INVESTIGATING THE EFFECTS OF NANOSTRUCTURED DIELECTRIC LITHIUM FLUORIDE AND PLASMONIC GOLD INTERLAYERS IN ORGANIC PHOTOVOLTAICS, INCLUDING THE USE OF IN-SITU IMPEDANCE SPECTROSCOPY
Materials Science and Engineering, PhD Dissertation, 2016
Assoc. Prof. Cleva W. Ow-Yang (Thesis Advisor), Prof. Dr. Yusuf Z. Menceloğlu, Assoc. Prof. Kürşat Şendur, Prof. Mustafa M. Demir, Asst. Prof. Çınar Öncel
Date & Time: August 1th, 2016 – 13.00 PM
Place: FENS G032
Keywords: organic photovoltaics, interface engineering, impedance spectroscopy, charge carrier dynamics, functional interfaces, plasmonic field enhancement
Organic solar cell performance can be limited by the problematic organic-inorganic interfaces between the active layer and the electrodes. One solution is the incorporation of nanostructured functional interlayers, which enable additional engineering control of these interfaces to improve charge collection efficiency, charge separation efficiency and photophysical interactions. Herein we demonstrated that solution processed dielectric LiF (sol-LiF) and plasmonic Au (sol-Au) nanostructuring on indium tin oxide (ITO) anode can be used to improve bulk heterojunction (BHJ) organic photovoltaic (OPV) device performance. We show that the surface work function of ITO thin film anodes can be tuned via the areal density of sol-LiF nanoparticles, and when incorporated into a BHJ OPV, enables the optimization of energy level alignment between the organic layers and ITO. In addition, we show that the electric field component of light can be strongly enhanced at the edges of sol-Au nanoparticles due to the excitation of localized surface plasmon resonances (LSPR). When incorporated into BHJ OPV devices, the important consequence of LSPR can be seen in the antenna-like behavior of the sol-Au nanoparticles, which re-emit the absorbed light into the BHJ active layer and increase the optical path length; in this manner, the increased number of photons in the BHJ active layer results in improved charge generation efficiency.
Although tuning organic solar cell performance can be achieved by the design of nanostructured interlayers between the active layer and the anode, elucidating the actual photophysical effects of such buried interfaces during device operation is a challenge, for which impedance spectroscopy (IS) analysis offers pivotal insight. Herein we have used IS to distinguish the effects of two different nanostructured interlayers, with sol-LiF and sol-Au nanostructures, on the charge generation/recombination and charge transport/collection kinetics in bulk heterojunction organic solar cells in detail. IS analysis revealed that a more favorable energy alignment with the hole transport layer (HTL) improved charge collection efficiency in devices containing an ITO anode modified by sol-LiF, by facilitating charge transport and extraction through decreased charge carrier transit lifetime. In the case of a sol-Au interlayer between ITO and the HTL, IS analysis revealed that plasmonic field enhancement within the active layer improved charge generation, although the increase in mobile charge carriers did not impact charge transit dynamics significantly. Moreover, our work underscores a key advantage offered by IS analysis—instead of tracking the multivariate OPV device characteristics of fill factor and short-circuit current, one may obtain a more detailed analysis of the underlying operating mechanisms to elucidate the specific contributions of nanostructured interlayers.