Internal photoemission in molecular junctions: parameters for interfacial barrier determinations

Area of Science:

• Materials Science
• Physical Chemistry
• Organic Electronics

Background:

• Molecular junctions are crucial for organic electronics.
• Understanding charge transport mechanisms is vital for device optimization.
• Internal Photoemission (IPE) is a significant process in molecular junctions.

Purpose of the Study:

• To investigate photocurrent spectra in large-area molecular junctions.
• To differentiate Internal Photoemission (IPE) from secondary photocurrent mechanisms.
• To analyze the influence of molecular structure and thickness on photocurrent.

Main Methods:

• Fabrication of large-area molecular junctions with partially transparent copper contacts.
• Illumination with UV-vis light to measure photocurrent spectra.
• Systematic variation of molecular layer thickness and structure.
• Analysis of photocurrent response to source intensity and layer thickness.

Main Results:

• IPE is dominant when the molecular layer does not absorb light, providing energy level alignment information.
• A secondary photocurrent mechanism is observed when the molecular layer absorbs light.
• IPE can be distinguished from the secondary mechanism by varying source intensity and molecular layer thickness.
• Photocurrent sign indicates transport through occupied (hole) or unoccupied (electron) molecular orbitals.

Conclusions:

• IPE is a valuable tool for characterizing interfacial energetics in intact molecular junctions.
• Experimental parameter variations allow clear differentiation between IPE and other photocurrent mechanisms.
• This study enables precise discrimination of electron versus hole transport in molecular devices.