Spectroscopic Investigations of Organic Semiconductor Thin Film Stabilities

Abstract

Organic semiconductors (OSCs) are of increasing interest as active elements for printable electronic devices such as thermoelectrics, light-emitting diodes, photovoltaics, and field effect transistors. However, two fundamental challenges continue to limit commercial success: low power conversion efficiencies and instability. Increased efficiency can be affected by increased conductivity. Conductivity increases are typically achieved by two mechanisms: blending a semiconducting polymer with a small molecule dopant for free charge carrier generation or by synthesizing the semiconducting molecule to have both electron donating (D) and accepting (A) components and therefore fine-tuned control of the electron density localization on the OSC. Instability of these systems can then be defined by physical, chemical, or de-doping type mechanisms. This dissertation will show that spectroscopy and chemical imaging are critical in understanding the stability of both types of more conductive OSC active layers. Regioregular poly(3-hexyl)thiophene (rr-P3HT) doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) was investigated as a model of small molecule doping. The acid/base chemistry of F4TCNQ, independent of rr-P3HT, was first characterized using IR spectroelectrochemistry to elucidate the impacts of its degradation products on the resulting IR spectra. FTIR and XPS were then used to show that there are two co-existing types of charge transfer (CT) states within F4TCNQ-doped rr-P3HT films, integer charge transfer (ICT) and partial charge transfer complex (CPX). The CPX state was found to be not only more thermodynamically favored, but that the rate and extent to which it is formed over the ICT can be controlled by the storage environment of the film, the dopant concentration, and the molecular weight of the polymer. The stability of F4TCNQ-doped rr-P3HT was also found to vary as a function of the annealing environment and temperature and ultimately resulted in the formation of HF4TCNQ-. The morphology of the film induced by different solvent processing methods was also monitored by synchrotron infrared nanospectroscopy, which provided critical insights into the chemical nature of crystallite formation in addition to monitoring charge transfer states in the morphological context. For the second mechanism, small push-pull molecules of A-D-A configuration with similar redox properties were synthesized by NREL and studied as a function of chemical stability towards photo-oxidizing conditions with XPS and UPS. This work showed that the chemical stability of these molecules was impacted by the accepting end-group functionality and that the extent of degradation chemistry observed on the end-groups was directly correlated to the chemistry observed in the core. Additionally, for 2 of the three molecules studied, the degradation chemistry observed could be mitigated with the inclusion of an additional electron acceptor within the film. Finally, preliminary work was done to study the reaction chemistries of OSCs to isolated atmospheric gases. Small molecule analogues of OSCs of interest were vapor deposited and exposed to incremental amounts of O2 with subsequently induced degradation chemistry monitored with in situ UHV Raman spectroscopy. The results of this preliminary work suggest the ability of this method to detect oxygenation of thiophenes and O2 induced structural distortions; however, future work needs to be done in the optimization of a new gas dosing apparatus.