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Crystallization Kinetics of Lead Halide Perovskite Film Monitored by In-situ Terahertz Spectroscopy Sae-June Park, Ah-Ram Kim, Jung Taek Hong, Ji-Yong Park, Soonil Lee, and Yeong Hwan Ahn J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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The Journal of Physical Chemistry Letters

Crystallization Kinetics of Lead Halide Perovskite Film Monitored by Insitu Terahertz Spectroscopy

S. J. Park, A. R. Kim, J. T. Hong, J. Y. Park, S. Lee, and Y. H. Ahn* Department of Physics and Department of Energy Systems Research, Ajou University, Suwon 16499, Korea

*

Corresponding Author: [email protected]

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ABSTRACT. Vibrational modes in the terahertz (THz) frequency range are good indicators of lead halide perovskite's crystallization phase. We performed real-time THz spectroscopy to monitor the crystallization kinetics in the perovskite films. First, THz absorptance was measured while the perovskite film was annealed at different temperatures. By analyzing the Avrami exponent, we observed an abrupt dimensionality switch (from 1D to 2D) with increasing temperature starting at approximately 90°C. We also monitored the laser-induced crystallinity enhancement of the pre-annealed perovskite film. The THz absorptance increased initially, then subsequently decayed over a couple of hours, although the enhancement factor varies depending on the film crystallinity. In particular, the Avrami analysis implied that the light-induced crystallization was assisted by the 1D diffusion processes. The activation photon energy was measured at 2.3 eV, which indicated that enhanced crystallization originated from the photo-induced structural change of residual lead iodide at the grain boundary.

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Over the past few years, lead halide perovskites (MAPbI3) have emerged as one of the most promising materials for use in high-performance solar cell devices due to their outstanding advantages. These advantages include high carrier mobility, band gap controllability, strong absorption across the solar spectrum, long hole and electron diffusion lengths, low cost, and simplicity of device fabrication.15

The perovskite structure is described by an ABX3 tetragonal structure, where A is a methylammonium,

B is a lead, and X is an iodide ion in the case of MAPbI3. This material has an optical bandgap of 1.5 – 2.3 eV, depending on the halide ion.5 Perovskite solar cells have attracted substantial attention as their efficiency has soared to 22.1%, despite being first developed in 2009.6 However, total cell efficiency is still lower than conventional silicon-based solar cells, and the key elements to improve cell efficiency lie in the development of optimized fabrication processes.7 Because cell efficiency is governed by material transport properties, enhancing the crystallinity of the film is imperative.8-10 Therefore, it is highly desirable to understand the underlying physical underpinnings of crystallization in the fabrication processes. The investigation into crystallization kinetics included a study of the crystallization mechanism (such as interfacial growth and diffusion-assisted crystallization, including dimensionality) and quantification of the crystallization activation energy.11-13 A thermal annealing process was adopted, as it is one of the most effective ways for a perovskite film to achieve a high-quality crystalline phase. The activation energy was determined by measuring the film's conductivity when annealed at different temperatures, yielding 0.2 – 0.4 eV.11-12 Recently, laser-induced crystallization effects have been studied using a strong near-IR laser, in which the improved crystallinity has been attributed to laser-induced thermal effects.14 Although light can produce a marked enhancement in the crystallinity of various amorphous materials, it has not been fully explored whether these changes are due to thermal effects or a photoinduced structural change.15-16 Up to now, the dynamical crystallization processes in perovskite films, either thermally assisted or induced by laser irradiation, have not been thoroughly understood.


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The crystallinity of perovskite has been widely investigated by X-ray diffraction (XRD) measurements;17-20 however, the slow measurement speed of this method (requiring more than 10 mins for each spectrum) limits its utility in studying crystallization dynamics. More recently, two-step crystallization dynamics have been studied with a real-time X-ray setup using a synchrotron radiation source, used immediately after combining a PbI2 thin film with a CH3NH3I solution.21 However, terahertz time-domain spectroscopy (THz-TDS) is a more powerful technique for monitoring target materials using their spectral fingerprints in the THz frequency range.22-24 THz spectroscopy is unique because it enables fast, non-contact, label-free, and non-destructive inspection of the target materials.22,24 More importantly, this method can provide useful information regarding the transient crystallization status of perovskite films because the halide perovskite MAPbI3-xClx films have representative phonon modes in the THz frequency range.25-26 Here, we developed in-situ THz-TDS to study the crystallization kinetics of lead halide perovskite films. We first monitored the dynamical phase transition of the perovskite film under thermal annealing processes. The THz absorption of the lead halide perovskite film was quantified as a function of annealing time at various temperatures and analyzed in terms of the Avrami exponent. We also investigated laser-induced crystallization of the perovskite film, in which we found an initial increase in crystallization followed by de-crystallization. In addition, the laser-induced change in the degree of crystallization was compared with transport properties by measuring the time-dependent photoconductivity. Finally, we studied the threshold photon energy of the photo-induced structural change by tuning the light wavelength. A schematic of in-situ THz spectroscopy for monitoring the crystallization kinetics of the perovskite film is shown in Figure 1a [also see the Methods section and Supporting Information Figure S1]. The lead halide perovskite film was fabricated via the solution-based spin coating method using MAPbI3 powder (purchased from One Solution Inc.), followed by a thermal annealing process. A scanning electron microscope image of the film is shown in Supporting Information Figure S2. We measured the ACS Paragon Plus Environment

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transient THz absorption peaks of the perovskite film to study the annealing effect on crystal structure formation. We also monitored the additional crystallization induced by an ultraviolet (UV) light source, as shown later. The annealing process, which is one of the key processes in the fabrication of lead halide perovskite solar cells, change the intermediate phase of the perovskite film to the tetragonal phase as illustrated in Figure 1b. Because the phonon modes of the tetragonal phase are in the THz region, they can be used as an efficient indicator for the film's crystallinity. Therefore, we could monitor the dynamical behaviors of crystal formation in-situ as we annealed the film. In Figure 1c, we show the THz absorption spectra for the MAPbI3 film, extracted from the transmission amplitudes in the THzTDS measurements. This film was annealed at 100°C for 15 min. We found two strong THz absorption peaks at 1 THz and 2 THz from the MAPbI3 film, which could be attributed to the motion of Pb–I–Pb bending and Pb–I stretching, respectively.25-26 Conversely, in the case of the Methylammonium (MAI) film (i.e., without Pb and I), we could not find noticeable absorption peaks in the THz frequency region. In addition, the two types of phonon modes decreased as iodide ions were replaced by chloride ions [Supporting Information Figure S3].


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Figure 1 (a) Schematic of in-situ THz spectroscopy for crystallization monitoring of a perovskite film under the thermal annealing and laser irradiation procedures. (b) Schematic illustration of the crystallization process of MAPbI3. (c) The THz absorptance spectrum of MAPbI3 (red) and MAI (black) films.

To begin with, we monitored transient crystallization with in-situ THz spectroscopy while we annealed the perovskite film at different temperatures. THz absorption spectra before and after the annealing process are shown in the supporting information and are supported by XRD data [Supporting Information Figure S4]. A representative 2D plot of in-situ THz absorptance, extracted from the transmission spectra, is shown in Figure 2a. We monitored the change in the absorption spectra (x-axis) as a function of the annealing time (y-axis). Initially, there was no noticeable absorption peak in the THz spectra. However, as we turned on the heater at t = 0, two peaks at 1 THz and 2 THz started to develop over time. The appearance of these peaks can be attributed to the formation of the perovskite crystal ACS Paragon Plus Environment

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structure as previously mentioned. Figure 2b shows a plot of THz absorptance as a function of time for the peaks at 1 THz (blue line) and 2 THz (red line). Both peaks increased until they were saturated, with a time constant of ~ 210 s, which is the time required to convert the intermediate phase to the tetragonal phase for a 300-nm-thick perovskite film.

Figure 2 (a) In-situ THz absorptance as a function of spectrum (x-axis) and time (y-axis) when we turned on the heater at 90°C at t = 0. (b) Plot of absorptance as a function of time at the 1 THz (blue) and 2 THz (red) peaks, extracted from (a).

Importantly, crystallization kinetics were studied at various temperatures as shown in Figure 3, using the transient data shown in Figure 2b. The phase transformation process was analyzed with the Avrami equation, which has been widely adopted to explain the kinetics of the transformation of the amorphous phase into the crystallized phase.13,27-29 According to the Avrami equation, the crystallized volume fraction is given by following relation: ACS Paragon Plus Environment

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݂ = 1 − expሾ−݇‫ ݐ‬௡ ሿ, where f is the volume fraction of crystallized perovskite crystal (which is proportional to the THz absorption), t is the time, k is the rate constant, and n is the Avrami exponent, which gives information regarding the mechanisms of crystallization. We plotted ln(−ln(1−f)) versus ln(t) in Figure 3a for annealing temperatures of 70ºC (blue) and 90ºC (red). The Avrami exponents were extracted by fitting the data, which yielded n = 1.05 and n = 2.02 for the 70ºC and 90ºC annealing temperatures, respectively. Surprisingly, as shown in Figure 3b, we found that the exponent changed abruptly from n = 1 to n = 2 at approximately 90ºC. In general, the thermally assisted crystallization process consists of initial nucleation followed by interfacial growth.30 Our results indicate that the interfacial growth dimensionality of the perovskite film changed from unidirectional (1D) to two dimensional (2D) as schematically illustrated in Figure 3b.13 This transition of dimensionality has not been reported for lead halide perovskite films during a thermal annealing process, which explains why 90ºC–100ºC annealing temperatures have been widely adopted. Above this temperature, the crystallinity degree of the film will be significantly improved due to a switch in the growth dimensionality. This has been reflected in the temperature-dependent crystallization studies as shown in the Supporting Information S3. From the transition behavior, we can achieve optimal crystallization without risking the thermal damage expected at higher temperatures.


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Figure 3 (a) ln(−ln(1−f)) versus ln(t) for annealing temperatures of 70ºC (blue circles) and 90ºC (red squares), where f is THz absorptance at 1 THz, normalized by the maximum value. Solid lines are fit to the data. (b) The Avrami exponent n as a function of annealing temperature extracted from (a).

We now turn to the crystallization enhancement and kinetics when the perovskite film was irradiated by a UV laser. Figure 4a shows a 2D plot of THz absorption with exposure to a UV (355 nm, ~ 2 W/cm2) laser as a function of spectrum (x-axis) and time (y-axis). We annealed the perovskite sample before UV exposure at the typical temperature condition of 100ºC for 15 min. From the results shown in Figure 4a, we could extract the transient absorptance at 1 THz and 2 THz as shown in Figure 4b. Surprisingly, the crystallization phase increased initially (< 10 min) by approximately 30% and degraded over a couple of hours as monitored by the peak intensity. Recently, laser-induced


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crystallization effects have been studied with a strong near-infrared (IR) laser (100 W/cm2) as previously mentioned; however, the effect of a UV laser has not been reported.14 Further, the UV intensity we used was much lower than that of an IR laser. We also note that we could not observe similar effects when we used a green laser (532 nm), even with a much higher intensity [Supporting Information Figure S5]. The UV-induced enhancement effects varied with the samples (from 0% to 30%), possibly depending on the initial crystallinity of the films, although they have been fabricated under the same conditions. It seems that the UV-induced crystallization process could be very promising for enhancement of cell efficiency, especially when crystallization is incomplete due to insufficient annealing procedures or exposure to humid air.31-32 Because the photon energy of a UV laser is much higher than that of an IR laser, it is imperative to address the contribution of photo-induced structural change to the enhancement of crystallinity and distinguish this effect from the thermal effects attributable to IR irradiation.14 As shown in Figure 4c, we extracted the Avrami exponent n from the results shown in Figure 4b. For the additional crystallization, we obtained n ~ 0.5, whereas n ~ −1 for UV-induced decrystallization. The exponent of 0.5 can be successfully interpreted to show that the additional crystallization process was mediated by the 1D diffusion process, which resulted in an increase of the crystal domain size in the films.13,27 This is completely different from the result of a thermal annealing process, in which the edge interface growth dominates, as shown above. Conversely, the UV induced decrystallization (n = −1) corresponds to simple exponential decay; in other words, the total crystallization volume on the film decayed as exp[−t/τ], where τ is the decay constant. The crystallinity decay is also an interesting phenomenon that has to be addressed in the future. For instance, additional XRD measurements (using a focused X-ray source to probe only the UV-irradiated area) could reveal the details on the structural damages, e.g., the decomposition of MAPbI3 into MAI and PbI2.


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Figure 4 (a) In-situ THz absorptance as a function of spectrum (x-axis) and time (y-axis) upon irradiation by a 355-nm laser. The laser was turned on at t = 0. (b) Plot of THz absorption at 1 THz (blue) and 2 THz (red) as a function of UV exposure time. (c) ln(−ln(1−f)) versus ln(t) for the peak at 1 THz in (b).

We also investigated the UV-induced change in the transport properties of the films in conjunction with the crystallization volume fraction obtained from THz-TDS. The time-dependent photocurrent (ip) was measured to investigate the laser-induced change in the perovskite photoconductor as schematically illustrated in the inset of Figure 5a (for a fixed bias of VSD = 100 mV). The perovskite film was ACS Paragon Plus Environment

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irradiated by a 355 nm laser under the same conditions used in Figure 4 (i.e., with a spot size of 1 mm and an intensity of 2 W/cm2). The time-dependent photocurrent in Figure 5a (red line) correlates roughly with the crystallization volume measured by the THz transmission in Figure 4b, except that ip decays faster than f. This is mostly because the UV light induced a reduction in the internal quantum efficiency in addition to the effect of the crystal volume reduction [Supporting Information Figures S6]. More importantly, enhancement of the initial photocurrent was as large as ~ 120%, which is much higher than that found with THz absorptance (~ 30%). This is because the film conductivity (σ) improved significantly above the percolation threshold as σ ∝ (f − fc)p, where p is the exponent and fc is the percolation threshold, yielding fc ~ 0.7 in our case [Supporting Information Figure S6]. For comparison, we studied the data collected when we illuminated the film with a wavelength 532 nm (green line) with the same intensity (~ 2 W/cm2) used for the 355-nm illumination. In the case of the 532 nm treatment, we could not observe a noticeable change in ip, in accordance with the THz results. To unravel the crystallization mechanisms further, we performed light-induced, photocurrent measurements by fine-tuning the light wavelength. Here, the perovskite film was annealed for 5 min (at 100°C), in which condition the crystallinity of the film is likely to be incomplete. This will magnify the laser-induced crystallization effects. In Figure 5b, the magnitude of photocurrent enhancement (∆ip) has been plotted as a function of the photon energy (EL). Surprisingly, we found distinct threshold behaviors of the crystallinity enhancement at EL = 2.3 eV (λL = 530 nm). We note that our film exhibited a continuous absorption spectrum with the typical bandgap of 1.55 eV and did not have a noticeable fingerprint around 2.3 eV [Supporting Information Figure S7]. The presence of a sharp transition in photon energy proves unambiguously that the dramatic crystallinity enhancement is not due to thermal effects. Recently, the automatic removal of the residuals due to the diffusion of MAI into PbI2 lattice (assisted by moist air) has been reported;33 however, our wavelength-dependent data rule out this possibility again. Instead, a photo-induced structural change is responsible for the enhancement.


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Importantly, the threshold photon energy matches very nicely with the bandgap of PbI2, which is 2.3 eV.34 Furthermore, the presence of a PbI2 phase at the grain boundary of the MAPbI3 film is reported, as schematically shown in Figure 5c,34 although details regarding the photodissociation process require further investigation. Including the results of the Avrami analysis in the THz measurements, we can conclude that light-induced crystallization was induced by the photo-induced structural change of residual PbI2 at the grain boundary into the MAPbI3 crystalline phase, assisted by 1D diffusion processes (as illustrated by Figure 5c). In other words, laser illumination induced stitching of the grain boundary, which dramatically increased the percolation of the perovskite film relative to that in thermal annealing processes. Our results suggest that the crystallization of the MAPbI3 layer is one of the crucial factors determining cell efficiency and that there is additional room for crystallinity enhancement when using advanced techniques such as light irradiation.

Figure 5 (a) Transient photocurrent at VSD = 100 mV as a function of laser exposure time for the 355 nm (red) and 532 nm (green) wavelengths. (Inset) Schematic of transient photocurrent measurement of the perovskite photoconductor upon UV illumination. (b) Photocurrent enhancement as a function of photon energy varied with the tunable light source. EA denotes


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the activation photon energy for the crystallinity enhancement. (c) Schematic of the light-induced crystallization enhancement, which describes the transformation of residual PbI2 into MAPbI3.

In conclusion, we demonstrated real-time monitoring of crystallization kinetics in lead halide perovskite films during a thermal annealing process followed by laser irradiation. THz spectra were measured in-situ as we annealed the perovskite film at different temperatures and discovered the perovskite structure transformed from an intermediate phase to the tetragonal phase. Importantly, we observed an abrupt switch in growth dimensionality with increasing temperature starting at approximately 90°C, determined by a change in the Avrami exponent from n = 1 to n = 2. In addition, we investigated the laser-induced phase transition of the perovskite film. The crystallization phase increased for 10 min and decayed over a couple of hours as monitored by THz absorptance. The initial increase was characterized by an Avrami exponent of n = 0.5, attributable to the additional crystallization assisted by the 1D diffusion processes. The photoconductivity of the perovskite devices increased as much as 120% when the crystalline volume fraction increased by approximately 30%, due to typical percolation threshold behaviors. Finally, we measured the activation energy of light-induced crystallization, which was 2.3 eV. This is a strong indicator that the UV/blue light induced the structural change of PbI2 residues at the grain boundary into the MAPbI3 crystalline phase. We expect our approaches will be very useful for improving the crystallinity of perovskite films and for optimizing cell efficiency.

Film Fabrication: We used a conventional solution-based spin coating method to fabricate the lead halide perovskite film in a nitrogen glove box. MAPbI3 powder (purchased from One solution) was added to a mixture of γ-Butyrolactone (GBL) and Dimethyl sulfoxide (DMSO) (7:3 v/v) and stirred at 60ºC for 12 h. The MAPbI3 solution was spin-coated on the quartz substrate (15 × 15 × 1 mm) treated with UV-ozone cleaning. The spin coating condition was 2000 rpm for 60 s. The toluene solution (150 µl) was introduced dropwise onto the substrate during the spin coating process.35 The sample was annealed on a hot plate at 100°C for 15 min. The thickness of the perovskite film was 300 nm. ACS Paragon Plus Environment

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In-Situ THz-TDS Measurements: We used an in-situ THz-TDS system to record the transmission of THz waves through the perovskite sample [Supporting Information Figures S1]. A linearly polarized THz pulse was produced from a photoconductive antenna (Gigaoptics, TeraSED) by illuminating a mode-locked femtosecond laser with an 800 nm wavelength. We applied high-frequency modulation to the photoconductive antenna bias (at 100 kHz), which enables us to achieve the signal-to-noise ratio sufficient for the high-speed measurement with an acquisition time of 5 s for each spectrum.36-37 The THz beam was focused on the perovskite sample with an area of ~ 1 mm2 under ambient conditions. The amplitude and phase of the transmitted THz field in the time domain were obtained by adjusting the time delay between the THz beam and the probe beam. We obtained the THz spectrum in the frequency domain by calculating the fast Fourier transform (FFT) of the transmitted THz field in the time domain. A ceramic heater with a 4-mm-diameter circular hole (Thorlabs Inc.) was incorporated into the THz TDS system to anneal the perovskite sample during measurement. The ceramic heater was in a direct contact with the substrate and the temperature at the sample surface was monitored by using an IR thermometer gun. The sample temperature reached a thermal equilibrium condition within 5 s after we turned on the heater. The THz-TDS system was enclosed by a nitrogen purging box to keep the perovskite film in a stable state. For the laser-induced annealing experiments, we illuminated a 355 nm UV laser (Cobolt Inc.) on the focused area of THz transmission in the perovskite film. The spot size of the UV laser was ~ 1 mm2. Time-dependent Photocurrent Measurements: We measured the transient photocurrent to investigate the UV response of perovskite devices. The perovskite-based photoconductors were fabricated on a quartz substrate. Two metal electrodes with thicknesses of 50 nm were defined on the perovskite film by metal evaporation through a shadow mask with a channel length of 30 µm and width of 1 mm. Lasers with 355 nm and 532 nm wavelengths were used to induce additional crystallization of the films that were previously annealed via thermal annealing. We also used a tunable light source from a Xenon lamp to fine tune the light wavelength (~ 1 W/cm2).


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ASSOCIATED CONTENT The Supporting Information is available free of charge. THz-TDS experimental setup, Percolation threshold behavior (PDF).

AUTHOR INFORMATION *

Corresponding Author: [email protected]

The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by the Midcareer Researcher Program (2014R1A2A1A11052108) through a National Research Foundation grant funded by Korea Government (MSIP) and by Human Resources Program in Energy Technology (20164030201380) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by Korea Government (MOTIE).

REFERENCES (1) Snaith, H. J. Perovskites: The emergence of a new era for low-cost, high-efficiency solar cells. J. Phys. Chem. Lett. 2013, 4 (21), 3623-3630. (2) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 2013, 342 (6156), 341-344. (3) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nat. Photon. 2014, 8 (7), 506-514. (4) Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. High charge carrier mobilities and lifetimes in organolead trihalide perovskites. Adv. Mater. 2014, 26 (10), 1584-1589. (5) McMeekin, D. P.; Sadoughi, G.; Rehman, W.; Eperon, G. E.; Saliba, M.; Hörantner, M. T.; Haghighirad, A.; Sakai, N.; Korte, L.; Rech, B., et al. A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells. Science 2016, 351 (6269), 151-155. (6) Research Cell Efficiency Records (NREL), http://www.nrel.gov/ncpv/ accessed on August 12, 2016. (7) Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J. C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A., et al. High-efficiency solution-processed perovskite solar cells with millimeter-scale grains. Science 2015, 347 (6221), 522-525. (8) Park, J. K.; Kang, J. C.; Kim, S. Y.; Son, B. H.; Park, J. Y.; Lee, S.; Ahn, Y. H. Diffusion length in nanoporous photoelectrodes of dye-sensitized solar cells under operating conditions measured by photocurrent microscopy. J. Phys. Chem. Lett. 2012, 3 (23), 3632-3638. ACS Paragon Plus Environment

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19


a

b c

20

Absorptance (%)

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MAI MAPbI3

15 10 5 0

0.5

1.0

1.5

2.0

Frequency (THz)

Figure 1 ACS Paragon Plus Environment

2.5

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a

b Absorptance (%)

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


10 5 0

fres = 1 THz

20 10 0 Heater fres = 2 THz On -10 -400 -200 0 200 400 600 800 Annealing time (sec)



ln(-ln(1-f ))

a

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n=2 70 C 90 C Fitting Fitting

-1

n=1 -2 0

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2

3

ln(t)

b

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1D 2

n

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1

2D 0

70

80

90

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120

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a

Frequency (THz) 1.0

2000

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UV Light On

15

10

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Time (sec)

1000

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4000

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Absorptance (%)

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b 20

c ln(-ln(1-f))

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1 THz 2 THz

15 10 5 0

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2

1

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0

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UV-induced crystallization

-1

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UV-induced decrystallization

2 lnt


4


a

c

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ip (nA)

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355 nm 532 nm

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60 40

EA

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Crystallization Kinetics of Lead Halide Perovskite ... - ACS Publications

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