Simplified charge separation energetics in a two-dimensional model for polymer-based photovoltaic cells

Area of Science:

• Materials Science
• Physical Chemistry
• Renewable Energy

Background:

• Understanding charge generation and separation in photoactive blends is key for efficient photovoltaic devices.
• Existing models often lack the detailed energetic and morphological considerations necessary for realistic descriptions.

Purpose of the Study:

• To extend a two-dimensional photovoltaic model for binary polymer and/or molecular photoactive blends.
• To provide a more realistic description of charge generation and separation processes by incorporating an energy model for exciton and geminate electron-hole pair configurations.

Main Methods:

• Developed a two-dimensional network model representing donor-acceptor morphology in a nanoscopic subvolume.
• Assigned energies to occupation states using ionization potential, electron affinity, and optical gap as inputs.
• Governed charge carrier dynamics via a linear master equation under stationary conditions.

Main Results:

• Simulations demonstrated the impact of energy parameters and donor-acceptor topology on short-circuit current versus active layer thickness.
• Identified an optimal photoactive film thickness for bulk heterojunctions based on kinetic factors.
• Observed space-charge limiting effects in nanometer-scale networks with high charge transfer driving force.

Conclusions:

• The refined model offers a more accurate representation of photovoltaic processes in complex blend systems.
• Optimal film thickness is kinetically determined and highly dependent on specific material energy parameters.
• Space-charge effects significantly influence performance in nanostructured donor-acceptor networks.