Study of Band Structure at the Zn(S,O,OH)/Cu(In,Ga)Se-2 Interface via Rapid Thermal Annealing and Their Effect on the Photovoltaic Properties

Abstract

This study focused on understanding the mechanisms of the photovoltaic property changes in Zn(S,O,OH)/ Cu(In,Ga)Se-2 solar cells, which were fabricated via annealing, using reflection electron energy loss spectroscopy (REELS), ultraviolet photoelectron spectroscopy (UPS), low temperature photoluminescence (LTPL), and secondary ion mass spectroscopy (SIMS). A pinhole-free Zn(S,O,OH) buffer layer was grown on a CIGS absorber layer using the chemical bath deposition (CBD). When the Zn(S,O,OH) film was annealed until 200 degrees C, the Zn OH bonds in the film decreased. The band gap value of the annealed film decreased and the valence band offset (CBO) value at the Zn(S,O,OH)/CIGS interface with the annealed film increased. Both results contribute to the conduction band offset (CBO) value at the Zn(S,O,OH)/CIGS interface and, in turn, yield a reduction in the energy barrier at the interface. As.a result of the annealing, the short circuit current (J(SC)) and quantum efficiency (QE) values (400-600 nm) of the cell increased due to the improvement in the electron injection efficiency. However, when the Zn(S,O,OH) film was annealed at 300 degrees C, the cell efficiency declined sharply due to the QE loss in the long wavelength region (800-1100 nm). The SIMS analysis demonstrated that the Cu content in the CIGS bulk decreased and the Cu element also diffused into CIGS/Mo interface. Through LTPL analysis, it was seen that the considerable drop of the Cu content in the CIGS bulk induced a 1.15 eV PL peak, which was associated with the transition from a deep donor defect to degrade the quality of the GIGS, bulk. Accordingly, the series.resistance (R-S) and efficiency of the cell increased.