Photovoltaic Characteristics of Solar Cells Based on Nanostructured ...

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We have designed solar cells based on nanostructured titanium dioxide sensitized by fluorescein sodium salt, and we have studied their photovoltaic and ...
Theoretical and Experimental Chemistry, Vol. 50, No. 2, May, 2014 (Russian Original Vol. 50, No. 2, March-April, 2014)

PHOTOVOLTAIC CHARACTERISTICS OF SOLAR CELLS BASED ON NANOSTRUCTURED TITANIUM DIOXIDE SENSITIZED WITH FLUORESCEIN SODIUM SALT W. A. Farooq,1 Amanullah Fatehmulla,1 F. Yakuphanoglu,2 I. S. Yahia,3,4 Syed Mansoor Ali,1 M. Atif,1 M. Aslam,1 and Walid Tawfik1,5

UDC 541.145

We have designed solar cells based on nanostructured titanium dioxide sensitized by fluorescein sodium salt, and we have studied their photovoltaic and impedance characteristics. Under AM1.5 illumination and optimal conditions, an open-circuit photovoltage of 0.56 V and a short-circuit current density of 72 µA/cm2 were achieved. An impedance spectroscopy study of the voltage dependence of the capacitance in the frequency range 2-5 kHz did not reveal any shift from positive to negative capacitance.

Key words: fluorescein sodium salt, solar cell, photovoltaic, nanostructure.

Solar energy has long been used by humans for many purposes, such as making fires and heating indoor spaces, but its use has reached a new level because of the development of photovoltaic cells in which dyes are used to sensitize inorganic materials (generally wide band gap semiconductors) [1, 2]. Photovoltaic research was stimulated by the discovery of the photoelectric effect, and led to creation of solar cells. The first cells were made in 1941, and since 1950 silicon-based cells have been widely used for various purposes [3, 4]. Depletion of natural fuel resources and the high burden imposed on the environment by existing technologies led to increased interest in solar energy. The high cost of silicon cells, preventing their wide use, has stimulated a search for new types of solar cells. Solar cells sensitized by dyes (DSSCs) or by quantum dots are considered today as an alternative to existing conventional silicon cells due to the low toxicity involved in their fabrication, their relatively low cost, and their lighter weight. Michael Gratzel and B. O’Regan [5-7] fabricated the first DSSCs of practical importance. A DSSC consists of a mesoporous semiconductor film, a sensitizer, and an electrolyte containing a redox couple (redox electrolyte). Widely used mesoporous semiconductors are TiO2, ZnO, Fe2O3, and SnO2, where nanostructured titanium dioxide, capable of good adsorption of a large number of sensitizer dye molecules, is the most widely used [8, 9]. The efficiency of the early cells was low, but over the past twenty years their performance has been considerably improved mainly by the choice of oxide materials and going to ________ 1 Department of Physics and Astronomy, College of Science, King Saud University, Riyadh 11451, Saudi Arabia. E-mail: [email protected]. 2 Department of Physics, Faculty of Science, Fýrat University, Elazýð, Turkey. 3 Nano-Science & Semiconductor Labs., Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt. 4 Department of Physics, Faculty of Science, King Khalid University, P. O. Box 9004, Abha, Saudi Arabia. 5 Department of Environmental Applications, National Institute of Laser NILES, Cairo University, Cairo, Egypt. ___________________________________________________________________________________________________ Translated from Teoreticheskaya i Éksperimental’naya Khimiya, Vol. 50, No. 2, pp. 119-124, March-April, 2014. Original article submitted December 7, 2013; revision submitted April 2, 2014. 0040-5760/14/5002-0121 ©2014 Springer Science+Business Media New York

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nanostructured materials [10-12]. Tricoli et al. [11], studying cells based on highly porous TiO2 films, showed that porosity is the main factor determining the efficiency of such cells. Lingyun Zhang and colleagues from Harbin Institute of Technology [13] built efficient DSSCs based on zinc oxide sensitized by a ruthenium complex (N719). In 2012, a considerable amount of research was devoted to development of this type of DSSC [14-16]. Natural pigments and synthetic dyes, absorbing in the red and infrared regions of the spectrum, can be efficient DSSC sensitizers due to their donor–acceptor structure. Such natural pigments as carotene, chlorophyll, and anthocyanin, are easily obtained from flowers, leaves of plants, and fruits. In DSSCs based on TiO2 sensitized by natural dyes, a photoconversion efficiency of 7.1% has been achieved [17]. In this research, we studied DSSCs based on nanostructured TiO2 sensitized by fluorescein sodium salt (Na-FL). The photovoltaic properties of the cell and also the voltage dependences of the capacitance, conductance, and resistance were studied over a broad frequency range.

EXPERIMENTAL In order to fabricate the electrodes, we used two equal sized plates of FTO conductive glass. The nanosized TiO2 powder (with particle size less than 100 nm) was made by Sigma. In order to prepare the active layer, we used a paste consisting of nanocrystalline TiO2, ethylene glycol, and ethanol, obtained by stirring the components at 50 °C. On the FTO surface, we deposited a layer of the TiO2-containing paste 3-5 µm thick, and annealed at 425 °C in air for 30 min. The films obtained were held in a Na-FL solution for 24 h, and then the films were washed with ethanol and dried in air. The counterelectrode was obtained by sputtering a platinum layer on the surface of an FTO plate at room temperature (SC-7620 sputter coater, Quorum Technologies, UK). A drop of electrolyte was placed between the TiO2-based photoanode and the counterelectrode, and then the electrodes were tightly clamped together. The surface morphology of the TiO2/FTO films was studied by atomic force microscopy (AFM) on a Park Systems XE100 microscope in non-contact mode. The current–voltage and capacitance–voltage characteristics were measured using a Keithley SC-4200 Semiconductor Characterization System. The films were illuminated by a Class BBA Solar Simulator; the light intensity was measured with a TM-206 solar power meter.

RESULTS AND DISCUSSION Structural Characteristics of TiO2/FTO Films. Atomic force microscopy is a powerful tool for studying surface morphology and roughness of TiO2/FTO films. Figure 1 represents the AFM image of a 4´4 µm2 section of the surface of a TiO2/FTO film. The mean titanium dioxide cluster size and the mean roughness of the film, determined from Fig. 1 using Park Systems XE1 image processing software, were ~152 nm and ~117 nm respectively. Photovoltaic Characteristics of the Cell. Figure 2a shows the current–voltage characteristics of the studied cell, obtained for different illumination intensities. As follows from the data presented, absorption of visible light by the cell leads to generation of charge carriers. The photocurrent and the photovoltage increase in proportion to the illumination intensity. Under AM1.5 illumination, corresponding to the average solar radiation density on the Earth’s surface, a cell with active surface area 0.26 cm2 achieves a short circuit photocurrent of 72 µA/cm2 and an open-circuit photovoltage of 0.56 V. Figure 2b shows the output power generated by the cell vs. the applied bias voltage. The maximum output power Pmax of the cell can be calculated using the following expression [18]: Pmax = IMVM,

(1)

where IM and VM are respectively the maximum photocurrent and the maximum photovoltage respectively. As follows from Fig. 2b, increasing the bias voltage V applied to the cell leads to an increase in the output power up to a certain value of the voltage, and then any further increase in V is accompanied by a decrease in the output power down to practically zero. The voltage corresponding to the maximum output power is shifted with increasing illumination intensity 122

Fig. 1. AFM 3D micrograph of a portion of the surface of a TiO2/FTO film.

Fig. 2. Current–voltage characteristics of the studied DSSC (a) and cell output power vs. bias voltage curves for different illumination intensities (b): 1) 20, 2) 40, 3) 60, 4) 80, 5) 100 mW/cm2.

toward higher values of V, and is 0.2 V (Pmax = 0.1·10–6 W) at 2 mW/cm2 and 0.35 V (12.5·10–6 W) at 100 mW/cm2. Figure 3a shows the variation in open circuit photovoltage Voc as the illumination increases. As follows from the data presented, there is no change in Voc in the illumination range 0-10 mW/cm2. After some threshold value is passed, we observe an increase in Voc practically in direct proportion to the illumination intensity [19]. For illumination above 55 mW/cm2, Voc asymptotically approaches a constant value. We know that when light is incident on a DSSC, current is generated due to injection of electrons from the photoexcited sensitizer to the conduction band [20-23]. The injected electrons then migrate toward the transparent electrode and enter the circuit, resulting in an emf. Traveling through the circuit, the electrons are returned to the system by conversion of the electrolyte components on the surface of the metal counterelectrode. The cycle is closed after regeneration of the oxidized form of the sensitizer [24]. Figure 3b shows the illumination dependence of the short circuit current Jsc of the studied DSSC. As follows from the data presented, the value of Jsc increases practically linearly as the illumination intensity increases. It was also established that the voltage Voc increases exponentially as Jsc increases (Fig. 3c), which is consistent with the familiar equation [25-27]: Voc =

nkT æ J sc ö + 1÷÷ ln çç q ø è J0

(2)

where n is the ideality factor of the diode characteristic; k is the Boltzmann constant; q is the charge; J0 is the reverse saturation current density. According to Eq. (2), Voc is proportional to the logarithm of Jsc and increases as the light intensity increases (Fig. 3b,c) [27]. Impedance Spectroscopy of the Studied DSSC. Impedance spectroscopy is an effective tool for studying the electrical properties of materials and their interfaces with electrodes having electron conductivity, and also the dynamics of bound or free charges in the bulk of such materials and at the interfacial region of their contact with the electrode material and

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Fig. 3. Illumination dependence of the open-circuit voltage Voc (a) and the short-circuit current Jsc (b) ; Jsc vs. Voc plot for the studied DSSC (c).

the electrolyte. In this work, we used this method to determine the capacitance–voltage characteristics in the frequency range 2-5 kHz. As follows from Fig. 4a, as the applied bias voltage increases from –3 V to +3 V, the capacitance of the DSSC increases, reaches a maximum at 0 V, and decreases as the voltage increases further toward more positive values. The capacitance remains positive as the measurement frequency varies (Fig. 4b), in contrast to some cells studied earlier [18] in which a transition from positive to negative capacitance was observed as the scanning frequency increased. Figure 4c shows the conductance vs. voltage plots for a cell based on titanium dioxide sensitized by Na-FL at different measurement frequencies (5 kHz to 5 MHz). When a small dc voltage is applied to the semiconductor, its conductance decreases due to majority charge carrier inversion in the surface layer relative to the majority carrier in the bulk of the semiconductor [28]. From Fig. 4c we see that as the applied bias voltage increases, at a frequency of 5 kHz the conductance increases, reaches a maximum at 0 V, and gradually decreases with further increase in voltage. At a frequency of 10 kHz, the 124

Fig. 4. Capacitance–voltage plots for the studied nanostructured DSSC at different frequencies: 1) 2, 2) 5, 3) 10, 4) 50, 5) 100, 6) 300, 7) 500, 8) 700, 9) 900 kHz; 10) 1, 11) 2, 12) 3, 13) 4, 14) 5 MHz. b) Capacitance of the DSSC, measured for different bias voltages and frequencies: 1) 900 kHz; 2) 1, 3) 2, 4) 3, 5) 4 MHz. c) Conductance–voltage plots obtained for the studied DSSC at different measurement frequencies: 1) 5, 2) 10, 3) 50, 4) 100, 5) 300, 6) 500, 7) 700, 8) 900 kHz; 9) 1, 10) 2, 11) 3, 12) 4, 13) 5 MHz. d) Plots of resistance as a function of voltage, obtained for the studied DSSC at different measurement frequencies: 1) 5, 2) 10, 3) 50, 4) 100, 5) 300, 6) 500, 7) 700, 8) 900 kHz; 9) 1 MHz.

initial rise in conductance and its decrease at positive values of the voltage are much less pronounced than at 5 kHz. At higher frequencies, we observe only a small variation in conductance with bias voltage. The series resistance Rs was calculated from the measured cell capacitance CM and its conductance GM. Figure 4d shows the dependence of Rs on the bias voltage (V) at different frequencies. The total conductance of the solar cell can be expressed by the following relation [29]: YMA = GMA + jwCMA.

(3)

The series resistance Rs was calculated from the total conductance using Eq. (4): Rs =

GMA 2 GMA

2 + w2C MA

.

(4)

The quantity Rs represents the real part of the impedance. As follows from the data presented in Fig. 4d, in the frequency range 5-10 kHz, the real part of the impedance (Rs) varies linearly with the increase in bias voltage, reaching a maximum at 0 V. Rs decreases with further increase in bias voltage. At frequencies above 10 kHz, there is practically no dependence of Rs on bias voltage.

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Therefore study of solar cells based on nanostructured TiO2 sensitized by fluorescein sodium salt has shown that under optimal conditions, they are characterized by a short circuit current density of 72 µA/cm2 and an open circuit voltage of 0.56 V under AM1.5 illumination. In studying the capacitance–voltage characteristics by impedance spectroscopy, we did not observe any shift from positive to negative capacitance. The photoelectric performance of such cells can be further improved by optimization of its fabrication conditions, in particular by varying the dye concentration and going to combinations of different dyes. This work was supported by the NPST program of King Saud University (project No. 11-NAN1464-02).

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