Doping and Stability of Organic Semiconductors

Abstract

Organic semiconductors (OSCs) have incredible prospects for next-generation, flexible electronic devices including bioelectronics, opto-electronics, energy harvesting and storage. They are flexible, biocompatible, printable and low-cost semiconductor materials with control over chemical tailorability which gives us control over device properties. These properties expand the functionality of electronics beyond the current era of silicon making devices such as solar windows, skin patch sensors, wearable electronics and foldable displays a possibility. However, they have lower conductivities compared to their inorganic counterparts, have a complex microstructure and are prone to degradation. Hence, to make better performing systems we need to better understand the underlying mechanisms of doping and degradation in these systems and their intimate connection to microstructure and charge transport. In this work we use the P3HT as our model OSC system to study the chemical and electrochemical doping mechanism.Central to the functionality of many electrochemical devices is the ability of conductive polymers to conduct both electrons and ions, a unique hybrid transport property. Understanding the molecule-level composition and structure controlling this collective charge transport in conductive polymer/electrolyte systems is required to take this field forward. We look at the collective, molecular structure factors that produce a heterogeneous electrochemical potential landscape of sub-populations with distinctive contributions to charge transfer, charge transport, and ultimately, device efficiency through a balance of both kinetic and thermodynamic principles to better understand the electrochemical doping mechanism. Further, we use a dopant F4TCNQ to p-type dope P3HT in the chemical doping mechanism. It has been shown in literature that this system either forms an integral charge transfer state (ICT) giving free charge carriers or a partial charge transfer state (CPX) forming traps based on processing techniques, with ICT being desirable. However, using a combination of spectroscopy, x-ray scattering and conductivity measurements we show that these states exist simultaneously and that their existence is co-related to the local density of states of the semiconductor matrix. Using these new insights in their doping mechanisms, we turn to then evaluate the stability of the doped P3HT-F4TCNQ system in terms of its thermal degradation mechanism in the presence of varying environmental conditions.