|M.Sc Student||Wing Dahvyd|
|Subject||Selection and Optimization of Transparent Conducting|
Oxides for Electrodes in Polymer Solar Cells
|Department||Department of Nanoscience and Nanotechnology||Supervisors||PROF. Nir Tessler|
|PROF. Avner Rothschild|
|Full Thesis text|
Organic photovoltaic (OPV) cells made from nanoscale phase separated blends of conjugated polymers and fullerenes have received much attention due to their ability to be produced by inexpensive, large area manufacturing techniques on flexible substrates. Power conversion efficiencies greater than 10% have been achieved by OPV cells, but there is room for improvement with regards to the cells' voltages.
Metal oxide interlayers are increasingly being used in OPV cells because they can improve open circuit voltages and fill factors. One way they can cause these improvements is by blocking charge carriers of the wrong type from entering an electrode, reducing leakage currents/surface recombination. At present few efficient hole blocking layers are known. Incidentally, numerous material requirements for these layers greatly reduce the number of candidate metal oxides. In this thesis, I present work on using pulsed laser deposited strontium titanate doped n-type with niobium (SrTiO3:Nb) as a hole blocking layer in OPV cells.
I deposited SrTiO3:Nb on various substrates, optimizing the deposition temperature and oxygen partial pressure, and the thickness deposited, in order to achieve electrodes with good transparency and electrical conductivity, and low surface roughness. The role of SrTiO3:Nb cathodes/interlayers in OPV cells was studied by using my samples as the bottom contacts in bulk heterojunction solar cells consisting of poly(3-hexylthiophene) (P3HT), and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), a model OPV system. OPV cells which had SrTiO3:Nb interlayers performed significantly better than cells without them, though it is unclear whether this is because SrTiO????3:Nb is a hole blocking layer or not. Unexpectedly, changes in the solar cells’ efficiencies due to different SrTiO3:Nb deposition conditions were overshadowed by a dramatic improvement in power conversion efficiency from 1.5x10-3% to 0.5% when these OPV cells remained under illumination for several hours (known as light soaking).
To investigate this light soaking effect, I constructed a series of single carrier metal-insulator-metal devices using either P3HT or PCBM and appropriately chosen contact materials. This made it possible to measure how SrTiO3:Nb affected hole transport and electron transport separately. Current-voltage measurements of the devices before and after light soaking were analyzed using dielectric conduction theory, including the effect of a built-in field. This analysis shows that the reason for the enhanced device efficiency is a rise in SrTiO3:Nb's Fermi energy level and a possible mechanism for the shift in SrTiO3:Nb’s Fermi energy level is suggested. Important considerations in selecting electrode materials for OPV devices are abstracted from this work.