Fabrication of cellulose paper-based counter electrodes for flexible dye-sensitized solar cells

Abstract

Here, we introduce the synthesis and deposition of organic/inorganic composite ink on cellulose paper using a rapid ultrasonic spray deposition approach that can be incorporated as a counter electrode (CE) in flexible dye-sensitized solar cells (FDSSCs). The composite ink comprised a copper indium sulfide (CuInS₂) nanostructure ink and dispersion of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) in water. Fabricated counter electrodes are biodegradable, environment-friendly, flexible, and economical and meet the requirements for sustainable green energy. To evaluate the catalytic activities and power conversion efficiencies of DSSCs, the produced CuInS₂/PEDOT:PSS composite ink-based CEs were compared with PEDOT:PSS counter electrodes. Cyclic voltammetry studies found that CuInS₂/PEDOT:PSS had a greater cathodic charge transfer current density (J_c) (−1.23 mA cm⁻²). Moreover, it was found that the potential separation values are small, which indicate a stronger catalytic activity than PEDOT:PSS counter electrodes. The observed exchange current density (J₀) was 3.98 mA cm⁻², while the limiting current density (J_lim) increased to 45.7 mA cm⁻², indicating a fast redox diffusion rate of the CuInS₂/PEDOT:PSS CE. The photovoltaic performances of CuInS₂/PEDOT:PSS and PEDOT: PSS-based DSSC's were measured and determined to be 5.66% and 4.41%, respectively, while the performance of CuInS₂/PEDOT:PSS FDSSC composed of cellulose paper was 1.06%.

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1. Introduction

As the world is currently facing energy crisis owing to the rapid growth of the global economy, population, urbanization, rising living standards, and exploitation of fossil fuel reserves, the primary matter confronting the planet at the moment is the production of efficient, and affordable energy.^1–4 Fossil fuels provide over 80% of the world's energy, although they are exhaustible and emit CO₂, a life-threatening environmental hazard and a greenhouse gas that leads to global warming. With the challenging issues of energy insecurity and environmental destruction, the astonishing growth of solar cells owing to their eco-friendly and sustainable features, has attracted interest in the growing demand for clean energy.^5–7

FDSSCs are an exciting innovation mainly due to tremendous features over the basic DSSC and Si-based modules, including reduced manufacturing cost, ease of processing, functionality, reasonable electrical efficiency, and other aspects.^8–14 In a typical DSSC, along with the working electrode (WE) and a redox electrolyte, the counter electrode (CE) also plays a crucial role in determining the overall cell power conversion efficiency.^15–18 The DSSC CE performs two essential roles: it supports the electrolyte reduction, which provides uninterrupted electricity generation, and acts as a means of transferring electrons from the outer circuit back to the electrolyte.^19,20 For CE to perform its function smoothly, it requires a catalytic material that can promote electrolyte reduction performance while also acting as an effective electron conductor.²¹ Platinum was chosen as the catalyst of choice in the first built DSSC because it can provide both properties.^22,23 Contrarily, the oil refinery, pharmaceutical, and automobile sectors all make extensive use of platinum. Thus, the price of platinum is astronomically high as a result of its tremendous demand. Compared to photovoltaics, Pt is less expensive in all well-established sectors mentioned above as large revenues are generated, lowering its price. However, lowering the cost of DSSC systems requires replacing pricey platinum and transparent conducting oxides with low-cost and high-efficiency materials.^18,24 This entails the use of a strong and active catalytic material as well as a conducting substrate with low sheet resistance.²⁵ Several alternatives to Pt have been reported in literature, comprising conducting polymers, oxides, carbon-based materials, and sulfides.^4,26–31

Poly-3,4-ethylenedioxythiophene polystyrenesulfonate (PEDOT:PSS) polymer has aroused special interest in the polymer industry due to the high conductivity, catalytic activity, and mechanical flexibility of PEDOT and the nonconductive PSS anion increases the solubility of the PEDOT polymer in organic solvents and water, despite this advantage, the addition of PSS reduce the overall conductive and electrocatalytic behavior towards I₃- reduction.^32–35 Here, ternary copper indium sulfide nanostructures are shown to play a distinctive role due to their good catalytic activity and stability.^36,37 As such, CuInS₂/PEDOT:PSS composite is a pronounced aspirant material for CE due to the unique properties of CuInS₂, PEDOT, and PSS. In literature, CuInS₂/PEDOT:PSS/conductive glass-based counter electrodes fabricated via different deposition approaches have been reported.⁸ This type of inorganic/organic composition is not fabricated in the paper-based substrate but other kinds of paper photo anodes, counter electrodes, and electrolytes have been reported in the literature, owing to their versatility in data storage, energy storage devices, writing, packing, and hygienic tissues. They are also affordable, thin, bendable, lightweight, and widely accessible, with excellent biocompatibility, and recyclability.³⁸ Paper's intrinsic compliance with printing has allowed the patterning of active inks on paper.³⁹ A clear path for scaling up the production process has been proven on paper using techniques such as blade, slot die, and spray coating.¹¹ Wang Bo and Lei L Kerr reported the first dye-sensitized paper solar cell in 2011. Photo-anode in DSSCs was made with Ni-coated box paper.¹² Hu, Liangbing, et al. (2013) found that cellulose nano-fibril nano-paper has better optical properties than ordinary paper. Nano paper is exceptionally transparent and scatters light in the forward direction because of its small nano-fibril size. In this case, the power conversion efficiency was 0.40%. Altering the nano paper during device assembly might boost cell performance. Transparent and conducting nano-fibrous paper can be used in optoelectronic applications.⁴

Here, we report the Pt metal-free counter electrodes, FDSSCs comprised of standard TiO₂ working electrode on ITO-PET substrate and flexible (organic/inorganic ink) counter electrode on cellulose paper substrate. Further, the synthesis and deposition of organic/inorganic composite ink on cellulose paper by a rapid ultrasonic spray deposition approach are also described which can be used as a flexible counter electrode in FDSSC. The composite ink comprised copper indium sulfide (CuInS₂) nanostructures ink and PEDOT:PSS 1.3 wt% dispersion in H₂O. Further, the electrocatalytic and photovoltaic measurements were performed to investigate the performance of the devices.

2. Experimental

2.1 Materials

An ITO-coated glass substrate (9 Ω per sq.), ITO-coated PET substrate, copper chloride 2 hydrate (CuCl₂·2H₂O), indium chloride tetrahydrate (InCl₃·4H₂O), and PEDOT:PSS 1.3 wt% dispersion in H₂O were obtained from Sigma-Aldrich. For preparing the counter electrode, guanidinium thiocyanate (GuSCN), iodine (I₂), 4-tert-butylpyridine (C₉H₁₃N), 1-methyl-3-propylimidazolium iodide (MPII), lithium perchlorate (LiClO₄), lithium iodide (LiI) and acetonitrile for electrolyte preparation, and, polyvinylpyrrolidone (PVP), ethanol, titanium isopropoxide (TTIP) and hydrochloric acid (HCl) for the working electrode were purchased from Sigma-Aldrich. Ethylene glycol (C₂H₆O₂), thiourea (CH₄N₂S), and acetylacetone were purchased from Merck. For the cellulose substrate, double A4-sized papers were used. N3 dye, which is basically cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′ dicarboxylato) ruthenium(II) was purchased from Sigma-Aldrich and used for the sensitization of the TiO₂ photoanode.

2.2 Synthesis of CuInS₂ nanostructured

The ternary CuInS₂ nanoparticles were synthesized by preparing a solution containing 1.70 g and 1.20 g of indium chloride tetrahydrate and copper chloride dehydrate, respectively, in 40 mL of ethylene glycol, as mentioned in previous work.⁴⁰ The mixture of chlorides in ethylene glycol was stirred for 60 min to obtain their complexes. A separate solution of 2.40 g of thiourea (CH₄N₂S) in 40 ml of ethylene glycol was prepared in a 50 ml beaker. The mixture was stirred until the solution became clear. After that, the solution of thiourea was added slowly to the prior solution. The final solution was stirred for 2 hours at 180 °C. During this, the color of the solution converted from colorless to pale yellow, brown, and eventually black as the temperature increased. The black solution was then cooled to room temperature, precipitated, and washed with ethanol to obtain the crude product. The product was then vacuum-dried for a few hours at 80 °C. The final product was annealed at 200 °C for 60 min to obtain a suitable phase for further usage.

2.3 Fabrication of counter electrodes (CEs)

CuInS₂ nanostructured suspension was formed by combining 20 mg CuInS₂ powder with 1 mL hexane and 13 mg thiourea with 5 mL ethanol. After that, the liquids were mixed and ultrasonicated for around 60 min, resulting in a homogeneous and stable suspension for future composite inorganic/organic ink synthesis. The suspension was combined with PEDOT : PSS in a 1 : 1 volume ratio. To prepare the stable CuInS₂/PEDOT:PSS ink using the ultrasonic spray deposition (USD) technique, the solution was agitated for 2 hours. The electrode surface for deposition was ultrasonically cleaned (deionized water, acetone, and ethanol) ITO-coated glass substrate (2 × 2 cm²) with an active area of 0.5 × 0.5 cm². A 3 ml syringe was filled with ink and inserted into a syringe pump. To atomize the ink on the substrate, the nozzle was set at a 2 cm distance. Pristine PEDOT:PSS and CuInS₂/PEDOT:PSS inks were consumed in the USD process for 3 min at a high power source with an 80 μl min⁻¹ spray rate, and the films were obtained following heat treatment at 120 °C for 3 hours on a hot plate. In DSSCs, the produced films were used as counter electrodes.

Furthermore, the USD process was carried out on ITO-coated polyethylene terephthalate (PET) and 80–120 mm cellulose paper substrates for FDSSCs using the same parameters, as used for the previously fabricated electrodes. Fig. 1 shows the electrode development process for DSSCs and FDSSCs at different substrates and inks.

Fig. 1 Schematics of counter electrodes prepared for DSSCs and FDSSCs from CuInS2/PEDOT:PSS and PEDOT:PSS inks.

2.4 Assembly of DSSCs

The working electrodes of the cells were fabricated on conducting FTO glass with the doctor blade technique, as mentioned in the literature.⁴¹ TiO₂ nanoparticles were prepared from TTIP and deposited on cleaned 2 × 2 cm² ITO glass with a 0.5 × 0.5 cm² active area. The above-formulated coating was dried at 80 °C for 10 min and calcined at 450 °C for 30 min.

The sensitization of the layer was performed in 0.3 M N3 dye solution in ethanol for 24 hours at room temperature and then washed with ethanol and dried in the ambient environment. Further, the working and CE were clamped and separated with a spacer. The electrolyte was prepared with 0.1 M GuSCN, 1.2 M MPII, 0.35 M iodine, and 0.5 M 4-tert-butylpyridine in acetonitrile, which was filled in the cell through the counter electrode hole.

To prepare the paper-based FDSSCs, the working electrode was fabricated on an ITO-PET substrate with a 0.5 × 0.5 cm² active area and further replaced the CEs with paper-based electrodes, as shown in Fig. S1 (ESI†).

2.5 Measurements and characterization

The crystal structure measurement of CuInS₂ nanomaterial was performed on a Bruker D8 ADVANCE X-ray diffractometer (XRD) using CuKα radiations and operating at 40 kV and 30 mA and scanned from 20° < 2θ < 70°. The optical properties, such as absorbance, were measured using a UV-Vis-NIR spectrophotometer. A TESCAN MIRA3 field-emission scanning electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS) was used to collect the surface and cross-section images and EDS spectra of the particles and films prepared using the ultrasonic spray deposition technique. The photo-current–voltage characteristics of DSSCs were taken using Newport Oriel Keithley PVIV 2400 source meter and solar simulator under illumination of 100 mW cm⁻². The active area of DSSCs was 0.25 cm². The open circuit voltage, short circuit current and fill factor were calculated using current density measurements.

Further electrochemical measurements of the counter electrode were performed on a CHI660E electrochemical workstation. Cyclic voltammetry (CV) was performed to measure the redox characteristics of (CuInS₂/PEDOT:PSS and PEDOT:PSS) counter electrodes towards tri-iodide reduction by assembling three electrodes at the 100 mV s⁻¹ scan rate where the prepared film acted as the working electrode; Ag/AgCl was used as the reference electrode, and a Pt wire was used as the counter electrode. The electrolyte solution contained 1 mM I₂, 10 mM LiI, and 100 mM LiClO₄ in acetonitrile.

The electrochemical impedance spectroscopy (EIS) of the DSSC counter was recorded in the frequency range from 100 kHz to 0.1 Hz with dummy cells consisting of two identical electrodes and the same electrolyte, which was used to fabricate the DSSCs. Further, the Tafel polarization curves were obtained using the same dummy cells used for EIS in a potential window of −0.6 V to 0.6 V.

3. Results and discussions

3.1 Investigation on CuInS₂ nanoparticles

Here, CuInS₂ nanoparticles were synthesized in ethylene glycol; usually, CuInS₂ nanoparticles are synthesized in olylamine, which is insoluble and immiscible in water. To produce stable ink for the USD process, the miscibility of materials was required. Poly (3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT:PSS) 1.3 wt% dispersion in water was used, as such, CuInS₂ nanoparticles were synthesized in ethylene glycol to prepare the stable CuInS₂/PEDOT:PSS ink.

For further investigation, the characterization of the prepared black powder was accomplished. For structural analysis, X-ray diffraction was performed between 20° and 70°, 2θ values. Fig. 2 shows the formation of chalcopyrite CuInS₂ nanoparticles without any impurities. The diffraction peaks are located at 27.90°, 32.30°, 45.35°, and 54.90° at 2θ values with (112), (200), (204), and (312) planes of CuInS₂ respectively. All the obtained diffraction peaks followed the standard tetragonal chalcopyrite (JCDS 85-1575) crystal phase.

Fig. 2 XRD spectra of synthesized CuInS₂ nanostructured powder and a theoretical structure model of CuInS₂ drawn through VESTA software, (b) absorbance spectra of the dispersed CuInS₂ nanopowder, (c) SEM image, and (d) energy dispersive spectra and elemental composition of CuInS₂ nanostructures.

The absorption spectrum of CuInS₂ NP dispersion recorded between 350 to 700 nm showed broad absorption in this range. The SEM images show the nano-size spherical shape morphology and homogenous growth of the NPs. EDS results indicate the presence of In, Cu, S, and O elements with approximately 1 : 1 : 2 atomic ratios of Cu, In, and S, respectively.

The conducting polymer PEDOT:PSS has recently attracted considerable attention as a relatively low-cost CE material for DSSCs owing to its favorable conductivity, remarkable electro-catalytic activity, and good film-forming ability. Despite this, PEDOT:PSS generates a smooth film with a small surface area, as illustrated in Fig. 4(a), which is inadequate in triiodide ion regeneration catalysis. To overcome this limitation, a composite ink composed of CuInS₂ nanoparticle ink and PEDOT:PSS 0.3 wt% dispersion in H₂O was fabricated and deposited in CEs to increase the active surface area, as shown in Fig. 4(b).

3.2 Catalytic behavior of CuInS₂/PEDOT:PSS film

After the investigation of CuInS₂ NPs, the inks of CuInS₂/PEDOT: PSS and PEDOT:PSS were prepared, as discussed in Section 2.3. The CV of CuInS₂/PEDOT: PSS/ITO-Glass-based counter electrode was performed to examine the cathodic current density (J_c) and peak-to-peak potential difference (ΔE_pp) because these parameters play an essential role in the investigation of the catalytic behavior of the electrodes. Generally, a higher value of J_c and smaller ΔE_pp reveal the better catalytic activity of the material. The cyclic voltammetry curves were obtained on the CuInS₂/PEDOT: PSS film using the three electrodes assembly operating at a scan rate of 20 to 120 mV s⁻¹ in a potential window from −1.0 to 1.5 V.

In the CV curve of CuInS₂/PEDOT:PSS/ITO counter electrode, we obtained two pairs of redox peaks. The positive peaks are termed anodic peaks, which are related to iodine and tri-iodide oxidation, whereas the negative peaks are named cathodic peaks and are associated with tri-iodide reduction. The redox reduction at the cathodic side is defined by eqn (1), and the positive one corresponds to the redox reaction per eqn (2)

I₃⁻ + 2e⁻ ↔ 3I⁻ (1)

3I₂⁻ + 2e⁻ ↔ 2I₃⁻ (2)

In dye-sensitized solar cells, the reduction reaction of I₃⁻ will occur at the counter electrode to complete the circuit conductivity and regenerate the dye molecules, therefore, it is crucial to examine the catalytic ability of I⁻/I₃⁻ at the negative potential side. Further, the ability of a counter electrode for I₃⁻ reduction in solar cells relates to the cathodic current density at a more negative potential.

A high value of J_c obtained from the CV measurements showed that the electrodes had increased active surface area and conductivity, which are more favorable for the reduction reaction of I₃⁻ and lift the value of J_sc and performance of DSSCs. Fig. S2 (ESI†) shows that the CV curve obtained at 100 mV s⁻¹ scan rate had a higher cathodic current density (−1.23 mA cm⁻²) and lower peak-to-peak potential difference (0.82 V) value as compared to other CV curves obtained at other scan rates.

3.3 Electrochemical investigation of the counter electrodes

The electrochemical properties of CuInS₂/PEDOT:PSS and PEDOT:PSS CEs were examined using the Tafel-polarization plots. At the electrolyte/counter electrode interface, the electron transfer kinetic exchange current density was directly measured from the Tafel polarization curve. As such, J₀ of the electron transfer kinetics was measured from the intersection of the linear anodic and cathodic curves at zero over potential, and the limiting current density (J_lim) was recorded from the y-interception of the diffusion zone. Generally, the higher J₀ supports the catalytic activity of the electrode and a similarly larger value of J_lim shows a fast redox diffusion rate. The calculated value of J₀ for CuInS₂/PEDOT:PSS was 3.98 mA cm⁻², which was higher than the PEDOT:PSS J₀ value of 1.90 mA cm⁻². Meanwhile, J_lim was also higher (45.7 mA cm⁻²).

Fig. 3 shows the CV curves of CuInS₂/PEDOT:PSS and PEDOT:PSS CEs fabricated on the ITO-glass substrate with iodide species. CuInS₂/PEDOT:PSS and PEDOT:PSS CEs produced two pairs of redox peaks, namely, anodic (positive) and cathodic (negative) peaks. The anodic peaks on the CV curves represent iodide and tri-iodide oxidation, while the cathodic peaks represent tri-iodide reduction, as mentioned in the previous section. There is an inverse relation between ΔE_pp and the charge transfer rate where a high J_c value reflects a strong catalytic behavior of the CE and a lower value of ΔE_pp indicates that the redox reaction occurred smoothly on the CE surface.

Fig. 3 (a) Tafel Polarization Curves and (b) cyclic voltammetry of CuInS₂/PEDOT:PSS and PEDOT:PSS counter electrodes.

As shown in Fig. 3, the J_c value of CuInS₂/PEDOT:PSS (J_c = −1.23 mA cm⁻²) is higher compared to the J_c value of PEDOT:PSS CE (J_c = −0.65 mA cm⁻²), while the ΔE_pp value of CuInS₂//PEDOT:PSS (ΔE_pp = 0.80 V) was lower than that of the PEDOT:PSS CE (ΔE_pp = 1.21V).

Both findings (ΔE_pp and J_c) for CuInS₂/PEDOT:PSS CE indicate an excellent catalytic activity because the composite material has a large redox activity toward I₃ and I₂ compared to PEDOT:PSS CE. Hence, the CE comprising CuInS₂ nanostructure-incorporated PEDOT:PSS rendered a synergistic catalytic effect compared with other CE.

Further, electrochemical impedance spectroscopy (EIS) measurements were performed to study the interfacial electrochemical properties and charge transfer phenomena of the CuInS₂/PEDOT:PSS and PEDOT:PSS CEs. Analysis was performed in the frequency range of 100 kHz to 0.1 Hz using the applied open circuit potential. Fig. 4(b) shows Nyquist plots for CuInS₂/PEDOT:PSS and PEDOT:PSS having dominant semicircle in the case of CuInS₂/PEDOT:PSS, in the presence of interfacial charge transport resistances in the fabricated CuInS₂/PEDOT:PSS and PEDOT: PSS-based devices. The Randles circuit model utilized for experimental data fitting is also depicted in Fig. 4(b). The semicircle, visible in the high-frequency range, indicates the charge-transfer resistance (R_ct) at the counter electrode/electrolyte interface, further, R_s denotes the sheet resistance, which is mainly due to the resistance of the specific substrate (conducting material) used for the fabrication of electrode and electrolyte diffusion resistance. The values of sheet resistance (R_s), interfacial charge-transfer resistance (R_ct), and other parameters are given in Table 1. It is clearly noticed that the interfacial charge transfer resistance R_ct of composite CE-based DSSC (175.6 Ω) is lower than the pristine PEDOT:PSS-based DSSCs (329.3 Ω), which elucidated the increased interfacial contact between the electrolyte and CuInS₂/PEDOT:PSS.

Fig. 4 (a) The Bode plot, and. (b) Nyquist plots of CuInS₂/PEDOT:PSS and PEDOT:PSS-based dummy cells consisting of two identical electrodes.

Table 1 Electrochemical parameters of the fabricated counter electrodes

Counter Electrode	R s (Ω)	R ct (Ω)	C (μF)	W	τ (ms)
CuInS2/PEDOT:PSS	8.685	175.6	6.027	0.00045	0.195
PEDOT:PSS	32.89	329.3	19.25	0.00736	1.324

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The total internal series resistance is equal to the sum of all these resistances (R_total). For DSSCs, made of CuInS₂/PEDOT:PSS and PEDOT:PSS, the total internal resistance was 184.5 Ω and 361.6 Ω, respectively. The performance of CuInS₂/PEDOT:PSS CE was superior to that of PEDOT:PSS CE. Further, the attained capacitance (C) of the composite-based DSSC was lower than that of the pristine PEDOT: PSS-based DSSC, as such, a higher value of C in the case of PEDOT: PSS-based device was obtained due to the broad absorption from the visible to NIR region, which upsurges the excitation of electrons responsible for the high electron concentration in the TiO₂ conduction band.

The existence of the interfacial charge transport resistance can be more inveterate from the phase angle shift in the Bode phase plots shown in Fig. 4(a). The electron relaxation lifetime (τ_e) was calculated from the peak frequency (f_max) of the Bode phase plots using the relation given in eqn (3),

It is noteworthy that the obtained electron relaxation lifetime of CuInS₂/PEDOT:PSS-based DSSC (0.195 ms) is lower than that of the PEDOT:PSS-based device (1.342 ms). As such, the higher τ_e of PEDOT: PSS-based DSSC resulted in a longer relaxation lifetime of electrons in the TiO₂ conduction band and hence induces higher recombination potentials in the device, which is responsible for the reduced FF in PEDOT: PSS-based DSSC in J–V measurements, as given in Table 2. K Ashok Kumar et al. reported similar electrochemical behavior for cobalt sulfide nanostructures in comparison with the Pt-based DSSCs and reported that cobalt sulfide has been proven to be very effective in catalyzing the redox electrolyte in dye-sensitized solar cells (DSSCs) and exhibits a great potential to replace the traditional noble metal platinum (Pt) counter electrodes in DSSCs.²⁸

Table 2 Photovoltaic parameters of DSSCs fabricated with CuInS₂/PEDOT:PSS and PEDOT:PSS-based CEs

Counter Electrode	V oc (V)	J sc (mA cm^−2)	FF	η (%)
CuInS2/PEDOT:PSS	0.76	12.96	0.57	5.66
PEDOT:PSS	0.77	10.52	0.54	4.41

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FE-SEM was used to evaluate the surface morphologies of CEs, and the photographs of both CEs composed of PEDOT:PSS and CuInS₂/PEDOT:PSS films are depicted in Fig. 4. From the top, it is clearly noticed that the PEDOT:PSS film has a smooth texture, which is detrimental to the electrocatalytic activities (Fig. 5a), and on the other hand, the CuInS₂/PEDOT:PSS film had a porous and irregular island-type topography that exhibited dual connectivity, namely, the continuity of CuInS₂ nanoparticles in the polymer, and the interconnectivity of pore channels. The nanoparticle continuity is preferable for charge transport within the layer, whereas porosity in the film is favorable for electrolyte permeation and redox pair dissemination in the electrolyte. Furthermore, the porous framework of the CuInS₂/PEDOT:PSS film helps to increase the counter electrode's active surface area by providing multiple reactive sites and, therefore, considerably enhancing CE's catalytic performance (Fig. 5b).

Fig. 5 FE-SEM top images of (a) PEDOT:PSS film, (b) CuInS₂/PEDOT:PSS film, and (c) cross-sectional view of CuInS₂/PEDOT:PSS.

In addition, Fig. 5c depicts the cross-sectional view of the CuInS₂/PEDOT:PSS film, which is estimated to be approximately 15 μm thick and exhibited the consistent thickness and high quality of the film, which is favorable for device performance and indicate the uninterrupted ultrasonic spray due to the stable ink on the overall active area of the CE's.

After the FE-SEM analysis, the presence and uniform incorporation of CuInS₂ nanostructures inside the PEDOT:PSS film were further investigated using energy-dispersive X-ray spectroscopy (EDS). Fig. 6 describes the PEDOT:PSS and CuInS₂/PEDOT:PSS electrode's EDS color mapping. Because, both CuInS₂ and PEDOT:PSS ink contain sulfur, the color mapping reveals the highest sulfur concentration. The EDS of a pristine PEDOT:PSS electrode is given in Fig. 6(a). Fig. 6(b) endorses the uniform nanostructure dispersion in the PEDOT:PSS electrode film.

Fig. 6 Energy dispersive X-ray Spectroscopy color mapping of (a) PEDOT:PSS and (b) CuInS₂/PEDOT:PSS film on the cellulose paper substrate.

To further elucidate the performance of CuInS₂/PEDOT:PSS and PEDOT:PSS as CEs in DSSCs, photocurrent density–voltage (J–V) characteristics were measured with a power source under the illumination of 100 mW cm⁻². Fig. 7 shows the J–V characteristic curves of DSSCs containing CuInS₂/PEDOT:PSS and PEDOT:PSS counter electrodes. The photovoltaic parameters, fill factor (FF), and power conversion efficiency (η), were calculated and all corresponding photovoltaic parameters are summarized in Table 1.

Fig. 7 Photocurrent Voltage measurements of DSSCs comprised with TiO₂/ITO based photo-anodes with N3 dye sensitization and CuInS₂/PEDOT:PSS/ITO, and pristine PEDOT:PSS/ITO-based counter electrodes.

The results show that the DSSC made with CuInS₂/PEDOT:PSS CE is more efficient compared to a cell made with the PEDOT:PSS counter electrode. Therefore, the performance of the PEDOT:PSS-based CEs can be significantly enhanced with the addition of CuInS₂ nanostructure. These results are consistent with the electrocatalytic measurements; the cell made with CuInS₂/PEDOT:PSS CE has smaller R_ct and the R_total values compared to PEDOT:PSS CE-based DSSCs. Hence, the performance of CuInS₂/PEDOT:PSS CE can be seen to be superior than that of PEDOT:PSS CE.

To further examine the benefits of the CuInS₂/PEDOT:PSS ink, we manufactured FDSSCs utilizing cellulose papers (CP1–CP3), and the ITO-PET substrate for counter electrodes produced with CuInS₂/PEDOT:PSS ink, all flexible CEs used N3-dye/TiO₂/ITO-PET flexible photoanodes. The fabrication process was the same one that was used for fabricating counter electrodes, as described in the previous section. The stability of the electrode is a major factor that affects the performance of the device. As such, the paper-based electrodes go through heat tolerance and electrolyte resistibility. To test heat tolerance, the CP electrodes were heated to 100 °C for 1 hour. It was discovered that CP1 electrode shrinkage occurred and that the effect was reduced with paper thickness. To test the percolation of electrolytes into paper electrodes, the electrolyte was poured on top of all the paper surfaces. It was observed that the electrolytes did not seep through to the P3 paper electrode. The P3 paper electrode was considered the most stable and ideal without surface burning, shrinking, and penetration of electrolytes. Fig. 8(a) shows the top view of the electrode surfaces without and with the deposition. Fig. 8(b) illustrates the stability data of the paper electrodes and these flexible electrodes are space-saving, thin, and easy to shape and roll.

Fig. 8 (a) Top view of the ITO-PET substrate-based counter electrode made with inorganic/organic ink using the USD procedure, (b) and (c) flexible and space-saving, (d) front view of cellulose paper as a substrate, (e) and (f) cellulose paper-based counter electrodes.

Fig. 8 shows the J–V characteristics of three flexible DSSCs made with paper-based counter electrodes. The PCE of the produced FDSSC based on the low-cost hierarchical structured CuInS₂/PEDOT:PSS/paper counter electrode is 1.06%, which is comparable to that of the Pt/paper-based DSSC.⁴² It is worth noting that for the paper-based FDSSCs, FF and V decrease due to the lower transparency, which enhances the recombination rates of the device. The presence of dual valences, Cu (Cu¹⁺ and Cu²⁺), raises the electro-catalytic activity of the LiI electrolyte, forcing electrons to pair with CB electrons. Furthermore, the presence of dual valence Cu ions causes intermediate trap levels between the conduction and valence bands, lowering the device's V_oc (Table 3).

Table 3 Photovoltaic parameters of FDSSCs fabricated with CuInS₂/PEDOT:PSS and PEDOT:PSS-based CEs

Counter Electrode	V oc (V)	J sc (mA cm^−2)	FF	η (%)
CuInS2/PEDOT:PSS/ITO-PET	0.71	10.27	0.54	3.99
PEDOT:PSS/ITO-PET	0.70	5.68	0.39	1.56
CuInS2/PEDOT:PSS/C Paper	0.70	2.18	0.35	1.06

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4. Conclusion

We demonstrated how inorganic/organic composite ink could be employed as CE material for FDSSCs. The tetragonal CuInS₂ NPs and PEDOT:PSS polymer were used to synthesize the composite ink, which was then utilized to prepare an economically efficient CE on ITO-PET and cellulose paper substrates in FDSSCs using an ultrasonic spray deposition technique. The inorganic/organic composite ink-based CEs exhibit higher electro-catalytic performance, as proven by all electrochemical studies, comprising cyclic voltammetry, Tafel polarization, and electrochemical impedance spectroscopy. According to the photovoltaic properties, CuInS₂/PEDOT:PSS CE can attain a PCE of 5.66% for DSSCs and 3.99%, 1.06% for CuInS₂/PEDOT:PSS/ITO-PET and CuInS₂/PEDOT:PSS/cellulose paper CEs, respectively, in FDSSCs. This work introduces a novel concept for the selection of CE materials for the first time on CP in FDSSCs and further expands the usage of composite ink in flexible electronics.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National University of Sciences and Technology (NUST), the technical support from the Solar Energy Research Lab, and the Advanced Energy Materials Lab.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2cp04358k

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