To Couple or not to Couple? Exploring Vibronic Coupling in Organic Semiconductors

Abstract

The Earth is heating up—an observation grounded not in speculation but in extensive scientific evidence, as illustrated in Figure 1. The accelerating rise in global temperature is not only a planetary signal but also a scientific and technological call to action. The last decade has witnessed record-breaking temperatures, rising sea levels, and a growing frequency of extreme climate events. Mitigating this crisis demands a fundamental shift in how we produce, distribute, and use energy. To limit warming below 2◦C, as outlined in the Paris Agreement, there is an urgent need to transition from fossil fuels to sustainable energy systems. Solar photovoltaics and energy-efficient lighting are central pillars of this transition. However, current silicon-based technologies, while mature, are constrained by inflexible processing, high material costs, and intrinsic efficiency ceilings. In contrast, organic semiconductors—both for solar cells and light-emitting devices—offer a path forward that is simultaneously low-cost, low-energy, and compatible with flexible substrates. Organic solar cells (OSCs) can be manufactured at scale using solution processing techniques, enabling roll-to-roll printing and deployment in environments where conventional rigid modules are impractical. Similarly, organic light-emitting diodes (OLEDs) provide high-efficiency lighting with reduced energy demands and tunable spectral properties—critical for both consumer electronics and off-grid lighting. Their lightweight nature and compatibility with transparent, biodegradable substrates further enhance their sustainability profile. Despite these advantages, organic materials are fundamentally limited by how their excited states—excitons and polarons—interact with nuclear motion. In particular, vibronic coupling governs many key performance metrics, including emission linewidth, radiative efficiency, charge separation rates, and non-radiative losses. The interaction between electronic excitations and vibrational degrees of freedom not only shapes energy conversion pathways but often dictates device efficiency limits. Controlling—or at the very least, understanding—this coupling is therefore essential. This thesis investigates the molecular and mesoscale origins of exciton–vibrational coupling in organic semiconductors using a combination of ultrafast spectroscopy, vibrational analysis, and molecular design. While each chapter presents a self-contained study, together they address a unified question: how can we suppress or exploit exciton–phonon interactions to enhance the efficiency of organic optoelectronic materials? The chapter 3 titled “Decoupling High-frequency vibrations from organic molecules” introduces a new design strategy for near-infrared (NIR) emitters by suppressing coupling to high-frequency stretching modes. Using radical-based emitters and impulsive vibrational spectroscopy, it is shown that transitions between non-bonding orbitals enable electronically allowed yet vibronically quiet emissions—offering a path to breaking the energy gap law at the molecular level. In contrast to crystalline systems, disordered hosts typically lack coherent lattice dynamics. However, the chapter 4 titled “Short-Range Exciton–Phonon Coupling in Disordered Molecular Solids” shows that even in amorphous hosts like CBP, low-frequency coherent phonons can emerge and couple to excitons in radical emitters. This interaction is proposed to provide alternate, efficient channels for radiative relaxation, especially under low-drive conditions. Charge separation at donor–acceptor interfaces is central to OSC function. The chapter 5 titled “Interplay of Vibrational Modes in Ultrafast Electron Transfer at a model heterojunction” investigates a model heterojunction with minimal energetic driving force and finds sub-15 fs charge transfer accompanied by vibrational wavepacket formation. The study reveals a mode-selective mechanism, where high-frequency vibrations launch transfer, while decoherence is governed by vibrational overlap across pathways. Narrowband emission is vital for high-colour-purity display applications. Subchapter-1 of the Chapter 6 titled “Vibrational decoupling and narrow emission from organic molecules” explores how structural rigidity and vibrational decoupling reduce both homogeneous and inhomogeneous broadening. The physical mechanisms underlying sharp emission are dissected through photophysical measurements and host–guest interaction analysis.