Improved photovoltaic performance of monocrystalline silicon solar cell through luminescent down‐converting Gd2O2S:Tb3+ phosphor

This work reports on efforts to enhance the photovoltaic performance of standard p‐type monocrystalline silicon solar cell (mono‐Si) through the application of ultraviolet spectral down‐converting phosphors. Terbium‐doped gadolinium oxysulfide phosphor and undoped‐gadolinium oxysulfide precursor powders were prepared by a controlled hydrothermal decomposition of a urea homogeneous precipitation method. The resulting rare‐earth element hydroxycarbonate precursor powders were then converted to the oxysulfide by annealing at 900°C in a sulfur atmosphere. The as‐prepared phosphors were encapsulated in ethylene vinyl acetate co‐polymer resin and applied on the textured surface of solar cell using rotary screen printing. Comparative results from X‐ray powder diffraction, field emission scanning electron microscopy, scanning transmission electron microscopy, and photoluminescence spectroscopy studies on the microstructure and luminescent properties of the materials are reported. We also compared the optical reflectance and external quantum efficiency response of the cells with and without a luminescent phosphor layer. The results obtained on the terbium‐doped gadolinium oxysulfide phosphor show clearly that the down‐conversion effect induced by the terbium dopant play a crucial role in enhancing the photovoltaic cells' performance. Under an empirical one‐sun illumination, the modified cells showed an optimum enhancement of 3.6% (from 16.43% to 17.02%) in conversion efficiency relative to bare cells. In the concentration range of 1 to 2.5 mg/mL, EVA/Gd2O2S (blank) composites also improve electrical efficiency, but not as much as EVA/Gd2O2S:Tb3+ composites.


| INTRODUCTION
The gradual increase in energy consumption and the approaching depletion of fossil fuels have initiated the search for clean and sustainable alternative energy sources. Photovoltaic (PV) cells can be used for the direct generation of electricity from solar radiation, with nearly zero-emission of greenhouse gases. Currently, the crystalline silicon (c-Si)-based solar cells are still dominating the global solar PV market because of their abundance, stability, and nontoxicity. 1,2 However, the conversion efficiency of PV cells is constrained by the spectral mismatch losses, non-radiative recombination and strong thermalisation of charge carriers. Generally, the c-Si solar cells are only able to absorb photons within a limited portion of solar spectrum, with ultraviolet (UV)-blue and most of the infrared (IR) region of light untapped. 3 According to the literature, around 47% of energy conversion is lost as lattice thermalisation, which is caused by the incident photons having energy above the band gap (E g = 1.12 eV). As a result, the maximum theoretical conversion efficiency for a single-junction c-Si solar cell with energy gap of 1.1 eV is limited to 30%. 4,5 Reducing these losses in c-Si solar cells may be achievable through spectrum modification by employing down-converting phosphors. [6][7][8][9] In a down-conversion (DC) process, a high-energy incident photon is absorbed by the DC phosphors and re-emitted as two or more lower energy photons at wavelengths where the silicon solar cells exhibit a strong spectral response. 10,11 A number of researchers have shown that there is significant enhancement in conversion efficiency of solar PV devices by integrating a down-converting or luminescent down-shifting (LDS) layer on the top surface of c-Si solar cells. [12][13][14][15][16][17][18][19] In recent years, rare-earth element doped-oxysulfides have been extensively investigated and studied because of their efficient performance as luminescent materials. Moreover, their high-conversion efficiency from X-ray wavelengths to visible light, low toxicity, high radiation stability, and easy preparation, which has seen their use in a wide range of applications such as emissive display devices, 20 optical temperature sensors, 21 oxygen storage, 22 X-ray detectors and scintillators. 23 Terbium-doped gadolinium oxysulfide (Gd 2 O 2 S:Tb 3+ ) is reported as the most commonly used phosphor in commercial X-ray intensifying screens. The high density (7.34 g/cm 3 ) and wide band gap (4.6-4.8 eV) enable it to effectively trap X-ray photons. 24 The intense green emission of the Gd 2 O 2 S:Tb 3+ phosphor under UV excitation implies its great potential of being used as an efficient spectral converter in an attempt to reduce the spectral mismatch losses while improving the overall light absorption in silicon solar cell. 25,26 The reason that a submicron Gd 2 O 2 S:Tb 3+ phosphor was chosen in place of a commercially available sample was two fold. Firstly, because of the surface texture of the mono-Si solar cell, commercial Gd 2 O 2 S:Tb 3+ phosphor particles with large grain size (average size around 3.5 μm) would not form a closepacked thin phosphor layer; instead, they would reflect some of the incident light and shade the cell because they are larger than the width and depth profile of the textured structures. Secondly, from our previous work with soft X-ray detectors using submicron Gd 2 O 2 S:Pr 3+ phosphor particles to form a thin phosphor layer, it was found that they gave a higher detection efficiency than a much larger commercial Gd 2 O 2 S: Pr 3+ sample. The higher detection efficiency of the smaller Gd 2 O 2 S: Pr 3+ particles was becuase of their ability to form a thin close-packed layer with very few "pinholes," thereby capturing more X-rays. 27 In the present work, submicron-sized Gd 2 O 2 S:Tb 3+ phosphor precursor powders were prepared by a urea hydrothermal homogeneousprecipitation method. 28,29 In addition, to encapsulate the phosphor particles on the textured surface of monocrystalline silicon (mono-Si) solar cell, ethylene vinyl acetate (EVA), with excellent optical properties, good thermal stability, and strong adhesion, was selected as the matrix and binder. 30 Hence, we believe that such a luminescent-composite layer could compensate for the low spectral response of silicon solar cell at the UV-blue wavelengths, thereby improving the conversion efficiency.
In this study, we demonstrated a low-cost effective luminescent layer comprising of an EVA/Gd 2 O 2 S:Tb 3+ mixture on the textured surface of commercial single-junction mono-Si solar cell through rotary screen printing. In addition to investigating the morphological and luminescent properties of the Gd 2 O 2 S:Tb 3+ particles, the PV current density-voltage (J-V) characteristics, optical reflectance, absorbance, and external quantum efficiency (EQE) response of the solar cell before and after coating were also obtained, in order to compare the cells' PV performance and examine the effectiveness of Tb-doped phosphors.

| Materials
The following chemicals were used in this work: gadolinium oxide

| Synthesis of Gd 2 O 2 S:Tb 3+ phosphor
The Tb 3+ -activated Gd 2 O 2 S phosphor particles were synthesized through a two-step method: the first step was the preparation of Tb 3+ -doped gadolinium hydroxycarbonate precursor (Gd (OH)CO 3 : Tb 3+ ) via a urea-based hydrothermal homogeneous precipitation method with minor modifications. 28,29,31 The second step was the sulfuration of the Gd (OH)CO 3 :Tb 3+ . In order to obtain the highest luminance in the Gd 2 O 2 S:Tb 3+ phosphor samples, the ratio of Tb 3+ ions was set to 2 mol% in respect with that of Gd 3+ molar concentration. 26  The solution was then heated until boiling, followed by the addition of urea (30 g). The solution was boiled until turbidity was observed at which point the solution was aged for 1 hour at the same temperature. The precipitates were then filtered at the pump while still hot, followed by washing twice with DI water (100 mL) and then dried in an oven at 100°C overnight. For sulfuration of the precursor powders, the as-prepared Gd (OH)CO 3 :Tb 3+ dry powder was thoroughly Hydrothermal decomposition of urea (rate 4% per hour at approximately 100°C) supplying reactants in a controlled manner, With increasing pH (less than pH 3), the cyanate ion rapidly reacts, The rare earth element ions are weakly hydrolysed in water, and the subsequent release of hydronium ions promotes urea decomposition, the resulting release of carbonate ions causes precipitation, once the concentration of reactants reaches critical supersaturation, The resulting (Gd (OH)CO 3 :Tb 3+ ) precursor powders were converted to the Gd 2 O 2 S:Tb 3+ phosphor by heating at 900°C in a sulfur atmosphere, 2.3 | EVA/Gd 2 O 2 S:Tb 3+ layer preparation and integration on the surface of solar cell For experimental purposes, the cell was previously cut into three equal pieces (15.6 × 5.2 cm) using a high-power laser machine (TMX90, CTR, UK). The as-prepared Gd 2 O 2 S:Tb 3+ phosphor powder was ready to be used for the layer coating process. In a typical procedure, a stoichiometric amount of phosphor powder was weighed and dispersed in p-xylene using an ultrasonic bath. A series of solutions containing the Gd 2 O 2 S: Tb 3+ /p-xylene with 1.0, 1.5, 2.0, 2.5, and 3.0-mg/mL particle concentrations was prepared and then separately mixed with various amounts of EVA shreds. When the solution became colloidal it was left at 30°C to 35°C under continuous stirring until uniform solutions were formed.
(The viscosity information of as-prepared EVA/p-xylene solution is provided in Figure S1, Supporting Information). Finally, the mixture solution was screen-printed on the textured surface of the solar cells

| Characterization
The J-V characteristics of solar cells with and without EVA/phosphor coatings were analysed using a Trisol Class AAA xenon flash solar simulator (OAI Trisol TSS156, USA) under one-sun illumination (100 mW/cm 2 ) and room temperature conditions. The solar simulator can cover the spectrum from 350 to 1200 nm, as shown in Figure 2 (the spectral mismatch error was estimated to be less than 1%). During    Table 1.    These results are consistent with the calculated data from the XRPD   measurements (see Table 1). Figure 6C displays the cross section image of a mono-Si cell after coating with EVA/Gd 2 O 2 S:Tb 3+ (2.0mg/mL particle density). It shows clearly the morphology of pyramidally textured silicon surface and the coated luminescent layer composed of EVA/Gd 2 O 2 S:Tb 3+ , with an average thickness of 1.56 μm.

| Morphology and structure analysis
(The variation of film thickness due to the change of EVA wt% are illustrated in Figure S2 and S3, Supporting Information). In addition, we have observed that some of the Gd 2 O 2 S:Tb 3+ particles are deposited onto the cell's surface through rotary screen printing, and most of the particles are well-encapsulated by the EVA binder.
The EDS (see Figure 8) from the cross-sectional scanning area of coated cells confirms that the deposited layers contain gadolinium (Gd), oxygen (O), sulfur (S), and terbium (Tb) elements. The additional intense peak in regarding to carbon (C) is from the EVA binder. Figure 9 shows the high angle annular dark field (HAADF) STEM images of the Gd 2 O 2 S:Tb 3+ particles fabricated in this work. In Figure 9A, the discrete particles of roughly hexagonal shape with apparently a smooth surface can be clearly observed. In Figure 9B, the light distribution image of a small cluster is seen, and the distribution of light appearing from part of the crystals is fairly uniform, where some areas are brighter. The overlay image (see Figure 9C) shows the high uniformity of visible light (green) emission from the submicron sized phosphor particles.   This phenomenon is due to the absence of doping agent (Tb 3+ ), and thus, the lack of certain UV photons for subsequent DC process. In such case, the improved light absorption is solely expected to benefit from the light scattering of the host particles, which is discussed in more details in Section 3.5.

| Electrical characteristics of coated solar cell
In addition, we have also observed that the enhancement contributed from both dispersed phosphor and host particles at different concentrations in EVA binder can be achieved (see Table 4 and Figure 11). As illustrated in Figure 11, in both cases, the enhancement coefficient started to drop when the particle density exceeded 2.0 mg/mL; and eventually, the conversion efficiency began to deteriorate when the particle concentrations were higher than around 2.7 mg/mL. The reduction in PCE at 3.0 mg/mL particle density may have been caused by the significant backward scattering arising from the serious particle aggregation. At higher concentration, the substantial absorption in the UV region by

| Optical characteristics of coated solar cell
To further validate the obtained results regarding the efficiency enhancement brought by the Gd 2 O 2 S:Tb 3+ phosphor particles, we performed reflectance measurements. Figure 12 shows the reflectance and absorption spectra for bare cell, solely EVA (15 wt%) binder-coated, EVA/Gd 2 O 2 S:Tb 3+ (2.0 mg/mL) and EVA/undoped- properties of all evaluated solely EVA coatings are presented and compared in Figure S4, Supporting Information).

| External quantum efficiency (EQE) characterization
External quantum efficiency measurements were performed to clarify the underlying mechanisms for the enhanced solar cell efficiency. The recorded EQE spectra are shown in Figure 14. The EQE value for binder-coated cells is fairly close to that of bare cells and is slightly higher at wavelengths of 300 to 380 nm, which coincides with the obtained reflectance results (see Figure 12A). Moreover, the solar cell coated with the EVA/Gd 2 O 2 S:Tb 3+ composite layer shows overall higher EQE value than the bare cell across the entire wavelengths range of 300 to 1100 nm, and a maximum relative enhancement over 19% in the UV regions was observed, indicating explicitly that the increase in photocurrent is mostly arisen from the enhanced absorption of UV photons by the Gd 2 O 2 S:Tb 3+ phosphors. Since a large proportion of electron-hole pairs derived from absorbing high energy photons usually situated near the solar cell surface, the photogenerated carriers subsequently consume and disappear easily through their recombination because of the surface defects, which may in turn give rise to inferior carrier collection possibilities. 16,17 Nevertheless, with the presence of the phosphor particles on the front surface of mono-Si solar cell, a greater number of photons can be absorbed closer to the depletion region for photo-current generation as soon as the UV photons are luminescent converted to the visible region of solar spectrum. With the effect of the internal electric field (E-field), the photo-generated electron-hole pairs will be instantaneously separated, and thus the enhancement in PV effect. As for the enhanced EQE response above 400 nm, the observed increase of EQE for both undoped-Gd 2 O 2 S and Gd 2 O 2 S:Tb 3+ cases are in consistent with the variation of corresponding reflectance spectra, as illustrated in Figure 12A. We consider that this enhanced behaviour particularly at short-wavelengths of UV-region (see Figure 12B and Figure 13), which did not disrupt the original DC functionality and uniform-light-scattering effect of the phosphor particles. Figure 14B plots the enhancement factor for coated cells with maximum enhancement in EQE, relative to those for a bare cell. The cells coated with EVA/Gd 2 O 2 S:Tb 3+ (2.0 mg/mL) achieved the maximal EQE enhancement factor (more than 1.15 at 300-400 nm and more than 1.03 at 400-1100 nm), followed by the cells coated with EVA/undoped-Gd 2-O 2 S (2.0 mg/mL) (more than 1 at 410-1100 nm), and then by cell coated with solely EVA (15 wt%) binder (more than 1 at 300-400 nm).

| CONCLUSIONS
In this work, we developed a simple and cost-effective luminescent  and is environmentally friendly. An EVA matrix provides promising host conditions for the phosphor particles. Finally, the rotary screen printing formed a relatively uniform layer and we believe that it can be applied to various types of photovoltaic devices and other phosphor powders.