Optical Properties of Lawsone dye doped with Zinc Oxide for the Fabrication of Dye Sensitized Solar Cell

Due to their availability of basic ingredients and affordable cost, natural dyes are important in many applications. One of such natural dyes, Lawsone which is naturally extracted from henna leaves. The Zinc Oxide (ZnO) was intermixed with lawsone dye to their matching of energy levels with Titanium dioxide (TiO2) enables efficient charge transfer phenomenon. The optical study illustrates Lawsone dye - Zinc Oxide (Ld-ZnO) composite to utilize in energy conversion application. The FTIR spectra demonstrates the presence of Zn-O, C-O, C=O and hydroxyl groups are evident from near 500, 1633 and around 3250 cm−1 respectively. The UV-Visible spectra reveal maximum absorbance around narrow and broad peak at 368 nm for Lawsone dye (Ld) and Ld-ZnO composite respectively. The lawsone dye has better emission property that causes recombination of charge carriers reduces the performance of Dye Sensitized Solar Cells (DSSCs). The DSSCs based on lawsone dye (Ld) and Ld-ZnO based material exhibits PCE of 0.12% and 0.17% respectively. This optical study suggests natural extraction of lawsone dye from henna leaves with suitable semi-conducting material was promising candidate to improve performance of DSSCs.


Introduction
The need for clean and renewable energy sources has intensified recently as the globe faces against the terrible effects of climate change and the depletion of finite fossil fuels.Among the many promising renewable energy technologies, dye-sensitized solar cells (DSSCs) have emerged as a viable choice for solar energy capture due to its affordability, environmental friendliness and efficiency [1].Plants carry out photosynthesis, which is replicated by the distinct photo electrochemical mechanism employed by DSSCs [2].This creative technique converts solar energy into electricity by using pigments that absorb light.The Lawsonia inermis plant's leaves are the source of the Lawsone dye, which has been utilized for generations in a variety of innovative and cultural endeavors.Given its historical relevance and artistic appeal, it is a compelling candidate for the field of renewable energy.However, because of its inherent spectrum limits and instability when exposed to UV light, the direct use of henna in DSSCs has demonstrated low efficiency [3].
Charge recombination is the process by which photo generated electrons and holes recombine before contributing to the electrical output is one of the primary challenges in DSSCs.The interfacial recombination between TiO2 and lawsone dye based DSSCs can result in considerable electron-hole recombination losses, resulting in decreased total cell efficiency.This problem can be lessened through establishing appropriate interfacial layers.It is well known that lawsone dye is sensitive to the environment, particularly to extended UV light exposure.Because of its sensitivity, the dye may eventually deteriorate, reducing light absorption and creating instability in the performance of DSSCs [4,5].Researchers have turned to nano-engineering techniques and coupled lawsone with semiconducting metal oxides such as zinc oxide (ZnO) to get around these issues and improve DSSC performance.A promising area of renewable energy technology is the integration of lawsone dye with zinc oxide in dye-sensitized solar cells [6].
This novel approach aims to study the unique optical properties of both lawsone dye and ZnO to improve the photo-electro-chemical efficiency of DSSCs.The exploration of charge transfer phenomenon in this hybrid material since the energy level of TiO2 NPs matching with Zinc oxide.The charges enable this to hold tremendous potential in mechanism of DSSCs.The potential synergy of lawsone and ZnO offers an approach to address the challenges faced by traditional silicon-based solar cells.This study will evolve into the fundamental principles, properties and potential applications of Lawsone-ZnO dye as photo-sensitizer in DSSCs, laying the groundwork for future advancements in solar energy conversion and paving the way for a more sustainable and environmentally conscious energy landscape.

Materials
The Henna leaves are collected from university campus garden.The ITO glass substrates, Zinc Oxide (ZnO), Titanium di-oxide (TiO2), Graphite was purchased from Sigma Aldrich.The ethylene glycol, acetone, isopropyl alcohol and distilled water were used as solvents.

Synthesis of Ld-ZnO (Lawsone dye-Zinc Oxide) Composite
The synthesis of Lawsone dye -Zinc Oxide (Ld-ZnO) prepared through natural dye extraction method.The small pieces of Henna leaves which is washed and dried.They were boiled at 120°C and dye solution was extracted through filtration process.The heat treatment of dye permits much more light absorption to use as dye sensitizer in DSSCs.The 20ml of Lawsone dye solution and 20 ml of aqueous Zinc oxide solution was intermixed through probe sonication for 30 minutes.The Lawsone dye -Zinc oxide (Ld-ZnO) composite synthesis as shown in scheme 1.

Fabrication of Dye sensitized solar cell
The ITO glass substrates were washed through ultra-sonication with distilled water, acetone and isopropyl alcohol each.The synthesized Titanium di-oxide (TiO2) NPs was deposited on ITO substrates through doctor-blading method that can dried at 350° C [7].The TiO2 coated glass substrates are dipped in solutions of lawsone dye with zinc oxide and Lawsone dye are separately for 30 hours in dark environment.The graphite coated on ITO that can acts as counter electrode.Both ITO substrates arrange one over other to fabricate DSSC for Lawsone dye with ZnO and lawsone dye by placing few drops of iodide electrolyte between them respectively [8].

FTIR Analysis
The intermolecular interactions are studied by Fourier Transform Infrared Spectrometer (ATR Alpha Bruker) model.It is an important tool to identify chemical bonding and stretching vibrational group measurements carried out in the range 450-4000cm -1 .The broad absorption peak appeared at around 3250 cm −1 was assigned to stretching vibration of hydroxyl (-OH) group and it is due to also the presence of water content in dye [9].An auxochrome is a functional group of molecules that connects to a chromophore that alters the wavelength of that the chromophore's absorption of light.The skeleton C=C stretching vibrations of an aromatic ring revealed from the peak at 1636 cm -1 [10].The magnified FTIR spectrum shows the presence of ZnO with lawsone dye alters the energy levels and also reveals together well blended in the composite.

Optical analysis
The light absorption properties of synthesized lawsone dye and Ld-ZnO composite are studied by using Perkin Elmer lambda 35 UV-Visible Spectrometer.As per the structure and FTIR analysis, the lawsone dye and its composite with zinc oxide consists of auxochrome functional group such as hydroxyl (-OH) present in it.These groups can cause green color of dye that enables broad absorption of light [11].The broad peak near 250 nm shows solvent content present in the dye [12].The more absorption of light from Ld-ZnO composite can promise to utilize this material in energy conversion applications.

Photo-voltaic study of Lawsone dye based DSSC
The figure 3 (a) reveals J-V characteristics was measured using class AAA Solar simulator.The above optical analysis evident that Ld and Ld-ZnO composite has efficient optical properties to use as sensitizer in fabricated DSSC.The light absorption from Lawsone dye and its composite can enable charge transport through iodide electrolyte, Titanium dioxide and graphite coated ITO substrate electrodes.Due to the device structure aligned with well blended energy levels of TiO2 and ZnO, the photon and electron interaction causes increase in photo-current in Ld-ZnO composite due to efficient charge transport takes place between them [13].The solar parameters of DSSC based on lawsone and its composite as reported in Table 1.The Ld-ZnO composite based DSSC has best performance due to its charge transport phenomenon of ZnO and TiO2 upon irradiation causes the high photo-current density.The graphite as counter have lower efficiency compared to other cathode can enhance efficiency to nearly 1% [14].The Ld-ZnO material is promising candidate to utilize as transport layer in energy conversion applications.

Conclusion
Lawsone is one of the pigments that is naturally derived from henna through the process of natural extraction.Lawsone and Zinc oxide were combined because of their similar energy levels of ZnO TiO2, which provide effective optical properties.C=C, C-C, and -OH interactions appear in the FTIR spectrum and are assigned at around 500 cm -1 , 1633 cm -1 , and approximately 3300 cm -1 respectively.The Lawsone dye (Ld) and Ld-ZnO composite exhibit maximal absorbance at a narrow and broad peak at 368 nm respectively according to the UV-visible spectra.Dye-sensitized solar cells (DSSCs) perform less efficiency due to weak binding between lawsone dye with TiO2 electrode.Lawsone dye (Ld) and Ld-ZnO-based materials used in the DSSCs show PCE values of 0.12% and 0.17% respectively.According to this optical investigation, a viable option to enhance the performance of DSSCs is the natural extraction of lawsone dye from henna plants using an appropriate semi-conducting material.

Figure 3 :
Figure 3: a) J-V characteristics and b) Mechanism of fabricated DSSCs.