Investigating the effects of nanostructured dielectric lithium fluoride and plasmonic gold interlayers in organic photovoltaics, including the use of in-situ impedance spectroscopy

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 photovoltaic performance. Herein we demonstrated that solutionprocessed dielectric LiF (sol-LiF) and plasmonic Au (sol-Au) nanostructuring on the 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 enables the optimization of energy level alignment between the organic layers and ITO. In addition, we show that the electric field component of incident light is 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, these sol-Au nanoparticles improved the efficiency of BHJ absorption by acting like antennas, enhancing charge carrier generation. Each of these interlayer types contribute to increased photocurrent generation. In order to distinguish the root cause of improvement, impedance spectroscopy (IS) analysis was applied to the modified OPVs in-operando. In the case of sol-LiF, more favorable energy level alignment engenders better charge collection. In the case of sol-Au, the improved charge generation rate occurs without perturbing the carrier extraction. Thus instead of tracking the multivariate OPV device characteristics, IS enables more detailed analysis of the underlying operating mechanisms to elucidate the specific contributions of nanostructured interlayers