1. Introduction

A hybrid of organic and inorganic halide perovskite has become the center of solar power-related research due to extreme efforts involved in preparing high-quality perovskite thin films [1,2], device structures [3,4], elemental optimization [5,6], and a basic understanding of charge transportation dynamics [7,8]. This hybrid was identified as a promising photovoltaic material in 2009 by Miyasaka et al. [9]. A testified PCE of ∼3.9% was achieved by introducing and integrating perovskite materials as a semiconductor into photovoltaic devices. Kim et al. [10] made substantial development in third-generation solar cells by introducing full solid-state perovskite solar cells (PSCs) in 2012. In the last decade, the PCE of PSCs has grown consistently, and it is now beyond the recently certified record value of 25.2% [11]. In wide-range applications, PSCs with different structures (e.g., mesoporous, planar, and inverted planar) have been fabricated and studied. Planar architecture receives much attention because of its easy fabrication, low processing costs and temperature, and comparatively low hysteresis over mesostructured devices [12–14]. Planar inverted structures allow searches for favorable inorganic charge-transporting materials [15] that can provide high PCE and good stability in ambient conditions [16] for robust construction. These inorganic materials cost lower than their counterpart organic materials. Organic lead iodide perovskite is prone to degradation in the presence of humidity and air due to its low formation energy [17]. Nevertheless, all structures can perform competently as long as the charge (electron and hole)-transporting layers of the device successfully separate and collect the charge carrier [18–20] from the perovskite absorber layer. Therefore, the charge-transporting layers play a crucial part in the operational principles of PSCs. They influence the device’s performance and provide a shield to the absorber layer to protect it from being exposed to the corresponding unfavorable atmosphere, and, thus, the layers contribute to a remarkably stable inverted PSC [15]. Currently, most high-efficiency PSCs are made from organic HTMs (e.g., poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) [21], 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-OMeTAD) [22], poly(triaryl amine) (PTAA) [23], and poly(3-hexylthiophene) (P3HT) [24], and these PSCs require additional doping and adhesive materials to increase the conductivity of organic HTMs [22,25]. These p-type HTMs are sensitive to environmental conditions due to their acidic and hygroscopic characteristics, especially when stability becomes a concern. The organic materials previously listed have clear advantages on device fabrication in terms of low processing temperature and easy process route (solution-based) and arbitrary tunable bandgap option. However, their high synthesis and purification costs and degradation must be taken into account for high-volume commercial production [15]. Deploying the perovskite precursor on organic polymers properly is also difficult due to the hydrophobicity of organic polymers. Moreover, designing, synthesizing, and aligning the bandgap of organic HTLs are tedious. These processes result in high production costs for large-scale manufacturing of PSCs [26]. Therefore, inorganic HTMs have become the center of attention for inverted PSCs, especially metal oxides such as NiO_x [27], CuI [28], CuSCN [29], Cu_xO [30], and MO₃ [31]. Using metal oxides, including MoO₃, WO₃, NiO_x, and Cr₂O₃ as HTM has promising outcomes such as improving the carrier transporting properties within solar cell devices. Metal oxides can also serve protection to the photo absorber layers against unfavorable environmental conditions [32,33] though these inorganic oxides still need to go further to catch up organic materials in terms of PCE. Among these metal oxides, NiOx is an ideal choice for holding an adequately large band gap (>3.7 eV) [34], allowing high transparency and suitable and favorable energy levels for effective hole transport. In conjunction with upper valence band maximum (VBM), this HTM can facilitate hole movement to the photoanode, unlike perovskite absorber layers. NiO_x also has a higher conduction band minimum (CBM) compared with perovskite absorber layers, which restrict electron movement to the photoanode of perovskite, thus providing good chemical stability [35]. Different approaches have been considered for the successful preparation of NiOx as an HTM in PSC. Seo et al. [36] deposited NiO_x as HTM by atomic layer deposition (ALD), which achieved 16.6% efficiency. Park et al. [37] used pulsed laser deposition (PLD), which produced 17.3% device efficiency. Both of these costly processes are known for small-scale deployment, and they may be unsuitable for complete device fabrication in commercial production. Deposition of Li-doped NiOx (LiNiO_x) through hot-casting method was introduced by Nie et al. and they achieved a highly efficient, hysteresis-free, and stable PSC with a PCE of approximately 18% [38]. Guo et al. [39] utilized thermolysis, which requires high temperature for nickel hydroxide to form NiO_x HTM layers for inverted PSCs, and this method achieved a PCE of 14.55%. Solution-processed NiO_x thin films were prepared by spin coating nickel (II) acetylacetonate in ethanol and HCl. The PSC obtained a PCE of 18.8%, retaining 16.92% of the initial PCE upon exposure to environmental conditions (>70% relative humidity) for 720 h [40]. Sol-gel processed NiO_x nanocrystals showed a PCE of 9.11% [41]. All these studies indicated the enormous potential of NiO_x as a successful HTM for PSC. The characteristic properties of NiOx are primarily influenced by NiO_x preparation methods, which allow for versatile fabrication of full device structure. Although the solution process generated expected results, the conversion of crystallize source to polycrystalline NiO_x demanded high processing temperatures, resulting in difficult production [16]. By contrast, Jung et al. [42] used a combustion method, a low temperature one, to achieve 17.74% efficiency, but it still required additional Cu-doping. Pre-synthesizing NiO_x NPs as HTMs is also complex and tedious, demanding a moderate amount of temperature [43] during synthesis. The material utilization efficiency of the solution process is lower with higher wastage compared with that of other methods [44]. Two important photovoltaic parameters (e.g., J_sc and FF) are affected substantially by using liquid processed NiOx films as HTM, reducing the overall performance of the PSC device [45]. Moreover, any residual solvents such as ethanol or methanol while preparing the precursor solution of HTM can decompose the perovskite layer [15].

A facile deposition method for NiO_x that offers low cost and high yield with commercial prospect for large-scale production is currently in demand. In the search of substitutional deposition method, physical vapor deposition techniques are also popular where sputtering has been mostly investigated [46] facilitating additional doping option. Magnetron sputtering has been used in PSC to prepare NiO_x as HTM by Aydin et al. [47] which exhibited 18.49% efficiency. Electron beam physical vapor deposition (EBPVD) technique is known alongside sputtering to prepare film of metal oxides in optical semiconductor film industries. Due to possessing a few numbers of deposition parameters and simple process, EBPVD is suitable alternative instated of other physical vapor techniques (Fig. 9, Supplementary Information, SI) such as sputtering or thermal evaporator for NiO_x deposition. Structural and morphological properties of films deposited by EBPVD can easily be manipulated. Besides, the required raw material volume in EBPVD is lower than that in solution process technique, thereby lowering material utilization. The slow as well as fast, both types of deposition rate of EBPVD allows it to have potential industrial applications, such as in semiconducting optical film and solar cell applications. The mean free path and source-to-substrate distance allow control of films’ properties. A very limited number of investigations has been performed using EBPVD to prepare NiO_x film as HTM for perovskite solar cell. Recently T. Abzieher et al. [48] reported competitive efficiency with electron-beam deposited NiO_x as HTM with complex inkjet-printed perovskite absorbers layer. Though additional oxygen gas was introduced to enhance NiO_x transmission profile which again push this simple system towards some difficulty in controlling oxygen amount. To overcome these issues, this study investigates a simple EBPVD technique to prepare NiOx films as HTM and effective electron blocking layer in PSC without insertion of any heteroatomic dopant given that physical vapor deposition (PVD) techniques are widely used in semiconductor film preparation for efficient and stable perovskite solar cell. Here, we deposited NiO_x as HTM for PSCs by EBPDV without providing any intentional heat treatment and achieved very close V_oc and FF values compared with the aforementioned solution techniques. This study focused on the simplest perovskite composition and one step perovskite deposition technique in fabrication of PSC devices with glass/FTO/NiO_x/CH₃NH₃PbI₃/PCBM/BCP/Ag structure (inverted assembled). The devices with as-deposited and annealed NiO_x film exhibited the highest PCE of 13.20% and 13.24%, respectively. This outcome is because of the successful matching to the effective Fermi level (E_f) of NiOx with corresponding valance band of FTO and CH₃NH₃PbI₃. Moreover, NiO_x provided such an optical transparency, which produced high J_sc in accordance with light absorption via the active layer of CH₃NH₃PbI₃. The stability of the fabricated devices using EBPVD NiO_x film as HTLs under ambient environment is investigated for 4 weeks’ time period. The devices extended their stability by 72.72% and 76.96% of the initial PCE for as-deposited and annealed NiO_x HTM, respectively, at aforementioned time. The outcome of this study revealed that a competitive PCE with inorganic HTM alongside organic HTM with stable PSC in environmental conditions was achievable with lower temperature processed EBPVD NiO_x films as HTM.

2. Experimental

2.1. Materials

All chemicals were purchased from commercial sources with high purity and used without further purification. Methyl ammonium iodide (CH₃NH₃I, >98%) was purchased from Tokyo Chemical Industry Co. (Japan), lead iodide (PbI₂,99%) was purchased from Sigma–Aldrich, NiO_x target (99.99%) was purchased from Kujondo Chemical Laboratories Co., LTD. (Japan), and PC₆₁BM (phenyl-C₆₁-butyric acid methyl ester) (99.5%) was purchased from Lumtec Co. (Taiwan). Other solvents such as gamma butyrolactone (GBL), dimethylsulfoxide (DMSO), chlorobenzene, hydochloric acid, acetone, bathocuproine (BCP) and zinc powder were purchased from Wako Pure Chemical Industries, Ltd. (Japan).

2.2 Deposition of NiOx film on the FTO substrate and fabrication of inverted PSCs

Fluorine doped in oxide (FTO, Pilkington, 14 Ω.cm) glasses (2.5 cm × 2.5 cm) were paternally etched by Zn powder and 2 M HCl solution, consecutively washed with detergent and deionized water, and dried with nitrogen blower. The dried substrates were then sonicated into acetone for 10 min by ultrasonic bath. The substrates were dried again with a nitrogen blower after sonication was completed. Before preparing inverted PSCs, UV/O₃ treatment of the dried substrates in UV-O₃ stripper was carried out at 115 °C for 15 min. On the patterned FTO glass substrate, NiO_x as HTL films were deposited from a 99.99% pure NiO_x pallet (1 cm²) by EBPVD technique. The deposition chamber pressure was approximately 4.6 × 10⁴ Pa during deposition. The substrates were deprived from any purposeful heating (room temperature) during deposition, and the deposition rate was adjusted to 0.15 nm/s to get around 20 nm-thick film. The HTM-coated substrates were taken into the N₂ glove box directly for further fabrication. Few of the HTM-coated substrates were then heated at 500 °C for 30 min in a muffle furnace (vacuum) for comparative study. The preparation of perovskite precursor solution has been described elsewhere [49]. In brief, 461 mg of PbI₂ was measured in a glass container, and 159 mg of CH₃NH₃I was measured in another different glass container. Subsequently, 78 mg of DMSO and 700 mg of GBL were used to make a clear solution of CH₃NH₃I. The CH₃NH₃I solution was added to the PbI₂ container and stirred at 55 °C overnight. During device fabrication, 50 µL of CH₃NH₃PbI₃ precursor solution was spread over NiO_x substrates. The spin coater was set into two different rotations and time interval with a 2 s slope. The first spin coating step was set at 1000 rpm for 10 s. The second spin step was set at 5000 rpm for 30 s immediately after 2 s slope in between. At the end of 14 s of the second spinning step, 500 µL of anhydrous toluene was dripped onto rotating substrate and annealed. The deposited film’s annealing process was distributed into two parts. First, the films were thermally treated for 10 min at 50 °C onto one hot-plate and then moved to another hot-plate set for annealing for 10 min at 100 °C. Consequently, 50 µL of PC₆₁BM solution (20 mg/mL in chlorobenzene) was spread over the CH₃NH₃PbI₃-deposited substrates, spin-coated for 30 s at 1000 rpm, and treated for 10 min at 80 °C. The BCP solution (150 µL) was dropped on the substrate and rotated for 30 s at 6000 rpm. After spinning was completed, the substrates were annealed at 80 °C (10 min). A 100 nm Ag back contact electrode was deposited on top of the BCP layer by a resistive thermal evaporator under a vacuum chamber pressure of 4.65 × 10⁻⁴ Pa. A customized patterned mask was used to attain the expected device structure of final PSCs.

2.3 Characterization

The crystalline structure of NiO_x and CH₃NH₃PbI₃ layers were characterized by X-ray diffraction (XRD) using an x-ray structure analyzer RINT-TTR III/N (Rigaku) with Cu Kα (k=1.5418 Å) radiation to determine 2θ values in the range of 10°–60° and 30°–90° for NiOx and perovskite film, respectively. The surface roughness of the deposited NiOx films were determined by atomic force microscopy (AFM, Nanosurf easyscan 2 AFM) in tapping configuration to a scanning area of 3 µm×3 µm. Surface morphologies of films were analyzed by field emission scanning electron microscope (FESEM, Hitachi SU6600, Japan) in low vacuum condition. Optical transmittance (T%) of NiO_x and CH₃NH₃PbI₃ films was measured by an optical spectrophotometer in a UV-near IR (JASCO V-570, Japan) equipped with a double-beam system with single monochromator wavelength ranging from 190 nm to 2500 nm. The ionization potential was measured by AC-3 with 2.0 nW light intensity and 0.05 interval (RIKEN KEIKI, JAPAN). The resistivity and mobility of the deposited NiO_x films were measured by HMS ECOPIA 3000. The magnetic field was 0.57 Tesla, and the probe current was set to 4 mA for all the samples. All these measurements were completed under ambient environmental conditions.

The photovoltaic parameters and performances of the fabricated PSCs were measured by a Keithley 2400 source meter with solar simulator (CEP-2000RP, Japan) under simulated AM 1.5 G and evaluated from the characteristic current density–voltage (J–V) arch. The computer-controlled solar illumination was set to 100 mW cm^-2 in air. Both the forward scan and reverse scan were set to start from −0.2 V to 1.2 V and vice versa. The external quantum efficiency (EQE) spectra were measured (300–850 nm wavelength) using an EQE system (Bunkoukeiki Co., CEP2000RR, Japan). The photo-illuminated active area of PSCs was 0.16 cm² (0.4 cm × 0.4 cm) without further protective measures, such as encapsulation. The actual thicknesses of the deposited films were determined by Dektak Veeco Surface profiler.

3. Results and discussion

The surface morphological and crystalline properties of NiO_x films were characterized by FESEM and XRD, respectively. The NiOx films were deliberately deposited onto Pilkington glass for XRD intensity. Figure 1(a) shows that the as-deposited NiOx films presented clear and distinct 2θ XRD peaks at 37.36°, 43.53°, 63.20°, 75.66°, and 79.67°, which were indexed to the (1 1 1), (2 0 0), (2 2 0), (1 1 3), and (2 2 2) planes of NiO_x [50], respectively agreeing with JCPDS cards no. 731523. The high-temperature annealed NiO_x films’ corresponding peak intensity was stronger than that of the as-deposited films, indicating the high crystallinity of the NiOx films once annealed. Moreover, Fig. 1(a) reveals that peaks of the 1 1 1 and 2 0 0 planes of the as-deposited films were almost in the same intensity, but the former dominated while annealed at 500 °C. The average crystallite size (Dp) of NiO_x films was measured with the Scherrer formula [51] expressed as follows.

 

Fig. 1. XRD data for as-deposited and annealed NiOx film on glass substrate.

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Therefore, EBPVD technique could facilitate the deposition of highly crystalline NiO_x films. Figure 2(a) illustrates the low-magnification SEM image of as-deposited NiO_x film on glass substrate. The NiOx film was well distributed, and no clustering was observed. Moreover, to investigate the distribution of NiO_x, the FESEM images of bare glass, FTO and NiOx film-coated FTO were obtained and shown in Fig. 2(b-c) and (d). The NiO_x was evenly dispersed as a film on FTO and perfectly replicated the morphology of FTO crystals as shown in the Fig. 2(d) inset. The grain boundaries were responsible for the surface roughness of FTO by nature.

 

Fig. 2. SEM images of (a) as-deposited; FESEM images of (b) as-deposited, (c) annealed NiO_x film on bare glass and (d) bare FTO; inset NiOx-coated FTO glass.

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The resistivity (ρ) and hole concentration (N_b) of the as-deposited and annealed NiO_x films were measured by the Hall effect and showed in Table 1.

Table 1. Summery of electrical properties of NiO_x films

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The as-deposited films showed ρ and N_b of 4.81×10³ Ω.cm and 2.01 × 10¹⁵/cm³, respectively. Improvements upon annealing of NiO_x films were observed. The resistivity (3.63) Ω.cm decreased by one orders of magnitude, and the carrier concentration (1.16 × 15¹⁶/ cm³) increased by one order of magnitude in high-temperature annealed NiOx compared with as-deposited films. But the carrier mobility of annealed NiO_x film was increased in a quiet number of folds comparing to as-deposited film. As shown in Fig. 3, the EBPVD processed NiO_x considerably leveled the FTO surface, which was demonstrated by reducing the root-mean-square roughness (R_q) of FTO. The Rq value of bare FTO was ∼25 nm, which reduced to ∼18 and ∼17 nm for the as-deposited and annealed FTO/NiO_x samples with the EBPVD method as expected, respectively. The uneven surface of HTM was expected to an extent because excess HTM layer smoothness might disturb Perovskite film adhesion on the HTM layer. The actual thicknesses of the deposited films were confirmed by surface profiler measurements, which revealed an average thickness of 18 nm.

 

Fig. 3. Topographic images by AFM of (a) bare FTO, (b) as-deposited FTO/NiOx film, and (c) annealed FTO/NiOx film; inset shows the 3D images.

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In an inverted structure of a PSC device, the TCO (FTO) layer is covered by the HTM initially while fabricating the device. Illuminated light first crosses the HTM layer before reaching the perovskite absorber layer. As a result, transparency is a major concern while choosing an HTM for PSC. Figure 4 presents the transmittance spectra of bare FTO, as-deposited NiO_x/FTO and annealed NiO_x/FTO in the region of 300–900 nm of solar magnetic radiation. A slight difference in bandgap between as-deposited and annealed NiO_x film might be recognised by the alterations of D_p, microstrain (ε) and dislocation densities (δ), resulting in lattice inconsistency and defects [53] of films. The absorption coefficient of the films increased at high annealing temperature. This performance could have possibly produced the increased density of states of holes at higher temperature annealing [54]. The results demonstrated that NiO_x-coated FTOs showed a negligible difference in light transmission compared with bare FTO. The excellent transparency properties of the as-deposited and annealed NiO_x HTLs are advantageous for fabricating inverted PSC because the absorber layer has maximum light for absorption.

 

Fig. 4. Transmittance of bare FTO and annealed FTO/NiO_x

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Since the device was fabricated in a p-i-n structure so maximum transparency was expected. A careful characterization suggested that upon the incremental thickness of NiO_x, the films’ bandgap reduced resulting lower transmission profile (Fig. 11, SI). It has been reported that 4% increment in films absorbance exhibits 1.0 mA cm⁻² loss of J_sc [48]. Meanwhile, for the comparison, 500 °C was proposed since at this temperature “interference oscillation” takes place inside the substrate which is the outcome of multiple reflection at NiO_x/substrate film’s boundary resulting smooth film formation [55]. High work function (φ), poor crystallinity, and poor dispersion were regarded as substantial drawbacks of low-temperature NiO_x films in producing improved PCEs [41]. We measured the ionization potentials (IPs) of the as-deposited (5.34 eV) and annealed (5.29 eV) NiO_x films by photoelectron yield spectroscopy (PYS) for further validation as shown in the Fig. 5(a) and Fig. (b) for as-deposited and annealed NiO_x films. The estimated Fermi level (E_f) should be 0.2 eV above the VMB of NiO_x [15], so the achieved work functions were 5.14 and 5.09 eV for as-deposited and annealed NiO_x, respectively.

 

Fig. 5. Workfunction determination of (a) as-deposited and (b) annealed NiOx film by PYS.

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As a result, the VBM (−5.4 eV) of CH₃NH₃PbI₃ demonstrated optimum band alignment configuration corresponding to the VBM of NiO_x (−5.29 eV for as-deposited or −5.34 eV for annealed) films, thus facilitating efficient charge carrier (hole) transfer from VB of the CH₃NH₃PbI₃ (−5.4 eV) layer to HTM NiO_x, as shown in the Fig. 6(b). It is not energetically possible to inject an electron from CB of CH₃NH₃PbI₃ into NiOx because NiO_x is a high band-gap semiconductor with CBM ∼−1.8 eV compared with CBM of CH₃NH₃PbI₃ (−3.9 eV). Figure 6(a) shows the full device geometry of fabricated PSC. By contrast, Fig. 7 shows the characteristic JV of the typical devices with as-deposited and annealed NiO_x HTLs.

 

Fig. 6. (a) Fabricated p-i-n PSC and (b) corresponding band diagram.

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Fig. 7. (a) J-V curves measured at solar simulator under simulated AM 1.5 G in this experiment at day 1 and 28th of fabrication; (b) EQE of the inverted planar PSCs fabricated with NiO_x HTLs for both as-deposited and annealed NiO_x film.

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The deposited NiO_x HTL films were annealed at 500 °C for 30 min, and PSC devices were fabricated following the same procedure for as-deposited NiO_x HTLs to evaluate the effect of high-temperature annealing. The corresponding photovoltaic parameters including V_oc, J_sc, FF, and PCE are presented in Table 2. Two other important parameters, namely, series resistance (R_s) and shunt resistance (R_sh), are recorded in Table 2, forming the J-V arcs. However, the thickness tuning of NiO_x films may differ in producing PSC devices, showing high performance depending on the deposition technique used. Therefore, the deposition technique of NiO_x films while fabricating PSC devices may alter characteristic properties such as crystallite size, plane orientation, band gap, and resistivity [56–58].

Table 2. Summary of fabricated PSC devices’ performance with NiO_x HTM at the 1st and 28th days of fabrication

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Under 100 mW/cm² illumination, the devices with as-deposited NiOx HTM films displayed a PCE of 13.20%, with J_sc of 17.90 mA/cm², FF of 77%, and V_oc of 0.95 V. The devices with annealed NiO_x HTM films reached a PCE of 13.24% with J_sc of 17.16 mA/cm², V_oc of 0.98 V, and FF of 0.78. The PCEs of the fabricated PSC devices were quite encouraging. The successful integration of the EBPVD NiO_x film as HTM were clearly pronounced by the PSC devices’ performance. Moreover, the charge carriers generated in the CH₃NH₃PbI₃ absorber layer were effectively transported to corresponding electrodes by selective contact to show those efficiencies. The data likewise disclose that the main photovoltaic parameters of the device based on as-deposited and annealed NiO_x HTL are almost same. Even their EQE (Fig. 7(b), both reaching 70% at 400-800 nm, also revels the identical outcomes which indicate EB deposition at low temperature has successfully managed to form quality NiO_x HTL film on top of FTO for better charge carrier transportation. However, V_oc of less than 1.0 V was observed, restricting FF and device performance. R_s refers to loss of internal carrier mobility [59], and R_sh refers to loss of photo-induced current due charge carrier recombination at the junction layer formed between layers of the device [60]. Both as-deposited and annealed devices showed a low R_s and high R_sh at the 1st day of fabrication, which were expected for such devices. On the contrary, R_s increased and R_sh decreased, affecting V_oc and FF and corresponding PCEs of inverted PSCs but still holding fair photovoltaic values after four weeks of deposition.

Figure 8(a) refers to the perfect XRD peaks with proper crystalline orientation of CH₃NH₃PbI₃ formation, but unchanged PbI₂ remained present. Figure 8(b) illustrates the light absorption spectra of CH₃NH₃PbI₃ film on as-deposited and annealed NiO_x films, indicating different absorption patterns in strong energy regions. The CH₃NH₃PbI₃ film on high temperature annealed NiO_x film showed better absorption in the solar spectrum range of 400–500 nm compared with CH₃NH₃PbI₃ film on top of the as-deposited NiOx film. The SEM image of the CH₃NH₃PbI₃ layer on NiO_x showed pinhole-free film formation. However, the 500–700 nm grain size of the CH₃NH₃PbI₃ layer possessed grain boundaries (Fig. 8(c)), which might cause electron-hole recombination at the junction interface. Meanwhile, it was expected that annealed NiO_x will exhibit high performance than as-deposited films due to possessing better carrier concentration. But consecutive increment carrier mobility in annealed film might be responsible for a minor improvement in PCE between both devices. The highest and average value of PCE, J_sc, V_oc and FF of as-deposited and annealed NiO_x film as HTM in PSC devices have been shown in the Table 3. Though the overall efficiency of both types PCEs almost identical, but the PSC devices with annealed NiOx as HTM provided very consistent performance while productivity was considered.

 

Fig. 8. (a) X-ray diffraction peaks of CH₃NH₃PbI₃; (b) UV-Vis spectra of FTO and PSC on as-deposited and annealed NiO_x; (c) SEM topography of CH₃NH₃PbI₃, and (d) cross-sectional FESEM image of the fabricated PSC.

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Table 3. Summary of the highest and average values of photovoltaic performances of PSC devices.

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To assess air (environmental) stability on NiO_x HTM device, we also measured the photovoltaic performance of those devices at the 28th day of fabrication. We measured and compared the PCE and other solar cell parameters of the devices. The devices were kept in 60%-70% relative humidity at 13 °C-25 °C in ambient environment, and no capping as well no intentional illumination was introduced during cell preservation. The measured JV of the fabricated PSCs on the 1st day are shown in Fig. 6(a). J_sc decreased from 17.90 mA/cm² on the 1st day to 16.39 mA/cm₂ on the 28th day of fabrication for the as-deposited NiO_x-based PSCs. Meanwhile, PSCs based on annealed-NiO_x/CH₃NH₃PbI₃/PCBM/BCP/Ag configuration showed reduced J_sc from 17.16 mA/cm² to 14.93 mA/cm² on the 1st and 28th days of measurement, respectively, followed by a decline in Voc and FF. The lower R_sh values of aged devices comparing with initial values of both fresh as-deposited and annealed NiO_x PSC suggested the presence of considerable recombination at NiOx interface upsetting those Voc. The R_sh was might be reduced due to presence of pin-hole introducing parasitic loss [61]. However, metal electrode penetration could also be responsible irreversible degradation especially at elevated temperature during measurement [55] which may lead to poor device performance later. As shown in Table 1, the devices showed a great stability retaining 72.72% and 76.96% of the initial PCE for the as-deposited and annealed NiO_x HTM, respectively, for the aforementioned time. Thus, EBPVD could successfully facilitate NiO_x HTM film formation for efficient and air-stable/ environment-stable PSC.

4. Conclusion

We demonstrated that EBPVD could form NiOx HTL films of superior quality using high-quality NiO_x pallet. The NiO_x HTL showed decent transparency, allowing a maximum gain of photon flux. The prepared NiO_x HTL exhibited rapid charge transfer to reduce carrier recombination and thus can be considered as an effective HTM. In addition, NiO_x possessed strong chemical stability, and its robustness toward environmental conditions was encouraging. Consequently, the as-deposited and annealed NiO_x HTL-based PSC devices showed PCE of 13.20% and 13.24%, retaining 72.2% and 76.96% of their primary PCE in ambient condition, respectively, after 672 hours of fabrication without any kind of intentional capping. It is clear evident that even without any post deposition heat treatment of electron-beam NiOx film can substantially provide a comparative device efficiency. Moreover, an 18 nm-thick NiO_x was sufficient for improved photovoltaic performance to ensure high material utilization efficiency and low-cost production. This study utilized EBPVD as an attainable means to achieve high-quality NiO_x inorganic HTM for efficient and stable PSCs. The EBPVD technique could produce well-dispersed, dense, high purity, and unwanted contamination-free NiO_x film at low temperature with high material utilization. However, the presence of surface recombination led to poor V_oc. This result suggested that the improvement of V_oc thorough HTM should be taken into account to minimize current loss due to interface and bulk recombination.

Appendix

 

Fig. 9. Working principal of EBPVD Technique.

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Fig. 10. EDX spectrum of FTO/NiOx films. Inset shows the weight% and atomic% of elements of as-deposited NiOx film.

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Fig. 11. Transmittance of EBPVD NiOx films on glass substrate. A clear decrease of transmission is observed in the higher energy shorter-wavelength when NiOx film thickness is increased and continue to exhibit lower transmission in the visible region of spectrum as well.

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Funding

Universiti Kebangsaan Malaysia (LRGS/1/2019/UKM-UKM/6/1).

Acknowledgments

The authors would like to acknowledge the Long Term Research Grant Scheme (LRGS) (LRGS/1/2019/UKM-UKM/6/1) grant funded by the University Kebangsaan Malaysia.

Disclosures

“The authors declare no conflicts of interest.”

References

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