Mark Dadmun

Control of morphology and function of low band gap polymer–bis-fullerene mixed heterojunctions in organic photovoltaics with selective solvent vapor annealing

Replacing PCBM with a bis-adduct fullerene (i.e. ICBA) has been reported to significantly improve the open circuit voltage (VOC) and power conversion efficiency (PCE) in P3HT bulk heterojunctions. However, for the most promising low band-gap polymer (LBP) systems, replacing PCBM with ICBA results in very poor short-circuit current (JSC) and PCE although the VOC is significantly improved. Therefore, in this work, we have completed small angle neutron scattering and neutron reflectometry experiments to study the impact of post-deposition solvent annealing (SA) with control of solvent quality on the morphology and performance of LBP–bis-fullerene BHJ photovoltaics. The results show that SA in a solvent that is selective for the LBP results in a depletion of bis-fullerene near the air surface, which limits device performance. SA in a solvent vapor which has similar solubility for polymer and bis-fullerene results in a higher degree of polymer ordering, bis-fullerene phase separation, and segregation of the bis-fullerene to the air surface, which facilitates charge transport and increases power conversion efficiency (PCE) by 100%. The highest degree of polymer ordering combined with significant bis-fullerene phase separation and segregation of bis-fullerene to the air surface is obtained by SA in a solvent vapor that is selective for the bis-fullerene. The resultant morphology increases PCE by 190%. These results indicate that solvent annealing with judicious solvent choice provides a unique tool to tune the morphology of LBP–bis-fullerene BHJ system, providing sufficient polymer ordering, formation of a bis-fullerene pure phase, and segregation of bis-fullerene to the air surface to optimize the morphology of the active layer. Moreover, this process is broadly applicable to improving current “disappointing” LBP–bis-fullerene systems to optimize their morphology and OPV performance post-deposition, including higher VOC and power conversion efficiency.

The impact of selective solvents on the evolution of structure and function in solvent annealed organic photovoltaics

This study examines the development of structure and performance in an organic photovoltaic (OPV) thin film comprised of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Specifically, the influence of P3HT and PCBM solubility in the solvents utilized for vapor annealing on the morphological properties and OPV performance of the resultant active layer is examined. The evolution of P3HT crystallinity as well as the growth of PCBM aggregates in the solvent annealed thin films were examined by Grazing Incidence Wide Angle X-ray Scattering (GIWAXS), Atomic Force Microscopy (AFM), and Energy Filtered Transmission Electron Microscopy (EFTEM). It is shown that P3HT crystallinity increases initially, then decreases with time for solvents that have a finite P3HT solubility. Alternatively, PCBM aggregates grow continuously with SVA, but are modulated by the PCBM solubility. High P3HT crystallinity and moderate PCBM phase separation correlates to improved power conversion efficiency (PCE). Hence, the relative P3HT and PCBM solubility plays a crucial role in choosing the best SVA time of different annealing solvents. Specifically, for samples annealed using solvents that prefer P3HT, PCE benefits from further SVA after the peak P3HT crystallinity time, which is ascribed to additional PCBM phase separation. On the other hand, solvents that prefer PCBM induce excess PCBM phase separation at longer SVA times, which limits exciton dissociation and PCE. EFTEM cross section images indicate that PCBM is distributed toward the bottom of the film, whereas SVA in a solvent with high PCBM solubility may induce PCBM to segregate towards the air surface, which benefits charge transport processes by preventing electron–hole recombination.

A Novel Reactive Processing Technique: Using Telechelic Polymers To Reactively Compatibilize Polymer Blends

Difunctional reactive polymers, telechelics, were used to reactively form multiblock copolymers in situ when melt-blended with a blend of polystyrene and polyisoprene. To quantify the ability of the copolymer to compatibilize the blends, the time evolution of the domain size upon annealing was analyzed by SEM. It was found that the most effective parameter to quantify the ability of the copolymer to inhibit droplet coalescence is K(rel)t(stable), the relative coarsening constant multiplied by the stabilization time. These results indicate that intermediate-molecular-weight telechelic pairs of both highly reactive Anhydride-PS-Anhydride/NH(2)-PI-NH(2) and slower reacting Epoxy-PS-Epoxy/COOH-PI-COOH both effectively suppress coalescence, with the optimal molecular weight being slightly above the critical molecular weight of the homopolymer, M(c). The effects of telechelic loading were also investigated, where the optimal loading concentration for this system was 0.5 wt %, as higher concentrations exhibited a plasticizing effect due to the presence of unreacted low-molecular-weight telechelics present in the blend. A determination of the interfacial coverage of the copolymer shows that a conversion of approximately 1.5-3.0% was required for 20% surface coverage at 5.0 wt % telechelic loading, indicating a large excess of telechelics in this system. At the optimal loading level of 0.5 wt %, a conversion of 15% was required for 20% surface coverage. The results of these experiments provide a clear understanding of the role of telechelic loading and molecular weight on its ability to reactively form interfacial modifiers in phase-separated polymer blends and provide guidelines for the development of similar reactive processing schemes that can use telechelic polymers to reactively compatibilize a broad range of polymer blends.

3D reconstruction of carbon nanotube networks from neutron scattering experiments.

UNLABELLED Structure reconstruction from statistical descriptors, such as scattering data obtained using x-rays or neutrons, is essential in understanding various properties of nanocomposites. Scattering based reconstruction can provide a realistic model, over various length scales, that can be used for numerical simulations. In this study, 3D reconstruction of a highly loaded carbon nanotube (CNT)-conducting polymer system based on small and ultra-small angle neutron scattering (SANS and USANS, respectively) data was performed. These light-weight and flexible materials have recently shown great promise for high-performance thermoelectric energy conversion, and their further improvement requires a thorough understanding of their structure-property relationships. The first step in achieving such understanding is to generate models that contain the hierarchy of CNT networks over nano and micron scales. The studied system is a single walled carbon nanotube (SWCNT)/poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate) ( PEDOT PSS). SANS and USANS patterns of the different samples containing 10, 30, and 50 wt% SWCNTs were measured. These curves were then utilized to calculate statistical two-point correlation functions of the nanostructure. These functions along with the geometrical information extracted from SANS data and scanning electron microscopy images were used to reconstruct a representative volume element (RVE) nanostructure. Generated RVEs can be used for simulations of various mechanical and physical properties. This work, therefore, introduces a framework for the reconstruction of 3D RVEs of high volume faction nanocomposites containing high aspect ratio fillers from scattering experiments.

Effect of chain structure on the miscibility of cellulose acetate blends: a small-angle neutron scattering study

The miscibility of cellulose ester blends with varying degree of substitution (DS) of acetates along the chain backbone has been investigated using small-angle neutron scattering. The difference in degree of substitution (ΔDS) between the two components in the blend was systematically varied from 0.06 to 0.63 where each blend was found to be a partially miscible, two-phase system. Miscibility between the two components initially decreases as ΔDS of the blends increases. The Flory interaction parameter, χ, concurrently increases with increasing ΔDS as a result of diminishing van der Waals forces between components. The cellulose acetates with lower degree of substitution, which contain more hydroxyl substituents, however, demonstrate greater miscibility even at higher ΔDS. This is interpreted to be the result of favorable hydrogen bonding between blend components that are possible in the presence of more hydroxyl groups. FT-IR data support this interpretation, indicating an increase in hydrogen bonding in a blend having a lower DS component. These results indicate that while an increase in structural differences between cellulose acetate blend components limits miscibility, the presence of hydroxyl groups on the chain promotes mixing. This competition accentuates the significant impact specific interactions have on blend miscibility for these copolymers.