Mechanism of Reversible Trap Passivation by Molecular Oxygen in

D3-CompuNet, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy. ACS Energy Lett. , 2017, 2 (12), pp 2794–2798. DOI: 10.1021/acsene...
7 downloads 11 Views 2MB Size
Download PDF
http://pubs.acs.org/journal/aelccp

Mechanism of Reversible Trap Passivation by Molecular Oxygen in Lead-Halide Perovskites Daniele Meggiolaro,†,‡ Edoardo Mosconi,*,†,‡ and Filippo De Angelis*,†,‡ †

Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, Via Elce di Sotto 8, 06123 Perugia, Italy D3-CompuNet, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy

‡

S Supporting Information *

ABSTRACT: The long lifetime and diffusion lengths of carriers and the defect tolerance of lead-halide perovskites are at the heart of their high efficiency in solar cell devices. Drastic photoluminescence quantum yield enhancements upon exposure of perovskites to molecular oxygen have been reported. Despite the general consensus about a direct role of oxygen in tuning the optical properties of perovskites, the microscopic origin of this effect is still under debate. On the basis of state-of-the-art density functional theory modeling, we propose a mechanism whereby oxygen effectively inactivates deep hole traps associated with iodide interstitials by forming moderately stable oxidized products. The small energy gain associated with trap passivation is in agreement with the reversibility of the process. Vibrational analysis points at the emergence of new active modes related to oxidized defect products, spanning the 300− 700 cm−1 frequency range, which may be probed experimentally.

T

opposite picture was also reported, however, whereby iodide oxidation in the precursor solution would lead to an understoichiometric reactants ratio and decreased solar cell performance.15 Also, the presence of O2 was associated with degradation of the perovskite, possibly through the formation of a perovskite-intercalated superoxide anion formed under light irradiation.16,17 Understanding the interaction of lead-halide perovskites with molecular oxygen is thus fundamental both for improving device performance and temporal stability. In a previous study, some of us reported that O2 molecules may only weakly interact with nondefective MAPbI3, with O2 incorporation into the perovskite lattice being endothermic by 0.17 eV.18 Fast O2 diffusion into MAPbI3 was recently proven and found to be mediated by defects.16 However, no models of how O2 would boost the PLQY in lead-halide perovskites have been reported, to our knowledge. Here we specifically focus on the interaction of defective MAPbI3 with O2 to understand the role of oxygen in possible trap annihilation. A high-level computational study based on hybrid density functional theory (DFT) incorporating spin−orbit coupling (SOC)19 revealed that a main defect introducing electronic states into the MAPbI3 band gap is interstitial iodine (Ii), which is stable in its +1 (acting as an electron acceptor through the +/0 transition, placed at 0.54 eV below the CB) and −1 (hole

he unusual optoelectronic properties of lead-halide perovskites are central to their outstanding performance in photovoltaic devices.1,2 Despite being solutionprocessed, these materials behave as close to defect-free semiconductors, both in single crystals and, even more surprisingly, in polycrystalline thin films. The apparently medium to low trap density observed in polycrystalline thin films and single crystals, ranging from ∼1016 to ∼1011 cm−3,3−5 respectively, is likely to be related to the unusual chemistry of the constituent lead and halide elements and to the soft nature of their chemical bonds. The photoluminescence quantum yield (PLQY) of leadhalide perovskites is usually taken as a proxy for the optoelectronic material quality. Higher PLQYs imply a reduction of nonradiative deactivation pathways, with longlived and long-transported charge carriers allowing efficient charge collection at selective contacts. An interesting observation that has been repeatedly reported is the drastic PLQY enhancement upon exposure of prototypical lead-halide perovskites (e.g., MAPbI3 and MAPbBr3) to dry air or oxygen.6−12 This observation indicates that controlled postsynthesis exposure to oxygen or mild oxidants could lead to trap annihilation and improved solar cell performance.13 Interestingly, the effect can be reversible over a few cycles.13 This suggests that the mechanism underlying such behavior should be related to a fairly weak chemical interaction between O2 and the perovskite. Impedance spectroscopy results also suggested a reversible effect of oxygen on MAPbI3 electronic properties, associated with a dedoping of the material.14 An almost © XXXX American Chemical Society

Received: October 2, 2017 Accepted: November 8, 2017

2794

DOI: 10.1021/acsenergylett.7b00955 ACS Energy Lett. 2017, 2, 2794−2798

Letter

Cite This: ACS Energy Lett. 2017, 2, 2794-2798

Letter

ACS Energy Letters

Figure 1. (a) Thermodynamic ionization levels calculated by HSE-SOC of Ii (black/gray lines) showing the energetics for trapping a hole by Ii− (0/−, black line), see calculated structure in b; or for trapping an electron by Ii+ (+/0, gray line, structure not shown); or for trapping two holes or two electrons by Ii− or Ii+, respectively (+/−, gray line). The red, blue, and green lines correspond to the energetics of the (0/−) Ii− transition after interaction with O2 to form the products shown in panels c−e. (b−e) Structure of Ii− and of its interaction products with 1/2 O2, O2, and 3/2 O2 along with the calculated energetics (ΔE, eV). Atom color code: yellow, Ii; purple, I; red, O; light blue, Pb. Methylammonium cations are shadowed in the background for clarity.

acceptor, 0/− transition 0.30 eV above the VB) charge states (Figure 1a). The +/− transition located roughly at mid gap is kinetically impeded by the two-electron nature of this process, which leads to a vanishingly small capture cross section. The presence of interstitial iodine in MAPbI3 was recently confirmed by a combined XRD and neutron diffraction study,20 with time-domain ab initio simulations confirming hole trapping at Ii−.21 Ii+ shows the trimer structure of an I3− molecule with one central I+ bounded by two I− on the sides,19 which are Pb-coordinated (not shown); Ii− shows two symmetric I− ions binding in a bridging coordination to two vicinal lead atoms (Figure 1b). Considering the chemical nature of interstitial iodine, O2 will likely interact with the electron-rich (hole accepting) Ii−, characterized by a high-lying HOMO, while the electron poor Ii+ mainly represents an electron acceptor because of its low-lying unoccupied state (corresponding to the I3− LUMO). We thus investigate the interaction of O2 with negative interstitial iodine employing a combination of scalar relativistic and hybrid-DFT calculations including SOC. We use scalar relativistic GGA-DFT calculations to evaluate geometrical structures and energetics of O2 reactions based on a 2 × 2 × 2 tetragonal supercell (384 atoms) at the experimental cell parameters. Hybrid HSE06-SOC22 calculations carried out on a smaller 2 × 2 × 1 tetragonal supercell are then used to assess the impact of O2 on the charge-trapping energetics of interstitial iodine. Because negative interstitial iodine is essentially an added iodide (oxidation state −1) to the perovskite lattice, we focus on the typical products of iodide oxidation, as summarized in the equations below:

I− + O2 → IO2− (iodite, 4 electron oxidation, iodine oxidation state +3)

(2)

I− + 3/2O2 → IO3− (iodate, 6 electron oxidation, iodine oxidation state +5) (3)

The +3 oxidation state of iodine is generally unstable, as it tends to disproportionate to the +1 and +5 states. We thus expect reaction 2 to be thermodynamically disfavored compared to reactions 1 and 3. The calculated energetics of reactions 1−3 are +0.02, + 0.13, and −0.05 eV, respectively (see Figure 1c−e and Table 1). In Table 1. Calculated Energetics, ΔE (eV), for the Oxidation of an Interstitial Ii− Compared to the Oxidation of a Lattice I− Species oxidation product

defect Ii− oxidation (eV)

lattice I− oxidation (eV)

IO− IO2− IO3−

0.02 0.13 −0.05

0.13 0.14 0.11

all the investigated cases, oxidation of Ii− is favored over the possible oxidation of a lattice iodide in the same supercell containing Ii−, with energy differences of up to 0.16 eV, see Table 1 and structures in the Supporting Information, implying that Ii− will be preferentially oxidized by interaction with O2 rather than lattice iodide. This is somehow expected based on the thermodynamic transitions of Figure 1a, showing that Ii− introduces a charge transition 0.3 eV above the VB; this difference approximately corresponding to the difference in oxidation potential of electrons residing in the defect or in the VB. The calculated structures of the reaction products are shown in Figure 1c−e and clearly correspond to formation of

I− + 1/2O2 → IO− (hypoiodite, 2 electron oxidation, iodine oxidation state +1) (1) 2795

DOI: 10.1021/acsenergylett.7b00955 ACS Energy Lett. 2017, 2, 2794−2798

Letter

ACS Energy Letters

Figure 2. Reaction products of iodide oxidation at MAI- (a) and PbI2-terminated (b) surfaces. In panel a, a surface apical lattice iodide is oxidized, while in panel b, a surface equatorial lattice iodide is oxidized. The IO3− oxidation products are highlighted by yellow circles. Atom color code: purple, I; red, O; light blue, Pb. Methylammonium cations are shadowed in the background for clarity.

coordinated IO−, IO2−, and IO3− within the perovskite lattice. As a matter of fact, while reactions 1 and 3 are almost thermoneutral, reaction 2 leading to iodine in the +3 oxidation state is slightly thermodynamically disfavored. In the interstitial IO− oxidation product, the interacting iodine and oxygen atoms are bonded at a distance of 2.08 Å and are both coordinated by two Pb ions lying in the perovskite ab crystal plane at distances of 3.29 and 2.26 Å, respectively. Similarly, in the IO2− species, two oxygen atoms are bonded to the interstitial iodine at distances of 1.97−2.00 Å. In this configuration, the IO2− molecule is aligned along the perovskite [111] direction with the oxygen atoms coordinated by two Pb ions placed in contiguous ab planes at distances of 2.30−2.35 Å. Finally, the IO3− product is made up by three oxygen atoms bonded to the interstitial iodine at mean distances of 1.90 Å and coordinated by three vicinal Pb ions in the ab plane with bond lengths between 2.46 and 2.65 Å. Quite surprisingly, all the oxidation products can be accommodated within the perovskite, thanks to the chemical flexibility of iodine in its various oxidation states and to the soft nature of the lead-halide lattice. Of particular note is the strong tendency of all structures to form Pb−O−Iox−Ired bond networks (Iox and Ired stand for iodine in positive and negative oxidation states, respectively) with the perovskite-bound IO3− showing an octahedral coordination of the central iodine atom (Figure 1e). To investigate the impact of the oxidized defects on the optoelectronic perovskite properties, we have calculated the thermodynamic ionization levels at the HSE06-SOC level for the considered species in Figure 1a. As can be noticed, oxidation of the negatively charged interstitial iodine leads to a drastic decrease of the energy associated with the deep 0/− transition which becomes shallow (≤0.1 eV above the valence band in all cases) with consequent diminished trapping activity and impact on the perovskite optoelectronic properties. We can furthermore predict that given the fairly low reaction energetics, the process of iodide oxidation could be easily reversed by, for example, keeping the material under vacuum, whereby the entropy gain associated with O2 release in the gas phase could overcome the moderate energetic stabilization of the IO3− oxidation product. Notably, the calculated energetics for iodide oxidation change drastically if one considers interaction with the perovskite surface, rather than bulk (Figure 2). The interaction of O2 with lattice iodide located at the surface (notice, we do not consider interaction with an interstitial defect here but with a lattice iodide; thus, the considered slab has neutral total

charge) is calculated to be strongly exothermic, possibly implying an irreversible interaction underlying a surfacemediated degradation mechanism. Iodide oxidation on the surface is calculated to be exothermic by ∼0.6−0.9 eV (Figure 2), to be compared to the ∼0.1 eV endothermic reaction calculated for the bulk (see Table 1). The passivation of surface traps induced by oxygen may also involve other defects, such as iodine vacancies and lead interstitials, whose neutral charged states are stabilized at the perovskite surface.23 As a further step, we calculated the local vibrational modes of the various bulk oxidation products and superimposed them to the full infrared (IR) phonon spectrum of MAPbI3 in Figure 3.

Figure 3. (a) IR phonon spectrum of MAPbI3 in the range up to 1000 cm−1. (b−d) Calculated vibrational modes of the considered Ii− oxidation products within the MAPbI3 bulk.

Our calculations clearly predict a series of new vibrational frequencies in the 100−700 cm−1 range. MAPbI3 is not absorbing (neither IR24 nor Raman25) in the 300−800 cm−1 range, thus suggesting a “transparent” region to be possibly investigated by vibrational spectroscopy. Three main classes of vibrational modes can be identified for the oxidation products: (1) low-frequency modes (up to 200 cm−1) mainly associated with bending of the interstitial iodine atoms and the inorganic sublattice, superimposed to native MAPbI3 modes; (2) middlefrequency modes (200−600 cm−1) associated with the I−O−I bending modes; and (3) high-frequency modes associated with the I−O stretching modes above 600 cm−1. Our simulated phonon spectra are fully consistent with the reported vibrational spectrum of IO3− salts,26 showing three groups of 2796

DOI: 10.1021/acsenergylett.7b00955 ACS Energy Lett. 2017, 2, 2794−2798

Letter

ACS Energy Letters bands at ∼120−205 cm −1 (Raman); ∼300−320 cm −1 (Raman); and ∼750−820 cm−1 (both IR and Raman active).26 Remarkably, the broad band spanning the ∼200−250 cm−1 range found in the experimental Raman spectra of MAPbI325,27,28 was assigned to degradation of the perovskite occurring during the measurements.27 Our calculations for the bulk MAPbI3 perovskite containing oxidation products clearly indicate several new transitions in the 200−300 cm−1 region, which we can associate to modes related to oxidation products. A weak band at ∼770 cm−1 was also observed along with the ∼250 cm−1 band in the Raman spectrum of the orthorhombic MAPbI3 phase,28 which is consistent with the presence of iodide oxidation products. In summary, we have proposed a possible mechanism for O2 interaction with lead-halide perovskites which may account for the reversible PLQY enhancement upon material exposure to O2. The oxidation of interstitial iodine is favored over that of lattice iodine, effectively inactivating a source of deep traps with levels located in the MAPbI3 band gap. The thermo-neutral or slightly exothermic oxidation reactions calculated for the perovskite bulk are consistent with a reversible interaction. Long-term exposure to O2 may, however, switch on oxidative damage reactions, initiated at surfaces, which may limit the material temporal stability. Vibrational frequency analyses indicate that the 200−300 and 600−800 cm−1 regions of the phonon spectrum could be further investigated to identify the signature of oxidation products.

in order to describe accurately the electronic structure of MAPbI3. By this approach a band gap for MAPbI3 of 1.58 eV was obtained, in agreement with the work by Du.19 The thermodynamic ionization levels ε(q/q′) have been evaluated by using the equation ε(q/q′) = [Ed(q) − Ed(q′)]/(q′ − q) − EV, where Ed(q) is the energy of the defect supercell in the charged state q and EV is the valence band edge of the pristine MAPbI3. Hybrid calculations have been performed on the optimized PBE structures in the 2 × 2 × 1 supercell by using norm-conserving pseudopotentials with the same number of electrons in the valence as those used in the PBE optimizations, except for Pb ions where a pseudopotential with 22 electrons has been used by including the 5s and 5p states into the valence. An energy cutoff on the wave functions of 40 Ry and 1 × 1 × 2 grids of k points in the Brillouin zone have been used.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00955. Optimized structures of lattice iodide oxidation products; full citation for ref 29 (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

■

*E-mail: [email protected]. *E-mail: fi[email protected].

METHODS All calculations were carried out by using the Quantum Espresso package.29 Geometry optimizations were carried out by the PBE functional30 on 2 × 2 × 2 and 2 × 2 × 1 tetragonal supercells at the experimental cell parameters.31 Electron−ion interactions were described by scalar relativistic ultrasoft pseudopotentials by including into the valence the 2s, 2p states for O, N, and C; 1s for H; 5s, 5p for I; and 6s, 6p, 5d for Pb ions. Plane-wave basis set cutoffs for the smooth part of the wave functions and the augmented density were 25 and 200 Ry, respectively. To simulate the perovskite surfaces, we have properly cut 2 × 2 slabs from the bulk tetragonal MAPbI3 crystal structure, which expose MAI- and PbI2-terminated surfaces. The exposed surfaces are the (001) ones, obtained from an optimized bulk structure showing an isotropic arrangement of the MA organic cations. Along the c axis an additional 10 Å of vacuum was used to simulate surface slabs. Calculations of both vibrational frequencies and thermodynamic ionization levels with the hybrid exchange-correlation functional have been carried out in the 2 × 2 × 1 perovskite supercell, in order to reduce the computational effort. To this aim, the equilibrium structures of the oxidized interstitials, i.e., IO−, IO2−, and IO3−, have been reoptimized in the smaller 2 × 2 × 1 supercell. Negligible variations (