Molecularly Resolved Electronic Landscapes of Differing Acceptor-Donor Interface Geometries

• Main Authors: Cochrane, Katherine A, Roussy, Tanya S, Yuan, Bingkai, Tom, Gary, Mårsell, Erik, Burke, Sarah A
• Format: Journal Article
• Language: English
• Published: 28-03-2023
• Subjects: Physics - Materials Science; Physics - Mesoscale and Nanoscale Physics
• DOI: 10.48550/arxiv.2303.16128

J. Phys. Chem. C 2018, 122, 15, 8437-8444 Organic semiconductors are a promising class of materials for numerous
electronic and optoelectronic applications, including solar cells. However,
these materials tend to be extremely sensitive to the local environment and
surrounding molecular geometry, causing the energy levels near boundaries and
interfaces essential to device function to differ from those of the bulk.
Scanning Tunneling Microscopy and Spectroscopy (STM/STS) has the ability to
examine both the structural and electronic properties of these interfaces on
the molecular and submolecular scale. Here we investigate the prototypical
acceptor/donor system PTCDA/CuPc using sub-molecularly resolved pixel-by-pixel
STS to demonstrate the importance of subtle changes in interface geometry in
prototypical solar cell materials. PTCDA and CuPc were sequentially deposited
on NaCl bilayers to create lateral heterojunctions that were decoupled from the
underlying substrate. Donor and acceptor states were observed to shift in
opposite directions suggesting an equilibrium charge transfer between the two.
Narrowing of the gap energy compared to isolated molecules on the same surface
are indicative of the influence of the local dielectric environment. Further,
we find that the electronic state energies of both acceptor and donor are
strongly dependent on the ratio and positioning of both molecules in larger
clusters. This molecular-scale structural dependence of the electronic states
of both interfacial acceptor and donor has significant implications for device
design where level alignment strongly correlates to device performance.