Correlation between grain composition and charge carrier collection in Cu(In,Ga)Se2 solar cells

Understanding the spatial composition inhomogeneity in CIGS solar cells can provide insight into a potentially large cause of decreased cell efficiency. We used synchrotron based nano x-ray fluorescence and nano x-ray beam induduced current to investigate the correlation between grain-to-grain compositional variations in Cu(In,Ga)Se₂ absorber layers and variations in carrier collection efficiency. We found compositional variations as large as 5 at. % and current variations as large as 10%. Grains with higher average copper concentration and lower indium concentration showed increased carrier collection than neighboring grains with lower copper and higher indium concentration. We also investigated the overall composition dependence of carrier collection in CIGS films and show that 30% gallium content devices show a correlation between charge carrier collection, while 60% content devices do not.