talk-icon
Description
Arun Kumar Mannodi Kanakkithodi1 Maria Chan1

1, Argonne National Laboratory, Argonne, Illinois, United States

Owing to their easy synthesis, tunable electronic properties and large absorption coefficients, lead halide hybrid perovskite semiconductors have emerged as attractive candidates for photovoltaic applications [1,2]. Intrinsic point defects, surface states, grain boundaries and external substituents play an important role in these materials in determining their solar cell efficiencies [3,4]. Here, we considered MAPbBr3-yCly perovskites (where MA = methylammonium and y = {0, 0.75, 1.5, 2.25, 3}) as parent semiconductors and used first principles computations to study intrinsic point defects, namely vacancy, self-interstitial and anti-site. In MAPbBr3-yCly perovskites, while the lattice constant decreases as y goes from 0 to 3, the band gap increases from ~ 2 eV for y = 0 to ~ 2.5 eV for y = 3 [1]. For defect calculations, the supercell approach was implemented in a density functional theory (DFT) framework to create 2x2x2 cells with one defect per 8 units (or 96 atoms) of MAPbBr3-yCly, achieving a satisfactorily dilute defect concentration [5]. By studying the defects in various charged states, charge transition levels were calculated for each and it was seen that while vacancy defects and cation/cation anti-site defects create shallow levels (i.e., close to the valence or conduction band), cation/anion anti-site defects create deeper levels in the band gap. The latter could act as non-radiative recombination centers, proving to be detrimental to solar cell performance [3,4]. Further, for different points in the calculated chemical ranges for MAPbBr3-yCly perovskite stability, the formation energy of each defect was estimated as a function of the fermi level, as it changes from valence to conduction band. The equilibrium fermi levels as determined by dominant acceptor-like and donor-like defects for each perovskite were seen to shift to the right with increasing Cl content, meaning the conductivity of MAPbBr3-yCly becomes more n-type with increasing y. While Br vacancy showed a low formation energy in Br-rich MAPbBr3-yCly perovskites owing to strong anti-bonding between Pb s and Br p orbitals, Cl vacancy showed a higher formation energy in perovskites with larger values of y. Lastly, the equilibrium growth conditions necessary for creating different intrinsic defects were determined; this helps us understand the electrical properties of the perovskite, and also paves a path for the study of suitable external substituent defects that can compensate for dominant intrinsic defects and thus change the properties.


REFERENCES

[1] M.D. Sampson et al., J. Mater. Chem. A. 5, 3578 (2017).
[2] M.T. Klug et al., Energy & Environ. Sci. 10, 236 (2017).
[3] W-J Yin et al., Appl. Phys. Lett. 104, 063903 (2014).
[4] T. Shi et al., Appl. Phys. Lett. 106, 103902 (2015).
[5] Freysoldt et al., Rev. Mod. Phys. 86, 253 (2014).

Tags