The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to study photoinduced charge separation in bilayers consisting of a smooth, transparent, 80 nm thick layer of anatase TiO 2 onto which poly(3-hexylthiophene) (P3HT) sensitizer layers have been spin-coated. Interfacial charge separation, resulting from excitation of the polymer in the visible, is found to persist well into the millisecond time domain. Photoconductivity action spectra have been measured between 420 and 700 nm for P3HT layer thicknesses, L, from ∼2 to 200 nm. Using this electrodeless technique, the bilayers could be irradiated from either the polymer ("front") or semiconductor ("back") side. On front-side irradiation at 540 nm (close to the absorption maximum of the polymer), the efficiency of charge separation per incident photon (IPCSE) initially increased to a maximum value of 0.8% for L ≈ 10 nm. For thicker layers the IPCSE gradually decreased, eventually to 0.1% for L ≈ 170 nm. On back-side irradiation the IPCSE increased over the first 10 nm to a value close to the maximum found for front-side irradiation, and decreased only slightly for further increase in layer thickness. Analytical expressions for the thickness dependence based on exciton diffusion with a Lambert-Beer excitation profile have been used to fit the experimental data. Best fits were obtained for an exciton diffusion length, Λ () (Dτ) with D the diffusion coefficient and τ the natural lifetime), of 5.3 or 2.6 nm depending on whether excitons were taken to be reflected or quenched at the polymer/gas interface, respectively. The IPCSE decreased at high light intensities; an effect that is attributed to the occurrence of exciton-exciton annihilation within the polymer layer.

Despite significant improvement in power conversion efficiencies of bulk-heterojunction solar cells, the mechanism of mobile charge carrier generation is still under debate. The time-resolved microwave conductivity technique is used to investigate the mobile charge carrier generation in blends of P3HT with monoPCBM and bisPCBM by varying the excitation wavelength from the visible to NIR and the temperature from 88 to 300 K. NIR excitation corresponds to the transition of an electron from the HOMO of the P3HT directly to the LUMO of the fullerene forming the charge transfer band (CT). From the results it is inferred that the binding energy between the electron and hole in the CT state is smaller than thermal energy at 88 K (7.8 meV) that is in large contrast to previously reported values of 0.3 eV. This is ascribed to efficient charge delocalization, which increases the mean distance between the electron and hole at the interface. For P3HT:bisPCBM, the yield of charge carries decreases by a factor of 3 on changing the wavelength from the visible to the NIR. This is attributed to recombination of CT states to triplet level of P3HT. However, as the yields for P3HT:PCBM and P3HT:bisPCBM are comparable on visible excitation, we conclude that for the latter blend formation of mobile charge carrier occurs primarily via a thermally nonrelaxed, hot CT state. This observation indicates that the excess energy involved in the exciton dissociation process is indeed important to avoid recombination to the triplet level and to achieve higher yields of charge carrier generation. On the basis of these findings, we suggest that the excess energy can be small as long as the triplet level of the polymer is located energetically higher than the CT state. This insight is of particular interest for the rational design of novel polymer/fullerene systems to achieve higher power conversion efficiencies.

Photoexcitation of the symmetrical molecules tetraphenylethylene and tetra-p-methoxyphenylethylene dissolved in saturated hydrocarbon solvents results in a transient increase in the dielectric loss of the solutions as monitored using the nanosecond time-resolved microwave conductivity (TRMC) technique. This provides direct evidence for the dipolar, or "zwitterionic", nature of the 'p* phantom state formed from SI by rotation around the central carbonsarbon bond.Dipole relaxation occurs mainly by charge inversion between the two energetically equivalent zwitterionic configurations, Z+, on a timescale of several picoseconds. A minimum dipole moment of ca. 7.5 D for the individual Z+ states is found.The fluorescence of TPE in alkane solvents has two decay components, one with a decay time less than 200 ps and a second with a decay time of 1.9 ns. The former (A,,, = 490 nm) is assigned to emission from the partially relaxed SI state prior to twisting. The latter (A, , , = 540 nm) is assigned to emission from a small, ca. 1%, concentration of the relaxed SI state in equilibrium with the 'p* state in saturated hydrocarbon solvents.

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