Scheme 1. Proposed Kinetic Model for the Dynamics of Charges in the Orthorhombic Phase.

Upon pulsed excitation, either excitons (green) or charges are formed. Electrons diffuse in all directions and are trapped at the surface with rate constant k_in. Trapped electrons, n_T, decay with rate constant k_TE back to the valence band by emitting a photon at ∼622 nm. In principle, it is also possible that the reverse happens: Holes are trapped rapidly at the surface and decay with mobile conduction band electrons.