A Time-Resolved Photophysical Study of Hybrid Organic-Inorganic Perovskite Photovoltaic Materials

The search for new photovoltaic materials has been driven by the combined need to exploit sources of energy that are clean and sustainable, while simultaneously doing it in a cost effective manner. In this light, hybrid organic-inorganic perovskites have recently emerged as an extremely promising photonic material, and their application as a functional photovoltaic layer has resulted in device efficiencies that rival long established silicon based technologies. Rapid progress in device efficiencies have occurred over the last years (22.1% being the current record). However, the simultaneous growth in basic studies of the material have not resulted in a conclusive understanding of the fundamental process that occur subsequent to photoexcitation. Hence, there is a pressing need to identify the pathways that currently limit device performance and provide direction for future work in materials and device engineering. Towards this goal, we investigate two perovskite compositions (CH3NH3PbI3 and (FAPbI3)0.85(MAPbBr3)0.15) using time-resolved (THz and electroabsorption) spectroscopic techniques. Chapter 1 and 2 provide a general introduction into the investigated system and the experimental techniques that have been used. In chapter 3, we detail time-resolved THz measurements and report on experimental evidence for carrier recombination through an indirect transition, as well as a direct recombination pathway that is present at higher carrier densities. We calculate temperature dependent carrier mobilities (at THz frequencies) and bimolecular recombination constants. Through which we identify phonon scattering as the primary limiting mechanism for carrier transport, and temperature dependent bimolecular recombination that is mediated by the relative mobility of the charge carriers. Analysis of the complex photoconductivity spectra using the Drude-Smith model revealed a large difference in carrier scattering between the two perovskite films that could be attributed to the significantly different morphologies. In chapter 4 we apply time-resolved electroabsorption spectroscopy (TREAS) to insulated CH3NH3PbI3 layers and investigate the macroscopic carrier transport dynamics under applied electric fields. Transport within the 40nm perovskite grain was discovered to be diminished by a factor ¿ 2 relative to the high frequency mobility obtained through THz spectroscopy. The averaged carrier mobility across the 280nm film length was reduced by a factor ¿ 4, due to the presence of grain boundaries and defects. Preliminary investigations also identified spectral signatures associated with carrier accumulation at the perovskite interface and delayed extraction at the contacts. Chapter 5 deals with complete solar cells formed using (FAPbI3)0.85(MAPbBr3)0.15 as the active layer and we report the first application of the TREAS technique to complete perovskite devices. Our results reveal that the improved morphology of the film results in film averaged mobilities that are near the intrinsic values obtained using THz spectroscopy. Analysis of transient absorption spectra revealed an electroabsorption signature, its dynamics can be correlated with the disassociation of a transient excitonic species to form free charge carriers.

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