Researchers from the Chinese Academy of Sciences (CAS) have developed a tandem solar cell based on a top perovskite solar cell and a bottom device relying on copper, indium, gallium and selenium (CIGS) using a coordination-engineering approach designed to improve the stability of the wide-bandgap perovskite material.
They used, in particular, an organic sulfur-containing compound known as bis(2-pyridylmethyl) sulfide (2PyS) to fine-tune the local coordination environment of lead (Pb) ions within the perovskite layer. This strategy helped limit defect formation, suppress halide ion migration, and reduce photoinduced phase segregation, addressing key stability challenges in wide-bandgap perovskite absorbers.
“A key origin of this instability is the presence of undercoordinated Pb defects and associated halide vacancies,” they explained. “These defects not only act as recombination centers but also disturb the local lattice environment and provide pathways for halide ion migration. Conventional post-treatment passivation strategies can reduce some defects after crystallization, but they often offer limited control over defect formation during the film growth process. As a result, photoinduced halide segregation remains difficult to suppress during long-term device operation.”
The top perovskite cell was built with an indium tin oxide (ITO) front electrode, a nickel oxide (NiOx) hole transport layer, a self-assembled monolayer (SAM) interfacial layer, a perovskite light-absorbing layer, a C60 fullerene electron transport layer, a tin oxide (SnO₂) buffer layer, an indium tin oxide (ITO) transparent electrode, and a silver (Ag) metal contact.
The subcell consisted of a molybdenum (Mo) rear electrode for charge collection, a CIGS absorber optimized for long-wavelength light harvesting, a cadmium sulfide (CdS) buffer layer for interface passivation, and zinc oxide-based window layers comprising intrinsic zinc oxide (i-ZnO) and aluminum-doped zinc oxide (Al:ZnO) for electron transport and transparent front contact formation.
The standalone CIGS bottom cell demonstrated a power conversion efficiency of 19.65% under full-spectrum illumination. When operated beneath the semi-transparent perovskite top cell, the available incident light was reduced due to spectral filtering, leading to a decrease in the subcell efficiency to 7.5%. As for the top cell, it achieved an efficiency 22.21%, while in a four-terminal (4T) tandem configuration, the integrated tandem device reached an overall efficiency of 29.71%. The devices were also found to retain more than 91% of their initial efficiency after 2000 hours of continuous operation.
This performance is described by the scientists as among the highest efficiencies reported for perovskite/CIGS four-terminal tandem solar cells to date. They also explained that 2PyS binds more strongly to lead iodide (PbI₂) than commonly used solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). This stronger interaction enables 2PyS to more effectively regulate the coordination environment of Pb ions during perovskite formation, thereby influencing the crystallization process.
“Moreover, in situ photoluminescence measurements further revealed that 2PyS modulates crystallization kinetics, suppresses rapid nucleation, and promotes more homogeneous film growth,” the research team concluded. “As a result, the modified perovskite films exhibit improved optoelectronic properties, reduced non-radiative recombination, enhanced structural integrity, and stronger phase stability under coupled illumination and thermal stress.”
The new cell concept was presented in “Coordination-regulated defect suppression enables stable wide-bandgap perovskites for efficient perovskite/CIGS tandem solar cells,” published in IPOScience.
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