DC photoconduction action spectra in thin film polymer crystals of 3 and 4BCMU polydiacetylene Original Research Article

Thin polydiacetylene single crystals with alkyl-urethane sidegroups, 3 and 4BCMU PDA, of macroscopic dimensions, have been prepared by surface electron beam polymerisation of bulk monomer crystals. Steady state photoconduction action spectra have been obtained for both the 3 and 4BCMU thin film polydiacetylene crystals. Analysis of this data has allowed determination of the band gap energy, EG for the two crystals. The analysis suggests an indirect, forbidden, band to band transition that is buried under a strong excitonic transition. It is found that the indirect gap, EG, is 2.23 eV±0.01 eV for 3BCMU and 2.21 eV±0.01 eV for 4BCMU. A bandwidth of approximately 1 eV may be inferred.