Coulomb barrier for charge separation at an organic semiconductor interface

Area of Science:

• Materials Science
• Physical Chemistry
• Photovoltaics

Background:

• Charge transfer (CT) excitons at donor-acceptor interfaces are critical for organic solar cell performance.
• Their physical characteristics, particularly binding energies, are not well understood.
• Understanding these properties is key to improving charge separation efficiency.

Purpose of the Study:

• To investigate the physical characteristics of CT excitons on a crystalline pentacene surface.
• To determine the binding energies of different CT exciton states (1s, 2s, 3s).
• To correlate exciton properties with their potential role in organic solar cells.

Main Methods:

• Utilized time-resolved two-photon photoemission spectroscopy.
• Probed CT excitons on a crystalline pentacene surface.
• Employed quantum mechanical modeling for comparison.

Main Results:

• Measured Coulomb binding energies for 1s, 2s, and 3s CT excitons as 0.43 eV, 0.21 eV, and 0.12 eV, respectively.
• Results align with quantum mechanical modeling predictions.
• The 1s CT exciton's high binding energy suggests it does not participate in photovoltaics.

Conclusions:

• Efficient charge separation in organic solar cells necessitates the involvement of hot CT excitons.
• The binding energy of CT excitons significantly impacts their role in photovoltaic processes.
• Further research into hot CT excitons is crucial for advancing organic solar cell technology.