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Ultrafast dynamics of excitons and charges in organic materials and semiconductor nanocrystals for optoelectronics

Speaker: 
Laurens D.A. Siebbeles (TU Delft)
Date: 
Wed, 30/10/2013 - 11:00am to 12:00pm
Location: 
S13-M01-11 (Conference Room)
Host: 
Yuan Ping Feng
Event Type: 
Seminars

The seminar will report studies of the behavior of electronic excited states (excitons) and excess charge carriers in conjugated polymers, covalent organic frameworks and semiconductor nanocrstals. These materials have fascinating optical and electronic properties that are of interest for applications in e.g. solar cells, photodiodes, light-emitting diodes, field-effect transistors and nanoscale molecular electronics.

We studied the mechanism of charge carrier photogeneration in blend films of conjugated polymers and electron accepting materials by ultrafast optical and terahertz spectroscopy. It will be discussed how material structure affects the spatial extent and motion of excitons and free charge carriers.1,2   

The generation of two or more excited states for the absorption of a single energetic photon is of interest for development of highly efficient (up to 44%) solar cells. Using time-resolved spectroscopy we found that this process of carrier multiplication occurs with high efficiency in films of PbSe quantum dots that are in strong electrical contact due to introduction of small linker molecules. The multiple charges produced by carrier multiplication are found to efficiently escape from recombination in case strong coupling between quantum dots makes them highly mobile.3-5

Covalent Organic Frameworks (COFs) are materials with great promise for application in optoelectronics. We carried out microwave conductivity measurements on charges moving in COFs consisting of phthalocyanine units that are strongly coupled by pi-pi stacking. On basis of the experimental results and  quantum mechanical calculations, it is inferred that charges move via a band-like mechanism with a mobility that can be as high as ~100 cm2/Vs.6

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