Xudong Xiao

Aligned ZnO/CdTe Core−Shell Nanocable Arrays on Indium Tin Oxide: Synthesis and Photoelectrochemical Properties

Vertically aligned ZnO/CdTe core-shell nanocable arrays-on-indium tin oxide (ITO) are fabricated by electrochemical deposition of CdTe on ZnO nanorod arrays in an electrolyte close to neutral pH. By adjusting the total charge quantity applied during deposition, the CdTe shell thickness can be tuned from several tens to hundreds of nanometers. The CdTe shell, which has a zinc-blende structure, is very dense and uniform both radially and along the axial direction of the nanocables, and forms an intact interface with the wurtzite ZnO nanorod core. The absorption of the CdTe shell above its band gap ( approximately 1.5 eV) and the type II band alignment between the CdTe shell and the ZnO core, respectively, demonstrated by absorption and photoluminescence measurements, make a nanocable array-on-ITO architecture a promising photoelectrode with excellent photovoltaic properties for solar energy applications. A photocurrent density of approximately 5.9 mA/cm(2) has been obtained under visible light illumination of 100 mW cm(-2) with zero bias potential (vs saturated calomel electrode). The neutral electrodeposition method can be generally used for plating CdTe on nanostructures made of different materials, which would be of interest in various applications.

Enhancement of low energy sunlight harvesting in dye-sensitized solar cells using plasmonic gold nanorods

This communication describes the use of Ag2S-encapsulated Au nanorods (AuNR@Ag2S) to enhance longer wavelength sunlight utility in dye-sensitized solar cells (DSSCs). We observed that the longitudinal plasmon resonance of AuNRs induces a remarkable increase of 37.6% in photocurrent generation at 600–720 nm. Optical characterizations indicate that the increased optical density and decreased light transmission as a result of AuNRs incorporation engender the striking improvement. With AuNR@Ag2S, the final power conversion efficiency (PCE) of the DSSC with a thin anode (6 μm) increases from 4.3% to 5.6%, which is comparable to that of a pure TiO2 anode based DSSC (5.8%) with a film thickness of 11 μm. Further, incorporation of AuNR@Ag2S into the thick anode leads to the PCE increasing to 7.1%.

Few-Layer MoSe2 Possessing High Catalytic Activity towards Iodide/Tri-iodide Redox Shuttles

Due to the two-dimensional confinement of electrons, single- and few-layer MoSe2 nanostructures exhibit unusual optical and electrical properties and have found wide applications in catalytic hydrogen evolution reaction, field effect transistor, electrochemical intercalation, and so on. Here we present a new application in dye-sensitized solar cell as catalyst for the reduction of I3− to I− at the counter electrode. The few-layer MoSe2 is fabricated by surface selenization of Mo-coated soda-lime glass. Our results show that the few-layer MoSe2 displays high catalytic efficiency for the regeneration of I− species, which in turn yields a photovoltaic energy conversion efficiency of 9.00%, while the identical photoanode coupling with “champion” electrode based on Pt nanoparticles on FTO glass generates efficiency only 8.68%. Thus, a Pt- and FTO-free counter electrode outperforming the best conventional combination is obtained. In this electrode, Mo film is found to significantly decrease the sheet resistance of the counter electrode, contributing to the excellent device performance. Since all of the elements in the electrode are of high abundance ratios, this type of electrode is promising for the fabrication of large area devices at low materials cost.

Conformational engineering of co-sensitizers to retard back charge transfer for high-efficiency dye-sensitized solar cells

We demonstrate that the post-adsorption of small molecules (a phenothiazine-based dye) on the porphyrin-sensitized TiO2 anode surface plays dual roles: (1) to greatly retard the back reaction between conduction-band electrons in TiO2 and the oxidized species (I3−) in the electrolyte and (2) to enhance the spectral response of solar cells. These two effects finally give rise to device efficiencies exceeding 10%, which are superior to those of individual dye-sensitized devices by either porphyrin (7.4%) or phenothiazine (8.2%) under the same conditions. Experimental analyses show that the incoming small molecules are adsorbed in the interstitial site of porphyrin dyes, forming densely surface packed molecules and thus impeding the I3− species from approaching the TiO2 surface. Since a broad range of ruthenium-based dyes and porphyrin-based photosensitizers possess relatively large molecular volumes, this method is anticipated to be applicable for further improving the energy conversion efficiency of devices sensitized by these two classes of dyes.

Recent progress in photocathodes for hydrogen evolution

Solar water splitting, which has been a topic of intensive research interest for several decades, is one of the promising approaches to utilize renewable energy to maintain the sustainable prosperity of our society. However, until now no mature photoelectrochemical cell has been used in practical large-scale applications because of the difficulties to satisfy all the harsh requirements, including high energy conversion efficiency, high stability and low cost. This feature article reviews the recent progress in developing photocathodes for photoelectrochemical cells for solar hydrogen production. Both the development of the p-type semiconductor light absorbers and the efforts to develop synergistic approaches to improve the overall performance of the photocathode are discussed.

Elucidating the Reaction Pathways in the Synthesis of Organolead Trihalide Perovskite for High-Performance Solar Cells

The past two years have witnessed unprecedentedly rapid development of organic–inorganic halide perovskite–based solar cells. The solution–processability and high efficiency make this technology extraordinarily attractive. The intensive investigations have accumulated rich experiences in the perovskite fabrication; while the mechanism of the chemical synthesis still remains unresolved. Here, we set up the chemical equation of the synthesis and elucidate the reactions from both thermodynamic and kinetic perspectives. Our study shows that gaseous products thermodynamically favour the reaction, while the activation energy and “collision” probability synergistically determine the reaction rate. These understandings enable us to finely tune the crystal size for high-quality perovskite film, leading to a record fill factor among similar device structures in the literature. This investigation provides a general strategy to explore the mechanism of perovskite synthesis and benefits the fabrication of high–efficiency perovskite photoactive layer.

The epitaxial growth of ZnS nanowire arrays and their applications in UV-light detection

Compared to random nanowires, vertically aligned nanowire arrays (NWAs) are especially useful as functional parts in future nanowire based devices. Here, vertically aligned ZnS NWAs have been grown on the GaAs (111)B substrates by metal–organic chemical vapor deposition. Ga was chosen as the catalyst and dopant of the ZnS nanowires during growth. The density of nanowires was controlled and their diameter was slightly tuned. The NWAs have been assembled into a photodetector by a low cost and effective method. An individual ZnS nanowire photodetector has also been fabricated for comparison. The two photodetectors are sensitive to UV but not to visible light. Under the same conditions, the photocurrent of the NWA photodetector is more stable, and five orders larger than that of the nanowire photodetector. In addition, the ZnS NWA photodetector has a fast response and an excellent repeatability, showing its potential for fast, visible-blind, UVA detection.

Limitation factors for the performance of kesterite Cu2ZnSnS4 thin film solar cells studied by defect characterization

In this work, photoluminescence (PL), admittance spectroscopy (AS) and drive-level capacitance profiling (DLCP) were performed to analyze the defect properties of a Cu2ZnSnS4 (CZTS) solar cell. Compared to a high efficiency CuInGaSe2 (CIGS) solar cell, the absorber of the CTZS device has larger potential fluctuation which can be attributed to the co-existence of a high concentration of deep acceptor (CuZn) and deep donor (ZnCu) defects. The density of the interface states in the CZTS device is also orders higher than that in the CIGS device. These high density defects (both in the bulk and at the CZTS/CdS interface) will induce a large loss in the open-circuit voltage (Voc), resulting in a lower performance of the CZTS device. We suggest that defect control can be a possible solution to reduce the potential fluctuation induced by acceptors. To overcome the potential fluctuation induced trapping effect for electrons by the ZnCu donors, a graded conduction band similar to CIGS will be good to eliminate electron localization.

Printable highly catalytic Pt- and TCO-free counter electrode for dye-sensitized solar cells.

Here we show that a counter electrode based on carbon network supported Cu2ZnSnS4 nanodots on Mo-coated soda-lime glass for dye-sensitized solar cells can outperform the conventional best electrode with Pt nanoparticles on the fluorine-doped SnO2 conducting glass. In the as-developed electrode, all of the elements are of high abundance ratios with low materials cost. The fabrication is scalable because it is conducted by a screen-printing based approach. Therefore, this research lays a solid ground for the large area fabrication of high-performance dye-sensitized solar cell at reduced material cost.

A low-temperature formation path toward highly efficient Se-free Cu2ZnSnS4 solar cells fabricated through sputtering and sulfurization

A novel low-temperature Cu2ZnSnS4 (CZTS) formation path using co-sputtered SnS2–ZnS–Cu precursors was employed for CZTS solar cell fabrication, which gave rise to a cell with a power conversion efficiency of 8.58%, a big step forward from the previous record of 6.77% reported by Katagiri et al. for this kind of solar cell. This method consists of a low-temperature annealing stage for CZTS phase formation followed by a short high-temperature annealing stage for grain growth and secondary phase removal. The employment of SnS2 as a precursor causes the CZTS phase to readily form at low temperature when SnS2 has not dramatically decomposed into volatile SnS. The two-stage process wisely separates the phase formation and crystal coalescence, which makes the fabrication of CZTS films more controllable. Furthermore, the demonstration of the low-temperature formation path provides new opportunities to fabricate highly efficient, cost-effective and environmentally friendly CZTS solar cells on low-weight and flexible substrates such as polyimides.