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Abstract
Since the first report, in 2009, on solar cells based on organic-inorganic metal halide perovskites as the active light absorber, intensive research on performance improvement of these types of solar cells has led to a dramatic rise in their power conversion efficiencies from 3-4% (in 2009) to the recently certified efficiency of 22.1% for laboratory-scale devices. Researchers from various fields of physics, chemistry, material science and engineering have been attempting to understand the opto-electronic properties of these attractive crystalline semiconductors to (i) improve and develop new fabrication techniques of the perovskite layer so as to optimise their photovoltaic characteristics; (ii) develop stable and energetically appropriate charge carrier selective contacts; (iii) understand the underlying reasons of the current-voltage hysteresis in these solar cells; and (iv) overcome their long-term operational stability and to lessen the toxicity issues of the perovskite layer.
In two separate sections, this thesis is dedicated to the understanding of both the optical characteristics of most studied photovoltaic perovskite semiconductors, with the focus being on the exciton binding energy influencing light absorption, and also to develop fast, reliable and standard characterization technique for electrical examination of the corresponding solar cells which is based on the luminescence properties of perovskites. An attempt has been made to cover many of the reported binding energy values for polycrystalline perovskite thin films (methylammonium lead triiodide and tribromide) and further, the methods employed in the literature are discussed. The absorption spectrum of these materials with and without the inclusion of excitonic effects is discussed in detail. The experimental values for the binding energies extracted from temperature-dependent absorption spectra of high-quality thin films are compared with theory. In the theory employed, the interaction of electrons and holes with an effective optical phonon was considered for the first time in these semiconductors to better estimate the theoretical values for the binding energy. Good agreement between theory and experiments was shown to be achievable.
Performance characterization of perovskite solar cells was initiated in this thesis by exploration of the generalized Planck s radiation law for planar structured devices both under light- and electrical-bias conditions (photoluminescence and electroluminescence, respectively). In hysteresis affected devices, light-soaking was found to be an essential prerequisite in order to be able to correlate luminescence intensity and the device internal voltage. Long-term behaviour of planar solar cells with methylammonium lead triiodide active layer was investigated using full device electroluminescence imaging and it was realized that interfacial deterioration even when the device is stored in the dark under glovebox conditions (devices only used for light current-voltage measurements) is the critical factor suppressing the fill factor. The shortcoming of using titanium dioxide and Spiro-OMeTAD as electron and hole selective contacts was elucidated.
In the last section, using photoluminescence and electroluminescence imaging, the immediate response and long-term electrical evolution of the perovskite bulk and titanium dioxide/perovskite and perovskite/Spiro-OMeTAD interfaces after the initial short and prolonged selective illumination of the device is elaborated. Perovskite structure buckling over time resulting in the interfacial decoupling at the titanium dioxide/perovskite is demonstrated and the associated mechanism is discussed in detail. The latter is explained by the intrinsic ion migration feature of methylammonium lead triiodide perovskite which is influenced principally due to the change in the internal electric field of the device.