Revealing a Discontinuity in the Degradation Behavior of

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Revealing a Discontinuity in the Degradation Behaviour of CHNHPbI during Thermal Operation 3

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Alessandra Alberti, Ioannis Deretzis, Giovanni Mannino, Emanuele Smecca, Salvatore Sanzaro, Youhei Numata, Tsutomu Miyasaka, and Antonino La Magna J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 2, 2017

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The Journal of Physical Chemistry

Revealing a Discontinuity in the Degradation Behaviour of CH3NH3PbI3 during Thermal Operation Alessandra Alberti,†,* Ioannis Deretzis,† Giovanni Mannino,† Emanuele Smecca,† Salvatore Sanzaro,†,‡ Youhei Numata,§ Tsutomu Miyasaka§ and Antonino La Magna† †

National Research Council-Institute for Microelectronics and Microsystems (CNR-IMM), Zona Industriale - Strada VIII n°5, Catania 95121, Italy ‡ Department of Mathematical and Computational Sciences, Physics and Earth Sciences, University of Messina, V. le F. Stagno d’Alcontres 31, Messina 98166, Italy § Graduate School of Engineering, Toin University of Yokohama, 1614 , Kuroganecho, Aoba, Yokohama 225-8503, Japan

ABSTRACT The advance of innovative photovoltaics based on hybrid perovskites is currently forced to face their stability and durability through the rationalization of the phenomena occurring into the lattice under conditions which mimic the material operation. In this framework, we study the structural modifications of MAPbI3 layers by in-situ structural and optical analyses upon recursive thermal cycles from 30°C to 80°C in different annealing environments. We reveal an acceleration of the material modification, above what expected, as the threshold of the tetragonal to cubic transition (~50°C) is surpassed. This produces discontinuities in the degradation rate, bandgap value and dielectric behavior of the MAPbI3 layer. The phenomenon is put in relationship with the order-disorder lattice modifications described by Car-Parrinello molecular dynamics calculations, and reveals that the action of species from humid air becomes largely more effective above 50°C for reasons related to the increased accessibility/reactivity of the lattice which, in turn, impacts on defects generation.

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INTRODUCTION Hybrid Lead Iodide perovskites are revolutionizing views and perspectives in all fields that can take advantage from their high electrical performance [1,2] and wide absorption capabilities (IRUV)[1], thereby attracting interest in photonics and microelectronics. Among others, the most intensively investigated perovskite has the methylammonium+ (MA+) as organic moiety with overall stoichiometry CH3NH3PbI3 (MAPbI3)[3,4,5,6,7,8,9,10,11,12]. However, the intrinsically low structural stability of the MAPbI3 layers risks to retard its wide-range applications in low cost/high yield device technologies[13]. To this end, a large effort is needed for the deep comprehension of the instability sources in relationship with boundary materials[14] and operation conditions, including temperature,[15] illumination[16] and external agents.[16,17,18,19] As a main origin of MAPbI3 degradation, the proton exchange between the organic (MA+) and the Inorganic (I-) moieties of the lattice cage is an intrinsic thermodynamically activated process[20,21] that occurs even in vacuum conditions.[5,22] Proton exchange with the consequent release of volatile species is further promoted by mediators (catalysts) such as water molecules [20,23] or oxygen[19]; thereby humid air is widely regarded as responsible for degradation and indeed requires mitigation actions[24,25]. Although the overall scenario is less obscure than years ago, the back-reaction of perovskites to the starting by-products (PbI2, MAI) needs to be further elucidated especially in proximity of the operation temperature. During the device exposure to heat sources (e.g under the sun), in fact, the effective temperature of the active layer can rise above 60°C .[17,25] This is not ineffective, since a polymorphic transition from a tetragonal to a cubic lattice is expected around 54°C in MAPbI3 layers.[15,22 ,26,27,28]


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In this paper, we intend to disentangle the combined effects of air and temperature on the CH3NH3PbI3 lattice structure under cycles of temperatures which mimic the thermal operation. We thus describe relevant structural and optical changes occurring at the atomic scale into the perovskite lattice during recursive thermal treatments under humid air and nitrogen environments in the range between 30°C and 80°C by using in situ X-ray diffraction (XRD) and Spectroscopic Ellipsometry (SE) analyses. Our description reveals the existence of a discontinuity in the degradation behaviour to be carefully assessed to prospect solutions for longer material durability.

RESULTS AND DISCUSSIONS

The MAPbI3 layer at RT has a tetragonal structure (Figure 1a, 1b, 1c, 1d) with negligible residual PbI2[16]. As often encountered,[15,20,29,30] the layer is highly textured preferentially exposing (001)/(110) planes as growth planes as sketched in figure 1c and represented by the rocking curve (black line) in Figure 1e.[20] This preferentiality causes the peaks at 2θ= 14.1° and at 2θ=28.4° to be predominant over all the other expected peaks. They are generated by the same family of crystallographic planes and are replica for multiple dspacing (002/110 vs. 004/220, respectively). The atomic distribution in a [001] textured lattice is sketched in Figure 1c, with special focus on the location of (211)/(103) planes since diagnostic of the tetragonal structure[20,22,28,31] The sample (fresh) has a brown colour and is made of large micrometer flat domains, as shown in the inset of Figure 1e

[22]

. The flatness and large size of the MAPbI3 domains are mutually

correlated and associated to the high texturing of the material, as effects of the growth


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procedure in presence of Cl species.[29] According to our previous works,[32,29] chlorine atoms do not take part in the perovskite lattice architecture; rather they have a role at the interface with the TiO2 substrate to trigger the texturing of the large MAPbI3 grains that provides the layer with a high degree of spatial uniformity. This represent the pre-requisite that makes bulk studies representative of the overall material behaviour. In the experiment, the MAPbI3 layer was forced to follow thermal cycles from RT to 80°C and back under controlled atmosphere while it was structurally and optically characterised in situ by XRD and SE. We used humid air (55±5%) under the threshold of hydrate phase formation[33,34]. The cycle was repeated 4 times; the sample stays at a given temperature for a time interval of 50 minutes. By XRD, the changes inside the large textured grains composing the MAPbI3 layer were followed by investigating the behaviour of their diagnostic 211/103 planes, whose diffracted intensity is expected to nullify in the cubic simmetry31 . To focus on them, we used specific acquisition methods called χ-scans, consisting in spanning the polar angle χ at the fixed 2θ angle (∼23.47°), moving from χ = 0 up to 85° (follow the arrows in Figure 1c, right panel). Since the textured domains occupy the large part of the layer volume, the analysis will provide a reliable overview on the sample structure. Consistently with the existence of the two main growth axes, two peaks are detected in the fresh sample at χ∼25° and χ∼72°(SI1), as expected on the basis of the domain structure and texturing. The raw peaks collected around χ=25° are shown in Figure 1f. The area underneath is instead represented in Figure 1g as a function of the annealing temperature. We firstly note that the peak area monotonically reduces with increasing the temperature, indicating an average


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continuous progression of the transition. This finding couples with the behavior of the main MAPbI3 peak at 2θ∼14.1° shown in SI2. As the peak disappears, the transition of the MAPbI3 lattice to cubic is completed, and this occurs above 50-60°C, according to the literature31. This structural transition has a certain degree of reversibility with T (this will be deeper explored in what follows) which reflects on the shape of the (002/110) rocking curves (Figure 1e), indicating that the material recovers its long-range bimodality also after the cooling down process. Note also that the cubic lattice is textured similarly to what encountered in the starting tetragonal lattice. The transition of the lattice to cubic implies a symmetrisation of the three crystallographic axes which leads the 002/110-like peaks to merge as shown in figure 1d (for the details see table 1 in SI). We correspondently measured a relative volume increase by 1.2%. The structural continuity in the lattice transition provides a clear insight on the fact that the average lattice configuration depends on the specific working temperature, with MAPbI3 experiencing intermediate configurations between the two extremes represented by the full tetragonal lattice (30°C) and the full cubic lattice (above 60°C). This scenario helps rationalising how the lattice in all is affected even by small changes in the operation temperatures.


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Figure 1. (a) overall diffraction pattern of the as-grown (fresh) MAPbI3 layer, with the diagnostic peak of the tetragonal lattice located at 2θ=23.47°; PbI2 is not detected in the layer; (b) a detail of the (004)/(220) peaks. (c) schematic of the main planes in the I4-mcm MAPbI3 lattice structure, with the intent of showing the inclination angle χ between the main growth axes and the diagnostic 211/103 peaks. The stereographic projection in the right panel represents the expected χ angles distribution in relationship with the main growth axes (001 and 110-type)


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within the domain volume, assuming a rotational degree of freedom along the azimuthal coordinate φ (black rings). Their loss indicates the lattice transition to cubic. (d) as an effect of the tetragonal to cubic transition, the (002)/(110) peaks merge into one due to the gained equivalency of the three crystallographic axes (we correspondently measured a volume expansion by 1.2%: see SI for the details); (e) rocking curves taken at the main peaks shown in d, before and after transition to cubic; (f) (211)/(103) peaks located at χ∼25.1° as a function of the annealing temperature and (g) their integrated area progressively reducing by approaching the cubic lattice arrangement . In this framework, we decided to go deeper in what occurs at the crossover of the polymorphic transition. To do that, we tailored a specific thermal cycle as represented in the upper panels of Figure 2. It allowed exploring the lattice state after each thermal treatment by returning to a control temperature (30°C), additionally elucidating the extent of the lattice reversibility over cycles. During the experiment across the different temperature the sample remains loaded into the analysis chamber, with the probed area thus unchanged. The results were represented in Figure 2 in terms of integrated area and gravity center (g.c.) of the main MAPbI3 peaks at 2θ∼14.1° collected during the cycle vs. time. We grouped the data as follows: data collected at the operation temperature in the left part of Figure 2 (upper panel: data taken at the red circles positions); and data collected at T=30°C (the control temperature) in the right part of figure 2 (upper panel: data taken at the red circles positions). The progressive shift of the gravity center of the 002/110 peak (Figure 2b) accounts for the thermal expansion of the MAPbI3 lattice plus a progressive merging of the two tetragonal peaks into one as the three crystallographic axes become equivalent in the “cubic” (or disordered tetragonal) symmetry. This is in agreement with the results published by Weller et al.[35] ; additionally, we have cyclically crossed the transition threshold pushing the working temperature up to 80°C. This allowed revealing unknown implications of the polymorphic transition on the lattice structure


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of the MAPbI3 layer, especially concerning the lattice stability. As a matter of fact, we revealed a discontinuity in the structural behaviour of the MAPbI3 layer as the material temperature surpassed 50°C. This was expressed by the downwards trend of the 002/110 peak area vs. T (Figure 2a) which is due to a progressive reduction of the MAPbI3 amount. This transformation is not reversible as shown in Figure 2c and d. Figure 2c, in particular, depicts the behaviour of the material after the layer was cooled back, from each annealing temperature, to 30°C and left under nitrogen conditions overnight. The data representation in the timescale provide evidence that the degradation process triggered at temperature above 50°C proceed during time even at 30°C in N2 (irreversible process). Those changes were associated to PbI2 generation by means of the data shown in Figure 2d (peak area taken at 2θ∼12.7°). The PbI2 generated at the end of the cycle corresponds to a MAPbI3 reduction by only 6% (see also inset in fig.1e). We emphasize that the measured reduction of the MAPbI3 amount along the explored range of temperature is highly reliable and representative of the overall material changes since the investigated area was not changed during analyses (sample unloaded). We additionally point out that the method is largely more sensitive than following the generation of PbI2 due to the huge volume contraction associated to the material degradation from MAPbI3 to PbI2 (VPbI2∼54%VMaPbI3). As a comparison and reference, it was verified that using Nitrogen during the annealing (same cycle but fresh sample) has instead a conservative role on the lattice structure within the used range of time[22] (data reported in SI). It signs a baseline for the thermodynamic behaviour of the material.


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Figure 2. XRD data on the thermal cycles in air: (a) and (b) are collected at T>30°C (see red circles in the upper left panel); the data in (c) and (d) are collected at T=30°C (see the coloured boxes in the upper right panel recalled in figure c). The acquisitions were repeated 4 times at the plateau of the steps (isothermal annealing). The sample was taken in nitrogen overnight. The arrows are guides for eyes. The MAPbI3 peak is a finer marker of the material degradation than the PbI2 peak due to the huge difference in the volume between the two (VPbI2∼54%VMaPbI3). In (d) the coloured circles refer to the line colours in the inset representing the raw data on the PbI2 amount at 30°C after annealing. Note the downward trend in the MAPbI3 peak area after the sample has experienced a temperature of 50°C (a) and the progress of the triggered degradation even at 30°C (c). The unaffected behaviour under the all cycle using dry nitrogen is reported in SI. A discontinuous behaviour (accelerated degradation) was not only observed on the structure of the layer (XRD) but also on its optical response. By Ellipsometric Spectroscopic measurements we evaluated the optical dielectric function and consequently the absorption


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coefficient of the layer during the cycle. Experiments in the literature based on optical constant calculations have been performed with the aim of studying the layer degradation[33,36,37] or the temperature effects near or below RT[38,39] ; instead here we explore the effect of temperatures in different environments near the real operation conditions of a cell.

The absorption coefficient vs. photon energy was represented in Tauc plots (Figure 3), and the energy gap (Eg) extracted taking into account that the material has a direct bandgap. The optical data confirms that Nitrogen has a conservative role (Figure 3a); thereby Eg has a normally expected behaviour with the operation temperature[15] and it is fully reversible (data superimposed at 30°C). Conversely, the use of humid air reflects on the tail of the Tauc plot close to the band-edge. It was observed to rapidly span over the starting value as the annealing temperatures overcomes 50°C (Figure 3b) and this causes a countertrend in the extracted Eg with respect to the behaviour in nitrogen, as shown in Figure 3c. Defects generation during annealing is expected due to the thermodynamic modification of the material (intrinsic process)[22,40,41] or due to interaction with catalytic species (extrinsic process)[5,22,,42,44]. Since we see that the Tauc tail close to Eg is modified by the insertion of defects states into the gap, as done by CH3NH2 and HI volatilization during degradation

[23,43],

, we explain the behaviour in

terms of promoted generation of defects above 50°C. We reasoned about this phenomenon in terms of favoured access/interaction/reactivity of external species with the MAPbI3 lattice, with special regard to the water which is well known to be able to enter the lattice as, among the others, reported in ref [44]. We employed ab initio molecular dynamics simulations within the Car-Parrinello scheme (see details in SI)[45] with the purpose of investigating the effect of temperature on the MAPbI3 cage. Figure 3d shows that at room temperature and after a period


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of ∼ 35 ps the MA+ ions acquired an ordered bidirectional orientation, pointing towards two out of the six faces of the cubic inorganic framework (considering the –NH3+ part of the ions). This scheme was preserved until the rest of the simulation period at room temperature, with the MA+ showing confined movements along the x-y plane, which shapes the tetragonal symmetry of the system.[26] When increasing the simulation temperature by 100K (in order to clearly operate within the cubic phase) we observed that the MA+ ions gradually acquired a zcomponent. We describe the transition of the tetragonal arrangement towards the cubic one by a progressive (see fig.1e and f) increase of the out-of-plane MA+ configuration component (along the z-axis) which brings the whole system from a partially ordered towards a disordered state. It was demonstrated [46] that pointing the positive charge of the dipole (NH3+) towards the surface of the layer promotes water molecules entering the MAPBI3 cage with extremely low activation barrier. The maximum of this configurational condition is achieved at the cubic phase. Percolation of water from the surface inside the perovskite cage is argued in ref 44. Thereby we conclude that the described change in the MA+ configuration at the cubic phase accounts for the increased accessibility and/or reactivity of the cage with external species [5,22,44]

, especially if they have a polar character (as water). The final effect is on defects

generation and degradation rate.


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Figure 3. (a,b) Tauc plots of the absorption coefficient α in different environmental conditions as a function of the simulated operation temperature; the change in the tail vs T. around Eg in the Tauc plots is ascribable to the presence of shallow defects states. (c) normalized energy gap trend (Eg at 30°C = 1.62 eV). Eg has a sudden reduction after annealing at 50°C. The insets represent ideal (left side) vs. defective (right side) lattice arrangements. (d) Orientation of the methylammonium ions (considering the –NH3+ direction) at room temperature (left) and at 395 K (right), integratedover a 5 ps interval of simulation time. The colour-scale is indicative of the z coordinate.


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To corroborate the existence of a relationship between threshold behaviour at the transition temperature and defects generation, we performed the critical points analyses of the MAPbI3 dielectric function. It is widely agreed that it has three critical points around 1.6, 2.5 and 3.1eV[39,49] which correspond to the excitations from the highest and second highest valence bands to the lowest conduction band split-off (E0 and E1) and from the doubly degenerate highest valence to the higher level split-off conduction band (E2) in the pseudocubic Brillouin zone, respectively. In particular, on the basis of ref 37, we know that small changes in the CH3NH3+ configuration, which impacts on the inorganic cage arrangement, induce large changes in the optical dielectric function. The critical points are indeed strictly related to the lattice structure (e.g. Pb-I-Pb bond angle, N-I distance) and can account for eventual (permanent) distortions/modifications due to defect formation. An excellent fit of experimental data in Figure 4a and b can be obtained by using different functions like Gaussian oscillators[47], Tauc-Lorentz oscillators [37,48,49], Tauc-Lorentz plus Lorentz oscillators[36] and PSTRI plus Gaussian functions[38,39]. Once the fitting has been done and the real and imaginary part of the optical dielectric functions ε have been calculated, we proceeded with the critical point analysis according to eq.

డమ ఌ డఠ మ

= ݊ሺ݊ − 1ሻ‫ ݁ܣ‬௜஍ ሺ߱ − ‫ ܧ‬+ ݅Γሻ௡ିଶ, where A, Φ, E and Γ are the

amplitude, excitonic phase, energy and broadening of the peak and n is the exponent that in our case assumes a value of -1, being all peaks related to excitonic optical transitions.

[39,50]

Further insights on the critical point analysis can be found in our ref 51.

A trend with a threshold as the temperature surpasses 50°C was observed in all peaks related to humid air conditions (penetration depth at E0 ∼250nm), as shown in detail in Figure 4c,d,e.


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In the figure, we also compare the data collected in air with those resulting from identical cycles in N2; they establish the reference behavior with T. The E2 peak, which is more sensitive to “surface” changes (penetration depth ∼25nm), undergoes in air a red shift as expected by moisture exposure due to I-related defects generation [37,51] (hydrated perovskites not observed by XRD). We therefore conclude that, under humid air, permanent deformations/modifications of the MAPbI3 cage (not ascribed to the working temperature itself as instead settled by the behavior under nitrogen) occurs mostly after the lattice is moved to cubic due to defects generation (permanent distortions). Our findings extend to warning to pay attention in respect to possible increment of atomic species entering the MAPbI3 lattice from boundary materials above 50-60°C (damaging and hysteresis issues) [52]. In positive, they additionally prospect some advantages in letting compliant/stabilising species to penetrate the perosvksite lattice at temperature above the transition threshold.


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Figure 4: . ψ (a) and Δ (b) at 70° incident angle for the fresh sample. (c), (d) and (e): temperature dependence of the interband critical point energies of the MAPbI3 layer for tetragonal and cubic phases during the thermal cycle. Note the threshold behaviour above 50-60°C, which corresponds to the crossover of the tetragonal to cubic transition To conclude, in the framework of the continuous transition which moves the MAPbI3 lattice from tetragonal to cubic we reveal that the configurational disorder gained by the lattice above 50°C impacts on the material degradation. The catalytic action of species in humid air results poorly effective on the MAPbI3 amount, energy gap and dielectric function up to 50°C. Instead, the accelerated changes of all the explored parameters above 50°C (discontinuity) suggests that that action becomes more effective in the cubic arrangement for reasons related to the increased accessibility/reactivity of the lattice with respect to external species. This


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accelerates defects formation which proceeds even after cooling down, thus producing a structural disorder that breaks the staring symmetry of the lattice and leads towards the irreversible collapse of the MAPbI3 architecture into PbI2. Moderate refrigerating methods are therefore warmly advocated to prevent the operation temperature from rising above 50°C. Our results also suggest the use of stabilising species deeply entering the lattice or the use of additives able to move upwards the transition temperature of the perovskite. EXPERIMENTAL METHODS Perovskite materials were deposited by solution processing on TiO2 compact layers[53] following the procedure reported in ref [22] (average thickness ∼350 nm). All the samples were stored in vials under a dry nitrogen atmosphere immediately after preparation to avoid triggering any degradation by humid air before the analyses. All the subsequent analyses were done in dark conditions. XRD patterns were collected by using a D8Discoved (Bruker AXS) diffractometer equipped with an Anton Paar heating stage to keep the samples at controlled temperatures (RT-80°C) in controlled atmospheres (55±5% hr air or dry nitrogen). Car-Parrinello molecular dynamics calculations

[45]

were performed to study the structural transitions of the MAPbI3

lattice from the tetragonal to the cubic phase. To this end, after a long simulation run at room temperature (∼ 50 ps), the target temperature was increased by 100° and the structural changes were observed for 10 ps. Computational details of the simulation setup can be found in a previous publication.26 A J. A. Woollam VASE Ellipsometer equipped with Autoretarder with a Instec Heating stage system attached was used to measure the changes in the optical constants in humid air (55±5% hr) or nitrogen in the range RT-80°C with an accuracy of less than 0.1°C. Once the model has been established using several angles, the analysis has been restricted to


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70° angle. For more details, see the Supporting Information file

SUPPORTING INFORMATION Details on the tetragonal to cubic transition; structural behaviour of the MAPbI3 layer under nitrogen environment; details on the method

AUTHOR INFORMATION Corresponding author *E-mail:[email protected]

Acknowledgements The authors wish to thank Corrado Bongiorno (CNR-IMM) for the stimulating discussions on the lattice stability; Aldo Spada, Alfio Nastasi and Nicolò Parasole (CNR-IMM) for technical support.


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References 

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Table of Contents Graphic

Upper panel: discontinuities in the structural and optical behaviour of a CH3NH3PbI3 layer at the tetragonal to cubic lattice transition, signing an acceleration in the material degradation. Lower panel: MA+ configuration changing from quasi planar (295 K) towards isotropic conditions above the transition temperature (395 K).


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Revealing a Discontinuity in the Degradation Behavior of

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