Generation and Diffusion of Photocarriers in Molecular Donor–Acceptor Systems: Dependence on Charge-Transfer Gap Energy

Here, we investigated photocarrier generation and diffusion characteristics in molecular-scale donor–acceptor charge-transfer (CT) systems. The photocarrier diffusion characteristics were measured on a series of mixed-stack CT compound crystals by the laser beam-induced current (LBIC) technique where the photocurrent is detected on the crystal surfaces as a function of either the laser illuminated position or the laser-modulation frequency. In the compounds with CT gap energy larger than 0.7 eV, the diffusion length of photocarriers reached larger than 10 μm. The dependence of diffusion length on the electric field and the laser-modulated frequency clearly indicates the direct generation of long-lived photocarriers without forming exciton. In contrast, the photocarrier diffusion was suppressed, and the diffusion length got smaller than 2 μm in the compounds with a gap energy smaller than 0.7 eV. We discuss that the electron–hole recombination becomes dominated when the CT gap energy is as small as the molecular reorganization energy. The results suggest that proper choice of donor–acceptor combination should promote efficient charge separation in organic photovoltaic cells (OPCs).