Charge carrier photogeneration, trapping, and space-charge field formation in PVK-based photorefractive materials

Abstract

We studied the dark conductivity ${(j}_{\mathrm{dark}}),$ the photoconductivity ${(j}_{\mathrm{photo}}),$ and the charge carrier photogeneration efficiency \ensuremath{\eta} of poly(N-vinylcarbazole)--based photorefractive (PR) materials with different glass-transition temperatures ${(T}_{g})$ and chromophore content $({\ensuremath{\rho}}_{\mathrm{CHR}}).$ Measurements were carried out at wavelengths similar to those used in degenerate four-wave mixing (DFWM) and two-beam coupling (2BC) experiments. Both thick (37 \ensuremath{\mu}m) and thin samples (\ensuremath{\approx}1 \ensuremath{\mu}m) were analyzed. Photoconductivity experiments at different temperatures show that both ${j}_{\mathrm{dark}}$ and ${j}_{\mathrm{photo}}$ are thermally activated. For ${j}_{\mathrm{dark}}$ the activation is not related to the glass-transition temperature of the blends, whereas photocurrents exhibit a universal behavior with respect to ${T}_{r}{=T}_{g}\ensuremath{-}T.$ The charge carrier photogeneration efficiency \ensuremath{\eta} was measured by xerographic discharge experiments. \ensuremath{\eta} was found to be independent of both ${T}_{g}$ and of ${\ensuremath{\rho}}_{\mathrm{CHR}}.$ The photoconductivity gain factor G defined as the number of charge carriers measured in photoconductivity in relation to the number of carriers initially photogenerated as determined by the xerographic experiments is used to compare the results of photoconductivity and xerographic discharge experiments. G is found to be much smaller than unity even for thin samples, which indicates that the mean free path of the photogenerated charge carriers is less than 1 \ensuremath{\mu}m at photoelectrical equilibrium. Using Schildkraut's model for the space-charge field formation in organic PR materials, trap densities ${T}_{i}$ of approximately ${10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ could be derived from G. The field and temperature dependence of ${T}_{i}$ is independent of ${\ensuremath{\rho}}_{\mathrm{CHR}}$ and might account for the universal ${T}_{r}$ dependence of ${j}_{\mathrm{photo}}.$ The estimated trap densities are used to calculate the first-order Fourier component of the space-charge field in the PR materials illuminated with a sinusoidal intensity pattern. Modifying Schildkraut's model so that the tilt between the applied electric field and the index of refraction grating is taken into account yields saturation fields of approximately 100 V/\ensuremath{\mu}m in agreement with findings from PR experiments. The dramatic decrease of the space-charge field when the temperature exceeds the glass-transition temperature of the blend is fully explained by a decrease in trap density. The fact that the trap density depends on the temperature with respect to ${T}_{g}$ and not on the absolute temperature suggests that the relevant traps are most likely of conformational nature.