Effect of broadband excitation ions in the luminescence of Ln.³+ doped SrF₂ nanophosphor for solar cell application
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Date
2015-06
Authors
Yagoub, Mubarak Yagoub Adam
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
SrF2:Pr3+-Yb3+ phosphor powder was previously investigated for down-conversion application
in solar cells. The rst surface, structural and optical characterization results
indicated that the Pr3+-Yb3+ couple requires a sensitizer for effective enhancement in
energy conversion. Broadband excitation ions of Ce3+ and Eu2+, that could be used
as sensitizers, were therefore doped and co-doped in the SrF2 crystal. Detailed characterizations
and investigations were then done on the surface, structure and optical
aspects to see the effect on the energy conversion.
Initially, the influence of different synthesis techniques on the surface, structure and
concentration quenching of Pr3+ doped SrF2 was studied. The singly doped SrF2:Pr3+
was prepared by the hydrothermal and combustion methods. Scanning electron microscope
(SEM) images showed different morphologies which was an indication that the
morphology of the SrF2:Pr3+ phosphor strongly depended on the synthesis procedure.
Both the SrF2:Pr3+ samples exhibited blue-red emission under a 439 nm excitation
wavelength at room temperature. The emission intensity of Pr3+ was also found to be
dependent on the synthesis procedure. The dipole-dipole interaction was found to be
responsible for the concentration quenching effects at high Pr3+ concentration in both
methods.
SrF2:Eu nano-phosphors were successfully synthesized by the hydrothermal method.
The crystalline size of the phosphors was found to be in the nanometre scale. The photoluminescence
and high resolution x-ray photoelectron spectroscopy (XPS) results
indicated that the Eu was in both Eu2+ and Eu3+ valance states. The presence of
Eu2+ and Eu3+ in the system largely enhanced the response of the Eu3+ under ultraviolet
excitation. Time of flight secondary ion mass spectrometry (tof-SIMS) results
suggested that the energy transfer between these two ions was likely occurred. The
relative photoluminescence intensity of the Eu2+ rapidly decreased with an increasing
laser beam irradiating time. This result would make the current Eu2+ doped SrF2 samples
unsuitable candidates for several applications, such as white light-emitting diodes
and wavelength conversion films for silicon photovoltaic cells. The effect of Ce3+ ions on the SrF2:Eu nano-phosphor was also studied. Ce3+ largely
enhanced the Eu3+ emission intensity via energy transfer mechanism. The calculated
energy transfer efficiency was relatively effcient at high Eu concentration. The results
suggested that Ce3+ may therefore be used as an efficient sensitizer to feed the Eu ions
in SrF2 host.
Eu2+ co-doped Pr3+, Yb3+ and Pr3+-Yb3+ couple in SrF2 were successfully prepared.
XPS confirmed that all Eu contents were in Eu2+ oxidation states. Initially, Eu2+
co-doped SrF2:Pr3+ was studied. From PL and decay curve results, an efficient energy
transfer was demonstrated in SrF2:Eu2+, Pr3+ phosphors. The energy transfer process
was effective until a concentration quenching between Pr3+ ions occurred. The results
proposed that Eu2+ could be a good sensitizer for absorbing the UV photons and hence
efficiently enhancing the Pr3+ emission intensity.
SrF2:Eu2+ (1.5 mol%) co-doped with Na+ (0.5 mol%) and various concentrations of
Yb3+ were also investigated. XRD results showed a mixture of the cubic SrF2 and
NaYbF4 phases. The NaYbF4 phase gradually formed with increasing Yb3+ doping
concentration. Emission spectra and the
fluorescence decay curve measurements were
utilized to demonstrate the cooperative energy transfer. Energy transfer occurred
subsequently from Eu2+ to Yb3+ followed by intense NIR emission. The energy transfer
was completed at high concentrations but the Yb3+ emission intensity was reduced as
a result of concentration quenching. In addition, from the photoluminescence data it
was evident that Na+ induced significant change to NIR emission.
The possibility of using the broadband absorption of Eu2+ to sensitize the Pr3+-Yb3+
down-conversion couple in SrF2 matrix was also investigated. The energy transfer
process was demonstrated by the decrease of Eu2+ and Pr3+ related photoluminescence
and lifetime with increasing Yb3+ concentration. Upon 325 nm excitation into the
5d levels of Eu2+, the samples yield intense near infrared emission corresponding to
Pr3+:4f-4f and Yb3+:4f-4f transition. Yb3+ emission was clearly observed only at high
Yb3+ concentrations after the emission intensity of Pr3+ was quenched. The PL lifetime
results of Eu2+ confirmed the the second-order cooperative energy transfer also occurred
between Eu2+ and Yb3+ ions.
Description
Keywords
Luminescence, Solar cells, Thesis (Ph.D. (Physics))--University of the Free State, 2015