Slot-die coated perovskite PV cell with precursor ink hits 22.54% efficiency

Scientists at Germany's Helmholtz-Zentrum Berlin für Materialien und Energie GmbH have developed a new precursor ink technique to improve film quality in halide perovskite solar cells. In the study “Ink Design Enabling Slot-Die Coated Perovskite Solar Cells with >22% Power Conversion Efficiency, Micro-Modules, and 1 Year of Outdoor Performance Evaluation,” which was recently published in Advanced Energy Materials, the researchers said that they used a formamidinium lead triiodide (FAPbI₃) precursor ink based on 2-methoxyethanol in the slot-die coating process.

Slot-die is a coating process used to deposit thin and uniform films with limited material waste and low operational costs.

“The slot-die coating is one of the most promising processes enabling sheet-to-sheet, continuous roll-to-roll coating, and solution-processable optoelectronic device technology such as organic electronic devices and polymer solar cells,” the team said, noting the industry commonly uses spin-coating for perovskite cell production.

The researchers said the new precursor ink method is aimed at preventing the so-called ribbing effects that usually arise in perovskite solar devices with slot-die and cause irregular shapes in the deposited wet perovskite films.

“Ribbing is a common effect observed at the downstream meniscus when the viscous and capillary forces between ink, slot-die, and substrate are unbalanced,” they stated.

They adjusted the ink viscosity via acetonitrile (ACN) with a 46% concentration. The compound acted as a co-solvent for the FAPbI₃ thin-film and improved its quality and layer homogeneity.

The German group utilized this technique in a small-area solar cell, which achieved a power conversion efficiency of 22.54%, which it said is the highest certified value for a slot-die-coated perovskite solar cell to date. The device also reached an open-circuit voltage of 1.088 V, a short-circuit current of 24.9 mA cm², and a fill factor of 83.1%. Germany's Fraunhofer ISE CalLab confirmed this result.

The team also built mini-modules with active areas of 12.7 cm² and an efficiency of 17.1%.

“A full year of outdoor stability testing with continuous maximum power point tracking on encapsulated devices is performed and it is demonstrated that these devices maintain close to 100% of their initial performance during winter and spring followed by a significant performance decline during warmer summer months,” said the researchers.

Looking forward, the academics plan to scale the technology to larger device areas and assess the performance under different long-term exposure conditions.