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

Abstract

A characteristic, reversible metastability is observed for Cu(In,Ga)Se2 thin films and ZnO/CdS/Cu(In,Ga)Se2 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 Cu(In,Ga)Se2 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 Cu(In,Ga)Se2 solar cells have a common origin.

Light soaking effect on photocurrent collection in (Zn,Mg)O/Cu(In,Ga)Se2 solar cells

“…In our recent preliminary experiments, a prominent increase in the carrier density of the ZMO film which affects the band diagram of the device was not observed after the LS. Experimental findings for later two mechanisms originated by the persistent photoconductivity of CIGS [10] and by Cu migration [11], respectively, reported by other research group supports the model presented in this paper. Thus, the CBO is not the origin of the LS effect but the trigger for the metastable behavior.…”

Anion vacancies as a source of persistent photoconductivity in II-VI and chalcopyrite semiconductors

“…Note that the initial ionized charge state here is different from the initial neutral state required in ZnO. Illumination [19,20,21,22,23] or electron injection [68] of such CuInSe [69]. Immediately after the optical excitation, the electron in the state occupies the shallow PHS rather than the DLS (viz.…”

Development of thin‐film Cu(In,Ga)Se2 and CdTe solar cells

“…[50][51][52] For the preparation of solar cells only slightly Cu-deficient compositions of p-type conductivity are suited. 53,54 Depending on the [Ga]/[In þ Ga] ratio, the bandgap of CIGS can be varied continuously between 1Á04 and 1Á68 eV, (Figure 3). The current high-efficiency devices are prepared with bandgaps in the range 1Á20-1Á25 eV, this corresponds to a [Ga]/[In þ Ga] ratio between 25 and 30%.…”