Broad NIR emission near c - Si band gap from Bi-doped Ba–Al metaphosphate glasses as promising solar spectral converter

A broad and intense visible as well as NIR luminescence is obtained from bismuth ions doped in alkali-free Ba–Al metaphosphate glasses. The glasses with varied bismuth oxide content have been prepared by melt quenching technique. With increase in bismuth concentration, the NIR emitting centers are found to be dominant, while at lower concentrations no NIR emission is perceived. The visible emission covers significant portion of the visible solar spectrum, while the NIR band obtained is much blue-shifted as compared to other host matrices. FTIR reflection spectroscopy studies elucidate that the dopant bismuth plays the role of a network modifier and weaken the network by formation of NBOs and P–O–Bi bonds at the expense of [PO₂] bonds. The excitation and emission spectra indicate the presence of bismuth in multiple valence state, mainly as Bi⁺, Bi²⁺, and Bi³⁺. The visible and NIR emissions are arising from different luminophore centers of dopant ions that are getting simultaneously excited.