Fabricated via thermal evaporation, the champion perovskite-perovskite-silicon triple-junction solar cell reached an efficiency of around 22% after 110 hours of fixed-voltage operation under ambient conditions without encapsulation.
A research team led by Germany’s Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) and the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia announced using thermal evaporation in the fabrication of an all-inorganic cesium lead halide perovskite (CsPbI₂Br) top cell in a perovskite-perovskite-silicon triple junction solar cell.
“This was the first time that CsPbI₂Br was employed as the top-cell material in a silicon-based triple-junction solar cells with perovskites, and also the first instance where thermal evaporation was used to fabricate the top cell in such a device architecture,” Yashika Gupta, lead author of the research, told pv magazine.
The CsPbI2Br composition is one of the most stable mixed-halide inorganic perovskite materials but issues with fabrication, such as light-induced halide segregation and the risk of solvent damage to underlying layers during top-cell deposition, have held it back from wider use.
“While all-inorganic perovskites like CsPbI₂Br offer excellent thermal stability, they had not yet been integrated into triple-junction devices mainly due to the high post-deposition annealing temperature, which would damage the other layers of the triple junction solar cell,” Gupta further explained.
To overcome these challenges, the research team developed a high bandgap, inorganic perovskite absorber using thermal evaporation at room temperature, eliminating the need for post-deposition annealing. “Notably, CsPbI₂Br was deposited at room temperature with no post-annealing, yielding films with a 1.88 eV bandgap and excellent photostability,” said Gupta, adding that the materials had “exceptional photostability,” that is, no halide segregation was observed even after 3 hours of continuous illumination.
Stability tests further demonstrated stability, with the device retaining performance after over a year of glove box storage and enduring more than 100 hours of fixed-voltage operation at the maximum power point without encapsulation in air.
The cell stack comprised a silicon heterojunction (SHJ) cell, indium tin oxide (ITO), a hole transport layer (HTL) made of poly(triarylamine) (PTAA), F-PEAI passivation (PFN), an absorber relying on the CsFAMA perovskite with an energy bandgap of 1.56 eV, a buckminsterfullerene (C60) electron transport layer (ETL), a tin oxide (SnOx) buffer layer, another ITO layer, a Spiro-TTB HTL, an evaporated CsPbI2Br perovskite layer with an energy bandgap of 1.88 eV, C60 as ETL, another SnOx buffer, ITO, silver (Ag), and magnesium fluoride (MgFx).
Physical vapor deposition (PVD) was used for the perovskite absorber, charge transport layers, and indium tin oxide (ITO). For the SnOx layers, atomic layer deposition was used.
“By keeping the substrate at room temperature during the whole process, we showed a pathway towards fully evaporated, solvent-free fabrication of multi-junction devices. This approach also aligns well with industrial requirements and opens up new possibilities for high-throughput solar manufacturing,” said Gupta.
The test results of the champion triple-junction device showed a power conversion efficiency of 21% under ambient conditions without encapsulation, and an open-circuit voltage of 2.83 V over an active area of 1 cm2. The PCE improved to ~22% after 110 hours of fixed-voltage operation under ambient conditions without encapsulation.
The details of the cell were presented in “Photostable inorganic perovskite absorber via thermal evaporation for monolithic perovskite/perovskite/silicon triple-junction solar cells,” published in Progress in Photovoltaics.
The research group has plans to further optimize evaporation to improve film crystallinity and tune the inorganic perovskite bandgap for better current matching in multi-junction cells, as well as seeking to make fully evaporated middle perovskite cells for integration in perovskite-perovskite-silicon triple-junction devices.