Electronic-Structure Effects on the Exciton Annihilation Radius in Organic Materials

Singlet–singlet annihilation (SSA) impacts the efficiency of organic optoelectronic devices at high current densities and is widely used to estimate exciton diffusion constants. However, current analyses rely on the poorly defined concept of the annihilation radius, introducing uncertainty into experiments and models. We present a theoretical model of SSA based on Förster resonance energy transfer, providing analytical expressions for SSA coefficients that distinguish between direct- and diffusion-assisted contributions. These expressions depend on molecular photophysical parameters accessible via electronic-structure calculations. Kinetic Monte Carlo simulations validate our model across diverse conditions. Applying the framework to three benchmark organic semiconductors, we explain their differing annihilation behaviors and propose a rigorous, physically meaningful definition of the annihilation radius consistent with the Smoluchowski theory. Beyond clarifying a long-standing conceptual gap, we identify the circumstances when neglecting this proper definition skews experimental estimates of exciton diffusion constants. Finally, we offer a molecular design objective to mitigate SSA, guiding the development of higher-performance organic materials.