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American researchers say the cell has a perovskite top with a transparent indium zinc oxide back contact and a commercially established cadmium telluride bottom device. They claim that the tandem champion cell has the potential to achieve 30% efficiency.
A research group at the University of Toledo (United States) has designed a four-terminal (4T) tandem solar cell with an upper device based on a tunable broadband perovskite absorber and a lower cell that uses a commercial technology of cadmium telluride (CdTe) narrow band absorber.
“Although much work has been done on perovskite-silicon, perovskite-CIGS, and perovskite-perovskite tandem solar cells, perovskite-cadmium telluride tandem solar cells are relatively unexplored,” the scientists explain. “Although the efficiency potential of CdTe-based tandems is probably lower than that of CIGS-based ones due to the higher bandgap of the CdTe bottom cell, the great commercial success of CdTe solar cells makes them a point of interest for research into thin film tandem applications.”
According to academics, a key element of the solar cell is the transparent back contact (TBC) technology used for the top tunable broadband perovskite cell. To construct these contacts, they used indium zinc oxide (IZO) as an alternative to indium tin oxide (ITO).
They prepared the IZO films using the radio frequency (RF) magnetron sputtering technique, which is a method of alternating the electrical potential of the current in a vacuum to RF environment.
They also explained that their efforts were directed at identifying the ideal thickness of IZO, as it plays a crucial role in improving the performance and optical transmittance of the semi-transparent perovskite top cell, by increasing the bandgap of the perovskite allowing more Long wavelength photons are transmitted and enter the bottom CdSeTe cell. In turn, this offsets a typical optical loss factor in a 4T tandem configuration.
The upper cell was constructed with a glass substrate and indium tin oxide (ITO), a hole transport layer (HTL) of nickel(II) oxide (NiOx), a layer of a phosphonic acid called carbazole methyl- substituted (Me-4PACz), the perovskite absorber, an electron transport layer (ETL) based on buckminsterfullerene (C60), a tin oxide (SnOx) buffer layer and the IZO back contact.
The bottom cell was designed with a glass and ITO substrate, a tin oxide (SnO2) ETL, a cadmium telluride (CdTe) absorber, a cadmium selenium telluride (CdSeTe) layer, a copper (CuSCN) and a gold metal contact.
Both cells were covered with an anti-reflective coating.
The best tandem cell configuration was achieved when the top cell absorber was adjusted to have an energy bandgap of 1.76 eV. With this value, the device achieved an overall energy conversion efficiency of 25.1%.
The top cell achieved an efficiency of 17.93%, an open circuit voltage of 1.315 V, a short circuit current 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 density of 11.15 mA cm2 and a fill factor of 76.0%.
“The result demonstrates the concept that the 4T perovskite-CdSeTe tandem configuration can be used to improve the efficiency of commercial CdSeTe thin film solar cells,” the researchers stated, adding that they are currently outlining a roadmap to increase the Device efficiency up to 30%. “Our analysis reveals that high-efficiency 4T perovskite-CdSeTe tandem solar cells are feasible with the future advancement of both photovoltaic cells.”
Details of the new cell design can be found in the study “ Four-Terminal Perovskite-CdSeTe Tandem Solar Cells: From 25% towards 30% Power Conversion Efficiency and Beyond ” % to 30% energy conversion efficiency and more), recently published in RRL Solar .
The University of Toledo has developed several types of CdTe solar cells in recent years. The devices include, among others, a 20% efficient cell based on a commercial tin(IV) oxide (SnO2) buffer layer, a 17.4% efficient device using a copper oxide layer -aluminum on the back side of the CdTe thin film, and a solar cell based on an indium gallium oxide (IGO) emitting layer and a transparent cadmium stannate (CTO) conductor as the front electrode. |
