Universal correlation between fibril width and quantum efficiency in diketopyrrolopyrrole-based polymer solar cells

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Diketopyrrolopyrrole (DPP)-based conjugated polymers are promising materials for organic solar cells.
• The morphology of the active layer significantly influences device performance.
• Understanding the relationship between microstructure and efficiency is crucial for optimizing organic photovoltaic devices.

Purpose of the Study:

• To establish a direct correlation between the fibrillar microstructure and external quantum efficiency (EQE) in DPP-based polymer solar cells.
• To investigate how polymer side chains and blend morphology affect charge generation.
• To determine the optimal fibril width for maximizing solar cell performance.

Main Methods:

• Synthesis of six DPP-based conjugated polymers with varying electron-rich segments and 2'-decyltetradecyl (DT) side chains.
• Fabrication of bulk heterojunction solar cells using DT-DPP polymers and [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) as the acceptor.
• Characterization of blend morphology using transmission electron microscopy (TEM) to determine fibril widths.
• Measurement of external quantum efficiencies (EQE) and power conversion efficiencies (PCE) for the fabricated solar cells.

Main Results:

• A clear correlation was observed between fibril width and EQE in DT-DPP:[70]PCBM blends.
• The highest EQEs (up to 60%) and PCEs (7.1%) were achieved with fibril widths below 12 nm.
• EQE decreased significantly for blends with wider fibrils (>12 nm) due to limited exciton diffusion.
• Fibril width was found to be inversely correlated with polymer solubility, with less soluble polymers yielding narrower fibrils.

Conclusions:

• The fibril width in the blend is a critical parameter determining the charge generation efficiency in DPP-based organic solar cells.
• A universal relationship exists between fibril width and EQE for this class of materials, suggesting morphology control is key.
• Optimizing polymer solubility is essential for achieving narrow fibrillar microstructures and high-performance organic solar cells.