Grain boundary barrier model can explain the beneficial effect of alkali doping in Cu(In,Ga)Se2 solar cells

Although the beneficial effect of alkali doping of Cu(In,Ga)Se₂ has been known for decades, there is still no agreement on its precise physical pathway. In this work we present a case for this effect being linked to the alkali-induced passivation of barriers at the grain boundaries (GBs). In this model, postulated earlier by, among all, C-S. Jiang and U. Rau, donor defects at the GBs result in downward band bending, creating energy barriers for holes and thus reducing the intergrain mobility, at the same time leading to the creation of depleted regions around GBs, decreasing apparent doping concentration. The effect of alkali doping would be through passivation of those donor defects, increasing both mobility and doping concentration.

Results of our systematic study on Cu(In,Ga)Se₂ thin films and solar cells doped with different concentrations of alkali metals (Na and K) point to the alkali effect leading to a simultaneous increase of both free hole concentration and hole mobility, irrespective of the type of alkali used. Additionally, the activation energy of conductivity – linked to the GB barrier height – decreased with an increase in alkali concentration. All of the above results are consistent with the grain boundary passivation model. To further test this hypothesis, experimental results were compared with SCAPS simulations of a multigrain CIGS thin film with varied concentration of donor defects located at the GBs. These simulations were in good quantitative agreement with experimental results with regards to conductivity, free hole concentration and GB barrier height.