pi-Conjugated Polymers for Electrochromic and Photovoltaic Applications

Date of Completion

January 2010

Keywords

Chemistry, Organic|Chemistry, Polymer

Degree

Ph.D.

Abstract

Electrochromism is a process by which a material can change its electronic-optical properties upon charge injection/removal. Conjugated polymers are an interesting class of electrochromic materials because of their color tunability, high optical contrasts, fast switching speeds, and processability. Poly(3,4-propylenedioxy)thiophenes (PProDOT) are a substantial subclass of materials in conjugated polymer electrochromics due to their high optical contract between the bleached and colored states. Common derivatives of this molecule are typically made at the beta position with respect to oxygen on the seven membered ring. PProDOTs with methyl and benzyl substituents (beta position with respect to oxygen) are two of the more successful due to their high contrast. We have found that there is a much more substantial effect when PProDOT is derivatized in the positions alpha to the oxygen. For example, two t-butyl groups with each placed alpha to the oxygen in PProDOT incurs a 200 nm shift in the lambda max (365 nm) compared to having two methyl groups with each placed alpha to the oxygen. The dimethyl derivative is blue in color whereas the di-t-butyl, dihexyl, diisopropyl is showing yellow, orange and red color respectively. The polymer of this new derivative, P13ProDOT-TB2 and P13ProDOT-Hex 2 is organic-soluble and can be processed by a variety of solution methods, including spray coating. Furthermore we have also studied some selenium based polymer for electrochromic application. Poly(3,4-propylenedioxy)selenophenes (PProDOS) is showing better optical contrast, stability and faster switching speed as compared to their sulfur analogs. ^ Low band gap conducting polymers (CPs) have relatively low absorption in the visible region, in their conducting states, making them promising candidates for optically transparent electrode, hole-injection layer for light-emitting diodes and suitable donor material for Photovoltaics. The monomer, Seleno[3,2- c]thiophene and Seleno[3,4-b]thiophene, were electrochemically and polymerized to produce new low band gap conducting polymer, poly(Seleno[3,2- c]thiophene) (PS32cT) and Poly(Seleno[3,4-b]thiophene) (PS34bT), having a low band gap of 1.03 eV and 1.50 eV respectively. Besides from the suitable energy gap, they also offer a good match of the absolute energy levels with the other materials in the photovoltaic device. The HOMO of the low band gap polymers agree with the work function of ITO and LUMO matches with the acceptor level of PCBM. This overlap is very important to the function of photovoltaic devices. ^ In a different approach we describe a new alternative route for the synthesis of thieno[3,4-b]thiophene, alkyl derivatives thereof, seleno[3,4- b]thiophene, and thieno[3,4-b]furan made from inexpensive starting materials, such as thiophene-2-carboxylic acid and furan-2-carboxylic acid. Such fused heterocycles are of great interest for low band gap organic semiconductors and applications including OLEDs, organic photovoltaic cells, and electrochromic applications. ^

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