Photocurrent generation in polymer-fullerene bulk heterojunctions

Area of Science:

• Materials Science
• Physical Chemistry
• Organic Electronics

Background:

• Photocurrent generation in conjugated polymer-fullerene blends is crucial for organic photovoltaics.
• Understanding charge carrier dynamics at the donor-acceptor interface is key to improving device performance.
• Bound electron-hole pairs (excitons) represent a significant loss pathway if not dissociated.

Purpose of the Study:

• To investigate the factors limiting photocurrent generation in polymer-fullerene blends.
• To model the dissociation efficiency of bound electron-hole pairs using Onsager's theory.
• To quantify the contribution of dissociated excitons to the short-circuit current.

Main Methods:

• Utilized a theoretical model based on Onsager's theory of geminate charge recombination.
• Analyzed the field and temperature dependence of photocurrent in poly(p-phenylene vinylene):[6,6]-phenyl-C61-butyric acid methyl ester (PPV:PCBM) blends.
• Quantified the dissociation efficiency of bound electron-hole pairs.

Main Results:

• The photocurrent is primarily governed by the dissociation efficiency of excitons at the donor-acceptor interface.
• Onsager's theory successfully explains the observed field and temperature dependencies of the photocurrent.
• At room temperature, only 60% of photogenerated bound electron-hole pairs dissociate and contribute to the current.

Conclusions:

• Exciton dissociation efficiency is a major limiting factor for photocurrent in PPV:PCBM photovoltaic devices.
• A significant portion of generated charge carriers are lost due to recombination before contributing to the current.
• Further optimization of donor-acceptor interfaces is necessary to enhance charge dissociation and improve solar cell efficiency.