Photo-physical Properties of Perovskites on Curved Surfaces

Organic-inorganic metal halide perovskites (MHPs) have recently attracted increasing attention as highly-efficient light-harvesting materials for photovoltaic applications. The technical ease of solution processing of these materials is one of their major advantages on the route towards fabrication of low-cost solar cells and optoelectronic devices. However, the precise control of crystallization and morphology of MHPs deposited from solutions, considered crucial for enhancing the final photovoltaic performance, still remains challenging.

Firstly, the PhD thesis focuses on novel and thus far unexplored issues of crystallization mechanisms and photo-physical characterization of polycrystalline MAPbI3 and MAPbBr3 coated via one-step solution processing onto the surface of elongated, cylinder-shaped, quartz substrates with unprecedentedly high curvatures (diameters of 80 to 1800 ÎŒm). The obtained results are then compared with the ones gathered for deposits of polycrystalline MAPbI3 and MAPbBr3 on planar substrates.

Secondly, the deposits of polycrystalline MAPbI3 and MAPbBr3 on cylindrical substrates are used to track the changes in photoluminescence (PL) and photocurrent (PC) of these materials in the presence of various gaseous media. In particular, the advantages offered by small cross-sectional dimensions and the cylindrical geometry of thus obtained deposits of MAPbI3 and MAPbBr3 made it possible to easily combine them with simple gas-flow observation chambers having both optical and electrical accesses. This approach enabled to perform simultaneous measurements of PL and PC for polycrystalline deposits of MAPbI3 and MAPbBr3 exposed to the precisely controlled flow of different gaseous media, including technologically important oxygen (O2) and nitrogen (N2). Thus, an insight could be gained into the role and importance of surface defects for the general optoelectronic properties, such as photo-brightening and photo-bleaching, as well as long-term stability of these materials.

Thirdly, a study of selective photo-bleaching of MAPbI3 under illumination with specific wavelengths is performed. For the first time this type of experiments were performed under precisely controlled atmosphere of either O2 or N2, additional insight could be gained on the influence of these gaseous media on the mechanism of selective, wavelength-dependent photo-bleaching in MAPbI3.

Lastly, a study of the photo-induced charge transfer in a model system of the MAPbI3/TiO2 interface is carried out. Specifically, a contactless technique based on a combination of low-temperature electron spin resonance (ESR) with in situ illumination is used to directly track the photo-carriers at the MAPbI3/TiO2 interface. In particular, this approach shows that the ESR signal intensity of paramagnetic defects in TiO2 (Ti3+-related centers) markedly changes upon illumination of the MAPbI3/TiO2 interface. It is then inferred that the presented novel methodology can be used to monitor the flow of light-excited electrons from MAPbI3 to TiO2.

Altogether, in addition to exploring morphological and photo-physical aspects of thin polycrystalline films of two archetypal MHPs, that is MAPbI3 and MAPbBr3, coated onto strongly curved substrates, this PhD dissertation also provides a novel approach for designing MHPs-based devices. In particular, it paves the way for designing sensitive differential gas sensors, which are expected to function under various environmental conditions.