[reaction: see text] Formation of a controlled fullerene mesophase within an organic host system has enabled us to create high-power conversion efficiency photovoltaics. This mesophase is formed using thermal gradients that provide a fluidic mobility of the fullerenes allowing for greater dispersion of nanocrystalline 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM) within regioregular poly(3-hexylthiophene) (P3HT). From this reorganization of the component materials in the matrix the overall efficiency of the system jumps dramatically from the roughly 2.4% to 5.2%.

A significant increase of the electrical conductivity of PEDOT:PSS films, brought about by the addition of dimethyl sulfate (DMS, (CH 3 ) 2 SO 4 ), while preserving the films' excellent flexibility and visible-light transparency, is reported. The electrical and morphological properties of the films were studied as a function of DMS concentration. At an optimal concentration of around 1:25 (DMS to PEDOT:PSS), the conductivity of the films is enhanced by a factor on the order of 1880 times that of pristine PEDOT:PSS films. Extensive spectroscopic measurements using absorbance, Raman, and FTIR techniques, as well as structural characterization by AFM microscopy, were performed. These measurements support the idea that the mechanism responsible for the conductivity enhancement is the partial replacement of the PSSsegments by SO 4 -2 anionic sulfates when a small amount of DMS is added to a PEDOT:PSS solution. This mechanism is associated with an increase of doping, and this doping can be understood in the following manner: due to that the SO 3 ions of the PSS segment only carry one negative charge, it is more probable for them to create polaronic states, whereas the SO 4 -2 ions are double charged, increasing the possibility of creating bipolaron carriers in the PEDOT backbone. In this way, the partial replacement of the PSSsegments by SO 4 -2 ions increases the bipolaron population by an ion exchange process, and, as a consequence, the doping level is increased.

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A density-functional theory for many-body lattice models is considered in which the single-particle density matrix ␥ i j is the basic variable. Eigenvalue equations are derived for solving Levy's constrained search of the interaction energy functional W͓␥ i j ͔. W͓␥ i j ͔ is expressed as the sum of Hartree-Fock energy E HF ͓␥ i j ͔ and the correlation energy E C ͓␥ i j ͔. Exact results are obtained for E C (␥ 12 ) of the Hubbard model on various periodic lattices, where ␥ i j ϭ␥ 12 for all nearest neighbors i and j. The functional dependence of E C (␥ 12 ) is analyzed by varying the number of sites N a , band filling N e , and lattice structure. The infinite one-dimensional chain and one-, two-, or three-dimensional finite clusters with periodic boundary conditions are considered. The properties of E C (␥ 12 ) are discussed in the limits of weak (␥ 12 Ӎ␥ 12 0 ) and strong (␥ 12 Ӎ␥ 12 ϱ ) electronic correlations, and in the crossover region (␥ 12 ϱ р␥ 12 р␥ 12 0 ). Using an appropriate scaling we observe that C (g 12 ) ϭE C /E HF has a pseudo-universal behavior as a function of g 12 ϭ(␥ 12 Ϫ␥ 12 ϱ )/(␥ 12 0 Ϫ␥ 12 ϱ ). The fact that C (g 12 ) depends weakly on N a , N e , and lattice structure suggests that the correlation energy of extended systems could be obtained quite accurately from finite-cluster calculations. Finally, the behaviors of E C (␥ 12 ) for repulsive (UϾ0) and attractive (UϽ0) interactions are contrasted.