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Perovskite semiconductors: From high performance devices to optical characterization


In just over a decade, perovskite semiconductors have shown immense promise as candidates for next generation opto-electronic technologies, attributed to broad absorption spectra, long recombination lifetimes, ambipolar charge carrier conductivity and impressive electron mobility. Perovskite photovoltaics(PV) with power conversion efficiencies > 23%, is challenging existing solar cell technologies and have revolutionized the field of solution processed solar cells. However, several drawbacks arising from perovskite stability and scalability have prevented them from achieving their full potential. The scope of this dissertation is to address some of these concerns, while also shedding light into the atypical physical properties of Perovskite semiconductors. The first part of the thesis deals with developing a rapid thoroughput and scalable fabrication route for perovskite thin film solar cells, utilizing an electro-hydrodynamic spraying route inspired from 'Marangoni flow' seen in nature. The fabricated perovskite solar cells had superior current-voltage characteristics and displayed an efficiency of > 16%, which at the time (2015) was one of the highest reported power conversion efficiencies. Additionally, the developed fabrication method requires lower amounts of precursor solvents compared to other scalable approaches, thus reducing Lead wastage and mitigating environmental issues often associated with perovskite solar cells.

Following this project, a range of low temperature, ultrafast optical spectroscopy techniques were used to probe the partial phase transitions in Perovskite thin films. This revealed charge migration pathways in perovskite thin films at low temperatures that prove detrimental for their device performance. In parallel, our experiments showed that the phase transition is associated with a crossover from excitonic to free charge carrier recombination in perovskite thin films.

The above results encouraged us to investigate nanocrystals of perovskite semiconductors, aiming to achieve better crystal phase stability at low temperatures. We successfully quenched partial crystal phase transitions at low temperatures in perovskite quantum dots utilizing ligand based surface modification techniques. These findings highlighted aspects of perovskite phase stability linked to nanoscale morphology, paving way for improved perovskite devices operating at low temperatures.

Lastly, the potential of perovskite quantum dots in spintronics applications were explored. We demonstrated optical spin polarization in perovskite quantum dots and measured their spin lifetimes using Hanle effect. In addition, Manganese ion dopants in Ruddlesden-Popper perovskite quantum dots exhibited polarized photoluminescence emission attributed to strong exchange interactions

In conclusion, we successfully developed high performing perovskite PV devices and demonstrated their plausible application in other opto-electronic technologies, while also shedding light into its interesting physical properties.

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