Upconverter and downconverter luminescent materials : progress and strategies towards highly efficient photovoltaic solar cells
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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.