Search

Silicon is the core element for microelectronics and conventional photovoltaics. The absorption and emission of photons in bulk Si are limited by its indirect electronic band gap that requires phonons for momentum conservation. In quantum confined low dimensional systems such as Si quantum dots (Si QDs), this momentum conservation becomes less stringent due to Heisenberg uncertainty principle. Si QD based materials provide the additional advantage of a tunable electronic band gap that is particularly important in the realisation of an all-Si tandem solar cell, a third generation photovoltaic device that aims to exceed the Shockley-Queisser Limit at low cost. The thesis investigation begins with the fabrication and analysis of single junction Si QD solar cells on quartz substrates, using Si QDs embedded in SiO2 matrix. The impacts of post-metallisation treatments such as phosphoric acid (H3PO4) etching, nitrogen (N2) gas anneal and forming gas (Ar: H2) anneal on the cells electrical and photovoltaic properties have been studied. The Si QD solar cells investigated in this work have achieved an open circuit voltage of 410 mV after various processes. Parameters extracted from electrical measurements suggest that the performance of the solar cell is strongly limited by poor carrier transport. This limiting factor can be partly eliminated by forming gas annealing. To enhance current transport in the cell, we propose a new nanostructure of Si QDs in SiO2/Si3N4 hybrid matrix as a possible improvement to the conventional Si nanostructure - Si QDs in SiO2 matrix . Initial work on the synthesis of undoped Si QDs in SiO2/Si3N4 shows the size and crystallization of the Si QDs can be controlled by factors including the annealing method and the thicknesses of the nanoscale layers, as evidenced in X-ray diffraction and Raman studies. Photoluminescence measurements indicate the apparent band gap of the material increases with reducing nanocrystal (NC) size, exhibiting the effect of quantum confinement. It is also shown that the conventional Si quantum dots in SiO2 matrix approach can lead to the formation of over-sized Si nanocrystals especially when doped with phosphorous, making the size-dependent quantum confinement less effective. By replacing the SiO2 tunnel barriers by the Si3N4 layers, the new material manages to constrain the growth of doped Si quantum dots effectively and enhances the apparent band gap. Besides, electrical characterization on Si QD/c-Si hetero-interface test structures indicates the new material possesses improved vertical carrier transport properties. To gain further understanding of the photon absorption and emission mechanisms of the new material, doped and undoped Si QDs in SiO2/Si3N4 matrix are characterised optically via spectroscopic and photoluminescence measurements. The values of optical band gap are extracted from interference-minimised absorption spectra using the Hishikawa s method and Tauc s analysis. Measurement results suggest that these nanocrystals exhibit transitions of both direct and indirect types. Lastly, we demonstrate all-Si light-emitting diodes (LEDs) based on boron-doped Si QD/c-Si p-n heterojunction structures, which show electroluminescence in the visible/infrared regions. The EL peaks at ~ 1.7 eV are a further evidence of enhanced electronic band gap of Si QDs due to quantum confinement. These LEDs are fabricated using the same materials and techniques developed in this thesis for Si QD cells. The electroluminescence spectra of these diodes can be modified by changing the quantum confining barriers from SiO2 to Si3N4. The results are the first demonstration of electroluminescence from boron-doped Si nanocrystals. The nanostructure material proposed in this thesis Si QDs in SiO2/Si3N4 hybrid matrix has exhibited improvements in a number of material properties, including nanocrystal size control, quantum confinement and current transport, over the conventional Si QDs in SiO2 matrix. Despite the necessity for further optimisation, this novel material is evidently a promising building block for the realisation of all-Si tandem solar cells.

Perovskite light-emitting diodes have recently broken the 20% barrier for external quantum efficiency. These values cannot be explained with classical models for optical outcoupling. Here, we analyse the role of photon recycling (PR) in assisting light extraction from perovskite light-emitting diodes. Spatially-resolved photoluminescence and electroluminescence measurements combined with optical modelling show that repetitive re-absorption and re-emission of photons trapped in substrate and waveguide modes significantly enhance light extraction when the radiation efficiency is sufficiently high. In this manner, PR can contribute more than 70% to the overall emission, in agreement with recently-reported high efficiencies. While an outcoupling efficiency of 100% is theoretically possible with PR, parasitic absorption losses due to absorption from the electrodes are shown to limit practical efficiencies in current device architectures. To overcome the present limits, we propose a future configuration with a reduced injection electrode area to drive the efficiency toward 100%., Perovskite light-emitting diodes have shown unexpected high external quantum efficiency of 20%, breaking the ray-optics limit. Here Cho et al. reveal that photon recycling is responsible for the enhancement and propose photonic structures to further improve the device efficiency.

Efficiency roll-off is a major issue for most types of light-emitting diodes (LEDs), and its origins remain controversial. Here we present investigations of the efficiency roll-off in perovskite LEDs based on two-dimensional layered perovskites. By simultaneously measuring electroluminescence and photoluminescence on a working device, supported by transient photoluminescence decay measurements, we conclude that the efficiency roll-off in perovskite LEDs is mainly due to luminescence quenching which is likely caused by non-radiative Auger recombination. This detrimental effect can be suppressed by increasing the width of quantum wells, which can be easily realized in the layered perovskites by tuning the ratio of large and small organic cations in the precursor solution. This approach leads to the realization of a perovskite LED with a record external quantum efficiency of 12.7%, and the efficiency remains to be high, at approximately 10%, under a high current density of 500 mA cm−2., Large drop in efficiency at high brightness has been holding back the development of various light-emitting diodes including halide perovskite. Here Zou et al. achieve high quantum efficiency of 10% under a high current density of 500 mA cm−2 in perovskite-based diodes by reducing luminescence quenching.

Solution-processed polymer organic light-emitting diodes (OLEDs) doped with triplet-triplet annihilation (TTA)-upconversion molecules, including 9,10-diphenylanthracene, perylene, rubrene and TIPS-pentacene, are reported. The fraction of triplet-generated electroluminescence approaches the theoretical limit. Record-high efficiencies in solution-processed OLEDs based on these materials are achieved. Unprecedented solid-state TTA-upconversion quantum yield of 23% (TTA-upconversion reaction efficiency of 70%) at electrical excitation well below one-sun equivalent is observed.

Electroluminescence in light-emitting devices relies on the encounter and radiative recombination of electrons and holes in the emissive layer. In organometal halide perovskite light-emitting diodes, poor film formation creates electrical shunting paths, where injected charge carriers bypass the perovskite emitter, leading to a loss in electroluminescence yield. Here, we report a solution-processing method to block electrical shunts and thereby enhance electroluminescence quantum efficiency in perovskite devices. In this method, a blend of perovskite and a polyimide precursor dielectric (PIP) is solution-deposited to form perovskite nanocrystals in a thin-film matrix of PIP. The PIP forms a pinhole-free charge-blocking layer, while still allowing the embedded perovskite crystals to form electrical contact with the electron- and hole-injection layers. This modified structure reduces nonradiative current losses and improves quantum efficiency by 2 orders of magnitude, giving an external quantum efficiency of 1.2%. This simple technique provides an alternative route to circumvent film formation problems in perovskite optoelectronics and offers the possibility of flexible and high-performance light-emitting displays.