Luminescence Decay Dynamics of a Down-Shifting Material ^†

Abstract

This study demonstrated luminescence decay dynamics of BaSiO₃:Eu²⁺, elucidating its potential as a spectral converting down-shifting material for improving the performance of dye-sensitized solar cells (DSSCs). Time-resolved photoluminescent (TRPL) measurements under excitation pulses of a picosecond pulsed light-emitting diode (EPLED) revealed complex decay dynamics described by a triple-exponential model. Average lifetime was in nanoseconds, which facilitated rapid emission of down-shifted photons, essential to mitigating reabsorption losses. The presence of a fast decay channel is crucial to minimizing photon reabsorption and maximizing the flux of visible photons transferred to the dye molecules of DSSCs to enhance photocurrent generation.

1. Introduction

The search for sustainable and clean energy has driven significant advancements in solar energy conversion technology. Among these, dye-sensitized solar cells (DSSCs) have emerged as a promising alternative to conventional silicon-based photovoltaics due to their cost-effectiveness, flexibility and good performance under low-light and indoor conditions [1,2,3,4]. However, despite these advantages, DSSCs face challenges in terms of low-light-harvesting capability, especially in the ultraviolet (UV) region of the solar spectrum. Most sensitizing dyes used in DSSCs exhibit minimal absorption in the UV, leading to suboptimal utilization of the full solar irradiance. To address this limitation, researchers have been exploring the use of down-shifting materials, which are phosphors that can absorb high-energy photons (UV) and re-emit them in the visible range where the dye molecules are more responsive [5,6,7]. Advances in material design, such as durability under environmental stress and process scalability, have been explored in structural composites and advanced materials’ long-term functional stability [8,9,10]. In this context, luminescent down-shifting materials for DSSCs must combine optical efficiency with structural and environmental stability. Among various luminescent materials, barium orthosilicate doped with divalent europium (BaSiO₃:Eu²⁺) has good optical and photostability properties. The BaSiO₃:Eu²⁺ was selected due to its broad visible emission well-matched to common DSSC dye absorption, chemical stability with low phonon energy of the silicate host, and favorable nanosecond-scale decay dynamics compared to other alternatives like the aluminate vanadate and titanate-based down-shifting materials. In terms of photostability [11,12], the material equally exhibits robust photostability under prolonged UV irradiation, making it suitable as a down-shifting material in photovoltaic applications, which can absorb high UV light and emit it in the visible region where DSSCs have good spectral responsivity for better device performance.

However, the application of BaSiO₃:Eu²⁺ in DSSCs centers on luminescent decay dynamics [13,14]. The excited-state lifetimes and the balance between radiative and non-radiative transitions govern the emission process in such phosphors. Luminescent decay lifetime reflects the average time [15] an excited electron remains in the excited 5d state before returning to the ground 4f state, either by emitting a photon or by transferring energy through non-radiative channels. This decay behavior affects the timing and probability of energy transfer [16] to adjacent dye molecules in the DSSC structure. In down-shifting applications, materials with faster decay lifetimes are generally preferable as they indicate efficient radiative transition and reduced energy losses. In materials like BaSiO₃:Eu²⁺, multi-exponential decay profiles are often observed due to the presence of multiple Eu²⁺ environments. This effect can enhance overall luminescent performance, due to its impact on emission efficiency and stability.

In this study, we analyzed the luminescent decay behavior of BaSiO₃:Eu²⁺ synthesized via a conventional solid-state technique. Surface morphology and elemental composition were analyzed by a scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. The luminescent properties were studied through photoluminescent (PL) excitation (355 nm) and PL emission (565). PL emission demonstrated strong broadband emission from approximately 460–650 nm attributed to the 4f⁶5d¹ → 4f⁷ electronic transition of Eu²⁺. Quantum yield analysis was carried out to determine the luminescence efficiency in terms of the ratio of the integrated emission spectrum to the integrated excitation spectrum of the material. Time-resolved PL measurements under excitation pulses of a picosecond pulsed light-emitting diode—EPLD—demonstrated complex decay dynamics, which can be described via a triple-exponential model. The average lifetime was estimated to be in nanoseconds, which facilitated rapid emission of down-shifted photons essential for mitigating reabsorption losses. The presence of a fast decay channel is crucial to minimizing photon reabsorption and maximizing the flux of visible photons transferred to the dye molecules of DSSCs to enhance photocurrent generation. Therefore, this research depicted the suitability of the down-shifting material for efficient energy transfer in DSSCs. The rapid luminescent decay revealed effective down-shifting of high-energy UV photons into lower energy levels in the visible range, thereby encouraging photon absorption for DSSC enhancement. The novelty of this work demonstrates that, unlike previous studies that primarily focused on the steady-state photoluminescence or device-level efficiency enhancement, this study uniquely emphasized time-resolved photoluminescence (TRPL) decay dynamics of BaSiO₃:Eu²⁺ and its direct relevance to luminescence down-shifting for DSSC applications. Specifically, this study provided a clear demonstration of how fast decay components mitigate photon reabsorption losses, which, to the best of our knowledge, has not been studied in prior Eu²⁺-doped silicate materials. It also explicitly established a link between nanosecond-scale average lifetime and enhanced photon flux transferred to DSSC dye molecules.

2. Materials and Methods

2.1. Experimental Procedure

The starting materials for the synthesis of Ba_(1−x)SiO₃:xEu²⁺ were barium carbonate (BaCO₃), silicon dioxide (SiO₂), 99.99%, and europium oxide (Eu₂O₃), 99.99%, all of analytical grade, purchased from Sigma-Aldrich chemical company, Johannesburg, South African branch. The europium (II)-doped barium silicate phosphor Ba_(1−x) SiO₃:xEu²⁺ (x = 0.03 mol) was synthesized by the conventional solid-state route. The choice of concentration (x = 0.03 mol = 3 mol %) of Eu²⁺ was due to the concentration quenching effect usually observed in Eu²⁺-activated silicate. Concentration beyond this threshold could lead to non-radiative energy transfer between activator ions, which can reduce emission efficiency. The workflow for the synthesis is shown in Figure 1. The specific experimental steps applied were as follows: first, the starting materials were initially weighted according to the nominal stoichiometry of Ba_0.97SiO₃:0.03Eu²⁺ phosphor; then, the powders were mixed evenly and milled thoroughly using a mortar and pestle. The chemical reaction that governed the stoichiometry calculation was as follows:

2BaCO₃ + 2SiO₂ + 2Eu₂O₃ 2Ba(_1−x)SiO₃:xEu²⁺ + 2CO₂ + 3O₂↑

The ground samples were placed in an alumina crucible and subsequently transferred into a tube furnace, where it was annealed at 900 °C for 4 h in vacuum. After re-grinding, the sample was sintered to 1300 °C for 4 h under an H₂ reducing atmosphere at approximately 100 standard cubic centimeters per minute (SCCM) in the same instrument. After cooling down the programmable furnace, the nominal compound of the BaSiO₃:Eu²⁺ phosphor was obtained. Microstructural analysis was performed to study the surface morphologies of the phosphors using the scanning electron microscope (SEM) model—JSM-7800F—JOEL Limited, Tokyo, Japan—with an electron beam voltage of 5.0 keV and further equipped with an energy-dispersive X-ray spectroscope (EDS) for elemental identification. Photoluminescent (PL) emission was performed using the Cary Eclipse Spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) and the emission lifetime measurement was carried out using the FLS980 spectrometer (Edinburgh Instruments, Scotland, UK). All characterizations were performed at room temperature, and all plots were created using Python 3.9.

2.2. Average Lifetime Estimation

The emission lifetime measurements were achieved using the FLS980 spectrometer (Edinburgh Instruments, Scotland, UK). The lifetime was recorded by the excitation with pulses of a picosecond pulsed light-emitting diode (LED), with the model number EPLED (360 nm, 100 kHz)—Edinburgh Instruments, Scotland, UK. The triple-exponential equation follows the following form [17,18,19]:

$$\mathrm{I}\left(\mathrm{t}\right)={\mathrm{A}}_1{\mathrm{e}}^{{\displaystyle \frac{-\mathrm{t}}{{\mathsf{\tau }}_1}}}+{\mathrm{A}}_2{\mathrm{e}}^{{\displaystyle \frac{-\mathrm{t}}{{\mathsf{\tau }}_2}}}+{\mathrm{A}}_3{\mathrm{e}}^{{\displaystyle \frac{-\mathrm{t}}{{\mathsf{\tau }}_3}}}+\mathrm{C}$$

(1)

where I is the luminescence intensity, t is the time, A_1, A₂ and A₃ are the fitting parameters, ${\mathsf{\tau }}_1$, ${\mathsf{\tau }}_2$ and ${\mathsf{\tau }}_3$ are the lifetimes for short, intermediate and long exponential components, and C is the constant offset or baseline. ${\mathsf{\tau }}_1$ is from the surface of Eu²⁺, ${\mathsf{\tau }}_2$ is near the surface and ${\mathsf{\tau }}_3$ is from the core of the phosphor particles. ${\mathsf{\tau }}_1$ is linked to the surface-related recombination, ${\mathsf{\tau }}_2$ is due to near-surface Eu²⁺ environments and ${\mathsf{\tau }}_3$ is attributed to the bulk lattice Eu²⁺ ions experiencing minimal non-radiative losses. The average lifetime (τ_av) can be estimated by Equation (2) [17,19].

$${\mathsf{\tau }}_{\mathrm{a}\mathrm{v}}={\displaystyle \frac{{\mathrm{A}}_1{\mathsf{\tau }}_1^2+{\mathrm{A}}_2{\mathsf{\tau }}_2^2+{\mathrm{A}}_3{\mathsf{\tau }}_3^2}{{\mathrm{A}}_1{\mathsf{\tau }}_1+{\mathrm{A}}_2{\mathsf{\tau }}_2+{\mathrm{A}}_3{\mathsf{\tau }}_3}}$$

(2)

The coefficient of determination (R²) was applied as a performance metric for measuring the goodness of the triple-exponential model. The value of R² ranges from 0 to 1. An R² close to 1 indicates that a significant portion of the variability in the measured data was captured by the model, suggesting a strong correlation between the predicted and actual values. R² is mathematically defined as follows [20]:

$${\mathrm{R}}^2=1-{\displaystyle \frac{{\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\left({\mathrm{y}}_{\mathrm{i}}-\widehat{{\mathrm{y}}_{\mathrm{i}}}\right)}^2}{{\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\left({\mathrm{y}}_{\mathrm{i}}-\overline{\mathrm{y}}\right)}^2}}$$

(3)

where n is the number of samples, ${\mathrm{y}}_{\mathrm{i}}$ is the actual value, $\widehat{{\mathrm{y}}_{\mathrm{i}}}$ is the predicted value, and $\overline{\mathrm{y}}$ is the mean of the actual value.

3. Results and Discussion

3.1. SEM Analysis

SEM is a crucial technique to gain insight into the morphology of nanomaterials. It provides insights into the shape, size, and surface morphology of the materials [21,22]. Surface morphology plays a critical role in the design and synthesis of phosphor materials, as it directly influences their optical and luminescent properties. The structural characteristics of the phosphor particles, such as their size, shape, and surface area, determine their efficiency in converting energy into light. Figure 2a shows the SEM image of the BaSiO₃:Eu²⁺-emitting phosphors. The surface morphology of the sample has small and large particle sizes of non-uniform agglomeration. The surface of the sample has irregular shapes, which means that the distribution of the particle sizes is not homogenous. As shown in Figure 2b, the average size of the phosphor was estimated to be 30.5 nm. Nanoparticles with a mean size of around 30.5 nm offer a good balance between surface area and optical functionality. While particle agglomeration may locally reduce effective surface area and introduce light scattering, the nanometer-scale particles (~30.5 nm) still provided sufficient surface interaction for efficient photon conversion. Furthermore, moderate agglomeration can enhance light trapping and scattering within the DSSC architecture, which may be beneficial for photon harvesting. Smaller particle agglomeration provides a greater surface area for dye adsorption, which is essential for maximizing light-harvesting efficiency [23,24] in DSSCs. Therefore, while some larger particles are present, their lower frequency distribution suggests that they are unlikely to significantly hinder charge transport.

3.2. EDS Analysis

EDS is a qualitative and quantitative analytical technique that provides information on the chemical composition of a sample for elements with atomic numbers (Z) greater than 3 [17]. By exciting the sample via bombarding with a focused electron beam, EDS can be conducted. This excitation causes the emission of X-rays from the sample. The emitted X-rays are collected by the X-ray detector, and the individual X-rays detected are converted into electrical energy. Therefore, the electrical signals produced in this process serve as fingerprints for identifying elements, which allows elemental analysis to be conducted on the sample. In this study, EDS was primarily employed for qualitative elemental confirmation rather than stoichiometric quantification, due to the known limitations of EDS for low-dopant-concentration detection. As shown in Figure 3, the prominent peaks in the spectrum corresponded to barium (Ba), silicon (Si), oxygen (O), and europium (Eu), all of which are anticipated elements in the compound. Multiple strong peaks attributed to barium are observed at approximately 0.7 keV, 4 keV, 4.5 keV, 5.2 keV, and 5.7 keV, which align with the Ba Lα and Lβ emission lines. This shows that barium is a major constituent, as expected from the BaSiO₃ matrix. A peak for silicon is visible around 1.75 keV, which is consistent with the presence of silicon from the orthosilicate (SiO₄) structure. Additionally, a distinct peak at approximately 0.5 keV confirms the presence of oxygen, which is a crucial constituent of the oxide network. Europium was also detected through several small peaks ranging between 1.1 keV and 8 keV. These peaks are characteristic of Eu M and L emission lines. Although Eu was present in a low concentration (3 mol%), the sensitivity of EDS qualitative analysis enabled its detection, indicating that the doping process was effective. The presence of europium peaks confirms that Eu²⁺ ions were effectively integrated into the host lattice, thereby acting as substitutes for Ba²⁺ due to their similar ionic radii.

3.3. Photoluminescent Study

Photoluminescence is the emission of light from a material under optical excitation, and the light emitted can be collected for spectral analysis. A material undergoing photo-excitation causes its electrons to move to the allowed excited states. The electrons in the process of returning to the ground state emit energy, which may include the emission of light (the luminescent/radiative process) or no emission (non-radiative process). While recording the photoluminescence, both the excitation and emission spectra are scanned. Figure 4a presents the excitation and emission spectra of the phosphor. A broad absorption band from 300 to 460 nm was observed (λ_em = 565 nm) in the phosphor peaking at 355 nm. This can be attributed to the electronic dipole allowing the 4f⁷ → 4f⁶5d¹ transition of the BaSiO₃: Eu²⁺ phosphor [18,19]. It also showed a broad emission band (λ_ex = 355 nm), and the Stokes shift was calculated to be >200 nm from the difference between the emission wavelength and the excitation wavelength, which showed that emission spectra were found at longer wavelengths [19]. The incident photons of the UV–blue wavelengths were absorbed by the BaSiO₃:Eu²⁺ phosphor and re-emitted within the visible wavelengths, approximately 460–650 nm, thereby demonstrating the luminescence down-shifting behavior of the material. Since DSSCs do not efficiently absorb the UV light, materials like BaSiO₃:Eu²⁺ can effectively capture the UV/blue light and re-emit it as visible light, which can then be absorbed by the dye molecules of DSSCs [25].

The photoluminescence quantum yield of the silicate-doped europium (II) with the reference was depicted in Figure 4b. The fluorescence emission spectrum of the material and the excitation spectrum of the reference were collected using the integrating sphere. Quantum yield estimation resulted in a value of 0.05, which denotes the ratio of the integrated photon emission with respect to the integrated excitation spectra of the material. The photoluminescent and quantum yield analysis positioned BaSiO₃:Eu²⁺ as a UV-absorbing down-shifting material that is suitable for DSSC applications. Also, a previous study on BaSiO₃:Eu²⁺ [14] showed strong overlap between the emission range of the material (460–650 nm) and the N719 dye absorption profile (360–600 nm), thereby facilitating efficient energy transfer potential. Through the mechanism of energy transfer, the absorbed UV photons are re-emitted to the visible wavelengths for better utilization in DSSC devices.

3.4. Photoluminescent Emission Lifetime

Time-resolved photoluminescent (TRPL) measurements performed under the excitation pulses of a picosecond pulsed light-emitting diode—EPLED—have provided significant insights into the emission dynamics of the material under study [26]. It emphasizes how the triple-exponential decay behavior and the presence of a fast decay channel in the down-shifting material, such as the BaSiO₃:Eu²⁺, facilitate energy transfer. By analyzing the time-resolved emission data, it was found that the photoluminescent decay does not follow a simple single-exponential behavior but is instead characterized by a complex decay dynamic that can be accurately described by a triple-exponential model, as shown in Figure 5. The coefficient of determination (R²) from the triple-exponential fit was 0.8331, which shows the goodness of fit. This triple-exponential decay implies the presence of triple emissive pathways or different populations of excited states within the material, each contributing to the overall photoluminescent signal with distinct lifetimes. The average photoluminescent lifetime was calculated to be approximately 567 nanoseconds, which positions the material in a regime where the emission occurs rapidly enough to be useful for practical applications like DSSCs. The observed average lifetime is comparable to and shorter than the lifetimes reported for Eu²⁺-activated silicate [27]. This relatively shorter lifetime supports faster emission and reduced reabsorption losses, which are crucial for down-shifting applications in DSSCs.

Table 1. Emission lifetime decay parameters.

${\mathit{\tau }}_1$ (ns)	${\mathit{\tau }}_2$ (ns)	${\mathit{\tau }}_3$ (ns)	${\mathit{\tau }}_{\mathit{a}\mathit{v}}$ (ns)	$\mathit{C}$	${\mathit{R}}^2$
85	86	815	567	5	0.8331

shows the emission lifetime decay parameters. The nanosecond-scale lifetime is especially beneficial in applications involving photon down-shifting, where higher-energy photons (such as ultraviolet or blue light) absorbed by the material are re-emitted at longer wavelengths, typically within the visible spectrum [28,29,30].

By ensuring that the luminescent centers emit photons quickly after excitation, the system effectively reduces the probability that these photons will be reabsorbed, thus enhancing the overall photon flux available for subsequent processes. In practical terms, this fast decay channel increases the efficiency with which down-shifted photons are transferred to adjacent functional layers, such as dye molecules in dye-sensitized solar cells (DSSCs). These solar cells rely on the absorption of visible photons by dye molecules to generate photocurrents; thus, maximizing the flux of these photons directly translates into improved device performance [31]. The interplay between the luminescent properties of the down-shifting material and the photovoltaic dye is crucial for optimizing photocurrent generation in DSSCs. The dye molecules are sensitized to absorb photons within a specific spectral range, often the visible region, and convert this absorbed light into electrical energy. However, many conventional photovoltaic materials have limited absorption in the ultraviolet region, which contains a significant portion of the solar spectrum. By incorporating a down-shifting layer that absorbs high-energy photons and re-emits them at longer wavelengths better matched to the dye’s absorption profile, the effective spectral utilization of the solar spectrum is improved [32,33,34].

4. Conclusions

This work explored the luminescent decay dynamics of BaSiO₃:Eu²⁺ and its application as a spectral down-shifting material to improve the performance of dye-sensitized solar cells (DSSCs). The DSSCs, although cost-effective and efficient in low-light conditions, absorb very low UV light. BaSiO₃:Eu²⁺ can address this limitation by absorbing high-energy UV photons and re-emitting them in the visible region of the spectrum, where DSSC dyes are more responsive. This study highlights that fast decay lifetimes are critical as they ensure efficient emission, minimize reabsorption losses, and enhance photon flux to the dye molecules. The material was synthesized using a solid-state route and characterized with SEM, EDS, photoluminescence (PL), and time-resolved photoluminescent (TRPL) spectroscopy. SEM revealed non-uniform particle morphology with an average particle size of approximately 30.5 nm, offering a balance between surface area and optical performance. EDS confirmed the successful incorporation of Eu²⁺ into the BaSiO₃ matrix. PL measurements showed strong broadband emission in the visible region, under UV excitation, confirming its down-shifting capability.

Time-resolved photoluminescent measurements demonstrated that BaSiO₃:Eu²⁺ exhibits complex triple-exponential decay dynamics, with an average lifetime of approximately 567 ns. The presence of a fast decay channel was particularly important, as it reduced photon reabsorption and increased the flux of visible photons transferred to DSSC dyes. While direct DSSC device integration was not performed in this study, we acknowledge this limitation, and the device-level integration will be implemented in future work. However, the novelty of this work illustrated that, unlike many previously reported down-shifting materials that primarily focused on steady-state photoluminescence enhancement, this study emphasized time-resolved luminescence decay dynamics as a key performance metric for DSSC-related down-shifting applications. BaSiO₃:Eu²⁺ exhibited a nanosecond-scale average lifetime with a distinct fast decay channel, which is crucial for minimizing photon reabsorption. These findings presented BaSiO₃:Eu²⁺ as an efficient down-shifting material that is capable of improving light harvesting and photocurrent generation in DSSC by converting otherwise underutilized UV light into useful visible photons.