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C5.03 - Excitonic Behavior in Thin Film PC60BM & PC70BM 
April 22, 2014   2:15pm - 2:30pm

Measuring the key photo-physical properties of PCxBM molecules and understanding charge transfer dynamics in polymer:PCxBM BHJ systems can better understand current successful devices for better future acceptor materials design. PCBM has already been shown to exhibit both singlet and triplet excitons, which both have the potential to change the performance of OPV devices. In particular, that singlet exciton photo-physics is relevant to the generation of charges from fullerene absorption, conventionally by first passing a hole onto a less electronegative molecule forming a charge transfer (CT) state before completely separating into radical charges. It is important to understand the kinetics of processes after the generation of the singlet exciton in order to predict which mechanisms dominate the route to successful separation. Germinate recombination can occur both in the singlet state and also after it has formed a CT state, which in addition to intersystem crossing (ISC) to triplet states, provide the major loss pathways. While it is not clear if triplets can act as a significant contribution to photocurrent is has already been shown that they play an important role in photostability. [1-5]In this work we use optical absorption and emission measurements to characterise the photophysical properties of PC60BM and PC70BM excitons. Using femto-second TAS measurements we are able to extend the understanding of formation of charges to the early time-scale both as a neat film and when suspended in an inert polystyrene (PS) matrix.[6] 1. Distler, A., et al., Effect of PCBM on the Photodegradation Kinetics of Polymers for Organic Photovoltaics. Chemistry of Materials, 2012. 24(22): p. 4397-4405.2. Schlenker, C.W., et al., Polymer Triplet Energy Levels Need Not Limit Photocurrent Collection in Organic Solar Cells. Journal of the American Chemical Society, 2012. 134(48): p. 19661-19668.3. Cook, S., et al., Singlet exciton transfer and fullerene triplet formation in polymer-fullerene blend films. Applied Physics Letters, 2006. 89(10): p. 101128-3.4. Benson-Smith, J.J., et al., Charge separation and fullerene triplet formation in blend films of polyfluorene polymers with [6,6]-phenyl C61 butyric acid methyl ester. Dalton Transactions, 2009(45): p. 10000-10005.5. Ohkita, H., et al., Radical ion pair mediated triplet formation in polymer-fullerene blend films. Chemical Communications, 2006(37): p. 3939-3941.6. Cook, S., et al., A photophysical study of PCBM thin films. Chemical Physics Letters, 2007. 445(4-6): p. 276-280.

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Keynote Address
Panel Discussion - Different Approaches to Commercializing Materials Research
Business Challenges to Starting a Materials-Based Company
Fred Kavli Distinguished Lectureship in Nanoscience
Application of In-situ X-ray Absorption, Emission and Powder Diffraction Studies in Nanomaterials Research - From the Design of an In-situ Experiment to Data Analysis