Development and application of chemically grafted polymer/quantum dot hybrids

Abstract

Chemical grafting of polymer chains to quantum dots (QDs) leads to the fabrication of polymer/QD hybrids. Due to their combined properties hybrids are appealing systems for applications in various fields, e.g. electronic devices and cellular imaging. This dissertation is focused on two aspects: the development and applications of semiconducting/QD polymer hybrids in lighting applications and the development of new multidentate anchor systems. Good color purity, ease of color tunability, simple fabrication process and applicability in flexible and transparent displays led to the increased interest in quantum dot light emitting diodes (QLEDs). In this work, semiconducting polymer/QD hybrids were investigated as the active layer in QLEDs. The factors affecting the performance of the hybrid devices were deduced and optimized. To study the correlation between device performance and the HOMO level of the polymer used three carbazole-based side-chain conjugated polymers with different electrochemical properties were developed. A low HOMO level proved to be a requirement for efficient hole injection into QDs and thus the fabrication of devices with good performances. Furthermore, to investigate the influence of active layer morphology on device performance hybrid QLEDs were compared to devices with physically mixed active layers. It was discovered that the homogeneous distribution of the QDs within the semiconducting polymer is essential for the fabrication of the devices with improved charge injection balance and suppressed QD photoluminescence quenching. Additionally, QD to polymer ratios were varied to obtain the ratio which led to the optimized electronic properties and thus to the device with the best performance. A requirement for the fabrication of stable QD/polymer hybrids is the strong coordination of polymer chains (ligands) to QD surfaces. The presence of multiple anchor groups is generally necessary to facilitate strong binding between the polymers and the QD surfaces. In this work, bidentate and tridentate lipoic acid-based anchor compounds which allow precise control of the quantity of the incorporated anchor groups were developed. The presence of amine and azide groups enabled the efficient introduction of anchor compounds to semiconducting and hydrophilic polymers using reactive ester and azide-alkyne chemistries. Finally, the successful synthesis of water-soluble QDs verified the anchoring ability of the new polymeric ligands.