Tuning morphology of hybrid organic/metal sulfide solar cells

This thesis explores the influence that morphology plays in hybrid organic/inorganic solar cells. This is studied for a range of different materials systems. A series of cadmium xanthate complexes were synthesised, for use as in-situ precursors to CdS nanoparticles in hybrid poly(3-hexylthiophene-2,5-diyl (P3HT)/CdS solar cells. The heterojunction morphology of these hybrid P3HT/CdS blends was found to be dependent on the ligand moiety of the precursor used. The formation of CdS domains was studied by time-resolved materials characterisation techniques and directly imaged using electron microscopy. A combination of transient absorption spectroscopy (TAS) and photovoltaic device performance measurements was used to show the intricate balance required between charge photogeneration and having percolated domains in order to effectively extract charges to maximize device power conversion efficiencies. An analogous method was also applied to a P3HT/Sb_2 S_3 system. Following on from the previous work, a non-toxic alternative to CdS and Sb2S3 was explored. Bismuth xanthates were thermally decomposed to form hybrid polymer/Bi_2 S_3 heterojunctions with two distinctly different morphologies. The bismuth xanthates were found to form nanorods in-situ, within the solid-state polymer matrix, as well as mesostructured arrays of Bi_2 S_3 rods that were later infiltrated with a polymer, using a two-step method. TAS was used to study the charge generation yield in both these systems and hybrid photovoltaic devices were also fabricated. Finally, through a collaboration with The Institute of Photonic Sciences (ICFO), TAS was used to study two separate organic semiconductor/Bi_2 S_3 BHJs. The first of which was a P3HT/Bi_2 S_3 nanoparticle blend solar cell. The charge generation yield in this system was investigated and then compared to a novel thiol-functionalised P3HT based block copolymer (P3HT-SH). Secondly, TAS was used to obtain a better understanding of the charge transfer at several interfaces in a vertically structured Bi_2 S_3 nanorod array that was filled with 2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (SPIRO).Open Acces

Multiscale Modeling and Simulation of Organic Solar Cells

In this article, we continue our mathematical study of organic solar cells (OSCs) and propose a two-scale (micro- and macro-scale) model of heterojunction OSCs with interface geometries characterized by an arbitrarily complex morphology. The microscale model consists of a system of partial and ordinary differential equations in an heterogeneous domain, that provides a full description of excitation/transport phenomena occurring in the bulk regions and dissociation/recombination processes occurring in a thin material slab across the interface. The macroscale model is obtained by a micro-to-macro scale transition that consists of averaging the mass balance equations in the normal direction across the interface thickness, giving rise to nonlinear transmission conditions that are parametrized by the interfacial width. These conditions account in a lumped manner for the volumetric dissociation/recombination phenomena occurring in the thin slab and depend locally on the electric field magnitude and orientation. Using the macroscale model in two spatial dimensions, device structures with complex interface morphologies, for which existing data are available, are numerically investigated showing that, if the electric field orientation relative to the interface is taken into due account, the device performance is determined not only by the total interface length but also by its shape

Photoelectrochemical water splitting and gas ionisation sensing using metal oxide nanostructures

Energy harvesting directly from sunlight has attracted tremendous attention owing to its great potential for low-cost and clean hydrogen production. However, the current photoconversion efficiency from nanostructured metal oxides remains low due to a number of factors, such as low surface area, limited light absorption, poor electron mobility and high electron-hole recombination. In this research, a number of approaches have been carried out to overcome these difficulties. Firstly, changing the morphology of nanomaterials will help to increase the effective surface area of the photoanodes. ZnO nanotubes were prepared and the photoelectrochemical measurements revealed an efficiency of 3 times higher than their nanorod counterparts. In addition, the combination of ZnO nanorods with a 3D metal substrate, stainless steel mesh, showed a further enhancement in the water splitting efficiency by two-fold when compared with that on a planar substrate. Secondly, the hybridisation of two different metal oxides was studied by creating a heterojunction to improve the charge separation, extending the light absorption and increasing the total surface area of the electrode. In this work, both urchin-like ZnO nanorod arrays on TiO2 hollow hemispheres and 1D BiVO4/ZnO nanorod films displayed synergistic enhancement in photoelectrochemical water splitting efficiency. Thirdly, doped ZnO nanostructures with different optical and/or electrical properties were tailored for photoelectrochemical water splitting and gas ionisation sensing applications. The photoelectrochemical water splitting performances of the doped ZnO nanostructures was improved by at least 27% due to increased light absorption. Conductive Y-doped ZnO nanorods were prepared and applied in gas ionisation sensor application. The measurements revealed that both the selectivity and sensitivity of Y-doped ZnO nanorods were enhanced with respect to undoped ZnO nanorods. Furthermore, the effect of UV illumination on gas sensitivity was also investigated. In summary, different approaches and namomaterials have been adapted and demonstrated in this thesis, for the design of specific photoanodes/electrodes for specific applications

An instrumental evaluation of selected metal functionalised semiconductors in the facilitation of photo-organic transformations.

Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.Throughout this research study, various strategies to design, synthesise and test the photo-reactivity of attenuated wide band gap semiconductors in alcohol oxidation studies have been explored. An alizarin red-sensitised zinc oxide photocatalyst which was stabilised in a silver-sodium electrolyte effectively facilitated a broad aromatic and aliphatic alcohol oxidation table with reported conversions ranging from 10 to ≥ 99 %. A systematic characterisation of the alizarin red-sensitised zinc oxide investigated the photoelectronic migrations across the alizarin red–zinc oxide interface and detected the transfer of electrons from the highest occupied molecular orbital of alizarin to the defect site of zinc oxide at 507 nm. Further studies were directed towards the development of a novel titanium dioxide semiconductor that was activated by visible light. Three attempted strategies (pseudo perovskites [Cu3TiO5, Ni3TiO5, and Mn3TiO5], silver functionalised cadmium sulfide and a heterojunction between cadmium sulfide and titanium dioxide) explored the possibility of lowering the band gap potential of wide band gap semiconductors through metal ion functionalisation (Cu, Ni, Mn, and Ag) and heterojunction principles for the purpose of finding applications in mediating alcohol oxidations. Whilst the three strategies were unable to demonstrate viable photocatalytic properties, the instrumental insight obtained during the process identified a suitable three-component semiconductor system (Cu/Pd-N-TiO2). Cu/Pd-N-TiO2 was extensively characterised with an array of instrumental techniques, thus developing an in-depth understanding of the photophysical properties that governed the photo-oxidative transformation of a range of cyclic alcohols and in the remediation of two dyestuffs typically associated with environmental contamination

SYNTHESIS AND DEVICE CHARACTERIZATION OF FUNCTIONALIZED PENTACENES AND ANTHRADITHIOPHENES

Research on pi-conjugated organic materials in the recent past has produced enormous developments in the field of organic electronics and it is mainly due to their applications in electronic devices such as organic field effect transistors (OFETs), organic light emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). The primary goal of this research work is to design and synthesize high performing charge transport organic semiconductors. One of the criteria for better performance of the organic thin film transistor (OTFT) is to have high uniform thin film morphology of the organic semiconductor layer on the substrate. The first project in this dissertation has been directed towards improving the thin film morphology of the functionalized pentacenes through liquid crystalline behaviour. The results have suggested the possibility of thermotropic liquid crystalline phases in 6,13-bis(diisopropylhexylsilylethynyl) pentacene which has no pi-stacking in its solid state and the presence of silyl group at the peri-position is crucial for the stability of the functionalized pentacenes. In the second project, i have investigated the effect of alkyl groups with varying chain length on the anthradithiophene chromophore on the performance of the charge transporting devices. Organic blend cell based on solution processable 2,8-diethyl-5,12-bis(triethylsilylethynyl) anthradithiophene has showed 1% power conversion efficiency and the performance is mainly attributed to the large crystalline phase segregation of the functionalized anthradithiophene from the amorphous soluble fullerene derivative matrix. OTFT study on alkyl substituted functionalized anthradithiophenes suggested the need of delegate balance between thin film morphology and the crystal packing. Third project has been directed towards synthesizing halogen substituted functionalized anthradithiophenes and their influence in the performance of OFETs. OTFT made of 2,8-difluoro-5,12-bis(triethylsilylethynyl) anthradithiophene produced devices with thin film hole mobilities greater than 1 cm2/Vs. The result suggested that the device is not contact limited rather this high performance OTFTs are due to the contact induced crystallinity of the organic semiconductor

Tuning the Properties of Isoindigo-Based Organic Semiconductors Through Structural Engineering

Solar power is one of the most prominent renewable energy technologies vying to replace fossil fuels. Traditionally, this field has been dominated by silicon solar cells; recent innovations, such as organic photovoltaic devices (OPVs), offer the possibility of lightweight, flexible solar power generation with a wide range of applications, from building façades to textiles. Unfortunately, organic solar cells suffer from low power conversion efficiencies. To overcome this problem, the design of new organic semiconductors has become an area of intense research. In engineering these materials, it is essential to develop strong links between molecular structure and the optoelectronic properties of the resulting semiconductors, such as optical band gap, extinction coefficient, frontier orbital energies, and charge carrier mobility. This thesis explores the relationship between structural differences in isoindigo derivatives, such as increasing electron deficiency, or increased molecular planarity, and differences in the resulting material’s optoelectronic properties. The first two sections of this thesis investigate the effects of heteroatom substitution on isoindigo-based semiconductors. Four target compounds were synthesized, each containing either electron-withdrawing nitrogen atoms, or electron rich alkoxy groups. The semiconductors were incorporated into the active layer of organic solar cells. Alkoxy substitution was shown to improve device efficiency; conversely, nitrogen substitution led to lowered device efficiency. In a follow-up study, it was shown that the azaisoindigo groups were capable of coordinating to a Lewis acid; this coordination caused a red-shift in the molecule’s S0S1 transition. The Lewis adduct was identified using UV/vis spectroscopy, NMR spectroscopy, and (TD)DFT calculations. It was then demonstrated that the coordination reaction could be performed with vapor phase Lewis acids. The third project in this thesis focuses on the synthesis of two isoindigo dimers. The first, bisisoindigo, is a ring-fused dimer of isoindigo. This was chosen to study the effects of increased planarity and conjugation length on the optoelectronic properties of isoindigo. Initially, both bisisoindigo and a donor-acceptor molecular semiconductor based on bisisoindigo were synthesized, characterized, and used in OPVs. Poor active layer morphology, due to aggregation of the bisisoindigo, led to low efficiencies in the OPVs. Following work on the ring-fused isoindigo structure, a second dimer was synthesized in which two isoindigo units were joined by a single bond; this design provides free rotation between isoindigo units. Both dimers, as well as isoindigo, were used as electron acceptors in a study of the effects of acceptor number and planarity in donor-acceptor copolymers. The acceptors were copolymerized with thiophene and terthiophene to yield a total of six polymers. Both the optoelectronic properties of these polymers, and their performance in OPVs, were compared to discover trends in donor-acceptor semiconductor properties with increasing acceptor content. Over the course of four major projects it has been demonstrated that altering the structure of the conjugated building block isoindigo has major effects on the optical band gap, orbital energies, and charge transport characteristics of the resulting organic semiconductors. The final chapter of this thesis will serve to link these projects in a general discussion of how the design of isoindigo-based organic semiconductors can be used to produce desired optoelectronic properties in the resulting materials. The thesis will conclude with a brief look towards the prospects of this area of research, including establishing the general applicability of these design strategies to organic semiconductors beyond those based on isoindigo