Designing a novel Eu²⁺-doped hafnium-silicate phosphor for an energy-down-shift layer of CsPbI₃ solar cells

Abstract

The excessive carbon discharge and low utilization efficiency of solar energy are still the biggest challenges in the world. A series of Eu²⁺ doped K₂HfSiO₅ phosphors were synthesized and applied in CsPbI₃ perovskite solar cells. Under the excitation of ultraviolet and near ultraviolet light in the range of 300–450 nm, K₂HfSiO₅:Eu²⁺ exhibits emission in the visible range of 450–650 nm. Due to their strong absorption in the ultraviolet region and high quantum efficiency, K₂HfSiO₅:Eu²⁺ phosphors could be used to fabricate an energy-down-shift layer for solar cells. When spin-coated with the layer, the value of the short-circuit current of the packaged CsPbI₃ solar cell increased, resulting in the enhancement of the power conversion efficiency by 2.19%. In addition, other basic properties of K₂HfSiO₅:Eu²⁺ phosphors such as photoluminescence, cathodoluminescence and stability have also been studied. All the results indicate that the energy-down-shift layer made using K₂HfSiO₅:Eu²⁺ phosphors has a positive effect on improving the performance of CsPbI₃ perovskite solar cells.

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1. Introduction

In recent years, a luminescence energy-down-shift (EDS) layer for solar cells has received much attention because of its obvious light conversion effect.^1–3 By converting the light that is useless for solar cells (ultraviolet (UV) or infrared (IR) light) into the useful visible or near infrared light, the EDS layers allow solar cells to produce more carriers, thus improving their power conversion efficiency (PCE).^4–6 Therefore, researching and developing EDS layers for solar cells are of great significance for the utilization of solar energy and clean energy.

Among several types of solar cells, CsPbI₃ perovskite solar cells have aroused interest due to their desirable band gap (about 1.73 eV), strong light absorption and outstanding photoelectric properties.^7–9 CsPbI₃ is considered to be one of the materials with the most potential as a new photovoltaic material for the next generation. According to some existing reports, inorganic quantum dots, carbon dots, up-conversion phosphors and metal clusters could serve as EDS layers for CsPbI₃ solar cells.^5,10–12 In these studies, although the performance of the solar cells has been enhanced to a certain extent, numerous shortcomings still exist. For example, the synthetic process of quantum dots is very complex, relating to many toxic raw materials which are harmful to human health and have a poor stability when set in an air environment. Besides, the photoluminescence quantum yield of most EDS layers made by up-conversion phosphors is very low, which affects the PCE of solar cells. Furthermore, though metal nano cluster luminescent materials have a high quantum efficiency, their fabrication cost is so high that they are not suitable for large-scale application. Hence, it is necessary for us to search for a novel light-conversion material which could be applied to the EDS layer for CsPbI₃ solar cells with low costs and convenience of preparation.

As is well known, the range of the solar spectrum is from UV to IR light. However, the CsPbI₃ solar cells cannot effectively utilize UV light because the photoelectric reaction in this region is weak. Instead, it has a strong photoelectric effect in the visible range (400–800 nm).^13,14 As a type of light conversion material, phosphors have many advantages such as suitable size, excellent luminous efficiency, preferable stability, intense absorption in the UV region and low energy consumption.^15–18 Consequently, phosphors with a down-conversion (DC) effect could better satisfy with the requirements of the EDS layer for CsPbI₃ solar cells.¹⁹

It is worth mentioning that Eu²⁺ doped phosphors generally exhibit a broad-band emission according to the 4f–5d transition, so that they have outstanding luminescence features.²⁰ Most part of Eu²⁺ activated phosphors which can be used in the EDS layer are nitrides such as CaAlSiN₃:Eu²⁺, SrSi₂O₂N₂:Eu²⁺ and (Ca,Sr)₂Si₅N₈:Eu²⁺, Tm³⁺; they all have a favourable luminescence performance.^21–23 However, it is very hard to synthesize nitrides because of their rigorous preparation conditions (high temperature and high pressure) and because the cost of their fabrication process is tremendously high. Silicate based phosphors are a superior choice; many of them have tremendous potential for photoluminescence and cathodoluminescence materials such as NaScSi₂O₆:Eu²⁺, RbNa₃(Li₃SiO₄)₄:Eu²⁺ and Sr₃SiO₅:Eu²⁺ because of their luminescence properties, splendid chemical and physical stability and so on.^24–26 Thus, when combining with polyvinyl pyrrolidone (PVP), the luminescence performance of the silicate phosphor will not be influenced due to its great stability. In addition, silicates are easily available and low cost, widely used in basic research studies.^27–29 In this work, we had prepared a K₂HfSiO₅:Eu²⁺ phosphor via replacing Zr in K₂ZrSiO₅:Eu²⁺ with Hf through the solid-state reaction. The highest internal quantum efficiency was nearly about 60% and the thermal stability was greatly improved.³⁰ Then the K₂HfSiO₅:Eu²⁺ phosphor was successfully applied onto the CsPbI₃ perovskite solar cells as an EDS layer. It is found that both short-circuit current (J_sc) and PCE are distinctly increasing. Meanwhile, the cathodoluminescence properties of the K₂HfSiO₅:Eu²⁺ phosphor were also explored, demonstrating that it has the potential for multifunctional application.

2. Experimental

Phosphor preparation

The K₂HfSiO₅:Eu²⁺ phosphor was fabricated through the high temperature solid-state method. The initial raw materials were K₂CO₃ (A.R), HfO₂ (≥99.9%), SiO₂ (A.R.) and Eu₂O₃ (99.99%). They were mixed together in a stoichiometric ratio, and 3 wt% H₃BO₃ was used as the flux. The mixture was ground homogeneously with alcohol for 20 minutes and then put into several alumina crucibles. The heating rate was set to 3 °C min⁻¹. After calcination in a tube furnace at 1120 °C for 6 h in a reduced atmosphere (H₂ : N₂ = 5 : 95), the products were taken out and cooled down to room temperature and ground again for further analysis.

EDS layer fabrication

The crude materials of the EDS layer were K₂HfSiO₅:0.02Eu²⁺ phosphor, PVP and ethanol. They were weighed according to the mass ratio of 1 : 10 : 50, and then PVP and ethanol were added into a beaker for stirring. After PVP dissolves, the K₂HfSiO₅:0.02Eu²⁺ phosphor was added into the beaker and stirring was continued for another 30 minutes. Finally, the EDS layer solution in the middle of the beaker was spin-coated on the near light side of the CsPbI₃ solar cells in a glove box at 2000 rpm for 30 s at room temperature.

Device fabrication

Precursor solution preparation: all experiments were carried out in a glove box that was filled with N₂. CsI and PbI₂ were weighed according to the stoichiometric ratio in the container, 0166 g DAMI and 1 ml DMF were weighed, respectively; DMF-d7 was added according to the volume ratio of 5%, and magnetic stirring was carried out for more than 6 h.

Packaged solar cell preparation: the precursor solution was spin coated on the TiO₂/FTO surface which was treated with peroxide plasma for 35 s at 3000 rpm. Then it was annealed at 210 °C for 10 min to obtain the CsPbI₃ film. After this, the device was assembled as an EDS layer/FTO/TiO₂/CsPbI₃ film/spiro-OMeTAD/electrode structure, and finally the spin coated CsPbI₃ device was obtained.

Measurements and characterization

The phase of the instrument of X-ray powder diffraction (XRD) was Bruker D2 PHASER X-ray diffractometer with Cu Kα radiation. The range of the measured 2θ was 10–80°, the set step length was 0.03° and the counting time was 0.1 s per step. The XRD Rietveld refinement result of the K₂HfSiO₅ host was obtained using the General Structure Analysis System (GSAS).³¹ Both the morphology and Energy Dispersive X-Ray (EDX) spectra were obtained using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Density functional theory (DFT) was used to calculate the band structure with the help of the Cambridge Serial Total Energy Packet (CASTEP) code.³² The diffuse reflectance spectra (DRS) were obtained using an UV-vis spectrophotometer (PE-Lambda 950) and BaSiO₄ powders were used as a reference. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra and decay curves were detected using a FLS-920T fluorescence spectrophotometer. At the same time, thermal stability was tested with the same PL equipment and a TAP-02 heating device was added. The cathodoluminescence (CL) properties of K₂HfSiO₅:Eu²⁺ phosphors were measured using a Mp-Micro-S instrument. The current density–voltage (I–V) curves were recorded via the illumination of an AM 1.5G (100 mW cm⁻²) solar simulator (Enlitech). The external quantum efficiency (EQE) spectra were measured using a Quantum efficiency tester (QE-R, Enlitech).

3. Results and discussion

Crystal structure and phase information

The XRD Rietveld refinement was introduced to acquire the detailed crystal structure of K₂HfSiO₅. For the sake of acquiring the phase information, K₂ZrSiO₅ was set as an initial model. The refinement results are shown in Fig. 1(a). The calculated data agree well with the experimental data and the refined factors are R_wp = 8.52%, R_p = 7.67% and χ² = 1.31, respectively. These parameters illustrated that the results of refinement have a referential value. The total refinement crystal data are shown in Table 1. It could be known that K₂HfSiO₅ is a monoclinic crystal system and its space group belongs to p121/c1(14), and the atom coordinates are listed in Table 2.

Fig. 1 (a) The K₂HfSiO₅ host Rietveld refinement result; (b) XRD patterns of K₂HfSiO₅:xEu²⁺ (0.00 ≤ x ≤ 0.035); and (c) the crystal structure sketch map of K₂HfSiO₅ and the different coordination surroundings of K⁺.

Table 1 Crystal data of K₂HfSiO₅

Formula	K2HfSiO5
Crystal system	Monoclinic
Z	8
V	975.51(32) Å^3
Space-group	P121/c1 (14)
P	4.97 g cm^−3
A	14.10(18) Å
B	5.67 Å
C	14.09 Å
A	90.00°
B	90.00°
Γ	119.98°
χ ^2	1.311
R wp	8.52%
R p	7.67%

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Table 2 Atomic coordinates of K₂HfSiO₅

Atom	Wyckoff	x/a	y/b	z/c
K1	4e	0.291(5)	0.778(6)	0.144(8)
K2	4e	0.075(3)	0.752(6)	0.126(4)
K3	4e	0.195(6)	0.221(2)	0.332(6)
K4	4e	0.430(6)	0.250(3)	0.071(7)
Hf1	4e	0.065(8)	0.739(3)	0.369(2)
Hf2	4e	0.434(2)	0.235(8)	0.309(4)
Si1	4e	0.161(3)	0.244(9)	0.088(6)
Si2	4e	0.338(4)	0.761(2)	0.394(6)
O1	4e	0.127(8)	0.102(4)	0.461(5)
O2	4e	0.289(9)	0.254(3)	0.150(9)
O3	4e	0.408(9)	0.789(13)	0.041(8)
O4	4e	0.212(9)	0.714(7)	0.362(5)
O5	4e	0.092(14)	0.303(9)	0.129(7)
O6	4e	0.378(9)	0.596(22)	0.338(15)
O7	4e	0.134(2)	0.934(7)	0.027(3)
O8	4e	0.363(1)	0.064(8)	0.392(6)
O9	4e	0.007(12)	0.474(4)	0.272(8)
O10	4e	0.492(10)	0.966(3)	0.263(1)

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According to the XRD patterns of K₂HfSiO₅:xEu²⁺ (0 ≤ x ≤ 0.35) samples shown in Fig. 1(b), with the increasing amounts of the introduced Eu ions, all the diffraction peaks are in accordance with the refinement result. The Eu²⁺ ions are considered to occupy the position of K⁺ because the values of the ionic radii of Eu²⁺ (1.17 Å, coordination number (CN) = 6; 1.2 Å, CN = 7) are close to those of K⁺ (1.38 Å, CN = 6; 1.46 Å, CN = 7), and much higher than those of Hf⁴⁺ (0.71 Å, CN = 6; 0.76 Å, CN = 7). It is worth noting in the partial enlarged patterns that the position of the diffraction peak at 33.3° gradually shifts to larger 2θ with Eu²⁺ doping. This phenomenon could be attributed to the fact that smaller radius (Eu²⁺) ions occupy the site of the larger ions (K⁺). According to the Bragg formula, when the smaller ions substituted the larger ones, the crystal lattice would shrink, leading to an increase of θ.³³ All the above results indicate that Eu ions are successfully doped into the K₂HfSiO₅ host and there is no impurity phase.

Fig. 1(c) displays the structure of K₂HfSiO₅. One [SiO₄]⁴⁻ tetrahedron is connected with another [HfO₆]⁸⁻ octahedron by sharing one O²⁻, and one [Hf₂O₁₁]¹⁴⁻ unit also consisted of two [HfO₆]⁸⁻ octahedra. A variety of polyhedron pairs constituting a 3D framework of K₂HfSiO₅ and the isolated K⁺ act as cations which could be substituted by doped ions. There are four different coordination surroundings of K⁺ (K1, CN = 6; K2, CN = 6; K3, CN = 7 and K4, CN = 7) and as listed in Table 3, the average bond lengths of K1–O, K2–O, K3–O and K4–O are 2.658 Å, 2.587 Å, 2.714 Å and 2.663 Å, respectively.

Table 3 Average bond lengths (Å)

Bond	Distance
K1–O(CN = 6)	2.658(6)
K2–O(CN = 6)	2.587(3)
K3–O(CN = 7)	2.714(9)
K4–O(CN = 7)	2.663(2)

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Fig. 2(a) reveals the SEM image of K₂HfSiO₅:0.02Eu²⁺ particles. The shape of the sample is irregular rod distribution, the ranges of the length and diameter are 10–15 μm and 2–5 μm, respectively. The SEM mapping image of the sample is shown in Fig. 2(b). It could be seen from the image that K, Hf, Si, O and Eu elements are detected and uniformly distributed on the selected particle surface. In addition, TEM and the High-Resolution Transmission Electron Microscope (HRTEM) graphs of K₂HfSiO₅:0.02Eu²⁺ are shown in Fig. 2(c). The stable crystal lattice could be seen clearly and the distance between two adjacent crystal planes is 0.4515 nm, which corresponds to the (111) crystal plane. The different element compositions of K₂HfSiO₅:0.02Eu²⁺ are exhibited in Fig. 2(d). There are no other elements in the sample (the C peak is attributed to the instrument) and it could be observed from the insight table that the stoichiometric ratio of each element has exactly constituted K₂HfSiO₅. These characterization methods demonstrated the successful preparation of K₂HfSiO₅:Eu.

Fig. 2 (a) SEM image of K₂HfSiO₅:0.02Eu²⁺; (b) SEM mapping (K, Hf, Si, O and Eu) of K₂HfSiO₅:0.02Eu²⁺; (c) TEM and HTEM images of one K₂HfSiO₅:0.02Eu²⁺ particle; and (d) EDX spectrum of K₂HfSiO₅:0.02Eu²⁺.

The calculated band structure of K₂HfSiO₅ is shown in Fig. 3(a) and (b) according to the refined crystallographic results. It could be observed that K₂HfSiO₅ has an indirect band;³⁴ the maximum position of the valence band (VB) is located at point B and the minimum position of conduction band (CB) is located at point G. The calculated bandgap of K₂HfSiO₅ is 4.091 eV. The diffuse reflectance spectrum (DRS) of the K₂HfSiO₅ host displayed in Fig. 3(c) was taken to judge whether the calculation value is reliable. The optical bandgap of the K₂HfSiO₅ host could be obtained using the following formulae:

(αhν)^1/2 = A(hν − E_g) (1)

Fig. 3 (a) and (b) Band structure of K₂HfSiO₅; (c) the DRS spectra of Eu²⁺ doped and host of K₂HfSiO₅ (the inset shows the relationship between photon energy and (αhν)^1/2 of the K₂HfSiO₅ host); (d) total and partial density of states of K₂HfSiO₅.

In these equations, α refers to the absorption co-efficient, hν is the incident photon energy, A is constant, and n = 1/2 corresponding to an indirect bandgap. As depicted in Fig. 3(c), the obtained optical bandgap of K₂HfSiO₅ is 4.125 eV. The value of the calculated bandgap is close to that of the optical bandgap, indicating the reliability of the calculation. This value illustrated that the 4f⁶5d ↔ 4f⁷ energy level transition of Eu²⁺ has little influence on the VB and CB of K₂HfSiO₅.³⁵

Fig. 3(d) displays the calculated total Density of States (DOS) and partial Density of States (PDOS) of K₂HfSiO₅. The graph shows that the upper VB is mainly composed of the p-states of O element. The lower VB (−10 eV to −30 eV) involves the K s, p-states, Hf p-states, Si s, p-states and O s-states. These characteristics indicate the strong sp-hybridization of O-Hf and O-Si, which could be frequently seen in silicate inorganic hosts.³⁶ The CB is chiefly composed of K p-states. The DOS shows that the formation of the CB and VB includes the participation of all elements. All the above results show that K₂HfSiO₅ could be used as a potential host of luminescent materials.

Photoluminescence properties

Fig. 4(a) shows the PL spectra and PLE spectra of K₂HfSiO₅:0.02Eu²⁺. When monitoring at 513 nm, a broad excitation band ranging from 250–480 nm has been detected, with a peak at 360 nm. Under 360 nm wavelength excitation, the K₂HfSiO₅:0.02Eu²⁺ phosphor shows a cyan-green light emission, exhibiting a wide emission band in the range of 400–650 nm with the maximum value located at 513 nm because of the 5d–4f energy transition of Eu²⁺.^37,38 However, several sharp peaks appear in the range of 550–650 nm, which could be attributed to the energy transition of Eu³⁺. In order to explore the reason of these peaks, the emission spectrum monitored at 394 nm is given in Fig. 4(b). It is found that the characteristic peak of Eu³⁺ was detected despite its intensity being weak. The sharp peaks at 570 nm, 586 nm, 598 nm, 615 nm and 650 nm correspond to the ⁵D₀–⁷F_j (j = 0, 1, 2, 3 and 4) energy transition of Eu³⁺, respectively. This result illustrates that a small amount of Eu³⁺ was not reduced to Eu²⁺ in K₂HfSiO₅:Eu²⁺. Owing to the existence of Eu³⁺, the K₂HfSiO₅:Eu phosphor is not suitable for light emitting diode (LED) devices. However, for the EDS layer, the characteristic emission of Eu³⁺ could increase the efficiency of the solar spectral conversion for solar cells.^39–41 The PL spectra of various concentrations of Eu²⁺ doped K₂HfSiO₅ are shown in Fig. 4(c). The optimum doping concentration is nearly about 2% with an internal quantum efficiency of 59.5% at this time. With the increase of Eu²⁺ concentration, the possibility of non-radiative energy transfer between Eu²⁺ was increasing, which would lead to concentration quenching.⁴² The inset photo shows the time-resolved PL (TRPL) spectra of K₂HfSiO₅:0.02Eu²⁺. It could be observed that the shape of TRPL spectra is consistent with that of PL spectra. Distinctly, different decay times have been detected when monitoring at the same wavelength, indicating that energy transfer occurs between different Eu²⁺ ions and further proving that there exist several luminescence centers in the K₂HfSiO₅:Eu²⁺ phosphor.⁴³

Fig. 4 (a) PLE and PL spectra of K₂HfSiO₅:0.02Eu²⁺ (the inset shows the phosphor image under 365 nm ultraviolet light); (b) PL spectrum of K₂HfSiO₅:0.02Eu²⁺ under 394 nm excitation; (c) the emission spectra of K₂HfSiO₅:xEu²⁺ (0.01 ≤ x ≤ 0.35) (the inset represents the TRPL spectra); and (d) the relative emission intensity of K₂HfSiO₅:xEu²⁺ (0.01 ≤ x ≤ 0.35) samples (the inset shows the curve of log(x) and log(I/x).).

There are three energy transfer methods among Eu²⁺ ions: radiation re-absorption, exchange interactions, or electric multipolar interactions.⁴² It could be observed from Fig. 4(a) that the emission and excitation bands have a little overlap area, so that the effect of re-absorption is small. The parameter of critical distance (R_c) has been introduced to further confirm which mechanism of energy transfer between the different Eu²⁺ in K₂HfSiO₅:0.02Eu²⁺. When R_c ≤ 5 Å, the energy transfer mechanism is exchange interactions, otherwise it may be electric multipolar interactions. The value of R_c could be calculated using the following equation:⁴⁴

V is the volume of the unit cell, Z is the number of formula units and x_c is the optimal doping concentration. In the case of K₂HfSiO₅:Eu²⁺, x_c = 0.02, Z = 8, and V = 975.51 ų. The calculated R_c is about 22.67 Å and this value is obviously larger than 5 Å, demonstrating that the electric multipolar interaction is the major energy transfer mechanism. The interaction type among Eu²⁺ activators could be analyzed by Dexter's theory using the following formula:⁴⁵

The factor x refers to the doping concentration of Eu²⁺, θ = 6, 8 or 10 representing the dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions, respectively. What's more, k and β are constants. Fig. 4(d) inset shows the fitting line of log (x) and log (I/x). The slope of this line represents θ/3. For K₂HfSiO₅:xEu²⁺ (0.01 ≤ x ≤ 0.035) samples, the calculated result of θ is close to 6, demonstrating that the energy transfer interaction type of K₂HfSiO₅:Eu²⁺ is dipole–dipole.

To further probe the reason for the concentration quenching of the K₂HfSiO₅:Eu²⁺ phosphor, Fig. 5 depicts the decay curves of the K₂HfSiO₅:xEu²⁺ (0.01 ≤ x ≤ 0.035) samples and all the decay curves could be fitted by the following formula[30]:

where A₁, A₂ and A₃ are constants, I is the emission intensity, t is the time and τ₁, τ₂ and τ₃ are the lifetimes. The average decay time of every sample could be obtained using the formula as follows:

Fig. 5 The decay curves of the K₂HfSiO₅:xEu²⁺ (0.01 ≤ x ≤ 0.035) samples.

According to this formula, the average decay times of different concentrations of Eu²⁺ doping samples are 512, 505, 486, 478, 468 and 455 ns, separately. With the increase of Eu²⁺, the decay time of K₂HfSiO₅:Eu²⁺ keeps decreasing. This could be ascribed to the fact that the distance between the activators becomes shorter with the increase of the Eu²⁺ doping amount. Therefore, non-radiative energy transfer is more likely to occur in K₂HfSiO₅:Eu²⁺, leading to concentration quenching of Eu²⁺.⁴⁶

The thermal stability of K₂HfSiO₅:Eu²⁺ is shown in Fig. 6(a). It could be found that the emission intensity decreased with the temperature increase from 25 °C to 200 °C; the position of emission almost doesn’t shift. Fig. 6(b) shows the relative emission intensity with a linear increase in temperature. When it reached 150 °C, K₂HfSiO₅:0.02Eu²⁺ could still retain about 81.2% of its initial emission intensity. This result shows that the thermal stability of K₂HfSiO₅:Eu²⁺ is apparently higher than that of Na₂HfSiO₅:Eu²⁺ (retains 45% at 150 °C) and K₂ZrSiO₅:Eu²⁺ (retains 19.4% at 150 °C) phosphors. To understand the reason why the K₂HfSiO₅:Eu²⁺ phosphor has great thermal stability, the activation energy (ΔE) could be calculated according to the Arrhenius equation:⁴⁷

where I₀ is the emission intensity at initial temperature, I_T represents the intensity at heating temperature, c and k are constant, and ΔE is the activation energy. As shown in the inset of Fig. 6(b), the calculated value of ΔE of K₂HfSiO₅:0.02Eu²⁺ is 1.7041 eV. The larger value of the activation energy might be one of the reasons for the excellent thermal stability of the K₂HfSiO₅:0.02Eu²⁺ phosphor.

Fig. 6 (a) Temperature dependent luminescence spectra of K₂HfSiO₅:0.02Eu²⁺ phosphor from 25 to 200 °C; (b) the relative emission intensity of K₂HfSiO₅:0.02Eu²⁺ at different temperatures (the inset shows the fitting activation energy of K₂HfSiO₅:0.02Eu²⁺); (c) the thermoluminescence (TL) curves of K₂HfSiO₅:0.02Eu²⁺ and K₂HfSiO₅ host; and (d) the energy level diagram of Eu²⁺ in K₂HfSiO₅.

The thermoluminescence curves of the K₂HfSiO₅ host and K₂HfSiO₅:Eu²⁺ are shown in Fig. 6(c). It could be obviously seen that there exists an electron trap at about at 50 °C in K₂HfSiO₅:Eu²⁺. The trap depth could be estimated using the following formula:

E_T refers to the depth of the energy trap, and T_m is the position of the TL peak. The calculated trap depth is about 0.64 eV at 50 °C. Based on the above result, the simulated energy level schematic diagram of Eu²⁺ in K₂HfSiO₅ is plotted in Fig. 6(d). The electron trap is considered to be close to the bottom of the conduction band. The electrons will be excited from the 4f⁷ ground state to the 5d state with 360 nm excitation. Some of the electrons could directly transit back to the ground state level, resulting in green emission, while other electrons in a higher excited state level could be trapped through the conduction band (procedure ①). Subsequently, as the temperature increases, the trapped electrons are released through the conduction band (procedure ②). The electrons return to the excited state and come to the ground state, leading to an increase of the emission intensity.⁴⁸ This is another reason for K₂HfSiO₅:Eu²⁺ possessing great thermal stability.

Cathodoluminescence properties

The cathodoluminescence (CL) performance is vital for the display of radar, television and oscilloscope.^49–51Fig. 7(a) and (b) show the SEM and CL mapping images of one K₂HfSiO₅:0.02Eu²⁺ particle. These graphs demonstrate that luminescence centers are distributed on the particle evenly.

Fig. 7 The SEM image (a) and CL mapping (b) of one K₂HfSiO₅:0.02Eu²⁺ particle; (c) the CL intensity of K₂HfSiO₅:0.02Eu²⁺ with various voltage (4–9 kV); and (d) the CL spectra of K₂HfSiO₅:0.02Eu²⁺ with different current (50–100 mA).

Fig. 7(c) represents the CL spectra of K₂HfSiO₅:0.02Eu²⁺ under increasing voltage. The shape of the CL spectra is similar to the PL spectra which are shown in Fig. 4(a). When the current remains at 60 mA, the CL intensity of K₂HfSiO₅:0.02Eu²⁺ continuously increases with the growing voltage (5–10 kV). This phenomenon could be explained by the electron penetration depth and the value could be calculated using the following formula:⁵²

In this case, A is the molecular weight of K₂HfSiO₅, Z indicates the atomic number, ρ refers to the density, and E is the loading voltage (kV). For K₂HfSiO₅:Eu²⁺, A = 364.77, Z = 207 and ρ = 4.97 g cm⁻³. The calculated electron penetration depths are 25.45 (6 kV), 92.31 (8 kV) and 307.69 (10 kV) nm, respectively. Moreover, as shown in Fig. 7(d), the CL intensity also increases with different currents varying from 50–100 mA. The deeper the electron penetration depth, the more luminescence centers detected, leading to an increase of the CL intensity.

The anti-degradation properties of K₂HfSiO₅:0.02Eu²⁺ have been tested under certain conditions (6 kV, 60 mA) shown in Fig. 8. Obviously, the CL intensity decreases slowly with the electron bombardment from 10 to 90 min. It is well known that graphite carbon will accumulate in the process of high current density electron beam irradiation. This accumulation will not only prevent low-energy electrons from reaching the phosphor particles, but also aggravate surface charging which leads to the decrease of CL intensity.⁵³ It can be observed from Fig. 8 that the CL intensity still remains about 80.12% of the initial intensity, indicating that the CL stability of K₂HfSiO₅:0.02Eu²⁺ is perfect. Meanwhile, the color stability and peak position of K₂HfSiO₅:0.02Eu²⁺ are almost unchanged. All the above results demonstrate that the K₂HfSiO₅:Eu²⁺ phosphor possesses outstanding CL properties and has potential application in CL devices.

Fig. 8 The anti-degradation properties and the relative intensity of K₂HfSiO₅:0.02Eu²⁺ under sustained electron bombardment (the top right corner represents the change of CIE coordinates and peak position).

Applications

Energy-down-shift layer for CsPbI₃ solar cells. The K₂HfSiO₅:Eu²⁺ phosphor was used to fabricate an energy-down-shift (EDS) layer for CsPbI₃ solar cells.^6,54–56 The morphology and particle size of K₂HfSiO₅:0.02Eu²⁺ phosphor are displayed in Fig. 9(a) and (b). In order to make the EDS layer have a better effect, the particle size was further reduced to an average value of 2.51 μm. Fig. 9(c) exhibits the feasibility of making the EDS layer using the K₂HfSiO₅:Eu²⁺ phosphor. The overlap of the excitation spectrum (green curve) and the strong absorption spectrum of solar cell device (red curve) illustrate that the K₂HfSiO₅:Eu²⁺ EDS layer has the ability to make the UV light from the sun light be effectively absorbed by CsPbI₃ solar cells due to the down-conversion effect.⁵⁷ The CsPbI₃ solar cells coated with the EDS layer have been packaged as shown in the schematic diagram in Fig. 9(d) according to the above analysis. The structure of the packaged CsPbI₃ solar cells with the K₂HfSiO₅:Eu²⁺ EDS layer is K₂HfSiO₅:Eu²⁺ EDS layer/FTO/TiO₂/CsPbI₃ film/spiro-OMeTAD/electrode. After packaging the spin-coated solar cells according to the schematic diagram and putting it under the irradiation of a 365 nm ultraviolet-lamp, they presented a cyan-green light as shown in the inset of Fig. 9(d), which is consistent with the emission color of K₂HfSiO₅:0.02Eu²⁺ phosphor.

Fig. 9 (a) The SEM image of K₂HfSiO₅:0.02Eu²⁺ particles for the energy-down-shift layer; (b) the average size of K₂HfSiO₅:0.02Eu²⁺ phosphor; (c) solar spectrum, emission spectrum of K₂HfSiO₅:0.02Eu²⁺ under 360 nm excitation, excitation spectrum monitored at 513 nm and the absorption spectrum of coated solar cell; and (d) schematic structure of CsPbI₃ solar cells coated with the EDS layer.

In order to more intuitively illustrate the successful application of the EDS layer on CsPbI₃ solar cells, the EDS layer was spin coated on FTO and characterized by SEM as shown in Fig. 10(a). According to the vertical view, the K₂HfSiO₅:Eu²⁺ phosphor in the EDS layer is evenly distributed in the film-forming agent PVP, and there is no obvious agglomeration of phosphor particles. The side view clearly shows that the EDS layer is a film with an average thickness of nearly about 2 μm. It could be seen from the reflection spectra in Fig. 10(b) that the reflectivity of the solar cells coated with the EDS layer has decreased. Therefore, it can be concluded that part of the UV light has been converted into visible light. The I–V curves are measured as shown in Fig. 10(c). When coated with this EDS layer, the J_SC shows an increasing tendency after coated with the EDS layer. To further verify the effect of the K₂HfSiO₅:Eu²⁺ phosphor EDS layer, the external quantum efficiency (EQE) spectra are shown in Fig. 10(d). In the range of UV light, the value of uncoated sample is lower than that of the coated sample. However, the EQE curves of the two samples are similar in other ranges of light. Therefore, it can be concluded that the increase of J_sc is related to the application of the energy transfer layer. PCE could be calculated from the parameters shown in Table 4:

Fig. 10 (a) SEM images of the EDS layer in vertical and side views; (b) reflection spectra of solar cells; (c) I–V curves of solar cells; (d) the EQE spectra of CsPbI₃ solar cells with coated and uncoated solar cells.

Table 4 Key I-V parameters of CsPbI₃ solar cells coated with and without the EDS layer

Sample	V oc (V)	J SC (mA cm^−2)	Fill factor (%)	PCE (%)	ΔPCE (%)
Bare cell	1.007	19.77	72.47	16.35	2.19
Coated cell	1.012	20.05	72.48	16.73

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This result shows that the CsPbI₃ solar cells coated with the EDS layer present a higher PCE value of 16.83%, with an amplification of 2.19% compared with that of the bare cell. These results indicate that K₂HfSiO₅:0.02Eu²⁺ could be used as an EDS layer to improve the PCE of CSPbI₃ solar cells. At the same time, it also provides the possibility for the combination of the luminescence material and perovskite solar cell technology.

4. Conclusions

In this work, a series of K₂HfSiO₅:xEu²⁺ (0.01 ≤ x ≤ 0.035) phosphors were successfully designed and synthesized for the first time. The structure, morphology and bandgap were investigated in detail. Under 360 nm excitation, the K₂HfSiO₅:0.02Eu²⁺ phosphor emits a high efficiency cyan-green light and the peak is located at 513 nm. After the feasibility analysis, we packaged the CsPbI₃ solar cells coated with the K₂HfSiO₅:0.02Eu²⁺ phosphor EDS layer and the major electrochemical parameters were increased. Under the sustained electron-beam excitation, the K₂HfSiO₅:0.02Eu²⁺ phosphor shows excellent anti-degradation properties and a perfect color stability. In addition, since the K₂HfSiO₅:Eu²⁺ phosphor has potential for multifunctional application, we can imagine that in future, we can use the K₂HfSiO₅:Eu²⁺ EDS layer to improve the photoelectric conversion rate of solar cells, and then apply the electric energy to cathodoluminescence devices fabricated using the K₂HfSiO₅:Eu²⁺ phosphor. All the above results offer us a novel way to achieve effective utilization of solar energy.

Conflicts of interest

There are no conflicts to declare.

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