Effect of broadband excitation ions in the luminescence of Ln.³+ doped SrF�?nanophosphor for solar cell application

[摘要] SrF2:Pr3+-Yb3+ phosphor powder was previously investigated for down-conversion applicationin solar cells. Therst surface, structural and optical characterization resultsindicated that the Pr3+-Yb3+ couple requires a sensitizer for effective enhancement inenergy conversion. Broadband excitation ions of Ce3+ and Eu2+, that could be usedas sensitizers, were therefore doped and co-doped in the SrF2 crystal. Detailed characterizationsand investigations were then done on the surface, structure and opticalaspects to see the effect on the energy conversion.Initially, the influence of different synthesis techniques on the surface, structure andconcentration 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 themorphology of the SrF2:Pr3+ phosphor strongly depended on the synthesis procedure.Both the SrF2:Pr3+ samples exhibited blue-red emission under a 439 nm excitationwavelength at room temperature. The emission intensity of Pr3+ was also found to bedependent on the synthesis procedure. The dipole-dipole interaction was found to beresponsible for the concentration quenching effects at high Pr3+ concentration in bothmethods.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 photoluminescenceand high resolution x-ray photoelectron spectroscopy (XPS) resultsindicated that the Eu was in both Eu2+ and Eu3+ valance states. The presence ofEu2+ and Eu3+ in the system largely enhanced the response of the Eu3+ under ultravioletexcitation. Time of flight secondary ion mass spectrometry (tof-SIMS) resultssuggested that the energy transfer between these two ions was likely occurred. Therelative photoluminescence intensity of the Eu2+ rapidly decreased with an increasinglaser beam irradiating time. This result would make the current Eu2+ doped SrF2 samplesunsuitable candidates for several applications, such as white light-emitting diodesand wavelength conversion films for silicon photovoltaic cells. The effect of Ce3+ ions on the SrF2:Eu nano-phosphor was also studied. Ce3+ largelyenhanced the Eu3+ emission intensity via energy transfer mechanism. The calculatedenergy transfer efficiency was relatively effcient at high Eu concentration. The resultssuggested that Ce3+ may therefore be used as an efficient sensitizer to feed the Eu ionsin 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 energytransfer was demonstrated in SrF2:Eu2+, Pr3+ phosphors. The energy transfer processwas effective until a concentration quenching between Pr3+ ions occurred. The resultsproposed that Eu2+ could be a good sensitizer for absorbing the UV photons and henceefficiently enhancing the Pr3+ emission intensity.SrF2:Eu2+ (1.5 mol%) co-doped with Na+ (0.5 mol%) and various concentrations ofYb3+ were also investigated. XRD results showed a mixture of the cubic SrF2 andNaYbF4 phases. The NaYbF4 phase gradually formed with increasing Yb3+ dopingconcentration. Emission spectra and the fluorescence decay curve measurements wereutilized to demonstrate the cooperative energy transfer. Energy transfer occurredsubsequently from Eu2+ to Yb3+ followed by intense NIR emission. The energy transferwas completed at high concentrations but the Yb3+ emission intensity was reduced asa result of concentration quenching. In addition, from the photoluminescence data itwas 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 transferprocess was demonstrated by the decrease of Eu2+ and Pr3+ related photoluminescenceand lifetime with increasing Yb3+ concentration. Upon 325 nm excitation into the5d levels of Eu2+, the samples yield intense near infrared emission corresponding toPr3+:4f-4f and Yb3+:4f-4f transition. Yb3+ emission was clearly observed only at highYb3+ concentrations after the emission intensity of Pr3+ was quenched. The PL lifetimeresults of Eu2+ confirmed the the second-order cooperative energy transfer also occurredbetween Eu2+ and Yb3+ ions.

[发布机构] University of the Free State