Lead-halide perovskites (LHP) have rapidly emerged as a contender for commercial photovoltaic applications. Among its many advantages, one of the most important is that LHPs can be paired with silicon solar cells, which currently dominate the photovoltaics market. This is because LHP absorb in the visible wavelength range, which is complimentary to silicon photovoltaics that absorb near-infrared light. By stacking LHP devices (with transparent electrodes on both sides) over silicon bottom cells, tandem photovoltaics with efficiencies exceeding silicon single-junction devices can be achieved. But a critical challenge is that the transparent top electrode is grown by sputter deposition, which damages the LHP absorber and the soft organic charge transport layers. The first part of the thesis explores the use of atmospheric pressure chemical vapour deposition (APCVD) as a new method for rapidly growing high-quality oxide buffer layers over the LHP device to protect the devices from sputter damage.

The second part of the thesis examines bismuth oxyiodide (BiOI) as an alternative to LHP. LHP are limited by the presence of lead, which is toxic and bio-accumulative. By contrast, bismuth-based compounds have little evidence of toxicity and the related BiOCl compound is used in medicines. BiOI was chosen because its electronic structure has important similarities to the electronic structure of LHPs, which is conducive to defect tolerance, and indeed BiOI has been found to be defect tolerant. However, BiOI has a layered structure, leading to anisotropic transport properties. In this thesis, there are three projects on BiOI. The first examines how the preferred orientation of BiOI, grown by chemical vapour deposition, could be controlled and what its effect is on photovoltaic performance. In the second project, a graphite electrode is developed to improve the stability of BiOI as a photocathode to produce hydrogen by water splitting. The third project examines the fundamental limitations of BiOI and how they could be overcome. By developing the growth of BiOI single crystals through chemical vapour transport, a series of correlative optical, electrical and structural measurements are made. It is shown that the carrier lifetime in BiOI is fundamentally limited by the interaction with phonons at room temperature, which limits the open-circuit voltage in photovoltaics, and increase the Urbach energy. This decay channel is switched off by cooling down the single crystal (below 100 K) to depopulate the two dominant optical phonons, resulting in the photoluminescence lifetime increasing from ≈1 ns to ≈10 μs. These results indicate that BiOI might not be suitable for room temperature photovoltaics, but has great potential as a non-toxic, air-stable radiation detector.