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Scientists from the University of Tokyo in Japan have developed an all-perovskite tandem solar cell that reaches 30.2% efficiency, as detailed in a study published in ACS Omega. The four-terminal device employs FAPbI3 nanoparticles along with a spectral splitting configuration, integrating a top cell with a 24.4% efficiency wide-bandgap and a bottom cell with a 21.5% efficiency narrow-bandgap.
This setup enhances light utilization by channeling various wavelengths to the most suitable subcells. The top cell is constructed on a glass and fluorine-doped tin oxide (FTO) substrate, featuring a tin oxide (SnO2) hole transport layer, the perovskite absorber, a Spiro-OMeTAD electron transport layer, and a gold metal contact. The bottom inverted cell uses a glass and FTO substrate, a Spiro-OMeTAD electron transport layer, the perovskite absorber, a buckminsterfullerene (C60) hole transport layer, a bathocuproine (BCP) buffer layer, and a silver metal contact.
FAPbI3 is commonly utilized in high-efficiency perovskite solar cells due to its bandgap of roughly 1.48 eV, which is near the optimal value for solar energy conversion. It facilitates strong light absorption and has enabled power conversion efficiencies exceeding 25% in experimental devices. However, a significant drawback is that the desired black alpha-phase is metastable and can degrade into a non-functional yellow phase, altering the material from a light-absorbing semiconductor into a wide-bandgap, inactive phase.
To counter this issue, the Japanese team employed FAPbI3 nanoparticles that were pre-synthesized using a hot injection method. The perovskite films were created through a solution spin-coating process on cleaned and UV-ozone-treated substrates in an inert environment. A precursor solution was made by dissolving PbI2 and formamidinium iodide (FAI) in a mixed solvent of dimethylformamide and dimethyl sulfoxide (DMF/DMSO), stirred until completely uniform. This solution was then spin-coated onto the substrates, followed by controlled thermal annealing to trigger crystallization, transforming the liquid precursor into a dense, crystalline FAPbI3 thin film with the desired photoactive alpha-phase.
Under standard testing conditions, the four-terminal tandem cell achieved a peak power conversion efficiency of 30.2%. The optimal performance occurred at a split wavelength of 775 nm, where the wide-bandgap top cell contributed 24.1% and the narrow-bandgap bottom cell added 6.1%. This wavelength aligns closely with the absorption edge of the top cell, allowing near-complete use of its spectral range. Beyond 775 nm, the top cell sees only a minor current increase, while the bottom cell experiences a much larger photocurrent loss, diminishing overall gains.
Corresponding author Satoshi Uchida noted that the primary benefit of spectral split two-junction, four-terminal solar cells is their capacity to cut losses from spectral mismatch while maintaining high efficiency. This is achieved by directing incoming light to the most appropriate subcell based on its wavelength. Additionally, the four-terminal design removes the need for current matching, enabling flexible combinations of solar cells with diverse compositions. If one subcell fails, the other can still produce power, offering a maintenance advantage.
For real-world application, conventional outdoor photovoltaic systems and integration with concentrator photovoltaics are seen as especially promising for this concept. Nevertheless, the high expense of dichroic mirrors required for spectral splitting remains a hurdle. Future practical deployment will require not only building on these findings but also investigating simpler designs, such as monolithic two-junction two-terminal devices and mechanically stacked two-junction four-terminal devices.
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