Photoactive DonorâAcceptor Composite ... - ACS Publications

Subscriber access provided by Fudan University

Article

Photoactive donor-acceptor composite nanoparticles dispersed in water Laurie Parrenin, Gildas Laurans, Eleni Pavlopoulou, Guillaume Fleury, Gilles Pecastaings, Cyril Brochon, Laurence Vignau, Georges Hadziioannou, and Eric Cloutet Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04496 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Photoactive donor-acceptor composite nanoparticles dispersed in water Laurie Parrenin,a,b,c Gildas Laurans,d,e Eleni Pavlopoulou,a,b,c Guillaume Fleury,a,b,c, Gilles Pecastaings,a,b,cCyril Brochon,a,b,c, Laurence Vignau,d,e Georges Hadziioannou,a,b,c and Eric Clouteta,b,c* a. Centre National de la Recherche Scientifique (CNRS), Laboratoire de Chimie des Polymères Organiques (LCPO), UMR 5629, Allée Geoffroy Saint Hilaire Bât B8, Pessac Cedex, F-33607, France b. Université de Bordeaux, Laboratoire de Chimie des Polymères Organiques (LCPO), UMR 5629, Allée Geoffroy Saint Hilaire Bât B8, Pessac Cedex, F-33607, France c. Institut Polytechnique de Bordeaux (INP), Laboratoire de Chimie des Polymères Organiques (LCPO), UMR 5629, Allée Geoffroy Saint Hilaire Bât B8, Pessac Cedex, F-33607, France d. Bordeaux INP, Laboratoire IMS, Ecole Nationale Supérieure de Chimie, Biologie et Physique, 16 Av. Pey Berland, 33607 Pessac, France e. Université de Bordeaux, Laboratoire IMS, Ecole Nationale Supérieure de Chimie, Biologie et Physique, 16 Av. Pey Berland, 33607 Pessac, France

KEYWORDS: PC71BM, PCDTBT, Nanoparticles, post-polymerization miniemulsification, Organic solar cells.

ACS Paragon Plus Environment

1

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 37

ABSTRACT: A major issue that inhibits large-scale fabrication of organic solar modules is the use of chlorinated solvents considered as toxic and hazardous. In this work, composite particles of

poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2’,1’,3’-benzothiadiazole]

(PCDTBT) and [6,6]-Phenyl C71 butyric acid methyl ester (PC71BM) were obtained in water from a versatile and a ready-to-market methodology based on post-polymerization miniemulsification. Depending on experimental conditions, size-controlled particles comprising both the electron-donor and the electron-acceptor were obtained and characterized using transmission electron microscopy (TEM), atomic force microscopic (AFM), small-angle neutron scattering (SANS), UV-visible absorption and fluorescence spectroscopies. Intimate mixing of the two components was definitely asserted through PCDTBT fluorescence quenching in the composite nanoparticles. The water-based inks were used for the preparation of photovoltaic active layers which were subsequently integrated in organic solar cells.

Introduction Due to environmental issues, clean and affordable alternatives to fossil fuels are becoming crucial. Harvesting the sun power by solar modules promises an inexhaustible energy source of lower ecological impact. The use of organic semiconductors and more particularly π-conjugated polymers into the active layer of organic photovoltaic solar cells (OPV) can be advantageous for nomadic applications requiring low cost, flexibility and lightweight.1 While the p-type polymer poly-(3-hexylthiophene) (P3HT) has been the OPV workhorse for more than 20 years, its limited absorption (i.e. only 20 % of the solar spectrum) has prompted the scientific community to focus on low band gap polymers.2 One way to create a low band gap polymer is to synthesize a


2

Page 3 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(co)polymer by alternating poor and rich electron subunits, thus lowering and raising the LUMO and HOMO levels, respectively. As a consequence, the solar energy is better harnessed leading to higher power conversion efficiency (PCE).3,4 Another challenge for the OPV industry is related to processing. Indeed, it is worth stressing that most of the π-conjugated polymers need to be processed using toxic organic solvents, mainly from the chlorinated family.5 To address this challenge, the solubility of these polymers can be enhanced either by introducing flexible side chains (which can detrimentally affect the electronic properties)6–8 or by synthesizing donor-acceptor (co)polymer particles dispersed in an environment-friendly phase (e.g. aqueous or oily non-chlorinated media). The second route is the main focus of this work. Beyond the interest in developing eco-friendly photoactive polymer inks it is also of paramount importance to design photoactive particles composed of both electron-donor and electron-acceptor species. Indeed the particle size should be tailored to comply with the domain size required for efficient exciton dissociation. Among the existing methodologies, emulsion and dispersion techniques allow a good control of the particle size in eco-friendly dispersed media. Different approaches have been considered to make (nano)particles (NPs) of π-conjugated polymers. For instance, polypyrrole,9 polyaniline10 and polythiophene derivatives such as poly(3,4-ethylenedioxythiophene) (PEDOT)11 have been synthesized by oxidative dispersion polymerization in dispersed media in water or alcohol. More recently, Kuehne et al12,13 proposed a new method for the preparation of polyfluorene and polythiophene nanoparticles in alcohol by dispersion polymerization. Based on their methodology we have previously reported the preparation of (poly[N- 9’-heptadecanyl-2,7carbazole-alt-5,5-(4,7-di-2-thienyl-2’,1’,3’-benzothiadiazole]) (PCDTBT) particles in alcohol.


3

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 37

Unfortunately low PCE was obtained after integration of these particles in organic solar cells due to the low PCDTBT molecular weight.14 Apart from dispersion polymerization, miniemulsion polymerization was reported by Baier et al15 in 2009 using a Glaser coupling for the formation of poly(arylene diethynylene) particles or by Hittinger et al16 in 2004 using a Sonogashira coupling for the formation of poly(p-phenylene ethynylene) particles. More recently, Li et al17 adopted a similar strategy to produce polymer nanoparticles via Suzuki cross-coupling. In all these examples, the polymers were synthesized in a bi-phasic system (e.g. toluene-in-water) using surfactants to obtain stable NPs dispersions. Furthermore, NPs of π-conjugated polymers can also be obtained by the so-called nanoprecipitation18,19 or post-polymerization emulsion20,21 techniques. Few studies deal with the nanoprecipitation methodology for photovoltaic applications since only a low solid content can be achieved. On the other hand, post-polymerization miniemulsification appears to be a promising route for the formation of NPs dedicated to organic photovoltaic, as highlighted by several research groups during the last years. Pioneer works of Kietzke et al.22 reported the postpolymerization

miniemulsification

of

poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-

(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) as an electron-acceptor and poly(9,9-dioctylfluoreneco-N,N'-bis(4-butylphenyl)-N,N'-diphenyl-1,4-phenylenediamine) (PFB) as an electron-donor. Integration of these particles in devices results in an external quantum efficiency (EQE) up to 1.5 % while the PCE could not be measured. Andersen et al.5 applied the same methodology to form poly[4,8-bis(2-ethylhexyloxy)benzo(1,2-b:4,5-b’)dithiophene-alt-5,6-bis(octyloxy)-4,7di(thiophen-2-yl)(2,1,3-benzothiadiazole)-5,5’-diyl],

poly[(4,4’-bis(2-ethylhexyl)dithieno[3,2-

b:2’,3’-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl]

and

poly[2,3-bis-(3-

octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] particles blended with PCBM,


4

Page 5 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

subsequently integrated into devices by roll-to-roll technique to reach a maximum PCE of 0.55 %. As today, the best performance using particles formed by post-polymerization miniemulsification was reported by Ulum et al.23 with a PCE of 2.5 % for a P3HT / Indene-C60 bisadduct (ICBA) system. Among the plethora of low band gap polymers, PCDTBT24 was selected in this work as its blend with PC71BM has been reported to show good photovoltaic performance with PCE up to 7.2%.25 Another advantage of this material is its relatively high glass transition temperature (Tg ~ 130 °C) which is a strong asset for operations at high temperature. Indeed, a low glass transition temperature can lead to macroscopic phase separation and, thus, decrease the solar cells’efficiency.26,27 Despite being one of the most performing low band gap polymer, no study of their processing into nanoparticles has appeared in literature so far, nor the integration of such nanoparticles in OPVs. In the present study, composite NPs of PC71BM and PCDTBT have been obtained via miniemulsification post-polymerization in water. The nanoparticles have been characterized by various techniques such as transmission electron microscopy (TEM), atomic force microscopy (AFM), small-angle neutron scattering (SANS), UV-visible absorption and fluorescence spectroscopies. Finally, the nanoparticle-containing inks were integrated into solar cells and their photovoltaic performance has been evaluated.

Experimental Part Materials. 9-(9-Heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester (97 %), 4,7-Bis(2bromo-5-thienyl)-2,1,3-benzothiadiazole (99+ %), Zinc acetate dihydrate (Zn(CH3COO)2,2H2O)


5

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 37

(99.9+ %), ethanolamine (NH2CH2CH2OH) (99 %), absolute ethanol (99.8 %), chloroform (99.5+ %) and deuterated sodium dodecyl sulfate (d-SDS) were purchased from Sigma Aldrich. Sodium Dodecyl Sulfate was purchased from Acros Organics (99 %). PC71BM was purchased from Solaris (99+ %). All chemicals were used as received unless otherwise stated. All the reactions were performed under an Argon atmosphere using standard schlenk techniques.

Synthesis Purification of the monomers The two monomers were purified as reported in the literature:28 9-(9-Heptadecanyl)-9Hcarbazole-2,7-diboronic acid bis(pinacol) ester was recrystallized twice in a mixture of methanol/ acetone (10:1) and 4,7-Bis(2-bromo-5-thienyl)-2,1,3-benzothiadiazole was recrystallized into odichlorobenzene.

Typical procedure for the polymerization of 9-(9-Heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester and 4,7-Bis(2-bromo-5-thienyl)-2,1,3-benzothiadiazole in bulk. PCDTBT was synthetized according to literature.28 In a flame-dried flask, 654.4 mg (0.966 mmol)

of

2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-N-9-heptadecanyl-carbazole,

458.8 mg (1.00 mmol) of 4,7-di(2′-bromothien-5′-yl)-2,1,3-benzothiadiazole, 4.2 mg (0.0046 mmol) of tris(dibenzylideneacetone)dipalladium(0) and 6.1 mg (0.020 mmol) of tri(otolyl)phosphine were dissolved in 10 mL of degassed toluene and 3.4 mL of degassed 20 % aqueous tetraethylammonium hydroxide. The reaction mixture was vigorously stirred and heated up to 95 °C at a heating rate of 1 °C per minute. After 3 h, bromobenzene (110 µL, 1.0 mmol) was added to the reaction which was kept at 95 °C for one hour. Then phenylboronic acid (120


6

Page 7 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

mg, 1.0 mmol) in hot toluene (3 mL) was added and reacted for another hour to allow the endcapping. The reaction mixture was poured in methanol / water (10:1) solution and the polymer was filtered. The crude material was washed with acetone and heptane using a Soxhlet during one day for each solvent. The remaining solid was extracted with chloroform (300 mL) for several days. Then the solvent was reduced using a rota-evaporator to about 50 mL and the polymer solution was precipitated in cold methanol (500 mL). The polymer was recovered by filtration and dried under vacuum to yield number average molecular weight (Mn) of 22000 g.mol-1 (as regards to PS standards) with a dispersity (Đ) of 2 as determined by size exclusion chromatography at high temperature in trichlorobenzene (see Figure S1 and Table S2 in Supporting Information).

Typical procedure for the formation of PC71BM/PCDTBT nanoparticles by postpolymerization miniemulsification The NPs were synthesized using a modified mini-emulsion method.29 PCDTBT and PC71BM were solubilized in 1 mL of CHCl3 (for NPs composed of only one of the component, the procedure is the same, starting with either PCDTBT or PC71BM solubilized in CHCl3). The CHCl3 solution was then introduced in a SDS aqueous solution (50 mg of SDS into 5 mL of MQwater) followed by sonication during 2 min at 150 W to form the miniemulsion. The samples were then heated to 70 °C for 40 min to allow the CHCl3 evaporation. In order to concentrate the sample and remove the excess of surfactant, the solution was then dialyzed with ultra-centrifuge dialysis tubes purchased from Sartorium (vivaspin20 - 10 kDa MWCO). The solution was placed into the dialysis tube and centrifuged at 4000 rpm for 7 min. The filtrate was then discarded and the remaining solution diluted with 5 mL of MQ-water.


7

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 37

The quantity of remaining SDS in the continuous phase was probed by surface tension measurements. The solution was washed until the surface tension reached at least 65 mN/m (at this point, the continuous phase contains less than 0.2 g/L of SDS, see Figure S3). The quantity of SDS stabilizing the particles was assessed by thermogravimetric analysis and represents around 5 wt% of the NPs (see Figure S4).

Characterization methods. Dynamic light scattering (DLS). DLS on diluted dispersion samples was performed on a Cordouan Particle Size Analyzer (Vasco). The autocorrelation function was analyzed using the Cordouan dispersion technology software algorithm to obtain number-weighted particle sizes. Size exclusion chromatography (SEC). SEC was performed in TCB at 150 °C with a flow rate of 1 mL.min-1 using a Agilent PL gel MIXED C column (pore size 5 µm) and a Agilent PL gel guard-column. The elution times were converted into molecular weights using a calibration curve based on low dispersity polystyrene standards. Thermogravimetric analysis (TGA). TGA measurements were carried out with TA Instruments Q50 from room temperature to 800 °C with a heating rate of 10 °C.min-1. Measurements were performed under a nitrogen atmosphere. UV-Visible and fluorescence spectroscopies. Absorption and emission spectra were acquired using a Shimadzu spectrophotometer UV-3600 and a Horiba Scientific Fluoromax-4 spectrofluorometer, respectively. Transmission electron microscopy (TEM). TEM images were obtained using accelerate voltage of 120 kV. Dispersed nanoparticles were cleaned by several centrifugation cycles before TEM analysis and deposited onto Formvar® grids. ForTEM analysis of the active layer, NPs were


8

Page 9 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

deposited on a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) film followed by a thermal annealing and then dipped into water in order to dissolve the underneath PEDOT:PSS layer. The active layer was then left to float and transferred onto the TEM grid. Zeta potential measurements. A Zetasizer Nano ZS from Malvern was used to determined the NPs Zeta potential using the Henry equation. Surface tensions and Surface energy measurements. Surface tensions were measured using the drop shape analysis technique on a pendant drop tensiometer (Krüss DSA-100) while the surface energies were determined by measuring contact angle between three solvents (water, diiodomethane and ethylene glycol) and the NPs film. The surface energies were calculated using Owens-Wendt model.30 Atomic force microscopy. A AFM Dimension FastScan (Bruker) was used in tapping mode to characterize the surface morphology of the samples. Silicon cantilevers (Fastscan-A) with a typical tip radius of 5 nm, a spring constant of 18 N m−1 and a cantilever resonance frequency of about 1.4 MHz were used. Small-angle neutron scattering. Dispersions were prepared with the typical procedure described above using d-SDS as surfactant. Samples were prepared by diluting the stock dispersion into solutions of varying H2O/D2O content, the final concentration being 2 wt%. Small-angle neutron scattering experiments were performed on the PAXY spectrometer of the Laboratoire Léon Brillouin (CEA-Saclay, France). Samples were measured using two different configurations: the first one with a sample-to-detector distance of D = 5 m and a neutron wavelength of λ = 8.5 Å to cover a scattering vector, q, range of 2.5×10-3 - 2.5×10-2 Å-1; the second one with D = 3 m and λ = 6 Å to cover a q range of 2×10-2 - 0.2 Å-1. The counting time at each position was 3600 s for the low q range and 1200 s for the high q range. Sample


9

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 37

transmission was measured at the largest sample-to-detector distance for each wavelength, and beam flux measurements were used to convert the scattering profiles to an absolute scale. Devices fabrication. Inverted solar cells have been fabricated with the following structure: Glass/ITO/ZnO/active

layer/MoO3/Ag,

the

active

layer

being

composed

of

either

PC71BM:PCDTBT NPs or PC71BM:PCDTBT in bulk hetero junction (BHJ). The Indium tin oxide (ITO)-coated glass substrates, purchased from Vision Tek, with a sheet resistance of ∼10 Ω.sq-1 were successively cleaned for 10 min in acetone, ethanol and isopropanol in an ultrasonic bath. The zinc oxide (ZnO) layer was spin-coated at 2000 rpm for 60 s followed by an annealing treatment at 180 °C for 1 hour in air leading to a 60 nm thick layer. The PC71BM:PCDTBT solutions (for BHJ) were prepared in chloroform with a concentration of 40 mg.mL-1 (1:4 weight ratio). The BHJ layers were spin-coated in a glove box at 2000 rpm during 60 s with a resulting PC71BM:PCDTBT thickness of 80 nm. Note that we opted for this thickness since this is reported to be the best-performing thickness for the active layers of the respective BHJ OPVs, as reported in literature.31 The NP layers were spin-coated in air at 600 rpm during 60 s followed by a 3000 rpm step for 3 s leading to a 80 nm thick layer. Once more, this thickness was targeted in order to allow for direct comparison with the reference BHJ devices. The films were dried at 100 °C for 2 min to evaporate the remaining water and then transferred into a vacuum chamber for the MoO3/Ag evaporation. MoO3 (10 nm) and Ag (80 nm) were successively thermally-evaporated at deposition rates of 0.1 nm.s-1 and 0.2-0.4 nm.s-1 respectively under secondary vacuum (10-6 mbar). The solar cells’


10

Page 11 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

active surface area is 10 mm2. The devices were characterized using a K.H.S. Solar Cell test-575 solar simulator with AM1.5G filters set at 100 mW.cm-2 with a calibrated radiometer (IL 1400BL). Labview controlled Keithley 2400 SMU enabled the current density-voltage (J-V) curves measurements. Results and discussion Synthesis of composite PC71BM:PCDTBT nanoparticles with tunable size. Particles of PC71BM:PCDTBT (80:20 NPs) with a tunable diameter (from 30 to 330 nm) were obtained via post-polymerization miniemulsification in water from a chloroform solution according to the procedure described in the experimental section. For this study, the ratio of PC71BM over PCDTBT was set at 80:20 wt% as the best photovoltaic results in the bulk heterojunction configuration were obtained for this weight fraction.32,33 As shown in Figure 1 adjustment of the NPs size could be obtained by varying the experimental parameters (see Table 1). The concentration of the active materials (entries 1, 2 and 3, Figure 1 TEM images 1, 2 and 3) has a direct impact on the size of the particles. By increasing the concentration of the stock solution from 5 mg.mL-1 to 50 mg.mL-1, the NPs size increased from 30 ± 20 nm to 330 ± 90 nm. This behavior was already reported by Landfester et al. for methyl substituted ladder-type poly(p-phenylene) (Me-LPPP) particles20 and Pras et al. for 2,7poly(9,9-dialkylfluorene-co-fluorenone) (PFFO) particles.34 The sonication time is another parameter allowing us to control the particle size.34 For instance, increasing sonication time results in smaller NPs (from 140 ± 30 nm for 2 min to 52 ± 10 nm for 5 min) (Table 1 entries 2 and 4, Figure 1 TEM images 2 and 4).


11

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 37

The sonication amplitude plays also a major role as already demonstrated by Pras et al. for PFFO particles;34 increasing the intensity from 150 W to 225 W results in a decrease of the NPs size from 140 ± 30 nm to 45 ± 5 nm (Table 1 entries 2 and 6, Figure 1 TEM images 2 and 6). Finally, the NPs size can be tuned by varying the amount of surfactant. In fact, increasing the amount of SDS leads to a higher surface coverage propensity, and thus to smaller particles (Table 1 entries 2 and 5, Figure 1 TEM images 2 and 5). However, the amount of surfactant has to be tuned for photovoltaic applications as such materials are insulating and could negatively impact the photovoltaic performance. As shown in Table 1, the NPs size dispersity drastically increases with their diameter which seems inherent to the miniemulsification process as higher concentration effects lead to coalescence phenomenon. Nonetheless, such dispersity could be beneficial for the application, since the small particles allow the production of more uniform films by filling the gaps between the bigger ones during the active layer deposition.35

Table 1. The size of the 80:20 NPs for various experimental conditions

Entry

[PC71BM:PCDTBT] in CHCl3 (mg/mL)

Time of sonication (min)

Probe intensity (W)

[SDS] in water (mg/mL)

Diameter (nm)*

1

5

2

150

10

30 ± 20

2

30

2

150

10

140 ± 30

3

50

2

150

10

330 ± 90

4

30

5

150

10

52 ± 10

5

30

2

150

30

55 ± 20

6

30

2

225

10

45 ± 5

*Determined by TEM analysis


12

Page 13 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 1. TEM images of the 80:20 NPs. The index at the bottom right corresponds to the entry number in Table 1.

A key feature of such NPs dispersions for further development concerns the ink stability until the final device fabrication. The dispersion stability was assessed using the zeta potential which was determined by measuring the electrophoretic mobility of the colloidal particles in solution upon voltage application. As the zeta potential depends on the magnitude of the electrostatic interactions between the particles, it is often used as a measure of the colloidal stability. The zeta potential of the composite particles was found around -58 ± 15 mV which allows us to consider our inks as stable; the repulsion forces being high enough to prevent aggregation upon ageing.36 This result was confirmed by DLS measurements since no agglomeration of the particles were noticed over 6 months (see Figure S5).


13

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 37

Note that the study presented in the rest of this paper focuses on the particles produced employing the conditions reported in the entry 6 in Table 1.

Optical characterization The optical properties of PC71BM, PCDTBT as well as the composite NPs were studied by UV-visible absorption and fluorescence spectroscopies. The absorption spectrum of the 80:20 NPs dispersed in water is presented in Figure 2a (solid line). The major peaks at 375 nm and 468 nm correspond to the absorption peaks of PC71BM, while the two “shoulders” at 392 nm and 548 nm result from the weak contribution of PCDTBT (only 20 wt% in the blend). A small discrepancy is observed between the 80:20 NPs spectrum and the one obtained from a mixture of 80 wt% PC71BM particles and 20 wt% PCDTBT particles (open circles in Figure 2a). Although the absorption peaks of the two components are apparent, their relative intensities are modified. This suggests that the two components are in intimate contact within the particles. To further support this outcome, we calculated the absorption spectrum that corresponds to a blend of PC71BM and PCDTBT particles, by adding the spectra of the pure particles in the same medium, taking into account the relative weight fractions (dotted line in Figure 2a, presented shifted with negative offset in order to allow for easier comparison). As expected, this spectrum is almost identical to the experimentally measured for the blend of particles, demonstrating that the PC71BM and the PCDTBT particles do not interact in this case. Figure 2b presents the fluorescence data recorded for the pure PCDTBT and PC71BM particles dispersed in water. An excitation wavelength of 395 nm was used, corresponding to the maximum absorption of PCDTBT. A broad emission peak is observed for PCDTBT at 700 nm, while PC71BM does not emit in this spectral range when excited at 395 nm. For the composite


14

Page 15 of 37

NPs the fluorescence is strongly quenched (open circles in Figure 2b). This result proves that the two components are in intimate contact within one single particle. To further support this statement, the spectrum acquired for mixture of 80 wt% PC71BM particles and 20 wt% PCDTBT particles is presented in the same figure (grey circles). As expected no quenching is observed since the two components coexist as separated particles. Additionally, the quenching of the emission of PCDTBT observed in the case of the composite NPs shows that an electron transfer takes place between the donor and the acceptor. This is a pre-requisite for an efficient OPV cells37 and demonstrates that the dispersion of the composite NPs in water can be considered as a photoactive ink.

Normalized Absorbance

1.2

a

1.0 0.8 0.6 0.4 0.2

Particles dispersed in H2O Composite particles Blend of donor and acceptor particles (experimental) Blend of donor and acceptor particles (calculated)

0.0 300

350

400

450

500

550

600

650

Wavelength (nm) 7 6 5

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

particles in water pure PCDTBT Composite 80:20 Mix 80:20 pure PC71BM

b

4 3 2 1 0 550

600

650

700

750

800

850

Wavelength (nm)

Figure 2. (a) Absorption spectra of the composite 80:20 NPs dispersed in water (solid line) and of the mixture of 80 wt% of PC71BM particles and 20 wt% of PCDTBT particles (white circles)


15

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 37

dispersed in water. The dot line corresponds to the calculated absorption spectrum for a blend of 80 wt% PC71BM particles and 20 wt% PCDTBT particles and is presented shifted in order to allow for easier comparison. (b) Emission spectra recorded for the pure PCDTBT particles (black circles), the pure PC71BM particles (solid line), the composite 80:20 NPs (white circles) as well as a mixture of 80 wt% PC71BM particles and 20 wt% PCDTBT particles (grey circles). All particles were dispersed in water. The excitation wavelength was set at 395 nm. Nanoparticle morphology The NPs synthesized using the same methodology show a core-shell structure38,39 where the higher surface energy compound segregates in the core of the particles.40 Such segregation behavior was explained by considering the difference of surface energies between the two components. Following this approach, the surface energies of pure PCDTBT and PC71BM films were measured at 37 mN.m-1 and 48 mN.m-1, respectively (see Table S6). As PCDTBT shows the lower surface energy, PCDTBT is expected to migrate at the external surface of the droplets during the NPs formation, thus forming the shell of the composite NPs. PC71BM:PCDTBT NPs were further characterized by contrast variation Small Angle Neutron Scattering (SANS) in order to gain more insights on the distribution of the two materials within the nanoparticles. Following the methodology reported by Richards et al. for P3HT based particles,41 80:20 NPs were dispersed into different H2O/D2O mixtures to probe their internal structure and spatial heterogeneity.42 Scattering profiles are presented in Figure 3 for 4 different solvent contrasts and the radius of gyration, Rg, was retrieved using Guinier analysis. The calculated apparent radius of gyration is dependent on the contrast with Rg ranging from 12.3 to 14.0 nm (see Table 2), underlining the heterogeneous spatial distribution of the components in the NPs (Rg would be independent of the scattering contrast for particles with a homogenous


16

Page 17 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

scattering length density). Stuhrmann plot was subsequently used in order to precisely characterize the internal heterogeneity of the NPs.43 Following this analysis, the quadratic dependence of Rg on ∆ρ (∆ρ = ρNP -ρS with ρNP and ρS the averaged NPs and solvent scattering length densities, respectively) can be expressed according to Equation 1.42 ܴ௚ଶ = ܴ௖ଶ +

α ௱ρ

−

β ௱ρమ

(1)

In which Rc is the radius of gyration of homogenous particles, α is the relative distribution of scattering length density radially from the particle center of mass and β is the distance of the center of mass of the particle to the center of mass of its heterogeneous components.41 As reported by Richards et al., the values of α and β allow to discriminate NPs internal heterogeneity; i.e. α and β being sensitive to the radial distribution of scattering length density and the NPs axisymmetry, respectively.41 Table 2. The radius of gyration of 80:20 NPs as a function of the H2O/D2O ratio. Fitting parameters α, β and Rc retrieved from Equation 1. H2O/D2O ratio

Rg (nm)

100/0

14.0 ± 3.1

85/15

13.2 ± 3.1

70/30

12.3 ± 2.8

55/45

13.2 ± 4.5

α

β (Å-2)

Rc (nm)

-0.047 ± 0.002

-3.9×10-8 ± 0.2×10-8

16.8 ± 2.3


17

Langmuir

10

I(q)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 37

0.1

100% H2O 85% H2O 70% H2O 55% H2O

1E-3 0.01

0.1 -1

q (Å )

Figure 3. SANS profiles of 80:20 NPs measured at four different H2O/D2O contrasts.

A uniform volume distribution of PC71BM and PCDTBT in NPs corresponds to zero α and β, while an enrichment of the lower contrast PCDTBT at the NPs surface would lead to a negative α value (assuming the SDS shell contribution is negligible due to the low SDS amount incorporated at the NPs surface). A non-zero β value is related to different spatial positions of the centers of mass and scattering length density in the NPs. Based on the fitting parameters obtained from the Stuhrmann plot, the dependence of Rg² on 1/∆ρ shows that the NPs present a non-axisymmetric distribution of the two components as a non-zero β value of -3.9×10-8 Å-2 is retrieved. The negative α value (α = -0.047) indicates that PCDTBT is preferentially segregated at the NPs surface in accordance with surface energy consideration. This preferential PCDTBT segregation at the surface of the NPs is also coherent with AFM observation on larger particles which exhibit a majority of PCDTBT at the surface (high phase contrast) with patches of PC71BM (low phase contrast) in Figure S7.

Film formation from inks of composite particles


18

Page 19 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

The photoactive inks composed of 80:20 NPs dispersed in water were used to form thin films by spin-coating on glass substrates. After deposition, the films were heated for 2 min at 100 °C in order to facilitate the water evaporation leading to a 80 nm thick layer. Thin film morphology was characterized by AFM and TEM. In Figure 4 AFM images recorded for the composite films after spin-coating are presented. A layer of closely packed particles is observed, showing that the smaller particles appear to fill in the voids between the larger particles. Yet, an important film roughness of 12 nm was retrieved as the film surface follows the particle shape with no apparent sintering. This issue could be detrimental for the application and will be discussed in the following section.

Figure 4. AFM topographic (left) and phase (right) images of the film made out of the composite 80:20 NPs.

Thermal annealing of PC71BM and PCDTBT composites films Even though thermal annealing leads to efficiency losses in photovoltaic devices incorporating PCDTBT:PC71BM BHJ,31,44 a thermal annealing step has been shown to be beneficial for the


19

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

formation of an active layer from NPs. Indeed, it allows a sintering of the NPs as well as the creation of more interfaces between the p- and n-components.27,45–48 A thermal treatment above the glass transition temperature of the materials is also expected to decrease the roughness induced by the NPs spin coating process (see Figure 4), thus limiting the occurrence of short circuits after the top electrode deposition. AFM and TEM characterizations were used to study the effect of thermal annealing at 130 °C and 140 °C (see Figure 5). Annealing treatment at higher temperatures leads to macro-phase separation which is detrimental for efficient exciton dissociation (Figure S8). After annealing at 130 °C, individual particles are still visible even if the beginning of a sintering process is apparent both in AFM and TEM images. Smoother and more homogeneous film surfaces (roughness of 4 nm) were retrieved after an annealing treatment at 140 °C. Both types of structural features (i.e. 130 °C and 140 °C) will be examined as regards to the photovoltaic properties.

Figure 5. (a-b) AFM and TEM images of 80:20 NPs after a 4 min thermal annealing at 130 °C and (c-d) at 140 °C. Integration of PC71BM:PCDTBT composites inks into solar cells


20

Page 21 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

The photoactive inks of PC71BM:PCDTBT NPs dispersed in water were finally integrated into solar cells using an inverted architecture as such devices are air-stable.49 The configuration used was based on a glass/ITO/ZnO/PC71BM:PCDTBT(NPs)/MoO3/Ag stack. After spin-coating, all active layers were dried at 100 °C during 2 min to evaporate traces of residual solvent while annealing at 130 or 140 °C for 4 min was performed for selected devices prior to thermal evaporation of MoO3 and Ag. A diode behavior exhibiting a photovoltaic effect was recorded in all cases, even for the very low-performing devices. As a comparison, we also prepared devices based on PC71BM:PCDTBT blends in chloroform which correspond to the normal BHJ architecture reported in literature.50,51 Note that the highperforming PCDTBT-based OPVs are usually prepared from o-dichlorobenzene or chlorobenzene solutions, while in this case the low-boiling point solvent chloroform was used, in order to mimic the fabrication processes followed for the nanoparticle-based devices. Table 3 summarizes the results obtained for the devices that incorporate three kinds of NPs: the 80:20 and the 60:40 composites, as well as the mixture of 80 wt% of PC71BM NPs and 20 wt% of PCDTBT NPs that were previously dispersed in water. When the composite 80:20 NPs are used, thermal annealing leads to an increase of the PCE by an order of magnitude (see Table 3 samples 1, 2 and 3) as opposed to BHJ-based solar cells (see Table 3 samples 10, 11 and 12). In the case of the BHJ devices upon annealing, the open-circuit voltage and the short-circuit current decreased and, consequently the efficiency decreased too. This implies that the NP structure may provide a more stable, from a thermodynamic point of view, alternative to the standard BHJ structure, as has been already reported by Feron et al39 for P3HT-based devices.


21

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

Based on the AFM characterization, this PCE enhancement for the NPs-based devices was attributed to the more pronounced sintering between the individual NPs after annealing at 130 °C and 140 °C and an improved intermixing of the donor and the acceptor materials. The PCE increase could be also attributed to a decrease of the SDS concentration at the surface of the film. Indeed, after annealing the surface energy of the NPs film decreased from 69 mN.m-1 to 38 mN.m-1 (see Table S9) which is compatible with a migration of the SDS molecules from the surface to the bulk of the film as already reported in literature.52 In fact, the increase of PCE upon annealing results from the concurrent increase of the shortcircuit current density (Jsc) and the open circuit voltage (Voc). Since overpassing the Tg of the polymer results in the merging of the particles and the formation of a more homogeneous film, it follows that a better connectivity between the PCDTBT and PC71BM domains is established. Actually, increasing temperature improves inter-diffusion of the two components and, thus, the number of interfaces between the donor and the acceptor increases. Consequently, exciton dissociation is enhanced and the Jsc is increased. On the other hand, the Voc increases from 0.06 V for the non-annealed active layers, to 0.3 V for those annealed at 130 °C, to 0.6 V for those annealed at 140 °C, approaching gradually the Voc value (Voc=0.86 V) reported for PCDTBT:PC71BM BHJ solar cells.51 We conclude that the morphological changes induced by annealing have a direct impact also on the Voc, probably due to the better interfaces between the donor and the acceptor induced by the homogenization of the film. For the three annealing procedures examined herein, the fill factor is practically the same, around 0.3. It is noteworthy that no device engineering has been performed to improve performance, since this work focuses on the material used to form the active layer.


22

Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

As far as the solar cells prepared with the composite 60:40 NPs are concerned, the thermal annealing has the same effect of increasing Jsc, Voc and consequently PCE (see Table 3 samples 4, 5 and 6). However, the efficiencies are lower compared to those obtained for the composite 80:20 NPs. The composition seems to have the same impact on the NP-based solar cells as on the BHJ-based, for which the 80:20 composition led to the best results.32,33 For the active layer formed by mixing 80 wt% PC71BM NPs and 20 wt% PCDTBT NPs, the efficiencies obtained are very low (see Table 3 samples 7, 8 and 9) even after annealing. Indeed, the mixture of particles led to individual p and n domains around 50 nm (equal to the size of the NPs) which is well above the exciton diffusion length and underlines the lack of connectivity between p- and n- domains. Only composite particles led to a measurable efficiency. Blended particles have much better efficiencies than mixed ones, in agreement with what has already been demonstrated in the literature for PFB:F8BT nanoparticles. 40 As a consequence, the best results using water-based NPs inks were obtained using the 80:20 NPs annealed at 140°C for 4 min as such treatment leads to the most favorable thin film structure. The lower performances obtained for the NP-based devices compared to the BHJ ones are probably due to the presence of the insulating and charged stabilizer (SDS). The sulfate group and its counter ion may inhibit charge transport acting as a trap and decreasing the charge mobility as reported in the literature.52 The use of a neutral surfactant could probably reduce charge trapping, however this kind of surfactant would provide only stabilization by steric hindrance and not electrosteric, with detrimental effects to particles stability. A compromise should be made, therefore we opted to use the ionic surfactants. In order to minimize undesirable effects the excess of SDS in the inks was removed after particles formation by dialysis. An


23

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

alternative that could eliminate the use of stabilizers or surfactants would be the use of the nanoprecipitation technique for the particles formation, however it is known to result in very low concentrations. 18,19 Table 3. Characteristics of the solar cells with different NPs nature (composite or mixture) and different ratio of PC71BM:PCDTBT, BHJ as a comparison, c stands for composite NP and m for mixed NP, the temperatures correspond to the annealing during 4 min.

Sample

Annealing temperature (°C)

Jsc (mA/cm)

Voc (V)

FF

PCE (%)

1

C 80:20 NP

/

1.52 (±0.75)

6.19E-02 (± 0.81E-02)

0.277 (± 0.026)

0.03 (± 0.02)

2

C (80:20) NP

130

2.56 (± 0.89)

3.14E-01 (± 0.52E-01)

0.296 (± 0.014)

0.32 (± 0.10)

3

C (80:20) NP

140

3.79 (± 0.70)

6.05E-01 (± 0.82E-01)

0.307 (± 0.003)

0.70 (± 0.24)

4

C (60:40) NPs

/

Photoactive DonorâAcceptor Composite ... - ACS Publications

Recommend Documents

Photoactive DonorâAcceptor Composite ... - ACS Publications