Scientists achieve 29.9% efficiency for solar cell

 29.9% efficiency achieved for four-junction perovskite-CSI tandem solar cell 

 National University of Singapore have achieved the highest efficiency ever recorded for a perovskite-CIS tandem solar cell at the research level to date. The researchers used methyldiammonium diiodide (MDADI) to reduce open-circuit voltage losses in the top perovskite cell. An international research team has fabricated a four-junction (4T) tandem solar cell based on perovskite and copper/indium/selenium (CIS) thin-film with a power conversion efficiency of up to 29.9%. The scientists said they used halide salt methyldiammonium diiodide (MDADI) for surface defect passivation, which reduced the open-circuit voltage loss in the device's top cell based on perovskite while improving the overall tandem cell efficiency and near-infrared (NIR) transmittance.

The scientists grew the top cell through a multi-stage co-evaporation process. The device was based on a substrate made of glass and indium tin oxide (ITO), a hole transport layer (HTL) made of nickel(II) oxide (NiOx), a perovskite absorber, a buckminsterfullerene (C60) electron transport layer (ETL), a tin oxide (SnOx) layer, an ITO layer and an ultrathin lithium fluoride (LiF) interlayer. “We then vary the optical pathways within the semi-transparent front cell to realize the control of the interference spectrum,” the team explained. “By matching the optical interference with the AM 1.5 G irradiance spectrum, the perovskite front cell achieves high average NIR transmittance (ANT) of 81.5% and an efficiency of 21.8%.”

The academics also explained that the MDADI-treated film can reach a much higher quasi-Fermi level splitting (QFLS) compared with the one without the treatment. “The pseudo current-voltage (J-V) plot shows that the MDADI-treated film reaches an implied open-circuit voltage of 1.29 V, whereas the untreated film reaches only 1.24 V, and there is a difference of around 50 mV”.  This top cell was then combined in a tandem device with a bottom cell made with a glass substrate coated with molybdenum (Mo), a CIS absorber, a cadmium sulfide (CdS) buffer layer, and a window layer of zinc oxide (i-ZnO). This device has an efficiency of 17.3%, an open-circuit voltage of 0.60 V, a short-circuit current density of 39.7 mA/cm2, and a fill factor of 72.9%. 

Tested under standard illumination conditions, the perovskite-CIS tandem cell achieved an efficiency of 29.9%, which the group said is the highest efficiency ever recorded for a perovskite-CIS tandem solar cell at the research level to date. The scientists described the device in the study “29.9%-efficient, commercially viable perovskite/CuInSe2 thin-film tandem solar cells,” which was recently published in Joule. “These findings indicate the vast opportunities for all thin-film-based perovskite/CIS tandem cells in diverse applications and pave the way for their commercialization,” they said. “The combination of superior performance and cost-effectiveness makes them an attractive prospect for the solar industry.”