Enhanced open-circuit voltages and efficiencies: the role of oxidation state of molybdenum oxide buffer layer in polymer solar cells

Molybdenum oxide (MoO_x) is widely used as a buffer layer in optoelectronic devices to improve the charge extraction efficiency. The oxidation state of MoO_x plays an important role in determining its electrical properties. However, there are few studies on the oxidation state to further guide the optimization of the MoO_x buffer layer. In this work, inverted-structured polymer solar cells (PSCs) with a MoO_x buffer layer were fabricated. Post-air annealing was used to control the cation valence state in MoO_x. X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM) and transient photocurrent (TPC) were employed to study the valence state, energy level, morphology of the MoO_x layers and the photovoltaic property and charge transfer efficiency of the devices. It was found that the oxidation state was effectively improved by the post-annealing process. As a result, the work function of MoO_x was raised and the hole mobility was improved. The open-circuit voltages and the efficiencies of PTB7-Th:PC₇₁BM based PSCs were enhanced from 0.77 V and 8.66% to 0.81 V and 10.01%, respectively. The results show that high oxidation state MoO_x provides optimized energy level alignment, reduced defects and better charge transfer efficiency, which is more in line with the requirement of buffer layer materials for optoelectronic applications.

The high oxidation state MoO_x buffer layer provides better level alignment and charge transfer efficiency, improving the open-circuit voltages and the efficiencies of polymer solar cells.     

Boosting light harvesting and charge separation of WO3via coupling with Cu2O/CuO towards highly efficient tandem photoanodes

Photoanodes based on semiconductor WO₃ have been attractive due to its good electron mobility, long hole-diffusion length, and suitable valence band potential for water oxidation. However, the semiconductor displays disadvantages including a relatively wide bandgap, poor charge separation and transfer, and quick electron–hole recombination at the interface with the electrolyte. Here we present a significantly improved photoanode with a tandem structure of ITO/WO₃/Cu₂O/CuO, which is prepared first by hydrothermally growing a layer of WO₃ on the ITO surface, then by electrodepositing an additional layer of Cu₂O, and finally by heat-treating in the air to form an exterior layer of CuO. Photocurrent measurements reveal that the prepared photoanode produces a maximum current density of 4.7 mA cm⁻², which is, in comparison, about 1.4 and 5.5 times the measured values for ITO/WO₃/Cu₂O and ITO/WO₃ ones, respectively. These enhancements are attributed to (1) harvested UV, visible, and NIR light of the solar spectrum, (2) accelerated charge separation at the heterojunction between WO₃ and Cu₂O/CuO, (3) better electrocatalytic activity of formed Cu_xO than pure Cu₂O, (4) formation of a protective layer of CuO. This study thus may lead to a promising way to make high-performance and low-cost photoanodes for solar energy harvesting.

A highly efficient photoanode with a tandem structure of ITO/WO₃/Cu₂O/CuO produces a photocurrent about 1.4 and 5.5 times the measured values for ITO/WO₃/Cu₂O and ITO/WO₃ ones, respectively.     

Modified top-down approach for synthesis of molybdenum oxide quantum dots: sonication induced chemical etching of thin films

A simple and modified top-down approach to synthesize molybdenum oxide (MoO_x: x = 2, 3) quantum dots (QDs) is proposed in this study. This modified approach involves the conversion of a bulk powder material into thin films followed by a sonication induced chemical etching process for synthesising QDs. X-Ray Diffraction (XRD) is used for crystal structural characterization of MoO_x thin films. The crystal structure properties of the MoO_x QDs are analysed by High Resolution Transmission Electron Microscopy (HRTEM) images and corresponding Selected Area Electron Diffraction (SAED) patterns. The optical band gap is estimated by Tauc''s plot from UV-Vis-NIR absorption spectra. The excitation dependent photoluminescence (PL) emission of MoO_x QDs as a function of acid concentration is investigated. The growth mechanism of QDs in different crystalline phases as a function of acid concentration is also exemplified in this work. The micro-Raman and Fourier Transform of Infrared (FTIR) spectra are recorded to analyse the vibrational spectrum of the molybdenum–oxygen (Mo–O) bonds in the MoO_x QDs.

A simple and modified top-down approach to synthesize molybdenum oxide (MoO_x: x = 2, 3) quantum dots (QDs) is proposed in this study.     

Impact of inkjet printed ZnO electron transport layer on the characteristics of polymer solar cells

In this paper, we demonstrate that zinc oxide (ZnO) layers deposited by inkjet printing (IJP) can be successfully applied to the low-temperature fabrication of efficient inverted polymer solar cells (i-PSCs). The effects of ZnO layers deposited by IJP as electron transport layer (ETL) on the performance of i-PSCs based on PTB7-Th:PC₇₀BM active layers are investigated. The morphology of the ZnO-IJP layers was analysed by AFM, and compared to that of ZnO layers deposited by different techniques. The study shows that the morphology of the ZnO underlayer has a dramatic effect on the band structure and non-geminate recombination kinetics of the active layer deposited on top of it. Charge carrier and transient photovoltage measurements show that non-geminate recombination is governed by deep trap states in devices made from ZnO-IJP while trapping is less significant for other types of ZnO. The power conversion efficiency of the devices made from ZnO-IJP is mostly limited by their slightly lower J_SC, resulting from non-optimum photon conversion efficiency in the visible part of the solar spectrum. Despite these minor limitations their J–V characteristics compare very favourably with that of devices made from ZnO layer deposited using different techniques.

In this paper, we demonstrate that zinc oxide (ZnO) layers deposited by inkjet printing (IJP) can be successfully applied to the low-temperature fabrication of efficient inverted polymer solar cells (i-PSCs).     

Barrier thickness dependence of MgxZn1−xO/ZnO quantum well (QW) on the performance of a p-NiO/QW/n-ZnO photodiode

An Mg_xZn_1−xO/ZnO quantum well (QW) structure, with various barrier (Mg_xZn_1−xO layer) thicknesses, was inserted into p-NiO/n-ZnO heterojunction photodiodes (HPDs) by using a radio-frequency magnetron sputtering system. The effect of various barrier thicknesses on the performance of QW-PDs was investigated. A band diagram shows that the QW-PD with 10 nm barrier layer presents a tunneling carrier transport mechanism, the UV- and visible-generated carriers tunnel through the thin barrier layer. Whereas the QW-PDs with thicker (≧25 nm) barrier layers show recombination-tunneling carrier transport. The visible-generated carriers are effectively confined within the well layer in the QW structure, causing the visible-response to be greatly reduced by more than 3 orders compared to that in the QW-PD with a 10 nm barrier layer. However, on further increasing the barrier thickness beyond 25 nm, the visible-response will no longer be reduced. In contrast, with decreasing the barrier thickness from 60 to 25 nm, the UV-response increases due to the overlap increase of the fundamental electron and hole wave function in the quantum well. Such a result drastically enhances the rejection ratio (320 nm/500 nm) from 264 for QW-PDs with a 10 nm barrier to 2986 for QW-PDs with a 25 nm barrier layer by a 11.3 ratio.

An Mg_xZn_1−xO/ZnO quantum well (QW) structure, with various barrier (Mg_xZn_1−xO layer) thicknesses, was inserted into p-NiO/n-ZnO heterojunction photodiodes (HPDs) by using a radio-frequency magnetron sputtering system.     

Construction of Ag-modified TiO2/ZnO heterojunction nanotree arrays with superior photocatalytic and photoelectrochemical properties

The work involves the preparation of TiO₂/ZnO heterojunction nanotree arrays by a three-step: hydrothermal, sol–gel, and secondary hydrothermal method, and then modification of Ag quantum dots (QDs). In the above process, the ZnO nanoparticles attached to the TiO₂ surface were subjected to secondary growth by a hydrothermal method to form a unique nanotree structure with TiO₂, followed by Ag quantum dot modification by quantum dot deposition. In summary, TiO₂/ZnO nanotree arrays are cited for the first time. The prepared Ag-modified TiO₂/ZnO heterojunction nanotree arrays were found to exhibit enhanced photoelectrochemical and photocatalytic properties. The photocurrent of the Ag-modified TiO₂/ZnO heterojunction nanotree arrays is increased by 8-fold compared to the pure TiO₂ nanorod arrays, the photocatalytic degradation rate within 180 minutes increased from 37% to 77% and the kinetic rate constants for the degradation of methyl orange were three times higher than the pure TiO₂ nanorod arrays. The improved performance is partly due to the introduction of the TiO₂/ZnO heterojunction nanotree arrays which provide Ag QDs with greater adhesion area. Localized surface plasmon resonance (LSPR) leads to an increase in the intensity of absorbed light due to the modification of Ag QDs. On the other hand the generation of the TiO₂/ZnO heterojunction decreases the forbidden band width, resulting in the redshift of the light absorption edge. Therefore, TiO₂/ZnO heterojunction nanotree arrays are expected to play an important role in solar cells and photocatalytic materials.

The work involves the preparation of TiO₂/ZnO heterojunction nanotree arrays by a three-step: hydrothermal, sol–gel, and secondary hydrothermal method, and then modification of Ag quantum dots (QDs).     

Effect of Ce doping on the structure–activity relationship of MoVOx composite metal oxides

A series of Ce-doped MoVO_x composite metal oxide catalysts were prepared by the rotary evaporation method. The effects of Ce doping ratio on the crystal phase composition, morphology and surface properties of the catalysts were investigated. The results show that the crystal phase composition of samples with different Ce doping content is also obviously different. When the doping amount is small, V_0.95Mo_0.97O₅ is the main crystal phase, while MoO₃ is dominant when the doping amount is large. The Ce-doped catalyst showed obvious rod-shaped morphology and its average single point pore diameter and the number of acidic sites increased. Compared to the un-doped MoVO_x, the pore size of the sample synthesized at a Mo/Ce atomic ratio of 10/1 exhibited an increase of 41.11 nm. In addition, the effect of Ce doping on the catalytic performance of MoVO_x was investigated with the selective oxidation of benzyl alcohol as a probe reaction. After doping, the MoVO_x catalyst showed improved benzyl alcohol conversion and selectivity to benzaldehyde. At a Mo/Ce atomic ratio of 10/1, the conversion rate of benzyl alcohol reaches 83.26%, which is 64.56% higher than that without doping, and the highest product yield can reach 76.47%.

Ce-doped MoVO_x with disperse rod-shaped exhibits excellent catalytic performance in selective oxidation of benzyl alcohol.     

Controlling the oxidation state of molybdenum oxide nanoparticles prepared by ionic liquid/metal sputtering to enhance plasmon-induced charge separation

Nanoparticles composed of molybdenum oxide, MoO_x, were successfully prepared by room-temperature ionic liquid (RTIL)/metal sputtering followed by heat treatment. Hydroxyl groups in RTIL molecules retarded the coalescence between MoO_x NPs during heat treatment at 473 K in air, while the oxidation state of Mo species in MoO_x nanoparticles (NPs) could be modified by changing the heat treatment time. An LSPR peak was observed at 840 nm in the near-IR region for MoO_x NPs of 55 nm or larger in size that were annealed in a hydroxyl-functionalized RTIL. Photoexcitation of the LSPR peak of MoO_x NPs induced electron transfer from NPs to ITO electrodes.

MoO_x NPs, prepared by sputtering Mo metal on a room-temperature ionic liquid (RTIL) followed by heating in air, produced anodic photocurrents with the excitation of their LSPR peak.     

Cation exchange synthesis of CuInxGa1−xSe2 nanowires and their implementation in photovoltaic devices

CuIn_xGa_1−xSe₂ (CIGS) nanowires were synthesized for the first time through an in situ cation exchange reaction by using CuInSe₂ (CIS) nanowires as a template material and Ga-OLA complexes as the Ga source. These CIGS nanowires maintain nearly the same morphology as CIS nanowires, and the Ga/In ratio can be controlled through adjusting the concentration of Ga-OLA complexes. The characteristics of adjustable band gap and highly effective light-absorbances have been achieved for these CIGS nanowires. The light-absorbing layer in photovoltaic devices (PVs) can be assembled by employing CIGS nanowires as a solar-energy material for enhancing the photovoltaic response. The highest power conversion efficiency of solar thin film semiconductors is more than 20%, achieved by the Cu(In_xGa_1−x)Se₂ (CIGS) thin-film solar cells. Therefore, these CIGS nanowires have a great potential to be utilized as light absorber materials for high efficiency single nanowire solar cells and to generate bulk heterojunction devices.

CuIn_xGa_1−xSe₂ (CIGS) nanowires were synthesized for the first time through an in situ cation exchange reaction by using CuInSe₂ (CIS) nanowires as a template material and Ga-OLA complexes as the Ga source.     

The synthesis of a Bi2MoO6/Bi4V2O11 heterojunction photocatalyst with enhanced visible-light-driven photocatalytic activity

A novel Bi₂MoO₆/Bi₄V₂O₁₁ heterostructured photocatalyst was successfully fabricated using a facile one-pot solvothermal method. This heterojunction consists of homogeneous dispersed Bi₄V₂O₁₁ nanocrystals anchored on the surface of Bi₂MoO₆ nanoflakes, endowing the heterojunction with nanosized interfacial contact. Based on the favorable interfacial contact, the band alignment at the heterojunction effectively facilitated photo-generated carrier transfer, which was verified by photoelectrochemical and photoluminescence measurements. Thereby, in contrast with pristine Bi₂MoO₆ and Bi₄V₂O₁₁, the as-synthesized heterojunction with nanoscale contact exhibited significantly enhanced photocatalytic activity towards the degradation of MB and the reduction of Cr(vi). In addition, the as-fabricated Bi₂MoO₆/Bi₄V₂O₁₁ heterojunction exhibited good cycling stability for MB degradation after 4 cycles. Finally, a plausible photocatalytic mechanism for MB degradation over the Bi₂MoO₆/Bi₄V₂O₁₁ heterojunction was discussed in detail. This work not only reports a highly efficient photocatalyst but also sheds light on the design and optimization of a heterojunction.

A Bi₂MoO₆/Bi₄V₂O₁₁ heterojunction exhibits remarkably enhanced photocatalytic activity for MB degradation and Cr(vi) reduction under visible-light illumination compared to its pristine samples.     

Formamidinium-based planar heterojunction perovskite solar cells with alkali carbonate-doped zinc oxide layer

We herein demonstrate n-i-p-type planar heterojunction perovskite solar cells employing spin-coated ZnO nanoparticles modified with various alkali metal carbonates including Li₂CO₃, Na₂CO₃, K₂CO₃ and Cs₂CO₃, which can tune the energy band structure of ZnO ETLs. Since these metal carbonates doped on ZnO ETLs lead to deeper conduction bands in the ZnO ETLs, electrons are easily transported from the perovskite active layer to the cathode electrode. The power conversion efficiency of about 27% is improved due to the incorporation of alkali carbonates in ETLs. As alternatives to TiO₂ and n-type metal oxides, electron transport materials consisting of doped ZnO nanoparticles are viable ETLs for efficient n-i-p planar heterojunction solar cells, and they can be used on flexible substrates via roll-to-roll processing.

Planar formamidinium perovskite solar cells have been fabricated with an alkali carbonate-doped zinc oxide layer.     

Facile synthesis of a novel WO3/Ag2MoO4 particles-on-plate staggered type II heterojunction with improved visible-light photocatalytic activity in removing environmental pollutants

A novel WO₃/Ag₂MoO₄ heterojunction has been synthesized through a facile precipitation method with Ag₂MoO₄ particles firmly deposited on the surface of WO₃ nanoplates, forming “particles-on-plate” type II heterojunction structures. This heterojunction exhibited improved photocatalytic activities for the degradation of rhodamine B (RhB), 4 chlorophenol (4-CP) and tetracycline hydrochloride (TC) under visible-light irradiation compared to pure Ag₂MoO₄ and WO₃. In addition, the heterojunction with 10 wt% Ag₂MoO₄ displays the best photocatalytic performance, which was about 2 times better than that of pure WO₃ or Ag₂MoO₄. The TC photodegradation rate reaches up to 91% within 90 min visible light irradiation. Furthermore, the photocatalytic efficiency of the Ag₂MoO₄/WO₃ heterojunction is 1.3 times higher than that of the mixture of the two individual photocatalysts. This remarkable enhanced photocatalytic performance results from the staggered bandgap between Ag₂MoO₄ and WO₃, which can suppress the recombination of electron–hole pairs efficiently. Moreover, based on the radical trapping experiment, the superoxide radical anions (·OH) and photogenerated holes (h⁺) are the crucial active oxidizing species.

A novel WO₃/Ag₂MoO4 heterojunction was synthesized and it showed excellent photodegradation performance and stability for several sorts of organic pollutants.     

A visible-light phototransistor based on the heterostructure of ZnO and TiO2 with trap-assisted photocurrent generation

Visible-light phototransistors have been fabricated based on the heterojunction of zinc oxide (ZnO) and titanium oxide (TiO₂). A thin layer of TiO₂ was deposited onto the spin-coated ZnO film via atomic layer deposition (ALD). The electrical characteristics of the TiO₂ layer were optimized by controlling the purge time of titanium isopropoxide (TTIP). The optimized TiO₂ layer could absorb the visible-light from the sub-gap states near the conduction band of TiO₂, which was confirmed via photoelectron spectroscopy measurements. Therefore, the heterostructure of TiO₂/ZnO can absorb and generate photocurrent under visible light illumination. The oxygen-related-states were investigated via X-ray photoelectron spectroscopy (XPS), and the interfacial band structure between TiO₂ and ZnO was evaluated via ultraviolet photoelectron spectroscopy (UPS). Oxygen-related states and subgap-states were observed, which could be used to generate photocurrent by absorbing visible light, even with TiO₂ and ZnO having a wide bandgap. The optimized TiO₂/ZnO visible-light phototransistor showed a photoresponsivity of 99.3 A W⁻¹ and photosensitivity of 1.5 × 10⁵ under the illumination of 520 nm wavelength light. This study provides a useful way to fabricate a visible-light phototransistor based on the heterostructure of wide bandgap oxide semiconductors.

Visible-light phototransistors have been fabricated based on the heterojunction of zinc oxide (ZnO) and titanium oxide (TiO₂).     

Modification of NiOx hole transport layer for acceleration of charge extraction in inverted perovskite solar cells

The modification of the inorganic hole transport layer has been an efficient method for optimizing the performance of inverted perovskite solar cells. In this work, we propose a facile modification of a compact NiO_x film with NiO_x nanoparticles and explore the effects on the charge carrier dynamic behaviors and photovoltaic performance of inverted perovskite devices. The modification of the NiO_x hole transport layer can not only enlarge the surface area and infiltration ability, but also adjust the valence band maximum to well match that of perovskite. The photoluminescence results confirm the acceleration of the charge separation and transport at the NiO_x/perovskite interface. The corresponding device possesses better photovoltaic parameters than the device based on control NiO_x films. Moreover, the charge carrier transport/recombination dynamics are further systematically investigated by the measurements of time-resolved photoluminescence, transient photovoltage and transient photocurrent. Consequently, the results demonstrate that proper modification of NiO_x can significantly enlarge interface area and improve the hole extraction capacity, thus efficiently promoting charge separation and inhibiting charge recombination, which leads to the enhancement of the device performances.

The NiO_x layer modified with NiO_x nanoparticles obtains surface property optimization and energy level modulation, thus improving charge transport and device performance.     

Improved performance and stability of perovskite solar cells with bilayer electron-transporting layers

Zinc oxide nanoparticles (NPs) are very promising in replacing the phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as electron-transporting materials due to the high carrier mobilities, superior stability, low cost and solution processability at low temperatures. The perovskite/ZnO NPs heterojunction has also demonstrated much better stability than perovskite/PC₆₁BM, however it shows lower power conversion efficiency (PCE) compared to the state-of-art devices based on perovskite/PCBM heterojunction. Here, we demonstrated that the insufficient charge transfer from methylammonium lead iodide (MAPbI₃) to ZnO NPs and significant interface trap-states lead to the poor performance and severe hysteresis of PSC with MAPbI₃/ZnO NPs heterojunction. When PC₆₁BM/ZnO NPs bilayer electron transporting layers (ETLs) were used with a device structure of ITO/poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA)/MAPbI₃/PC₆₁BM/ZnO NPs/Al, which can combine the advantages of efficient charge transfer from MAPbI₃ to PC₆₁BM and excellent blocking ability of ZnO NPs against oxygen, water and electrodes, highly efficient PSCs with PCE as high as 17.2% can be achieved with decent stability.

Perovskite solar cells with PC₆₁BM/ZnO nanoparticles bilayer electron-transporting layers were achieved with a power conversion efficiency of 17.2% and decent stability.

Organic–inorganic hybrid perovskite solar cells (PSCs) have recently attracted tremendous attention because of their excellent photovoltaic efficiencies.^1–4 Since the initial results published in 2009 with efficiencies about 4% using a typical dye-sensitized solar cell structure with liquid electrolyte,⁵ significant progress has been made in device performance through developing high quality film processing methods,^6–10 tuning the perovskite composition,^11–15 optimizing the device architectures^16,17 and synthesizing new hole/electron transport materials.^18–21 Recently, a certified record power conversion efficiency (PCE) of 22.7% was achieved.²² Despite of the success in obtaining dramatically improved PCE, there are certain concerns about the stability and cost towards commercialization. For the state-of-the-art PSCs, perovskites are susceptible to degradation in moisture and air, thus the charge transport materials should prevent the perovskite from exposure to such environments.^20,23–25 One the other hand, PSCs also suffer from the high cost of widely used organic charge transport materials such as 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD), phenyl-C_61/71-butyric acid methyl ester (PC_61/71BM).^3,18,26 As alternatives, inorganic materials such as CuSCN,²⁷ CuI,²⁸ CuGaO₂,²⁰ and NiO_x ^29,30 which can be acted as hole transport materials and ZnO,^31,32 SnO₂ ^12,33,34 and TiO₂ ^10,35 which can be acted as electron transport materials are widely studied. Among them, metal oxide nanoparticles (NPs) are very promising in replacing the organic counterparts due to the high carrier mobilities, superior stability, low cost and solution processability at low temperatures.^16,31,33The perovskite/ZnO NPs heterojunction has been demonstrated much better stability than perovskite/PCBM,²³ however it shows lower PCE compared to the state-of-art devices based on perovskite/PCBM heterojunction.^36–38 Thus in this paper, we systematically studied the charge transfer and recombination at CH₃NH₃PbI₃ (MAPbI₃) and ZnO NPs or PC₆₁BM interfaces and tried to fabricate devices with high PCE and super stability simultaneously. We demonstrated that insufficient charge transfer from MAPbI₃ to ZnO NPs and significant interface trap-states lead to the poor performance and severe hysteresis of PSCs based on MAPbI₃/ZnO NPs heterojunction, while the devices based on MAPbI₃/PC₆₁BM show high PCE and negligible hysteresis due to the efficient charge transfer from MAPbI₃ to PC₆₁BM and less recombination at the interface. On the other hand, the MAPbI₃/ZnO NPs devices show excellent stability in air because of the excellent capping ability of ZnO NPs while the stability of MAPbI₃/PC₆₁BM devices is very poor. Thus, we fabricated the PSCs with bilayer electron-transporting layers (ETLs) with the device structure of ITO/poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA)/MAPbI₃/PC₆₁BM/ZnO NPs/Al, trying to combine the advantages of efficient charge extraction ability of PC₆₁BM and excellent blocking ability of ZnO NPs against oxygen, water and electrode, and finally device with PCE as high as 17.2% was achieved with decent stability.     

Synthesis of 2D MoS2(1−x)Se2x semiconductor alloy by chemical vapor deposition

Alloying/doping in two-dimensional (2D) materials is emerging as an increasingly important strategy due to the wide-range bandgap tunability and versatility of these materials. Monolayer 2D transition metal dichalcogenide (TMD) alloy has been investigated both theoretically and experimentally in recent years. Here, we synthesized a bilayer MoS_2(1−x)Se_2x semiconductor alloy via the chemical-vapor deposition technique. The as-grown triangular MoS_2(1−x)Se_2x flakes with size of roughly 10 μm were observed by optical microscope and scanning electron microscope (SEM). The 1.4–1.9 nm thickness of the samples, as measured by AFM, means that bilayer MoS_2(1−x)Se_2x alloys were grown. The characteristic Raman modes related to Mo–S and Mo–Se vibrations were observed in the Raman spectrum. Two emission peaks were respectively found, corresponding to the A and B excitons in the photoluminescence (PL) spectrum. XPS measurements confirmed the Se doping of the alloy. The first-principles calculation results show a contraction of the band gap value with the increase of Se doping in the MoS₂ lattice. Compared with monolayer MoS_2(1−x)Se_2x alloy, the band bending effect is more obvious, and the bilayer MoS_2(1−x)Se_2x alloy still shows the direct band gap luminescence characteristic, which has certain guiding significance for the growth of two-dimensional materials and for device preparation.

Alloying/doping in two-dimensional (2D) materials is emerging as an increasingly important strategy due to the wide-range bandgap tunability and versatility of these materials.     

Photocatalytic properties of Cu-containing ZnO nanoparticles and their antifungal activity against agriculture-pathogenic fungus

In this work, nanoparticles (NPs) of ZnO, ZnO with Cu incorporated at 2 and 30 wt%, and CuO were prepared by the hydrothermal method. X-ray diffraction pattern (DRX) analysis showed that ZnO with high Cu incorporation (30 wt%) generates the formation of a composite oxide (ZnO/CuO), while X-ray photoelectron spectroscopy (XPS) of the Cu (2 wt%) sample indicated that Cu is incorporated as a dopant (ZnO/Cu_2%). The samples with Cu incorporated had enhanced visible light absorption. Methyl orange (MO) dye was used to perform photocatalytic tests under UV radiation. The antifungal activity of the NPs was tested against four agricultural phytopathogenic fungi: Neofusicoccum arbuti, Alternaria alternata, Fusarium solani, and Colletotrichum gloeosporioides. The ZnO/Cu_2% nanoparticles showed adequate photocatalytic and high antifungal activity in comparison to pure oxides and the composite sample.

In this work, nanoparticles (NPs) of ZnO, ZnO with Cu incorporated at 2 and 30 wt%, and CuO were prepared by the hydrothermal method.     

Designing a novel visible-light-driven heterostructure Ni–ZnO/S-g-C3N4 photocatalyst for coloured pollutant degradation

In this study, photocorrosion of ZnO is inhibited by doping Ni in the ZnO nanostructure and electron–hole recombination was solved by forming a heterostructure with S-g-C₃N₄. Ni is doped into ZnO NPs from 0 to 10% (w/w). Among the Ni-decorated ZnO NPs, 4% Ni-doped ZnO NPs (4NZO) showed the best performance. So, 4% Ni–ZnO was used to form heterostructure NCs with S-g-C₃N₄. NZO NPs were formed by the wet co-precipitation route by varying the weight percentage of Ni (0–10% w/w). Methylene blue (MB) was used as a model dye for photocatalytic studies. For the preparation of the 4NZO-x-SCN nanocomposite, 4NZO NPs were formed in situ in the presence of various concentrations of S-g-C₃N₄ (10–50% (w/w)) by using the coprecipitation route. The electron spin resonance (ESR) and radical scavenger studies showed that O₂⁻ and OH free radicals were the main reactive species that were responsible for MB photodegradation.

Ni-doped ZnO/S-g-C₃N₄ nanocomposites were formed as a novel heterostructure photocatalyst.     

Localized incorporation of cesium ions to improve formamidinium lead iodide layers in perovskite solar cells

For the perovskite solar cells with formamidinium lead iodide (FAPbI₃) as a light harvester, cesium ions (Cs⁺) can be used to stabilize the perovskite crystal structure of FAPbI₃. However, the incorporation of Cs⁺ ions usually reduces the grain size and degrades the crystallization of FAPbI₃ layers, and this is harmful to the photovoltaic performance of solar cells. In this work, we incorporate Cs⁺ ions into FAPbI₃ layers using the interfacial doping method, which is different from the mixed solution doping method in previous reports. Elemental analysis indicates that Cs⁺ dopants cannot be detected at the outer surfaces of perovskite layers, and the majority of Cs⁺ dopants should be localized in the vicinity of TiO₂/perovskite interfaces, which is remarkably different from the distribution of Cs⁺ dopants in the perovskite layers prepared using the mixed solution doping method. It is found that interfacial doping method can avoid the blue shift of the light absorption edge and can improve the crystallization of FAPbI₃ layers. For the optimized conditions, Cs_xFA_1−xPbI₃ solar cells prepared using the interfacial doping method achieve a power conversion efficiency (PCE) of 17.1%, which is better than the PCE of Cs_xFA_1−xPbI₃ devices prepared using the mixed solution doping method.

An interfacial doping method leads to a localized profile of dopants at interfaces, which results in improved photovoltaic performance.     

n–n ZnO–Ag2CrO4 heterojunction photoelectrodes with enhanced visible-light photoelectrochemical properties

In this study, ZnO nanorods (NRs) were hydrothermally grown on an Au-coated glass substrate at a relatively low temperature (90 °C), followed by the deposition of Ag₂CrO₄ particles via a successive ionic layer adsorption and reaction (SILAR) route. The content of the Ag₂CrO₄ particles on ZnO NRs was controlled by changing the number of SILAR cycles. The fabricated ZnO–Ag₂CrO₄ heterojunction photoelectrodes were subjected to morphological, structural, compositional, and optical property analyses; their photoelectrochemical (PEC) properties were investigated under simulated solar light illumination. The photocurrent responses confirmed that the ability of the ZnO–Ag₂CrO₄ heterojunction photoelectrodes to separate the photo-generated electron–hole pairs is stronger than that of bare ZnO NRs. Impressively, the maximum photocurrent density of about 2.51 mA cm⁻² at 1.23 V (vs. Ag/AgCl) was measured for the prepared ZnO–Ag₂CrO₄ photoelectrode with 8 SILAR cycles (denoted as ZnO–Ag₂CrO₄-8), which exhibited about 3-fold photo-enhancement in the current density as compared to bare ZnO NRs (0.87 mA cm⁻²) under similar conditions. The improvement in photoactivity was attributed to the ideal band gap and high absorption coefficient of the Ag₂CrO₄ particles, which resulted in improved solar light absorption properties. Furthermore, an appropriate annealing treatment was proven to be an efficient process to increase the crystallinity of Ag₂CrO₄ particles deposited on ZnO NRs, which improved the charge transport characteristics of the ZnO–Ag₂CrO₄-8 photoelectrode annealed at 200 °C and increased the performance of the photoelectrode. The results achieved in the present work present new insights for designing n–n heterojunction photoelectrodes for efficient and cost-effective PEC applications and solar-to-fuel energy conversions.

ZnO NRs hydrothermally grown on Au coated glass substrate, followed by deposition of Ag₂CrO₄ particles via SILAR route. The content of the Ag₂CrO₄ particles on the ZnO NRs were controlled by changing the number of SILAR cycles.