First Evidence of CH3NH3PbI3 Optical Constants Improvement in a

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First Evidence of CHNHPbI Optical Constant Improvement in N Environment in the Range 40-80 °C 2

Giovanni Mannino, Alessandra Alberti, Ioannis Deretzis, Emanuele Smecca, Salvatore Sanzaro, Youhei Numata, Tsutomu Miyasaka, and Antonino La Magna J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00764 • Publication Date (Web): 21 Mar 2017 Downloaded from http://pubs.acs.org on March 24, 2017

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The Journal of Physical Chemistry

First Evidence of CH3NH3PbI3 Optical Constant Improvement in N2 Environment in the Range 40-80 °C

Giovanni Mannino1, Alessandra Alberti1*, Ioannis Deretzis1, Emanuele Smecca1, Salvatore Sanzaro1, Youhei Numata2, Tsutomu Miyasaka2 and Antonino La Magna1

1

CNR-IMM Zona Industriale, Strada VIII 5, 95121, Catania, Italy

2

Graduate School of Engineering, Toin University of Yokohama, 1614, Kuroganecho, Aoba,

Yokohama 225-8503, Japan

ABSTRACT We study the optical response of CH3NH3PbI3 layers to light solicitation of the material under different environmental gas and temperature conditions. The measurements were performed in non-reactive (Ar or N2) and reactive (O2 or humid air) in the range 40-80 °C crossing the tetragonal-cubic transition (∼50 °C). With respect to truly inert Ar, the use of N2 not only assures the reversibility of the optical constants during thermal cycles but also improves the optical response of the material. While in N2 and Ar atmospheres the optical parameters of the material can be recovered at the end of the cycle, on the opposite the presence of humidity in the air causes the absorption coefficient to monotonically and inexorably decrease in the whole visible range, especially after the lattice has moved to cubic. The use of N2 thus represents an effective strategy to improve the absorption under thermal operation conditions.

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INTRODUCTION A great attention is progressively rising in the literature to face the stability of photoactive hybrid perovskites during simulated operation conditions, including temperature illumination.

2,3

1

and

The efforts are mostly devoted to the comprehension of the mechanisms

governing the back-reaction of perovskites to the starting reactants (PbI2, MAI) that pilot the devices to failure. Since several variables are involved (temperature, water, oxygen, light illumination, UV, interfaces, texture, grain boundaries, etc.)

2,4,5

standard experimental

conditions have been explored in the literature in order to try to discriminate and to weigh the different contributions. In this respect, a clear evidence of the disruptive effect of combining light with oxygen on the degradation of CH3NH3PbI3 layers to yellow PbI2 was recently argued 6 as even dominant. The experiments were carried out under light with the UV component intentionally filtrated, and the results were related to reactive singlet O2– species generation through photo-electrons capture by oxygen molecules, that adversely acts in the deprotonation of methyl-ammonium cations. The awareness of the oxygen attack when combined with light adds up to our previous observation

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on a thermodynamic degradation

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of the material due its

intrinsic tendency of exchanging protons between the organic (MA+) and the Inorganic (I-) moieties of the lattice cage. Degradation phenomena are consequently observed even in vacuum conditions.7,9 Proton exchange events are further promoted by water molecules acting as efficient proton mediators;7,10 thereby degradation is accelerated by humidity.11,12 To counteract the extrinsic agents against material disruption, several sealing solutions were proposed.13,14 Nonetheless, solutions able to push the technology to a large stability during time and in operation conditions still need to additionally pass through the full comprehension of the degradation mechanisms and the related driving forces.


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The role of UV photons in the CH3NH3PbI3 degradation process is also controversial. Degradation was, in this case, attributed to the photo-catalytic action of the TiO2 interfaces. To mitigate this effect, some passivating coverages were proposed as blocking layers;

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moreover,

high surface exposure by mesoporous TiO2 layers was demonstrated to restrain the effects of degradation by activated oxygen species 6 as they ensure more efficient extraction of the photogenerated carriers from the CH3NH3PbI3 layer. 15 Another main concern is the effect of real operation temperature of the material. During the device exposure to the sunlight, the effective temperature of the active layer can reach high temperature between 60 °C and 80 °C.

4,14

This is effective,

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since a polymorphic transition

from a tetragonal to a cubic lattice is expected around 54 °C in MAPbI3 layers.

1,9,17,18,19

It was

recently shown that the device can slightly benefit from an operation temperature around 60 °C, 1 although a certain degree of non-reversibility of the cell parameters (especially the Voc) was reported by the same authors. We conclude that an exhaustive explanation of the implications of the thermal operating conditions at the lattice scale is still lacking as it can complement with the results on devices, with special regards to the role of the environmental conditions (humidity, oxygen etc). To date the experiments aimed at the extraction of optical constants have been performed to study the layer degradation20,21,22 or the temperature effects near or below room temperature23,24 ; but none of them is performed at temperatures near the real operation conditions of a cell under the sun.


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EXPERIMENTAL SECTION

Sample preparation Perovskite materials were deposited by solution processing in a dry room with a monitored humidity by a Cl-assisted one step deposition on glass substrates. A 40 wt% solution of PbCl2 and methylammonium iodide (MAI) (ratio 1:3) in dimethylformamide (DMF) was prepared at 70°C under stirring for 1 h. During spin coating, 1 ml of toluene was dripped onto the substrate as anti- solvent based treatment to form a uniform homogenous film. After the substrates were placed on a hot plate to complete the evaporation of the solvent the perovskite layer formed was stored in vials under a dry N2 atmosphere immediately after preparation to avoid interactions with chemically reactive species and especially humid air containing H2O (Figure S1)

Analytical techniques Spectroscopic ellipsometry A J. A. Woollam VASE Ellipsometer equipped with Autoretarder with an Instec Heat stage system attached was used to measure the changes in the optical constants of the perovskite in the range RT-80°C with an accuracy of less than 0.1°C. This monochromator-equipped instrument is the best option to to truly separate the contributions due to surface or bulk effects depending on the light absorption coefficient. The sample was kept in a closed chamber with an overpressure of an Ar, O2 or N2 to investigate the layer in different ambient conditions. Prior to performing the cycles, a fresh sample was measured at 60°, 65° and 70° incident angles to establish a


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consolidated optical model of optical constants based on accurate fit in the spectral range 1-5 eV. Being the sample deposited on glass, some backside reflection is possible when the photon energy exceeds ∼1.5 eV. However, the removal of backside reflection from the glass substrate has been ensured by rear surface roughening via sandblasting. The thickness of the perovskite, ∼500nm) was determined by verifying that n obeys the Cauchy equation in the transparent region. This value was kept constant afterwards for assessing a consistent optical model based on Kramers–Kronig oscillator model. It is widely agreed that CH3NH3PbI3 dielectric function has three critical points around 1.6, 2.5 and 3.1 eV 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 Brillion zone of the cubic phase, respectively, that are reflected to the zone center in the lower symmetry tetragonal and orthorhombic phases. However, the fitting functions used to reproduce the experimental data shape largely differ and a variety fit have been performed by using only Gaussian oscillators 25, only Tauc-Lorentz oscillators, 22,26,27 Tauc-Lorentz plus Lorentz oscillators21 and by PSTRI plus Gaussian functions.

23,24

In this work, we initially explored all the three approaches based on

multi Tauc-Lorentz, multi-Gaussians or 2 PSTRI plus Gaussians functions but we concluded that the figure of merit describing quantitatively the quality of these fits, the mean square error (MSE), is almost identical. It has to be pointed out that in the end all of these methods have very similar number of fitting parameters regardless of the specific function used and thus the MSE is strictly related to this number rather than the function used. In our case, an additional parameter has been introduced to take into account the sample morphology. Micrometer large domains are very flat with surface roughness ( 3.1 eV) and during thermal


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cycles. The lack of UV-promoted optical modifications was experimentally verified in inert environment and in the specific illumination conditions (75W with the intensity further reduced by the monochromator). Thereby, excluding effects from UV, we focused on temperature and environment effects under illumination in the whole 1-5 eV range. In Figure 1a we schematically depict the thermal cycle, in a given atmosphere (Ar, N2, O2, humid air), consisting of a sequence of isothermal processes, namely: 120 min at 30 °C, 120 min at a selected operation temperature and subsequent cooling back to 30 °C, with soak time of 120 min. The operation temperature is chosen in the range 40-80 °C. The humidity in the air has been measured to be 55±5% while Ar, N2 and O2 are dry gases. We have cut from a single large sample (2.5 cm x 2.5 cm) a quarter of it; thereby from a unique perovskite preparation process we have obtained 4 fresh sample, one for each gas environment. During each isothermal annealing, the optical measurements were repeated 4 times. The sample was left overnight, regardless of the environment used during the thermal cycle, at a controlled temperature of 25 °C in dry N2 atmosphere with the aim of evaluating if the optical changes recorded are purely temperature effects or irreversible structural changes triggered inside the material. During the cycle the sample was never unloaded, and therefore the probed area is unchanged.


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Figure 1 Schematic view of the thermal cycle used to probe CH3NH3PbI3 degradation. (a) The thermal cycle consists of 120 min at 30 °C, 120 min at a higher temperature and 120 min at 30 °C, in a given atmosphere (N2 or humid air) and a recovery at room temperature in N2. Ellispsometric data ψ (b) and ∆ (c) at 70° incident angle for the fresh sample (orange) and the sample subjected to thermal cycle in air (violet). The solid lines show the fitting result calculated using model as described in text. (d) The absorption coefficients as calculated from eq. 1. The penetration depth at ∼1.6 eV (the band edge) is ∼250 nm, while at 3.5 eV is ∼25 nm.

Figure 1b,c show the experimental amplitude (tan ψ) and phase (∆) of the polarized reflected light from the on CH3NH3PbI3 for the fresh sample and the sample subjected to the thermal cycle up to 80 °C in air (see dots in Figure 1a), which are used to represent the largest


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variation we measured in all the experiments. This large spread of values is due to a combination of temperature and of the action of chemically reactive species, as it will be disentangled in what follows. It can be seen that the calculated spectra show excellent fitting to the experimental spectra (solid lines). However, the fit diverges from spectroscopic ellipsometric data very slightly at E ≥ 4.0 eV, probably due to fine rough surface structure. 22 The changes in the dielectric constants εଵ and εଶ , calculated from the fitting of Ψ and Δ, reflect on the absorption coefficient of the materials, which is the parameter tightly connected to the device operation. Thereby, we here investigate in details the variation of the absorption coefficient in Ar, N2, O2 and humid air during heating in the range of cell thermal operation conditions. Figure 1d shows the absorption coefficient as calculated from eq. 1 ߙ=

ଶఠ ඨ ௖

ට(ఌభమ ାఌమమ )ିఌభ ଶ

Eq. 1

where ε1 and ε2 are obtained from the optical model, ω is the frequency and c the light speed. The direct bandgap at E0 = 1.61 eV, and two absorption peaks at E1 = 2.5 eV and E2 = 3.4 eV appears in absorption spectra (Figure 1d). 23 We performed the critical point analysis of optical constants (Figure 1), as reported in Refs. 23 and 28, in order to describe the system modification. The second derivative of real and imaginary part of the complex dielectric function have been fitted using the following equation: డమ ఌ

డఠ మ

= ݊(݊ − 1)‫ ݁ܣ‬௜஍ (߱ − ‫ ܧ‬+ ݅Γ)௡ିଶ

Eq. 2

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. The fit was performed simultaneously for the real and imaginary part of the


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dielectric constant using a least-squares procedure, 24,28 Figure 2 shows the second derivatives at the three critical points and the fitting according to Eq. 2, in the fresh sample (orange curves) and in the most degraded sample at the end of the thermal cycle in air (blue curves) similarly to what we have done with the absorption coefficient in Figure 1d. From these fit we extracted the peak and broadening of the critical point reported in Figure 3 as a function of the temperature and annealing atmosphere. E0

300 200

Fresh

(a)

E1

40

(b)

10

0

0

-100 -2 2 2

ε1 ε2

-200 -300 180 120

After The rmal cycle in air

(d)

60 0

-10

-20

-20 -40 30

-30

(e)

40

20

(f)

20

10 0

0

-60 -120 -180 -240 1.5

(c)

20

100 0

E2

40 30

20

de /dω (eV )

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-10

1.6

1.7

ε1

-20

ε2

-30

1. 8

2.0

-20 -40 2.2

2.4

2.6

2.8

2.8

3.0

3. 2

3.4

3.6

E nergy (eV )

Figure 2: Critical point analysis. Second derivatives as calculated from the optical model (symbols) and the corresponding fit (lines) as calculated from Eq. 2 for at the critical points E0, E1 and E2 for the fresh sample (orange) and the sample subjected to the overall thermal cycle in air (blue).


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In order to focus the attention on eventual thermally induced structural modifications on the MAPbI3 (e.g. induced defects) and to avoid pure thermal effects (e.g. due to lattice thermal vibrations), we have reported in Figure 3a,b,c,d,e,f the position and broadening of all the peaks collected at 30 °C at the end of each soaking process in the range 40-80 °C in the different gas environment. In the range of 2.8-3.2 eV the light penetration depth is ∼25 nm so the red-shift and narrowing of the E2 peak are very likely associated to the loss of iodine species and PbI2 formation on the topmost part of the sample (see SEM analyses in S1). Iodine-related defects have effect on E2 as reported hereafter by Density functional theory (DFT) calculations. Both E1 and even more E0 peaks are instead the mark of bulk properties and in this case, in agreement to other works,

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we observe a peak blue-shift and widening in air describing lattice modification

inside the material. The most interesting aspect is that shift and width variations coherently appears above ∼50°C. Oxygen represents an intermediate case because it affects the near surface layer as air does (Figure 3c), but not the deeper layer which remains unaffected as in the case of nitrogen and argon. Finally, the extent of the peak shift and width variations increases with energy but, although the absolute value is small particularly for E0 and E1, anyhow it is comparable to that recently reported by Jiang and co-workers

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when considering a similar

temperature interval of about 50 °C.


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Figure 3: Temperature dependence of the interband critical point energies (a-b-c) and broadening (d-e-f) of CH3NH3PbI3 for tetragonal and ortho-rhombic phases during the thermal cycle (see Figure 1d). Note the threshold behavior above 50 °C, which corresponds to the crossover of the tetragonal to cubic transition. A picture of the effect of the cycle on the overall structure of the CH3NH3PbI3 layer in the final conditions as depicted in Figure 1a, is provided by the X-ray diffraction analysis in Figure 4 (blue line). The diffraction analysis is taken in a region wherein CH3NH3PbI3 and PbI2 can simultaneously contribute and is performed on sample in all gas atmospheres conditions used (the pattern in Ar is not distinguishable from that in N2). This allows exploring eventual


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structural degradation of the materials.

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The comparison highlights that, N2, Ar and O2 do not

sensibly affect the lattice structure in its average during the cycle (their pattern is not different from that of the as-deposited layer; the small PbI2 peak evidenced in the semilog-scale is supposed to come from preparation) whereas air conditions (thereby more properly humidity) have generated an incremental amount of PbI2 into the material.

Figure 4: X-ray diffraction analyses of the layers at the end of the thermal cycles in different atmospheres. The result in Ar is identical to that in N2. Note the semi-log scale of the inset. The small PbI2 amount in the O2 and N2 treated layers does not differ from that in the starting material (not reported for simplicity). A relative increase of the PbI2 amount is observed only at the end of the cycle (5 days) in air (see the inset).

Material changes impact on applications as described by the behaviour of the absorption coefficient, which is a key-parameter to rate the solar cell performances. From Figure 1d two


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different ranges can be identified wherein the spread among fresh and degraded layer is maximum, namely a first range from 2.1 eV to 3.1 eV, which is E1 critical point related (E1rel), and a second range from 3.1 eV to 4.5 eV, which is E2 critical point related (E2rel). A third range, namely 1.6-2.1 eV, has been considered but not reported for simplicity since it provides similar insights to those offered by the 2.1-3.1 eV range. Thereby, we have calculated the integral of the absorption coefficient in these two intervals to elucidate the absorption coefficient variation. Note that the lower energy interval contains E1 and the upper interval extends in the deep UV (>3.1 eV) where important interband transitions occur (E2). The variation of the absorption coefficient reported in Figure 5a,b makes well clear the environment-driven degradation process and the role of the temperature. We note that a temperature increase from 30 °C to any temperature, even just incremented by 10 °C, produces (reproducibly) a decrease of the absorption coefficient that, in some cases, can be recovered as the sample is cooled back to 30 °C. This recovery, which mainly occurs in Ar and N2 (and in O2 atmospheres to some extent) forms a typical V-shape that characterizes the low-high-low temperature thermal cycle (Figure 5a). On the contrary, as the atmosphere is chemically adverse (humid air), the recovery is partial or even nullified by a monotonic inexorable decrease of the absorption coefficient (Figure 5b). The action of humid air is so adverse that even during the nights (between two measurements) done in the most conservative conditions (room temperature+N2), the absorption coefficient continues decreasing. This implies that the degradation process, once triggered by water molecules, cannot be stopped and proceeds in cascade to the involvement of deeper layers of the material.


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1.04

(a) 2.1-3.1 eV

(b) 3.1-4.5 eV

1.02

Abs Coeff In tegral (norm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.00 0.98 0.96 0.94 0.92

Ar, N 2 O2 Air

0.90 30 4030

3050 30

3060 30

30 7030

30 8030

3040 30

3050 30

30 6030

30 7030

3080 30

Temperature (°C)

Figure 5: Absorption coefficient of CH3NH3PbI3 in Ar (black), N2 (red), O2 (green) and air (blu) integrated in the (a) 2.1-3.1 eV range (E1 critical point related) and (b) 3.1-4.5 eV range (E2 critical point related) as a function the thermal cycle reported in Figure 1a.

We used as benchmark the variation of the absorption coefficient in Ar, since in a truly inert atmosphere the variation of the absorption coefficient and the V-shape are arguably ascribed to purely temperature effects on the CH3NH3PbI3 lattice. In both Ar and N2 atmosphere, the absorption coefficient closely follows the temperature increase and comes back to the reference state (30 °C) after any high temperature cycle (the two high points of the V have identical heights), even at 80 °C, and this characterises the reversibility of the optical behaviour of the material under thermal cycles. Additionally, a steady temperature in N2 for many hours produces a slight increase of the absorption coefficient which is clearly visible in the data at 30 °C, especially in the E2rel (3.1-4.5 eV) region overnight. The positive effect of N2 on the


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absorption coefficient leaves open the possibility of a kind of interaction between the N2 molecules and the perovskite lattice 29,30. The use of O2 represents an intermediate case which mainly impacts, differently from the use of Ar, on the variation extent of the absorption coefficient (especially above the 50 °C) and slightly in terms of recovery. The oxygen alone at high temperature induces a significant transient change in the absorption but it recovers completely almost always (except for the range 3.1-4.1 eV above 50 °C) when the sample is cooled back. This effect produces a symmetrical Vshape but deeper than in inert gas atmospheres. As the extreme case, air introduces dramatic changes of optical constants and consequently in the perovskite absorption. In this high adverse condition,

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a temperature threshold is

identified at ∼50 °C, consistently with the results on the critical points analyses, above which the absorption coefficient does not recover at all and the V-shape becomes strongly asymmetrical. Our choice to separate the range in two parts highlights this effect that is indeed particularly dramatic in E2 (Figure 5b). From the figure additionally emerges that 50 °C signs a not-recovery threshold of the optical behaviour and the rapid worsening of the response of the material. This was associated to an acceleration of the degradation rate above the threshold of 50 °C. Since the use of pure (100%) dry O2 atmosphere conditions has not this disruptive effect, we argue that the air action can be mostly attributed to humidity. To reinforce the deleterious action of humidity, it is observed that the absorption coefficient continues decreasing overnight, even though the sample is left in N2 at room temperature, which in principle could help to slight increase the absorption coefficient. This attests that irreversible changes were triggered by the temperature on the CH3NH3PbI3 lattice which produce a progressive reduction of the absorption capability of the light in the layer.


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To elucidate the degradation mechanism in the different environmental conditions, we have calculated the electronic structure of CH3NH3PbI3 based on supercell calculations and the density functional theory (DFT) (Figure 6). The calculations were performed with the planewave QUANTUM ESPRESSO code31. We used the Perdew-Burke-Ernzerhof (PBE) implementation32 of the generalized gradient approximation for the description of the exchangecorrelation functional along with scalar relativistic ultrasoft pseudopotentials. In the case of Pb, 5d semicore electrons were explicitly included in the valence group. The structural and electronic proprerties of CH3NH3PbI3 were computed for a pseudo-cubic (2×2×2) supercell, which allows for a more realistic description of the electronic structure as compared to unit cell calculations. The lattice parameter was set to 6.279 Å from XRD measurements33. All atoms were allowed to fully relax. Convergence was achieved with a plane-wave cutoff kinetic energy of 35 Ry, an augmented charge density cutoff of 280 Ry and a (4×4×4) Monkhorst-Pack grid34 for the sampling of the Brillouin zone. Figure 6 shows the projected density of states for the system, denoting the atomic contributions in the total electronic structure. In accordance with previous reports,

27,35

the PDOS reveals a strong I character for the upper valence band and major Pb

contributions for the lower conduction band. States that have a methylammonium origin are too far away for the range of optical interest. We then consider the plausible interband transitions that give rise to the E0, E1 and E2 peaks of the absorption spectrum, noting that these should be mainly direct and quantitatively pronounced for the peaks of the PDOS spectrum. We note that the E0 transition can be only attributed to electronic excitations from the maximum of the valence band (maxV) to the minimum of the conduction band (minC). The E1 peak has an energetic range that can either involve transitions from maxV towards the PDOS peak above minc, or similarly, from states bellow maxV towards minC. On the contrary, the E2 transition


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uniquely involves transitions from deeper states bellow maxV towards the lower conduction band. It is important to note here that in the case of the E2 peak, transitions from the top of the valence band towards the second PDOS peak of the conduction band are indirect and therefore should marginally contribute to the optical spectrum. Within this scheme, the E2 transition becomes diagnostic for the presence of iodine in the material, as the high PDOS peak at ∼0.8 eV below maxV has primarily an iodine character. Consequently, the higher sensitivity of the E2 peak to the degradation of the material in air and O2 can be directly attributed to a gradual loss of iodine content from the sample.

7,8,9

This aspect is consistent with our previous studies, which

have indicated a degradation path through the creation of MAI and HI defects. 8,9

Figure 6: Projected density of states as a function of energy for CH3NH3PbI3, showing the contributions of each element in the total electronic structure. Calculations are based on the (2×2×2) pseudocubic supercell shown in the inset.


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Figure 7a,b shows the thermodynamic behaviour of the CH3NH3PbI3 lattice during isothermal annealing. Overall, the annealing time induces a very small modification (≈2‰) compared to 2-8% due to the annealing temperature as in Figure 4. However, the most important feature is the opposite lattice behaviour in N2 and all other environments, i.e. the N2 produces of the absorption coefficient while all other environments reduce it in both ranges (Figure 7a,b). The time dependent decrease of the integrated absorption coefficient in presence of Ar testifies that a thermodynamic change of the lattice occurs, and this is in agreement with what previously shown.

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Water adds up a catalytic effect which speeds up the downward trend and causes the

observed threshold behaviour. Water entering the MAPbI3 cage is expected to depend on the surface termination

35

and on the MA+ orientation,

12

and therefore to be more effective as the

MA+ gain a rotational degree of freedom. Since this occurs as the MAPbI3 lattice moves to the cubic arrangement, 17 the observed threshold behaviour above 50 °C is clearly linked to the phase transition. Deep water interaction with the perovskite lattice causes defects formation, as indicated by relating the optical behavior of the material to our DFT calculations, where the E1 behavior and even more the important lowering of the E2 peak are clearly correlated to the loss of iodine (iodide-related defects) content from the sample (Figure 6). On the contrary, and consistently with what already discussed, we highlight, as in Figure 5, that the optical parameters slightly improve by keeping the sample in N2 environment during the cycle. The positive effect of N2 on the absorption coefficient leaves open the possibility of a kind of interaction between the N2 molecules and the perovskite lattice. 29,30


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Figure 7: Absorption coefficient during an isothermal cycle of 2h at 50-60-70 °C of CH3NH3PbI3 in dry Ar (black), dry N2 (red), dry O2 (green) and humid air (blu) integrated in the (a) 2.1-3.1 eV range (E1rel) and (b) 3.1-4.5 eV range (E2rel) (b).

CONCLUSIONS We used an approach based on spectroscopic ellipsometry in order to finely describe the optical changes occurring in a planar CH3NH3PbI3 layer during thermal cycles used to mimic the thermal operation conditions of the material under the sun. We elucidated that chemical species present in the environment of the material play a major role in the optical response. The behaviour under Ar conditions represents a reference and show a certain degree of reversibility in the limit of the unavoidable thermodynamic intrinsic degradation of the material. Water molecules contained in humid air, more effectively than oxygen atoms alone, add up with catalytic actions on the cage equilibrium and cause a dramatic inexorable degradation of the optical response of the material especially above the threshold of the tetragonal to cubic


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transition. With this action, defects are formed in the layer (I-related defects) by a process which starts from the surface and rapidly involves in cascade deeper layers of the material. In this respect, nitrogen plays a double role in protecting the surface of the layer and even positively impacting on the bulk behaviour. We reveal, in fact, that nitrogen not only assures the recovery of the optical response of the material after annealing in the whole explored range (energy and temperature) but even improve the absorption coefficient. The role of nitrogen at the atomic scale merits further insights to rationalize the material behavior and propose practical stabilizing solutions.

ASSOCIATED CONTENT Supporting information Scanning electron microscopy, Spectroscopic ellipsometry.

AUTHOR INFORMATION Corresponding author *[email protected]; Tel +39 095 5968 236 (A. A.)

Notes The authors declare no competing financial interest.


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TOC graphic


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First Evidence of CH3NH3PbI3 Optical Constants Improvement in a

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