No evidence for passivation effects of Na and K at grain boundaries in polycrystalline Cu In,Ga Se2 thin films for solar cells

Thin film solar cells based on Cu In,Ga Se2 absorber layers have reached conversion efficiencies of well above 20 . One key of this success is the incorporation of alkali metals such as Na and K into the surface and the volume of the Cu In,Ga Se2 thin film. The present work discusses the impact of Na and K on the grain boundary GB properties in Cu In,Ga Se2 thin films, i.e., on the barriers for charge carriers, amp; 934;b, and on the recombination velocities at the GBs, sGB. The authors first revise the physics connected with these two quantities as well as their impact on the device performance, and then provide values for the barrier heights and recombination velocities from the literature. The authors measured sGB values by means of cathodoluminescence analysis of Na K free CIGSe layers as well as on CIGSe layers on Mo sapphire substrates which were submitted to only NaF or only KF PDTs. Overall, passivating effects on GBs by neither Na nor K can be confirmed. The GB recombination velocities seem to remain on the same order of magnitude, in average about 103 104 cm s, irrespective of whether Cu In,Ga Se2 thin films are Na K free or Na K containin