CHAPTER-3 FABRICATION OF SOLID- LIQUID JUNCTION SOLAR CELL USING TMDCS CRYSTALS

Transcription

1 CHAPTER-3 FABRICATION OF SOLID- LIQUID JUNCTION SOLAR CELL USING TMDCS CRYSTALS 72

2 3.1 INTRODUCTION: As the solid- liquid junction solar cells require only electron donating material and electron accepting material, it is easy to fabricate these solar cells. In these cells, photoexcitation of semiconductors would lead us to the generation of photovoltage and photocurrent [1, 2]. Solid- Liquid junction solar cell is a two electrode- electrolyte system having two interfaces. A change in the electrode potential due to the illumination of electrode-electrolyte interface of such cells is called photoelectrochemical (PEC) effect. Thus, in PEC solar cells the charge transfer across semiconductor- electrolyte interface in dark or illumination result in flow of current through the junction of electronically conducting solids and the ionically conducting liquids. In the present investigation, TMDCs crystals are used for the preparation of solar cell. This work is centered on the grown crystals of MoSe 2 and WSe 2 material as these are corrosion resistive, it is interesting to observe the photoconversion behavior of these materials. Different electrolytes and different counter electrodes like platinum, copper, aluminium, and graphite have been used for this purpose. Various intensities of illumination applied on the semiconducting electrode. The photoconversion is measured by the different parameters like open circuit voltage, short circuit current, photoconversion efficiency, fill factor, shunt resistance, series resistance etc. 3.2 BASIC EQUIPMENTS OF PHOTOELECTROCHEMICAL SOLAR CELL: For the fabrication of PEC solar cells, three basic components are needed. In this cell, photo anode and reference electrode (metal cathode) are immersed in electrolyte solution. These basic three components are: Semiconducting Electrode Electrolyte Solution Reference or counter Electrode The experimental arrangement of PEC solar cell using these equipments is shown in Fig

3 Fig. 3.1 Experimental set-up for the PEC solar cell 74

4 3.3 REQUIREMENTS OF SEMICONDUCTING ELECTRODE: The overall performance of PEC solar cells mainly depends on the type of material chosen for the fabrication of semiconducting electrode and its parameters. Some of the important aspects related to the chosen semiconducting material and a PEC solar cell, which are to be carefully looked while the fabrication process has been given below: [1] A semiconducting material of direct band gap is more suitable than the indirect band gap material. [2] The width of space charge layer must be large. [3] The energy band gap of the semiconducting electrode should be near to the maxima of the incident solar radiation. It controls the photo response behavior. [4] To keep away from any existence of potential barrier which reduces the flow of carriers, the back contact of the semiconducting photoelectrode should be ohmic. [5] For the higher absorption of incident radiation, the optical absorption co-efficient of the semiconducting material should be high. [6] The stability of the semiconducting material which is used in the fabrication of photoelectrode should be high. [7] The diffusion length of minority carriers within the semiconducting materials should be as long as possible to enhance the transport of photogenerated carriers. [8] The series resistance of the semiconducting material should be as small as possible. The series resistance of the semiconducting material is the major source of total resistance of the PEC solar cell. [9] The Thickness and area of the photoelectrode should be large for higher absorption of incident radiation. 75

5 3.3.1 PREPARATION OF PHOTOELECTRODE: The preparation of photoelectrode is most important part for the fabrication of photoelectrochemical solar cell. Fig. 3.2 shows the structure of semiconducting photoelectrode. The photoelectrodes have been fabricated from TMDCs (MoSe 2 & WSe 2 ) crystal. In present case, a glass rod is used to prepare the photoelectrode. It consist 0.6 cm diameter, 11 cm length & narrow bore of 0.08 cm. One end of this narrow bored glass rod was flattened by a hot gas blow. The flat portion of the rod was used as a supporting base for proper mounting of semiconducting crystal. The conducting copper wire (30 SWG) was passed through the narrow bore and flattened at one end (which is towards the flat portion of the glass rod) to provide electrical contact and support the crystal. The semiconducting electrodes made from TMDCs crystals grown in such a way that the contacting material (highly conducting silver point) provides good ohmic contact between the copper wire and the back side of the crystal. The assembly was kept in an oven for few hours at temperatures less than 100ºc for getting good adhesion and ohmicity of this back contact. After proper setting of crystal on the copper wire, the back side of the semiconducting crystal as well as the edges of the crystals was insulated/covered with highly insulating epoxy (araldite) in such a way that the front area is left uncovered for the exposure to incident radiation. The prepared semiconducting electrode has been shown in Fig. 3.2 (a) & 3.2 (b). 76

6 Fig. 3.2 Semiconducting Electrode 77

7 (a) (b) Fig. 3.2 (a) & (b) Photograph of Semiconducting photo electrode 78

8 3.4 DESCRIPTION OF ELECTROLYTE: Gerischer [2, 3] have reported the interesting electronic properties of the electrolyte. As indicated by them, oxidized and reduced species present in the electrolyte solution do not contain the same energy. These ionic species facilitates to shift the holes produced by the light from photoelectrode to the counter or reference electrode. When considering the electronic levels, the oxidized species have empty electronic levels. So, they behave as acceptors. In the case of reduced species, which are donor corresponding to the full electronic level. The distribution of electronic levels of the redox system is shown in Fig.3.3. As shown in equation 3.1, the electronic structure is concentration dependent, thus a concentration change induces a shift of the redox potential (E redox ). This phenomenon is shown by the famed Nernst equation; E f redox = E 0 redox+ (RT/zF). ln(c Ox /C Red ) Where, E 0 redox = Standard redox potential, C Ox = Concentrations of the oxidized species C Red = Concentrations of the reduced species, R= Gas constant, Z=Number of electrons involved in the reaction F= Faraday constant. The concept of energy levels in the electrolyte is quite similar to the model of energy states in the solid material. Here, E f redox is equivalent to the Fermi energy level of the semiconductor. If the semiconductor electrode is brought in contact with the redox electrolyte solution, the equilibrium condition is attained by the electron transfer at the surface and the Fermi level of the semiconductor adjusts with each other. It turns out in a barrier height which depends on the type of the solution species and particular semiconductor. 79

9 E Oxidized (empty) E O Ox E O Redox E O Red Reduced (Occupied) D (E) Fig.3.3 Energy Diagram of electrolyte 80

10 3.5 PROPERTIES AND SELECTION OF APPROPRIATE ELECTROLYTE: PROPERTIES OF THE ELECTROLYTE: [1] The electrolyte should be transparent to the incident solar radiation. So that the absorption of light would become minimum by the electrolyte. [2] For the negligible ohmic losses, the ionic conductance of the electrolyte should be large. [3] Photo and thermal stability of oxidized, reduced species and other solvents should be good against incident radiation. [4] The charge transfer rate due to redox systems of the electrolyte at semiconducting electrode and counter electrode of a PEC solar cell should be high. [5] The price and the toxicity of the electrolyte should be low. [6] The electrolyte should be less reactive with the semiconducting electrode and counter electrode. [7] To acquire high current densities, the oxidized species, reduced species and sustaining ion concentration in electrolyte should be adequate SELECTION OF THE PROPER ELECTROLYTE: Selection of the appropriate electrolyte is significant part of the fabrication because it actually is a source for electrochemical reactions leading to the photo effects. Oxidized and reduced species of the electrolyte solution should be ionic in nature, which help in the transfer of photogenerated carrier from the semiconducting photoelectrode to the counter electrode. Many reports are available for the compatibility of TMDCs photoelectrodes in different electrolytic solution [8-15]. 81

11 List of Prepared Electrolyte used for the present research work: I M I M NaI M Na 2 SO 4 II M I M NaI M Na 2 SO drops of H 2 SO 4 III M I M NaI M Na 2 SO ml water IV M I M KI M K 2 SO 4 V M I M KI M K 2 SO 4 VI M I M NaI M Na 2 SO 4 VII M I M NaI M Na 2 SO 4 VIII. 25 ml ( M I M NaI M Na 2 SO 4 ) + 75 ml water 3.6 THE COUNTER ELECTRODE: For the better performance of PEC solar cell, the counter electrode is necessary equipment. The regenerative processes must be satisfied by the counter electrodes. There is no chemical change occurs because of the counter electrodes in oxidized electrolyte species [4-6]. This could not affect the chemical composition of electrode [7]. The requirements for the counter electrode to get the high efficiency PEC solar cell and for better performance are given as below: [1] The counter electrode must have large area. [2] For the reduction process, it should have low potential. [3] When a counter electrode is deep into the electrolyte, the half cell potential of the counter electrode should be identical with the half cell potential of the semiconductor electrode. [4] The counter electrode should not react with electrolyte solution. It should be less reactive. [5] The counter electrode should not be costly. In the present case, Platinum, Graphite, Aluminum and Copper have been used as the counter electrode 82

12 3.7 PARAMETERS OF PEC SOLAR CELL: There are so many parameters which affects the performance of PEC solar cell. Many reports have been given for the parameters of solar cell. These parameters have been measured by the experiments. These parameters includes short circuit current, open circuit voltage, fill factor, efficiency etc. These parameters are listed below, [1] Short circuit current [2] Open circuit voltage [3] Fill factor [4] Photoconversion Efficiency [5] Flat band potential [6] Quantum efficiency [7] Series resistance [8] Shunt resistance SHORT CIRCUIT CURRENT (I sc ): When the terminals of the solar cell are shorted, the current becomes maximum, this produced current is known as Short circuit current. When a photon is absorbed in a solar cell, it produces an electron-hole pair, which is detached by the junction and then transported to the external circuit [41, 42]. The short circuit current depends on the band gap of the material. A material having large band gap will absorb less number of photons as compared to the materials with low band gap. Therefore, the short circuit current will increase with decrease in band gap energy. Equation 3.2 represents the short circuit current. exp V OC KT Where, I 0 = Reverse saturation current T= Operating temperature K= Boltzman constant V OC = Open circuit voltage 83

13 3.7.2 OPEN CIRCUIT VOLTAGE (V oc ): Open circuit voltage V oc is the maximum voltage that can be obtained from a solar cell when its terminals are left open. If the energy of absorbed photon is higher than the band gap energy, it stimulates an electron from the valance band to the conduction bend, lifting up its potential energy by an equal amount to E g. If there are no potential drops in the solar cell then the electrostatic potential V= - E g /q should be the maximum possible voltage that can be obtained from the solar cell. Equation 3.3 represents the open circuit voltage of PEC solar cell. if the band gap of the material is greater, the open circuit voltage of the solar cell is higher. ln kt/e = Volts (at T=300K) I L = Intensity of illumination FILL FACTOR (F.F.): The fill factor (FF) is characterized as the squareness of the I-V curve and is primarily related to the resistive losses in a solar cell [10]. In ideal case, its value can be 100% corresponding to square I-V curve. But it is not feasible to have square I-V. There are always some losses which reduce the value of fill factor. It can be represented as shown in equation 3.4 or 3.5. F. F J V J V F. F P J V

14 3.7.4 PHOTOCONVERSION EFFICIENCY (η %): Photoconversion efficiency is defined as the ratio of electrical power generated and the optical power incident on the cell. It can be calculated using the following formula; η V J I L 100% Where, J I /Area J mp = The current density at maximum power point V mp =The voltage at maximum power point I L =Intensity of incident illumination QUANTUM EFFICIENCY ( q ): For the monochromatic radiation by the illumination of intensity, the performance of PEC solar cell can be evaluated by the following formula, η N U N U For the ideal cell, the quantum efficiency is 100%. Low conversion efficiency can be found out for practical solar cell FLAT BAND POTENTIAL (V fb ): This parameter is defined as the potential applied to the semiconductor electrode with reference to the standard electrode so that the energy in bands in the semiconductor remains flat even if one approaches the interface. This parameter usually depends on the relative positions of the Fermi levels of the semiconductor and the Fermi levels of the electrolyte. If the flat band potential increases, the open circuit voltage also increases up to certain extent. 85

15 3.7.7 SERIES RESISTANCE (R s ): The series resistance R s regarded as conductivity i.e. mobility of the specific charge carriers in the relevant transport medium. The mobility can be affected by space charges and traps or other barriers. In semiconducting material, R s also increase with a longer traveling distance of the charges in e.g. thicker transport layers. The series resistance can be calculated through different methods e.g. slope method, area method etc. The series resistance can be evaluated by the slopes of the I - V curves near I = 0, using the equation. R V I I SHUNT RESISTANCE (R sh ): The shunt resistor R sh is due to recombination of charge carriers near the dissociation site (e.g Donor/Accepter interface) Provided the series resistor R s is at least one order of magnitude lower than R sh, it may also include recombination further away from the dissociation site e.g. near the electrode. R sh can be evaluated by slope method PHOTOVOLTAGE- PHOTOCURRENT CHARACTERISTICS OF TMDCS BASED PEC SOLAR CELLS: For the fabrication of PEC solar cell, TMDCs has been used as a semiconducting material and the I 2 /I - electrolyte in the present investigation [16-43]. The photoconversion characteristics of PEC solar cell using MoSe 2 crystal and WSe 2 crystal has been measured in the present investigation. It is observed that the efficiency of the cell is increases with the increase in intensity of illumination. Photoconversion characteristics can be found in earlier research [26-86

16 47]. Moreover, the short circuit current and open circuit voltage are increasing with intensity of illumination. Study of solar conversion devices have been reported by the different researcher [. Fig. 3.4 Schematic view of V- I characteristics 3.9 VARIATION OF SHORT CIRCUIT CURRENT AND OPEN CIRCUIT VOLTAGE WITH INTENSITY OF INCIDENT ILLUMINATION: The absorption of incident illumination (photons) by the semiconducting materials produces electron-hole pairs in the semiconductor. In n-type semiconductor, the holes are minority carriers which take part in the oxidation process at the semiconductor- electrolyte interface. at the same time, the reduction process occurs at the platinum counter electrode. As a whole, the complete redox process leads to the generation of the photocurrent/photovoltage in the external circuit [7]. Most commonly, the photocurrent flowing in the external circuit depends on two basic processes occurring within a PEC solar cell. 87

17 (I) The photgeneration of carriers and their efficient transport within the semiconducting material (working as photoelectrode) (II) The electrochemical kinetics, which include the oxidation / reduction process occurring at the two electrodes. The variation of photocurrent with the intensity of light demonstrates a linear or a nonlinear behavior depending upon the rates of these two reactions. If the oxidation/reduction process is rapid and more efficient than the photogeneration of carriers within the semiconductor, all the photogenerated carriers reaching the interface take part in the oxidation/reduction process with the ionic species. This specifies that all the photogenerated carriers are being absorbed by the ionic species which results into a linear increase in the photocurrent as the intensity of incident illumination increases [11-15, 17]. But, if the oxidation / reduction process does not respond to the number of photogenerated carriers. (May be due to the limitations of the electrolyte), the photocurrent in the output circuit shows a non- linear dependence on the intensity of incident illumination [18-25]. The variation of short circuit current with the intensity of incident illumination has also been evaluated. The short circuit current increases with the increase in the input intensity. The increase in short circuit current is linear up to certain intensity VARIATION OF PHOTOCONVERSION EFFICIENCY WITH THE INTENSITY OF INCIDENT ILLUMINATION: The variation of photoconversion efficiency of MoSe 2 /I 2 /I - /Pt PEC solar cells with different intensities of incident illumination has been measured in the present investigation. In ideal solar cell, as the intensity of incident illumination increases, the photoconversion efficiency also increased [9, 11, 14]. The photoconversion efficiency is mostly dependent on the open circuit voltage V oc and short circuit current I sc VARIATION OF FILL FACTOR WITH THE INTENSITY OF INCIDENT ILLUMINATION: The fill factor is a parameter, which decides the quality of the solar cells. In the present thesis, this parameter has been evaluated for MoSe 2 and WSe 2 PEC solar cells at different 88

18 intensities of illumination and for the different counter electrodes. The relationship of Fill factor with the intensity of illumination has been observed. This can be almost seen from the squareness of the Vph- Iph characteristics observed, as FF is a measure of the "squareness" of the I-V curve, a solar cell with a higher voltage has a larger possible fill factor since the "rounded" portion of the IV curve takes up less area. The maximum theoretical fill factor from a solar cell can be determined by differentiating the power from a solar cell with respect to voltage and finding where this is equal to zero IDEALITY FACTOR OF PEC SOLAR CELLS: The ideality factor can be represented as the equation given below: From this equation we can say that the slope of lni L V oc graph can be used to calculate the value of ideality factor of PEC solar cell. For the ideal solar cell, the ideality factor is unity EFFECT OF TEMPERATURE ON THE PARAMETERS OF PEC SOLAR CELL: In light of the above discussions, it has been deliberated to observe the effect of temperature on the overall behavior of PEC solar cells and with this view the operating temperature of TMDCs based PEC solar cells has been varied. The photoconversion characteristics have been investigated at different temperature. Butler has shown that the increase in temperature leads to the absorption coefficient of the semiconducting material which, in turn, increases the photogeneration of carriers consequently leading to the enhancement of the photocurrent. According to the rajeshwar et.al, the increase in the short circuit current can be attributed to both the temperature induced charges in the electrical and optical properties of semiconducting material and the corresponding changes in the charge and potential distribution across the semiconductor- electrolyte interface [5, 10, 16]. The variation of photoconversion efficiency and the fill factor of MoSe 2 and WSe 2 based solar cells with temperature have been investigated in the present investigation. 89

19 3.14 EFFECT OF WAVELENGTH OF INCIDENT ILLUMINATION ON PEC SOLAR CELL: If the semiconductor electrode is illuminated by the monochromatic light of specific wavelength, some of the important parameters of PEC solar cells like monochromatic efficiency and quantum efficiency as well as the parameters of semiconductors like the absorption coefficient and the diffusion length of minority carriers can be evaluated. The photocnoversion characteristics of MoSe 2 and WSe 2 based PEC solar cell have been investigated for the monochromatic illumination of different wavelengths at 10mW/cm 2 obtained from the lamp using standard potential EFFECT OF OPTICAL DENSITY OF ELECTROLYTE ON PERFORMANCE OF PEC SOLAR CELL: The absorbance (also called optical density) of a material is a logarithmic ratio of the radiation falling upon a material, to the radiation transmitted through a material. In physics, the term spectral absorbance is used interchangeably with spectral absorptance or absorptivity. Absorptance is the ratio of the radiation absorbed by a surface to that incident upon it. Total absorptance refers to absorptance measured over all wavelengths. Spectral absorptance refers to absorptance measured at specified wavelengths. In this case it has a slightly different meaning: the fraction of radiation absorbed at specific wavelengths. The amount of light transmitted through a material diminishes exponentially as it travels through the material. Since the absorbance of a sample is measured as a logarithm, it is directly proportional to the thickness of the sample and to the concentration of the absorbing material in the sample. Some other measures related to absorption, such as transmittance, are measured as a simple ratio so they vary exponentially with thickness and concentration of the material. The optical density of various electrolytes has been measured by the colorimeter using standard filters. The optical densities of the different electrolyte solutions for the different wavelength have been measured by the digital colorimeter (EQUIP-TRONICS, EQ-650-A) (Fig.3.5) at Chemistry Department, R.R. Mehta science college, Palanpur. 90

20 Fig.3.5 Photgraph of digital colorimeter 91

21 3.16 REFERENCES: [1] R. J. Pathak, Ph. D. thesis, Sardar Patel University, Vallabh vidyanagar, (2002) [2] H. Gerischer, In physic chemistry- An Advanced Treatise, Ed. H. Eyring, D. Henderson and W. Jost, [Acadmic press, New York], Vol. 9A (1970). [3] H. Gerischer, Semiconductor liquid junction solar cells ed. A Heller (The electrochemical society, New York, (1977). [4] H. Tributsch, Ber. Bunsenges. Phys. Chem., 82, (1978) 169. [5] Hans S. Rauschenbach, Solar Cell Array Design Handbook, (1980) [6] P. A. Crossley, G. T. Noel, and M. Wolf, RCA Astro-Electronics Division, Princeton, New Jersey (1968). [7] E. Bucher, Appl. Phys., 17 (1978)1. [8] J. D. Sandstrom, Records of the 6th IEEE Photovoltaic Specialists Conference, (1967). [9] Parrott, J.E., The saturation photovoltage of a pn junction, IEEE Trans. Electron. Devices, ED-21 (1974) 84. [10] Lindmayer, J., COMSAT Tech. Rev., 2 (1972) 105. [11] Chai, Y.G., and W.W. Anderson, Appl. Phys. Lett., 27 (1975) 183. [12] D.E. Carlson, IEEE Int. Electron Devices Meeting, Technical Digest, Washington, D.C., (1977). [13] Fahrenbruch, A.L., F. Buch, K. Mitchell, and R.H. Bube, Conference Record of the Eleventh IEEE Photovoltaic Specialists Conference, 1975, p

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