Bi 3+ Ion as a Performance Curator in a Y 2 Ge 2 O 7 (Yb-Er) Spectral Convertor: Proliferation of Sensitization Range and Competence

Optoelectronics lies at the core of numerous technological devices. The growth of the optoelectronics field largely relies on functional phosphors in which multipolar interactions between the phosphor center and the activator/sensitizer play a profound role in determining the device efficiency. Though the robustness, tunable lattice, and dielectric nature of germanates render them as frontrunner host materials for optoelectronics, yet their potential as the host of an optically active center has been largely underexplored. The present work reports first-time exploration of a yttrium pyrogermanate system to host an efficient upconverting Yb3+-Er3+ couple with Bi3+, introduced as a supplementary codopant to tailor system polarity and performance. Meticulous analysis by different techniques established the formation of the tetragonal Y2Ge2O7 phase with a homogeneous distribution of the luminescent center (Er3+) and sensitizer (Yb3+, Bi3+). DFT calculations in conjunction with experimental optical parameters reveal that while inclusion of Bi3+ preserves the geometrical symmetry of the host lattice, concurrently it enhances polarity in the neighbourhood of Er3+ ions and overall ionicity of host. This facilitates stronger multipolar interactions between the luminescent center and sensitizers along with enhancment in the probability of different electronic transitions of Er3+. Detailed analysis of relative intensity ratios of the green emission of Er3+ ions indicated that Y2Ge2O7:Yb/Er and Bi3+ codoped Y2Ge2O7:Yb/Er are suitable for temperature sensing in the 300-400 K range. Notably, inclusion of Bi3+ in Y2Ge2O7:Yb/Er not only enhances near-infrared-induced visible upconversion of Er3+ but simultaneously it also opens up the window of UV-to-visible downconversion by additional sensitization. In summary, the polar nature and dual mode luminescence of the "Bi3+ incorporated Y2Ge2O7:Yb/Er phosphor" make it a promising candidate for phosphor converted light-emitting diode application and spectral conversion application for boosting solar cell efficiency.