Improved Carrier Separation Research Articles

Cobalt-phosphate modified Fe-Zn0.2Cd0.8S/CuSbS2 heterojunction photoanode with multiple synergistic effect for enhancing photoelectrochemical water splitting

Carrier separation efficiency and visible light response range were key factors limiting the development of photoelectrochemical (PEC) water splitting. In this paper, multiple synergistic mechanisms of metal ion doping-heterojunctions and heterojunction-cocatalysts were proposed to broaden the visible light response range and improve carrier separation efficiency. The Fe-Zn0.2Cd0.8S/CuSbS2/Co-Pi was firstly synthesized by hydrothermal and electrochemical deposition, to study the multiple synergistic effects of the composite. Compared with the initial Zn0.2Cd0.8S (2.83 eV), the optical band gap of Fe-Zn0.2Cd0.8S/CuSbS2 was 2.44 eV, which was due to the synergistic effect of ion doping and heterojunction to improve its optical properties. The photocurrents of the ion doping and the heterojunction used alone were 0.24 mA/cm2 and 0.27 mA/cm2 at 0.9 V vs. RHE, respectively, and the photocurrent using the synergistic effect of the ion doping and the heterojunction was 0.31 mA/cm2 at 0.9 V vs. RHE. On the basis of this, the synergistic effect of the heterojunction-cocatalysts made the photocurrent 0.35 mA/cm2 at 0.9 V vs. RHE, which was 1.75 times of the initial Zn0.2Cd0.8S. Fe-Zn0.2Cd0.8S/CuSbS2/Co-Pi had the best photo-corrosion resistance, which was corroded to 94% at 6000 s, compared to Fe-Zn0.2Cd0.8S/CuSbS2 (85%) and Zn0.2Cd0.8S (78%). This multiple synergistic design not only enhances the PEC properties of Zn0.2Cd0.8S, but also expands the approach of constructing the ternary sulfide photoanode.

Photocatalytic Properties Dependent on the Interfacial Defects of Intergrains within TiO2 Mesocrystals.

The regulation of interfacial defects of nanocrystals aligned orderly in a superstructured photocatalyst is an effective approach to improve the photocatalytic performance. However, the synthesis of ordered self-assemblies with abundant interfacial defects and reactive sites is hard to achieve, and applications are limited due to the unclear physicochemical properties, which result from the unique mesocrystalline microstructures. It is reported herein that the photocatalytic properties depend on the interfacial defects of intergrains in anatase TiO2 mesocrystals (TMCs). Research reveals that the photocatalytic activity largely depends on defects, such as lattice distortion and oxygen vacancies, which are located at highly aligned interfaces of intergrains within TMCs. Moreover, the mesocrystalline TiO2 photocatalysts exhibit higher photocatalytic performance in organic degradation and hydrogen evolution, compared with single crystals and polycrystals; this can be ascribed to an appropriate amount of interfacial defects at the intergrains and improved carrier separation efficiency through the highly oriented interfaces. In addition, the photocatalytic performance of the TMCs could be further improved through regulation of defects by undergoing an annealing process under a redox atmosphere. This work can provide an avenue to defect engineering for improved photocatalytic performance.

Direct Visualization of Grain Boundaries in 2D Monolayer WS2 via Induced Growth of CdS Nanoparticle Chains

AbstractTo date, wafer‐scale synthesis of two‐dimensional (2D) materials are well achieved by chemical vapor deposition, but the obtained monolayers typically have multidomains with electrical and optoelectronic properties affected by grain boundaries and domain sizes. When these 2D materials are used as the growth templates, these boundaries would also provide unknown influences to the successive heterostructure formation for extended applications. Here, for the first time, direct visualization of grain boundaries in monolayer WS2 film can be realized by the growth of CdS nanoparticles. Specifically, CdS is found to first preferentially nucleate and form as nanoparticle chains along WS2 grain boundaries in a random manner, independent of the grain boundary characteristics. Due to electron scattering and type II band alignment at the WS2–CdS heterojunction, WS2 reduces in its mobility while becoming enhanced in its electron concentration. Notably, the WS2–CdS heterostructure also yields improved carrier separation and collection for the photodetection performance enhancement. All these results can facilitate the detailed evaluation of crystalline grains‐related information of 2D materials and provide thorough understanding on the effect of these overgrown CdS on underlying WS2 monolayers, being extremely important to further optimize and enable their functionalities for advanced device applications.

The alterations of carrier separation in kesterite solar cells

Cu2ZnSn(S,Se)S4 (CZTSSe) thin films have attracted attention as low-cost absorber materials for solar cells; however, further studies are required to develop flexible solar cells from this material and to achieve a high power conversion efficiency. Toward this objective, this work investigated eight types of precursors applied on flexible Mo foil substrates, some of which also contained a layer of NaF. Secondary phases, defects, and defect clusters were different in the various samples, and the surface electrical characteristics of the CZTSSe absorber layer varied accordingly. In contrast to those in the CIGS-based cells, defects and defect clusters generated in the CZTSSe absorber layer caused an upward band bending-like band structure to form at the grain boundaries (GBs), thereby forming an intra-grain (IG) current path. By improving carrier separation, a flexible CZTSSe thin-film solar cell was developed on a Mo foil substrate with a power conversion efficiency of 7.04%. Thus, the efficiency of CZTSSe thin-film solar cells could be increased through carrier separation measures that enabled the collection of holes toward the GBs and of electrons toward IGs.

Black phosphorus quantum dot/g-C3N4 composites for enhanced CO2 photoreduction to CO

The development of low cost, metal free semiconductor photocatalysts for CO2 reduction to fuels and valuable chemical feedstocks is a practically imperative for reducing anthropogenic CO2 emissions. In this work, black phosphorus quantum dots (BPQDs) were successfully dispersed on a graphitic carbon nitride (g-C3N4) support via a simple electrostatic attraction approach, and the activities of BP@g-C3N4 composites were evaluated for photocatalytic CO2 reduction. The BP@g-C3N4 composites displayed improved carrier separation efficiency and higher activities for photocatalytic CO2 reduction to CO (6.54 µmol g−1 h−1 at the optimum BPQDs loading of 1 wt%) compared with pure g-C3N4 (2.65 µmol g−1 h−1) . This work thus identifies a novel approach towards metal free photocatalysts for CO2 photoreduction.

Realizing the regulated carrier separation and exciton generation of Bi24O31Cl10via a carbon doping strategy

A novel carbon doping strategy induced by an organic salt is successfully developed. Improved carrier separation and exciton binding energies (Eb) are achieved on C-doped Bi24O31Cl10, which provides excellent photocatalytic CO2 reduction and persistent 1O2 generation.

Boosting photoresponse in silicon metal-semiconductor-metal photodetector using semiconducting quantum dots.

Silicon based metal-semiconductor-metal (MSM) photodetectors have faster photogeneration and carrier collection across the metal-semiconductor Schottky contacts, and CMOS integratibility compared to conventional p-n junction photodetectors. However, its operations are limited by low photogeneration, inefficient carrier-separation, and low mobility. Here, we show a simple and highly effective approach for boosting Si MSM photodetector efficiency by uniformly decorating semiconducting CdSe quantum dots on Si channel (Si-QD). Significantly higher photocurrent on/off ratio was achieved up to over 500 compared to conventional Si MSM photodetector (on/off ratio ~5) by increasing photogeneration and improving carrier separation. Furthermore, a substrate-biasing technique invoked wide range of tunable photocurrent on/off ratio in Si-QD photodetector (ranging from 2.7 to 562) by applying suitable combinations of source-drain and substrate biasing conditions. Strong photogeneration and carrier separation were achieved by employing Stark effect into the Si-QD hybrid system. These results highlight a promising method for enhancing Si MSM photodetector efficiency more than 100 times and simultaneously compatible with current silicon technologies.

Interlayer-I-doped BiOIO3 nanoplates with an optimized electronic structure for efficient visible light photocatalysis.

The success in the synthesis of Bi-based layered photocatalysts with high photocatalytic activities has triggered intensive studies. Herein, we prepared interlayer-I-doped BiOIO3 nanoplates by a facile method. Interestingly, it was found that I atoms were doped into the BiOIO3 interlayers instead of substituting for the lattice atoms based on theoretical and experimental results. The interbedded I atoms endowed BiOIO3 with an extended light response from the UV to the visible region by narrowing the bandgap and generating a middle level. The enhanced oxidation capability via positive-shifting the valence band position and improved carrier separation efficiency via forming charge delivery channels at the adjacent two layers can be achieved simultaneously. As expected, I-intercalated BiOIO3 with an optimized electronic structure demonstrated outstanding NO removal ability under visible light irradiation, much superior to pure BiOIO3. The present success in fabricating interlayer-I-doped BiOIO3 would open a promising route to prepare other Bi-based layered semiconductors with efficient visible-light photocatalysis.

Reduced monoclinic BiVO4 for improved photoelectrochemical oxidation of water under visible light

A monoclinic BiVO4 film was grown on a transparent conducting substrate for photoelectrochemical oxidation of water. A photocurrent up to 2.3 mA cm(-2) under visible light (λ > 420 nm) was achieved after treating the sample simply by electrochemical reduction followed by NaBH4. The high photocurrent is believed to be due to the improved carrier separation and transportation as a result of increased donor density.

Carrier dynamics and design optimization of electrolyte-induced inversion layer carbon nanotube-silicon Schottky junction solar cell

Carrier dynamics of the electrolyte-induced inversion layer carbon nanotube-silicon Schottky junction solar cells is explored by numerical simulations. Operation mechanisms of the solar cells with and without the electrolyte-induced inversion layer are presented and compared, which clarifies the current flow mechanisms in a solar cell with an induced inversion layer. A heavily doped back contact layer can behave as a hole block layer. In addition to lowering contact resistance and surface recombination, it is particularly useful for improving carrier separation in an electrolyte-induced inversion layer solar cell or a metal-insulator-semiconductor grating solar cell.

Eco-friendly visible-wavelength photodetectors based on bandgap engineerable nanomaterials

The development of visible-wavelength photodetectors based on eco-friendly and bandgap engineerable nanomaterials represents an important and promising direction in photodetector study. This highlight reviewed the recent progress in the design and construction of photodetectors based on the environmentally friendly binary metal chalcogenides and bandgap-tunable ternary or quaternary compounds such as InSe, In2Se3, ZrS2, Sb2Se3, and CuInSe2etc. Most photodetectors exhibit high sensitivity, fast photoresponse and good stability, benefiting from increased carrier mobility or improved carrier separation. We believe that the continuous effort in this direction will produce promising and practicable photodetectors and may open up new insight into the design of controllable photodetectors and their potential applications in new fields.