Use of a new organic complexing and buffer agent for Zn(S,O) deposition for high-efficiency Cu(In,Ga)Se₂-based solar cells

Chemical-bath-deposited (CBD) Zn(S,O) is one of the most studied and promisingCd-free buffer layers forCu(In,Ga)Se2 (CIGSe)-based solar cells and has already demonstrated its potential to lead to high-efficiency solar cells. However, CBD Zn(S,O) presents some inconveniences compared with a classic CBD cadmium sulfide (CdS) buffer. Indeed, Zn(S,O) deposition time, important ammonia concentration, and metastable behavior of the final devices are obstacle to its generalized use. A new complexing agent, i.e., morpholine, has been mixed to ammonia as a buffer and complexing agent. From a theoretical study, the mix of these leads to the formation of a majority zinc-morpholine complex instead of the classic zinc-ammonia one. In situ studies have shown an increase in the deposition rate, allowing us to reduce the concentration of the other reactants. Scanning electron microscopy and X-ray photoelectron spectroscopy measurements have shown that the deposited material presents equivalent covering properties and sulfur incorporation ([S]/([S] + [O]) ratio) to the one determined using standard formulations. Finally, CIGSe-based devices have been realized using the optimized CBD Zn(S,O) as a buffer layer. Higher efficiencies than CdS-buffered reference aremeasured, and no metastability is observed.