Lucy Whalley1
Northumbria University1
The unusual defect chemistry and physics of lead halide perovskites has attracted significant attention as point defects in these materials are associated with a range of processes including ion diffusion, hysterisis and degradation.<sup>1,2</sup> While the high voltages and light-to-electricity conversion efficiencies indicate a low rate of non-radiative electron-hole recombination relative to other photovoltaic materials, understanding non-radiative capture processes remains crucial for the development of high-efficiency devices with increased stability.<br/><br/>The most stable materials employ mixed cations on the perovskite A-site, however the reason for this improved performance is not fully understood. While there has been a number of computational defect studies for the single cation compounds, most commonly methylammonium (MA) lead iodide, less is known about the impact of cation mixing on the defect physics of these materials.<br/>We will present results from the quantum chemical simulations of single cation (MA) and mixed cation (Ma/Cs) materials. We will consider both the pristine materials and the materials with an iodine interstitial defect, which has been established as an active site for non-radiative charge trapping.<sup>3</sup><br/><br/>First we will demonstrate that cation mixing on the A-site leads to increased octahedral tilting in the minimum energy (relaxed) structures of the pristine materials. This supports previous results which show that octahedral tilts are "locked-in" for the mixed-cation compounds.<sup>4</sup> We will then analyse the quantum mechanics of carrier trapping at the iodine interstitial. We will show that the lattice relaxation after charge capture is also mediated through octahedral tilting of the inorganic cage, and so can be tuned through A-site composition. Using a computational framework for calculating non-raditaive capture rates from first-principles<sup>3,5</sup> we will quantify the impact that cation mixing has on the rates of electron and hole capture at the iodine interstitial.<br/><br/>Finally, we will demonstrate that due to the rotational motions of the A-site molecular cation after charge capture, the commonly used approximation of linear interpolation for constructing the potential energy surface cannot be applied to this system. Instead, we will discuss the use of Kabsch interpolation and anharmonic potential energy surfaces for systems where rotational motion is significant. Our results highlight the interplay between halide perovskite composition, structural dynamics and defect physics, and we expect these findings to be more widely applicable to systems where dynamic octahedral tilting is evident.<br/><br/>[1] Ni, Z. et al. (2022) <i>Nat. Energy </i><b>7 </b>65–73<br/>[2] Yihua Chen<i> </i>et al. (2020) <i>J. Appl. Phys.</i> <b>128</b> 060903<br/>[3] L. Whalley et al.<i> </i>(2021) <i>J. Am. Chem. Soc.</i> <b>143</b> 9123–9128<br/>[4] D. Ghosh et al. (2017) <i>ACS Energy Lett.</i> <b>2</b> 2424–2429<br/>[5] Kim et al. (2020) <i>J. Open Source Softw. </i><b>5</b> 2102.