Field-dependent photocarrier generation in crystalline polydiacetylenes

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Abstract
The electric field dependence of the photocarrier generation rate has been measured for crystalline polydiacetylenes (PTS and DCH) in the range 105V cm-1. For E4V cm-1it follows the predictions of the one-dimensional Onsager theory, above 104V cm-1a superlinear increase with no indication of saturation is noted. The high-field behaviour is interpreted in terms of field-assisted ionisation of Frenkel excitation states that are degenerate with conduction band states. Applicability of the 1D Onsager theory to treat geminate pair dissociation is consistent with the recently developed concept of strong electron-phonon interaction leading to short carrier mean free paths. The microscopic anisotropy of the carrier mobility parallel and perpendicular to the polymer chain direction is found to be greater than 104, larger than inferred from macroscopic conduction experiments.