Field Effect Passivation in Perovskite Solar Cells by a LiF Interlayer

Abstract

as higher PCEs have been achieved due to lower parasitic absorption losses, [4,5] and their low-temperature processing enables a wider choice of bottom cells.The device stability and PCE of silicon/perovskite tandem solar cells are crucially determined by the quality of the contact layer interfaces, [6,7] as they are found to limit especially the open circuit voltage (V OC ) and fill factor. Charge carrier selective contacts and their interfaces toward MHP absorbers are thus a field of recent and ongoing research. [8][9][10] The charge carrier transport across interfaces depends on two main parameters: The energy level alignment of both materials forming the interface, and the density of defect states at the surfaces and interfaces.The fullerene C 60 has become the standard electron transport layer (ETL) for p-i-n MHP solar cells. Similarly, it has also been applied in silicon/perovskite tandem solar cells with a p-i-n structure for the perovskite top cell, yielding the highest-efficiency devices in their class. [10] However, the obtained open circuit voltage for p-i-n MHP solar cells is found to be limited by the electron selective interface due to non-radiative recombination losses, [11,12] which raises interest in thoroughly studying the energetic formation of the perovskite/C 60 interface, including the energy level alignment and density of gap states. The insertion of an ultra-thin (≈1 nm) LiF interlayer between the perovskite and C 60 has been shown to significantly increase the device V OC by reducing the non-radiative recombination while keeping a highThe fullerene C 60 is commonly applied as the electron transport layer in highefficiency metal halide perovskite solar cells and has been found to limit their open circuit voltage. Through ultra-sensitive near-UV photoelectron spectroscopy in constant final state mode (CFSYS), with an unusually high probing depth of 5-10 nm, the perovskite/C 60 interface energetics and defect formation is investigated. It is demonstrated how to consistently determine the energy level alignment by CFSYS and avoid misinterpretations by accounting for the measurement-induced surface photovoltage in photoactive layer stacks. The energetic offset between the perovskite valence band maximum and the C 60 HOMO-edge is directly determined to be 0.55 eV. Furthermore, the voltage enhancement upon the incorporation of a LiF interlayer at the interface can be attributed to originate from a mild dipole effect and probably the presence of fixed charges, both reducing the hole concentration in the vicinity of the perovskite/C 60 interface. This yields a field effect passivation, which overcompensates the observed enhanced defect density in the first monolayers of C 60 .

Shallow Defects and Ultralong Photoluminescence Decay Times up to 280 µs in Triple-Cation Perovskites

“…However, interfacing the perovskite layer with C60 substantially lowers ∆EF possibly by introducing additional interfacial defects as suggested by refs. 23,24 and consistent with previous reports on steady state 25 and transient photoluminescence 15,26 . OAI modification can effectively passivate the perovskite/C60 interface defects, as samples with the perovskite/C60 interface show a stronger enhancement in ∆EF after OAI modification than stacks without C60.…”

Overcoming C60-induced interfacial recombination in inverted perovskite solar cells by electron-transporting carborane

“…However, the non-radiative recombination at the perovskite/C 60 interface and the involved defect states remain fundamentally poorly understood 9 – 12 . Recent results in this regard have shown that recombination occurs predominantly across the interface 12 , likely due to a small energy level offset between the conduction band minimum of the perovskite and the lowest unoccupied molecular orbital (LUMO) level of C 60 13 , or charge transfer states at the interface 12 , 14 . Thus, it is critical to improve the energy alignment, passivate defects on the perovskite surface or repel minority carriers from the interface.…”

Rationalization of passivation strategies toward high-performance perovskite solar cells

“…By utilizing CFSYS, the field-effect passivation ability of the LiF interlayer at the perovskite/C 60 interface was demonstrated, which resulted in a mild dipole effect and fixed charges that lowered the hole concentration in the vicinity of the interface. 131…”

“…67,126 The constant final state yield spectroscopy (CFSYS) was recently applied to investigate the electronic interface formation and energy level alignment between perovskite and ETL in inverted PCSs, in order to confirm field-effect passivation. 131 This characterization is achieved by applying UPS with specific excitation energy and additionally varying the incident photon energy in the near-UV range and measuring the excited photoelectrons per absorbed photon (yield) at one constant final state (fixed kinetic energy). Through directly observing the Fermi edge in the CSFYS spectra, the measurement-induced surface photovoltage effects in the photoactive layer stacks and the energetic positions to the actual surface Fermi level can be accounted.…”