In this project funded by the Macromolecular, Supramolecular and Nanochemistry Program of the Chemistry Division, Wei You of University of North Carolina at Chapel Hill and Jason Locklin of University of Georgia will explore chemistry for the controlled surface-initiated polymerization of thiophene in order to solve the problem of replacing polystyrene sulfonate - poly(2,4-ethylenedioxythiophene) / indium-tin-oxide layers in organic photovoltaic cells. The approach is to study the mechanism of Kumada-type coupling polymerizations of thiophene in solution and from surfaces in order to uncover the proper conditions for controlled growth of polythiophene brushes from surfaces, to extend this chemistry to thienothiazole monomers, and then to probe the interfacial properties and morphologies of these active layers on electrodes using a variety of thin film characterization techniques and device architectures. Conjugated polymer brushes as the interfacial layer may offer a homogeneous film with uniform thickness; improved contact properties between the electrode and the active layer; and reduced barriers to charge injection. The broader impacts involve interdisciplinary training of undergraduate and graduate student researchers via the collaboration, integrating these research concepts into courses on organic electronics and soft materials, and participating in outreach activities to underrepresented minorities, high school students, and the general public.
This work will enhance our fundamental understanding about synthesizing thin films of conducting polymers and their photochemical behavior. Ultimately, such work could lead to the development of organic electronic materials, low cost solar cells, light emitting displays, and chemical and biological sensors.
Goals An inter-institutional collaboration involving four research groups (You at UNC Chapel Hill, and Locklin at UGA) have explored conjugated polymer brushes as interfacial layers for polymer solar cells, and the intrinsic conductivity of these conjugated brushes. The goals of this proposal are multi-fold: (a) to further understand and manipulate the growth of conjugated polymer brushes, together with their electrical and optical properties; (b) to utilize them as the hole transporting layer for polymer solar cells; (c) to optimize both polymer brushes and conjugated polymers for solar cells in order to achieve better performance. Intellectual Merit Specifically, through this collaboration, the You group (UNC Chapel Hill) achieved the following (not exhaustive, only the highlights): 1. The Locklin group (UGA) used the SI-KTCP synthetic strategy to generate uniform P3MT layers on ITO, and we used it to test the viability of these conjugated polymer brushes in replacing PEDOT:PSS. When the P3MT interfacial layer was employed as the hole transport layer (HTL) for solution-processed BHJ polymer solar cells with a typical configuration of ITO/P3MT/polymer:PCBM/Ca/Al, we were able to obtain cell efficiencies as high as 5% based on doped thin P3MT interfacial layers. 2. In a close collaboration with the Locklin group (UGA), we have successfully grown conjugated polymer brushes with (4-bromobenzyl)phosphonic acid as the anchoring group attaching to the bottom electrode ITO (tin doped indium oxide) and 3-methylthiophene as the monomer via the surface initiated Kumada catalyst-transfer polycondensation (SI-KCTP). A protected thiol end capping group was added at the end of the growth of conjugated polymer brushes, which allows the attachment of the top electrode (Au) via transfer printing. 3. In a close collaboration with the Locklin group (UGA), we have also successfully varied the chain length of such conjugated polymer brushes, and have successfully conducted initial electrical measurements on these length-variable conjugated polymer brushes with metal (ITO)-molecule (brushes)-metal (Au) junctions. Broader Impacts The broader impacts have been significant, including the following. 1. Research results: The fundamental understanding we have gained in the past three years with conjugated polymer brushes has not only disclosed new insights into surface-bound metal-mediated cross-coupling reactions as described below, but also led to improvements in the interfacial properties and morphology control in organic/polymer solar cells (OPVs). Using polymer brushes as interfacial modifiers will impact other related fields of organic electronics, such as transistors (OFETs), light emitting diodes (OLEDs), and fuel cells. 2. Education and training: this project has been a great opportunity to train graduate students and post doc fellows as it involves experts from different research fields. This project also helped students and post docs to develop research and communication skills and understand challenges involved in other fields and also how to work with researchers from different fields to achieve specific goals. 3. Publication and presentation: some of the results obtained in this work have been published in peer reviewed journals, and the participants (students, postdocs, and faculty members) have given research presentations in various venues. 4. Outreach to general public, and K-12: UNC team has participated in the UNC Science Expo (2013 and 2014) to show the importance of renewable energy. In addition, UNC team also participated in the Climate LEAP program and ran a few lab tours for high school students in the summer of 2012, 2013, and 2014.
