טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentSoreni-Harari Michal
SubjectTailoring the Interface in Quantum Dots-Polymer
Devices
DepartmentDepartment of Electrical Engineering
Supervisor Professor Nir Tessler
Full Thesis textFull thesis text - English Version


Abstract

The integration of semiconductor nanocrystals in optoelectronic devices paved the way to the development of near-infra red (NIR) spectral range applications. The technologically important NIR range is required in telecommunications, in applications such as light emitting diodes, photodetectors, and solar cells.

The electronic processes governing the functionality and performance of these optoelectronic devices are charge transfer and energy transfer taking place between the components in the active layer. These are dependent upon the blend morphology, the relative position of the energy levels of the host polymer and the guest NCs, and the NC-NC crosstalk.

Therefore, the focus of this study is the interface of the components that constitute the active layer in devices: the polymer-NC interface or the NC-NC interface.  The interface of a colloidal NC consists of a surfactant layer that is necessary for its solution-based synthesis and is also pertinent to its chemical processability. However, the original capping ligands used for the synthesis are considered to shield a NC from its neighboring NCs, and to inhibit charge transport. Thus, our goal was to tailor the NCs interface through surface capping layer modifications in order to implement the full potential of NCs-based devices. 

We showed for the first time tuning of the electronic level positions with respect to the vacuum level in colloidal semiconducting NCs using surface ligand exchange. By performing detailed electrochemical as well as scanning tunneling spectroscopy measurements on InAs particles capped with different ligands, we showed that the organic surface capping layer of the inorganic NC takes part and affects its electronic system. The tuning effect resulted in shifting the energy level alignment from type I towards type II based on the energy levels determination. This effect was demonstrated in several prototype devices showing improved photovoltaic device performance upon the exchange.

Another effort was to modify the interface in NCs-based films in order to allow efficient charge transport that is essential for optoelectronic devices. By fabricating InAs nanocrystals field effect transistors, we found that the ON/OFF ratio can be improved from ~5 all the way to ~105 through the applied procedures. The interface manipulation affected the surrounding matrix of the inorganic core, the inter-particle distance, and the nanocrystals order in the 3D array. Films with enhanced charge transport qualities retained their quantum confined characteristics throughout the procedure, thus making them useful for optoelectronic applications.