Alessandro Boccolini

Self-absorption in upconverter luminescent layers: impact on quantum yield measurements and on designing optimized photovoltaic devices

This Letter details a theoretical investigation of self-absorption within an upconverter (UC) material, consisting of trivalent erbium (Er3+)-doped hexagonal sodium yttrium fluoride (β-NaYF4) and its implications on two experimental situations: the case of a quantum yield measurement, and on the effective performance in a UC-enhanced photovoltaic (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. It also has been found that the external photoluminescence quantum yield (ePLQY) measured through an integrating sphere may result in an underestimation with respect to the performance that the UC material could achieve in a UC-PV device. Finally, it has been found the optimal thickness and the molar concentration of Er3+ ions are inversely proportional, suggesting that an optimal number (1.3-2.9·10(17)) of Er3+ ions should be contained within the UC layer.

Highly efficient upconversion in Er3+ doped BaY2F8 single crystals: Dependence of quantum yield on excitation wavelength and thickness

This manuscript presents a study of the upconversion (UC) in barium yttrium fluoride (BaY2F8) single crystal doped with trivalent erbium ions (Er3+) under excitation of the 4I(13/2) level at three different wavelengths: 1493 nm, 1524 nm and 1556 nm. The resulting UC emission at around 980 nm has been investigated and it has been found that a thickness optimization is required to reach high quantum yield values, otherwise limited by self-absorption losses. The highest external photoluminescence quantum yield (ePLQY) measured in this study was 12.1±1.2 % for a BaY2F8:30at%Er3+ sample of thickness 1.75±0.01 mm, while the highest internal photoluminescence quantum yield (iPLQY) of 14.6±1.5 % was measured 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. Finally, the losses due to self-absorption were estimated in order to evaluate the maximum iPLQY achievable by the upconverter material. The estimated iPLQY limit values were ∼19%, ∼25% and ∼30%, for 10%, 20% and 30% Er3+ doping level, respectively.

Neural network identification of people hidden from view with a single-pixel, single-photon detector

Light scattered from multiple surfaces can be used to retrieve information of hidden environments. However, full three-dimensional retrieval of an object hidden from view by a wall has only been achieved with scanning systems and requires intensive computational processing of the retrieved data. Here we use a non-scanning, single-photon single-pixel detector in combination with a deep convolutional artificial neural network: this allows us to locate the position and to also simultaneously provide the actual identity of a hidden person, chosen from a database of people (N = 3). Artificial neural networks applied to specific computational imaging problems can therefore enable novel imaging capabilities with hugely simplified hardware and processing times.

Progress Towards Enhancing the Performance of c-Si Photovoltaic Devices via Up-and Down-Conversion

Spectral conversion can improve the performance of photovoltaic devices by matching the wavelengths of incident light to the bandgap of the solar cell. This work reports on materials optimization, novel characterization techniques and device approaches.

Optimization of thickness in luminescent down-conversion layers for photovoltaic applications

An optical model is presented determining the constraints imparted by self-absorption on the luminescence emitted from down-conversion materials. The model clarifies the origin of the disparity between theoretical and experimental efficiencies reported for some materials.