A facile and low-cost route for producing a flexible hydrogel–PANI electrolyte/counter electrode applicable in dye-sensitized solar cells (DSSC)
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In this work, hybrid hydrogels were prepared based on poly(acrylic acid-co-acrylamide) hydrogels (Gel) and self-assembled nanostructured polyaniline (PANI). These hybrid hydrogels were prepared by in situ UV-assisted aniline polymerization during hydrogel network formation in a controlled interfacial reaction. They were then tested as flexible quasi-solid electrolytes with a counter electrode in dye-sensitized solar cells. This electrolyte-counter electrode system was prepared on top of a titanium dioxide layer deposited on a transparent and conductive anode electrode (ITO). Three different nanostructured Gel–PANI hybrid hydrogels were fabricated by employing different PANI concentrations and used for constructing electrolyte–counter electrode systems that displayed conductivities between 0.003 S/cm and 0.02 S/cm. Dye-sensitized solar cells (DSSC) that had the configuration ITO/TiO2/Gel–PANI/ITO were prepared and tested using natural dye extracted from Myrciaria cauliflora. The energy conversion efficiency was evaluated under AM1.5 (100 mW/cm2) light irradiation and was found to reach values above 2.0%. The device’s energy conversion efficiency was found to be higher than those of others with the same natural dye. The process proved to be fast and facile, and it utilized an inexpensive natural dye.
KeywordsSolar cell DSSC Myrciaria cauliflora Hydrogel PANI
Massive use of mobile devices and the spread of wearable technologies lead to constant searches for mobile power sources or sources incorporated into the devices. In this sense, flexible and low-cost solar cells can be widely employed due to the vast presence of energy radiated by the sun.
Dye-sensitized solar cells (DSSC), which are composed of a nanocrystalline semiconductor oxide film electrode, dye sensitizers, electrolytes, a counter electrode and a transparent conducting substrate, have the potential to develop low-cost photovoltaic (PV) devices. Like plants that are effective in capturing energy over a wide range of light conditions, they work in lower light levels than do traditional PV. Thus, the outdoor environments of hospitals, airports, bus/train station windows and surfaces have become low cost energy sources .
Typically, dye-derived nanocrystalline titania (TiO2) films have been used as photoanode, platinum-coated substrate as a counter electrode, and between them exists an electrolyte solution of I3−/I− in an organic solvent, forming sandwiched solar cells . To reduce the tri-iodide ions of the liquid electrolyte, the counter electrode must have high electrocatalytic activity. In addition to platinum coating on transparent conductive oxide substrate, carbon is usually used. Nevertheless, the high cost of platinum has triggered its replacement by other cheaper materials . Conductive polymers, such as polyaniline (PANI), have been investigated as an alternative due to their properties including high conductivity, good stability, catalytic activity and low cost [4, 5].
Long-term operation of liquid electrolytes is usually restricted due to their sealing issues and volatility of the organic solvent. The development of quasi-solid-state electrolytes, such as gel structures, constitutes another approach to avoid the problems of liquid electrolytes. Conductive polymers such as PANI, polypyrrole, polythiophene, polyphenylene vinylene and polyacetylene are potential candidates for quasi-solid-state electrolytes .
DSSCs based on natural sensitizers (NDSSC) have become a topic of significant research because of their urgency and importance in the energy conversion field. Moreover, they offer advantages over more conventional approaches, such as easier fabrication, lower-cost and being constituted by nontoxic materials .
Anthocyanins are a viable alternative to red dyes from natural sources. They are soluble in water, which facilitates their incorporation into aqueous systems and may replace artificial red colorants such as Ponceau 4R, erythrosine and Bordeaux .
A good source of anthocyanin pigments is the fruit of the jabuticabeira. The jabuticaba (Myrciaria cauliflora) is a native Brazilian plant that is located mainly in several states of the southeastern region of the country . As plants that are effective in capturing energy in a wide range of lighting conditions, dye-sensitized solar cells operate at lower light levels than traditional silicon solar cells.
The aim of this work was to develop a quasi-solid electrolyte with inside counter electrode based on an ionic hydrogel and PANI produced in one step over an ITO conductive glass substrate coated with TiO2 nanoparticles, which were sensitized using natural dye from Myrciaria cauliflora as a photoactive electrode. Current density–voltage measurements were performed in darkness and under illumination conditions. Furthermore, the morphological, structural, electrical and thermal properties of the counter electrode were also investigated.
2 Materials and methods
Aniline was purchased from Merck. Acrylic acid (AA), polyethylene glycol diacrylate (Mw ~ 700 g mol−1, PEGDA), acrylamide (AAm), 37% hydrochloric acid, ammonium persulfate, 2-hydroxy-2-methylpropiophenone (97%), poly(vinyl alcohol) (Mw ~ 30,000 g mol−1, 98% hydrolyzed, PVA), and titanium (IV) oxide nanopowder (primary particle size < 100 nm, ≥ 99.5% trace metals basis) were purchased from Sigma-Aldrich (St. Louis, MO, USA) and were used as received. Natural dye was extracted from the skin of Myrciaria cauliflora.
2.2 Construction of NDSSC-type solar cell
Poly(ethylene terephthalate) (PET) substrates coated with indium-doped tin oxide (ITO) (60 Ω/sq) with dimensions of 1 cm by 1.5 cm were used, in which one of the edges had a deposited aluminum contact of 2 mm, thus possessing an active area of 1.3 cm2. They were previously washed with deionized water and with 2% neutral detergent, then heated to 50 °C for 1 h with moderate magnetic stirring. After rinsing with deionized water at the same temperature for 1 h, substrates were oven dried at 28 °C.
The substrates were then coated with titanium dioxide nanoparticles by preparing a slurry containing 1 g titanium dioxide nanoparticles and 0.3 g 99% acetic acid. The slurry was deposited on the substrate using a glass rod and dried at room temperature. The following procedures were used to produce hydrogel films on these substrates that will act as quasi-solid electrolyte with inside counter electrodes.
2.3 Synthesis of poly(acrylamide-co-acrylic acid) hydrogels
Hydrogels were prepared by formulating an aqueous solution with 77 wt% AA, 19 wt% AAm, 2 wt% PEGDA and 2 wt% photoinitiator. The final concentration of the reactants in water was 0.6 g ml−1. The solution was then cast on glass slides and kept under ultraviolet radiation, using a light source consisting of three 8 W UVA lamps and a UVC lamp for 5 min. Samples were washed with water and dried under vacuum at room temperature to yield free standing hydrogel films.
2.4 Incorporation of PANI in the hydrogels by interfacial oxidative UV-assisted polymerization of aniline
Hydrogels containing PANI were prepared by combining two different solutions: (1) the first solution (solution A) was prepared following the same procedure described above. However, aniline was added in the final solution in concentrations of 2, 4 and 6 wt%, while an aqueous solution with a concentration of 1 mol L−1 of HCl was used instead of pure water. (2) A second (solution B) solution was prepared by dissolving 12 wt% of PVA in a 1 mol L−1 aqueous solution of HCl. Solution B was allowed to stir for 15 min. Thereafter, 2, 4 and 6 wt% ammonium persulfate (PANI initiator) were added to yield solutions with different concentrations of ammonium persulfate to match solutions A, which were prepared with 2, 4 and 6 wt% aniline, respectively. The obtained solutions were stirred vigorously for another 15 min.
Solution B was placed on an ITO-PET substrate to yield a 1 mm thick film. After that, solution A was carefully poured over it. The visual initiation of the green-colored PANI formation at the interface between the two solutions was observed after 3 min of initial contact. The system was then photopolymerized by illuminating with the UV lamps. Free standing films of the hydrogels containing PANI were also obtained by repeating the same procedure described above on glass slides instead of ITO-PET substrates. These free standing films were used in the characterization procedures.
Concentrations of components (% w/w) in hydrogel precursor solution (solution A) and ammonium persulfate in PVA solution (solution B)
solution (solution A)
Concentration of the components
12 wt% PVA solution (solution B)
Concentration of the components
AA + AAM + PEGDA + Photoinitiator
The dye was obtained by maceration of 100 g Myrciaria cauliflora (jabuticaba) bark and mixture with 100 ml ethanol. The obtained solution was kept at 5 °C for 24 h before vacuum filtration.
2.5.1 Conductive atomic force microscopy (I-AFM)
AFM images were obtained by an AFM (Park Systems, Inc., XE-70) in conductive mode (I-AFM), which allows mapping of the surface of the samples by measuring the current flowing from the polarized sample in contact with a cantilever tip used as a probe. Since this current is of small magnitude, it is necessary to use a current amplifier before the image is processed. The DLCPA-200 external amplifier with variable gain adjustment between 103 and 1011 V coupled to the I-AFM Measurement Module was used.
2.5.2 Scanning electron microscopy (SEM)
Images of the surfaces of the films were obtained by scanning electron microscopy using a mini Sputter (PI Supplies-Sputter coater) system for their metallization and the JEOL JSM-6360LV and Quanta FEG 3D FEI scanning electron microscopes.
2.5.3 Differential scanning calorimetry (DSC)
DSC curves were obtained by using the EXSTAR DSC 7020 equipment. The DSC analyses were performed under a nitrogen atmosphere, with the first heating run starting at − 20 °C and ending at 130 °C, followed by a second run from − 20 to 250 °C with a rate of 10 °C/min.
2.5.4 Fourier transform infrared spectroscopy (FTIR) and UV–Vis spectroscopy
FTIR spectra of the samples were collected after keeping them dried in a vacuum oven at room temperature for 24 h. The attenuated total reflection technique (ATR) was employed, the accessory for which was coupled to the Thermo Scientific spectrophotometer model Nicolet 6700. UV–Vis spectra were collected by UV-2600 spectrophotometer, Shimadzu Corporation, Japan, model TCC-24OA.
AFM nanoindentation measurements (Park Systems, Inc., XE-70) were performed to estimate the elastic modulus (E) of the gel–PANI films. The procedure involved printing a mark on the surface of the materials using the AFM cantilever, with a tip with known geometry employed as indenter. A constant tip shift velocity of 1 μm/s was used until the determined maximum force of 1.6 μN was reached and maintained constant for 0.5 s. Throughout the process, tip displacement was measured as a function of force. The nanoindentations of the films were performed by attaching free standing films of the gel–PANI onto a glass slide. AFM nanoindentation measurements were performed underwater so that the hydrogel–PANI samples could be studied in their full swollen state. The water used in the nanoindentation had pH = 7 and resistivity of 1 MΩ/cm. Measurements were performed after a stabilization time underwater of 45 min. The procedure for estimating the elastic modulus from AFM nanoindentation measurements was developed by Oliver and Pharr  and is incorporated into Park Systems Corporation’s XEI 1.8.9 software, which is used to obtain values of at least 30 nanoindentations per sample.
2.5.6 Conductivity measurement by the 4-point method
Free standing films of gel–PANI were submitted for analysis of their conductivity through the four-probe system, using a Keithley 2400 electrometer conjugated to the Signatone GMX45 four-point system.
2.5.7 Solar cell characterization
The cells were characterized by obtaining current versus voltage curves (I × V) in the dark and under solar spectral irradiance of AM1.5 (100 mW/cm2) using the combination of full spectrum LED lamps with sample temperature control at 25 °C. This equipment was calibrated using a calibrated commercial 10 W module. A circuit based in MOSFET IRF 640 and controlled by the National Instruments myDAQ system was used as a variable resistance simulator and to apply voltage in dark conditions. The obtained parameters were:
Im: Electrical current at maximum power point, Vm: Electrical voltage at maximum power point, Isc: Course current circuit (v = 0), Voc: Open circuit voltage (I = 0), Pm: Maximum power.
3 Results and discussion
3.1 Fabrication of hydrogel-PANI and NDSSC
3.2 Morphology of hydrogels containing PANI
SEM and I-AFM images of hydrogel samples with different concentrations of PANI
The surface of the hydrogel with no PANI (Gel:PANI 0) did not show any major features that could be related to heterogeneities, such as second phases, voids or cracks, which suggests that the UV-promoted polymerization of the hydrogel had occurred as designed. In Gel:PANI 2 samples with the lowest percentages of aniline and ammonium persulfate, it can be clearly observed that the employed synthesis conditions led to a controlled growth of the PANI particles to yield needle-like structures with mean diameter of 70 nm and length of 100-200 nm, as revealed by I-AFM imaging. This special nanostructured morphology is a result of the PANI polymerization confined in a 2D environment, which is represented by the interfacial region in which both viscous solutions interact. Special PANI morphologies due to particular processing conditions have already been observed and reported by others . I-AFM was also useful in confirming that the observed needle-like structures in the SEM image have high electrical conductivity and thus are related to PANI.
SEM and I-AFM images of Gel:PANI 4 and Gel:PANI 6 with increased concentrations of PANI with respect to Gel:PANI 2 show the same trend, in which controlled polymerization growth of the conducting polymer led to special morphologies due to the presence of particular environments, such as the interface of the two precursor solutions and the hydrogel structure, that act as templates for the polymerization of PANI. I-AFM images of Gel:PANI 2, Gel:PANI 4 and Gel:PANI 6 show that higher concentrations of aniline led to shifts in morphology of the PANI phases from needle-like (nanosticks) to long fibers/ribbons and sheets (for the Gel:PANI 6).
3.3 Characterization of hydrogels containing PANI
The DSC curve of the commercially available PANI in Fig. 5a shows the typical behavior of the material , displaying an endothermic event between 125 and 150 °C corresponding to the loss of the HCl dopant of the material and the start of polymer degradation above 250 °C. In hydrogel samples with PANI, this endothermic event associated with the loss of the HCl dopant shifted to higher temperatures, which indicates that interactions between acrylic acid moieties and PANI would require higher levels of energy to extract the dopant from the PANI structure.
The first derivatives of DSC curves are shown in Fig. 5b to reveal the glass transition temperatures (Tg) (according to ASTM E1356–08) of the hydrogels, which is approximately 5 °C. The Tg of PANI could not be observed due to the presence of highly aromatized chains. In Fig. 5b, the relationship between the increase in the percentage of PANI and higher Tg values can be observed: Gel:PANI 2, Gel:PANI 4 and Gel:PANI 6 presented glass transition temperatures of 9, 12 and 16 °C, respectively. This finding confirms that the interactions between the hydrogel and the conductive polymer reduce the overall mobility of the hydrogel chains and function like physical cross-links .
3.4 FTIR and UV–Vis spectroscopy results
The relative intensity of the C=O stretch band of the hydrogel in respect to PANI bands was reduced for samples with higher concentrations of PANI incorporation as evidence of the presence of PANI in these samples. A shift of the C=O peak of the hydrogel to lower wavenumbers was also observed, which indicates a strong interaction with PANI, an effect that has also been observed by other authors [14, 15]. This result together with the DSC results that showed higher temperatures for the loss of the HCl dopant supports the hypothesis of the secondary doping effect of PANI by acrylic acid units of the hydrogel, as reported by others .
Oxidation index (Rox) and doping values (Rd) for films
Rox = I1496/I1600
Rd = I1155/I1496
1.02 ± 0.06
1.34 ± 0.09
0.82 ± 0.10
1.09 ± 0.12
0.98 ± 0.06
1.17 ± 0.10
All samples had values of Rox near 1, which indicates the dominant presence of polyaniline in its emeraldine phase. The highest doping level (Rd) was observed for the Gel:PANI 02, which is the sample with the lowest concentration of PANI; Gel:PANI 02 displayed PANI as thin needle-like structures (also called: nanosticks) that have the highest surface area/volume ratio, which seem particularly useful for improving the ability to have PANI acid doped.
3.5 Nanoindentation measurements
Figure 8a shows force versus displacement curves that were obtained during the nanoindentation measurements. Measurements were performed both in dry and wet conditions (an AFM liquid cell was used for the underwater-wet-measurements). In Fig. 8a, only the underwater measurements for hydrogels with different amounts of PANI are shown for clarity. The results revealed that the incorporation of PANI led to stiffer samples, i.e., for nanoindentation forces of 1.6 µN, the penetration of the AFM tip was much shallower for hydrogels with higher concentrations of PANI.
Results in Fig. 8a were used to estimate the elastic modulus values of the hydrogels, as exhibited in Fig. 8b. As expected, the elastic moduli of swollen (wet) hydrogels were much lower than the elastic moduli for dry hydrogels of the same composition, as a consequence of chain relaxation due to the interaction between water and hydrogel chains. It is also observed in Fig. 8b that the elastic moduli of samples with higher concentrations of PANI were much higher than the modulus of the hydrogel with no PANI (Gel:PANI 0), due to the rigid ring-rich macromolecular structure of PANI, as well as to the intense interactions between PANI and hydrogels that restrict chain mobility and enhance stiffness.
3.6 Electrical conductivity of hydrogels containing PANI
Conductivity of hydrogel samples with PANI
Standard deviation (S/cm)
It was observed that the percolation factor was the most significant influence in this type of measurement, since the Gel:PANI 2 sample with the lowest concentration of PANI, in the form of nanosticks, was not able to significantly influence the overall conduction values because the conducting regions of PANI were not interconnected.
On the other hand, hydrogel samples with larger amounts of self-assembled PANI nanofibers, such as Gel:PANI 4 and Gel:PANI 6, displayed higher values of conductivity even with low percentages of PANI due to the higher interconnectivity between the conducting regions. The obtained results demonstrated the effectiveness of the developed technique to prepare highly conductive hydrogels containing nanostructured PANI, since conventional PANI blends only tend to have effective conductivity between 10 and 40% PANI content .
3.7 Solar cell characterization
Current versus voltage data and efficiency of NDSSC-type solar cells with quasi-solid electrolytes with inside counter electrodes based on hydrogels with different amounts of PANI
Isc (mA cm−2)
Im (mA cm−2)
Pm (mW cm−2)
It can be seen that the open-circuit voltage values of Gel:PANI 4 and GEL:PANI 6 samples are within the range expected in the literature, i.e., a value of approximately 650 mV. On the other hand, short-circuit current and efficiency (> 2%) were higher than the average results reported in the literature, which demonstrates that the hydrogels with the nanostructured PANI were able to reduce current losses by recombination, which usually occurs between oxidation and reduction of iodine ions. This increase in efficiency can be related to the fact that the electrolyte is within the counter electrode in the developed NDSSC and, at the same time, the counter electrode also contains nanostructured PANI which is recognized as a powerful catalyst for the reduction of the I3− ions. These features of the NDSSC have probably allowed the reduction of the necessary average path of the iodine ions between the TiO2 layer sensitized with the dye and counter electrode.
In Table 5, values of Voc and Isc can be explained based on the fact that, on charging the material, the electrons are induced and injected from I− ions into the photo-oxidized dye, and further the produced I3− are reduced on the counter electrode . Therefore, higher contents of PANI inside quasi-solid electrolytes can increase the I3− reduction rate, leading to higher values of Isc. Moreover, charge recombination within the DSSC can also increase the Voc.
Also in Table 5, higher values of fill factor (FF) for samples with high contents of PANI confirmed the tendency of lower resistance in series in comparison with the shunt resistance due to improved generation and transfer of charges to the electrodes.
The possibility of having both energy conversion and storage in DSSC containing cations such as Li+ has been discussed recently . In our case, nanostructured PANI could connect the counter electrodes that would reduce the capacity of storage. Therefore, the DSSCs lifespan may be improved. More studies should be performed to prove it.
In this work, the preparation of hybrid hydrogels, in which a nanostructured PANI was polymerized concomitantly with the formation of the hydrogel network by combining a surface-based polymerization with UV-assisted reaction, was demonstrated. SEM and conductive AFM images showed that the morphology of the PANI phase within the hydrogels shifted from needle-like to fiber-, ribbon- and sheet-like morphologies when higher concentrations of PANI were used. FTIR, DSC and UV–Vis results indicated that a strong interaction between PANI and the hydrogel chains and a secondary doping of PANI by the acrylic acid moieties were promoted. Nanoindentation measurements demonstrated that PANI was able to enhance the elastic moduli of both wet and dry hydrogels. Moreover, the electrical conductivity of the hydrogels was significantly improved by the incorporation of PANI with low percolation thresholds. Natural dye-sensitized solar cells were fabricated by using the developed hydrogel containing PANI as a quasi-solid electrolyte with inside counter electrode and the shell of Myrciaria cauliflora (jabuticaba) as a dye. Solar cell efficiencies above 2% were measured, which indicated that hydrogels containing PANI can act as quasi-solid electrolytes, as well as that counter electrodes can enhance diffusion and reduction of iodine ions. Therefore, hydrogels with nanostructured PANI were shown to be good candidates for quasi-solid electrolytes that can also act as counter electrodes for flexible solar cells, such as those based on natural dye sensitizers.
The authors acknowledge the financial support from CNPq, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES, Finance Code 001) and FAPEMIG.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- 2.Al-bahrani MR, Xu X, Ahmad W, Ren X, Su J, Cheng Z, Gao Y (2014) Highly efficient dye-sensitized solar cell with GNS/MWCNT/PANI as a counter electrode. Mater Res Bull 59:272–277. https://doi.org/10.1016/j.materresbull.2014.07.029 CrossRefGoogle Scholar
- 4.Li Q, Tang Q, Chen H, Xu H, Qin Y, He B, Liu Z, Jin S, Chu L (2014) Quasi-solid-state dye-sensitized solar cells from hydrophobic poly(hydroxyethyl methacrylate/glycerin)/polyaniline gel electrolyte. Mater Chem Phys 144:287–292. https://doi.org/10.1016/j.matchemphys.2013.12.032 CrossRefGoogle Scholar
- 10.Blythe T, Bloor D (2008) Electrical properties of polymers, 2nd edn. Cambridge University Press, CambridgeGoogle Scholar
- 13.Reed EW (2010) Synthesis of a directionally-conductive polyaniline hydrogel composite by vapor deposition, Master Thesis University of LouisvilleGoogle Scholar
- 17.Mittal V (2017) Functional polymer blends: synthesis properties and performance. CRC Press, Boca RatonGoogle Scholar