Mechanisms of LiF Interlayer Enhancements of Perovskite Light-Emitting Diodes

Abstract

The use of LiF as a thin interlayer between the electron transport layer and cathode has played a pivotal role in remarkable advances in perovskite LEDs (PeLEDs); however, the mechanism behind the effect of LiF remains to be fully understood. Here, we report a combined experimental and computational study, from which we ascribe the benefits of a LiF interlayer to the migration of dissociated Li into the cathode and dissociated F into the anode. Electronic device simulations reveal that the former improves electron injection by lowering the Schottky barrier height, while the latter reduces the barrier width. These reduce turn-on voltage and improve current density and charge balance in LEDs. We fabricate PeLEDs with and without the LiF interlayer and link these materials and electronic phenomena to the device light-current− voltage characteristics. X-ray photoelectron spectroscopy obtained in sputter profiling of PeLEDs corroborates the dissociation of LiF.

Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction

“…Our comparison strongly suggests that the utilization of a LiF interlayer reduced the stability. As described in other works (58)(59)(60)(61), the decrease in stability might be caused by deterioration of the electrodes and C 60 interface upon migration of Li + and Fions. We would like to note that it is important to declare the mismatch-conditions because the utilization of a NIR-poor spectrum would lead to a silicon limited cell and thus to a higher stability (see supplementary text).…”

Field Effect Passivation in Perovskite Solar Cells by a LiF Interlayer

“…[7,10,11] Several ideas are reported in the literature with LiF either chemically passivating defects in the perovskite or the C 60 , creating a dipole, or even (partially) dissociating with the individual ions diffusing throughout the layer stack. [12][13][14][15] However, the exact mechanisms behind this are still poorly understood. Furthermore, since device stability deteriorates with the implementation of this LiF interlayer, [10] a better understanding of the underlying mechanisms can guide the search for other interlayers, which might yield similarly beneficial effects without sacrificing stability.…”

Optimizing Performance and Operational Stability of CsPbI3 Quantum-Dot-Based Light-Emitting Diodes by Interface Engineering

“…Although the precise mechanism behind the enhanced electron injection of bilayer cathode is still unknown, plausible explanations are related with the metal-induced chemical reduction of the EIL and the subsequent doping of the ETL layer, or by charge injection via quantum tunnelling. 21,31,[34][35][36][37][38] It is important to note that Liq is a metal-organic semiconductor and LiF is an insulator, and therefore, a small change in LiF thickness could severely affect the formation of band bending zone and restrict electron injection. [38][39][40] Therefore, an efficient LED operation also requires tuning of EIL/metal interface beside ETL/emitter or HTL/emitter interfaces.…”

“…21,31,[34][35][36][37][38] It is important to note that Liq is a metal-organic semiconductor and LiF is an insulator, and therefore, a small change in LiF thickness could severely affect the formation of band bending zone and restrict electron injection. [38][39][40] Therefore, an efficient LED operation also requires tuning of EIL/metal interface beside ETL/emitter or HTL/emitter interfaces. So far, no direct comparison of such bi-layers, for example, metal-organic complex Liq/Ag with insulator compound LiF/Al has been made, in terms of their charge-injection performance and corresponding EQE of LEDs.…”

“…To overcome this issue, thin buffer layers (also often reported as electron injection layer, EIL) are employed with the metal electrode (bilayer cathode) to reduce the cathode work function. , Typical examples include inorganic alkali/alkaline metal/compounds LiF/Al ,, or LiF/Au, Cs 2 CO 3 /Al, , CsF/Al, CaF/Al, Ca/Al, or Ca/Ag, organic alkali/alkaline metal complex 8-quinolinolato lithium (Liq)/Al , or Liq/Ag, 8-quinolinolato sodium (Naq)/Al, cesium quinoline-8-oxide (Csq)/Al, and so on. Although the precise mechanism behind the enhanced electron injection of bilayer cathode is still unknown, plausible explanations are related with the metal-induced chemical reduction of the EIL and the subsequent doping of the ETL or by charge injection via quantum tunneling. ,,− It is important to note that Liq is a metal–organic semiconductor and LiF is an insulator, and therefore, a small change in LiF thickness could severely affect the formation of a band-bending zone and restrict electron injection. − Therefore, an efficient LED operation also requires tuning of EILs/metal interfaces beside ETLs/emitter or HTLs/emitter interfaces. So far, no direct comparison of such bilayers, for example, metal–organic complex Liq/Ag with insulator compound LiF/Al has been made, in terms of their charge-injection performance and corresponding EQE of LEDs.…”