Colloidal quantum dots based hybrid LEDs and photovoltaics

Colloidal quantum dots (QDs) have proven themselves as excellent light emitters and strong absorbers. This work aims to use QDs to enhance the performance of light emitting diodes (LEDs) and photovoltaics. Photonic quasi crystals (PQC) are used to bring QDs close to quantum wells (QWs) of InGaN LED. This thesis demonstrates the colour conversion effective quantum yields of 123% and 110% for single colour QDs and white LEDs respectively. High colour conversion quantum yield was made possible through efficient coupling of QDs and QWs by both radiative and non-radiative resonant energy transfer (RET). Existence of RET between QWs and QDs is demonstrated using the time resolved photoluminescence spectroscopy. The PQC LED module with current tunable submodules was inkjet printed with different colours of QDs. Reproducibility and correlated colour temperature tunability of colour tunable module using inkjet printing is also demonstrated. Lead sulfide (PbS) QDs as a superficial layer on Si solar cell has shown the absolute and relative photon conversion efficiencies of 1.37% and 20% respectively. This improvement in photon conversion was achieved through luminescent down shifting and RET from QDs to underlying silicon. The PbS QD at the surface also serves as a refractive index matching layer thus the light scatters, increasing the coupling of photon into the Si solar cell. CdSe/ZnS (core/shell) QDs were also hybridised on the planar Si solar cell and optical enhancements were investigated. It was demonstrated that the QD layer can also serve as an efficient refractive index layer and optimum thickness was found through dispersing QDs at different spin speeds. Finally, PQC was used to bring CdSe/ZnS QDs in proximity to metallurgical junction of the Si solar cell. For optimised QD layer, the relative enhancement of Jsc was found to be 17.5%. It was shown that by increasing the air fill fraction of the PQC solar cell, efficient light trapping can be achieved. On hybridisation with CdSe/ZnS QDs, short circuit current of 31.67mA/cm2 was demonstrated. When the comparison was drawn between the champion hybrid PQC solar cell and planar device, the hybrid PQC showed an absolute and relative Jsc enhancement of 9mA/cm2 and 41% respectively

Encapsulated textile organic solar cells fabricated by spray coating

Solution based processes such as screen printing and spray coating are established processes for fabricating organic solar cells (OSCs) on flexible polymer substrates. However, realizing a flexible solar cell on a textile substrate remains a significant challenge due to the properties of the textile itself, which can present an absorbent, rough and fibrous surface. The textile also limits processing temperatures which can reduce functional materials performance. In this work, we demonstrate an optimized fabrication approach using entirely spray coating to fabricate textile OSCs with a power conversion efficiency (PCE) of 0.4 %. An interface layer is first deposited on the standard woven textile that forms a smooth supporting layer for the subsequent spray coated functional layers. A top encapsulation layer is deposited on top of the fabricated textile OSCs, which improves the durability and life time of the OSCs is evidenced by cyclic bending test

Data for 'Encapsulated Textile Organic Solar Cells Fabricated by Spray Coating'

This data supports the paper Li, Y., Arumugam, S., Krishnan, C., Charlton, M., &amp; Beeby, S. (2018). Encapsulated Textile Organic Solar Cells Fabricated by Spray Coating. Solution based processes such as screen printing and spray coating are established processes for fabricating organic solar cells (OSCs) on flexible polymer substrates. However, realizing a flexible solar cell on a textile substrate remains a significant challenge due to the properties of the textile itself, which can present an absorbent, rough and fibrous surface. The textile also limits processing temperatures which can reduce functional materials performance. In this work, we demonstrate an optimized fabrication approach using entirely spray coating to fabricate textile OSCs with a power conversion efficiency (PCE) of 0.4 %. An interface layer is first deposited on the standard woven textile that forms a smooth supporting layer for the subsequent spray coated functional layers. A top encapsulation layer is deposited on top of the fabricated textile OSCs, which improves the durability and life time of the OSCs is evidenced by cyclic bending test.</span

Photonic crystal based control of directionality in GaN based LEDs

LED surface structuring has been widely used to increase light extraction[1]. Due to the high refractive index of the thick GaN epitaxy layers, most emitted light becomes trapped and reabsorbed by the epitaxial layers. While random structuring can effectively scatter trapped light out of the LED, it gives little control over the resulting beam-shapef[2]. Photonic crystals however provide a means to simultaneously improve light extraction efficiency and control beam directionality. Furthermore, P-side up LEDs normally utilize a transparent top contact layer in order to allow top light emission whilst maintaining good electrical properties. In this paper we investigate a novel photonic crystal LED configuration with a nontransparent metal top contact layer, and cylindrical holes etched through the top contact layer and deep into the underlying epitaxy. In this novel configuration light emission is only possible from the etched holes giving rise to extreme beam steering effects. We utilize broadband spectroscopic reflectometry to experimentally investigate beam shape and optical properties from fabricated devices. We observe a range of achievable beam patterns with extreme deviations from the normal Lambertian. We investigate the effect of square and triangular photonic crystal lattices on beam directionality.</p

Data And Code For PhD Thesis Hybrid Solar Cells and Hybrid LEDs utilising Photonic Quasi Crystals and Colloidal Quantum Dots

Data And Code For University of Southampton PhD Thesis &quot;Hybrid Solar Cells and Hybrid LEDs utilising Photonic Quasi Crystals and Colloidal Quantum Dots&quot; The data is sorted by the figure to which it relates. The code is sorted in a folder structure by language and purpose.</span

FDTD study of anti-reflective properties of photonic crystal slabs in silicon

Nanostructuring for the purpose of reflectance reduction has been widely investigated for Silicon based solar applications. Bare Silicon surfaces reflect between 50 and 60 % of the incident light and are thus unsuitable for absorbing significant amounts of sunlight. A typical approach to addressing this is to use an anti-reflective coating on top of the Silicon which reduces reflectance via destructive interference. Since this interference is mainly dependent on the thickness of film this type of anti-reflection layer can only be optimized for a certain wavelength and thus is inherently limited. To reduce the reflectance over a broad range of wavelengths a structuring based approach is necessary. A common approach to implementing this is by wet etching the top surface of a crystalline solar cell to create pyramid structures based on the crystalline dependence of the etching process. Since this approach exploits the crystalline structure it is most suited for crystalline Si. Dry etching based nanostructuring can offer a high level of control over the resulting structure with the crystalline dependence being less concern. One approach is to etch cylindrical holes arranged in a periodic fashion into the top surface of the device to create a photonic crystal lattice. Here we present a systematic analysis of a photonic crystal slabs in Silicon and how the geometry affect the reflectance of the device. Lumerical's FDTD solution is used to vary the pitch, diameter and depth of the cylindrical holes making up the Photonic Crystal structure. The analysis reveals that air fill fraction and hole depth are the most significant determinants of the overall reflectance.</p

Hybrid photonic crystal LED renders 123% color conversion effective quantum yield - Raw data

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FAPbBr3 perovskite quantum dots as a multifunctional luminescent-downshifting passivation layer for GaAs solar cells

Solar cells based on GaAs often include a wide-bandgap semiconductor as a window layer to improve surface passivation. Such devices often have poor photon-to-electron conversion efficiency at higher photon energies due to parasitic absorption. In this article, we deposit FAPbBr3 perovskite quantum dots on the AlInP window layer of a GaAs thin-film solar cell to improve the external quantum efficiency (EQE) across its entire absorption range, resulting in an 18% relative enhancement of the short-circuit current density. Luminescent downshifting from the quantum dots to the GaAs device contributes to a large effective enhancement of the internal quantum efficiency (IQE) at shorter wavelengths. Additionally, improved surface passivation of the window layer results in a 14–16% broadband increase of the IQE. These mechanisms combined with increased overall photon collection (antireflective effects) results in a doubling of the EQE in the ultraviolet region of the solar spectrum. Our results show a promising application of perovskite nanocrystals to improve the performance of well-established thin-film solar cell technologies.</p

High symmetry nano-photonic quasi-crystals providing novel light management in silicon solar cells

Reduction of surface reflection loss is crucial for high efficiency next generation Si solar cells. Surface texturing provides a viable method to reduce loss over the full solar bandwidth. Previous studies have concentrated on simple moth-eye silicon pillar arrays protruding from the surface. Using FDTD simulation methods, we undertake a systematic investigation into performance benefits provided by complex semi-random photonic quasi-crystal surface patterning methodologies whereby arrays of air holes are etched deep into the solar cell surface. In contrast to other studies we carefully investigate the effect of lattice symmetry, systematically comparing performance of simple 6-fold symmetric triangular photonic crystal patterning to 12 fold symmetry photonic quasicrystal patterning and infinitely symmetric 2D Fibonacci patterning. We optimize key geometric parameters such as lattice pitch, hole size and etch depth to maximize optical performance for each lattice type. 12 fold photonic quasi crystal lattice is found to provide best overall anti-reflectance performance providing a solar-corrected average reflectance of 8.3% for a hole depth of 1.5 µm and 300 nm diameter, in comparison to 36.4% for a bare silicon solar cell surface. Practical feasibility of the optimal designs is demonstrated by fabrication of physical prototypes consisting of arrays of nm scale air-holes etched into the surface of a silicon slab fabricated Using e-beam lithography and ICP/RIE etching. FDTD Simulation methodology is validated by convergence studies as well as comparison to optical measurements on these fabricated devices. Furthermore, in contrast to previous studies we provide an in depth analysis of the physical mechanisms responsible for reduction in surface reflection, determining the parameter space where conventional Gaussian optical processes such as effective refractive index, refraction and Fresnel reflection dominate, vs parameter space where sub wavelength photonic crystal scattering effects play the main role. We finish up with an analysis of electrical performance for the optimal designs to further validate real world performance. Taking electrical performance into account we determine that infinite-symmetry 2D Fibonacci patterning far outperforms lower symmetry 12 fold and triangular arrangement. We believe that this is the first in depth investigation into 2D Fibonacci patterning in silicon solar cells

Direct UV written planar Bragg gratings that feature zero fluence induced birefringence

Direct UV writing is a planar fabrication process capable of simultaneously defining waveguides and Bragg gratings. The technique is fully computer controlled and uniquely uses a small focused spot ~7 μm in diameter for direct writing exposure. This work investigates its use to achieve phase trimming and Bragg grating definition in silica-on-silicon lithographic waveguides. It is observed that birefringence control using direct UV writing can be made independent of exposure fluence with this technique through tailoring substrate stress. The result is demonstrated experimentally and supported theoretically using finite element analysis