Researchers at University of North Carolina at Chapell Hill and University of Rochester have developed a novel hot gas-assisted method that could improve the fabrication of narrow bandgap (NBG) perovskite films for tandem solar cells. This strategy, combined with an anti-oxidation material added in the film, could increase the solar cells' carrier recombination lifetime (i.e., the time it takes for excess charge carriers to decay).
The researchers explained that all-perovskite tandem perovskite solar cells have the potential to reduce the cost of photovoltaic systems, due to their potential to reach a higher efficiency than their single-junction counterparts, while maintaining the solution fabrication processes. They said that compared to single junction perovskite modules, the application of tandem structures, which have much smaller photocurrents but higher photovoltage, can also reduce the cell-to-module efficiency derate and enable the realization of higher module efficiencies for monolithically interconnected modules in a series.
In all-perovskite tandem solar cells, both the wide bandgap (WBG) and narrow bandgap (NBG) perovskite layers are deposited using a method called blade coating which is a scalable coating technique that entails applying an excess of coating material to a substrate and then removing some using a blade, until one reaches the desired coating.
The researchers in the recent work devised a new blade coating strategy that could be very beneficial for the fabrication of NBG perovskite films. In contrast with other typically employed strategies, their technique utilizes a hot gas.
"To create our NBG perovskite films, we developed a hot gas-assisted blade coating strategy to achieve high-quality, large-area, and thick films", said University of North Carolina at Chapel Hill's Jinsong Huang. "The hot gas accelerated the drying of the high boiling point solvents to solidify the as-coated wet film, preventing the microscale solution flows. In addition, a reducing agent benzylhydrazine hydrochloride (BHC) was introduced to prevent Sn2+ and iodide oxidization during film deposition and, more importantly, to withstand the air exposure during module fabrication."
Using their new blade coating method, Huang and his team were able to suppress unfavorable processes occurring during the coating of their NBG perovskite film. The resulting film was then used to create all-perovskite tandem solar cells with a remarkable efficiency of 21.6%, with a 14.3 cm2 aperture area, which corresponded to an active area efficiency of 23%.
"Our innovative hot-gas assisted blade coating method enables the high-throughput fabrication of large area, high quality NBG films for all-perovskite tandem solar cells," Huang added. "On the other hand, scaling up all-perovskite tandem solar modules is challenging due to the degradation of the narrow band gap subcell during module processing in an ambient condition. In this work, the reducing agent BHC enables module fabrication in an ambient environment, which is a critical step toward industrialization."
In the future, the results if this work could contribute to the industrialization and up-scaling of efficient all-perovskite tandem solar cells that are more affordable. Meanwhile, the researchers plan to use their method to develop more efficient and stable solar modules with larger surface areas.