• 
  Kafedjiska, I.; Levine, I.; Musiienko, A.; Maticiuc, N.; Bertram, T.; Al-Ashouri, A.; Kaufmann, C.A.; Albrecht, S.; Schlatmann, R.; Lauermann, I.: Advanced Characterization and Optimization of NiO_x:Cu-SAM Hole-Transporting Bi-Layer for 23.4% Efficient Monolithic Cu(In,Ga)Se₂-Perovskite Tandem Solar Cells. Advanced Functional Materials 33 (2023), p. 2302924/1-19

10.1002/adfm.202302924
Open Access Version

Abstract:
The performance of five hole-transporting layers (HTLs) is investigated in both single-junction perovskite and Cu(In, Ga)Se 2 (CIGSe)-perovskite tandem solar cells: nickel oxide (NiOx,), copper-doped nickel oxide (NiOx:Cu), NiOx+SAM, NiOx:Cu+SAM, and SAM, where SAM is the [2-(3,-6Dimethoxy-9H-carbazol-9yl)ethyl]phosphonic acid (MeO-2PACz) self-assembled monolayer. The performance of the devices is correlated to the charge-carrier dynamics at the HTL/perovskite interface and the limiting factors of these HTLs are analyzed by performing time-resolved and absolute photoluminescence ((Tr)PL), transient surface photovoltage (tr-SPV), and X-ray/UV photoemission spectroscopy (XPS/UPS) measurements on indium tin oxide (ITO)/HTL/perovskite and CIGSe/HTL/perovskite stacks. A high quasi-Fermi level splitting to open-circuit (QFLS-Voc) deficit is detected for the NiOx-based devices, attributed to electron trapping and poor hole extraction at the NiOx-perovskite interface and a low carrier effective lifetime in the bulk of the perovskite. Simultaneously, doping the NiOx with 2% Cu and passivating its surface with MeO-2PACz suppresses the electron trapping, enhances the holes extraction, reduces the non-radiative interfacial recombination, and improves the band alignment. Due to this superior interfacial charge-carrier dynamics, NiOx:Cu+SAM is found to be the most suitable HTL for the monolithic CIGSe-perovskite tandem devices, enabling a power-conversion efficiency (PCE) of 23.4%, Voc of 1.72V, and a fill factor (FF) of 71%, while the remaining four HTLs suffer from prominent Voc and FF losses.