The four-terminal tandem device is based on FAPbI₃ nanoparticles and a spectral splitting design, combining an upper cell with a wide band gap of 24.4% and a lower cell with a narrow band gap of 21.5% to achieve an efficiency of 30.2%. The system improves light utilization by sending different wavelengths to optimized subcells.
Researchers at the University of Tokyo in Japan have used a novel to create a tandem solar cell made entirely of perovskite a light-absorbing layer deposition technique using formamidinium lead iodide (FAPbI3) nanoparticles.
FAPbI3 is widely used in high efficiency perovskite solar cells due to its band gap of about 1.48 eV, which is close to the ideal value for solar energy conversion. It enables strong light absorption and has helped achieve energy conversion efficiency of more than 25% in research equipment. However, the main limitation is that the desired black α phase is metastable and can transform into a nonfunctional yellow phase. This has serious consequences for the performance of solar cells, as it immediately changes the material from a light-absorbing semiconductor to an inactive phase with a large band gap.
To address this, researchers typically use mixed cations, additives and interface engineering to stabilize the material and improve durability. The Japanese scientists used FAPbI3 nanoparticles pre-synthesized by hot injection method for perovskite film formation using a two-step method. FAPbI₃-based perovskite layers were fabricated using a solution spin coating process on cleaned and UV-ozone treated substrates under inert conditions. A precursor solution was prepared by dissolving PbI2 and formamidinium iodide (FAI) in a mixed solvent of dimethylformamide-dimethyl sulfoxide (DMF/DMSO) and stirring until completely homogeneous.
The solution was then spin-coated onto substrates, followed by controlled thermal annealing to induce crystallization of the perovskite film. This process converted the liquid precursor into a dense, crystalline FAPbI3 thin film with the desired photoactive α-phase.
The four-terminal tandem device (4T) is built with a wide bandgap (WBG) cell at the top with an efficiency of 24.4% and a narrow bandgap (NBG) cell at the bottom with an efficiency of 21.5% and an inverted structure. The two cells were integrated into a four-terminal spectral splitting architecture using dichroic mirrors that separate light at selected wavelengths. This optical design reportedly minimizes losses while allowing efficient use of the solar spectrum across both cells.
The top cell is built with a glass substrate and fluorine-doped tin oxide (FTO), a hole transport layer (HTL) of tin oxide (Sno2), the perovskite absorber, a Spiro-OMeTAD electron transport layer (ETL), and a gold (Au) metal contact. The bottom inverted device was fabricated with a glass and FTO substrate, a Spiro-OMeTAD ETL, the perovskite absorber, a buckminsterfullerene (C60) HTL, a bathocuproin (BCP) buffer layer and a silver (Ag) metal contact.
Image: University of Tokyo
“The main advantage of spectrally split two-junction, four-terminal solar cells lies in their ability to reduce losses caused by spectral mismatch while achieving high efficiency,” said corresponding author Satoshi Uchida. pv magazine. “This is achieved by directing incident light to the most appropriate subcell based on its wavelength. Furthermore, due to the four-terminal configuration, there is no limitation on current tuning, allowing flexible combinations of solar cells with a wide range of compositions. Furthermore, even if one subcell experiences a failure, the other can continue to generate power, providing an advantage from a maintenance perspective.”
Tested under standard lighting conditions, the four-terminal cell achieved a maximum energy conversion efficiency of 30.2%. The best performance was obtained at a split wavelength of 775 nm, with the top WBG cell contributing 24.1% and the bottom NBG cell contributing 6.1%. This wavelength closely matches the absorption edge of the top cell, making almost full use of the spectral range. Above 775 nm, the top cell gains only a small increase in current, while the bottom cell loses significantly more photocurrent, reducing the overall gain.
“Overall, our research shows that carefully chosen spectral splitting wavelengths enable very high efficiencies in both four-terminal and two-terminal perovskite solar cell architectures,” says Uchida.
“In terms of practical application, conventional outdoor photovoltaic systems and integration with photovoltaic concentrators are considered particularly promising for our solar cell concept,” he continued. “On the other hand, the high cost of dichroic mirrors used for spectral splitting remains a challenge. For future practical implementation, it will be important not only to build on the findings of this study, but also to investigate simplified architectures such as monolithic two-terminal, two-terminal devices and mechanically stacked, two-terminal, four-terminal devices.”
The tandem device was presented in “Fully perovskite four-terminal spectrally splitting solar cells of 30% PCE with FAPbI3 Wide band gap perovskite fabricated by nanoparticle technology”, published in ACS Omega.
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