A Kinetic Monte Carlo Model to Understand Limiting Regimes of Photocurrent and Interfacial Charge Transfer in Organic Semiconductor–Electrolyte Interfaces

Abstract Organic semiconductor–electrolyte interfaces enable a wide range of applications, spanning photoelectrochemistry, energy conversion, and bioelectronics. Predictive modeling of the complex interplay among light‐charge conversion, charge transport, and redox reaction kinetics is essential for advancing specific applications, but an in‐depth understanding of the polymer/water interfaces (PWIs) under optical excitation remains incomplete. This work presents a multiphysics kinetic Monte Carlo simulation to quantitatively reproduce the photoelectrochemical behavior of a prototypical PWI between poly(3‐hexylthiophene) (P3HT) and phosphate‐buffered saline. The simulation model enables nanometer‐scale resolution of space‐charge regions and cross‐interface electron‐solute interactions. The study reveals that the electrochemical current can be limited by the transport of minority charge carriers in the organic electrode when light is absorbed from the opposite side of the PWI; moreover, it can be further limited by the inverted Marcus region in the P3HT/O ₂ system. Overall, results demonstrate that Marcus‐Gerischer theory is inadequate for describing the current‐potential curves, necessitating the particle‐based simulations with molecular Marcus theory. By bridging microscopic kinetics with device‐level performance, this work deepens the mechanistic understanding of interfacial charge transfer and current flow across the PWI, providing valuable design principles for next‐generation biohybrid devices.