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C: Energy Conversion and Storage; Energy and Charge Transport

Impact of Hydroxyl Groups Boosting Heterogeneous Nucleation on Perovskite Grains and Photovoltaic Performances Hyebin Kim, Jungyun Hong, Chaewon Kim, Eul-Yong Shin, Mi Jung Lee, Yong Young Noh, Byoung Choo Park, and Inchan Hwang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05374 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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The Journal of Physical Chemistry

Impact of Hydroxyl Groups Boosting Heterogeneous Nucleation on Perovskite Grains and Photovoltaic Performances Hyebin Kim, a Jungyun Hong, a Chaewon Kim, b Eul-Yong Shin, c Mijung Lee, b Yong-Young Noh,c Byoungchoo Park,* d and Inchan Hwang* a a Department of Electronic Materials Engineering, Kwangwoon University, Seoul 01897, Republic of Korea b School of Advanced Materials Engineering, Kookmin University, Seoul 02707, Republic of Korea c Department of Energy and Materials Engineering, Dongguk University, Seoul 04620, Republic of Korea d Department of Electrophysics, Kwangwoon University, Seoul 01897, Republic of Korea

*Corresponding author. E-mail address: [email protected], [email protected]

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ABSTRACT: Surface energy is a key factor in controlling the kinetics of nucleation and growth of perovskite, which are crucial for the formation of high quality films and the photovoltaic efficiency of solar cells. It has been reported that substrate-wettability and perovskite grain size are to be compromised with necessity, as promoted heterogeneous nucleation that occurs on a hydrophilic surface reduces the grain size for a two-step deposition method. Herein, the increase in grain size on hydrophilic surfaces in the presence of hydroxyl groups and the direct correlation between the perovskite grain formation and photovoltaic performance are investigated. The surface energy of the hole transport layer in planar p-i-n type perovskite solar cells is modulated by the introduction of polymer surfactant additive, poly(ethylene glycol) tridecyl ether (PTE). Perovskite films deposited on a hydrophilic surface by a two-step method contains small grain size, leading to a reduction in photovoltaic performance. In contrast, surface hydroxyl groups were found to induce the preferential (110) orientation and large grain size in the perovskite films deposited by means of a one-step method. Nucleation and growth mechanisms are proposed to explain those different behaviors of the dependence of grain size on surface energy. The enlarged perovskite grains on hydrophilic surfaces lead to the efficiency improvement owing to the increase in the short-circuit current and fill factor. Our study highlights that the increase size of grains with high crystallinity can be achieved even with accelerated heterogeneous nucleation on hydrophilic substrate surface.


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1. INTRODUCTION Organic-inorganic hybrid halide perovskite semiconductors have received considerable attention because of excellent performance for the optoelectronic applications such as photovoltaics and light-emitting diodes. Perovskite solar cells have been rapidly developed with a certified efficiency of over 22%.1 Such a steep development arises from the advantages of the versatile solution-processibility.2-7 The achievement of high efficiency requires excellent film quality, with respect to film coverage and low roughness, of perovskites simultaneously with large grain sizes and thus small interfacial area between grains that result in high charge carrier mobility by reducing defect and trap sites in the bulk of perovskite films.8-10 A vital factor in acquiring pin-hole free films is the kinetic control of nucleation, growth and crystallization of the perovskites when precursor solutions are transformed to the solid-state film while spin-coating.1114

Various approaches including adding additives into the precursor solution,15-19 anti-solvent

engineering,20-21 different precursor solution concentrations,22-23 temperature control,24-25 and surface energy control26-27 have been attempted to enlarge perovskite grains and consequently photovoltaic efficiency. A compromise between good film quality and large grain size limits the efficiency of perovskite solar cells, and therefore a careful balance between them has to be taken into account to attain high efficiency.28-29 Large grains can be observed by having fast growth rate and/or a low density of nuclei. However, this often gives rise to a rough surface and poor film coverage by creating pin-holes in perovskite films. To avoid these undesirable phenomena, additives could be added into the precursor solution to retard the crystallization speed.30-33 Other effective way to have a good film quality is to drip anti-solvent while spin coating,34-35 in order to reach a supersaturation state quickly, so called a fast deposition-crystallization method,36 and


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consequently to create a high density of nuclei, leading to dense and packed grains, but with small size. Surface energy modification also is an attractive pathway to manage the nucleation reaction kinetics. It was demonstrated that the hydrophobic substrate surface results in large grain size and thus high photovoltaic efficiency using a two-step spin coating method, but in this case, film coverage can be poor due to low substrate-wettability.29 Poly(ethylene glycol) tridecyl ether (PTE) consists of an alkyl chain at the end and a hydroxyl chain at the other end. This additive, therefore, can play a role of a surfactant to modify interfacial properties such as surface energy by anchoring one terminal chain on the surface of the layer and thus having an orientation normal to the surface. This surfactant additive has been used as an interfacial layer between indium tin oxide (ITO) and titanium dioxide (TiO2) layers, acting as a passive layer.37 The PTE interfacial layer has been introduced to improve the electron selectivity for extraction, resulting in a higher efficiency for the inverted organic solar cells.38 The application of this surfactant additive is not limited to the electrode interfacial layer. When it is introduced into the organic bulk-heterojunction and the interfacial layer between the donor and acceptor layer in the bilayer-structured device, it plays a role in capping the donor or acceptor domains, leading to a reduction in recombination of charge carriers due to orientational effects of dipole moments.39-40 The interfacial engineering between perovskite and charge transport interlayers by incorporating various kinds of functional groups can enhance the photovoltaic performance.41 For instance, the strong coordinative interaction between the titania surface and the phosphonic acid groups improves infiltration of the perovskite within the mesopores of TiO2.42 Surface passivation is another approach to improve optoelectronic performance via a reduction in electronic trap site by utilizing the halogen bonding43 or the coordinate bonding between the


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undercoordinated lead ions and the nitrogen atoms in the amino group44. In this article, we demonstrate the impact of surface energy modified by the surface hydroxyl groups from PTE on grain size and crystallinity of the perovskite film deposited by one-step and two-step methods. The planar p-i-n type perovskite solar cells were fabricated with a solution-processed nickel oxide (NiOx) as a hole transport material.45-47 Previous studies show the effect of substratewettability with diverse materials used as an interlayer, but this contains other factors including energy levels, electrical conductivity and surface roughtness that can affect photovoltaic performance.29, 48-49 We found that whilst PTE gives little change in chemical compositions and electronic properties of the NiOx hole transport layer, the surface energy of the NiOx layer can be effectively tuned by a small amount of PTE added into the NiOx precursor solution. This enables us to investigate a direct link between grain size and photovoltaic performances. More importantly, the behavior of perovskite grain size versus contact angle of the underlying layer was found to conversely differ for the two deposition methods. We discuss nucleation and growth mechanisms to explain the surface energy dependent grain size and photovoltaic performance.

2. EXPERIMENTAL METHODS Materials Nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O, 99.998 %), monoethanolamine (MEA, 99.0 %), anhydrous N,N-Dimethylformamide (DMF, 99.8 %) and anhydrous chlorobenzene (CB, 99.8 %) were purchased from Sigma Aldrich. Lead(II) iodide (PbI2, 99.9985 %) and methylammonium iodide (CH3NH3I, MAI) were purchased from Alfa Aesar and Dyesol, respectively. Phenyl-C61-butyric acid methyl ester (PC60BM) was purchased from Nano-C and A colloidal suspension of ZnO nanoparticles was obtained from Nanograde (N-10). PTE was


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purchased from Sigma Aldrich and was baked in the vacuum oven at 150 °C for 12 hours before use and other materials were used as received without further purification. NiOx film preparation with PTE Nickel acetate tetrahydrate was dissolved in ethanol at 0.1 M and monoethanolamine (MEA) was added into the solution at a 1:1 molar ratio of Ni and MEA. PTE dissolved in ethanol was added into the NiOx precursor solution with different concentrations from 0 to 1.5 wt%, keeping the resulting NiOx precursor solution concentration fixed. The precursor solution was vigorously stirred for 4 hours at 70 °C prior to spin-coating onto ITO substrates at 4000 rpm for 45 s, and then the spin-coated films were annealed at 300 °C for 1 hour in ambient conditions. Perovskite device fabrication ITO/glass substrates were sonicated sequentially in acetone and isopropyl alcohol for 10 min. Oxygen plasma treatment was carried out 5 min. For a single-step method, the perovskite precursor solution was prepared by mixing 650 mg PbI2 and 225 mg CH3NH3I in a molar ration of 1:1 in 1 ml anhydrous DMF and stirred overnight at 70 °C. The precursor was filtrated with a 0.2 µm PTFE filter. Then, the perovskite solution was spin-coated at 5000 rpm for 55 s. Anhydrous CB solvent (50 µl) was dropped onto the substrate 5-6 s after starting spin-coating. For a two-step deposition method, 461 mg PbI2 was dissolved in 1 ml DMF solution and stirred for overnight at 70 °C. 20 mg/ml CH3NH3I was dissolved in IPA at room temperature. The PbI2 solution was spin-coated at 5000 rpm for 30 s, then dried for 5 min. After loading the CH3NH3I solution on the dried PbI2 layer for 20 s, films were spin-coated at 2000 rpm for 30 s. The spin-coated film was dried at room temperature for 5 min, followed by annealed at 100

°C for 20 min. After cooling to room temperature, PC60BM (20 mg/ml in CB) and ZnO nanoparticle were spin-coated at 2000 and 4000 rpm for 1 min and 40 s, respectively. Finally, Ag electrode (100 nm) was thermally deposited on top of the ZnO layer inside a vacuum chamber


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(~1 ×10-6 torr). The active area of the device is 5.25 mm2. All the procedure without NiO film preparation was performed in a glove box. Characterization Current density-voltage (J-V) characteristics with a forward scanning direction, from short circuit to open circuit, were recorded using a Keithley 2401 under AM 1.5 G (100 mW/cm2) illumination using a solar simulator (Oriel Sol3A Class AAA Solar Simulators, Newport) with a xenon lamp. A standard silicon reference cell was used to calibrate the light source to 1 sun. All devices were measured in ambient air without encapsulation. The surface morphologies of the perovskite films were acquired by scanning electron microscopy (SEM, JSM-7610F, JEOL) at an accelerating voltage of 5 kV. We analyzed the scanning electron microscopy (SEM) images using ImageJ software to estimate the grain size of perovskite films. X-ray diffractometer (XRD, D/MAX-2500V, Rigaku) was used to investigate the crystallization of perovskite layers. X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) using a PHI 5000 VersaProbe II from ULVAC-PHI were performed in the analysis ultra-high vacuum chamber at a base pressure of 1 × 10-6 Pa, equipped with a monochromatic X-ray source (Al Kα, 1486.6 eV), the He I photon line (hν = 21.22 eV) from a He-discharge lamp as the excitation source. The overall resolutions were 50 meV for XPS and 25 meV for UPS. The water contact angle was analyzed using a Phoenix300. Absorption spectra were collected by using a UV-vis spectrophotometer (HP8453, HP). Photoluminescence measurements were carried out using a Cary Eclipse Fluorescence Spectrophotometer. The excitation wavelength was 485 nm. Thermal behavior of PTE was investigated with Thermogravimetry/differential thermal analyzer (Seiko Exstar 6000, TG/DTA 6100) under nitrogen atmosphere with a heating rate of 10 °C/min


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3. RESULTS AND DISCUSSION Thermal stability of PTE was investigated with the TG-DTA curves (Figure S1) to ensure that PTE remains after annealing at the temperature of NiOx crystallization. No weight loss in a broad temperature range, accompanied by an endothermic reaction around 100 °C, associated with the removal of adsorbed water occurs. This indicates that our prepared PTE does not contain water molecules that could be physisorbed onto the hydroxyl groups. In addition, it is found that PTE is not vaporized in the range of temperature investigated, as there are no endothermic processes accompanying weight loss. Instead, the sample weight begins to decrease from 250 °C, accompanied with an exothermic peak at 323 °C, which we attribute to the decomposition of PTE. That is, PTE molecules may be in part decomposed at 300 °C at which the films are thermally annealed for NiOx crystallization. According to our TG-DTA data, 74% of our sample is survived even at 300 °C. Therefore, although some PTE is decomposed, but PTE molecules still remain in the NiOx film.

Figure 1. Water contact angle of films based on NiOx:PTE with different PTE concentrations. Photographs of the water contact angles are exhibited in Figure S2.

The wettability of the hole transport layer in a p-i-n type perovskite solar cell is of significance because it has a strong influence on the grain and film formation of perovskite. This substrate


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wettability can be investigated by contact angle measurements. The contact angle θ is determined by the balance of the surface energies at interfaces, cos(θ) = (γ1 – γ2)/ γ3, where γ1 is the substrate surface energy, γ2 the interfacial energy between solution and substrate, and γ3 the solution surface energy. High substrate surface energy, γ1, therefore, reduces the contact angle and improves the hydrophilicity of the hole transport layer. Figure 1 shows that the contact angle of a water droplet on the hole transport layer based on blends of NiOx and PTE at different concentrations. The photographic images of a water droplet on the surface of NiOx films with and without PTE are presented in Figure S2. The contact angle for a NiOx film without PTE is 34.64°, and decreases to 17.62° with increasing PTE concentration up tp 1.2 wt%. This suggests that the surfactant PTE is perhaps aligned at the surface of the NiOx film having the hydroxyl end groups are directed outward toward air, which is likely to occur due to dipole orientation as reported in the literature.38-40 However, it is found that the contact angle increases with PTE concentrations higher than 1.2 wt%. Further increase in PTE concentration leads to increase in contact angle, 26.05° for 1.5 wt% and 30.44° for 3.0 wt%. This parabolic-like behavior of contact angle as a function of surfactant concentration is often found.50-52 The increase in contact angle at high PTE concentrations can be explained by the fact that the PTE surfactant starts to form a multilayer and thus reduces the density of hydroxyl groups exposed to air, making the NiOx surface back to hydrophobic. It would be of interest to extensively investigate the exact structural formation of this PTE surfactant in the NiOx film to make sure our speculation right. In any case of PTE alignment and layer structures, however, the reduction (increase) of contact angle (surface energy) at low PTE concentrations can be explained only by the changes in the density of the hydroxyl groups of PTE placed at the surface. The fact that contact angle increases


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again passing through the minimum with excess PTE concentration directly evidences the reduction of the hydroxyl groups exposed toward air at the surface. The influence of PTE blended in the NiOx films (NiOx:PTE) on the chemical composition of the surface was investigated with XPS spectra. Figure 2 displays the XPS spectra at Ni 2p3/2 and O 1s core levels of a NiOx film in the absence and presence of PTE inside the films investigated. The peak at the binding energy of 853.7 eV represents Ni2+ in Ni-O octahedral bonding configuration in cubic rock-salt NiO, and the peak at 855.4 eV is due to the formation of Ni2O3 and Ni(OH)2.46, 53 The peak at 860.8 eV is ascribed to a shakeup process in the NiO structure.45, 54

Figure 2(a) shows that PTE has nearly no influence on the change of chemical composition at

the surface. No change of chemical composition by PTE is identified also by the O 1s XPS spectra shown in Figure 2(b). The peak at 529.1 eV is ascribed to the lattice O atoms of NiO, while the peak at 530.9 eV is due to the existence of Ni2O3 and Ni(OH)2.55 No significant change in XPS peaks indicates that the hydroxyl groups of PTE are free from chemical bonding to NiOx, and are ready for the interaction with the post-deposited perovskite layer spin-coated on top of NiOx:PTE films. The interaction between the hydroxyl group and perovskite will be discussed later with XRD data.


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Figure 2. XPS spectra of (a) Ni 2p3/2 and (b) O 1s core levels for NiOx surfaces with and without PTE.

Figure 3. UPS spectra of the NiOx and NiOx:PTE films (a) in the high binding energy cut-off and (b) in the onset regions.


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The valence band structure and work function of NiOx and NiOx:PTE films was analyzed by UPS. Figure 3 show the onset of high binding energy cut-off of the UPS spectra and valence band UPS spectra for NiOx and NiOx:PTE films. The work functions of the both of NiOx and NiOx:PTE films are determined to be 4.7 eV, as derived from the high binding energy cut-off values (Figure 3(a)). The valence band states were found to be close to the Fermi energy level with 0.5-0.6 eV as shown in Figure 3(b), indicating the p-type character of NiOx. The ionization energies, therefore, are determined to be 5.2-5.3 eV. Those values are in agreement with the previous work.56-57 Those UPS spectra indicate that there is no meaningful change in energy levels by PTE content and support that PTE does not change the electronic structure of NiOx.

Figure 4. (a) XRD patterns of the perovskite films spin-coated on the films of NiOx, NiOx:PTE with 0.8 wt% PTE and NiOx:PTE with 1.5 wt% PTE. (b) The (110) and (310) diffraction peak intensities of the perovskite film with different PTE concentrations. (c) The FWHMs of the (110) and (310) peaks with different concentrations of PTE.


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Whilst PTE makes little change in chemical compositions and energy levels of the NiOx layer, we found that it influences the grain size and crystallinity of the perovskite film, which will be discussed later. The advantage of having surface hydroxyl groups is that the interaction with PbI3- and PbI42- in the perovskite precursor solution can promote the assembly of PbI2 and a better interfacial contact with dense and packed perovskites at the interface as other work pointed out.55, 58 The main concern of having surface hydroxyl groups might be the decomposition or inhibited formation of perovskites due to the presence of water molecules that can be adsorbed on the hydroxyl groups.59-60 However, we note that for our prepared samples there exist no significant amount of water molecules as discussed with thermographic analysis for PTE, which means that such indirect disadvantages of surface hydroxyl groups cannot be expected. The crystallinity of the CH3NH3PbI3 perovskite films deposited on the NiOx film with different PTE concentrations was analyzed by XRD spectra (Figure 4). We limit the PTE concentration up to 1.5 wt% for the discussion of perovskite film quality and photovoltaic performance as a further increase will detrimentally reduce the electronic properties of NiOx due to the insulating properties of PTE. The three most prominent XRD peak observed at 14.2°, 28.6° and 31.9° are assigned to the (110), (220) and (310) crystal planes of the tetragonal phase, respectively.61-62 Other small peaks also are assigned, as shown in Figure 4(a).63-65 It is found that the diffraction peak corresponding to the (110) plane is strongest when the PTE concentration was 0.8 wt% where the surface hydroxyl groups are most among our prepared films. The peak intensity for the (310) plane is not significantly altered by the presence of PTE content. Furthermore, the full width at half maximum (FWHM) of the (110) peak slightly reduces when the PTE concentration is 0.8 wt% but that of the (310) peak rather increases. These XRD results indicate that the


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hydroxyl groups lead to the (110) orientation preference with enhanced crystallinity of the perovskite film.

Figure 5. SEM images of CH3NH3PbI3 perovskite films deposited on the NiO and NiO:PTE films. The lower panels are histograms corresponding to the size distribution of grains shown in the top panels. The average size of grains without PTE, with 0.8 wt% PTE, and with 1.5 wt% PTE is 225, 325, 256 nm, respectively.

Table 1. Perovskite grain size determined from the SEM images shown in Figure 5. The percentage of grain size over 300 nm is highest for 0.8 wt% PTE. The number of grains calculated for average is 100.

>300 nm

> 400 nm

> 500 nm

Without PTE

19%

6%

1%

PTE 0.8 wt%

53%

31%

15%

PTE 1.5 wt%

29%

4%

0%


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The SEM images of perovskite films are presented in Figure 5. The histograms corresponding to the lateral size of grains are shown in the lower panels. The average size of grains for the perovskite film on NiOx is 225 nm, and for the perovskite film on NiOx:PTE with 0.8 wt% is increased to 325 nm. With a higher PTE concentration, 1.5wt%, it is found that the grain size rather decreases to 256 nm. Those average grain sizes are changed in accordance with the surface energy of the NiOx layer. Table 1 clearly shows that the number of large grains is increased as the surface of the NiOx film becomes hydrophilic. More than 50% of grains are larger than 300 nm for the hydrophilic surface with 0.8 wt%, while the relatively hydrophobic surface without PTE gives 19% of grains larger than 300 nm. Changes in grain size with different substratewettability support that heterogeneous nucleation takes part in the formation of perovskite grains. According to the heterogeneous nucleation theory, it is interesting to observe larger grains formed on the hydrophilic surface, because one would expect that grains become small when many nuclei are produced. Given that the Gibbs free energy change for homogeneous nucleation is identical, which meets our case study as we only modulated the surface energy, the Gibbs free energy change for heterogeneous nucleation should be smaller for a lower contact angle as it is reduced by a factor of (2+cosθ)(1-cosθ)2/4.66-68 Consequently, this results in a higher density of nuclei on the surface and the grain growth would be expected to be ceased when the grains are impinged, leading to smaller size of grains. This was experimentally demonstrated for the twostep spin-coated perovskites in the literature.29 In the two-step deposition case as nucleation and growth processes separately occur and perovskite grains are grown with isopropyl alcohol, if many nuclei are generated on the hydrophilic surface the size of perovskite grain would be limitedly grown. Indeed, when we deposited the perovskite films on the surface with different surface energies using a two-step method, the grain size was found to decrease from 344.8 nm to


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291.1 nm as the NiOx layer becomes hydrophilic with adding 0.8 wt% PTE (Figure S3). With 1.2 wt% PTE, the grain size becomes large again, 312.9 nm. This trend is in agreement with the literature.29

Figure 6. (a) J-V characteristics of CH3NH3PbI3 perovskite photovoltaic devices with different concentrations of PTE in NiOx layers. (b) Histograms corresponding to the power conversion efficiencies of the perovskite solar cells with the NiOx and 0.8 wt% PTE blended NiOx layers.

As expected, for the perovskite films with the largest average grain size, the power conversion efficiency (PCE) is found to be highest among the devices studied (Figure 6a). The efficiency of the best device fabricated by using a one-step method was 15.53% and the average efficiency was 14.43%, showing an improvement as compared to the device without PTE, 13.28% for the best device and 12.38% for the average. The efficiency of our reference devices with undoped


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nickel oxide as a hole transport material is in agreement with the previous studies reported by other research groups.46,

69-70

As the hysteresis of the J-V characteristics for p-i-n structured

perovskite devices with nickel oxide is negligible, J-V characteristics were recorded in a forward scan direction. The J-V characteristics of the best device for other different PTE concentrations are exhibited in Figure S4, and the table S1 summarizes their photovoltaic parameters. As compared between the best devices, the short-circuit current (Jsc) of the device with 0.8% PTE is 20.03 mA/cm2 and fill factor (FF) is 0.76, both of which are increased compared to the device without PTE, 18.25 mA/cm2 and 0.68 fill factor, while the open-circuit voltage (Voc) remains the same, ~1.00 V. The enhancement in short-circuit current and fill factor is attributed to better charge transport properties due to reduced grain boundaries. Figure 6b displays the histograms corresponding to the efficiencies of 20 devices for the NiOx and NiOx:PTE layers. A clear distinction in efficiency of the two devices indicates that PTE in NiOx film enhances photovoltaic efficiency.

Figure 7. Correlation between PCE (black solid square), average perovskite grain size (blue open square) and contact angle of the hole transport layer (a) for one-step deposited and (b) for two-step deposited perovskite solar cells.


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The J-V characteristics of perovskite devices fabricated using a two-step method are exhibited in Figure S5. The short-circuit current is reduced from 18.9 mA/cm2 to 16.47 mA/cm2 and thus the power conversion efficiency is lowered from 12.95% to 11.35 % (Table S2), as the NiOx surface becomes hydrophilic (from without PTE to 0.8 wt% PTE blended). These low power conversion efficiencies in overall compared to the one-step fabricated perovskite solar cells might be due to incomplete CH3NH3PbI3 conversion of perovskite near the bottom layer. Figure 7 shows the photovoltaic performance has a direct link to the perovskite grain size, and the conversely different behavior of grain size and PCE as a function of contact angle of the NiOx layer. It is worth reminding that the introduction of PTE to NiOx modifies the wettability only. With increasing contact angle (becoming more hydrophobic), perovskite grain size increases for two-step deposited perovskite films but decreases for one-step deposited perovskite films, and so do PCEs. It should be noted that a two-step method separates nucleation and growth kinetics for the formation of perovskite. The PbI2 nucleation occurs when the PbI2 solution is solidified while spin coating, and this acts as nucleation seeds for CH3NH3PbI3 perovskite crystallization when CH3NH3I is diffused. Therefore, the number of perovskite nucleation seeds is limited by that of PbI2 nuclei already formed before the diffusion of CH3NH3I solution for perovskite growth. However, for a one-step method, the nucleation and growth process simultaneously occur during solidification, as reported by the previous studies on the concentration-dependent grain size showing that a higher concentration of a precursor solution rather enables to form grains with larger size due to the fact that growth dominantly occurs compared to nucleation.23,

71-72

Therefore, the control of grain size has to be more carefully

considered as the grain size is modulated not only by nucleation kinetics, but also by various


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other aspects including the timing of nucleation and growth processes and the possibility of further perovskite growth by the fabrication condition after spin coating.

Figure 8. Schematic illustration of our proposed mechanisms on perovskite nucleation and growth via a one-step coating. During spin coating, nucleation and early growth processes coexist. Subsequently, grains become coarsened owing to the residual DMF remaining residual DMF upon thermal annealing, despite dense and packed grains with small sizes at the stage of early growth.

The proposed formation mechanism of one-step deposited perovskites on a hydrophilic surface using a one-step method is schematically illustrated in Figure 8 and its description is as follows. The reduced Gibbs free energy on a hydrophilic surface leads to a higher nuclei density owing to accelerated heterogeneous nucleation kinetics, and grains are limitedly grown during spin coating with anti-solvent engineering. At this stage, grains are small but so packed and dense enough to impinge each other. In contrast to a two-step method that utilizes isopropyl alcohol in which perovskites are almost insoluble, perovskite films right after spin coating using a one-step method may contain a residue of DMF that can dissolve perovskite even after the spin-coating step. Upon thermal annealing, as a result, the perovskite grains can be further grown and become coarse by fusion that involves small grains and perhaps also large nuclei placed between grains, leading to the formation of larger grains. However, if less nuclei are produced on a relatively


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hydrophobic surface, it is likely that perovskite grains are not packed enough to be grown as large as grains on a hydrophilic surface through interconnection during thermal annealing. When perovskite films are deposited using a one-step method, more dense and packed nucleus and grains can be beneficial for the formation of large grains at the final stage, and this can be achieved on a hydrophilic surface.

4. CONCLUSIONS In summary, we have described the mechanism of the enlargement in grain size of perovskite deposited on a hydrophilic NiOx surface. The surface energy of the hole transport layer was modulated by the introduction of the polymer surfactant additive, PTE, into the NiOx precursor solution, in order to control the heterogeneous nucleation. The fact that surface energy modulation affects grain size indicates that there is a significant contribution of heterogeneous nucleation to the formation of perovskite grains. High crystallinity with a preferential (110) orientation and large grain size are realized on hydrophilic substrate surfaces containing the hydroxyl groups, leading to an enhancement in the short-circuit current and fill factor, and thus in power conversion efficiency. The direct link between grain size controlled by surface energy and photovoltaic performance is investigated for both one-step and two-step deposited perovskites. Our results highlight that for a one-step method a large number of nuclei that can be generated on a hydrophilic surface are beneficial for the formation of large grains after annealing with help of residual solvent remaining. We believe that our proposed mechanism can open a new pathway toward the improvement in photovoltaic performance of perovskite solar cells by realizing both the large grains and good film coverage on a hydrophilic substrate surface.


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Supporting Information TG-DTA curves for PTE (Figure S1). photographic images of water contact angles on the surface of NiOx with different PTE concentrations (Figure S2). SEM images of the perovskite films deposited using a two-step method. (Figure S3) J-V characteristics of the perovskite solar cells fabricated by using a one-step method (Figure S4) and a two-step method (Figure S5) with their summarized photovoltaic parameters, respectively (Tables S1 and S2).

Author Information Corresponding Author *E-mail address: [email protected], [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements This research was supported by the Basic Science Research Program through the National Research

Foundation

of

Korea

(NRF),

funded

by

the

Ministry

of

Education

(2016R1D1A1B03932615) and by the Korea Government(MEST) (2017R1A2A1A17069729). I.H. acknowledges Research Grant from Kwangwoon University in 2018.

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Figure 1. Water contact angle of films based on NiOx:PTE with different PTE concentrations. Photographs of the water contact angles are exhibited in Figure S2. 67x50mm (300 x 300 DPI)


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Figure 2. XPS spectra of (a) Ni 2p3/2 and (b) O 1s core levels for NiOx surfaces with and without PTE. 66x104mm (300 x 300 DPI)



Figure 3. UPS spectra of the NiOx and NiOx:PTE films (a) in the high binding energy cut-off and (b) in the onset regions. 75x46mm (300 x 300 DPI)


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Figure 4. (a) XRD patterns of the perovskite films spin-coated on the films of NiOx, NiOx:PTE with 0.8 wt% PTE and NiOx:PTE with 1.5 wt% PTE. (b) The (110) and (310) diffraction peak intensities of the perovskite film with different PTE concentrations. (c) The FWHMs of the (110) and (310) peaks with different concentrations of PTE. 166x94mm (300 x 300 DPI)



Figure 5. SEM images of CH3NH3PbI3 perovskite films deposited on the NiO and NiO:PTE films. The lower panels are histograms corresponding to the size distribution of grains shown in the top panels. The average size of grains without PTE, with 0.8 wt% PTE, and with 1.5 wt% PTE is 225, 325, 256 nm, respectively. 170x82mm (300 x 300 DPI)


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Figure 6. (a) J-V characteristics of CH3NH3PbI3 perovskite photovoltaic devices with different concentrations of PTE in NiOx layers. (b) Histograms corresponding to the power conversion efficiencies of the perovskite solar cells with the NiOx and 0.8 wt% PTE blended NiOx layers. 69x106mm (300 x 300 DPI)



Figure 7. Correlation between PCE (black solid square), average perovskite grain size (blue open square) and contact angle of the hole transport layer (a) for one-step deposited and (b) for two-step deposited perovskite solar cells. 83x57mm (300 x 300 DPI)


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Figure 8. Schematic illustration of our proposed mechanisms on perovskite nucleation and growth via a onestep coating. During spin coating, nucleation and early growth processes coexist. Subsequently, grains become coarsened owing to the residual DMF remaining residual DMF upon thermal annealing, despite dense and packed grains with small sizes at the stage of early growth. 361x93mm (300 x 300 DPI)


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