Investigating the effect of alkali metals on the structural & optoelectronic properties of hexafluorozirconate red phosphors A2ZrF6 (A = Cs, K, Na) using first-principles calculations: A prospect for warm-white LEDs (w-LEDs) applications

Abstract

To meet the energy challenges of the near future, the development of environment-friendly and energy-efficient phosphors for luminescence devices like light-emitting diodes (LEDs) is the main quest of researchers nowadays. In this connection, traditional lighting devices like halogen lamps, fluorescent lamps, backlights of liquid crystal displays (LCDs), and incandescent lamps can be replaced by state-of-the-art next-generation lighting technology developed using phosphor-converted white LEDs. Hereby, we have presented a systematic study on the structural and optoelectronic properties of Zr-based phosphors A2ZrF6 (A = Cs, K, Na) for potential photoluminescence and photovoltaic applications like LEDs. The GGA and GGA + U schemes are used for exchange and correlation energy potentials treatment to provide a comparative study on the density functional theory (DFT) based first-principles calculations. The GGA + U approach improves bandgap values, confirming the well-known flaw of GGA-based DFT methods in underestimating band gaps in strongly correlated systems. Indirect bandgaps are found with the values 2.05, 6.415, and 7.648 eV for Cs2ZrF6, K2ZrF6, and Na2ZrF6, respectively. The epsilon 2(omega) spectra show that Cs2ZrF6 absorbs the maximum number of incident photons in the visible region, whereas K2ZrF6 and Na2ZrF6 absorb the maximum of the incident photons in the near UV region. From the R(omega) spectra, we can note that A2ZrF6 (A = Cs, K, Na) are the weak reflector of incident photons in the visible and IR regions. In contrast, up to 40% of the incident photons are reflected on higher energies (above 12.0 eV).