arXiv subjects

Haibo Xue

Publications and source records attributed to Haibo Xue.

4 recordsLinked to original sources

arXiv2022

Defects in Halide Perovskites: Does It Help to Switch from 3D to 2D?

Ruddlesden-Popper hybrid iodide 2D perovskites are put forward as stable alternatives to their 3D counterparts. Using first-principles calculations, we demonstrate that equilibrium concentrations of point defects in the 2D perovskites PEA$_2$PbI$_4$, BA$_2$PbI$_4$, and PEA$_2$SnI$_4$ (PEA: phenethyl ammonium, BA: butylammonium), are much lower than in comparable 3D perovskites. Bonding disruptions by defects are more detrimental in 2D than in 3D networks, making defect formation energetically more costly. The stability of 2D Sn iodide perovskites can be further enhanced by alloying with Pb. Should, however, point defects emerge in sizable concentrations as a result of nonequilibrium growth conditions, for instance, then those defects hamper the optoelectronic performance of the 2D perovskites, as they introduce deep traps. We suggest that trap levels are responsible for the broad sub-bandgap emission in 2D perovskites observed in experiments.

cond-mat.mtrl-sci↗

arXiv2022

Compound Defects in Halide Perovskites: A First-Principles Study of CsPbI$_3$

Lattice defects affect the long-term stability of halide perovskite solar cells. Whereas simple point defects, i.e., atomic interstitials and vacancies, have been studied in great detail, here we focus on compound defects that are more likely to form under crystal growth conditions, such as compound vacancies or interstitials, and antisites. We identify the most prominent defects in the archetype inorganic perovskite CsPbI$_3$, through first-principles density functional theory (DFT) calculations. We find that under equilibrium conditions at room temperature, the antisite of Pb substituting Cs forms in a concentration comparable to those of the most prominent point defects, whereas the other compound defects are negligible. However, under nonequilibrium thermal and operating conditions, other complexes also become as important as the point defects. Those are the Cs substituting Pb antisite, and, to a lesser extent, the compound vacancies of PbI$_2$ or CsPbI$_3$ units, and the I substituting Cs antisite. These compound defects only lead to shallow or inactive charge carrier traps, which testifies to the electronic stability of the halide perovskites. Under operating conditions with a quasi Fermi level very close to the valence band, deeper traps can develop.

cond-mat.mtrl-sci↗

arXiv2021

The thermodynamic trends of intrinsic defects in primary halide perovskites: A first-principles study

Defects in halide perovskites play an essential role in determining the efficiency and stability of the resulting optoelectronic devices. Here, we present a systematic study of intrinsic point defects in six primary metal halide perovskites, MAPbI$_3$, MAPbBr$_3$, MAPbCl$_3$, FAPbI$_3$, CsPbI$_3$ and MASnI$_3$, using density functional theory calculations with the SCAN+rVV10 functional. We analyse the impact of changing anions and cations on the defect formation energies and the charge state transitions levels and identify the physical origins underlying the observed trends. Dominant defects in the lead-iodide compounds are the A$^+$ cation interstitials (A = Cs, MA, FA), charge-compensated by I$^-$ interstitials or lead $({2-})$ vacancies. In the lead-bromide and -chloride compounds, halide vacancies become relatively more prominent, and for MAPbBr$_3$, the Pb$^{2+}$ interstitial also becomes important. The trends can be explained in terms of the changes in electrostatic interactions and chemical bonding upon replacing cations and anions. Defect physics in MASnI$_3$ is strongly dominated by tin $({2-})$ vacancies, promoted by the easy oxidation of the tin perovskite. Intrinsically, all compounds are mildly p-doped, except for MASnI$_3$, which is strongly p-doped. All acceptor levels created by defects in the six perovskites are shallow. Some defects, halide vacancies and Pb or Sn interstitials in particular, create deep donor traps. Although these traps might hamper the electronic behavior of MAPbBr$_3$ and MAPbCl$_3$, in iodine-based perovskites their equilibrium concentrations are too small to affect the materials' properties.

cond-mat.mtrl-sci↗

arXiv2021

First-principles calculations of defects in metal halide perovskites: a performance comparison of density functionals

Metal halide perovskite semiconductors have outstanding optoelectronic properties. Although these perovskites are defect-tolerant electronically, defects hamper their long-term stability and cause degradation. Density functional theory (DFT) calculations are an important tool to unravel the microscopic structures of defects, but results suffer from the different approximations used in the DFT functionals. In the case of metal halide perovskites, qualitatively different results have been reported with different functionals, either predicting vacancy or interstitial point defects to be most dominant. Here, we conduct a comprehensive comparison of a wide range of functionals for calculating the equilibrium defect formation energies and concentrations of point defects in the archetype metal halide perovskite, MAPbI$_3$. We find that it is essential to include long-range Van der Waals interactions in the functional, and that it is vital to self-consistently optimize structure and volume of all compounds involved in the defect formation. For calculating equilibrium formation energies of point defects in MAPbI$_3$ and similar metal halide perovskites, we argue that the exact values of the chemical potentials of the species involved, or of the intrinsic Fermi level, are not important. In contrast to the simple Schottky or Frenkel pictures, we find that the dominant defects are MA and I interstitials, and Pb vacancies.

cond-mat.mtrl-sci↗