Persistent photoconductivity in Cu(In,Ga)Se2 heterojunctions and thin films prepared by sequential deposition

Abstract

A characteristic, reversible metastability is observed for ${\mathrm{Cu(In,Ga)Se}}_{\mathrm{2}}$ thin films and ${\mathrm{ZnO/CdS/Cu(In,Ga)Se}}_{\mathrm{2}}$ heterojunctions. Annealing at 80 °C leads to a decrease of the dark conductivity of the thin films by up to a factor of 2 at room temperature and several orders of magnitude when measured at lower temperatures. By exposure to light, the initial state can be re-established. This reenhancement of the dark conductivity can be looked at as persistent photoconductivity. Admittance measurements at ${\mathrm{Cu(In,Ga)Se}}_{\mathrm{2}}$ heterojunction solar cells display a reversible shift of the activation energy of a distinct dielectric loss peak ranging from 70 to about 160 meV upon illumination or annealing at 80 °C, respectively. We propose that both phenomena as well as commonly observed light-soaking effects of ${\mathrm{Cu(In,Ga)Se}}_{\mathrm{2}}$ solar cells have a common origin.