Abstract
We have demonstrated efficient solution-processed small-molecule solar cells based on a novel molecular donor, DTS(PTTh2)2. A record power conversion efficiency (PCE) is achieved for small-molecule organic photovoltaics: PCE=7% under AM 1.5 G irradiation (100 mW cm–2) from bulk heterojunction (BHJ) composites of DTS(PTTh2)2:PC70BM (donor to acceptor ratio of 7:3) with short circuit current (Jsc) of 14.4 mA cm–2, open circuit voltage (Voc) of 0.78 V and fill factor (FF) of 59%. These high values were obtained by using remarkably small percentages of solvent additive (0.25% v/v of diiodooctane, DIO) during the film forming process. Transmission electron microscopy was used to directly image crystalline DTS(PTTh2)2 domains in BHJ films and investigate changes with the varying concentrations of the solvent additive. These results provide innovative guidelines for the realization of high performance solution-processed organic solar cells fabricated using molecular materials. The final step in the preparation of p-DTS(PTTh2)2 involves end capping of the PT-DTS-PT core with hexyl bithiophene units via a microwave assisted Stille cross coupling reaction. Methyl transfer (instead of hexylbithiophene transfer) can occur leading to the formation of (MePT)DTS(PTTh2. Trace impurities of (MePT)DTS(PTTh2) in BHJ solar cells fabricated from synthesis batches of p-DTS(PTTh2)2 significantly influence the photovoltaic properties, causing a ~50% reduction in efficiency and affecting all of the relevant device parameters (Jsc, Voc and FF). From a broader perspective, despite molecular design, the suitability of a material for efficient devices is often only determined by trial and error in the device processing laboratory. As shown by the data, promising materials found to be unsuitable for device applications may suffer from highly dilute impurities that act to increase carrier recombination.The mechanism of charge transfer in blended films of organic molecules relevant to solar energy production is a central problem in the field. In particular, the structure property relationship regarding the process is of wide interest. We show that the timescale of ultrafast charge transfer between a small molecule donor and a fullerene acceptor is affected by the use of processing additives during film formation, decreasing to less than 50 fs. In light of these results, a model is put forth which describes ultrafast and high yield charge transfer as the result of excited state delocalization. Co-authors on this work are as follows: G. C. Bazan, Y. Sun, G. C. Welch, WL Leong L. G. Kaake, D. Moses, C. J. Takacs.
Biography
Widely known for his pioneering research in and the co-founding of the field of semiconducting and metallic polymers, Professor Heeger is also the recipient of numerous awards, including the Nobel Prize in Chemistry(2000), the Oliver E. Buckley Prize for Condensed Matter Physics, the Balzan Prize for the Science of New Materials, the President's Medal for Distinguished Achievement from the University of Pennsylvania, the Chancellor's Medal from the University of California, Santa Barbara, and honorary doctorates from universities in the United States, Europe and Asia. He is a member of the National Academy of Science (USA), the National Academy of Engineering (USA), the Korean Academy of Science and the Chinese Academy of Science. Prof. Heeger has more than 800 publications in scientific journals and more than 50 patents. He founded UNIAX Corporation in 1990; UNIAX was acquired by DuPont in 2000. Prof. Heeger is a co-founder and serves on the Board of Directors Konarka Technologies Inc. He is co-founder and Chairman of CBrite Inc. in Santa Barbara. He is co-founder and Vice-Chairman of Cynvenio (micro fluidics for cell sorting and related areas) and Cytomx Therapeutics (novel technology for targeted drug delivery). He has long been interested in and a fan of theatre. Prof. Heeger has participated in the production of three Broadway plays: "In the Heights" (2008 Best Musical and still ruinning), "West Side Story" (revival currently running) and "Barefoot in the Park" (a revival in 2007 --- did not survive the critics!).