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Relating electronic structure to charge transport in organic semiconductors for opto-electronic applications

Vojtech Nádaždy (Institute of Physics, SAS)

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Abstract:
Distribution of the density of electronic states (DOS) in organic semiconductors determines their electron transport and opto-electronic properties, which are broadly utilized in organic solar cells, organic light emitting diodes, organic field effect transistors, etc. The knowledge of DOS distribution is a prerequisite for optimizing their performance. However, a suitable experimental method for direct DOS measurement in the entire bandgap from HOMO to LUMO was lacking. To overcome this problem, the Energy-resolved Electrochemical Impedance Spectroscopy (ER-EIS) has been recently developed at the Institute of Physics SAS.

In the lecture, first the principles of the ER-EIS method will be introduced. Subsequently, its potential will be exemplified on films of several conjugated polymers and films of organic donor-acceptor blends. The results will demonstrate that the ER-EIS technique does not only allow the mapping of DOS distribution over five orders of magnitude and over a wide energy range of 7 eV, but can also delineate the changes in DOS that occur upon blending. The knowledge of DOS enables us to interpret the relationship between a distinct conformational defect and the respective charge transport response in the conjugated polymer. This demonstrates how charge-carrier mobility in organic semiconductors can be controlled via molecular conformation. Furthermore, the interfacial binding energy of the charge-transfer state in the blend can be determined to identify the mechanisms, which reduce the activation energy required to separate the interfacial electron–hole pair at the polymer/fullerene interface. A quantitative correlation between the charge transport and energetic disorder in conjugated polymers probed by DOS is determined as well.

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