Impact of annealing on Cu(In,Ga)Se2 solar cells with Zn(O,S)/(Zn,Mg)O buffers

The influence of annealing in air on high-quality Cu(In,Ga)Se2 (CIGS) thin-film solar cells with Zn(O,S)/(Zn,Mg)O buffers is investigated. We have grown CIGS with an in-line co-evaporation process and the Zn(O,S) buffer layer by chemical bath deposition (CBD). After post-annealing in air, the open-circuit voltage VOC of our cells with CBD Zn(O,S) buffer increases significantly. The enhanced VOC agrees with an increased acceptor density and a reduced space charge width extracted from capacitance-voltage measurements. This finding correlates with a higher sodium concentration after air annealing as measured with secondary ion mass spectrometry (SIMS) at the CIGS/buffer interface and in the Zn(O,S) layer itself. Furthermore, we could detect a decrease of the S/(S + O) ratio in the Zn(O,S) film with sputtered neutral mass spectrometry after different tempering steps. The reduced S/(S + O) ratio implies a decrease in the bandgap energy Eg of Zn(O,S). A reduction of Eg was also extracted from transmission measurements for Zn(O,S) films on quartz glass substrates. This result could explain the observed enhanced short-circuit current density after annealing due to an optimized conduction band offset. In addition, an in-diffusion of zinc from the Zn(O,S) buffer into the CIGS absorber after annealing was detected with SIMS.

► Annealing of Cu(In,Ga)Se2/Zn(O,S)/(Zn,Mg)O solar cells was investigated.
► The Zn(O,S) buffer layer was solution grown.
► The bandgap energy of Zn(O,S) decreases due to oxygen incorporation.
► Higher open-circuit voltage can be explained by increased sodium at the interface.
► Zinc diffuses from Zn(O,S) buffer into Cu(In,Ga)Se2 absorber.