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

The decay of the transient Photo-ElectroMotive-Force U(Photo-E.M.F.) of org anic dye pigments can be described as a biexponential process, even if the signals change the sign of U(t) within the decay. The dependencies of the f our kinetic parameters U-1(0); U-2(0); k(1) and k(2) on wavelength and inte nsity of the exciting laser flash or additional continuous illumination wer e investigated using N,N'-Dimethylperylene-tetracarboxylic-bisimide (MePe) and Copper(II)-phthalocyanine (CuPc) as pigments dispersed in polyvinyl but yral (PVB) layers. In agreement with the trap concept of MOTT 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 detailed information about the role of traps available for photogenera ted charge carriers can be derived from time resolved Photo-E.M.F. experime nts under additional continuous illumination of the sample by poly- and mon ochromatic light. The results show, that charge carriers involved in Photo- E.M.F. measurements will be infuenced mainly by shallow traps (DeltaE in th e meV-range). Occupation of traps by charge carriers generated under continuous illuminat ion results in an increase of the decay rate for the faster process. This p oints out, that its parameters U-1(0) and k(1) may be attributed to a parti al Photo-E.M.F. generated near the surface of the pigment particles.