CVD graphene as interfacial layer to engineer the organic donor-acceptor heterojunction interface properties

Area of Science:

• Materials Science
• Organic Electronics
• Surface Chemistry

Background:

• Organic solar cells (OSCs) rely on efficient charge generation and transport at organic donor-acceptor heterojunctions.
• Indium-tin-oxide (ITO) is a common transparent conductive electrode, but its surface properties can limit heterojunction performance.
• Surface modification strategies are crucial for optimizing the interface in OSC devices.

Purpose of the Study:

• To investigate the use of chemical-vapor-deposited (CVD) graphene as a surface modifier for ITO electrodes in OSCs.
• To understand how CVD graphene influences the molecular orientation and electronic properties of organic donor-acceptor heterojunctions.
• To enhance the performance of inverted organic solar cells through interface engineering.

Main Methods:

• Fabrication of inverted organic solar cells with bare ITO and CVD graphene-modified ITO electrodes.
• In situ near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy to analyze molecular orientation.
• In situ ultraviolet photoelectron spectroscopy (UPS) to study energy level alignment.
• Device performance characterization.

Main Results:

• CVD graphene induces a transition in organic heterojunctions from a standing to a less-standing molecular configuration.
• The less-standing configuration enhances charge transport perpendicular to the electrode surface.
• Graphene modification leads to better energy level alignment between organic layers and electrodes.
• Improved charge collection efficiency was observed in devices with CVD graphene-modified ITO.

Conclusions:

• CVD graphene is an effective surface modifier for ITO electrodes in organic solar cells.
• Graphene's ability to template molecular orientation optimizes charge transport and collection.
• This interface engineering approach significantly enhances the performance of inverted organic solar cells.