Quantum Optoelectronics

Abstract:

We present a comprehensive investigation of the polarization properties of non-polar a-plane InGaN quantum dots (QDs) and their origin with statistically significant experimental data and rigorous k.p modelling. The unbiased selection and study of 180 individual QDs allow us to compute an average polarization degree of 0.90, with a standard deviation of only 0.08. When coupled with theoretical insights, we show that a-plane InGaN QDs are highly insensitive to size differences, shape anisotropies, and indium content fluctuations. Furthermore, 91% of the studied QDs exhibit a polarization axis along the crystal [1-100] axis, with the other 9% polarized orthogonal to this direction. When coupled with their ability to emit single-photons, a-plane QDs are good candidates for the generation of linearly polarized single-photons, a feature attractive for quantum cryptography protocols.

Abstract:

Systematic tuning of chemical and physical structure allows fine control over the desired electronic and optical properties of many molecular materials, including conjugated polymer semiconductors. In the case of physical structure, molecular orientation via liquid crystalline alignment allows access to fundamental optical anisotropies and the associated refractive index modification offers great potential for the fabrication of photonic structures. In this paper, we report on the use of photoalignment to orient the liquid crystalline conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole), specifically involving two-photon infrared laser writing of patterns in an azobenzene sulphonic dye photoalignment layer. These patterns are transferred into the overlying film by thermally orienting the polymer chains in their nematic phase and are then frozen in place by quenching the film to room temperature as a nematic glass. Optimization of the laser power and scan speed allows features to be achieved with linewidths down to 1 um or less. Photoluminescence (PL) peak anisotropy values reach PL_para / PL_perp = 13 for laser writing, compared with PL_para / PL_perp = 9 for polarized UV-LED exposure of the same azobenzene sulphonic dye alignment layer. The two approaches also result in different film microstructures as evidenced by characteristic changes in PL spectra. The anisotropic PL spectra provide information on the emissive excited states that complements previous studies on non-oriented poly(9,9-dioctylfluorene-co-benzothiadiazole) and related copolymers, also suggesting two emissive state but with more complex spectral signatures than previously considered.

Abstract:

With increasing global surface temperatures due to greenhouse gas emissions from human activity, the global society is in urgent need of pivoting the energy system towards low or zero-emission energy sources. Photovoltaic solar energy has the potential to deliver substantial emission reductions and is expected to make up a major portion of the energy mix in the remainder of the 21st century. Over the last decade, halide perovskite materials have shown great promise in bolstering and diversifying photovoltaic solar technologies, as well as the potential for efficient and colour-pure light-emitting diodes.

Leaps of progress have been made on halide perovskite-based single-junction and tandem solar cell technologies, but triple-junction solar cells, which theoretically could provide an even higher power conversion efficiency, are still in the infancy. For mainly perovskite-based triple-junctions to reach the highest realistic power conversion efficiencies, top cell bandgaps in the range 1.95 to 2.05 eV are required. Herein, we review the achievements and progress that have been made on the topic of perovskite triple-junction solar cells and the important wide-bandgap (2 eV) top cell. We then go on to characterize our FAPb(Br_0.7I_0.3)₃-based 2.0 eV single-junction solar cells. We identify the main reasons for V_OC-loss in these cells and look into the effect of halide segregation on the performance. Finding that nonradiative recombination in the bulk is the main cause of V_OC-loss and that halide segregation may be significantly more detrimental to the J_SC than the V_OC. Further, we investigate halide segregation and degradation trends in isolated films of this 2 eV material under the influence of different atmospheres. We observe untypical wavelength-shifts in the photoluminescence spectrum which may be linked to iodide-depletion under light exposure.

Finally, we improve the performance of our green perovskite LEDs by introducing a novel composite p-contact comprising the organic hole-transporting polymer TFB deposited by solution processing and Al₂O₃ deposited by atomic layer deposition. Due to the different chemical reactivity of the trimethyl aluminium precursor of the ALD process towards the oxide surface groups and the TFB polymer, the Al₂O₃ has the dual function of being able to insulate any bare patches of the underlying transparent conducting oxide electrode, which were not adequately covered during the TFB processing, and intergrow in the porosities of the TFB layer and cause a swelling, further blocking any potential nonradiative recombination sites.