PHOTOPHYSICAL STUDIES OF HALIDE PEROVSKITE AND ORGANIC SEMICONDUCTORS

Abstract

Metal halide perovskites (MHPs) came in the spotlight in 2012 after their fascinating photovoltaic, light emitting and photodetection properties were discovered. At first, they were only used for absorber in dye-sensitized solar cells, but soon the field of their application expanded. Within a very limited time, single-cell perovskite photovoltaic devices have displayed over 25% efficiency. Unfortunately, thin films of these materials possess very high defect densities. These materials also contain intrinsic faults and abnormal behaviour of grain boundaries that hinders their performance in diverse applications. For example, polycrystalline perovskite solar cells fabricated through thin film deposition have achieved a highest power conversion efficiency (PCE) of 25.6%, which is very low compared to the conceptual Shockley−Queisser limit (SQL) of ∼30.5% for a single-junction MAPbI3 solar cell which is due to inbuilt carrier recombination in the bulk and surface. In perovskite materials the lowest lying phonon modes, polarons and excitons energetics lie in the THz range. For this reason, we used Time-Resolved Terahertz (THz) Spectroscopy (TRTS) with a fixed pump delay and varying the probe delay time using frequency interval of8i8 0.3 to 2.5 THz to investigate the characteristics of photoconductivity in solution-processed MAPbI3 and MAPbBr3 perovskite films at room temperature. We used the Drude-Smith model to describe the frequency-resolved photoconductivity spectra and quantified the mobility at different delay times. We find that the photogenerated charge carriers are free charge carriers with carriers decaying at the expense of time. We also obtained a fraction of carriers being back-scattered off LO phonon and defect states that were inflicted upon room temperature processing of the films. Nowadays, perovskite single crystals (PSCs) are trending on account of their probable application in attenuating the challenges and as only realizable method to attain theoretical SQL by acutely improving fill factors. These single crystals (SCs) possessed large carrier diffusion lengths, low trap density, and long-range order in the crystal lattice, these all are very optimistic features to get rid of imperfections in lattice and lessen the loss mechanisms. So, much amount of research has been done in this direction in recent years. The photophysical, morphological and charge carrier dynamics properties of these PSCs were discovered to be much better from those of their polycrystalline counterparts, thus placing them in the centre of a new area of research . Single crystals possess continuous crystal lattices. Prominently, lack of grain boundaries leading to fewer defects in them. Because of their monocrystalline characteristics, they exhibit surprising optoelectronic features which depend upon the physical properties and appearance of crystallites. Recently it has been demonstrated that perovskite single-crystals can overcome the obstacles faced by their poly-crystalline counterparts because of their tremendous optoelectronic properties. Thus, because of the absence of grain boundary defects and other limiting conditions, perovskite single crystals can be useful for developing high performing optoelectronic devices. In this regards, all inorganic perovskite nanocrystals also came into limelight for many outstanding qualities. They have diffusion length lower than single crystallites and higher exciton binding energy which make them ideal candidate for LED application. Lead (Pb) is a very harmful metal for living organisms and has a very long lasting and adverse effect in living organisms. So, researchers are trying to gradually move towards lead-free perovskites comprised of other benign elements which have similar optoelectronic properties like Pb but less toxic. But, due to some serious drawbacks, PSCs didn’t get enough success in optoelectronic applications in terms of reliability and reproducibility. For this purpose, researchers have tried dimensional tuning of lead-free perovskite materials, from single-crystals (SCs) to nanocrystals (NCs). Here in this thesis, different applications of perovskite materials with dimensional tuning have been discussed, for example, energy storage application of PSC and photovoltaic application of NCs. The present thesis is divided into seven chapters.