|Ph.D Student||Magen Osnat|
|Subject||Simulation and Fabrication of Hybrid Organic/Inorganic|
|Department||Department of Electrical Engineering||Supervisor||Professor Nir Tessler|
|Full Thesis text|
Hybrid ZnO/organic solar cells, in which ZnO nanowires (NWs) serve as the electron acceptor material and organic semiconductor is infiltrated in between the NWs, are theoretically ideal for efficient excitonic light harvesting. However, by now no such efficient SCs were reported, and very low efficiencies were demonstrated. In this work, we are investigating such hybrid solar cells both via theoretical calculation and fabrication.
In the theoretical section, we have implemented a 2D simulation tool that solves simultaneously for the internal electric field and the charge carriers distribution in a given device structure and a given set of physical processes, with a special care taken at the interfaces between dissimilar material, such as hybrid organic/inorganic.
We have used the simulation tool for analyzing organic/ZnO diodes, fabricated by our collaborators, and revealed the role of surface states at the junction. Experimental results showed that introducing a dopant into the ZnO layer improves the efficiency of charge injection into the organic component, without modifying the magnitude of the injection barrier. The simulation tool was thus used for examining the role of traps (local states) at the ZnO, which are hardly accessed experimentally. It was found that simulating surface traps at a single energy level (Et) would give a good fit to the measured J-V characteristics, with a minimal number of fitting variables, and that adding the dopant increase the traps’ energy level thus making it shallower. UV photoelectron spectroscopy (UPS) measurements supported our findings.
We have also used the simulation tool for revisiting solar cells’ blocking-layers (BLs) and their effects on the cell’s performance. The open circuit voltage of solar cells is a critical property which, in many thin film devices, is found to be much lower than the theoretical limit associated with the material’s absorption energy gap. A known method for improving the open-circuit voltage is that of adding charge blocking layers (a.k.a. charge selective or charge transport layers). However, such interfacial layers are often found to decrease the fill factor (FF) or even cause undesired S-shape in J-V curves. Here, we explain how such layers enable significant Voc increase (even above the built-in voltage) and show that the layers’ resistance is responsible for the problematic behavior observed.
The third aspect examined was the losses introduced by metallic electrodes. We discuss the inclusion of pinning at the integer charge transfer state with and without using the image charge potential. The explicit introduction of the image potential creates band-gap narrowing at the contact thus affecting both dark leakage current and photoconversion efficiency.
In the second part, we will survey our attempts for fabricating hybrid ZnO(NWs)\organic solar cells, in collaboration with Prof. Taleb Mokari’s group at Ben-Gurion University of the Negev. Timewise, this sub project was the first and longest. However, in the bottom line, it was not successful, thus we summarize mainly the points that could help someone starting a similar task.
The thesis ends with summary and conclusions.