A band model for synthetic dopa-melanin

To determine a band model for synthetic melanin obtained by autooxidation of L-dopa (3,4-dihydroxyphenyl-L-alanine), dependence of dark current on temperature in the range 298-333 degrees K was measured as well as dependence of optical absorption coefficient on wave-length in the range 250-800 nm. It was found that up to 311 degrees K thermal activation energy equals 0.1 eV and above 313 degrees K it equals 0.78 eV. The first value is connected with the band of localized states at the Fermi level. Optical gap, determined from optical absorption measurements is equal to 1.40 eV. The estimated value of sigma o, assuming the value of thermal coefficient of activation-energy to be gamma = 5 X 10(-4) eV/degrees K, is 2 X 10(-6) omega -1 cm-1 for 0.1 eV and 5 X 10(2) omega -1 cm-1 for 0.78 eV. Density of states in the valence band is N(EV) = 8 X 10(21)/cm3 X eV, and in the band of localized states at the Fermi level N(EF) = 3 X 10(13)/cm3 X eV.