Sputtered Nickel Oxide Research Articles

Magnetron sputtered nickel oxide with suppressed interfacial defect states for efficient inverted perovskite solar cells

Widely used spin-coated nickle oxide (NiOx) based perovskite solar cells often suffer from severe interfacial reactions between the NiOx and adjacent perovskite layers due to surface defect states, which inherently impair device performance in a long-term view, even with surface molecule passivation. In this study, we developed high-quality magnetron-sputtered NiOx thin films through detailed process optimization, and compared systematically sputtered and spin-coated NiOx thin film surfaces from materials to devices. These sputtered NiOx films exhibit improved crystallinity, smoother surfaces, and significantly reduced Ni3+ or Ni vacancies compared to their spin-coated counterparts. Consequently, the interface between the perovskite and sputtered NiOx film shows a substantially reduced density of defect states. Perovskite solar cells (PSCs) fabricated with our optimally sputtered NiOx films achieved a high power conversion efficiency (PCE) of up to 19.93% and demonstrated enhanced stability, maintaining 86.2% efficiency during 500 h of maximum power point tracking under one standard sun illumination. Moreover, with the surface modification using (4-(2,7-dibromo-9,9-dimethylacridin-10(9H)-yl)butyl)phosphonic acid (DMAcPA), the device PCE was further promoted to 23.07%, which is the highest value reported for sputtered NiOx based PSCs so far.

Factors Impacting the Photostability of a Roll‐to‐Roll Processed PEDOT Derivative for Flexible Electrochromic Devices

AbstractFor the use of polymeric electrochromic devices (ECDs) in architectural or automotive applications, the photostability of the electrochromic (EC) polymer is crucial for long‐term durability. In this study, the photostability of the side‐chain modified poly(3,4‐ethylenedioxythiophene) (PEDOT‐EthC6)thin films deposited by roll‐to‐roll slot‐die coating on indium tin oxide coated polyethylene terephtalate (PET‐ITO) is investigated and characterized by UV–vis and IR spectroscopy. Residual iron salt on the EC polymer layer, necessary for the in situ polymerization, and the variation of storage times between processing steps have no influence on the degradation rate during light exposure. The photostability of the thin films is 30% better in an inert atmosphere than in ambient conditions. Different long‐pass filters are applied to enhance the photostability in the colored and the bleached state. The position of the absorption edge strongly affects the photostability and visual appearance/color of the PEDOT‐EthC6 electrodes. The analysis of the CIE L*a*b* color coordinates reveals that a trade‐off between stability and a desirable color is necessary. To prevent UV degradation of the EC polymer in flexible ECDs, sputtered nickel oxide is chosen as the counter electrode material. The hybrid ECD shows no sign of degradation and loss of electrochromic performance after 350 h of light exposure.

Influence of film thickness on structural, optical, and electrical properties of sputtered nickel oxide thin films.

Utilizing radio frequency magnetron sputtering, we successfully fabricated nickel oxide thin films with different thickness (from 80 to 270 nm), and conducted an in-depth examination of their structural, morphological, optical, and electrical properties. The crystal structure and surface roughness were determined using x-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. The XRD analyses showed that the films were composed of cubic nickel oxide, exhibiting a notable orientation along the (200) direction. This crystal texture partially increased when the film thickness reached 270 nm. In addition, a direct correlation between film thickness and crystallite size was observed, with the latter increasing as the former did. AFM analysis provided insights into the surface morphology, revealing metrics like the bearing area, 3D surfaces intersections, and statistical properties of surface height. These insights underscore the relationship between film thickness and surface properties, which in turn influence the overall electrical, and prominently, optical properties of the films. Employing transmittance UV-visible spectroscopy, we characterized the optical behavior of these films, noting a proportional increase in refractive index with film thickness. Additionally, resistivity was observed to increase concomitantly with film thickness. In conclusion, the deposition process's film thickness acts as a pivotal parameter for fine-tuning the structural, morphological, and optical properties of nickel oxide thin films. This knowledge paves the way for optimizing nickel oxide-based devices across various applications. RESEARCH HIGHLIGHTS: We synthesized and characterized of p-type semiconducting NiO thin films sputtered on substrates by using RF magnetron sputtering with different thickness. Advanced crystalline structures and fractal features extracted from XRD and AFM analysis. The 2D and 3Dsurface analysis of the samples indicates a complex structure with an imperfect self-similarity that suggests a multifractal structure. We represented graphically the relative representation of higher geometric objects in the AFM image. We attributed the optical and electrical properties of the samples to the crystallite size, and the concurrent reduction in oxygen vacancies and crystalline defects within the films.

Over 19% Efficiency Perovskite Solar Modules by Simultaneously Suppressing Cation Deprotonation and Iodide Oxidation.

Perovskite solar cells (PSCs) based on sputtered nickel oxide (NiOx) hole transport layer have emerged as promising configuration due to their good stability, cost-effectiveness, and scalability. However, the adverse chemical redox reaction at the NiOx/perovskite interface remains an ever-present problem that has not yet been well solved. To address this issue before, the problems that cation deprotonation and iodide oxidation that occurred in precursor solution easily result in the interfacial chemical reaction should be prevented. Hence, we report an efficient strategy to simultaneously suppress the interfacial reaction and stabilize the precursor solution by incorporating a reducing and weakly acidic stabilizer, l-ascorbic acid (l-AA). l-AA can reduce I2 generated in the precursor solution and during the interfacial reaction to I-. Furthermore, the protons ionized by adjacent enol hydroxyl groups in l-AA effectively impede the deprotonation of organic cations in the precursor solution as well as at the NiOx/perovskite interface resulting from the chemical reaction. Attributing to the improved crystallization of the perovskite film and the suppression of the interfacial reaction by l-AA, the inverted PSC based on such good light absorber achieves an impressive power conversion efficiency (PCE) of 22.72% along with a high open-circuit voltage of 1.19 V. Notably, further introducing l-AA into the large-area solar modules by the slot-die coating method in air enables a remarkable PCE of 19.17%, which reaches one of the highest PCEs reported for inverted perovskite solar modules (PSMs) (active area >50 cm2) to date. l-AA located at the buried interface also forms a barrier layer that can prevent undesirable chemical reactions at the NiOx/perovskite interface, significantly enhancing the device stability of solar cells and PSMs. These findings in our work provide important guidance for improving the NiOx/perovskite interface and the fabrication of highly efficient, low-cost, and large-area PSMs.

Toward Efficient and Fully Scalable Sputtered NiOx‐Based Inverted Perovskite Solar Modules via Co‐Ordinated Modification Strategies

Sputtered nickel oxide (NiOx) has become one of the most promising inorganic hole transport layers for p–i–n perovskite solar cells (PSCs) due to its appealing features such as its robust nature, low material cost, and easy integration to tandem structures and large‐area applications. However, the main drawback with NiOx‐based PSCs is typically low open‐circuit voltage (VOC) due to the inferior energy‐level alignment, low charge mobility, and high recombination at the interface. Herein, two types of phosphonic acid self‐assembled monolayers (SAMs) deposited by blade coating as an interfacial layer to modulate the sputtered NiOx/perovskite interface properties are used. While sputtered NiOx serves as a conformally coated hole selective layer, the ultrathin SAM interlayer facilitates the hole extraction and minimizes the energy loss at the interface. Co‐ordinately introduced stabilizing additive, namely octadecyl 3‐(3,5‐di‐tert‐butyl‐4‐hydroxyphenyl)propionate (I‐76), further improves the device performance of NiOx/SAM‐based PSCs, resulting in VOC of 1.14 V and a power conversion efficiency of 21.8%. By applying these strategies for perovskite module upscaling, aperture area module efficiencies of 19.7%, 17.5%, and 15.5% for perovskite minimodules of 4, 16, and 100 cm2 are demonstrated, corresponding to active area module efficiencies of 20.4%, 18.0%, and 16.4%, respectively.

Enhancing Efficiency of Inverted Perovskite Solar Cells by Sputtered Nickel Oxide Hole‐Transport Layers

Perovskite solar cells (PSCs) are now approaching their theoretical limits and the optimization of the auxiliary layers is crucial for fully exploiting the potential of perovskite materials. In this study, NiOx as a hole‐transport layer (HTL) for inverted p–i–n PSCs is focused on. Sputtered NiOx is an attractive p‐type HTL owing to its facile processing, wide energy bandgap that prevents electron transfer, high transparency, stability, and effective hole extraction. Despite substantial research on sputtered NiOx, the relationship between the carrier concentration and work function is still unclear. In this study, the use of sputtered NiOx as a widely compatible HTL and the effect of its thickness on PSC device performance are investigated. Inverted PSCs with the optimal 10 nm thick NiOx achieve a remarkable power conversion efficiency of 20.54%, which is the highest reported to date for sputtered NiOx‐based PSCs. Furthermore, PSCs with various NiOx thicknesses demonstrate similar performances, demonstrating the excellent versatility of NiOx for use with different perovskite absorbers. The devices exhibit excellent thermal and photostability, retaining 97% of their initial power conversion efficiency at 65 °C and 1 sun illumination for 350 h. Sputtered NiOx HTLs have great potential for use with diverse perovskite compositions and PSC structures.

Uniform Coverage Functional Layers Enable High‐Efficient Flexible Perovskite Solar Modules with an Outstanding Fill Factor

Uniform and pinhole‐free functional layers are essential for high‐efficient flexible perovskite solar modules (FPSMs). However, the poor wettability of self‐assembled monolayers (SAMs) with carbazole bodies and phosphonic acid binding groups usually leads to porous large‐area perovskite films. Herein, a layer of nickel oxide nanoparticles is inserted between sputtered nickel oxide and the SAM to increase the surface energy, and thus simultaneously improving the wettability and nucleation of the perovskite during vacuum‐quenching method. Following this strategy, the best flexible perovskite solar cells with an active area of 0.09 cm2 achieve a power conversion efficiency (PCE) of 21.97% under 1 sun illumination. In addition, FPSMs with the same structure exhibit a high fill factor approach 80% and reach a champion PCE of 19.71% on 58.14 cm2 active area and a certified PCE of 18.17% on 61.26 cm2 aperture area, which is the highest recorded PCE to the best knowledge. Furthermore, the optimized interface also enhances the adhesion between hole‐transport layer and perovskite, and the encapsulated device retains 95% of its initial PCE after 1000 cycles under a 2 cm bending radius.

Effect of RF power on structural, optical and electrical properties of sputtered nickel oxide

In this study, we focus on the effect of the radio frequency (RF) sputtering power on the structural, optical and electrical properties of NiO thin films, deposited on glass substrates in the O2/Ar atmosphere. It was observed that by increasing the RF power from 50 to 300 W, the crystallite size increases from 5.5 to 8.28 nm then decreases for 350 W. The thickness increases from 40 to 800 nm as the power increases from 50 W to 300 W, and decreases to 450 nm for 350 W. Raman spectroscopy analyses confirmed the formation of the NiO phase for all RF powers. SEM images exhibited homogenous and no porous surfaces. Besides, the optical transmittance decreases with increasing the RF power, the band gap is in the range 2.68–3.15 eV and the Urbach energy decreases from 8.06 to 2.04 eV with a minimum obtained for 250 W. The minimum electrical resistivity was obtained for the RF power 50 W.

Effect of substrate temperatures on structural, optical and dispersion energy parameters of RF sputtered nickel oxide thin films

Thin films of nickel oxide (NiO) have been widely identified as a promising ion storage layer in electrochromic devices owing to their excellent electrochemical properties. There is, however, a lack of accurate description of their optical parameters, which are quite important to explore the optical and electrochromic characteristics of NiO. In the present investigation, nickel oxide (NiO) thin films have been deposited using radio frequency (RF) magnetron sputtering at different substrate temperatures (Tsub) viz. 100, 200 and 300 °C. The consequences of substrate temperature on the structural, morphological, optical, and vibrational properties were examined. The XRD pattern showed increased crystallinity with an increase in substrate temperature. Surface morphology revealed an ultrafine and smooth morphology. The average optical transmittance varied between 64% and 85%. Shrinkage in the energy bandgap was observed with the change in substrate temperature. The values of optical parameters like refractive index (n), extinction coefficient (k), the real part of the dielectric constant (ε1), the imaginary part of the dielectric constant (ϵ2), and porosity of NiO films were found to change with an increase in substrate temperature. The effect of substrate temperature on dispersion energy parameters (Eo and Ed) derived from the Wemple-DiDomenico model was calculated and discussed in detail. Room temperature photoluminescence studies revealed peaks along 3.39 eV (365 nm) and 2.95 eV (420 nm) nm as a consequence of band-band PL phenomena for nickel oxide and defects. In the Raman spectrum, one predominant peak around 560 cm−1 corresponds to the one-phonon LO mode, and another minor peak at 1100 cm−1 owing to the two-phonon LO mode were observed, which arises from Ni–O vibrations. The observed results reveal the potential application of the NiO films in optoelectronic and electrochromic device applications.

Effect of surface treatment of sputtered nickel oxide in inverted perovskite solar cells

Nickel oxide (NiOx) is promising hole transport material for efficient and stable lead halide perovskite solar cell (PSC) due to its promising optoelectronic properties and superior moisture resistivity. Herein, we investigate the effect of surface treatment of sputtered NiOx film using ethanol (NiOx-EtOH) in device parameters of PSC. Ethanol treatment of NiOx facilitates the growth of high-quality perovskite film (larger grain size, better crystallinity, and faster carrier injection). The device with NiOx-EtOH demonstrated the champion power conversion efficiency of ∼13.52% (with open circuit voltage (VOC) ∼1.075 V) of 1 cm2 large-area device. While the device with pristine NiOx has a device efficiency of 11.78% (with VOC ∼0.99 V). This report corroborates that the PSC with NiOx-EtOH improves device performance owing to hydrophilicity, defect passivation of sputtered NiOx surface, and attenuation of recombination in the perovskite bulk.

NiOx thickness dependent improvement of NiOx/Perovskite interface for inverted planar perovskite solar cells

Sputtered nickel oxide (NiOx) as hole transport layer (HTL) could easily realize the large-area uniformity, as well as reduce the fabrication cost for metal-halide perovskite solar cells (PSCs). However, nonradiative recombination losses at the interface between NiOx and perovskite (PVK) lead to an open-circuit voltage (Voc) deficit, which is an important limiting factor for the improvement of power conversion efficiency (PCE). To address this situation, we report a strategy of improving the energy band matching at the NiOx/PVK interface by the simple thickness control of NiOx. Combined experimental and numerical simulation results indicate that the suitable thickness reduces the valence band barrier at NiOx and PVK interface and expedites the charge carrier collection. We achieved a PCE of 16.1% via an optimized NiOx thickness of PSCs, which is 2.2% and 1.7% absolute higher than that of 10 and 30 nm thickness of NiOx-based PSCs, respectively. Overall, these findings provide critical insights for improved hole transport at the interface between the NiOx and perovskite thin film, and the fabrication of highly efficient, robust, and large-area perovskite optoelectronic devices.

Impact of heat treatment on the physical properties of sputtered nickel oxide thin films containing molybdenum

The effect of annealing the substrate on the structural and optical properties of the nickel oxide thin films containing a fixed ratio of molybdenum was investigated. Energydispersive X-ray spectroscopy (EDX) revealed that the Mo ratio is 4.41 wt%. X-ray diffraction (XRD) revealed that the formed films comprise a phase of NiO0.96. The average crystallite size, dislocation density, and strain function were calculated. Atomic force microscopy (AFM) was employed to investigate the morphology and surface roughness. With the increase in the substrate temperature from 298 K to 673 K, the optical band gap values varied from 3.72 eV to 3.58 eV.

NiOx‐Seeded Self‐Assembled Monolayers as Highly Hole‐Selective Passivating Contacts for Efficient Inverted Perovskite Solar Cells

Self‐assembled monolayers (SAMs) have emerged as effective carrier transport layers in perovskite (PVK) solar cells because of their unique ability to manipulate interfacial property, as well as simple processing and scalable fabrication. However, the defects and pinholes derived from their sensitive adsorption process inevitably deteriorate the final device performance. Herein, a sputtered nickel oxide (NiOx) interlayer is used as a seed layer to promote the adsorption of the [2‐(3,6‐dimethoxy‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (MeO‐2PACz) SAM on the indium tin oxide (ITO) substrate. The promoted adsorption is attributed to the enhanced tridentate binding between MeO‐2PACz and NiOx relative to the conventional bidentate binding between MeO‐2PACz and ITO. In addition, the NiOx modification can simultaneously improve the passivation ability and hole‐selectivity of the MeO‐2PACz, provide a favorable energy‐level alignment at the ITO/PVK interface, and prevent a direct contact between PVK and ITO. As a consequence, this NiOx‐seeded MeO‐2PACz hole transport layer enables a significantly enhanced power conversion efficiency of 19.9% in comparison with 18.4% of the control device. This work provides an effective strategy to improve the performance of the SAM‐based photoelectric device.

The growth, properties and application of reactively sputtered nickel oxide thin films in all thin film electrochromic devices

Nickel oxide (NiOX), widely used as ion storage layer in all thin film electrochromic devices (ATF-ECDs), was deposited by reactive direct current (DC) magnetron sputtering at different oxygen fluxes (FO2). The reactive DC magnetron sputtering processes were finely monitored and controlled by the target voltages. The growth model and material properties were systematically investigated. The electrochromic properties for NiOX thin films and corresponding ATF-ECDs were studied as well. The optimal ATF-ECD (SLG/ITO/NiOX/Ta2O5/WO3/ITO) was achieved with a NiOX ion storage layer reactively sputtered at a FO2 of 15 sccm. The electrochromic mechanisms was discussed in details.

Deposition Rate and Electrochromic Properties of Ni Oxide Thin Films Deposited By Sputtering Using Water Vapor As a Reactive Gas

Ni oxide is well known electrochromic (EC) materials. And nickel hydroxide (Ni (OH)2) and oxyhydroxide (NiOOH) are considered to be electrochemically more active than nickel oxide (NiO). Reactive sputtering is one of the most commonly used techniques for obtaining compound thin films, however, deposition rate of compound thin film is low. We have reported that the deposition rate of sputtered nickel oxide thin film increased remarkably by substrate cooling using a liquid nitrogen and injection of water vapor (H2O) onto the target surface in argon (Ar) atmosphere [1]. In this study, we studied the effect of the direction of water vapor injection and the substrate temperature on deposition rate, composition and EC characteristics of the Ni oxide thin films.Ni oxide thin films were prepared by an RF magnetron sputtering system at substrate temperatures (Ts) of -80 °C and -170 °C. Ar and H2O were used as sputtering gas, and the flow rates of Ar and H2O were kept constant at 2.5 cc/min. H2O was injected onto the substrate or target surface. RF power and sputtering gas pressure were constant at 50 W and 50 mTorr, respectively. Film thickness was measured using a stylus profiler, and deposition rate was calculated using the film thickness and sputtering time. Crystal structure and chemical bonding state of the films were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR), respectively. Optical and electrical properties of the films were studied by UV-Vis spectroscopy and four probe method. EC properties of the films were evaluated using cyclic voltammetry (CV) and chronoamperometry (CA) in a 1 M KOH aqueous electrolyte.Figure 1 shows the deposition rate of Ni oxide thin films deposited at Ts=-80 °C and -170 °C with the H2O injection onto the substrate or target surface. At Ts=-80 °C, high deposition rates of 30-35 nm/min were obtained regardless of the direction of H2O injection. The deposition rates are much higher than that obtained by conventional reactive sputtering at room temperature (4nm/min) [1]. In contrast, the deposition rate varied from 21 nm/min for H2O injection to substrate 38 nm/min (the highest deposition rate) for H2O injection to target at Ts=-170 °C. Highly transparent films with a transmittance above 80% were obtained regardless of the direction of H2O injection at Ts=-80 °C, however, the transmittances of the films deposited at Ts=-170 °C were low. From the four probe method measurement, the thin film deposited at Ts=-170 °C with H2O injection to substrate was found to be a mixed film of metal Ni and Ni oxide with a resistivity of 3.3×10-4Ωcm. The films deposited at Ts=-80 °C with H2O injection to substrate or target and the film deposited at -170 °C with H2O injection to target were considered to be NiO or Ni(OH)2. CV and CA measurements showed that the film deposited at Ts=-80 °C with H2O injection to target had good cycle stability and large transmittance change of 58% at a wavelength of 600nm.In summary, the deposition rate, chemical composition, and EC properties of the Ni oxide films were found to be strongly dependent on the substrate temperature and the direction of H2O injection. The films deposited with H2O injection to target at Ts=-170 °C showed the highest deposition rate and that deposited at Ts=-80°C with H2O injection to target showed the largest transmittance change.REFERENCES[1] Y. Yokoiwa et al, Jpn. J. Appl. Phys. 58 (2019) 055504. Figure 1

Sputtered Nickel Oxide Thin Films on n-Si(100)/SiO2 Surfaces for Photo-Electrochemical Oxygen Evolution Reaction (OER): Impact of Deposition Temperature on OER Performance and on Composition before and after OER

Magnetron sputtered nickel oxide thin films deposited on the native oxide of crystalline n-Si(100) wafers are studied in dependence of the substrate deposition temperature (600 °C, 400 °C, 200 °C, and room temperature) using X-ray and synchrotron excited photoemission spectroscopy as well as cyclic-voltammetry under illumination. We show that the substrate temperature during nickel oxide sputtering governs the composition of the pristine NiOx film and the OER performance. Two dedicated nickel oxide species are found with Ni2+ corresponding to stoichiometric NiO while Ni3+ indicates an oxygen rich NiOx (x > 1) phase. With decreasing deposition temperature, the ratio of Ni3+/Ni2+ in the pristine NiOx film increases. Information depth dependent synchrotron related photoemission spectroscopy further suggests that oxygen rich NiOx is found on top of the surface and at the grain boundaries. The OER onset potential improves from 1.55 V to 1.1 V in correlation to an increasing Ni3+/Ni2+ ratio in the pristine NiOx film and an increasing emission from a nickel oxyhydroxide phase (h-NiOx) after photo-assisted cyclic-voltammetry in alkaline solution. Upon electrochemical treatment, a reconditioning process is observed with the formation of h-NiOx that consists of Ni(OH)2 and NiOOH, while NiOx disappears.

Utilizing compliance current level for controllability of resistive switching in nickel oxide thin films for resistive random-access memory

In this paper, we investigate two different resistive switching mechanisms in sputtered nickel oxide thin film which result from using the forming compliances of 15 μA and 15 mA. X-ray photoelectron spectroscopy material analysis results verify the nickel and oxygen chemical shifts, while energy-dispersive X-ray spectroscopy results confirm oxygen content changes after the forming process. Moreover, two markedly different resistive switching mechanisms, those of unipolar and bipolar, are accomplished in a single nickel oxide thin film. To clarify the two resistance switching mechanisms in the Pt/NiO/TiN device, a size effect measurement was conducted to determine the filament-type conduction. Moreover, the device capability to act as a memory device utilizing these two switching behaviors was determined, and a retention test for reliability was also carried out over 125 °C. Finally, the switching mechanism was confirmed by current fitting and sampling reset methods, and an appropriate physical model is presented. We anticipate this model can provide an insight into further future applications of RRAM devices.

Effect of different surface treatments of sputtered NiOX on the photovoltaic parameters of perovskite solar cells: a correlation study

Understanding the surface chemistry of sputtered (sp) nickel oxide (NiOx) hole transporting layer and its influence on the perovskite photovoltaic properties is crucial in aiding improved device performance. Herein, the influence of (1) as-deposited, (2) 2-propanol (IPA) dipping, (3) UV-ozone and (4) plasma treated sp-NiOx on the perovskite device performance is compared. Surface treatment not only changes the work function and surface wettability of sp-NiOx but also enhances the perovskite quality and its device performance. Perovskite device with plasma treatment yielded the best efficiency owing to hydrophilic sp-NiOx surface, deep-lying valance band, and suppressed recombination in bulk of perovskite.

Evaluation of sputtered nickel oxide, cobalt oxide and nickel–cobalt oxide on n-type silicon photoanodes for solar-driven O2(g) evolution from water

Thin films of nickel oxide (NiOx), cobalt oxide (CoOx) and nickel–cobalt oxide (NiCoOx) produced integrated, protected Si (111) photoanodes integrated, protected Si photoanodes that did not require deposition of a separate heterogeneous electrocatalyst for water oxidation.

Effect of RF Magnetron Sputtered Nickel Oxide Thin Films as an Anode Buffer Layer in a P3HT:PCBM Bulk Hetero-Junction Solar Cells

Effect of RF Magnetron Sputtered Nickel Oxide Thin Films as an Anode Buffer Layer in a P₃HT:PCBM Bulk Hetero-Junction Solar Cells