Group III Elemental Composition Dependence of RbF Postdeposition Treatment Effects on Cu(In,Ga)Se₂ Thin Films and Solar Cells

Abstract

The effects of RbF postdeposition treatment (RbF-PDT) on Cu­(In,Ga)­Se2, CuInSe2, and CuGaSe2 thin films and solar cell devices are comparatively studied. Similar to the effect of the KF postdeposition treatment (KF-PDT), Cu­(In,Ga)­Se2 and CuInSe2 film surfaces show significant pore formation resulting in a rough surface morphology with RbF-PDT, whereas this is not the case for In-free CuGaSe2. The device properties of the In-containing and In-free Cu­(In,Ga)­Se2 solar cells also show contrasting results, namely, Cu­(In,Ga)­Se2 or CuInSe2 devices show an increase in the open circuit voltage (V oc) and fill factor (FF) values and almost constant or a slight decrease in the short-circuit current density (J sc) values with RbF-PDT, whereas CuGaSe2 devices show no significant improvements in the V oc and FF values but a substantial increase in the J sc values. These results suggest that the alkali effects on the Cu­(In,Ga)­Se2 film and device properties strongly depend on the group III elemental composition in the Cu­(In,Ga)­Se2 films as well as alkali-metal species.

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“…CIGSe (and CISe) films showed pore‐formation in the surface region and device properties showed improvements in V oc and FF values upon RbF‐PDT . In contrast, the surface morphology of CGSe films showed no prominent pore formation upon RbF‐PDT when the amount of RbF supplied was small, although when a large amount of RbF is supplied a rough surface morphology develops but the surface modification is somewhat different from that observed in CIGSe and CISe films as shown in Figure . Namely, the size of pores formed in the CGSe surface region is much smaller than that in the CISe and CIGSe surfaces.…”

“…HLS treatment is generally known to increase the carrier concentration in CIGSe and decrease the depletion width . On the other hand, CGSe cells showed the opposite behavior to In‐containing CIGSe cells, namely, a decrease in the carrier concentration and an increase in the depletion width upon HLS treatment . Although the detailed mechanism behind this result require further investigation, differences in the defect formation physics in CISe and CGSe may be the chief reason.…”

“…The pore formation observed in CIGSe (CISe) film surfaces upon RbF‐ or KF‐PDT is attributable to alkali‐In‐Se compound phase formation in the surface region . The formation of the RbInSe 2 compound phase in CIGSe and CISe film surfaces has been directly observed by transmission electron microscopy measurements .…”

“…GGI indicates [Ga]/([Ga] + [In]) composition ratio. Detailed film growth and device fabrication process conditions can be found elsewhere . All films were grown by the three‐stage process at a substrate temperature of 350 and 550 °C during the first, and second and third stages, respectively.…”

“…This result is attributable to an increase in the depletion width in the CGSe cell upon RbF‐PDT. It should be noted here that heat‐light soaking (HLS) treatment, which is one of the techniques that effectively activates metastable acceptors in CIGSe, leads to further improvements in the V oc and FF values of CIGSe cells grown with RbF‐PDT . HLS treatment is generally known to increase the carrier concentration in CIGSe and decrease the depletion width .…”