Light Management Investigation of Transparent DSSC based on Ln3+ ion doped BaO-ZnF2-B2O3-TeO2 Glass (Ln3+ = Dy3+/Sm37Eu3+) as UV Down-conversion Material

Solar irradiance to electrical energy conversion could be achieved via photoelectric effect using solar cells device. However, not all solar wavelengths could be captured and converted by the active layer of solar cell. The absorption limitation associated with the bandgap energy of solar cell active layer in the ultraviolet region (high photon energy) and infrared region (low photon energy) leads to 70% energy loss. Introducing a material that can convert higher photon energy to lower photon energy that is suitable with the bandgap energy of the solar cell provides a solution to this problem. In the present work, glass materials based on BaO, ZnF2, B2O3, TeO2, and Ln2O3 (Ln2O3 = Dy2O3, Sm2O3, and Eu2O3) were developed using conventional melt and quenching technique and applied as down-conversion (DC) element in dye-sensitized solar cell (DSSC). The absorption spectra of Z907 as dye photosensitizer was measured as well as the absorption spectra of DC glass. The DC glass emission spectra were also investigated to know the compatibility between the absorption of solar cell and the emission band of the DC glass. The current-voltage of the DSSC was measured while placing the DC glass on top of the solar cell device. The electrical parameters, such as power conversion efficiency, fill factor, short-circuit current, and open-circuit voltage, were then determined to analyze the effect of DC glass application on the performance of DSSC. DC glass with 1.5Eu produced an efficiency of 2.03%, showing the best result among other lanthanide ions.


Introduction
Light management in solar cells (SCs) typically deals with manipulation of solar light for maximizing the photogenerated current through various mechanisms, such as up-conversion (UC), down-conversion (DC), tandem/graded structures, scattering, luminescent solar concentrator, anti-reflection, and surface plasmon resonance.These techniques could be used as a solution for addressing the absorption loss in the ultraviolet and infrared regions known as a spectral mismatch.Up-conversion or down-conversion mechanism could be achieved using trivalent lanthanides ions (Ln³⁺) due to their unique energy level for energy conversion from UV to IR region and vice versa.Meanwhile, the use of antireflection coating can enhance the light absorption of SCs and diminish the energy loss in the visible region [1,2].In another report, surface plasmon resonance technique has been reported using metallic nanostructured materials to improve light harvesting by more capturing more light [3].Among light management technology, down-conversion technology using trivalent lanthanide ion (Ln³⁺) has attracted many interests.In the down-conversion process as presented in figure 1, photon with higher energy from the incident light will excite electrons in the ground state and promote them to a higher energy level.Afterward, the electrons in the higher level will return to the ground level by releasing two photons with lower energy.When the energy of the light emission is identical to the bandgap energy of the active layer in SC, the absorption will increase and consequently higher efficiency will be achieved [4].Furthermore, DC glass not only increases the light absorption of SCs but also protects the SCs from external damage such as heat, UV irradiation as well as environmental disruption when installed outdoors.As reported by X. Yang et al., the application of Eu³⁺ ion DC glass for silicon SCs shows the strongest emission at 612 nm when excited at 362 nm [6].It was reported that the conversion of light from ultraviolet to the visible region coincided with silicon solar cell absorption.As a result, the power conversion efficiency (PCE) increased by around 0.25% with the addition of DC glass.Meanwhile, T. M. Hierro et al., developed Eu(III) complex in the polymeric film as Si-based solar cells down-converter.From their experiment, the improvement of external quantum efficiency of around 78% has been achieved when the down-converter was placed above the SCs device [7].
To the best to our knowledge, the development of DC material for silicon-based solar cell has been widely investigated [6][7][8].However, the application of DC material for improving the performance of other types of the solar cell is still rarely reported.In particular, the utilization of DC-based-Ln³⁺ iondoped glass materials integrated with dye-sensitized solar cells (DSSC) has not been reported.DSSC has been intensively explored due to their high potential as a future transparent solar cell with inexpensive development, simple manufacturing, and high energy conversion efficiency [5].The configuration of DSSC consists of two semiconductor film electrodes including a counter electrode and a working electrode.A mesoporous TiO₂ nanoparticle layer as electron transport medium is coated on the working electrode.Meanwhile, platinum (Pt) is coated on the counter electrode for accelerating electron transfer towards electrolyte to undergo redox reaction.DC material based on glass is suitable as host matrix due to the good solubility of Ln³⁺ ions and could be easily adjusted.In the present work, we combined DC glass based on Dy³⁺/Sm³⁺/ Eu³⁺ ion doped barium oxide, zinc flouride, borate oxide, and tellurite oxide on top of transparent DSSC.The electrical properties of the solar cells were then characterized by I-V measurement to obtain various electrical parameters such as fill factor (FF), efficiency (), etc.

Experimental Details
The working electrode and counter electrode of transparent DSSC for the present work were manufactured using a screen-printing technique.The FTO glass known as fluorine-doped tin oxide glass (Great cell, 15/sq sheet resistance) was cut around 1.5 × 1.0 cm² followed by cleaning in ultrasonic bath for two cycles: 15 min in deionized water and Teepol, then 15 min in isopropyl alcohol.After cleaning, the FTO glasses were dried at room temperature using blow dryer.The blocking layer and TiO₂ layer were deposited on the working electrode where the active area was around 0.5 × 0.5 cm².Meanwhile, the platinum paste was printed on an area of around 1.0 × 1.0 cm² at the counter electrode side.Each layer was similarly treated following dried at 120 °C in the oven for 30 min and then heated at 500 °C for 180 min.The working electrode was immersed into TiCl₄ solution on the hotplate at 70 °C for 30 min and then dried.Afterward, the working electrode was dipped into a dye solution (20 mg Z907 at 100 mL ethanol) for 24 h.The TiO₂ film thickness was found 9.6 ± 0. ) and 464 nm ( 7 F0 → 5 D2) for Eu 3+ ion.Furthermore, the glass matrix host shows broad absorption in the ultraviolet region from 200 to 400 nm, while the control DSSC generates the board absorption in the ultraviolet to visible region from 200 -700 nm.The photoluminescence of Dy 3+ /Sm 3+ /Eu 3+ ion doped BaO-ZnF2-B2O3-TeO2 glass is presented in figure 4 along with the absorption spectrum of Z907 dye represented by the black solid line.The luminescence of Dy 3+ ion under ex = 385 nm is generated by electron transition from 4 F9/2 level that is collapse to 6 H15/2, 6 H13/2, 6 H11/2 and 6 H9/2 level belonging to 482, 575, 664, and 755 nm.The wavelength of 575 nm is the strongest emission light of Dy 3+ :glass.For Sm 3+ ion, the emission maxima that are studied under ex = 398 nm were found at 562, 600, 644, and 703 nm due to electron transition out of 4 G5/2 level that goes down to 6 H5/2, 6 H7/2, 6 H9/2, and 6 H11/2 level, respectively.Meanwhile, the emission maxima of Eu 3+ ion under ex = 388 nm come from electron transition of 5 D0 → 7 F1, 7 F2, 7 F3, and 7 F4 level that is located at 588, 613, 649, and 702 nm, respectively.The Z907 dye absorption band is represented by the black solid line of figure 4. The dye solution has an absorption range between 330 to 800 nm, where the absorption maxima is located at 374 and 520 nm.As shown in figure 4, almost all emissions produced by the DC glass are located near the edge of Z907 dye absorption, indicating low absorption possibilities.
The I-V curves of control DSSC and DSSC integrated with DC glass parameters are shown in figure 5, while the current density at maximum power, short-circuit current density, maximum power, opencircuit voltage, fill factor, and efficiency are presented in table 2. The DC glass was installed on the top of the DSSC for I-V measurement as shown in figure 2. The open-circuit voltage values for all samples are relatively the same, i.e. 0.62 V, while the short-circuit current densities of DSSC were found to decrease with the utilization of DC glass.As presented in table 2, almost the DSSC parameter value decreased with the integration of DC glass.It can also be seen in figure 6 that the incident photon-tocurrent conversion efficiency (IPCE) at 300 -800 nm decreased when DC glass was arranged on top of the solar cell device.The IPCE reduction could be due to the strong absorption by the Ln 3+ ions from the ultraviolet to infrared region as shown in figure 3. Thus, most of the solar light will be absorbed by the DC layer, so-called parasitic absorption, which leads to reduction in photogeneration and hence loss in efficiency.Furthermore, non-radiative (NR) transition could also occur when an electron is excited by initial light and goes up to an excited state then falls to the lower level generating phonon instead of photon energy.The non-radiative transition occurs through 4 F7/2→ 4 F9/2 transition for Dy 3+ ion, 4 I 13/2 → 4 G 5/2 transition for Sm 3+ ion, and 6 P 7/2 → 4 F 9/2 transition for Eu 3+ ion [11][12][13].The NR transitions will reduce the light emission of the DC glass, thus lowering the generated light that could be absorbed further by the DSSC.
Although the use of DC glass overall has not been able to improve the DSSC efficiency, we found a few improvements (0.38%) of IPCE at the wavelength of 760 nm for DSSC coupled with Dy 3+ :glass due to the Dy 3+ ion emission in the wavelength of 755 nm as presented the emission spectra in figure 4. A previous study shows that the application of DC glass integrated with silicon and gallium phosphide (GaP) solar cell produced an enhanced efficiency [6,14].However, a contrary result was reported by J. Liu, etc., where the reduction efficiency was occur due to the utilization of LaVO4:Dy nanocrystal as a DC layer integrated by DSSC-based-N3 dye from 3.9% (without DC layer) to 3.7% (with DC layer) [15].Further investigation for glass composition and rare earth dopants in DC glass for increasing DSSC efficiency are needed such as Eu 3+ /Yb 3+ co-doped SrAl2O4 [8], SiNx: Tb 3+ /Yb 3+ [16], Pr 3+ /Yb 3+ co-doped gallo-germanate glass [17], etc.Indeed, the absorption band examination of other solar cell active layer  types that have broad absorption wavelengths such as crystalline or polycrystalline silicon [6], PTB7:PC71BM [18], PCDTBT: PC71BM [19], or dyeA [20] is also required to find the most suitable active layer for Eu 3+ ion based-DC glass.

Conclusion
The application of down-conversion glass based on Dy 3+ /Sm 3+ /Eu 3+ ion doped BaO-ZnF 2 -B 2 O 3 -TeO 2 glass for light management in transparent DSSC has been investigated.The DC glass was fabricated using conventional melt and quenching method, while the DSSC was fabricated using screen-printing technique.The DC glass is placed on top of the DSSC where the absorption maxima of DC glass are found in the UV to IR region.The photoluminescence spectra of DC glass are found at 482 to 755 nm that go over the edge of Z907 dye absorption.The reduction in DSSC efficiency was found with the utilization of DC glass, which is due to the spectral mismatch between the dye absorption band and the DC glass emission band as well as the parasitic absorption of DC glass.However, DSSC coupled with Dy 3+ :glass shows the IPCE enhancement 0.38% from control DSSC at 755 nm due to the energy conversion from 385 nm to 755 nm as shown by emission spectra.In the future, the emission band of DC glass should be optimized further in order to fit the solar cell absorption band by varying the glass composition either ion doping or modification of the host material.Further investigation is also needed for combining DC glass with other types of solar cells, such as c-Si or organic photovoltaic to find the most suitable solar cell for Ln 3+ ion based-DC glass.

Figure 1 .Figure 2 .
Figure 1.DC glass and solar cell configuration along with the downconversion mechanism of Ln³⁺ ion.

Figure 4 .
Figure 4. Emission spectra of Dy 3+ /Sm 3+ /Eu 3+ ion doped BaO-ZnF 2 -B 2 O 3 -TeO 2 glass as DC glass along with the absorption spectra of Z907 dye represented by the black solid line.

Figure 5 .
Figure 5. I-V curves of control and DSSC coupled with DC glass irradiated by 500 W/m 2 simulated solar.

Table 1 .
[9,10]asured three times by digital thickness gauge from Mitutoyo Corp. High-performance electrolyte from Greatcell was filled into DSSC after two electrodes were combined by pressing them together with a hot press for 30 s at 120 °C.The DuPont Surlyn® was used as sealant to keep both electrodes at certain distance and keep the electrolyte from leaking.The DC glass was prepared according to previous research[9,10]using the melt and quenching technique.About 15 g raw material of 30BaO + 10 ZnF₂ + 30 B₂O₃ + (30-Ln³⁺:glass composition for the down-conversion layer of transparent DSSC.The absorption and transmittance spectra of Dy³⁺/Sm³⁺/Eu³⁺ ion doped BaO-ZnF₂-B₂O₃-TeO₂ glass from 200 -1100 nm are presented in figure3.The absorption maxima originate from Dy³⁺, Sm³⁺, and Eu³⁺ ion energy levels.The absorption maxima were centered at 449, 800, 894, and 1089 nm because of electron transition through 6 H15/2 ground level to 4 I15/2, 6 F5/2, 6 F7/2, and 6 F9/2 level for Dy³⁺; 400, 470, x)TeO₂ + xLn₂O₃ (xLn₂O₃ = 1 mol% for Dy₂O₃; 1 mol% for Sm₂O₃; and 1.5 mol% for Eu₂O₃) was prepared and the mass of each compound was presented in table 1.The Dy³⁺:glass, Sm³⁺:glass, and Eu³⁺:glass were cut and polished around 1.5 × 1.0 × 0.3 cm³.The absorption spectra were collected using Optical Maya spectroscopy with Deutrium and Halogen as lamp sources.Meanwhile, the photoluminescence spectra of DC glass were observed using Cary Eclipse Fluorescence Spectrophotometer (Agilent technology).A solar simulator from Oriel Production, USA was used to measure IV curves and the electrical parameters were collected using Keithley SMU2400 source meter unit.The DSSC configuration used during the I-V measurement is presented in figure2along with DC glass on the top of the device.Data recording was conducted by a personal computer using KlickStart as the interface program.Photovoltaic parameters such as fill factor, efficiency, open-circuit voltage, and short-circuit current were obtained during this characterization.