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Understanding the Role of the Mesoporous Layer in the Thermal Crystallization of a Meso-Superstructured Perovskite Solar Cell Daniel Ramirez, Mario Alejandro Mejía Escobar, Juan F. Montoya, and Franklin Jaramillo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02808 • Publication Date (Web): 05 Apr 2016 Downloaded from http://pubs.acs.org on April 6, 2016

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The Journal of Physical Chemistry

Title: Understanding the Role of the Mesoporous Layer in the Thermal Crystallization of a Meso-Superstructured Perovskite Solar Cell Author(s), and Corresponding Author(s)* Daniel Ramirez, Mario Alejandro Mejía Escobar, Juan F. Montoya and Franklin Jaramillo*

Daniel Ramirez, Mario A. Mejía Escobar, Dr. Juan F. Montoya and Prof. Franklin Jaramillo Affiliation: Centro de Investigación, Innovación y Desarrollo de Materiales – CIDEMAT, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Colombia *E-mail: [email protected] Tel: (+574) 2196680

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ABSTRACT Perovskite solar cells (PSCs) have been extensively studied in recent years due to their unexpected properties and low-temperature processing. In terms of morphology, the annealing conditions are crucial and highly determinant on the performance of the devices. Here, it is important to know the heat transfer in order to prevent detrimental effects in the photovoltaic performance principally produced by low crystallization and localized excessive thermal stress in the synthetized perovskite. In this work, differential scanning calorimetry (DSC) was used to reveal the thermal transitions occurring during crystallization of a mesoporous alumina-based PSC. We found that when the mixed-halide perovskite (CH3NH3PbI3-xClx) is crystallized in presence of a mesoporous layer, the heat transfer flux is affected and therefore the perovskite formation shifts to higher temperatures causing that the infiltrated perovskite crystallizes differently to the capping layer. DSC analysis also indicated that when low annealing temperatures were used the perovskite did not present good crystallinity while at high temperatures the thermal stress generated on the infiltrated perovskite promoted low efficiencies. Finally, the optimal crystallization conditions were found to be 100 °C during 90 minutes, with a short post-annealing at 130 °C for 10 minutes, which promoted a photoconversion efficiency up to 10.89%.

INTRODUCTION The recent discovery of organic-inorganic perovskites (e.g.,CH3NH3PbI3 and the mixedhalide perovskite CH3NH3PbI3-xClx) as adequate light absorber materials for photovoltaic applications has brought the attention of the scientific community in the last few years.

1-5

Perovskite materials have good semiconductor properties and almost all the characteristics required in a solar cell, such as, adequate band-gap (nearly the ideal), high extinction coefficient, high mobility and diffusion length.

6-9

Another important aspect is the variety of

deposition techniques that can be used to produce high quality films of this material, ranging 2 ACS Paragon Plus Environment

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from thermal evaporation solution processes.

13

10

, vapor assisted solution process

11

to one-step

12

and two-step

The low amount of materials and low cost associated to the fabrication

of the devices make this technology very attractive. PSCs can be obtained in both, a planar architecture where all the layers are compact thin films or in a mesoscopic configuration where a scaffold material is also present. Typically, the mesoporous materials are TiO2, 14,15 Al2O3, 16,17 or ZrO2. 18 These meso-structurated layers can help during the crystallization of the perovskite material when solution processing techniques are used because they provide a straightforward way to define the crystal orientation, grain size and unit cell volume. 19

Many studies have focused on controlling the morphology of the perovskite films by randomly adjusting the annealing conditions, using solvent engineering

22

20

the organic/inorganic precursor ratio,

and additives within the precursor solution.

23,24

21

or

In the case of

thermal annealing conditions of the perovskite layer in both planar and mesoporous architectures, it has been claimed that the optimal crystallization conditions are nearly 100 °C, with annealing times ranging from 30 minutes up to 120 minutes.

25

Most of the

crystallization conditions are based on finding the adequate annealing time and temperature to achieve high efficiency devices, therefore an empirical approach through a set of optimization experiments is typically adopted, which then serve to explain the result by characterization techniques such as X-ray diffraction (XRD) 20,28,29

26,27

and scanning electron microscopy (SEM).

Recently we adopted differential scanning calorimetry (DSC) as a new technique to

follow up PSCs crystallization.

30

This technique was found to be very useful to identify the

optimal crystallization conditions for planar based architectures. In this work, we expanded the use of DSC in order to understand the crystallization of CH3NH3PbI3-xClx in a mesosuperstructured architecture. Additionally, we reproduced the mesoscopic architecture in an aluminum pan by using Al2O3 nanoparticles as scaffold material during the DSC analysis. Our efficiency results highly correlated to the thermal transitions occurring during the perovskite 3 ACS Paragon Plus Environment


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formation as indicated by the DSC thermograms. This calorimetric study led us to obtain efficiencies up to 10.89% when the crystallization process included isothermal annealing at 100 °C for 90 minutes and recrystallization transition at 130 °C for 10 minutes. Low efficiency results were correlated to poor crystallization of the perovskite and high residual thermal stresses when lower (110 °C) were used, respectively, as shown in the thermal analysis simulation.

EXPERIMENTAL SECTION Device fabrication. Devices were fabricated on fluorine-doped tin oxide (FTO) coated glass (Pilkington, 7 Ω/square). Initially, FTO was removed from regions under the anode contact by etching the FTO with zinc powder and 2M HCl. Substrates were then cleaned sequentially in neutral soap, acetone, isopropanol, and UVO. Then, the titanium oxide layer (c-TiO2) was fabricated by spin-coating a mildly acidic solution of titanium isopropoxide in ethanol anhydrous (350 µL in 5 mL of ethanol with 0.016M HCl). The mesoporous Al2O3 scaffold was deposited by spin coating a colloidal dispersion of < 50 nm Al2O3 nanoparticles at 20 wt% (Aldrich) diluted with isopropanol at a volume ratio of 2:1 at 4000 rpm for 1 minute. The wet film was then heat-treated at 150 °C for 10 minutes, yielding a m-Al2O3 layer with a thickness of 320±30 nm. The perovskite layer was then deposited by spin-coating a precursor solution of methylammonium iodide and lead chlorine (3:1 molar ratio; 0.88 M lead chloride and 2.64 M methylammonium iodide) in N,N-dimethylformamide (DMF) at 4000 rpm for 45 seconds. Under these conditions, all samples had a perovskite capping layer with a thickness of 250±20 nm. The films were then annealed at different temperatures according to the transitions found in the calorimetric curve. Annealing temperatures between 90 °C and 120 °C with and without recrystallization at 130 °C were evaluated in this study. Then, the holetransporting layer was deposited via spin-coating a 8.5 wt% of 2,2′,7,7′-tetrakis-(N,N-dipmethoxyphenylamine)9,9′-spirobifluorene (Spiro-OMeTAD) in chlorobenzene (CB), with 4 ACS Paragon Plus Environment

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additives of 4-tert-butylpyridine and lithium bis(trifluoromethanesulfonyl) imide. Spin coating was carried out at 2000 rpm for 60 seconds. Devices were then left overnight in a desiccator in order to dope the Spiro-OMeTAD via oxidation. 31 Finally, to complete the devices 120 nm silver electrodes were thermally evaporated under vacuum (≈10 −6 Torr) at a deposition rate around ≈0.1 nm s−1.

Characterization. A DSC Q200 equipment from TA Instruments was used to perform dynamic experiments under nitrogen atmosphere at both heating and cooling rates of 10 °C min-1. Perovskite solution was deposited by drop casting onto the pans that already contained the mesoporous Al2O3. This process was done inside a glove box filled with nitrogen and then the pans were sealed with hermetic lids. Prior to start the scans, a pinhole was made to the lid in order to allow the evaporation of the gases (solvent and by-products). Structural information was obtained directly from the thin films in a PANalytical diffractometer in the range of 2θ=10° to 50° in a Bragg–Brentano geometry, using Cu Kα (1.5408 Å) and radiation with a step size of 0.04° and a speed of 2° per minute. Optical absorption was measured in the range of 400-800 nm using a Cary 300 Agilent spectrometer. The electrical characterization of the devices was performed using a 4200SCS Keithley system at a voltage swept speed around 500 mV s-1 in combination with an Oriel sol3A sun simulator, which was calibrated to AM 1.5G standard conditions using an oriel 91150 V reference cell. An Oriel IQE 200 was used to determine the external quantum efficiency. Surface roughness and thickness of the different layers was obtained using a Bruker DektakXT profilometer. Films coverage percentage was calculated using Image J software.

RESULTS AND DISCUSSION A DSC analysis was implemented to study the crystallization process of the perovskite material when a mesoporous layer is present. The volume of Al2O3 nanoparticles dispersion 5 ACS Paragon Plus Environment


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and perovskite precursor solution was carefully adjusted in order to reproduce the structure of the fabricated devices as shown in Figure 1a. The transversal EDS profile showed the presence of lead and iodine and the absence of aluminum from the surface of the sample up to 15 µm, indicating that this region corresponds to a capping layer of perovskite. Going further than 15 µm the concentration of lead and iodine decreased and the concentration of aluminum increased, therefore, this region can be attributed to the perovskite material infiltrated into the mesoporous Al2O3 layer. Under these conditions, the ratio of infiltrated to capping layer of perovskite thickness is about 1, which was taken to simulate the geometry relation of the solar cell devices and the thermal simulation experiment (discussed later). Two heating and cooling cycles were done in the DSC measurements, the first heating and cooling cycle are related to the evaporation of the solvent because no thermal processes corresponding to the perovskite were observed (see Figure S1). The second heating and cooling cycles are presented in Figure 1b. Two endothermic and one exothermic transitions were found during the heating step, which have been previously identified according to the literature as follows: the first endothermic peak with an onset temperature of about 65 °C is assigned to the direct formation of perovskite from PbI2 and CH3NH3I or the sublimation of organo-chloride compounds formed as intermediate reaction products according to the mechanisms reported in the literature. 32,33 In our case this transition was found at higher temperatures (onset 82.2 °C and maximum at 90.8 °C) probably because the presence of the insulator mesoporous Al2O3 layer retards the energy transfer and therefore temperature of the thermal transition. The second larger peak with an onset of about 85 °C corresponds to the formation of the α-phase perovskite, while the high temperature exothermic peak is related to a recrystallization process. We also found the second endothermic transition at higher temperatures (Tonset= 96.1 °C) but surprisingly the peak split into two peaks with maximums at 106.1 °C and 112.7 °C. It is known that the Al2O3 is a very stable oxide, where no thermal transitions taking place in this range of temperature. For this reason, these two endothermic peaks must 6 ACS Paragon Plus Environment

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correspond to a second formation step of the α-phase (cubic) perovskite. This observation is in concordance with the fact the perovskite was present as an infiltrated mesoporous layer and a capping layer. This agrees to the fact that in Figure 1d just one endothermic peak was observed at this temperature corresponding to the infiltrated perovskite with no capping layer. Finally, the exothermic recrystallization was found at 132.5 °C, which is in good agreement with the literature. 30 During the cooling step two exothermic peaks were found at 118.4 °C and 74.3 °C, which in the first case corresponds to a complete recrystallization and in the second case to the phase transition α to β (tetragonal). On the other hand, we decided to perform the same EDS and DSC measurements to a full infiltrated perovskite (Figure 1c and 1d) in order to determine the effect of just the mesoporous layer. Figure 1c showed the presence of lead, iodine and aluminum in the whole range of the EDS profile, suggesting that there is not capping layer and the mesoporous Al2O3 was fully infiltrated. In Figure 1d a broad exothermic peak starting at about 90 °C and centered at 111.1 °C was identified, indicating a single perovskite transition, while the recrystallization peak remained almost constant at 131.9 °C. During cooling, the same transitions of the perovskite with capping layer were observed, but in the case of the phase transition, it occurred at lower temperatures, ratifying once again the thermal delaying effect of the mesoporous layer.



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Figure 1. Transversal EDS profiles of DSC pans corresponding to a) mesoporous Al2O3/infiltrated CH3NH3PbI3-xClx with capping layer and c) mesoporous Al2O3/infiltrated CH3NH3PbI3-xClx without capping layer. b) and d) in situ differential scanning calorimetric analysis from solution on the DSC pans for samples a) and c), respectively.

According to the previous perovskite transition temperatures, we carried out various annealing programs as shown in Figure 2. Concretely, four different annealing ramps with sustained temperatures of T1 (90oC), T2 (100 oC), T3 (110 oC), and T4 (120 oC) were evaluated for perovskite films with capping layer, as shown in Figure 2a. Additionally, a set of samples (T5 to T8) were submitted to the same annealing programs followed by a “flash” annealing procedure by heating the samples at 130 oC during the last 10 minutes of recrystallization as shown in Figure 2b. The top view SEM micrographs reported in the inset of Figure 2 revealed different perovskite crystal domain sizes depending on the annealing conditions. In general, the crystal domain size increased from hundreds of nanometers to 8 ACS Paragon Plus Environment

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several micrometers as the annealing temperature increased. This tendency was followed for the samples without (Figure 2a) and with flash annealing process (Figure 2b), where very similar morphology and crystal domains were observed for samples annealed at the same plateau temperature (for instance samples T4 and T8). These observations demonstrate that high sustained temperature lead to coalescence perovskite crystals forming bigger crystalline domains in the capping layer. It can also be observed that the recrystallization process evidenced by DSC at 130 °C did not have any effect on the final morphology. Therefore, it is more important the initial heating step to observe changes in the capping layer. On the other hand, the surface coverage of perovskite capping layer (Figure S2) for all samples was quite similar being 70±15% the average. This value is clearly higher than other previous reports (28%) for the same type of cells, 19 but is very low compared to a planar perovskite solar cell where the surface coverage is around 97%. 29 EDS elemental analysis (Figure 2c) were carried out for samples with capping layer in order to confirm the formation of perovskite inside and over the mesoporous structure. This analysis was performed in the zone of m-Al2O3 fully infiltrated by perovskite and the zone of perovskite capping layer (inset of Figure 2c). The iodine to lead atomic ratio of perovskite in both zones was about 3, independently on the annealing conditions. These results confirm that perovskite samples in a thin m-Al2O3 scaffold are composed of infiltrated perovskite and capping layer perovskite.



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Figure 2. Thermal processing protocol of CH3NH3PbI3-xClx perovskite solar cells. (a) Time vs. temperature profile at four plateau temperatures. (b) Thermal annealing profiles of perovskite samples submitted to flash annealing at 130 oC for 10 minutes. Insets in a) and b): Top view SEM micrographs of the perovskite films with capping layer for each annealing condition. The equivalent surface coverage percentage of perovskite capping layers calculated from optical microscopy is showed in the supporting information (Figure S2). (c) Iodine to lead atomic ratio of perovskite films calculated from EDS elemental analysis in the fully infiltrated perovskite zone and in the capping layer zone. Inset: SEM micrographs showing the zones analyzed by EDS.

Figure 3a and Table 1 summarizes the photovoltaic performance reached with the perovskites synthetized under the proposed annealing methodologies of Figure 2. Typical I-V curves in reverse and forward scan are shown in Figure S3. The most efficient devices were obtained under annealing at T5 and T6 conditions. Photovoltaic performance of the best devices is illustrated in Figures 3b and 3c. Under the optimal annealing conditions, the average 10 ACS Paragon Plus Environment

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photovoltaic conversion efficiency was 8.98% (best 10.89%), which is in the state-of-the-art for the considered meso-superstructured device.

16,17,34

However, photovoltaic efficiencies as

high as 14% were recently reported using a pre-spin coating process.

35

These results suggest

that the more appropriate crystallization is reached at sustained temperatures ≤100 °C followed by a short recrystallization process at 130 °C. With relation to the other annealing conditions, low efficiencies were obtained when no recrystallization was performed, remarking a clear correlation between the annealing temperatures and the observed thermal transitions in the DSC curve. Therefore, in order to get devices with high PCE, it is required to perform long annealing at temperatures ≤100 °C which lead to the formation of the α-phase perovskite. Then, a short recrystallization at 130 °C is required in order to allow the transformation from the α to the β phase perovskite. Even though 100 °C is found to be outside the peak associated to the formation of the α-phase perovskite, this transition was able to occur due to the prolonged 90 minutes annealing. In general, it could be inferred that perovskite needs to be slowly crystallized in order to achieve high quality layers. This statement is in good agreement with the absorption spectrum showed in Figure 3d, which suggests that the more appropriate crystallization is reached at 100 °C, one of the reasons may be due to the highest optical absorption achieved for curves T2 and T6. We presume that perovskite need slow crystallization because it can suffer from mechanical stress due to confinement into the Al2O3 mesoporous layer. To confirm this hypothesis we performed a thermal stress simulation using finite element analysis that will be discussed later in Figure 5.



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Figure 3. a) Relation between the calorimetric curve and photovoltaic performance of the fabricated devices. Values in parentheses represent the best device efficiencies. b) I-V curve of the best device annealed at T5 program. c) I-V curve of the best device annealed at T6. Insets in b) and c): EQE spectra of the best devices. d) Absorption spectra for the samples obtained at the different annealing conditions. Absorption for T1 annealing is not shown because the sample synthesized under this condition had a premature degradation.

Table 1. Photovoltaic parameters of the fabricated devices at different annealing conditions. Annealing condition

Jsc (mA/cm2)

Voc (mV)

FF (%)

PCE (%)

Best

1.09

424.02

31.72

0.15

Average

0.67±0.52

169.85±148.22

20.07±12.82

0.05±0.05

Best

3.24

3.06

81.03

0.01

Average

1.06±1.56

2.38±3.62

14.80±28.08

1.98x10-3 ±3.12x10-3

T1 (90 °C)

T2 (100 °C)


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T3 (110 °C)

T4 (120 °C)

T5 (90 – 130 °C)

T6 (100 – 130 °C)

T7 (110 – 130 °C)

T8 (120 – 130 °C)

Best

4.32

781.60

46.44

1.57

Average

0.98±1.82

192.15±355.86

9.82±18.58

0.30±0.59

Best

3.27

1.14

3.45

0.01

Average

0.85±1.27

0.43±0.59

2.11±3.11

1.95x10-3 ±4.46x10-3

Best

7.09

916.97

72.03

4.69

Average

6.64±0.73

920.30±16.90

70.91±1.16

4.33±0.40

Best

16.05

1009.07

67.23

10.89

Average

13.60±2.96

993.34±24.28

65.46±4.95

8.98±2.54

Best

1.44

1.56

10.18

0.23

Average

0.75±0.77

2.60±3.45

12.45±14.95

0.09±0.10

Best

0.59

1.56

21.48

1.96x10-3

Average

0.15±0.28

0.58±0.81

10.05±15.16

5.93x10-3 ±8.80x10-4

XRD elemental analysis were carried out in order to understand the relation between DSC spectra, crystal structure and photovoltaic performance. As shown in Figure 4a and 4b, the XRD spectra of a perovskite films with and without capping layer present a major peak at 2θ = 14.1°, which are consistent with the (110) plane of tetragonal perovskite.

36

The full XRD

spectra (Figure S4a) of both samples also showed peaks at 28.5° and 31.9o, which are consistent with the (220) and (310) planes of tetragonal perovskite. Details about sample preparation and thickness profiles for both samples can be seen in the supporting information (Figure S5). Despite both samples had the same peaks, Gaussian fitting revealed that the film without capping layer showed a symmetric peak (Figure 4a) while the sample with capping layer presented an asymmetric peak (Figure 4b) that can be deconvoluted by the presence of two different grain sizes corresponding to the infiltrated perovskite and the capping layer, as shown in Figure 4c. (XRD spectra for the samples of Figure 4c are also shown in Figure S4). This result is in good agreement with DSC spectra reported in Figure 1 where it was shown that sample without capping layer has one single peak corresponding to the formation of the perovskite, while the sample with capping layer presented a split peak for the same endothermic transition. Analysis of the major peak from plane (110) for samples with capping layer allowed us to calculate the crystallite size by employing the Scherrer equation and the 13 ACS Paragon Plus Environment


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crystallinity of the perovskite films by analysis of the peak intensity. As shown in Figure 4c, the crystallite sizes were almost constant for all the annealing conditions. Analysis of the deconvoluted peak from plane (110) enabled us to calculate a crystal size of around 30 nm for the infiltrated perovskite and 90 nm for the capping layer at all annealing conditions, which is in good agreement with previous studies.

19

Figure 4c showed that peak intensity from (110)

plane is almost constant (black points at T3, T5, T6 and T7) for samples exposed to annealing at sustained temperatures ≤110 oC, irrespective if they were submitted to flash annealing or not. Therefore, sustained temperatures ≤110 oC lead to perovskites with almost the same degree of crystallinity. This result is also in agreement with DSC spectra reported in Figure 1, which showed that a minimum temperature of 120 oC is necessary to achieve the endothermic transition corresponding to the formation of the α phase for perovskite films with and without capping layer. XRD spectra of a perovskite film annealed at 110 oC (Figure S4b) presented a peak at 12.1o that correspond to PbI2, confirming the incomplete crystallization of perovskites annealed at sustained temperatures ≤110 oC. Despite the samples below 110 oC may have residual PbI2, the flash annealing at 130 oC was enough to complete its transformation (see as an example Figure S4d). The size of the crystalline domains played a critical role in the devices. Those obtained at processing conditions T5, T6 and T7 showed small crystalline domains as compared to T4 and T8 (higher intensity from Figure 4c). For the last two conditions the larger crystalline domain size probably gave as a result higher thermal stress creating permanent mechanical deformation in the perovskite, contrary, to those observed for conditions T5, T6 and T7 (similar to T3). Such small domains did not affect the mechanical structure of the perovskite layer and were in the limit to have a good device performance.


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Figure 4. Perovskite crystallinity analysis. (a) XRD spectra of a perovskite film without capping layer showing Gaussian fitted and experimental peak at 14.1o assigned to the (110) plane of tetragonal perovskite.(b) XRD spectra of a perovskite film with capping layer. c) Relative intensity of the (100) peak and crystallite size of perovskite films with capping layer annealed at some temperatures profiles depicted in Figure 2.

In order to validate the thermal stress hypothesis that can suffer the perovskite layer, a 2D finite element model was proposed considering a mesoporous layer infiltrated with perovskite and a capping layer of the same composition. Alumina particles ≤60 nm were considered to conform a mesoporous layer of 320 nm. Additionally a capping layer of 200 nm was used in the proposed model. The final diagram considering nodes and meshes is showed in the supporting information (Figure S6). 15 ACS Paragon Plus Environment


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Concerning to the mathematical model, equations (1) and (2) were used as a simplified model to describe the heat transfer and thermal stress into the meso-superstructured configuration respectively. Equation (1) describes the 2D heat transfer in non-steady state, where k is the thermal conductivity, ρ is the bulk density and Cp is the calorific capacity. On the other hand, equation (2) defines the volumetric stress change with the temperature change. Here, E is the Young´s Modulus and αv is the volumetric thermal expansion coefficient. All parameters mentioned above for hybrid perovskite and alumina are summarized in the Table 2. Some of them were taken from the literature and other ones were calculated, as in the case of the calorific capacity (Cp) for the mixed-halide perovskite (see supporting information, Figure S7). To the best of our knowledge this is the first experimental report of Cp = f(T) for the CH3NH3PbI3-xClx hybrid perovskite.

k

δ Temp δ Temp δ Temp +k = ρ *Cp δx δy δt

(1)

∆σ = E * α v *(Temp − Troom )

(2)

Table 2. Properties of alumina and perovskite used in the 2D finite element model. Material

Parameter

Nomenclature

Value

Units

k_Al2O3

35

W m-1 K-1

ρ_Al2O3

3950

Kg m

Heat capacity 37

Cp_Al2O3

80

Young’s Modulus 37

E_Al2O3

300

Volumetric thermal expansion 37 coefficient

ߙ௩ _Al2O3

8x10

mm K

k_pvkt

0.5

Wm K

Density 26

ρ_pvkt

4160

Kg m-3

Heat capacity *

Cp_pvkt

232.5+0.16T

E_pvkt

7

GPa

ߙ௩ _pvkt

1.57x10-4

M m-1 K-1

Thermal conductivity 37 Density Alumina

37

Thermal conductivity

Perovskite CH3NH3PbI3-xClx

38

-3

J mol-1 K1

-6

GPa -1

-1

-1

-1

-1

Young’s Modulus

39

Volumetric thermal expansion coefficient 40

J mol K

-

1

* Calculated from modulated DSC as explained in the supporting information. The thermal stress simulation results are shown in Figure 5. The most relevant observations are summarized in order to explain the photovoltaic performance under the tested annealing


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methodologies for the crystallization of the perovskite. It is important to remark that we performed our analyses based on the conditions without the flash annealing step because as pointed out before, the main thermal stresses are generated in the prolonged thermal annealing step (i.e., T1 T2, T3 and T4 conditions). As showed in Figure 5a, the higher thermal stress values are localized into the meso-superstructured film due to the high thermal expansion of perovskite inside the mesoporous Al2O3 scaffold, which easily can surpass its size (Average Pore Size of m-Al2O3 is 26 nm

19

). These generated stresses are transferred to the alumina

nanoparticles and then again to the perovskite when it is slowly cooled. With respect to the capping layer, the suffered thermal stress is lower than in the alumina scaffold and independent of the annealing conditions. This was expected because perovskite can grow and expand freely on top of the alumina layer. This behavior was observed before in the SEM images of the Figure 2. If the produced stress inside the mesoporous layer surpasses the ultimate tensile stress (UTS) of the perovskite, the performance of the devices can be decreased because inevitably this hybrid material could mechanically break and hence the charge transport would be dramatically affected. Additionally, this formation of cracks can promote charge accumulation and therefore low current density. For this reason UTS was calculated from Vickers hardness (VH) values reported in the literature, approximation: VH ≈ 3*UTS.

41

39

using the mathematical

The estimated UTS is shown in Figure 5b as a dotted line.

The results suggested that the thermal stress generated in the capping layer (point z) and the corresponding interface with the alumina scaffold (point y) did not surpass the UTS under all studied annealing conditions. In the region x of the mesoporous alumina, the average thermal stress was bellow of UTS for T1 and T2 conditions. With respect to the other annealing conditions (T3 and T4) the thermal stress was critical and surpassed the UTS of the material, indicating that the perovskite could have been broken, promoting the detrimental effects mentioned above. This results are in good agreement with the photovoltaic performance for 17 ACS Paragon Plus Environment


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T6, which did not surpass the UTS of the material and was also well crystallized by the flash annealing at 130 oC.

Figure 5. Thermal stress simulations at different conditions of annealing without recrystallization at 130 °C. T1 (90 °C), T2 (100 °C), T3 (110 °C) and T4 (120 °C). a) Stress distribution in the active layer (Al2O3 + perovskite). b) Maximum thermal stress into alumina scaffold (x), alumina/capping layer interface (y) and capping layer (z). Altogether, our results indicated that the crystallization conditions are very critical to achieve a high quality perovskite in the meso-superstructured architecture. Although it is important to achieve complete crystallization of the perovskite layer, this process must not be 18 ACS Paragon Plus Environment

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accomplished at high temperatures (>100 oC) because the thermal stress could cause the infiltrated perovskite to break. However, if low temperatures (

Understanding the Role of the Mesoporous ... - ACS Publications

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