|Ph.D Student||Frankenstein Shefa Hadar|
|Subject||Atomic Layer Deposition for Fabrication and Characterization|
of Organic Electronic Devices
|Department||Department of Materials Science and Engineering||Supervisor||Professor Gitti Frey|
Atomic layer deposition (ALD) is a surface limited technique that produces highly conformal and smooth films at relatively low deposition temperatures. Therefore, ALD is an extensively used technique for depositing thin films for a variety of applications.
Exposing polymer films to ALD sequences can result in both surface and sub-surface deposition. The deposition inside the polymer film is termed vapor phase infiltration (VPI) and depends on the type and properties of the polymer including its functional groups, crystallinity etc., and the ALD processing conditions including the temperature, type of precursors, number of cycles etc.
Semiconducting polymers are promising materials for various electronic devices due to their processability and property tunability. Accordingly, organic light emitting diodes, organic thin film transistors and organic solar cells (OSCs) became the subject of intense research and development in the last 30 years. This, combined with the ability of these materials to conduct not only electronic charges, but also ions, paved the way for a relatively new and fast-growing field: interfacing organic electronics with biology. Yet, this young field is still lacking fundamental understanding of processes occurring in the organic mixed ionic electronic conductors (OMIECs) so that new material and devices can be developed.
In this dissertation, I studied the conditions for surface vs sub-surface ALD deposition of ZnO on/in polymer films and utilized the ability to control the growth location for improving OSCs efficiencies and to visually characterize OMIECs film morphology.
In the first part, I show, for the first time, that judicious selection of materials and processing conditions allow the use of ALD to deposit thin conformal ZnO electron transporting interlayers (ETLs) on top of the polymer active layer in OSCs. OSCs with ALD-ZnO ETLs exhibited higher photocurrent densities (Jsc), but lower open circuit voltages (Voc) compared to OSCs with the common solution deposited ZnO nanoparticle interlayer. To recover the Voc, I introduced a fluorinated phosphonic acid additive to the active layer. The additive migrates to the film surface, interacts with the ZnO and passivates traps, effectively improving the energy level alignment and increasing Voc.
In the second part, I use VPI to visualize the morphology of OMIEC films composed of a polymer blend and study structure-property relationships. The common semiconducting poly(3-hexylthiophene-2,5-diyl), P3HT, that conducts holes, was blended with different molecular weights (Mw) of the common polymer electrolyte polyethylene oxide, PEO, that conducts ions. I found that the high Mw PEO supports phase separation leading to P3HT connectivity and crystallinity that encourages good electronic conductivity. Low Mw PEO, on the other hand, supports intermixing and high P3HT:PEO interfacial area that encourages good ionic-electronic coupling. Combining VPI with HRSEM, enabled visualization of the pure PEO domains, characterized by lamellar structure, and to confirm the OMIEC morphology.
Overall, in this research, I showed that by controlling the ALD conditions and judiciously selecting the organic materials, it is possible to utilize ALD either for processing and improving device efficiencies or for characterizing and studying the morphologies of the active layer in various organic electronic devices.