Influence of the dispersivity of charge transport on the holographic properties of organic photorefractive materials

The holographic properties of the photorefractive polymer composite PSX:DMN PAA:TNF are determined as a function of the grating spacing by use of two-b eam coupling and degenerate four-wave mixing measurements. The phase shift was quantified with the grating translation technique. In particular, an al most constant mean drift length of the charge carriers of about 0.5 mu m wa s obtained. The resulting discrepancies between the standard theory of crys tals, which is generally used as an approximation for amorphous organic mat erials, and the experimental data are discussed and attributed to the diffe rent mechanisms of charge transport in amorphous organic materials. To veri fy our conclusions we performed time-of-flight and degenerate four-wave mix ing measurements on the two photorefractive organic glasses DRDCTA:DOP:C-60 and DRDCTA:EHMPA:C-60, which differ in the degree of dispersivity of charg e carrier transport and, hence, the involved trap energy distributions. Com paring the behavior of the two materials under investigation, we obtained a qualitative relationship between dispersivity and a theoretically unexpect ed drop in the diffraction efficiency: The larger the dispersivity the earl ier the drop in the efficiency. (C) 1999 American Institute of Physics. [S0 021-9606(99)01747-X].