Henna (Lawsonia inermis L.) Dye-Sensitized Nanocrystalline Titania Solar Cell

Abstract

Low-cost solar cells have been the subject of intensive research activities for over half century ago. More recently, dye-sensitized solar cells (DSSCs) emerged as a new class of low-cost solar cells that can be easily prepared. Natural-dye-sensitized solar cells (NDSSCs) are shown to be excellent examples of mimicking photosynthesis. The NDSSC acts as a green energy generator in which dyes molecules adsorbed to nanocrystalline layer of wide bandgap semiconductor material harvest photons. In this paper we investigate the structural, optical, electrical, and photovoltaic characterization of two types of natural dyes, namely, the Bahraini Henna and the Yemeni Henna, extracted using the Soxhlet extractor. Solar cells from both materials were prepared and characterized. It was found that the levels of open-circuit voltage and short-circuit current are concentration dependent. Further suggestions to improve the efficiency of NDSSC are discussed.

1. Introduction

A solar cell is a photonic device that converts photons with specific wavelengths to electricity. Materials presently used in photovoltaics (PVs) are mainly semiconductors including, among others, crystalline silicon, III–V compounds, cadmium telluride, and copper indium selenide/sulfide [1–3]. Low-cost solar cells have been the subject of intensive research work for the last three decades. Amorphous semiconductors were announced as one of the most promising materials for low-cost energy production. Most recently dye-sensitized solar cells (DSSCs) emerged as a new class of low-cost energy conversion devices with simple manufacturing procedures. Incorporation of dye molecules in some wide bandgap semiconductor electrodes was a key factor in developing photoelectrochemical solar cells. O'Regan and Grätzel [4] and Nazerruddin et al. [5] succeeded for the first time in producing a dye-sensitized solar cell (DSSC) using a nanocrystalline TiO₂ film with novel Ru bipyridl complex. It was shown that DSSCs are promising class of low-cost and high-efficiency solar cell based on organic materials [1].

The overall DSSC efficiency was found to be proportional to the electron injection efficiency in wide bandgap nanostructured semiconductors. This finding has escalated research activities over the past decade. ZnO₂ nanowires, for example, have been developed to replace both porous and TiO₂ nanoparticle-based solar cells [6]. Also, metal complex and novel man-made dyes have been proposed [7–9]. However, processing and synthesization of these dyes is complicated and costly process [10–16]. Moreover, development or extraction of photosensitizers with absorption range extended to the near IR is greatly desired. We found that our environment provides natural, nontoxic, and low-cost dyes sources with high absorbance level of UV, visible, and near IR. Examples of such dye sources are Bahraini Henna (Lawsonia inermis L.). This work provides further investigation on the first reported operation of Henna (Lawsonia inermis L.) as a dye sensitizer of nanostructured solar cell [17, 18].

In this work we describe first the preparation of nanostructured TiO₂ layer, followed by the process of dye extraction and staining of the nanostructured TiO₂ layer. In the second part we describe the solar cell preparation using two types of Henna extracts, namely, the Bahraini and Yemeni Henna extracts. In the third part we examine the structural, optical, and electrical characterization of the dye-sensitized solar cells manufactured from the above extracts.

2. Structure and Operation of Dye-Sensitized Solar Cells

Following the description in [18, 19] the operating principle of dye-sensitized solar cells is shown schematically in Figure 1. The cell is composed of four elements, namely, the conducting and counter conducting electrodes, the nanostructured TiO₂ layer, the dye molecules, and the electrolyte. The transparent conducting electrode and counterelectrode are coated with a thin conductive and transparent layer of tin dioxide (SnO₂). Nanocrystalline TiO₂ is deposited on the conducting electrode (photoelectrode) to provide the necessary large surface area where dye molecules are adsorbed. Upon absorption of photons, dye molecules are excited from the highest occupied molecular orbitals (HOMOs) to the lowest unoccupied molecular orbital (LUMO) states as shown schematically in Figure 1. This process is represented by (1). Once an electron is injected into the conduction band of the wide bandgap semiconductor nanostructured TiO₂ film, the dye molecule (photosensitizer) becomes oxidized (2). The injected electron is transported between the TiO₂ nanoparticles and then gets extracted to a load where the work done is delivered as an electrical energy (3). Electrolyte containing I⁻/I₃₋ redox ions is used as an electron mediator between the TiO₂ photoelectrode and the carbon electrode. Therefore, the oxidized dye molecules (photosensitizer) are regenerated by receiving electrons from the I⁻ ion redox mediator that get oxidized to I₃₋ (Tri-iodide ions). This process is represented by (4). The I₃₋ substitutes the internally donated electron with that from the external load and gets reduced back to I⁻ ion, (5). Therefore, generation of electric power in DSSC causes no permanent chemical change or transformation:

Figure 1 

Schematic diagram illustrating the structure and operation principle of a dye-sensitized cell.

Excitation process,𝑆+photon⟶𝑆^∗(1)

Injection process,𝑆^∗+TiO₂⟶𝑒⁻⁽_TiO2)+𝑆⁺(2)

Energy generation,𝑒⁻⁽_TiO2)+C.E.⟶TiO₂+𝑒⁻⁽_C.E.)+electricalenergy(3)

Regeneration of dye,𝑆⁺+32I⁻⟶𝑆+12I³⁻(4)

𝑒⁻ Recapture reaction,12I³⁻+𝑒⁻⁽_C.E.)⟶32I⁻+C.E.(5)

In fact, a smaller energy separation between the HOMO and LUMO is desired to ensure absorption of low energy photons in the solar spectrum. This is analogous to inorganic semiconductors energy bandgap (𝐸_𝑔). Therefore, the photocurrent level is dependent on the HOMO-LUMO levels separation. To enhance electron injection into the conduction band of TiO₂, one must use a sensitizer with the largest energy separation of LUMO and the bottom of the TiO₂ conduction band. Furthermore, for the HOMO level to effectively accept the donated electrons from the redox mediator, the energy difference between the HOMO and redox chemical potential must be more positive. Finally, the maximum potential produced by the cell is determined by the energy separation between the electrolyte chemical potential (𝐸_redox) and the Fermi level (𝐸_𝐹) of the TiO₂ layer as shown in Figure 1.

3. Experimental

Nanostructured TiO₂ films were prepared following the procedure detailed in [18]. A suspension of TiO₂ is prepared by adding 9 mL of nitric acid solution of PH 3-4 (1 mL increment) to 6 g of colloidal P25TiO₂ powder in mortar and pestle. While grinding, 8 mL of distilled water (in 1 mL increment) is added to get a white-free flow-paste. Finally, a drop of transparent surfactant (any clear dishwashing detergent) is added in 1 mL of distilled water to ensure coating uniformity and adhesion to the transparent conducting glass electrode. The ratio of the nitric acid solution to the colloidal P25TiO₂ powder is a critical factor for the cell performance. If the ratio exceeds a certain threshold value the resulting film becomes too thick and has a tendency to peel off. On the other hand, a low ratio reduces appreciably the efficiency of light absorption.

Soxhlet Extractor is used for the extraction of dyes solution from 80 g of Bahraini Henna and 84 g of Yemeni Henna (powder) where 100 mL of methanol is used in each extraction process. Different concentrations have been prepared from the collected extract. The light harvesting efficiency (LHE) for each concentration has been calculated from the measured absorbance using dual-beam spectrophotometer. The electrical characteristic and parameters of the assembled solar cells were determined and presented in Table 1.

Table 1 

Measured and calculated parameters of assembled Bahraini and Yemeni Henna solar cells.

Doctor blade method was employed by depositing the TiO₂ suspension uniformly on a cleaned (rinsed with ethanol) electrode plate. The TiO₂ film was allowed to dry for few minutes and then annealed at approximately 450°C (in a well ventilated zone) for about 15 minutes to form a porous, large surface area TiO₂ film. The film must be allowed to cool down slowly to room temperature. This is a necessary condition to remove thermal stresses and avoid cracking of the glass or peeling off the TiO₂ film. Investigation of the formation of nanoporous TiO₂ film was confirmed by scanning electron microscope SEM. After that, theTiO₂ nanocrystalline layer was stained with the dye for approximately a day and then washed with distilled water and ethanol to ensure the absence of water in the film after removal of the residual dye. The counterelectrode is coated with graphite that acts as a catalyst in redoxing the dye. Both the photo- and the counterelectrodes are clamped together and drops of electrolyte are applied to fill the clamped cell. The electrolyte used is iodide electrolyte (0.5 M potassium iodide mixed with 0.05 M iodine in water-free ethylene glycol) containing a redox couple (traditionally the iodide/triiodide I⁻/I₃₋ couple). The measurements of open-circuit voltage and short-circuit current have been performed under direct sun illumination at noon time. Neither UV or IR cutoff filters nor antireflection (AR) coatings on the photoelectrode have been used.

4. Results and Discussion

Bahraini and Yemeni Henna extracts have been prepared at different concentrations. Optical measurements show that these extracts posses interesting optical properties and demonstrate the effectiveness of sensitizing the nanocrystalline TiO₂ layer. Both the Bahraini and Yemeni Henna extract dye solutions of different concentrations have been optically characterized by measuring the absorbance using the dual-beam UV-VIS spectrophotometer (Shimadzu, model UV-3101). Typical examples of light harvesting efficiency (LHE = 1−10^−𝐴, where 𝐴 is the absorbance) measurements are shown in Figure 2. Bahraini Henna extracts were found to exhibit higher LHE than those of corresponding concentration of Yemeni Henna. The high light harvesting efficiency of Henna extracts in the UV, visible, and near IR region of the spectrum suggests that they are promising natural dye sensitizer for single-junction DSSC. The color of the highly concentrated extract (80 g in 100 mL of methanol) is dark green, and when it is diluted the degree of the green color enhanced. Moreover, as the concentration of Henna extract is increased the film texture becomes more greasy and sticky. The color of the highly concentrated Yemeni Henna extract (84 g in 100 mL of methanol) is dark green golden, and when it is diluted the golden degree enhanced.

(a)

(b)

Figure 2 

Light harvesting efficiency versus wavelength for Bahraini Henna extracts and Yemeni Henna extract of different concentrations. Data are given in g per 100 mL of methanol.

The TiO₂ films were first annealed, and their nanostructure properties were then examined by SEM measurements. Figure 3 shows the SEM measurements carried out on the TiO₂ layer. It shows that after sintering the TiO₂ film becomes nanocrystalline. X-ray diffraction measurements on our samples confirmed the formation of nanocrystalline TiO₂ particles of sizes less than 50 nm [17]. The formation of nanostructured TiO₂ film is greatly affected by TiO₂ suspension preparation procedures and the annealing temperature. It was found that a sintered TiO₂ film at temperatures lower than the recommended 450°C resulted in solar cells that generate unnoticeable electric current even in the μA domain. Moreover, TiO₂ film degradation in this case is fast and cracks form after a short period of time when the cell is exposed to illumination.

Figure 3 

SEM images of TiO2 film after annealing.

Figure 4 shows the I-V characteristics of NDSSC sensitized with Yemeni Henna and Bahraini Henna. Because the 80 g Bahraini Henna and 84 g Yemeni Henna extracts are highly concentrated and posses sticky texture, the electron injection efficiency into the nanocrystalline TiO₂ film deteriorated and hence lower values of the measured photocurrent are obtained. In other words, the use of highly concentrated extract introduces series resistance 𝑅_𝑠 in the solar cell mainly due to the path traversed by the photogenerated electrons. At lower concentrations where Henna extract viscosity is similar to that of the solvent, the cell I-V characteristics show a better operation, a much lower series resistance effect, and a higher efficiency. Dye concentration has remarkable effect on the magnitude of collected photocurrent. Highly diluted extracts reduce the magnitudes of photocurrents and cell efficiencies.

Figure 4 

I-V characteristic of Henna extracts cells.

Table 1 illustrates the electrical properties of Henna photovoltaic cells. Due to light reflection and absorption by the conductive photoelectrode and the scattering nature of the nanostructured TiO₂, the measured transmittance of the photoelectrode shows that only an average of 10% of the solar spectrum (AM 1.5) is useful. Despite the variation of Bahraini Henna extract concentration the cells produced almost the same open-circuit voltage 𝑉_OC. However, the short-circuit current 𝐼_sc reflected variations due to Henna extract concentration. The Yemeni Henna extracts solar cells produced almost the same level of the short circuit current, but open circuit voltage varies with the concentration. It turns out that highly concentrated Henna extracts do not exhibit ideal I-V characteristics even though they possesses 100% light harvesting efficiency in the UV and visible parts of the electromagnetic spectrum.

Investigations showed that there are many factors affecting the performance of the natural-dye-sensitized photovoltaic cells. Dye structure must own several carbonyl (C=O) or hydroxyl (−OH) groups to enable dye molecule chelating to the Ti (IV) sites on the TiO₂ surface [20]. For example, extracted dye from California blackberries (Rubus ursinus) has been found to be an excellent fast-staining dye for sensitization. on the other hand, dyes extracted from strawberries lack such complexing capability and hence not suggested to be used as natural dye sensitizer in NDSSCs [10, 21]. Since not all photons scattered by or transmitted through the nanocrystalline TiO₂ layer get absorbed by a monolayer of chelating Henna dyes molecules, the incorporation of energy relay dyes might help enhancing the light harvesting efficiency. A remarkable enhancement in absorption spectral bandwidth and 26% increase in power conversion efficiency have been accomplished with some sensitizers after energy relay dyes have been added [22].

The fact of the dependence of both hole transport and collection efficiency on the dye-cation reduction and I⁻/I₃₋ redox efficiency at counterelectrodes is to be taken into account [23], and the redoxing electrolyte must be chosen such that the reduction of I₃₋ ions by injection of electrons is fast and efficient (see Figure 1). As a matter of fact, besides limiting cell stability due to evaporation, liquid electrolyte inhibits fabrication of multicell modules, since module manufacturing requires cells be connected electrically yet separated chemically [24, 25]. Another important factor playing major role in enhancing the cell's efficiency is the thickness of the nanostructured TiO₂ layer which must be less than 20 μm to ensure diffusion length of the photoelectrons be greater than that of the nanocrystalline TiO₂ layer. To increase photogenerated electron diffusion length, studies suggest replacing the nanoparticles film with an array of single-crystalline nanowires or nanosheets in which electrons transport increase by several orders of magnitude [6, 26–28]. Significant successes have been achieved in improving the photoconversion efficiency of solar cells based on CdSe quantum dot light harvesters supported with carbon nanotube network [29, 30]. This is accomplished by incorporating carbon nanotubes network in the nanostructured TiO₂ film and accordingly assisting charge transport process. Consequently, an appreciable improvement in the photoconversion efficiency of the DSSC was obtained.

Finally, dark current in DSSC is mainly due to the loss of the injected electron from nanostructured TiO₂ to I₃₋ (the hole carrier in solution electrolyte). Reduction of dark current enhances the open-circuit voltage of the cell, and one successful way to suppress dark current is to use coadsorbates on the nanostructured TiO₂ surface. Therefore, to achieve further improvement of the performance of DSSC cell many researchers have suggested replacing the liquid electrolyte with a solid state one that provides a better sealing of the cell [31, 32] and fabrication of large area modules with efficiency above 12%. The success of Sony in 2010 to present a DSSC module with efficiency close to 10% is one of the driving achievements toward fabrication of outdoor larger area modules that can be integrated in green buildings. Also, as presented by Dyesol, development of flexible transparent substrate (polymer based) will transform DSSCs industry toward mass production and commercialization of single-cell or minimodules for indoor and personal appliances.

5. Conclusion

In this work we studied the optical and photovoltaic properties of sensitized nanostructured TiO₂ with two types of Henna (Lawsonia inermis L.) extracts. Henna dyes were extracted using Soxhlet extractor technique. It was shown that both types of Henna extracts exhibit high level of absorbance in the UV, visible, and near-infrared region of the solar spectrum. However, a higher light harvesting efficiency of Bahraini Henna, as compared to Yemeni Henna, was obtained. This fact is reflected in a higher photocurrent and open-circuit voltage and consequently a higher efficiency of the Bahraini Henna based photovoltaic cell. Extract concentration was found to influence remarkably the magnitude of the collected photocurrent. High concentration of Henna extract introduces a series resistance that ultimately reduces the collected photocurrent. On the other hand, diluted extracts reduce the magnitude of the photocurrent and cell efficiency. Optimization of the preparation conditions of the nanocrystalline TiO₂ layer and the choice of the redoxing electrolyte are found to be determining factors for the performance of the dye-sensitized photovoltaic cell.

Acknowledgments

The authors are greatly indebted to the University of Bahrain for financial support. Also, they would like to express their thanks to Dr. Mohammad S. Hussain (National Nanotechnology Center King Abdulaziz City for Science and Technology (KACST)) for providing the SEM measurements.