Researchers from EPFL, Tianjin University, Nanjing Tech University, The University of Tokyo, Shanghai University and Toyota Motor Corporation have used ionic liquids (ILs) with halide anions as additives to improve the performance and stability of a perovskite solar cell.

Ionic liquid-based perovskite solar cell with 22.86% efficiency imageImage from study in Cell Reports Physical Science

Ionic liquids are viewed as a "greener" alternative to organic solvents due to their lower volatility and flammability, as well as to their wide liquid-state window.

The team's work built on previous research showing that ILs doping containing iodide anions or cations with fluorous chains did not influence the overall 3D structure of perovskite films, with the stability of the solar cells being increased by the formation of the hydrophobic surface.

“As ILs are gaining increased popularity as additives for perovskite solar cells because of their attractive physical properties, such as non-volatility and high thermal and electrochemical stability, the chemical mechanism of the passivation process is an urgent issue that has been long overdue in being tackled,” the scientists said.

Three different ILs, ([C1CNmim]Cl, [C3CNmim]Cl, and [(C3CN)2im]Cl), were paired with a chloride anion and utilized as additives for a perovskite precursor solution known as (Cs0.08FA0.8MA0.12)Pb(I0.88Br0.12)3. Chloride salts were used in the chloride anion to stabilize the perovskite crystal lattice. The three resulting perovskite films, according to the researchers, exhibited improved uniformity and clear grain morphology with bigger crystal size compared to reference films without ILs.

The films were integrated in a solar cell architecture consisting of a a fluorine-doped tin oxide (FTO) substrate, a titanium dioxide (TiO) layer, a Tin(IV) oxide (SnO2) layer, the perovskite layer, a hole transporting material made of phenethylammonium iodide (PEAI), and a layer made of gold (Au).

“For all PSC devices, the perovskite absorber layer was prepared via a one-step spin-coating method,” the scientists explained. “All of the perovskite films were treated with PEAI prior to the deposition of the HTMs, a recent but widely used technique that results in increased performance.”

The cells were tested under standard illumination conditions and the device based on [C3CNim]Cl achieved the highest power conversion efficiency of 22.86%, an open-circuit voltage of 1.16 V, a short-circuit current of 24.03 (mA cm−2), and a fill factor of 0.818. The reference cell without ILs, by comparison, achieved an efficiency of 20.94%, an open-circuit voltage of 1.09 V, a short-circuit current of 24.31 of mA cm−2, and a fill factor of 0.793. The other two cells fabricated with [C1CNim]Cl and [(C3CN)2im]Cl achieved efficiencies of 21.82% and 22.45, respectively.

With the best performing cell, the Swiss-Japanese team fabricated a mini module with a size of 7 cm × 6.5 cm that reportedly showed an efficiency of 20.9%, which compares to 18.54% in a reference module without the IL-based device. The panel also showed remarkable stability, with around 95% of its initial efficiency being retained after 1,000 hours of operation.