Author: MUHAMMAD MUJAHID
Dissertation title: Machine learning-based diagnosis of cancer and fibrosis with second harmonic generation microscopy
Fields of science: Physics N 002
Scientific supervisor: Prof. Habil. Dr. Steponas Ašmontas
Defence of the dissertation: 15 October 2025
SUMMARY: Transient photovoltage in single-junction perovskite solar cells exhibits two oppositely polarized components: a rapid negative part driven by hot carriers resulting from carrier heating and a slower positive contribution caused by the traditional generation of electron-hole pairs. This behaviour of the two components indicates that the hot carriers have adverse effects on photovoltage, thereby lowering the overall power conversion efficiency of single-junction perovskite solar cells. Stacking wide-bandgap perovskite cells on top of narrow-bandgap cells (e.g., silicon) allows the top cell to absorb high-energy photons and pass the low-energy photons that were absorbed. Tandem solar cells reduce thermalisation losses and increase the practical utilisation of the solar spectrum, resulting in increased total efficiency. Carrier lifetimes can be increased and recombination losses reduced by lowering defect density (particularly upon alloying perovskites with other metals) and by interface optimization to improve both the voltage and current outputs of perovskite solar cells. The existence of potential barriers caused by band bending at charge transport layers further contributes to the development of the hot carrier thermoelectromotive force, which is detrimental to the device's efficiency. The elimination or weakening of these possible barriers, such as flat-band conditions, removes the thermoelectromotive force and thereby enhances the device's performance. The addition of tin (Sn) in perovskite films decreases the bandgap. It contributes to the creation of non-radiative defects, which increase the recombination rates on the surface and significantly decrease the photoluminescence lifetime. These kinds of structural modifications deteriorate the photoelectric behavior of perovskite solar cells and reduce their total power conversion efficiency relative to those composed of pure lead-based perovskites.