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).