Complete Report - University of New South Wales

Many lanthanide energy levels are below about 10000 cm -1 (with emission wavelengths less
than about 1000nm) making them potential UC luminescent centres for silicon PV-UC devices.
Unfortunately, closely spaced energy levels result in very large non-radiative relaxation.
Investigation of the Dieke diagram suggests that the only promising co-doped ETU system
that may improve the spectral robustness of a silicon PV-UC device is one based on Dy 3+
and Er 3+ ions. This is illustrated in Figure 4.5.19. The 6 H 7/2 and 6 F 9/2 levels of Dy 3+ can
be excited using 1100-1120nm photons, the 6 H 9/2 and 6 F 11/2 by 1290-1320nm photons,
the 6 H 9/2 by 1700-1730nm photons. These excited ions quickly non-radiatively relax down
to the 6 H 13/2 energy level. This level can also be excited by 2800-2900nm photons, but the
number of photons at these wavelengths from the solar spectrum is negligible. Nevertheless,
the 6 H 13/2 level becomes highly populated from the non-radiative relaxation from the higher
levels. This energy level is resonant with the 4 I 11/2 - 4 I 13/2 of Er 3+ . This implies that energy
transfer up-conversion (ETU) from the 4 I 13/2 to the 4 I 11/2 from the Er 3+ ions is likely if the
Er 3+ are in their fi rst excited state. This fi rst excited state can occur with 1480-1580 nm
photons (in the NaYF 4 host). Due to the rapid non-radiative relaxation of higher energy levels
in the Dy 3+ ions, and non-resonant conditions between the Dy3+ and Er3+ ions, higher order
UC processes or cross-relaxation are unlikely. A signifi cant amount of the sub-bandgap NIR
spectral irradiance can be utilised in this co-doped system.
Figure 4.5.19: ETU between Dy 3+ and
Er 3+ ions. Solid up arrows represent
GSA. The wavy lines represent rapid nonradiative
relaxation. The dotted arrows
represent the ETU mechanism. The solid
down arrow represents the resulting UC
luminescence.
In the above case, the Dy 3+ ions ‘downshift’
higher energies to lower ones which
better match the resonant conditions of
the Er 3+ ions. Although this is an energy
loss mechanism, improved UC responses
will most likely result.
Quantum dots (QDs) could also be used to down-shift higher energy photons to lower energy
photons which are within the absorption range of the UC phosphors. ZnS-passivated CdSe
and PbSe quantum dots have ideal absorption and emission properties for PV-UC QD devices
and are readily commercially available (for example from Evident Technologies). These
quantum dots have large EQE’s (over 50% for the CdSe) and high indices of refraction
compared to the phosphors, making them better matched for Si devices. Figure 4.5.20
shows typical absorption and emission spectra for PbSe quantum dots. The energy transfer
will probably occur by radiative emission (from the quantum dot) followed by absortpion by the
UC phosphor. Direct coupling between the PbSe and UC phosphors resulting in ETU may also
occur and may prove to be a dominant UC mechanism.
84