Perovskite-silicon tandem solar cell treated with n-Butanol achieves 29.4% efficiency

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A group of researchers led by the Nanjing University in China has designed a perovskite-silicon tandem solar cell with a new solvent engineering strategy that utilizes n-butanol (nBA), which is also called normal butyl alcohol and is a primary clear, colorless alcohol used as a cleaning agent in many industries, including electronics manufacturing.

The scientists explained that nBA offers low polarity and saturation vapor pressure and ensures that the typical detrimental effects of moisture in perovskite cell fabrication in an ambient environment can be significantly reduced.

The perovskite film of the tandem cell's top device had an active area of 0.049 cm2 and was fabricated via co-evaporation and blade-coating techniques, which the research group said meets the requirements for large-area fabrication of the perovskite films.

“It is worth noting that the second step was implemented in air to match the realistic production environment,” it also explained, noting that nBa replaced common ethanol and isopropyl alcohol, which negatively affect film uniformity. “Both the polarity and evaporation rate of the solvent have a joint effect on H2O absorption levels. In this view, nBA emerges as the optimal solvent for our specific requirements.”

The nBa-based film was found to have improved charge collection, due to the larger grain sizes minimizing recombination, compared to control films developed with conventional solvents.

The researchers built the top cell with a glass-coated indium tin oxide (ITO) absorber, a nickel (II) oxide (NiO) layer, a hole transport layer (HTL) with a self-assembled monolayer (SAM), a perovskite absorber with an energy bandgap of 1.68 eV, an electron transport layer based on buckminsterfullerene (C60) and a tin oxide (SnOx) buffer layer, and a copper (Cu) electrode.

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Tested under standard illumination conditions, this device achieved a power conversion efficiency of 20.8%.

The top device was then integrated in a tandem cell with an active area of 1.044 cm2 integrating a bottom heterojunction silicon solar cell. This cell achieved an efficiency of 29.4%, an open-circuit voltage of 1.83 V, a short-circuit current density of 20.45 mA cm−2, and a fill factor of 78.63%.

The team was also able to certify a 28.7% efficiency for the tandem cell and 26.3% for a device with an aperture area of 16 cm2. “The encapsulated device retained 96.8% of the initial output after 780 h of maximum power point tracking,” the academics added. “Additionally, we have showcased the potential for commercial scaling by achieving a conversion efficiency of 25.9% for 16 cm² devices fabricated via slot-die coating.”

They presented the cell in the paper “Solvent engineering for scalable fabrication of perovskite/silicon tandem solar cells in air,” published in nature communications. “This solvent engineering strategy demonstrates the feasibility of commercial perovskite-silicon tandem solar cells,” they concluded.

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