Spectral dependence and Hall effect of persistent photoconductivity in polycrystalline Cu(In,Ga)Se2 thin films

We investigate persistent photoconductivity in polycrystalline $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ thin films by measuring the transient behavior of their electrical conductivity under and after illumination. Characteristic nonexponential transients of the conductivity under and after illumination extend over more than four orders of magnitude in time. From this result, we conclude that the persistent photoconductivity in $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ cannot be explained by the mesoscopic barrier model. Rather, it is a microscopic effect which can be understood in the frame of a large lattice relaxation model. Investigation of the conductivity under monochromatic subband gap illumination determines the minimum photon energy for the direct excitation of the persistent photoconductivity to be approximately 0.6 eV. Hall effect measurements identify the persistent photoconductivity as a hole density effect in the bulk of the $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ grains, consistent with the lattice relaxation model.