Optoelectronic properties of hybrid metal halide perovskites: from nanoscale to devices

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Anthropogenic climate change is currently one of the biggest challenges facing our society. In order to mitigate the detrimental effects of burning fossil fuels and releasing CO2 into the atmosphere, we must switch to clean, renewable sources as quickly as possible. Solar energy is one of the most promising options available because it has the potential to completely meet our entire global energy requirements. Although the solar energy industry has seen rapid development in recent years, challenges still remain to increase the efficiency of photovoltaic devices while decreasing or maintaining costs. Hybrid metal halide perovskite thin films are promising materials for achieving these goals as high efficiency devices which rival existing technology can be easily synthesized via solution processing methods using inexpensive, earth-abundant materials. However, current state-of-the-art perovskite materials struggle to maintain long-term stability under ambient conditions. Two-dimensional, Ruddlesden-Popper phase perovskites have demonstrated superior stability, and have thus attracted much attention in the perovskite community, although photovoltaic devices made with these materials have not been able to achieve the same high efficiencies as their 3D counterparts. As it is currently unknown whether this inefficiency is due to intrinsic limitations of the material or to extrinsic factors fixable with improved processing procedures, a comprehensive study of the fundamental optoelectronic properties in these materials is desperately needed. This research will fill this knowledge gap by fully characterizing the optoelectronic properties of 2D perovskites in order to determine their ultimate viability for use in solar cells. This task is far from straightforward, however, as many competing factors can limit efficient charge transport in these materials. In addition to exhibiting high exciton binding energies, 2D perovskites also demonstrate increased doping density and decreased crystallinity associated with their thin-film microstructure, all of which previous work has shown to limit charge-carrier diffusion lengths (Milot et al, Nano Lett, 2016). The challenge for understanding the optoelectronic properties in these materials is being able to isolate the effects of the intrinsic properties (e.g excitonic effects) from extrinsic properties such as doping density and crystallinity which could be altered with improved processing methods. As many of the extrinsic properties can further change with incorporation into solar cells, this problem is nontrivial. To address this issue, this research will pioneer a new approach by studying optoelectronic properties from single crystals to devices in order to gain a full picture of intrinsic properties and determine how they are affected by extrinsic factors including microstructure and solar cell inclusion. To best enable comparisons, it will utilize THz and photoluminescence (PL) spectroscopy, two of the most versatile techniques for the analysis of optoelectronic properties including charge-carrier mobility and recombination dynamics. It will further harness the versatility of these two techniques by combining THz scattering near-field optical microscopy (THz-SNOM) and time-resolved PL microscopy analyses for the first time, adding the capability of nanoscale spatial resolution to the existing capabilities for ultrafast time resolution. Through comparison with conventional measurements of photovoltaic power conversion efficiencies, it will identify pathways to improvement in device fabrication. The greater understanding of the optoelectronic properties of 2D perovskites that this research presents will lead directly to the development of high efficiency solar cells to meet our energy needs.