The electronic structure of the [Zn(S,O)/ZnS]/CuInS2 heterointerface – Impact of post-annealing

Origin of Interface Limitation in Zn(O,S)/CuInS2-Based Solar Cells

Copper indium disulfide (CuInS₂) grown under Cu-rich conditions exhibits high optical quality but suffers predominantly from charge carrier interface recombination, resulting in poor solar cell performance. An unfavorable "cliff"-like conduction band alignment at the buffer/CuInS₂ interface could be a possible cause of enhanced interface recombination in the device. In this work, we exploit direct and inverse photoelectron spectroscopy together with electrical characterization to investigate the cause of interface recombination in chemical bath-deposited Zn(O,S)/co-evaporated CuInS₂-based devices. Temperature-dependent current-voltage analyses indeed reveal an activation energy of the dominant charge carrier recombination path, considerably smaller than the absorber bulk band gap, confirming the dominant recombination channel to be present at the Zn(O,S)/CuInS₂ interface. However, photoelectron spectroscopy measurements indicate a small (0.1 eV) "spike"-like conduction band offset at the Zn(O,S)/CuInS₂ interface, excluding an unfavorable energy-level alignment to be the prominent cause for strong interface recombination. The observed band bending upon interface formation also suggests Fermi-level pinning not to be the main reason, leaving near-interface defects (as recently observed in Cu-rich CuInSe₂) as the likely reason for the performance-limiting interface recombination.

Soft X-ray Spectroscopy of a Complex Heterojunction in High-Efficiency Thin-Film Photovoltaics: Intermixing and Zn Speciation at the Zn(O,S)/Cu(In,Ga)Se2 Interface

The chemical structure of the Zn(O,S)/Cu(In,Ga)Se₂ interface in high-efficiency photovoltaic devices is investigated using X-ray photoelectron and Auger electron spectroscopy, as well as soft X-ray emission spectroscopy. We find that the Ga/(Ga+In) ratio at the absorber surface does not change with the formation of the Zn(O,S)/Cu(In,Ga)Se₂ interface. Furthermore, we find evidence for Zn in multiple bonding environments, including ZnS, ZnO, Zn(OH)₂, and ZnSe. We also observe dehydrogenation of the Zn(O,S) buffer layer after Ar⁺ ion treatment. Similar to high-efficiency CdS/Cu(In,Ga)Se₂ devices, intermixing occurs at the interface, with diffusion of Se into the buffer, and the formation of S-In and/or S-Ga bonds at or close to the interface.

Study of band structure at the Zn(S,O,OH)/Cu(In,Ga)Se2 interface via rapid thermal annealing and their effect on the photovoltaic properties

This study focused on understanding the mechanisms of the photovoltaic property changes in Zn(S,O,OH)/Cu(In,Ga)Se2 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 °C, the Zn-OH bonds in the film decreased. The band gap value of the annealed film decreased and the valence band offset (VBO) 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 (JSC) 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 °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 CIGS bulk. Accordingly, the series resistance (RS) and efficiency of the cell increased.