Researchers in China have developed a four-junction tandem perovskite/CIGS cell. They claim it is the most efficient device of its kind with an n-i-p structure thus far. They used a novel surface passivation strategy to improve the perovskite cell efficiency and fill factor.

Researchers from the Wuhan University in China have fabricated a four-junction tandem (4T) solar cell based on perovskite and copper, indium, gallium and selenium (CIGS), through a novel surface passivation technique that uses guanidine bromide (GABr).

They tested GABr in mixed solvents combining isopropyl alcohol (IPA) and toluene (TL), which they said can efficiently passivate interface and grain boundary defects by minimizing the IPA solubility of the perovskite surface. They compared the mixing of IPA with ethyl acetate (EA), chlorobenzene (CB), and toluene (TL) to dissolve GABr, and further optimized the concentration of GABr and the mixing ratio of the two solvents.

“Our experiment discovered that when IPA and TL were mixed at a concentration of 2 mg mL−1 and dissolved at a ratio of 1:1, the mixed solvents assisted treatment may not only minimize perovskite surface solubility, but also efficiently passivate interface and grain boundary defects, reduce non-radiative recombination of the film surface, and avoid poor heterogeneous carrier transmission,” they said.

This passivation strategy was used to build the perovskite cell for the 4T device. The cell achieved a power conversion efficiency of 22.7%, an open-circuit voltage of 90 mV, a short-circuit current density of 24.2 mA cm–2, and a fill factor of 80.6%. For comparison, a reference cell was fabricated without the novel passivation technique, and it achieved an efficiency of 20.1% and a fill factor of 77.9%.

By replacing the metallic back electrode with indium tin oxide (ITO) transparent back contact based on molybdenum oxides (MoOx), the scientists built a semitransparent perovskite solar cell with an efficiency of 18.3%. The cell was then combined with a 17.5% CIGS cell in a 4T device with an n-i-p structure.

“Impressively, this is the most efficient 4T perovskite/CIGS tandem solar cell of the n-i-p structure ever reported,” they said. “The mixed solvent passivation strategy demonstrated here, hopefully, will open new avenues for improving perovskite solar cells’ efficiency and stability.”

They presented their findings in “Mixed Solvents Assisted Post-Treatment Enables High-Efficiency Single-Junction Perovskite and 4T Perovskite/CIGS Tandem Solar Cells,” which was recently published in Advanced Science.