Solid-State Infrared Upconversion in Perylene Diimides Followed by Direct Electron Injection.

In this contribution we demonstrate a solid-state approach to triplet-triplet annihilation upconversion for application in a solar cell device in which absorption of near-infrared light is followed by direct electron injection into an inorganic substrate. We use time-resolved microwave photoconductivity experiments to study the injection of electrons into the electron-accepting substrate (TiO2) in a trilayer device consisting of a triplet sensitizer (fluorinated zinc phthalocyanine), triplet acceptor (methyl subsituted perylenediimide), and smooth polycrystalline TiO2. Absorption of light at 700 nm leads to the almost quantitative generation of triplet excited states by intersystem crossing. This is followed by Dexter energy transfer to the triplet acceptor layer where triplet annihilation occurs and concludes by injection of an electron into TiO2 from the upconverted singlet excited state.

High charge carrier mobility and efficient charge separation in highly soluble perylenetetracarboxyl-diimides.

In this communication we report on the synthesis and charge mobility of highly soluble perylenebisimid derivatives. We show that introduction of alkylester side chains results in compounds combining a high solubility with charge mobilities up to 0.22 cm(2) V(-1) s(-1). These materials are therefore interesting as an electron acceptor for solution-processed organic photovoltaics.

Determination of Singlet Exciton Diffusion Length in Thin Evaporated C60 Films for Photovoltaics.

C60 is used as an electron acceptor in small molecule photovoltaic devices in combination with various electron donors. The transport of excitons, i.e., bound electron/hole pairs, is an important factor determining the efficiency of such devices. Here we investigate the exciton diffusion length in C60 with the electrodeless time-resolved microwave conductance (TRMC) technique. Bilayers of 30 nm Zn-phthalocyanine with a C60 layer with variable thickness are prepared by physical vapor deposition. Analysis of the photoconductance with an exciton diffusion model yields a diffusion length of 7 nm, and the mobility of holes along Zn-Phthalocyanine stacks is close to 1 cm(2)/(V s). From analysis of the rise and decay of the TRMC transients, we attribute the photoconductance to diffusion and dissociation of singlet excitons rather than triplets. The energy transfer rate between C60 molecules exceeds 8 × 10(10) s(-1). Exciton diffusion cannot be described by the Förster model due to the close proximity of the molecules.