Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

Fig. 1. Mechanisms of downshifting and down-conversion in Tb 3+ , Yb 3+ and crossrelaxation mechanisms within
Tb 3+ iob and Tb 3+ , Yb 3+ ions pair
Y 4
Al 2
O 9
is one of the four crystalline phases in the Y 2
O 3
–Al 2
O 3
system with the formation of monoclinic, maintaining Y 2
O 3
:Al 2
O 3
ratio of 2:1. Other phases are cubic garnet with ratio of 3:5
(Y 3
Al 5
O 12
, abbreviated YAG), orthorhombic perovskite with ratio
of 1:1 (YAlO 3
, abbreviated YAP) and a metastable hexagonal structure
phase YAH, with the same stoichiometry as YAM, observed
during the synthesis by soft chemistry methods. Crystals grown
by standard Czochralski method crack during cooling because of
phase transition in YAM. In our investigations the modified solgel
method was used to obtain YAM:Tb 3+ +Yb 3+ samples, in form
of nanopowders. The samples were obtained in the Institute of
Electronic Materials Technology in Warsaw. The measurements
were performed using Photon Technology International spectrophotometer.
The properties of Tb 3+ activated YAM hosts were subject of
few investigations. In case of YAM: Tb 3+ +Yb 3+ system the main
routes for down-conversion and downshifting are presented in
the Fig. 1. the 5 D 4
Tb level plays critical role in down-conversion
process. The energy of the Tb 3+ 5 D 4
→ 7 F 6
transition is twice larger
than the energy of the Yb 3+ 2 F 5/2
→ 2 F 7/2
transition. This suggests
a possibility of efficient energy transfer between 5 D 4
level of single
Tb 3+ ion and 2 F 5/2
levels of two Yb 3+ ions. The emission spectrum
of Yb 3+ ion possesses two distinct peaks at 976 nm and 1026
nm, which suit transitions from Stark sub-levels of the 2 F 5/2
level.
This emission may be excited by direct excitation of Yb 3+ ion at
908 nm or via down-conversion of excitation of Tb 3+ ion at about
274 nm. Such wavelength corresponds to spin-allowed transition
of electron from 4f 8 to 4f 7 5d 1 configuration [4]. This down-conversion
mechanism is the most desirable due to emission from Yb 3+
coinciding with the range of high spectral response of the standard
crystalline silicon cells.
The down-conversion is not the only mechanism occurring in
Tb 3+ +Yb 3+ doped system. Another way of relaxation of exited 4f 7 5d 1
configuration is by emission o light at wavelengths corresponding
to transitions from 5 D 3
→ 7 F J
and longer wavelength emissions from
5
D 4
→ 7 F J
(J=6,5,4,3,2). The strongest of those emissions occurs in
case of 5 D 4
→ 7 F 5
transition, which gives UV irradiated terbium materials
a distinct green glow. Spectrum of above mentioned emissions
are presented in Fig. 3, along with excitation spectrum of the
5
D 4
→ 7 F 5
emission. Shape of the spectrum, again with dominant
feature at around 274 nm confirms that both Yb 3+ NIR and Tb 3+ VIS
emissions primarily result from the same UV excitation and are
thus competitive processes. From point of view of obtaining maximally
efficient down-conversion process emissions from higher
energy 5 D 3
level to 7 F J
multiplets are particularly unfavorable, since
they provide pathway of depopulation of this level before excited
electrons non-radiatively decay to the lower 5 D 4
level, where
they have chance to take part in the cooperative energy transfer
Fig. 2. Excitation spectrum of Yb 3+ IR emission at 1026 nm (black) and
emission spectrum from excitation of Tb 3+ ion at 274 nm indicates
energy transfer from Tb 3+ to Yb 3+ ions
114
Fig. 3. Emission spectrum resulting from excitation of Tb 3+ ion at 274
nm and excitation spectrum of green emission at 544 nm
Elektronika 6/2012