Charge injection and doping zone engineering of light-emitting electrochemical cells for sports performance monitoring
Chee, Kenji Jian Zhi
Date of Issue2017
School of Materials Science and Engineering
This thesis first explains the need for light emitting markers as an inexpensive means of biomechanical motion analysis. With analysis of the current sports analysis landscape, flexible organic lighting devices based on conjugated emissive polymers with low turn-on voltage and colour tunability will be deemed as a wearable light marker for different sports and weather conditions. Light-emitting electrochemical cell (LEEC) in comparison to organic light-emitting diodes possesses a combination of advantages. It consists of an active layer blend of conjugated polymer, ion transporter and salt sandwiched between two electrodes that offer simple single layer solution processability, independent of electrode work function and optimized carrier injection due to doped regions based on electrochemical doping of organic polymers. Electrolytes within the active layer of LEEC determines the performance of the active layer. Salt and ion conductors are the constituents of the electrolyte and especially the p doped regions having higher conductivity and larger doped area point toward a dominant character of the anion. Bis(trifluoromethylsulfonyl)imide (TFSI-) anion due to its low binding energy, high number of resonance states, high ionic conductivity on the order of 10-4 S cm-1 provides efficient doping, lowered turn-on voltages and enhanced maximal brightness of the device ( ~ 100 cd/m2). Li+ and BMIM+ cations have been used to revalidate this concept with coherent findings. Complexation between TFSI- anion and polymer was confirmed via X-ray Photoelectron Spectroscopy with strong correlations in Photoluminescence (PL) and Absorption (Abs) studies. Based on this, we have revealed a novel mechanism which explains recombination zone centering in the active layer that enhances brightness and lowers the turn-on voltage. Facile quantum dot (QD) incorporation into this active layer has effected Förster Resonance Energy Transfer facilitating voltage induced wavelength tuning to harness the properties of the QD. Further lowering of turn-on voltage is observed with enhanced brightness and turn-on voltage dependence on the spectral integral overlap between the donor PL and acceptor Abs spectra. Flexible LEEC testing in ambient reveals the device dependence on the mechanical properties of the transparent conductive electrodes and stable operating voltages of such devices.