Effects of Hole Transporting Layers and Surface Ligands on Interface Energetics and Photovoltaic Performance of Methylammonium Lead Iodide Perovskites

10/19/2020 - 2:00pm to 3:00pm
Speaker(s) / Presenter(s): 
So Min Park

Abstract: Organic metal halide perovskites are promising materials for various optoelectronic device applications such as light emitting diodes (LED) and photovoltaic (PV) cells. Perovskite solar cells (PSCs) have shown dramatic increases in power conversion efficiency over the previous ten years, far exceeding the rate of improvement of all other PV technologies. PSCs have attracted significant attention due to their strong absorbance throughout the visible region, high charge carrier mobilities, color tunability, and ability to make ultralight weight devices. However, organic metal halide perovskites still face several challenges. For example, their environmental stability issue must be overcome to enable widespread commercialization. Meeting this challenge involves material and interface development and optimization throughout the whole PV device stack. Fundamental understanding of the optical properties, electrical properties, interfacial energetics, and device physics is key to overcome current challenges with PSCs. In this dissertation, we report a new family of triarylaminoethynyl silane molecules as hole transport layers (HTLs), which are in part used to investigate how the PV performance depends on the ionization energy (IE) of the HTL and provide a new and versatile HTL material platform. We found that triarylamoniethynyl silane HTLs show comparable PV performance to the state-of-the art HTLs and demonstrated that different processing conditions can influence IE of methylammonium lead iodide (MAPbI3).

Surface ligand treatment provides a promising approach to passivate defect states and improve the photoluminescence quantum yield (PLQY), charge-carrier mobilities, material and device stability, and photovoltaic (PV) device performance of PSCs. Numerous surface treatments have been applied to PSC thin films and shown to passivate defect states and improve the PLQY and PV performance of PSCs, but it is not clear which surface ligands bind to the surface and to what extent. As surface ligands have the potential to passivate defect states, alter interface energetics, and manipulate material and device stability, it is important to understand how different functional groups interact with the surfaces of PSC thin films. We investigate a series of ligand binding groups and systematically probe the stability of the bound surface ligands, how they influence energetics, PLQYs, film stability, and PV device performance. We further explore ligand penetration and whether surface ligands prefer to remain on the surface or penetrate into the perovskite. Three variations of tail groups including aryl groups with varying extents of fluorination, bulky groups of varying size, and linear alkyl groups of varying length are examined to probe ligand penetration and the impact on material stability.

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