Nanostructured Polymer Films for Electronics



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Nanostructured polymer films are used as active materials in polymer-based photovoltaics, and as sacrificial materials for building nanoscale semiconductor devices. The research work described in this thesis addresses both topics.

First, we used a simple approach to cross-link the conjugated polymer donor using electron-beam lithography or proximity ion beam lithography. Spectroscopy measurements show light absorption properties in cross-linked polymer, therefore aromatic groups responsible for charge generation and transport are retained after the lithographic process. Gradient and nanostructured devices were fabricated to study their optoelectronic properties. Nanostructured devices showed improved device efficiency due to increase in interfacial area for charge dissociation. Cross-linked polymers are resistant to heat and solvent processing which allows the study of active layer morphology by deconstructing the nanostructured device. Structure-property relationships are established using these simple model systems. In order to have a complete understanding of cross linked polymer, effects of radiation on molecular ordering, polymer crystal orientations and charge-carrier mobility were investigated. Cross-linked polymer system has reduced degree of crystallinity and hole mobility by almost 50%.

Second, we investigate the effects of surface and substrate interactions on self assembly of block copolymer lamellae in thin films. The aim is to identify conditions where lamellar domains are oriented perpendicular to the substrates. Thin films of diblock and triblock copolymers were prepared on neutral substrates, which were ordered by thermal annealing. Thin films were evaluated by optical microscopy, scanning force microscopy and grazing incidence small angle X-ray scattering (GISAXS). Domain orientations in diblock copolymer films were sensitive to annealing temperature, quality of neutral substrate and film thickness. Thin films of triblock copolymers have perpendicular domain orientation for all conditions studied due to increase in the end-segmental conformational entropy. Triblocks are much easier to orient than diblock coploymer films but both these films are associated with high density of "tilt" defects. Studies were done on diblock copolymer films prepared on different neutral substrates of varying grafting densities of polymer brushes to understand the inplane and out-of-plane ordering. In-plane order improves with increases in the brush grafting density and film thickness. However, while out-of-plane order improves with increases in brush grafting density, it is always reduced in thicker films where the free energy penalty for bending is small. These studies demonstrate that the optimal film thickness for block copolymer lithography is equal to the lamellar periodicity.



Polymer-based solar cell, Block copolymer self-assembly


Portions of this document appear in: Moungthai, Suchanun, Nikhila Mahadevapuram, Paul Ruchhoeft, and Gila E. Stein. "Direct patterning of conductive polymer domains for photovoltaic devices." ACS applied materials & interfaces 4, no. 8 (2012): 4015-4023. And in: Vu, Thai, Nikhila Mahadevapuram, Ginusha M. Perera, and Gila E. Stein. "Controlling domain orientations in thin films of AB and ABA block copolymers." Macromolecules 44, no. 15 (2011): 6121-6127. Reproduced with permission.