This thesis explores a range of photoactive metal halide perovskite materials for use in photovoltaic applications. These materials are of huge interest due to their outstanding optoelectronic properties which result in high photovoltaic power conversion efficiencies. In particular, this thesis discusses perovskites with stoichiometry ABX₃ where A is a singly charged cation, for example methylammonium (MA), B is predominantly lead (Pb²⁺), and X is iodide (I-) and/or bromide (Br^-). At present the commercial applications of these materials are limited by the chemical instability of the A-site cation. In this thesis, the effect of chemical substitution of the A-site is investigated as a way to increase the stability of the perovskite material. Full replacement with the inorganic cation caesium (Cs⁺) is shown to significantly improve the chemical stability. However, the inorganic lead halide perovskites with ideal bandgaps for photovoltaic applications exhibit structural instability. Routes to achieve both chemical and structural stability for these perovskites are discussed. Consequently, this thesis represents pioneering work in the field of inorganic halide perovskites and will greatly assist the development of stable inorganic perovskite materials for optoelectronic applications such as tandem photovoltaics and LEDs. Chapters 1 and 2 of this thesis present the motivation for perovskite materials to be used in solar cells, along with relevant background information about these materials and solar cell operation in general. Chapter 3 details the methods utilised in the experimental results chapters which follow. The first experimental results chapter, Chapter 4, shows how incorporation of Br^- in place of I^- in CsPbI₃ leads to increased ambient stability of the perovskite structure, and the first solar cells with CsPbI₂Br as the absorbing photovoltaic material are reported. Chapter 5 remedies the deficit of information about the optoelectronic properties of the CsPbI_3-xBr_x (0 ≤ x ≤ 3) perovskites through magneto-optical measurements on thin-films. These measurements raise questions about the room temperature perovskite structure of the CsPbI_3-xBrx compositions with small x, previously thought to be cubic perovskite, which is shown in Chapter 6 to be an orthorhombic perovskite polymorph. This finding motivates preliminary work presented in Chapter 7 aimed at chemical stabilisation of this orthorhombic perovskite polymorph. Finally, Chapter 8 summarises the work presented in this thesis, and recommends further research for the development of stable perovskite materials for photovoltaics.

The study explored the classroom practices of Natural Sciences teachers when teaching the Matter and Material strand in senior phase schools in the Siyabuswa circuit. The following research questions were explored: What is the nature of the teacher’s teacher knowledge when teaching Matter and Material strand in the senior phase schools? What is the nature of the teacher’s instructional strategies when teaching the Matter and Material strand in the senior phase schools? How does the teacher’s teacher knowledge and instructional strategies shape the classroom interactions and discourse? The qualitative case study approach was employed, wherein three teachers participated. Interviews and observation were used for data collection. The findings revealed that Natural Science teachers lack content knowledge and inadequate Subject Matter Knowledge which influences their instructional strategies, hence their classroom interactions and discourses. Recommendations were made to the Department of Education and government to look into the factors that influence the teaching of Natural Sciences as far as teacher’s knowledge and contextual factors are concerned. / Irhubhululo leli liphathelene nomfundisi nakafundisa ngekumbeni isifundo se Natural Sciences khulu khulu sitjheje amakghono kunye nobukgwari I Matter and Material strand emabangeni aphakathi (Senior Phase) isiyingi sange Siyabuswa. Imibuzo erhujululiweko netsengiweko ngelandelako: inzindzolwazi lomfundisi lingangani lokha nakafundisa isifundo se Matter and Material strand emabangeni aphakathi (Senior Phase)? Anjani amakgono womfundisi nakafundisa / nokwethula isifundo se Matter and Material strand emabangeni aphakathi (Senior Phase)? Ingabe ilwazi neendlela zokufundisa zomfundisi ziletha njani ukuzwisisa nokuragela phambili kwesifundo ngekumbeni? Irhubhululo elingeneleleko mayelana nokufuna ilwazi ngesihloko esingehla lisetjenzisiswe. Kusetjenziswe abafundisi abathathu kulelirhubhulo. Ukuhlolwa kwelwazi babuzwe imibuzo begodu baphoswa ilihlo elibukhali ukubuthelela inzinzolwazi. Imiphumela iveze pepeneneni bona abafundisi be Natural Sciences bayatlhayela ngelwazi kunye nelwazi elingeneleleko mayelana nesifundo, lokho kunomthelela omumbi ngendlela abathula ngayo ilwazi kunye nendlela yokuzwisisa kwabafundi. UMnyango wezeFundo kunye norhulumende bayelelisiwe bona batjheje amaphuzu anemithelela emimbi lokha nakufundiswa isifundo se Natural Sciences khulu khulu inzinzo lwazi lomfundisi kunye neenqabo zokufunda. / Die studie is die ondersoek na klaskamer praktyke van Natuurwetenskaponderwysers wanneer onderrig gegee word in Materie-en Materiale vesel in senior fase skole, in die Siyabuswa omgewing. Die volgende navorsingsvrae was ondersoek: Wat is die aard van die onderwyser se onderrigkennis wanneer Materie-en Materiale vesel in senior fase skole onderrig word? Wat is die aard van die onderwyser se onderrigstrategie wanneer Materie-en Materiale vesel in senior fase skole onderrig word? Hoe vorm die onderwyser se kennis-en onderrigstrategieë die klas se interaksies en diskoers? Die kwalititatiewe gevallestudie benadering was toegepas waartydens drie onderwysers deelgeneem het en die versamelde data gebruik was. Die bevindinge het die Natuurwetenskaponderwysers se gebrek aan inhoudskennis en onvoldoende Vakkennis, wat hul onderrigstrategieë nadelig beïnvloed uitgewys, vandaar hul klaskamer en interaksie diskoerse. Aanbevelings is gemaak aan die Departement van Opvoeding en die Regering om ondersoek in te stel na wat die onderrig van Natuurwetenskappe benadeel in so verre dit die kennis van die onderwysers betref. / Science and Technology Education / M. Ed. (Specialisation in Natural Science Education)