Raman Spectroscopic Investigations of the Interfacial Chemistry of Solid-State Organic Thin Films with Vapor Deposited Metals for Organic Photovoltaics

Abstract

This dissertation is focused on the elucidation of the reaction chemistry that governs the low work function metal/organic interface found in organic photovoltaics (OPVs). To this end Raman spectroscopy was used in ultra-high vacuum to study Ag, Mg, Ca, and Al metal vapor deposition on pyridine, C₆₀, and graphene. In an effort to understand the interfacial reaction chemistry of complex organic molecules with metal an approach of systematic deconstruction is used where by small molecules, in this case pyridine can be used to gain insight into the chemistry of various chemical functionalities with minimal spectral complication. In the Ag/pyridine system no reaction was observed and the integrity of the film was preserved with spectral enhancement being the only result. This enhancement is achieved via a weak Ag--N bonding interaction. For the other three metals (Mg, Ca, and Al) a great deal of fascinating reaction chemistry can be observed initiated in each case by metal-to-organic electron transfer resulting in the formation of pyridyl radical anions. Once radicals are formed the reaction pathways for each metal diverge resulting in different specific reaction products. In the case of Mg the pyridyl radicals undergo reductive dimerization and yield 4,4'-bipyridine. For Ca the pyridyl radicals follow two pathways either losing a hydride to form the diradical pyridyne or through a pathway of ring opening degrade into amorphous carbon. These results highlight the vast differences possible for reaction chemistry between metals and organics even for simple molecules. Buckminsterfullerene (C₆₀) and fullerene derivatives are ubiquitous to the field of OPVs, thus an understanding of their metal/organic interfacial chemistry is of critical importance to unlocking the full potential of devices. In a similar manner to what can be observed for Ag/pyridine systems the Ag/C₆₀ system shows little more than surface enhancement effects due to a lack of any substantial reactivity and Mg, Ca, and Al exhibit metal-to-organic charge transfer forming C60 anion radicals. These anion radical react to form an as of yet unidentified reaction product in the case of all three reactive metals and in the case of Al these reaction products further degrade forming amorphous carbon. The understanding of this chemistry can be directly correlated to device data found in the literature and provides insight into the formation of interfacial gape states at the metal/organic interface of OPVs. Due to its unique electrical properties and high degree of mechanical stability graphene is starting to play a significant role in the development of OPVs. Because graphene is being used in contact with vapor deposited metal it is of relevance to understand the chemistry that occurs at this interface. While deposition of Ag onto graphene again shows no reaction and only enhancement the enhancement leads to the identification of unique defects in the graphene lattice namely carbon vacancies and C--C bond rotations which lead to Stone-Wales defects which are likely a result of the graphene growth method. Mg, Ca, and Al show strong evidence for n-type doping of electrons into the graphene film due to their work functions being lower than graphene. This data highlight the stability of graphene showing that even though it undergoes a similar metal-to-organic electron transfer as seen with C₆₀ and pyridine there is no further compromise of the films molecular structure.