Understanding oxygen vacancy defects of molybdenum oxide for applications in organic photovoltaics

• Main Author: Dasgupta, Binayak
• Other Authors: School of Physical and Mathematical Sciences
• Format: Final Year Project
• Language: English
• Published: 2012
• Subjects: DRNTU::Science::Physics::Descriptive and experimental mechanics; DRNTU::Science::Physics::Optics and light; DRNTU::Engineering::Electrical and electronic engineering::Nanoelectronics
• Online Access: http://hdl.handle.net/10356/49153
• Institution: Nanyang Technological University

In this work, we investigated the role of defects in the anode buffer layer or organic bulk heterojunction solar cells. We have conducted studies on defect generation via both sputtering and annealing using x-ray photoelectron spectroscopy (XPS). We found that the stability of annealed defects were much higher than the sputtered defects under both oxygen plasma and ambient exposure. This is attributed to the dimerization of Mo-Mo atoms upon annealing creating more stable defects and thus suitable used in latter part of this study. Subsequently, we conducted systematic studies of the defect generation and electronic structure of MoO3 under different annealing conditions and correlated this with the device performance in the OPV cell. We found that increase in annealing temperature generally increases the defect densities that tend to reduce both the ionisation potential and work function of the MoO3. OPV performances of devices with generated defects are typically improved. This can generally be attributed to the enhancement in hole extraction at the anode. The correlation of the characterization and electrical data shows that the improvements can be attributed to an increased density of defects and also an improvement in the charge injection barrier through the energy alignment studies. The observations of the change in current is suitably explained using a space change limited current model and is consistent for both dark and photogenerated current.