Novel Insights on Fundamental Photophysics of Non-Fullerene Acceptors used in Organic Photovoltaic Cells.

<p><strong>Organic photovoltaics (OPVs) have garnered significant attention as a next-generation solar technology, owing to their mechanical flexibility, synthetic versatility, and compatibility with low-cost fabrication techniques. While early research was predominantly centered on optimizing donor polymers, the advent of non-fullerene acceptors (NFAs)—especially the high-performing Y-series—has dramatically advanced device efficiencies and broadened the scope of fundamental photophysical studies. With these advancements, critical questions have emerged regarding how molecular packing in the solid state governs excited-state dynamics, and how these processes ultimately influence device performance. In parallel, a new class of NFA molecules inspired by fullerene architectures is being developed, yet their excited-state behavior remains largely unexplored. This thesis addresses these gaps by first systematically linking nanoscale morphology to ultrafast photophysical behavior, and ultimately to photovoltaic operation, using a suite of time-resolved spectroscopic techniques and advanced data analysis methodologies.</strong></p><p>Chapter 3 focuses on the intrinsic photophysical properties of pristine Y6 thin films, with an emphasis on understanding how crystal packing governs the generation of charge-transfer (CT) states. By fabricating thin films with distinct morphologies—co-facial and end-on packing—we demonstrated that co-facial dimers facilitate ultrafast CT state formation, as evident from the sub-picosecond dynamics observed in transient absorption (TA) measurements. This behavior was absent in end-on packed films, which showed no CT signatures. The as-cast film, which contained a mixture of both packing motifs, exhibited intermediate behavior. Our findings strongly suggest that the ultrafast CT generation in Y6 arises specifically from its ability to form co-facial stacks, highlighting the importance of morphological control in optimizing CT dynamics.</p><p>Chapter 4 builds on these insights by probing how subtle changes in microstructure, induced via solvent additives, influence electronic coupling in Y6 thin films. Using sub 50 fs TA spectroscopy, we extracted the transient electroabsorption (TEA) signal and demonstrated that films treated with CN additives exhibit faster TEA rise times and more pronounced second-derivative lineshapes, consistent with stronger intermolecular CT delocalisation. These features were suppressed in DIO-treated films, which showed slower TEA kinetics and first-derivative-like spectral shapes. The strong correlation between TEA characteristics and device performance in PM6:Y6 blends processed under identical conditions suggests that TEA is a powerful spectroscopic probe for evaluating and predicting device-relevant morphologies. This chapter establishes a direct connection between ultrafast measurements on pristine acceptor films and macroscopic charge generation efficiency in donor:acceptor blends.</p><p>Chapter 5 extends the scope to integrated perovskite–organic solar cells (IPOSCs), where the organic subcell functions as a charge transport layer. Here, ternary donor:NFA:PCBM blends were explored, and their excited-state dynamics were investigated using TA spectroscopy with selective excitation of the NFA. The inclusion of selenium-substituted NFAs such as W-SeSe-Cl improved blend uniformity and accelerated hole transfer dynamics by nearly threefold. Furthermore, the donor polymer was found to suppress parasitic recombination at the NFA:PCBM interface. These results underscore the role of NFA design in efficiently mediating charge separation between the donor and PCBM in tandem device architectures, providing new strategies for optimizing charge transport layers in hybrid solar cells.</p><p>Chapter 6 explores the excited-state dynamics of a new class of flattened C60 oligomers, which represent potential next-generation acceptors due to their broadband absorption properties. Transient grating photoluminescence (TGPL) spectroscopy revealed hot emission from the S1 state with a lifetime of approximately 300 fs, indicating the presence of rapid non-radiative decay pathways. To capture even faster processes, sub-3.5 fs TA spectroscopy was employed, uncovering an additional ultrafast component at 30 fs, followed by relaxation into a vibrationally hot ground state. Complementary computational studies attributed these dynamics to torsional motions that reduce the S1–S0 energy gap and enable an avoided crossing, thereby facilitating efficient internal conversion. To probe the impact of molecular rigidity, TGPL measurements were repeated in a polymer matrix, where constrained motion resulted in slower excited state relaxation. These results emphasize the critical role of conformational freedom in dictating decay dynamics and highlight the importance of structural rigidity for stabilizing long-lived excited states in fullerene-inspired acceptor design.</p><p>Taken together, this thesis demonstrates cohesive strategies for decoding the interplay between molecular packing, excited-state behavior, and solar cell performance. From fundamental investigations on model systems to practical applications in device-relevant blends, the work underscores the power of ultrafast spectroscopy as both a diagnostic and predictive tool.</p>