A dipole interaction model for the molecular second hyperpolarizability

A dipole interaction model (IM) for calculating the molecular second hyperpolarizability, gamma, of aliphatic and aromatic molecules has been investigated. The model has been parametrized from quantum chemical calculations of gamma at the self-consistent field (SCF) level of theory for 72 molecules. The model consists of three parameters for each element p: an atomic polarizability, an atomic second hyperpolarizability, and an atomic parameter, Phi(p), describing the width of the atomic charge distribution. The Phi(p) parameters are used for modeling the damping of the interatomic interactions. Parameters for elements H, C, N, O, F, and Cl were determined, and typical differences between the molecular gamma derived from quantum chemical calculations and from the IM are below 30% and on average around 10%. As a preliminary test, the dipole interaction model was applied to the following molecular systems not included in the training set: the urea molecule, linear chains of urea molecules, and C-60. For these molecules deviations of the IM result for the molecular gamma from the corresponding SCF value were at most around 30% for the individual components, which in all cases is a better performance than obtained with semiempirical methods

Analyzing poly(3-hexyl-thiophene):1-(3-methoxy-carbonyl)propyl-1-phenyl-(6,6)C61 bulk-heterojunction solar cells by UV-visible spectroscopy and optical simulations

A nondestructive method for assessing the thickness of the photoactive layer in poly(3-hexyl-thiophene):1-(3-methoxy-carbonyl)propyl-1-phenyl-(6,6)C61 (P3HT:PCBM) solar cells is reported. In the approach the absorption spectrum of the solar cell as derived by optical simulations is fitted to the corresponding measured spectrum, varying only the P3HT:PCBM layer thickness. Within the 50–250 nm thickness range, a linear correlation between the position of a certain spectral minimum and the P3HT:PCBM layer thickness is shown, based on simulated absorption spectra. As an initial application, absorption spectra for 240 P3HT:PCBM solar cells prepared at four different spin-coating speeds were recorded, and the average P3HT:PCBM layer thickness estimated for each spin-coating speed. The simulated fraction of light absorbed in the P3HT:PCBM layer of the solar cells is compared with the P3HT:PCBM absorption spectra measured for films spin coated on simpler substrate types. The latter spectra cannot account for the light harvested in the photoactive layer of P3HT:PCBM solar cells because of substantial optical interference in the solar cells. The measured short circuit current densities Jsc for the solar cells vary with the spin-coating speed in a manner confirmed by optical simulations of the maximal short circuit current densities. The measured efficiencies follow the same pattern. On average the measured Jsc is 1–2 mA/cm2 below the simulated maximal short circuit current densities. Based on the resemblance of the measured and simulated absorption spectra such difference can be attributed to recombination exclusively. © 2007 American Institute of Physics.</em