Temperature Assisted Nucleation and Growth To Optimize Perovskite

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Temperature Assisted Nucleation and Growth to Optimize Perovskite Morphology at Liquid Interface: A Study by Electrochemical Impedance Spectroscopy Priya Srivastava, Anukul Prasad Parhi, Rahul Ranjan, Soumitra Satapathi, and Monojit Bag ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00818 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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ACS Applied Energy Materials

Temperature Assisted Nucleation and Growth to Optimize Perovskite Morphology at Liquid Interface: A Study by Electrochemical Impedance Spectroscopy †

Priya Srivastava, Anukul P. Parhi,

†

†,ǁ

Rahul Ranjan,§ Soumitra Satapathi,

*,†

Monojit Bag,

*,†,⊥

Department of Physics, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667,

India. ǁ

Department of Physics, Indira Gandhi Institute of Technology Sarang, Dhenkanal, Odisha 759146, India. §

Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India. ⊥

Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India.

ABSTRACT: In this work, methylammonium lead tri-iodide perovskite has been synthesized by spin coating of lead acetate-trihydrate and methylammonium iodide precursor on pre-heated substrate. Significant difference in film morphology has been observed as the temperature was

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varied from room temperature to 120 °C prior to spin coating.

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Nucleation and growth

mechanism is revisited to find out the optimum substrate temperature for fabricating uniform perovskite films and is attributed to the fast homogeneous nucleation followed by delayed growth process. Electrochemical impedance spectroscopy measurement at perovskite-liquid electrolyte interface reveals the impact of film morphology on the anomalous diffusion behavior observed at low frequency regime.

KEYWORDS: Methylammonium lead tri-iodide, Substrate Temperature, Nucleation, Growth, Equilibrium Melting Point, Electrochemical Impedance Spectroscopy, Perovskite-Electrolyte Interface

MAIN TEXT: Hybrid organic-inorganic perovskite (HOIP) materials are widely used in many optoelectronic applications including solar photovoltaics,1 light emitting diodes2 and lasers, optical sensors and detectors,3 and memory devices.4 The key advantages of HOIPs are high absorption coefficient,5 ambipolar charge mobility,6 large carrier diffusion length,7 tunable bandgap8 as well as ease of solution processability.3 There have been plethora of research on perovskite photovoltaic to optimize device efficiency by material processing,9,10 device structure and interface engineering11 and band-gap engineering.12 Understanding photo-physical properties as well as charge transport at various interfaces are thus important to synthesize high quality materials with optimum device structure for better performance. Analyzing each of the interfaces by various characterization techniques including electrochemical impedance spectroscopy (EIS)13,14 in a solid state active device geometry is not only difficult to decipher but sometime is misleading as well since there may be multiple processes occurring at similar time scale at


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multiple interfaces. Therefore a new set of characterization is required for interpreting photophysical properties in HOIPs.15 To simplify the analysis of the perovskite materials under photo-excitation EIS should be carried out in perovskite-liquid electrolyte interface.15,16 Recently, H. Y. Hsu reported liquid junction photoelectrochemical solar cell with high open circuit voltage (VOC) of 1.05 V.16 Due to high VOC provided by MAPbI3 perovskite under illumination, it can also be a good candidate for photocatalytic/photoelectrochemical water splitting applications.17–19 Charge concentration as well as types of charge carrier accumulation at the perovskite-liquid interface have been extracted from the Mott-Schottky relation. Recently, Li et. al. have measured the flat band potential and p-type carrier density for a spin coated and a spray coated films of methylammonium lead tri-iodide (MAPbI3) perovskite.15 A space charge layer is observed when analyzing the EIS data at a frequency range of > 1 kHz. At a frequency below 1 kHz a complex diffusion behavior was observed for both the films. However, the spray coated film showed a higher degree of diffusion at the low frequency regime compared to that of spin coated film. The origin of anomalous diffusion in spray coated film is not discussed in detail. Therefore, a complete study is needed to understand the perovskite-liquid electrolyte interface as a function of perovskite morphology. The opulence of research is also going on to get the more uniform, full coverage with crystalline films which is essential for high performance devices.20–23 The study of perovskite morphology on different substrate temperature has been reported by Tidhar et.al. for efficient solar cell24 and Zheng et.al. for Plannar Hetero-junction Perovskite Solar Cell25. Tidhar et. al. reported that the perovskite films were more uniform and had better coverage when faster nucleation was induced at higher temperature. Zheng et. al. studied Volmer Weber Growth mechanism induced


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thermally and reported that thermal energy would overcome the energy barrier of the precursor solutions on the substrate and facilitate the formation of uniform perovskite films. In this letter, we have synthesized MAPbI3 from lead acetate and methylammonium iodide (at 1:3 molar ratio) precursor at different substrate temperature to control the morphology and measured EIS under illumination. Nucleation and growth model is used to analyze perovskite film formation as a function of substrate temperature varied from room temperature (RT) to 120 °C. Thin film XRD, Field Emmision Scanning electron microscopy (FESEM), atomic force microscopy (AFM) and EIS measurement have been performed and analyzed to measure the crystal structure, grain boundaries and the surface roughness, and demonstrated that the diffusion phenomenon observed at the low frequency regime strongly depends on the perovskite morphology. MAPbI3 film was prepared by single step deposition method from lead acetate and methylammonium iodide precursor in N, N-Dimethylformamide (DMF) solvent. The detail of the fabrication procedure is given in the supporting information. A significant variation in the film morphology is observed as shown in Figure 1(a)-(e). Considering the macroscopic picture of the film formation, we observed that when the precursor solution was spun onto the substrate at different temperature a phase transformation occurred from the liquid (L) to the crystalline (β) phase. In this kind of phase transformation, the free energy of the two phases plays an important role. The variation of free energies of the two phases is shown in Figure 2(a).26 At the equilibrium melting point Tm , the free energies of both the phases are equal. Above this temperature Tm , the free energy of the β phase is more than the free energy of the L phase, which indicates that the change in free energy (∆g)> 0 for the transformation from L to β phase and the transformation is not favorable under this condition. Below Tm, the free energy of β phase is less than that of L phase and the value of ∆g< 0 which is favorable for the transformation to proceed


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spontaneously. Therefore, for the spontaneous phase transformation the temperature must be below Tm. From the DSC curves, the transition at around 120 °C has been assigned to the melting of the amorphous phase of intermediate complex possibly formed due to the incomplete reaction of lead acetate-trihydrate with MAI to the crystalline phase of MAPbI3 (see SI Figure S1c). Therefore, substrate temperature closed to 100 °C satisfies this condition. Moreover, the substrate temperature closed to Tm results in slow reaction rate due to very small change in free energy. This is a favorable condition for uniform film formation as the slow transformation allows the newly formed nuclei to spread homogeneously over the substrate before growth takes place. Therefore, pre-heating the substrate provides the temperature close to Tm for uniform perovskite film formation. At room temperature (RT) film is highly non-uniform and only islands are formed. At 60 °C film is more uniform than that of RT. In both cases there is some residual lead acetate present in the film [see Figure 1(f) & Table S1] Moving on to the microscopic picture, once the condition for the spontaneous occurrence of the phase transformation i.e. ∆g Tm) i.e. about 120 °C, the temperature required for the growth of the nucleated particles is provided during the spin coating itself. So there is a chance that growth can occur soon after the nucleation resulting in heterogeneity in the film formation due to insufficient time lag between nucleation and growth.


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W. Zhang and co-worker found that using a non-halide lead source such as lead acetate (PbAc2) results in fast crystal growth of the perovskites allowing to obtain ultra-smooth and almost pinhole-free perovskite films instead of PbCl2 and PbI2.29 The basic idea they reported is that the better crystallization of perovskites occurs as the excess organic component is driven out of the film easily with increased enthalpy. The advantage of using PbAc2 source over the lead halide source is that the by-product of reaction in case of PbAc2 is methylammoniumacetate (CH3NH3Ac) which is more volatile to that of lead halide sources.30 Therefore by providing temperature during the process of crystallization itself volatile by-product CH3NH3Ac is driven out at a faster rate leading to fast crystallization. However, at higher substrate temperature of about 120 °C, an increase in roughness is observed in AFM image analysis. This may be attributed to the fact that at higher temperature DMF evaporates too fast, which leads to the faster solute extraction from the precursor solution and this results in irregular and more vertical growth instead of horizontal layer formation [Figure 3(a)-(b)]. To study the impact of morphology on the interfacial properties of perovskite-liquid electrolyte interface, we have carried out EIS measurement under illumination at a bias voltage equal to the open circuit voltage to minimize the space charge effect in the bulk perovskite film. Experimental detail of EIS measurement is given in the supporting information. A high frequency (> 10 kHz) semicircle corresponds to the perovskite/ITO interface is observed in all the devices. The low frequency response (< 100 Hz) is significantly different for different substrate temperature (SI Figure S5). At first we model the low frequency response as a simple RC circuit as shown in the supporting information (SI Figure S6) with the low frequency capacitance in the order of µF. However we observed a poor curve fitting (except the device fabricated at 100 °C substrate temperature) in the low frequency regime as it contains ion


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diffusion phenomenon originating from the electrode/liquid-electrolyte interface.15 Therefore one should expect Warburg-like diffusion phenomenon at the interface. But it does not follow Warburg-type diffusion either. A similar characteristic has also been observed by Li et. al. 15. We therefore argue that the diffusion is modified either due to the presence of ions in the HOIP active layers or due to the morphology itself (SI Figure S4). It has been observed from the AFM image analysis as shown in Figure 3(a) and (b) is that the surface r.m.s. roughness (SI Figure S5 (c)) decreases from 91 nm at room temperature to 26 nm at 80 °C and it further increases to 103 nm at 120 °C. This trend is also confirmed from the SEM image analysis as shown in Figure 1(a)-(e). However at 80 °C substrate temperature although the roughness is minimal, there are cracks through which liquid electrolyte can penetrate into the HOIP active layer. Therefore diffusion phenomenon is predominating at low frequency regime. On the other hand at 100 °C substrate temperature we observed continuous film formation resulting in minimal diffusion as the experimental result fits well with the RC circuit model where the simple capacitance is replaced with constant phase element (CPE) and the exponent is close to 0.87. This result indicates that the origin of anomalous diffusion at low frequency is not only due to the ionic conductivity presence in HOIP but also from the morphology of the active layer. Interestingly the open circuit voltage (VOC) is highest (1.018 V) for the device fabricated at 100 °C among all the devices followed by the device fabricated at 120 °C (VOC ~ 0.963 V). Device fabricated at RT shows the lowest VOC (0.784 V) among all (see Table S2). Further increasing the substrate temperature increases the surface roughness and hence increases the diffusion behavior despite continuous film formation. We have added a constant phase element (CPE4) in series with the resistance R4 to model the anomalous diffusion behavior at the low frequency regime. The exponent turns out to be 0.7 which is significantly low to be called as capacitance. Further


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analysis is thus needed to understand the anomalous diffusion behavior at low frequency regime for the devices fabricated at low (below Tm) as well as high (above Tm) substrate temperature. In conclusion we have fabricated perovskite active layers at different substrate temperature to control the morphology and proposed a model for the nucleation and growth based on the substrate temperature during the spin coating. EIS measurement indicates that the diffusion at the low frequency regime significantly deviates from the Warburg-like diffusion due to not only the presence of ionic conductivity in the HOIP layer but also the morphology of the active layer. Device fabricated at 100 °C substrate temperature seems to have better film coverage with optimum roughness giving rise to high open circuit voltage under illumination. This study therefore indicates the importance of perovskite morphology on the device performance. Current research is under way to study the impact of different charge transport layers as well as the composition of perovskite materials on the behavior of low frequency anomalous diffusion. ASSOCIATED CONTENT Supporting Information is included. A PDF file containing experimental details, Schematic experimental set up and diagram, EDAX data, supporting figures of PXRD, DTA/TGA/DTG, DSC, UV-Vis, film image, EIS, Tables of EIS model paramters.

AUTHOR INFORMATION Corresponding Author *(Monojit Bag) Email: [email protected] *(SoumitraSatapathi) Email: [email protected] Present Addresses


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ǁ

Department of Physics, Indira Gandhi Institute of Technology Sarang, Dhenkanal, Odisha 759146, India. Funding Sources The work is partially supported by SERB grant, Department of Science and Technology, INDIA under award no. ECR/2016/001530 dated 24/03/2017. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT PS acknowledge Prof. Raju K Gupta from Indian Institute of Technology Kanpur for getting the XRD characteriazation and fruitful discussion. SS acknowledge Solar Energy Research Initiative (SERI) Grant (DST/TMD/SERI/S147(G)) from DST, INDIA.

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Figure 1.(a) – (e) FESEM image of perovskite film fabricated at different substrate temperature. The scale is 10 µm. Inset: zoomed FESEM image of each film with the scale bar 1µm. (f) XRD of perovskite film fabricated at different substrate temperature. Peaks indicated by red dotted lines (~14°, ~28.5° and ~32°) correspond to the characteristic MAPbI3 peak. Peak indicated by blue dotted line (~12.6°) corresponds to lead iodide peak and black dotted line (~7.9°) corresponds to lead acetate peak.


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bT m

gL Tm Temperature

c 120 100 80

I

∆T

gβ

U

Temperature

Free Energy

a

Temperature (°C)

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Nucleation Rate (I) Growth Rate (U)

Spin coating

Annealing

∗ ∗ Nucleation ∗

Growth

∗

60

∗ RT 0

15

∗

RT 60 ° C 80 ° C 100 ° C 120 ° C

35

635

Time (s) Figure 2.(a) Schematic variation of free energy of a material in liquid (gL) and crystalline form (gβ) as a function of temperature. (b) Temperature dependence of nucleation rate (I) and growth rate (U). (adopted from ref. 26) (c) Expected temperature variation (not true scale) of the substrates during spin coating and annealing (assuming Newtonian cooling during spin coating and at thermal equilibrium during annealing). Asterisk (∗ ∗) indicates the nucleation point where yellow colored films turn dark colored.


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Figure 3.(a) AFM image (2D) of perovskite film fabricated at different substrate temperature. The scan area is 10 µm × 10 µm. (b) corresponding 3D image of each device. (c) Bode (|Z|) and phase (θ) plot of EIS measurement at different substrate temperature. Symbols represent experimental data whereas lines represent corresponding fit data.


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Temperature Assisted Nucleation and Growth To Optimize Perovskite

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