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New study uncovers charge transport role in organic materials

Organic semiconductors hold great promise as active elements in solar cell or optoelectronic devices. Elucidation of their charge transport mechanism, which defines device performance, was under the scope of an EU-funded team.
New study uncovers charge transport role in organic materials
Within ELECTROMAT (Electronic transport in organic materials), researchers worked to develop a theoretical and computational framework that links the atomic structure of the organic material to its electrical properties.

Organic semiconductors may be sensitive to polaronic effects, meaning that they cannot effectively transport charge. With this in mind, the first task was to identify the nature of charge carriers in organic crystals. A detailed study on electron-phonon coupling in polyacene crystals showed that such coupling is not strong enough to lead to polaron formation.

Next, the ELECTROMAT team developed a methodology to simulate charge transport in organic crystals. The method involved use of ab-initio calculations that took into account the quantum nature of both phonons and electrons. Data regarding carrier mobility with increasing or decreasing temperature were in good agreement with experimental data in polyacene organic crystals. Theoretical data also matched experimental in quantum dot crystals. In such solids, the team also found that small polaron hopping is the relevant conduction mechanism.

Depending on the processing conditions, conjugated polymer materials can have a complex structure; some parts are arranged in an orderly manner and others form a tangled mess. The role of the interface between crystalline and amorphous regions in charge transport is not well understood. Considering two different interface types, researchers found that charge transport takes place through crystalline domains. The amorphous domains act as high barriers for charge carriers. Contrary to what happens in organic crystals from small molecules, there was no formation of trap states at the interface.

Through simulations, researchers also gained thorough understanding of charge transport in highly disordered and ordered conjugated polymers.

Are grain boundaries in organic crystals detrimental to charge transport? The team found that grain boundaries introduce trap states within the material bandgap. Their spatial positions and energies can be predicted solely from the geometrical arrangement of molecules near the boundary. In addition, wave functions of these states are localised on closely spaced pairs of molecules from across the boundary.

Organic materials offer a low-cost alternative for use in solar cells, field-effect transistors, light-emitting diodes and even polymer lithium car batteries. ELECTROMAT insights into charge transport in such materials are expected to have a major socioeconomic impact.

Related information


Charge transport, organic materials, organic crystals, polaron, grain boundaries
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