In-Depth Understanding of Loss Mechanisms in High Performing Polymer:Non-fullerene Acceptor Bulk-Heterojunction Organic Solar Cells

Even though significant breakthroughs with unprecedented power conversion efficiencies (PCEs) approaching 18% have been achieved for polymer:non-fullerene acceptor (NFA) organic solar cells (OSCs) recently, not many studies in the literature have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these novel systems. The common knowledge in the OSC community is that to achieve high performances, there needs to be a minimization of the voltage losses due to charge generation, recombination, and energetic disorder, as well as an optimal control of the bulk heterojunction morphology for beneficial charge transport and extraction. In practice, it is extremely challenging to obtain such parameters simultaneously and even more complex to delineate device photophysics in polymer:NFA blends comprehensively. Furthermore, tracing origins of the differences in device photophysics to the subtle differences in energetics and morphology can be particularly complicated. This dissertation encompasses four studies that unify approaches to understand the complicated device photophysics of organic semiconductor devices. Firstly, a systematic study of a series of polymer:NFA blends is conducted to unify and correlate the cumulative effects of i) voltage losses ii) charge generation efficiencies, iii) non-geminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics are established. Secondly, the device photophysics in the high performing PM6:Y6 blend system is investigated. This blend system is found to exhibit low voltage losses coupled with moderate non-geminate recombination and exceptional extraction that can explain its high efficiencies of over 15%. Thirdly, a study is conducted to understand the role of morphology in the key operating processes of high performing polymer:NFA organic solar cells. Varying polymer molecular weight fractions is used as a tool to exert fine control over the interfacial and bulk morphology. This study provides an avenue to understand two fundamental and complex questions that are relevant to the OSC community: i) the role of the nature of the D:A BHJ interface on charge generation and recombination processes, and ii) the factors affecting charge extraction and transport in OSCs. The results from this work provide recommendations on the significant bulk and interfacial morphological features that are critical in optimizing charge generation, recombination, and extraction processes to give high performances, expediting the pathway to the commercialization of OSCs in the near future. Lastly, the DOS distribution widths of two structurally unique organic semiconducting polymers are characterized by using a combination of techniques that have not be explored together. These include, temperature-dependent current density-voltage (J-V) measurements, Kelvin probe measurement (KP) of band bending, and energy-resolved electrochemical impedance spectroscopy (ER-EIS). A quantitative correlation between energetic disorder from band‐bending measurements and charge transport is established, providing direct experimental evidence that charge‐carrier mobility in disordered materials is compromised due to the relaxation of carriers into the tail states of the DOS. Distinction and quantification of locally ordered and disordered regions of thin films at an atomic level was achieved using solid‐state NMR spectroscopy