In this work we explore how an electrolyte additive (fluorinated ethylene carbonate – FEC) mediates the thickness and composition of the solid electrolyte interphase formed over a silicon anode in situ as a function of state-of-charge and cycle. We show the FEC condenses on the surface at open circuit voltage then is reduced to C-O containing polymeric species around 0.9 V (vs. Li/Li+). The resulting film is about 50 Å thick. Upon lithiation the SEI thickens to 70 Å and becomes more organic-like. With delithiation the SEI thins by 13 Å and becomes more inorganic in nature, consistent with the formation of LiF. This thickening/thinning is reversible with cycling and shows the SEI is a dynamic structure. We compare the SEI chemistry and thickness to 280 Å thick SEI layers produced without FEC and provide a mechanism for SEI formation using FEC additives.

With the use of neutron reflectometry, we have determined the thickness and chemistry of the solid-electrolyte interphase (SEI) layer grown on a silicon anode as a function of state of charge and during cycling. We show the chemistry of this SEI layer becomes more LiF like with increasing lithiation and more Li−C−O−F like with delithiation. More importantly, the SEI layer thickness appears to increase (about 250 Å) as the electrode becomes less lithiated and thins to 180 Å with increasing Li content (Li 3.7 Si). We attribute this "breathing" to the continual consumption of electrolyte with cycling.

The epitaxial growth and preferred molecular orientation of copper phthalocyanine (CuPc) molecules on graphene has been systematically investigated and compared with growth on Si substrates, demonstrating the role of surface-mediated interactions in determining molecular orientation. X-ray scattering and diffraction, scanning tunneling microscopy, scanning electron microscopy, and first-principles theoretical calculations were used to show that the nucleation, orientation, and packing of CuPc molecules on films of graphene are fundamentally different compared to those grown on Si substrates. Interfacial dipole interactions induced by charge transfer between CuPc molecules and graphene are shown to epitaxially align the CuPc molecules in a face-on orientation in a series of ordered superstructures. At high temperatures, CuPc molecules lie flat with respect to the graphene substrate to form strip-like CuPc crystals with micrometer sizes containing monocrystalline grains. Such large epitaxial crystals may potentially enable improvement in the device performance of organic thin films, wherein charge transport, exciton diffusion, and dissociation are currently limited by grain size effects and molecular orientation.

No abstract

The attractive optoelectronic properties of conducting polymers depend sensitively upon intra-and inter-polymer chain interactions, and therefore new methods to manipulate these interactions are continually being pursued. Here, we report a study of the isotopic effects of deuterium substitution on the structure, morphology and optoelectronic properties of regioregular poly(3-hexylthiophene)s with an approach that combines the synthesis of deuterated materials, optoelectronic properties measurements, theoretical simulation and neutron scattering. Selective substitutions of deuterium on the backbone or side-chains of poly(3-hexylthiophene)s result in distinct optoelectronic responses in poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) photovoltaics. Specifically, the weak non-covalent intermolecular interactions induced by the main-chain deuteration are shown to change the film crystallinity and morphology of the active layer, consequently reducing the short-circuit current. However, side-chain deuteration does not significantly modify the film morphology but causes a decreased electronic coupling, the formation of a charge transfer state, and increased electron-phonon coupling, leading to a remarkable reduction in the open circuit voltage.

InstrumentationThermal Analysis. Standard thermal gravimetry experiments were performed on a TA Instruments Q5000IR TGA. Samples were heated in platinum pans from ambient temperature to 600.0°C at 20.0°C/min. A TA Instruments Q1000 Differential Scanning Calorimeter (DSC) was used to evaluate thermal transitions of the (co)polymers. Samples (~ 3-8 mg) were prepared in standard aluminum pans/lids and were first heated from ambient temperature to 160.0°C at a ramp rate of 20.0°C/min. Samples were subsequently cooled to -50.0°C at 25.0°C/min and finally heated to 160.0°C at 20.0°C/min. Glass transition temperatures are reported from the second heating as the mid-point of the heat flow derivative curve. The DSC was calibrated using indium standard (In; melting point, T m, In = 156.6 °C; provided by TA Instruments) according to the manufacturer's recommendation, which includes baseline and temperature calibrations.Additionally, standard thermal gravimetry experiments were performed on a TA Instruments Q5000IR TGA. Samples were heated in platinum pans from ambient temperature to 600.0°C at 20.0°C/min. Raman Microspectroscopy. Raman spectroscopy of thin polymer films were performed using a Renishaw 100 confocal micro-Raman system equipped with a CCD detector. A 632.8 nm HeNe laser was focused to 2 µm spot size with a 50x objective. Raman spectra were acquired using a 60 s integration time. Atomic ForceMicroscopy. A Digital Instruments Dimension 3100 atomic force microscope (AFM) was used in tapping mode to obtain height images of 1000 µm lines of a PGMA 73 -b-PVDMA 174 copolymer spin-coated from solution in CHCl 3 at a concentration of 0.75% wt and annealed under vacuum at 110 °C. The micropattern was made by photolithographic techniques. 1The amplitude set-point and proportional and integral gains were adjusted for each sample assuring optimal image quality. All measurements were done at a scanning rate of 0.5 Hz using silicon nitride cantilevers. An area of 8 µm × 8 µm at the edge of the pattern was initially surveyed in order to obtain a direct comparison of layer thickness values obtained by AFM and by ellipsometry. Then, a 2 µm × 2 µm area on the polymer layer was sampled, which allowed the film's topography and roughness to be examined.

A conducting diblock copolymer of PS-b-P3HT was added to serve as a compatibilizer in a P3HT/PCBM blend, which improved the power-conversion efficiency from 3.3% to 4.1% due to the enhanced crystallinity, morphology, interface interaction, and depth profile of PCBM.

Recent work has shown that poly(3-hexylthiophene) (P3HT) and the surface-functionalized fullerene 1-(3-methyloxycarbonyl)propyl(1-phenyl[6,6])C(61) (PCBM) are much more miscible than originally thought, and the evidence of this miscibility requires a return to understanding the optimal morphology and structure of organic photovoltaic active layers. This manuscript describes the results of experiments that were designed to provide quantitative thermodynamic information on the miscibility, interdiffusion, and depth profile of P3HT : PCBM thin films that are formed by thermally annealing initial bilayers. It is found that the resultant thin films consist of a 'bulk' layer that is not influenced by the air or substrate surface. The composition of PCBM in this 'bulk' layer increases with increased PCBM loading in the original bilayer until the 'bulk' layer contains 22 vol% PCBM. The introduction of additional PCBM into the sample does not increase the amount of PCBM dispersed in this 'bulk' layer. This observation is interpreted to indicate that the miscibility limit of PCBM in P3HT is 22 vol%, while the precise characterization of the depth profiles in these films shows that the PCBM selectively segregates to the silicon and near air surface. The selective segregation of the PCBM near the air surface is ascribed to an entropic driving force.