Alan Heeger 2011-2012 Seminar Series
February 29, 2012
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.