Spectroscopic Studies on the Impact of Defects in Hybrid Organic-Inorganic Perovskites on Charge Carrier Dynamics and Photovoltaic Device Performance

Abstract

Photovoltaic devices based on hybrid organic-inorganic perovskites have seen a historic rise in efficiency over the past decade. Relatively low cost of fabrication has allowed a wide range researchers to make meaningful contributions. This class of materials is fascinating in that it acts in some ways like well-known semiconductor systems, but exhibits certain key differences, which provides ample opportunity for future fundamental studies. The work described in this dissertation includes the spectroscopic characterization of charge carriers in methylammonium lead iodide and the use of this characterization, along with time-resolved spectroscopy, to better understand the influence of carrier dynamics on solar cell device performance. We observed that a decrease in device fill factor corresponded to an increase in trap state density and a change in the trapping rate constant, implying that the different film fabrication methods lead to a change in the nature of the trap states present in the film. Delving further into understanding the influence of trap states on the excited state dynamics in this system, we performed pump-push-probe spectroscopy and transient absorption spectroscopy with a variable wavelength sub-gap pump pulse to study both the energetics and kinetics of trapping processes in methylammonium lead iodide films. Here we utilized a low energy push pulse to delocalize trapped electrons back into the conduction band and then modeled the resulting recombination based on changes in trapped and free electron densities. From these studies, we were able to add experimental observations to the body of knowledge on the electronic structure of hybrid organic-inorganic perovskites. We also showed how dynamics in the early time periods (ps-ns) following photoexcitation can influence perovskite solar cell performance.