Engineering CIGS grains qualities to achieve high efficiency in ultrathin Cu(InxGa1-x)Se2 solar cells with a single-gradient band gap profile

Reducing the Cu(InxGa1-x)Se-2 (CIGS) thickness is an effective way to reduce the material use and increase manufacturing throughput. However, it is still a challenge to obtain high efficiency in the ultrathin CIGS solar cell. Here, the CIGS solar cell with a 1.3 mu m-thickness-CIGS was synthesized via a three-stage co-evaporation method. The obtained CIGS solar cells were characterized by capacitance-voltage, capacitance-frequency, secondary ion mass spectrometry, X-ray fluorescence, transmission electron microscope, and electron beam induced current techniques. By optimizing the grain size, interface quality, and the Ga gradient in the ultrathin CIGS solar cell, the highest efficiency reached to 11.72% without any light trapping and anti-reflecting coating techniques. Compared with the typical CIGS solar cell with a thickness of 2.3 mu m, the ultrathin CIGS solar cell showed a higher open-circuit voltage due to formation a back electrical field. The grain boundaries were found to be beneficial to the carrier's separation and transport. The ultrathin CIGS solar cell had good ability to resist ion bombardment, suggesting its potential application in the space devices. Our results provide a strategy to achieve high-efficiency ultrathin CIGS solar cells.