Understanding and controlling morphology evolution via DIO plasticization in PffBT4T-2OD/PC₇₁BM devices

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Bulk-heterojunction (BHJ) organic solar cells offer a promising renewable energy solution.
• Optimizing the nanostructure morphology of the active layer is crucial for efficient charge generation and transport.
• Additives are often employed during device fabrication to tune morphology and enhance performance.

Purpose of the Study:

• To investigate the effect of 1,8-diiodooctane as a co-solvent on the power conversion efficiency (PCE) of BHJ photovoltaic devices.
• To elucidate the mechanism by which 1,8-diiodooctane influences film morphology and device performance.
• To determine the optimal conditions for the removal of 1,8-diiodooctane post-fabrication.

Main Methods:

• Fabrication of PffBT4T-2OD/PC71BM BHJ photovoltaic devices with and without 1,8-diiodooctane.
• Ellipsometry and ion beam analysis to monitor the removal of 1,8-diiodooctane.
• Small-angle neutron scattering (SANS) to characterize the evolution of nanostructure morphology.
• Power conversion efficiency measurements to quantify device performance.

Main Results:

• Inclusion of 1,8-diiodooctane increased PCE from 7.2% to over 8.7%.
• 1,8-diiodooctane acts as a transient plasticizer, promoting domain coarsening and improving morphology.
• Complete removal of 1,8-diiodooctane was achieved via thermal annealing at temperatures ≤100°C.
• An interplay between additive evaporation rate and domain coarsening rate was observed.

Conclusions:

• 1,8-diiodooctane significantly enhances BHJ photovoltaic device efficiency through a plasticization mechanism.
• Controlled evaporation and thermal annealing are key to optimizing morphology and performance.
• This study provides insights into additive-assisted morphology control for improved organic solar cell efficiency.