Effects of Alkali Elements on Copper Indium Gallium Aluminum Selenide Flexible Solar Cells Fabricated on Polyimide Substrates

Polycrystalline Cu(InGaAl)Se (CIGAS) thin films were prepared on polyimide (PI) foils by depositing aluminum (Al) and CIGS precursor layers. Three ceramic CIGS quaternary targets with different sodium (Na) contents were used for investigating the influences of alkali doping at an annealing temperature of 500 °C. The Al concentration was enriched at the front interfaces of absorber films with different Na doping amounts after annealing. Na in the precursor layer diffused to both interfaces during the annealing process, most Na diffused into the Mo layer, and Na existed in the annealed film as compounds Na Se and Na SeO . An appropriate amount Na element could be beneficial for grain growth in the region beneath the surface. Low Na doping had no significant effect on the crystallization property. High Na doping effected the diffusion of the Cu Se liquid phase and reduced the grain size. On the basis of good crystallization, the passivation effect of Na can effectively improve the performance of cells. A certified power conversion efficiency of 16.19% of a CIGAS flexible solar cell with a 0.54 cm active area has been achieved.