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

Thin polydiacetylene single crystals with alkyl-urethane sidegroups, 3 and 4BCMU PDA, of macroscopic dimensions, have been prepared by surface electro n beam polymerisation of bulk monomer crystals. Steady state photoconductio n action spectra have been obtained for both the 3 and 4BCMU thin film poly diacetylene crystals. Analysis of this data has allowed determination of th e band gap energy, E-G for the two crystals. The analysis suggests an indir ect, forbidden, band to band transition that is buried under a strong excit onic transition. It is found that the indirect gap, E-G, is 2.23 eV +/- 0.0 1 eV for 3BCMU and 2.21 eV +/- 0.01 eV for 4BCMU. A bandwidth of approximat ely 1 eV may be inferred. (C) 1999 Elsevier Science B.V. All rights reserve d.