|dc.description.abstract||In this thesis, the downconversion (DC) and upconversion (UC) luminescence properties
of rare earth doped materials were investigated for the spectral conversion of
part of the solar spectrum, in order to enhance the performances of silicon photovoltaic
devices. Significant progress were achieved regarding the understanding of
loss mechanisms which limit the photoluminescence quantum yield (PLQY) of those
materials. Achieving high PLQY values is of key importance for the successful realisation
of DC- or UC-enhanced photovoltaic devices.
It was found that, in high absorbing materials, the PLQY can be reduced drastically
by the self-absorption effect. The constraints imparted by this loss mechanism
on the optical performances of the luminescent materials were determined using a
one dimensional optical model developed by the author. The model was also experimentally
validated via spectroscopic characterisation of a downconverting co-doped
Ce3+/Yb3+ borate glass.
The role of self-absorption within an hypothetical DC-enhanced photovoltaic
(DC-PV) device was investigated to find out the physical performance limitations
of the device. Moreover, an UC material consisting of Er3+-doped hexagonal sodium
yttrium fluoride (β-NaYF4) was theoretically investigated to look at the implications
of self-absorption on two experimental situations: the case of a PLQY measurement,
and on the effective performance in a UC-enhanced photovoltaic (UC-PV) device.
The study demonstrates that an optimization of the thickness is essential in order to
reduce the effect of self-absorption and maximize the possible additional photocurrent
that could be harvested, and that the optimal thickness takes different values
depending on the case considered.
As a major progress, an UC material consisting in barium yttrium
fluoride (BaY2F8) single crystal doped with Er3+ was optically characterised resulting in
a measured external photoluminescence quantum yield (ePLQY) of 12.1± 1.2 % for
a BaY2F8:30at%Er3+ sample of thickness 1.75 ±0.01 mm, and a measured internal
photoluminescence quantum yield (iPLQY) of 14.6± 1.5 % in a BaY2F8:20at%Er3+
sample with a thickness of 0.49± 0.01 mm. Both values were obtained under excitation
at 1493 nm and an irradiance of 7.0± 0.7 Wcm-2. The reported iPLQY
and ePLQY values are among the highest achieved for monochromatic excitation in
this research field. Finally, the losses due to self-absorption were estimated in order
to evaluate the maximum iPLQY achievable by this promising UC material. The
estimated iPLQY limit values were ~19%, ~ 25% and ~30%, for 10%, 20% and 30%
Er3+ doping level, respectively.
The self-absorption model clarifes the origin of the disparity between the theoretical
and the experimental PLQY reported for some materials. The results from
this work assist with the design and implementation of DC and UC layers for photovoltaic
devices, as well as providing a framework for optimization of luminescent
materials to other fields of optics and photonics.||en_US