טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentShamieh Basel
SubjectTuning the Work Function of Non-Reactive Metal Electrodes
for Organic Solar Cells
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Gitti Frey


Abstract

The future of global renewable energy is one of the most important subjects undertaken by the scientific community. Solar energy is an excellent choice for an environmentally friendly renewable source. In recent years, organic Solar Cells (OSC) have matured into a credible alternative to established inorganic photovoltaic technologies. The advantages of organic electronics include flexibility, good processability, and low-power operation, making them a promising supplementary technology to conventional electronics. The architecture of the most efficient OPV devices comprises an interpenetrating network of an electron donor, often a conjugated polymer such and electron acceptor sandwiched between two electrodes and mediated by interlayers.


In all organic electronic devices, regardless of the application, charges are transferred across an organic/electrode, generally a metal, interface. Often, ultra-thin layers are introduced between the organic semiconductor and the metal to align the energy levels and reduce energy barriers. The interlayers introduce a dipole that modifies the metal’s effective work function and tunes the interfacial electrochemical potential. Interlayers deposited on the bare substrate are easy to process and characterize, however, the top electrode/organic interface is “buried” under the metal and hence challenging to manipulate and characterize. Recently, we developed a methodology to self-generate interlayers at the buried interface that is simple, versatile and compatible with large-modules processing techniques.


This research utilizes the self-forming interlayer to investigate the energy level alignment at the buried organic/silver and correlate it to the device performance. To alter the electrochemical environment at the interface we direct the migration of additives towards the right interface relying on physical principles of phase separation such as surface energy and chemical affinity. The selected additive molecules will consist a dipole to tune the effective work function, be soluble in organic solvents so that they can blend with the active layer, and include a thiol end-groups to induce their migration to the interface during silver deposition.


The result of a continuous interlayer formation of a judiciously chosen additive or additive blend will be a sensitively modified energy level alignment at the organic/silver interface. In parallel to inducing energy level alignment at the interface, the self- formed interlayers enhance significantly the ambient stability and reduce thermal degradation of un-encapsulated OPV devices. Understanding and controlling the electrochemical potential at the buried organic/metal while protecting the device from degradation caused by operational conditions is of great significance to industrial processing and real-life applications of all organic electronic devices.