Charge Carrier Transport in Solution Processed Organic Semiconductor Thin Films

Understanding and controlling the flow of charge carriers lies at the heart of today's society as it is the basis for the successful operation of electronic devices. Here, the charge transport properties of organic semiconductor thin films will be investigated. The first part of this work focusses on the fundamentals of charge transport in such films starting with investigations into metal-insulator semiconductor capacitors. A robust and widely applicable approach to measure the charge carrier mobility in semiconductors will be presented next, enabling a novel way to measure this important transport parameter with minimal influence of the commonly occurring injection barriers. Further investigations into the local electrical properties of a transistor under operation condition are carried out. The second part of this thesis is focused on detailed studies into two material classes bearing potential applications in the field of organic electronics. It is shown that the prototypical organic semiconductor Poly(para-phenylene) indeed possesses semiconducting properties in its unsubstituted form. The materials crystallinity was further improved through careful precursor design. Cinnamic acid derivatives are further shown to cross-link under electron radiation. This finding is applied in a semiconducting polymer which preserves its semiconducting properties even after electron beam patterning

Spatiotemporal Measurement of Arterial Pulse Waves Enabled by Wearable Active-Matrix Pressure Sensor Arrays

Wearable pressure sensors have demonstrated great potential in detecting pulse pressure waves on the skin for the noninvasive and continuous diagnosis of cardiac conditions. However, difficulties lie in positioning conventional single-point sensors on an invisible arterial line, thereby preventing the detection of adequate signal amplitude for accurate pulse wave analysis. Herein, we introduce the spatiotemporal measurements of arterial pulse waves using wearable active-matrix pressure sensors to obtain optimal pulse waveforms. We fabricate thin-film transistor (TFT) arrays with high yield and uniformity using inkjet printing where array sizes can be customizable and integrate them with highly sensitive piezoresistive sheets. We maximize the pressure sensitivity (16.8 kPa(-1)) and achieve low power consumption (10(1) nW) simultaneously by strategically modulating the TFT operation voltage. The sensor array creates a spatiotemporal pulse wave map on the wrist. The map presents the positional dependence of pulse amplitudes, which allows the positioning of the arterial line to accurately extract the augmentation index, a parameter for assessing arterial stiffness. The device overcomes the positional inaccuracy of conventional single-point sensors, and therefore, it can be used for medical applications such as arterial catheter injection or the diagnosis of cardiovascular disease in daily life

Solubility Modulation of Polyfluorene Emitters by Thermally Induced (Retro)-Diels–Alder Cross-Linking of Cyclopentadienyl Substituents

For cost-efficient organic electronic devices, the consecutive deposition of active layers by solution-based processes is a key benefit. We report a synthetic approach enabling solubility reduction of bis­(cyclopentadienyl)-substituted polyfluorenes as emissive layers in organic light-emitting diodes (OLEDs). Thermally induced retro-Diels–Alder reaction liberates free cyclopentadiene as “protecting group” and pending cyclopentadienyl units, which cross-link the polymer strands upon cooling via [4+2] cycloadditions. The activation temperature is tuned in the range of 180–250 °C through alkyl, alkoxy, or ester linkages. Ultimately, macrocyclic self-protected bis­(cyclopentadienylene) moieties avoid extrusion of volatile cyclopentadiene during activation. The solvent resistance of the emissive layers after cross-linking is examined by absorption spectroscopy and white light scanning interferometry. The influence of the desolubilization procedure on the performance of solution-processed OLEDs is investigated

Influence of Interfacial Area on Exciton Separation and Polaron Recombination in Nanostructured Bilayer All-Polymer Solar Cells

The macroscopic device performance of organic solar cells is governed by interface physics on a nanometer scale. A comb-like bilayer all-polymer morphology featuring a controlled enhancement in donor–acceptor interfacial area is employed as a model system to investigate the fundamental processes of exciton separation and polaron recombination in these devices. The different nanostructures are characterized locally by SEM/AFM, and the buried interdigitating interface of the final device architecture is statistically verified on a large area <i>via</i> advanced grazing incidence X-ray scattering techniques. The results show equally enhanced harvesting of photoexcitons in both donor and acceptor materials directly correlated to the total enhancement of interfacial area. Apart from this beneficial effect, the enhanced interface leads to significantly increased polaron recombination losses both around the open-circuit voltage and maximum power point, which is determined in complement with diode dark current characteristics, impedance spectroscopy, and transient photovoltage measurements. From these findings, it is inferred that a spatially optimized comb-like donor–acceptor nanonetwork alone is not the ideal morphology even though often postulated. Instead, the energetic landscape has to be considered. A perfect morphology for an excitonic solar cell must be spatially and energetically optimized with respect to the donor–acceptor interface