The King Abdullah University of Science and Technology and the Fraunhofer Institute for Solar Energy Systems report producing perovskite-silicon tandem solar cells with open-circuit voltages exceeding 1.9 V as the result of a two-step hybrid evaporated/blade-coated process for perovskite films.

Researchers from King Abdullah University of Science and Technology (KAUST) in Saudi Arabia and Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) in Germany have fabricated perovskite-silicon tandem solar cells with an open-circuit voltage of 1.9 V and 27.8% power conversion efficiency using a new two-step hybrid evaporated/blade-coating for applying the perovskite on the silicon sub-cell. 

The development is one of the results of an 18-month collaboration between Fraunhofer ISE and KAUST. “It is the culmination of combining Fraunhofer ISE’s extensive expertise in the hybrid evaporation/wet-chemical method with KAUST’s experience in the upscaling of perovskite deposition via blade-coating,” the research’s lead author, Er-Raji told pv magazine.

In the paper “Coating dynamics in two-step hybrid evaporated/blade-coated perovskites for scalable fully-textured perovskite/silicon tandem solar cells,” published in EES Solar, the research team outlined a lab method based on hybrid evaporation/spin-coating with a new two-step hybrid evaporation/blade-coating approach on thin films, single junction and perovskite-silicon tandem solar cells, reporting the experimental results along with theoretical considerations, including the analysis of the influence of fluid mechanisms involved in the blade-coating process.

“Combining experimental results with theoretical considerations on meniscus formation, we comprehensively analyze the influence of fluid mechanisms involved in the blade-coating process and find that the final perovskite film properties can be controlled through two main properties: wet film thickness and solvent’s evaporation rate,” the researchers said.

The learnings about the dynamics during blade-coating could eventually be transferred by the researchers to the even more scalable slot-die-coating process, according to Juliane Borchert, leader of the Perovskite Materials and Interfaces group at Fraunhofer ISE in a statement.

“A key novelty of this work was the identification of a direct correlation between the blade speed and the perovskite conversion rate – a crucial parameter in the hybrid method that quantifies the transformation of evaporated inorganic material into the perovskite photoactive phase,” said Er-Raji.

“Our observations revealed that when the coating speed was lower than the solvent evaporation rate, rapid solvent evaporation compromised the conversion process. Conversely, in the Landau-Levich regime, the conservation of a wet film allowed full infiltration of organic precursors into the inorganic scaffold, ensuring complete conversion,” he added. The Landau–Levich equation is widely used in solar research to predict the thickness of wet layers deposited on substrates by dip-coating

The group said it was also able to transfer the use of crystallization additives into the hybrid evaporation/blade-coating process to enhance grain size that it had previously demonstrated without requiring adjustments in concentration or annealing recipe, according to Er-Raji.

The process reportedly enabled a “solution volume that was eight times lower than that used in the hybrid evaporation/spin-coating method.”

“Finally, we conducted the first outdoor stability test for a tandem solar cell using scalable perovskite film fabrication on silicon bottom cells with industry-relevant texturing. The one-month operational stability assessment indicated the need for a more robust perovskite bulk quality, which we aim to improve through compositional engineering in future studies,” said Er-Raji.

Looking ahead at future research along these lines, Er-raj said, “Now that we have gained a deeper understanding of the coating dynamics, we will leverage these insights to refine the perovskite composition, aiming to enhance the operational stability of our devices.”  Further work is planned on interfacial passivation to minimize defect-induced non-radiative recombination and extend the stability, as well as scalable wet-chemical approaches, ink-jet printing and spray-coating processes.

The research team also included researchers from the University of Freiburg,