Doctoral Candidate
Materials Science & Engineering

BS, Brown University, 2002 (materials engineering)

Cook Hall 3070
2220 Campus Drive
Evanston, IL 60208-3108

(847) 461-5950 office
(847) 467-6415 lab
(847) 491-3010 facsimile

Email Marina

Marina Sofos

Self-Assembled Oligo(Phenylene-Vinylene) Based Lyotropic Liquid Crystals

The goal of my work is to utilize the powerful, bottom-up approach of molecular self-assembly to organize luminescent molecules into well-ordered nanostructures to improve their performance in organic electronic devices.  

Conjugated molecules provide a viable alternative to traditional inorganic semiconductors due to their easy-solution-based processing, mechanical flexibility, and chemical tunability.  One of the key challenges, however, is that disordered polymeric networks contain large numbers of defects leading to trapping sites, which hinder charge transport, and consequently, device performance.  Phenylene vinylene is a particularly attractive chromophore for use in optoelectronics, due to its luminescent efficiency and stability.  Our group has developed a series of tri-block amphiphiles consisting of oligo(phenylene vinylene) (OPV) asymmetrically end-substituted with a hydrophilic poly(ethylene glycol) (PEG) block and a hydrophobic alkyl block.   A number of applications can potentially take advantage of the self-assembling nature of these luminescent amphiphiles, which exhibit both thermotropic and lyotropic liquid crystalline (LC) phases.  Solar cells, which are promising, inexpensive sources of renewable energy, could benefit from self-assembling systems.  Towards improving charge carrier mobility in heterojunction solar cells, the supramolecular nanostructure generated by the hole-conducting OPV is an attractive template in which to successfully assemble electron-acceptor materials in an easy and efficient manner.   

Figure 1: Self-supporting gel of OPV amphiphile.

Previously, in the Stupp group, P. Braun, et. al., developed a novel templating approach in which a lyotropic LC phase formed by a non-ionic organic amphiphile in water was used to mineralize the II-VI semiconductor, CdS (among others).   In my work, the lyotropic LCs formed by the OPV-amphiphiles is being utilized to template and direct the growth of the electron-transporting II-VI semiconductor, CdSe.  A low-temperature technique is required for the mineralization of CdSe within the OPV LC framework due to the limited temperature range in which mesophase is stable (50 oC).  To accomplish this, CdSe films will be electrochemically deposited from precursors introduced into the aqueous solutions used to form the lyotropic OPV LC, and so, the layered structure of the OPV could provide appropriate aligned pathways for the efficient movement of charge carriers to the electrodes.  Alignment of the director axis of the LC will be necessary to arrange the alternating layers of CdSe and OPV perpendicularly to the substrate and electrodes.  I am also investigating the incorporation of the electron-acceptor, C60, pre-formed, into the hydrophobic alkyl layers of the OPV LC structure.

Figure 2: Schematic of hybrid solar cell device.

Publications:

Hulvat, J. F.; Sofos, M; Tajima, K; Stupp, S. I. "Self-Assembly and Luminescence of Oligo(p-Phenylene Vinylene) Amphiphiles"  J. Am. Chem. Soc. 2005, 127(1), 366 - 372.