A spin-boson theory for charge photogeneration in organic molecules: Role of quantum coherence

TL;DR

This paper models charge photogeneration in organic molecules as a quantum heat engine, revealing ultrafast coherent dynamics and a highly efficient photo-to-charge conversion process driven by quantum entanglement.

Contribution

It introduces a spin-boson quantum heat engine model to analyze ultrafast charge photogeneration, highlighting the role of quantum coherence and entanglement in efficiency.

Findings

Transient energy flow shows a two-stage process with coherence first, then steady current.

Maximum conversion efficiency reaches 93% under optimal conditions.

Quantum entanglement sustains high efficiency in the process.

Abstract

The charge photogeneration process in organic molecules is investigated by a quantum heat engine model, in which two molecules are modeled by a two-spin system sandwiched between two bosonic baths at their own temperatures. The two baths represent the photon emission source and the phonon environment, respectively. We utilize the time-dependent density matrix renormalization group algorithm to investigate the ultrafast quantum thermodynamical processes of the model. We find that the transient energy flow through the two spins behaves a two-stage effect: The first stage shows a coherent dynamics which represents the ultrafast delocalization and dissociation of the charge-transfer state, and in the second stage a steady current is establish. The photo-to-charge conversion is highly efficient with the maximum efficiency being $93\%$ with optimized model parameters. The survival entanglement between the two spins is found to be mostly responsible for the hyper efficiency.