Engineering a Ga-Gradient by One-Step Sputtering to Achieve Over 15% Efficiency of Cu(In,Ga)Se Flexible Solar Cells without Post-selenization.

A one-step sputtering process using a quaternary target has been demonstrated to be a simple route to form Cu(In,Ga)Se (CIGSe) absorber without post-selenization; however, the lack of a Ga-grading structure in the CIGSe absorber confines its efficiency. Here, we demonstrate a one-step cosputtering process to control the Ga profile in the CIGSe absorber on flexible stainless steel substrates. Special attention was paid to the formation of second phases and their effects on the cell performance. Although the normal Ga-grading and efficiency enhancement could be achieved by cosputtering of CIGSe and GaSe targets, high-energy ion bombardment during the sputtering process might cause the decomposition of the GaSe target, leading to the formation of GaO in the CIGSe absorber, which gradually degraded the device performance. We replaced the GaSe target with a stoichiometric CuGaSe target for cosputtering, which can further enhance the cell efficiency due to the elimination of GaO. However, when the Ga content at the back side of CIGSe is further increased by raising the deposition power of the CuGaSe target, the phase separation of CuGaSe may take place, resulting in the formation of CuSe and CuGaSe at the back side of the CIGSe absorber; therefore, the recombination at the back side is increased. By cosputtering a CIGSe target with a Cu-deficient CuGaSe target, we can suppress the formation of second phases and achieve designable normal grading, leading to the highest efficiency of 15.63% without post-selenization on flexible substrates.