Broadband shifting luminescence in Cr3+/Yb3+ codoped Y3Al5O12 thin films by pulsed laser deposition

Yinzhen Wang; Ning Li; Pingping Duan; Junyong Deng; Liaolin Zhang; Benli Chu; Qinyu He

doi:10.3788/COL201513.060701

Abstract

{Cr}^{3 +} / {Yb}^{3 +}

codoped

Y_{3} {Al}_{5} O_{12}

(i.e., YAG) thin films are prepared by pulsed laser deposition. The films are characterized by X-ray diffraction, atomic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and photoluminescence spectra. Excitation at 446 or 587 nm, a broadband emission in the range of 610–800 nm, and an intense near-infrared at 1030 nm are obtained, showing cooperative energy transfer from

{Cr}^{3 +}

{Yb}^{3 +}

ions in the

{Cr}^{3 +} / {Yb}^{3 +}

codoped YAG thin films; energy transfer efficiency is 71%. The YAG films may have potential application to enhance the efficiency of silicon solar cells.

Recently, the rare earth (RE)-ion-doped down-conversion (DC) materials have attracted intensive interest due to their promising application as spectral convertors to improve photovoltaic conversion efficiency. The theoretical limit for the conversion efficiency can be increased from 30% to 39% by applying an ideal down-converting layer on top of Si solar cells^[1]. Various trivalent RE ions are possible, such as $\Pr^{3 +}$ , ${Ho}^{3 +}$ , ${Er}^{3 +}$ , ${Tb}^{3 +}$ , and ${Tm}^{3 +}$ with f–f transition^[1–9]. In order to make better use of sunlight, RE ions with f–d transition, such as ${Ce}^{3 +}$ , ${Eu}^{2 +}$ , and ${Yb}^{2 +}$ ^[10–12]; metal ions, such as ${Cr}^{3 +}$ and ${Bi}^{3 +}$ ^[13–15]; the host^[16] were chosen as the energy donors for ${Yb}^{3 +}$ to realize the broadband spectral conversion. At present, most studies in DC are carried out in powders, crystals, glass ceramics, and glass. For practical application, a down-converting layer should be placed on the front of the p–n junction of solar cells^[17]. For powders, the scattering of the incident light becomes a negative factor, which will greatly weaken the absorption of the incident solar light. Regarding crystals, glass ceramics and glass are very fragile and it has big residual stress in the growth processing. On the contrary, it is easy to control film thickness, high transmittance, and also doping concentration of RE ions. Thin-film phosphors can be prepared by many deposition techniques such as pulsed laser deposition (PLD)^[18]. In this work, we first explored the efficient broadband energy transfer (ET) from ${Cr}^{3 +}$ to ${Yb}^{3 +}$ ions in $Y_{3} {Al}_{5} O_{12}$ (i.e., YAG) thin films by PLD. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra show that ET happened between ${Cr}^{3 +}$ and ${Yb}^{3 +}$ ions. The decay lifetime of ${Cr}^{3 +}$ and ET efficiency of the films were investigated. These results showed that the efficient DC of ${Cr}^{3 +} - {Yb}^{3 +}$ codoped YAG films might be a good way to improve the efficiency of silicon-based solar cells.

The ${Cr}^{3 +} / {Yb}^{3 +}$ codoped YAG target was prepared by conventional solid-state reaction methods using analytical grade $Y_{2} O_{3}$ , ${Al}_{2} O_{3}$ , ${Yb}_{2} O_{3}$ , and ${Cr}_{2} O_{3}$ powders as the starting materials. These powders were weighted according to the molecular formula $Y_{2.88} {Cr}_{0.02} {Yb}_{0.1} {Al}_{5} O_{12}$ . The starting powders with designed stoichiometric quantities were ball milled for 24 h, then dried and calcinated at 1500°C for 10 h. The resulting powders were pressed into disk pellets and sintered at 1400°C for 6 h. The as-prepared target showed a YAG crystalline phase in the X-ray diffraction (XRD) pattern. The $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ film was grown by PLD on Si(100) substrate at room temperature. PLD process was performed in a vacuum chamber, vacuum degree of $10^{- 5} Pa$ , oxygen pressure of 6.2 Pa, and laser pulse energy power of 300 mJ. After the deposition, the $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films were annealed at 1000°C for 120 min.

XRD patterns of the samples were recorded with a Rigaku D/max-IIIA X-ray diffractometer ( $Cu - K α 1$ , $λ = 0.15405 n m$ ). The surface morphology of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films was studied by atomic force microscopy (AFM; Digital Instrument Nanoscope IIIa) and transmission electron microscopy (TEM). The chemical state of the films was studied by X-ray photoelectron spectroscopy (XPS) using an ESCALAB250 system. Optical spectroscopy, including emission, excitation spectra, and the decay curves of the specimens, was recorded with an Edingburgh FLS920 fluorescence spectrophotometer at room temperature.

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Figure 1 shows the XRD patterns of as-grown and annealed $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ thin films deposited on Si(100) substrates. There is no diffraction peak of as-deposited $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ thin films, indicating that the $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ thin films are amorphous. After annealing at 1000°C, diffraction peaks are clearly observed; main diffraction peaks can be well-assigned to the standard JCPDS card of YAG (JCPDS No. 79-1891) indicating the pure cubic YAG phase with polycrystalline structure. No obvious shifting of peaks or second phase can be detected at the current doping level, indicating that the introduction of ${Cr}^{3 +}$ and ${Yb}^{3 +}$ ions does not change the crystal structure.

Figure 1.XRD patterns of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ thin films.

Figure 2 shows an AFM image of the $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ thin films annealed at 1000°C. AFM images of the films show a smooth surface with an average grain size of around 16 nm, which is consistent with XRD results. The roughness of the surface of the film is about 8.32 nm.

Figure 2.AFM image of the $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ thin film.

Figure 3 shows the XPS spectrum of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ thin film. The XPS survey spectrum confirms the presence of Al, O, Y, Cr, Yb, and C (from carbon pollution).

Figure 3.XPS spectrum of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ film.

Figure 4(a) shows the cross sectional transmission electron micrograph of the $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ thin films; the thickness of the film was 116 nm. Figure 4(b) shows a high-resolution transmission electron micrograph (HRTEM), clear lattice can be seen, but there are obvious boundaries, such as the red line which marks the approximate location of the grain boundaries, indicating the presence of a polycrystalline sample. The interplanar spacing of the (400) plane is 0.30 nm. Figure 4(c) presents the corresponding selected area electron diffraction (SAED) pattern; SAED patterns show the (400), (420), and (440) rings typical of the polycrystalline YAG structure.

Figure 4.TEM images of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ thin films.

Figure 5 shows the excitation spectra of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films. The excitation spectra show two broad excitation bands at 446 and 595 nm by monitoring ${}^{2}E \to {}^{4}{A_{2}}$ transition of ${Ce}^{3 +}$ at 670 nm and ${}^{2}{F_{5 / 2}} \to {{}^{2}F}_{7 / 2}$ transition of ${Yb}^{3 +}$ at 1026 nm; the 446 and 595 nm excitation bands are ascribed to ${}^{4}{A_{2}} \to {{}^{4}T}_{1}$ transition and ${}^{4}{A_{2}} \to {{}^{4}T}_{2}$ transition of ${Cr}^{3 +}$ , respectively. Figure 6 shows the visible emission spectra of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ . The emission spectrum of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films shows a broadband emission in the range of 610–800 nm with a maximum emission at 688 nm assigned to ${Cr}^{3 +} {}^{2}E \to {{}^{4}A}_{2}$ transition under 446 and 587 nm excitation. Figure 7 shows the near-infrared (NIR) emission spectra of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ . The NIR emission peak centered at 1030 nm originated from ${}^{2}{F_{5 / 2}} - {{}^{2}F}_{7 / 2}$ transition of ${Yb}^{3 +}$ ions can be clearly observed under the excitations of ${Cr}^{3 +} : {}^{4}{T_{1}}$ or ${}^{4}{T_{2}}$ energy levels at 446 or 587 nm, which indicates the ET from ${Cr}^{3 +}$ to ${Yb}^{3 +}$ . For ${Yb}^{3 +}$ single-doped sample, no emission at 900–1100 nm was detected excited by 466 or 587 nm. In addition, the NIR emission spectrum of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films excited with simulated sunlight illumination (400–800 nm) is the same as the emission spectrum excited with xenon lamp at 446 or 587 nm of ${Cr}^{3 +}$ ; sunlight can be directly applied as a light source for the solar cells’ down-converting layer.

Figure 5.Excitation spectra of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films.

Figure 6.Visible emission spectra of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films.

Figure 7.NIR emission spectra of $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films.

ET from ${Ce}^{3 +}$ to ${Yb}^{3 +}$ can be further clarified with the help of decay curves. Figure 8 shows decay curves of 688 nm emission of ${Cr}^{3 +}$ under 466 nm excitation. For the ${Cr}^{3 +}$ single-doped sample, the decay curve of ${Cr}^{3 +}$ exhibits a double-exponential feature. With ${Yb}^{3 +}$ doping, the lifetime decreases rapidly and the lifetime of ${Cr}^{3 +}$ decreases, implying the introduction of the extra decay pathway, i.e., cooperative energy transfer (CET) from ${Cr}^{3 +}$ to ${Yb}^{3 +}$ . Figure 9 illustrates the schematic energy levels for ${Cr}^{3 +}$ ion and ${Yb}^{3 +}$ ions in YAG. The ${Cr}^{3 +}$ ion is first excited from ${}^{4}{A_{2}}$ ground state to ${}^{4}{T_{1}}$ and ${}^{4}{T_{2}}$ excitation state, ${}^{4}{T_{1}}$ followed nonradiative relaxation down to the ${}^{4}{T_{2}}$ and to further relaxation to ${}^{2}E$ level, or directly from ${}^{4}{T_{2}}$ to ${}^{2}E$ , then pass to nearly ${Yb}^{3 +} : {}^{2}{F_{5 / 2}}$ energy level, i.e., ${Cr}^{3 +} : {}^{4}{T_{2}} \to Yb : {}^{2}{F_{5 / 2}}$ and ${Cr}^{3 +} : {}^{4}{T_{1}} \to Yb : {}^{2}{F_{5 / 2}}$ . Subsequently, the radiative transition of ${Yb}^{3 +} : {}^{2}{F_{5 / 2}} \to {{}^{2}F}_{7 / 2}$ occurs, generating 1030 nm photons.

Figure 8.Decay curves of ${Cr}^{3 +}$ at 688 nm with the excitation at 466 nm for the $YAG : {Cr}^{3 +}$ and $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films.

Figure 9.Energy-level diagram and CET mechanism of ${Cr}^{3 +}$ and ${Yb}^{3 +}$ in $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films.

From the decay curves in Fig. 8, the transfer efficiency $η_{E T}$ can be calculated using^[2] $η_{E T} = 1 - \frac{\int I_{y {Yb}^{3 +}} d t}{\int I_{0 {Yb}^{3 +}} d t},$ (1)where $I$ denotes intensity, and $y$ stands for the ${Yb}^{3 +}$ concentration. The obtained ET efficiency for YAG samples is 71%, which is higher than ${Cr}^{3 +} - {Yb}^{3 +}$ codoped YAG crystals (60%)^[13], indicating $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films can as convertors to enhance the solar cell efficiency.

In conclusion, $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films are investigated as a promising material to enhance the energy conversion efficiency of silicon-based solar cells. Excitation spectra, emission spectra, and decay measurements are performed to prove the CET from ${Cr}^{3 +}$ to ${Yb}^{3 +}$ ions. This evidence implies that $YAG : {Cr}^{3 +} / {Yb}^{3 +}$ films are promising candidates for improving the efficiency of silicon-based solar cells by means of DC.