Steady-State Photoconduction in Amorphous Organic Solids

By applying various experimental techniques, e. g., transient absorption in a strong electric field and delayed field collection of charge carriers, it has been proven that in neat conjugated polymers singlet excitations can dissociate into pairs of free charge carriers in a strong electric field. Random walk theory has been developed to treat this process analytically. At variance of conventional 3D Onsager theory it is assumed that an exciton with finite lifetime can first transfer endothermically an electron to an adjacent site, thereby generating a charge transfer state whose energy is above the energy of that of the initial exciton. In a second step the latter can fully dissociate in accordance with Onsager's concept Brownian motion. The results indicate that, depending on the energy required for the first jump, the first jump contributes significantly to the field dependence of the dissociation yield. Disorder weakens the temperature dependence of the yield dramatically and precludes extracting information on the exciton binding energy from it. The chapter will also address the problem of photogeneration in donor-acceptor blends used in organic photovoltaic cells and emphasize the crucial role of geminate recombination of the electron-hole pair at the internal interface.