Chemical Composition of Additives That Spontaneously Form Cathode Interlayers in OPVs

Interlayers between the active layer and the electrodes in organic devices are known to modify the electrode work function and enhance carrier extraction/injection, consequently improving device performance. It was recently demonstrated that chemical interactions between the evaporated electrode and interlayer additive can induce additive migration toward the metal/organic interface to spontaneously form the interlayer. In this work we used P3HT:PEG blends as a research platform to investigate the driving force for additive migration to the organic/metal interface and the source of the work function modification in OPVs. For this purpose PEG derivatives with different end groups were blended with P3HT or deposited on top of P3HT layer, topped with Al or Au evaporated electrodes. The correlation between the additive chemical structure, the <i>V</i>_oc of corresponding devices, and the metal/organic interface composition determined by XPS revealed that the driving force for additive migration toward the blend/metal interface is the chemical interaction between the additives’ end group and the deposited metal atoms. Replacing the PEG additives with alkyl additives bearing the same end groups has shown that the Al work function is actually modulated by the PEG backbone. Hence, in this work we have identified and separated between structural features controlling the migration of the interlayer additive to the organic/metal interface and those responsible for the modification of the metal work function

Mechanism of Metal Oxide Deposition from Atomic Layer Deposition inside Nonreactive Polymer Matrices: Effects of Polymer Crystallinity and Temperature

Atomic layer deposition (ALD) is conventionally used to deposit smooth and conformal coatings from the gas phase onto surfaces. ALD onto organic films, however, may lead to precursor infiltration into the sample and subsurface deposition. Hence, ALD into polymer films could be used for the preparation of inorganic-in-organic nanocomposite materials. However, harnessing this approach requires deep understanding of the mechanisms that govern the infiltration, nucleation, and <i>in situ</i> growth with respect to the processing and properties of the organic matrix. Here we investigate the effect of matrix crystallinity and growth temperature on the deposition into nonreactive polymer matrices (i.e., polymers that do not bear functional groups which interact with the ALD precursors). This is done by exposing films of a nonreactive polymer, poly­(3-hexylthiophene-2,5-diyl) (P3HT), with different extents of crystallinity, to ALD cycles of ZnO precursors at different deposition temperatures. In the case of polymer matrices that chemically react with the precursors, the amount of inorganic phase uptake is a result of the interplay between precursor diffusion and matrix reactivity. However, using absorption measurements and high-resolution scanning electron microscopy, we show that, in the case of nonreactive polymer matrices, the inorganic uptake is significantly affected by the rate of nucleation which is determined by the retention of the precursors in the matrix. Furthermore, we find that the retention in the film is facilitated by the presence of crystalline domains, probably due to physisorption of the precursor molecules. This retention-dependence mechanism is further supported by temperature dependence and deposition in amorphous/semicrystalline bilayers. We find that the precursors diffuse through the top amorphous layer but ZnO is deposited strictly in the bottom semicrystalline layer due to the preferred retention. Revealing the general growth mechanism in nonreactive polymer matrices offers new approaches for nanoscale engineering of hybrid materials with an eye toward creating inorganic–organic heterostructures for organic electronic device applications

Dynamics of Additive Migration to Form Cathodic Interlayers in Organic Solar Cells

Migration of additives to organic/metal interfaces can be used to self-generate interlayers in organic electronic devices. To generalize this approach for various additives, metals, and organic electronic devices it is first necessary to study the dynamics of additive migration from the bulk to the top organic/metal interface. In this study, we focus on a known cathode interlayer material, polyethylene glycol (PEG), as additive in P3HT:PC₇₁BM blends and study its migration to the blend/Al interface during metal deposition and its effect on organic solar cell (OSC) performance. Using dynamic secondary ion mass spectroscopy (DSIMS) depth profiles and X-ray photoelectron spectroscopy surface analysis (XPS), we quantitatively correlate the initial concentration of PEG in the blend and sequence of thermal annealing/metal deposition processes with the organic/Al interfacial composition. We find that PEG is initially distributed within the film according to the kinetics of the spin coating process, i.e., the majority of PEG accumulates at the bottom substrate, while the minority resides in the film. During electrode evaporation, PEG molecules kinetically “trapped” near the film surface migrate to the organic/Al interface to reduce the interfacial energy. This diffusion-limited process is enhanced with the initial concentration of PEG in the solution and with thermal annealing after metal deposition. In contrast, annealing the film before metal deposition stalls PEG migration. This mechanism is supported by corresponding OSC devices showing that <i>V</i>_oc increases with PEG content at the interface, up to a saturation value associated with the formation of a continuous PEG interlayer. Presence of a continuous interlayer excludes the driving force for further migration of PEG to the interface. Revealing this mechanism provides practical insight for judicious selection of additives and processing conditions for interfacial engineering of spontaneously generated interlayers