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.