Ultrasonically sprayed-on perovskite solar cells-effects of organic cation on defect formation of CH3NH3PbI3 films

Current Applied Physics 19 (2019) 1427–1435

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Ultrasonically sprayed-on perovskite solar cells-effects of organic cation on defect formation of CH3NH3PbI3 films

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Nakorn Henjongchoma, Nopporn Rujisamphanb,c, Teneng Stanilius Ntiab, Pisist Kumnorkaewd, I-Ming Tangb, Vittaya Amornkitbumrunge, Thidarat Supasaia,∗ a

Department of Materials Science, Faculty of Science, Kasetsart University, 10900, Bangkok, Thailand Nanoscience and Nanotechnology Graduate Program, Faculty of Science, King Mongkut's University of Technology Thonburi, Bangkok, 10140, Thailand c Theoretical and Computational Science Center (TaCS), Faculty of Science, King Mongkut's University of Technology Thonburi, Bangkok, 10140, Thailand d National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, 111 Thailand Science Park, Phahonyothin Rd, Khlong Nueng, Khlong Luang, Pathumthani, 12120, Thailand e Integrated Nanotechnology Research Center, Department of Physics, Faculty of Science, KhonKaen University, KhonKaen, 40002, Thailand b

A R T I C LE I N FO

A B S T R A C T

Keywords: Ultrasonic-sprayed perovskite Surface photovoltage Defect states

Methylammonium lead iodide (CH3NH3PbI3) based perovskite having low degrees of the disorder is of great interest for optoelectronic and photovoltaic applications. In this work, a layer of CH3NH3PbI3 was successfully prepared using an ultrasonically sprayed-nebulous method. Changes in structural and optical properties alongside with photo-induced charge separation and transportation behavior were systematically studied. The surface photovoltage spectra reveal a significant reduction of the density of deep defect states as the organic content was increased. It was observed that the measured values of Urbach energies decrease from 33.36 to 28.24 meV as the amount of organic content was increased to an optimum value. The best perovskite solar cells obtained using the sprayed-on approach exhibited a Jsc of 16.54 mA/cm2, a Voc of 0.99 V, and a FF of 62.4, resulting in an overall PCE of 10.09%.

1. Introduction Recently methylammonium lead trihalide semiconducting material (CH3NH3PbX3, X = I, Cl, or Br) has received much attention in the research field of photovoltaics. This is due to its excellent optoelectronic properties such as a direct bandgap (~1.5 eV) [1], a long electron-hole diffusion length (1 μm) [2], low-exciton-binding energy [3,4], and a high charge carrier mobility [5]. In addition, the optical bandgap can be tuned by varying the composition of the halide anions. This last feature leads to light absorption over a broad range [6] making this class of materials suitable for use as the top cells in the narrow bandgap harvesters needed in the multi-junction solar cells based on silicon or on copper indium gallium selenide (CIGS) [7–9]. The new approach (using a perovskite-tandem system) has pushed up the power conversion efficiency (PCE) closer to the Shockley–Queisser limit [10]. Moreover, the superiority of the solution process ability makes it a more suitable pathway for the industrial production of perovskite solar cells (PSCs). This is attributed to a lower processing cost, more simplified device architectures [11–14] and better surface/interface engineering techniques, all needed for achieving high-quality perovskite semiconducting

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films [15–17]. The best PCE reported so far has been limited to a small device area. Increasing the device area leads to a drastic drop in PCE [18–20]. The most prominent technique used for device fabrication has been the spincasting method. However, this technique is non-scalable. In hopes of improving the throughput and scalability, a novel approach which is capable of producing high-quality perovskite films having the wellcontrolled thickness and large substrate area is studied and implemented. Typically, the perovskite active layers have been fabricated predominantly by an one-step deposition method in which both precursors in the appropriate stoichiometric amounts are mixed into in the same solution and then spin-coated into a thin film, and by a two-step deposition method or sequential deposition process, in which the two precursor solutions are prepared separately. One precursor usually lead iodide (PbI2) is deposited first, followed by the other, either by spin casting or immersion of the PbI2 spin-coated thin film into the next precursor solution methylammonium iodide (MAI) [21,22]. Recently, much effort has been made towards the production of high quality, and scalable perovskite layers. Some of the new techniques implemented are roll-to-roll printing [23], doctor blade coating [24], inkjet printing

Corresponding author. E-mail address: [email protected] (T. Supasai).

https://doi.org/10.1016/j.cap.2019.09.010 Received 15 February 2019; Received in revised form 20 July 2019; Accepted 19 September 2019 Available online 19 September 2019 1567-1739/ © 2019 Published by Elsevier B.V. on behalf of Korean Physical Society.

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up where the nitrogen gas was flowed softly to get rid of any solvents remaining and to minimize the defects in the perovskite crystal [32].

[25], vapor deposition [26–28], and spray deposition [29,30]. In this work, we report on a sequential sprayed nebulous deposition method. This approach has the potential to be scale-up the fabrication of the perovskite layer having full surface coverage within a controllable nitrogen environment. This involves the use of an ultrasonic vibrator to generate an ultra-fine aerosol of PbI2 and MAI. A layer of PbI2 was first deposited on the heated patterned (F-doped SnO2) FTO coated glass substrate by ultrasonic-sprayed technique, followed then by the deposition of atomized MAI, generated by the ultrasonic mist maker. This initiates a chemical reaction which results in the perovskite structure. In this manner, controllable reactions between PbI2 and MAI occur slowly. This allows for the tracking of precursor-to-perovskite transition at various stages of the reaction process. The evolution of the perovskite structure along with the changes in the optical properties, the photo-induced charge generation, and the charge separation behaviors were systematically investigated. This made it possible to gain insights into correlations between the structure-properties and the responses. This would help in the rational design of devices with optimal properties.

2.2. Sprayed-on CH3NH3PbI3 characterization The evolution of the crystal structure in the sprayed-on CH3NH3PbI3 films as a function of the reaction time was tracked by using X-ray diffraction method (D8 Bruker diffractometer), utilizing the CuKα radiation source (1.5418 Å). All XRD measurements were performed at 2θ in the range of 10–50° with an increment step of 0.02°. The surface morphology of the sprayed-on CH3NH3PbI3 films was investigated by the scanning electron microscopy (SEM, model Quanta 450 FEI) with an operating voltage of 30 kV. Before the SEM imaging, a thin layer of Au was sputtered on the samples to minimize the charging effect. The absorption spectra were obtained using a UV–Vis spectrometer (PerkinElmer Lambda model 650) with a 60 mm integrating sphere component in the wavelength range of 300–900 nm. The film was placed in front of the integrating sphere for the absorption measurement. The photoinduced charge separation and charge transport behavior on the deposition parameters of the CH3NH3PbI3 films were investigated using a modulated surface photovoltage (SPV) spectroscopy. The SPV signals which are proportional to the photo-induced charge separation in space, and can provide information regarding charge migration were detected with a high impedance buffer (20 GΩ) and a double-phase lock-in amplifier (Elektron-Manufaktur Mahlsdorf, Germany), consisting of a 100W halogen light source accompanied by a quartz prism monochromator chopped with a modulation frequency of 15 Hz.

2. Experiment 2.1. Sprayed-on CH3NH3PbI3 A layer of CH3NH3PbI3 perovskite was prepared by a sequentially sprayed nebulous deposition technique, the details of which are described elsewhere [31]. In brief, an ultrasonic generator was used to generate the precursor aerosols of PbI2 and MAI. A stream of the aerosolized precursor aerosols flowed towards the reaction zone using nitrogen gas (N2) as a carrier. The substrate holder, two inches in diameter, was attached to a heating source containing a thermocouple. A Kapton sheet was placed on the ultrasonicator container to separate the precursor solution and the ultrasonic mist maker. This prevents corrosion and contamination. A layer of PbI2 was first prepared on the substrate by ultrasonic-sprayed nebulous technique, and sequentially, a stream of an ultra-fine aerosol of MAI was deposited on the deposited PbI2 layer to form the perovskite structure. Before the PbI2 film deposition, the FTO coated glass substrates (purchased from Bangkok Solar, Thailand) were ultrasonically cleaned with detergent, acetone, ethanol, and deionized water sequentially for 15 min. The substrates were then dried in nitrogen gas. The cleaned FTO substrates were then treated with oxygen plasma for 20 min to improve the adhesion and the hydrophilic properties. To prepare the PbI2 precursor solution, PbI2 powder (Sigma-Aldrich, 99.9%) was dissolved in N, N-Dimethylformamide (DMF, Qrec, AR) and stirred at 70 °C for 1 h. A thin layer of PbI2 was obtained by nebulously spraying the PbI2 precursor solution onto the substrate for 30 min. During this process, the N2 gas flow rate, precursor concentration, and deposition temperature were kept constant at 2 L/min, 0.1M (46.2 mg/ml), and 70 °C, respectively. The MAI powder (purchased from GrateCell Solar) was dissolved in anhydrous ethanol (Sigma-Aldrich 99.9%) to form 0.1M (15.9 mg/ml) MAI precursor solution. After the PbI2 deposition, a stream of MAI aerosol was brought into contact with the PbI2 layer where interdiffusion and reaction of the MAI with PbI2 layer occurred, inducing the perovskite phase formation. To note, the substrate temperature has a critical impact on the chemical reaction of the PbI2 and MAI. So before the deposition of the MAI aerosol, the PbI2 layer was allowed to cool to at room temperature. This act was done in order to minimize the evaporation of the solvent since ethanol had been used as the solvent for the dissolution of MAI. Here, the N2 flow rate of the stream of MAI aerosol was kept at a lower constant ratio of 1 L/min for its generation. The deposition time of MAI aerosol on the PbI2 layer was varied from 5, 10, 15, 20, and 30 min in different runs. This allowed us to study the evolution of the crystallization and of the phase transformation of the CH3NH3PbI2 crystal structure. The prepared CH3NH3PbI3 films were annealed at a moderate temperature of 100 °C for 10 min inside the set

2.3. Perovskite solar cells and characterizations To realize a solar cell device in which the light-harvesting layer was fabricated by the sprayed-on method, the n-i-p structure was used. For electron charge, selective contact, a compact layer of tin oxide (Alfa, 15% weight% in H2O colloidal dispersion) was spin-casted on an FTO coated substrate at 6000 rpm for 40s, and annealed at 150 °C for 90 min. For hole charge selective contact, 100 mg Spiro-OMeTAD (purchased from Ossila), 39.4 μL 4-tert-butylpylpyridine (TBP), 22.7 μL lithium bis(trifluoromethanesulfonyl)imide (1.5 M in acetonitrile) in 1.087 mL chlorobenzene (CB) was deposited onto the perovskite at 3000 rpm for 30 s. The sample then was kept in a desiccator overnight before depositing silver (Ag) electrode. The 0.16 cm2 device area is defined as the overlap region between the top and the bottom electrodes. Spin-casted perovskite films were also prepared using one step anti-solvent method in a nitrogen-purged glovebox. The details could be found elsewhere [33]. The current-voltage (J-V) curves were measured with the forward and reverse scans at a step of 50 mV under the continuous illumination of 100 mW cm−2 using “LCS-100” sun simulator. Incident photon to converted electron (IPCE) spectra of the solar cells were characterized by using a tunable-xenon-light-source system equipped with a monochromatic instrument (Newport Cornerstone 130). The light was chopped at 80 Hz. The spectra were collected in the wavelength ranging from 300 to 850 nm. 3. Results and discussions 3.1. Structural properties Fig. 1(a) shows the XRD patterns of the sprayed-on CH3NH3PbI3 films at different time intervals. The structural evolution of the CH3NH3PbI3 structure concerning MAI spraying is observed. Initially, the solid-state complexes of PbI2 (sample with PbI2 layer only), reveal the characteristic central peak at 12.7° assigned to the reflection from the preferential orientation of (001) plane of PbI2. This peak by the crystallographic database (no.7–235), shows that the sprayed-on PbI2 1428

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reaction time of 5 min. This suggests that the dynamics of the intercalation process of the organic cations of MAI into the lead plumbate complexes occurred vigorously, resulting in a fast formation CH3NH3PbI3 crystal. Recent work on the study of the perovskite formation dynamics reveals that much longer reaction time of the perovskite transformation could take place due to the inter diffusion of solid-state formed MAI into the PbI2 layer [35]. It should be noted that the aerosolized forms of MAI have a greater ability to accelerate the intercalation dynamics than that of the solid-state precursor form of MAI. This would lead to a shorter reaction time between the MAI and PbI2, resulting in a more rapid final phase transition into perovskite crystals. It is also important to mention here that as the spraying time was increased to 10 and 15 min, the intensity of the perovskite peaks also increases significantly and becomes sharper as is seen in Fig. 1(a). Meanwhile, those belonging to PbI2 (2θ ~12.7°) as noted by “PB” decreased. This is ascribed to the increased crystallization (and formation) of the perovskite layer, induced by the organic content as the reaction time was prolonged. Interestingly, as the reaction time was further increased to 20 min, the PbI2 characteristic peak was observed to disappear. Remarkably, the splitting of the perovskite peaks becomes more pronounced, as is clearly illustrated in the insets of Fig. 1(b). The absence of the PbI2 phase, suppressed by the MAI as the reaction time was increased to 20 min proves that a complete transformation into the perovskite phase has occurred. The observed splitting of peaks confirms the tetragonal crystal structure in the sprayed-on perovskite samples. Comparatively, the sample fabricated by the conventional one-step spin casting approach did not reveal such peaks-splitting, as illustrated in Fig. 1(c). This could be ascribed to the fact that the crystallization and growth of the perovskite crystals are crucially affected by the kinetics of nucleation and thermodynamics of the chemical reactions, such as the reaction time, reactant concentration and the reactant phases [36,37]. The tetragonal lattice parameters of the samples determined by the Bragg's equation show the slight difference from a = b = 8.839 Å, c = 12.602 Å to a = b = 8.820 Å, c = 12.586 Å, as the reaction time was increased from 5 to 20 min. This is due to well-packed crystal lattice and the enhanced crystallinity of the crystal. As a remark, the values of the calculated lattice parameters are slightly smaller than those of the bulk perovskite reported in the literature [38]. This could be due to the induced stress and strain in the films during the perovskite formation [39]. By adopting the Williamson-Hall method [40], we calculated the strain in the perovskite films. We noticed a small variation from 1.80 × 10−3 to 1.84 × 10−3 corresponding to the films prepared within the reaction time 5–15 min. A significant decrease of 1.72 × 10−3 was observed when the exposure time was increased to 20 min. The obtained value of the strain during the phase transformation (reaction time for 5–15 min) is relatively more substantial than that of the sample with a complete conversion (reaction time for 20 min). This is possibly due to the significant lattice mismatch between the PbI2 and perovskite crystal, yielding some distortions in the CH3NH3PbI3 structure. However, the lowest value of the strain was found in the sample with a complete perovskite phase conversion since there was no foreign structure of PbI2 presented, and hence a relax strain. The reported values of the lattice parameters and the spontaneous strain for the PbI2 bulk are a = b = 4.558 Å, c = 6.986 Å and 1.2 × 10−3, respectively [41]. Murali et al. reported a correlation of the average strain of the perovskite CH3NH3PbBr3 single-crystal powder, which greatly depends on the annealing temperatures [42]. They found that the strains can vary from 0.5 × 10−3 to −2.5 × 10−3 as the annealing temperature changes. Furthermore, the characteristic peaks of the dihydrated methylammonium lead iodide ((CH3NH3)4PbI6·2H2O) at lower diffraction angles of 11.4°, 11.5°, and 11.6° are observed to be more pronounced (Fig. 1(a), marked by “DH”) [43], as the spraying time is increased to 30 min. On the other hand, the perovskite footprint vanishes, indicating the disappearance of the perovskite domain. The diffraction peaks are in agreement with the pattern, as reported by

Fig. 1. (a) X-ray diffraction (XRD) patterns of as-prepared PbI2 layer (0 min), and of sequentially sprayed MAI for 5, 10, 15, 20, and 30 min. The “DH,” “PB” and “PS” indicated in the XRD patterns correspond to dihydrated (CH3NH3)4PbI6·2H2O, PbI2 and CH3NH3PbI3, respectively. (b) The XRD pattern of the CH3NH3PbI3 corresponding to 20 min of reaction time, with respectively enlarged views of the planes (110), (220) and (222). (c) The XRD pattern of the spin-casted CH3NH3PbI3 film for comparison.

layer has a hexagonal structure for P3m1 space group [32]. We note that lower intensity reflections from the FTO characteristic peaks (2θ ~ 26.6°, 33.7° and 37.8°) are also seen. When the reaction time was increased to 5 min, the XRD feature shows additional diffraction peaks at 14.1°, 23.6°, 28.5°, and 31.9° (marked by “PS” in Fig. 1(a)) which correspond to the (110), (211), (220), and (310) planes, respectively. All the peaks are in perfect agreement with the calculated X-ray diffraction peaks of CH3NH3PbI3 with tetragonal structure (space group I4/mcm) as listed in the database reported in the literature [34]. Considering the main characteristic perovskite peaks (14.1°, 28.5°, and 31.9°), we observed the appearance of the peaks splitting, showing the existence of a typical tetragonal structure in the sprayed-on perovskite films. The XRD results indicate that the structure of CH3NH3PbI3 initiates its formation after it is sprayed onto the PbI2 layer within a short

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540 nm corresponding to the band gap energy of 2.30 eV, which is that of PbI2. It should be noted that the value of the band gap of PbI2 strongly depends on the method of fabrication [46,47]. Upon exposing the PbI2 layer to MAI aerosol for 5 min, a new absorption edge at a wavelength of approximately 800 nm seems to appear, while the absorption edge of PbI2 remains evident. The absorption edge appearing at ~800 nm corresponds to the band gap of the CH3NH3PbI3 perovskite. This result is consistent with the XRD data, which shows a much sharper PbI2 peak. As seen, the sample shows a relatively higher absorption due to the predominant existence of the PbI2 phase in the sample, even though the solar radiation in the wavelength range of 400–550 nm is relatively low. It is worth mentioning that in the solar radiation spectrum, 22% of the photon flux is in the wavelength range of 400–550 nm and 42% in the range 500–900 nm. When increasing the spraying time further to 10 min, a stronger absorption edge was seen at ~800 nm. The decreased absorbance in the region relating to PbI2 is due to the reduction of the PbI2 phase in the sample. A similar trend is observed in the sample, whose exposure time was 15 min. However, it has a better absorption when compared to that of the 10 min exposuretime sample in the whole range of wavelength (ultraviolet to visible region). This can be assigned to the better conversion to the perovskite phase. Increasing the exposure time to 20 min, the sample exhibits a steep absorption edge at 800 nm with the highest absorbance being between 600 and 800 nm. This steepness of the band edge corresponds to a lower degree of disorder parallel with the lower Urbach absorption tail states in the forbidden gap. When the sample was further exposed to MAI by increasing the reaction time to 30 min, the absorption edge corresponding to the band gap of CH3NH3PbI3 perovskite vanished, but another one appears around ~400 nm. This new feature is due to the formation of the dehydrate [45], which is also seen in our XRD data. A similar observation on the influence of the MAI contents on the optical properties of perovskite was reported by Halder et al. [45]. They found out that when the molar ratio of MAI in PbI2 precursor solution was increased, the film exhibited an absorption peak at ~375 nm, which is close to our observed value. So, it is worthy to note that the MAI organic cation has a crucial impact on the optical band gap of the perovskite. Ke et al. reported that an increase in the organic concentration of MAI in PbI2 solution could induce a transition from direct to indirect band gap in the CH3NH3PbI3 films [48]. Motta et al. simulated the influence of the orientation of MAI molecules on the electronic structure of CH3NH3PbI3. They stated that the band transition from direct to indirect CH3NH3PbI3 could be attributed to the variation in the orientation of MA molecules [49].

Leguy et al. [44]. A similar observation was investigated in perovskite films prepared by the spin-coating method, where the molar concentration of MAI was higher than that of PbI2 [45]. The excess MAI converts the CH3NH3PbI3 components to hydrated species of monohydrate (CH3NH3PbI3·H2O) and dihydrate ((CH3NH3)4PbI6·2H2O) by absorbing water molecules from the environment. Note the structure of monohydrated perovskite is completely different from that of the dihydrated with relatively lower diffraction peak at 2Ɵ = 8.47 [44]. The structural formation for both monohydrated and dihydrated perovskite complexes through a hydration reaction are proposed as follows:

4(CH3 NH3)PbI3 + 4H2 O↔ 4[(CH3 NH3)PbI3⋅H2 O] ↔(CH3 NH3) 4 PbI6⋅2H2 O+ 3PbI2 + 2H2 O

3.2. Optical properties We noticed that even though an extremely low peak intensity of the dihydrated perovskite is seen for the 20 min, the perovskite crystal lattice in the film is the dominant one. The optical property of the samples was investigated using the UV–vis spectrophotometer. The absorbance spectra of the sprayed-perovskite as a function of the exposure time of the MAI aerosol are shown in Fig. 2(a). The inset presents the corresponding photographs of the sprayed films as a function of the reaction times. The change in color of the samples is dependent on the spraying time. Looking at the initially sprayed-on PbI2 layer, (yellow sample), the absorption edge appears at a wavelength of

3.3. Optical band gap The optical band gap of our samples at the various times of exposure, as determined from the Tauc's equation is as shown in Fig. 2(b). The determination of the band gap (Eg) is estimated from the extrapolation of the linear portion of the plot of (αhν )2 versus hν to zero photon energy, where α and hν are the absorption coefficient and the photon energy, respectively. Our calculated band gap lies in the range 1.58–1.60 eV depending on the exposure time. The experimental values of Eg for CH3NH3PbI3 films reported are in the range 1.5–1.61 eV depending on the deposition method and measurement technique [50,51]. Likewise, 1.48 eV was reported for the bulk CH3NH3PbI3 [52]. A more quantitative analysis on the absorption tail, known as the Urbach tail energy (Eu) which defines the degree of disorder in the material can be deduced from the absorption coefficient. The calculated Urbach energy was obtained from the inversion of the absorption edge slope of the plot of the absorption coefficient in a logarithmic scale versus the photon energy [53]. The inset presents the estimated Urbach energy of the samples. Our evaluated value of Eu gradually decreases from 33.36 to 28.24 meV as the exposure time increases. This is due to the reduction of the distributed tail states inside the forbidden gap during the perovskite crystal growth. Landi et al. reported a correlation

Fig. 2. (a) UV–Vis absorption spectra of the sprayed-on PbI2 and of sequentially sprayed MAI on the PbI2 layer as a function of reaction time. The inset shows the corresponding photographs of the sprayed-on CH3NH3PbI3 layer. (b) The plot of (αhν )2 versus hν for the CH3NH3PbI3 at various reaction times. The inset presents the determination of the Urbach tail energy of the corresponding films from the absorption coefficient. 1430

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Fig. 3. Top-view SEM images of (a) sprayed-on PbI2 and of sequentially sprayed MAI on the PbI2 at the various reactions of time (b) 5 min, (c) 10 min, (d) 15 min, (e) 20 min and (f) 30 min. Insets show the corresponding cross-sectional views of the films.

perovskite (~1 μm) occurs in for the sample after being exposed for 20 min and is consistent with the XRD data of Fig. 1(b), which shows a more intense and sharper diffraction peak of the (110) plane. The thermodynamics, the nucleation, and the dynamic process of the crystal growth of the perovskite particles are theoretically based on Gibbs free energy (known as self-assembly) [55,56]. The nucleation and crystallization of the perovskite particles are found to be strongly influenced by the concentrations MAI, the temperature (both substrate and MAI solution) and the reaction time [57–59]. When the reaction time (spraying duration) was increased to 20 min, the films were relatively thicker and more compact with fewer voids in the interior of the film. Also, a very sharp interface appeared between the perovskite layer and the substrate, suggesting that negligible interdiffusion of the perovskite particles across the interface had occurred. As the spraying time was further increased to 30 min, the cuboidal feature appears to become more spherical with the crystal size becoming smaller than those formed when the spraying was 15 or 20 min. The decrease in the crystal size along with the reduction in the film thickness is due to an overabundance of MAI, which would inhibit the crystal growth and cause the perovskite structure to transform that of dihydrated perovskite. This result is also in line with the XRD data which shows no characteristic peaks of perovskite only sharp peaks of dihydrated perovskite.

between Eu (estimated from the quantum efficiency spectra) and the grain size of the CH3NH3PbI3 [54]. The value of Eu was estimated to be 31 meV for the relatively larger grain (dgrain ∼370 nm) and increased to be 37 meV as the grain sizes decreased to 150 nm. This is because the larger grains provide a longer diffusion length and fewer trap densities.

3.4. Changes in surface morphology The change in the surface morphology of the sequentially sprayed CH3NH3PbI3 films at the different times of exposure of the MAI aerosol on the PbI2 layer with the corresponding cross-sectional views is shown in Fig. 3. As seen in Fig. 3(a), a dense layer of PbI2 was successfully deposited on the substrate via the ultrasonic-sprayed nebulous deposition method. The film shows a planar morphology and appears to have full coverage on the substrate with a uniform thickness of about 0.66 μm. Upon the deposition of MAI aerosol on PbI2 layer for 5 min, the appearance of the film is different from that of the sprayed-on PbI2 layer as seen in Fig. 3(b). The film shows grain facets and cuboidal features. Partial coverage and non-uniform morphology are observed. The cuboid shape suggests the presence of a tetragonal crystal structure of perovskite. This is also in confirmation with our XRD data, as it shows the peak-splitting at (002) plane and (110) plane. The thickness of the film is approximately two times larger than that of the sprayed-on PbI2 layer. This is due to expansion in volume during intercalation of MAI into the PbI2 framework, forming the perovskite structure. As the exposure time was increased to 10 min, the film displays more surface coverage, and the cuboid particles appear to have a similar average grain size, similar to that of the five min-sprayed time sample. The thickness of the film is also seen to increase. This evidence suggests that the perovskite crystals appear to have a preferential direction of nucleation along the c-axis. When the reaction time increases to 15 min, the perovskite crystal size increases. The distribution of the particle size, however, becomes non-uniform. The larger crystal size of the

3.5. Surface photovoltage (SPV) Changes in the charge dynamic behavior upon the phase conversion were studied using modulated surface photovoltage (SPV) spectroscopy. The phase evolution, which is influenced by the amount of MAI reacting with PbI2, shows significant impact on charge separation and charge transport behavior. In this work, modulated SPV spectra were operated under an atmosphere with a fixed capacitor arrangement, with an insulating mica sheet (30–40 μm thick) placed between the sample 1431

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Fig. 4. (a) Modulated SPV spectra of the in-phase (filled circles) and 90° phase-shifted (open circles) signals of sequentially sprayed MAI on PbI2 at different time intervals. (b) Spectra of the PV amplitude of sequentially sprayed MAI on PbI2 at various intervals of time. (c) Ratio dependence of the PV amplitude measured at 1.1 eV and 1.8 eV against the reaction time; inset shows the percentage of phase composition of PbI2 and CH3NH3PbI3 at different reaction times as characterized by XRD. The SPV spectra of the spin-casted CH3NH3PbI3 film are also presented.

energy of the SPV signals is generally lower than the absorption edge of the material. Since the onset of the SPV signal is connected to the mobility edge, the onset would approach the band gap energy for an ideal semiconducting material. The SPV signals detected below the band gap are therefore attributed to charge separation from the defect states existing in the sub-bandgap. Upon spraying MAI for 5 min, the Xsignal shows a negative sign, suggesting preferential separation of photo-generated electrons towards the external surface (top surface) which corresponds to a p-type doped semiconductor with a depletion region at the surface. It is found that the dominant onset of the X signal occurs at a photon energy of 2.38 eV, corresponding to the band gap of PbI2. This result agrees with the XRD data showing dominant PbI2 phase. When MAI was sprayed for 10 min, the X-signal at a photon energy of 1.6 eV increases from −26 μV to −114 μV while the Y-signal decreases from −67 μV to −31 μV. The increase in X-signal with a corresponding decrease in Y-signal is indicative of the relatively faster response of charge separation/transportation about the modulation of light. It should be noted that for shorter reaction time (i.e., 5 and 10 min) the samples show the same sign for the X- and Y-signals. This suggests the presence of two mechanisms of charge separation, i.e., most electrons and some holes are being separated towards the external surface [60]. As the reaction time was increased further to 15 min, a change in the sign was observed in the Y-signal, whereas the X-signal remains unchanged. The X-signal increases to −130 μV and the Y-signal amounted to 21 μV (at a photon energy of 1.6 eV). The opposite sign of

and the electrode thus forming a capacitor. The SPV signals measured with respect to the ground were regulated with a high impedance gain buffer and were collected by a double-phase lock-in amplifier. The inphase (X-signal) and the 90° phase-shifted concerning the modulated light signal (Y-signal) provide evidence of the direction of separation of photo-generated charge carriers and of the response time of the sample on the modulated light, respectively. The SPV spectra amplitude is defined as the square root of the sum of the squares of the X and Y signals. The positive (negative) X-signals and negative (positive) Ysignals give evidence of preferential movement of electrons (hole) towards the internal surface and holes (electron) towards the external surface. The opposite sign of the in-phase and the 90° phase-shifted signals is an indication of only one mechanism of charge separation, and the relaxation of SPV signals caused by recombination [60]. Moreover, the in-phase and the 90° phase-shifted signals are related to the fast or slow response of the photo-generated charge separation and recombination, respectively, concerning modulation period in the on and off illumination. Fig. 4(a) shows a full view of the spectra of the in-phase and 90° phase-shifted SPV signals when the MAI was separately sprayed on PbI2 films for 5, 10, 15, and 20 min. To investigate how the deposition method plays a role in the mechanisms of charge separation and transportation, the SPV spectra of the spin-casted CH3NH3PbI3 films are also comparatively presented. It is noticed that all samples show the onset of SPV signals at about 1.46 eV, which is lower than the band gap energy of CH3NH3PbI3. It is important to point out here that the onset 1432

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X- and Y-signal is evidence of only one mechanism of charge separation and relaxation due to recombination processes. This involves a preferential charge separation; for example, electrons towards the top surface and holes towards the substrate. Further increasing the spraying time to 20 min causes an increase in the X-signal by a factor of 2 (−290 μV at 1.6 eV) when compared to the 15 min exposure time. The Y-signal becomes negative again between 2.0 and 3.0 eV. The change of the sign could be caused by the change in surface electronic properties induced by the surface chemical reactions [61]. The increase in the X-signal concomitant with the reduction in Ysignal indicates that there is fast response in the charge separation and transportation during the period of the modulation of the signal. In the case of spin-casted samples, the X- and Y-signals exhibit the same sign in a whole spectrum. The X- and Y-signals are found to reach the maximum value of −156 μV and of −38 μV, respectively at a photon energy of 1.64 eV. The relatively stronger decrease of the Xsignals could be connected to relaxation of the photo-generated charge being governed by trap states at the surface. Fig. 4(b) shows the spectra of the PV amplitude of sequentially sprayed MAI on the PbI2 at various time intervals and of the spin-casted sample. The PV amplitude is proportional to a distance from the center of electrons and holes being separated, and the number of separated charges. As seen in this figure, all samples exhibit aspects of separation and recombination processes with a sharp onset of the PV signal at the photon energy of 1.46 eV. This is what expected for having CH3NH3PbI3 characteristic. Only the sample with an exposure time of 5 min exhibits a change in the PV signal at a photon energy of 2.38 eV indicative of the presence of PbI2 phase. These results are in line with our XRD data showing the existence of the CH3NH3PbI3 phase in all samples and the presence of the PbI2 dominant phase for the five min-sprayed samples. Interestingly, after spraying MAI on PbI2 layer for 20 min, the PV amplitude increased by a factor of ~5. This indicates that stronger separation of photo-generated charges is found at excitations above the band gap energy (~1.55 eV). The PV amplitude at photon energies below the band gap appears to be in the same range with the 5 min sprayed sample. This is substantial evidence of the passivation of defects during the phase evolution. Quantitative analysis on deep defect states related to the phase evolution at various reaction periods was carried out by quantifying the ratio of the PV amplitude measured at 1.1 eV and 1.8 eV (R1.1eV/R1.8eV). Fig. 4(c) shows the dependence on the value of R1.1eV/R1.8eV on the reaction time together with the percentage of phase evolution as characterized by XRD. As a remark, the value for the spin-casted sample is also presented for comparison. The value of R1.1eV/R1.8eV for the spin-casted sample is 1.2–2.2 larger than that of the sprayed-on sample. This value decreased sharply at the beginning of the reaction time and then gradually decreased with prolonged reaction time. The reduction of the value of the R1.1eV/R1.8eV related to the decrease in the density of deep defect states in the material's bandgap could be attributed to an enhanced perovskite phase formation with an increase in reaction time, as seen in the inset of Fig. 4(c). The deep electronic states in the perovskite CH3NH3PbI3 could have originated from imperfections arising from the crystal lattice during the process of growth. For example, point defects due to the antisite substitution of PbI or IPb and/or caused by impurities along the grain boundaries [54,62,63]. Landi et al. pointed out that at a temperature above the structural phase transition from orthorhombic to tetragonal phases (at 160 K), the recombination kinetics are found to be strongly influenced by a capture of the deep electronic states, and this, therefore, has a crucial impact on the device performance [54].

Fig. 5. The best J-V characteristic curves under illumination alongside EQE and integrated Jint spectra of the device fabricated by the spin-coated and sprayedon process (a), and statistical distributions of Voc, Jsc, FF, the corresponding PCE (b).

The integrated values of the current density are 20.99 mA/cm2 and 15.86 mA/cm2 for the spin-coated and sprayed-on devices, respectively. The mismatch of about 5–9% is found when compared to the J-V measurements. Statistical distributions of open-circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and the corresponding PCE of the sprayed-on and spin-coated PSCs are also presented in Fig. 5(b). The summary of these device parameters, including the values of series (Rs) and shunt (Rsh) resistance is presented in Table 1. Although the spin-coated device shows a PCE of 15.07%, the best device of the sprayed-on one exhibits a PCE of 10.09% (on-average efficiency of 7.20%), with Voc of 0.99 V (on-average of 0.93 V), Jsc of 16.54 mA/cm2 (on-average 14.50 mA/cm2) and FF of 62.4 (on-average 52.64). The relatively low Jsc and FF were expected due to the obstacle to charge collections at their respective electrode. This could be due to the excessively high recombination when the light absorber is relatively thick or to parasitic effects at the interfaces. If we take into account the possibility of the photon collection in a visible range up to 800 nm, photocurrent density as high as 26 mA/cm2 could be possible. Optimizing the perovskite thickness, including a study on the device's stability, is essential for achieving the above and is in our progress.

3.6. Performance of PSC To better appreciate the role of the sprayed-on films on the device performance, we have constructed a regular n-i-p solar cell. Fig. 5(a) shows the best J-V characteristic of the sprayed-on and spin-coated devices and their EQE and the integrated current density (Jint) spectra. 1433

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Table 1 Summary of device parameters for the CH3NH3PbI3 perovskite solar cells fabricated by the spin-coating and ultrasonically sprayed-on methods. Preparation method

Voc (V)

Jsc (mA/cm2)

FF

Rs (Ω cm2)

Rsh (kΩ cm2)

spin-coating sprayed-on

0.98( ± 0.02) 0.93( ± 0.04)

21.10( ± 0.72) 14.72( ± 1.29)

68.49( ± 2.47) 52.60( ± 6.34)

52.6( ± 6.7) 75.0( ± 12.0)

2.10( ± 0.83) 1.56( ± 0.85)

a

PCE (%)

a

14.43( ± 0.64) 7.23( ± 1.55)

Each result was measured from at least 20 devices.

4. Conclusions

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In conclusion, polycrystalline methylammonium lead iodide perovskite films were successively deposited on FTO coated glass substrate by an ultrasonic-sprayed nebulous deposition method. With this approach, a layer of PbI2 was first sprayed on the substrate then, sequentially by spraying the nebulized MAI. The amount of MAI content was investigated to have a crucial impact on the defect states during the evolution in the perovskite phase. Increasing the amount of MAI by varying the spraying time, a phase change occurs, with an optimum at 20 min, as confirmed by the XRD and UV–vis data. The Urbach energy (Eu), which relates to the degree of disorder, decreases from 33.36 to 28.24 meV as the reaction time was increased from 5 to 20 min. This showed a reduction of the distributed tail states in the forbidden band gap. Additionally, when the amount of MAI was increased, the SPV spectra revealed a substantial reduction of the deep defect states along with an increased in the PV amplitude signals at photon energies above the bandgap, suggesting passivation of the defects during the perovskite phase formation. We present that the ultrasonically sprayed-aerosol deposition technique has potentials for fabricating scalable perovskite layer with high crystallinity and low degree of disorder. Here, the perovskite solar cells with a sprayed-on approach showed the Jsc of 16.54 mA/cm2, Voc of 0.99 V, and FF of 62.4, outputting an overall PCE of 10.09%. Conflicts of interest The authors declare no conflicts to declare. Acknowledgment This work was supported by the Thailand Research Fund (MRG6282111), the co-funding from Electricity Generating Authority of Thailand (EGAT) and National Science and Technology Development Agency (NSTDA) (grant # P-17-51209), and by King Mongkut's University of Technology Thonburi through the “KMUTT through the Research Center of Excellent”. T.S is grateful to Kasetsart University Research and Development Institute (grant no.228.61 phase#2) for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cap.2019.09.010. References [1] T. Miyasaka, Chem. Lett. 44 (2015) 720. [2] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J.P. Alcocer, T. Leijtens, L.M. Herz, A. Petrozza, H.J. Snaith, Science 342 (2013) 341. [3] K. Galkowski, A. Mitioglu, A. Miyata, P. Plochocka, O. Portugall, G.E. Eperon, J.T.W. Wang, T. Stergiopoulos, S.D. Stranks, H.J. Snaith, R.J. Nicholas, Energy Environ. Sci. 9 (2016) 962. [4] K. Tanaka, T. Takahashi, T. Ban, T. Kondo, K. Uchida, N. Miura, Solid State Commun. 127 (2003) 619. [5] C. Wehrenfennig, M. Liu, H.J. Snaith, M.B. Johnston, L.M. Herz, Energy Environ. Sci. 7 (2014) 2269. [6] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S. Il Seok, Nano Lett. 13 (2013) 1764. [7] J.P. Mailoa, C.D. Bailie, E.C. Johlin, E.T. Hoke, A.J. Akey, W.H. Nguyen,

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