Structure-Property Correlations in Heterojunction Organic Solar Cells Across Material Systems Via Synchrotron X-Ray Techniques

Obaid Alqahtani

doi:10.7273/000005303

The power conversion efficiency (PCE) of organic solar cells (OSCs) is rising, surpassing 18%. The low cost, flexibility, and uniquely tunable properties of organic materials set OSCs as a promising integrable and competitive renewable technologies. However, transferring high-performing ink-printed OSCs from the lab-scale to industrial scale remains challenging. For example, the optimal device thicknesses that are not easily achievable via large-scale printing methods like roll-to-roll printing and are rather prone to pin-hole defects. Another issue facing most of the record high-performing systems with non-fullerene acceptors (NFAs) is the need for optimization with solvent additives. Processing additives invoke many obstacles when transferring to an industrial scale, such as solvent residuals often leading to undesirable film structures and mechanical properties incompatible with printing methods. Although wide varieties of organic materials have been synthesized and tested for OSCs applications, the pool of candidates of efficient and scalable materials is very low. Understanding and control of OSC device nanostructure, which is responsible for their photo-electrical properties are essential criteria of selection regarding their industrial viability. Multimodal characterization and holistic analysis of device performance and nanostructure of bulk heterojunction (BHJ) OSCs as functions of device fabrication variables, however, are required for judging the scalability of a material or processing condition.This work aims to thoroughly investigate structures of a selected variety of OSCs systems with unique attributes or novel materials. Soft X-ray scattering and spectro-microscopy are excellent characterization tools due to their proven sensitivity to material contrast. Those tools are used with X-ray diffraction and other electron-based techniques for more accurate multimodal analyses of the studied OSC nanostructures. By correlating nanomorphology with device functionalities, conclusions can be drawn pertaining to the viability of material types and fabrication procedures. One of the investigated OSCs systems (polymer:fullerene) maintains relativity high PCE even with an active-layer thickness of 1µm critical for commercial scale-up. We have discovered that sharp donor-acceptor (D-A) interfacial widths significantly reduces charge recombination. This finding might be key to successfully commercializing printed thick OSCs. Another focus of this work is investigating impacts of solvent additives on device performance and morphology in NFA OSCs. Results of studying two polymer:NFA systems show high device performance and morphology sensitivity to the most prevalent solvent additive chloronaphthalene (CN). The findings suggest that additive-free methods and carefully designed NFA molecules will be essential to industrial-scale fabrication of stable NFA OSCs. Interestingly, CN is commonly used to optimize device performance in most of the state-of-the-art NFA systems, namely PM6:Y6 (NFA) OSCs. Our results and previous literature show that PM6:Y6 system is also susceptible to typical concentration of CN as the optimizing solvent additive. In the last study of this dissertation, an alternative of CN, known as phenylnaphthalene (PN), was used to optimize device performance in PM6:Y6 OSCs. A thorough examination of device performance and nanostructure of PM6:Y6 devices as a function of PN concentration shows that: PN is a potential eco-friendly (non-halogen) candidate to replace CN as a processing solvent additive with less concerns regarding sensitivity to the additive concentration. Also, we find that the optoelectrical charge generation processes in PM6:Y6 OSCs can be fine-tuned using the PN solvent additive. The findings of OSC structure-property relationships in this dissertation give insights into some key aspects to be considered during material synthesis and device fabrication for successful printing of efficient OSCs. Finally, combinations of characterizing OSC device performance via holistic methods and probing their morphology via multimodal synchrotron X-ray techniques seem essential to realize the potential of emerging binary NFA OSCs and ternary systems.

Structure-Property Correlations in Heterojunction Organic Solar Cells Across Material Systems Via Synchrotron X-Ray Techniques

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