Effect of hot-casted NiO hole transport layer on the performance of perovskite solar cells

Solar Energy 188 (2019) 609–618

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Effect of hot-casted NiO hole transport layer on the performance of perovskite solar cells

T

Abdullah Al Mamuna, Tanzila Tasnim Avaa, Tarek M. Abdel-Fattahb, Hyeon Jun Jeongc, ⁎ Mun Seok Jeongc, Seonhye Hand, Hargsoon Yoond, Gon Namkoonga, a

Department of Electrical and Computer Engineering, Old Dominion University, Applied Research Center, 12050 Jefferson Ave, Newport News, VA 23606, USA Applied Research Center Thomas Jefferson National Accelerator Facility, Department of Molecular Biology and Chemistry, Christopher Newport University, Newport News, VA 23606, USA c Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea d Center for Materials Research, Norfolk State University, 700 Park Avenue, Norfolk, VA 23504, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Perovskites Nickel oxide Interface Light soaking Hysteresis

NiO is extensively studied as a hole transport layer in perovskite solar cells but syntheses of NiO precursor involves toxic chemicals and time-consuming processes. Moreover, the synthesized NiO contains surface defects acting as trapping sites at the NiO/perovskite interfaces, resulting in poor charge extraction, hysteresis and light soaking. In this manuscript, we developed a non-toxic methodology for NiO precursor solution by using a simple mixture of NiO powder and HCl in an air environment. In addition, a new hot-casting technique was developed to successfully fabricate densely-packed, less defective NiO films. Interestingly, the hot-casting temperature was found to significantly affect morphology, film coverage and surface defects of NiO films. When a hot-casting temperature was below 100 °C, non-uniform NiO films were sparsely formed on the FTO surface and were characterized by defects in the form of hydroxyl groups and water on the surface. Such defective NiO films resulted in severe hysteresis and light soaking effect due to the trapped charges at the defective NiO/perovskite interface of perovskite solar cells. In contrast, when the hot-casting temperature was 120 °C, the NiO film formed densely-packed morphologies, covering the FTO surface. Furthermore, this film exhibited an ordered chemistry with strong Ni-O octahedral bonding and facilitated charge extraction at NiO/perovskite interface, resulting in a negligible hysteresis and light soaking. Finally, this non-toxic and simple method of fabricating NiO film will assist further development of perovskite solar cells.

1. Introduction Owing to remarkable optoelectronic properties such as high extinction coefficient, high charge carrier mobility, and long carrier diffusion length, organic-inorganic hybrid perovskite solar cells (PSCs) have shown rapid development in their power conversion efficiency (PCE) reaching over 22% (Jiang et al., 2017; Yang et al., 2017). At present, the two device architectures of the PSCs include a mesoporous or planar heterojunction structure that achieves the PCE of 22.1% and 21.6%, respectively (Jiang et al., 2017; Yang et al., 2017). However, the planar heterojunction structure is considered as an effective alternative to the mesoporous device architecture. This is because the mesoporous architectures typically require the high annealing temperature (> 450 °C) during the fabrication process (Sun et al., 2014). Planar heterojunction PSCs can be fabricated with either n–i–p or p–i–n

⁎

structure due to the ambipolar semiconducting characteristic of perovskite (Heo et al., 2013). Unfortunately, the n–i–p planar cells designed with TiO2 as electron transport layer (ETL) and (2,2′,7,7′-tetrakis (N,N-di-pmethoxyphenylamine)9,9′-spirobifluorene) (spiro-OMeTAD) as hole transport layer (HTL) exhibited a serious hysteresis, poor stability in moisture and temperature, and reduced photocurrent-voltage (J-V) characteristics in the PSCs (Unger et al., 2014; Zhao et al., 2017). In contrast, inverted planar heterojunction PSCs with p–i–n structures attracted considerable attention due to less hysteresis than n–i–p type planar structures (Namkoong et al., 2018). In the inverted planar PSCs, poly(3,4-ethylenedioxythiophene) polystyrene sulphonate (PEDOT:PSS) is widely used as HTL. However, because of its acidic and hygroscopic nature, organic HTL needs to be replaced by an inorganic one. So far, many inorganic materials such as CuSCN, MoO3, V2O5, CuI,

Corresponding author. E-mail address: [email protected] (G. Namkoong).

https://doi.org/10.1016/j.solener.2019.06.040 Received 27 February 2019; Received in revised form 3 June 2019; Accepted 15 June 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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and the lack of hydroxyl group. Particularly, PSCs with NiO prepared at hot-casting temperature of 120 °C showed the highest PCE with negligible light soaking and hysteresis.

and NiO have been used as alternative to organic PEDOT: PSS HTL (Chen and Park, 2018). In particular, NiO stands out as the most promising candidate for efficient HTL owing to its high optical transmittance, easy processability, a wide band gap, suitable energy level alignment with perovskites, and excellent electron blocking ability (Yin et al., 2017; Islam et al., 2017). Various fabrication processes were developed to fabricate NiO thin film such as sol-gel spin casting (Zhu et al., 2014), thermal decomposition of Ni(OH)2 (El-Kemary et al., 2013), sputtering (Islam et al., 2017), and pulse laser deposition (Park et al., 2015). Recently, Jung et al. fabricated a Cu-doped NiO HTL to enhance the electrical conductivity, achieving a PCE of 17.74% (Jung et al., 2015). In addition, alternative doping using lithium or magnesium for NiO also led to improved performance of PSCs with a PCE of 18.3% (Chen et al., 2015a, 2015b). Furthermore, room-temperature solution-processed colloidal nanoparticles (NPs) were used to produce a smooth and pinhole-free NiO thin film that produced a PCE of 14.5% (Zhang et al., 2015). Typical chemicals to prepare for NiO precursor solution include nickel nitrate, nickel (II) acetylacetonate, nickel chloride hexahydrate, hydrazine monohydrate, 2-methoxyethanol, and ethanolamine (Yin et al., 2017; Zhu et al., 2014; El-Kemary et al., 2013; Tang et al., 2018). It is noteworthy that all these solution processes require time-consuming processes ranging from hours to days before the solution is ready to be spin-coated onto the substrate. For instance, Zhu et al. dissolved nickel (II) acetylacetonate in ethanol with diethanolamine and sealed the solution for overnight at 70 °C, followed by a post-annealing at 150 °C for 45 min to make the sol-gel precursor (Zhu et al., 2014). In addition, solution-based NiO fabrication required for use of highly toxic chemicals such as ethylenediamine and hydrazine monohydrate, which exhibit acute toxicity via oral, dermal, or inhalation. Therefore, these toxic chemicals should be processed in a glove box filled with inert gas. An alternative to avoid the use of toxic chemicals was to exploit dry coating processes such as sputtering and pulsed laser deposition. However, these processes are costly and thus unsuitable for large scale production. Another critical issue regarding NiO thin film is the presence of surface defects acting as trap states in the solar cell structure, which results in detrimental hysteresis and light soaking (Flynn et al., 2016). The defects in NiO films are typically present in the form of hydroxyl groups originating from the oxygen deficiency or unconverted Ni(OH)2 (Zhu et al., 2014; Tang et al., 2018; Sun et al., 2018). The defective Ni(OH)2 creates a deeper valance band, which impedes the hole transport from perovskite to NiO and also alters the work function of NiO (Seo et al., 2016). In order to overcome these issues, mesoscopic (mp) NiO or mp-Al2O3 is typically employed between NiO and perovskite to remove morphological defects of the film (Wang et al., 2014). However, the presence of thicker mesoporous layer reduced the transparency of the film, which consequently reduced the performance of PSCs. In this study, we present a facile and inexpensive process for depositing NiO thin films that can be used as the hole transport layer for PSCs. Specifically, our method involves a simple mixture of commercially available NiO powder and HCl solution, which does not require for acute toxic chemicals such as ehylenediamine, or hydrazine monohydrate during the synthesis process. The chemical mixing time requires for less than 15 min in an air environment and the NiO precursor can be used immediately for the deposition of NiO film. We also developed a new hot-casting technique as an efficient methodology to fabricate densely packed NiO thin film for PSCs. Specifically, the hotcasting temperature was controlled to produce planar and less defective NiO films. Comparative characterizations for fabricated NiO films were performed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), photoluminescence (PL), and time-resolved photoluminescence (TRPL). Interestingly, when a hot-casting temperature was above 100 °C, NiO film formed densely-packed morphologies completely covering FTO surface with an ordered chemistry with strong Ni-O octahedral bonding

2. Experimental details 2.1. NiO precursor preparation NiO precursor was prepared by mixing 0.72 g of NiO (SigmaAldrich) with 2 mL of HCl (Alfa Aesar, 36%) and then stirred at 75 °C for 15 min. After mixing, the solution is filtered by a 0.45 µm filter. 2.2. Fabrication of solar cells Patterned FTO (SnO2/F, ∼8 Ω/sq−1, Aldrich) substrates were cleaned using mucosal, de-ionized water, methanol, acetone, and isopropanol (IPA) sequentially. Each solution was heated at 100 °C and then put into ultrasonic bath for 10 min to clean the substrate. The substrate was then dried with nitrogen and heated at 120 °C for 20 min to completely evaporate all residues. NiO thin film was prepared using a hot-casting technique. For that, FTO substrates were kept at different hot-casting temperatures ranging from room temperature to 120 °C. The prepared NiO precursor solution was then immediately deposited on the hot substrate by spin coating at 2000 rpm for 60 s in order to maintain the substrate temperature. NiO thin film was then heated to 350 °C for 15 min. Perovskite (CH3NH3PbI3-xClx) precursor was prepared by mixing lead iodide (PbI2, Sigma-Aldrich, 99%) and methylamine hydrochloride (MACl, Sigma-Aldrich) at a ratio of 1:1 before adding N, N-dimethylformamide (DMF, Sigma-Aldrich, anhydrous, 99.8%) to get 11 wt% concentration. The perovskite film was fabricated using a hot-casting technique in which the substrates were kept at 180 °C and the precursor solution at 70 °C (Nie et al., 2015; Namkoong et al., 2018; Namkoong et al., 2017). The electron transport layer consists of PCBM (Nano-c) dissolved in di-chlorobenzene (Sigma-Aldrich) and was spin coated onto the perovskite film in a nitrogen filled glove box at 1250 rpm. C60 mixed carbon and silver contact were deposited by using E-beam evaporator. Therefore, the resultant perovskite solar cells consisted of FTO/NiO/MAPbI3-xClx/PCBM/C60:C/Ag. The thickness was measured by a cross-section secondary electron microscope (SEM) and the measured thickness of the perovskite layer was about 300 nm. The PCBM and C60 mixed carbon layers were also measured to be about tens of nanometers and the thickness of Ag metal was about 150–180 nm. 2.3. Characterization The morphologies of NiO films were imaged by using a high-resolution field emission scanning electron microscope (FE-SEM) (Hitachi SU8010). Topographic image of perovskite and cross section SEM of PSCs were collected by JEOL JSM-6400LV at an accelerating voltage of 15 kV. Steady-state and time-resolved photoluminescence (PL) measurements were recorded using Horiba FluoroLog-3 spectrofluorometer and a time-correlated single photon counting (TCSPC) with a 450 nm solid-state laser. Transmission spectra were collected by using Perkin Elmer Lambda 45 spectrophotometer. For surface defects and chemistries of NiO films, X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi) was utilized by using monochromatic AlKa 1486.7 eV X-ray source on an area of approximately 15 mm in diameter. The electron take-off angle was fixed at 0, the vacuum pressure was below 1 × 10−9 mbar, and a constant analyzer mode was used during the spectra acquirement at a pass energy of 20 eV with a step size of 0.05 eV. The binding energy scale was calibrated using the hydrocarbon contamination on the surface based on the C 1s peak at 284.0 eV. For UPS measurement, He lamp (HeⅠ21.22 eV) was used for UV radiation and vacuum pressure was 10−8 mbar. The UPS machine was calibrated by fitting a complementary error function to the resolution to Fermi 610

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the NiO films that were measured by a high-resolution scanning electron microscopy (HRSEM). Fig. 3(a) shows FTO-coated glass slide consisted of irregular pyramid shaped FTO crystals, which is in good agreement with a previous report (Chen et al., 2015a, 2015b). The crystal size of FTO crystals ranged from tens of nanometers to hundreds of nanometers. When the NiO precursor solution was spin-casted at hotcasting temperatures of RT and 50 °C, irregular pyramidal shapes remained, as shown in Fig. 3(b) and 3(c). Since these irregular morphologies are not ideal for use as electrodes for solar cell applications, a variety of methods have been developed to achieve uniform coverage of the FTO (Chen et al., 2015a, 2015b; Yao et al., 2017). For instance, Chen et al. inserted mesoporous Al2O3 on top of ultrathin NiO film (Chen et al., 2015a, 2015b). Also, Yao et al. employed Cu doped mesoporous-NiO to obtain a good surface coverage and uniform film of NiO (Yao et al., 2017). In our case, the ultra-flat and compact NiO was obtained by simply controlling the hot-casting temperature of FTO/ glass slide. Remarkably, when a hot-casting temperature was increased to 70 °C, a densely packed NiO film started to sparsely form on the FTO surface, as shown in Fig. 3(d). When a hot-casting temperature was further increased to 100 and 120 °C, a densely packed NiO film on the FTO surface was obtained, as shown in Fig. 3(e) and (f). The grain size of NiO was about 50 nm. To find out more deeply about the topographic morphology of NiO films, atomic force microscopy (AFM) of NiO films was measured, as shown in Fig. S1. It is found that the surface roughness of FTO substrate was 29.28 nm, which is in good agreement with previous report (Chiappim et al., 2016). However, upon the deposition of NiO at RT on FTO substrate, the roughness was somewhat decreased to 25.74 nm. When the hot casting temperature of the NiO film was increased to 70 °C, the surface roughness of the NiO-70 °C film was reduced to 20.77 nm. With increasing the NiO hot casting temperature to 120 °C, NiO-120 °C film has the lowest surface roughness of 14.76 nm. This indicates that NiO-120 °C film has the most flat morphology compared to other NiO films consistent with SEM measurements, as shown in Fig. 3. We also investigated UV–VIS transmisttance of NiO films fabricated at different hot-casting temperatures, as shown in Fig. S2. It is found that the transmittance of NiO thin films fabricated by a hot-casting technique at RT and 120 °C are comparable to that of FTO. Moreover, to find out the thickness and coverage of NiO films, the cross-sectional SEM images were measured, as shown in Fig. S3. It is clearly seen from cross-sectional images that at the hot-casting temperatures below 100 °C, NiO film was sparsely deposited on FTO substrates, which is in good agreement with SEM and AFM in Fig. 3 and Fig. S1. Conversely, when a hot casting temperature of 120 °C was used, NiO layers about 50 nm thick covered the irregular pyramidal FTO surfaces. To confirm the presence of Ni and O in the as-prepared NiO thin films, the elemental compositional analysis was performed using energy dispersive X-ray spectroscopy (EDX). Fig. 4(a) and (b) show the EDX analysis of NiO films prepared on FTO substrates with two different hotcasting temperatures of RT and 120 °C, namely NiO-RT and NiO-120 °C films. The analysis confirms the presence of Ni and O (Yin et al., 2017; Ukoba et al., 2018) in both NiO films while negligible amount of Cl was observed for both NiO films. This testifies the successful transformation from NiO powder dissolved in HCl to NiO thin film. However, it is noted that NiO-RT film exhibited negligible amount of Ni concentration, compared to that of NiO-120 °C film. This might be due to the very thin NiO deposition on the FTO when a lower hot-casting temperature was applied, as judged by the SEM image of Fig. 3. To further understand the surface chemistries and defects of NiO films, X-ray photoelectron spectroscopy (XPS) measurements were performed. It is known that the XPS analysis of oxides and hydroxides of Ni is very difficult as it holds the position of a first-row transition metal in the periodic table. The difficulty in understanding the electronic structures are attributed to the complexity of their 2p spectra due to asymmetric peaks, complex multiplet splitting, shake-up process, final state effects, and binding energy overlapping (Seo et al., 2016;

edge of a polycrystalline gold specimen. The photocurrent density (J) vs. voltage (V) curves were obtained in ambient air by using a Keithley 2400 source meter under AM 1.5G illumination at 100 mW/cm2 provided by a solar simulator (Newport 69907). One sun illumination was calibrated using the NREL-calibrated, KG-2 filtered Si diode. A 450 W Xenon lamp was used as a light source and the lamp remained on for 30 min before starting the photocurrent measurement to stabilize the light intensity. For Forward scanning (FS), J–V curves were measured from −0.05 V to 1.15 V, and for reverse scanning (RS), J–V curves were measured from 1.15 V to −0.05 V. The scan rate is kept constant at 200 mV/s. To measure the J-V characteristics, a mask was designed whose aperture was 10 mm2. The mask covered all edges of the device. Moreover, the back side of the substrate was kept in dark as device is inserted inside the sample stage. During the measurement, the sample stage was kept in a fixed position. 3. Results and discussion Fig. 1 illustrates a quick, inexpensive, and non-toxic synthesis method for a NiO precursor solution. In this process, the NiO precursor was prepared by mixing 0.72 g NiO powder (Sigma-Aldrich) with 2 mL HCl, followed by the stirring at 300 rpm on a hot plate. When the mixed solution was heated at 75 °C for about 15 min, it turned into a blackish green1 color with cloudy vapor. Heated solution was further filtered using a 0.45 µm PVDF membrane filter, resulting in a clear green solution. In this process, excess amount of NiO was used with HCl so that HCl converts into NiCl2. For the prepation of NiO thin film, a hotcasting approach developed for fabricating perovskite thin films (Nie et al., 2015; Al Mamun et al., 2017a, 2017b; Namkoong et al., 2016) has been adapted in this study. Fig. 1(b) schematically illustrate a hotcasting technique for fabricating NiO films. Our approach involves casting NiO precursor solution onto the FTO/glass slides that were preheated at various temperatures from room temperature (RT) to 120 °C and subsequently spin-coated (60 s) to obtain a uniform NiO film. Thereafter, the hot-casted films were heat-treated at 350 °C for 15 min to promote thermal decomposition of NiCl2 and evaporation of HCl. The possible general mechanism in our hot-casting technique can be described by a series of steps as follows 75 °C

NiO + 2HCl → NiCl2 + H2 O NiCl2 + 6H2 O

heating

→

NiO + 2HCl↑ + 5H2 O↑

(1) (2)

In an effort to understand the formation of NiO precursor solution (green solution in Fig. 1), we conducted thermodynamic modeling of the chemical system containing NiCl2 (the product of reaction of NiO with HCl as in Eq. (1)) in aqueous solutions (or Ni2+ ions). All the thermodynamic modeling calculations refer to the bulk solutions only. In the thermodynamic modeling of the aqueous or Ni2+ ions, we define the system to be investigated by the following main components: Ni2+, NiCl+, NiCl2, Ni(OH)+, Ni(OH)2, and Ni(OH)3−. Fig. 2a shows that concentrations of all possible nickel species can be formed at different pHs. As shown in Fig. 2b, the Ni2+ ions form weak complexes with Cl− ions in the acidic pH range. However, with an increase in the pH, Ni2+ ions form stable solid precipitates of Ni(OH)2 in the pH range of 7–14. On the basis of thermodynamics modeling, the pH range of 6–8 can produce many nickel species, which may lead to contaminated NiO film after hot-casting (Eq. (2)). Below pH 6 is the optimum pH for maintaining soluble Ni species to obtain a uniform NiO film. In our experimental conditions an acidity was below 6. Fig. 3 represents the surface morphologies of NiO films hot-casted at two different temperatures of RT and 120 °C. Notably, it is found that the hot-casting temperature significantly affected the morphology of 1 For interpretation of color in Fig. 1, the reader is referred to the web version of this article.

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Fig. 1. (a) Schematic of synthesizing NiO precursor solution and (b) a hot-casting technique to fabricate NiO thin film. For a hot-casting, FTO/glass substrates were heated at different temperatures ranging from room temperature to 120 °C.

Fig. 2. Thermodynamic modeling diagrams showing (a) the log concentration of all possible nickel species vs pH and (b) the distributions of the fraction nickel species vs pH.

2013; Peck and Langell, 2012). When a hot-casting temperature increased from RT to 120 °C, the intensity of shoulder peak (856 eV) noticeably decreased, which can be attributed to change in chemical states of film surface. Typically, hydroxylation is a common phenomenon for oxide materials, such as NiO. In our process, we fabricated NiO film in ambient environment. Therefore, the probability of hydroxylation might be high. To probe the amount of surface hydroxylation and water physisorption, XPS spectra of O1s were further analyzed using Gaussian function. It was found that O1s spectra mainly consisted of three main peaks, as shown in Fig. 4(d) and (f). The middle peak is assigned to surface hydroxyl groups at 531.6 eV and the two side peaks are ascribed to the lattice oxygen at 529.5 eV and adsorbed water at 533.4 eV (Seo et al., 2016; Mossanek et al., 2011; Marrani et al., 2013; Peck and Langell, 2012). It is evident from Fig. 4(d) that NiO-RT film is characterized by the dominant presence of hydroxyl group and water. In contrast, the NiO-120 °C film showed a very strong lattice oxygen peak with drastically reduced hydroxyl and water peaks. Therefore, it is

Mossanek et al., 2011; Marrani et al., 2013; Peck and Langell, 2012). Fig. 4(c) and (e) show the XPS spectra of Ni 2p3/2 and O 1s core levels of NiO films deposited at two different hot-casting temperatures of RT and 120 °C. The measured XPS spectra of the Ni 2p3/2 on both films were deconvoluted to account for the chemical disorders and defects. It was found that both NiO films contained the main peak (854 eV), the shoulder peak (856 eV), and its satellite peaks (861 eV, 865 eV). However, the key differences in Ni 2p3/2 spectra of both samples were the relative peak intensities at 854 eV and 856 eV, respectively. Note that, the peak centered at 854 eV corresponds to Ni2+, which is a feature of the standard Ni–O octahedral bonding configuration of the cubic NiO rock salt (Seo et al., 2016; Mossanek et al., 2011; Marrani et al., 2013; Peck and Langell, 2012). In contrast, the shoulder peak centered at 856 eV was initially assigned to the Ni3+ ions in the literature. However, it is recently interpreted as the combination of multiple chemical states of screening and surface states rather than initially assigned Ni3+ defects in NiO (Seo et al., 2016; Mossanek et al., 2011; Marrani et al., 612

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Fig. 3. SEM images of (a) bare FTO substrate and NiO thin films prepared on FTO substrates using different substrate temperatures of (b) room temperature, (c) 50 °C, (d) 70 °C, (e) 100 °C and (f) 120 °C.

S6. Although both films showed large grainy morphologies, it is found that perovskite layer on NiO-120 °C film exhibited larger grain size, as shown in Fig. S6. This might be due to the flatter surface of NiO-120 °C film, compared to that of NiO-RT film. In addition, cross-sectional SEM images of perovskite films with different NiO (RT and 120 °C) films were measured and shown in Fig. S7. It is found that there was no significant effect of NiO films on the thickness of perovskite film. The thickness of both perovskite films was measured to be ∼300 nm. The current-voltage (J-V) curves of perovskite solar cells with NiO fabricated at different hot-casting temperatures were measured as a function of light illumination time and their extracted photovoltaic parameters were plotted in Figs. S8 and S9, respectively. Fig. 6(c) and (d) showed devices with NiO-RT and NiO-120 °C films. It is found that the better device performance was obtained from NiO-120 °C film with a PCE of ∼ 15.34%, whereas the NiO-RT film showed the poor performance. Fig. 6(c) and (d) illustrate the light soaking effect on the device performance with NiO-RT and NiO-120 °C films and the corresponding photovoltaic parameters are summarized in Table 1. Interestingly, perovskite solar cells with NiO-RT layer showed very strong light soaking effect. An initial power conversion efficiency (PCE) was measured to be 9.17% and increased by ∼49% to –13.64% after 10 min light illumination. Note the continuous increase in VOC from 0.88 V to 1.02 V after 10 min light illumination. It is well known that the VOC is very sensitive to the interfacial defects and is determined by the difference in quasi-Fermi levels of electrons and holes. Therefore, it is speculated that a continuous increase in VOC might be the gradual passivation of defective NiO-RT film during light soaking. In contrast, perovskite solar cells with NiO-120 °C film showed negligible light soaking effect, which might be due to the ordered and smooth morphology of NiO-120 °C film, as shown in Fig. 6(d). However, in the J-V characteristic, the unwanted S-shape was observed. Typically, S-shaped J-V characteristics of solar cells might be related to imbalanced charge carrier mobility, charge carrier blocking layer, and poor charge carrier extraction (Tress et al., 2011; Ecker et al., 2012). In our previous report, PEDOT:PSS HTL based solar cells do not show S-shape in J-V characteristics (Al Mamun et al., 2017a, 2017b). This is because PEDOT:PSS has high mobility and conductivity that are typically on the order of μ = ∼10−2 cm2 V−1 s−1 (Rutledge and Helmy, 2013) and σ = ∼1 S cm−1 (Kanwat et al., 2018). However, NiO thin films exhibited lower mobility and conductivity than PEDOT:PSS by one or two orders of magnitude. The reported mobility and conductivity of NiO are on the order of μ = ∼10−3 cm2 V−1 s−1 and

believed that the higher the hot-casting temperature, the lower the surface defects of the hydroxyl and water are formed on the NiO thin films. To probe energy bandgap alignment, ultraviolet photoelectron spectroscopy (UPS) measurement was carried out for both NiO-RT and NiO-120 °C films, as shown in Fig. 5(a). Both NiO films were calculated to have a Fermi energy of −4.76 eV (vs vacuum) and a valence band maximum (VBM) of −5.36 eV (see Fig. S4). This is in a good agreement with reported values of NiO thin film (Sun et al., 2018). As a comparison, we also investigated the charge quenching of both NiO films using steady-state and time-resolved PL measurements. For charge quenching studies, perovskite device structures composed of FTO/NiO (RT or 120 °C)/perovskite/PCBM/C60: carbon were designed to facilitate photo-generated electrons and holes to respective transport layers. Interestingly, NiO-120 °C film exhibited lower PL peak intensity compared with NiO-RT film, inferring better charge extraction, as shown in Fig. 5(b). Fig. 5(c) shows time-resolved PL decays that were measured by using a time correlated single photon counting system (TCSPC) and fitted by a bi-exponential decay function of I(t) = A1exp(−t/ τ1) + A2exp(−t/τ2), where τ1 and τ2 are fast and slow decay lifetimes and A1 and A2 are corresponding weight fractions. It should be noted that the quantitative analysis of fast and slow decays reveals the charge extraction dynamics. Particularly, it is well known that in the presence of charge transport layers, the fast decay process results from the better charge extraction of photo-generated charge carriers to the charge transport layers. In contrast, the slow decay is typically considered to be due to a radiative decay. Therefore, the relative charge extraction efficiency of the devices can be evaluated by comparing the fast decay. Quantitative analyses showed that the fast decay lifetimes of NiO-RT and NiO-120 °C layers were 6.42 ns and 3.26 ns, respectively. This clearly indicates that the perovskite solar cells with NiO-120 °C layer exhibited faster and more efficient hole extraction capability, which might be attributed to lower interfacial defects between NiO and perovskite film identified by XPS. In order to further explore the influence of NiO as the HTL, perovskite solar cell devices were fabricated with an inverted p-i-n architecture of glass/FTO/NiO/Perovskite/PCBM/C60:C/Ag (Fig. 6(a)). The corresponding band diagram of the device structure is also depicted in Fig. 6(b). The VBM of perovskite films was calculated to be −5.4 eV (Fig. S3), which is well aligned with the VBM of NiO film. The surface morphology of perovskites on NiO (RT and 120 °C) films are shown in Fig. 613

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Fig. 4. EDS spectrum of (a) NiO-RT and (b) NiO-120 °C film. XPS spectra of (c, d) Ni 2p3/2 and (e, f) O 1s spectra of NiO thin films were prepared by using different hot-casting temperatures of RT and 120 °C, respectively.

σ = ∼10−3 S cm−1, respectively (Yao et al., 2017). Thus, S-shaped J-V curves of perovskite solar cells may originate from poor charge carrier mobility and low intrinsic electrical conductivity of NiO. However, by using various dopants such as copper, lithium, and silver, the mobility and conductivity of NiO can be improved (Yao et al., 2017; Xia et al., 2018). For instance, Cu-doped NiO films greatly improved carrier mobility and conductivity to μ = 1.09 × 10−2 cm2 V−1 s−1 and σ = 3.01 × 10−3 S cm−1 from μ = 3.05 × 10−3 cm2 V−1 s−1 and σ = 5.59 × 10−4 S cm−1, respectively (Yao et al., 2017). Fig. 7 shows J-V characteristics with forward (lower → higher voltage) and reverse (higher → lower voltage) scans at a sweep rate of 0.2 V/s. Fig. 7(a) shows J-V curve of NiO-RT based cells exhibiting

severe hysteresis upon light illumination. After continued light soaking, its hysteresis decreased but did not completely disappear, as shown in Fig. 7(b). The observed hysteresis of NiO-RT based perovskite solar cells might originate from the defective NiO that causes different rates of trapping/detrapping of photogenerated charges upon applied bias (Meloni et al., 2016; Coll et al., 2015). Note that hysteresis-free solar cells were obtained with the PEDOT:PSS hole transport layer and PCBM/C60 electron transport layer (Al Mamun et al., 2017a, 2017b). In contrast, perovskite solar cells with NiO-120 °C film showed negligible hysteresis upon light soaking, as shown in Fig. 7(c) and (d). The anomalous J–V hysteresis behavior (Chen et al., 2016; Tress et al., 2015; Sanchez et al., 2014) has been observed in a variety of PSCs 614

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Fig. 5. (a) UPS spectra, (b) steady-state PL and (c) TRPL decays of NiO film prepared at hot-casting temperatures of RT and 120 °C. An inset shows fast and slow decay lifetimes and corresponding weight fractions.

2014). This process will make defects non-active. Previously, our group reported that that defects at grain boundaries and interfaces have a substantial effect on the charge extraction and hysteresis effects (Namkoong et al., 2018). The passivation of such defects led to the absence of hysteresis of perovskite solar cells. Thus, it is believed that the defects present in NiO-RT film were passivated during light soaking, which facilitated the charge transfer to electrode and consequently eliminated the photocurrent hysteresis. We also performed a stability test of perovskite solar cells where current-voltage and current-time measurements were measured. The normalized current-time measurement of the best performing perovskite solar cells with NiO-120 °C HTL was conducted at a bias voltage

regardless of their device architectures. Typically, the hysteresis of inverted p-i-n structure is less severe than that of normal n-i-p structure, but it can be changed under certain circumstances. The hysteresis of perovskite solar cells can be influenced by many factors including trapping and de-trapping (Snaith et al., 2014; Almora et al., 2015; Cojocaru et al., 2015; Jena et al., 2015; Kim et al., 2015), ferroelectric polarization of the perovskite layer (Frost et al., 2014; Wei et al., 2014), and ion migration of related defects under applied bias (Eames et al., 2015; Yang et al., 2015). Particularly, under continuous illumination, perovskites will undergo structural modifications that photo-chemically activate mobile ionic species, subsequently diffusing towards defect sites, well-known as the prominent light soaking effect (Shao et al.,

Fig. 6. (a) Device structure consisting of glass/FTO/NiO/perovskite/PCBM/C60:C/Ag, (b) corresponding energy band alignments, and light soaking effect of photovoltaic performance of perovskite solar cells with NiO films prepared with (c) NiO-RT film and (d) NiO-120 °C film. 615

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Table 1 Photovoltaic parameters of inverted planar PSCs prepared with NiO-RT film and NiO-120 °C film.

NiO (RT)

NiO (120 °C)

Time (min)

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

0 1 2 5 10 0 1 2 5 10

22.18 21.88 22.00 22.08 22.21 22.31 22.42 22.51 22.42 22.31

0.88 1.01 1.03 1.04 1.02 1.08 1.08 1.08 1.08 1.07

0.47 0.55 0.58 0.59 0.60 0.62 0.63 0.63 0.64 0.64

9.17 12.12 13.12 13.43 13.64 14.98 15.18 15.35 15.33 15.34

of 0.6 V close to Vmax in a nitrogen environment. A good steady-state efficiency was observed, as shown in Fig. S10. We also carried out air stability tests of unencapsulated perovskite solar cells fabricated with NiO- and PEDOT:PSS-HTLs in atmospheric environments with relative humidity of 40%. It is clearly seen from Fig. 8 that NiO based perovskite solar cell (PSCs) exhibited better stability than the reference PEDOT:PSS based solar cell. Particularly, PEDOT:PSS based solar cells were rapidly degraded within 24 h, while NiO based solar cells were also degraded but the degradation rates were much slower. However, the use of polymer encapsulation on top of ETL layer substantially improved the air stability of NiO-based PSCs, which retained 92% of the initial performance for 336 hrs. This indicates that the dominant degradation of unencapsulated NiO-based perovskite solar cells may be due to the degradation of perovskite surfaces, ETL layers and metal

Fig. 8. Normalized photo conversion efficiency (PCE) showing air stability of perovskite solar cells with PEDOT:PSS and NiO HTLs. Improved air stability was observed with polymer encapsulation.

electrode rather than NiO layer (Al Mamun et al., 2018; Kato et al., 2015). 4. Conclusion In summary, we have reported a facile and low-cost method of preparing NiO precursor solution that can be quickly prepared without using toxic chemicals like ethylenediamine and expensive equipment. In addition, we also developed a hot-casting technique that can readily

Fig. 7. Current-voltage characteristics with forward and reverse scans of perovskite solar cells prepared with NiO-RT film and NiO-120 °C films (a,c) right after light illumination and (b,d) after 10 min light soaking, respectively. 616

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produce ordered, densely-packed and smooth morphologies of NiO thin film. Particularly, it is found that hot-casting temperature below 100 °C resulted in non-uniform morphologies of NiO films that were sparsely covered on the FTO surfaces. In addition, NiO-RT films were characterized by defects in the forms of hydroxyl groups and water. The presence of surface defects of NiO-RT films manifested as trap states in the solar cell structure, which resulted in severe hysteresis and light soaking phenomena. Conversely, using hot-casting temperature of 120 °C produced NiO thin film with low defects and an absence of hydroxyl groups. In addition, perovskite solar cells with NiO-120 °C film exhibited better interface with less recombination loss. Consequently, perovskite solar cells with NiO-120 °C film showed negligible hysteresis and light soaking effects with improved PCE. This work will further be helpful in fabricating stable perovskite solar with NiO based hole transport layer.

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