Elucidating Black α-CsPbI3 Perovskite Stabilization via PPD Bication-Conjugated Molecule Surface Passivation: Ab Initio Simulations

The cubic α-CsPbI3 phase stands out as one of the most promising perovskite compounds for solar cell applications due to its suitable electronic band gap of 1.7 eV. However, it exhibits structural instability under operational conditions, often transforming into the hexagonal non-perovskite δ-CsPbI3 phase, which is unsuitable for solar cell applications because of the large band gap (e.g., ∼2.9 eV). Thus, there is growing interest in identifying possible mechanisms for increasing the stability of the cubic α-CsPbI3 phase. Here, we report a theoretical investigation, based on density functional theory calculations, of the surface passivation of the α-, γ-, and δ-CsPbI3(100) surfaces using the C6H4(NH3)2 [p-phenylenediamine (PPD)] and Cs species as passivation agents. Our calculations and analyses corroborate recent experimental findings, showing that PPD passivation effectively stabilizes the cubic α-CsPbI3 perovskite against the cubic-to-hexagonal phase transition. The PPD molecule exhibits covalent-dominating bonds with the substrate, which makes it more resistant to distortion than the ionic bonds dominant in perovskite bulks. By contrasting these results with the natural Cs passivation, we highlight the superior stability of the PPD passivation, as evidenced by the negative surface formation energies, unlike the positive values observed for the Cs passivation. This disparity is due to the covalent characteristics of the molecule/surface interaction of PPD, as opposed to the purely ionic interaction seen with the Cs passivation. Notably, the PPD passivation maintains the optoelectronic properties of the perovskites because the electronic states derived from the PPD molecules are localized far from the band gap region, which is crucial for optoelectronic applications.

Table S2: Calculated crystallographic parameters of CsPbI 3 bulk perovskites on its α, γ and δ , modeling with different supercells.Lattices constants (a 0 , b 0 and c 0 ), angles (α 0 , β 0 and γ 0 ), volume divided by the number of fundamental units (V 0 / f .u.) and minimal energy divided by the number of fundamental units (E tot / f .u.).The unit cell of δ phase cannot be measured in terms of unitary cell of α-cubic phase (1 × 1 × 1), however since δ phase has the same stecheometry as other phases, we have divided its volume and total energy by 4.
Table S8: Effective Bader charge analysis for α-CsPbI , and δ -CsPbI 3 -(1 × 1 × 1) bulk perovskites.Qi is the average of effective Bader charge for species i = Cs, Pb, and I calculated from the difference between the number of valence electrons and the Bader charge at a given atom.The last column presents the net charge calculated from the formula unit stoichiometry.All effective charges are reported in units of electron charge (e). )

Figure S5 :
Figure S5: PPDI molecules in gas phase.Upper panels show the initial geometries and the bottom panels show the optimized geometries.

Figure S7 :Figure S8 :
Figure S7: The ten relaxed configuration of the PPD/α-CsPbI 3 (100)-(2×1) layers, ordered from lowest to highest energy.The numbers at the top of the crystal structures are the relative energies compared to the lowest energy black-phase material.

Figure S9 :Figure S10 :
Figure S9: The ten relaxed configuration of the PPD/γ-CsPbI 3 (100)-( √ 2× √ 2) layers, ordered from lowest to highest energy.The numbers at the top of the crystal structures are the relative energies compared to the lowest energy black-phase material.

Figure S11 :
Figure S11: The ten relaxed configuration of the PPD/δ F -CsPbI 3 (100)-(1×1) frozen layers, ordered from lowest to highest energy.The numbers at the top of the crystal structures are the relative energies compared to the lowest energy yellow-phase material.

Table S1 :
Computational details of the selected PAW-PBE projectors: number of valence electrons (Z val ), electronic configuration of valence states (Valence), and maximum recommended cutoff energy (ENMAX).

Table S3 :
Local parameters of CsPbI bulk perovskites.Average effective coordination number on Pb atom (ECN Pb av ), average Pb -I -Pb angles (θ PbIPb av ) and average of distance between Pb and I (d PbI av ).

Table S4 :
Electronic band gaps (E g ) of α, γ and δ phases of CsPbI-based perovskites measured by different experimental methods reported.

Table S5 :
Rigid shift of the band gap calculated by means of the hybrid functional HSE (χ HSE ) and using the irreducible k-points of the k-mesh of 4 × 4 × 4 for the α-CsPbI

Table S6 :
Rigid shift of the band gap calculated by means of the hybrid functional HSE (χ HSE ) and using the irreducible k-points of the k-mesh of 3 × 3 × 2 for the γ-CsPbI 3 -( The specific mixing coefficient of exact exchange (α XX ) used in Hartree-Fock calculations is also indicated.Since we have direct band gap, the band gap rigid shift on k Fpoint is indicated in boldface text.

Table S7 :
Rigid shift of the band gap calculated by means of the hybrid functional HSE (χ HSE ) and using the irreducible k-points of the k-mesh of 5 × 2 × 1 for the δ -CsPbI 3 -(1 × 1 × 1) bulk perovskite.The specific mixing coefficient of exact exchange (α XX ) used in Hartree-Fock calculations is also indicated.Since we have direct band gap, the band gap rigid shift on k F -point is indicated in boldface text.

Table S10 :
Nearest distances between H and N atoms (d N−H ) and H and I atoms (d H−I ) of the two NH 3 groups of PPD, PPD 2+ and PPDI molecules.All values are reported in Å.

Table S11 :
Effective Bader charge analysis of PPD molecule in gas phase.Q is the Bader charge, Z val is the number of valence electrons and Q e f f = Z val − Q is the effective Bader charge.The atoms that make up the NH 3 groups are indicated in boldface.All values are reported in units of electron charge (e).

Table S12 :
Effective Bader charge analysis of PPD 2+ molecule in gas phase.Q is the Bader charge, Z val is the number of valence electrons and Q e f f = Z val − Q is the effective Bader charge.The atoms that make up the NH 3 groups are indicated in boldface.All values are reported in units of electron charge (e).

Table S13 :
Effective Bader charge analysis of PPDI putative global minima configuration (PGMC) molecule in gas phase.Q is the Bader charge, Z val is the number of valence electrons and Q e f f = Z val − Q is the effective Bader charge.The I atoms and those forming the NH 3 groups are indicated in boldface.All values are reported in units of electron charge (e).

Table S14 :
Effective Bader charge analysis of PPDI with higher energy configuration (HEC) molecule in gas phase.Q is the Bader charge, Z val is the number of valence electrons and Q e f f = Z val − Q is the effective Bader charge.The I atoms and those forming the NH 3 groups are indicated in boldface.All values are reported in units of electron charge (e).

Table S15 :
Rearrangement following the minimum energy of the several configuration of the PPD/α-CsPbI 3

Table S17 :
Relative energy (∆E) of the several configuration of the PPD/α-CsPbI 3

Table S18 :
Surface formation energy (E S F ) of the several configuration of the x/α-CsPbI 3 (100)-