Structure-property relationships in organic photovoltaic devices

• Main Author: Tam, Dexter Teck Lip
• Other Authors: Li Hairong
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/51177
• Institution: Nanyang Technological University

The purpose of this project is to understand structure-property relationship of organic materials in the field of organic electronics, especially in organic photovoltaics (OPV). This was carried out by systematically varying the acceptor moiety in donoracceptor (D-A) type electron donating polymers. Two bottlenecks for obtaining efficient OPV devices have been identified; the typically high bandgap and low charge mobility of organic materials. To answer these, nitrogen based fused tricyclic heteroacene structures were designed, synthesized and used as the acceptor moiety in D-A type polymer backbones. These nitrogen based heteroacenes not only act as strong electron acceptors but also promote quinoid formation which favors low bandgaps. The fused tricyclic structures provide strong intermolecular interaction through ! – ! stacking which is essential for good charge mobility. These acceptor moieties were characterized both experimentally and theoretically in order to better understand the changes produced by varying the chemical structures. The results can be extrapolated to their polymers, making the comprehension of these complex polymer systems easier. The systematic change of the acceptor moiety, together with variation of aromatic side groups, resulted in tunable energy levels, bandgaps, light absorption profile, solubilities and polymer configurations. Most of these polymers show low optical bandgap in the range of 1.16 eV to 1.59 eV, with the exception of two polymers that show higher bandgaps due to their twisted polymer backbones. OPV devices of these polymers, blended with PC71BM, were fabricated and characterized. The results show that device performance is strongly dependent on the energy levels and polymer configuration. Other factors include aromatic side groups and length of solubilizing group. Understanding these structure – property relationships can help in rational design of materials not only for OPV but also other organic electronics application.