The role of traps in the interpretation of Photo-EMF parameters of organicdye pigments

The decay of the transient Photo-ElectroMotive-Fozce U (Photo-E.M.F.) of or ganic dye pigments can be described as a biexponential process [1] even if the signals change the sign of U(t) within the decay. U(t) = U-1(0) exp(-k(l)t) + U-2(0) exp(-k(2)t) The dependencies of the four parameters U-1(0), U-2(0), k(1) and k(2) on wa velength and intensity of the exciting laser flash were investigated using Copper(II)-phthaiocyanine (CuPc) and N,N'-Dimethylperylene-tetra-carboxylic -bsimide (MePe) as pigments dispersed in polyvinyl butyral (PVB) layers. In agreement with the trap concept of MOTT [2,3] the type of conductivity ( n- or p type photoconduction) of MePe depends on the energy of the exciting light. CuPc does not show such a change of the photoconduction. More detai led information about the role of traps available for photogenerated charge carriers - electron and hole (e/h) - can be derived from time resolved Pho to-E.M.F. experiments under additional continuous illumination of the sampl e by poly- and monochromatic light. The results show, that charge carriers involved in Photo-E.M.F. measurements will be influenced mainly by shallow traps (DeltaE in the meV-range). Occupation of traps by charge carriers gen erated under continuous illumination results in an increase of the decay ra te for the faster process. This points out, that its parameters U-1(0) and k(1) may be attributed to that Photo-E.M.F, generated near the surface of t he pigment particles.