Photoelectrochmeical Cell Based on Natural Pigments and ZnO Nanoparticles
Getachew Yirga1, Sisay Tadesse3, Teketel Yohannes2
1Department of Physics, Haramaya University, Dire Dawa, Ethiopia
2Materials Science Program, Addis Ababa University, Addis Ababa, Ethiopia
3Department of Chemistry,Hawassa University, Hawasa, Ethiopia
To cite this article:
Getachew Yirga, Sisay Tadesse, Teketel Yohannes. Photoelectrochmeical Cell Based on Natural Pigments and ZnO Nanoparticles. Journal of Energy and Natural Resources. Vol. 5, No. 1, 2016, pp. 1-10. doi: 10.11648/j.jenr.20160501.11
Abstract: Natural pigments extracts fromBougainvillea spectabilis, Carissa Ovata, Hibiscus sabdariffa, Amarathus iresine herbisti, Beta vulgaris, are used as natural sensitizers for a dye sensitized solar cell (DSSC). ZnO nanoparticles were synthesized using sol-gel method and the size of the nanoparticle was determined using effective mass approximation model. Devices were Assembled using ZnO nanoparticles and natural sensitizers. DSSCs based on ZnO nanoparticles and ethanol extract of Amarathus iresine sensitizers have shown relatively better conversion efficiency of 0.039. Incident photon to current conversion efficiency (IPCE), short circuit current density (Jsc) and open circuit voltage (Voc) were measured for all the sensitizers.
Keywords: Natural Dyes, Electrolytes, Solar Cells, Titanium Dioxide (TiO2)
The development of dye sensitized solar cells (DSSCs), which have derived inspiration from photosynthesis, has opened up exciting new possibilities and prototypes for producing solar photovoltaics possibly at lower cost. Early DSSC designs involved transition metal coordinated compounds (e.g., ruthenium polypyridyl complexes) as sensitizers because of their strong visible absorption, long excitation lifetime, and efficient metal-to-ligand charge transfer. Because of these, such type of DSSC’s are highly effective with maximum efficiency of 11% [1,2]. The costly synthesis and undesired environmental impact of those prototypes call for cheaper, simpler, and safer dyes as alternatives. Natural pigments, including chlorophyll, carotene, and anthocyanin, are freely available in plant leaves, flowers, and fruits and fulfill these requirements. Experimentally, natural dye sensitized TiO2 solar cells have reached an efficiency of 7.1% and high stability . Higher efficiency over 8.0% has been obtained using similar synthetic organic dyes .
Natural dyes have become a viable alternative to expensive and rare organic sensitizers because of its low cost, easy attainability, abundance in supply of raw materials and no environment threat. Various components of a plant such as the flower petals, leaves and bark have been tested as sensitizers. The nature of these pigments together with other parameters has resulted in varying performance. In this study pigments Bougainvillea spectabilis, Carissa Ovata, Hibiscus sabdariffa, Amarathus iresine herbisti, and Beta vulgaris were extracted as sensitizers and as a semiconductor ZnO and TiO2 were used. All the photoelectrochemical performance was measured and characterized for all the above sensitizers and semiconductors.
The solar cell efficiency was determined by its current-voltage (J-V) characteristics under standard illumination conditions. A standard solar spectrum of air mass 1.5 (AM 1.5) with an intensity of 1000 W/m2 is used for characterization. The Voc is the difference in potential between the two terminals in the cell under light illumination when the circuit is open. Jsc is the photocurrent per unit area (mA/cm2). The degree of the squared shape of the J-V curve is given by the fill factor (FF), which measures the ideality of the device and is defined as the ratio of the maximum power output per unit area to the product of Voc and Jsc.
The solar to electric power conversion efficiency η is given by the ratio of the maximum extractable power to the incident solar power (Pin) given by Equation 2.
Where Pin is the incident power, Pout is the output power, FF is the fill factor, η is the efficiency, Jsc is the short circuit current density, and Voc is the short circuit voltage. IPCE is one of the fundamental measurements of the performance of the solar. It is also known as the external quantum efficiency and describes how efficiently the light of a specific wavelength is converted to current that is (electrons out) / (photons in). The IPCE can be calculated according to the following equation.
Jsc is the short circuit current density, λ is the wavelength of the incident light and Pin is the intensity of the incident light. The factors determining the IPCE can be expressed as:
Where the light harvesting efficiency of the sensitized oxide layer, is the electron injection efficiency from the sensitizer into the oxide, and is the electron collection efficiency.
2. Materials and Experimental Methodology
2.1. Synthesis of ZnO Nanoparticles
Zinc acetate (BDH), sodium hydroxide pellets (Scharlau), and polyethylene glycol (Applied Science) were used as received. All the materials were first cleaned and rinse with distilled water and dried. All the chemicals were weighed with analytical balance and mixed in cleaned round bottom flask. For the sol-gel method 2.7 g of zinc acetate dehydrate and 0.5 g of polyethylene glycol (PEG) were taken and dissolved in in 250 ml of distilled water separately. 2 g of sodium hydroxide was dissolved in 500 ml of distilled water with vigorous stirring. The sodium hydroxide solution was added to the zinc acetate solution drop wise and the mixture was refluxed for 8 hrs. at 120°C using the setup shown in Fig 1.
The obtained ZnO solution was centrifuged to solid matter and solution. The solid matter was washed first by distilled water repeatedly, finally dried in Furnace to obtain ZnO. Diagrammatically the whole experiment can be summarized below. After preparing the colloidal nanoparticles optical absorption was done using Gensys-2 PC spectrometer to determine the size of colloidal nanoparticles. About 5 ml of the colloid were taken for measurement. Here for measurement the colloid nanoparticles were taken as soon as they reach the final temperature. We record the UV-visible absorption spectra between 275 and 500 nm. The solvent was used as a blank solution.
2.2. Natural Dyes Extraction
To select the dye that has good absorption in the visible region, fresh fruits, leaves, and flowers of plants including, Beta vulgaris, Bougamvillea, Amarathus iresine herbisiti, Hibiscus sabdariffa, and Carissa ovata collected. The collected plant was dried at room temperature in a shade to prevent pigment degradation (see Fig.3). After drying for about 2 months the samples are completely dried in an oven at 700C to avoid some moisture from it. Then after, the samples were crushed with Micro Plant Grinding machine to produce the powder of the respective plant materials. The dye extraction from the powder was done as follows; 2 g of each powder sample was taken and soaked in 50 ml of ethanol for extracting using ethanol and in 50 ml water for extracting with water in separate bottle.
The solution is stored at room temperature for about 6 hours to dissolve the powder completely. Then the solution was filtered with glass filter to separate the solid from the pure liquid. After filtration the extracted pigment in different solvent are shown in Fig. 4.
The pigment shown in Fig.4 are ready for soaking the electrode inside it, and also used for measuring the UV-vis absorption spectra. The absorption measurement was carried using the Gensys-2 PC spectrometer for each extract.
2.3. Fabrication of Dye-sensitized Solar Cells
The materials and the fabrication method employed for the fabrication of dye-sensitized Photo electrochemical cells based on ZnO nanoparticles are described below.
2.3.1. Substrate Cleaning
The substrate cleaning is believed to be a key process that influences the final performance of the devices. A significant effect on the photovoltage behavior can be observed experimentally depending on the extent of cleaning . For this reason; TCO glass substrates have been thoroughly cleaned before film deposition. The glass cleaning protocol was, first cleaned with Acetone then with isopropanol finally with Ethanol. In all case the cleaning process was done in ultrasonic bath for about 20 minute in each solvent.
2.3.2. ZnO Paste Formation
30 mg of polyethylene glycol (dispersing agent) and 10 ml of distilled water was mixed. 1.8 g of ZnO powder continuously grinded down by using procelain mortar to break down the aggregated particle. Then 2.5 ml of the PEG solution was slowly added to the powder and completely mixed with each other by using the mortar. Finally the paste was ready for deposition.
2.3.3. Deposition of ZnO Films
The simplest and most widely used method for depositing ZnO paste on a substrate is the so called doctor blade method. The technique is also known as slot coating in its mechanized version. It uses a hard squeegee, or doctor blade, to spread a portion of ZnO paste onto the glass (see Fig.5).
With the conductive side facing up, we apply four parallel strips of tape on the edges of the glass plate, covering about small portion of glass. After making the paste ready for deposition we apply the portion of paste near the top edge of the TCO glass between the two pieces of tape with the help of a glass rod, then, the paste spread across the plate with the support of the tape on all sides. The preparation of electrode was completed by firing the deposited layer. The organic solvent burns away, leaving the ZnO nanoparticles sintered together. This process ensures electrical contact between particles and good adhesion to the TCO glass substrate.
2.3.4. Electrode Sensitization
Natural dyes were used for electrode sensitization in the course of this thesis. Before immersion in the dye solution, films were warmed up to higher temperature (800C) to minimize the water vapor content inside the porous of the semiconductor electrode. The sensitization was always performed at room temperature. The best method of adsorbing a natural sensitizer to the oxide layer is by dipping the electrode in a solution of the dye already prepared. After sensitization, the films were rinsed in the same solvent (ethanol, water) as employed in the dye solution.
2.3.5. Quasi-Solid State Electrolyte
The polymer gel electrolyte was prepared according to the method developed by L. Fan et al as described below . 0.9 M of 1-ethylene-3-methyl immidazolium iodide (EMIM-I) was added into acetonitrile (Aldrich) under stirring to form a homogeneous liquid electrolyte. In order to obtain a better conductivity, 0.5 M of sodium iodide (BDH) was dissolved in the above homogeneous liquid electrolyte, and then 0.12M iodine and 35 %( w/w) of PVP (Aldrich) were added. Then, the resulting mixture was heated at 70 -800C under vigorous stirring to dissolve the PVP polymer, followed by cooling down to room temperature to form a gel electrolyte.
2.3.6. Coating Counter Electrodes
The poly (3, 4-ethylenedioxythiophene) (PEDOT) film for the counter electrode was formed by electrochemical polymerization of 3, 4-ethylenedioxy-thiophene (EDOT) (Aldrich), in a three electrode one-compartment electrochemical cell. The electro- chemical cell consisted of a pre cleaned ITO-coated glass working electrode, platinum foil counter electrode and quasi Ag/AgCl reference electrode dipped in LiClO4 (Aldrich) acetonitrile (sigma-Aldrich) solution. The solution used for the polymerization contained 0.1 M EDOT and 0.1 M LiClO4 in acetonitrile (Sigma-Aldrich). The monomer was used as received. The polymerization was carried out potentiostatically at +1.8 V. At this potential, the electrode surface becomes covered with blue-doped PEDOT film. The film was then rinsed with acetonitrile and dried in air.
2.3.7. Assembly of DSSCs for Characterization
Here the sensitized electrode was washed by the solvent of the dye then by ethanol and dried using hair dryer to dry the electrode, and then the non-covered part of the film by the paste was covered by a tape spacer in all side by leaving some place for electrical contact. By facing the active sides of the photoanode and the cathode, the two electrodes are pressed together after putting the quasi electrolyte on the photoanode. Then the devices are ready for characterization.
3. Results and Discussion
3.1. ZnO Nanoparticles Size Determination
Fig 6 is the plot of absorbance versus wavelength of the colloidal ZnO nanoparticles. From the absorption maxima the corresponding wavelength λ = 366 nm was obtained which is used for calculation of energy bandgap of the nanoparticle.
Using this wavelength the energy bandgap of the nanoparticle Eg was calculated using Equation 1:
Where m0,, and are the rest mass of electron, effective masses of electron, and effective mass of hole respectively [6,7,8]. Rearranging Equation 5 and solving for R gives:
For Eg(nano) of 3.39eV, the radius of nanoparticle, R was obtained to be 5.7 nm. Finally it is possible to say that the nanoparticle prepared by the method which was mentioned in the experimental part has an approximate average particle size of 11.4 nm.
3.2. Optical Absorption Measurements
The absorption spectrum of the prepared dye was measured by using the Genesys-2 PC spectrometer. Here first, some amount of the final dye solution was put in the quartz cuvette, and the absorbance versus wavelength measurement was taken for each sample of ethanol and H2O extract. The results of absorbance measurement of each sample was shown in Fig. 7.
In water extract of Bougainvillea spectabilis [Fig. 7 (a)] two peaks were found: the first one around 485 nm which can be associated to the presence of indicaxanthin (which is a type of betaxanthin found in plant pigment), while the second one at 531 nm is attributable to the betacyanin pigment. The pigment extracted from Carissa Ovata using water extracted shows an absorption peak at 553 nm [Fig. 7 (b)]. In water extract, the spectra show an absorption peak in the region of 520 - 550 nm which is the peak of anthocyanin containing dyes.
This is because of the diverse pigmentation from orange to red, purple, and blue pigment which are found in anthocyanin containing pigment and shows an absorption in the visible region (approximately 490 - 550 nm) .
Water extract of Hibiscus sabdariffa shows absorption peak at 519 nm. The peak ascertains the presence of anthocyanin pigment [Fig. 7 (c)]. In water extract of Amarathus iresine herbisti [Fig. 7 (d)] (λmax= 532 nm) anthocyanin containing pigment were observed in the absorption spectra. Beta vulgaris showed an intense absorption peaks in the region 400 - 600 nm [Fig. 7 (e)]. Here also water extract shows strong absorption peak of betalians, which are at 470 nm and 533 nm due to the mixed contributions of the yellow-orange betaxanthins, and of the red-purple betacyanines at around 480 nm and 540 nm respectively .
Ethanol extract of Bougainvillea spectabilis shows an absorption peak at 664 nm which has also an absorption peak below 500 nm [Fig. 8 (d)]. From these peak it is clearly shown that ethanol extract contain both chlorophyll a and b which have an absorption peak in between 400 - 500 nm and 600 - 700 nm . The pigment extracted from Carissa Ovata [Fig. 8 (c)] using ethanol shows an absorption peak at 664 nm. In ethanol extract of Carissa ovata the extracted pigment was contain chlorophyll pigment which shows the characteristics absorption peak of chlorophyll.
Both water and ethanol extract of Hibiscus sabdariffa [Figs. 7(c) and 8(b))] shows absorption peaks at 549 nm and 519 nm respectively. The peaks are associated with presence of anthocyanin pigment. Beta vulgaris [Fig. 8 (a)] showed an intense absorption peaks in the region 400 - 600 nm. Here also both water and ethanol extract shows strong absorption peak of betalians, at 470 nm and 533 nm because of the mixed contributions of the yellow-orange betaxanthins and the red-purple betacyanines (480, and 540 nm respectively)  Ethanol extract of Amarathus iresine herbisti (λmax= 433, 464, 664 nm) indicates the presence of chlorophyll which absorb most of the blue and red light [Fig. 8 (e)].
3.3. Current Density Versus Voltage Characteristics of ZnO Based DSSCs
The J-V characteristic of all sensitizers were measured and plotted for analysis and made comparison as shown below. Figure 9 (b) is ethanol extract of Beta vulgaris, the current density decreases as compared to the water extract [Fig. 10 (d)], but the reverse is true for the open circuit voltage. This is due to the low injection rate of electron into the semiconductor conduction band which decreases the short circuit current.
Water extract of Hibiscus sabdariffa [Fig. 10 (e)] has showed better current than the corresponding ethanol extract [Fig. 9 (c)] with some increment in short circuit current density.
In Fig. 10 (c), (d) and Fig. 10 (b), (e) both water and ethanol extract almost they have the same open circuit voltage but the current density relatively higher for ethanol than the water extract which means that there is only a difference in electron injection efficiency. Most of the natural dye which have a good and a broader absorption in the visible spectrum are expected to show a good rectification of the J-V curve that is responsible for good current density and power conversion efficiency. In these studies ethanol extract of Carissa ovata, Bougainvillea spectabilis, and Amarathus iresine herbisti shows a better rectification which results relatively good photoelectrochemical performance for ethanol extract than the water extract. In ethanol and water extract of Hibiscus sabdariffa, Bougainvillea spectabilis, and Carissa ovata, during socking, it was clearly an observed that, the film starts to dissolve.
This is because of protons derived from sensitizers make the dye solution relatively acidic which leads to the dissolution of the ZnO colloid.
The low local pH at the surface of ZnO during dye sensitization leads to the dissolution of Zn2+ ions from the ZnO surface [Fig. 11 (2)]. These Zn2+ ions form complexes with the dye [Fig. 11 (3)], which accumulate in the pores of the semiconductor film. It is assumed that only dye molecules directly attached to the ZnO surface can inject efficiently electrons and contribute to the photocurrent . Therefore, Zn2+/dye complexes, in spite of absorbing light, do not inject electrons. So that the low current as well as low power conversion efficiency may arises due to the above reasons. The instability of ZnO in acidic dyes results due to its amphoteric nature. In general, in a solution, the surface of the oxide is predominantly positively charged at a pH below the point of zero charge and negatively charged above this value, while the point of zero charge of metal oxides is defined as the pH at which the concentrations of protonated and deprotonated surface groups are equal.
For example the ZnO sensitization process with Ru-complex dye, the (pH = 5) is much lower than the point of zero charge of ZnO (≈ 9). That means that the ZnO surface is positively charged. Thus, the protons adsorbed on the ZnO surface will dissolve the ZnO. Table 1 Summarized the performance of the DSSCs in terms of short-circuit photo-current (Jsc), open-circuit voltage (Voc), fill factor (FF), and energy conversion efficiency (η) compared to those of other extracts.
The efficiency of cell sensitized by the Amarathus iresine herbistii extracted with ethanol was the best among the others. This is due to broader absorption range of the sensitizers, higher interaction between ZnO nanocrystaline film and the pigment extracted from Amarathus iresine herbistii which leads to a better charge transfer. The current density and open circuit voltage it has higher value than the others which are 0.29 mAcm-2 and 0.29 V respectively. The fill factor also shows best value of 48.5% for ethanol extract of Amarathus iresine herbisti.
|Sensitizers||Solvent||Jsc(mAcm-2)||Voc||FF (%)||η (%)|
As reported in literature,  that the extracting solvent can affect the DSSCs performance. The efficiency of the DSSCs was found to increase immensely when ethanol was used for extracting pigments . In this study, similar finding was also obtained. This might be due to the fact that our extracted pigments are more soluble in ethanol as a result; the aggregation of dye molecules decreases as expected. Better dispersion of dye molecules on the oxide surface could exactly improve the efficiency of the system.
3.4. Photocurrent Action Spectra of ZnO Based DSSCs
The Photo action spectra [Figs. 12 and 13] provided further insights on the photoelectrochemical behavior of natural dyes. All the following figures shows the incident photon to current conversion efficiency (IPCE) spectra of ZnO electrodes sensitized with all of our extracted natural dyes as a function of wavelength. Some of the IPCE spectra of the organic dyes adsorbed on the ZnO electrodes are broader than the absorption curves of the dyes in solution.
Ethanol extract of Bougainvillea spectabilis [Fig. 12(c)], Carissa ovate [Fig. 12(b)], Hibiscus sabdariffa [Fig. 12(d)], Beta vulgaris [Fig. 12(e)], and Amarathus iresine herbisti [Fig. 12(a)], shows broad IPCE spectrum up to 610,720, 430, 460, 550, 710 nm, respectively [Fig. 12]. Water extract of Bougainvillea spectabilis, Carissa ovata, Hibiscus sabdariffa, Jacaranda mimosifolia, Beta vulgaris, and Amarathus iresine herbisti from 300 up to 550, 730, 700, 460, 550, 650 nm, respectively [Fig. 13], which showed some sensitization effect, but there spectrum is dominated by the spectrum of the ZnO nanoparticles. The IPCE of ZnO sensitized by carisa ovate and Hibiscus sabdariffa shows a red shift on the absorption onset for water extract than ethanol extract.
Low IPCE results in either inefficient light harvesting efficiency (LHE) by the dye, or inefficient charge injection into ZnO, or inefficient collection of injected electrons. A low LHE can be due to a low dye absorption coefficient over the solar spectrum, a low dye concentration, a thin ZnO film to support large concentration of adsorbed dye which absorbs a significant fraction of the incident light, insufficient light scattering within the film, absorption of light by ZnO or the redox electrolyte, and dye degradation .
Low ηinj can be due to dye desorption, dye aggregation or the excited state levels of the dye lying below the conduction band edge of ZnO, the presence of the surrounding electrolyte, the wavelength of the exciting photon. Low ηcc is due to competition between fast recombination of photo-injected electrons with the redox electrolyte or oxidized dye and electron collection. The formation of Zn2+/dye complex can agglomerate which comes from dissolution of the nanostructured film form a thick covering layer instead of a monolayer, and is therefore inactive for electron injection which also limit the incident photon to current conversion efficiency. In IPCE of H2O extract of Carissa ovata, both H2O, and ethanol extract of Amarathus iresine shows additional spectral peaks which are a quite strong evidence of aggregate formation . The presence of aggregated dyes or non-injecting dyes at the surface affects all parameters. In all DSSCs device studied a small value of the incident to photon conversion efficiency, short circuit current, and fill factor were obtained that leads to small solar to electricity conversion efficiency .
In view of the instability of ZnO in acidic dyes, the development of new types of photosensitizers for use in ZnO DSSCs is a subject that has already been widely investigated. These photosensitizers are expected to be chemically bonded to the ZnO semiconductor, be charge transferable with high injection efficiency, and be effective for light absorption in a broad wavelength range. New types of dyes should be developed with the aim of fulfilling these criteria. For our study some of the chlorophyll extract shows relatively good efficiency for ethanol extract than water extract.
In water extract of natural dye, increased socking time of the film will leads to the dissolution of ZnO nanoparticle on the ITO, this is because of the extraction solvent (water) possesses an acidic nature, protons derived from dissolution of the dye makes the dye solution relatively acidic and this may be one of the reason for decrease in the short-circuit photocurrent. Also upon long socking time, formation of aggregate are formed which can be center of charge recombination and decrease the photovoltaic effects.
Generally, natural dyes suffer from low Voc, and Jsc which leads lower power conversion efficiency than an equivalent commercial N719 sensitized cell. This is because of most of natural dyes didn’t completely attached to the ZnO, and TiO2 nanoparticles, even upon washing by solvent to avoid aggregation from the socked film, the sensitizers leave the surface of the film, this is due to, more of the natural dye attached with film of the nanoparticles by physical attaching or by a weak bonding force.
In this work, ZnO nanoparticle was successfully synthesized which is one of the major semiconductors for DSSCs, using polyethylene glycol as dispersing agent. The average particle size of these semiconductor nanoparticles was determined using an approximation model called the effective mass model. From optical absorption spectra the maximum absorption λmax was determined which was used for finding the energy band gap of the nanoparticles. From the effective mass approximation which relates the radius of nanoparticle with its energy bandgap the size of ZnO nanoparticle was determined to be 1.14 nm.
DSSCs device are fabricated using ZnO nanoparticles for the six sensitizers and their photovoltaic performance were determined. For ethanol extract of Bougainvillea spectabilis, Carissa ovata, Amarathus iresine herbistii, Beta vulgaris, and Hibiscus sabdariffa, efficiency of 0.0066%, 0.0031%, 0.039%, 0.0039%, and 0.0043% respectively were obtained. The efficiency of water extract of Bougainvillea spectabilis, Carissa ovata, Amarathus iresine herbistii, Beta vulgaris, and Hibiscus sabdariffa were found 0.0016%, 0.003%, 0.0062%, 0.0077%, and 0.0104% respectively.
In general, ZnO nanoparticles based DSSCs shows low photoelectrochemical performance as compared to commercial TiO2 based DSSCs. Some of the limiting factor for this was insufficient attachment of natural dyes with the nanoparticles, formation of aggregation between the nanoparticles up on film formation, low injection rate, low regeneration of electron, and formation of Zn2+ /dye complex. Therefore, modification of ZnO aggregates with a very effective shell material on ZnO, preventing the generation of Zn2+/dye agglomerates will be a good method for improving the stability of ZnO nanoparticles.