Investigation of up and down-converting rare earth ions doped ZnTiO3 for photovoltaic applications

Abstract

We are living in an age where the demand for energy is growing rapidly. This means that supplies to easily accessible oil and natural gas is unlikely to keep up with the demand as times goes on. The world will have to use energy more efficiently and increase its use of other sources of energy. This study is aiming at developing materials that will improve the power conversion efficiency of photovoltaic cells by using up and down-converting phosphor materials. ZnTiO3-Zn2TiO4 composite and ZnTiO3 phosphors doped with Er3+,Yb3+, Eu3+ and Al3+, which display up and down-converted luminescence were synthesized by a simple high temperature conventional solid state reaction method. The structure, particle morphology, absorption, photoluminescent properties and elemental distribution were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis-NIR absorption spectrometer, photoluminescence (PL) spectroscopy and time of flight secondary ion mass spectroscopy (TOF-SIMS), respectively. ZnTiO3-Zn2TiO4 composite doped with different concentration of Er3+ ions was synthesized via solid state chemical reaction method at 1100 ℃. The X-ray diffraction (XRD) confirmed the crystallization of both the hexagonal ZnTiO3 and cubic spinel Zn2TiO4 phases of the composite. The SEM images of ZnTiO3-Zn2TiO4:Er3+ composite showed that the particle morphology was made up of faceted hexagons. Furthermore, the ZnTiO3-Zn2TiO4:Er3+ phosphors were excited in the near-infrared (NIR) region using a laser diode with a wavelength of 980 nm and displayed both green and red up-conversion emission bands in the visible range at 543, 553, 650 – 670 nm. These emission bands correspond to 2H11/2, 4S1/2→ 4 I15/2 and 4F9/2→ 4 I15/2 transitions of Er3+ ions. However, the interaction mechanisms involved in the upconversion process of ZnTiO3-Zn2TiO4:Er3+ phosphor is discussed with the help of an energylevel schematic diagram and the number of the photons involved in the up-conversion luminescence process were of a double photon mechanism. The decay lifetimes were studied by fitting the luminescence decay curve with a single-component exponential decay. Er3+ and Yb3+ incorporated zinc titanate (ZnTiO3) phosphor powders were synthesized using conventional solid-state reaction method at 800 ℃. A ZnTiO3:Er3+,Yb3+ phosphor that resembled an ecandrewsite single phase with space group R-3 (148) was obtained, as proven by X-ray diffraction (XRD). The SEM image showed a surface morphology composed of agglomerated irregular shaped particles. The energy band gap of ZnTiO3 was engineered by incorporating different concentration of the dopant ions. After irradiating ZnTiO3:Er3+with a 980 nm laser beam, the phosphor up-converted the photon energy to display green and red emissions in the visible range that were positioned at 527, 545 and 665 nm. Enhancement of the luminescence intensity of ZnTiO3:Er3+ phosphor was achieved by variation of Er3+ concentration. Co-doping with Yb3+ ions proved to be effective in enhancing the luminescence intensity of the optimized Er3+ ion emission and new emission bands at 410 and 480 nm, through an energy transfer mechanism were observed. The enhancement of the lifetime of the up-conversion luminescence was also achieved by co-doping ZnTiO3:Er3+ phosphor with Yb3+ ion. The energy transfer mechanisms involved in Er3+ - Yb3+ co-doped ZnTiO3 phosphor was illustrated and discussed in detail. The ZnTiO3:Er3+, Yb3+ thin films were successfully deposited by pulsed laser deposition (PLD) by varying the silicon (100) substrate temperature. The distribution of the ions in the films was investigated and the TOF-SIMS showed that the ions were homogeneously distributed throughout the ZnTiO3 host lattice which indicated a successful incorporation of the Er3+ and Yb3+ ions. The optical response of the phosphors revealed that the reflectance percentages of the ZnTiO3:Er3+, Yb3+ vary with the silicon substrate temperature due to the differences in the thickness and morphological roughness of the thin films. The ZnTiO3:Er3+, Yb3+ thin films also exhibited up-conversion emission from Er3+ electronic transitions, with violet, blue, green and red emission lines at 410, 480, 525, 545 and 660 nm from 2H9/2 → 4 I15/2, 4F7/2 → 4 I15/2, 2H11/2 → 4 I15/2, 4S3/2 → 4 I15/2 and 4F9/2 → 4 I15/2 transitions, respectively. These up-conversion emissions were enhanced by increasing the silicon substrate temperature during the deposition. ZnTiO3 host co-doped with Eu3+ and Al3+ was synthesized by solid state reaction to convert the UV photons to visible photons. Charge compensation effects of Al3+ incorporated ZnTiO3:Eu3+ as a co-dopant ion was reported in detail. The structural and morphological characterization show that the addition of Eu3+ and Al3+ does not affect the phase formation and the surface morphology of the host. The visible emission intensity of Eu3+ ions for an optimal concentration of 2 mol% under 395 nm excitation, was enhanced by incorporating Al3+. The energy level diagram showing the charge compensation mechanism was proposed for the co-doped system.