Development of N- and P- Types of Semiconducting Polymers

In the past three years, our research project sponsored partially by AFOSR has been focused on developing low bandgap semiconducting polymers as both the donor and acceptor materials for BHJ solar cells. Research effort were also devoted to establish structure-property relationship and develop approaches to optimize light harvest, charge separation and charge collection, thus the overall power conversion efficiency (PCE). We have made very important contributions to the field and developed a series of new semiconducting polymers for highly efficient ternary polymer solar cells. We also discovered cooperative plasmonic effect of dual precious metal nanoparticles in enhancing PCE of BHJ solar cells. Multiwall carbon nanowire were introduced to significantly enhance the charge collection. These results have had significant impact to the area. Several polymers developed in our lab are now widely investigated around world. Seven of the papers published are highly cited as indicated by SCI. Accomplishments: In the past three years, we made significant progress in four research directions in OPV solar cells. 1. Establishing structure property relationshipbetween Low Bandgap Polymers and Their Organic Solar Cells Performance. A key challenge of the development of organic photovoltaic devices is obtaining a predictive understanding of the relationship between polymeric structure of the donor material and device performance. Among the factors that may influence solar energy conversion, the nature of electron donating and accepting materials and the morphology of the composites play the crucial roles in determining the final performance of the devices. In recent years, fullerene derivatives such as [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) have been widely adopted as electron acceptors due to their low lying energy levels and relatively high electron affinity and mobility. It was also found that addition of a small amount of high boiling point solvent, generally1,8-diiodooctane (DIO), can reliably improve the morphology of most of the composite systems. Thus, we focus our main effort on understanding the structure/property correlation of electron donor materials in order to develop innovative strategies for achieving high performance solar cells. In previous studies, a linear correlation of the calculated dipole moment change between the ground and excited states Δμge of single repeating units in charge transfer polymers was established, suggesting a larger local Δμge could defray a part of exciton binding energy, leading to a higher power conversion efficiency (PCE). We further synthesized a new low bandgap polymers incorporating an artificial sweetener derivatives, N-alkyl, 3-oxothieno[3,4-d]isothiazole 1,1-dioxide (TID). Semi-empirical calculations on the local dipole moment change between ground and excited states (∆μge) in the repeating units of the new polymer indicate that the replacement of the carbonyl by a sulfonyl group leads to larger ∆μge values. The resulting polymers exhibit diminished power conversion efficiency (PCE) from bulk heterojunction (BHJ) solar cells with PC71BM as acceptor, which extends the correlation between PCE and ∆μge of