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.