Effect of the use of inorganic-based photoluminescent materials in solar energy devices application

The dynamic processes that take place in life require energy. The main convenient source of energy even in living tissue are hydrocarbons. This is due to the high-carbon based energy source that eventually converts fossil fuels. The preferred fuels as the primarily and industrial fuel contribute to the observed negative effects which emit pollutants to our environments and result to global warming. To mitigate such energy dependence, solar radiation has been exploited to produce clean energy through the use of photovoltaic cells. Nevertheless, natural radiation varies depending on the season of the year. This study investigated the properties of inorganic materials that support fluorescence after the source of light has been withdrawn. The minerals Sb/Ca/Mn/Ag were prepared hydrothermally to cultivate luminescent properties. The resulting products were characterized using Fourier Transformed Infrared (FTIR) spectroscopy and then applied to extend radiation in a photoactive material of a photocell. Optimal values of the prepared luminescent substance were established to produce the best generated potential in the fabricated solar cells when the natural radiation was withdrawn. Established receptive layer composites (KI/I 2 : CX) were inserted into a molding dice on top of the photosensitive layer and joined together by pressing. The ensued inorganic luminescent cells optical characteristics were observed under UV radiation (320 - 400 nm) wavelength which produced sequential varying colorations from brown, light blue, blue to red before diminishing gradually as the incident radiation was removed. The open circuit potential voltage (V OC ) and current density (J SC ) generated parameters were observed. The (0.45, 0.4, and 0.086) V residue potential resulted due to shifting of IR by SbNO3, Sb-PO4, Sb-CO3 and Sb-O luminescent materials. The dopants had promising properties for radiation delay in photo voltaic devices application.


Introduction
The excess use of fossils has resulted in global warming that has resulted leading to erratic climate change with erratic rainfall patterns and rapid evaporation of water from the soil. This makes life uncomfortable and a challenge to grow food crops or any other vegetation in most areas. To mitigate this, other options have to be exploited such as the use of solar radiation. This can be achieved by the use of renewable energy. This study reports on the fabrication of a cost effective and environmentally friendly solar cell using titanium dioxide to provide photovoltaic solar energy as an alternative energy source.
Sequential progression of inorganic materials has actualized into notable evolution in photovoltaics. Their high performance and low-cost solution-based processes used for their preparation technology attest to their projected future utilization. Solar radiation is not constant at all times of the day and also differs depending on the season of the year. The net result is reduction in potential energy caused by such changes. Spectral losses in solar energy devices contribute to low responses and their achievable efficiencies. Direct dependence of normal solar irradiation using the existing solar dependent devices is thus affected. To increase their efficiency, spectral radiation need to be enhanced by use of materials which have potential to change the photo activity of some specific materials by altering their bang gaps. This happens due to introduction of defects in the material and shifts incident radiation, at a certain wavelength and emitted at a longer wavelength. That property was employed to extend supply of energy for a longer time constant after the source of natural radiation is withdrawn [1]. Reports by [2] noted that antimony and manganese doped calcium phosphors has the ability to transform ultraviolet radiations to delayed emission for a prolonged length of time. This enhances electroluminescence from milliseconds to several hours [2]. Calcium compounds which glow in minimum radiation forms double-activated crystal structures that accommodate Sb 3+ ions and Mn 2+ ions into their apatite lattice sites [2]. The radiation absorbed by Sb 3+ ions in the crystal lattice is partly transferred to Mn 2+ ions leading to high electroluminescence with a narrow band gap [1]. When illuminated with UV radiation, the impurities introduced energy levels which downshifted solar radiation in the energy gap between valance and conduction bands [3]. That downshifted radiation stimulated electrons towards the conduction band which were trapped into the d-donor levels for a while until they escaped to the valence band thus emitting lower energy photons [4].
Reports by [5,6] indicated that the absorbed energy is transferred to the neighboring luminescent impurities and emitted after a specific time of delay. According to [7], calcium, antimony, manganese and silver persistently glow after withdrawal of an incident radiation and thus find a large scientific application that require afterglow. Calcium and antimony compounds are widely applied to manufacture of battery anodes because they exhibit superior cycling stability with high charge retention capacity of 87.7 % [8]. A report by [3] noted that mixtures of the phosphor material at varying ratios yielded different afterglow durations. [3] noted that the inorganic impurities with antimony and calcium compound introduced defects which alter the band gaps of the host materials [9]. This study considered the suitably of preparing antimony and calcium to accommodate manganese and silver ions to enhance radiation delay for an extended period after solar radiation diminishes.

The envisioned problem
The uneven irradiance resulted from natural sources experienced by solar radiation devices limits their expected performance in their times of operations [10]. Enhancing their capacity with varied band gap expand their absorption range bandwidth. That increase the length their desired optical radiation is made available to the host material due to fluorescence after the incident natural radiation is withdrawn and lengthen the duration needed for operation

Material and methods
The study applied experimental processes to analyze the organic photo luminescent material and the doped host material.

Chemicals and reagents
To obtain the fluorescent properties of the Sb/Mn/Ag doped calcium hydroxyapatite, the inorganic 0.22 g Sb, 12 g [Ca (OH)2], 0.2 g (MnO4) and 0.04 g of AgNO3 were weighed into a (250 ml) beaker and 20 ml of dilute phosphoric acid (H3PO4) was added drop wise. To make Ag NPS, a solution of AgNO3 dispersed in triethylamine was treated with a solution hydrazine. The solution was thoroughly stirred using a stirring rod as 10 ml of aqueous ammonium solution was added drop wise adjusting the pH of the system to 8. The stirring continued until a homogeneous phas1e was formed. The mixture was then placed into an oven at a temperature of 140 0 C for 48 hours. The resultant was allowed to cool to room temperature then washed with acetone to remove ionic impurities to make it ready for use.

Instrumentation
All the material used in this study were characterized using a Shimadzu FTIR 2014 spectrometer. The schematic presentation of the equipment used is as shown in Figure1.

Figure 1 FTIR spectrometer (for IR absorption measurement)
The instrument consists of a source, interferometer, sample compartment, detector, amplifier, signal processor, and a computer. The source generates radiation which passes the sample through the interferometer and reaches the detector.

Fabrication of the cell doped with the different luminescent materials
This was achieved by taking (0.3) g TiO2 finely ground photo active material mixed with varied masses of inorganic Sb/Ca/Mn/Ag material respectively in a molding dice and pressed at a constant pressure of 10000 psi. The anode layer preparation was done by molding with the pressing dice at constant (0.18/Cx) g mass ratios of I2 KI/Cx mixture and disposed on the previously molded photo active disc. The resulted layers of the luminescent photo active cells were then carefully removed and placed on a conductor to enable measurement of their I-V characteristics. Figure 2 shows a schematic presentation of the luminescent solar cell.

I-V Characterization
The I-V characterization of the cells was carried out and the assembly for the experiment is as shown in Figure 3.

V-T Characterization
A stopwatch was incorporated besides the set to determine the time of potential decay when the set was transferred to a darkroom. Figure 4 shows the circuit diagram for the potential-time decay characterization.

Figure 4 Potential Decay Characterization Circuit
The equations tabulated in Table 1 were applied to calculate the derived parameters of the luminescent solar cell.

Results and discussion
The inorganic complex semiconductor-metal (sc/m) hybrid has nanostructure interface sheath with reduced electronic structure atoms forming defects with charge carriers traps dominated by varied Ag nanoparticles (NPs) and random physical interactions interfaces. The silver nanoparticles were made the reduction of silver nitrate by hydrazine hydrate in water ionic liquid micro emulsion. The resultant material was washed thoroughly with water the acetone. The absence of organic matter was confirmed using an FTIR spectrometer whose characteristics were as provided in Figure  5.

Chemical characterization of the inorganic (Sb/Ca/Mn/Ag) luminescent material
The dry Sb/Ca/Mn/Ag inorganic powder subjected to IR radiation generated by a Shimadzu FTIR 2014 spectrometer initiated a series of frequency bands shown in Figure 5.  Figure 5 shows a spectrum of Sb/Mn/Ag for the frequencies that were assigned to transfer of energy by Sb-NO3, Sb-PO4 and Sb-CO3 and Sb-O centres. The signals of low intensity due to free nitrate ion were observed to resonate at 1356 -828 cm −1 while that due to carbonate ion were observed at 1690 cm −1 .These results indicated that antimony bonding with different anions delayed radiations as evidenced by their respective resonant frequencies [11]. This is due to the splitting of d-orbitals groups resulting to d→d transitions which are relatively weak hence infrared active [11]. By extension, their results noted that varied colours resulted from delayed relaxation of excited electrons. Separate studies by [5,6] noted that different particle's momentum resulted to transfer of energy to the neighboring particles as illustrated by the following equations.
The equations show that there was energy transfer by the photo luminescent particles. That delayed relaxation was exploited in this study with a view to increase spectral traps in the SbNO3, Sb-PO4, Sb-CO3 and Sb-O ions and released later to prolong interaction of radiation with the titanium dioxide (TiO2) photo fluorescent material. A study of the delay in photo activity after withdrawal was carried out to investigate the potential delay by the doped TiO2 solid solar cells.

Response of Sb/Ca/Mn/Ag luminescent materials luminescence material on radiation delay
Varying masses of inorganic (Sb/Mn/Ag) material were thoroughly mixed with separate (TiO2) (1.00 g) mass photoactive material each as presented in Table 2. From the respective mixtures, seven photo active cells were fabricated and assigned labels (Z1, Z2, Z3, Z4, Z5, Z6, and Z7). Receptive layers were made using (0.48 g) of graphite-iodine (Cx /I/KI) for each photo cell whose ratios were 0.3 g, 0.17 g and 0.01 g respectively.

Fluorescent Solar Cells
The resulting cells were exposed to natural radiation and variation of potential was observed. The data obtained was presented in Table 3.
The cells were then transferred to a dark room and variation of potentials monitored for an interval of one hour was as shown in Table 4. The results from Table 3 and 4 are presented graphically as shown in Figure 6 and 7 respectively. and PCE respectively. The buildup of charge rate was observed to be proportional to mass of the (Sb/Ca/Mn/Ag) dopants with varied potentials. However, cell (Z1, (0 g)) void of (Sb/Ca/Mn/Ag) inorganic fluorescent material generated the lowest (0.266 V) (VOC) with (0.075) µA short circuit current with (0.305 and 6.085E-3%) FF and PCE respectively. This was attributed to the radiation due to the lack of enhancement for generation of high charge carrier density. The (FF and PCEs) derived parameters due to (0-0.06) g (Sb/Ca/Mn/Ag) fluorescent dopant resulted to {(0.372 and 4.544E-3 %), (0.484 and 6.639E-3 %) and (0.482 and 9.182E-3 %)}. This was attributed to the corresponding the quantity of charge per generated with time which is also referred to as charge rate density delivered to the external circuit. A study by [12] noted that fluorescent dopants mass ratios was proportional to the excited charge carriers density. Similar results reported by [13] indicated that dielectric constant of solvent material was proportional to the excited electron holes pairs density which determined the ensued ionic dissociation. Imbalance between dopant and the host material affected the potential with time delay which were the parameters of interest due to reduction of the intensity of excitation energy [14]. That enhancement further supported time delay in potential (voltage) as observed from Figure 7. The P-T characteristics in diminished radiation shows that (0.052, 0.065, 0.086, and 0.076 V) -(VOC) delay resulted from {Z4,b (0.03) g, Z5,b (0.04) g, 64,b (0.05) g and Z4,b (0.06) g} fluorescent solar cells realized due to the (Sb/Ca/Mn/Ag-TiO2) complex. Dielectric constant (the property of a material to insulate) of the dispersing TiO2 host material was thought to be proportional to the resulted density electron holes pairs which determined the ensued ionic dissociation. A similar study by [12] reported that reduction in charge carriers density gave rise to instantaneous downturns of potential profiles. Imbalance between the dopant and perovskite species affected the parameters of interest resulting from the intensity of excitation energy [14]. The introduced signal was absorbed as {(Sb-NO3) at These results were verified when the antimony ions bonded to different anions was subjected to U-Vis in a darkroom as they resulted to delayed emissions with varied observed colours. That concurred with findings of [15] noted that absorption peaks of the ligands of C-N and C-C ionic bonds appeared at 1082 and 1039 cm −1 respectively. According to [16,17] the bands between 1650-2100 and 800-1000 cm -1 were due to (Sb-O) ionic bond while that between 1250 -1650 cm −1 was contributed by the phosphate ion. [11] reported that splitting of d-orbitals groups differed in energy resulting to d→d orbital transitions which are relatively weak hence IR active that is it resonated with the infra-red region. Reports by [11] noted that varied colours result from delayed relaxation of excited electrons. That difference in wavelength occurred due symmetric and asymmetric stretching bonds of oxygen atoms coordinating with metallic ions monodentate and bidentate bridges modes, within the crystal structure in the complex [18].

Conclusion
The study observed that the inorganic luminescent material transferred energy to the neighboring species resulting to radiation delay by Ca 2+ , Sb 2+ , Mn 2+ and Ag + ions due to existence radiation after withdrawal of the spectral radiation. The defect traps were introduced by the doping material which resonated at 1356 -828 cm −1 while that due to carbonate ion resonated at 1690 cm −1 . The absorbed infrared radiation was shifted to longer wavelengths (1593, 1353, 1402, and 1385) cm −1 by the complex with the respective anions. The band at (460 -448) cm −1 and 434 -426 cm −1 were due to Sb-O transferring energy to Mn centres. These results indicated that antimony ionic bonding with different anions contributed to delayed radiations as evidenced with their respective resonant frequencies emitting varied colours. The ligands of C-N and C-C ionic bonds were observed to resonate at 1082 and 1039 cm −1 respectively. That down conversion (to lower energy) gave rise to potential delay of 0.086V. The inorganic dopants had promising properties for radiation delay in photo voltaic devices application.

Compliance with ethical standards
Acknowledgments A lot of gratitude to Dr. Jane Mburu, Chemistry Department, Kenyatta University, for her support with inorganic luminescent materials synthesis equipment and space and their chemical characterization. My sincere appreciation to Mr. Fred Mundimba of Physics department, Kenyatta University, for the P -T and I-V instruments he accorded to me.

Disclosure of conflict of interest
Authors declare their sincere lack of knowledge of any known competing financial or personal relationships that could have appeared to influence the work reported in this research article.