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Developed by German scientists, the triple junction cell is based on a perovskite top cell with an energetic bandgap of 1.84 eV, a perovskite middle cell with a bandgap of 1.52 eV, and a silicon bottom cell with a bandgap. of 1.1 eV. The device achieved an open circuit voltage of 2.84 V, a short circuit current of 11.6 mA cm-2, and a fill factor of 74%.A group of researchers led by the German Karlsruher Institut für Technologie (KIT) has manufactured a perovskite-perovskite-silicon triple junction solar cell that has achieved a record efficiency of 24.4%.
To date, perovskite-based multijunction photovoltaic cells using three, four, or even more junctions have underperformed monolithic perovskite-based double-junction solar cells.
“The key challenges in the processing of triple junction architectures are the sequential processing of high-quality perovskite thin films in the increasingly complex multilayer architecture, the light management and current adaptation of the monolithically interconnected subcells, as well as the development of low-loss tunnel/recombination junctions,” Ulrich W. Paetzold, director of KITs new generation photovoltaic group, told pv magazine . “We highlight that, to date, the most critical junction is the middle perovskite subcell, as it is processed on top of the lower Si cell and has to support the subsequent processing of the upper wideband perovskite (WBG) cell. its acronym in English).
In the study “ Triple-junction perovskite- perovskite-silicon solar cells with power conversion efficiency of 24.4%,” published in Energy & Environmental Science , Paetzold and his colleagues explained that the cell was based on a perovskite top cell with an energetic bandgap of 1.84 eV, a perovskite middle cell with a bandgap of 1.52 eV, and a silicon bottom cell. with a bandgap of 1.1 eV.
The bottom cell had a thickness of 200 µm. It was etched with potassium hydroxide and based on electron-collecting poly-Si-on-oxide (POLO) bonds. For the center and top devices, the scientists used one of the most promising halide perovskites: a-formamidinium lead iodide, known as a-FAPbI3. Recombination junctions were formed with sputtered indium tin oxide (ITO) layers.
“ITO also serves as an anchoring oxide for the sequential hole transport layer (HTL), especially for NiOx/self-assembled monolayer (SAM) dual HTLs,” the academics explained. “In both perovskite subcells, a double HTL based on a combination of sputtered nickel(II) oxide (NiOx) and carbazole (2PACz) is used, which offers excellent extraction of charge carriers, a robust barrier to precursor solvents of perovskite and very good performance for the devices.”
Tested under standard lighting conditions, the triple junction cell achieved a power conversion efficiency of 24.4%, an open circuit voltage of 2.84 V, a short circuit current of 11.6 mA cm-2, and a 74% filling. According to the research group, this is the highest efficiency recorded to date in this type of triple junction devices.
The cell was also able to retain 96.6% of the initial efficiency in dark storage by aging at 85 °C for 1,081 h.
“By leveraging optical simulations and experimental optimizations in triple junction, current mismatch was minimized and current generation was maximized,” Paetzold said. “The key to this achievement was our development of a high-performance intermediate perovskite subcell, employing a high-quality pure alpha-phase stable FAPbI3 thin film.”
Paetzold also explained that the medium perovskite subcell is beneficial for providing adequate medium bandgap, very good thermal stability, excellent interfaces with both recombination junctions, and a low density of defects and pinholes.
“Our study opens the door to a new era of high-efficiency triple-junction photovoltaics based on perovskite,” he concluded. |
