Diffusion of Alkali Metals in Polycrystalline CuInSe₂ and Their Role in the Passivation of Grain Boundaries

Abstract

The behavior of alkali atom point defects in polycrystalline CuInSe2 is studied. In this work, three grain boundary models, one coherent twin boundary and two twin boundaries with dislocation cores, are considered. Total energy calculations show that all alkali metals tend to segregate at the grain boundaries. In addition, the segregation of alkali atoms is more pronounced at the grain boundaries with the dislocation cores. The diffusion of alkali metals along and near grain boundaries is studied as well. The results show that the diffusion of alkali atoms in the grain boundary models is faster than within the bulk. In addition, the ion exchange between Na and Rb atoms at the grain boundaries leads to the Rb enrichment at the grain boundaries and the increase of the Na concentration in the bulk. While the effects of Na and Rb point defects on the electronic structure of the grain boundary with the anion-core dislocation are similar, Rb atoms passivate the grain boundary with the cation-core dislocation more effectively than Na. This can explain the further improvement of the solar cell performance after the RbF-postdeposition treatment.

Alkali Atoms Diffusion Mechanism in CuInSe2 Explained by Kinetic Monte Carlo Simulations

“…[60] This show that the high efficiency of solar cell after the PDT could be due to the combined effect of both lighter and heavier AM atoms. From these simulations, it is evident that the diffusion of alkali atoms in the CISe lattice depends primarily on their atomic radii.…”

“…The positive effect of heavier AM atoms could be due to the passivation of GBs or improving the electronic structure of interfaces. [60] This show that the high efficiency of solar cell after the PDT could be due to the combined effect of both lighter and heavier AM atoms.…”

“…For Na, the diffusion of an interstitial atom has a comparable energy barrier (0.69 eV) to the Na-Cu ion exchange (0.59 eV), whereas the diffusion of a Rb i atom has a larger migration barrier (1.2 eV) compared to the Rb-Cu ion-exchange mechanism (0 eV). [60] By decreasing the temperature, the solubility of external atoms in CISe decreases. The differ- ences between the diffusion mechanisms of Na and Rb atoms can explain the higher concentration of Na atoms in the GI compared to Rb atoms.…”

“…[40,42,43] Our formation energy calculations also showed the tendency of AM atoms to segregate close to GBs. [60] By decreasing the temperature, the solubility of external atoms in CISe decreases. At the same time the concentration of intrinsic defects also decreases.…”

Grain Boundaries in Cu(In, Ga)Se2: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

“…This trend was recently also found in DFT calculations, where the density of states in the bandgap at GBs was found to be strongly reduced by alkali elements at the GBs, and the more, the heavier the alkali element was. [ 179 ] The strongest passivation was found when placing Cs at the GB. [ 180 ] For the recorded cell with 23.35% with a CsF PDT, the V oc loss was even reduced to 350 mV.…”