Solution-Processed Pnictogen Halide Thin Film Semiconductors

Optoelectronics - electronic devices which interact with light - are a vital technology in the modern age. Solar power (photovoltaics) is a key ingredient in the modern push towards a carbon-neutral future, as well as providing reliable electrical generation in remote communities. Photoconductors and photodiodes enable light sensing and image capture, broadly applied across myriad devices today. The production of silicon used for optoelectronics is expensive and energy-intensive, so it is desirable to find alternative materials to incorporate into these technologies. Metal halide perovskites have been found to exhibit very remarkable properties in recent years, which has spurred a wave of investigation into related metal halide material families. This thesis focuses on a few related pnictogen halide semiconducting materials, and their processing into thin films for optoelectronic uses. Antimony(V) compounds have been seldom reported in the literature. These materials can form perovskite-like structures with semiconducting properties and substantially deeper band gaps than other antimony halide compounds. Development of these bromoantimonates is the first primary theme of the thesis. One such material, NEPSbBr6 , was reported to achieve very impressive photovoltaic results. Unfortunately this compound is highly unstable in thin films, an observation commonly occurring in Sb(V) materials. In this study, a broad variety of organobromoantimonate compounds are synthesised in order to identify candidate materials with superior properties. The thermal degradation behaviour and optical properties of each material are evaluated in order to select the best prospects. Tauc analysis found the materials to possess optical band gaps ranging from 1.6-2.5 eV, showing a substantial degree of band gap tunability may be achieved within the bromoantimonate family. This approach results in the selection of two candidate materials, with comparable bandgaps to the best example in the literature, and significantly enhanced thermal stability. Processing bromoantimonate materials into Abstract thin films is a significant challenge, as they are liable to debrominate (degrade) during conventional thin film processing. To overcome this obstacle, a new methodology for fabrication of bromoantimonate thin films is presented, termed post-deposition bromination (PDB). PDB fabrication is demonstrated to produce superior quality thin films and is applied to the new materials, enabling the first report of thin films of bromoantimonate compounds other than NEPSbBr6 in the literature. PDB provides a fast and flexible method for thin film processing of this material family. Along with bromoantimonates, this thesis also studies the bismuth chalcohalide family of materials. The dimensionality of these compounds BiOI and BiSI is more limited, which causes significant anisotropy in their electronic properties. As such, control and modification of the crystalline orientation of the materials in thin films has great import for their application. A newly reported method for the production of BiOI, hydrolysing BiI3 in a water bath, is thoroughly explored. Modification of this hydrolysis bath method with chemical additives allows for control over the growth of BiOI grains in the film, producing consistent lateral orientation of the material. This synthetic method is extended to other bismuth halides. These methods are applied to create BiOI photodetectors incorporating both random and aligned thin films. BiSI thin films are synthesised by the reaction of BiOI with H2S gas. This allows for various parameters which are controlled in the BiOI precursor film to be inherited by the BiSI product thin films. This synthetic system is thoroughly characterised, identifying a consistent method for production of BiSI and the related sulfohalide Bi13S18I2 . Both of these materials are applied to lateral photodetector devices. BiSI photodetectors display promising performance characteristics, so a systematic study of the optimisation of BiSI PDs is presented. The highly crystalline, oriented thin films produced from BiOI films treated with water bath additives are found to enable a remarkable increase in performance, with a record responsivity of 547 A W−1 achieved.