A simplified model system is used to compute the rates of interfacial charge separation (CS) and recombination (CS) in the P3HT/PCBM blend (poly(3-hexylthiophene) and [6,6]-phenyl-C 61 -butyric acid methyl ester) used in bulk heterojunction solar cells. The absolute charge-transfer rates of CS (k CS ) and CR (k CR ) processes were calculated to be 1.50 Â 10 11 and 1.93 Â 10 9 s -1 , respectively, from the Marcus-Levich-Jortner rate equation, in reasonable agreement with the range of available experimental values for a model containing a six thiophene rings chain and a single PCBM molecule. A detailed discussion of the inaccuracy intrinsic in the evaluation of all quantities entering the rate expression (equilibrium energy, electronic coupling, and internal and external reorganization energies) is provided together with a discussion of the sensitivity of the computed rate to these quantities. A variety of DFT methods is used to evaluate the states energy of the system (TDDFT, calculation with background charges, and unrestricted DFT), and it was found that unrestricted calculations of the lowest triplet state can describe with good accuracy the equilibrium energy and geometry of the charge-transfer states. A physically plausible range for the external reorganization energy is computed with a continuum model, and it is shown that a more accurate evaluation of this quantity is not essential.

Low lying excited states of the fullerene anion promote a faster charge separation in organic solar cells containing fullerene derivatives as electron acceptors. Alternative electron acceptors, not based on fullerenes but that share the same property, can be easily designed. On the other hand, it is unlikely for a generic electron acceptor to replicate this fullerene characteristic by chance.

Visible light excitation of [Cu(I)(dmp)(2)](BArF), where dmp is 2,9-dimethyl-1,10-phenanthroline and BArF is tetrakis(3,5-bis(trifluoromethylphenyl))borate, in toluene produces a photoluminescent, metal-to-ligand charge-transfer (MLCT) excited state with a lifetime of 98 +/- 5 ns. Probing this state within 14 ns after photoexcitation with pulsed X-rays establishes that a Cu(II) center, borne in a Cu(I) geometry, binds an additional ligand to form a five-coordinate complex with increased bond lengths and a coordination geometry of distorted trigonal bipyramid. The average Cu-N bond length increases in the excited state by 0.07 A. The transiently formed five-coordinate MLCT state is photoluminescent under the condition studied, indicating that the absorptive and emissive states have distinct geometries. The data represent the first X-ray characterization of a molecular excited state in fluid solution on a nanosecond time scale.

The differences between the introduction of chlorine and fluorine atoms to small-molecule acceptors were deeply investigated. From the single-crystal structures of three molecules, the Cl-substitution intervention into the molecular configuration and packing mainly lies in three aspects as follows: single molecule configuration, one direction of the intermolecular arrangement, and three-dimensional (3D) molecular packing. First, the introduction of the chlorine atom in IDIC-4Cl leads to a more planar molecular configuration than IDIC-4H and IDIC-4F because of the formation of a molecular interlocked network induced by the strong Cl•••S intermolecular interactions. Second, IDIC-4Cl shows the closest π−π stacking distance and the smallest dihedral angle (0°) between adjacent molecules to form ideal J-aggregation, which should be beneficial for charge transportation between different connected molecules in this direction. Finally, the interlocked interactions between Cl and S atoms lead to a highly ordered 3D molecular packing, in which the end groups will form an ideal overlapped packing among different molecules, whereas the other two analogues with H or F show less ordered packing of their 1,1dicyanomethylene-3-indanone ending groups. Organic solar cells based on IDIC-4Cl show the highest power conversion efficiency (PCE) of 9.24%, whereas the PCEs of IDIC-4H-and IDIC-4F-based devices are 4.57 and 7.10%, respectively.