Probing Buried Interfaces within Organic Diodes Using Electromodulated Transmittance Spectroscopies

Abstract

We used two electrically-modulated transmittance spectroscopies on various device structures to gain better understand the physics built into interfaces within organic diode structures. In one line of investigation we noninvasively probed the buried interface between the transparent indium tin oxide (ITO) electrode and the polymer in functional, solution-coated organic polymer diodes. We systematically modified ITO electrode substrates with various dipolar phosphonic acids that form self-assembling monolayers (SAMs) to yield various effective work functions. We used electroabsorption spectroscopy to measure corresponding changes to the built-in potential across the polymer layer, which indirectly gives information about the ITO/SAM/polymer interface. This indirect probe revealed an interface that greatly deviates from typical observations of ambient-processed electrode-organic interfaces, appearing to behave more like "clean" electrode-organic interfaces that typically require ultra-high vacuum process conditions. In the second line of investigation we use charge modulation spectroscopy (CMS) to look at charge transfer to-and-from plasmonic Ag nanoprisms (AgNPs). The AgNPs were previously shown to increase absorption within bulk heterojunction semiconductor blends used in organic photovoltaics (OPVs), but a spectroscopic fingerprint indicated that some portion of photogenerated free charges where transferring to the AgNPs. Using CMS, we verified that AgNPs which were not electrically insulated from the organic photoactive layer materials were being chemically reduced, posing a potential recombination pathway in OPV devices which would incorporate the nanoparticles. The two electromodulated spectroscopies probed buried interfaces within functional devices and device-relevant structures, both indirectly and directly, which stands out from many surface science techniques which either require exposed interfaces or which may alter the chemical composition upon probing. We gained previously unseen insight into two materials systems which can be used to improve the power conversion efficiency in organic photovoltaics.