LESIMS Laboratory, Department of physics, Badji Mokhtar’s, University, Annaba, Algeria.
Received: 11/08/2013 Accepted: 02/10/2013
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The electrical transport in nanotube is extremely sensitive to local electrostatic environment due to their small size, large surface to volume ratio and high mobility. Among them, Oxide Zinc and Titanium are friendly for environment and promised materials. Dye sensitized solar cell (DSSC) is the only solar cell that can offer both the flexibility and transparency but its conversion efficiency is affected by physical and morphological parameters, like thickness, series resistance (Rs), ideality factor (n), saturation current (Is), shunt resistance (Rsh) and photocurrent (Iph) during elaboration as well as their normal use. This paper presents a simulation of photovoltaic characteristics of ZnO nanotube dye-sensitised solar cells and TiO2 nanostructure, by extracting the solar cell parameters which influence directly on solar cell output: conversion efficiency, fill factor, the short circuit photocurrent densities Isc and open-circuit voltage Voc. Furthermore, we review the relationship between geometry and output parameters
Nanotube, Titanium,oxide Zinc, simulation, photovoltaic, physical parametres
In recent decades, dye-sensitized solar cells (DSSCs) have attracted much attention as next-generation solar cells due to their remarkably high conversion efficiency combined with their ease and low cost of manufacturing. It is based on solar light harvesting of a sensitizing dye (SD) attached to a wide band gap semiconductor. The process of conversion of solar energy to electrical energy in a DSSC involves SD adsorbed on the surface of wide band gap n-type metal oxide semiconductor nanoparticles  (typically TiO2, ZnO, SnO2, Nb2O5, etc).
The oxide material of choice for many of these systems has been TiO2. Its properties are intimately linked to the material content, chemical composition, structure and surface morphology. From the point of the material content and morphology, two crystalline forms of TiO2 are important, anatase and rutile (the third form, brookite, is difficult to obtain). Anatase is the low temperature stable form and gives mesoscopic films that are transparent and colorless [2,3,4,5].
The application of ZnO in excitonic solar cells, XSCs, (organic, dye sensitized and hybrid) has been rising over the last few years due to its similarities with the most studied semiconductor oxide, TiO2. ZnO presents comparable band gap values and conduction band position as well as higher electron mobility than TiO2. It can be synthesized in a wide variety of nanoforms applying straight forward and scalable synthesis methodologies [6-10]. Particularly, the application of vertically-aligned ZnO nanostructures it is thought to improve contact between the donor and acceptor material in organic solar cells (OSCs), or improve electron injection in dye sensitized solar cells (DSSCs).
Since the invention of dye sensitized solar cells (DSSC) , some significant progresses have enabled to reach an efficiency higher than 11% for this type of solar cells . Until now, the major part of the photoanode DSSC research has mainly focused on TiO2 nanostructures. However, ZnO semiconductor can be a good alternative as it can exhibit several advantages in comparison with TiO2 semiconductor, such as a direct band gap (3.37 eV), higher exciton binding energy (60 meV) compared to TiO2 (4 meV), and higher electron mobility (200 cm2 V-1 s-1) over TiO2 (30 cm2V-1 s-1) for similar band gap energy levels [13,14]. Furthermore, several types of ZnO nanostructures with different geometries can easily be grown, such as nanoparticles, nanowires, nanorods, or nanobelts [15-18]. Nevertheless, the solar energy conversion efficiencies are twice lower for ZnO based solar cells than for TiO2 based solar cells for some reasons which are still unidentified.
These work, present a comparative study between ZnO nanotube and TiO2 nanostructure dye-sensitized solar cell (DSSC) by extracting the solar cell parameters which influence directly on solar cell output: conversion efficiency, fill factor, the short circuit photocurrent densities Isc and open-circuit voltage Voc. Furthermore, the geometry parameters of the basic material play an important role in physical parameters and output parameters, so, we review the relationship between output parameters and geometry parameter.
Solar Cell Model
The model of the solar cells for which the superposition principle is applicable can be represented by expressed as Eq.(1) which includes light-generating current source (Iph), series resistance (Rs) and shunt resistance (Rsh).
Iph, Is, n, Rs and Gsh (=1/Rsh) being the photocurrent, the diode saturation current, the diode quality factor, the series resistance and the shunt conductance, respectively. Ip is the shunt current and β=q/kT is the usual inverse thermal voltage.
Measurement data provided by the manufacturer include the I-V curve, open circuit voltage (Voc), shunt circuit current (Isc), and voltage (Vmpp), current (Impp), and power (Pmax) at the maximum power point (MPP), measured at given Temperature under a standard AM 1.5 solar spectrum and irradiance of 100 mW/cm2. To establish the solar cell model in Eq. (1), five parameters of the solar cell equation: Rs, Rsh, n, Is and Iph must be extracted using data provided by the manufacturer.
• For most practical illuminated solar cells we usually consider that Is<<Iph, the photocurrent can be given by the approximation Isc ≈ Iph, where Isc is the short-circuit current. This approximation is highly acceptable and it introduces no significant errors in subsequent calculations  .
• The shunt conductance Gsh is evaluated from the reverse or direct bias characteristics by a simple linear fit . The calculated value of Gsh gives the shunt current Ip = GshV.
•The determination of ideality factor and the series resistance, Our measured I-V characteristics are corrected considering the value of the shunt conductance as obtained and for V+RsI>>kT, the current voltage relation becomes:
By writing relation (2) in its logarithmic form .
And (V0, I0) is a point of the I-V curve.
The linear regression of equation (3) gives n and Rs, but when Rs is low or when multiple conduction processes intervene, the determination of Rs become difficult, so, to get a better accuracy we consider a set of Ii-Vi data giving rise to a set of X-Y values, with i varying from 1 to N. Then, we calculate X and Y values for I0 = Ii0 and I=Ii0+1 up to I=IN. This gives (N-1) pairs of X-Y data. We start again with I0 = Ii0+1 and I = Ii0+2 up to IN and get (N-2) additional X-Y data, and so on, up to I0 = IN-1. Finally, we obtain N(N-1)/2 pairs of X-Y data that means more values for the linear regression.
The most interesting point with this method is the fact that we do not have any limitation condition on the voltage hop and it is very easy to use.
At last, The saturation current Is was evaluated using a standard method based on the I-V data by plotting ln(Iph–Icr) versus Vcr equation (4). Note that Icr-Vcr data were the corrected current voltage I-V data taking into account the effect of the series resistance where I-V are the measured current-voltage data.
When we plot ln (Ic) where (Ic=Iph-Icr) versus Vcr, it gives a straight line that yields Is from the intercept with the yaxis.
The current-voltage (I-V) characteristics of TiO2 nanostructures DSSC is taken from the work of Jingbin et al. (2010) and The current-voltage (I-V) characteristics of ZnO nanotube is taken from the work of Fang Shoa et al. . The two characteristics correspond to the higher photovoltaic performance, where η=6.00%, FF=58.33%, Isc=15.25mAcm-2 and Voc=0.67V for TiO2 nanostructures, and for ZnO nanotube η=1.18%, FF=0.58%, Isc=3.24mAcm-2 and Voc=0.68V.
The extracted parameters obtained using the method proposed here for the Dey-Sensitized solar cell based on TiO2 nanostructures and ZnO nanotube are given in Table 1, and to test the quality of the fit to the experimental data, the percentage error is calculated as follows:
Where Ii,cal is the current calculated for each Vi, by solving the implicit Eq.(1) with the determined set of parameters ( Iph, n, Rs, Gsh, Is). (Ii, Vi) are respectively the measured current and voltage at the ith point among N considered measured data points.
Statistical analysis of the results has also been performed. The root mean square error (RMSE), the mean bias error (MBE) and the mean absolute error (MAE) are the fundamental measures of accuracy. Thus, RMSE, MBE and MAE are given by:
N is the number of measurements data taken into account.
Satisfactory agreement is obtained for most of the extracted parameters. Statistical indicators of accuracy for the method of this work are shown in Table 1.
Figures 1 and 2 show the plot of I-V experimental characteristics and the fitted curves derived from equation (1) with the parameters shown in Table 1 for Dey-Sensitized solar cell based on TiO2 nanostructures and ZnO nanotube.
Extracting solar cells parameters is a vital importance for the quality control and evaluation of the performance of the solar cells, this parameters are: series resistance, shunt conductance, saturation current, the diode quality factor and the photocurrent. A simple method for extracting the solar cell parameters, based on the measured current-voltage data. The method has been successfully applied to dye-Sensitized solar cell based on TiO2 nanostructures and ZnO nanotube under different temperatures.
Good agreement is observed for the different DSSCs, especially for the TiO2 nanostructures solar cells with statistical error less than 1%, and 2% for ZnO nanotube DSSC solar cells respectively, which attribute mainly to lower parasitic losses, where we can observe a low series resistance 0.025923Ω compared to 0.383441 Ω for TiO2 nanostructures and ZnO nanotube solar cell respectively.
The interesting point with the procedure described herein is relation between the results, where the five solar cells parameters extractions explain the difference in conversion efficient of the different DSSCs.
Figs. 3-4 show the effect of the geometry parameter (thickness) on output parameters: FF, Isc, Voc, η of dyesensitized solar cells based on ZnO nanotube with different lengths. The shortcircuit photocurrent densities (Isc) obtained with nanotubes of 0.7, 1.5, 2.9 and 5.1 μm lengths were 0.68, 1.51, 2.50 and 3.24 mA cm−2, respectively. The highest photovoltaic performance of 1.18% (open-circuit voltage Voc = 0.68 V and fill factor FF = 0.58) was achieved for the sample of 5.1 μm length. This efficiency is attractive, taking into account that the film thickness is only 5.1 μm and no scattering layer is added. The Voc of the DSSC decreases upon increasing the length of the ZnO nanotubes, which is possibly related to the increase in the dark current which scales with the surface area of the ZnO film as shown by Jingbin et al.
Figs. 5-6 show the effect of the geometry parameter (thickness) on output parameters: FF, Isc, Voc, η of dyesensitized solar cells based on TiO2 nanostructure grown in NaOH solution with different concentrations (0.5, 1, 3, and 10 M) and with different lengths. The shortcircuit photocurrent densities (Isc) obtained with nanostructures of 11.68, 15.11, 24.10 and a (too thick to be measured) μm lengths were 10.26, 15.25, 10.2 and 3.71 mA cm−2, respectively.
The highest photovoltaic performance of 6.00% (open-circuit voltage Voc = 0.67 V and fill factor FF = 58.33) was achieved for the sample of 15.11 μm length..
The two DSSCS ZnO nanotube and TiO2 nanostructure used (N719) dye sensitizer, The photon-current conversion efficiencies of DSSC using 0.7, 1.5, 2.9 and 5.1 μm length ZnO nanotubes were 0.32%, 0.62%, 0.83% and 1.18%, which were much lower than those of TiO2 nanostructure DSSCs (i.e. 4.40%, 6.00%, 4.84% and 1.44%). So, the difference between the photovoltaic performance for these two type DSSCs is observed, where a high photovoltaic performance is given by TiO2 Nanostructured with different thickness and over the range of NaOH concentrations, conversion efficient increased from 4.40% at 0.5M to a maximum value of 6.00% at 1 M, which correspond to a high short-circuit photocurrent densities 15.25 mA cm-2. Compared with ZnO nanotube DSSC, where the higher conversion efficient is 1.18% correspond to high short-circuit photocurrent densities 3.24 mA cm-2. The cause of this difference is the based materials properties (ZnO and TiO2), the method of fabrication and the different condition of measured I-V characteristics of temperature and illumination. Where, ZnO is not an easy material, it’s properties of which are greatly influenced by external conditions like synthesis methods, temperature, testing atmosphere air, vacuum or illumination, for example, minimal changes in the shape of the ZnO (nanoparticules, nanorods, nanotips, etc), can produce different properties which in turn, affects the interface with any organic semiconductor or dye molecule.in order, Titanium dioxide is a fascinating material, with a very broad range of different possible properties, which leads to its use in application as different as toothpaste additive.
In this contribution, a simple comparative study between experimental and simulation works to improve the deysensitized solar cell performance of two DSSCs based on TiO2 nanostrucures and ZnO nanotube, under differents condition of temperature. An evaluation of the physical parameters of solar cell: series resistance (Rs), ideality factor (n), saturation current (Is), shunt resistance (Rsh) and photocurrent (Iph) from measured current-voltage characteristics by using a numerical method proposed by the authors is presented by comparing the obtained results of tow DSSCs, than we present the effect of geometric parameters on the output parameters which are: the conversion efficient, the fill factor, the short-circuit photocurrent and the open-circuit voltage, where we observe a high photovoltaic performance for TiO2 nanostrucures with maximun conversion efficient 6% compared to 1.18% for ZnO nanotube. Good resuts are given by the differents DSSCs, and specialy for on dey-sensitized TiO2 nanostrucures, which justify the experimental work.
We would like to thank all the members of LESIMS laboratory, specially our supervisor Pr Djamel Eddine MEKKI for his advices and help and my wife Nadia Nehaoua for her comprehension and support.