Preliminary study: Potency metal complexes with UV-active ligands as dye sensitizer in Dye Sensitized Solar Cells

The use of metal complexes as a dye sensitizer in DSSC (Dye-Sensitized Solar Cells) provides many promising opportunities. Complex compounds with ligands that actively absorb UV rays have the potential to be applied in technology solar cells in areas rich in UV rays such as Indonesia. This research aim is to investigate the potential of several complex compounds with UV active ligands and apply them as dye sensitizer in DSSC. Ligand phenanthroline (phen), and morin will be synthesized with several transition metals. The synthesized metal complexes were characterized by spectrophotometer UV-VIS (for all metal complexes) and spectrophotometer FTIR (for morin complexes). The performance of metal complexes as dye sensitizer will be analyzed from the current and voltage obtained and in this research was limited to the phenanthroline complex compound because the research was still in the ongoing process. The highest current produced by [Ni(phen)3]2+ with 0.452 mA while the highest voltage produced by complex [Fe(phen)3]2+ with 711 mV.


Introduction
Metal complexes or complex compounds, or coordination compounds, are chemical compound consisting a central atom (usually a metal ion) that bonded with a ligand in covalent coordination bonding [1].Different character for each complex compound including molecular geometry, ligandexchange phenomenon, photophysical reactions, redox and catalytic properties, the unique electronics and stereochemistry make complex compound be the best candidate for modern molecular electronic technology applications such as dye sensitizer materials in DSSC (Dye Sensitized Solar Cells) [2], [3].
The utilization of metal complexes as dye sensitizer promises many opportunities due to the ease in modification of the structure and functional groups of the molecules.Performance DSSC is highly depend on the structure and functional features of the dye sensitizer molecule, especially the absorption spectra, stability, molecular orbital energy levels, and how easy the dye sensitizer bound to the semiconductor surface [4].Up to the present, the development of dye sensitizer materials has focused on their ability to absorb all sunlight (panchromatic), and still rare focused on their ability to absorb UV light.Whereas the geographical location of Indonesia, which is in the equatorial line, makes it get more UV light than other country.Therefore, this research will focus on the utilization metal complexes with good ability in absorbing UV rays.
In this research, metal complexes will be synthesized by utilizing ligands that have a good ability to absorb UV light and transition metal as a central atom.Some natural dyes have selective properties to harvest UV photons, and when viewed from their structure, they can act as ligands.However, these compounds have not been widely studied and applied as dye sensitizers.In this study, we introduce phenanthroline (phen) and morin compounds as ligands.These compounds are organic compounds that have many phi electrons (π electrons), conjugated double bonds, and also non-bonding electrons so that they have UV active properties [5].Phenanthroline is an organic compound that can absorb UV light at wavelengths around 210-260 nm (UV B), mainly because its aromatic ring contains pi electrons that can absorb UV energy [6].Morin is a flavonoid compound found in plants.Morin also has UV absorption abilities.The UV absorption wavelength of morin generally ranges from 330 -365 nm (UV A) [7][8].
The synthesized metal complexes were characterized using spectrophotometer UV VIS to describe the electronic transition process and spectrophotometer FTIR to study the functional groups and metalligand bonds of the metal complexes.In this study, all metal complexes have been characterized using spectrophotometer UV VIS.However, only morin complexes has been characterized using spectrophotometer FTIR, and only phenanthroline complexes has been undergo photovoltaic assay because the research is still in the finishing process.The photovoltaic assay was carried out by measuring the current and voltage generated by DSSC cells that use synthesized metal complexes as dye sensitizers.

Synthesis metal complexes with UV-active ligands
2.1.1.Synthesis metal complexes with ligand phenanthroline.Metal-phen complexes were synthesized using a mole ratio metal to ligand = 1:3.10 mL ion metal solution [Fe(II), Co(II), Ni(II)] 10 -2 M was reacted with 30 mL phenanthroline 10 -2 M [9].Furthermore, the mixture was refluxed at 76 ˚C for two hours until the volume was one-third from the initial volume.Then, the solution was transferred into a glass beaker and allowed to stand overnight at room temperature until a precipitate formed.The precipitate formed was then filtered, washed with hot ethanol, and then dried [10].
2.1.2.Synthesis metal complexes with ligand morin.Metal-morin complexes were synthesized using a mole ratio metal to ligand = 1:3 (for Fe(III) and Co(II)) and 1:2 (for Zn(II)).Metal salt and morin solid was weighed based on the mole ratio and each dissolved by ethanol.After that, the metal ion solution was added to the morin solution so that the color of the solution changed.Then, the mixture was refluxed at 70 ˚C for two hours [11].The mixture was allowed to stand for 24 hours, then filtered using a Buchner funnel and recrystallized several times with ethanol to produce yellow crystals.After that, the metal complexes were dried at room temperature [12].

Characterization of synthesized metal complexes using spectrophotometer UV-VIS
All synthesized metal complexes were measured using a spectrophotometer UV-VIS at wavelengths of 200-800 nm to study their electronic transition profiles.

Characterization of synthesized metal complexes using spectrophotometer FTIR
The synthesized metal complexes were measured using spectrophotometer FTIR at wavenumber 4000-250 cm -1 to determine the functional groups and bonds formed between metals and ligands.

Photovoltaic assay
2.4.1.Coating process of TiO2 solution on FTO glass.FTO glass measuring 2.5 cm x 2.5 cm with a thickness of 1 mm was soaked for 24 hours with ethanol to remove impurities.Then it was weighed to determine the mass of the FTO glass before coating.Furthermore, the TiO2 coating area was formed with the help of tape on the side of the FTO glass until an area of 2 × 2 cm was formed.On the other hand, as much as 7 grams of anatase TiO2 powder were dissolved in 30 mL of ethanol and stirred until a paste was formed.Then the paste was added to 10 drops of Triton X-100 and stirred for 1 hour.Then the clean FTO glass was dipped in TiO2 paste evenly.Then it was lifted and dried at 155 ˚C for 30 minutes and then dipped back into the TiO2 paste.This drying is intended so that TiO2 paste can stick more optimally.This immersion is done several times using the same method in order to obtain the same TiO2 layer thickness results.The immersion is also called the dip coating method.Furthermore, FTO glass-coated TiO2 was calcined at 450 ˚C for 30 minutes.Before and after the TiO2 coating FTO glass is weighed to control the homogeneity of the FTO glass coating.

Preparation electrolyte solution.
The electrolyte solution was prepared by mixing KI with I2 solution.A total of 0.83 g of KI was dissolved in 1 mL of distilled water and stirred.After KI dissolved, 0.1270 g of I2 was added little by little until I2 dissolved completely and add distilled water until the volume becomes 10 mL [13].
2.4.3.Preparation of a working electrode.The working electrode was made by immersing FTO glass coated with TiO2 in a 1000 ppm metal complexes for 24 hours, and then the FTO glass was taken and dried at room temperature.Then the FTO glass is stored in a dark box to avoid scratches and sunlight [14][15].
2.4.4.Preparation of a counter electrode.FTO glass was coated with graphite using a graphite pencil by shading the surface of the glass evenly, and then it was coated with carbon from candle light to form a carbon layer on the surface of the FTO glass.
2.4.5.Assembling of solar cell device.The DSSC that has been assembled in the form of a sandwich between working electrode and counter electrode and the series was connected to a multimeter.A counter electrode was connected to the positive pole (cathode) and the working electrode is connected to the negative pole (anode).The DSSC that has been assembled is then exposed to direct sunlight, with the working electrode at the top.Then the current and voltage were measured using a multimeter.

The result of synthesis metal complexes with UV-active ligands
In this study, complex compounds or metal complexes that selectively absorb UV light have been synthesized by reacting transition metal ions as central atoms and UV-active compounds as ligands.The UV-active ligands used were phenanthroline (phen) and morin.In phen ligand, metal complexes were synthesized using Fe(II), Co(II) and Ni(II) metal ions with a metal to ligand ratio of 1:3.On the other hand, for morin ligand, metal complexes were synthesized using Fe(III) and Co(II) metal ions with a metal to ligand ratio of 1:3, and Zn(II) metal ions with a metal to ligand ratio of 1:2.Phen and morin are bidentate ligands and capable donating two pairs of free electrons to the central atom to form octahedral complexes (Fe, Co, and Ni) and square planar complexes (Zn) [16]- [18].The postulates offered (based on literature [6], [19] [20]) for the structure and metal-ligand bonding in the complex compound are shown in Figure 1.Whereas the solid of synthesized metal complexes is shown in Figure 2. The synthesized metal complexes were characterized using spectrophotometer UV-VIS and spectrophotometer FTIR.UV-VIS spectrophotometer is used to describe the electronic transition process that occurs in metal complexes, while FTIR spectrophotometer is used to study the functional groups and metal-ligand bonds of metal complexes.The results of characterization of metal complexes using UV-VIS spectrophotometer are shown in Figure 3 and Figure 4, while the results of characterization of metal complexes using FTIR spectrophotometer are presented in Figure 5 and Table 1.

Figure 3. The absorption spectra of dye metal-phen complexes
Based on Figure 3, it can be seen that the metal-phen complexes produce a different absorption band when compared to the absorption band of the phenanthroline ligand.This indicates that the metal-phen complexes have been successfully formed.In the phen ligand shows two maximum absorption bands at wavelengths of 230 and 264 nm.This proves that the phen ligand is UV-active and able to absorb UV B light with a wavelength range of 280-325 nm and UV C light with a wavelength range of 100-280 nm [6].After being complexed with Fe(II) to form the [Fe(phen)3] 2+ , there is a very significant wavelength shifted at 510 nm, which indicates the occurrence of the MLCT (Metal to Ligand Charge Transfer) phenomenon [20].The existence of MLCT phenomenon is very beneficial for dye sensitizer application because it can increase the ability of the complex compound in terms of light harvesting from the sun.However, with no absorption in the UV region, the [Fe(phen)3] 2+ complex compound has less potential if applied as a UV-selective dye sensitizer in DSSC.The same thing is also experienced by the [Co(phen)3] 2+ complex compound, which does not show an absorption peak in the UV region, so this complex compound also has less potential to be applied as a UV-selective dye sensitizer in DSSC.On the other hand, the [Ni(phen)3] 2+ complex compound shows an absorption peak at a wavelength of 341 nm.This shows that the [Ni(phen)3] 2+ complex compound has the potential to be applied as a UVselective dye sensitizer in DSSC because it can absorb UV-A light with a wavelength range of 315-400 nm.  Figure 4 shows that the morin ligand and the [Zn(morin)2] 2+ complex compound show similar absorption bands, namely the appearance of absorption bands at 269 nm and 400 nm wavelengths.This shows that the [Zn(morin)2] 2+ complex compound has the potential to be applied as a UV selective dye sensitizer because it can absorb UV C light with a wavelength range of 100-280 nm and UV A light with a wavelength range of 315-400 nm.The [Co(morin)3] 2+ complex compound also provides absorption bands at two peak wavelengths, namely 262 nm and 402 nm.This indicates that the [Co(morin)3] 2+ complex compound also has the ability to act as a UV absorber.The [Fe(morin)3] 3+ complex compound experiences a shift in the bathochromic wavelength because the morin ligand has been substituted with Fe(III) metal ions [21].This complex compound shows an absorption peak at a wavelength of 295 nm and is able to absorb UV B light with a wavelength range of 280-325 nm [22].
In the next step, the synthesized solids were characterized using a FTIR spectrophotometer to study the functional groups and metal-ligand bonds of the complex compounds formed.The results of the FTIR characterization of metal-morin complexes are presented in Figure 5 and Table 1.Based on the results of FTIR characterization, the formation of metal-morin complexes can be seen in the difference in spectra between morin ligands and complex compounds.The intensity of free carbonyl (C=O) in the morin ligand is higher than when it is bound to metal.This indicates that there is a bond formed between the carbonyl group on the ligand and the metal atom.In addition, the O-H absorption band on the morin ligand is also higher than that on the morin-metal complex compound.This indicates a bond formed between the hydroxyl group on the morin ligand and the metal atom.In the [Fe(morin)3] 3+ complex compound, the metal-ligand bond appears at wavelengths of 308.61 cm -1 and 331.76 cm -1 .In the [Co(morin)3] 2+ complex compound, the metal-ligand bond appears at waves 339.47 cm -1 and 455.20 cm - 1 .In the [Zn(morin)3] 3+ complex compound, metal-ligand bonds appear at wavelengths of 300.9 cm -1 and 354.9 cm -1 .The results of this characterization strongly support the postulates of the structure and metalligand bonds in metal-morin complex compounds where metals bind to ligands in carbonyl and hydroxyl groups (Figure 1).  Figure 6 and Table 2 show that the highest current produced by each DSSC cell is strongly influenced by the incoming light from sunlight measured on the light meter.The higher the incoming light, the higher the current value produced.This is because the current in the DSSC cell is strongly influenced by the photons absorbed by the dye sensitizer in the DSSC cell [14].When compared among the three metal-phen complex compounds, the highest current is produced by the [Ni(phen)3] 2+ complex compound, which is 0.452 mA.This is because only the [Ni(phen)3] 2+ complex compound has the ability to absorb UV-A light with a wavelength range of 315-400 nm.This phenomenon proves that the most visible light in Indonesian sunlight is UV light, so the best DSSC technology is to utilize UV-selective dye sensitizers.When entering days 14, 21, and 28, the current generated by all DSSC cells has decreased.This is because the KI3 electrolyte solution used has begun to evaporate.
The next DSSC cell test is to measure the voltage against the day.The results of the voltage measurement comparison can be shown in Figure 7 and Table 3.The measurement data shows that, as with the current measurement, the higher the intensity of sunlight measured on the light meter, the higher the voltage value produced; namely, on the 6 th day, the voltage produced by phen was 558 mV, the complex compound [Fe(phen)3] 2+ was 711 mV, the complex compound [Co(phen)3] 2+ was 677 mV, and the complex compound [Ni(phen)3] 2+ was 683 mV.Voltage in DSSC is formed as the electrons flow from the photoanode to the counter electrode through the external circuit.This voltage is the potential difference between the oxidized dye molecules in the photoanode and the reduced iodine ions in the electrolyte.The voltage drives the flow of electrons, creating an electric current in the external circuit.When compared to the three complex compounds, the [Fe(phen)3] 2+ complex compound produces the highest voltage due to factors such as the very significant difference between the bandgap energy of TiO2 and the [Fe(phen)3] 2+ complex compound [26][27].

Conclusion
This research proves that complex compounds with UV active ligands are proven to be able to absorb UV A, UV B, or UV C rays and have the potential to be utilized as dye sensitizers in DSSC and applied to UV-rich areas such as Indonesia.The [Ni(phen)3] 2+ complex compound proved to be able to produce the highest current of 0.452 mA, while the [Fe(phen)3] 2+ complex compound proved to be able to produce the highest voltage of 711 mV.

Figure 4 .
Figure 4.The absorption spectra of dye metal-morin complexes

Table 1 .
Wavenumber detail of functional group of metal-morin complexes In this research, current and voltage measurements of DSSC cells were carried out for 7 consecutive days, and measurements were continued once a week on the 14 th , 21 st , and 28 th days.The measurements were carried out in outdoors under direct sunlight at 09.00-10.00WIB.In this measurement process, four DSSC cells with different working electrode compositions were measured under the same conditions.The four working electrodes are working electrodes with dye sensitizer ligand phen, complex compound [Fe(phen)3] 2+ , [Co(phen)3] 2+ , and [Ni(phen)3] 2+ .The results of the current measurement comparison can be shown in Figure6and Table2.Graph of current measurement against day on DSSC cells 8 Figure 6.

Table 2 .
Current measurement results against day in DSSC cells