Deep Red Phosphors Based on 3d Transition Metals for Energy Conversion from UV to Visible Rays

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Reflecting the recent increasing concern about an energy issue, solar cells have been developing for several decades. Silicon is the major solar cell material with the band gap energy of ~1.17 eV. The optical absorption band corresponds to ~1100 nm and the radiations with wavelengths shorter than it are absorbed to generate photovoltaic power. The spectral sensitivity characteristics do not match the solar spectrum and energy conversion of the UV and blue radiations to longer wavelength side will improve further the efficiency of the Si solar cells. This work presents deep-red phosphors based on lithium aluminates and related compounds using 3d transition metals as the luminescent center. Deep-red emission by 3d transition metal phosphors has been much attracted because it can be attained with quite few rare-earth phosphors. LiAlO2 and LiAl5O8 were chosen for phosphor host materials, which were prepared through a solid state reaction method from lithium and aluminum sources of LiF and Al2O3. The luminescent center of Fe3+ or Mn4+ was added in a form of Fe(NO3)3·9H2O or MnO2. The mixed powder in a Pt crucible was heat treated in air at 800-1300 °C. It was found that the lithium source LiF brought several unusual phases in the products. Metastable α-LiAlO2 appeared at 800 °C and below when a mixture of LiF and Al2O3 was treated with an excess a LiF condition of LiF/Al = 3. It was good contrast to the fact that the a-phase obscurely crystalizes at 500-700°C with the broad peaks when LiOH·H2O was used as a lithium source. α-LiAlO2 transformed to stable γ-LiAlO2 at 800 °C or above. On the other hand, a mixture at LiF : Al2O3 = 1 : 2 resulted in a novel host material with an unknown composition. Although the XRD pattern itself has been reported as JCPDS #38-610, the details of the crystal structure and the actual composition have never been disclosed. The composition estimated in this work with EDS and XPS was Al4LiF0.1O6.45. A disordered spinel structure was assigned for ALFO by Rietveld analysis. The luminescent spectra are compared among α-LiAlO2:Fe3+, γ-LiAlO2:Fe3+ and ALFO:Fe3+ in Fig. 1, and between LiAl5O8:Mn4+ and ALFO:Mn4+ in Fig. 2. α-LiAlO2:Fe3+, γ-LiAlO2:Fe3+ and ALFO:Fe3+ showed rather broad PL with the peak tops between 650 and 800 nm. Their peak positions are slightly different to each other and the emission peak of ALFO:Fe3+ resides at the shortest wavelength side. The excitation bands are in the UV region and the maxima are around 250 nm. On the contrary, the Mn4+ luminescent center changes its properties depending on the host materials. Deep red emission at 661 nm was much enhanced in ALFO:Mn4+, whereas LiAl5O8:Mn4+ exhibited only faint PL. Acknowledgement: This work was performed under the Cooperative Research Program of "Network Joint Research Center for Materials and Devices (20181151). Figure 1