Study of the Nb₂O₅ Insertion in ZnO to Dye-sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSC) have received much attention as an alternative to silicon-based solar cells, due to various advantages. Zinc oxide (ZnO) is an n-type semiconductor employed as photoanode on DSSC. The decrease of charge recombination is an efficient strategy capable of improving the photovoltaic performance of the device. In this perspective, Nb₂O₅ was added on ZnO solar cells. The oxides were characterized by the X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and X-ray fluorescence (XRF). The photovoltaic parameters were obtained through J-V plots and photocronoamperometry. The results showed that the niobium oxide obtained presented orthorhombic crystal structure and DSSC with the addition of niobium oxide showed better efficiency, of 1.42% when compared the device with only ZnO.

Keywords:
DSSC; charge recombination; photovoltaic device

1. Introduction

Dye-sensitized solar cells (DSSC) or Grätzel cells have attracted increasing attention due to mainly low cost, relatively high efficiency, easy fabrication, and flexibility. DSSC is dispositive of solar energy conversion into electrical energy, which involves a semiconductor oxide/dye interface^1,2,3.

Recently, ZnO, with similar band gap (3.37 eV) to the TiO₂ (3.2 eV), appears as an alternative material for the fabrication of high-efficiency DSSC^4,5,6. The ZnO present higher electron mobility and electronic conductivity when compared to TiO₂, such faster electron transport should in principle favor efficient collection of injected electrons at the device electrode, thereby minimizing interfacial charge recombination losses to either dye cations or oxidized redox species in the electrolyte and improving device performance^7,8.

In an attempt to improve electric conduction and minimize the dark current, several materials have been explored for this purpose, such as Al₂O₃, Nb₂O₅, ZrO₂, SnO_2, In₂O₃ and CeO₂^9,10. In photovoltaic devices the Nb₂O₅ forms an energy barrier due to its energy gap being larger when compared to other semiconductors, causing the increase of the open circuit potential, thus suggesting the reduction in the speed of the reactions of charge recombination^11,12,13. For that reason Nb₂O₅ layers have also been used as blocking layers of other oxides, preventing electron back-transfer¹⁴. Therefore, the present work aims to study ZnO/Nb₂O₅ the mixture in dye-sensitized solar cells.

2. Material and Methods

2.1 Obtaining and characterization of niobium pentoxide (Nb₂O₅)

The Nb₂O₅ were prepared using the Pechini method adapted from previous procedures reported in the literature¹⁵. Started with ethylene glycol under constant stirring under 70 °C and was added the citric acid until complete dissolution. The ammoniacal complex of niobium (NH₄H₂[NbO(C₂O₄)₃].nH₂O) was added slowly and the solution remained under constant stirring for 30 min. After that sample, it was calcined by 4 h at 350 °C and macerated and then was calcined by 700 °C by 4 h with a heating rate of 2 °C/min. The characterization was made in a Fourier transform double-beam infrared spectrophotometer, Agilent Technologies, Cary 600 Series FTIR spectral band spectral range from 2500 cm^-1 to 400 cm^-1, with a 10% KBr insert. For the structural identification of the oxides, the X-ray analysis was performed in the range of 10-75°, on the Bruker D2 Phaser DRX apparatus, with CuKα radiation of 1.54 Å at 30 kV, 10 mA, scanning speed of 0.07° s^-1 and LynxEye detector.

2.2 Preparation of the zinc oxide (ZnO) films with niobium pentoxide (Nb₂O₅)

The films were prepared by adding the Nb₂O₅ to commercial ZnO (SIGMA-ALDRICH, 99+%) in the proportion of 5% m/m. The percentage was confirmed with X-ray fluorescence (XRF), on the X-MET7500 series handheld XRF analyzer in 40 kV and 10 mA. For the preparation of films was used 3.0 g of the oxides mixture (2.85 g ZnO and 0.15 g Nb₂O₅), 0.1 mL of polyethylene glycol, 0.1 mL of Triton X-100, 0.1 mL acetylacetone 99.5% and 4 mL of deionized H₂O. The deposition was performed by the doctor blade method adapted to FTO conductive glass. After deposition the film was brought to the temperature of 450 ° C so that all the organic material was removed and only the oxide (ZnO/Nb₂O₅) deposited in the FTO.

2.3 Composition of the dye-sensitized solar cell (DSSC)

The cell scheme is shown in Figure 1.

Figure 1
Schematic of a DSSC with dye N719.

Sensitization of the sample was made with immersion in dye di-tetrabutylammonium cis-bis(isothiocyanate)bis (2,2' - bipyridyl - 4,4'- dicarboxylate) ruthenium(II) (N-719, Sigma Aldrich) at 24 h. Platinum was used as the counter electrode, platinum was electrodeposited using K₂PtCl₆ and cyclic voltammetry about conductive glass substrate on fluorine-doped tin oxide (FTO; ca.7 Ω sq^-1) FTO and the electrolyte was an iodide/triiodide solution. The cells were produced in a sandwich with an area of 0.2 cm², the anode being the FTO coated with oxide films/N-719 and the cathode an FTO deposited with platinum.

3. Results and Discussion

3.1 Characterization of niobium pentoxide

The DRX confirm the production of niobium pentoxide by Pechini method with high crystallinity and free of impurities and the orthorhombic crystalline structure to Nb₂O₅, according to PDF 96-210-6535 of ICDD^16,17,18. In general, the crystallization conditions of each Nb₂O₅ structure depends on the starting materials, synthesis methods and heat treatment conditions. The T-Nb₂O₅ (orthorhombic structure) was obtained heating the oxide to 700°C. In particular, the T-Nb₂O₅ net parameters are: a = 6.17Å; b = 29.32Å; c = 3.94Å, and its crystalline structures consist of 4 × 4 blocks of corner-shared NbO₆ octahedra, with connected blocks sharing the edges of the octahedron^15,16.

Figure 2 presents the FTIR spectrum of ZnO and a mix of ZnO/Nb₂O₅.

Figure 2
FTIR spectrum for the ZnO and the mix of ZnO/Nb₂O₅.

Through analysis of FTIR shown in Figure 2, a band is observed in the region of 832-997 cm^-1 that is attributed stretching of the Zn-O band was observed that when Nb₂O₅ is added that band is shifted and intensified. A similar behavior for both diagrams is observed since bands respectively appearing in the 677-906 cm^-1 region can be attributed to stretching of Zn-O and Nb-O bonds. Already a band in 1629-1230 and 1378-1507 cm^-1 for a mixture of oxide and only ZnO can be attributed assigned to the asymmetric stretching modes of oxygens present in sample^18,19,20,21.

Figure 3 shows the X-ray fluorescence (XRF) spectrum.

Figure 3
XRF spectra for mixture ZnO/Nb₂O₅.

The spectrum (Figure 3) presents the characteristic peaks of Zn(K_α) in 8.6 keV, Zn(K_β) in 9.6 keV and Nb(K_α) and Nb(K_β) in 16.5 keV and 18.6 keV respectively and was possible to confirm the percentage of Nb₂O₅, 4.70 ± 0.03 %, on insertion in ZnO, 95.25 ± 0.12 %, disregarding the presence of oxygen (the equipment does not read the oxygen element).

3.2 Photochronoamperometry (PCA) measurement

The photocronoamperometry technique was used to evaluate the current density in samples with only ZnO and addition of Nb₂O₅ and also provides information on the stability of the solar cell. In the PCA investigation, light is irradiated on the device for a given period and is then interrupted, followed by monitoring of the short-circuit current⁸.

Figure 4 shows the PCA curves of solar cell compounds ZnO and mix ZnO/ Nb₂O₅.

Figure 4
Photochronoamperometry for solar cells under 100 mW cm² illumination with the light on and off in the 60 s range.

The current density curves (Figure 4) showed decay during illumination, indicating the device is diffusion-limited. As the electrolyte, which regenerates the dye, stay in the reduced state the oxidized dye molecules have to wait for more redox pairs to diffuse into the photo-anode^21,22. Over time, an increasing number of dye molecules are left in the oxidized state and, therefore, a decrease in the photocurrent is observed. The cells studied presented similar behaviors, in which the film with the only ZnO presented a lower current density, 2.701 mA cm^-2, in relation to the film with the mixture of the oxides. Where the ZnO/Nb₂O₅ film obtained the current density at the beginning (60 s) of 4.56 mA.cm^-2 after 600 s it presented a current density of 3.02 mA.cm^-2, already the film with only ZnO J= 1.46 mA.cm^-2.

3.3 Characterization J_sc vs V_oc curves of solar cells

Figure 5 is shown the curves JxV of the ZnO and mix ZnO/Nb₂O₅.

Figure 5
Curves J-V obtained under lighting of 100 mW cm^-2 in the solar cells based on ZnO/Nb₂O₅ films and only ZnO.

The current density as a function of potential (J-V) and gives the photoelectrochemical parameters for the calculation of the efficiency of the system, using Equation 1.

Equation 1 $\mathit{\eta }=\left(\frac{{\mathit{J}}_{\mathit{sc}}\times {\mathit{V}}_{\mathit{oc}}\times \mathit{FF}}{{\mathit{I}}_{\mathit{o}}}\right)\times 100\%$

Where J_sc is equivalent the short-circuit density, FF, the fill factor, V_oc is open-circuit potential and I_o is the flux of photons irradiated, I_o = 100 mW cm^-2. The values of J_sc and V_oc are obtained by considering the points where the voltage and current are respectively zero in this curve. The current density (J_sc) is related to the absorption of the dye in a semiconductor oxide; already open circuit potential is related to the processes of charge recombination¹⁸.

Table 1 presents the parameters of the solar cells drawn from the J-V curves (Figure 5) and from Equation 1.

Table 1
Photovoltaic parameters of dye-sensitized solar cells N719.

DSSC	Jsc/mA.cm-2	Voc/V	FF	η/%
ZnO	2.701	0.569	0.369	0.57
ZnO/Nb2O5	3.995	0.704	0.504	1.42

In Figure 5 was observed that the cell with the addition of Nb₂O₅ presented the highest values of J_sc, this being 3.995 cm^-2. The current produced by the cell is associated with the electron injection mechanism and the charge transport of the carriers^1,18. The addition of Nb₂O₅ increased the efficiency values, being the cell containing Nb₂O₅, presenting efficiency of e de η = 1.42%, while the cell constituted only by ZnO showed the efficiency of 0.57%. The increase in efficiency values indicates that the addition of niobium changes the electrical properties of zinc oxide^22-24.

Table 1 shows that the cell with addition of Nb₂O₅ presents an increase in the value of V_oc in relation to the cell using the only ZnO, indicating that when adding Nb₂O₅ occurs an increase of the lifetime of the electron as consequence of the decrease of the recombination processes²⁴. The lowest value of circuit potential in ZnO cell is attributed the oxygen vacancies where the excited electrons that were released from the dye to the ZnO will be deactivated to fill those energy levels that are located between the conduction band of zinc oxide, reducing the photocurrent and the efficiency of the solar cell. When added to the Nb₂O₅ these vacancies are decreased by increasing the in the value of Voc increasing consequently the lifetime of the electron^2-23.

4. Conclusion

The analysis of X-ray diffraction, X-ray fluorescence and infrared spectroscopy incated that the particles of niobium oxide was formed and are present in the mixture with ZnO.

The addition of Nb₂O₅ in ZnO on DSSC provided an effective improvement in the photovoltaic parameters. PCA showed that the current density value was twice as highest (ZnO/Nb₂O₅, J=3.02 mA.cm^-2) as the ZnO cell (J=1.46 mA.cm^-2). The efficiency value of the ZnO/Nb₂O₅ cell presented an increase approximately of 200% to the cell ZnO. Others parameters as J_sc, V_oc and FF presented in the J-V curve indicate that the presence of niobium oxide minimized the charge recombination processes and that the electrical properties of the zinc oxide were also altered.

5. Acknowledgments

This study was financed in part by the Coordenação de Aperfeiçoamento de Nível Superior - Brasil (CAPES) - Finance Code 001.

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Publication Dates

• Publication in this collection
  01 July 2019
• Date of issue
  2019

History

• Received
  08 Dec 2018
• Reviewed
  18 Apr 2019
• Accepted
  19 May 2019