CuCrO2 Films Research Articles

Communication—Annealing Strategies for Spray Deposited Precursor Films of p-Type CuCr1-xMgxO2

Annealing of sprayed pure and Mg doped CuCrO2 thin films by high intensity, short time light irradiation leads to a single delafossite phase at comparatively low temperatures compared with traditional furnace annealing. P-type crystalline undoped and Mg-doped CuCrO2 films were obtained within few minutes by annealing with halogen lamp between 550 °C and 650 °C in Ar atmosphere. Transport properties of Mg-doped thin films were comparable to furnace annealed samples despite much shorter annealing time. The results demonstrate that post-annealing of chemically deposited samples using light irradiation is an effective and fast method for obtaining transparent conducting delafossite thin films.

IoT-enabled surface-active Pd-anchored metal oxide chemiresistor for H2S gas detection

The Internet of Things (IoT) has a crucial role in advancing various fields such as Industry 4.0, Big Data, and Machine-to-Machine technologies. All systems continuously gather information on various parameters such as temperature, speed, pressure, health data, environmental conditions, and consumption. Considering this, we've developed a novel fabrication method for H2S gas sensor prototypes. These sensors are based on noble metal-functionalized on metal oxide semiconductor (MOS) chemiresistors. The fabrication process involved growing CuCrO2 sensing thin film on SiO2/Si substrates using the RF sputtering method. Subsequently, Pd nanoparticles, known for their excellent gas-sensing catalyst properties, were functionalized onto the CuCrO2 films using DC sputtering with varying sputtering times of 3, 6, 9, and 12 s. Nanorice morphology boosts gas absorption, capturing more target molecules. A 9 s Pd sputtering time greatly improved H2S sensing over other gases. CuCrO2 film with Pd showed the highest 72.3% response to 50 ppm H2S, detecting down to 0.5 ppm. These results were achieved at the optimal working temperature of 150 °C. After optimizing parameters, we transferred the technology to develop a sensor module for the prototype with IoT integration. The prototype sensor connects to NODEMCU-ESP8266 Wi-Fi, which links to a smartphone through a mobile hotspot.

O, N co-doped CuCrO2 as efficient hole transport layer for high-performance ultraviolet photodetectors

The optoelectronic properties and film-formation characteristics of CuCrO2 are optimized through oxygen (O) and nitrogen (N) co-doping using the space-limited domain annealing method in ambient air. After O, N co-doping at 100 °C, the conductivity of CuCrO2 is enhanced due to a decrease in trap-state density and an increase in hole concentration, thereby facilitating carrier transport. With a work function (WF) of 5.24 eV and an electron affinity (χ) of 2.29 eV, the CuCrO2 films serves as both a hole transport layer (HTL) and an electron-blocking layer, playing a pivotal role in enhancing photogenerated holes extraction while suppressing nonradiative charge recombination at the heterojunction interface. Consequently, the responsivity and detectivity of the device with FTO/TiO2 NRs/O, N co-doped CuCrO2/Au structure reach 188 mA W−1 and 3.14 × 1013 cm Hz1/2 W−1 in self-powered mode, respectively. Simultaneously, the rise/decay time is reduced to just 31.2/32.1 ns. Furthermore, after applying a bias of − 2 V, the photodetector demonstrates significantly improved performance with a responsivity of up to 5 × 105 mA/W (365 nm, 0.97 mW cm−2) and an apparent quantum efficiency of ∼30,000%. These results highlight the promising potential of utilizing O, N co-doped CuCrO2 HTL for constructing high-performance optoelectronic devices.

Diffusion-assisted molecular beam epitaxy of CuCrO2 thin films

Using molecular beam epitaxy (MBE) to grow multielemental oxides (MEOs) is generally challenging, partly due to difficulty in stoichiometry control. Occasionally, if one of the elements is volatile at the growth temperature, stoichiometry control can be greatly simplified using adsorption-controlled growth mode. Otherwise, stoichiometry control remains one of the main hurdles to achieving high-quality MEO film growths. Here, we report another kind of self-limited growth mode, dubbed diffusion-assisted epitaxy, in which excess species diffuses into the substrate and leads to the desired stoichiometry, in a manner similar to the conventional adsorption-controlled epitaxy. Specifically, we demonstrate that using diffusion-assisted epitaxy, high-quality epitaxial CuCrO2 films can be grown over a wide growth window without precise flux control using MBE.

A comparative study of the antibacterial properties of copper-based transparent oxides at the solid–liquid interface

This study investigates the antibacterial properties of CuCrO2 (CCO) films fabricated by sputter deposition followed by calcination. The effects of the doped magnesium and the film fabrication in nitrogen atmosphere on both CCO crystallinity and antibacterial properties are compared. The antibacterial properties are evaluated using the plate counting method, and the crystallinity of the films are analyzed by X-ray diffraction. The results show the proliferation of Escherichia coli colonies that can be suppressed within 3 h, with magnesium-doped CCO showing the best antibacterial properties of all samples. This is explained by the formation of CuO byproduct during fabrication.

Spray Pyrolysis Deposition of CuCrO2 Films as Promising Inorganic Hole Transport Layers for Highly Efficient Perovskite Solar Cells

Recently, CuCrO2 films have been successfully introduced as effective hole‐transporting materials in perovskite solar cells (PSCs). However, the long reaction time (≈50–60 h) at the high temperature of hydrothermal synthesis and the complexity of conversion from nanoparticles to uniform films hinder the application of CuCrO2 nanoparticles in PSCs. Herein, a simple, low‐cost, and highly reproducible method for the spray pyrolysis deposition (SPD) of Cu‐deficient CuCrO2 films is reported. The resulting CuCrO2 films exhibit advantages in terms of transparency, roughness, and electrical properties compared to their extensively studied NiO x counterparts. Meanwhile, the deep‐lying valence‐band‐maximum of CuCrO2 films matches very well with that of perovskite materials, minimizing the charge‐extraction barrier. As a result, the excellent electrical properties and energetically favorable energy‐level alignment of the CuCrO2 film allow efficient charge transport from the perovskite layer to the hole transport layer (HTL). Consequently, the optimized CuCrO2‐based device delivers an outstanding power conversion efficiency of 18.86% versus 16.80% for NiO x ‐based devices. This work demonstrates the validity of a CuCrO2 film derived from the SPD method as an HTL for highly efficient PSCs.

Highly Conductive and Visibly Transparent p-Type CuCrO2 Films by Ultrasonic Spray Pyrolysis.

The development of high-performing p-type transparent conducting oxides will enable immense progress in the fabrication of optoelectronic devices including invisible electronics and all-oxide power electronics. While n-type transparent electrodes have already reached widespread industrial production, the lack of p-type counterparts with comparable transparency and conductivity has created a bottleneck for the development of next-generation optoelectronic devices. In this work, we present the fabrication of delafossite copper chromium oxide p-type transparent electrodes with outstanding optical and electrical properties. These layers were deposited using ultrasonic spray pyrolysis, a wet chemical method that is fast, simple, and scalable. Through careful screening of the deposition conditions, highly crystalline, dense, and smooth CuCrO2 coatings were obtained. A detailed investigation of the role played by the deposition temperature and the cation ratio enabled the properties of the prepared layers to be reliably tuned, as verified using X-ray diffraction, X-ray photoelectron spectroscopy, optical spectroscopy, Hall effect measurements, and electron and atomic force microscopies. We demonstrate record conductivities for solution-processed CuCrO2, exceeding 100 S cm-1, and we also obtained the highest value for two separate figures of merit for p-type transparent conducting oxides. These performances position solution-deposited CuCrO2 as the leading p-type transparent-conducting oxide currently available.

Optimized Stoichiometry for CuCrO2 Thin Films as Hole Transparent Layer in PBDD4T-2F:PC70BM Organic Solar Cells.

The performance and stability in atmospheric conditions of organic photovoltaic devices can be improved by the integration of stable and efficient photoactive materials as substituent of the chemically unstable poly (3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS), generally used as organic hole transport layer. Promising candidates are p-type transparent conductive oxides, which combine good optoelectronic and a higher mechanical and chemical stability than the organic counterpart. In this work, we synthesize Cu-rich CuCrO2 thin films by aerosol-assisted chemical vapour deposition as an efficient alternative to PEDOT:PSS. The effect of stoichiometry on the structural, electrical, and optical properties was analysed to find a good compromise between transparency, resistivity, and energy bands alignment, to maximize the photovoltaic performances., Average transmittance and bandgap are reduced when increasing the Cu content in these out of stoichiometry CuCrO2 films. The lowest electrical resistivity is found for samples synthesized from a solution composition in the 60–70% range. The optimal starting solution composition was found at 65% of Cu cationic ratio corresponding to a singular point in Hackee’s figure of merit of 1 × 10−7 Ω−1. PBDD4T-2F:PC70BM organic solar cells were fabricated by integrating CuCrO2 films grown from a solution composition ranging between 40% to 100% of Cu as hole transport layers. The solar cells integrating a film grown with a Cu solution composition of 65% achieved a power conversion efficiency as high as 3.1%, representing the best trade-off of the optoelectronic properties among the studied candidates. Additionally, despite the efficiencies achieved from CuCrO2-based organic solar cells are still inferior to the PEDOT:PSS counterpart, we demonstrated a significant enhancement of the lifetime in atmospheric conditions of optimal oxides-based organic photovoltaic devices.

Out of stoichiometry CuCrO2 films as a promising p-type TCO for transparent electronics

Optimal deposition conditions to synthesize highly conductive and transparent CuCrO2 films out of stoichiometry and Cu2O + CuCrO2 nanocomposite films by AA-MOCVD at a low temperature and atmospheric pressure.

Pulsed-laser epitaxy of metallic delafossite PdCrO2 films

Alternate stacking of a highly conducting metallic layer with a magnetic triangular layer found in delafossite PdCrO2 provides an excellent platform for discovering intriguing correlated quantum phenomena. Thin film growth of delafossites may enable not only the tuning of the basic physical properties beyond what bulk materials can exhibit, but also the development of novel hybrid materials by interfacing with dissimilar materials, yet this has proven to be extremely challenging. Here, we report the epitaxial growth of metallic delafossite PdCrO2 films by pulsed laser epitaxy (PLE). The fundamental role of the PLE growth conditions, epitaxial strain, and chemical and structural characteristics of the substrate is investigated by growing under various growth conditions and on various types of substrates. While strain plays a large role in improving the crystallinity, the direct growth of epitaxial PdCrO2 films without impurity phases was not successful. We attribute this difficulty to both the chemical and structural dissimilarities with the substrate and volatile nature of the PdO sublayer, which make nucleation of the right phase difficult. This difficulty was overcome by growing CuCrO2 buffer layers before PdCrO2 films were grown. Unlike PdCrO2, CuCrO2 films were readily grown with a relatively wide growth window. Only a monolayer thick buffer layer was sufficient to grow the correct PdCrO2 phase. This result indicates that the epitaxy of Pd-based delafossites is extremely sensitive to the chemistry and structure of the interface, necessitating near perfect substrate materials. The resulting films are commensurately strained and show an antiferromagnetic transition at 40 K that persists down to as thin as 3.6 nm in thickness. This work provides key insights into advancing the epitaxial growth of the broader class of metallic delafossites for both studying the basic physical properties and developing new spintronic and computing devices.

Raman scattering measurements of phonon anharmonicity in the delafossite CuGa1‐xCrxO2 (0 ≤ x ≤ 1) films

AbstractRaman spectra of the delafossite CuGa1‐xCrxO2 (0 ≤ x ≤ 1) solid solution films have been measured in the temperature range from 80 to 823 K. Raman phonon modes are observed at 224 (Ag), 328 (A2u), 396 (Eg), and 757 (A1g) cm−1 for the CuGaO2 film, and phonon modes of the CuCrO2 film are around 211 (Ag), 324 (A2u), 458 (Eg), and 712 (A1g) cm−1, respectively. The A1g phonon mode has a distinct redshift with increasing Cr composition, which indicates that the interatomic potential becomes weaker between Cu and O atoms. Except Raman modes at the Brillouin zone center, the additional phonon modes in the spectral range of 500−670 cm−1 were observed due to Raman selection rules being relaxed by the crystal defects. With increasing temperature, the A1g phonon mode undergoes a larger phonon softening but a smaller line broadening relative to that of the Eg phonon mode, which indicated the anisotropic behavior for the delafossite CuGa1‐xCrxO2 films. The nonlinear temperature dependence of the phonon frequency and linewidth is an indication of nonharmonic lattice dynamics. The Raman shift and the linewidth of the Eg and A1g optical modes as a function of temperature were studied as an experimental evidence of the combined effect of lattice expansion and anharmonic phonon–phonon interaction. And the three‐phonon anharmonic coupling plays the dominant role in the anharmonicity behavior. The phonon anharmonicity behavior of the CuGaO2 and CuCrO2 films is slightly different. The larger Γ0 is intrinsic for the A1g mode of the CuGa0.8Cr0.2O2 film due to the broadening by the crystalline defects, which perturb the translation symmetry of the harmonic crystal, making the momentum conservation rule for phonon scattering partially relaxed.

Effect of Ni doping on the structural, electrical, and optical properties of transparent CuCrO2 films grown using pulsed laser deposition

In this study, the structural, electrical, and optical properties of CuCr1−xNixO2 epitaxial films (x = 0, 0.01, 0.03, 0.05), which exhibited p-type properties, were investigated. The (001)-oriented epitaxial films were deposited on c-plane α-Al2O3 substrates using pulsed laser deposition at a growth temperature of 700 °C and oxygen pressure of 10 mTorr. The optical energy band gap of the CuCr0.95Ni0.05O2 film was determined to be 3.22 eV. The hole carrier concentration of the CuCrO2 film increased from 5.1 × 1014 to 2.2 × 1017 cm−3 upon doping with 5 at% Ni. Based on Hall measurement and X-ray photoelectron spectroscopy results, it was suggested that the substituted Ni2+ dopants at Cr3+ sites formed an acceptor level without any charge compensation with Cu2+ and/or Cr4+.

Highly efficient tandem photoelectrochemical solar cells using coumarin6 dye-sensitized CuCrO2 delafossite oxide as photocathode

In this study, we introduce a new concept for the highly efficient tandem p-n photoelectrochemical cell consisting of coumarin6 organic dye-anchored p-type CuCrO2 delafossite semiconductor as photocathodes coupling with traditional n-type TiO2 based photoanodes. Also, we have systematically studied the photovoltaic performance of tandem cells as a function of post-annealing of CuCrO2 films before sensitization. Hydrothermally synthesized CuCrO2 nanocrystals exhibited a high surface area with small crystallite size of 12 nm, phase-pure and well-crystalline after the optimized post-annealing conditions. The experimental results indicated that the optimal post-annealing temperature was 450 °C for 1 h due to the larger active surface area, lower Rct, and higher Jsc and Voc values. This tandem cell, fabricated by employing the CuCrO2 photocathodes, iodide-based redox mediator and a coumarin6 organic dye, afforded an impressive efficiency of 2.33% with Voc of 813 mV, Jsc of 4.83 mA cm−2, and fill factor of 0.59. The obtained parameters are acceptably high in comparison to NiO photocathode-based tandem cells previously reported in literature under similar experimental conditions. Therefore, this work opened the way for developing highly-efficient tandem photoelectrochemical solar cells.

Optical and structural properties of CuCrO2 thin films on c-face sapphire substrate deposited by reactive RF magnetron sputtering

Reactive sputtering with Ar/N2 mixture gas was introduced to improve stoichiometry of p-type transparent CuCrO2 films, and effects of N2 partial pressure ratio (αN) on optical and structural properties were investigated. Film composition was changed from Cu rich (i.e. Cr poor) to Cr rich (i.e. Cu poor) by N2 addition, and the stoichiometric film was grown at αN of about 20%. Although N atoms were not incorporated into the films from analyses of crystal structure and chemical bonding state, both transmittance from visible to near-infrared wavelength and crystallinity were improved at αN up to 10%. These improvements were attributed to suppression of the CuO formation and promotion of the O-Cu-O dumbbell bonds formation. This was confirmed by the decrease of diffraction intensity from CuO and the increase of vibrational intensity corresponding A1g mode. From these results, it can be considered that N atoms decreased not only Cu but also excess O in the film. At αN of 20% or above, transmittance at wavelength of 450nm and crystallinity deteriorated. This is supposed that excessive N2 addition probably generated both O and Cu deficiencies. As a result, it was found that slightly Cu-rich composition is suitable to obtain high-transparency CuCrO2 thin films for practical use.

Effects of annealing time on the structural and optoelectronic properties of p-type conductive transparent Cu–Cr–O films

Cu–Cr–O films were prepared by DC magnetron co-sputtering using Cu and Cr targets on fused silica substrates. The as-deposited Cu–Cr–O films were amorphous, semi-transparent, and insulated. After annealing at 800 °C for 30–240 min, the surface roughness and optical transmittances of the resultant films increased, but their electrical resistivity, direct optical band gap (Eg) values, and compressive residual stress decreased. The as-deposited amorphous Cu–Cr–O films crystallized to the delafossite structure of the CuCrO2 phase and showed p-type conductivity. The maximum optical transmittance of the films was as high as 80 % at a visible wavelength of 740 nm. In addition, the electrical resistivity of the films decreased from 34.72 to 14.73 Ω cm as annealing time increased from 30 to 120 min but became saturated after 240 min. These results indicate that the CuCrO2 films obtained in this work show promising optoelectronic properties under a suitable annealing time.

Control of p-type conduction in Mg doped monophase CuCrO2 thin layers

This work aims to clarify the origin of hole conduction in undoped and Mg-doped CuCrO2 oxide in order to have the possibility of controlling it by corresponding growth parameters. A chemical spray pyrolysis procedure for the deposition of p-type semiconductor thin films is described. The as-deposited films were amorphous. The formation of highly crystalline CuCrO2 and Mg-doped CuCrO2 films with a single phase delafossite structure was realized by annealing between 600 °C and 960 °C in a nitrogen atmosphere.The carrier concentration and the point defects of the samples are calculated by using the developed Kroger method of quasi-chemical reactions. p-type conductivity was predicted and observed in the undoped and Mg doped CuCrO2 sample, and with n ~ 1018 cm−3 carrier concentrations for 4%Mg doping. The electrical resistivity for a 4% Mg doped sample was 1.4 Ω·cm with a Seebeck coefficient of +130 μV K−1 at 40 °C. By electroparamagnetic resonance spectroscopy Cr3+ and Cu2+ related defects were studied.

Investigation of structural, morphological, optical, and electrical properties of CuCrO2 films prepared by sol–gel method

High-quality p-type CuCrO2 thin films have been deposited on (001) sapphire substrates by the sol–gel method which is low cost, easy setup, large area coating, and mass production. The sol–gel derived films were pretreated at 400 °C for 20 min in air and then annealed at 600–1000 °C for 1 h in flowing nitrogen gas (N2). The influence of annealing temperature on the structural and optoelectronic properties of CuCrO2 thin films was systematically investigated. The microstructure, surface morphology, optical, and electrical properties of the films was characterized by XRD, FE-SEM, UV–Vis spectrometer, and Hall effect measurement. It shows that single-phase delafossite–CuCrO2 films were all highly c-axis oriented; the surfaces were smooth and consist of fine particles, whose grain sizes change with annealing temperatures. The maximal transmittance of CuCrO2 films can reach 90 % in the visible region with the optical band gaps ranging between 3.15 and 3.2 eV. The lowest resistivity was obtained with annealing at 800 °C, and a resistivity of 92.85 Ω cm was achieved at room temperature. The results suggest that the temperature in the annealing process plays a primary role in the formation of delafossite–CuCrO2 thin films. High-quality p-type CuCrO2 thin films have been deposited on (001) sapphire substrates by the sol–gel method annealed at 600–1000 °C for 1 h in flowing nitrogen gas (N2). The optimum values obtained for CuCrO2 film with the thickness of ~142 nm were ρ = 92.85 Ω cm and T = 90 %, indicating that CuCrO2 film may be a candidate for transparent applications.

Effect of annealing temperature on structural, optoelectronic properties and interband transitions of CuCrO2 nanocrystalline films prepared by the sol–gel method

Transparent conductive CuCrO2 oxide films were prepared on sapphire substrates by the sol–gel method using copper (II) acetate monohydrate and chromium (III) nitrate hydrate as raw materials. The highly c-axis orientation and optical transparency (60–80%) in the visible region were obtained. The microstructure, vibration modes, optical and electrical transport properties as a function of annealing temperatures have been systematically discussed. With increasing annealing temperature, the crystalline quality and the c-axis orientation of CuCrO2 films were improved, which induced the increase of Raman phonon amplitude with the decrease of the linewidth. But the film compactness and density decrease with the increase of average particle size. In order to study the intrinsic mechanism of the optical response behavior, a three-phase layered structure (air/film/substrate) was applied to simulate the transmittance spectra of CuCrO2 films. Three electronic transitions can be uniquely assigned and it is found that the direct transitions are the dominant effect for CuCrO2 optical properties. The temperature dependence of electrical conductivity was studied and the electrical conduction mechanisms of the thermal activation behavior and the three-dimensional variable range-hopping transport were observed. In addition, the conductivity of CuCrO2 films decreases with increasing annealing temperature induced by its microstructure and the grain boundary scattering. Thus, the film annealed at 600 °C has the largest electrical conductivity of 0.49 S m−1 at room temperature. The present results could be crucial and provide theoretical support for future applications of p-type semiconductor and optoelectronic devices.

Formation and characterization of p-type semiconductor CuCrO2 thin films prepared by a sol–gel method

This study adopted the sol–gel method to synthesize p-type semiconductor CuCrO2 films and analyzed the effects of an annealing treatment, under a controlled argon atmosphere by changing the temperature and time, on the phase transformation, micro- and nano-structure, composition, and semiconductor properties of thin films. In the Cu–Cr–O phase transformation system, CuO, Cr2O3, and CuCr2O4 were the intermediate phases of the reaction for forming CuCrO2: in the metastable state reaction process, the composite phases changed into a single phase, CuCrO2; in the stable-state reaction process of CuCrO2, carbon elements of precursors were released and eliminated; and finally the optoelectronic properties of the CuCrO2 thin film were adjusted and changed. The CuCrO2 thin film possessed cell- and polygon-like shaped microstructures. The carbon content in the CuCrO2 film decreased, so the copper, chromium, and oxygen contents increased accordingly. The optical band gap of CuCrO2 thin film increased from 2.81eV to 3.05eV, while the resistivity decreased. The nanoscale crystal was identified which also of the delafossite CuCrO2 structure. Using the sol–gel method to prepare the CuCrO2 thin films, an appropriate annealing temperature and time were helpful in forming the single-phase CuCrO2; the decrease of precursor elements in the thin film could enhance the band gap and the conductivity of the material.

Towards delafossite structure of Cu–Cr–O thin films deposited by reactive magnetron sputtering: Influence of substrate temperature on optoelectronics properties

Cu–Cr–O thin films were co-sputtered from metallic Cu and Cr targets in the presence of a reactive argon–oxygen gas mixture. Evolution of the coatings composition as a function of the discharge current dissipated on each target allowed to obtain convenient composition. The films deposited at ambient temperature (without intentional heating of substrate) were initially amorphous and need to be annealed at different temperature under vacuum to achieve delafossite structure. In this case, the delafossite structure presented a thermal instability to the annealing temperature. CuCr2O4 and CuO as the secondary phases were clearly detected by XRD analysis, and could evidently affect the CuCrO2 films optoelectronic properties. Cu–Cr–O thin films were also deposited at high temperatures (substrate temperature varied from 923 to 1083 K) to investigate the influence of substrate temperature on the structure, morphology and optoelectronic properties of the films. Relatively, raising the substrate temperature up to 1033 K was better to obtain CuCrO2 delafossite phase and improved their optoelectronic properties. The optimum conductivity and transparency were achieved for the film deposited at about 1033 K with figure of merit of 6.18 × 10−13 Ω−1 (σ ≈ 1.34 × 10−2 S cm−1 and the maximum visible transmittance up to 43%).