Thaer Kassar

Technische Physik
Staudtstr. 1
D-91058 Erlangen

Room 0.141

Tel.: +49 9131 85-28318
Fax: +49 9131 85-25182

Doctoral candidate


B.Sc. in Mechanical Engineering (Materials Science and Engineering), University of Aleppo, 2003-2008

M.Sc. in Materials Science Exploiting Large-scale Facilities (MaMaSELF), University of Turin and Ludwig Maximilian University of Munich, 2010-2012

My Research

Solution processed bulk heterojunction polymer solar cells are becoming viable technology for sustainable energy generation due to their rapidly increasing efficiencies and their potential as a low-cost, printable, and flexible renewable energy source.

Little is known about the structure formation of the blend films and how it can be controlled; ex. the mixing of conjugated polymers and fullerene derivatives. It was thought that they separate into pure domains. However, recent reports suggest that intercalation of fullerenes between the side chains of conjugated polymers and the formation of bimolecular crystals are possible. To optimize such devices, one needs to identify the influencing parameters on the molecular assembly and nanostructure formation of the interpenetrating polymer-fullerene self-assembled network. We investigate them here during the drying process. For this purpose, our working group has built a dedicated experimental setup consisting of a cell with an integrated doctor blading coater to perform in situ measurements which allow real time observation of the evolution of the blend structure during solvent evaporation.

Fig. 1: Upper picture: The in-situ drying cell mounted on the sample stage in front of the detector tube. Lower picture: The doctor-blade applicator during coating inside the cell.

To access the 3D structure of the studied thin films from the molecular to the mesoscale, we probe the inner structure through the modern methods of grazing incidence X-ray diffraction (GIXD) and grazing incidence small angles X-ray scattering (GISAXS). Grazing incidence geometry of the incident X-ray with respect to the sample surface is used to enhance the scattered intensity and limit it to the near surface region.


Fig. 2: Snapshot of two dimensional grazing incidence x-ray diffraction (GIXD) pattern taken with the main instrument of our research group “GANESHA SAXSLAB”. The sample is final dried P3HT: PCBM blend film (~120nm) processed by doctor-blading over silicon wafer.


We exploit classical SAXS to study the solutions before coating them. We also study ternary cells which overcome the absorption limitation of organic semiconductors and how solvent mixtures, processing additives, thermal/solvent annealing and multilayer stack build-up of the device affect the final dried blend structure.



Fig. 3: Snapshot of two dimensional grazing incidence small angles x-ray scattering (GISAXS) pattern of P3HT:CA19:PCBM blend film showing its Yoneda and specular peaks.