Ultrafast Dynamics of Photoinduced Charge Generation and Recombination in Hexabenzocoronene Molecular Semiconductors

摘要

Harnessing solar energy as an alternative resource continues to stimulate organic photovoltaic (OPV) research with the potential for low cost fabrication on mechanically flexible substrates.Significant efforts have been devoted towards improving OPV efficiency above 10%; however, challenges still remain to understand, control, and optimize the photon-to-current conversion within the photoactive layer. In the typical organic photoactive film, which is comprised of electron donors (D) and acceptors (A), light is absorbed to form excitons (electron-hole pairs, or excited states) that migrate through the solid as an electrically neutral entity, where only energy is transferred in this process. Several deactivation pathways exist each with a characteristic timescale: i) Collisions with the lattice that results in heat formation (non-radiative decay (ps), ii) The release of energy through photons (fluorescence (ns), and iii) Intersystem crossing to a triplet excited state (sub-ps) and iv) Exciton dissociation into a free electron and hole (ps) within an electron donor and acceptor. The preferred pathway, which would maximize efficiency, involves exciton diffusion to the interface where dissociation via electron transfer occurs to form an ion pair state (or free electron and hole). Recombination of the electron and hole through geminate recombination of a bound pair, non-geminate recombination of a free pair, or trap assisted recombination are undesired energy wasting pathways that often occur become of the large Coulomb attraction between the electron and hole. Deconvolution of these competing pathways in blended films is often complicated by disorderd morphologies or incompatibility between D and A that create charge traps and reduce charge carrier mobility, even after high temperature annealing.