Methylammonium fragmentation in amines as source of localized trap levels and the healing role of Cl in hybrid lead-iodide perovskites

The resilience to deep traps and localized defect formation is one of the important aspects that qualify a material as a suited photoabsorber in solar cell devices. Here we investigate by ab initio calculations the fundamental physics and chemistry of a number of possible localized defects in hybrid methylammonium lead-iodide perovskites. Our analysis encompasses a number of possible molecular fragments deriving from the dissociation of methylammonium. In particular, we found that in stoichiometric conditions both ammonia and methylamine molecules present lone-pair localized levels well within the perovskite band gap, while the radical cation ${\mathrm{CH}}_2{\mathrm{NH}}_3^{+}$ observed by EPR after irradiation injects partially-occupied levels into the band gap but only in $p$-type conditions. These defects are thus potentially capable of significantly altering absorption and recombination properties. Amazingly, we found that additional interstitial Cl is capable of removing these localized states from the band gap. These results are consistent with the observed improvement of photoabsorption properties due to the Cl inclusion in the solution processing.

Figure 1

(a) Density of states of bulk ${\mathrm{MPbI}}_3$. The 0 energy is set to the valence band maximum. To better visualize the molecular levels, they are plotted in separate panels, one for each atomic type. Top panel: total DOS (green). Bottom panels: projected DOS on atomic states of methylammonium molecule. HC and HN are hydrogens bound to C and N, respectively; black and red lines refer to $s$-type and $p$-type states. (b) scheme of methylammonium orbital levels, as inferred for the calculated DOS.

Figure 2

Atomic and electronic structure of the perovskite after ${\mathrm{CH}}_3{\mathrm{NH}}_3\to {\mathrm{CH}}_3+{\mathrm{NH}}_3$ (a) dissociation process. (a): Atomic structure in close-by configuration: ${\mathrm{CH}}_3$ migrates onto a ${\mathrm{PbI}}_4$ plaquette to form a iodomethane molecule (${\mathrm{CH}}_3\mathrm{I}$, the CI dimer indicated by cyan and red balls), while ${\mathrm{NH}}_3$ remains close to the initial A-site of the perovskite. (b): corresponding DOS. Upper panel: total DOS (green); lower panels: atom-resolved DOS for ${\mathrm{CH}}_3\mathrm{I}$ and ${\mathrm{NH}}_3$ molecules (orbital contribution labels are the same as the DOS in Fig. 1). We take the VBM as the reference 0 energy. ${\mathrm{E}}_F$ is placed at the middle between the highest occupied level and the CBM.

Figure 3

Structural optimization of the ${\mathrm{CH}}_3{\mathrm{NH}}_3\to {\mathrm{CH}}_4+{\mathrm{NH}}_2$ dissociation (c); (a) ${\mathrm{NH}}_2$ migrates into a ${\mathrm{PbI}}_4$ plaquette to form a iodoamine like complex (${\mathrm{NH}}_2-\mathrm{I}$), and methane (${\mathrm{CH}}_4$) remains stuck to the starting M site. (b): corresponding DOS, and atomic projected DOS. Orbital types and labels are the same as in Fig. 2. Reference 0 energy is placed at the VBM. (c) and (d): schemes of the molecular levels, as inferred from the atomic-projected DOS.

Figure 4

Simulation of the ${\mathrm{CH}}_3{\mathrm{NH}}_3\to {\mathrm{CH}}_3{\mathrm{NH}}_2+\mathrm{H}-\mathrm{I}$ dissociation: one H migrates into a ${\mathrm{PbI}}_6$ octahedron to form hydrogen iodide (HI), leaving a stable methylamine molecule (${\mathrm{CH}}_3{\mathrm{NH}}_2$) close to the original M-site. (a): Atomic resolved DOS in the full energy range. The 0 reference energy is placed at the VBM; ${\mathrm{E}}_F$ is placed at the middle between occupied and empty states. Orbital types and labels are the same as in Fig. 1. (b) DOS in a small energy interval around the band gap, to better visualize the molecular levels overlapping with the valence bands (see text). (c): Band structure along the $\mathrm{\Gamma }-Z$ direction. The flat red band is due to the N($2p$) lone-pair levels of methylamine. The blue band is related to H-I empty levels. The dashed line indicates the level of the conduction band bottom.

Figure 5

(a) Structure of the ${\mathrm{CH}}_2{\mathrm{NH}}_3-\mathrm{I}$ complex. The I ion is displaced from its equilibrium position to bind to the deprotonated fragment. (b) electronic structure of the ${\mathrm{CH}}_2{\mathrm{NH}}_3-\mathrm{I}$ complex, illustrated by the DOS projected on ${\mathrm{CH}}_2{\mathrm{NH}}_3$ and I sites. States are filled up to the valence band minimum (VBM) which is placed at 0. Except for the H-I peak close to the conduction band there are no other defect states within the gap. Five distinct peaks (see text) are present in the projected DOS of ${\mathrm{CH}}_2{\mathrm{NH}}_3$; C-derived $s-p$ states are strongly hybridized with the I $5p$ states.

Figure 6

Scheme of the stable fragments considered in the present work under stoichiometric conditions. For each configuration the molecular levels closer to the band gap region are also indicated. In particular, amine groups display localized lone-pair HOMO and LUMO orbitals well into the band gap which are eventually removed after addition of interstitial Cl.

Figure 7

Density of states close to the VBM for two ${\mathrm{MPbI}}_3$ supercells containing one methylamine at different hole doping concentrations. Top-panel: a bulk ${\mathrm{MPbI}}_3$ with one methylammonium substituted by one methylamine (i.e., one H subtracted) in the 192-atom supercell. This corresponds to a hole doping concentration of $2.74\times {10}^{20}{\mathrm{cm}}^{-3}$. Bottom panel: a bulk ${\mathrm{MPbI}}_3$ plus a M-vacancy plus a M-to-methylamine substitution (2 holes per supercell). The 0 reference energy is placed at the VBM and the Fermi level value ${\mathrm{E}}_F$ has been computed with the linear tetrahedron method.

Figure 8

(a) Atomic structure of the ${\mathrm{CH}}_2{\mathrm{NH}}_3^{+}-\mathrm{I}$ complex in $p$-type conditions. (b) Electronic structure of the complex illustrated by the DOS projected on the atomic orbitals of the molecules. The Fermi level energy ${\mathrm{E}}_F$ is computed with the linear tetrahedron method.

Figure 9

(a) Atomic structure obtained upon structural optimization after inclusion of one Cl in a bulk-like MPbI3 supercell; (b) atomic structure upon structural optimization with Cl included in a supercell after M dissociation in methylamine plus hydrogen iodide. In both cases, Cl ends up in the octahedral cage, binding with Pb atoms. (c) DOS relative to the system displayed in (b). The upmost panel is the total DOS, lower panel the DOS of the methylamine molecule, resolved per atoms. As usual red and black colors represent $p$ and $s$ states. (d): Band structure for the same system along $\mathrm{\Gamma }-\mathrm{Z}$. The green line is the HOMO of methylamine, the blue line a HI state, both laying within the band gap before Cl inclusion [Fig. 5].

Figure 10

Schematic representations of the effects of $p$-type doping; (left) the molecular level of the methyl-type fragment is able to trap one hole and it forms a singly occupied level right in the gap; (right) the lone pair of the methylamine fragment is always doubly occupied, and shifts below the Fermi level in $p$-type conditions.