Designing Novel Poly(oxyalkylene)-Segmented Ester-Based

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Applications of Polymer, Composite, and Coating Materials

Designing Novel Poly(oxyalkylene)-segmented Ester based Polymeric Dispersants for Efficient TiO2 Photoanodes of Dye-sensitized Solar Cells Yow-An Leu, Yen-An Lu, Min-Hsin Yeh, Po-Ta Shih, Sheng-Yen Shen, Kuo-Chuan Ho, and Jiang-Jen Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09852 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 14, 2018

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ACS Applied Materials & Interfaces

Designing Novel Poly(oxyalkylene)-segmented Ester based Polymeric Dispersants for Efficient TiO2 Photoanodes of Dye-sensitized Solar Cells Yow-An Leua,b§, Yen-An Lub,c§, Min-Hsin Yehd*, Po-Ta Shiha, Sheng-Yen Shena, Kuo-Chuan Hoa,b,c*, and Jiang-Jen Lina,c* a

Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617,

Taiwan b

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan

c

Advanced Research Center for Green Materials Science and Technology, National Taiwan

University, Taipei 10617, Taiwan d

Department of Chemical Engineering, National Taiwan University of Science and Technology,

Taipei 10607, Taiwan

KEYWORDS: dye-sensitized solar cells, photoanode, polymeric dispersants, poly(oxyalkylene), titanium oxide nanoparticle.

1 Environment ACS Paragon Plus

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ABSTRACT A family of new polymeric dispersants, branched poly(oxyethylene)-segmented esters of trimellitic anhydride adduct (polyethylene glycol-trimethylolpropane-trimellitic anhydride, designated as PTT), were synthesized and used to homogeneously disperse TiO2 nanoparticles. The weight fraction of poly(oxyethylene)-segment in the dispersants and the molecular architecture in favoring the branched shape are two predominant factors for designing the effective dispersants. In particular, the poly(oxyethylene) block of 1,000 g/mol from PEG1000 as the starting material and total molecular weight of 12,000 g/mol have constituted the polymeric dispersants for the best performance for homogenizing TiO2 nanoparticles. The dispersant structures were characterized by using Fourier-transform infrared spectroscopy (FTIR), acid value (AV) determination, and gel permeation chromatography (GPC). The TiO2 dispersibility was evaluated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The synthesized dispersants were used to homogenize the as-prepared TiO2 and then fabricated into films of photoanodes for dye-sensitized solar cells (DSSCs). The ultimate performance of DSSC was measured to be 8.17±0.13% for the device efficiency (η) which was significantly higher than the conventional TiO2 photoanode at η = 7.14±0.12%. The photoanode film was characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) for surface area, and dye loading amount measurements. The kinetics of photogenerated electron in the photoanode, including electron lifetime and electron transit time of the film, was studied via electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS).


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INTRODUCTION Efforts on improving the efficiency of dye-sensitized solar cells (DSSCs) have been continuously made in recent years to demonstrate their advantages of high photoelectric conversion efficiency and relatively simple fabrication.1-4 The semiconductive oxide film in the photoanode of DSSC possesses the essential functions of the adsorbed dye and transporting the photogenerated electrons from dye molecules to an external circuit. The properties of the semiconductive oxide film including surface area, thickness, nanoparticle interconnection, pore size distribution and aggregates formation may strongly affect the performance of photoanode and eventually the power conversion efficiency of the device.5 In the field of DSSCs, titanium dioxide (TiO2)6-8 is commonly utilized to achieve the highest efficiency among all semiconductive oxides, such as ZnO, SnO2, and Nb2O5 etc. Nevertheless, several issues, such as shape, particle size, and crystalline structure, should be concerned with fabricating the TiO2 film as the photoanode to achieve the high efficiency of the corresponding device. Also, the morphology, surface area, and pore size distribution of TiO2 film for dye adsorption and redox couple penetration are essential contributing factors and strongly influenced by the chemical composition of TiO2 paste.9-11 The organic binders are commonly used as the additive in the TiO2 paste to fabricate a crack-free TiO2 film and further improve the adhesion to the substrate without delamination. On the other hand, a dispersant is required to disperse TiO2 particles in homogeneity to avoid their inherent agglomeration and loss of packing density. In considering the dispersion of individual TiO2 nanoparticles through the sol-gel type of TiO-R-O-Ti intermediate bindings during the dehydration process, the dispersant was



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designed to compose of the hydroxyl (-OH) functionality for suiting the affinity for the surface of TiO2. The presence of the added organics may aid the releases of the tensile stress built up in the TiO2 film during sintering process and avoid the occurrence of crack and delamination.9, 12-13 Two common organic binders are polyethylene glycol (PEG)14-19 and ethyl cellulose (EC),20-22 with the chemical structures illustrated in Scheme S1. PEGs are the functionalities of poly(oxyethylene) (POE) segment and termini of hydroxyl (OH). The end-capped-OH functionalities have the affinity for binding on the surface of TiO2 nanoparticles while the POE segment could function to wrapping around the particles and simultaneously generating the solvation in polar mediums, such as water, ethanol, etc. The effects of the amount

14-16

and molecular weight

17-19

of PEG were

investigated previously. As for ethyl cellulose with the structure of hydroxyl and ethyl ether functionalities in the repeated glucose units, TiO2 was allowed to disperse into a paste and evaluated for DSSC performance, as reported by Gemeiner et al.20 The fabrication of high quality and crack-free TiO2 film in single printing by adding 0.8 wt% of graphene oxide as an auxiliary binder in the conventional TiO2/EC paste was reported for improving DSSC efficiency of 7.70%.23 Karthick et al. had optimized the DSSC power conversion efficiency of 6.77% by applying poly(vinylpyrrolidone) (PVP) in photoanode.24 Park et al. proposed the functionalized epoxidized soybean oil as the binder for TiO2 pastes and claimed a superior performance to the conventional PEG binder.25 In addition, for further improving the porosity and surface area of the corresponding TiO2 film, graft copolymers with bulky branch which could also act as the template for preparing the ordered mesoporous TiO2 film. Kim et al. reported that POEM based


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amphiphilic graft copolymers of PVC-g-POEM could act both as the template and the polymeric binder for fabricating the organized mesoporous TiO2 films.26-27

Scheme 1 Synthetic scheme for the branched poly(oxyethylene)-segmented esters of trimellitic

anhydride

adduct

(polyethylene

glycol-trimethylolpropane-trimellitic

anhydride, designated as PTT) with partial cross-linking. On the other hand, the dispersant is introduced in the paste formulation to homogenize TiO2 nanoparticles by overcoming the high surface energy of the individual nanoparticles to inhibit secondary agglomeration inherently. The drawback of the particle agglomeration may lead to a TiO2 photoanode with low packing density and eventually diminished the needed surface area for adsorbing dye molecules.

28

The low-molecular-

weight surfactants, such as alkylphenol ethoxylates based Triton™-X100, were also



utilized in the TiO2 paste formulation.28-31 The representative water-soluble nonionic surfactant of Triton™ X-100 is also structurally illustrated in Scheme S1, with an average of 9 to 10 oxyethylene units as the hydrophilic block and a hydroxyl terminus on the hydrophobic octylphenol tail. Nam et al. introduced five different functionalized derivatives as the dispersants into the formulation of TiO2 paste. The introduction of carboxylic acid (-COOH) functionality in the structure of the dispersants could lead to the improvement of TiO2/dye packing density and the enhancement of DSSC device efficiency from 7.89% to 8.56%.28 In another example, the addition of acetic acid in the TiO2 paste was also documented.32 As inspired from literatures regarding the increased porosity and surface area of TiO2 based photoanode after calcination, a new polymeric dispersant of branched poly(oxyethylene)-segmented esters with trimellitic anhydride adduct (polyethylene glycol-trimethylolpropane-trimellitic anhydride, designated as PTT) was designed in this study. Since the TiO2 film formation and corresponding morphological control are largely influenced by the viscoelastic properties of polymer additive in TiO2 paste under the hightemperature annealing process. Herein, a family of new polymeric dispersants were systematically synthesized and used to homogeneously disperse TiO2 nanoparticles for fabricating the highly efficient TiO2 based photoanode in the dye-sensitized solar cells (DSSCs). The structural versatility in POE length, molecular shape of branching, molecular weight, and relative molar ratio of –COOH to –OH termini of PTT dispersants are considered. The as-synthesized dispersant structures were characterized by using Fourier-transformed infrared spectrometry (FTIR), acid value (AV) determination, and gel permeation chromatography (GPC). The effects of molecular weight (i.e., the extent


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of branching) and POE length on the dispersibility of TiO2 were evaluated. Further investigations using dynamic light scattering (DLS) and transmission electron microscopy (TEM) were performed. After optimizing the structure and molecular weight of PTT, the ultimate performance of the corresponding DSSC efficiency (η) was recorded at 8.17±0.13%, which was significantly higher than that of the DSSC with the conventional PEG

based

TiO2

photoanode

(η=7.14±0.12%).

Furthermore,

the

kinetics

of

photogenerated electron in the photoanode, including electron lifetime and electron transit time, was studied via electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). This study offers new type polyester based dispersants, not only for the successful introduction in the field of DSSCs, but also for the suitable use in other industrial applications. EXPERIMENTAL SECTION Materials p-Toluene sulfonic acid (PTSA) and polyethylene glycols (PEG) with various molecular weights of 600 (PEG-600), 1000 (PEG-1000), and 2000 (PEG-2000) were purchased from Alfa Aesar. PEG with molecular weight of 20,000 (PEG-20000), anhydrous LiI, I2, and acetonitrile (AN) were purchased from Merck. Nitric acid (HNO3, 65%), lithium perchlorate (LiClO4), isopropyl alcohol (IPA, 99.5%), and 2–methoxyethanol (≥99.5%) were obtained from Sigma-Aldrich. 2-methoxyethanol, trimethylolpropane (TMP), titanium(IV) isopropoxide (TTIP, +98%), 4-tert-butylpyridine (tBP, 96%), and tertbutanol (t-BuOH, 99.5%) were acquired from Acros. Commercial products of cis-



bis(isothio-cyanato) bis(2,2’–bipyridyl–4,4’–dicarboxylato ruthenium(II) bis-tetrabutyl ammonium (N719 dye) and 1,2-dimethyl-3-propylimidazolium iodide (DMPII), were received from Solaronix S. A. 3-methoxypropionitrile (MPN, 99%) was obtained from Fluka. Trimellitic anhydride (TMA) was obtained from Tokyo Chemical Industry Co. Ltd. Tetrahydrofuran (THF) was obtained from J.T. Baker, Fisher Scienctific. Synthesis of branched and poly(oxyethylene)-segmented esters Poly(oxyethylene)-segmented esters (PTT) were synthesized from diol, triol, and triacid as starting materials with the molar ratio of 3:1:3. PEGs with various molecular weights, namely PEG-600, PEG-1000, and PEG-2000, were selected as the diols, TMP as the triol, and TMA as the triacid monomer. The details of synthetic procedures are exemplified in the followings and the synthetic scheme is representatively illustrated in Scheme 1. For the synthesis of PTT with PEG-1000-block and branched structure (designated as PTT-B1000), PEG-1000 (60 g, 60 mmol) was added into a 250 mL of three-necked round-bottomed flask reactor, equipped with a mechanical stirrer, a nitrogen inlet-outlet lines, a Dean-Stark trap, and a thermometer. Under agitation, TMP (2.68 g, 20.0 mmol) was added into the reactor. After that, the mixtures were heated to 70 oC to allow a well mixing. TMA (11.53 g, 60 mmol) was then added with an aliquot of PTSA in the amount of 0.1 wt% to the reactants. The reactant mixtures were gradually heated to 220 oC to remove the generated water from the ester formation through the trap. During the esterification at 220 oC for 3h, the reaction was monitored by taking the samples periodically and analyzed its characteristic absorption peaks, acid value (AV), and molecular weight (Mw) one by one. The high conversion of the equivalents of TMP could lead to a gel product, which was avoided by carefully monitoring the reaction progress before reaching the excessive conversion of acid and hydroxyl groups which could be insoluble (yield > 95%). The PTT-B1000 product


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was crystalline solid with slightly yellowish at ambient temperature. The FTIR spectra of PTT formation under various states are shown in Figure 1a, showing the characteristic absorptions of 1726 cm-1 for ester formation under the elevated temperature, the disappearance of anhydride ring at 1781 and 1838 cm-1, and the appearance of carboxylic acid at 1713 cm-1.33-34 At 220 oC, the maximum formation of the ester functionality was identified during the progression of the reaction. Various kind of PEGs, including PEG-600 (36 g, 60 mmol) and PEG-2000 (120 g, 60 mmol), were employed as the diols in mixing with the corresponding amounts of TMA and TMP for generating the PTT analogs of PTT-B600 and PTT-B2000, respectively (yield > 90%). Preparation of TiO2 paste and photoanode The TiO2 paste and corresponding photoanodes were prepared according to the procedures reported previously.35-36 In brief, TTIP (72 mL) was added to HNO3 (430 mL, 0.1M) to form hydrolyzed TiO2 solution. It was heated with stirred by sol-gel process (at 88 °C for 8 h). After cooling to room temperature, the resultant colloid was filtered and heated in an autoclave (at 240 o

C for 12 h). The TiO2 colloid was thus concentrated to 13.0 wt%. An amount of PEG or as-

synthesized PTT series of polymeric dispersants (10 wt% with respect to TiO2) was added to the TiO2 colloid to obtain a paste consisting of nanocrystalline TiO2 particles. All of the key parameters for preparing the TiO2 pastes have been fixed in this study (mixing time: 24 h, temperature: 25 oC, concentration of TiO2: 13 wt%, concentration of dispersant: 10 wt% with respect to TiO2), except the type of dispersants. The dispersibility of each PTT based polymeric dispersants with various molecular weights as well as the degree of esterification were characterized by dynamic light scattering (DLS) for evaluating the particle size distribution of TiO2 particles. For fabrication of TiO2/PEG and TiO2/PTT-B1000 photoanodes, the cleaned FTO/glass was treated with a solution containing TTIP and 2-methoxyethanol at the weight ratio



of 1:3 as the pretreatment. A TiO2 layer was pasted on the thus obtained FTO conductive glass (TEC-7, 10 Ω sq−1, NSG America, Inc., NJ, U.S.A.) using the above-obtained colloidal solution by a doctor-blade technique and follow by the annealing of the TiO2 film at 500 °C for 30 min. The different film thickness of TiO2/PEG and TiO2/PTT-B1000 photoanodes were obtained by controlling the coating process. The PEG or PTT-B1000 was intended to prevent the film from cracking during the drying process and further control the morphology of the film. The treatment with the solution of TTIP was intended for obtaining a good mechanical contact between the conducting surface of FTO conductive glass and the TiO2 film. TiO2 film was patterned in proper active area (0.4 × 0.4 cm2) on FTO substrate. Assembly of DSSC The TiO2/PEG and TiO2/PTT-B1000 photoanodes were immersed in an AN/tBuOH (v/v = 1:1) solution, containing N719 dye (~3 × 10-4 M) for 18 h to obtain sensitized photoanodes. The DSSC was assembled with the sensitized photoanode and the sputtering Pt counter electrode by keeping a gap of 60 µm between them with Surlyn® (Solaronix) spacer. An electrolyte containing DMPII (0.6 M), LiI (0.1 M), I2 (5 × 10-2 M), and TBP (0.5 M) in AN/MPN (v/v = 1:1) was injected into the cell through a hole between the two electrodes by capillarity. Characterization Fourier transform infrared spectroscopy (FTIR) was recorded on a Perkin-Elmer SpectrumOne with the range of 1,000-4,000 cm-1. The acid value (AV) which represents carboxylic acid functionality was measured by the titration with a normalized 0.1 N KOH solution using a phenolphthalein indicator. Molecular weight distribution of assynthesized PTT based polymeric dispersants was obtained by using a gel permeation


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chromatography (GPC), which was performed on a Waters apparatus (515 HPLC pump, 717 auto-sample, and 2410 refractive index detector). The Waters Stygel column set was used for analyzing the relative molecular weights under a 1.0 mL/min flow rate of THF, calibrated against the polystyrene standards. The average particle size and bimodal distribution were measured by dynamic light scattering (DLS) equipped with a particle size analyzer (90 Plus, Brookhaven Instrument Corp.) and a 15 mW solid-state laser (λ=675 nm). The diluted TiO2/PEG and TiO2/PTT-B1000 paste were observed using a transmission electron microscope (TEM, JOEL JEM-1230 electron microscope, operated at 100 kV and photographed with a Gatan DualVision CCD Camera) for evaluating the TiO2 dispersibility. The thermal properties of the PTT-B1000 and PEG were analyzed by thermogravimetric analysis (TGA, Perkin-Elmer Pyris 1) in the air. The temperature was increased from 100 to 500 oC at a heating rate of 10 oC min-1. The surface morphology of the prepared photoanodes as well as their film thicknesses were characterized using a scanning electron microscopy (SEM, NovaTM Nano SEM 230). The surface area, pore diameter, and pore volume of the TiO2 photoanode film were measured by N2 adsorption/desorption isotherms (Micrometrics Instruments ASAP 2010) with BrunauerEmmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) analysis methods. The crystalline structure of the TiO2 photoanodes was analyzed using X-ray diffraction (XRD, Shimadzu SD-D1) with Cu Kα radiation (35 kV, 30 mA). The concentration of the desorbed dye was calculated from ultraviolet-visible absorption spectra (UV-Vis spectrophotometer, V-570, Jasco). The photocurrent density-voltage (J-V) curves were recorded with the Autolab potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, Utrecht, the Netherlands).



Incident photo-to-current conversion efficiency (IPCE) curves were obtained at the shortcircuit condition. The light source was a class A quality solar simulator (PEC-L11, AM1.5G, Peccell Technologies, Inc.); the light was focused through a monochromator (Oriel Instruments, model 74100) onto the photovoltaic cell. Electrochemical impedance spectroscopy (EIS) were obtained by the above-mentioned electrochemical station equipped with an FRA2 module under a illumination of 100 mW cm-2. The frequency range 65 kHz to 10 mHz was explored. The operated voltage was set at the open-circuit voltage (VOC) of the DSSC between the CE and the WE, starting from the short-circuit condition; the corresponding AC current amplitude was fixed at 10 mV. The impedance spectra were analyzed using an equivalent circuit model. Intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) were carried out using the same Autolab potentiostat/galvanostat electrochemical workstation. Green light emitting diode (LED) as a light source (λ = 530 nm) was applied with LED driver (LD80083, Autolab, Eco-Chemie, Utrecht, the Netherlands) as a source supply. The LED provides both AC and DC components of the illumination. The frequency range was set from 200K to 0.1 Hz and 1000 to 0.1 Hz for IMPS and IMVS, respectively.


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Figure 1 (a) Characteristic FTIR absorptions of PTT formation from anhydride to acid to ester functionalities, (b) reaction profile of molecular weight and acid titration for PPT-B1000 under various esterification times; the photo of as-synthesized PTT-B1000 product and gelled product were also shown as the inset.



RESULTS AND DISCUSSION Reaction profile of PTT: ester formation from anhydride and triol Owing to the possibility of TMP crosslinking under the equal equivalent reaction of a trifunctional carboxylic acid, hydroxyl diol, and triol, the branched PTT dispersants were synthesized under a careful control of the conversion to avoid an unwanted gelled product. During the esterification, the samples were taken periodically and monitored for the AV and MW, as shown in Fig. 1b and Table S1. For the synthesis of PTT-B1000 as the reaction progresses from 0.5 to 2 h, the analyses of AVs decreased from 87.1 to 54.0 mg KOH/g and the MW increases from 3,600 to 12,000 g/mol, accordingly. AV was the indication of acid disappearance during the formation of ester products from acid and hydroxyl reactants. The esterification occurred under the considerable rate at 220 oC for the formation of an ester linkage between anhydride/carboxylic acid and PEG-1000/TMP. The esterification was accompanied with an increase in product MW and viscosity, as well as with a decrease in AV. After 3 h, the esterification led to the product with a highly branched structure before reaching a gelled product. As the comparison, other two PTT series of polymeric dispersants were also prepared from PEG-600 and PEG-2000 for the designated as PTT-B600 and PTT-B2000, respectively. Detail analytic results of PTT-B600 and PTT-B2000 under various reaction times are shown in Fig. S1 and then summarized in Table S1. It was noticed that the gelled products appeared for PTT-B600 and PTT-B2000 once the reaction time was higher than 1 and 7 h, respectively. The difference of gelling time for PTT series of polymeric dispersants is consistent with the molecular weight of PEG, since the shorter PEG exhibits higher reactivity for reaching the gelling point easily. Accordingly, the relative reaction rate for PTT was revealed, namely PEG600 > PEG-1000 > PEG-2000, under the same monomer compositions. The esterification time


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and the titration of AV were adopted as the two controlling parameters in avoiding a gelled product. Dispersion of TiO2 with PTT series of polymeric dispersants The series of PTT based polymeric dispersants were applied to TiO2 dispersion. The particle size and volume distribution of the PTT dispersed TiO2 paste were analyzed by using dynamic light scattering (DLS). As shown in Fig. 2 and Table 1, the pristine TiO2 without dispersant was highly aggregated with a bimodal peak of 910/6000 nm (77/23) particle size and an average diameter of 2070 nm. The high surface energy of TiO2 nanoparticles tends to self-aggregate into particle cluster formation or inhomogeneous sizes. By adding the conventional PEG binder (Mw~20,000 g/mol, 10 wt% with respect to TiO2), the bimodal particle size distribution appeared around 1520 nm (100). Moreover, the precipitates or ill-dispersed appearances were also observed for the pristine TiO2 paste and PEG dispersed TiO2 paste, as shown in the inserted pictures of Fig. 2a and Fig. 2b. On the other hand, the well-dispersed TiO2 paste was obtained by using PTTB1000 with various MW as shown in Fig. 2c. For further understanding the molecular weight effect of the series of PTT-B1000 on TiO2 dispersibility, the particle size distribution of the corresponding TiO2 pastes was evaluated. PTT-B1000 with esterification time of 0.5 h (Mw~ 3,600 g/mol) could disperse TiO2 paste at bimodal particle size distribution of 180/640 nm (36/64) and 470 nm of the mean diameter. Further increase the molecular weight of PTT-B1000 could lead to narrow down the particles size distributions of TiO2 nanoparticles. For the case of PTT-B1000 with higher conversion (2h, Mw~12,000 g/mol), the best dispersibility with bimodal particle size distribution of 87/320 nm (50/50) and the average diameter of 170 nm for TiO2 paste was achieved.



However, our results show that further esterification of PTT-B1000 (> 2h) would lead to the formation of insoluble and gelled product. According to our experimental results, the branching effect on PTT based polymeric dispersants appeared to favor the TiO2 dispersion.

Figure 2 Particle sizes and corresponding bimodal distribution of (a) TiO2 without dispersant, (b) TiO2 with PEG, (c) TiO2 with PTT-B1000 based polymer dispersants (Mw=3,600~12,000 g/mol). The photos of corresponding TiO2 pastes was also shown as the inset.


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Table 1 Particle sizes and the corresponding distribution of TiO2 dispersion without dispersant or with different polymer dispersants. Samples

MW (g/mol)

Mean diameter (nm)

Particle size (nm) and distribution (%; bimodal peaks)

w/o dispersant

-

2070

910/5980-(77/23)

PEG

20,000

1520

1520-(100)

PTT-B600

16,100

760

140/810-(7/93)

PTT-B1000

12,100

170

87/320-(50/50)

PTT-B2000

11,000

220

88/390-(57/43)

For discussing the diol effect on PTT based polymeric dispersants for TiO2 dispersion, the dispersibility of TiO2 paste with PTT-B600 and PTT-B2000 under various esterification times was studied in the following section. The particle size distribution and an average diameter of TiO2 pastes with PTT-B600 and PTT-B2000 under various esterification times are shown in Fig. S2 and summarized in Table S1. All of these assynthesized dispersants exhibited reasonable dispersibility for TiO2 nanoparticles in comparing with the conventional PEG (Fig. 2b). In three series of the PTT based polymeric dispersants, the high degree of branching structure was generally favored for enhancing the TiO2 dispersion. For the cases of PTT-B2000 with esterification times from 0.5 (Mw~3,600 g/mol) to 3 h (Mw~11,000 g/mol), the particle size distribution of TiO2 paste can be gradually enhanced from 180/750 nm (9/91) to 88/390 nm (57/43), respectively. However, further exceeding esterification (> 3 h) of PTT-B2000 with higher Mw (>11,000 g/mol) may diminish the dispersibility of TiO2 paste, as shown in Table S1. This phenomenon can be well explained by the high conversion of branching-rich PTT based polymeric dispersants as accompanied by the decrease of -COOH and -OH terminal functionalities, which could affect the dispersibility of TiO2 paste. In other



words, the effectiveness of TiO2 dispersion was mainly determined by the balance of branching number and terminal functionality in the PTT structures. Comparing to conventional PEG, the PTT series of polymeric dispersants can be easily tailored in the branched molecular shape of POE segments, hydroxyl and carboxylic acid terminal groups. The dispersibility of TiO2 paste with commercial PEG and PTT based polymeric dispersants with optimized Mw is listed in Table 1. The case without adding any dispersant is also listed for comparison. Among the three PTT based polymeric dispersants with different PEG lengths in the structure, the PTT-B1000 exhibited the best dispersibility for TiO2 paste with a bimodal distribution of 87/320 nm (50/50) and a mean diameter of 170 nm. The corresponding performance was a slightly better than the lengthy PTT-B2000 but superior to that of shorty PTT-B600 dispersant, with respect to the particle size analysis. Furthermore, the feature of the as-prepared TiO2 films with PEG and PTT series of polymeric dispersants was investigated by N2 adsorption/desorption isotherms with Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) analysis methods, as shown in Fig 3 and summarized in Table 2. The TiO2 films with PTT series of dispersants showed fine pore size and large surface area, implying a high compact packing for the TiO2 nanoparticles. To compare among PTT-B600, PTT-B1000, and PTT-B2000, the higher surface area was selected as the benchmark for judging the dispersibility of TiO2 nanoparticles, as summarized in Table 2. Among these PTT series dispersants, the highest surface area of 94.66±0.40 m2 g-1 was achieved (TiO2/PTTB1000). Therefore, it is reasonable to assume that TiO2/PTT-B1000 may exhibit the highest value of dye-uptake when fabricating dye-sensitized solar cells.


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Figure 3 (a) N2 adsorption-desorption isotherms and (b) pore size distribution of TiO2/PEG, TiO2/PTT-B600, TiO2/PTT-B1000, and TiO2/PTT-B2000. 19 Environment ACS Paragon Plus


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Table 2 Pore size, pore volume, porosity and specific surface area of TiO2 with different polymer dispersants, which calculated from N2 adsorption/desorption isotherms.

Samples

Pore sizea (nm)

Pore volume (cm3 g-1)

Porosityb (%)

BET surface area (m2 g-1 )

TiO2/PEG

16.9

0.3961

60.6

79.16±0.35

TiO2/PTT-B600

12.2

0.3578

58.1

89.09±0.57

TiO2/PTT-B1000

12.1

0.3509

57.7

94.66±0.40

TiO2/PTT-B2000

12.0

0.3587

58.3

90.78±0.59

TiO2/PEG

16.9

0.3961

60.6

79.16±0.35

a

The pore size of these samples was estimated by using the face-centered cubic close packing model of spheres with identical diameters b The porosity of these samples was calculated by the following equation: porosity (P) = Vp /(ρ-1 + Vp), where Vp = the specific cumulative pore volume (cm3 g-1) and ρ-1 = inverse of the density of anatase TiO2 (ρ-1 = 0.257 cm3 g-1).

For further examining the dispersibility of as-proposed PTT based polymeric dispersants with the optimized molecule weight and structure, the TiO2 paste with conventional PEG and optimized PTT-B1000 (Mw~12,000 g/mol) as the dispersant was examined by TEM, as shown in Fig. 4. The case without adding any dispersant was also shown as the comparison. As shown in Fig. 4a and b, the pristine TiO2 nanoparticles without dispersant were in the form of flocculation by self-aggregation as measured over 900 nm in the cluster. On the other hand, the conventional PEG as the TiO2 dispersant offered only a minor improvement for subsiding the serious aggregation of TiO2 nanoparticles, observed in Fig. 4c and d. The aggregation force was substantially overcome by using optimized PTT-B1000 dispersant as indicated in Fig. 4e and f, showing less of particle clusters or aggregates. The synthesized PTT dispersant with the PEG segments and the functionalities of hydroxyl and carboxylic acid termini in the branched structure could effectively anchor on the TiO2 nanoparticle and further interact


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with others. Meanwhile, the ester linkage and the POE segments in the PTT polymer can also facilitate the solubility in water. Overall, the branching structure of the PTT dispersants ultimately affected the homogeneity of the TiO2 dispersion in water as well as the corresponding film after water removal.

Figure 4 TEM images of TiO2 paste with (a, b) no dispersant, (c, d) PEG, (e, f) PTT-B1000 at different magnifications.



Characterization of TiO2 film with PEG and PTT-B1000 In the fabrication of TiO2 film as the photoanodes, the addition of organic dispersant affected the binding properties through Ti-O-R-O-Ti (R=organics) bonding during the sol-gel dehydration and the annealing process. Similar to the formation mechanism of PEG or EG as the traditional binder for the TiO2 thin film, the covalent bond would form once the dehydration reaction proceeds at the interface of PTT (end-capped COOH or end-capped OH) and TiO2 (Ti-OH) during the calcination process. The polymeric dispersant can be totally removed after calcination at high temperature (~500 oC). By evaluating the result of thermal gravimetric analysis (TGA), as shown in Fig. 5a, the degradation temperature of PEG and PTT-B1000 dispersant was at 450 and 465 oC, respectively. Accordingly, the process of removing the dispersant was designed at 500 °C for 30 min for obtaining the mesoporous films with fine pore size and high surface area. Moreover, the similar crystalline orientation of TiO2 nanoparticles with characteristic diffraction peaks at 2θ around 25.3, 37.8, and 48.1° for corresponding to the anatase crystal planes of (101), (004), and (200), accordingly, were observed for both of TiO2/PEG and TiO2/PTT-B1000 films by XRD (Fig. 5b). No obvious difference indicated the PTT-B1000 could not only inhibited the grain growth, but also retained the intrinsic properties of TiO2 nanoparticles during sintering procedure. For further characterizing of the corresponding films, SEM was utilized to examine the surface morphology with different magnifications. In Fig. 5c, TiO2/PEG film with numerous cracks implied the polymeric dispersant did not efficiently associate the TiO2 nanoparticles. On the other hand, the crack-free morphology with well-interconnected TiO2 nanoparticles and fine pore size was particularly observed for the TiO2/PTT-B1000 film, as presented in Fig. 5d. The observation was correlated to the fine dispersion of TiO2 in water as well as to the corresponding film after water removal. All of above results suggested that PTT-B1000 exhibited the promising


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dispersibility in PTT series of polymeric dispersants. Hence, the PTT-B1000 was selected for comparing with commercial PEG to further fabricate highly efficient TiO2 photoanode in dyesensitized solar cells (DSSCs).

Figure 5 (a) TGA plot and (b) XRD patterns for TiO2/PEG and TiO2/PTT-B1000 films; SEM image of films of (c) TiO2/PEG and (d) TiO2/PTT-B1000; magnified images of the corresponding film are shown in the inset. Photovoltaic performances of DSSCs with TiO2/PEG and TiO2/PTT-B1000 based photoanode The photoanodes fabricated from TiO2/PEG and TiO2/PTT-B1000 with optimized film thickness were studied in DSSCs at the beginning. As exhibited in Fig. S3 and Table S2, the photovoltaic



Page 24 of 36

parameters of photocurrent density (JSC) and solar-to-electricity conversion efficiency (η) for both of devices were increased in proportion to the TiO2 film thickness increases since the issue of dye loading in the photoanodes. However, transportation of photo-generated electron in photoanode was suffered by the thicker TiO2 film with longer transport distance. After optimizing the thickness of TiO2/PEG and TiO2/PTT-B1000 films, the efficiencies of DSSC with photoanode of TiO2/PEG and TiO2/PTT-B1000 were obtained with 7.14±0.12 and 8.17±0.13% for the optimized thickness of 28.94 and 25.84 µm, respectively, as shown in Fig. 6a. Corresponding photovoltaic parameters of DSSC with photoanode of TiO2/PEG and TiO2/PTTB1000 were also listed in Table 3. The high efficiency of DSSC with photoanode of TiO2/PTTB1000 was attributed to the relatively higher JSC since its higher surface area, as observed in the BET analysis, for increasing the amount of dye loading. This result was further confirmed by the dye loading amounts, as shown in Fig. 6b. Higher absorption spectra of N719 dye for the case of TiO2/PTT-B1000 based photoanode suggested that more dye molecules (2.67 x 10-7 mol cm-2) were contained in the corresponding TiO2 film, as compared to that of TiO2/PEG based photoanode (1.91 x 10-7 mol cm-2).

Table 3 Photovoltaic parameters of the DSSCs with the photoanode of TiO2/PEG and TiO2/PTTB1000 films, measured at 100 mW cm–2 (AM 1.5G). The results of electrochemical impedance spectroscopic (EIS) and dye loading for the corresponding films are also listed in the table. TiO2 photoanodea

JSC -2 (mA cm )

Rct2 (Ω)

ZW (Ω)

Dye loading (10-7 mol cm-2)

0.71±0.02 16.2±0.1

0.62±0.03 7.14±0.12 20.6

11.0

79.16±0.35

TiO2/PTT-B1000 0.71±0.01 18.4±0.4

0.63±0.01 8.17±0.13 16.5

9.70

89.09±0.57

TiO2/PEG a

VOC (V)

FF

η (%)

Average values and standard deviations from three independent cells.


Page 25 of 36 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


Figure 6 Characterization of the DSSCs with sensitized photoanodes of TiO2/PEG and TiO2/PTT-B1000 films: (a) J-V curves, (b) absorption spectra of N719 dye on the corresponding photoanodes, (c) IPCE, and (d) EIS spectra. All of the photovoltaic measurements for the corresponding DSSCs were obtained under an illumination of 100 mW cm-2 (AM 1.5). The PTT-B1000 derived films provides a well interconnected TiO2 nanoparticle matrix with high surface area for efficiently absorbing the dye and eventually boosting the η up to 8.17±0.13% in comparison with 7.14±0.12% by the conventional PEG as the binder. Incident photon-to-current conversion efficiency (IPCE) is defined as the number of electrons in the external circuit at a given wavelength over the number of incident photons 37. As in Fig. 6c, the IPCE was measured to show the broad curves, covering almost the entire visible spectrum from 400 to 800 nm with the maximum of 74% and 83% at 530 nm for the DSSCs with photoanode of TiO2/PEG and



Page 26 of 36

TiO2/PTT-B1000, respectively. The higher IPCE value was consistent with its higher dye loading as mentioned above. In order to understand the interfacial charge transfer process, electrochemical impedance spectroscopy (EIS) analysis was applied. Nyquist plot is shown in Fig. 6d with the inlet of the corresponding equivalent circuit. Each plot was of three semi-circle measurements in different ranges of frequencies representing the resistance at the counter electrode/electrolyte interface (Rct1), photoanode/electrolyte interface (Rct2) and the resistance of Warburg diffusion of I-/I3- in the electrolyte (ZW), respectively. The values of Rct1 were calculated; they showed a similar value for both DSSCs due to the same sputtered Pt was used as the counter electrode. The ZW was not obvious and overlapped with RCT2 due to the thin spacer in the devices. The Rct2 value of TiO2/PTT-B1000 (16.5 Ω) was smaller than that of TiO2/PEG (20.6 Ω), which was well explained by the well-connected TiO2 nanoparticles for the PTTB1000 dispersant and the improvement of electron transportation. Electron transit and recombination time in DSSC For further understanding the charge transfer dynamics in both of TiO2 films, intensitymodulated

photocurrent

spectroscopy

spectroscopy (IMVS) were introduced

38-39

(IMPS)

and

intensity-modulated

photovoltage

. As shown in Fig. 7, the plots of the charge transfer

dynamic parameter as the function of light intensity were illustrated and corresponding kinetic parameters were listed in Table 4 for detail. Electron transit time (τd) and electron lifetime (τr) were extracted from IMPS and IMVS by using the following Equation (1) and (2), respectively:

τ

d

τ

r

=

1 2πf d

(1)

=

1 2πf r

(2)


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Electron diffusion coefficient (Dn) and electron diffusion length (Ln) were obtained according to Equations (3) and (4), respectively:

l2 Dn = 2.35τ d

Ln = Dnτ r

(3)

(4)

where l is the film thickness of TiO2; in this study, the film thickness of TiO2/PEG and TiO2/PTT-B1000 were well controlled in 28.94 and 25.84 µm, respectively.

Figure 7 Electron transport and recombination kinetics for the DSSCs with photoanodes of TiO2/PEG and TiO2/PTT-B1000 films: (a) electron transit time constant, (b) electron lifetime constant, (c) effective diffusion length, and (d) charge-collection efficiency as the function of the incident light intensity for 530 nm LED light sources.



Page 28 of 36

Table 4 Kinetic parameters of electron transport and recombination of the DSSCs with the photoanode of TiO2/PEG and TiO2/PTT-B1000 films. TiO2 photoanode

τd (ms)

τr (ms)

Dn (µm2 s-1)

Ln (µm)

ηcoll (%)

TiO2/PEG

7.97

50.3

4.47 x 104

47

84.1

TiO2/PTT-B1000

1.00

50.3

28.3 x 104

119

98.0

As shown in Fig. 7a, TiO2/PTT-B1000 photoanode showed a shorter electron transit time than TiO2/PEG under different light intensity. The well-interconnected TiO2 nanoparticles and packing density could fasten the transit of the photo-generated electrons. As the electron lifetime represents the electron survival without recombining with I3- electrolyte, nearly the same value was observed at a given light intensity, implying the similar overall recombination reaction. Although the fast electron transit in TiO2/PTT-B1000 film eases the possibility of recombination, the increase sites for the recombination reaction were provided since the higher surface area of the corresponding film as evidenced by the BET surface area and dye loading amount. The overall same electron lifetime was then obtained. Moreover, the TiO2/PTT-B1000 film showed a larger diffusion coefficient (Dn) and larger electron diffusion length (Ln) than the TiO2/PEG film. Overall, the charge-collection efficiency (ηcoll), the sum of the photo-induced electrons and their recombination, could be determined according to Equation (4):

η

coll

=1 −

τt τn

(4)

The ultrahigh ηcoll was recorded to 98% for DSSC with TiO2/PTT-B1000 based photoanode, much higher than that with TiO2/PEG one (ηcoll~84%) since their difference in the feature of electron transit and charge recombination.


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CONCLUSIONS The ester-functionalized polymeric dispersants were designed, synthesized, and evaluated for TiO2 dispersion. The uses of the selected dispersant as the binder allowed ones to prepare the homogeneously distributed TiO2 nanoparticles in the paste or thin film form. The evaluation of the photoanode film for the use in DSSC gave an enhanced device efficiency of 8.17±0.13%, superior to the conventional TiO2/PEG analog (η = 7.14±0.12%). The increase in DSSC efficiency was attributed to the polymeric dispersant which was designed to have the poly(oxyethylene)-block of 1000 g/mol segmental weight in the branched ester structure of 12,000 g/mol average molecular weight. By using the dispersant in 10 wt% with respect to TiO2, the inherently aggregated TiO2 was well-dispersed in a homogeneous manner by showing the fine particle sizes that were lowered from the originally 2100 nm to 170 nm in diameter. The homogeneity and fine particle distribution ultimately contributed to the efficiency of contacting dye and upgraded the DSSC performance. ASSOCIATED CONTENT Supporting Information Related characterization of materials is described in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION *Corresponding Author E-mail: [email protected], Tel: +886-2-2737-6643 (M.H. Yeh) E-mail: [email protected], Tel: +886-2-2366-0739 (K.C. Ho); 29 Environment ACS Paragon Plus


E-mail: [email protected], Tel: +886-2-3366-5316 (J.J. Lin) Author Contributions § These authors contributed equally.

All authors have given approval to the final version of the manuscript. Funding Sources The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This work was financially supported by the “Advanced Research Center for Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 105-2221-E-002-229-MY3, 1062218-E-002-038, 107-3017-F-002-001).


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