Efficiency of molecular doping of conjugated polymers

Licentiate thesis, 2022

In recent years, organic semiconductors have become known as a promising alternative to traditional inorganic materials. Conjugated polymers are one of the most studied classes of organic semiconductors. Molecular doping is a powerful tool to tune their optical, electronic and –possibly– even the mechanical properties. The addition of molecular dopants introduces positive or negative charges to the polymer, so called polarons. However, factors such as dopant aggregation, the energetic offset between the polymer and dopant, as well as less-than-integer charge transfer limits the efficiency of the charge transfer process. Additionally, each polaron-dopant ion pair is strongly bound. Hence, only a fraction of holes is able become free charges that contribute to electrical transport. In order to develop materials that show improved doping efficiency, it is paramount to measure the number of (bound and free) polarons.

This thesis explores different methods that allow to assess the polaron density and concepts to improve the efficiency of molecular doping. Spectro-electrochemistry combined with chronoamperometry can be used to measure the polaron attenuation coefficient. If the polaron attenuation coefficient is known, it is possible to estimate the polaron density using UV-vis-NIR spectroscopy. Fourier transfer infrared (FTIR) spectroscopy can be used to estimate the polaron density for polymers doped with quinodimethane type monomeric dopants. Further it is shown that electron paramagnetic resonance (EPR) spectroscopy can be used to estimate polaron density and confirm that the dopant Mo(tfd-COCF3)3 can undergo two charge transfer events, i.e. double doping, with a low ionization energy conjugated polymer.