Reversible Dimensionality Tuning of Hybrid Perovskites with Humidity

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Reversible Dimensionality Tuning of Hybrid Perovskites with Humidity: Visualization and Application to Stable Solar Cells Sumit Kumar Sharma, Chinmay Phadnis, Tapan Kumar Das, Akash Kumar, Balasubramaniam Kavaipatti, Arindam Chowdhury, and Aswani Yella Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04115 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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Chemistry of Materials

Reversible Dimensionality Tuning of Hybrid Perovskites with Humidity: Visualization and Application to Stable Solar Cells Sumit Kumar Sharmaa, Chinmay Phadnisb, Tapan Kumar Dasc, Akash Kumard, Balasubramaniam Kavaipattid, Arindam Chowdhuryb, Aswani Yellaa,c*

a. Center for Research in Nanotechnology and Science, Indian Institute of Technology Bombay, Powai, Mumbai, India-400076 b. Department of Chemistry, Indian Institute of Technology Bombay, Powai Mumbai, India-400076 c. Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai, India-400076 d. Department of Energy Science & Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, India-400076

Corresponding Author e-mail: [email protected]

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ABSTRACT

Hybrid perovskites have attracted much attention as a promising photovoltaic material in the past few years. Typically these hybrid perovskites like methyl ammonium lead halides (MAPbX3) undergo dimensionality reduction from 3-D to 0-D and finally to PbX2 upon continuous moisture exposure. Our current study shows that 0-D perovskite related structures exhibit reversible transformation from transparent state to colored 3-D state upon exposure to humidity. Fluorescence imaging of individual microcrystals revealed that the structural phase transition could be visualized in solid state, where in the shape of the crystals transform to cubic crystals. The plausible reason for this transformation is proposed to be a dynamic dissolution and recrystallization of the excess methyl ammonium halide (MAX) with varying humidity. The thermal and the moisture stability were found to be greatly enhanced in the transformed 3-D perovskite. Excellent device stability was also demonstrated when the devices were kept under moist (~70 %RH) conditions.

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Recently perovskite materials have taken the solar cell research by storm with their rapid improvement in performance, with power conversion efficiencies rising from 3.8%1 to more than 22%2 within a few years. Halide perovskite structures, consisting of organic-inorganic counterparts, with most common one being methyl ammonium lead iodide (MAPbI3), are a subject of great interest because of their remarkable optoelectronic properties, easy solution based fabrication processes, low cost and ease of incorporation into a wide variety of device architectures.3-14 Typically, the processing and handling of these perovskite solar cells require inert atmosphere and the devices usually does not survive in hot and humid conditions, which hamper the real life applications of the perovskite solar cells. Though high power conversion efficiencies were obtained, the instability of MAPbX3 to ambient atmosphere is still an open problem.15-26 Among the various factors affecting performance, regulation of humidity has received lot of attention because of its adverse impact on perovskite material properties and the device characteristics like power conversion efficiencies and stability. Higher relative humidity is found to be detrimental to the device performance of MAPbX3 based perovskite solar cells.15-23 Kamat et al. carried out a detailed study of the effect of humidity on the MAPbI3 perovskites and showed that these 3-D perovskites get hydrated to 0-D MA4PbI6.2H2O upon exposure to moisture in dark, which in turn form PbI2 under the light.15 Later, Leguy et al. reported that the transformation of MAPbI3 actually takes place through an intermediate monohydrate MAPbI3.H2O which then converts to the dihydrate crystals (MA4PbI6.2H2O) under humid conditions. These dihydrate crystals undergo irreversible degradation to PbI2.16 This observation was also supported from the studies on the perovskite crystallization by Kanatzidis and group19. Lead iodide is also frequently witnessed in the degraded perovskite solar cells. Overall, the crystallographic phase transformation from MAPbI3 to PbI2 through 0-D MA4PbI6.2H2O is ACS Paragon Plus Environment

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attributed to be the main reason for the degradation of the perovskite solar cells in humid conditions. Till date, the effect of the humidity is found to be unidirectional, resulting in the formation of PbI2 upon long exposures.15-26 Very recently, it has been reported that the nanocrystals of Cs4PbBr6 can be converted to CsPbBr3 by the extraction of CsBr with water.27-31 However the conversion of the nanocrystals were uni-directional and performed by the mediation of the ligands. To date all such transformations are uni-directional and only shown in the case of nanocrystals and the reverse conversions required external intervention of PbBr2 or CsBr. In the case of the films, unidirectional dimensionality reduction was observed from CsPbBr3 to CsPb2Br5.32 Herein, we show a reversible transformation of 0-D MA4PbX6.2H2O (where X = Br, I) to 3-D MAPbX3 perovskite upon exposure to humidity, and the transformed perovskites remain unaffected to humidity for more than 1000 h in terms of their absorption and photovoltaic performance in comparison with the as prepared 3-D perovskites. We have carried out a detailed study of the changes that manifest in the optical absorption, crystal structure, film morphology and photoluminescence characteristics of 0-D when exposed to varying humidity. The crystal structures of the 3-D and 0-D perovskite related structures are shown in the Figure 1a and 1b respectively. The films of 0-D perovskites are prepared by controlling the reactant ratios of PbBr2 and MABr in 1:4 respectively. The films turned reddish orange upon heating to 55 oC. Upon cooling to room temperature at 40 %RH, the film turned transparent. To check the influence of humidity on the 0-D perovskites, these films were kept at room temperature and controlled humid atmosphere (10, 40, 60, and 80% RH). Humidity was controlled to approximately ±5 % by passing N2 gas through the water bubbler kept at different temperatures ranging from RT to 40 oC. Figure 1c, d shows the photographs of the films observed at varying humid atmospheres for the 3-D perovskite and 0-D perovskites, respectively. Clear color variation was observed with the change in RH, which could be observed with the naked eye. ACS Paragon Plus Environment

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Around 40 %RH, the 0-D film was transparent and when the humidity is reduced to ~10 %RH, color of the film changed to yellow, and interestingly when the RH is increased to ~60%, color of the film changed to orange in color. Further increase in the humidity didn’t result in any visible change. UV-Vis absorption spectra of the films were collected after maintaining the humidity in the chamber to a desired value, using water vapor and N2 gas (fig.1e). Upon reducing the RH to 10%, slight increase in the absorbance was observed in the visible region in the range of 350-530 nm. Increasing the RH to 60% and above, the same film resulted in a striking increase in absorbance, with two most prominent peaks at 400 nm and 530 nm. Also a red shift in the absorption edge of ~30 nm is observed. Interestingly, we observe the same phenomena upon halide exchange. In this paper, we have carried out analogous studies by replacing bromide with iodide and we observe similar results by varying the halide (Fig. 1f). The absorption spectrum extended across the whole of the visible region ranging from 350-780 nm with the moisture exposure in the case of iodide. Figure S1 shows the photographs showing the transformation of the 0-D MA4PbI6 to 3-D MAPbI3 with varying humidity.


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Figure 1 (a) Crystal structures of the 0-D and 3-D perovskites (c,d) Photographs of the films under different RH conditions for the 3-D MAPbBr3 and 0-D MA4PbBr6 respectively (e) UV-Vis absorption spectra of 0-D MA4PbBr6 (f) UV-Vis absorption spectra of 0-D MA4PbI6 in different RH conditions.

Another interesting feature is that the whole process is completely reversible and the film transforms to transparent film upon reducing the RH to 40%. Figure S2 of the supporting


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information shows the UV-Vis spectra recorded for 40 cycles with increasing and decreasing humidity. Remarkably, even after 40 cycles, no change in the original absorption intensity was observed. The transformation takes place very fast and the change in the absorption could be observed in few seconds. Figure S3 shows the change in the absorption at 520 nm with time as the humidity is changed from 40% to 80 %RH. Previously it has been reported that the 0-D perovskite upon heat treatment results in the formation of the 3-D perovskite by dehydration.33 The absorption spectrum of the 0-D high humid phase is extended to the red by 30 nm in comparison with the 55 oC heat-treated sample irrespective of the halides. Figure S4 shows the comparison of the absorption spectra of the 0-D high humid phase and the 55 oC heat-treated sample for both the halides. Conditions of high humidity resulted in phase pure MAPbBr3 with the absorption onset matching perfectly with the MAPbBr3 structure, however the heat treated sample doesn’t have the same degree of coupling between the PbBr6 octahedra. To better understand the effect of humidity on the 0-D perovskite films and to completely appreciate the underlying phenomena, X-ray diffraction patterns were recorded by passing the water vapor along with N2 as a carrier gas. Figure 2a shows the diffraction patterns obtained for the 0-D perovskite when the film is treated with the altering humid conditions. The 0-D MA4PbBr6 doesn’t have a reference ICSD pattern, so we have simulated the XRD pattern for the same using CrystalMaker Software suite, based on the space group of MA4PbI6. Details of the estimation of the initial values of the lattice parameters of MA4PbBr6 from that of the reference ICSD file of MA4PbI6 are given in the supplementary information. At around 40 %RH, all the reflections could be assigned to the formation of the 0-D MA4PbBr6. Upon reducing the humidity to 10%, slight difference was observed with respect to that of the pattern obtained at 40 %RH. The reflections from the MA4PbBr6 still exists at 10 %RH, along with new reflections corresponding to MAPbBr3. The starting values of the simulated MA4PbBr6 pattern were then used for Rietveld refinement of the XRD patterns, which allowed us to estimate the weight ACS Paragon Plus Environment

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fraction of the two. We find that 18.9 wt% MAPbBr3 exists along with MA4PbBr6. The results of the Rietveld refinement are summarized in the table S1 and the fitting of the patterns are shown in the fig S5 in the supporting information. Upon increasing the RH to 60 %, remarkable difference in the X-ray diffraction pattern was observed. Interestingly, all the new reflections observed could be attributed to the formation of 3-D MAPbBr3. The most striking reflections corresponding to the (100) and (200) planes of MAPbBr3 started to appear after exposure to 60 %RH and converted completely to 3-D perovskite upon water vapor treatment (RH~80%). This is completely unexpected as the 0-D perovskite, which is reportedly the hydrated phase, upon treatment with water vapor resulted in the formation of 3-D perovskite (supposedly the dehydrated phase) all without any change in the temperature. This moisture-induced transformation could take place by a two-step process under the MABr rich conditions. Since water is known to react with the methyl ammonium ions, in the initial step, excess humidity accelerates the exclusion of methyl ammonium ions from the 0-D MA4PbBr6. The initially decoupled PbBr6 octahedra in 0-D MA4PbBr6 upon the exclusion of methyl ammonium ions, results in the coupling and reorganization of the lattice to form 3-D MAPbBr3 perovskite. So, dynamic equilibrium exists between the MA4PbBr6.2H2O and MAPbBr3 in the presence of moisture through the following reaction MA4PbBr6.2H2O = MAPbBr3 + 3MABr + 2H2O. Forward reaction proceeds in a much faster rate in the presence of humidity as observed due to the dissolution of MABr. The released MABr could be in proximate to the MAPbBr3, which upon reducing the humidity, the dynamic equilibrium shifts towards the formation of MA4PbBr6.2H2O. To further understand the effect of humidity on the optical properties of the 0-D perovskites, steady state and time resolved photoluminescence spectra were collected, by subjecting the films to various humid conditions. Figure 2b shows the steady state photoluminescence of the 0-D ACS Paragon Plus Environment

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films with varying humidity. Interestingly, the 0-D perovskite films of both the halides are transparent at room temperature, but they exhibit intense and Stokes shifted photoluminescence in comparison to the 3-D perovskite films. Standard Gaussian peak centered at 525 nm and 720 nm were observed in the case of bromide and the iodide based 0-D perovskites respectively (Fig. S6). Decreasing the RH to 10% resulted in a blue shift of ~20 nm in the photoluminescence and a reduction in the intensity could be observed (Fig. 2b). This is probably a result of the decrease in the confinement of the excitons as a result of the formation of minor amount of MAPbBr3. Increasing the RH to above 60% resulted in red shifted luminescence with lower intensity. The red-shifted PL peak position matches well with that of the MAPbBr3 phase band-to-band recombination, which also supports the XRD data obtained. Further increase in the RH to above 80% resulted in the reduced photoluminescence intensity. To ensure that the bright PL observed at 40% RH is reproducible, we have carried out the absorption, PL and PL excitation spectrum for the same sample (fig. 2(c)). At 40% RH, though the sample is visibly transparent, a tail exists until 550 nm indicating that a certain degree of ordering exists between the PbBr6 octahedra in the 0-D MA4PbBr6.2H2O. From the PLE spectrum it is observed that a dip exists at 310nm, where a sharp peak is found in the absorption spectrum. This indicates that the PL observed at 40% RH is not resulting from the MA4PbBr6.2H2O but due to the existence of the certain degree of ordering of the PbBr6 octahedra similar to the observations made previously.34-38 As the humidity is slightly increased to 50% RH, the color of the film turned pale orange and under those conditions, the three spectra were again recorded. Upon increasing the humidity, the absorbance of the film increased but a similar behavior was observed in the PL, PLE (Fig. S7). With the further increase in the humidity to 80% RH, the intensity in the PLE spectrum has reduced drastically and doesn’t exhibit a dip indicating the absence of pure MA4PbBr6.2H2O. To confirm that the emission is indeed coming for the degree of ordering that exists, wavelength dependent emission studies were carried out, which clearly show that the maximum emission is ACS Paragon Plus Environment

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not observed at the wavelength where there is a maximum in the absorption spectrum.

Figure 2 (a) X-ray diffraction patterns obtained with varying humidity MA4PbBr6. (b) Steady state photoluminescence spectra of the 0-D perovskite films MA4PbBr6. (c) Absorption, PL, PL excitation spectrum of the 0-D perovskite film at 40% RH. (d) Excitation wavelength dependent PL spectrum of the 0-D perovskite film at 40% RH.

To further decipher the photoluminescence characteristics during the phase transformation from 0-D to 3-D perovskite structures, fluorescence microscopy study was carried out by altering humidity. The samples were grown, on a coverslip with lower concentrations (0.2 M) of precursors, which ensured that the formed crystals are well separated. Figure 3 shows a series of fluorescence microscopic images (30 x 30 mm2), for spatially segregated microcrystal film of MA4PbBr6 under varying RH in the environment. Each image was acquired using a CCD at 50 ms exposure, following laser excitation (405 nm, 1.2 W/cm2) of an identical area of the sample at each ambient RH. The ambient RH was increased from 15% to ~80% and then reverted back to ACS Paragon Plus Environment

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15% for PL imaging measurements, to check for the reversibility in the shape, size and emission characteristics of microcrystals.

Figure 3 PL intensity images of spatially separated MA4PbBr6 perovskite microcrystals with varying RH of the environment.

It is evident from the Figure 3 that the microcrystals re-structure from irregularly shaped disks to square-shaped disks with increase in RH. However, after reaching maximum humidity value, with the decrease in humidity, each microcrystal become unstructured resulting in a loss of their cubic shape. We find that such remarkable transformation in the shape of each individual crystal occur at ~60% RH. Moreover, the regular shape, obtained at ~80% RH, disappears slightly below 60% RH indicating that the transformation is essentially reversible. It is intriguing that individual microcrystals undergo reversible phase transformation from 0-D perovskite to 3-D perovskites, and such transformation in the shape happens in the absence of any solvents. Further, apart from shape, the size of the crystals are also reduced with increasing RH, demonstrating the dissolution of excess MABr from the initial crystals with increasing moisture. On the other hand, the PL emission intensity decreased gradually with increase in RH, which eventually recovers with the reduction in humidity. Fig. S8 shows the PL intensity images of ACS Paragon Plus Environment

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MA4PbBr6 microcrystals in different humid conditions and the video (Video S1) depicting the reversibility is also shown in the supporting information. To elucidate the effect of humidity on emission characteristics of these perovskite microcrystals, spectrally resolved photoluminescence (PL) microscopy measurements were performed under various ambient moisture contents. The PL emission spectra (λex = 405 nm) from a representative entire individual (MA)4PbBr6 microcrystal at different RH is presented in Figure S9. As demonstrated in Fig. S9 inset, the emission intensity increased initially (from 15%) with increase in RH and then decreased drastically at around 60% RH. Whereas, with decrease in RH (from 80%), the intensity remained almost stable till 50%, after which it increased incessantly. The emission intensity not only increased in this situation, but displayed higher value at lowest RH. Besides, the PL peak position is found to be shifted towards red side of the energy spectrum. The red shift in emission maxima is moderate (~4 nm; ~3 meV) from 15% to 50% but after 50% RH the emission red shifts by ~ 25 nm (~112 meV) for 60% RH. Interestingly, the emission maxima start to blue shift with further decrease in RH and eventually attain initial value (~520 nm) at lowest RH of 15%.

Figure 4 SEM micrographs of the 0-D perovskite films MA4PbBr6 after treating the film with different humid atmospheres. (a-c) Initial film prepared at 40 %RH (d-e) Increasing the RH to 80%


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The clear change in the shape and size of the crystals from the fluorescence imaging prompted us to carry out scanning electron microscopy (SEM) studies of the films. The morphological changes were observed for the same film by increasing the humidity exposure. Figure 4 shows the top-down scanning electron micrographs of the as prepared 0-D films and then exposing the same film to humidity. The micrographs clearly indicate that definite structural changes happened after treating the film with water vapor. At 40% RH (initially prepared), the films exhibited uniform coverage with some fine particles on top. However, upon increasing the RH to ~80%, the film is still shows uniform coverage but the morphology of the particles has transformed to cubic shaped particles. Since the complete solid-state transformation is taking place, we presume that the methyl ammonium bromide is present at the grain boundaries when the films are exposed to humidity, which upon reduction of humidity results in the decoupling of the PbBr6 octahedra. The complete reversibility and the instantaneous phase transformation from 0-D to 3-D perovskite (i.e., transparent to colored phase) upon exposure to humidity makes it suitable for the application in smart windows. To study the aptness of this phase transformation for the smart window application, photovoltaic devices are made with mesoporous TiO2 as the electron transport layer and spiro-OMeTAD as hole conducting layer. The 0-D perovskite is coated on to the mesoporous TiO2 layer, which in turn is used as the absorber layer.


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Figure 5 (a) J-V curves obtained using the 0-D perovskite as the absorber at 40%RH and 65 %RH. (b) EQE of the device at 65 %RH. (c) Stability studies of the devices under constant illumination at 65% RH.

Figure 5a shows the photovoltaic performance of the device utilizing MA4PbBr6 as the absorber layer. The transparent 0-D perovskite device exhibited open circuit potential of 828 mV, a short circuit current density of 1.2 mA/cm2 and a fill factor of 25.5%, corresponding to a power conversion efficiency of 0.25%. The same device after exposing to humid atmosphere with an RH of nearly 65% under constant 1 sun illumination resulted in the phase transformation, which is visible to naked eye (figure S10 of supporting information). The same device after the phase transformation resulted in open circuit potential of 1068 mV, a short circuit current density of 7.9 mA/cm2 and a fill factor of 55%, corresponding to a power conversion efficiency of 4.7%. This corresponds to ~20 fold increase in the power conversion efficiency after the exposure to ACS Paragon Plus Environment

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moisture (RH~60%) under full sun illumination. In order to validate the obtained short circuit current density, external quantum efficiency (EQE) measurements were carried out on the best devices. Figure 5b shows the EQE data obtained at 60% RH for the best device. The EQE of these devices spanned the range of 300 to 580 nm. The Jsc obtained by integrating the product of the AM1.5G photon flux and the IPCE spectra are in reasonable agreement with the Jsc derived from the J-V curves. Under the same conditions with 65% RH, the devices made with the MAPbBr3 exhibited slightly lower efficiencies and resulted in an open circuit potential of 970 mV, short circuit current density of 7.8 mA/cm2 and a fill factor of 50% corresponding to a power conversion efficiency of 3.8%. When the devices were prepared using the 0-D iodide counterpart the morphology of the film formed is not uniform (Fig. S11) and the device efficiencies obtained were similar to the bromide counter part. It is worthwhile to point out that the 3-D perovskite obtained from the humidity assisted 0-D perovskite displayed excellent photo- and thermal stability, even compared to the films with direct 3-D perovskites. Figure S12 shows the comparison of the absorption spectra of the 0-D perovskite after exposure to 60 %RH and after light soaking for 48 h and heat treatment at 60 oC at 60 %RH. No noticeable difference was observed in both the conditions after light soaking and heat treatment at 60 oC, indicating better stability of the transformed 3-D perovskite films. The moisture stability of the corresponding devices were tested at room temperature with a humidity of about 65%. As shown in the figure 5c, the devices with 0-D perovskite retained over 95% of its initial efficiency after 250hrs. In contrast the devices with pure perovskite film degraded under the humid conditions retaining only

Reversible Dimensionality Tuning of Hybrid Perovskites with Humidity

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