Amy S. Ferreira

7.7% Efficient All-Polymer Solar Cells.

By controlling the polymer/polymer blend self-organization rate, all-polymer solar cells composed of a high-mobility, crystalline, naphthalene diimide-selenophene copolymer acceptor and a benzodithiophene-thieno[3,4-b]thiophene copolymer donor are achieved with a record 7.7% power conversion efficiency and a record short-circuit current density (18.8 mA cm(-2)).

Electrochemical Oxidation of Ferrocene: A Strong Dependence on the Concentration of the Supporting Electrolyte for Nonpolar Solvents

The estimation of the driving force for photoinduced charge-transfer processes, using the Rehm-Weller equation, requires the employment of redox and spectroscopic quantities describing the participating electron donor and acceptor. Although the spectroscopic data are usually obtained from diluted solutions, the redox potentials are most frequently obtained from electrochemical measurements conducted in concentrated electrolyte solutions. To correct for the differences in the media, in which the various types of measurements are conducted, a term, based on the Born equation for solvation energy of ions, is introduced in the Rehm-Weller equation. The Born correction term, however, requires a prior knowledge of the dielectric constants of the electrolyte solutions used for the redox measurements. Because of limited information for such dielectrics, the values for the dielectric constants of electrolyte solutions are approximated to the values of the dielectric constants of the corresponding neat solvents. We examined the validity of this approximation. Using cyclic voltammetry, we recorded the first one-electron oxidation potential of ferrocene for three different solvents in the presence of 1-500 mM supporting electrolyte. The dielectric constants for some of the electrolyte solutions were extracted from fluorescence measurements of a dimethylaminonaphthalimide chromophore that exhibits pronounced solvatochromism. The dielectric constants of the concentrated electrolyte solutions correlated well with the corresponding oxidation potentials. The dependence of the oxidation potential of ferrocene on the electrolyte concentration for different solvents revealed that the abovementioned approximation in the Born correction term indeed introduces a significant error in the estimation of the charge-transfer driving force from redox data collected using relatively nonpolar solvents.

Long-Lived Photogenerated States of α-Oligothiophene−Acridinium Dyads Have Triplet Character

Photoinduced processes, leading to charge-transfer states with extended lifetimes, are of key importance for solar-energy-conversion applications. Utilizing external heavy-atom effect allowed us to photogenerate long-lived transients of electron donor-acceptor dyads. For an electron acceptor and a principal chromophore of the dyads, we selected N-methylacridinium, and for electron donors thiophene, bithiophene, and terthiophene were selected. While the photoinduced charge transfer, mediated by the investigated dyads, occurred in the picosecond time domain, the lifetime of the transients extended to the microsecond time domain. We ascribed the relatively long lifetimes to the triplet character of the observed transients. An increase in the size of the donor lowered the energy of the charge-transfer states of the dyads. When the energy level of the acridinium triplet lies below the energy level of the charge-transfer state, the locally excited triplet accounted for the long-lived transient. For the conjugates with charge-transfer states lying below all other excited states, the long-lived transients were, indeed, the charge-transfer species.

Dependence of the quality of adhesion between poly(dimethylsiloxane) and glass surfaces on the conditions of treatment with oxygen plasma.

Treatment with oxygen-containing plasma is an essential step for the fabrication of devices containing components of polydimethylsiloxane (PDMS). Such oxidative treatment chemically modifies the surface of PDMS allowing it to permanently adhere to glass, quartz, PDMS and other silica-based substrates. Overexposure of PDMS to oxidative gas plasma, however, compromises its adhesiveness. Therefore, regulation of the duration and the conditions of the plasma treatment is crucial for achieving sufficient surface activation without overoxidation. Using a semiquantitative ternary approach, we evaluated the quality of adhesion ( QA) between flat PDMS and glass substrates pretreated with oxygen plasma under a range of different conditions. The quality of adhesion manifested good correlation trends with the surface properties of the pretreated PDMS. Examination of the QA dependence on the treatment duration and on the pressure and the RF power of the plasma revealed a range of oxidative conditions that allowed for permanent adhesion with quantitative yields.

Long-lived photoinduced polaron formation in conjugated polyelectrolyte-fullerene assemblies

Photoinduction of long-lived polarons Photosynthetic complexes and organic photovoltaics can rapidly create separated charges upon photoexcitation. However, unproductive charge recombination often occurs in the human-made system. This is in part because the charge acceptor and donor structures are much larger. Huber et al. created aqueous micelles that pair conjugated polyelectrolyte charge donors with fullerene acceptors at a much smaller interface. They observed the photoinduced formation of polarons—stable pairs of separated charges—with lifetimes of several days. Science, this issue p. 1340 An aqueous micelle can stabilize separated charges created by photoinduced electron transfer for several days. The efficiency of biological photosynthesis results from the exquisite organization of photoactive elements that promote rapid movement of charge carriers out of a critical recombination range. If synthetic organic photovoltaic materials could mimic this assembly, charge separation and collection could be markedly enhanced. We show that micelle-forming cationic semiconducting polymers can coassemble in water with cationic fullerene derivatives to create photoinduced electron-transfer cascades that lead to exceptionally long-lived polarons. The stability of the polarons depends on the organization of the polymer-fullerene assembly. Properly designed assemblies can produce separated polaronic charges that are stable for days or weeks in aqueous solution.

Beyond PCBM: methoxylated 1,4-bisbenzyl[60]fullerene adducts for efficient organic solar cells

Organic solar cells have been based mostly on conjugated polymers and the classic fullerene derivative PCBM and are characterized by modest open circuit voltages (Voc). Increasing Voc requires fullerene acceptors with higher LUMOs than PCBM. To date, most fullerene derivatives synthesized for this purpose either do not achieve the high photocurrent afforded by PCBM or show relatively poor compatibility with the next-generation low bandgap conjugated polymers used in high-efficiency organic solar cells. Here, we report the facile synthesis of methoxylated 1,4-bisbenzyl fullerene adducts and their application as efficient electron acceptors in conjugated polymer-based solar cells. The methoxy groups are found to be essential to increasing the LUMO levels, and accordingly the Voc, of the devices compared to the parent 1,4-bisbenzyl fullerene, and more importantly, to PCBM. The best fullerene 1,4-bisadduct provides a ∼20% enhancement in power conversion efficiency over PCBM when used with the classic crystalline polymer P3HT. When used in combination with a higher-performance low bandgap polymer, PTB7, the bisadduct both increases the device open-circuit voltage and maintains the high photocurrent provided by the more traditional PCBM. We also examine 10 different 1,4-fullerene bisadducts and show that the photovoltaic device performance is strongly influenced by the number and relative position of the methoxy substituents on the benzyl addends: moving a single methoxy substituent by one position on the benzyl rings can change the device efficiency by over a factor of 2.

Structure and Conductivity of Semiconducting Polymer Hydrogels

Poly(fluorene-alt-thiophene) (PFT) is a conjugated polyelectrolyte that self-assembles into rod-like micelles in water, with the conjugated polymer backbone running along the length of the micelle. At modest concentrations (∼10 mg/mL in aqueous solutions), PFT forms hydrogels, and this work focuses on understanding the structure and intermolecular interactions in those gel networks. The network structure can be directly visualized using cryo electron microscopy. Oscillatory rheology studies further tell us about connectivity within the gel network, and the data are consistent with a picture where polymer chains bridge between micelles to hold the network together. Addition of tetrahydrofuran (THF) to the gels breaks those connections, but once the THF is removed, the gel becomes stronger than it was before, presumably due to the creation of a more interconnected nanoscale architecture. Small polymer oligomers can also passivate the bridging polymer chains, breaking connections between micelles and dramatically weakening the hydrogel network. Fits to solution-phase small-angle X-ray scattering data using a Dammin bead model support the hypothesis of a bridging connection between PFT micelles, even in dilute aqueous solutions. Finally, time-resolved microwave conductivity measurements on dried samples show an increase in carrier mobility after THF annealing of the PFT gel, likely due to increased connectivity within the polymer network.