16]. Perylene diimide (PDI) as a typical n-type organic semiconductor has good light-harvesting property, thermal and chemical stability, and strong electron-accepting ability. However, the PDI cores tend to aggregate and form large crystal domains in blended films because they have good planarity and strong intermolecular interaction. To solve this problem, some strategies have been used to modify PDI cores by side chain substitution on imide nitrogen atoms and/or substitution on bay-regions of PDIs [17e21]. In particular, PDI dimers, connecting two PDI units with a single bond or aromatic moiety at bay-positions or NeN bond at imide positions [22e27], and star-shaped PDIs [28] form twisted structures and suppress the self-aggregation and over-crystallization. Since the first PDI-based polymer acceptor for all-polymer solar cells was reported by Zhan et al., in 2007 [29], intensive effort has been devoted to searching for more suitable PDI polymer acceptors to improve PCEs [30e32]. Recently, PCEs of up to 4.4% were achieved for PDI polymer acceptors based OSCs [32].
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Fig. 1. Molecule structure of compounds 3 and 5.
In this contribution, we report the synthesis of a small molecule (compound 3) and a polymer (compound 5) based on perylene diimide and thienylenevinylene (Fig. 1). Both small molecule and polymer exhibited broad absorption (300e800 nm) and suitable energy levels matched with those of the low-bandgap polymer donor PBDTTT-C-T. We used compound 3 and compound 5 as an acceptor and PBDTTT-C-T as a donor to fabricate fullerene-free OSCs. We carried out comparative studies of the absorption, energy levels, charge transport, morphology and photovoltaic properties of the small molecule and polymer. 2. Experimental section 2.1. Measurements and characterization 1
H NMR spectra were recorded using a Bruker AVANCE 300 MHz or 400 MHz spectrometer. 13C NMR spectra were recorded using a Bruker AVANCE 600 MHz spectrometer. Mass spectra were recorded using Bruker Daltonics Biflex III MALDI-TOF Analyzer in the
MALDI mode. Elemental analysis was performed using a Flash EA1112 analyzer. UVevis absorption (solution in chloroform, thin film using quartz substrate) was measured using a Jasco V-570 spectrophotometer. Electrochemical measurements were conducted under nitrogen in a solution of tetra-n-butylammonium hexafluorophosphate (0.1 mol L1) in CH3CN employing a computer-controlled CHI660C electrochemical workstation, glassy carbon working electrode coated with compound 3 or compound 5 films, a platinum-wire auxiliary electrode, and an Ag/AgCl reference electrode. The potentials were referenced to a ferrocenium/ ferrocene (FeCpþ/0 2 ) couple using ferrocene as an external standard. Thermogravimetric analysis (TGA) measurements were performed using a Shimadzu thermogravimetric analyzer (Model DTG-60) under flowing nitrogen gas at a heating rate of 10 C min1. The nanoscale morphology of the blended films was observed using an atomic force microscope (AFM) (NanoMan VS, Veeco, USA) in the tapping mode. The gel permeation chromatography (GPC) measurements were performed using a Waters 515 chromatograph connected to a Waters 2414 refractive index detector, using THF as
Scheme 1. Synthetic routes to compounds 3 and 5.
S. Dai et al. / Dyes and Pigments 114 (2015) 283e289
285
Table 1 Thermal, optical and electrochemical data of compounds 3 and 5. Compound
3 5 a b
Fig. 2. TGA curves of compounds 3 and 5.
an eluent and polystyrene standards as calibrants. Three Waters Styragel columns (HT2, 3, 4) connected in series were used. Fourier transform infrared spectroscopy (FT-IR) was performed using a Bruker Tensor 27 with a thin KBr pellet. 2.2. Fabrication and characterization of the photovoltaic cells Photovoltaic cells were fabricated with the structure of ITO/ PEDOT:PSS/PBDTTT-C-T:acceptor/Ca/Al. The patterned indium tin oxide (ITO) glass (sheet resistance ¼ 15 U ,1) was pre-cleaned in an ultrasonic bath with acetone and isopropanol, and treated in an ultraviolet-ozone chamber (Jelight Company, USA) for 30 min. A thin layer (30 nm) of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT: PSS, Baytron PVP AI 4083, Germany) was spin-coated onto the ITO glass, and then baked at 150 C for 15 min. A blend solution of PDI acceptor: PBDTTT-C-T in chloroform (CF) or o-dichlorobenzene (o-DCB) was spin-coated on the PEDOT: PSS layer from. The thickness of the photosensitive layer was measured using AFM. The calcium layer (ca. 20 nm) and the aluminium layer (ca. 80 nm) were subsequently evaporated onto the surface of the photosensitive layer under vacuum (ca. 105 Pa) to form the negative electrode. The active area of the device was 4 mm2. An XES-70S1 (SAN-EI Electric Co., Ltd.) solar simulator (AAA grade, 70 70 mm2 photobeam size) coupled with AM 1.5 G solar
Td ( C)
426 447
opt a
lmax (nm)
Eg
Solution
Film
524 626
530 632
1.75 1.61
(eV)
HOMO (eV)
LUMO (eV)
Egec b (eV)
5.58 5.70
3.72 3.67
1.86 2.03
Estimated from the absorption edge in films. Obtained from electrochemistry.
spectrum filters was used as the light source, and the optical power at the sample was 100 mW cm2. A 2 2 cm2 monocrystalline silicon reference cell (SRC-1000-TC-QZ) was purchased from VLSI Standards Inc. The currentevoltage (IeV) measurement of the devices was conducted using a computer-controlled Agilent B2912A Precision Source/Measure Unit. The incident photon to converted current efficiency (IPCE) spectrum was measured using a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd.). The light intensity at each wavelength was calibrated using a standard single crystal Si photovoltaic cell. Hole-only or electrononly diodes were fabricated using the architectures: ITO/PEDOT: PSS/PBDTTT-C-T: acceptor/Au for holes and Al/PBDTTT-C-T: acceptor/Al for electrons. Mobilities were extracted by fitting the current densityevoltage curves using the MotteGurney relationship (space charge limited current). 2.3. Materials The materials and solvents were purchased commercially and used without any purification unless stated otherwise. Toluene and tetrahydrofuran (THF) were distilled from sodium benzophenone under nitrogen before use. Compound 1 was purchased from Suna Tech Inc. Compound 2 was synthesized according to the reported procedure [33,34], while compound 4 was synthesized according to the reported procedure [29]. 2.4. Synthesis 2.4.1. Compound 3 Compound 1 (0.3 mmol, 230 mg) and compound 2 (0.15 mmol, 78 mg) were dissolved in anhydrous toluene (20 cm3), then Pd(PPh3)4 (0.023 mmol, 27 mg) was added. The solution mixture was stirred at 110 C for 48 h under argon atmosphere. The mixture was cooled to room temperature, and then extracted with CHCl3.
Fig. 3. UVevis absorption spectra of compounds 3 and 5 in chloroform solution (a) and in thin film (b).
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Fig. 4. Cyclic voltammograms of compounds 3 and 5 (a) and energy alignment (b).
The organic phase was dried over anhydrous MgSO4. After removing the solvent, the residue was purified using column chromatography on silica gel employing CHCl3 as an eluent, affording a purple black solid (117 mg, 48.7%). MS (MALDI-TOF): m/ z 1561 [MþH]þ. FT-IR (KBr, cm1): 1705 (C]O, stretching band), 1656 (C]C, stretching band). 1H NMR (400 MHz, CDCl3): d 9.38 (d, J ¼ 8.4 Hz, 2H), 8.63-8.57 (m, 2H), 8.53-8.42 (m, 2H), 8.39-8.35 (m, 2H), 8.29-8.18 (m, 4H), 7.10-6.99 (m, 6H), 4.50 (t, J ¼ 5.2 Hz, 4H), 4.18-4.10 (m, 8H), 2.12-2.05 (m, 4H), 1.72-1.67 (m, 4H), 1.39-1.32 (m, 34H), 1.13 (t, J ¼ 7.2 Hz, 6H), 0.96-0.88 (m, 26H). Anal. Calc. for C98H104N4O10S2: C 75.35; H 6.71; N 3.59. Found: C 75.19; H 6.74; N 3.61.
with chloroform, and the extract was concentrated before it was precipitated with methanol again, affording a dark solid (110 mg, 38.5%). GPC: Mn ¼ 8962, Mw ¼ 15628, Mw/Mn ¼ 1.74. FT-IR (KBr, cm1): 1703 (C]O, stretching band), 1663 (C]C, stretching band). 1 H NMR (300 MHz, CDCl3): d 8.66 (br, 2H), 8.32 (br, 4H), 7.10-7.04 (br, 6H), 4.14 (br, 4H), 2.00 (br, 2H), 1.22 (br, 92H). 13C NMR (150 MHz, CDCl3): d 163.6, 145.3, 143.0, 142.1, 135.6, 134.2, 133.7, 133.0, 129.9, 129.2, 128.6, 128.0, 127.0, 125.3, 123.3, 122.4, 120.9, 53.6, 45.0, 36.9, 32.1, 31.9, 30.3, 29.9, 29.6, 26.7, 22.9, 14.3. Anal. Calc. for C82H112N2O4S2: C 78.54; H 9.00; N 2.23. Found: C 77.27; H 8.54; N 2.26. 3. Results and discussion
2.4.2. Compound 5 A mixture of compound 2 (83 mg, 0.16 mmol), compound 4 (195 mg, 0.16 mmol) and Pd(PPh3)4 (11 mg, 0.009 mmol) in anhydrous toluene was heated under reflux for 48 h under Ar. 2-(Tributylstannyl)thiophene (2.6 mm3, 0.008 mmol) was added, and the mixture was refluxed for another 12 h. Then bromothiophene (1.6 mm3, 0.016 mmol) was added, and the mixture was refluxed for additional 12 h. After allowed to cool to room temperature, the reaction mixture was poured into methanol (200 cm3). The isolated crude product was then subjected to soxhlet extraction with methanol, acetone, and hexane. Finally, the product was extracted
3.1. Synthesis and characterization Two compounds 3 and 5 were synthesized via Still coupling reactions between either bromo- (compound 1) or dibromosubstituted PDI (compound 4) and 1,2-dithienylethene ditin (compound 2) in one pot (Scheme 1). Compound 3 is soluble in chloroform (ca. 5 mg mm3 at 60 C), while compound 5 is readily soluble in common organic solvents such as dichlorobenzene and chloroform at room temperature. The thermal properties of compounds 3 and 5 were investigated using thermogravimetric
Fig. 5. JeV characteristics (a) and IPCE curves (b) of PBDTTT-C-T: compound 3 and PBDTTT-C-T: compound 5 (1:1, w/w) devices.
S. Dai et al. / Dyes and Pigments 114 (2015) 283e289 Table 2 Device data at different PBDTTT-C-T: 3 and PBDTTT-C-T: 5 weight ratios without or with DIO. A D/A (w/w) Solvent DIO (v/v, %) VOC (V) JSC (mA cm2) FF (%) PCE (%)a 3 3 3 5 5 5 5 5 5 a
1:1 1.5:1 1:1.5 1:1 1.5:1 1:1.5 1:1 1:1 1:1
CF CF CF DCB DCB DCB DCB DCB DCB
/ / / / / / 1 3 5
0.84 0.83 0.79 0.74 0.71 0.72 0.72 0.76 0.72
2.68 2.31 2.60 1.73 1.37 1.75 1.76 3.29 2.21
30.8 29.6 29.5 32.9 30.3 29.8 33.4 40.0 42.9
0.69 0.57 0.61 0.42 0.29 0.38 0.43 1.00 0.69
(0.67) (0.56) (0.60) (0.39) (0.27) (0.35) (0.41) (0.93) (0.64)
Average PCE in brackets.
PBDTTT-C-T: 3 PBDTTT-C-T: 5
DIO (v/v, %) / 3
carbon working electrode in a CH3CN solution containing 0.1 M [nBu4N]þ[PF6] using a potential scan rate of 100 mV s1. The two compounds show irreversible oxidation waves and quasi-reversible reduction waves (Fig. 4(a)). The HOMO and LUMO values were estimated from the oxidation and reduction potentials, respectively, assuming the absolute energy level of FeCpþ/0 to be 4.8 eV. 2 Compound 5 exhibits slightly down-shifted HOMO level (5.70 eV) than compound 3 (5.58 eV), while compound 5 exhibits slightly up-shifted LUMO level (3.67 eV) than compound 3 (3.72 eV) (Table 1). Fig. 4(b) shows the energy alignment for compounds 3, 5 and PBDTTT-C-T. 3.4. Organic solar cells
Table 3 Hole and electron mobilities of PBDTTT-C-T: 3 and PBDTTT-C-T: 5 (1:1, w/w). Blend
287
mh (cm2 V1 s1) 4
1.51 10 3.72 105
me (cm2 V1 s1) 5.02 105 6.91 105
analysis (TGA). Two compounds have excellent thermal stability with decomposition temperatures (Td, 5% weight loss) of 426 and 447 C (Fig. 2). 3.2. Absorption spectra The absorption spectra of compounds 3 and 5 in chloroform solution and in thin film are shown in Fig. 3. In solution, the small molecule shows two maximum absorption peaks at 394 and 524 nm and a shoulder peak at 570 nm due to the pep stacking and aggregation. The extinction coefficient of compound 3 is 5.3 104 M1 cm1. In solution, the polymer shows three absorption bands peaked at 392, 490 and 626 nm. Compared with the small molecule, the polymer exhibits a broader absorption. Relative to their solutions, the films of compounds 3 and 5 exhibit red-shifted absorption. The optical bandgaps of compounds 3 and 5 estimated from the absorption edge in the films are 1.75 and 1.61 eV, respectively (Table 1).
We used compounds 3 and 5 as the acceptor materials, and PBDTTT-C-T [35] as the donor material to fabricate BHJ solar cells. PBDTTT-C-T and compound 3 or compound 5 have complementary absorptions. The LUMO gap and HOMO gap between PBDTTT-C-T (HOMO ¼ 5.11 eV, LUMO ¼ 3.25 eV) and compound 3 or compound 5 are large enough to allow efficient exciton dissociation [36]. The difference between HOMO energy level of the donor and LUMO energy level of the acceptor is ca. 1.4 eV, which is beneficial to obtaining an open circuit voltage (VOC) in OSCs [37]. Fig. 5 shows the JeV and IPCE characteristics of PBDTTT-C-T: compound 3 and PBDTTT-C-T: compound 5-based devices under AM 1.5G illumination at an intensity of 100 mW cm2. The weight ratio of donor to acceptor (D/A) to some extent affects VOC, short circuit current density (JSC), fill factor (FF) and PCE of the compound 3- and compound 5-based devices (Table 2); the best PCE is 0.69% and 0.42%, respectively, when the D/A weight ratio is 1:1. The addition of 1,8-diiodooctane (DIO) additive leads to enhancement in performance of the compound 5-based devices; when the volume ratio of DIO is 3%, the best PCE of 1.00% is achieved. However, the addition of DIO exerts no obvious effect on the PCE of the compound 3-based devices. From IPCE curves (Fig. 5(b)), both the acceptor and the donor contribute to the generation of photocurrent of the devices. The two blended films exhibit broad photoresponse from 300 nm to 800 nm. 3.5. Measurements of hole and electron mobility
3.3. Electrochemistry The electrochemical properties of compounds 3 and 5 were investigated by cyclic voltammetry (CV) using films on a glassy-
The measurements of hole and electron mobilities of blended films were carried out using the method of space charge limited current (SCLC) [38]. Hole-only or electron-only diodes were
Fig. 6. J-V characteristics measured under dark for (a) hole-only and (b) electron-only devices based on PBDTTT-C-T: compound 3 and PBDTTT-C-T: compound 5 (1:1, w/w) blend films.
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Fig. 7. AFM height images (a and c) and phase images (b and d) of PBDTTT-C-T: compound 3 blend films without DIO (a and b) and with 3% DIO (c and d), and AFM height images (e and g) and phase images (f and h) of PBDTTT-C-T: compound 5 blend films without DIO (e and f) and with 3% DIO (g and h) spin-coated on ITO/PEDOT: PSS substrates. The scan size for all images is 1 mm 1 mm.
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fabricated using the architectures of ITO/PEDOT:PSS/PBDTTT-C-T: acceptor/Au for holes and Al/PBDTTT-C-T: acceptor/Al for electrons. The hole and electron mobilities of PBDTTT-C-T: acceptor blend films are listed in Table 3. The hole mobilities of PBDTTT-C-T: compound 3 and PBDTTT-C-T: compound 5 are 1.51 104 and 3.72 105 cm2 V1 s1, respectively. The electron mobility of PBDTTT-C-T: compound 3 (5.02 105 cm2 V1 s1) is comparable to that of PBDTTT-C-T: compound 5 (6.91 105 cm2 V1 s1). The more balanced electron and hole transport of PBDTTT-C-T: compound 5 partially accounts for a higher efficiency relative to PBDTTT-C-T: compound 3 (Fig. 6). 3.6. Blended film morphology The morphology of the BHJ active layer was examined using atomic force microscope (AFM) in the tapping mode. Fig. 7 shows the AFM height images and phase images of PBDTTT-C-T: compound 3 and PBDTTT-C-T: compound 5 (1:1, w/w) blended films without or with 3% DIO. We found that the addition of 3% DIO led to larger phase separation and bigger root-mean-square (RMS) roughness for both compounds 3 and 5. When we add 3% DIO, PBDTTT-C-T: compound 3 blended film exhibits a typical cluster structure with many aggregated domains, and the RMS increases from 0.71 nm to 11.1 nm. When 3% DIO is added, PBDTTT-C-T: compound 5 blended film shows bicontinuous interdigitated network with an increased RMS roughness from 1.25 to 3.27 nm, leading to higher JSC and FF, and thus achieving better PCE. 4. Conclusions Small molecule and polymer acceptors based on perylene diimide and thienylenevinylene were synthesized. Both small molecule and polymer exhibit strong and broad absorption in the visible region. Compared with the small molecule 3, polymer 5 shows slightly down-shifted HOMO levels, up-shifted LUMO levels, and reduced optical band gaps. Solution processed organic solar cells based on PBDTTT-C-T/compound 3 and PBDTTT-C-T/compound 5 (1:1, w/w) exhibit PCEs up to 0.69% and 1.00%, respectively. The better PCE of compound 5-based devices is attributed to bicontinuous interdigitated network morphology and balanced hole and electron mobilities of the blended films. Acknowledgements This work was supported by the 973 Program (No. 2011CB808401), the NSFC (No. 91433114, 21025418, 51261130582), and the Chinese Academy of Sciences. References € sel M, Søndergaard RR, Jørgensen M. 25th anniversary [1] Krebs FC, Espinosa N, Ho article: rise to power e OPV-based solar parks. Adv Mater 2014;26:29e39. [2] Lin Y, Li Y, Zhan X. Small molecule semiconductors for high-efficiency organic photovoltaics. Chem Soc Rev 2012;41:4245e72. [3] Zhao X, Zhan X. Electron transporting semiconducting polymers in organic electronics. Chem Soc Rev 2011;40:3728e43. [4] Peet J, Heeger AJ, Bazan GC. “Plastic” solar cells: self-assembly of bulk heterojunction nanomaterials by spontaneous phase separation. Acc Chem Res 2009;42:1700e8. [5] Li G, Zhu R, Yang Y. Polymer solar cells. Nat Phot 2012;6:153e61. [6] Li YF. Molecular design of photovoltaic materials for polymer solar cells: toward suitable electronic energy levels and broad absorption. Acc Chem Res 2012;45:723e33. [7] Cheng Y-J, Yang S-H, Hsu C-S. Synthesis of conjugated polymers for organic solar cell applications. Chem Rev 2009;109:5868e923. [8] You J, Dou L, Yoshimura K, Kato T, Ohya K, Moriarty T, et al. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat Commun 2013;4:1446.
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