Entropy and disorder enable charge separation in organic solar cells

TL;DR

This paper demonstrates that entropy and disorder significantly facilitate charge separation in organic solar cells by reducing the Coulomb barrier, enabling near-unity efficiency in charge dissociation.

Contribution

It reveals that disorder-induced entropy effects can eliminate or greatly reduce the charge-separation barrier, challenging traditional models of charge binding.

Findings

Disorder reduces the Coulomb barrier to about $k_B T$ or less.

Charges can separate spontaneously without thermodynamic binding.

Entropy effects are crucial even with localized, thermalized carriers.

Abstract

Although organic heterojunctions can separate charges with near-unity efficiency and on a sub-picosecond timescale, the full details of the charge-separation process remain unclear. In typical models, the Coulomb binding between the electron and the hole can exceed the thermal energy $k_\mathrm{B}T$ by an order of magnitude, suggesting that it is impossible for the charges to separate before recombining. Here, we consider the entropic contribution to charge separation in the presence of disorder and find that even modest amounts of disorder have a decisive effect, reducing the charge-separation barrier to about $k_\mathrm{B}T$ or eliminating it altogether. Therefore, the charges are usually not thermodynamically bound at all and could separate spontaneously if the kinetics otherwise allowed it. Our conclusion holds despite the worst-case assumption of localised, thermalised carriers, and is only strengthened if mechanisms like delocalisation or `hot' states are also present.