Understanding the Role of Reduced Graphene Oxide in the Electrolyte

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Understanding the Role of Reduced Graphene Oxide in the Electrolyte of Dye Sensitized Solar Cells Paulo Ernesto Marchezi, Gabriela Gava Sonai, Marcelo K. Hirata, Marco A. Schiavon, and Ana F. Nogueira J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07319 • Publication Date (Web): 23 Sep 2016 Downloaded from http://pubs.acs.org on September 27, 2016

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The Journal of Physical Chemistry

Understanding the Role of Reduced Graphene Oxide in the Electrolyte of Dye Sensitized Solar Cells

Paulo E. Marchezi1, Gabriela G. Sonai1, Marcelo K. Hirata1, Marco A. Schiavon1,2 and Ana F. Nogueira1*

1

Laboratório de Nanotecnologia e Energia Solar (LNLS), Chemistry Institute, University of

Campinas (UNICAMP), P. O. Box 6154, 13083-970, Campinas, São Paulo, Brazil. 2

Department of Natural Science, Federal University of São João del-Rei (UFSJ), 36301-160, São

João del-Rei, Minas Gerais, Brazil. *Corresponding author: E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT One of the major drawbacks in dye-sensitized solar cells (DSSC) is related to the use of a liquid electrolyte, which limits durability and stability. Part of this problem can be solved by replacing the liquid electrolyte by a polymer or gel electrolyte, although the open circuit potential (VOC) of the solar cells is affected. In this work, the role of the reduced graphene oxide (RGO) added to a gel electrolyte in order to improve the efficiency of DSSCs is discussed in detail. The gel polymer electrolyte is composed of poly(ethylene oxide) (PEO), -butyrolactone (GBL), LiI, I2 and different concentrations of RGO. The best solar cell using 0.5 wt% of RGO delivered an efficiency of (5.07±0.97)%, with the highest values of Isc and VOC. RGO sheets are acting as a multipurpose component in the electrolyte. The recovery of the VOC values can be related to the removal of polyiodide species from the photoanode surface by interaction with the RGO sheets. The increase in the Isc is assigned to the enhancement in the diffusion of I3- species, and by the reduction of the electron transfer resistance in the counter electrode.


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Introduction From all the emerging photovoltaic technologies, dye sensitized solar cells (DSSCs) are the first devices to reach acceptable efficiency values to be introduced into the market. DSSCs have leadership in number of researchers, industry cooperation, solar panel prototypes, testing and products. Since the first demonstration in 1991 by O´Regan and Grätzel1 and after two decades of research and development, DSSCs with iodine/triiodide (I-/I3-) liquid electrolyte have achieved light to electric power conversion efficiencies over 11% using ruthenium dyes,2 more than 12% based on zinc porphyrin dyes,3 and > 10% based on metal-free organic dyes.4 Recently, through the molecular engineering of zinc porphyrin sensitizers,5 Grätzel and co-workers have reported DSSC with 13% power conversion efficiency. The new sensitizers feature the prototypical structure of a donor–π-bridge–acceptor, maximize electrolyte compatibility and improve lightharvesting properties.6 DSSC efficiency has continued to increase during the last years, achieving the record PCE of 14%.7 However, there remain several problems that limit the mass production and compromise the long-term stability of DSSCs, such as leakage and evaporation of the liquid electrolyte, corrosion of the metal-based current collectors (silver, copper, etc.) by iodine, and so on. Many efforts have been made to find alternative electrolytes and/or to introduce new concepts to develop novel electrolyte-free DSSCs. Solid-state dye sensitized solar cells (ss-DSCC) based on hole transport materials (HTM) such as the macromolecule 2,2',7,7'tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD) have reached efficiencies close to 7%.5,8 However, the low pore filling by the HTM material, high rate of charge recombination and parasitic absorption losses limit the efficiency and stability of these devices. 9–11


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Our group was the pioneer to introduce polymer electrolytes in DSSCs in the middle of the 90’s12-14. The first DSSC assembled in our laboratory have employed a polymer electrolyte based on poly(epichlorohydrin-co-ethylene oxide) copolymer (P(EO/EP)), using the pair iodine/iodide and achieved an efficiency of only 0.22%.15 In 2001, after introducing changes in the electrolyte preparation, and in the titania film, an efficiency of 1.6% was attained in our group16, demonstrating that this type of solid-state electrolyte had potential to achieve even higher efficiencies. At this time, we have already observed that solar cells employing polymer electrolytes were more stable after long time of light soaking. Further works using polymer plasticizers and other small molecules additives to the electrolyte provided DSSC with efficiencies up to 4.4%.17,18 Although conductivity values similar to liquid electrolytes were attained, those values are in a great extension, a consequence of the ionic transport or mobility of the positive charged species, such as Li+ and Na+, and not related to the anionic species, which are detrimental to the dye cation regeneration (regeneration of the dye cation relies on the reaction with the iodide species in the standard electrolyte). Besides, when replacing the polymer by a gel electrolyte the gain in photocurrent (due to an increase in the ionic conductivity) was always followed by a drop in the VOC values. In a previous work, we assigned this effect to the presence of higher polyodide species that cause an increase in the charge recombination.17 Many works have been published about gel electrolytes employing different polymers.14,19 Wang et al. prepared a quasi-solid-state electrolyte using poly(vinylidenefluoride-co-hexafluoropropylene (PVDF-HFP) and achieved more than 6% of efficiency.20 Ileperuma et al. reported a DSSC using a poly(acrylonitrile) PAN-based polymer gel electrolyte, which achieved a high efficiency of 7.23%.21 More recently, Dissanayake et al. reported a mixed cation polyethylene oxide (PEO)-


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based electrolyte for dye-sensitized solar cell with efficiency up to 4.44%.22 This area was recently reviewed by our group in reference 14. Reduced graphene oxide (RGO) is a solution-processable carbon nanostructured material, which differs from graphene in several aspects, in chemical composition, optical and electronic properties. RGO is less conducting than pristine graphene due to residual oxygen-containing functional groups such as carbonyl, hydroxyl or epoxy, which still persist in its structure. The electron-accepting ability presented by RGO sheets is still an outstanding property that can be used to enhance electron transport properties when RGO is incorporated in inorganic matrices to form nanocomposites23-26. Our group has reported interesting results combining RGO sheets with TiO2 nanoparticles and their applications in methylene blue dye photodegradation,27 photocatalysis for water splitting,28 CO2 photorreduction29 and as electron transport layer in organic solar cells (OPV).30,31 Herein, we describe the preparation and characterization of a novel nanocomposite polymer gel electrolyte based on reduced graphene oxide (RGO) and a PEO copolymer, and its application in DSSCs. The role of the RGO in the nanocomposite electrolyte is explored using several electrochemical techniques, time resolved and Raman spectroscopies, thermal analysis and rheology. This work provides a scientific understanding of how RGO sheets are acting in the electrolyte, based on their interaction with the polymer matrix and polyiodide species.

Experimental Section GO was synthesized from natural graphite powder using a modified Hummers and Offeman’s method.32 Graphite powder (10 g, Micrograf 99507 UJ, Nacional de Grafite Ltd.) was added to a mixture of concentrated H2SO4 (60 mL), K2S2O8 (5 g) and P2O5 (5 g). The resultant solution was


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heated to 80 °C and was stirred for 6 h. The mixture was then diluted with distilled water, filtered, and washed with water until the rinse water pH turned neutral. The pre-oxidized graphite powder was added to a mixture of concentrated H2SO4 (200 mL) and NaNO3 (5 g) under vigorous stirring, and the solution was cooled to 0 °C. Next, 15 g of KMnO4 was added gradually under stirring and the temperature of the mixture was kept below 20 °C. The mixture was stirred at 40 °C for 4 h, and then diluted with distilled water (2 L) in an ice bath. The reaction was terminated by the addition of 30% (v/v) H2O2 solution (50 mL) and a brilliant yellow product was formed. The mixture was filtered and washed with 3% (v/v) H2SO4 solution. The GO product was suspended in distilled water to give a viscous and brown dispersion, which was subjected to dialysis to completely remove metal ions and acids. RGO was prepared by a solution of GO in ethanol (22 wt%), which was sonicated with tetrabutylammonium hydroxide (TBAH, 0.087 mol L-1, Sigma-Aldrich) for 30 minutes. After, hydrazine hydrate (0.37 mol L-1, Merck, N2H4·H2O 50% v/v in H2O) was added into solution under stirring at room temperature. Then, the solution was kept under sonication at room temperature for 2 h. The resulting fine precipitate, RGO, was isolated by repeated centrifugation using a mixture of ethanol and diethyl ether and was dried in a vacuum oven at room temperature for 5 h. In order to prepare the gel electrolyte, the copolymer Poly (ethylene oxide-co-2-(2methoxiethoxy) ethylglicidyl ether-co-allylglycidyl ether) [P(EO/EM/AGE)] (MM = 2.15×106 g mol-1, Daiso Co. Ltd.) was dissolved in GBL at the concentration of 85 wt% related to the copolymer by stirring for 24 hours at room temperature. After that, RGO was added in three concentrations (0.1, 0.3 and 0.5 wt%), together with LiI (98%, Merk) and I2 (99,5% Merk), at concentrations of 20% and 2%, respectively (relative to the total mass (copolymer + GBL)). The


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solutions remained sealed and stirred for 24 hours. RGO concentrations higher than 0.5 wt% were tested but they were unstable after sonication due to graphene precipitation. Dye sensitized solar cells were assembled with 0.25 cm2 of active area. A TiO2 suspension (DSL 18NR-T, DyeSol Ltd.) was deposited by the doctor blading technique onto the FTO substrates (Hartford Glass Co. Inc., sheet resistance: 15 Ω sq-1). The films were heated to 450 ºC for 30 min., giving a 6 µm layer. The electrodes were immersed in a 2.5×10−4 mol L−1 solution of the

dye,

[cis–bis(isothiocyanate)

bis

(2,2-bipyridyl-4,4-dicarboxylate]ruthenium(II)bis-

tetrabutylammonium (N719 Dye, Solaronix) in t-butanol : acetonitrile (1:1) for 18 h at room temperature. Afterwards, the electrodes were washed with ethanol. The electrolyte deposition was done inside a vacuum chamber33. Subsequently, Pt counter electrodes were sealed onto the top of the gel electrolyte film. J-V curves under illumination of 100 mW cm−2 were measured under standard AM 1.5 conditions using a Xe (Hg) lamp as light source and filters. Viscoelastic measurements (HAAKE Rheometer, RheoStress RS1) were performed as a function of shear rate. Oscillatory frequency sweep tests were done maintaining the fixed shear rate of 10-1 2-1 and varying the frequency of 10-1 to 102 Hz. The differential scanning calorimetric curves (DSC, TA Instruments 2100) were obtained using constant flow of Argon (100 ml min-1) according to: (a) Heating to 100 ºC at a rate of 10 ºC min-1; (b) 5 min isotherm at 100 ºC; (c) cooling from 100 ºC to -100 ºC with rate of 10 ºC min-1; (d) 5 min isotherm at -100 ºC; (e) Heating up to 100 ºC with a rate of 10 ºC min-1. The registered curves are related to the second heat. The thermogravimetry (TGA, TA Instrument 2950) was performed from room temperature to 500 ºC with a heating rate of 10 ºC min-1, under constant Argon flow (100 ml min-1). X-ray diffraction (XDR) was measured in a Shimadzu XRD-7000 operating in scan mode with λ = 1.54086 Å (CuKα) radiation generated at 30 kV and 30 mA. For electrolytes, Raman


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spectroscopy (micro Raman XploRA, Horiba) was performed using a He-Ne laser at 638 nm. For GO and RGO samples, Raman spectroscopy (T6400, Jobin Yvon Horiba) was performed using a He-Ne laser in 632.8 nm. For conductivity measurements, electrolyte solutions were dropped onto a stainless steel disk (area of 1.0 cm2) using a 0.05 mm-thick Teflon® spacer. Conductivity values were calculated from electrochemical impedance spectroscopy (EIS) data, using an Autolab PGSTAT 12 in the frequency range of 10 to 103 Hz and applying alternating current (AC) amplitude of 10 mV. Cyclic voltammetry (CV) measurements were performed according to Papageorgiou et al.34, in a potential range from -0.70 to 0.70 V (Autolab PGSTAT 30). Prior to characterization, the samples were maintained in a desiccator containing P2O5 for four days to eliminate the residual water. Transient absorption measurements were performed using N2 dye laser as excitation pulse (pump beam at 540 nm, 1 Hz), and a tungsten lamp as continuous light source (probe beam at 800 nm).29 The wavelength of the excitation pulse was determined according to dye absorption spectrum of N719.13

Results and Discussion The XRD patterns of the graphite, pre-oxidized graphite, GO and RGO samples are depicted in Figure S1 (Supporting information). Pre-oxidized graphite sample showed an intense peak in the same region of the graphite characteristic peak35 2θ = 26.4º, which is assigned to the (002) plane with an interplanar distance of 3.37 Å. However, this peak is wider and the broadening is an indication of the pre-oxidation of the graphite. This process changes graphite’s crystal structure, increasing the disorganization of the sheets by the insertion of some functional groups between


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the lamellae of graphite. After the chemical oxidation of the graphite the peak position shifts to 2θ = 12.06º and the interplanar spacing increases to 7.34 Å. The increase in the interplanar distance in the GO sample is related to the presence of functional groups, and also intercalation of water molecules between graphite plates after the oxidation step36. The subsequent reduction of the GO to RGO resulted in a shift and broadening of the diffraction peak at 2θ = 24.2º. However, reduction of the GO samples with hydrazine did not fully restore the crystalline structure of the graphene sheets, suggesting the formation of RGO with few sheets "stacked". Figure 1(a) shows the Raman spectra obtained for graphite, GO and RGO samples. The Raman spectrum for the sample of GO and RGO exhibits two vibrational modes at 1350 cm-1 and 1575 cm-1, corresponding to the D and G bands, respectively. The G band is related to the E2g vibration mode of sp2 carbon domains and can be used to explain the degree of graphitization, whereas the D band is associated with structural defects and partially disordered structures of the sp2 domains37. The intensity ratio between the D and G bands (ID/IG) is often used as a parameter to correlate the disorder degree in the carbonaceous structures.38 In the Raman spectrum of GO, the G band is broadened and shifted to 1598 cm-1, while the intensity of the D band at 1350 cm-1 increases substantially (ID/IG = 0.98) when compared to the graphite (ID/IG = 0.62). These phenomena can be attributed to the significant decrease in size of the in-plane sp2 domains due to severe oxidation.39 The ID/IG intensity ratio is slightly larger (ID/IG = 1.07) for RGO when compared to GO sample. This change suggests a decrease in the average size of the sp2 domains formed during the reduction with hydrazine. It is reasonable to consider that the reduction of GO causes fragmentation along the reactive sites and yields new graphitic domains, which led to RGO sheets being smaller, but more numerous. The small size of the RGO sheets results in a


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large number of edges. The edges will act as defects, consistent with the increase in the D band intensity40.

Figure 1: (a) Raman spectra of GO and rGO samples; (b) Raman spectra of the nanocomposites gel polymer electrolytes containing different amounts of rGO (in all samples, the amount of GBL, LiI and I2 is constant and equal to 85, 20 and 2 wt%, respectively (relative to the total mass (copolymer + GBL). Excitation laser at 632.8 nm.

Figure 1(b) shows the Raman spectra of the gel polymer electrolytes containing different amount of RGO (the amount of salt, polymer and GBL were kept constant). The polyiodide species exhibit distinct modes of vibration when compared to free ions, ion pairs or ion clusters41. The iodine acts as a Lewis acid (electron acceptor) and coordinates with electron donors. This results in the population of molecular orbital with anti-bonding character and consequently leads to lowering of the bond order; the constant force decreases and shifts the stretching band of 180 cm-1 (solid iodine) to 145-165 cm-1.42-46 The Raman spectra show two intense bands at 111 and 147 cm-1. The more intense band at 111 cm-1 has been associated with asymmetric vibration of I3- and the band at 147 cm-1 is assigned to


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the stretching of the bonds in the anionic I5- polyiodide specie with C2v symmetry.24 We analysed the ratio between these two bands and the results are summarized in Table 1. As the amount of RGO increases in the electrolyte, the proportion of the I5- polyiodide also increases. The formation of higher polyiodide species as I5- is favoured by the good interaction between the graphene sheets and the iodine molecules.47 The resulting polyiodide molecules are adsorbed onto the RGO sheets by interactions with the existing π bonds.50 The proposed mechanism is that the I2 molecules accept electrons from the graphene sheets and the resulting anions react with the excess of I2 to form I3-, I5- and so on48. These results will have an important correlation with the open-circuit voltage of the solar cells and we will return to this discussion in the next sections.

Table 1: Ratio of the absorption bands of vibrations related to the ions I3- and I5- species Electrolyte

I3-/I5-

Gel/0.0wt%RGO

0.73

Gel/0.1wt%RGO

0.69

Gel/0.3wt%RGO

0.61

Gel/0.5wt%RGO

0.62

The glass transition temperature (Tg) values of electrolytes containing different concentration of RGO were summarized in Table 2 and corresponding DSC curves shown in Figure S2. We observe that the Tg values of the copolymer decrease as the amount of RGO is increased in the electrolyte (salt and GBL concentrations remain unchanged). This unexpected result might be a


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consequence of a positive interaction between the RGO sheets and the copolymer that facilitates the segmental motion of the polymer chains in the presence of the carbonaceous material.

Table 2: Glass transition temperature (Tg) values obtained from the DSC curves (Figure S2) for electrolytes prepared with different amounts of RGO compared to the pure copolymer. Electrolyte

Tg / ºC

Pure P(EO/EM/AGE)

–59

Gel/0.0wt%RGO

–74

Gel/0.1wt%RGO

–76

Gel/0.3wt%RGO

–83

Gel/0.5wt%RGO

–85

To further understand the RGO effect in the viscoelastic properties, the rheological behaviour of the system was investigated (Figures S3 and S4). We observe a decrease in viscosity upon an increase of RGO concentration in the gel electrolyte. This data corroborates with Tg values, and although, at first glance, we could expect an opposite trend, both experimental results confirm that RGO sheets are acting as a plasticizer, increasing the mobility of the polymer chains. The viscoelastic response of the electrolytes is represented by the storage modulus (G') and loss modulus (G") on the frequency applied (Figure S4). At low frequency values, G' values are lower than G", implying that the samples are more stable in the rest state. At the point of intersection between the two curves, there is a reversal in the flow behaviour. In the case of the values of G" are higher than G', we can conclude that the liquid-like behaviour of the fluid is prevailing on the elastic nature. Thus, the addition of RGO sheets decreases the viscous nature of the electrolytes.


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TGA curves of the nanocomposite gel polymer electrolytes prepared with different amounts of RGO are shown in Figure S5. For all electrolyte samples, we have observed multiple-steps of the weight loss, beginning at temperatures lower than 100 ºC, which is indicative of the presence of residual water in the gel electrolyte. At this temperature, GBL and iodine losses are also expected to occur. For the electrolyte samples containing RGO we observe more continuous processes of degradation, which are supposed to be due to the degradation of RGO debris. The TGA curves show that the temperature at which we observe the major weight loss related to the copolymer degradation (320 ºC) was not significantly changed up on the addition of RGO. Figure S6 exhibits the Nyquist diagrams for the nanocomposite electrolyte gel samples containing 0.1, 0.3 and 0.5wt% of RGO, and the ionic conductivity values obtained from the analysis of these diagrams are summarized in Table 3.

Table 3: Ionic conductivity values calculated for the nanocomposite gel electrolytes prepared with 0.1, 0.3 and 0.5wt% of RGO in relation to the polymer and GBL weight. Electrolyte

Ionic conductivity (σ) (S cm−1)

Standard deviation

Gel/0.0wt%RGO

5.8 ×10−3

1.1×10-3

Gel/0.1wt%RGO

5.9 ×10-3

7.6×10-4

Gel/0.3wt%RGO

6.5 ×10-3

6.5×10-4

Gel/0.5wt%RGO

6.5 ×10-3

7.5×10-4

From this analysis, all electrolyte samples exhibited ionic conductivity values which are in the same order of magnitude, ~10-3 Scm-1. These values are expected in gel electrolyte samples with


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corresponding high amount of liquid/additive or plasticizers. The ionic conductivity of the samples is practically not altered by the presence of the RGO sheets. Cyclic voltammetry (CV) provides information about the oxidation and reduction processes of the species in the electrolyte. Using this technique it is possible to assess the effect of addition of RGO in the electroactivity of the redox couple, which in our case is the iodide/tri-iodide pair. As shown in Figure 2, the addition of RGO in the electrolyte leads to small displacement in the Epa and Epc values (Table 4). Besides, one can observe an increase in the values of the anodic and cathodic peak current, suggesting that the mobility of the redox species increases as the concentration of RGO increases in the electrolyte.

Figure 2: Cyclic voltammograms of gel nanocomposites polymer electrolyte with different amounts of RGO. The potential ranged from -0.7 to 0.7 V with rate of 30 mV s-1.

The limiting current was determined by steady-state voltammetry measurements placing the electrolyte between two platinum electrodes and by applying a reduced scanning rate (5 mV s-1).


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Steady state conditions were reached, since the system does not present hysteresis effect (Figure 3)49. The cathodic limiting current reflects the maximum rate at which the oxidized species can reach the electrode surface to be reduced50. According to Equation 3, the limiting current density (ilim) is proportional to the diffusion constant of species I3- ( )51,52.

= [ ]

(3)

values calculated from equation (3) displayed on Table 4 show that the addition of RGO in the electrolyte increases the diffusion coefficient of the ionic species. This result corroborates with the decrease in the Tg values and with the rheology data that indicates that the system becomes less viscous after the incorporation of the carbonaceous material. From these results, we have also estimated (Eq. 3) the diffusion limiting current of the cells considering the distance between the electrodes (20µm). The results are also presented in Table 4. By comparing these values with the Isc of the devices (Table 5) one can see that they have reached the values of the estimated limiting current.


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Figure 3: Steady-state current–voltage curves for the electrolytes with RGO using two Pt electrodes sandwiched with the electrolyte. The scan rate was 5 mV s-1.

Table 4: Influence of concentration of RGO on anodic (Eap) and cathodic (Ecp) potential peaks, anodic (iap) and cathodic (icp) current peaks, the diffusion coefficient ( ) of the I3- species, and the diffusion limiting current estimated for the cells (ilim cell). Electrolyte

/10-6 cm2 s−1

Eap/V

iap/mA

Ecp/V

icp/mA

ilim cell/mA

Gel/0.0wt%RGO

0.18

6.4

–0.18

–6.5

4.46

13.51

Gel/0.1wt%RGO

0.19

8.1

–0.19

–8.1

4.85

14.69

Gel/0.3wt%RGO

0.20

10.6

–0.20

–10.7

4.66

14.11

Gel/0.5wt%RGO

0.27

11.8

–0.27

–11.8

5.20

15.74


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DSSC were assembled with the gel nanocomposite electrolytes containing different concentrations of RGO. For comparison, a standard device was also assembled using a commercial liquid electrolyte (EL-HPE, Dyesol). When analyzing the data shown in Figure 4 and Table 5, we observe that the addition of RGO sheets in the electrolyte provided an increase in short-circuit photocurrent (ISC). We also observe that, when comparing the devices containing RGO, the VOC values increases from 0.60 to 0.65 V. The best solar cell efficiency was achieved by the DSSC using 0.5 wt% of RGO in the electrolyte (5.07%). We could not obtain electrolyte samples with higher amount of RGO due to its low dispersion. The solar cell parameters indicate that the RGO influences the performance of DSSC based on polymer gel electrolytes.

Figure 4: Photocurrent–voltage curves for the DSSCs using gel polymer electrolyte with different amounts of RGO, compared with a DSSC using standard liquid electrolyte. The curves were obtained in AM 1.5 conditions (Intensity of 100 mWcm-2)


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Table 5: Electrical parameters for the DSSCs using gel polymer electrolyte with different amounts of RGO, compared with a DSSC using standard liquid electrolyte. VOC / V

ISC / mAcm−2

FF

η/%

Gel/0.0%RGO

0.60±0.08

12.63±3.79

0.56±0.10

3.99±0.27

Gel/0.1%RGO

0.64±0.06

13.43±3.22

0.55±0.05

4.62±0.79

Gel/0.3%RGO

0.65±0.05

13.90±2.78

0.52±0.05

4.59±0.55

Gel/0.5%RGO

0.65±0.04

15.46±3.31

0.51±0.06

5.07±0.97

Liquid electrolyte

0.74±0.02

15.43±3.08

0.61±0.05

6.97±1.08

Electrolyte

When we changed from classical polymer electrolytes to gel polymer electrolytes we can found more polyiodides species, as I3- and I5- due to increased mobility of ions present in solution.18

Error! Bookmark not defined.

These species are large, have low mobility and they tend to be

located near the photoelectrode. As electron acceptors, they promote recombination reactions between the electrons and the electrolyte, reducing the values of VOC. The incorporation of RGO in the gel polymer electrolytes acts in the opposite direction, increasing the values of VOC. This behaviour is related to the strong interaction of the polyiodide with the RGO sheets as described by Kalita et al.47 Thus, this positive interaction allows the removal of polyiodide ions far from the photoelectrode interfaces, keeping them adsorbed on the surface RGO sheets, recovering the VOC values. In the dark, DSSCs behave like a discharging capacitor53. The impedance of the dark reaction is related to electron transfer from the conduction band of TiO2 film (surface states) to I3- or other polyiodide ions present in this interface. EIS is a useful technique in DSSC research to obtain information about the internal resistances of the cell, which is directly related to the cell


18

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performance54. Figure 5 shows the Nyquist plots of the solar cell measured in the dark, by applying a potential difference equal to VOC cell. Qualitatively, the larger is this semicircle, in the dark, the greater is the charge transfer resistance to the electrolyte, and consequently, the recombination is less pronounced55. We observe that RGO sheets act by reducing recombination at the TiO2/electrolyte interface, which confirms the increase in VOC values observed previously.

Figure 5: Nyquist diagrams obtained by means of electrochemical impedance spectroscopy technique of solar cells mounted with electrolytes containing 0.0%; 0.1%; 0.3% and 0.5% of RGO in relation to polymer and GBL mass. The measurements were performed by applying a potential difference equal to the Voc value of the cell and in dark conditions.

Once explained the increase in VOC values, we continue our discussion involving the increase in the photocurrent values in our devices. This effect was already reported in the literature, but a detailed explanation about how the RGO sheets are promoting such increment is scarcely discussed56. Figures S7 to S10 shows the Nyquist and Bode diagrams of the assembled solar cells. The response at high frequencies is related to the counter electrode/electrolyte interface


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(R1), the middle frequency response is related to the photoelectrode/electrolyte interface (R2) and the response at low frequencies can be associated with the diffusion process of the electrolyte species (O)57. Considering this hypothesis, we used the equivalent circuit shown in Figures S7 to S10, represented by Rs[C(R1O)](R2O), to adjust the impedance spectra using the model software developed by Boukamp58. The element Rs is the cell series resistance, R1 is the of charge transfer resistance from the electrode interface, O is the Warburg resistance for diffusion in a finite space, R2 is the resistance of the photoelectrode/electrolyte interface. The capacitor, C, is related to the counter electrode interface and Q is a constant phase element, regarding the porous TiO2 photoanode interface57. All calculated parameters are shown in Table 6. As summarized in Table 6, it can be observed that there is an increase in the solar cell series resistance as RGO is added, which implies in a decrease of the FF values. However, the resistances related to the diffusion of I3- species (O) decrease with the addition of RGO. The parameters obtained from Warburg element, O, where used to calculate the diffusion coefficients (Equation 457). The values for increase with RGO incorporation, in accordance with the decrease of the viscosity of the gel electrolyte (Figure S3 and S4) and previous electrochemical characterization.

= ,

(4)

where d is the distance between two electrodes (20 µm) and B is a parameter obtained from the fitting of the impedance results57. As previously calculated, there is an increase in the values of with addition of RGO. The calculated values are in the same order of magnitude of our previous values (Table 4) and follow the same behavior.


20

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We also observed a gradual decrease in R1 values, indicating that the addition of RGO also facilitates charge transfer processes in the counter electrode. This effect may be related to the accumulation of RGO near the surface of the counter electrode, acting thus as a catalyst for the triiodide reduction.55 Measurements of transient absorption spectroscopy (TAS) (Figure S11), suggest that no effect on the dye regeneration, which implies that the increase in ISC value is not related to the acceleration of the dye regeneration process. The increase in the Isc is ascribed to be related to the enhancement in the diffusion of I3- species, and also by the reduction of the electron transfer resistance in the counter electrode (R1).

Table 6: Parameters obtained by adjusting the Nyquist diagrams shown in Figure 6 using the equivalent circuit Rs [C(R1O)](R2O) with the method of Boukamp58. Diffusion coefficients were calculated using Equation 4. O 2

Electrolyte χ /10

-4

Rs/Ω C1/µF R1/Ω

/10-6

Q

Y01/S

B/s1/2

R2/Ω

Y02/mFsn-1

n

cm2s-1

0.0% rGO 2.55

14.8 22.9

4.77

0.189

1.16

21.0

1.16

0.97

3.0

0.1% rGO 1.33

22.2 1.87

4.50

0.157

0.92

22.5

1.23

0.92

4.7

0.3%rGO

1.62

18.0 1.62

4.18

0.164

0.96

19.0

1.38

0.90

4.3

0.5%rGO

5.29

16.6 1.63

3.45

0.093

0.92

22.9

0.94

0.92

4.7

Conclusions The incorporation of reduced graphene oxide (RGO) sheets in DSSCs based on polymer or gel electrolytes is a promising strategy to overcome the limitations that occur after the


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Page 22 of 31

replacement of the liquid electrolyte, e.g. lower photocurrent and open circuit values. This work unveils the role of RGO sheets when this type of carbonaceous structure is incorporated in the DSSC’s electrolyte. Solar cells assembled with the nanocomposite electrolyte have achieved an efficiency of 5.07%. The increase in the open-circuit potential, when compared to the devices without RGO, can be related to the interaction of the higher polyiodides with the RGO sheets. Such positive interaction removes partly these species from the photoanode, as consequence, the recombination reactions in the photoelectrode are minimized and the VOC values are recovered. The major effect, however, is on the ISC values of the devices after the incorporation of RGO in the electrolyte. We demonstrated here using several techniques that the diffusion coefficient of the I3- and other ionic species are increased and this is a consequence of the plasticizer effect of the carbon sheets on the polymer chain mobility. However, we cannot exclude the positive effect in the reduction of the electron transfer resistance at the counter electrodes. Further studies have to be carried out in order to test the stability of the DSSC based on the nanocomposite electrolyte as well, to find alternatives to increase the VOC values, which is the major loss when polymer or gel electrolytes based on iodide/iodine are employed.

Supporting Information. X-ray diffraction of graphite, pre-oxidized graphite, GO and RGO

(Figure S1); DSC curves of pure copolymer, compared with gel polymer electrolytes containing different amounts of RGO (Figure S2); Viscosity versus shear rate diagram (Figure S3); Frequency sweep essay for the gel electrolytes (Figure S4); Thermogravimetric curves of gel polymer electrolytes containing different quantities of RGO (Figure S5); Nyquist diagrams for


22

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the nanocomposites electrolyte gel samples containing different quantities of RGO (Figure S6); Nyquist and Bode diagrams obtained by electrochemical impedance spectroscopy of solar cells mounted with electrolytes containing different quantities of RGO (Figures S7 to S10); Normalized curves of the transient absorption decay respect to electrolytes with different amounts of RGO and the inert electrolyte (Figure S11).

Acknowledgments The authors acknowledge the financial support from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Instituto Nacional de Eletrônica Orgânica (INEO) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

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Understanding the Role of Reduced Graphene Oxide in the Electrolyte

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