Excitons and charges at organic semiconductor heterojunctions

Area of Science:

• Organic electronics
• Photovoltaics
• Materials science

Background:

• All-organic heterojunction solar cells demonstrate high quantum efficiencies and improving power conversion efficiencies.
• Strong Coulomb interactions and large exchange energies pose challenges for efficient charge generation and separation in organic semiconductors.

Purpose of the Study:

• Investigate the mechanisms limiting charge generation efficiency in polymer-polymer solar cells.
• Analyze charge separation, recombination, and sweep-out processes in efficient polymer-fullerene devices.
• Explore singlet exciton fission in pentacene/C60 heterojunctions for enhanced charge generation.

Main Methods:

• Transient optical spectroscopy to observe charge dynamics.
• Analysis of intersystem crossing mediated by proton hyperfine interaction.
• Study of singlet exciton fission and charge dissociation at organic heterojunctions.

Main Results:

• Identified intersystem crossing to a triplet state as the cause of low charge generation efficiency in a polymer-polymer system, with a 250 meV activation barrier for charge separation.
• Observed charge separation, recombination, and sweep-out dynamics in efficient polymer-fullerene devices.
• Reported singlet exciton fission in pentacene leading to triplet excitons for dissociation at a C60 heterojunction.

Conclusions:

• Understanding charge generation and separation mechanisms is crucial for optimizing organic solar cell performance.
• Proton hyperfine interaction plays a role in limiting charge separation via intersystem crossing.
• Singlet exciton fission offers a potential pathway for enhancing charge generation in organic photovoltaics.