Furtak Research Projects

Charge Transfer Optimization for Organic Electronics

Organic/inorganic composites represent a new and important class of electronic materials. They are being developed in applications ranging from organic LEDs and solar cells to biological sensing and fuel cells. Often, interfacial properties such as wetting of the inorganic by the organic, the ordering of the organic by the inorganic, and the charge transfer energetics at the interface control the performance of hybrid devices. The goal of this project is to develop and characterize surface monolayer functionalization strategies for inorganic materials which improve the performance of organic/inorganic hybrids. The emphasis is on metal oxides and particularly ZnO.

The figures present an example of how surface modification can improve the morphology of hybrid ZnO nanowire/polymer solar cells. In organic solar cells, optical absorption in the polymer creates excitons. The excitons do not naturally dissociate to form free carriers, but, introducing a heterointerface between the polymer and a second material allows dissociation to occur. The short exciton diffusion length means these interfaces must be no more than a few tens of nanometers apart. The common method for achieving this is to create a bulk heterojunction by the natural phase separation between an organic electron acceptor and the polymer. This leads to a bicontinuous network in which the polymer regions are close to the organic acceptor. Collection of carriers after dissociation, however, requires hopping from one nanoparticle to another, hence the mobility and morphology of the electron acceptor network has a major impact on device performance.

An alternative approach involves ZnO nanowire array/polymer blends to create both closely spaced interfaces and a natural conduit for electron collection. For this approach to be effective several issues must be resolved. For example, it must be possible to infiltrate the nanowire network with polymer so the required heterointerfaces form. This has proven to be a challenging. Nanorods grow preferentially in the (0001) direction exposing a polar end face which typical polymers used in organic solar cells do not wet. As the upper image shows, when polymer is spun onto untreated wire arrays it tends to sit on the surface of the array. Heating to near the glass transition temperature has been shown to improve intercalation, but it also increases disorder in the polymer which hurts performance. Treating the ZnO with phenyltriethoxysilane (PTES) creates a surface which the polymer more easily wets and results in some interpenetration of the nanowire array byt the polymer at room temperature (bottom picture).

This research is a partnership with Reuben Collins and is sponsored by the National Science Foundation. The work involves collboarations with David Ginley and Dana Olson at NREL, as well as with Cecile Ladam at the Norwegian University of Science and Technology (NTNU). Students who are currently involved with the project include graduate students Darick Baker, Thomas Brenner, and Gang Chen, as well as undergraduates Shirley Moore, Casey Smith, Mickey Wilson, Ryan Crisp, Quintin Sheridan, and Ryan Crawford.