Xiangyi Shen received her Bachelor's degree in Chemical Biology from Changzhi University in 2021. She is currently a master's student at the Institute of Advanced Materials (IAM) & School of Flexible Electronics (Future Technologies), Nanjing Tech University. Her current research focuses on perovskite solar cells with charge transport layers of metal oxides fabricated by magnetron sputtering.
Xinwu Ke received his Bachelor's degree in Applied Chemistry from Qilu University of Technology in 2022. He is currently a master's student at the Institute of Advanced Materials (IAM) & School of Flexible Electronics (Future Technologies), Nanjing Tech University. His current research focuses on vacuum-deposited perovskite solar cells.
Yingdong Xia received her B.E. degree in macromolecular science and engineering from Hebei University in 2006 and the Ph.D. degree in polymer chemistry and physics from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, in 2011. She then conducted her postdoc research at Georgia Institute of Technology and Wake Forest University during 2011–2013. She is now a Professor at Nanjing Tech University. Her current research interest is focused on organic light-emitting diodes, perovskite light-emitting diodes, and solar cells.
Qingxun Guo received his Bachelor's degree from the Jilin University in 2012, and Ph.D. degree in Polymer Chemistry and Physics from the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2017. He engaged in OLED panel design for two years at TCL China Star Optoelectronics Technology Co., Ltd (CSOT) and then joined the Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology as a Postdoctoral Researcher in 2019. He is currently an associate professor at Nanjing Tech University. His research focuses on metal halides for light-emitting diodes and solar cells.
Yonghua Chen received a bachelor's degree in Chemistry from Inner Mongolia University in 2006 and a Ph. D. in Polymer Chemistry and Physics at the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, in 2011. Afterward, he spent time at Wake Forest University (2 years) and Case Western Reserve University (2 years) as a postdoctoral researcher. He is currently a full professor at Nanjing Tech University. His research interests are organic and organic/inorganic hybrid optoelectronic materials and devices for energy conversion.1. Introduction
With the rapid development of human society, the energy demand is increasing year by year. At present, the predominant energy in people's daily life is still fossil fuels, which are non-renewable, polluting the environment, and not conducive to the development of the concept of carbon neutrality. Solar energy, with its universal green and sustainable energy characteristics, will be the central component of the future energy system. Perovskite solar cells (PSCs) are emerging as the third-generation technology with the advantages of high efficiency and low cost compared to the traditional solar cell technology such as monocrystalline, polycrystalline silicon, copper indium gallium selenide (CIGS) and cadmium telluride (CdTe)[1−4]. In the laboratory, the efficiency of perovskite solar cells has reached 26.7%[5], surpassing that of crystalline silicon cells. Nonetheless, there is still significant room for improvement in their commercialization and large-scale manufacturing processes.
Inverted perovskite solar cells have attracted widespread attention due to their low-temperature manufacturing process, low hysteresis, and good device stability. The hole transport layer (HTL) plays an essential role in inverted perovskite solar cells. It not only promotes the extraction and transportation of photogenerated holes, but also serves as a protective barrier against moisture and oxygen, thereby enhancing the stability of the solar cell. Currently, the predominant HTLs for PSCs are organic materials, such as self-assembled monolayers (SAMs), poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS), and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA)[6−9]. As shown in Figs. 1(a) and 1(b), these materials are usually expensive, unstable, and unsuitable for mass production[10−14]. In contrast, NiOx film with p-type semiconductor properties, shows the advantages of high mobility, excellent stability, low cost, and the compatibility of mass production, which makes it the ideal candidate as the efficient and stable HTL for high-performance PSCs. In addition, its outstanding chemical stability, excellent light transmittance, and economical material costs have shown tremendous application potential in all kinds of optoelectronic devices such as ultraviolet detectors, gas sensors, electrochromic display materials, and laser diodes, demonstrating that NiOx thin film plays a crucial role in modern electronic and optoelectronic technology[15−17].
Various methods have been developed for the deposition of NiOx thin films, such as the spin coating of NiOx nanoparticles, pulsed laser deposition, magnetron sputtering, electron beam evaporation, and atomic layer deposition[18−22]. Among them, magnetron sputtering is a more mature technique for thin film preparation. It is characterized by high tunability, dense film, good repeatability, and smooth film surface, which is conducive to industrial production. Therefore, magnetron sputtering NiOx is widely used in solar cells, especially in mass-produced photovoltaic modules.
In this comprehensive review, we summarized the optoelectronic properties of sputtered NiOx thin films and systematically analyzed the magnetron sputtering process, highlighting its impact on the characteristics of NiOx films. We reviewed the recent advancements in perovskite solar cells (PSCs) utilizing sputtered NiOx as a hole transport layer (HTL). Particular attention was given to critical interface issues between sputtered NiOx and perovskite, along with potential modification strategies to enhance device performance, as shown in Fig. 2. Finally, we discussed the current challenges faced by sputtered NiOx and explored future opportunities for its application. We believe this review will draw greater attention to the field of sputtered NiOx PSCs and inspire broader applications of NiOx in advanced optoelectronic devices.
2. Properties and the deposition mechanism of NiOx thin films
2.1 Structure and properties of NiOx
NiOx is a widely studied transition metal oxide, possessing a typical NaCl face-centered cubic crystal structure, as shown in Fig. 3(a)[23]. When NiOx exists in an ideal stoichiometric ratio, it exhibits excellent insulating properties with low intrinsic conductivity (10−4 S∙cm−1)[24]. However, the chemical composition of nickel oxide often does not strictly adhere to the stoichiometric ratio. Typically, excess oxygen penetrates the NiOx lattice leading to the oxidization of Ni2+ to Ni3+, which enhances the film conductance. This is also followed by the formation of nickel vacancies, which in turn promotes the migration of holes, as shown in Fig. 3(b)[25]. The energy level of Ni-3d is close to that of O-2p6. Therefore, it is expected that the Ni-3d orbitals can hybridize strongly with the O-2p6 orbitals, as shown in Fig. 3(c)[26]. This deviation from the ideal stoichiometric ratio endows NiOx with inherent p-type semiconductor properties. NiOx possesses a deep valence band maximum (VBM) of more than 5.0 eV and a wide bandgap of 3.4−4.0 eV[27]. It exhibits excellent transparency in the near-ultraviolet and visible spectra, with transmittance greater than 80% in the 460−750 nm range[28]. The opto−electrical properties of NiOx can be easily tuned by altering the parameters for the fabrication process of NiOx thin films. For example, Kwon and colleagues studied the properties of NiOx films deposited at different temperatures and atmospheres, and found that their resistivity increased with increasing temperature, suggesting that sputtering temperature is one of the main factors determining the photoelectric properties of NiOx films. Modulation of the photoelectric properties of NiOx films can also be achieved by doping with other metal ions. For example, Dewan et al. changed the electrical conductivity of NiOx films by doping with different amounts of Zn, as shown in Fig. 3(d)[29].
The fabrication of NiOx thin films through methods like nanocrystal growth, chemical synthesis, or magnetron sputtering may result in different grain morphologies and sizes within the films[30−32]. The SEM morphology of NiOx deposited on the FTO surface was characterized in Fig. 3(e), with the grain size generally less than 10 nm[28, 31]. A characteristic XRD pattern of a standard NiOx thin film is marked by three prominent diffraction peaks at 2θ angles of 37.2°, 43.2°, and 62.8°, aligning with the (111), (200), and (220) crystallographic planes, respectively, as shown in Fig. 3(f)[33]. Generally, NiOx films exhibit similar diffraction angles regardless of morphology, growing preferentially along the (111) crystal plane[28, 34]. However, the intensity and lattice strain of the peaks can be tuned by sputtering temperature and power. For instance, when employing high sputtering power, the film experiences lattice distortion, which leads to a minor shift of the diffraction peak towards a higher angle[35, 36].
2.2 Deposition mechanism of NiOx thin films
Due to its excellent hole transport properties and stability, NiOx has attracted considerable attention for the fabrication of high-performance PSCs. Various deposition methods have been explored to realize high quality NiOx thin films.
2.2.1 Magnetron sputtering
Magnetron sputtering is an efficient physical vapor deposition (PVD) technique used for the fabrication of various functional thin films. The thin film deposition process generally involves the use of an accelerating electric field to bombard the target material with charged ions. Atoms or molecules on the surface of the target material are sputtered out and then deposited onto the substrate, enabling crystal growth. During the crystal growth process, the substrate used for growth is placed on the anode, and the target material made from the deposition material is placed on the cathode, as shown in Fig. 4(a). When a high voltage is applied between the cathode and anode, the sputtering gas (such as argon) between the two electrodes is ionized, resulting in a glow discharge phenomenon. The positively charged ions produced by gas ionization are accelerated by the electric field and strike the cathode target, causing atoms in the target material to gain enough energy. When their kinetic energy exceeds the binding energy, they detach from the lattice points in the form of free atoms. These free atoms continue to collide with surrounding atoms, forming a cascade of collisions. When this process extends to the surface of the target material with kinetic energy higher than the surface binding energy of the target material atoms, they are sputtered out from the surface of the target material. The sputtered atoms reach the anode substrate at a certain speed and direction, gradually grow and crystallize, and ultimately form a thin film. In the sputtering of NiOx, inert gases such as argon and oxygen are commonly used as sputtering gases. By precisely adjusting the sputtering pressure power, gas ratio and flow, and annealing temperature, thin films with different optoelectronic properties can be achieved.
2.2.2 Spin-coating
The spin-coating technique is a commonly used method for thin film preparation, as shown in Fig. 4(b)[37]. The precursor solutions used in this method are mainly commercial NiOx nanocrystals or synthesized by sol-gel method. NiOx nanocrystals are highly prone to agglomeration, which leads to a degradation of the quality of the formed film. The resulting large leakage current is not favorable for improving the performance of PSCs[38]. The sol-gel method usually uses nickel salts (e.g., nickel nitrate) as precursors[39, 40]. The thickness and composition of the films can be easily controlled by adjusting the solution concentration, pH, and heat treatment conditions. However, the density of the film may be affected by the preparation conditions, leading to the formation of holes. Higher heat treatment temperatures and times are required to ensure the complete transformation of the gel into NiOx films. Park et al. used a relatively neutral ammonium salt as a stabilizer in the preparation of NiOx films by the sol-gel method and achieved a PCE of 19.91% of the resulting PSCs, which could be maintained at 97% of the initial PCE after 800 h[41]. However, spin-coating is not conducive to large-area processes.
2.2.3 Spray pyrolysis
This method involves spraying a solution containing NiOx precursors (typically nickel nitrate solution) onto a heated substrate using a sprayer. The substrate temperature is usually between 200 and 600 °C. The solution rapidly evaporates on the substrate, leaving a solid film that is then thermally decomposed at high temperatures to form a NiOx thin film. Fig. 4(c) displays the deposition process of the spray pyrolysis method at various temperatures and droplet sizes. This method has the advantage of simple equipment and low operational costs, making it suitable for thin film preparation on large-area substrates. However, it typically requires precise control of the thermal treatment conditions to ensure the quality and performance of the thin film[42]. For example, Scheideler et al. fabricated PSCs based on spraying NiOx substrate at a high temperature of 300 °C, realizing the enhanced efficiency of large-area devices[43].
2.2.4 Electrodeposition
The electrodeposition of NiOx thin film involves immersing the working electrode (substrate) in an electrolyte solution and applying an electric current to reduce the NiOx precursor (such as Ni(NO3)2) on the surface of the working electrode, forming a NiOx thin film, as shown in Fig. 4(d)[44]. Electrochemical deposition parameters, such as deposition potential, time, and electrolyte composition, have a significant effect on film formation. Small variations in these parameters may lead to significant differences in crystallinity, surface roughness, defect density, and film quality[45].
2.2.5 Pulsed laser deposition (PLD)
PLD involves the process that utilizes a high-power pulsed laser beam to irradiate the target material after absorbing the laser energy. This increases the temperature of target materials, leading to the melting, vaporization, and splattering of the target. Material escaping from the surface of the target forms a plasma, which then deposits on the substrate to form a thin film, as shown in Fig. 4(e)[46]. Park et al. fabricated highly transparent (111)-oriented NiOx thin films using the PLD technique[47]. The resulting PSCs showed good hole extraction efficiency and suppressed electron leakage current. The resulting device showed an extremely high fill factor of 81.3%.
2.2.6 Atomic layer deposition (ALD)
ALD technology is based on self-limiting chemical reactions that involve alternating cycles of different chemical vapor deposition (CVD) processes, depositing a single atom layer with each cycle. For NiOx thin films, typical reactions involve a nickel source (such as nickel organic compounds) and an oxidant source (such as ozone or water vapor). This method enables precise atomic-level control over the film thickness, allowing for the formation of uniform films on substrates with complex shapes. The resulting films showed high purity and good crystallinity. However, the deposition rate is relatively slow, necessitating meticulous control over gas flow rates and reaction durations, and placing stringent demands on equipment specifications. Fig. 4(f) illustrates the process of depositing NiOx thin films by ALD, depicting a schematic of the change in the oxidation state of Ni and the presence of oxygen vacancies in the film[48]. Park et al. used different oxidants to deposit NiOx thin films through the ALD technique, demonstrating the correlation between the oxidizing power of the oxidant in the ALD process and the surface oxidation state of the ALD-NiOx film[49]. PSCs based on ALD-NiOx film achieved a PCE of 19%.
Compared to the other deposition methods, NiOx thin films prepared by magnetron sputtering have the following unique advantages, such as large area uniformity, high denseness, and precise and controllable thickness. Therefore, magnetron sputtering is considered to be one of the most preferred technologies for the mass fabrication of large-scale NiOx thin films, as well as numerous other functional layers including TiO2, SnO2, etc.
3. Regulating the properties of NiOx thin film by magnetron sputtering
Magnetron sputtering is now widely used in NiOx thin film fabrication, where the physical properties of NiOx thin films can be easily tuned. During the sputtering process, parameters such as the sputtering pressure, the sputtering angle formed between the sputtering target and the substrate, substrate temperature, sputtering atmosphere, sputtering power, etc. can all affect the physical and chemical properties, as well as the crystal morphology of the NiOx thin films. This leads to significant differences in the performance of the resulting PSCs. Therefore, selecting the appropriate deposition parameters in sputtering NiOx thin films is crucial for high-performance perovskite solar cells.
3.1 The sputtering pressure
The sputtering pressure determines the content of the sputtering gas in the chamber. As shown in Fig. 5(a), when the pressure is relatively low, the collision frequency between the product atoms and the sputtering gas ions is lower, leading to a longer mean free path. Therefore, atoms will arrive on the substrate with high kinetic energy, forming a dense thin film. On the contrary, when the sputtering pressure increases, there are more sputtering gas ions in the chamber. This increases collisions and leads to a lower deposition rate[50]. The sputtering pressure also affects the grain size of the resulting film. Under low-pressure conditions, fewer numbers of plasma are produced, leading to the aggregation or coarsening of uncharged nuclei in the gas phase and the formation of larger clusters, which increases the roughness of the film[51]. At high pressures, the particles are charged with Coulomb forces that inhibit aggregation growth, reducing the roughness, as shown in Fig. 5(b). Therefore, the sputtering rate and grain size can be increased by varying the chamber pressure leading to the reduced scattering of charge carriers at grain boundaries and improved carrier mobility.
The transmittance of the NiOx thin film can also be modulated by adjusting the chamber pressure. As shown in Fig. 5(c), the transmittance of the NiOx film in the spectral range of 400 to 800 nm gradually increases with the increase of the chamber pressure, which helps to minimize optical losses while enhancing the light absorption capacity of perovskite materials. This change of transmittance can be attributed to the formation of Ni3+ ions, which act as color centers in the NiOx film. A higher ratio of Ni3+ to Ni2+ results in a lower average visible light transmittance. Fig. 5(d) demonstrates the variation of the Ni3+/Ni2+ ratio in the film at different pressures. However, this transformation promotes the transfer of holes from Ni3+ to Ni2+ sites, thereby improving the conductivity of NiOx thin films, as shown in Fig. 5(d)[52−54]. The Ni3+/Ni2+ ratio could also be regulated by tunning the oxygen pressure during the sputtering process. With a high oxygen partial pressure, more Ni2+ would be oxidized to Ni3+, thereby increasing the ratio of Ni3+ to Ni2+ and the film conductivity. Therefore, the content of Ni3+ needs to be controlled to balance the transmittance and conductivity of the film according to different needs.
3.2 Annealing temperature
The substrate annealing temperature is also a key factor affecting the optoelectronic properties of sputtered NiOx thin film. As the substrate temperature gradually increases, atoms arriving on the substrate absorb more energy. This results in a higher migration rate of atoms on the surface of the thin film, thereby promoting their rearrangement and the formation of a more orderly crystal structure and larger grain size. Specifically, as the substrate temperature rises, the originally amorphous film will gradually be converted to a polycrystalline state, along with changes in morphology. Reddy et al. conducted an in-depth study of the crystallographic orientation characteristics of Cu-doped NiOx thin films at different substrate temperatures[55]. At room temperature (RT), the film exhibited a broader XRD diffraction peak, indicating its relatively lower degree of crystallinity. When the substrate temperature was below 200 °C, the NiOx thin film mainly exhibited an orientation of the (220) crystal plane. However, as the temperature further increased to 300 °C, the adsorbed atoms gained additional energy, leading to a significant change in the crystallographic orientation of the film, from the (220) crystal plane to the more favorable (111) crystal plane, and the grain size also increased accordingly, as shown in Fig. 5(e). This has an important impact on the electrical conductivity and optoelectronic performance of the thin film.
The annealing temperature has a significant impact on the carrier mobility of NiOx thin films, primarily due to the grain boundary scattering mechanism. Generally, the mobility increases as the carrier concentration or grain size increases. However, when the carrier concentration exceeds 1020 cm−3, ionized impurity scattering becomes the dominant mechanism, which leads to a decrease in mobility as the carrier concentration further increases. Specifically, an appropriate annealing temperature helps to reduce grain boundary scattering, allowing carriers to move more freely and improving mobility. But when the carrier concentration is too high, even with larger grain sizes, the impact of ionized impurity scattering becomes significant enough to dominate the scattering process of carriers, negating the positive effects of increased grain size and resulting in reduced mobility[56]. Therefore, optimizing the annealing temperature and controlling the carrier concentration is crucial for achieving high carrier mobility in thin film materials.
High-temperature annealing could also promote the oxidization of Ni2+ into Ni3+. Zheng et al. systematically studied the impact of annealing temperature on the content of Ni3+ existence forms (NiOOH, Ni2O3) in the film[57]. After post-annealing at temperatures above 250 °C, Ni(OH)2 is oxidized and dehydroxylated to form NiOOH, which then dehydrates to create the high-temperature stable oxide Ni2O3. This effectively increased the carrier density of the film (from 3.84 × 1011 to 3.79 × 1013 cm−3) and reduced its resistivity (from 1.95 × 103 to 4.71 × 102 Ω·cm).
3.3 O2 partial pressure
NiOx with an ideal stoichiometric ratio is not electrically conductive and the addition of oxygen to the sputtering atmosphere directly affects the Ni/O ratio in the film. NiOx films are usually prepared by either direct sputtering of NiOx targets or sputtering of Ni metal followed by oxidation in an oxygen atmosphere. The deposition quality of sputtered films is influenced by the ratio of O2 to Ar gas flow. Hwang et al. investigated the impact of varying oxygen incorporation levels from 0% to 100% on the Ni3+/Ni2+ ratio, observing an increase from 1.32 to 2.63, as shown in Fig. 5(f). This enhancement in the Ni3+/Ni2+ ratio effectively boosted the hole carrier concentration within the NiOx thin film. The presence of nickel and oxygen vacancies significantly influences the electrical characteristics of the material. By regulating the oxygen ratio between 66%−100%, a carrier concentration within the semiconductor-optimal range of 1015 to 1018 cm−3 can be achieved. Furthermore, concentrations exceeding 1018 cm−3 are found to be appropriate for the fabrication of transparent conductive oxides (TCO).
Oxygen content also affects the crystallinity and grain size of the films. Hwang et al. observed a slight increase in grain size when the oxygen partial pressure was increased[58]. In contrast, Tuna et al. demonstrated that the preferred orientation and peak strength of sputtered NiOx polycrystalline films varied with oxygen content[59]. By adjusting the oxygen partial pressure to a range of 40% to 50% during the sputtering process with a nickel metal target, the grain size could be significantly reduced, from 15.03 to 4.62 nm.
3.4 P-type doping
NiOx, as a wide bandgap semiconductor, has a relatively low intrinsic electrical conductivity. Doping can increase the concentration of charge carriers (electrons or holes), thereby improving the material's conductivity. Magnetron sputtering doping typically involves two methods: co-sputtering and target doping. Co-sputtering introduces doping elements directly during the sputtering process by controlling the composition of the sputtering targets. By adjusting the sputtering power, the relative content of doping can be precisely controlled to avoid adverse effects of excessive doping, such as lattice distortion or increased carrier scattering. In contrast, target doping involves directly incorporating doping elements into the target material for sputtering.
The common type of sputtering doping usually involves the doping of cations with the same charge as Ni2+, such as copper (Cu2+), zinc (Zn2+), etc. These elements can replace the position of Ni2+, thus affecting the concentration of charge carriers and conductivity. Huang et al. investigated the effect of Cu doping on the properties of NiOx thin films. When the doping level was low, the lattice symmetry remained unchanged, generating negatively charged defects, which improved the hole mobility of the NiOx film. Through EDS research, the optimal doping amount of Cu in NiOx was determined to be 5.7 at%. After Cu doping, the valence band of the NiOx film becomes more aligned with that of the perovskite, as shown in Fig. 5(g)[60]. They also explored the doping of NiOx films with cobalt (Co), a modification that altered the optical bandgap and Fermi level of the film, resulting in a 25% improvement in the device's PCE[61]. In addition, lithium (Li), magnesium (Mg), and silver (Ag) have also been proven to be excellent p-type doping elements[20, 62, 63]. The choice of doping elements should be carefully considered for their potential effects on the chemical and thermal stability of the material, particularly under high-temperature conditions or within specific atmospheric environments.
Doping with cations of different valences, such as aluminum (Al3+) or lithium (Li+), may lead to charge compensation mechanisms, such as the creation of additional oxygen vacancies or Ni interstitials. As shown in Figs. 5(h) and 5(i), Zhang et al. used Al-doping in NiOx to increase the electrical conductivity of the thin film from 0.51 to 2.56 cm2·V−1·s−1, which significantly enhanced the current of the device[64].
4. NiOx as the HTL for efficient PSCs
Research on planar PSCs based on inverted p−i−n structure is flourishing. NiOx, with its ideal bandgap position, excellent hole extraction efficiency, superior stability, and great potential for large-scale production at low temperatures, has been regarded as the most promising HTL for fabricating high-performance PSCs. Table 1 and Fig. 6(a) summarized the structure and efficiency data of PSC devices that used sputtered NiOx as HTL. As shown, NiOx HTL with excellent performance, was widely used in various perovskite material systems. In recent years, the PCE with NiOx HTL has increased rapidly. As shown in Fig. 6(b), starting from 12% in 2016, the efficiency of small-area inverted PSCs with sputtered NiOx films increased to 23% by tuning the sputtering parameters (pressure, oxygen partial pressure, temperature, etc.), doping various dopants (e.g., Li, Cu, and Mg) into the NiOx films, and modifying the NiOx films with the introduction of different organic or inorganic interface modifying materials.
| Device structure | Sputtering parameters | Jsc (mA/cm2) |
Voc (V) |
PCE (%) |
Publication date |
Ref. |
| FTO/NiOx/Cs0.17FA0.83Pb(I0.8Br0.2)3/C60/BCP/Cu | Ar/14 sccm/1.32 W/cm2 | 18.79 | 1.05 | 15.71 | 2020 | [52] |
| ITO/NiOx/MeO-2PACz/MAPbI3/AZO/Ag | 3.5 Pa | 20.1 | 1.11 | 16.25 | 2022 | [65] |
| FTO/NiOx/MAPbI3/PCBM/BCP/Ag | Ar&O2/1.3 Pa, 100 W | 20.57 | 1.05 | 16.29 | 2018 | [66] |
| FTO/Cu:NiOx/CH3NH3PbI3/ZnO/Ag | Ar : O2 = 35 : 5 sccm/1.5 Pa, Ni : Cu = 300 : 15 W, | 23.05 | 1.03 | 16.51 | 2020 | [67] |
| ITO/NiOx /MAPbI3/PCBM/BCP/Ag | Ar/20 sccm/0.4 Pa, 1.97 W/cm2, |
20.65 | 1.07 | 17.6 | 2018 | [68] |
| ITO/Mg:NiOx/MAPbI3/PCBM/ZnMgO/Al | Ar&O2/500 Pa, 80 W | / | / | 18.5 | 2017 | [20] |
| ITO/NiOx/MA0.65FA0.35PbI3/PCBM/BCP/Ag | / | / | / | 18.7 | 2020 | [57] |
| ITO/NiOx/spiro-TTB/MAPbI3/PCBM/BCP/Ag | 2.7 Pa, 20 W | 22.3 | 1.08 | 19.5 | 2023 | [54] |
| FTO/NiOx/NiyN/MAPbI3/PCBM/BCP/Ag | Ar : N2 = 15 : 45 sccm/0.4 Pa | / | / | 19.8 | 2022 | [69] |
| ITO/NiOx/MeO-2PACz/ Cs0.05(FA0.83MA0.17)0.95Pb(I0.82Br0.18)3 /C60/BCP/Ag |
Ar : O2 = 20 : 2.2 sccm/0.4 Pa, 80 W | 22.3 | 1.10 | 19.9 | 2021 | [70] |
| ITO/NiOx/N719/ Cs0.05MA0.15FA0.80Pb(I0.85Br0.15)3/C60/BCP/Ag |
/ | 23.2 | 1.13 | 20.3 | 2021 | [71] |
| ITO/NiOx/Me-4PACz/ Cs0.2FA0.8Pb(I0.94Br0.053)3/LiF/C60/BCP/Cu |
/ | 24.1 | 1.14 | 21.8 | 2024 | [72] |
| ITO/NiOx/ Cs0.05MA0.16FA0.79Pb(Br0.16I0.84)3/PCBM/BCP/Ag |
Ar/100 sccm /0.37 Pa, 200 W | 24.12 | 1.19 | 22.72 | 2024 | [73] |
| ITO/NiOx/Me-4PACz/TEACl/ Cs0.1FA0.9PbI2.855Br0.145/TEACl/LiF/C60/BCP/Cu |
/ | 24.4 | 1.16 | 23.0 | 2023 | [74] |
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NiOx possesses high chemical stability, thermal stability, and photostability. It does not easily react with moisture and oxygen in the environment, which helps to maintain the long-term performance of the HTL and reduce the degradation of cell performance caused by chemical degradation. NiOx can maintain its structure and performance stability within a wide temperature range. Fig. 6(c) demonstrates the long-term stability of NiOx and MeO-4PADBC-based PSCs at different temperatures. MeO-4PADBC-based PSCs degraded to 65% of the starting level after 1200 h of operation at 65 °C. In contrast, PSCs based on NiOx exhibited much enhanced thermal stability, allowing PSCs to operate at 65 °C for 1200 h while still maintaining more than 80% of their initial efficiency[18].
Large-area deposition of NiOx by magnetron sputtering is particularly suitable for the mass production manufacturing of PSCs. As shown in Fig. 6(d), PCEs of 19.7%, 17.0%, and 15.5% on large-area modules of 4, 16, and 100 cm2, have been achieved based on NiOx HTL, respectively[72]. The current efficiency of PSC preparation using magnetron sputtering nickel oxide is still low. Stacking is an effective way to improve efficiency, and researchers have employed tandem device structures with silicon-perovskite or perovskite-perovskite combinations. Currently, the efficiency of crystalline silicon-perovskite tandem solar cells has reached 34.6%, which is much higher than the single-junction perovskite solar cells[5]. In these structures, the NiOx-based connecting layer of tandem PSC, satisfied both the requirements for carrier transport and high transmittance, ensuring effective charge transfer and greater optical utilization, as shown in Fig. 6(e). Mao et al. used NiOx as the HTL to achieve a certified efficiency of 28.84% for perovskite/silicon tandem cells on an area of 1.2 cm²[75].
Magnetron sputtering technology has demonstrated great potential for applications in the fabrication of flexible electronic components such as flexible displays and wearable devices, as shown in Fig. 6(f)[76]. To prevent damage to the fragile substrate, the process requires thin film deposition at low temperatures. Magnetron sputtering enables the deposition of high-quality films at low temperatures. This not only protects the flexible substrate, but also significantly improves the life and stability of the electronic device. While the performance of current NiOx-based perovskite solar cells still falls behind the highest standards of the most efficient devices, they hold significant potential for advancement. With continued optimization, it is anticipated that a significant leap in device performance can be realized, aligning with the requirements for commercial-scale manufacturing.
5. Interface issues between NiOx and perovskite
Despite the significant advantages in reproducibility and stability when using NiOx prepared by magnetron sputtering as an HTL, there are still challenges and difficulties on the road to the practical application and commercialization of NiOx-based PSCs. Currently, the highest PCE achieved by magnetron sputtering NiOx is 23%, which is significantly lower than that of the best-performing perovskite devices. This is mainly due to the energy level mismatch between NiOx and perovskite layers, as well as the unavoidable interfacial reaction problem. These challenges not only limit the further improvement of the performance of PSCs but also pose an obstacle to their mass production and marketing.
5.1 Misalignment of energy levels
NiOx is a wide bandgap p-type semiconductor material, with a deep VBM of ~5.6 eV, which could not perfectly match that of the perovskite. Energy level mismatch in PSCs leads to carrier recombination between the transport layer and perovskite, reducing effective charge collection. Open circuit voltage mismatch with quasi-fermi energy levels leads to voltage loss and poor device performance. There is a certain energy level barrier between NiOx and perovskite, which hinders further enhancement of device performance. This issue could be overcome by adjusting the sputtering parameters of NiOx, such as sputtering power, atmosphere, and post-treatment steps. Niu et al. lowered the Fermi level of NiOx by adjusting the oxygen doping in NiOx, making the energy level of NiOx more compatible with perovskite[77]. The introduction of O2 could suppress oxygen vacancies. This improved the open circuit voltage of the device by 0.09 V. Li et al. altered the energy level of NiOx thin films by changing the doping amount of Mg[20]. This led to a better energy level alignment between NiOx and perovskite, with an improved fill factor and short-circuit current. The resulting PSC showed an enhanced PCE of 18.5%, with good uniformity over an area of 100 square centimeters. Xia et al. co-doped NiOx thin films with Li and Ag[78]. This co-doping method could better align the energy levels of NiOx with perovskite and endow the film with higher conductivity and hole mobility. The PCE reached 19.24%, after modification with excellent stability, which remained at 95% of the initial efficiency even after 30 days without packaging.
Utilizing an interfacial modification layer is another effective strategy to mitigate energy level barriers. Li et al. anchored MeO-4PADBC onto NiOx to minimize the energy level mismatch at the interface[18]. As illustrated in Fig. 7(a), the energy level discrepancies between NiOx and perovskites with three distinct bandgaps were all optimized following the MeO-4PADBC interface modification. Consequently, the device's PCE was improved to 25.6% at a bandgap of 1.53 eV, as depicted in Fig. 7(b).
5.2 Unfavorable interfacial reactions
The interfacial chemical reactions between NiOx and the perovskite layer may also lead to adverse redox reactions, which could generate harmful by-products. These by-products can capture charge carriers, leading to severe non-radiative recombination and a decrease in efficiency. In addition, these interfacial reactions may also induce the degradation of materials, posing a threat to the long-term stability of the PSCs. Wang et al. demonstrated that the interfacial reactions between NiOx and the perovskite layer were mainly attributed to the presence of Ni≥3+ point defects in NiOx[73]. These highly oxidized nickel ions possess high chemical reactivity and are prone to chemical reactions. Ni≥3+ defect sites may also exhibit Bronsted acid-base characteristics, readily undergoing chemical reactions with A-site cations in the perovskite structure. These interactions are poised to set off a chain of transformations, including the deprotonation of A-site cations, the conversion of iodide ions (I−) into iodine molecules (I2), and the formation of Ni≥2+OxH complexes at the interface. As shown in Fig. 7(c), these processes underscore the intricate chemical reactions taking place at the perovskite interface. These complexes form a high-energy potential barrier at the interface that severely hinders the effective extraction of holes, affecting the hole transport efficiency of the solar cell. Chemical reactions at the interface not only hinder the effective extraction of holes, but may also introduce additional defects in the perovskite layer. These defects may become non-radiative recombination centers and further reduce the efficiency of PSCs. They introduced L-ascorbic acid (L-AA) as an interfacial blocking layer to suppress the interfacial reactions, and the protons from the ionization of the adjacent enol hydroxyl groups suppressed the deprotonation of the organic cations[73]. Mann et al. introduced Zn3N2 and confirmed that the interfacial proton redox process can be suppressed by increasing the ratio of Ni2O3 (Ni3+) to NiO (Ni2+) while reducing the content of NiOOH (Ni≥3+)[79]. The efficiency enhancement can also be achieved by incorporating an excess of A-site ion into the perovskite precursor solution. The dark J−V curves and bright J−V curves of PSCs with excess A-site ion after perovskite degradation are shown in Fig. 7(d)[80]. These chemical reactions may also disrupt the chemical stability of the interface, making the device more susceptible to environmental factors such as humidity, O2, and temperature.
Therefore, interfacial modification is pivotal for refining energy level alignment, facilitating charge transfer, and mitigating trap-related issues, thereby enhancing the device's efficiency and stability.
6. Interface modification of between NiOx and perovskite
To alleviate the problems of energy level mismatch and interfacial reactions, the NiOx preparation parameters should be adjusted, such as increasing the sputtering gas pressure, reducing the oxygen content in the atmosphere, and lowering the annealing temperature, in order to decrease the concentration of Ni≥3+ point defects. For instance, lowering the annealing temperature can effectively reduce the oxidation of Ni2+ to Ni≥3+ within the film and mitigate the formation of defect states[57]. Introducing an interfacial modification layer with improved chemical stability and interfacial compatibility between NiOx and the perovskite layer to isolate the Ni≥3+ point defects in NiOx from the perovskite layer, could also minimize direct chemical interactions. Besides, incorporating specific chemical substances into the perovskite precursor solution to passivate the active sites on the NiOx surface could reduce the potential for interfacial reactions. Up to now, incorporating interfacial modification materials (e.g. SAMs, CsBr, N719) has been the predominant strategy for enhancing chemical stability at the interface.
6.1 Self-assembled monolayers (SAMs) modification
SAMs modification technology is a strategy for fine chemical modification of material surfaces. This single-molecule material usually consists of three parts, the hole-transporting aromatic amine terminal group, the anchoring group adsorbed on the substrate, and the bridging group connecting the two. By carefully designing the molecular structure and functional groups of SAMs, better anchoring and faster carrier migration can be achieved[18].
The primary molecular anchoring groups for SAMs are phosphonic acid and silane groups, as illustrated in Fig. 8(a). These groups can be anchored to NiOx, effectively mitigating surface defects and trap states, thereby minimizing carrier recombination at the interface. For example, Tang et al. used SAMs with trimethoxysilane groups for tridentate anchoring on indium tin oxide substrates, which improved the operational stability of PCE[81]. In addition, the uniform protective film formed by the SAMs on the NiOx surface effectively reduced the direct contact between NiOx and the perovskite layer. This approach could avoid unfavorable chemical reactions caused by Ni≥3+ point defects and improve the stability of the interface, leading to enhanced device efficiency and stability. By modifying NiOx with these SAMs, PSCs were able to maintain 98.9% of their initial PCE after 1000 h of wet heat testing. The high density of Ni vacancies on the NiOx surface increases the O dangling bonds and reacts with perovskite to affect the properties of PSCs. Cao et al. introduced phosphorylcholine chloride into Me-4PACz. The terminal phosphate groups cause the molecules to align vertically on the NiOx surface. The presence of Cl− causes the NiOx lattice to expand and inhibits the formation of Ni3+ defects, as shown in Fig. 8(b)[82]. Sun et al. introduced [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) between the NiOx and perovskite layers[70]. It formed a stronger tridentate anchorage on the NiOx surface, serving as an interfacial layer that prevented reactions between NiOx and perovskite. At the same time, NiOx helped to prevent the formation of pinholes and molecular aggregation that might be induced by the ultra-thin layer of MeO-2PACz. The contact potential difference (CPD) data obtained by scanning Kelvin probe microscopy (SKPM) showed that the presence of the NiOx layer could promote the adsorption of MeO-2PACz molecules as shown in Fig. 8(c). In addition, the CPD values of ITO/NiOx/MeO-2PACz are lower than those of the pure ITO/MeO-2PACz interface. Suggesting that the introduction of NiOx improves the work function (WF) of the MeO-2PACz molecule. This was further confirmed by Alghamdi et al., as shown in Fig. 8(d)[65]. This innovative dual-hole interface engineering not only optimized the energy level alignment, but also greatly enhanced the stability of the interface, leading to the enhancement of PCE from 11.9% to 17.2%. As shown in Fig. 8(e), Jin et al. found that after treatment with NaIO4, the hydroxyl density of the NiOx film increased, making it easier to form P−O covalent bonds, which significantly increased the coverage of SAMs[83]. The increased hydroxyl density and the formation of P−O covalent bonds further enhance the coverage of SAMs on the surface of NiOx, contributing to the improvement of interfacial stability and device performance.
6.2 Alkali metal halide modification
The passivation of PSCs with alkali metal halides (AMHs) utilizes halides of alkali metals such as sodium (Na), potassium (K), and cesium (Cs) to blanket the perovskite material's surface, forming a protective passivation layer. This layer not only insulates the perovskite from detrimental environmental influences but also effectively mitigates internal defects within the material, thereby optimizing the overall performance of the solar cell.
Modification of NiOx using AMHs is an effective method to enhance the crystal ordering of perovskite films and thus reduce the defect density. Chen et al. showed that the introduction of a NaCl buffer layer on NiOx helps to form a more ordered interface between NiOx and perovskite, effectively reducing the formation of defects on the surface of perovskite films, as shown in Fig. 9(a)[84]. Theoretical calculations and experimental data analysis have demonstrated that the NaCl-treated samples had higher built-in potentials and reduced densities of surface and bulk phase trap states. Not only the PCE of the device is improved, but also the diffusion of ions is effectively suppressed. Devices based on NaCl-treated NiOx thin-film achieved a significant increase in the open-circuit voltage (Voc) from 1.07 to 1.15 eV, with a PCE close to 21%. After 150 days of air aging, the modified device still maintained 94.9% of its initial efficiency, showing excellent long-term stability.
AMHs can also enhance the quality of perovskite crystals. Pant et al. modified the NiOx/CH3NH3PbI3 interface with various AMHs and found that the incorporation of a CsBr interlayer suppressed the reaction between NiOx and A-site ions, reducing the residual PbI2, as shown in Fig. 9(b)[85]. CsBr also reduced tail states in NiOx and passivated surface defects. This ultimately enhanced the overall charge collection performance at the NiOx/CH3NH3PbI3 interface, improving device efficiency and long-term operational stability. Zhang et al. demonstrated that the insertion of a CsBr buffer layer could alleviate the interfacial stress caused by lattice mismatch, slightly promoting crystal growth and increasing the VBM of NiOx from 5.41 to 5.84 eV (Figs. 9(c) and 9(d))[86].
6.3 Other modification materials
In addition to SAMs and AMHs, many other passivation (e.g. N719, AlOx, NiyN) materials had also been used to modify the NiOx films. They could optimize the energy level alignment and charge transport, so as to improve the performance and stability of the devices.
Zhumagali et al. employed an interfacial passivation strategy using the organic metal dye molecule N719 with carboxyl groups. As illustrated by the electrostatic potential (ESP) diagram of the N719 molecule in Fig. 9(e), where the carboxyl, thiocyanate, and carboxylate groups on the side chains are highly electron-rich. Consequently, these groups could provide strong intermolecular binding, especially to the electron-deficient interface of NiOx and perovskite. The N719 dye has been widely used as an absorbent material in the mesostructured structure of dye-sensitized solar cells, which was adsorbed on the surface of metal oxides. The presence of both Lewis basic and acidic groups in a single molecule of N719 enables it to coordinate simultaneously with the Ni≥3+ and O-dangling bonds on the surface of the NiOx film. With favorable electrostatic potential at both interfaces, this dye molecule effectively assists in the extraction of charges from the perovskite. The WF and VBM of NiOx were adjusted to align with the energy levels of the perovskite. This modification resulted in an average increase of 70 mV in the VOC of perovskite solar cell devices. Its application in silicon/perovskite tandem solar cells increased the efficiency from 23.5% to 26.2%[71]. Similar to N719, many other effective organic molecular interfacial materials, such as Spiro-TTB, VNPB, PTAA, MeO-4PADBC, etc. had been reported to effectively modify NiOx[18, 54, 87, 88].
AlOx is also an effective inorganic interfacial modification material, which can suppress the interfacial reaction between NiOx and perovskite. For instance, Yang et al. employed the ALD technique to deposit a 0.2−0.5 nm thick AlOx film on NiOx. By harnessing the strong acidity of AlOx, the deprotonation process was suppressed, greatly reducing the redox reactions between NiOx and perovskite. The presence of aluminum vacancies and oxygen vacancies intensified the effect of negative fixed charges, increased the electrostatic potential, and diminished interfacial recombination losses, as shown in Fig. 9(f). Ultimately, devices passivated with AlOx exhibited exceptional stability, with negligible performance loss in terms of PCE after 2000 h of MPP tracking under one sun illumination at 85 °C[89].
In addition to the aforementioned passivation materials, Itzhak et al. applied a NiyN passivation layer to the NiOx surface through sputtering with a Ni target in a blended atmosphere of Ar and N2. This method is non-destructive to the NiOx layer and offers a dual-layer safeguard during the device fabrication process. Firstly, the NiyN layer protected the NiOx from being reduced to Ni2+ by Ar plasma, which compromises the conductivity of NiOx. Secondly, it passivated the interface between NiOx and the perovskite, preventing the degradation of the perovskite and maintaining the stability of the device. This dual effect increased the efficiency of the perovskite solar cell device from 17.4% to 19.8%[69].
7. Conclusions and outlook
The advent of perovskite solar cells (PSCs) has revolutionized the photovoltaic industry with their remarkable power conversion efficiencies and potential for low-cost manufacturing. This review has provided an in-depth analysis of the role of nickel oxide (NiOx) as a hole transport layer (HTL) in PSCs, particularly focusing on NiOx films prepared by magnetron sputtering. The comprehensive evaluation of NiOx spans its synthesis, structural and optoelectronic properties, and its application in PSCs, offering valuable insights into its role in enhancing device performance.
NiOx, with its high carrier mobility, excellent stability, and suitability for large-scale production, has emerged as a promising candidate for HTLs in PSCs. The sputtering process allows for precise control over film thickness and uniformity, which are critical for achieving high device efficiencies. However, despite the significant advancements, several challenges need to be addressed to fully harness the potential of sputtered NiOx in PSCs. (1) Interfacial issues: One of the primary challenges with magnetron sputtering NiOx is the interfacial compatibility between NiOx and perovskite layers, including energy level misalignment and interfacial reactions. Poor interface quality can degrade the performance and stability of PSCs. To address this, more effective modification strategies and materials should be explored to enhance the interface's stability and charge transfer characteristics. (2) Process control: The magnetron sputtering process necessitates meticulous management of parameters, including pressure, temperature, and deposition rate, to ensure the uniformity and quality of the NiOx films. Minor deviations in these parameters can lead to significant variations in film characteristics. The influence of these parameters on film properties is still not yet fully understood, future studies can leverage a suite of advanced characterization techniques to systematically probe these effects. By doing so, we can refine the sputtering process, achieving exquisite control over film properties and, consequently, enhancing the power conversion efficiency and stability of PSCs. (3) Scalability: While magnetron sputtering offers a route to large-area deposition, the scalability of the process and the translation from laboratory to industrial production present significant challenges. Overcoming these challenges is essential for the widespread adoption of NiOx-based PSCs.
Looking ahead, the future of NiOx in PSCs appears promising. By focusing on interfacial compatibility, process optimization, and scalability, the ongoing research and development efforts are expected to address the current challenges and pave the way for more efficient, stable, and scalable perovskite solar cell technology.
Acknowledgments
This research was financially supported by the Natural Science Foundation of China (62288102, 22379067, T2441002, 6220514, and 5230226), the National Key Research and Development Program of China (2023YFB4204500), the Jiangsu Provincial Departments of Science and Technology (BE2022023, BK20220010, and BZ2023060), and the Open Project Program of Wuhan National Laboratory for Optoelectronics (2021WNLOKF003).
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