Researchers from King Abdullah University of Science and Technology (KAUST) have reported four-terminal perovskite/perovskite/silicon triple-junction tandem solar cells, with the device structure comprising a perovskite single-junction top cell and monolithic perovskite/silicon tandem bottom cell.
The cells reportedly yielded a 31.5% power conversion efficiency, which the team said is the highest efficiency ever reported for perovskite-based 4-T and triple-junction tandem solar cells. The key feature of the cell is the hole transport layer of the top perovskite cell, which was engineered with self-assembled monolayers.
The team said that processing compatibility may be a significant challenge when depositing so many layers as are needed in this configuration, and the recombination junction between the perovskite top and perovskite mid cell may be especially challenging.
The scientists said that the key feature of the new tandem device was the hole transport layer (HTL) of the top cell, which was engineered with self-assembled monolayers (SAMs) based on a compound known as poly(9,9-bis(3′-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9 dioctylfluorene))dibromide (PFN-Br), which was in turn combined with sputtered nickel oxide (NiOx).
The top perovskite solar cell was built with a substrate made of glass and indium tin oxide (ITO), the HTL based on carbazole (2PACz), PFN-Br, and NiOx, an absorber relying on a perovskite material known as Cs0.1FA0.9PbI1Br1.96Cl0.04, an electron transport layer based on buckminsterfullerene (C60) and tin(IV) oxide (SnO2) layer, an ITO electrode, a silver (Ag) metal contact, and an anti-reflective coating based on magnesium fluoride (MgF2).
This device achieved a power conversion efficiency of 14.3%, an open-circuit voltage of 1.402 V, a short-circuit current density of 12.5 mA/cm2, and a fill factor of 81.5%
The mid perovskite device and the bottom silicon cell were integrated into a monolithic perovskite/silicon tandem cell which achieved an efficiency of 17.2%, an open-circuit voltage of 1.783 V, a short-circuit current density of 11.6 mA/cm2, and a fill factor of 83.0%.
The team found that all devices were able to retain 80% of their initial efficiency after 1,500 minutes of maximum power point tracking, which they described as a “significant” result for the 2.0 eV-bandgap organic−inorganic perovskite material they chose for the top cell.
According to the team, the HTL exhibited a wide distribution in all PV parameters, with one-third of all devices being fully shunted. They said that this fully shunted performance originates from the not fully covered perovskite films in the device, attributed to the hydrophobic surface of 2PACz anchored ITO substrates.
