On the Photogeneration of Charge Carriers in Quasi-One-Dimensional Semiconductors: Polydiacetylene

Experimental data are presented showing that recombination of free carriers occurs in the bulk of mm size samples of PDA-TS single crystals under steady-state excitation. The results, combined with the shape of the photocurrent i dependence on field strength, E, lead to the following conclusions: 1) the quantum yield of free carriers ϵ does not depend on the field strength at E < 103 V/cm but increases at higher E; 2) ϵ depends also on the diffusion length L of free carriers before recombination in the bulk of the crystal: ϵ ∼ L −1. The latter accounts for the unusual type of i dependence on I, i ∼ I″′ where m varies with the intensity of the light I in the range 1 ≤ m ≤ 2/3. The properties of geminate diffusive pairs of charges of opposite signs are responsible for the photogeneration of free carriers. These pairs are as large as L ≅ 10−5 cm which is much higher than Onsager's radius rc. The diffusive pairs are responsible also for the magnetic field effect on the photocurrent observed.