Dye-sensitized solar cell employing chitosan-based biopolymer electrolyte

Conductivity and transport properties of a cost-effective and environment friendly chitosan based solid biopolymer electrolytes which form mechanically robust thick film, have been reported here. A maximum ionic conductivity of ∼ 10-4 S/cm has been achieved by optimizing the concentrations of the salt (LiClO4) and the plasticizer (EC) in the biopolymer electrolyte. Ion transport properties of the biopolymer electrolytes are studied from Raman spectroscopy. A dye-sensitized solar cell (DSSC), with a sandwich structure, is fabricated with chemically synthesized ZnO (∼ 60 nm) as the nanoporous semiconductor material coated with Rose Bengal dye as a photosensitizer, the chitosan biopolymer as electrolyte and platinum as counter electrode. Linear Sweep Voltammetry analysis of the DSSCs illustrates the photovoltaic performance of these cells. Without any external addition of redox couple in the biopolymer electrolytic system, a maximum short-circuit current density of JSC = 0.556 mA/cm2 and open-circuit voltage Voc = 0.605 V with power conversion efficiency 0.051 % is achieved by the DSSC.


Introduction
The rapid worldwide consumption of fossil resources (such as coal, oil, gas) and the undesirable consequences of environmental pollution are emerging drastically which lead to the development of green, sustainable and highly efficient electrochemical energy storage and energy conversion devices such as batteries [1][2], supercapacitors [3][4] and dye-sensitized solar cells (DSSC) [5][6].Polymer electrolytes have attracted huge attention among the researchers worldwide since the past few decades for their good electrochemical properties along with the improved safety which ensures their potential applications in the energy industry.
Dye-sensitized solar cells (DSSCs) are photovoltaic (PV) cells that convert any visible light into electrical energy.DSSCs have attracted great interest for their low fabrication cost and simple structure, compared to the silicon-based p-n junction photovoltaic devices [7][8].A standard nanostructured DSSC 1291 (2023) 012014 IOP Publishing doi:10.1088/1757-899X/1291/1/012014 2 (first developed by O'Regan and Gratzel) comprises of a wide band gap nanocrystalline semiconductor film (TiO2) coated with ruthenium dye complex on a transparent conducting glass electrode (ITO or FTO), a redox couple electrolyte (I -/I3 -) and a counter electrode.The basic operation of the DSSC involves the injection of photo-excited electrons from dye molecules into the conduction band of the semiconductor electrode that subsequently diffuse toward the transparent conducting glass substrate, leaving oxidized dye molecules behind.The oxidized dye molecules are regenerated by oxidizing I − to I3 -ions in the electrolyte.The electrons finally reach the counter electrode through external circuit where I3 -reduced to I -and make the circuit complete.Recently, ZnO which has a similar band gap (~ 3.3eV) like TiO2, appears to be an alternative semiconductor material for the fabrication of high efficiency DSSCs [9].Comparing with TiO2, ZnO has much higher electron diffusivity and mobility [10][11] which provides a direct conduction pathway for rapid collection of electrons at the substrate of the photoanode.
The electrolytes play a very important role in getting highly efficient photo-electrochemical DSSCs [12].Various biopolymers such as starch, chitosan, cellulose etc. have attracted considerable interest owing to their biodegradability, non-toxicity, cost-effective and environmentally friendly properties [13][14][15].Hence the use of these materials can contribute in minimizing the environmental waste, and thereby lowering the cost of the energy conversion devices.Among all the natural biopolymer hosts, chitosan which is composed of two main groups i.e., N-acetyl glucosamine and glucosamine units linked by -1,4-glycosidic bonds, has attracted considerable attention of researchers in the development of SPE since the past few decades.The presence of polar functional groups in chitosan such as hydroxyl and amine can give the material a dipole moment that affects the dielectric polarization.Hence, intermolecular interactions are created between these polar groups of chitosan and alkali metal salt ions, leading to the solvation of salt.Also, chitosan has some other outstanding features such as its abundant availability, low cost of extraction, non-toxicity, biodegradability, biocompatibility and excellent mechanical strength.Hence, chitosan can be considered as a fascinating polymer host for developing polymer electrolyte.But chitosan has very low conductivity [16] which can be tuned by doping with suitable ionic salts and plasticizers.The most popular ionic salt used for incorporating charge carriers in the polymer matrix is lithium perchlorate (LiClO4) [16][17][18].Ethylene carbonate (EC) has been chosen as plasticizer since it has high dielectric constant which may assist in the dissociation of salt by weakening the Coulombic force between its cations and anions and thus can tailor the ionic conductivity and provide mechanical stability to the film [19].
In this article, the conductivity and transport properties of the chitosan based solid biopolymer electrolytes (SPE) containing different concentrations of salt (LiClO4) and plasticizer (EC) are reported at room temperature.The ion transport parameters in the biopolymer electrolytes are studied by Raman spectroscopy.A dye-sensitized solar cell (DSSC) is assembled with Zinc Oxide (ZnO) nanoparticles as the semiconductor material, Rose Bengal (RB) dye as the photosensitizer and chitosan-based biopolymer electrolyte.The constructed photovoltaic cells are characterized using linear sweep voltammetry (LSV) analyses and the power conversion efficiencies of the cells are estimated for the chitosan biopolymer electrolytes containing different concentrations of LiClO4 and EC.So far as found in literature, this is the first report of direct application of the chitosan biopolymer electrolyte without any external introduction of redox couple (I -/I3 -), in photovoltaic cells.

Synthesis of Zinc Oxide (ZnO) nanoparticles
ZnO nanoparticles are synthesized in de-ionized water ambiance and (CH3COO)2Zn.2H2O is taken as Zn 2+ source.ZnO nanoparticles are prepared by following the protocol as reported by Mondal et al [20].The prepared nanoparticles were characterized by X-ray diffraction (XRD) and UV-VIS Spectroscopy.XRD was done by X-ray diffractometer (model: Bruker D8 Advance) using the CuKα radiation of wavelength 1.5406 Å.The optical transmission spectrum of the ZnO nanoparticles has been studied in the wavelength region 200 -800 nm by using UV-VIS a diffuse reflectance spectrophotometer (Shimadzu, UV 2401PC).

Synthesis of chitosan-based biopolymer electrolytes
The solid biopolymer electrolytes are synthesized by solution casting technique.Chitosan (0.5 g) is dissolved in 50 ml 1% aqueous acetic acid solution.Then different weight percentages of LiClO4 (40, 60, 80 wt.%) as ionic salt and ethylene carbonate (EC) (30, 45, 50 wt.%)as plasticizer are added in the chitosan solution and stirred continuously until complete dissolution.All the resulting homogeneous solutions are cast in separate polypropylene petridishes and allowed to dry at room temperature for the thin polymer films to form.The solid biopolymer electrolyte films (SPE) are cut into suitable sizes and characterized using computer interfaced impedance analyzer (Agilent 4294A-Precision Impedance Analyzer) in the frequency range between 40 Hz and 2 MHz at room temperature, by sandwiching them between two gold electrodes, each of diameter 1 cm, at room temperature.Raman spectroscopy measurements were performed in backscattering geometry using LabRAM HR (JobinYvon) spectrometer equipped with a Peltier-cooled charge-coupled-device (CCD) detector.

Development of the dye-sensitized solar cell (DSSC)
The photoanode of the dye-sensitized solar cell (DSSC) was fabricated by depositing ZnO nanoparticles on indium tin oxide (ITO) glass and thereafter, coating it with 10 mM Rose Bengal (RB) dye solution using drop casting technique and dried for 24 hours at room temperature.The chitosan-based biopolymer electrolyte was then sandwiched between the ITO/ZnO/RB (photoanode) and platinum (Pt) (counter electrode).Linear Sweep Voltammetry (LSV) responses of the photovoltaic cells are performed with AUTOLAB PG STAT 12 Potentiostat/Galvanostat (Eco Chemic, Netherlands).The photovoltaic performance of the DSSC is measured from current-voltage (I-V) characteristics under light illumination (intensity 100 mW/cm 2 ) using the Keithley 2635B source meter interfaced with a computer.

Structural characterization: X-ray diffraction (XRD)
ZnO nanoparticles have been characterized using XRD.The diffraction peaks of ZnO nanoparticles as illustrated in Fig. 1(a), exhibit a single-phase wurtzite structure (space group p63mc, JCPDF #36-1451).The crystallite size (D) and microstrain () of ZnO nanoparticles have been calculated from Williamson and Hall's modified Scherrer's formula given by [21], The inset of the Fig. 1a shows the  cos  vs sin  plot revealing the average size of the ZnO nanoparticles as 58 nm.

UV-Vis Spectroscopy
UV-VIS absorption experiments are performed to investigate the optical band gap of ZnO.The fundamental absorption, which corresponds to the transition from the valence band to the conduction band, can be used to determine the optical band gap of a material.The absorption coefficient for direct band gap systems, can be related with the incident photon energy by the following equation,

Transport properties: Raman Spectroscopy
The Raman spectra for CLx (0 ≤ x ≤ 80) and CL80Ey (30 ≤ y ≤ 50) SPEs are shown in Fig. 3 in the range of 250 cm -1 to 1500 cm -1 .Raman analysis is carried out to study the interactions of the chitosan biopolymer with LiClO4 salt and EC plasticizer for the estimation of transport parameters such as number density of free charge carriers responsible for the ionic conduction in the CLx (40 ≤ x ≤ 80) and CL80Ey (30 ≤ y ≤ 50) SPE systems.Raman deconvolution technique has been adopted to determine the ion transport parameters in the SPE systems.The number density of free charge carrier (n) in the SPE system is directly determined by deconvoluting the Raman spectrum in the range of 900 cm -1 to 950 cm -1 [22].Consequently, the ionic mobility (µ) and the diffusion coefficient (D) of the ions are calculated using the Eqs.5 and 6, respectively.The peak close to 919 cm -1 is assigned due to the free ions and peak close to 927 cm -1 is assigned to contact ion pairs in the SPE system [22].The deconvoluted Raman spectra in the range 900 cm -1 to 950 cm -1 for for CLx (40 ≤ x ≤ 80) and CL80Ey (30 ≤ y ≤ 50) SPEs are shown in Fig. 4. The percentage of free ions (  ) is estimated as, where,   is the area under the peak corresponding to the free ions and   is the area under the peak corresponding to the contact ion pairs [18].The percentage of free ions (  ), as shown in Fig. 5, follows the conductivity trend (Fig. 2) of the SPEs.Thus, the main contribution in conductivity enhancement of these SPEs are due to the free ions, rather than the contact ions.The values of n, µ and D are calculated using the Eqs. 4, 5 and 6 respectively, as given below [18],

𝜇 = 𝜎 DC 𝑛e
(5) where, M is the number of moles of LiClO4 salt present in each SPE, Vtotal is the total volume of the SPE and DC is the DC conductivity obtained from impedance spectroscopy.The calculated values of the transport parameters n, µ and D for the SPE systems are shown in Fig. 6.It is noteworthy to mention that the highest conducting SPE, CL80E45 exhibits the maximum mobility (3.12510 -6 cm 2 V -1 s -1 ) as well as the maximum number of charge carriers (4310 19 /cm 3 ).LSV responses of the DSSCs assembled with CLx (40 ≤ x ≤ 80) and CL80Ey (30 ≤ y ≤ 50) biopolymer electrolytes are shown in Fig. 8.An increase in current density is noticed under illumination, compared to that in the dark for both the sets of polymer electrolytes.This indicates the photovoltaic performance of the cells.The current has been produced due to the diffusion of monomer units of chitosan in the polymer electrolyte on the application of voltage.The apparent diffusion coefficients (′) of the chitosan monomer units which are assumed to be responsible for the redox reactions in the photovoltaic cells, have been calculated using the following equation [23][24], ′ =     0 (7) where, ′, the diffusion coefficient, has unit in cm 2 /s,   is the limiting current density in A/cm , δ is the ZnO/RB film thickness in cm, n is the number of electrons required for the redox reaction to happen, F is the Faraday constant which is equal to 9.6510 4 C/mol and  0 is the initial molar concentration of chitosan monomer unit in moles/mL.The limiting current densities (  ) and the apparent diffusion coefficients ( ′ ) of the chitosan monomer units for CLx (40 ≤ x ≤ 80) and CL80Ey (30 ≤ y ≤ 50) biopolymer electrolytes based DSSCs are presented in Table 1.It is revealed that the diffusion coefficient (′) of monomer unit of chitosan increases in presence of light for each working electrode (photoanode).This is due to removal of the products like -CHO unit and H + ion from the electrode surface, which facilitates the diffusion of another -CH2OH unit of another monomer to the surface.In presence of light, the oxidation and the associated product removal occurs quickly, with an effect of increased diffusion.Moreover, the local temperature of the electrode surface may increase in presence of light and thus the diffusion coefficient which is proportional to the absolute temperature, increases.In the DSSCs, the working electrode (photoanode) absorbs incident photon energy from the illuminated light and thereafter the electrons in the RB dye molecules become excited from ground state to the excited state (D * ).The excited photoelectrons are then injected into the conduction band (CB) of ZnO which results in oxidation of the RB dye photosensitizer (D + ) (Fig. 7).These The fill factor () and efficiency (ƞ) are calculated by, where, Pmax is the maximum power output and Pin is the input power.The fill factor () is a measure of the quality of the dye-sensitized solar cell and its power conversion efficiency (ƞ) represents the percentage of solar (light) power that the cell can transform to electrical power.The DSSC performance parameters for CLx (40 ≤ x ≤ 80) and CL80Ey (30 ≤ y ≤ 50) biopolymer electrolytes based DSSCs are listed in Table 2.It is found that the power conversion efficiency (ƞ) of the photovoltaic cells follows the conductivity trend of the biopolymer electrolytes used to construct the cells.The DSSC fabricated with CL80 biopolymer electrolyte exhibits an efficiency of 0.037%, which is increased to 0.051 % for CL80E45 biopolymer electrolyte based DSSC.This observation is analogous to that reported by Buraidah et al [25] with chitosan-polyethylene oxide polymer blend electrolytes by adding iodine crystals as a redox couple.The highest conducting biopolymer electrolyte, CL80E45 based DSSC demonstrates a large short-circuit current density (Jsc) of 0.556 mA/cm 2 even without any introduction of external redox couple.This experimental result may suggest that the redox couple is intrinsic within the chitosan biopolymer complex which results an increased current density under illumination.

Conclusions
Solid biopolymer electrolytes based on chitosan with LiClO4 as a supplier of charge carriers and ethylene carbonate (EC) as a plasticizer are prepared using solution-casting technique.Introduction of salt and the plasticizer helps to tune the ionic conductivity of chitosan from ~ 10 -9 S/cm to ~ 10 -4 S/cm at room temperature.The transport properties of the SPEs are estimated by Raman deconvolution technique.All the ion transport parameters follow the trend of ionic conductivity of the biopolymer electrolytes.The highest conducting SPE (CL80Ey with  = 45) shows the maximum charge carrier mobility (3.125  10 - 6 cm 2 V -1 s -1 ) and highest number of mobile charge carriers (43  10 19 /cm 3 ).Dye-sensitized solar cells (DSSC) are developed with CLx (40 ≤ x ≤ 80) and CL80Ey (30 ≤ y ≤ 50) biopolymer electrolytes respectively with ITO/ZnO/RB as the working electrode and platinum as the counter electrode.Without any external addition of redox couple in the electrolyte, the maximum power conversion efficiency of 0.051% is obtained in CL80Ey (with  = 45) biopolymer electrolyte based DSSC, which also shows highest short-circuit current density Jsc = 0.556 mA/cm 2 and an open-circuit voltage Voc = 0.605 V.It is found that the power conversion efficiencies of the DSSCs follow the conductivity trend of the biopolymer electrolytes used to fabricate the cells.LSV responses of the cells confirm the photovoltaic performance of the DSSCs.The current is produced due to the diffusion of monomer units of chitosan in the polymer electrolyte on the application of voltage.
1291 (2023) 012014 IOP Publishing doi:10.1088/1757-899X/1291/1/0120144 where, g E is the band gap of the material.The band gap of ZnO has been evaluated using this relationship, in the standard manner from 2 ) (  h versus  h plot as illustrated in Fig. 1b.The calculated value of optical band gap of ZnO nanoparticles is 3.32 eV.

Fig. 1 .h
Fig. 1.(a) XRD spectrum of ZnO and the inset figure shows the Williamson-Hall plot, (b) Plot of

3. 3 .
Ionic conductivity studies: Impedance Spectroscopy The Cole-Cole plots of the chitosan based solid biopolymer electrolytes (SPE) with different concentrations of salt (LiClO4) (henceforth denoted as CLx, where x = 40, 60, 80 wt.percentages of the polymer) and plasticizer (EC) (henceforth denoted as CL80Ey, where y = 30, 45, 50 wt.percentages of the polymer) are shown in Figs. 2 (a) and (b) respectively.The DC ionic conductivity (DC) of the SPE is calculated using the relation, DC = L/RbA where, L is the SPE thickness, A is the effective contact area of the electrode and the SPE surface and Rb is the bulk resistance.Rb is determined from the high frequency intercept on the real Z axis of the Cole-Cole plot.The variation of DC conductivities for different LiClO4 and EC concentrations are shown in the insets of Figs. 2 (a) and (b) respectively.It is found that the ionic conductivity increases on increasing the LiClO4 concentrations in the SPE system and CL80 shows the highest conductivity value of ~ 10 -5 S/cm at room temperature (300 K).This indicates that the mobile charge carrier ions (Li + ) play a significant role for the enhancement of conductivity.To increase the mobility of Li + further, different concentrations of EC as plasticizer, have been added in the matrix of the biopolymer electrolyte CL80.It is observed that the ionic conductivity is increased by one order of magnitude (from 10 −5 to 10 −4 S/cm) on addition of EC in the CPL80Ey (y = 45).This enhancement in the ionic conductivity may be attributed to the increase in salt dissociation on the incorporation of EC in the SPE.Further increase in EC content (y > 45) decreases the conductivity of the SPE by inhibiting the fast ionic transport through salt recrystallization.

Fig. 2 .
Fig. 2. Cole-Cole plots of chitosan based solid biopolymer electrolytes (SPE) with different (a) salt (LiClO4) and (b) plasticizer (EC) concentrations.Insets of (a) and (b) shows the variation of DC conductivities for different LiClO4 and EC concentrations, respectively.

Fig. 5 .
Fig. 5. Percentage of free ions as a function of (a) LiClO4 and (b) EC concentrations.

Fig. 6 .
Fig. 6.Ion transport parameters as a function of (a-c) LiClO4 and (d-f) EC concentrations.