We report a rapid method of depositing phosphonic acid molecular groups onto conductive metal oxide surfaces. Solutions of pentafluorobenzyl phosphonic acid (PFBPA) were deposited on indium tin oxide, indium zinc oxide, nickel oxide, and zinc oxide by spray coating substrates heated to temperatures between 25 and 150 °C using a 60 s exposure time. Comparisons of coverage and changes in work function were made to the more conventional dip-coating method utilizing a 1 h exposure time. The data show that the work function shifts and surface coverage by the phosphonic acid were similar to or greater than those obtained by the dip-coating method. When the deposition temperature was increased, the magnitude of the surface coverage and work function shift was also found to increase. The rapid exposure of the spray coating was found to result in less etching of zinc-containing oxides than the dip-coating method. Bulk heterojunction solar cells made of polyhexylthiophene (P3HT) and bis-indene-C60 (ICBA) were tested with PFBPA dip and spray-modified ITO substrates as well as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS)-modified ITO. The spray-modified ITO solar cells showed a similar open circuit voltage (VOC) and fill factor (FF) and a less than 5% lower short circuit current density (JSC) and power conversion efficiency (PCE) than the dip- and PEDOT:PSS-modified ITO. These results demonstrate a potential path to a scalable method to deposit phosphonic acid surface modifiers on metal oxides while overcoming the limitations of other techniques that require long exposure and post-processing times.
The influence of planar organic linkers on thermal boundary conductance across hybrid interfaces has focused on the organic/inorganic interaction energy, rather than on vibrational mechanisms in the molecule. As a result, research into interfacial transport at planar organic monolayer junctions has treated molecular systems as thermally ballistic. We show that thermal conductance in phosphonic acid (PA) molecules is ballistic, and the thermal boundary conductance across metal/PA/sapphire interfaces is driven by the same phononic processes as those across metal/sapphire interfaces without PAs, with one exception. We find a more than 40% reduction in conductance across Henicosafluorododecyl phosphonic acid (F21PA) interfaces, independent of metal contact, despite similarities in structure, composition and terminal group to the variety of other PAs studied. Our results suggest diffusive scattering of thermal vibrations in F21PA, demonstrating a clear path toward modification of interfacial thermal transport based on knowledge of ballistic and diffusive scattering in single monolayer molecular interfacial films.
Direct deposition of barrier films by atomic layer deposition (ALD) onto printed electronics presents a promising method for packaging devices. Films made by ALD have been shown to possess desired ultrabarrier properties, but face challenges when directly grown onto surfaces with varying composition and topography. Challenges include differing nucleation and growth rates across the surface, stress concentrations from topography and coefficient of thermal expansion mismatch, elastic constant mismatch, and particle contamination that may impact the performance of the ALD barrier. In such cases, a polymer smoothing layer may be needed to coat the surface prior to ALD barrier film deposition. We present the impact of architecture on the performance of aluminum oxide (Al2O3)/hafnium oxide (HfO2) ALD nanolaminate barrier films deposited on fluorinated polymer layer using an optical calcium (Ca) test under damp heat. It is found that with increasing polymer thickness, the barrier films with residual tensile stress are prone to cracking resulting in rapid failure of the Ca sensor at 50 °C/85% relative humidity. Inserting a SiNx layer with residual compressive stress between the polymer and ALD layers is found to prevent cracking over a range of polymer thicknesses with more than 95% of the Ca sensor remaining after 500 h of testing. These results suggest that controlling mechanical properties and film architecture play an important role in the performance of direct deposited ALD barriers.
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