Researchers at the University of Toledo have designed a four-terminal (4T) tandem solar cell with a top device that uses a perovskite absorber with a tunable wide-bandgap and a bottom cell based on a commercially established narrow-bandgap absorber technology made of cadmium telluride (CdTe).
Perovskite-cadmium telluride tandem solar cells are relatively unexplored compared to other tandems. While the efficiency potential of CdTe-based tandems is lower than CIGS-based tandems due to the higher bandgap of the CdTe bottom cell, the broader commercial success of CdTe solar cells makes them interesting to investigate for thin-film tandem applications.
The scientists said a key element of the solar cell is the transparent back contact (TBC) technology used for the top tunable wide-bandgap perovskite cell. For the construction of these contacts, they used indium zinc oxide (IZO) as an alternative to well-established indium tin oxide (ITO).
They prepared the IZO films through the radio frequency (RF) magnetron sputtering technique, which is an approach involving alternating the electrical potential of the current in a vacuum environment at RFs.
They also explained that their efforts were aimed at identifying the ideal IZO thickness, as this plays a crucial role in improving the performance and optical transmittance of the semitransparent perovskite top cell by increasing the perovskite bandgap allowing more long-wavelength photons to transmit and enter the CdSeTe bottom cell. In turn, this compensates for a typical optical loss factor in a 4T tandem configuration.
The top cell was constructed with a substrate made of glass and indium tin oxide (ITO), a hole transport layer (HTL) made of nickel(II) oxide (NiOx), a layer made of a phosphonic acid called methyl-substituted carbazole (Me-4PACz), the perovskite absorber, an electron transport layer (ETL) relying on buckminsterfullerene (C60), a tin oxide (SnOx) buffer layer, and the IZO back contact.
The bottom cell was designed to have a substrate made of glass and ITO, an ETL made of tin oxide (SnO2), a cadmium telluride (CdTe) absorber, a cadmium selenium telluride (CdSeTe) layer, a copper thiocyanate (CuSCN) HTL, and a gold metal contact.
Both cells were covered with an anti-reflecting coating.
The best tandem cell configuration was achieved when the absorber of the top cell was tuned to have an energy bandgap of 1.76 eV. With this value, the device reached an overall power conversion efficiency of 25.1%.
The top cell was found to achieve an efficiency of 17.93%, an open-circuit voltage of 1.315 V, a short-circuit current of density of 17.11 mA cm2, and a fill factor of 79.7%. The bottom cell showed an efficiency of 7.13%, an open-circuit voltage of 0.842 V, a short-circuit current of density of 11.15 mA cm2, and a fill factor of 76.0%.
These result demonstrates the concept that 4T perovskite–CdSeTe tandem configuration can be used to improve the efficiency of commercial CdSeTe thin-film solar cells, according to the researchers, adding they are currently outlining a roadmap to increase the device’s efficiency to 30%.
