Fig. 1. Schematic diagrams of Na incorporation pathways: (a) pre-deposition, (b) co-deposition, (c) post-deposition on alkali-free substrate.

Fig. 2. The observed changes of CIGS material properties due to Na incorporation: (A) enhanced carrier density (via Na doping or grain boundary passivation), (B) gallium (Ga) segregation, (C) changes in crystallographic orientation. (Reproduced with permission [69]. Copyright 2012, Institute of Electrical and Electronics (IEEE)).

Fig. 3. The modification of CIGS surface induced by KF PDT leads to an improved heterojunction quality between CIGS and CdS in a shorter CdS deposition time, allowing a significant gain in cell efficiency from 18.7 % to 20.4 %. (Reproduced with permission [109]. Copyright 2015, Institute of Electrical and Electronics (IEEE)).

Fig. 4. Schematic of diffusion pathways of Na and Rb by NaF- and RbF-PDT in CIGS absorbers: (a) diffusion of Rb into Na-free CIGS absorber along grain boundaries as well as grain interiors, (b) diffusion of Rb into Na-doped CIGS absorber; here, Rb tends to diffuse into grain boundaries instead of grain interiors compared to (a), (c) diffusion of Na into Rb-free CIGS absorber along grain boundaries as well as grain interiors, and (d) diffusion of Na into Rb-doped CIGS absorber; here, diffusion of Na into grain boundaries is hindered by Rb and therefore Na tends to diffuse into the grain bulk instead of grain boundaries compared to (c). (Reproduced with permission [120]. Copyright 2018, American Institute of Physics (AIP).).

Fig. 5. Schematic diagrams of Cs diffusion pathways and possible mechanisms in CIGS absorbers evaporated on Na-contained SLG substrates under different situations: (a) without CsF-PDT, (b) and (c) with CsF-PDT. (Reproduced with permission [14]. Copyright 2020, Elsevier B.V.).

Fig. 6. Schematic diagram of the ion-exchange mechanism induced by heavier alkali elements and the possible formation of secondary alkali-In-Se2 (AlkInSe2) compounds and charged defects at GBs. (Reproduced with permission [129]. Copyright 2019, Springer Nature).

Fig. 7. Phase diagrams for (a) Cu - In - Se, (b) Li - In - Se, (c) Na - In - Se, (d) K - In - Se, (e) Rb - In - Se, and (f) Cs - In - Se systems. (Reproduced with permission [21]. Copyright 2017, American Chemical Society (ACS)).