•The divalent Zn cation with low-toxicity, high-richness, good stability is successfully introduced to the perovskite films.•The addition of Zn effectively suppressed the notorious hysteresis, making the PCE value more reliable.•The approach provides a possibility to achieve the less-Pb perovskite layer by optimizing the fabrication process of films.The high efficiency and low-toxicity are the two main objectives for all perovskite solar cells (PSCs) towards large scale commercialization. Herein, ecology-benign Zn was introduced to partially replace the noxious Pb in the typical Pb-based perovskite absorber layer owing to its low-toxicity, high-richness in the crust, suitable valence states, and good stability against oxidation and reduction. It is found that the addition of Zn with a certain amounts significantly improves the crystalline quality, enlarges the grain sizes in the perovskite thin films, reduces the non-radiative recombination loss and lengthens the carrier lifetime, leading to the improved photovoltaic performance and eliminated hysteresis. It is demonstrated that Zn doping is an effective strategy to replace Pb to achieve superior solar cell performance with reduced Pb usage.Download high-res image (297KB)Download full-size image

•Developed a controlled monolayer-by-monolayer Pt deposition using a dual buffer strategy.•The present Pt group metal loading is 25 times lower than the U.S. DOE 2017 target value.•This development may pave a way to fabricate superior Pt catalysts with the minimal Pt usage.A controlled monolayer-by-monolayer deposition process has been developed to fabricate Pt coating on carbon fiber paper with complex network structures using a dual buffer (Au/Ni) strategy. The X-ray diffraction, electrochemical quartz crystal microbalance, current density analyses, and X-ray photoelectron spectroscopy results conclude that the monolayer deposition process accomplishes full coverage on the substrate and that the thickness of the deposition layer can be controlled on a single atom scale. This development may pave a way to fabricate superior Pt catalysts with the minimal Pt usage. In fact, the present Pt group metal loading is 25 times lower than the U.S. DOE 2017 target value.A controlled monolayer-by-monolayer deposition process has been developed to fabricate Pt coating on carbon fiber paper with complex network structures using a dual buffer strategy. This development may pave a way to fabricate superior Pt catalysts with the minimal Pt usage. In fact, the present Pt group metal loading is 25 times lower than the U.S. DOE 2017 target value.Download high-res image (162KB)Download full-size image

Photodetectors (PDs), especially those that respond in the infrared region, are highly desirable and have a wide range of applications ranging from cell phones, cameras, and home electronics to airplanes and satellites. Herein, we designed and fabricated PDs based on air-stable α-CsPbI3 QDs and an up-conversion material (NaYF4:Yb,Er QDs) using a facial low temperature spin-coating method. When the α-CsPbI3 QDs are surface-modified using NaYF4:Yb,Er QDs, their optical response is extended to the NIR region to allow broadband application from the UV to visible to NIR region (260 nm–1100 nm). The optoelectronic properties and compositional stability of the devices were also studied in detail. From the results, the PDs are capable of broad-bandwidth photodetection from the deep UV to NIR region (260 nm–1100 nm) with good photoresponsivity (R, 1.5 A W−1), high on/off ratio (up to 104) and very short rise/decay time (less than 5 ms/5 ms). It was found that the photoresponsivity performance of the PDs in this work is better than that of all the other previously reported perovskite QD-based PDs with a lateral device structure. Furthermore, the device performance shows very little degradation over the course of 60 days of storage under ambient conditions. The combination of remarkable stability, high performance broad-bandwidth photodetection, and easy fabrication suggest that these QDs are a very promising semiconducting candidate for optoelectronic applications.

The electron transport layer (ETL), which also serves as the hole-blocking layer, is a key component in planar perovskite solar cells (PSCs). The commonly used ETL is an anatase-TiO2 (an-TiO2) film due to its excellent optical transmittance, chemical stability and semiconducting characteristics. Nevertheless, its rough surface and plenty of surface defects often lead to a substandard perovskite film and large J–V hysteresis. Herein, a novel low-trap-density ETL is developed by surface modification of the an-TiO2 film using small-molecular ITIC. As a result, the device efficiency has been dramatically increased from 17.12% to 20.08%, entering the league of the highest planar-type perovskite cells. Moreover, the J–V hysteresis has been significantly reduced. Further investigation shows that the ITIC smoothens the TiO2 surface, passivates defects or dangling bands parasitizing the TiO2 surface, and optimizes the device band alignment. In addition, it is demonstrated that the thin ITIC promotes the formation of high quality, uniform perovskite films with better surface coverage and large grain size, implying that there is a synergistic effect between the low-trap-density ITIC and high-mobility TiO2 in improved PSC performance.

Inorganic lead halide perovskite nanocrystals (CsPbCl3 NCs) with excellent ultraviolet (UV) light absorption, high carrier mobility, long carrier diffusion lengths, and long-term stability are good candidates as smart materials for transparent optoelectronic devices. In this study, transparent UV photodetectors (PDs) based on CsPbCl3 NCs were fabricated for the first time. The optimized device exhibited visible light transmittance approximately 90%, strong absorption of UV light in the wavelength from 300 nm to 410 nm, good photoresponsivity (1.89 A W−1), and a high on/off ratio (up to 103). Meanwhile, the rise and decay response times of the device were less than 41 ms and 43 ms, respectively. Furthermore, we performed detailed analysis of the effects by employing CsPbCl3 NCs in assembled films and final devices using various characterization methods. The simple fabrication and remarkable UV photodetection capabilities of CsPbCl3 NCs make them promising semiconducting candidates in optoelectronic applications.

Sustainable hydrogen generation via electrocatalytic/photocatalytic water splitting has been widely regarded as the most promising energy carrier and has attracted extensive attention. However, a considerable hydrogen evolution reaction (HER) always involves the rare noble metals. Herein, we report a new HER candidate, an Fe-doped NiS2 (Fe–NiS2) nanosheet, with the performance of high activity and electrochemical stability. We chose the sulfidation of Ni(OH)2 under mild calcinated temperature for Fe–NiS2 formation. The theoretical and experimental results suggest that the Fe3+ doping into the surface lattice of the NiS2 (002) facet lowers the activation energy of H2 generation. The synthesized Fe–NiS2 sample shows good electrocatalytic HER performance with a low Tafel slope of 37 mV dec−1 and a small overpotential of 121 mV at 10 mA cm−2. Moreover, Fe–NiS2 gives considerable stability with a negligible loss of HER value l after a reaction of 1100 min. The Fe–NiS2 sample is also in situ loaded onto the surface of CdS nanorods to act as a co-catalyst for photocatalytic H2 generation with the result of 3.2 mmol h−1 g−1 hydrogen evolution under visible light, 46 times higher than with bare CdS. This work can help us to design new electrocatalysts for water splitting; it also provides a good understanding of the hydrogen evolution pathway.

The effect of maximum incident light absorption, conversion and utilization by a semiconductor on solar fuel generation was investigated in this study. Sub-15 nm g-C3N4–TiO2 (CN–TiO2) was synthesized through a hydrothermal process at a relatively high temperature. Three samples with different TiO2 sizes, i.e. 9, 12 and 15 nm, were obtained by changing the pH of solution and named CN–TiO2-9, CN–TiO2-12 and CN–TiO2-15. Based on the Mie scattering law, the nano-sized heterojunction samples can achieve almost 100% incident light absorption without reflection. Characterization results from XRD and FTIR indicate that the samples are composed of protonated g-C3N4 and anatase TiO2. Further results from TEM images provide information on the size of the synthesized hybrid samples. It is established that the two components together show sub-15 nm particle size. The nano-sized heterojunction delivered considerable solar-to-hydrogen conversion efficiency with the apparent quantum yield (AQY) of 6.9% under 405 nm visible light irradiation. Moreover, it is interesting to find that the AQY values do not decrease when increasing the incident photon flux. The large absorption cross-section area and the prolonged lifetime of photogenerated carriers of the sub-15 nm CN–TiO2 heterojunction are the origin of the high photon-to-electron conversion.

•E-beam evaporated Nb2O5 film can be directly used as ETL for large area rigid and flexible PSCs with decent PCEs.•Nb2O5 shows negligible light absorption in Vis-IR region and suitable for solar cell application.•Nb2O5 film shows efficient carrier extraction from excited perovskite film and less interface recombination.E-beam evaporated Nb2O5 film is directly used as an effective electron transport layer (ETL) for perovskite solar cells without needing any posttreatment. The effect of Nb2O5 thickness on optical and electronic properties of the perovskite layer deposited thereupon are studied in detail. It is found that 60 nm thick Nb2O5 ETL delivers the best photovoltaic performance with PCE as high as 18.59%. In particular, e-beam evaporated Nb2O5 is found to be advantageous in large area flexible perovskite solar cells, with larger area cells showing comparable Jsc and Voc values as smaller area devices, and the PCE loss is mainly caused by increased series resistance leading to reduced FF. With proper cell design to limit the resistance and associated FF loss, it is expected that the large area cells should present respectable FF and PCEs as their smaller area devices.Download high-res image (259KB)Download full-size image

Regulating the temperature during the direction contact and intercalation process (DCIP) for the transition from PbI2 to CH3NH3PbI3 modulated the crystallinity, crystal grain size and crystal grain orientation of the perovskite films. Higher temperatures produced perovskite films with better crystallinity, larger grain size, and better photovoltaic performance. The best cell, which had a PCE of 12.9%, was obtained on a film prepared at 200 °C. Further open circuit voltage decay and film resistance characterization revealed that the larger grain size contributed to longer carrier lifetime and smaller carrier transport resistance, both of which are beneficial for solar cell devices.

•NiFe2O4/α-Ni(OH)2 was synthesized through one-pot simple hydrothermal method.•NiFe2O4/α-Ni(OH)2 showed the enhanced OER performance compared to Ni(OH)2.•The photocatalytic co-catalyst ability of NiFe2O4/α-Ni(OH)2 was better than PtOx.Water splitting has been intensively investigated as a promising solution to resolve the future environmental and energy crises. The oxygen evolution reaction (OER) of the photo- and electric field-induced water splitting limits the development of other reactions, including hydrogen evolution reaction (HER). Fe, Ni and NiFe (hydro) oxide-based catalysts are generally acknowledged among the best candidates of OER catalysts for water splitting. Herein, we developed a one-pot simple hydrothermal process to assemble NiFe2O4 nanoparticles onto the α-Ni(OH)2 nanosheets. The first formed NiFe2O4 under high temperature and pressure environment induces and assists the α-Ni(OH)2 formation without any further additives, because the distance between the neighboring Ni atoms in the cubic NiFe2O4 is similar to that in the α-Ni(OH)2 {003} facets. We have synthesized a series of NiFe2O4/α-Ni(OH)2 compounds and find that the overpotential decreases with the increase of Ni(OH)2 content while the OER kinetics stays unchanged, suggesting that Ni(OH)2 plays a major role in overpotential while NiFe2O4 mainly affects the OER kinetics. The obtained NiFe2O4/α-Ni(OH)2 compounds is also found to be a promising co-catalyst for the photocatalytic water oxidation. In fact, it is even more active than the noble PtOx with acceptable stability for the oxygen generation.

•PL of the ITO film is enhanced significantly by the Au NPs, so do the macroscopic conductance and the microscopic C-AFMI–V characteristics.•The surface work function of the ITO film (φs) is changed by the Au NPs, the longer time the Au NP deposition, the larger φs.•When the Au NP/ITO sample is illuminated, its φs downshifts.•The Jsc of the solar cells is increased by 20.78% by the Au NPs.Gold nanoparticles (Au NPs) were prepared on the top of the indium tin oxide (ITO) window layer in thin-film silicon solar cells using a magnetron sputtering method. In comparison with the pure ITO film, the samples with the Au NPs show higher optical transmittance. It is found that the photoluminescence (PL) intensities from the ITO films are enhanced significantly by the Au NPs, so do the macroscopic conductance and the microscopic conductive atomic microcopy (C-AFM) current–voltage (I–V) characteristics. It also appears that the surface work function (φs) is changed by the Au NPs, the longer time the Au NP deposition, the more Au NPs on surface, the larger φs. When the sample is illuminated, its φs downshifts due to the local surface plasmon resonance (LSPR) excited by the illumination on the Au NPs. It is found that the short circuit current density (Jsc) of the solar cells is increased by as much as 20.78% for the optimized solar cell configuration.

•TCOs with different surface energy, will affect the perovskite crystal grain size.•FTO shows the fastest electron extraction and ITO is the slowest.•AZO possesses the largest charge carrier recombination resistance and ITO has the smallest one.The effect of transparent conductive oxide (TCO) on the performance of compact-layer free perovskite solar cells has been investigated. The results show that the perovskite film formation, charge transfer and charge recombination are affected by the TCO layer. All common TCOs, including ITO, FTO and AZO, show some beneficial and detrimental characters for solar cell application. Finally, the compact-layer free perovskite solar cells based on them show similar performance.

•CuO@ZnO core–shell cubes with controlled shell thickness were synthesized using a facile two-step solution route.•CuO@ZnO cubes offers 2.6 times higher response comparing to the pristine CuO sensor.•The p-type conducting behavior of the CuO@ZnO cubes was revealed.CuO@ZnO core–shell cubes with controlled shell thickness were synthesized using a facile and environment-benign two-step solution route. A bending behavior was observed when electrical testing was conducted on the CuO@ZnO core–shell heterostructure coating, demonstrating formation of heterojunction barrier. Moreover, the CuO@ZnO appears to be a superior material for p-type sensor applications, it offers 2.6 times higher response, faster recovery rate and better selectivity comparing to the pristine CuO sensor. The enhanced gas sensing performance can be attributed to the coating of ZnO shell and improvement of the effectively electrical contacts between heterojunction materials.

•We synthesis MoS2/sulfur and nitrogen co-doped graphene (SNG) nanocomposite via a two-step hydrothermal route.•S and N atoms doped into the graphene network can further enhance conductivity of the nanocomposite.•MoS2/SNG nanocomposite exhibits excellent activity and stability in the HER.Cost-effective materials for electrocatalytic water splitting are key to renewable energy research. In this work, MoS2/sulfur and nitrogen co-doped reduced graphene oxide (SNG) nanocomposite was produced via a two-step hydrothermal process. The formation of nanocomposite was confirmed by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM). It is found that the MoS2/SNG nanocomposite exhibits much higher catalytic activity for the hydrogen evolution reaction (HER) relative to MoS2 and MoS2/RGO catalysts, as manifested by much larger cathodic current density with smaller overpotential of −120 mV and lower Tafel slope of 45 mV dec−1. It is believed that the outstanding performance is attributed to the high electronic conductivity of SNG for fast charge transport. This study highlights the significance of the strong electronic coupling effect between the MoS2 and SNG in the enhancement of HER electrocatalytic activity.The MoS2/SNG nanocomposite exhibited excellent catalytic activity for HER due to the strong electronic coupling effect between MoS2 and SNG.

•CuO thin films were prepared by PLD technique.•n-Type CuO thin films can be obtained at oxygen pressure of 8–12 Pa.•CuO thin film deposited at 5 Pa has the lowest resistivity of 9.5×101 Ω cm.CuO thin films were deposited on glass substrates by pulsed laser deposition (PLD) and the influence of oxygen (O2) pressure on the structural and electrical properties of the CuO films was studied. X-ray diffraction (XRD) measurements reveal that the films deposited at 3×10−3 Pa, 3–5 Pa, and 8–12 Pa show a single Cu2O phase, mixed Cu2O–CuO phase, and single CuO phase, respectively. The preferential orientation of CuO films varies from (111) to (002) as the O2 pressure increases from 5 to 8 Pa. Hall effect measurements and gas-sensing property study demonstrate that CuO films deposited at 3–5 Pa show p-type conductivity while those deposited at 8–12 Pa show n-type conductivity. The CuO film deposited at 5 Pa has the highest carrier concentration of 5.0×1017 cm−3 and the lowest resistivity of 9.5×101 Ω cm.

Color-tuned perovskite films have been recognized as a promising candidate for building integrated photovoltaics; bright, colorful displays; and component cells in multijunction solar cell applications. In this paper, four representative color-tuned perovskite films with chemical formula CH3NH3PbBrxI3–x (x = 0, 1, 2, and 3) are successfully prepared by using a technique that combines the advantages of direct contact lead halide film with hot methylamine halide powder and intercalcation processes. The energy-dispersive X-ray spectrometry results indicate that the Br/I ratio is controlled as desired. The scanning electron microscopy imaging shows very uniform films with good surface coverage on the substrate. The highest power conversion efficiency of the perovskite solar cells with the four different compositions are 12.76%, 6.84%, 4.12%, and 3.53%, respectively.

•We fabricate gallium@graphene core–shell nanoparticles using a PLD technique.•Ga NP cores are completely encapsulated by the multi-layer graphene shell.•Graphene shell can contribute extra enhancement to the SERS signal.•Our study provide a new method to fabricate synergistic SERS substrates.We report our successful fabrication of a new gallium@graphene core–shell (Ga@G) nanostructure consisting of graphene grown on the surface of Ga nanoparticles (NPs) and its application as a high-performance substrate for surface enhanced Raman scattering (SERS). To our knowledge, this is the very first report to fabricate Ga@G NPs, particularly using a pulsed laser deposition (PLD) technique. The Raman results demonstrate that higher substrate temperature is beneficial for the formation of better quality graphene. Transmission electron microscopy (TEM) images reveal that Ga NP cores are completely encapsulated by the multi-layer graphene shell. More importantly, Ga@G NPs show significantly higher Raman signal for the absorbed Rhodamine 6G (R6G) molecules as compared to using the bare Ga NPs, validating the Ga@G NPs as a new type of SERS substrate.

Spectroscopic ellipsometry (SE), high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and optical transmittance measurements were used to study and establish a correlation between the open-circuit voltage (Voc) of solar cells and the p-layer optical band gap (Ep). It is found that the ellipsometry measurement can be used as an inline non-destructive diagnostic tool for p-layer deposition in commercial operation. The analysis of ellipsometric spectra, together with the optical transmittance data, shows that the best p-layer appears to be very fine nanocrystallites with an Ep 1.95 eV. HRTEM measurements reveal that the best p-layer is composed of nanocrystallites ~9 nm in size. It is also found that the p-layer exhibits very good transmittance, as high as ~91.6% at ~650 nm. These results have guided us to achieve high Voc value 1.03 V for thin film silicon based single junction solar cell.

•Lateral match of front and back textures in Si film solar cell was proposed.•Influence of textures lateral position on light absorption was studied.•Optimum textures lateral position for maximum light absorption was obtained.We systematically study the influence of the relative lateral position of the front and back periodic textures on the light absorption in microcrystalline Si thin film solar cells by finite difference time domain method. We show that there is an optimum lateral position match between the periodic front and back textures, which allows maximum light absorption to be obtained in the Si thin film solar cells. A relative lateral shift between the front and back periodic textures breaks the symmetry of the conformal cell structure, which can result in more wave modes in the Si thin film solar cells.

•ZnO nanorods with a range of diameter were synthesized by a one-step solvothermal method.•Studied the kinetics and proposed a growth mechanism.•ZnO nanorods prepared with 20% ethanol in solvent are Zn-rich – a beneficial factor to gas sensing response.•ZnO nanorods (a20) have the best crystallinity as revealed in XRD analysis. Gas sensor fabricated using a20 gave the best alcohol vapor response.ZnO nanorods with a range of diameter were synthesized by a one-step solvothermal method via adjusting ethanol content in solvents. Material characterization has included XRD, SEM, PL and XPS. It was found that diameter of the ZnO nanorods can be regulated by adjusting ethanol content in solvent. When the ethanol content was increased from 10% to 50%, average diameter of the resultant ZnO nanorods decreased from 360 to 220 nm monotonically. It is also found that ZnO nanorods (a20) prepared with 20% ethanol in solvent has the best crystallinity as revealed in XRD analysis. PL and XPS analyses suggest that the ZnO nanorods are Zn-rich – a beneficial factor to gas sensing response. Gas sensor fabricated using a20 gave the best alcohol vapor response.

An one-step solvothermal synthesis technique has been developed to prepare uniform ZnO nanorods for gas sensor applications. Material characterization has included X-ray diffraction (XRD), scanning electron microcopy (SEM) and photoluminescence (PL). It has been found that ZnO nanorods prepared with ethanol solvent not only exhibit cleaner and smoother surfaces, larger crystallite size, reduced strain, smaller diameter and more donor-related surface defects, but also gave better gas sensing performance comparing to the ZnO nanorods prepared with pure water solvent. It was found that at alcohol level of 500 ppm, such sensor showed the response of 142, among the highest reported values achieved for ZnO nanorod sensors.

•Cu2S/brass-mesh has been successfully developed as a semi-transparent counter electrode.•Achieving reliable dual-side illumination for the first time in CdS sensitized solar cells.•Electrochemical study shows that Cu2S brass-mesh counter electrode has high electrocatalytic activity.Cu2S/brass-mesh has been successfully developed as a counter electrode (CE) in a CdS quantum dot sensitized solar cell (QDSSC) to attain power conversion efficiency (PCE) of 1.81% under front-side light illumination, equivalent to 2.01% for Cu2S/brass-sheet CE based QDSSC. Furthermore, the Cu2S/brass-mesh CE achieved a PCE of 0.48% upon light illumination from CE side for the first time. It appears that the Cu2S/brass-mesh CE can be used to fabricate cost-effective dual-side illuminated QDSSCs for light-weight applications.Cu2S/brass-mesh has been successfully developed as a semi-transparent counter electrode in CdS sensitized solar cells for reliable dual-side illumination for the first time.

Sustainable hydrogen generation via electrocatalytic/photocatalytic water splitting has been widely regarded as the most promising energy carrier and has attracted extensive attention. However, a considerable hydrogen evolution reaction (HER) always involves the rare noble metals. Herein, we report a new HER candidate, an Fe-doped NiS2 (Fe–NiS2) nanosheet, with the performance of high activity and electrochemical stability. We chose the sulfidation of Ni(OH)2 under mild calcinated temperature for Fe–NiS2 formation. The theoretical and experimental results suggest that the Fe3+ doping into the surface lattice of the NiS2 (002) facet lowers the activation energy of H2 generation. The synthesized Fe–NiS2 sample shows good electrocatalytic HER performance with a low Tafel slope of 37 mV dec−1 and a small overpotential of 121 mV at 10 mA cm−2. Moreover, Fe–NiS2 gives considerable stability with a negligible loss of HER value l after a reaction of 1100 min. The Fe–NiS2 sample is also in situ loaded onto the surface of CdS nanorods to act as a co-catalyst for photocatalytic H2 generation with the result of 3.2 mmol h−1 g−1 hydrogen evolution under visible light, 46 times higher than with bare CdS. This work can help us to design new electrocatalysts for water splitting; it also provides a good understanding of the hydrogen evolution pathway.

The electron transport layer (ETL), which also serves as the hole-blocking layer, is a key component in planar perovskite solar cells (PSCs). The commonly used ETL is an anatase-TiO2 (an-TiO2) film due to its excellent optical transmittance, chemical stability and semiconducting characteristics. Nevertheless, its rough surface and plenty of surface defects often lead to a substandard perovskite film and large J–V hysteresis. Herein, a novel low-trap-density ETL is developed by surface modification of the an-TiO2 film using small-molecular ITIC. As a result, the device efficiency has been dramatically increased from 17.12% to 20.08%, entering the league of the highest planar-type perovskite cells. Moreover, the J–V hysteresis has been significantly reduced. Further investigation shows that the ITIC smoothens the TiO2 surface, passivates defects or dangling bands parasitizing the TiO2 surface, and optimizes the device band alignment. In addition, it is demonstrated that the thin ITIC promotes the formation of high quality, uniform perovskite films with better surface coverage and large grain size, implying that there is a synergistic effect between the low-trap-density ITIC and high-mobility TiO2 in improved PSC performance.