טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentYaacobi-Gross Nir
SubjectColloidal Semiconductor Nanocrystals based Solar Cells
DepartmentDepartment of Electrical Engineering
Supervisor Professor Nir Tessler
Full Thesis textFull thesis text - English Version


Abstract

Colloidal semiconductor Nanocrystals are promising materials for low-cost and high efficiency solar cells. Their unique optoelectronic properties as well as simple and safe solution phase syntheses and film fabrication, suggests that NC based solar cells could allow low cost solar conversion. The quantum confinement of charge carriers within the NCs leads to distinctive thermal and optical properties that may also be used to overcome the efficiency limits governing traditional solar cells in so-called 3rd generation solar cells.    

Nevertheless, charge confinement is a mixed blessing also limiting the efficiency of current NCs based solar cells. The confinement energy of the NCs needs to be overcome in order for excitons to be efficiently dissociated as well as for charge transport between adjacent NCs. This study is focused on various ways to overcome the exciton binding energy in a NCs solid to allow exciton dissociation to charge carriers.

Following Soreni-Harari et al., revelation of energetic level tuning of NCs using ligand exchange, we demonstrate the application of such tuning to introduce a type-II heterojunction between organic polymer and NCs solid. The heterojunction allows better exciton dissociation exhibits two orders of magnitude improvement in device performance at the near infrared region.  We also propose a novel approach where an all NCs device was formed with an active layer made from a single batch of NCs. The heterojunction in this work is formed using an energy shift induced by different capping ligands covering the NCs.

Following the above promising results, we investigate the effect of mixed capping layer on the properties of NCs. We found that by attaching two different molecules to a nanocrystal one can induce electric fields large enough to significantly alter the electronic and optoelectronic properties of the quantum dot. This electric field is created within the nanocrystals due to a mixture of anchor groups ligands. Namely, by varying the composition of the capping layer, the strength of the electric field can be fine-tuned. We also demonstrate that the use of mixed ligands induced electric field dramatically enhances the charge generation efficiency nanocrystals based solar cells, thus improving the overall cell’s efficiency.

Finally we demonstrate that the intrinsic energy level alignment between two different types of NCs could be harnessed to overcome the exciton binding energy of both materials. We systematically study different ways combining such NCs of different surface chemistry and different sizes in order to improve solar cells efficiencies.