Spectroscopic Tools for Studies of Voltage-Based Degradation of Organic Semiconductors: From Simple Films to Device-like Architectures

Abstract

Organic semiconductors (OSCs) have recently become of interest for inclusion in various organic electronics due to their advantageous properties. OSCs are the active elements in printable and flexible electronic devices such as thermoelectrics, light-emitting diodes, photovoltaics, and field-effect transistors. Despite the beneficial properties of OSCs, hurdles remain for widespread commercial adoption. Specifically, organic photovoltaics have historically had comparably lower power conversion efficiencies and stabilities than their inorganic counterparts. While recent progress has made significant strides in closing the power conversion efficiency gap, greater device lifetimes are still necessary to make them practical for commercial applications. The power conversion efficiency problem has been addressed by synthesizing semiconducting molecules that contains both electron-donating and accepting components, which leads to fine-tuned control of the electron density localization on the OSC, which affects the various electronic properties such as bandgap to better absorption properties. However, the instability issue is more complex, with both physical and chemical components that often coincide. This dissertation will show that spectroscopy and device-like conditions are critical in understanding the chemical degradation of active layer components in organic photovoltaics. To investigate the degradation of donor-acceptor active layer materials, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2- ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl ]] (PTB7) was used as a model system in studies with preogressively increasing device-like architectures. The initial investigation into the effect of applied voltage on the degradation of the donor materials was investigated with Fouriier transform infrared spectroscopy (FTIR), electrical characterization, and x-ray photoelectron spectroscopy (XPS) to understand the role of atmospheric exposure in degradation of PTB7 in the absence of light. FTIR and XPS results show clear evidence of degradation through the introduction of oxygen-containing functional groups into the polymer backbone. This degradation was explained through the formation of intermediate charge transfer complexes beginning with contact charge transfer, forming a charge transfer complex (CTC) that then leads to a full charge transfer (CT) that allows oxygen to be made into superoxide anion radical, O_2^(.-). This species is proposed as the active degradant due to limited formation pathways of other reactive oxygen species (ROS) without illumination. Similar degradation does not occur under identical conditions in an inert nitrogen environment for the same applied voltage. Superoxide-induced degradation also follows a distinct pathway, leading to the formation of unique degradation products that differ from those previously reported from photooxidation of PTB7. Over sufficiently long time periods, film degradation occurs in the dark under applied voltage, with the extent of degradation enhanced in the presence of water vapor in the degradation environment. To better understand the role of applied voltage in the degradation of PTB7, films were exposed simultanesously to both light and voltage. This exposure also leads to degradation, but the products exhibit contributions from both applied voltage and photooxidation. Despite differences in the degradation time scale, hours for illumination and hundreds of hours for applied voltage, when combined, the presence of both applied voltage and illumination alters the degradation pathway from that observed with illumination only. This is an interesting finding, as degradation for the voltage-only conditions relies on superoxide anion as the ROS, whereas singlet oxygen is thought to be the ROS under illumination. Based on the difference in degradation timescale, singlet oxygen is the ROS more likely to degrade the film. Despite having different predominat ROS active, applied voltage alters the degradation from photoillumination only and highlights the complex interplay of device operation and degradation. A final series of studies adopted a full device-like architecture to add relevant additional interfaces and structure to the model system. Changes in degradation relative to the simplistic model systems described above were observed, although similar degradation products are formed. This convergence to similar products highlights the complexity of performing simplified analysis of devices as standalone films. Also, it highlights the need for stability testing not to be performed at either the open circuit potential or in the absence of an electrical load that an organic photovoltaic will experience when operating. The additional interfaces and structure affect the degradation extent and products, thus providing an additional reason device degradation studies must account for more device-like architectures and operational conditions to understand the complex degradation chemistry of organic photovoltaics.