A THEORETICAL AND EXPERIMENTAL STUDY OF CHARGE TRANSPORT IN ORGANIC THERMOELECTRIC MATERIALS AND CHARGE TRANSFER STATES IN ORGANIC PHOTOVOLTAICS

Date: 
10/23/2020 - 9:00am to 10:00am
Location: 
Zoom
Speaker(s) / Presenter(s): 
Ashkan Abtahi

Abstract: Application of organic electronics have increased significantly over the past two decades. Organic materials can be used in flexible devices with cheaper cost of fabrication, yet in most cases the devices suffer from poor performance and stability. Investigating doping mechanism, charge transport and charge transfer in such materials can help us to understand the origin of these issues and later resolve them. In this dissertation, organic materials are used in three different device structures to investigate charge transport and charge transfer. Chemically doped pi-conjugated polymers are promising materials to be used in thermoelectric (TE) devices, yet their application is limited by their low performance. Blending two polymers is a simple way to change the properties of the TE devices. Here we used a simple analytical model to calculate TE properties of polymer blend by taking into account for energetic disorder, energetic offset between two polymers and localization length which proposed TE performance of polymer blend can exceed the individual ones at specific blends of two polymers. We showed these improvements are achievable by experimentally testing TE properties of selected polymer blends. Further, to investigate the doping mechanism in polymers, we used organic electrochemical transistors to investigate the effect of anion size on polaron delocalization and the thermoelectric properties of single polymers. This device structure allowed us to control the charge carrier concentration with minimizing the effects on the film morphology.

In organic photovoltaics (OPVs), upon fluorination of donor molecules the performance of device increases in most cases. So, we investigated the charge transfer state energy between the electron donor anthradithiophene (ADT) and the electron acceptor C60 upon halogenation of the ADT molecule. Interfacial energetics and charge transfer state energies between donor and acceptor are crucial to performance of these devices. We probe interfacial energetics of donor/acceptor interfaces with Ultraviolet photoelectron spectroscopy (UPS) charge transfer state energies with sensitive External Quantum Efficiency (EQE) setup both in bilayer and bulk heterojunction device structure. These measurements coupled with DFT calculations allowed us to explain the effects of halogenation on the OPV devices characteristics. Investigating charge transfer states energies, charge transport and doping mechanism in organic materials allow us to improve the performance of organic based electronics and also propose new applications for these family of materials.

 

 

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