Compound Defects in Halide Perovskites: A First-Principles Study of CsPbI₃

Abstract

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.

Trends for Acceptor Dopants in Lead Halide Perovskites

“…The case of CsPbI 3 is intermediate between CsPbCl 3 and MAPbI 3 : native defects are more likely to compensate the acceptors in CsPbI 3 than in CsPbCl 3 , but the compensation is not as significant as that in MAPbI 3 . Our results are very similar to previous reports on native defects in CsPbI 3 . ,, Though the Pb/I antisites (Pb I and I Pb ) are important NRR centers in CsPbI 3 , they are too high in energy to compensate Na Pb acceptors under the I-rich conditions we are interested in. In contrast, Pb Cs and I i have lower formation energies than Na Pb when E F is low under I-rich conditions but only when the Fermi level is within 0.25 eV of the VBM.…”

“…Our results are very similar to previous reports on native defects in CsPbI 3 . 23,37,38 Though the Pb/I antisites (Pb I and I Pb ) are important NRR centers in CsPbI 3 , 23 they are too high in energy to compensate Na Pb acceptors under the I-rich conditions we are interested in. In contrast, Pb Cs and I i have lower formation energies than Na Pb when E F is low under I-rich conditions but only when the Fermi level is within 0.25 eV of the VBM.…”

Defect Quantification in Metal Halide Perovskites Anticipates Photoluminescence and Photovoltaic Performance

“…74,75 These new midgap cation defect states introduced in the excess FAI film suggest the presence of point defects where the overall coordination environment of the central heavy metal cations is changed so that, in the defect, it is easier to reduce (or farther from vacuum), for example where Pb replaces an I site (i.e., Pb I ) as discussed in previous modeling studies. 71,76 The spectroelectrochemical data confirms that cation and anion defects demonstrate energetic distributions much like "Urbach tails" that have been recently proposed for perovskite optoelectronic materials. 61 For the excess PbI 2 active layers, anion defects are below our detection limit (below 10 14 cm −3 ), while cation defects are distributed energetically similar to the stoichiometric films as demonstrated in Figure 4.…”

Precrystallization of the Cs2PbI2Cl2 Intermediate Phase for MA- and Br-Free Wide Bandgap Perovskite Solar Cells

“…17,18,22,26 However, this process typically requires annealing temperatures higher than 185 °C. 18,22 Higher annealing temperatures increase the concentration of point defect states (as calculated by the first-principles density flooding theory (DFT)), 27 which can reduce the efficiency of the device and increase open-circuit voltage loss. 28−30 This indicates that decreasing the annealing temperature of the perovskite film can effectively decrease the defect density.…”

“…Mixing A-site cations in compositional engineering is essential to boost thermal and structural stabilities and inhibit halide segregation. , Moreover, the high Cs content at the A-site rather than Br at the X-site is preferable in improving the phase stability of the perovskite layer . Cs-rich FA 1– x Cs x PbI 3 ( x > 0.3) perovskites with high Cs content at the A-site exhibit promising phase stability, excellent thermal stability (lack of volatile MA components), , and tunable bandgap making them suitable for addressing the issue of photoinduced halide segregation. , However, achieving high-quality Cs-rich FA 1– x Cs x PbI 3 ( x > 0.3) with superior PV performance remains challenging due to the large lattice mismatch and different phase transition temperatures between FAPbI 3 and CsPbI 3 perovskites. ,, The Cs 4 PbI 6 intermediate method is the most commonly used method for preparing Cs-rich FA 1– x Cs x PbI 3 perovskites, which involves forming Cs 4 PbI 6 and an FA-rich perovskite phase in the raw film, followed by a solid-state ion-exchange reaction to obtain the target perovskite and eliminate excess FAI. ,,, However, this process typically requires annealing temperatures higher than 185 °C. , Higher annealing temperatures increase the concentration of point defect states (as calculated by the first-principles density flooding theory (DFT)), which can reduce the efficiency of the device and increase open-circuit voltage loss. − This indicates that decreasing the annealing temperature of the perovskite film can effectively decrease the defect density.…”