Synthesis of bismuth halides and chalcohalides for optoelectronic applications
Creators: name: Farooq, Sidra
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340301
subject: Inorganic materials (incl. nanomaterials)
classificationCode: 400910
subject: Photovoltaic devices (solar cells)

description: Recently, a great interest has emerged for lead halide perovskites as a leading contender in optoelectronic applications due to their excellent light absorption properties and low cost fabrication. The highest efficiency for a single photovoltaic device based on perovskites has been recorded at 25.8%, which rivals commercial silicon solar cells. Beside the applications in photovoltaics, perovskites have been used in high performing photodetectors and light emitting devices. The major obstacle in commercialization of these materials is lead toxicity and instability in ambient conditions. Research into alternative materials with similar composition has led scientist to bismuth based compounds. Bismuth-based semiconductors, especially halides, chalcohalides and perovskites have seen encouraging breakthroughs in the fields of optoelectronics.

This thesis focuses on the synthesis and characterisation of bismuth halides and chalcohalides for use as the absorber layers in photovoltaics and photodetectors. Although these compounds have promising optoelectronic properties, the applications of these materials in the devices hinges on fabrication of high quality thin films. However, fabrication of compact films with large grains has been challenging as the solution chemistry of the solvents and precursor solution impacts the morphology of this materials. Gaining control over the crystallization and optimizing the film morphology will benefit the device integration, and consequently help facilitate the incorporation of this material into devices, or as precursors for derivative compounds such as bismuth halide perovskites.

In this project, I have explored a new combination of solvent and specific additives for solution processing of BiI3 thin films. The technique is a simple, one step method to form compact, uniform film with optimal morphology. It allows a complete control over morphology of the films by varying annealing temperature, concentration of solutions, amount and type of additives in the solution. The relationship between synthesis methods and morphological/structural properties is then elucidated by analysing the performance of BiI3-based optoelectronic devices (photodetectors and solar cells).

Further, the knowledge obtained from the fabrication of BiI3 thin films is then utilized in the formation of bismuth chalcohalides thin films. To accomplish this, an entirely new method for the fabrication of bismuth chalcohalide thin films (BiOI and BiSI) is developed. This method does not involve the use of any ligands or counter ions at any point during fabrication. Consequently, the semiconductor thin films produced are highly pure and free of carbon residues and other contaminants. Building on this, BiSI thin films are integrated into photodetectors, which show outstanding performances and high stability. The measured direct ~1.55 eV band gap of BiSI accommodates optical sensing over the full visible spectrum. The performance of these BiSI photodetectors is the best value reported to date across chalcohalide materials of any type. The dynamics of photocurrent generation are demonstrated to be dominated by photoconductive gain. These results cement BiSI as an exciting candidate for high performance photodetector applications and encourage further work in BiSX (X=Cl, Br, I) materials for optoelectronics. 

Lastly, I present a comprehensive investigation of the conversion from BiOI to two phases of bismuth chalcohalides - BiSI and Bi13S18I2. Pure bismuth sulfide iodide thin films at relatively lower temperatures are demonstrated for both phases. A detailed study of structural, morphological, optical and electronic properties is conducted to fully characterize the properties of these two materials, with a focus on iodine deficient phase - Bi13S18I2. After exploring the parameter space of the BiSI to Bi13S18I2 conversion, the iodine deficient phase is incorporated in photodetectors, demonstrating infrared detection up to 1200 nm. This study is the first to report responsivity and detectivity of Bi13S18I2 in photodetectors.