Synthesis and Application of Low-band Gap Conjugated Materials for Organic Photovoltaics
Assist. Prof. Muhammet Erkan Kose
Department of Chemistry and Biochemistry
North Dakota State University
Fargo, ND 58108, USA
Organic photovoltaics offer the use of organic conjugated systems as an alternative to silicon and other inorganic materials. Organic materials are much cheaper than their inorganic counterparts. Easy processing conditions as well as deposition on flexible substrates make organic materials attractive in photovoltaic technology. The latest organic photovoltaic devices with novel sun-light absorbing materials have yielded power conversion efficiencies exceeding 8% under solar illumination. It is clear that new donors and/or acceptors are required in order to reach high efficiencies needed for commercialization of OPV technology. The goal here is to utilize novel donor materials with donor optical band-gaps around 1.5 eV or less for more efficient sun-light harvesting that would yield high power conversion efficiencies in organic solar cells. Synthesis of a low band-gap material is just one of the parameters that should be optimized among the other factors such as carrier transport and recombination, exciton diffusion, and bulk morphology. A set of star-shaped molecules with benzothiadiazole and benzoxadiazole groups in the arms were designed and synthesized as a low band gap material for use in organic photovoltaic devices. The effect of structural variation on carrier mobility and device efficiency will be discussed. The photovoltaic properties of four donor-acceptor polymers utilizing thieno[3,4-c]pyrrole-4,6-dione (TPD) as acceptor comonomer will also be introduced. The theoretical calculations provide insights to the photophysical properties of these polymers as well as an explanation for the planarization of the conjugated backbone due to hydrogen bonding interactions between the TPD and the neighboring thiophene units. Last but not least, an experimental methodology will be introduced for accurate measurement of exciton diffusion in conjugated materials along with related simulations and calculations.