RESUMO
In order for a photovoltaic cell to function, charge carriers produced by photoexcitation must fully dissociate and overcome their mutual Coulomb attraction to form free polarons. This becomes problematic in organic systems in which the low dielectric constant of the material portends a long separation distance between independent polaron pairs. In this paper, we discuss our recent efforts to correlate the role of density of states, entropy, and configurational and energetic disorder to the open-circuit voltage, VOC, of model type-II organic polymer photovoltaics. By comparing the results of a fully interacting lattice model to those predicted by a Wigner-Weisskopf type model we find that energetic disorder does play a significant role in determining the VOC; however, mobility perpendicular to the interface plays the deciding role in the eventual fate of a charge-separated pair.
RESUMO
Recent experiments by Rao et al. (Nature, 2013, 500, 435-439) indicate that recombination of triplet charge-separated states is suppressed in organic polymer-fullerene based bulk-heterojunction (BHJ) photovoltaic cells exhibiting a high degree of crystallinity in the fullerene phase relative to systems with more disorder. In this paper, we use a series of Frenkel-exciton lattice models to rationalize these results in terms of wave-function localization, interface geometry, and density of states. In one-dimensional co-linear and co-facial models of the interface, increasing local energetic disorder in one phase localizes the interfacial triplet charge-transfer ((3)CT) states and increases the rate at which these states relax to form lower-energy triplet excitons. In two dimensional BHJ models, energetic disorder within the fullerene phase plays little role in further localizing states pinned to the interface. However, inhomogeneous broadening introduces strong coupling between the interfacial (3)CT and nearby fullerene triplet excitons and can enhance the decay of these states in systems with higher degrees of energetic disorder.
RESUMO
Toward quantitative description of protein aggregation, we develop a computationally efficient method to evaluate the potential of mean force between two folded protein molecules that allows for complete sampling of their mutual orientation. Our model is valid at moderate ionic strengths and accounts for the actual charge distribution on the surface of the molecules, the dielectric discontinuity at the protein-solvent interface, and the possibility of protonation or deprotonation of surface residues induced by the electric field due to the other protein molecule. We apply the model to the protein lysozyme, whose solutions exhibit both mesoscopic clusters of protein-rich liquid and liquid-liquid separation; the former requires that protein form complexes with typical lifetimes of approximately milliseconds. We find the electrostatic repulsion is typically lower than the prediction of the Derjaguin-Landau-Verwey-Overbeek theory. The Coulomb interaction in the lowest-energy docking configuration is nonrepulsive, despite the high positive charge on the molecules. Typical docking configurations barely involve protonation or deprotonation of surface residues. The obtained potential of mean force between folded lysozyme molecules is consistent with the location of the liquid-liquid coexistence, but produces dimers that are too short-lived for clusters to exist, suggesting lysozyme undergoes conformational changes during cluster formation.
