The Physics of Twin Boundary Termination in Cu(In, Ga)Se2 Absorbers

The grain boundaries (GBs) in the absorber of a Cu(In,Ga)(S,Se)2 (CIGS) solar cell play a vital role in its efficiency and cells with polycrystalline absorbers exhibit high conversion efficiency (>23%). Previous investigations confirm that the traits of GBs in CIGS are directly connected to their composition. However, such a relationship cannot be established for twin boundaries (TBs). This is because although electron beam-induced current (EBIC) highlights the existence of both electrically inactive and active TBs, atom probe tomography is not able to detect composition fluctuations in a very limited volume (one or two monolayers). Therefore, herein, high-resolution scanning transmission electron microscopy at TBs to investigate their differences and correlate them with their traits is used. It is found that the electrically neutral TBs are cation–anion-terminated boundaries, whereas the electrically beneficial TBs are cation–cation terminated. Density functional theory results show that the formation of point defects next to cation–cation TBs is more favorable compared to the case with cation–anion TBs. The presence of Cu vacancies can result in a passivated TB and a hole-depletion region next to the cation–cation TBs, and consequently a better electron transport, as the bright contrast observed in the EBIC map suggests.