Xiaofeng Xu

Hydrophilic Cu2ZnSnS4 nanocrystals for printing flexible, low-cost and environmentally friendly solar cells

Using Cu2ZnSnS4 (CZTS) nanocrystal-based ink (via a solvothermal route) and roll-to-roll printing, CZTS films are prepared on a Mo-coated Al foil, and then flexible solar cells with a structure of Al foil/Mo/CZTS/ZnS/i-ZnO/ITO/Al–Ni and a power conversion efficiency of 1.94% are constructed, in which all the materials are low-cost and environmentally friendly.

In situ synthesis of P3HT-capped CdSe superstructures and their application in solar cells

Organic/inorganic hybrid solar cells have great potentials to revolutionize solar cells, but their use has been limited by inefficient electron/hole transfer due to the presence of long aliphatic ligands and unsatisfying continuous interpenetrating networks. To solve this problem, herein, we have developed a one-pot route for in situ synthesis of poly(3-hexylthiophene) (P3HT)-capped CdSe superstructures, in which P3HT acts directly as the ligands. These CdSe superstructures are in fact constructed from numerous CdSe nanoparticles. The presence of P3HT ligands has no obvious adverse effects on the morphologies and phases of CdSe superstructures. Importantly, higher content of P3HT ligands results in stronger photoabsorption and fluorescent intensity of CdSe superstructure samples. Subsequently, P3HT-capped CdSe superstructures prepared with 50 mg P3HT were used as a model material to fabricate the solar cell with a structure of PEDOT:PSS/P3HT-capped CdSe superstructures: P3HT/Al. This cell gives a power conversion efficiency of 1.32%.

In situ preparation of CuInS2 films on a flexible copper foil and their application in thin film solar cells

The in situ preparation of semiconductor films on a flexible metal foil has attracted increasing attention for constructing flexible solar cells. In this work, we have developed an in situgrowth strategy for preparing CuInS2 (CIS) films by solvothermally treating flexible Cu foil in an ethylene glycol solution containing InCl3·4H2O and thioacetamide with a concentration ratio of 1u2006:u20062. The effects of solvothermal temperature, time and concentration on the morphology and phase of the CIS films are investigated. Solvothermal temperature has no obvious effect on the morphology of the final films, but higher temperature is favorable for the growth of CIS films with higher crystallinity. Reactant concentration plays a significant role in controlling the morphology of CIS films; if InCl3·4H2O concentration is relatively low (≤0.042 M), single-layered CIS films can be produced, which are composed of high ordered potato chips shaped nanosheets, otherwise, it prefers to form a double-layered film, for which the lower layer is similar CIS ordered nanosheets while the upper layer is composed of flower shaped superstructures. A possible mechanism of the CIS films is also investigated. UV-vis measurements show that all these CIS films possess a direct bandgap energy of 1.48 eV, appropriate for the absorption of the solar spectrum. Finally, single-layered CIS films on Cu foil were employed for fabricating flexible solar cells with a structure of Cu foil/CuInS2/CdS/i–ZnO/ITO/Ni–Al, and the resulting cells yield a power conversion efficiency of 0.75%. Further improvement of the efficiencies of the solar cells can be expected by optimizing the morphology, structure and composition of the CIS films, as well as the fabrication technique.

Fabrication of ZnO/CdS/Cu2ZnSnS4 p–n heterostructure nanorod arrays via a solution-based route

ZnO/CdS/Cu2ZnSnS4 p–n heterostructure nanorod arrays were fabricated through a novel solution-based method using ZnO nanorod arrays as the template. Typical ZnO/CdS/Cu2ZnSnS4 heterostructure nanorod arrays consist of ZnO/CdS core/sheath nanorods and Cu2ZnSnS4 nanocrystallines grown on the primary ZnO/CdS core/sheath nanorods. In the ZnO/CdS/Cu2ZnSnS4 heterostructure nanorods arrays, ZnO core and CdS sheath are single crystalline, while Cu2ZnSnS4 nanoparticles are polycrystalline. The possible formation mechanism of ZnO/CdS/Cu2ZnSnS4 heterostructure nanorod arrays is also discussed in detail. The ZnO/CdS/Cu2ZnSnS4 heterostructure nanorod arrays possess a broad and enhanced optical absorption from the ultraviolet to the near-infrared region. A good rectification characteristic is observed in the I–V curve of ZnO/CdS/Cu2ZnSnS4 nanorod, which confirms that the ZnO/CdS/Cu2ZnSnS4 nanorod is a p–n heterostructure. Micro-Raman spectroscopy was used to investigate the vibrating properties of ZnO/CdS/Cu2ZnSnS4 heterostructure nanorod arrays. The results indicate that the ZnO/CdS/Cu2ZnSnS4 p–n heterostructure nanorod arrays may shed light on new opportunities for nanoscale optoelectronic devices.

In Situ Atom Scale Visualization of Domain Wall Dynamics in VO2 Insulator-Metal Phase Transition

A domain wall, as a device, can bring about a revolution in developing manipulation of semiconductor heterostructures devices at the atom scale. However, it is a challenge for these new devices to control domain wall motion through insulator-metal transition of correlated-electron materials. To fully understand and harness this motion, it requires visualization of domain wall dynamics in real space. Here, domain wall dynamics in VO2 insulator-metal phase transition was observed directly by in situ TEM at atom scale. Experimental results depict atom scale evolution of domain morphologies and domain wall exact positions in (202) and (040) planes referring to rutile structure at 50°C. In addition, microscopic mechanism of domain wall dynamics and accurate lattice basis vector relationship of two domains were investigated with the assistance of X-ray diffraction, ab initio calculations and image simulations. This work offers a route to atom scale tunable heterostructure device application.

Negative capacitance switching via VO2 band gap engineering driven by electric field

We report the negative capacitance behavior of an energy band gap modulation quantum well with a sandwich VO2 layer structure. The phase transition is probed by measuring its capacitance. With the help of theoretical calculations, it shows that the negative capacitance changes of the quantum well device come from VO2 band gap by continuously tuning the temperature or voltage. Experiments reveal that as the current remains small enough, joule heating can be ignored, and the insulator-metal transition of VO2 can be induced by the electric field. Our results open up possibilities for functional devices with phase transitions induced by external electric fields other than the heating or electricity-heat transition.

Formation of Ge Nanosheets Decorated Hierarchical ZnSe/GeSe Nanowire Heterostructures

We report on the fabrication of Ge nanosheets decorated ZnSe/GeSe hierarchical heterostructure nanowires via a simple one-step thermal evaporation of ZnSe and Ge powder. A typical Ge nanosheets decorated ZnSe/GeSe hierarchical heterostructure nanowire consists of ZnSe/GeSe bi-coaxial nanowire, secondary nanostructures of GeSe nanobrushes are grown on the primary ZnSe/GeSe bi-coaxial nanowire, Ge nanosheets are decorated on the GeSe nanobrushes. The possible growth mechanism of the novel hierarchical nanowire heterostructures is discussed. In addition, the vibrating properties of Ge decorated ZnSe/GeSe hierarchical heterostructure nanowires are studied by using Raman spectroscopy.

Growth, structural and vibrating properties of CdSe–Ge, CdSe–Ge–CdSe, CdSe–Ge/Ge, Ge–GeSe heterostructure nanowires and GeSe nanobelts

CdSe–Ge, CdSe–Ge–CdSe heterostructure nanowires, CdSe–Ge biaxial nanowire core/polycrystalline Ge sheath heterostructures, Ge–GeSe biaxial nanowires and GeSe nanobelts were grown via a simple one-step thermal evaporation of different molar ratios of CdSe and Ge, respectively. The CdSe and Ge subnanowires in CdSe–Ge biaxial nanowires (or triaxial nanowires) and the Ge and GeSe subnanowires in Ge–GeSe biaxial nanowires are single crystalline. A good epitaxial relationship exists in the interface between CdSe and Ge in CdSe–Ge biaxial nanowires and in the interface between Ge and GeSe in Ge–GeSe biaxial nanowires. Two sides of CdSe subnanowires in the CdSe–Ge–CdSe triaxial nanowire have an obvious differential in microstructure is just induced by the view angle. A structural model for the crystallographic relationship between CdSe and Ge in CdSe–Ge biaxial nanowires is given. The possible growth mechanism of CdSe–Ge based heterostructure nanowires is proposed as the co-growth mechanism. The vibrating properties of CdSe–Ge based heterostructure nanowires were investigated by micro-Raman spectroscopy. We observe a LO mode of CdSe, a LO (TO) mode of Ge and a LO and TO mode of GeSe in the five different nanostructures have the different wave-number shift in comparison with that of the responding bulk counterpart, respectively.

Fabrication of single-crystal/phase Cu2ZnSnS4 nanorods via a two-step spin coating route

In this study, we develop a two-step spin coating route without a template and use it to fabricate Cu2ZnSnS4 nanorods. Cu2ZnSnS4 nanorods with a diameter of about 30 nm were nearly perpendicular to the molybdenum-coated soda lime glass substrate. The results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy confirm that the as-synthesized products are single-crystal/phase Cu2ZnSnS4 nanorods. The effects of various factors on the formation of Cu2ZnSnS4 nanorods are also investigated. Hall effect measurement reveals that the Cu2ZnSnS4 nanorod film has p-type conductivity.

Vanadium dioxide thin films for smart windows: optical design and performance improvement

The high quality vanadium dioxide (VO2) thin films have been fabricated successfully on sapphire by a simple novel sputtering oxidation coupling (SOC) method. Transmittance spectra of vanadium dioxide film have been measured between 25 °C and 90 °C. The thin film samples exhibit a good insulator-metal transition near room temperature. The optical constants of VO2 thin film samples were derived by fitting the transmittance spectra using the Drude-Lorentz model. In order to improve the transition efficiency, the thin film thickness was optimized by the optical design. The calculated results with different thin film thickness show that VO2 thin film with 84 nm owns a maximums value of the transition efficiency. This research will promote VO2 thin film optical performance improvement for the smart windows.