Influence of Side Chain Position on the Electrical Properties of Organic

Article pubs.acs.org/Macromolecules

Influence of Side Chain Position on the Electrical Properties of Organic Solar Cells Based on Dithienylbenzothiadiazole-altphenylene Conjugated Polymers Francesco Livi, Natalia K. Zawacka, Dechan Angmo, Mikkel Jørgensen, Frederik C. Krebs, and Eva Bundgaard* DTU Energy, Technical University of Denmark, Frederiksborgvej 399, DK-4000, Roskilde, Denmark S Supporting Information *

ABSTRACT: Seven conjugated copolymers, based on dithienylbenzothiadiazole and benzene, have been synthesized with side chains placed in different position along the conjugated backbone. An additional polymer with a small modification of the investigated backbone was also included in the study. Alkoxy and alkyl side chains were considered, depending on the aromatic ring they were anchored to. Our goal was to perform an extensive study, by evaluating the possible anchoring positions of the same backbone, in order to demonstrate the huge influence of the position of side chains on a well performing polymer backbone for polymer solar cells. All the polymers were roll slot die coated under ambient conditions on flexible ITO-free plastic substrates to give inverted polymer solar cell devices with an upscaled active area of 1 cm2. The best characteristics were found for the polymer carrying alkoxy side chains on the benzene ring where power conversion efficiencies of up to 3.6% were achieved. All studied materials were prepared with an objective of low-cost starting materials, simple synthesis, and simple processing conditions which was most successful for the polymer P5. The polymer P7 containing fluorine atoms showed excellent performance under constant illumination and high temperature (exhibiting stable photovoltaic properties even after 670 h under conditions similar to ISOS-L-2 lifetime protocol). This makes P7 a good candidate for further upscaling and device optimization. The photovoltaic performance results were corroborated with full optical and morphological characterization of the conjugated polymers. We conclude that the determination of the best anchoring position for the side chains is the most rational starting point for the optimization of a polymer with a potential for large-scale fabrication of polymer solar cells.

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INTRODUCTION Much effort has been put into the development of low-cost fabrication of polymer solar cells (PSC) during the past years.1−3 Commercial feasibility of PSC requires reducing the cost of all involved materials4 and their processing costs as much as possible. In PSC, this implies the use of low-cost and flexible substrates (plastic), in addition to relatively cheap and accessible materials including polymers whose preparation requires few synthetic steps. Indeed, the fabrication of indium tin oxide (ITO)-free PSC is now possible and flexible solar cells can be produced in large industrial scale now and in the near future.5 Poly(3-hexylthiophene) (P3HT) was for a decade the mostemployed conjugated polymer in the active layer for plastic photovoltaic devices. However, focus on improving the efficiency for PSC led to the development of a new class of polymer materials, the donor−acceptor copolymers with a lower band gap, which have gradually replaced P3HT as the donor material in the active layer for PSC. P3HT still remains the reference material for studies of efficiency, stability, cost optimization, etc. Recently, we studied 100 different low band gap polymers based on variations in donor and acceptor units6 on a low-cost ITO-free electrode known as Flextrode.7 Among © XXXX American Chemical Society

them, the conjugated polymer based on 4,7-di(thiophen-2yl)benzo[c][1,2,5]thiadiazole (DTBT) and phenylene (P), PPDTBT (see Figure 1), stood out as a promising polymer backbone, which needs to be further developed as it demonstrated good power conversion efficiency (PCE) in flexible PSC, reaching efficiencies of up to 2.4%6 (better than P3HT under the conditions of the study), and simplicity in synthesis requiring only a few steps. Polymers based on such backbone structures have already been prepared and employed as the active layer in spin-coated PSC. However, in all these cases, alkoxy side chains were anchored just to the benzene ring, and the only additional modification was the presence of fluorine in the benzothiadiazole ring. In particular, polymers a− e shown in Figure 1 (part 1) have already been synthesized and tested in spin-coated devices achieving good to excellent PCE (b, d, and e):8−10 Nguyen et al. reached efficiency up to 7.18% for e without any post-treatments.9 The polymer b was also employed for roll-coated all-polymer tandem PSC together with solution-processed metal electroReceived: March 20, 2015 Revised: April 30, 2015

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DOI: 10.1021/acs.macromol.5b00589 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. (1) Polymers based on PPDTBT which have been studied as active layers in PSC (materials spin-coated onto ITO-glass substrates). (2) Polymers based on PPDTBT studied in this work, roll-coated on PET substrates (an additional polymer, with a modified backbone, has been studied in this work; see Figure 4).

des.11 Taking into consideration the application potential of polymer b, we decided to perform a study on structural modifications on the main polymer backbone through introduction of various side chains in different positions (see Figure 1, part 2). This narrows down the conditions for a further optimization. When optimizing a conjugated polymer for organic electronics, a compromise between a high degree of conjugation and good solubility must as a first step be found (i.e., find the best side chain anchoring position). When the best anchoring position has been determined, the impact of different lengths of simple side chains (i.e., alkyl or alkoxy), either straight or branched, can be investigated. Once the optimal length of the side chain has been determined, a further step in the optimization of a conjugated polymer could be the variation of the side chain type (into others than alkyl or alkoxy) or the end group functionalities of the polymer. Furthermore, the presence of different elements anchored to either side chains or the polymer backbone can be evaluated in order to investigate any influences on the polymer morphology. The final step would be to vary the heteroatoms that are a part of the heterocyclic rings by substitution. All the above approaches have been reported in the literature. Nonbonding interactions were found between vicinal heteroaromatics, therefore affecting the planarity by minimizing dihedral angles12 or between heteroaromatics and other substituent such as oxygen13 and the most popular fluorine.14,15 Side chains, usually in the form of alkyl or alkoxy substituents, affect not only the solubility of the polymer but also the performance of polymer:fullerene PSC, according to their nature and the resulting morphology of the blend.16−20 Indeed, other chemical groups,21 elements,22,23 or side chains different from alkyl and alkoxy types24−27 have also been shown to influence the photovoltaic properties of conjugated polymers. Increasing fluorine content in conjugated polymers can affect the photovoltaic performances,28 even though it has been shown in a few cases not to be a benefit.29 Further modifications were also possible: heavy atoms were included in conjugated backbones giving narrowed band gaps,30,31 side chains were modified with terminal groups such as siloxy functionalities, resulting in a larger out-of plane hole mobility32 and polymers

end-capped with different groups with an influence on their PCE.33 All these chemical modifications are just examples of what should be considered when a specific polymer backbone is subjected to an optimization scheme. In this paper we focus on the first step of the optimization of PPDTBT where the optimum position of the side chain is determined followed by a second step where the optimal side chain type is found. Both branched and linear side chains have been employed at the best anchoring position since they can have different effect (positive or negative) on the photovoltaic performances of materials, depending on their nature.34−38 The functionalization of polymer backbones with solubilizing side chains must be performed in a position where the amount of induced twisting of the backbone is minimized as also suggested by Uy et al.39 The anchoring position on the polymer backbone has been studied for certain polymer backbones; however, no clear trends have been found and the choice of the best position seems to be polymer dependent.39 The photovoltaic performance was used as determinant for the best side chain position in polymers based on DTBT and benzo[2,1-b:3,4-b′]dithiophene (BDT), PBDTDTBT series, and the best efficiency was found in the case of least steric hindrance and maximum planarity. In case of PBDTDTBT it was found to be best when the polymer was alkylated in the four positions of the two thienyl groups.40 Polymers based on the same DTBT unit and alkylidene fluorene (AF), PAFDTBT, underwent a similar study. Alkyl chains were kept anchored at AF, and different possibilities of functionalization of DTBT were investigated. The maximum PCE was recorded when alkoxy chains were anchored just to benzothiadiazole (BT), leaving thiophene rings unsubstituted.41 Polymers based on bithiophene units (bT), BT and thieno[3,2b]thiophene (ThT), PbTBTbTThT, performed better when the 3-position of the DTBT system was not functionalized with any side chains and side chains were anchored to the ThT system instead.42 DTBT has been widely investigated, and from all the above studies it clearly emerges that the 3-position must be side chain free in order to prevent steric hindrance; however, in the case of polymers based on DTBT and cyclopentadithiophene (CPDT), PDTBTCPDT, higher PCEs were B


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Macromolecules

Figure 2. Polymer backbones investigated for the best side chain anchoring position (thick arrows indicate the best side chain anchoring position, while thin arrows with a cross indicate anchoring positions that gave lower performance of the final devices).40−46

The side chain position and its effect on the PCE are the main objective of this study. We investigate the optimal placement of substituents in a known backbone focusing on the thin film properties and the device functionality. Since few low band gap materials show an acceptable PCE in large scale processed PSC, a detailed study on modifications of a goodperforming backbone is fundamental to understand the structure−performance correlation and for a real development of this research area. Simple structures with the optimal side chain positioning can yield high performing materials, meaning that while designing candidate materials for PSC, interactions between different rings must be taken into account.

reached with 3-substitution rather than 4-substitution of DTBT (even though the best performance was observed with no side chains on DTBT).43 The most effective anchoring position has also been found on polymers based on two thiophene rings (TT), BT, DTBT, an additional thiophene (T) and carbazole (C), PTTBTDTBTTC. Carbazole was functionalized with the same alkyl chain in all the polymers while the benzothiadiazole unit was kept unsubstituted.44 The thiophene rings of the TTBTDTBTT unit were functionalized with EH chains in different positions (except the thiophene unit between the two BT rings), yielding five polymers. Large differences in PCE were observed ranging from 0.6% to 5.1%. The best performances were achieved for the polymer having chains on the external thiophene moieties (in the position with adiacent thiophenes, rather than carbazole) of the TTBTDTBTT unit.44 Silafluorene-based (SiF) and 5,6difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTDFBT) copolymers, PSiFDTDFBT, were also studied in terms of best-anchoring position, and when silafluorene was functionalized with an alkoxy chain, the best results were found.45 Finally, an extensive study on the anchoring position and side chain types was conducted on copolymers based on 11,12-difluorodibenzo[a,c]phenazine (DFDBP), benzo[1,2b:4,5-b′]dithiophene (BDT) and thiophene bridges, PTDFDBPTBDT. The best photovoltaic properties were accomplished when thiophenes were functionalized with hexyl side chains just on the position close to 11,12difluorodibenzo[a,c]phenazine unit while a α-hexylthiophene was anchored to the BDT unit.46 See Figure 2 for the abovedescribed investigated polymer backbones.

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EXPERIMENTAL SECTION

Materials. All reagents were purchased from Sigma-Aldrich Chemical and used as received, unless they were employed as monomers, i.e., 1,4-dibromobenzene, which was recrystallized from EtOAc. [60]PCBM (PV-A600) was purchased from Merck Chemicals, PEDOT:PSS (Orgacon EL-P 5010) was from Agfa, thermally curable Ag (PV-410) was from DuPont, UV-curable adhesive (DELO, Katiobond LP 655) was from DELO, and P3HT (OS1200) was from Plextronics. Characterization of the Polymers. Size exclusion chromatography (SEC) was performed using a KNAUER chromatograph, in HPLC-grade mode with chloroform at room temperature. Two gel columns in succession with pore diameters of 104 and 105 Å, both with dimensions of 25 mm × 600 mm, were employed. The detectors used for the characterization consisted of refractive index and diode array UV−vis types. SEC was run at 70 °C with 1,2-dichlorobenzene as eluent for P7 and at 150 °C with 1,2,4-trichlorobenzene as eluent for P1 and P4. Polystyrene standards were used to determine the molecular weight. NMR experiments were carried out in CDCl3 on a C


Article

Macromolecules 500 MHz Bruker spectrometer. AFM characterization was performed on a N8 NEOS instrument, tapping mode (Bruker Nano GmbH, Herzogenrath, DE), and the software SPIP 6.2.0 was used for analyzing the images and processing the data. UV−vis absorption spectroscopy experiments were carried out on a Shimadzu UV-3600 PharmaSpec spectrometer. Cyclic voltammetry and differential pulse voltammetry (DPV) were performed on BioLogic potentiostat/galvanostat electrochemical workstation (Figures S1−S15). The experimental setup consisted of three electrodes: a carbon electrode was used as the working electrode, and two platinum electrodes were used as counter and quasi reference electrodes. The supporting electrolyte was a solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in anhydrous acetonitrile. Polymers were dissolved in chloroform, and films were deposited onto the working electrode from the solutions. The redox couple Fc/Fc+ was used as reference, and the recorded electrochemical waves were calibrated accordingly and converted to the vacuum level (4.8 eV). The onset values of oxidation potentials (first oxidation potential) were used to estimate the HOMO levels (DPV). The optical band gap and the electrochemical HOMO levels were combined to calculate the LUMO levels. The method of the space charge limited current method (SCLC) was adopted47 to calculate the hole mobility, by the use of a custommade program. A solution of each polymer in chlorobenzene (30 mg/ mL) was prepared and stirred overnight at 60 °C. The polymer solution was spin-coated at 500 and 700 rpm (devices of each thickness were prepared) on top of a PEDOT:PSS layer previously coated on ITO slides at 2800 rpm. ITO slides were cleaned in advance by ultrasonication in 2-propanol for 40 min. After spin-coating of the active layer the samples were annealed for 5 min at 120 °C. An I−V curve in the dark was recorded for each sample, first from −1 to 2 V and later from −1 to 4 V, and the thickness of the layer was measured on a Dektak profilometer. X-ray diffraction (XRD) patterns of rollcoated films were recorded using Cu Kα radiation source (λ = 1.5418 Å) in the range of 2θ (3 ≤ 2θ ≤ 24) with a scanning rate of 0.02 deg/s on a Bruker D8; d-spacing was calculated according to Bragg’s law. Additionally, PET substrate and the machine holder underwent the same XRD characterization, in order to discriminate between the samples peaks and those due to the substrate and the background. For AFM and UV−vis, materials were roll-coated onto a PET/ZnO layer, while only PET was used in the case of XRD study. Device Fabrication and Testing. The devices were fabricated on a lab-scale mini-roll coater.48 PSC were fabricated on Flextrode substrate, an ITO-free semitransparent substrate composed of PET/ Ag-grid/PEDOT:PSS/ZnO.7 The active layer (solution of polymer: [60]PCBM, 1:1.5 (w:w), in 1,2-dichlorobenzene with a concentration of 40 mg/mL) was slot die coated at 70 °C. The flow rate of the solution and the web speed were adjusted in order to have a thickness of ca. 420 nm. The back PEDOT:PSS was subsequently slot die coated on the active layer, and a silver current collecting comb structure was screen-printed on top. The devices were annealed for 5 min at 110 °C in an oven. All processing was carried out in ambient conditions using coating and printing techniques directly transferrable to roll-to-roll large-scale processing. The device structure is shown in Figure 3. The I−V curves of the PSC were measured with a Keithley 2400 source meter in the voltage range of −1 to 1 V with 50 mV step size and 20 ms step time under a solar simulator (Steuernagel) calibrated with a reference photodiode providing 1000 W m−2 AM1.5G. The cells were tested under 40 ± 10% of relative humidity. The cells were encapsulated between glass slides with an UV-curable adhesive (DELO LP655) for the lifetime study, which was run according to the following parameters: AM1.5G, 1200 W m−2, 130 ± 5 °C, 40 ± 10% relative humidity. I−V curves were recorded every 5 min from −1 to 1 V with 25 mV steps. The devices remained open circuit between measurements. UV barrier foils were not employed; hence, the devices would most likely undergo UV-induced photodegradation. Synthesis of the Monomers. All the reactions were carried out under an Ar atmosphere. The solvents were dried and degassed prior to use. Monomers 1, 2, 5, 6, 10, 11, 12, 4,7-bis(4-dodecylthiophen-2yl)benzo[c][1,2,5]thiadiazole (12-S), 1,4-bis(4-dodecylthiophen-2-yl)-

Figure 3. Schematic view of the roll-coated PSC. benzene (10-S), and (4-dodecylthiophen-2-yl)trimethylstannane (9-S) were synthesized according to literature reports.49−60 Monomers 3, 7, and 9 were synthesized following general synthetic methods reported below. Dibromination of benzene-based materials and thiophenebased materials and dialkoxylation of benzene rings were performed according to methods reported in the literature.8,57,59 The synthetic routes of the monomers are reported in the Supporting Information (Scheme S1). General Method for Stille Small Molecule Dicoupling. 1 equiv of the dibrominated compound and 2.5 equiv of the monostannylthiophene-based compoundeither (4-dodecylthiophen-2-yl)trimethylstannane or tributyl(thiophen-2-yl)stannanewere dissolved in anhydrous toluene. 0.18 equiv of tri(o-tolyl)phosphine and 0.03 equiv of tris(dibenzylideneacetone)dipalladium(0) were added, and the mixture was refluxed overnight. The mixture was then allowed to reach rt and filtered thorugh Celite and concentrated under vacuum. Hexane and methanol were added to the residue, and the phases were separated. The hexane phase was washed with methanol (×3), dried over MgSO4, and filtered. The solvent was evaporated, and the product was purified by recrystallization if solid, otherwise by chromatography (hexane/ethyl acetate mixtures depending on the product). If the product was not soluble in hexane, after toluene had been evaporated, the resulting oil or solid was washed several times with MeOH and worked up as reported above. 5,5′-(2,5-Bis(dodecyloxy)-1,4-phenylene)bis(3-dodecylthiophene) (11-S). 11-S was synthesized from the coupling of 1 equiv of 1,4dibromo-2,5-bis(dodecyloxy)benzene (6) and 2.5 equiv of (4dodecylthiophen-2-yl)trimethylstannane (9-S). Yellow solid, 81% yield. 1H NMR (500 mHz, CDCl3, ppm): δ 7.37 (d, J = 1.4 Hz, 1H), 7.20 (s, 2H), 6.91 (d, J = 1.4 Hz, 2H), 4.07 (t, J = 6.5 Hz, 4H), 2.63 (t, J = 7.7 Hz, 4H), 1.8 (p, J = 6.6 Hz, 4H), 1.66 (p, J = 7.5 Hz, 4H), 1.56−1.50 (m, 4H), 1.39−1.23 (m, 68H), 0.88 (dt, J = 7.1, 6.8 Hz, 12H). 13C NMR (126 MHz, CDCl3): δ 149.63, 143.27, 139.26, 127.08, 123.37, 120.56, 113.14, 70.00, 32.20 (2C), 32.15, 30.92, 30.84, 29.97 (2C), 29.93, 29.92 (2C), 29.91, 29.89, 29.81, 29.77, 29.74, 29.70, 29.63 (2C), 26.60, 22.95 (2C), 14.36 (2C). General Method for Dilithiation/Stannylation. 1 equiv of thiophene-based compound was dissolved in anhydrous THF, and the resulting mixture was cooled to −78 °C before 2.5 equiv of n-BuLi were added slowly. If the employed compound contained benzo[c][1,2,5]thiadiazole, LDA, instead of n-BuLi, was used. When n-BuLi (or LDA) addition was complete, the reaction mixture was stirred for 1−3 h until all starting material was dilithiated (checked by 1H NMR). 3 equiv of trimethyltin chloride were added to the reaction mixture, kept at −78 °C, before the mixture was allowed to reach rt and stirred overnight. The reaction was then quenched with water, and hexane was added (when side chains were not present on the product, diethyl ether was used instead of hexane). The phases were separated, and the organic phase was washed with a large amount of water (×3). Hexane was evaporated under vacuum to yield the distannyl product. In case the product was a solid, if possible, a recrystallization was performed (MeOH or MeOH/CHCl3). 4,7-Bis(4-dodecyl-5-(trimethylstannyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (3). 3 was synthesized from 4,7-bis(4-dodecylthD


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Macromolecules iophen-2-yl)benzo[c][1,2,5]thiadiazole (12-S). Red oil, 88% yield. 1H NMR (500 mHz, CDCl3, ppm): δ 8.09 (s, 2H), 7.82 (s, 2H), 2.69 (t, J = 7.8 Hz, 4H), 1.69 (p, J = 7.7 Hz, 4H), 1.42−1.24 (m, 36H), 0.89 (t, J = 6.7 Hz, 6H), 0.45 (s, 18H). 13C NMR (126 MHz, CDCl3): δ 152.88, 152.24, 144.77, 134.72, 130.15, 126.04, 125.86, 33.28, 32.45, 32.17, 29.94 (2C), 29.92, 29.91, 29.87, 29.84, 29.61, 22.95, 14.38, −7.54 (3C). 1,4-Bis(4-dodecyl-5-(trimethylstannyl)thiophen-2-yl)benzene (7). 7 was synthesized from 1,4-bis(4-dodecylthiophen-2-yl)benzene (10S). Yellow oil, 85% yield. 1H NMR (500 mHz, CDCl3, ppm): δ 7.57 (s, 4H), 7.30 (s, 2H), 2.60 (t, J = 7.8 Hz, 4H), 1.62 (p, J = 7.7 Hz, 4H), 1.36−1.27 (m, 36H), 0.90−0.86 (t, J = 6.8 Hz, 6H), 0.40 (s, 18H). 13C NMR (126 MHz, CDCl3): δ 152.35, 149.38, 133.64, 132.16, 126.22, 125.81, 33.26, 32.38, 32.18, 29.95, 29.92, 29.91 (2C), 29.85, 29.83, 29.62, 22.95, 14.37, −7.61 (3C). ((2,5-Bis(dodecyloxy)-1,4-phenylene)bis(3-dodecylthiophene-5,2diyl))bis(trimethylstannane) (9). 9 was synthesized from 5,5′-(2,5bis(dodecyloxy)-1,4-phenylene)bis(3-dodecylthiophene) (11-S). Yellow solid, 91% yield. 1H NMR (500 mHz, CDCl3, ppm): δ 7.57 (s, 2H), 7.23 (s, 2H), 4.09 (t, J = 6.3 Hz, 4H), 2.65 (t, J = 7.6 Hz, 4H), 1.92 (p, J = 6.5 Hz, 4H), 1.69−1.57 (m, 8H), 1.42−1.30 (m, 68H), 0.91 (td, J = 7.2, 7.1 Hz, 12H), 0.43 (s, 18H). 13C NMR (126 MHz, CDCl3): δ 151.07, 149.43, 144.96, 132.22, 128.30, 123.13, 113.00, 69.79, 33.23, 32.43, 32.14 (2C), 29.93 (6C), 29.88, 29.88, 29.86, 29.85, 29.85, 29.84, 29.59, 29.58, 26.75, 22.90 (2C), 14.30 (2C), −7.75 (3C). Synthesis of the Polymers. All the reactions were carried out under an Ar atmosphere. The solvents were dried and degassed prior to use. The synthetic routes of the polymers are reported in the Scheme 1. General Method for Stille Polymerization. 1 equiv of each monomer was dissolved in anhydrous toluene (50 mg of monomers/mL). 0.18 equiv of tri(o-tolyl)phosphine and 0.03 equiv of tris(dibenzylideneacetone)dipalladium(0) were added before the mixture was refluxed. The reaction mixture was precipitated after 48 h into MeOH. The solid was collected, and a Soxhlet extraction with MeOH (48 h) and hexane (24 h) was performed. The polymer product was then extracted with hot chloroform and precipitated into MeOH, filtered, and dried under vacuum, with final yields ranging from 30% to 73%. The polymers were characterized by 1H NMR, SEC chromatography, and UV−vis spectroscopy. Poly[(phenylene)-alt-(5,6-bis(tetradecyloxy)-4,7-di(thiophen-2yl)benzo[c][1,2,5]thiadiazole)] (P1). Purple solid, 72% yield. Numberaverage molecular weight (Mn) = 13 kDa, polydispersity index (PDI) = 1.7. 1H NMR (500 mHz, CDCl3, ppm): δ 8.55 (br, 2H), 7.77 (br, 4H), 7.50 (br, 2H), 4.21 (br, 4H), 2.09−0.84 (br, 54H). Poly[(phenylene)-alt-(4,7-bis(4-dodecylthiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (P2). Purple solid, 32% yield. Mn = 11 kDa, PDI = 1.2. 1H NMR (500 mHz, CDCl3, ppm): δ 8.08 (br, 2H), 7.90 (br, 2H), 7.62 (br, 4H), 2.82 (br, 4H), 1.76 (br, 4H), 1.41−1.21 (br, 36H), 0.87 (t, J = 6.0 Hz, 6H). Poly[(2,5-bis(dodecyloxy)phenylene)-alt-(4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (P3). Blue solid, 54% yield. Mn = 3 kDa, PDI = 1.3. 1H NMR (500 mHz, CDCl3, ppm): δ 8.17 (br, 2H), 7.92 (br, 2H), 7.70−7.66 (m, 2H), 7.37−7.31 (m, 2H), 4.19 (br, 4H), 1.99 (br, 4H), 1.63 (br, 4H), 1.43−1.20 (m, 32H), 0.90−0.84 (m, 6H). Poly[(phenylene)-alt-(4,7-bis(3-dodecylthiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (P4). Red solid, 35% yield. Mn = 10 kDa, PDI = 1.7. 1H NMR (500 mHz, CDCl3, ppm): δ 7.73−7.70 (br, 6H), 7.39 (br, 2H), 2.73 (br, 4H), 1.71 (br, 4H), 1.30−1.19 (br, 36H), 0.8−0.86 (m, 6H). Poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(4,7-di(thiophen-2yl)benzo[c][1,2,5]thiadiazole)] (P5). Blue solid, 31% yield. Mn = 16 kDa, PDI = 1.5. 1H NMR (500 mHz, CDCl3, ppm): δ 8.18 (br, 2H), 7.93 (br, 2H), 7.69 (br, 2H), 7.37 (br, 2H), 4.10 (br, 4H), 2.01 (br, 2H), 1.68 (br, 4H), 1.44−1.23 (m, 44H), 0.86−0.82 (br, 12H). Poly[(2,5-bis(dodecyloxy)phenylene)-alt-(4,7-bis(3-dodecylthiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (P6). Magenta solid, 30% yield. Mn = 16 kDa, PDI = 1.5. 1H NMR (500 mHz, CDCl3, ppm): δ 7.72 (br, 2H), 7.62 (br, 2H), 7.33 (br, 2H), 4.16 (t, J = 6.5 Hz, 4H),

2.73 (br, 4H), 1.96 (p, J = 6.6 Hz, 4H), 1.72 (br, 4H), 1.63−1.57 (m, 4H), 1.42−1.23 (m, 68H), 0.87 (dt, J = 6.9, 6.8 Hz, 12H). Poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (P7). Blue solid, 69% yield. Mn = 14 kDa, PDI = 1.9. 1H NMR (500 mHz, CDCl3, ppm): δ 8.38−7.20 (br, 6H), 4.10 (br, 4H), 2.01−0.85 (br, 62H). Poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-4,7-bis(3-hexyl[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole] (P8). Dark-green solid, 73% yield. Mn = 23 kDa, PDI = 1.2. 1H NMR (500 mHz, CDCl3, ppm): δ 8.05 (br, 2H), 7.86 (br, 4H), 7.55 (br, 2H), 7.30 (br, 2H), 4.05 (br, 4H), 2.94 (br, 4H), 1.99−1.95 (m, 2H), 1.80 (p, J = 7.9 Hz, 4H), 1.64−1.22 (m, 84H), 0.93−0.83 (m, 18H).

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RESULTS AND DISCUSSION In this study, seven polymers with side chains of different nature and/or in distinct positions have been synthesized: P1− P7 (see Figure 4). Additionally, a modification of the backbone by incorporation of two thiophene rings with a hexyl side chain was included in one of the polymers (P8). In order to simplify the synthesis and to create only limited alteration to the system, the side chains have been first introduced in one of the aromatic rings (thiophenes, benzene, or benzothiadiazole); P1−P5, P7, and later, in more, P6 and P8. The synthesized polymers are shown in Figure 4. The different polymers have been chosen according to the simplest starting materials. The side chain length is a parameter which could be optimized in a further study, and therefore, the choice was made according to the number of aliphatic carbon atoms that provide good solubility in organic solvents such as chloroform, chlorobenzene, etc. (at least two chains with 12, 14, or 16 carbon atoms each). Since the impact of the side chain nature (alkoxy or alkyl) may be polymer backbone dependent, either alkoxy or alkyl chains have been chosen according to the ease of preparation and purification of the corresponding materials. Alkyl chains were anchored to thiophene rings while alkoxy chains have been chosen for benzothiadiazole and benzene. Following the same logic, linear dodecyl alkyl chains for thiophene, dodecyl alkoxy chains for benzene, and tetradecyl alkoxy chains for benzothiadiazole were chosen according to both cost and availability yielding P1−P4. Despite the low Mn of P3, the polymer shows the lowest band gap among P1−P4. Therefore, the branched chains, 2hexyldecyl (HD), were introduced in P3, and the resulting P5 was compared with the linear chains polymer counterpart. To investigate the effect of the functionalization of more than on type of aromatic ring, both benzene with straight alkoxy chains and thiophenes with alkyl chains were functionalized, giving P6. Finding that P5 was the best performing polymer, additional changes were applied. These included (1) the presence of 4,7-dibromo-5,6-difluorobenzo[c][1,2,5]thiadiazole (DFBT) as acceptor monomer, instead of 4,7-dibromobenzo[c]-1,2,5-thiadiazole (BT), yielding P7; (2) the presence of two additional alkylthiophenes, keeping the same OHD side chain on the benzene ring, yielding P8. The monomers were synthesized according to literature reports, and the monomer synthesis was designed in order to keep the use of toxic organotin compounds at a minimum. The polymers were characterized by SEC chromatography, and their molecular weights were in the range of 10−23 kDa, except P3 (see Table 1). P3 showed a low Mn (3 kDa) probably due to the presence of impurities in the ((2,5-bis(dodecyloxy)-1,4-phenylene)bis(thiophene-5,2-diyl))bis(trimethylstannane) organometallic monomer due to difficult stannylation and purification of its precursor. Therefore, P3 was synthesized through an inverse E


Article

Macromolecules Scheme 1. Synthesis of the Conjugated Polymers P1−P8 and Their Colors in ODCB Solution

functionalization of the monomers (Scheme 1), but it showed a low Mn as well (3 kDa). P1 and P7 aggregated with CHCl3 as eluent for SEC; therefore, the molecular weight of P7 was determined with 1,2-dichlorobenzene (ODCB) while 1,2,4-

trichlorobenzene was necessary for P1. P4 did not have a good solubility neither in CHCl3 nor in ODCB, so 1,2,4trichlorobenzene was used as eluent in this case as well. The enhanced reactivity of DFBT compared to BT resulted in a F


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Macromolecules

Figure 4. Polymers P1−P8 synthesized and studied in this report.

Table 1. Molecular, Physical, and Electronic Properties of the Polymers P1−P8

a

polymer

Mn

PDI

λonset solution

λonset film

optical band gap

HOMO levela

LUMO levelb

P1 P2 P3 P4 P5 P6 P7 P8

13 11 3 10 16 16 14 23

1.7 1.2 1.3 1.7 1.5 1.5 1.9 1.2

635 593 654 556 658 584 697 657

646 629 733 591 711 621 700 760

1.92 1.97 1.69 2.10 1.74 2.00 1.77 1.63

−5.05 −5.23 −5.19 −5.13 −5.00 −5.16 −5.18 −4.90

−3.13 −3.26 −3.50 −3.03 −3.26 −3.16 −3.41 −3.27

hole mobilityc (cm2/(V s)) 9.92 3.51 5.59 6.87 2.00 1.95 7.34 2.39

× × × × × × × ×

10−4 10−4 10−3 10−4 10−3 10−3 10−3 10−3

Estimated from DPV. bCalculated from the optical band gap and HOMO level. cP3HT exhibited a hole mobility value of 8.78 × 10−4 cm2 V−1 s−1.

UV−vis spectra of the polymer solutions in ODCB were recorded as well as polymer−[60]PCBM blend films (see Figure 5). All the polymers absorb light in two different regions of the spectrum; i.e., π−π* transitions at lower wavelength and charge transfer bands at higher wavelength were observed. Two main class of polymers emerged from the analysis of their UV absorption: one group with intense π−π* transitions and lowwavelength charge transfer absorptions and the other group with red-shifted charge transfer absorptions and weaker π−π* transitions. Clearly, dodecyl side chains on thiophene appear to lower the conjugation length of the backbone, resulting in less planar structures, observed by blue-shifted absorptions; i.e., P1, P3, P5, P7, and P8 had a lower band gap when compared to P2, P4, and P6. When benzene rings were functionalized with alkoxy side chains, it resulted in lower band gaps. For P8, which had the same functionalization but with additional 3hexylthiophenes, the lowest band gap was observed, in addition to a considerable red-shift of the film absorption profiles, compared to those of ODCB solution spectra. A significant redshift of the absorption profiles of the films compared to the corresponding solutions were also observed for P3 and P5. All the above-described red-shifts between solution and film absorption are probably due to intramolecular nonbonding interactions, most likely promoted by the oxygen part of alkoxy groups on benzene rings.9 According to UV−vis characterizations, alkyl chains on thiophene rings gave distorted

higher yield of P7 (69%) compared to P5 (31%). P8, the only polymer obtained from thiophene−thiophene couplings, showed high Mn (23 kDa) and yield (73%). The polymers and their blends with phenyl-C61-butyric acid methyl ester ([60]PCMB) (1:1.5 w:w) were further characterized with UV− vis and electrochemical characterizations to investigate the optical band gap and HOMO−LUMO levels (see Table 1). Most studies in the literature show PSC prepared with spincoated films of the active material blended with fullerenes on glass/ITO substrates, and the characterization as well as the morphological studies are carried out with films on those substrates. However, in this study, since all the synthesized polymers are studied as candidates for ITO-free PSC, active layer films required for optical and morphological analyses were prepared in order to mimic the characteristics they have in PSC. The morphology of the films seems to depend on the chosen fullerene acceptor, since different XRD patterns have been reported with the same polymer but different fullerenes.8,9 The morphology in the film may depend also on the substrate, meaning that differences could be noticed between films deposited on glass and those coated on plastic substrates. However, keeping in mind the prospective of a large-scale PSC production, also the characterization and analyses have to be carried out on plastic substrates, in order to study the properties of polymeric films on substrates other than glass. G


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Figure 5. Normalized UV−vis spectra of the polymer solutions in ODCB (a) and of blend films (b): P1, dashed turquoise; P2, gray; P3, green; P4, dashed violet; P5, black; P6, purple; P7, blue; P8, red.

system, allowing for more noncovalent interactions. Furthermore, these results provide an additional, even if indirect, proof that oxygen (in the alkoxy chains) and fluorine (as benzothiadiazole substituent) may act as promoters for the formation of nonbonding interactions in PPDTBT. Computational studies were performed by Nguyen et al. on PPDTBT,9 leading to similar conclusions. In our group we also showed that a methylene group directly attached to a benzene ring prefers a torsion angle of about 90°, while similar alkoxy substituents prefer angles of about 0°.62 The presence of both alkoxy chains and fluorinated benzothiadiazole on P7 are in agreement with its highest hole mobility. Additionally, P3 showed a higher hole mobility than P5 and this could be explained by the presence of a straight rather than a branched chain which, as reported in the literature, could yield a more organized structure.16 The presence of alkoxy groups anchored to the benzene ring, either branched or linear, even in the presence of alkylated thiophenes in the backbone, was beneficial since both P6 and P8 showed a higher hole mobility value, if compared to P2 and P4. PSC were prepared with low-cost materials in order to study the effect of the different side chains and their position (see Table 2) on photovoltaic performances. The polymer: [60]PCBM ratio was kept constant at 1:1.5 (w:w). Blends were prepared from ODCB, and the thickness of the active layers was kept at 420 nm for all PSC devices prepared. The PSC were prepared with an inverted device geometry comprising Ag/PEDOT:PSS/ZnO/polymer:[60]PCBM/PEDOT:PSS/Ag. All devices were fabricated under ambient conditions using a laboratory roll-coater and using only coating and printing conditions.48 All the inks were additive-free, and optimization of the devices has not been carried out since a comparison of the different materials in the same conditions was the purpose of this work. At least three solar cells were tested for each polymer, and the size of the active area for the devices was ∼1 cm2. A consistent difference in terms of efficiency was observed according to the position of the side chains in the backbone (Table 2). In general, it was observed that dodecyl alkyl side chains on the thiophene rings resulted in nonoperational devices; tetradecylalkoxy side chains on benzothiadiazole increased the performance a little, and finally, when −OHD

backbones, while alkoxy chains on benzene rings gave more planar structures. However, if additional thiophenes are part of the backbone, the presence of alkyl chains seems to have no influence on the planarity of the resulting conjugated polymer (P8). A similar trend can be found in the energy levels. The alkyl chain position on thiophenes (3- or 4-position of DTBT system) influenced the energy levels of the polymers: P2 and P4 exhibited different energy levels, and P2 (thiophene functionalized in the 4-position of DTBT) showed the lowest HOMO energy level among all the polymers. The presence of additional alkyl-functionalized thiophenes (with short and linear chains) in the backbone yielded the highest HOMO energy level in P8 instead. Deep HOMO energy levels were measured for P2, P4, and P6, which have long alkyl side chains anchored on thiophenes. We ascribed the decrease in the HOMO energy level of these to a weaker electron-donating character of alkyl substituents compared to alkoxy groups (P1, P5, and P8). In the case of the acceptor modifications, i.e., the presence of fluorine atoms on BT (DFBT) in P7, a decrease in the energy levels was observed, when compared to those observed for P5. Benzene functionalization showed that straight, rather than branched, alkoxy chains on benzene rings result in lower band gap and energy levels. Hole mobility studies on polymer-only devices were carried out on ITO/PEDOT:PSS/Polymer/Au spin-coated devices, following a method based on SCLC.47 Polymer-only devices were chosen in order to have information on the polymers rather than their PCBM blends (see Table 1). Also, the devices were spin-coated according to methods reported in the literature47 instead of performing the studies on a flexible substrate. P3HT was tested as reference. Most of the polymers showed a higher hole mobility than P3HT (see Table 1), except for P2 and P4, where dodecyl side chains were anchored to the thiophene rings. These showed relatively low hole mobilities (3.51 × 10−4 and 6.87 × 10−4 cm2 V−1 s−1, respectively). Considering the same backbone, hole mobility and crystallinity should be directly related,61 and hole mobility measurements are an additional proof that anchoring side chains to the thiophene rings resulted in more distorted structures, reducing nonbonding interactions between different aromatic rings. The highest hole mobility values were observed for P3 and P7, probably due to the presence of the less distorted DTBT H


Article

Macromolecules Table 2. Photovoltaic Performancesa and Morphological Characteristics of the Polymers Voc

Isc

FF

PCE

PCE av

RMSb

dc

polymer

(V)

(mA)

(%)

max (%)

(%)

(nm)

(Å)

P1 P2 P3 P4 P5d P5e P5f P6 P7 P7g P8

0.74 0.60 0.53 0.31 0.74 0.70 0.74

−2.26 −0.27 −0.64 −0.041 −9.59 −8.57 −6.48

36 26 31 25 51 43 41

0.61 0.041 0.10 0.0030 3.6 2.6 1.9

0.55 0.038 0.091 0.0026 3.3 2.5 1.8

3.95 0.33 0.51 0.40 0.42

19.2

0.72 0.74 0.63

−5.61 −4.75 −5.13

55 57 37

2.2 2.0 1.2

1.9 1.9 1.1

(compared to P5) suggests that an increase in Voc could be achieved with an optimization of the devices. Different batches of P5 with different Mn were synthesized, and the important dependence of P5 on the molecular weight was observed: when the Mn of P5 was 11 kDa, just 5 kDa lower than the other P5 batch of Mn 16 kDa, the PCE dropped by 1% (see Table 2). Higher performance of the larger Mn batch was mainly due to an increase in FF and a slight increase in short current density (Isc) and Voc. The influence of Mn on photovoltaic performances of conjugated polymers is wellknown, and it has just been shown for P7 as well by Kang et al.63 The two best performing polymers, P5 and P7, were also tested with another solvent mixture (see Table 2). A nonhalogenated solvent mixture (o-xylene/tetralin) was chosen in agreement with industrial relevant large-scale production protocols, where halogenated solvents must be avoided. When P5 was roll-coated from a nonhalogenated solvent, the Isc dropped significantly, whereas P7 maintained almost the same performances. On the other hand, the Voc increased slightly for both polymers (in the case of P5, considering the Mn = 11 kDa batch) when roll-coated from nonhalogenated solvent mixtures. The resulting P7 film was more homogeneous than the one deposited from ODCB, and more importantly, the resulting PCE reached 2% with the low-cost, simple, and scalable materials and halogen-free solvents. To further investigate the morphology of the different polymer:[60]PCBM blends, AFM images were recorded on slot die coated films. Most of the films resulted in very smooth and uniformly distributed surfaces with limited variations (see Table 2). Both smooth and rough films can be beneficial for PSC: a good adhesion (smooth films) or an increased contact area (rough films) at the active layer/electrode interfaces can improve photovoltaic performances of PSC.64,65 In our case, RMS values were found to be close to 1 nm for all polymers, except for P1 and P7, where the functionalization of benzothiadiazole influenced the topography images, which is evident even in a more pronounced manner in their phase images (Figures S16−S23). From our results, it is evident that both rough (P1, P7) and smooth films (P5, P8) produced working devices, even though the highest PCEs were achieved with P5, whose blend films formed a smooth layer. XRD analysis has been carried out for all the samples (Figure S25). XRD signals correspond to the out-of plane diffraction patterns, and it was observed that most of the samples were amorphous. P5 and P7 showed reflection peaks in previous works,9 but we did not experience the same for roll-coated P5 and P7 onto PET substrates. This difference could be ascribed both to the presence of different fullerene acceptors ([60]PCBM was used in our work and [70]PCBM was used by Nguyen et al.9) and/or different substrates/film formation (roll-coating on PET versus spin-coating on Si substrates9). However, two polymer blends gave reflections, both (100) types. They are P3, at 2θ = 4.41 degrees, and P8, at 2θ = 4.60 degrees. Interchain d spacing for P3:[60]PCBM and P8: [60]PCBM blends was 20 and 19.2 Å, respectively. Two parasubstituted benzene rings with straight alkoxy chains in P3 may result in a certain degree of packing, when compared to the −OHD chains (P5). Additionally, noncovalent bond interactions between the other heteroaromatic rings could yield a less-distorted structure allowing the intercalation of [60]PCBM. The modification of the polymer backbone with additional thiophenes carrying n-hexyl chains has clearly an

0.88 2.20 0.31

20.0

a

P3HT was tested under the same conditions employed for the polymers (polymer:[60]PCBM ratio of 1:1.5 (w:w), ODCB and 420 nm thick film) giving a PCE max of 1.8%. bRoot-mean-square roughness from AFM studies. cInterchain d-spacing from XRD experiments. dBatch of 16 kDa. eBatch of 11 kDa. fBatch of 11 kDa, o-xylene/tetralin mixture. go-Xylene/tetralin mixture.

chains were anchored to the benzene rings, the best performances were achieved. P6 did not result in any functional PSC, and extremely low performances were measured for both P2 (0.04%) and P4 (0.003%). The high band gaps of P2, P4, and P6 may explain their poor device performances. P3 showed low performance (0.1%) which is ascribed to a low Mn. P1 showed a higher efficiency (0.6%), if compared to the previous materials. However, the low performance could probably be due to benzothiadiazole functionalization since the alkoxy electron-rich substituents lower the acceptor character and thus results in a higher band gap. P5, P7, and P8 all showed PCE above 1%: they all underwent benzene ring functionalization, and this influenced the photovoltaic characteristics of the corresponding devices. The influence of branched chains on the voltage has been reported in the literature,16 and indeed, introduction of branched chains on benzene consistently increased the open circuit voltage (Voc) from 0.53 to 0.74 V, even though P5 did not show a deeper HOMO energy level than P3. Usually, lower HOMO energy levels should produce higher Voc in polymers with similar structures and in the presence of the same acceptor,16 but the low Mn of P3 does not allow for a rational comparison on the photovoltaic properties between P3 and P5. P5 showed the highest efficiency reaching 3.6%, an exceptionally high value for low-cost ITO-free ambient fabricated PSC. The modification of the backbone of P5 yielding P8 did not improve the photovoltaic performances of the polymer and showed a lower fill factor (FF). P7 showed lower performances, compared to P5, in contrast to what is reported in the literature.9 P7 film, when prepared from ODCB, resulted in a nonuniform film, and higher performances may be reached for this polymer if an optimization of the fabrication conditions was evaluated (i.e., solvent optimization, film thickness, and polymer/acceptor ratio). Except for the lowperforming polymers (P2, P3, and P4), the Voc values were in fairly good agreement with the HOMO energy levels of the polymers. P1 and P5 showed similar HOMO energy levels, and this resulted in the same appreciably high Voc value of 0.74 V. In the case of P8, a rather low Voc of 0.63 V was the result of a higher HOMO energy level. P5 and P7 showed very similar Voc values; however, the deeper HOMO energy level of P7 I


Macromolecules impact on the organization of the corresponding blend film. The consistent red-shifted absorption of P8 film compared to that of the ODCB solution, observed in UV experiments, suggests that a rearrangement of the polymer could occur. Additional thiophenes with straight and short chains are part of the backbone in P8; therefore, with two more aromatic rings in the polymer backbone, the bulky −OHD branched side chains anchored to the benzene rings are not as close to each other as in all the other polymers (P5, P7), and the intercalation of [60]PCBM may be possible. Lifetime studies, based on stronger light intensities and higher temperature than those of ISOS-L-2,66 were carried out for the best performing polymers P5, P7, and P8. Extreme conditions were employed for all the samples in order to evaluate the maximum stress onto the material, and the cells were encapsulated without UV filters. With these studies the stability of P5 was examined, and the influence of both fluorine (P7) and additional thiophenes in the backbone (P8) on the stability of the final devices was investigated. P5, P7, and P8 were all compared with P3HT (see Figure 6), which underwent lifetime studies under the same conditions as the low band gap polymers.

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CONCLUSION

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ASSOCIATED CONTENT

Dramatic differences were detected in the investigated backbone (PDTBT) when the position and nature of side chains was varied. We assume that varying the anchoring position of the side chains, intramolecular interactions, and structural rearrangements occur accordingly, and important effects have been observed in terms of physical, morphological, and photovoltaic properties of the conjugated polymers. In fact, alkoxy chains anchored to the benzene ring, rather than alkyl side chains anchored to thiophene or alkoxy side chains anchored to benzothiadiazole (nonfunctionalized DTBT system), yield more planar structures. Most importantly, much higher performances were accomplished when the benzene moiety was functionalized rather than left side chainfree. This is probably because benzene is a heteroatom-free aromatic ring, and less noncovalent interactions are available when benzene units are alkoxy chain-free. We believe that such studies will bring better understanding of the relationship between structure and photovoltaic performances of conjugated polymers. Future work should progress with aspects of low cost in mind, and the synthetic chemists should optimize simple polymer backbones in order to gain better performances. PPDTBT has been confirmed to be a good performing polymer for PSC, and the performance of P5 has been considerably increased since our last work. P7 should be optimized for flexible substrates due to its high potential as stable material. Additionally, important modifications of benzene functionalization should be considered as next optimization step, eventually after having evaluated the optimum side chain length (a longer or shorter branched chain may be required for better performances or higher Mn). P8 showed a reasonable PCE, and therefore the modification of PDTBT backbone should also be investigated in future works. We believe that, when designing new materials (or when modifying the existing ones) for organic electronics, the effects of substituents of a certain polymeric backbone are as important as the backbone itself, as demonstrated with this work.

S Supporting Information *

Figure 6. Lifetime studies of P5 (black), P7 (blue), P8 (red), and P3HT (green).

Synthetic scheme of monomers synthesis, CV and DPV voltammograms, AFM images, I−V curves of P1−P5 and P7−P8 (roll-coated from ODCB), XRD patterns and 1H NMR spectra of the intermediates, monomers, and polymers. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00589.

The cells were placed under the solar simulator (1.2 sun) and kept at a temperature of about 130 °C for 28 days, recording PCE every 5 min. P3HT had a clear, exponential decay, after an initial increase in PCE. P5 and P7 experienced an initial, dramatic efficiency loss. This loss is limited in the case of P8. P5 clearly showed a low stability, even though still better than P3HT. P7, apart from the initial loss, was extremely stable, and its efficiency remained constant, even after 28 days. This is a considerable result, and despite its lower absolute performance, it demonstrates that P7 is a stable and efficient low band gap polymer for PSC. The degradation behavior was caused by a concomitant drop of the Voc, Isc, and, in more pronounced, FF. The above conditions were chosen in order to prove the impressive stability of P7. P8 was not as stable as P7, since a decrease in terms of efficiency was observed. However, P8 can still be considered a stable material. It is impressive how the presence of fluorine in P7 or two thiophene rings carrying a hexyl side chain in P8 could influence the lifetime of these materials. The lifetime study suggested that P7 and P8 may be of more interest than P5 for a large-scale PSC production.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (E.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from The Villum Foundation Young Investigator Program (project “Materials for Energy Production”). Partial financial support was also received from the Danish National Research Foundation (project “Danish Chinese Center for organic based solar cells with morphological control”). J


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Macromolecules

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