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

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.


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 TiO 2 (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 TiO 2 , 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 Al 2 O 3 , Nb 2 O 5 , ZrO 2 , SnO 2, In 2 O 3 and CeO 2 9,10 . In photovoltaic devices the Nb 2 O 5 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 Nb 2 O 5 layers have also been used as blocking layers of other oxides, preventing electron back-transfer 14 . Therefore, the present work aims to study ZnO/Nb 2 O 5 the mixture in dye-sensitized solar cells.

Obtaining and characterization of niobium pentoxide (Nb 2 O 5 )
The Nb 2 O 5 were prepared using the Pechini method adapted from previous procedures reported in the literature 15 3 ].nH 2 O) 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.

Preparation of the zinc oxide (ZnO) films with niobium pentoxide (Nb 2 O 5 )
The films were prepared by adding the Nb 2 O 5 to commercial ZnO (SIGMA-ALDRICH, 99+%) in the proportion of 5% m/m. The percentage was confirmed with X-ray

Composition of the dye-sensitized solar cell (DSSC)
The cell scheme is shown in Figure 1.
depends on the starting materials, synthesis methods and heat treatment conditions. The T-Nb 2 O 5 (orthorhombic structure) was obtained heating the oxide to 700°C. In particular, the T-Nb 2 O 5 net parameters are: a = 6.17Å; b = 29.32Å; c = 3.94Å, and its crystalline structures consist of 4 × 4 blocks of corner-shared NbO 6 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/Nb 2 O 5 . 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 K 2 PtCl 6 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 cm 2 , the anode being the FTO coated with oxide films/N-719 and the cathode an FTO deposited with platinum.

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 Nb 2 O 5 , according to PDF 96-210-6535 of ICDD 16,17,18 . In general, the crystallization conditions of each Nb 2 O 5 structure 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 Nb 2 O 5 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. 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 Nb 2 O 5 , 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).

Photochronoamperometry (PCA) measurement
The photocronoamperometry technique was used to evaluate the current density in samples with only ZnO and addition of Nb 2 O 5 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 current 8 .    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/Nb 2 O 5 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 . 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.

Characterization J sc vs V oc curves of solar cells
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 recombination 18 . Table 1 presents the parameters of the solar cells drawn from the J-V curves ( Figure 5) and from Equation 1. In Figure 5 was observed that the cell with the addition of Nb 2 O 5 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 Nb 2 O 5 increased the efficiency values, being the cell containing Nb 2 O 5 , 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][23][24] . Table 1 shows that the cell with addition of Nb 2 O 5 presents an increase in the value of V oc in relation to the cell using the only ZnO, indicating that when adding Nb 2 O 5 occurs an increase of the lifetime of the electron as consequence of the decrease of the recombination processes 24 . 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 Nb 2 O 5 these vacancies are decreased by increasing the in the value of Voc increasing consequently the lifetime of the electron 2-23 .

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 Nb 2 O 5 in ZnO on DSSC provided an effective improvement in the photovoltaic parameters. PCA showed that the current density value was twice as highest (ZnO/Nb 2 O 5 , J=3.02 mA.cm -2 ) as the ZnO cell (J=1.46 mA.cm -2 ). The efficiency value of the ZnO/Nb 2 O 5 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.

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