Hidden structures in domain interfaces in organic semiconductors
Hidden structures in domain interfaces in organic semiconductors

A team from the Lawrence Berkeley National Laboratory in the US has revealed hidden structures in domain interfaces within thin films that hamper the performance of organic semiconductors. Due to their use in light emitting diodes (LEDs), field effect transistors, and photovoltaic cells, understanding these interfaces and their intermolecular and electronic structure of the semiconductors has become increasingly important.

Large-scale organic electronics manufacturing requires solution processing to offer a highly scalable and cheaper alternative to silicon-based devices. In terms of small-molecule organic semiconductors, solution processing results in crystalline domains with high charge mobility, with the interfaces between these domains impeding charge transport, thus degrading device performance. However, this new study, published in Nature Communications [Wong et al. Nat. Commun. (2015) DOI: 10.1038/ncomms6946], could have resolved this problem.

Using transient absorption (TA) microscopy to isolate a unique signature of a hidden domain interface within an especially high-performing solution-processed organic semiconductor called TIPS-pentacene, a tangle of randomly oriented nanocrystallites was found to be kinetically trapped in the interfaces during solution casting. As team leader Naomi Ginsberg points out, “If the interfaces were neat and clean, they wouldn't have such a large impact on performance, but the presence of the nanocrystallites reduces charge-carrier mobility.”

By providing a key intermediary in the feedback loop of device optimization by characterizing the microscopic details of the films that go into the devices, as well as in inferring how the solution casting could have created the structures at the interfaces, it is hoped this breakthrough could suggest ways of altering the fine balance of solution casting parameters to produce more functional films, and also find uses as a diagnostic for solution processing of small-molecule films in organic electronics.

The TA microscopy on a self-fabricated optical microscope generated focal volumes a thousand times smaller than usual for traditional TA microscopes, as well as deploying multiple different light polarizations to isolate interface signals not seen in either of the adjacent domains. This produced a predictive factor to scalable and affordable solution processing of these materials to minimize discontinuities and maximize charge-carrier mobility.

The team now hopes to explore how the interfacial structure changes as casting conditions are altered or the films annealed, as well as other strategies for ultrafast microscopy below the optical diffraction limit to resolve heterogeneities in the excited state properties of organic semiconducting, and other new optoelectronic materials that cannot presently be observed.