TiO₂/Pt-Free DSSC produced with Nb₂O₅ and Polyaniline: a preliminary study

Abstract

Solar energy offers an eco-friendly alternative to address global energy challenges. This study develops a cost-effective dye-sensitized solar cell (DSSC) using niobium pentoxide (Nb₂O₅) as a photoanode and a polyaniline (PANI) composite as a counter electrode, replacing traditional titanium dioxide (TiO₂) and platinum (Pt). Nb₂O₅ was synthesized via the Pechini method, and PANI through polymerization. The components were assembled using the Doctor Blading method in a sandwich configuration. Characterization revealed Nb₂O₅ with a band gap of ~3.0 eV and an orthorhombic morphology, demonstrating potential as a TiO₂ substitute. DSSCs with composite electrodes outperformed those with pure PANI or niobium pentoxide electrodes. The PANI/Nb1 composite (1% Nb) achieved a V_oc of 0.69 V and a current density of 0.075 mA cm⁻². These findings highlight the promise of Nb₂O₅-enhanced PANI for efficient, low-cost solar energy conversion.

Keywords:
Gratzel Cell; semiconductor oxides; photovoltaic system; niobium pentoxide

INTRODUCTION

Solar energy, an abundant renewable source, offers a viable alternative to solve energy crises without environmental repercussions ¹. Given Brazil geographical position and high solar incidence rates, solar cells, particularly photovoltaic cells, emerge as key to a sustainable future due to their potential implementation in remote locations without the need for extensive transmission networks ².

Photovoltaic cells can be categorized into first, second, and third generations ³. First-generation cells typically use silicon materials in composition, second-generation cells employ the use of thin inorganic films deposited on conductive substrates, and third-generation cells include quantum dot sensitized solar cells (QDSC), perovskite solar cells (PKSC), and dye-sensitized solar cells (DSSC) ^4)-(5. DSSC has gained prominence due to its use of renewable energy, simple manufacturing processes, and high efficiency in converting solar energy to electrical energy ^1),(6)-(7. These cells typically consist of a photoanode (PE), a counter electrode (CE), a photosensitizer, and an electrolyte ^{6), (8)-(10}.

In a DSSC, the photoanode usually comprises a semiconductor oxide and a photosensitive dye that facilitates electron flow for generating an electric current ⁵. Titanium dioxide (TiO₂) is commonly used as the nanocrystalline semiconductor oxide ¹. However, its high recombination rate and cost encourage the exploration of alternative materials. The counter electrode (CE) in a DSSC transfers electrons from the external circuit to the electrolyte, facilitating the regeneration of the oxidized dye materials ^5),(11 and often uses platinum in composition due to its catalytic properties ^6),(8),(12. Therefore, low-cost alternatives such as conductive polymers, oxides, and carbon-based materials are being investigated for CE applications ¹.

Polyaniline (PANI) is a conductive polymer gaining attention for its technological applications, offering electrical, optical, and electrochemical properties, along with simple and cost-effective synthesis methods ^13)-(14. PANI has been utilized in various applications, including diodes, supercapacitors, corrosion-resistant coatings, batteries, dye-sensitized solar cells, sensors, and more ^13),(15)-(23. One method to enhance PANI properties is by forming composites with inorganic materials ¹². Composites are materials derived from two or more components that exhibit improved properties compared to their individual constituents ²⁴. Niobium pentoxide (Nb₂O₅), a metal oxide known for its chemical stability and n-type semiconductor properties with a band gap ranging from 3.1 to 4.0 eV ²⁵, presents itself as a candidate for PANI composite formation, combined with the production costs, due to the high niobium reserves in Brazilian soil.

This study aims to produce and characterize a low-cost DSSC comprising a Nb₂O₅ photoanode and a PANI/ Nb₂O₅ composite CE, thereby offering a TiO₂ - and Pt-free DSSC alternative.

EXPERIMENTAL

For the synthesis of Nb₂O₅ nanoparticles, the following reagents were used: ammonium niobium oxalate ((NH₄NbO(C₂O₄)₂(H₂O)), citric acid, and ethylene glycol. For the production of the photoanodes, the following materials were used: acetylacetone, polyethylene glycol, Triton X, distilled water, and the dye cis-bis di-tetrabutylammonium (2,2’-bipyridyl- 4,4’-dicarboxylato) ruthenium (II) (Aldrich^®). In the synthesis of polyaniline, the following reagents were employed: aniline, ammonium persulfate ((NH₄)₂S₂O₈), and hydrochloric acid (HCl).

Nanoparticles of Nb₂O₅ were obtained using the Pechini method ²⁶. Initially, 5g of ammonium niobium oxalate was dissolved in a 1 mol L⁻¹ citric acid solution under magnetic stirring at 70 °C. Ethylene glycol was then added, and the temperature was raised to 100 °C for 30 minutes. Subsequently, the solution was calcined in a muffle furnace at 400 °C for 2 h to remove the polymer and further calcined at 700 °C for 2 h to eliminate organic residues.

PANI was synthesized by polymerizing aniline. Specifically, 2 mL of aniline was dissolved in 20 mL of 1 mol L⁻¹ HCl solution under magnetic stirring and maintained in an ice bath (-5 °C) in a round-bottom flask. Separately, 6 g of ammonium persulfate was dissolved in 80 mL of the HCl solution and slowly added to the flask containing aniline. The system was kept under the same conditions for 2 h ²⁷. After this period, the precipitate formed was filtered using a Büchner funnel under vacuum and washed with the HCl solution.

The oxide photoanodes were prepared by mixing 2 g of Nb₂O₅ with 32 µL of acetylacetone, 332 µL of polyethylene glycol, 32 µL of Triton X, and 632 µL of distilled water ²⁸. The resulting paste was deposited onto the surface of a fluorine-doped tin oxide (FTO~7 Ω sq^-1) conductive glass using the Doctor Blading method. After deposition, the films were sintered in a muffle furnace at 450 °C for 30 min. Once cooled, the photoanodes were immersed in a solution of cis-bis di-tetrabutylammonium (2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium (II) (N719) at a concentration of 2.5×10⁻⁴ mol L⁻¹ for 24 h to sensitize the photoanodes.

Four types of counter electrodes were prepared: Nb100, Nb0, PANI/Nb1, and PANI/Nb5. The Nb100 counter electrode consists solely of Nb₂O₅, while Nb0 consists solely of PANI. PANI/Nb1 and PANI/Nb5 are composites of PANI/Nb₂O₅ with 1% and 5% Nb₂O₅ (w/w), respectively. All counter electrodes were also prepared using the Doctor Blading method ²⁸.

The PANI/Nb₂O₅ composites were prepared by dissolving 2 g of PANI in 20 mL of 2-methylpyrrolidone under magnetic stirring for 24 hours. After this period, 5 mL of this solution was removed, and the appropriate percentage of Nb₂O₅ relative to the volume of the PANI solution was added. The PANI/Nb₂O₅ composite was stirred for an additional 30 minutes before being deposited onto FTO plates.

The solar cells were assembled in a sandwich format, with the photoanode and counter electrode, adding the electrolyte between the FTO glass plates, each with an area of 0.2 cm². The electrolyte was produced using: 0.5 mol L^-1 of tert-butylpiridine, 0.6 mol L^-1 of tetrabutylammonium iodide, 0.1 mol L^-1 lithium iodide, and 0.2 mol L^-1 of resublimed iodide solubilized in metoxypropionitrile.

The morphological analyses of the samples were conducted using the SEM (Scanning Electron Microscopy) technique. The samples were deposited on carbon tape and sputter-coated with a thick layer of gold to enhance conductivity. SEM images were acquired using an electron acceleration voltage of 15 kV.

The X-ray diffractograms of the samples were obtained using a Bruker XRD D2 Phaser instrument, operating in the region of (2θ) 10 ° to 80 °, using the 30.0 kV Cukα radiation and a current of 10 mA (λ = 0.1506 nm).

Fourier transform infrared (FTIR) spectroscopy was performed using a Shimadzu IRAffinity model, operating in the range of 400-4000 cm^-1. For such analysis, samples were prepared as tablets in 1 wt% KBr.

The energy of the forbidden band of Nb₂O₅ (gap) was obtained by means of the Kubelka-Munk method, using UV-Vis spectroscopy data by reflectance obtained through a UV-3600i Plus spectrophotometer in the absorption range between 190 and 1100 nm.

Solar devices were analyzed in a Zennium Zahner potentiostat, with a solar simulator at 100 mW cm^-2, AM 1.5 G provided by a Xenon lamp. The potential of cells was measured by open-circuit potential curves, and the current and cell stability by photochronoamperometric curves, performed with light on/off. The cell interfaces were studied by Electrochemical Impedance Spectroscopy (EIS), with a LED lamp of 530 nm, with a frequency range of 10 kHz - 0.01 Hz and an amplitude of 10 mV.

Figure 1 presents the morphological and structural analysis of the Nb₂O₅ and PANI. Figure 1a demonstrates the XRD diffractograms of Nb₂O₅ particles, showing some characteristic narrow and well-defined peaks. In addition, the peaks (001), (180), (181), (002) were cataloged using Match Software in comparison with the crystallographic record PDF 30-0873, referring to the orthorhombic phase of Nb₂O₅²⁹.

Figure 1:
XRD diffractogram to Nb₂O₅ in A, SEM images in b: (i) Nb₂O_5, (ii) pure PANI, FTIR in C: (ii) pure PANI and (iii) PANI/Nb₂O₅.

Figure 1b presents Scanning Electron Microscopy (SEM) micrographs of polyaniline (PANI) and Nb₂O₅ (the magnified images have been included in the supplementary material for further clarification). The Nb₂O₅ particles (Figure 1bi) exhibit an irregular morphology, which can lead to the formation of aggregates. In contrast, the PANI (Figure 1bii) also shows an irregular structure resulting from the aggregation of smaller particles.

Figure 1c displays the Fourier-transform infrared (FTIR) spectra of PANI (Figure 1ci) and the PANI/Nb₂O₅ composite (Figure 1cii). There is no significant difference between the spectra of PANI and PANI/Nb₂O₅. The spectra reveal a band at 3427 cm^-1 corresponding to the N-H bond. Additionally, bands at 1664 cm^-1 and 1497 cm^-1 can be associated with the stretching of C=C and C=N bonds present in the quinonoid and benzenoid rings of PANI ^10)-(12. Bands at 1109 cm^-1 and 1293 cm^-1 are also observed, which can be attributed to the in-plane bending vibration of the C-H bond in the aromatic ring and the stretching of the C-N bond, respectively ^16),(30.

The optical properties of the Nb₂O₅ samples were evaluated by diffuse reflectance spectroscopy in the VIS-NIR region using a UV-Vis-NIR spectrophotometer. The diffuse reflectance spectra were converted to the Kubelka-Munk function, F(R), using the following equation:

$F\left(R\right)=\frac{{\left(1-R\right)}^2}{2R}$ (A)

Where R is the reflectance. This approach allows for the estimation of the band gap energy by plotting [F(R)hν]ⁿ versus photon energy (hν), where n is 1/2 for indirect transitions and 2 for direct transitions ³¹. The band gap was determined by extrapolating the linear portion of the Tauc plot to the energy axis. The absorbance spectrum (Figure 2a) offers qualitative information regarding the electronic transitions, while the diffuse reflectance data were further processed using the Kubelka-Munk function to estimate the optical band gap (Figure 2b).

Figure 2:
Absorbance vs wavelength for Nb₂O₅ films in A with Egap extrapolation in the Tauc plot from the Kubelka Munk function in B.

A plot of [F(R)hν]^1/2 vs Energy was constructed, which is suitable for indirect allowed transitions ³¹. The energy band gap (Eg) value can be determined from the intercept of the resulting linear region with the x-axis. As observed in Figure 2B, for Nb₂O₅, an energy gap of 3.0 eV was obtained, similar to that of TiO₂, the most commonly used oxide, suggesting that niobium pentoxide is a good candidate for titanium substitution ³¹.

In Figure 3 are depicted the open circuit potential (V_OC) curves for the cells analyzed. The potential parameter in dye solar cells could be defined as the difference between the E_redox from electrolyte and the E_fermi Level of scaffold oxide; however, as demonstrated by Wu and co-workers (2018), the counter electrode may affect the potential (V) ¹¹.

Figure 3:
Open circuit potential curves (V_oc) to dye solar systems produced with Nb₂O₅ and PANI at solar incidence ot 100 mW cm^-2 and 1.5 AM.

The voltage reduction of the theoretical parameter comes from the overall overpotential of CE, due to the mass transfer and kinetic process at such an interface. So, a good counter electrode should present amazing electrocatalytic properties for the reduction of the redox couple and good conductivity. In addition, some materials could be incorporated in conduction substrates to speed up the reaction $\left(I^{-}+2e^{-}\to 3I^{-}\right)$.

It is important to note that PANI/Nb1 exhibited a higher V_OC compared to the other samples, suggesting better conductivity and electrocatalytic properties of the counter electrode. In contrast, higher concentrations of Nb₂O₅ (PANI/Nb5 and Nb100) led to a decrease in cell potential, as the characteristics of polyaniline were overshadowed by those of the semiconductor oxide. Additionally, the potential of the PANI/Nb1 system remained stable up to 0.69 V, which is a promising result, as a potential of 0.7 V was reported by O’Brien and Gratzel as being associated with the most efficient solar devices ^32)-(33. Figure 4 depicts the photochronoamperometric curves for the systems analyzed.

Figure 4:
Photochronoamperometric curves to dye solar systems produced with Nb₂O₅ and PANI at solar incidence ot 100 mW cm^-2 and 1.5 AM.

In Figure 4, it is observed that all the systems developed were photosensitive, since with light intensity insertion, it was noted that the current increased, and in the light off condition, a zero current flowed in the device ³⁴.

In accordance with Figure 3, the cell PANI/Nb1 presented superior current density (0.075 mA cm^-2), since, as discussed, the conductivity and photocatalytic properties of polyaniline counter electrode were improved by 1% of Nb₂O₅ insertion. Nb100 has not presented increased potential, as suggested by the low conductivity of Nb₂O₅ materials. Regarding the PANI/ Nb0 condition, despite the good conductivity of PANI material, it was also observed that low current density (j = 0.02 mA cm^-2), indicated by the low uniformity due to the difficulty of PANI solubilization, showed that the electrodeposited materials may present superior performance ¹¹.

Some keys to enhancing the photoconversion of solar energy to electricity are the counter electrode uniformity, newly coated methodologies, and size reduction of niobium scaffold oxide ^{11),(28),(35),(36}. To verify the charge transportation process in solar cells interfaces, the electrochemical impedance spectroscopy was performed and depicted in Figure 5.

Figure 5
Electrochemical impedance spectroscopy curves for dye solar cells produced with different amounts of Nb₂O₅ in counter electrodes at solar incidence of 530 nm by a LED lamp, with 10 mV.

EIS spectra provide valuable information about the charging process at the electrolyte/interface in dye-sensitized solar cells. Higher frequencies (greater than 10 Hz) offer insights into the counter electrodes, while lower frequencies reveal details about the FTO/Nb₂O₅/electrolyte interfaces (below 10 Hz) ³⁶. When the arcs are well separated in the impedance spectra, it is a significant characteristic of highly efficient systems, as it indicates distinct and well-defined charge transfer processes occurring at each interface. In the cells analyzed, it was observed that Nb₂O₅ effectively separated electron collection both at the working electrode and the counter electrode, as indicated by the presence of two distinct time constants in the system. This behavior was not observed in PANI/Nb0, suggesting that bare polyaniline alone is not a suitable material for counter electrode production in DSSCs, as only a single time constant was obtained ^29)-(36.

For the PANI/Nb1 and PANI/Nb2 samples, a clear response at high frequencies (~166 Hz) is observed, corresponding to the charge transfer processes occurring at the counter electrode. Additionally, a second time constant appears at lower frequencies, 0.48 Hz for PANI/ Nb1 and 0.66 Hz for PANI/Nb2, characterizing a clear separation of time scales between the counter electrode and the working electrode. In contrast, the sample without niobium (Nb0) does not exhibit a well-defined feature in the high-frequency region, indicating a less efficient charge transfer at the counter electrode. This suggests that the incorporation of niobium into polyaniline enhances the electronic conductivity and facilitates charge transfer processes at the counter electrode interface.

Using the low frequency region, it is possible to calculate the electron lifetime to dye solar cells, using Equation B(f) representing the frequency at low region). In addition, the parameter is related to the recombination reaction, and superior values are required for efficient solar devices ^29)-(36.

${ԏ}_n=\frac{1}{2\pi f}$ (B)

To the PANI/Nb0 cell, a ԏ_n = 0.014 s was obtained, and with PANI insertion (PANI/Nb1), the time value was shifted to ԏ_n = 0.332 s, indicating that improvements in the counter electrode enhance electron flow and charge transfer at the working electrode interface, thereby increasing the electron lifetime and overall device performance. This demonstrates the effectiveness of polyaniline modifications in producing a low-cost and efficient device.

CONCLUSION

This study demonstrates a promising approach for developing low-cost dye-sensitized solar cells, using Nb₂O₅ as a photonoanode and PANI/Nb₂O₅ counter electrodes, replacing the traditional titanium and platinum materials applied in such devices.

Characterization results indicated that Nb₂O₅ exhibited a 3.0 eV energy gap, irregular morphology, and an orthorhombic phase when produced by Pechini route at 700 °C. When applied in a DSSC, the counter electrode containing 1% Nb in PANI achieved an open-circuit potential of 0.69 V with a current density of 0.075 mA cm^-2. The open-circuit potential was comparable to that of a standard TiO₂-based dye-sensitized solar cell, suggesting a notable enhancement in performance relative to other configurations.

Overall, this approach indicates that the combination of PANI with Nb₂O₅ indicates a cost-effective solution for efficient solar energy conversion, especially in countries as Brazil, which have significant niobium resources.

DATA AVAILABILITY

Research data are only available upon request.

ACKNOWLEDGEMENTS

The authors thank the “Laboratório Multiusuário da UTFPR Campus Londrina (LabMulti)” for the analyses. The authors also thank the “Laboratório Multiusuário da UTFPR Campus Campo Mourão” for the FTIR analysis.

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Edited by

• (AE: Rosa I. M. Rubi)

Publication Dates

• Publication in this collection
  03 Nov 2025
• Date of issue
  2025

History

• Received
  03 Dec 2024
• Reviewed
  09 May 2025
• Reviewed
  09 July 2025
• Accepted
  16 July 2025