Understanding the Role of Lithium Doping in Reducing Nonradiative Loss in Lead Halide Perovskites

Abstract

Adding alkali metal into lead halide perovskites has recently been demonstrated as an effective strategy for reducing nonradiative loss. However, the suggested role of the alkali metal is usually limited to surface passivation, and the semiconductor doping effect is rarely discussed. Here, the mechanism of lithium doping in the photocarrier recombination in solution-processed methylammonium lead halide films is investigated by photoluminescence and photoelectron spectroscopies. It is demonstrated that lithium doping weakens the electron-phonon coupling and acts as donor in perovskites, which provide solid evidence that lithium enters the lattice rather than just in the surface region. The n-type doping creates free electrons to fill the trap states in both the bulk and surface regions, leading to suppressed trapping of photocarriers and reduces nonradiative recombination.

Keywords: doping; perovskites; photoluminescence; recombination.

Figure 1

a) XRD patterns of a series of Li‐doped MAPbI₃ films. The peaks labeled by asterisk can be indexed to tetragonal perovskite. Dashed line represents the diffraction of PbI₂. b) XPS Li1s core level spectra of a series of Li‐doped MAPbI₃ films. c) XPS Li1s core level spectra of the Li‐10 sample after Ar⁺ ion etching for 30, 60, and 90 s. The curves are vertically separated for clarity.

Figure 2

PL properties of the Li‐doped MAPbI₃ films. a) Steady‐state room temperature PL spectra. b) PL decay traces. Temperature‐dependent PL spectra of the c) undoped and d) Li‐5 perovskite films. Dashed lines represent the evolution of PL peak wavelengths. e) Arrhenius plot of the thermal quenching of PL intensity. Solid lines represent the fitting results using Equation (1).

Figure 3

a) Temperature‐dependent PL energy of the undoped (Li‐0) and Li‐doped (Li‐5) MAPbI₃ films. Solid lines represent fitting curves using Equation (2). b) UPS spectra of a series of Li‐doped MAPbI₃ films. Inset shows the derived work function using the energy of He I (21.2 eV) as the reference.

Figure 4

a) Log–log plot of the PL intensity as a function of excitation power. The decrease of slope indicates the reduced density of empty trap states. b) Schematic to illustrate recombination mechanisms of the Li‐doped MAPbI₃ films. The doping of Li creates free carriers to fill the trap states, thus suppressing the trapping of photocarriers and improving the radiative recombination.