[Paper Review] Energy Efficient Dissociation of Excitons to Free Charges

Maxim Tabachnyk, Samuel Smith+5 more|arXiv (Cornell University)|Apr 12, 2017

Organic Electronics and Photovoltaics31 references3 citations

TL;DR

This study demonstrates that endothermic charge separation in excitonic systems can proceed with near-unity efficiency and minimal voltage loss by leveraging entropic gain, overcoming a 220 meV Coulomb barrier in pentacene/C60 within 50 ps. The work reveals that excitonic photovoltaics face no fundamental efficiency disadvantage compared to systems generating free charges directly, with only 400 meV effective open-circuit voltage loss relative to the triplet exciton energy.

ABSTRACT

In photovoltaics (PVs) and artificial photosynthetic systems based on excitons, the question of how energy must be lost in overcoming the Coulomb barrier, converting excitons to free charges, is fundamental, as it determines the achievable open-circuit voltage or over-potential1-3. Here, using transient and steady-state optical spectroscopy we study a model system, pentacene/C60, where pentacene triplet excitons are dissociated to form charge transfer states, which we show to be degenerate in energy with the triplet excitons. We directly track these charge transfer states undergoing efficient long-range separation to free charges within 50 ps, despite a significant Coulomb barrier that we measure to be 220 meV. We model this endothermic charge separation to be driven by entropic gain. Our results demonstrate that endothermic charge separation can proceed rapidly with near unity efficiency, overcoming Coulomb barriers greater than 200meV via a gain in entropy, thus minimising loss of open-circuit voltage or over-potential. In the pentacene/C60 system this leads here to an effective VOC loss of only 400meV with respect to the energy of the triplet exciton. This suggests that excitonic photoconversion systems have no fundamental disadvantage compared to systems which generate free changes directly upon photoexcitation.

Motivation & Objective

• To understand the energy loss mechanisms in excitonic photovoltaics during exciton-to-free-charge conversion.
• To investigate whether endothermic charge separation can occur efficiently despite significant Coulomb barriers.
• To determine the role of entropy in driving charge separation in model organic heterojunctions.
• To quantify the open-circuit voltage loss in excitonic systems relative to the initial exciton energy.
• To assess whether excitonic systems are fundamentally disadvantaged compared to direct free-charge-generating systems.

Proposed method

• Transient and steady-state optical spectroscopy were used to track the dynamics of triplet excitons and charge transfer states in pentacene/C60 heterojunctions.
• Energy level alignment between triplet excitons and charge transfer states was measured to confirm degeneracy.
• Time-resolved spectroscopy captured charge separation dynamics on the 50 ps timescale.
• The Coulomb barrier was quantified experimentally as 220 meV using spectroscopic analysis.
• A thermodynamic model was applied to show that entropic gain drives endothermic charge separation.
• The effective open-circuit voltage loss was calculated as 400 meV relative to the triplet exciton energy.

Experimental results

Research questions

• RQ1 Can endothermic charge separation occur efficiently in organic heterojunctions despite a large Coulomb barrier?
• RQ2 What is the role of entropy in enabling charge separation when the process is energetically uphill?
• RQ3 How does the open-circuit voltage loss in excitonic systems compare to the energy of the initial exciton?
• RQ4 Is there a fundamental efficiency disadvantage in excitonic photovoltaics compared to systems that generate free charges directly?
• RQ5 What is the timescale of long-range charge separation following exciton dissociation in pentacene/C60?

Key findings

• Charge separation in pentacene/C60 occurs within 50 ps despite a 220 meV Coulomb barrier, indicating rapid dynamics.
• The charge transfer state is degenerate in energy with the triplet exciton, enabling efficient population transfer.
• Entropy gain drives the endothermic charge separation process, minimizing energy loss.
• The effective open-circuit voltage loss is only 400 meV relative to the triplet exciton energy, significantly lower than expected.
• The system achieves near-unity quantum efficiency in converting excitons to free charges.
• The results demonstrate that excitonic systems are not fundamentally limited in efficiency by Coulomb barriers when entropy is harnessed.