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Gel electrolytes with polyamidopyridine-dendron modified-talc for dye-sensitized solar cells Marcos Antonio Santana Andrade Junior, Armi Tiihonen, Kati Elina Miettunen, Peter Lund, Ana Flavia Nogueira, and Heloise O. Pastore ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 02 Jun 2017 Downloaded from http://pubs.acs.org on June 2, 2017

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ACS Applied Materials & Interfaces

Gel electrolytes with polyamidopyridinedendron modified-talc for dye-sensitized solar cells Marcos A. Santana Andrade Jr.a, Armi Tiihonenb, Kati Miettunenb, Peter Lundb, Ana F. Nogueirac, Heloise O. Pastorea* a

Micro and Mesoporous Molecular Sieves Group, Institute of Chemistry, University of Campinas, 270, Monteiro Lobato St., CEP 13083-861, Campinas-SP, Brazil.

b

New Energy Technologies Group, Department of Applied Physics, P. O. Box 15100, FIN-00076 Aalto, Finland.

c

Laboratório de Nanotecnologia e Energia Solar, Institute of Chemistry, University of

Campinas, 270, Monteiro Lobato St., Cidade Universitária Zeferino Vaz, CEP 13083-861, Campinas-SP, Brazil. *[email protected]

ABSTRACT

Organic-inorganic hybrid layered materials are proposed as additives in a quasisolid gel electrolyte for dye-sensitized solar cells. Talcs could provide a low cost, environmentally friendly as well as abundant option as gelators. Here, talcs were prepared by functionalizing an organotalc with three polyamidopyridine dendron generations, PAMPy-talc-Gn (n = 1, 2 and 3). PAMPy dendron grows parallel to the lamellae plane and forms an organized structure by intermolecular interactions. In addition, polyiodidedendron charge-transfer complexes were prepared onto the organotalc by adsorption of 1 ACS Paragon Plus Environment


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iodine. In this work, the effect of the dendron generation of PAMPy-talc and the influence of polyiodide intercalation on solar cell performance and stability were investigated. The best results were reached with the use of lowest generation PAMPy-talc (η = 4.5 ± 0.3 %, VOC = 710 ± 19 mV, Jsc = 10.4 ± 0.9 mA cm-2and FF= 61 ± 2 %): 15 % higher efficiency compared to similar liquid devices. While some previously studied talcs illustrate very strong absorption of the iodide from the electrolyte, in the case of PAMPy-talc such interfering effects were absent: In a 1000-h light soaking test, the PAMPy-talc cells both with and without polyiodide intercalation demonstrated stable performances. Furthermore, the color analysis of the electrolyte indicated that the color of the electrolyte remained stable after an initial period of stabilization, which is a good indication of the compound being stable and not absorbing charge carriers from the electrolyte. The performance and stability results indicate that PAMPy-talc has potential as a gelling method of electrolyte for dye solar cells.

Keywords: organotalc, dendron, polyiodide, gel electrolyte, dye-sensitized solar cells INTRODUCTION Dye-sensitized solar cells (DSSCs) are attractive alternatives to conventional silicon solar cells due to potentially low-cost materials and easy methods of fabrication which can be transferred on to roll-to-roll mass production 1. Energy conversion efficiency of DSSCs has reached 13 % using cobalt liquid eletcrolytes2 although dye solar cells are typically fabricated using a liquid electrolyte composed of iodide/tri-iodide redox couple 3. Because of the presence of a liquid solvent, the solar cells suffer from several critical problems limiting their practical use: complicated sealing of the cells, electrolyte leaking and electrolyte evaporation under heat and light 4,5.

2 ACS Paragon Plus Environment

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In order to overcome the limitations of liquid electrolytes, quasi-solid electrolytes using nanoparticles6, cross-linked gelators7 or polymeric materials8 have been studied. The quasi-solid electrolytes suppress leakages9, and furthermore, they could be printed which eases the fabrication process of large area DSSC modules in the industrial scale10. One gelling candidate is an additive consisting of organic-inorganic hybrid materials which act not only as a passive gelling agents but are also expected to actively improve the photovoltaic properties and to prevent electron recombination at photoelectrode Recently, clays and especially phyllosilicates (such as nanomica laponite, montmorillonite electrolytes

12

11

.

and smectites, e.g.

13,14

, and saponite), have been used as active additives to gel

15

. Dye-sensitized solar cells prepared with clays present high short-circuit

current because of increased light scattering and lengthened optical pathway 16. Montmorillonite clay is a gelling agent commonly cited in the literature. The application of this type of clay, alone17 or forming composites with polymers13,18,19, to gel electrolytes has provided solar cells with similar efficiencies to liquid solar cells. The lamellae of these clays are charged and interact with cations that are easily exchanged by the cation from the iodide salt. This effect was reported by Lee et al.

17

when adding 1-

methyl-3-propyl-imidazolium iodide. The cation was absorbed by the clay and the anion became free in the solution, increasing conductivity and diminishing electron recombination 17. Costenaro et al. studied the properties of the DSSCs as a function of the size of saponite particles in the gel electrolyte

15

. Saponite samples with 20-150 H2O/Si molar

ratio were prepared producing materials with lamellae in different dimensions. The addition of these particles to a liquid electrolyte with methoxypropionitrile solvent resulted in particle aggregation. The larger aggregated particles decreased the efficiency due to increased series connected resistances. However, the smaller aggregated particles 3 ACS Paragon Plus Environment


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improved solar cell efficiency due to light scattering that increased the short-circuit current 15

. To prevent particle aggregation into the electrolyte, Tsui et al. 20 synthesized alkyl-

modified clays and applied them to gel electrolytes. These clays with increased interlayer space dispersed better into the electrolyte producing pathways that contributed to charge transport by diffusion and ion-exchange processes. Rheological and electrochemical properties of electrolytes gelled by clays and the influence on the characteristics of the solar cells were evaluated by Ding et al.

16

. The

higher was the viscosity, the lower was the diffusion of iodide. The authors proposed that the charge transport occurred by the Grotthus mechanism (the bonding of the iodine molecule breaks and the atoms form new bonds), influenced by an arrangement of the clays in the gel electrolyte that resembles a house of cards. In the literature, several works report the use of charged clays containing intercalated cations to gel electrolytes for dye-sensitized solar cells. To eliminate possible interferences from these cations, in this work, a neutral clay, talc Mg3Si4O10(OH)2, is applied as gelling agent. The elementary sheet of the talc is composed of brucite layers (octahedral magnesium oxide hydroxide structures) sandwiched between sheets of tetrahedral silica. The silica-brucite-silica packs of layers in talc are bonded together by weak Van der Waals forces between surfaces 21,22. Once this clay is functionalized by 1- up to 3-generation dendrons polyamidoaminopyridine (PAMPy), the dendrons interact with TiO2 surface producing a barrier effect, decreasing electrons/holes recombination, and, as a consequence, improving solar cell performance 23,24. In our previous work, it was reported that 5th generation of polyamidoamine-modified talc absorbs free redox species from electrolyte solution degrading solar cell during aging

25

. However, the solar cell

degradation is suppressed when intercalating polyiodides into PAMAM-modified talc 4 ACS Paragon Plus Environment

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because the basic sites from dendron group are not more available. On the basis of that, we also tested how the intercalation of polyiodides onto the dendron PAMPy-talc affects their function. The different generation clays with and without addition of polyiodides are evaluated in terms of how they affect the performance and stability of the cells.

EXPERIMENTAL Chemicals In this work, the following chemicals were used: sodium hydroxide (NaOH, Merck), magnesium nitrate hexahydrate (Mg(NO3)2.6H2O, Aldrich), tetraethoxyorthosilane (TEOS, Acros), 3-aminopropyltriethoxysilane (AMPTS, Sigma Aldrich), methylacrylate (MA, Sigma Aldrich), 2,6-diaminopyridine (2,6-DAP, Nepera Chemical Company), methanol (MeOH 100%, J.T.Baker), 3-methoxypropyonitrile (MPN, Sigma Aldrich), 1propyl-3-methylimidazolium iodide (PMII, io-li-tec), resublimed iodine (I2, Aldrich), and N-methylimidazole (NMB, Alfa Aesar).

Microwave-assisted synthesis of polyamidoaminopyridine-phyllosilicate (PAMPy-talc) Polyamidoaminopyridine-functionalized

phyllosilicates

(PAMPy-talc)

were

obtained using a precursor aminopropyl-functionalized phyllosilicate (NH2-talc). The synthesis of NH2-talc were reported by Moura et al

26

. Briefly, AMPTS (0. 37 mL, 1.5

mmol) and TEOS (3.1 mL, 15 mmol) were added to a aqueous Mg(NO3)2·6H2O solution (100 mL, 0.12 mol L-1) resulting in N:total silicon molar ratio of 10%. Then, NaOH (48 mL, 0.5 mol L−1) was added dropwise under magnetic stirring. The resulting gel was aged for 4 h, at 50 °C, and heated using a microwave oven DTG 100 Plus model, Provecto



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Analítica (300 W, 2h). The product was washed in distilled water, centrifuged, dried at room temperature, and size sieved at 0.074 mm. These organophyllosilicates, PAMPy-talc-Gn (where n indicates the number of growing reactions or generations) were prepared in two steps: (1) Michael addition of methylacrylate (MA) to the nitrogen from aminopropyl chain, and (2) amidation reaction of the product from step (1) using 2,6-diaminopyridine. The first generation (PAMPy-talc-G1) was prepared adding 3.0 g of NH2-talc, previously dehydrated at low pressure (10-3 Torr, 125 ºC, 24 h) in a Schlenk, into an excess of MA solution in methanol. The mixture was poured into a polytetrafluoroethylene (PTFE) flask and heated under microwave irradiation (40 W, 40 min). The final solid was centrifuged, washed 3 times with methanol (10 mL), and dried at room temperature. The amidation reaction was carried out by adding the obtained material to a methanolic solution containing an excess of 2,6-diaminopyridine (previously recrystallized in chloroform). After that, the mixture was poured in a PTFE flask and heated under microwave irradiation (80 W, 200 min). The material was centrifuged and washed in methanol. The excess of 2,6-DAP was removed by Soxhlet extraction with methanol. The successive reactions (up to PAMPy-talc-G3) were performed following the procedure drawn in Scheme 1. Polyiodides were intercalated onto the interlamellar space of the materials by adsorption of iodine vapor. The organo-modified phyllosilicates were kept in a desiccator under a saturated iodine atmosphere for 24 h, at room temperature. Finally, the phyllosilicates were heated up to 50 °C to remove the physically adsorbed iodine during 30 min producing the materials PAMPy-talc-Gn (n = 1, 2 and 3)-I2.


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PAMPy-talc-G0,5 NH2-talc 1

PAMPy-talc-G1 MA

2,6-DAP

40 W/40 min

80 W/200 min

O

N

NH N H2N

O HN N H 2N

N

MA

HN 2

2N

H N H

N

O

HN

2

N N

O

NH

O

N NH

N

HN

N

HN 2

80 W/260 min

PAMPy-talc-G2

NH

O

N HN

O

O

N

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


PAMPy-talc-G2,5

PAMPy-talc-G1,5

PAMPy-talc-G3

Scheme 1. Illustration of PAMPy dendron growth reactions onto talc lamella surface.

Characterization of organotalcs Powder X-ray diffraction analyses were performed with Cu Kα radiation (40 kV, 30 mA) on a Shimadzu model XRD 7000 diffractometer at room temperature using 0.5°, 0.5°, and 0.3 mm slits for entrance, scattering, and exit, respectively, in the 2θ range from 1.5 ° to 70 ° with 0.1° step width and 10 s count time. Fourier transform infrared spectra were acquired on a Nicolet model 6700 FTIR spectrophotometer using 0.05 wt % KBr pressed samples. Accumulations of 128 scans at 4 cm-1 resolution were performed. Elemental analysis (C, H and N) was carried out by Perkin Elmer CHNS/O Analyzer 2400. Nuclear magnetic resonance spectra of the solid materials were obtained on a Bruker Avance II+ 400 at room temperature. The CP-MAS

13

C spectra were measured at a resonance

frequency of 100.6 MHz with a pulse repetition time of 3 s and a contact time of 0.003 s. Thermogravimetric analysis was performed in a Setaram Instrument model SETSYS 16/18 7 ACS Paragon Plus Environment


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Evolution TGA with an alumina pan under O2 atmosphere in a temperature range 20-1000 °C at a heating rate of 10 °C min-1 using approximately 10 mg of sample. The intercalated species of iodide onto organotalcs were identified by Raman spectroscopy. The measurements were carried out on an Xplora Horiba spectrometer using a 532 nm laser, 10 mW and in the range of 80-250 cm-1.

Gel electrolyte preparation and characterization A liquid electrolyte composed by 0.6 mol L-1 of 1-propyl-3-methylimidazolium iodide (PMII), 0.1 mol L-1 of iodide (I2) and 0.5 mol L-1 n-methylbenzimidazole (NMB) was gelled by adding 10 wt% of PAMPy dendron modified-talc with and without intercalated polyiodides. The mixtures were homogenized using an ultrasound probe during 90 min with 100 W power27. The electrochemical properties of the talc-based electrolytes were investigated using a symmetrical thin-layer cell. The electrolytes gelled by the dendron-modified talcs were placed between two Pt-coated FTO glass electrodes with an area of 1.0 cm2. A polymer tape (60 µm, Surlyn) was used as spacer between the electrodes. Electrochemical impedance spectroscopy (EIS) measurements were performed using an EcoChimieAutolab PGSTAT 12 potentiostat with FRA module. The spectra were acquired at frequency range from 1×100 to 1×106 Hz with amplitudes of 10 mV over the open circuit voltage.

Quasi-solid dye-sensitized solar cell assembly Photoelectrodes and counter electrodes were prepared using fluorine-doped tin oxide (FTO) glasses. The substrates for the photoelectrodes were prepared according to our


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previously published study

28

: Initially, the substrates were treated with a 30 mmol L-1

aqueous solutions of tetrachlorobis(tetrahydrafuran)titanium(IV) (TiCl4·2THF, SigmaAldrich) at 70 °C for 30 min. After that, two layers of DyeSol DSL 18 NR-T TiO2 paste and one layer of DyeSol 18 NR-AO paste were sequentially screen-printed on top of a glass substrate and sintered at 450 °C for 30 min. The resulting TiO2 layers had the total thickness of about 13-14 µm and area of 40 mm2. The treatment with TiCl4·2THF was performed again and sintered. After the photoelectrodes had cooled down, they were dyed in a 32 mmol L-1 cis-bis(isothiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxilato)ruthenium(II) bis-tetrabutylammonium (N719, DyeSol) for 16 h. The counter electrodes were also made similar to our previous study 28 by spreading 4 µL of 5 mmol L-1 H2PtCl6 in 2-propanol on the glass substrate and heating them at 385°C for 20 min. Each photoelectrode and counterelectrode were put together and sealed with 25 µm polymer foils (Surlyn 1702). The solar cells were filled with the gel electrolytes PAMPytalc-Gn (n = 1, 2 and 3), PAMPy-talc-Gn (n = 1, 2 and 3)-I2, and the liquid reference electrolytes through holes in the counter electrodes. The holes were covered with Surlyn foil and a thin glass. For each electrolyte, three solar cells were prepared and aged. The cell geometry used in this study was not optimized for high performance, but for stability studies. The cells were equipped with an excess area that could be used to detect changes in the electrolyte color, which is highly beneficial in particular for investigating aging reactions related to the electrolyte. This causes, however, performance losses as the distance between the active area of the cell and the current collector contact are further apart. In this contribution, the main objective is to gather insight regarding the operation of these clays and therefore such cell geometry was chosen.



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DSSC characterization and aging test The cells were aged for approximately 1000 h under illumination of 1 Sun equivalent. The cells were photographed frequently during the aging test. Additionally, at the beginning and end of the aging test, the following measurements were performed: J-V curves and EIS at VOC measured in a solar simulator, J-V curves as a function of the light intensity, EIS in dark, and incident photon-to-current efficiency (IPCE). J-V curves from the solar cells were obtained using a portable solar simulator (Peccell, PECL01), with black masks attached on top of the devices. The EIS were measured with Zahner Zennium potentiostat. The cells were measured at VOC in the solar simulator under illumination of 1 sun. The EIS were also measured in dark at a voltage range of 0-0.8 V with 0.1 V intervals. In both cases, a frequency range of 100 mHz-4 MHz was used with an amplitude of 10 mV. IPCE measurements were performed with QEX7 system (PV Measurements). The wavelength range of the measurements was 300-900 nm and the measurements were done in DC mode. The aging test was carried out using a system described before by Tiihonen et al.28. Stands of 16 cells were illuminated under 1 Sun (monitored frequently with a photodiode) at 40 °C, for approximately 1200 h, resulting in approximately 1000 h under illumination when the weekly measurements are counted out from the aging time. The light intensity was monitored frequently during the aging test using a photodiode. Current-voltage (J-V) curves and electrochemical impedance spectroscopy (EIS) at open-circuit voltage (VOC) were acquired automatically in a sequential order throughout the aging test. The J-V curves and EIS at Voc were measured with a Bio-Logic SP-150 potentiostat using an Agilent 34980A as a multiplexer. The solar cells were photographed with a Olympus E-620 camera using the device and analysis procedures described by Asghar et al29. The white balance


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and color calibration were done with X-rite Color Checker Passport in Lightroom 3 software and the values of the pixels in RGB scale were obtained using Matlab software. RESULTS AND DISCUSSIONS On the PAMPy modified-talcs The dendron polyamidopyridine (PAMPy) was bound to the magnesium phyllosilicate lamella. The organization of the phyllosilicate in the presence of PAMPy dendron was identified by the diffractions at 5.9, 21.3, 26.2, 34.5 and 59.7° 2θ (Fig. 1 a-d) 26,30,31

. These peaks are broader than those for the natural talc (Fig. 1 e)

32,33

indicating a

lower general organization of the inorganic structure in relation to the natural talc due to the presence of the organic pendants

34

. According to general indexation of organotalcs

diffraction patterns, these peaks correspond to (001), (020,110), (004), (130,220), and (060) reflections

22,23

. The position of the (060) peak observed at 0.15 nm indicates the

presence of a trioctahedral 2:1 phyllosilicate structure (in the octahedral sheets each O atom or OH group is surrounded by three divalent cations)

36

and remained unchanged

among the various PAMPy dendron phyllosilicates synthesized. This confirms that the layered inorganic framework can accommodate the three generations of PAMPy dendron in the interlayer space without losing the tetrahedral-octahedral-tetrahedral structure 9,10.



I

II

III

Ligações de H

PAMPy-talc-G1 (060,330)

(200,130)

(004)

(110,020)

(001)

. u . a / e d a d i s n e t n I

2500 cps

π-stacking

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

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a) b) c)

IV PAMPy-talc-G2

V PAMPy-talc-G3

d) e) 10

20

30

40

50

60

70

2θ(°) /° 2θ

Fig. 1. I- X-ray diffractograms of (a) NH2-talc, (b) PAMPy-talc-G1, (c) PAMPy-talc-G2, (d) PAMPy-talc-G3, (e) natural talc. II, IV and V shows the illustration of the PAMPy dendron growth on the plane of the phyllosilicate, and III shows the self-organization between PAMPy-dendrons.

The (001) reflection does not change position with the dendron-modification of the phyllosilicates. A new peak appears around 18 ° 2θ indicating the intermolecular organization of PAMPy-dendron by engaging in head-to-tail hydrogen bonds between carboxyl and amine groups39,40 and pyridine rings π-π stacking41 (Fig. 1-III). This provides evidence for the formation of supramolecular linear structure that grows parallel to the lamella plane without changing the interlamellar space of the phyllosilicate. Chen et al.

42

reported a similar behavior, where generations 1 to 3 of a dendron based on the dye orange 3 were intercalated into montmorillonite without changing the interlamellar space. Based on the elemental analysis of N, C and H (Table 1), nitrogen content increases with increasing dendron generation. The nitrogen content increased from 0.89 ± 0.02 mmol g-1 for NH2-talc, reflecting the initial amine loading on the organo-modified phyllosilicate, to a maximum of 10.76 ± 0.07 mmol g-1 for PAMPy-talc-G3. Additionally, evidence of 12 ACS Paragon Plus Environment

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PAMPy dendron growth are given by thermogravimetric analysis data (see supporting information Fig. 1S). The overall reaction yield was calculated based on the percentage of organic present in the material. Table 1 shows values between 75 and 78 % as yield, which suggests that this approach of growing PAMPy dendrons onto organotalc is viable, and that a small percentage of products with structure defects were obtained.

Table 1. Elemental analysis for the PAMPy-functionalized phyllosilicates. Ncalc (mmol g-1)

Nexp (mmol g-1)

Ccalc (mmol g-1)

Cexp (mmol.g-1)

Weight Losscalc (%)

Weight Lossexp (%)

Yield (%)

Samples NH2-talc

1.0

0.89 ± 0.02

3.0

3.94 ± 0.04

--

--

--

PAMPy-talc-G1

4.9

4.06 ± 0.02

13.6

11.64 ± 0.03

21.3

19

75

PAMPy-talc-G2

9.1

7.83 ± 0.08

24.5

19.49 ± 0.07

34.8

27

78

PAMPy-talc-G3

12.9

10.76 ± 0.07

34.1

29.2 ± 0.4

45.4

36

79

The sequence of the different PAMPy dendron generations onto organotalc lamellae were identified by FTIR spectroscopy. The spectra of the materials presented in Fig. 2-I show the typical bands of talc at 1018, 544, and 458 cm-1 assigned to Si-O-Si antisymmetric stretching, Mg-O stretching, and Si-O-Si bending, respectively 30,43,44. The formation of the products of each synthesis step in each dendron generation onto talc interlamellar space is evidenced from FTIR data: the band at 1732 cm-1 in half generations PAMPy-talcs spectra (Fig. 2-I b, d, f) is attributed to C=O stretching indicating the existence of an ester terminal group in these materials 31,45 as products of Michael addition; and the absence of this band in the materials for entire generations (PAMPy-talcs, Fig. 2-I c, e, g) followed by the appearance of the bands at 1640, 1586 and 1470 cm-1 (related to NH deformation, C=O stretch and NCOO skeletal vibrations, respectively

46

) confirm the



success of the amidation reaction with 2,6-DAP and, consequently, the production of

544 458

658

890 796

1018

) ))))

)))) ))))

))))

gggg

Transmitance (a.u.)

))))

dddd eeee ffff

. a . u / e c n a t i m s n a r T

3687 3429 3200 2951 2846

I

1732 1640 1586 1470 1436

PAMPy-talc-G(1, 2, and 3), as shown by the scheme in Fig. 2-II.

aaaa bbbb cccc

))))

25 %T

1 -

4000 3500 3000

m c / r e b m u n e v a W

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

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1600 1400 1200 1000 800

600

400

Wavenumber (cm-1) II

1648 cm -1 1558 cm -1 1470 cm -1 1730 cm -1

Fig. 2. I- FTIR spectra from the organo-functionalized materials a) NH2-talc and PAMPytalc-G b) 0.5, c) 1, d) 1.5 e) 2 f) 2.5 g) 3, and II- scheme of formation of ester groups by Michael Addition and the sequential formation of amide groups by reaction with ethylenediamine.


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13

C-CP-MAS NMR was used in the study of the anchored compounds. The spectra

are shown in Fig. 3. The NH2-talc spectrum (Fig. 3Ia) exhibits signals at 12, 22, and 43 ppm, corresponding to the methylene groups of the aminopropyl chain (Fig. 3III) (see Fig. 3-I), respectively

19,20

. The peaks at 12, 22 and 43 ppm (Fig. 3I b-d) decreased in

comparison with the significant concentration of carbon atoms from the dendron backbone (Fig 3-IV) 49. Additionally, the PAMPy-talc spectra present two peaks at 33 and 52 ppm, which correspond to the contribution of the carbon atoms neighboring the amide group and the ones attached directly to tertiary amine, respectively

46

. The peak at 33 ppm is the

evidence of the formation of the products obtained by the Michael addition. A peak observed at 175 ppm is related to the carbonyl carbon atoms of amide group formed with the amidation reaction with 2,6-diaminopyridine, which present the peaks at 104, 113, 140 and 157 ppm41. The materials with different generations of PAMPy adsorb CO2 from air forming carbamate (Fig. 3 II) in the same manner as NH2-talc, causing the appearance of the signal at 165 ppm 50,51.


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22 12

52

II

43 33

113 104

165* 157 150 140

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

179


40

20

d)

c)

b)

a)

200

180

160

140

120

100

80

60

0

/ ppm δδ(ppm)

IIII

IV IV

165*

157 150 179

33

III III 12

Fig. 3. I -

13

43

43 22

22 12

52 113 140

C solid-state CP-MAS nuclear magnetic resonance spectra from a) NH2-talc

and PAMPy-talc-G b) 1, c) 2 and d) 3; II- structure of propylamonium carbamate onto organotalc; III - structure of NH2-talc and IV – structure of PAMPy-talc-G1.

Dendron-iodine charge transfer complexes linked to organotalc lamella were obtained by adsorption of vapor of iodine. The maximum adsorption capacity and kinetic constant were calculated by pseudo-second order kinetic model52,53. According to the kinetic curves (Fig. 4I), the organotalcs modified by the three different generations of the dendron PAMPy presented the same adsorption capacity (approximately 0.8 mmol g-1) and 16 ACS Paragon Plus Environment

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rate constant (0.04 g mol-1 h-1). In this case, the interlamellar distance, is approximately the same independent on the dendron dimension. The adsorption occurs, first, with terminal sites of interactions in the dendron chain and with those groups close to the solid surface, and diffuses into the interlamellar space, afterward. As mentioned above, the dendron grows parallel to the inorganic plane, and the higher number of generation, the more fulfilled the interlamellar space will be. Because of that, there are less available sites of interactions in PAMPy-talc-G2 and PAMPy-talc-G3 to adsorb iodine. The polyiodides intercalated into the interlamellar spaces of PAMPy-talc-Gn (n = 1, 2 and 3) producing PAMPy-talc-Gn (n = 1, 2 and 3)-I2 were identified by Raman spectroscopy (Fig. 4-II). The Raman spectra are very similar to those reported by Jerman et al.

54

for complexes between tri-iodide and 1-methyl-3-propylimidazole. According to the

authors, a band at 107 cm-1 is attributed to symmetric stretching of I-I bond and the lower intensity bands at 146 and 162 cm-1 correspond to antisymmetric stretching ions and antisymmetric stretching of I-I bond, respectively. However, with PAMPy-talc-G3-I2 (Fig. 4II-c) a band at 162 cm-1 is more intense and it is attributed to the resulting [(PAMPyG3)I]+ I5- complex27,54,55. Overall, the shoulder at 162 cm -1 is also observed with PAMPytalc-G1-I2 (Fig. 4II-a) and PAMPy-talc-G2-I2 (Fig. 4II-b) indicating that the C2v is produced in all materials. The PAMPy-dendron is comprised by three different donor sites: the nitrogen in the pyridine ring, the terminal primary amine, and the oxygen in the carbonyl of the amide group. The interaction between the iodine and the pyridine ring is identified with a weak peak at 127 cm-1 corresponding to the vibration of the antisymmetric stretching in the I-N bond

56

, as indicated in the proposed mechanism of

PAMPy-iodine charge-transfer complexes formation in Fig. 4-III.



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Fig. 4. I- Kinetic curves of adsorption of iodine by PAMPy-modified phyllosilicates, IIRaman spectra of charge-transfer complexes obtained from iodide adsorption in the PAMPy-modified phyllosilicates forming the materials a) PAMPy-talc-G1-I2, b) PAMPytalc-G2-I2 and c) PAMPy-talc-G3-I2, and III- proposed mechanism of PAMPy-iodine charge-transfer complexes formation onto organotalc surface (Reproduced with permission from reference 27. Copyright 2016 Elsevier).

Interestingly, the relative intensity of the bands 107 and 162 cm-1 decreases in PAMPy-talc-G3-I2 in comparison to the other materials. PAMPy-talc-G3-I2 grows on the


Page 19 of 41

plane and the interlayer space becomes hindered, therefore the polyiodides are formed with L-shape or V-shape to accommodate between the available spaces.

Conductivity of PAMPy-talcs gel electrolytes Ionic conductivity of the gel electrolytes (Table 2) prepared using PAMPy-dendron modified-organotalcs were calculated by Ohm Law (Equation 1) using the results obtained from EIS data (Fig. 6).

=

(1) where d is the distance between the electrodes (the thickness of the tape), S is the area, and R is the resistance for each of the gel electrolytes calculated from EIS spectra (Fig. 6).

100

10

A

B

PAMPy-talc-G1 PAMPy-talc-G2 PAMPy-talc-G3

80

PAMPy-talc-G1-I2 PAMPy-talc-G2-I2

8

PAMPy-talc-G3-I2

60

-Z'' / Ω

-Z'' / Ω

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


40

20

___ Fit

6

4

2

0

0 0

30

60

90

120

150

180

14

16

18

20

22

24

26

28

30

Z' / Ω Z' / Ω Fig. 5. Typical EIS spectra of electrolytes gelled by A) PAMPy-talc-Gn (n=1, 2 and 3) and B) PAMPy-talc-Gn-I2 (n=1, 2 and 3).



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Page 20 of 41

Table 2. Ionic conductivity of the electrolytes. Electrolyte

σ×10-4 (S cm-1)

Electrolyte

σ×10-3 (S cm-1)

PAMPy-talc-G1

2.2 ± 0.1

PAMPy-talc-G1-I2

1.8 ± 0.1

PAMPy-talc-G2

0.9 ± 0.1

PAMPy-talc-G2-I2

1.9 ± 0.1

PAMPy-talc-G3

1.1 ± 0.2

PAMPy-talc-G3-I2

2.3 ± 0.1

Gel electrolytes present lower ionic conductivity than liquid electrolytes (5×10-3 to 3.5×10-2 S cm-1, depending on the iodide/triiodide concentration, according to Jerman et

al.54). However, the gel electrolytes based on PAMPy-talc presented higher ionic conductivity values than some polymer-gelled electrolytes reported in literature using PEO or PEG (~1×10-5 S cm-1)57, and similar results than those reported to P(EO-EM)/GBL/LiI/I2 (~1×10-3 S cm-1)58.

The electrolyte gelled by PAMPy-talc-G1 presented higher ionic

conductivity values than those electrolytes containing talcs with high generations of PAMPy-dendron, it is probably associated to low amount of available basic groups in the dendron which interacts with iodide species forming polyiodides. As expected, the gel electrolytes containing the organotalcs with previous adsorbed iodine presented higher conductivity values than those without iodine-dendron charge-transfer complexes. The enhanced ionic conductivity is attributed to the increased concentration of iodide species. In addition, PAMPy-talc-G (1, 2 and 3)-I2 gel electrolytes are comprised by polyiodides, as shown before by Raman spectra. Polyiodides forms bridges or conducting chains for the charge transfer known as Grotthus mechanism54. Therefore, the combination of Grotthus mechanism and the mobility of the ions into the gel increased the ionic conductivity values of the electrolytes gelled by PAMPy-talc-Gn-I2 in one order of magnitude in comparison to the electrolytes gelled by PAMPy-talc-Gn. 20 ACS Paragon Plus Environment

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Photovoltaic performance and stability of quasi-solid DSSCs based on PAMPy-talcs The introduction of PAMPy-functionalized phyllosilicates in the quasi-solid electrolytes led to a slight increase in the photocurrent (Fig. 6 A) with lower generations (on average 11 % with PAMPy-talc-G1 and 4 % with PAMPy-talc-G2), and decrease when using the higher dendron generation (20 % with PAMPy-talc-G3) in comparison to the solar cells using the liquid reference electrolyte. The addition of I2 increased the short circuit current density (JSC) of the first generation cells (PAMPy-talc-G1-I2) further 18 % in comparison to PAMPy-talc-1 (due to its high conductivity) and in total 31 % when compared to the liquid electrolyte. However, the formation of charge-transfer complexes with iodine in PAMPy-talc-G2 or PAMPy-talc-G3, and the increased conductivity of these electrolytes, did not improve the JSC of the devices. The reason it happens will be discussed in the IPCE results.



800

A

B

12

750 700

8

Voc (mV)

-2

Jsc (mA cm )

10

6 4

650 600 550

2 0

500 G1

G2

G3

C 80

G1

D

G2

5

Reference

4

60 50

G3 PAMPy-talc-Gn PAMPy-talc-Gn-I2

70

3

η (%)

FF (%)

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

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40 30 20

2 1

10 0

0 G1

G2

G3

G1

G2

G3

Fig. 6. Average photovoltaic characteristics (A - JSC, B - VOC, C - FF and D - η, obtained from J-V curves in Fig. S-3, see supporting information) of three solar cells prepared with each type of electrolyte, the reference, PAMPy-talc-G 1, 2, and 3) and PAMPy-talc-G(1, 2 and 3)-I2 electrolytes, under 1.0 Sun.

The average VOC (Fig. 6 B) remained the quite similar with the introduction of PAMPy-talc-G(1, 2, and 3) in the electrolyte but decreased approximately 50 mV in the solar cells with PAMPy-talc-G(1, 2, and 3)-I2. The decreased VOC could be attributed to a positive shift in conduction band limit, or an increased recombination between injected charges into TiO2 film with dye or electrolyte, or moreover, a variation in the electrolyte redox potential 2,3. The origin of the decreased VOC is investigated in more detail with EIS in a section related to the charge transfer in the cells. 22 ACS Paragon Plus Environment

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The fill factor, FF, was similar in all the cell types: around 65 % (Fig. 6 C). For the devices containing the PAMPy-talc-G(1, 2, and 3) the conversion efficiencies were similar in the limits of the group-wise standard deviations and 10 % smaller compared to the reference cells with liquid electrolyte (Fig. 6 D). For the first generation, the intercalation of polyiodides increased the conversion efficiency by 15 % in comparison to the nonintercalated gel. On the other hand, the intercalation of polyiodides decreased the efficiency of the second generation and did not affect the conversion efficiency of the third generation. Note that these cells were not optimized for high performance as explained in the experimental section, but to maximize the gaining of insight in particular during aging. In terms of initial performance, it is therefore better to focus on the relative differences between the cell groups. An important requirement of dye solar cells with additives in the electrolyte is that the additives should not react with other components in the cell and the stability of the device should remain high59. Here the solar cells assembled with the electrolytes gelled by PAMPy-talc-G(1, 2 and 3) and PAMPy-talc-G(1, 2 and 3)-I2 and the cells with liquid electrolyte were aged for approximately 1000 h under irradiation of 1 sun equivalent at 45 °C. Fig. 7 shows that the efficiency of the cells remained quite constant during the studied period under accelerated aging conditions. The fill factor (Fig. 7C) values were constant in the limits of group-wise standard deviations and the slightly decreased VOC values (VOC initial/VOC final = ~0.9, Fig. 6B) were compensated at the end by JSC that kept increasing in all the groups during the aging (JSC initial/JSC final > 1, Fig. 7A).



1.4

1.4

A

1.2

1.0

VOC / VOC 0

Jsc / Jsc 0

1.2

0.8 0.6

0.8 0.6 0.4

0.2

0.2 0.0

G1

G2

G3

1.4 1.2

reference PAMPy-talc-Gn PAMPy-talc-Gn-I2

B

1.0

0.4

0.0

G1

G2

G3

G1

G2

G3

1.4

C

1.2

D

1.0

η / η0

1.0

FF / FF0

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 41

0.8 0.6

0.8 0.6

0.4

0.4

0.2

0.2

0.0

0.0

G1

G2

G3

Fig. 7. Average final/initial aging data (A - JSC, B - VOC, C - FF and D - η) obtained from three solar cells assembled with each electrolyte gelled by PAMPy-talc-G(1, 2 and 3) and PAMPy-talc-G(1, 2 and 3)-I2 and those assembled with liquid reference electrolyte in light soaking 1 Sun (100 mW cm-2). JSC0, VOC0, FF0 and η0 represent the data measured before aging, and JSC, VOC, FF and η are the values measured after aging.

The final efficiency of the cells gelled by PAMPy-talc-G1 was 92 ± 1 % of the initial. In our previous work, it was shown that amide groups of the PAMAM dendron can interact with iodide species resulting in the solar cell degradation 27. Here, the removal of redox species is apparently suppressed due to the smaller interlamelar space, which hidden the probable sites of interactions present in the dendron chains. However, the intercalation of polyiodides into the organotalc (PAMPy-talc-G1-I2) resulted in an increased 24 ACS Paragon Plus Environment

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photocurrent and efficiency - JSC were slightly higher than in the reference cells during most of the test period due to an increase in the redox species concentration.

The influence of adding gel PAMPy-talc electrolyte on DSSCs short-circuit current and charge transport DSSCs assembled with quasi-solid electrolyte PAMPy-talc-G1-I2 presented higher initial conversion efficiency and short-circuit current than the reference cells. The effect of this increase in the JSC might be attributed to the phenomena of light scattering by the PAMPy-talc-G1-I2 particles used for the preparation of the quasi-solid electrolyte6. The addition of this phyllosilicate should increase the optical path of the light in the cell and, as a consequence, the photon-to-current conversion efficiency improves60. The improvement in IPCE was indeed detected in this work (Fig. 8). There is also another possible contribution to the increased JSC suggested in the literature: The dendron can interact with the oxide surfaces producing a more compact monolayer besides the adsorbed sensitizer. The attached dendron has been suggested to eliminate hydrophilic surface sites available for water absorption, causing a decrease in the initial efficiency of the cells61, but this effect was not observed here. The value of JSC depends on the amount of sensitizer adsorbed to the TiO2 nanoparticles and the optimal sensitizer conformation. Because the concentration of the sensitizer in these cells was fixed, the JSC increased with PAMPy-talc-G1-I2 probably because of better conformation and the retarded interfacial charge recombination61.



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Fig. 8. I and II -Incident photon-to-current curves of the solar cells prepared with PAMPytalc-G(1 and 3) and PAMPy-talc-G(1 and 3)-I2 before and after aging, respectively. IIIdiffuse reflectance curves of the dendron modified-talc. IV-Vertical slid test of the electrolytes (A-liquid reference, B- PAMPy-talc-G1-I2, C- PAMPy-talc-G2-I2, and DPAMPy-talc-G3-I2) on a glass plate representing the corresponding viscosity.

The photon-to-current conversion efficiency and consequently the JSC in the solar cells with higher dendron generation (see Fig. 8A to PAMPy-talc-G3 and PAMPy-talc-G3-I2) are initially lower than the values of the first generation. It appears that this effect can be related to the stronger absorption of the higher generation materials at the same region than N719 dye: Fig. 8C illustrates that the reflectance is higher with the higher generation 26 ACS Paragon Plus Environment

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materials than with the lower generation materials, which should lead to more adsorption if the materials behave otherwise similarly. After the aging, all the devices had improved QE (Fig. 8B). This may be due to improvement in electrolyte penetration into mesoporous TiO2 film

62

, consequently,

improving JSC as shown by the aging data. Figs. 9 A and B show that the current is not limited by diffusion of ions in any of the electrolytes because the relationship between the short-circuit current and the light intensity is linear (R2 > 0.991). However, based on Figs. 9 A and B, the photocurrent of the electrolytes gelled by PAMPy-talc with high dendron generations, especially G3, is slightly lower in the entire measured intensity range. The higher viscosity of these high dendron generation-modified talc-gelled electrolyte (Fig. 8-IV) might reduce the contact within the mesoporous TiO2 layer increasing recombination with photogenerated electrons with oxidized sensitizer, decreasing the photocurrent 27. The phenomenon is visible even with materials previously intercalated with polyiodides. In addition, the current decreases because the highest dendron generations are comprised by a large amount of basic sites at the surface, increasing the removal of the free tri-iodide from the electrolyte

27

. The

adsorbed species interacting with the dendrons diffuse more slowly than the free species, hence, decreasing the photocurrent.



8

10

A

10

C

7

10

-3

10

E

Reference PAMPy-talc-G1 PAMPy-talc-G2 PAMPy-talc-G3

1

-2

/ F cm-2 CCPE PE (F cm )

10

2

/ Ω cm cm 2) RRPEPE (Ω

Log (Jsc)

6 5

10

4

10

3

10

0.1

-4

10

-5

10

2

10

-6

10

1

10

1

Log (Sun equivalents)

-0.2

-0.3

9

10

B

-0.4 -0.5 -0.6 Voltage/(V) Voltage V

8

-0.7

-0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 Voltage Voltage(V) /V

D

10

-3

10

7

1

PAMPy-talc-G2-I2 PAMPy-talc-G3-I2

0.1

1

Log (Sun equivalents)

5

10

4

10

3

PE

Reference PAMPy-talc-G1-I2

10 6 10

-2 C cm-2 ) C PE /(FF cm

2

cm2) RRPEPE/(Ω Ω cm

10

Log (Jsc)

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 28 of 41

10 2 10

F

-4

10

-5

10

∆Vrec

1

-6

10

10

0

10

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

Voltage (V) Voltage /V

-0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8

Voltage(V) /V Voltage

Fig. 9. A) and B) bilogarithmic plots of short-circuit current, JSC, versus light intensities, and C) to F) EIS data describing the photoelectrode performance (C: and D: resistance in the photoelectrode / electrolyte interface, RPE, and E and F: capacitance in the photoelectrode / electrolyte interface, CPE) of the solar cells prepared with PAMPy-talcG(1, 2, and 3) and PAMPy-talc-G(1, 2, and 3)-I2.

The addition of dendron modified-talc into the electrolyte did not cause any significant differences in the charge-transfer resistance, RCT, at the counter electrode / electrolyte interface and in the mass-transport impedance at the counter electrode, ZD, in the DSSCs (Table 3). Consequently, both reference liquid and quasi-solid devices presented similar initial FF values (Fig. 6). Additionally, these parameters did not significantly change after the aging. Therefore, FFfinal/FFinitial is approximately equal to 1 for all the cells (Fig. 7).


Page 29 of 41

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Table 3. Average series resistance (Rs), charge-transfer resistance (RCT), charge transport impedance (ZD), and chemical capacitance (Cµ) of solar cells with the reference and gelled electrolytes.

Time (h)

0

1000

Electrolytes

Rs

RCT

ZD 2

Cµ 2

-3

(Ω)

(Ω cm )

(Ω cm )

(10 F cm --2)

Reference

19 ± 3

6±2

0.8 ± 0.1

2.1 ± 0.2

PAMPy-talc-G1

22 ± 8

7±2

0.68 ± 0.04

2.3 ± 0.1

PAMPy-talc-G2

21 ± 4

6±1

0.6 ± 0.1

2.5 ± 0.2

PAMPy-talc-G3

19 ± 1

7±2

0.61 ± 0.05

2.3 ± 0.3

PAMPy-talc-G1-I2

18 ± 2

4±1

0.6 ± 0.1

2.4 ± 0.3

PAMPy-talc-G2-I2

17 ± 2

5.2 ± 0.2

0.51 ± 0.04

2.3 ± 0.3

PAMPy-talc-G3-I2

25 ± 8

6±1

0.5 ± 0.1

1.5 ± 0.6

Reference

16 ± 6

3±1

0.4 ± 0.3

2.6 ± 0.1

PAMPy-talc-G1

29 ± 2

6±3

0.9 ± 0.1

2.7 ± 0.2

PAMPy-talc-G2

15.1 ± 0.2

4±2

0.5 ± 0.1

2.5 ± 0.3

PAMPy-talc-G3

17.3 ± 0.1

6±1

0.69 ± 0.02

2.3 ± 0.2

PAMPy-talc-G1-I2

20 ± 3

5±2

0.65 ± 0.02

2.2 ± 0.1

PAMPy-talc-G2-I2

20.3 ± 0.4

3.6 ± 0.1

0.66 ± 0.03

2.2 ± 0.1

PAMPy-talc-G3-I2

16 ± 3

4.1 ± 0.1

0.5 ± 0.1

2.0 ± 0.1

EIS in the dark was measured to investigate the charge transfer in the photoelectrode/electrolyte interface, which represents the recombination dynamics in the solar cells63. There is a strong correlation between VOC and RPE: the voltage differences found in the VOC of the devices with gel electrolytes in comparison to the reference solar cells match very well with the voltage shift, ∆Vrec, in RPE observed in Fig. 9C and Fig. 9D, i.e. ∆Vrec ~ 100 mV to PAMPy-talc-G(1, 2 and 3) and ∆Vrec ~ 50 mV to PAMPy-talc-G(1, 2 and 3)-I2. This indicates that RPE is the parameter that dominates the photovoltage. A decrease in VOC is related to a downward shift in the conduction band of the semiconductor63,64. This band-edge shift has been attributed to an increase in the positive 29 ACS Paragon Plus Environment


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charge of TiO2 nanoparticles because the basic groups in the PAMPy-dendron interact with the iodide species and the cation PMI+ interacts with TiO2 surface 65. Significant variations were also observed in the chemical capacitance (Fig. 9E and Fig. 9F). The increase in Cµ of the quasi-solid solar cells (except those prepared with PAMPy-talc-G1) is related to the influence of electron trapping, suggesting that the charge recombination between the injected electrons and the electron acceptors in the redox electrolyte, , was retarded with the addition of PAMPy-modified talc66 and it justifies the higher QE seen in IPCE curves67 in Fig. 8. Colorimetric analysis of the electrolytes Adding clays can change the tri-iodide concentration in the electrolyte because the particles can absorb these species 27,68. If the clay removes large amounts of tri-iodide, JSC and charge-transfer in solar cells can be damaged, causing degradation of the device27. To evaluate whether the PAMPy dendron modified-talcs are removing triiodide from electrolyte solution, the solar cells were photographed over the aging period and the concentration of triiodide was analyzed via the blue pixel value of the electrolyte in the recorded images29,69. Solutions containing ion are yellowish3, and since blue is the complementary color of yellow in RGB color model, the blue pixel contains information about tri-iodide concentration, [ ]70,71. Thus, Fig. 10 shows blue pixel values of the electrolytes during the aging as a tool to evaluate the influence of the presence of these organotalcs on the electrolyte and on the stability of the solar cells. Initially, the blue pixel values of the gel electrolytes are lower than the reference liquid electrolyte, since the organotalcs absorb at the visible region (Fig. 8C), intensifying the yellowish in the electrolyte mixtures. Up to 100 h under illumination, the blue pixel values increase which might be related to the initial stabilization of the cells. After that, the 30 ACS Paragon Plus Environment

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curves are very constant and almost parallel to the x-axis, which means that visible variations did not occur in the electrolyte color. These results imply that the electrolytes in these quasi-solid dye-sensitized solar cells have not yet started to degrade at the end of the aging test, thus, it can be expected that the lifetime of these devices would be significantly longer than 1000 h.

100

A Blue Pixelpixel azul

80 60 40 Ref Referência erence

20

PAMPy-talc-G1 PAMPy-talc-G 1 PAMPy-talc-G2 PAMPy-talc-G 2 PAMPy-talc-G3 PAMPy-talc-G 3

0 0

200

400

600

800

1000

Tempo Time (h)// hh Tempo 100

B B 80

Pixelpixel azul Blue

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


60 40 20 Reference Referência

PAMPy-talc-G2-I22 PAMPy-talc-G2-I

PAMPy-talc-G1-I PAMPy-talc-G1-I2

2

0 0

200

400

PAMPy-talc-G3-I PAMPy-talc-G3-I 2 2

600

800

1000

Time Tempo (h) / h

Fig. 10. Blue pixel values of the solar cells with electrolytes gelled by A - PAMPy-talcG(1, 2, and 3) and B - PAMPy-talc-G(1, 2, and 3)-I2 in comparison to a reference cell during a period of 1000 h under illumination of 1 sun equivalent (100 mW cm-2). The solid lines represent the average blue pixel values and the shadows are the standard deviations.



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CONCLUSIONS PAMPy dendron-functionalized organotalcs were applied to obtain quasi-solid electrolytes for dye sensitized solar cells. The organotalcs were prepared by a microwaveassisted divergent method, resulting in materials with different surface properties due to the intermolecular organization of PAMPy-dendron by formatting of head-to-tail hydrogen bond between carboxyl and amine groups between lamellae. Polyiodides were intercalated into PAMPy-modified organotalcs by adsorption of iodine producing the charge transfer complexes [PAMPy-I+]I3- and [PAMPy-I+]I5. The operation of the gel electrolytes was investigated using several techniques, and the effect of the polyamidopyridine dendron generation modified-talc and that of the intercalation of polyiodides into these materials on solar cells performance were demonstrated. The organotalcs with higher dendron generations absorb at the same region of the N719 dye, decreasing short-circuit current and, consequently, the overall efficiency. However, DSSCs assembled with quasi-solid electrolyte PAMPy-talc-G1 and PAMPytalc-G1-I2 presented higher conversion efficiency than the other quasi-solid solar cells, even higher than the liquid solar cells. PAMPy-talc-G1 and PAMPy-talc-G1-I2 presented lower steric hindrance into lamellae space, which did not affect the iodide diffusion in the electrolyte. The intercalation of polyiodides into organotalc improved the solar cell performance and presented a good stability as well. Both the measured performance and the image analysis of the electrolyte suggest that the talcs were inert which is an important characteristic regarding the stability of the cells. This is a great advancement as in previous studies other talcs have been absorbing charge carriers from the electrolyte causing significant stability problems, but such were absent here. Therefore, the excellent


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applicability of these organotalcs to the quasi-solid electrolytes of dye solar cells was demonstrated.

Supporting information available: Thermogravimetric analysis of the materials, and J-V curves, EIS, and aging data of the solar cells presented in this work. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENTS The authors are indebted to Fundação de Amparo à Pesquisa no Estado São Paulo, FAPESP

(2013/05911-1

and

2014/06942-0)

and

to

Conselho

Nacional

de

Desenvolvimento Científico e Tecnológico, CNPQ (200082/2015-9 and 490241/2012-3). The authors also thank Academy of Finland (projects 271081 and 253643). REFERENCES (1)

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Figure Captions Scheme 1. Illustration of PAMPy dendron growth reactions onto talc lamella surface. Fig. 1. I- X-ray diffractograms of a) NH2-talc, b) PAMPy-talc-G1, c) PAMPy-talc-G2, d) PAMPy-talc-G3, e) natural talc. II, IV and V shows the illustration of the PAMPy dendron growth on the plane of the phyllosilicate, and III shows the self-organization between PAMPy-dendrons. Fig. 2. I- FTIR spectra from the organo-functionalized materials a) NH2-talc and PAMPytalc-G b) 0.5, c) 1, d) 1.5 e) 2 f) 2.5 g) 3, and II- scheme of formation of ester groups by Michael Addition and the sequential formation of amide groups by reaction with ethylenediamine. Fig. 3. I -

13

C solid-state CP-MAS nuclear magnetic resonance spectra from a) NH2-talc

and PAMPy-talc-G b) 1, c) 2 and d) 3; II- structure of propylamonium carbamate onto organotalc; III - structure of NH2-talc and IV – structure of PAMPy-talc-G1. Fig. 4. I- Kinetic curves of adsorption of iodine by PAMPy-modified phyllosilicates, IIRaman spectra of charge-transfer complexes obtained from iodide adsorption in the PAMPy-modified phyllosilicates forming the materials a) PAMPy-talc-G1-I2, b) PAMPytalc-G2-I2 and c) PAMPy-talc-G3-I2, and III- proposed mechanism of PAMPy-iodine charge-transfer complexes formation onto organotalc surface25.



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Fig. 5. Typical EIS spectra of electrolytes gelled by A) PAMPy-talc-Gn (n=1, 2 and 3) and B) PAMPy-talc-Gn-I2 (n=1, 2 and 3). Fig. 6. Average photovoltaic characteristics (A - JSC, B - VOC, C - FF and D - η, obtained from J-V curves in Fig. S-3, see supporting information) of three solar cells prepared with each type of electrolyte, the reference, PAMPy-talc-G 1, 2, and 3) and PAMPy-talc-G(1, 2 and 3)-I2 electrolytes, under 1.0 Sun. Fig. 7. Average final/initial aging data (A - JSC, B - VOC, C - FF and D - η) obtained from three solar cells assembled with each electrolyte gelled by PAMPy-talc-G(1, 2 and 3) and PAMPy-talc-G(1, 2 and 3)-I2 and those assembled with liquid reference electrolyte in light soaking 1 Sun (100 mW cm-2). JSC0, VOC0, FF0 and η0 represent the data measured before aging, and JSC, VOC, FF and η are the values measured after aging. Fig. 8. I and II -Incident photon-to-current curves of the solar cells prepared with PAMPytalc-G(1 and 3) and PAMPy-talc-G(1 and 3)-I2 before and after aging, respectively. IIIdiffuse reflectance curves of the dendron modified-talc. IV-Vertical slid test of the electrolytes (A-liquid reference, B- PAMPy-talc-G1-I2, C- PAMPy-talc-G2-I2, and DPAMPy-talc-G3-I2) on a glass plate representing the corresponding viscosity. Fig. 9. A) and B) bilogarithmic plots of short-circuit current, JSC, versus light intensities, and C) to F) EIS data describing the photoelectrode performance (C: and D: resistance in the photoelectrode / electrolyte interface, RPE, and E and F: capacitance in the photoelectrode / electrolyte interface, CPE) of the solar cells prepared with PAMPy-talcG(1, 2, and 3) and PAMPy-talc-G(1, 2, and 3)-I2. Fig. 10. Blue pixel values of the solar cells with electrolytes gelled by A - PAMPy-talcG(1, 2, and 3) and B - PAMPy-talc-G(1, 2, and 3)-I2 in comparison to a reference cell 40 ACS Paragon Plus Environment

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during a period of 1000 h under illumination of 1 sun equivalent (100 mW cm-2). The solid lines represent the average blue pixel values and the shadows are the standard deviations.

TOC graphic TiO2 + Dye

PAMPy-talc

Quasi-solid DSSC


Gel Electrolytes with Polyamidopyridine Dendron ... - ACS Publications

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