Post-treatment-Free Solution-Processed Non-stoichiometric NiO(x) Nanoparticles for Efficient Hole-Transport Layers of Organic Optoelectronic Devices

A regulation strategy of self-assembly molecules for achieving efficient inverted perovskite solar cells

Self-assembled monolayers (SAMs) have been successfully employed to enhance the efficiency of inverted perovskite solar cells (PSCs) and perovskite/silicon tandem solar cells due to their facile low-temperature processing and superior device performance. Nevertheless, depositing uniform and dense SAMs with high surface coverage on metal oxide substrates remains a critical challenge. In this work, we propose a holistic strategy to construct composite hole transport layers (HTLs) by co-adsorbing mixed SAMs (MeO-2PACz and 2PACz) onto the surface of the H2O2-modified NiOx layer. The results demonstrate that the conductivity of the NiOx bulk phase is enhanced due to the H2O2 modification, thereby facilitating carrier transport. Furthermore, the hydroxyl-rich NiOx surface promotes uniform and dense adsorption of mixed SAM molecules while enhancing their anchoring stability. In addition, the energy level alignment at the interface is improved due to the utilization of mixed SAMs in an optimized ratio. Furthermore, the perovskite film crystal growth is facilitated by the uniform and dense composite HTLs. As a result, the power conversion efficiency of PSCs based on composite HTLs is boosted from 22.26% to 23.16%, along with enhanced operational stability. This work highlights the importance of designing and constructing NiOx/SAM composite HTLs as an effective strategy for enhancing both the performance and stability of inverted PSCs.

Electron Deficient Ir-O Bonds Promote Heterogeneous Ir-Catalyzed Anti-Markovnikov Hydroboration of Alkenes under Mild Neat Conditions

Tuning electronic characteristics of metal-ligand bonds based on reaction pathways to achieve efficient catalytic processes has been widely studied and proven to be feasible in homogeneous catalysis, but it is scarcely investigated in heterogeneous catalysis. Herein, we demonstrate the regulation of the electronic configuration of Ir-O bonds in an Ir single-atom catalyst according to the borane activation mechanism. Ir-O bonds in Ir1/Ni(OH)x are found to be more electron-poor than those in Ir1/NiOx. Despite the mild solvent-free conditions and ambient temperature, Ir1/Ni(OH)x exhibits outstanding performance for the hydroboration of alkenes, furnishing the desired alkylboronic esters with a turnover frequency value of ≤3060 h-1 and 99% anti-Markovnikov selectivity, which is significantly better than that of Ir1/NiOx (42 h-1). It is further proven that the more electron-poor Ir-O bonds as active centers are more oxidative and so benefit the activation of the H-B bond in the reductive pinacolborane.

Boosting Cu─In─Zn─S-based Quantum-Dot Light-Emitting Diodes Enabled by Engineering Cu─NiOx/PEDOT:PSS Bilayered Hole-Injection Layer

The imbalance of charge injection is considered to be a major factor that limits the device performance of cadmium-free quantum-dot light-emitting diodes (QLEDs). In this work, high-performance cadmium-free Cu─In─Zn─S(CIZS)-based QLEDs are designed and fabricated through tailoring interfacial energy level alignment and improving the balance of charge injection. This is achieved by introducing a bilayered hole-injection layer (HIL) of Cu-doped NiOx (Cu─NiOx)/Poly(3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT:PSS). High-quality Cu─NiOx film is prepared through a novel and straightforward sol-gel procedure. Multiple experimental characterizations and theoretical calculations show that the incorporation of Cu2+ ions can regulate the energy level structure of NiOx and enhance the hole mobility. The state-of-art CIZS-based QLEDs with Cu─NiOx/PEDOT:PSS bilayered HIL exhibit the maximum external quantum efficiency of 6.04% and half-life time of 48 min, which is 1.3 times and four times of the device with only PEDOT:PSS HIL. The work provides a new pathway for developing high-performance cadmium-free QLEDs.

Evaporated Nickel Oxide Films with Slow Annealing and Interface Modification for Perovskite Solar Cells

Electron-beam-evaporated nickel oxide (NiOx) films are known for their high quality, precise control, and suitability for complex structures in perovskite (PVK) solar cells (PSCs). However, untreated NiOx films have inherent challenges, such as surface defects, relatively low intrinsic conductivity, and shallow valence band maximum, which seriously restrict the efficiency and stability of the devices. To address these challenges, we employ a dual coordination optimization strategy. The strategy includes low heating rate annealing of NiOx films and using an aminoguanidine nitrate spin coating process on the surfaces of NiOx films to strategically modify NiOx films itself and the interface of NiOx/PVK. Under the synergistic effect of this dual optimization method, the quality of the films is significantly improved and its p-type characteristics are enhanced. At the same time, the interface defects and energy level alignment of the films are effectively improved, and the charge extraction ability at the interface is improved. The combined treatment significantly improved the efficiency of inverted PSCs, from 17.85% to 20.31%, and enhanced device stability under various conditions. This innovative dual-coordinated optimization strategy provides a clear and effective framework for improving the performance of NiOx films and inverted PSCs.

Intercalated PtCo Electrocatalyst of Vanadium Metal Oxide Increases Charge Density to Facilitate Hydrogen Evolution

Efforts to develop high-performance electrocatalysts for the hydrogen evolution reaction (HER) are of utmost importance in ensuring sustainable hydrogen production. The controllable fabrication of inexpensive, durable, and high-efficient HER catalysts still remains a great challenge. Herein, we introduce a universal strategy aiming to achieve rapid synthesis of highly active hydrogen evolution catalysts using a controllable hydrogen insertion method and solvothermal process. Hydrogen vanadium bronze HxV2O5 was obtained through controlling the ethanol reaction rate in the oxidization process of hydrogen peroxide. Subsequently, the intermetallic PtCoVO supported on two-dimensional graphitic carbon nitride (g-C3N4) nanosheets was prepared by a solvothermal method at the oil/water interface. In terms of HER performance, PtCoVO/g-C3N4 demonstrates superior characteristics compared to PtCo/g-C3N4 and PtCoV/g-C3N4. This superiority can be attributed to the notable influence of oxygen vacancies in HxV2O5 on the electrical properties of the catalyst. By adjusting the relative proportions of metal atoms in the PtCoVO/g-C3N4 nanomaterials, the PtCoVO/g-C3N4 nanocomposites show significant HER overpotential of η10 = 92 mV, a Tafel slope of 65.21 mV dec-1, and outstanding stability (a continuous test lasting 48 h). The nanoarchitecture of a g-C3N4-supported PtCoVO nanoalloy catalyst exhibits exceptional resistance to nanoparticle migration and corrosion, owing to the strong interaction between the metal nanoparticles and the g-C3N4 support. Pt, Co, and V simultaneous doping has been shown by Density Functional Theory (DFT) calculations to enhance the density of states (DOS) at the Fermi level. This augmentation leads to a higher charge density and a reduction in the adsorption energy of intermediates.

NiOx Nanoparticles Hole-Transporting Layer Regulated by Ionic Radius-Controlled Doping and Reductive Agent for Organic Solar Cells with Efficiency of 19.18

Nickel oxide (NiOx ) has garnered considerable attention as a prospective hole-transporting layer (HTL) in organic solar cells (OSCs), offering a potential solution to the stability challenges posed by traditional HTL, PEDOT:PSS, arising from acidity and hygroscopicity. Nevertheless, the lower work function (WF) of NiOx relative to donor polymers reduces charge injection efficiency in OSCs. Herein, NiOx nanoparticles are tailored through rare earth doping to optimize WF and the impact of ionic radius on their electronic properties is explored. Lanthanum (La3+ ) and yttrium (Y3+ ) ions, with larger ionic radii, are effectively doped at 1 and 3%, respectively, while scandium (Sc3+ ), with a smaller ion radius, allows enhanced 5% doping. Higher doping ratios significantly enhance WF of NiOx . A 5% Sc3+ doping raises WF to 4.99 eV from 4.77 eV for neat NiOx while maintaining high conductivity. Consequently, using 5% Sc-doped NiOx as HTL improves the power conversion efficiency (PCE) of OSCs to 17.13%, surpassing the 15.64% with the neat NiOx . Further enhancement to 18.42% is achieved by introducing the reductant catechol, outperforming the PEDOT:PSS-based devices. Additionally, when employed in a ternary blend system (D18:N3:F-BTA3), an impressive PCE of 19.18 % is realized, top-performing among reported OSCs utilizing solution-processed inorganic nanoparticles.

Quenching Detrimental Reactions and Boosting Hole Extraction via Multifunctional NiOx /Perovskite Interface Passivation for Efficient and Stable Inverted Solar Cells

Nickel oxide (NiOx ) is one of the most promising hole transport materials for inverted perovskite solar cells (PSCs). However, its application is severely restrained due to unfavorable interfacial reactions and insufficient charge carrier extraction. Herein, a multifunctional modification at the NiOx /perovskite interface is developed via introducing fluorinated ammonium salt ligand to synthetically solve the obstacles. Specifically, the interface modification can chemically convert detrimental Ni≥3+ to lower oxidation state, resulting in the elimination of interfacial redox reactions. Meanwhile, interfacial dipole is incorporated simultaneously to tune the work function of NiOx and optimize energy level alignment, which effectively promotes the charge carrier extraction. Therefore, the modified NiOx -based inverted PSCs achieve a remarkable power conversion efficiency (PCE) of 22.93%. Moreover, the unencapsulated devices obtain a significantly enhanced long-term stability, maintaining over 85% and 80% of the initial PCEs after storage in ambient air with a high relative humidity of 50-60% for 1000 h and continuous operation at maximum power point under one-sun illumination for 700 h, respectively.

Arresting Ion Migration from the ETL Increases Stability in Infrared Light Detectors Based on III-V Colloidal Quantum Dots

III-V colloidal quantum dots (CQDs) are of interest in infrared photodetection, and recent developments in CQDs synthesis and surface engineering have improved performance. Here this work investigates photodetector stability, finding that the diffusion of zinc ions from charge transport layers (CTLs) into the CQDs active layer increases trap density therein, leading to rapid and irreversible performance loss during operation. In an effort to prevent this, this work introduces organic blocking layers between the CQDs and ZnO layers; but these negatively impact device performance. The device is then, allowing to use a C60:BCP as top electron-transport layer (ETL) for good morphology and process compatibility, and selecting NiOX as the bottom hole-transport layer (HTL). The first round of NiOX -based devices show efficient light response but suffer from high leakage current and a low open-circuit voltage (Voc) due to pinholes. This work introduces poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) with NiOX NC to form a hybrid HTL, an addition that reduces pinhole formation, interfacial trap density, and bimolecular recombination, enhancing carrier harvesting. The photodetectors achieve 53% external quantum efficiency (EQE) at 970 nm at 1 V applied bias, and they maintain 95% of initial performance after 19 h of continuous illuminated operation. The photodetectors retain over 80% of performance after 80 days of shelf storage.

Advances in Hole Transport Materials for Layered Casting Solar Cells

Huge energy consumption and running out of fossil fuels has led to the advancement of renewable sources of power, including solar, wind, and tide. Among them, solar cells have been well developed with the significant achievement of silicon solar panels, which are popularly used as windows, rooftops, public lights, etc. In order to advance the application of solar cells, a flexible type is highly required, such as layered casting solar cells (LCSCs). Organic solar cells (OSCs), perovskite solar cells (PSCs), or dye-sensitive solar cells (DSSCs) are promising LCSCs for broadening the application of solar energy to many types of surfaces. LCSCs would be cost-effective, enable large-scale production, are highly efficient, and stable. Each layer of an LCSC is important for building the complete structure of a solar cell. Within the cell structure (active material, charge carrier transport layer, electrodes), hole transport layers (HTLs) play an important role in transporting holes to the anode. Recently, diverse HTLs from inorganic, organic, and organometallic materials have emerged to have a great impact on the stability, lifetime, and performance of OSC, PSC, or DSSC devices. This review summarizes the recent advances in the development of inorganic, organic, and organometallic HTLs for solar cells. Perspectives and challenges for HTL development and improvement are also highlighted.

Structured hybrid photodetectors using confined conducting polymer nanochannels

We design and fabricate hybrid organic inorganic perovskite photodetectors that utilize hole transport layer poly(3,4-ethylene dioxythiophene):poly (styrenesulfonate) PEDOT:PSS confined in alumina nanocylinders. This structural asymmetry in the device where the alumina nanopore template is partially filled with PEDOT:PSS provides features that improve certain device characteristics. The leakage component of the current in such devices is considerably suppressed, resulting in enhanced responsivity and detectivity. The funneling aspect of the photogenerated charge carrier transit ultimately leads to fast detectors as compared to conventional perovskite detectors.

A pyridine-capped quaterthiophene as an alternative to PEDOT:PSS, processable from organic solvents and without acidity, for more stable electronic devices

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a material that has become ubiquitous in the field of organic electronics. It is most commonly used as a hole transport layer (HTL) in optoelectronic devices and can be purchased commercially in various formulations with different properties. Whilst it is a most convenient material to work with, there are stability issues associated with PEDOT:PSS that are detrimental to device stability and these are due to the acidic nature of the PSS component. In this paper, we present a molecular, non-acidic alternative to PEDOT:PSS. The parent structure is composed of a quater(3,4-ethylenedioxythiophene) unit capped either side of the short chain with two pyridine units. This compound, termed (BEDOTPy)2, can be prepared chemically and electrochemically to give doped materials with a choice of counteranions. Further functionalisation via quaternisation at the nitrogen atoms allows for modification of solubility and film-forming properties. The conductivity of the doped samples can reach up to 3.75 S cm-1. The materials are non-acidic and are therefore attractive alternatives to PEDOT:PSS for device applications. We demonstrate an OLED device using the compound (BEDOTPy-EtOH-I)2PF6 as an HTL, and compare the device performance to one made with PEDOT:PSS. Due to the non-acidic nature of the molecular material, the corresponding OLED device does not show a drop in luminance over time, whereas a loss of performance is observed for the device containing PEDOT:PSS over a short period. These results are presented to introduce the parent compound (BEDOTPy)2 as an attractive alternative to PEDOT:PSS, which can be easily modified chemically to provide a plethora of potential compounds with tunable properties.

Synthesizing and formulating metal oxide nanoparticle inks for perovskite solar cells

The perovskite solar cell has commercial potential due to the low-cost of materials and manufacturing processes with cell efficiencies on par with traditional technologies. Nanomaterials have many properties that make them attractive for the perovskite devices, including low-cost inks, low temperature processing, stable material properties and good charge transport. In this feature article, the use of nanomaterials in the hole transport and electron transport layers are reviewed. Specifically, SnO2 and NiOx are the leading materials with the most promise for translation to large scale applications. The review includes a discussion of the synthesis, formulation, and processing of these nanoparticles and provides insights for their further deployment towards commercially viable perovskite solar cells.

Experimentally revealed and theoretically certified synergistic electronic interaction of V-doped CoS for facilitating the oxygen evolution reaction

Since electrocatalytic oxygen evolution (OER) is a four-electron transfer reaction with very slow kinetics, there is great competition to develop cheap, durable and efficient catalysts for oxygen evolution. A molecular model is designed for density functional theory (DFT) simulation calculations to guide the experiment, and this hypothesis is fully supported by the experimental data. Herein, regulating the composition and morphology of the bimetallic VCo and MoCo sulfide and monometallic sulfide nanoparticles (NPs) at the oil-water interface was achieved via a one-step hydrothermal method for efficient and durable OER electrocatalysts. Compared to CoS and MoCoS, the VCoS NPs show superior OER performance. By adjusting the atomic composition ratio of the VCoS nanoparticles, the VCoS NPs (1 : 2 : 1.5 mole ratio) showed a significant OER overpotential η = 255 mV (10 mA cm-2), and their outstanding stability was demonstrated after 48 h of continuous testing. The CoS and MoCoS NPs were also tested for comparison. Density functional theory (DFT) calculations showed that appropriate V doping (VCoS) can heighten the density of states (DOS) of the Fermi level, which generates more charge density and reduces the intermediate adsorption energy. This study not only provides efficient and powerful integrated catalysts, but also details a DFT calculation model guided by experiments to regulate the oxygen insertion technology, thus leading to the design of ideal materials and enabling more far-reaching applications in electrocatalysis.

Low-Temperature Chemical Bath Deposition of Conformal and Compact NiOX for Scalable and Efficient Perovskite Solar Modules

A scalable and low-cost deposition of high-quality charge transport layers and photoactive perovskite layers are the grand challenges for large-area and efficient perovskite solar modules and tandem cells. An inverted structure with an inorganic hole transport layer is expected for long-term stability. Among various hole transport materials, nickel oxide has been investigated for highly efficient and stable perovskite solar cells. However, the reported deposition methods are either difficult for large-scale conformal deposition or require a high vacuum process. Chemical bath deposition is supposed to realize a uniform, conformal, and scalable coating by a solution process. However, the conventional chemical bath deposition requires a high annealing temperature of over 400 °C. In this work, an amino-alcohol ligand-based controllable release and deposition of NiOX using chemical bath deposition with a low calcining temperature of 270 °C is developed. The uniform and conformal in-situ growth precursive films can be adjusted by tuning the ligand structure. The inverted structured perovskite solar cells and large-area solar modules reached a champion PCE of 22.03% and 19.03%, respectively. This study paves an efficient, low-temperature, and scalable chemical bath deposition route for large-area NiOX thin films for the scalable fabrication of highly efficient perovskite solar modules.

All-Oxide Transparent Photodetector Array for Ultrafast Response through Self-Powered Excitonic Photovoltage Operation

Can photodetectors be transparent and operate in self-powered mode? Is it possible to achieve invisible electronics, independent of the external power supply system, for on-site applications? Here, a ZnO/NiO heterojunction-based high-functional transparent ultraviolet (UV) photodetector operating in the self-powered photovoltaic mode with outstanding responsivity and detectivity values of 6.9 A W-1 and 8.0 × 1012 Jones, respectively, is reported. The highest IUV /Idark value of 8.9 × 104 is attained at a wavelength of 385 nm, together with a very small dark current value of 9.15 × 10-12 A. A large-scale sputtering method is adopted to deposit the heterostructure of n-ZnO and p-NiO sequentially. This deposition instinctively forms an abrupt junction, resulting in a high-quality heterojunction device. Moreover, developing a ZnO/NiO-heterojunction-based 4 × 5 matrix array with an output photovoltage of 4.5 V is preferred for integrating photodetectors into sensing and imaging systems. This transparent UV photodetector exhibits the fastest photo-response time (83 ns) reported for array configurations, which is achieved using an exciton-induced photovoltage based on a neutral donor-bound exciton. Overall, this study provides a simple method for achieving a high-performance large-scale transparent UV photodetector with a self-powered array configuration.

Bio-inspired artificial synapse for neuromorphic computing based on NiO nanoparticle thin film

The unprecedented need for data processing in the modern technological era has created opportunities in neuromorphic devices and computation. This is primarily due to the extensive parallel processing done in our human brain. Data processing and logical decision-making at the same physical location are an exciting aspect of neuromorphic computation. For this, establishing reliable resistive switching devices working at room temperature with ease of fabrication is important. Here, a reliable analog resistive switching device based on Au/NiO nanoparticles/Au is discussed. The application of positive and negative voltage pulses of constant amplitude results in enhancement and reduction of synaptic current, which is consistent with potentiation and depression, respectively. The change in the conductance resulting in such a process can be fitted well with double exponential growth and decay, respectively. Consistent potentiation and depression characteristics reveal that non-ideal voltage pulses can result in a linear dependence of potentiation and depression. Long-term potentiation (LTP) and Long-term depression (LTD) characteristics have been established, which are essential for mimicking the biological synaptic applications. The NiO nanoparticle-based devices can also be used for controlled synaptic enhancement by optimizing the electric pulses, displaying typical learning-forgetting-relearning characteristics.

Multilevel resistive switching in stable all-inorganic n-i-p double perovskite memristor

Memristors are promising information storage devices for commercial applications because of their long endurance and low power consumption. Particularly, perovskite memristors have revealed excellent resistive switching (RS) properties owing to the fast ion migration and solution fabrication process. Here, an n-i-p type double perovskite memristor with "ITO/SnO₂/Cs₂AgBiBr₆/NiO_x/Ag" architecture was developed and demonstrated to reveal three resistance states because of the p-n junction electric field coupled with ion migration. The devices exhibited reliable filamentary with an on/off ratio exceeding 50. The RS characteristics remained unchanged after 1000 s read and 300 switching cycles. The synaptic functions were examined through long-term depression and potentiation measurements. Significantly, the device still worked after one year to reveal long-term stability because of the all-inorganic layers. This work indicates a novel idea for designing a multistate memristor by utilizing the p-n junction unidirectional conductivity during the forward and reverse scanning.

High-Performance NiO/TiO2/ZnO Photovoltaic UV Detector

The ultraviolet (UV) photodetector has found many applications, ranging from optical communication to environmental monitoring. There has been much research interest in the development of metal oxide-based UV photodetectors. In this work, a nano-interlayer was introduced in a metal oxide-based heterojunction UV photodetector to enhance the rectification characteristics and therefore the device performance. The device, which consists of nickel oxide (NiO) and zinc oxide (ZnO) sandwiching an ultrathin dielectric layer of titanium dioxide (TiO₂), was prepared by radio frequency magnetron sputtering (RFMS). After annealing, the NiO/TiO₂/ZnO UV photodetector exhibited a rectification ratio of 10⁴ under UV irradiation of 365 nm at zero bias. The device also demonstrated a high responsivity of 291 A/W and a detectivity of 6.9 × 10¹¹ Jones at +2 V bias. Such a device structure provides a promising future for metal oxide-based heterojunction UV photodetectors in a wide range of applications.

Enhancing the Performance of Quantum Dot Light-Emitting Diodes Using Solution-Processable Highly Conductive Spinel Structure CuCo2O4 Hole Injection Layer

Charge imbalance in quantum-dot light-emitting diodes (QLEDs) causes emission degradation. Therefore, many studies focused on improving hole injection into the QLEDs-emitting layer owing to lower hole conductivity compared to electron conductivity. Herein, CuCo₂O₄ has a relatively higher hole conductivity than other binary oxides and can induce an improved charge balance. As the annealing temperature decreases, the valence band maximum (VBM) of CuCo₂O₄ shifts away from the Fermi energy level (E_F), resulting in an enhanced hole injection through better energy level alignment with hole transport layer. The maximum luminance and current efficiency of the CuCo₂O₄ hole injection layer (HIL) of the QLED were measured as 93,607 cd/m² and 11.14 cd/A, respectively, resulting in a 656% improvement in luminous performance of QLEDs compared to conventional metal oxide HIL-based QLEDs. These results demonstrate that the electrical properties of CuCo₂O₄ can be improved by adjusting the annealing temperature, suggesting that solution-processed spinel can be applied in various optoelectronic devices.

Suppressing Nickel Oxide/Perovskite Interface Redox Reaction and Defects for Highly Performed and Stable Inverted Perovskite Solar Cells

The inorganic hole transport layer of nickel oxide (NiO_x ) has shown highly efficient, low-cost, and scalable in perovskite photovoltaics. However, redox reactions at the interface between NiO_x and perovskites limit their commercialization. In this study, ABABr (4-(2-Aminoethyl) benzoic acid bromide) between the NiO_x and different perovskite layers to address the issues has been introduced. How the ABABr interacts with NiO_x and perovskites is experimentally and theoretically investigated. These results show that the ABABr molecule chemically reacts with the NiO_x via electrostatic attraction on one side, whereas on the other side, it forms a strong hydrogen bond via the NH₃ ⁺ group with perovskites layers, thus directly diminishing the redox reaction between the NiO_x and perovskites layers and passivating the layer surfaces. Additionally, the ABABr interface modification leads to significant improvements in perovskite film morphology, crystallization, and band alignment. The perovskites solar cells (PSCs) based on an ABABr interface modification show power conversion efficiency (PCE) improvement by over 13% and maintain over 90% of its PCE after continuous operation at maximum power point for over 500 h. The work not only contributes to the development of novel interlayers for stable PSCs but also to the understanding of how to prevent interface redox reactions.

Partially delocalized charge in crystalline Co-S-Se/NiOx nanocomposites for boosting electrocatalytic oxygen evolution

Although reasonably specified and adjustable preparation of nanostructures with specific morphologies, ordered chemical compositions and electronic structures involving advanced energy chemistries is an important research area, high-efficiency, stable, and low-cost electrocatalysts for water splitting are greatly desirable and challenging. In this study, partially delocalized Co-S-Se ultra-thin nanosheets are obtained via a liquid-liquid interface-mediated strategy at an oil-water interface. These Co-S-Se ultra-thin nanosheets exhibit different-sized lamellar structures and have an average thickness of 0.83 nm. The ternary ultra-thin Co_0.45S_0.38Se_0.17 nanosheets demonstrate excellent performance for the OER, accompanied by an overpotential of 290 mV (1.52 V vs. RHE) at 10 mA cm^-2, and a Tafel slope of 74.5 mV dec^-1. In the meantime, the catalyst recombined with a stoichiometry NiO_x catalyst to form a composite interface, which also exhibited a good OER performance, with an overpotential of 260 mV at 10 mA cm^-2 and a smaller Tafel slope of 53.9 mV dec^-1. The nanosheets can rearrange the electronic density near the metal catalytic centers and increase the electron transfer. DFT calculations indicate that the partially delocalized charges can improve electrocatalytic performances, demonstrating modulated electroreduction properties. Due to the special atomic and electronic structure of the ternary transition metal alloy chalcogenide, the compound has great potential for energy storage, which will help in the rational design and synthesis of high-efficiency electrocatalysts.

A Perspective on Perovskite Solar Cells: Emergence, Progress, and Commercialization

With rapid progress in light-to-electric conversion efficiencies, perovskite solar cells (PSCs) have exhibited great potential as next-generation low-cost, efficient photovoltaic technology. In this perspective, we briefly review the development of PSCs from discovery to laboratory research to commercializing progress. The past several decades have witnessed great achievement in device efficiency and stability due to tremendous research efforts on compositional, process, and interfacial engineering. Regarding commercial applications, we expound the merits and disadvantages of PSCs compared to the existing silicon photovoltaic technologies. Although PSCs promise solution processability and low manufacturing cost, their limited stability and element toxicity should to be addressed on the path to commercialization. Finally, we provide future perspectives on commercialization of PSCs in the photovoltaic marketplace. It is suggested that PSCs will be more promising in low-cost modules and tandem configurations.

Highly Sensitive and Durable Organic Photodiodes Based on Long-Term Storable NiOx Nanoparticles

Organic optoelectronic devices that can be fabricated at low cost have attracted considerable attention because they can absorb light over a wide frequency range and have high conversion efficiency, as well as being lightweight and flexible. Moreover, their performance can be significantly affected by the choice of the charge-selective interlayer material. Nonstoichiometric nickel oxide (NiO_x) is an excellent material for the hole-transporting layer (HTL) of organic optoelectronic devices because of the good alignment of its valence band position with the highest occupied molecular orbital level of many p-type polymers. Herein, we report a simple low-temperature process for the synthesis of NiO_x nanoparticles (NPs) that can be well dispersed in solution for long-term storage and easily used to form thin NiO_x NP layers. NiO_x NP-based organic photodiode (OPD) devices demonstrated high specific detectivity (D*) values of 10¹²-10¹³ jones under various light intensities and negative biases. The D* value of the NiO_x NP-based OPD device was 4 times higher than that of a conventional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based device, an enhancement that originated mainly from the 16 times decreased leakage current. The NiO_x NP-based OPD device demonstrated better reliability over a wide range of light intensities and operational biases in comparison to a device with a conventional sol-gel-processed NiO_x film. More importantly, the NiO_x NP-based OPD showed long-term device stability superior to those of the PEDOT:PSS and sol-gel-processed NiO_x-based devices. We highlight that our low-temperature solution-processable NiO_x NP-based HTL could become a crucial component in the fabrication of stable high-performance OPDs.

Silver-doped nickel oxide as an efficient hole-transport layer in polymer light-emitting diodes

In this study, silver-doped nickel oxide (NiO:Ag) was successfully synthesized by a sol-gel method and spin-coated on indium titanium oxide (ITO) as a hole-transport layer for polymer light-emitting diodes (PLED). After the calcination of the NiO:Ag/ITO substrate at 300°C for 1 h, stable conductive regions and the mean work-function on the NiO:Ag/ITO surface reached 89.43% and 5.53 eV, respectively, which were greater than those on a conventional poly [3,4-ethylenedioxythiophene] polystyrene sulfonate (PEDOT:PSS)/ITO surface. When NiO:Ag (300°C)/ITO was used as an anode window substrate for PLEDs, the enhancement factor for the average current efficiency in the current-density range of 20-50 mA/cm² and electroluminescence intensity at an applied bias of 8.0 V were 4.60 and 2.55 times, respectively, in comparison with those of PLED based on a conventional PEDOT:PSS/ITO anode.

Recent Advances in Hole-Transporting Layers for Organic Solar Cells

Global energy demand is increasing; thus, emerging renewable energy sources, such as organic solar cells (OSCs), are fundamental to mitigate the negative effects of fuel consumption. Within OSC's advancements, the development of efficient and stable interface materials is essential to achieve high performance, long-term stability, low costs, and broader applicability. Inorganic and nanocarbon-based materials show a suitable work function, tunable optical/electronic properties, stability to the presence of moisture, and facile solution processing, while organic conducting polymers and small molecules have some advantages such as fast and low-cost production, solution process, low energy payback time, light weight, and less adverse environmental impact, making them attractive as hole transporting layers (HTLs) for OSCs. This review looked at the recent progress in metal oxides, metal sulfides, nanocarbon materials, conducting polymers, and small organic molecules as HTLs in OSCs over the past five years. The endeavors in research and technology have optimized the preparation and deposition methods of HTLs. Strategies of doping, composite/hybrid formation, and modifications have also tuned the optical/electrical properties of these materials as HTLs to obtain efficient and stable OSCs. We highlighted the impact of structure, composition, and processing conditions of inorganic and organic materials as HTLs in conventional and inverted OSCs.

Flexible Ni/NiOx-Based Sensor for Human Breath Detection

We developed a simple methodology to fabricate an Ni/NiOx-based flexible breath sensor by a single-step laser digital patterning process of solution-processed NiOx thin-film deposited using NiOx nanoparticle ink. Laser-induced reductive sintering phenomenon enables for the generation of three parts of Ni electrodes and two narrow NiOx-sensing channels in between, defined on a single layer on a thin flexible polymer substrate. The Ni/NiOx-based breath sensor efficiently detects human breath at a relatively low operating temperature (50 °C) with fast response/recovery times (1.4 s/1.7 s) and excellent repeatability. The mechanism of the gas-sensing ability enhancement of the sensor was investigated by X-ray photoelectron spectroscopy analysis. Furthermore, by decoupling of the temperature effect from the breathing gas, the response of the sensor due to the temperature alone and due to the chemical components in the breathing gas could be separately evaluated. Finally, bending and cyclic bending tests (10,000 cycles) demonstrated the superior mechanical stability of the flexible breath sensor.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

CuI/Spiro-OMeTAD Double-Layer Hole Transport Layer to Improve Photovoltaic Performance of Perovskite Solar Cells

The hole transport layer (HTL) is one of the main factors affecting the efficiency and stability of perovskite solar cells (PSCs). However, obtaining HTLs with the desired properties through current preparation techniques remains a challenge. In the present study, we propose a new method which can be used to achieve a double-layer HTL, by inserting a CuI layer between the perovskite layer and Spiro-OMeTAD layer via a solution spin coating process. The CuI layer deposited on the surface of the perovskite film directly covers the rough perovskite surface, covering the surface defects of the perovskite, while a layer of CuI film avoids the defects caused by Spiro-OMetad pinholes. The double-layer HTLs improve roughness and reduce charge recombination of the Spiro-OMeTAD layer, thereby resulting in superior hole extraction capabilities and faster hole mobility. The CuI/Spiro-OMeTAD double-layer HTLs-based devices were prepared in N2 gloveboxes and obtained an optimized PCE (photoelectric conversion efficiency) of 17.44%. Furthermore, their stability was improved due to the barrier effect of the inorganic CuI layer on the entry of air and moisture into the perovskite layer. The results demonstrate that another deposited CuI film is a promising method for realizing high-performance and air-stable PSCs.

Inverted Polymer Solar Cells with Annealing-Free Solution-Processable NiO

Nickel oxide (NiO) offers intrinsic p-type behavior and high thermal and chemical stability, making it promising as a hole transport layer (HTL) material in inverted organic solar cells. However, its use in this application has been rare because of a wettability problem caused by use of water as base solvent and high-temperature annealing requirements. In the present work, an annealing-free solution-processable method for NiO deposition is developed and applied in both conventional and inverted non-fullerene polymer solar cells. To overcome the wettability problem, the typical DI water solvent is replaced with a mixed solvent of DI water and isopropyl alcohol with a small amount of 2-butanol additive. This allows a NiO nanoparticle suspension (s-NiO) to be deposited on a hydrophobic active layer surface. An inverted non-fullerene solar cell based on a blend of p-type polymer PTB7-Th and non-fullerene acceptor IEICO-4F exhibits the high efficiency of 11.23% with an s-NiO HTL, comparable to the efficiency of an inverted solar cell with a MoO_x HTL deposited by thermal evaporation. Conventionally structured devices including this s-NiO layer show efficiency comparable to that of a conventional device with a PEDOT:PSS HTL.

Evaluation of Ni-Based Flexible Resistance Temperature Detectors Fabricated by Laser Digital Pattering

Temperature sensors are ubiquitous in every field of engineering application since temperature control is vital in operating, testing and monitoring various equipment systems. Herein, we introduce a facile and rapid laser digital patterning (LDP) process to fabricate low-cost, Ni-based flexible resistance temperature detectors (RTDs). Ni-based RTDs are directly generated on a thin flexible polyimide substrate (thickness: 50 µm) by laser-induced reductive sintering of a solution-processed nonstoichiometric nickel oxide (NiO_x) nanoparticle thin film under ambient conditions. The shape of RTDs can be easily adjusted by controlling computer-aided design (CAD) data without using the physical patterning mask while the sensitivity (temperature coefficient of resistance (α) ~ 3.52 × 10⁻³ °C⁻¹) of the sensors can be maintained regardless of shape and size of the sensor electrodes. The flexible Ni-based RTDs can operate over a wide temperature range up to 200 °C with excellent repeatability. Additionally, the Ni-based RTDs respond quickly to the temperature change and can operate in corrosive environments including water and seawater. Moreover, the Ni-based RTDs show a superior mechanical and electrical stability with a negligible resistance change up to a radius of curvature of 1.75 mm. Finally, a tape-pull test demonstrates the robust adhesion of Ni-based RTDs on the substrate.

Highly efficient self-powered perovskite photodiode with an electron-blocking hole-transport NiOx layer

Hybrid organic-inorganic perovskite materials provide noteworthy compact systems that could offer ground-breaking architectures for dynamic operations and advanced engineering in high-performance energy-harvesting optoelectronic devices. Here, we demonstrate a highly effective self-powered perovskite-based photodiode with an electron-blocking hole-transport layer (NiOx). A high value of responsivity (R = 360 mA W-1) with good detectivity (D = 2.1 × 1011 Jones) and external quantum efficiency (EQE = 76.5%) is achieved due to the excellent interface quality and suppression of the dark current at zero bias voltage owing to the NiOx layer, providing outcomes one order of magnitude higher than values currently in the literature. Meanwhile, the value of R is progressively increased to 428 mA W-1 with D = 3.6 × 1011 Jones and EQE = 77% at a bias voltage of - 1.0 V. With a diode model, we also attained a high value of the built-in potential with the NiOx layer, which is a direct signature of the improvement of the charge-selecting characteristics of the NiOx layer. We also observed fast rise and decay times of approximately 0.9 and 1.8 ms, respectively, at zero bias voltage. Hence, these astonishing results based on the perovskite active layer together with the charge-selective NiOx layer provide a platform on which to realise high-performance self-powered photodiode as well as energy-harvesting devices in the field of optoelectronics.

Improved efficiency and carrier dynamic transportation behavior in perovskite solar cells with CuInS2 quantum dots as hole-transport materials

Inorganic quantum dot (QD)-based hole-transport materials (HTMs) have proved their potential in perovskite solar cells (PSCs). In this work, CuInS2 quantum dots (CIS QDs) were applied as HTMs for PSCs with the architecture of TiO2/Cs0.17FA0.83Pb(Br0.2I0.8)3/HTM/Au. By optimizing the preparation process, a high-quality perovskite thin film could be obtained. When the speed was 5000 rpm, the speed acceleration was 3000 rpm per s and heat treated at 150 °C, the perovskite film had low surface roughness (15.26 nm) and obvious grain boundary. The photoelectric conversion efficiency (PCE) of PSCs was greatly improved from 2.83% to 12.33% utilizing CIS QDs at an optimal concentration and with surface ligands as HTMs. Surface ligands can control the size and shape of CIS QDs, and thus affect the performance of PSCs. The carrier dynamic transportation behaviour at the CIS/perovskite interface was studied, which showed that CIS QDs as HTMs in PSCs can strongly quench the fluorescence and increase the photobleaching recovery rate. Therefore, CIS QDs are promising inorganic HTMs for the fabrication of PSCs.

Chemical vapour deposition (CVD) of nickel oxide using the novel nickel dialkylaminoalkoxide precursor [Ni(dmamp′)2] (dmamp′ = 2-dimethylamino-2-methyl-1-propanolate)

Nickel oxide (NiO) has good optical transparency and wide band-gap, and due to the particular alignment of valence and conduction band energies with typical current collector materials has been used in solar cells as an efficient hole transport-electron blocking layer, where it is most commonly deposited via sol–gel or directly deposited as nanoparticles. An attractive alternative approach is via vapour deposition. This paper describes the chemical vapour deposition of p-type nickel oxide (NiO) thin films using the new nickel CVD precursor [Ni(dmamp′)₂], which unlike previous examples in literature is synthesised using the readily commercially available dialkylaminoalkoxide ligand dmamp′ (2-dimethylamino-2-methyl-1-propanolate). The use of vapour deposited NiO as a blocking layer in a solar-cell device is presented, including benchmarking of performance and potential routes to improving performance to viable levels.

We describe CVD of nickel oxide (NiO) thin films using a new precursor [Ni(dmamp′)₂], synthesised using a readily commercially available dialkylaminoalkoxide ligand (dmamp′), which is applied to synthesis of a hole transport-electron blocking layer.

NiOx nanoparticles obtained from hydrothermally treated NiC2O4 as an electron blocking layer for organic photodetectors

A room-temperature p-type NiO_x film synthesized from a NiC₂O₄ precursor via hydrothermal treatment is employed as an electron blocking layer (EBL) to fabricate organic photodetectors (OPDs). A simple and efficient calcine process at 375 °C in air decomposes the NiC₂O₄ particles into NiO_x, removes organic components and crystal water, and releases CO₂ gas. Our experimental results indicate that this gaseous by-product prevents the agglomeration of NiO_x, which yields smaller nanoparticles (5-10 nm). The formation of an EBL at room temperature improves device performance. After optimization, the performance parameters obtained, including dark current density, responsivity, specific detectivity and response, are 1.13 × 10^-7 A cm^-2, 0.74 A W^-1, 3.86 × 10¹² Jones, and 0.5/8 ms, respectively. Additionally, the dark current is reduced by more than an order of magnitude after the insertion of the NiO_x layer. The proposed simple and easy method for producing an EBL could be beneficial for the commercial low-temperature and large-area preparation of OPDs.

Establishing Multifunctional Interface Layer of Perovskite Ligand Modified Lead Sulfide Quantum Dots for Improving the Performance and Stability of Perovskite Solar Cells

While organic-inorganic halide perovskite solar cells (PSCs) show great potential for realizing low-cost and easily fabricated photovoltaics, the unexpected defects and long-term stability against moisture are the main issues hindering their practical applications. Herein, a strategy is demonstrated to address the main issues by introducing lead sulfide quantum dots (QDs) on the perovskite surface as the multifunctional interface layer on perovskite film through establishing perovskite as the ligand on PbS QDs. Meanwhile, the multifunctions are featured in three aspects including the strong interactions of PbS QDs with perovskites particularly at the grain boundaries favoring good QDs coverage on perovskites for ultimate smooth morphology; an inhibition of iodide ions mobilization by the strong interaction between iodide and the incorporated QDs; and the reduction of the dangling bonds of Pb²⁺ by the sulfur atoms of PbS QDs. Finally, the device performances are highly improved due to the reduced defects and non-radiative recombination. The results show that both open-circuit voltage and fill factor are significantly improved to the high values of 1.13 V and 80%, respectively in CH₃ NH₃ PbI₃ -based PSCs, offering a high efficiency of 20.64%. The QDs incorporation also enhances PSCs' stability benefitting from the induced hydrophobic surface and suppressed iodide mobilization.

Embedded Nickel-Mesh Transparent Electrodes for Highly Efficient and Mechanically Stable Flexible Perovskite Photovoltaics: Toward a Portable Mobile Energy Source

The rapid development of Internet of Things mobile terminals has accelerated the market's demand for portable mobile power supplies and flexible wearable devices. Here, an embedded metal-mesh transparent conductive electrode (TCE) is prepared on poly(ethylene terephthalate) (PET) using a novel selective electrodeposition process combined with inverted film-processing methods. This embedded nickel (Ni)-mesh flexible TCE shows excellent photoelectric performance (sheet resistance of ≈0.2-0.5 Ω sq^-1 at high transmittance of ≈85-87%) and mechanical durability. The PET/Ni-mesh/polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS PH1000) hybrid electrode is used as a transparent electrode for perovskite solar cells (PSCs), which exhibit excellent electric properties and remarkable environmental and mechanical stability. A power conversion efficiency of 17.3% is obtained, which is the highest efficiency for a PSC based on flexible transparent metal electrodes to date. For perovskite crystals that require harsh growth conditions, their mechanical stability and environmental stability on flexible transparent embedded metal substrates are studied and improved. The resulting flexible device retains 76% of the original efficiency after 2000 bending cycles. The results of this work provide a step improvement in flexible PSCs.

Dual Functionalization of Electron Transport Layer via Tailoring Molecular Structure for High-Performance Perovskite Light-Emitting Diodes

Great progress in modification and optimization of emission layer (EML) in perovskite light-emitting diodes (PeLEDs) results in a significant improvement in device efficiency. However, so far, less attention has been paid to the exploration of hole/electron injection and transporting layers to maximize the utilization of charge carriers for efficient and stable PeLEDs. At present, low electron mobility of electron transport layer (ETL) causes an unbalanced charge injection, and the defects at the ETL/perovskite interface limit the formation and utilization of generated excitons. Here, a series of compounds (BPBiTP, BPBiPN, and BPBiPA) flanked by diphenyl-1H-benzo[d]imidazole end groups have been developed as ETL materials, where the bridging units (benzene, naphthalene, anthracene) are manipulated to achieve dual functionality, namely, the high charge carrier mobility and effective passivation of perovskite surface. The coordinating end groups effectively reduce the trap state at the interface of ETL and EML due to their strong nucleophilic quality. H-aggregation of anthracene units and large transfer integral in BPBiPA lead to its superior electron mobility of 8.4 × 10^-4 cm² V^-1 s^-1 in the solid state, over 1 order of magnitude higher than that of the typical one (TPBi). Consequently, green PeLEDs with a maximum external quantum efficiency (EQE) of 19.7%, reduced efficiency roll-off, as well as extended operational lifetime have been achieved without any outcoupling technique. Our result demonstrated that optimization of ETL materials via improving both passivation capability and electron mobility is a powerful strategy for producing high-performance PeLEDs.

22% Efficiency Inverted Perovskite Photovoltaic Cell Using Cation-Doped Brookite TiO2 Top Buffer

Simultaneously achieving high efficiency and high durability in perovskite solar cells is a critical step toward the commercialization of this technology. Inverted perovskite photovoltaic (IP-PV) cells incorporating robust and low levelized-cost-of-energy (LCOE) buffer layers are supposed to be a promising solution to this target. However, insufficient inventory of materials for back-electrode buffers substantially limits the development of IP-PV. Herein, a composite consisting of 1D cation-doped TiO2 brookite nanorod (NR) embedded by 0D fullerene is investigated as a top modification buffer for IP-PV. The cathode buffer is constructed by introducing fullerene to fill the interstitial space of the TiO2 NR matrix. Meanwhile, cations of transition metal Co or Fe are doped into the TiO2 NR to further tune the electronic property. Such a top buffer exhibits multifold advantages, including improved film uniformity, enhanced electron extraction and transfer ability, better energy level matching with perovskite, and stronger moisture resistance. Correspondingly, the resultant IP-PV displays an efficiency exceeding 22% with a 22-fold prolonged working lifetime. The strategy not only provides an essential addition to the material inventory for top electron buffers by introducing the 0D:1D composite concept, but also opens a new avenue to optimize perovskite PVs with desirable properties.

Efficient Flexible Perovskite Solar Cells Using Low-Cost Cu Top and Bottom Electrodes

Perovskite solar cells (PSCs) are promising technology for flexible photovoltaic applications because of the low cost and good flexibility of the halide perovskite materials. Nevertheless, the use of transparent conductive oxides (TCOs) and noble metals (e.g., Au and Ag) as PSC electrodes is very costly, and TCOs are too brittle for flexible applications. How to fabricate flexible PSCs (FPSCs) with cost-effective and soft electrode materials remains to be a big challenge. Herein, we report the first study of FPSCs using low-cost Cu electrodes. Both the transparent bottom electrode and the opaque top electrode are fabricated with Cu. FPSCs made with such Cu electrodes acquire a champion efficiency of 13.58% (J_sc of 17.79 mA cm^-2, V_oc of 1.031 V, and FF of 74.07%), which retains over 90% after 1000 cycles of bending at a small radius of curvature of 5 mm. The device shows negligible changes in Voc and FF after storage for 10 weeks without encapsulation.

Structural and Electrical Investigation of Cobalt-Doped NiOx/Perovskite Interface for Efficient Inverted Solar Cells

Inorganic hole-transporting materials (HTMs) for stable and cheap inverted perovskite-based solar cells are highly desired. In this context, NiO_x, with low synthesis temperature, has been employed. However, the low conductivity and the large number of defects limit the boost of the efficiency. An approach to improve the conductivity is metal doping. In this work, we have synthesized cobalt-doped NiO_x nanoparticles containing 0.75, 1, 1.25, 2.5, and 5 mol% cobalt (Co) ions to be used for the inverted planar perovskite solar cells. The best efficiency of the devices utilizing the low temperature-deposited Co-doped NiO_x HTM obtained a champion photoconversion efficiency of 16.42%, with 0.75 mol% of doping. Interestingly, we demonstrated that the improvement is not from an increase of the conductivity of the NiO_x film, but due to the improvement of the perovskite layer morphology. We observe that the Co-doping raises the interfacial recombination of the device but more importantly improves the perovskite morphology, enlarging grain size and reducing the density of bulk defects and the bulk recombination. In the case of 0.75 mol% of doping, the beneficial effects do not just compensate for the deleterious one but increase performance further. Therefore, 0.75 mol% Co doping results in a significant improvement in the performance of NiO_x-based inverted planar perovskite solar cells, and represents a good compromise to synthesize, and deposit, the inorganic material at low temperature, without losing the performance, due to the strong impact on the structural properties of the perovskite. This work highlights the importance of the interface from two different points of view, electrical and structural, recognizing the role of a low doping Co concentration, as a key to improve the inverted perovskite-based solar cells’ performance.

Metal oxide heterojunctions using a printable nickel oxide ink

Wide band gap metal oxides are ideally suited for inorganic optoelectronic devices. While zinc oxide is a commonly used n-type material, there is still a lot of ongoing work for finding suitable p-type oxides. In this work, we describe a two-step route to formulate a stable and conducting p-type nickel oxide (NiO) nanofluid. NiO nanoparticles were synthesised using a bottom-up wet chemical approach and dispersed in ethylene glycol to form a nanofluid. The viscosity and surface tension of the nanofluid were optimised for printing. The printing was done using an extrusion-based direct writer. The NiO nanofluid was printed onto an aluminum-doped zinc oxide layer and annealed at different temperatures. Electrical characterisation of the junction was used to extract the junction barrier for carriers across the interface. The resulting heterojunction was found to exhibit rectifying behaviour, with the highest rectification ratio occurring at an annealing temperature of 250 °C. This annealing temperature also resulted in the lowest junction barrier height, and was in excellent agreement with theoretically predicted values. The development of a printed p-type ink will help in the realisation of oxide-based printed electronic devices.