S. M. Huang

Large-scale growth of Cu2ZnSnSe4 and Cu2ZnSnSe4/Cu2ZnSnS4 core/shell nanowires

We present a fast and simple protocol for large-scale preparation of quaternary Cu(2)ZnSnSe(4) (CZTSe), as well as CZTSe/Cu(2)ZnSnS(4) (CZTS) core/shell nanowires using CuSe nanowire bundles as self-sacrificial templates. CuSe nanowire bundles were synthesized by reacting Cu(2 - x)Se nanowire bundles with sodium citrate solution. CZTSe nanowires were prepared by reacting CuSe nanowire bundles with Zn(CH(3)COO)(2) and SnCl(2) in triethylene glycol. X-ray diffraction (XRD) and selected area electron diffraction studies show that stannite CZTSe is formed. The formed CZTSe nanowire bundles have diameters of 200-400 nm and lengths of up to hundreds of micrometers. CZTSe/CZTS nanocable bundles with similar morphologies were grown by the addition of some elemental sulfur to the reaction system for growth of CZTSe bundles. The stannite CZTSe/kesterite CZTS core/shell structure of the grown nanocables was confirmed by XRD and high-resolution transmission electron microscope investigation. The influence of S/Se molar ratio in the reaction system on the crystallographic structures and optical properties of CZTSe/CZTS nanocables was studied. The obtained CZTSe/CZTS core/shell nanocable bundles show broad and enhanced optical absorption over the visible and near-infrared region, which is promising for use in photovoltaic applications.

Improvement in performance of GaN-based light-emitting diodes with indium tin oxide based transparent ohmic contacts

InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) with indium tin oxide (ITO) and Ni/Au p-contacts were fabricated. ITO (500 nm) and Ni/Au (2 nm/9 nm) films were deposited onto p-GaN epitaxial layers by an e-beam evaporation system. For the LEDs using in situ annealed ITO and Ni/Au films as p-contacts, the forward voltage at 20 mA was 3.5 and 3.2 V, respectively. Under the same amount of injection current, the LED with in situ annealed ITO p-contact had higher output electroluminescence (EL) intensity and larger light output power. The EL intensities and the light output power of ITO LEDs were enhanced by 85% and 60%, respectively, at 20 mA. As a result, the light output and power conversion efficiency of ITO LEDs on GaN were greatly improved at high injection currents. The fabricated LEDs were subjected to a stress test at 30 mA and 55 °C and showed a very small degradation of optical power (<1% decrease) for 24 h. The light output of MQW LEDs keeps 80% of the original value after 1000 h stressing. Therefore, the fabricated LED devices have demonstrated a good reliability.

Graphene nanosheet counter-electrodes for dye-sensitized solar cells

Graphene nanosheets (GNs) have been investigated as a counter electrode for dye-sensitized solar cells (DSCs). Mesoporous TiO2 films are prepared from the commercial TiO2 nano-powders by screen-printing technique on fluorine-doped tin oxide (FTO) slides. GNs are applied to substitute for platinum as counter-electrode materials. GN films are screen printed on FTO glass using a paste based on GNs dispersed in a mixture of terpineol and ethylcellulose. GN counter-electrodes were prepared by annealing the GN films at different temperatures. A metal-free organic dye (indoline dye D102) is used as a sensitizer. Morphological and electrochemical properties of the formed counter-electrodes are investigated by scanning electronic microscopy and electrochemical impedance spectroscopy (EIS), respectively. The electronic and ionic processes in platinum and GNs based DSCs are analysized and discussed. A conversion efficiency of 2.94 % has been obtained for GNs based DSCs. It is found that the quality of the GN counter-electrode and the photovoltaic performance are strongly affected by the annealing temperature of GN materials.

STUDY OF CHEMICAL BATH DEPOSITION OF ZnS THIN FILMS WITH SUBSTRATE VIBRATION

An improved chemical bath deposition (CBD) technique has been provided to prepare zinc sulfide (ZnS) thin films on glass substrates deposited at 80–82°C using a mixed aqueous solution of zinc sulfate, ammonium sulfate, thiourea, hydrazine hydrate, and ammonia at the alkaline conditions. Both the traditional magnetic agitation and the substrates vibration by hand frequently were done simultaneously during the deposition. The substrates vibration reduced the formation and residence of gas bubbles on the glass substrates during growth and resulted in growth of clean ZnS thin films with high quality. Ammonia and hydrazine hydrate were used as complexing agents. It is found that hydrazine hydrate played an important role in growth of ZnS films. The structure and microstructure of ZnS films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and UV-vis spectroscopic methods. The XRD showed a hexagonal structure. The formed ZnS films exhibited good optical properties with high transmittance in the visible region and the band gap value was estimated to be 3.5–3.70 eV.

Influence Of Cadmium Salts On A Modified Chemical Bath Deposition Of Cadmium Sulfide Thin Films

A simply modified bath deposition technique has been successfully used to deposit uniform CdS thin films using cadmium chloride or cadmium acetate as the cadmium ion source, and thiourea as the sulfur source on glass substrates. Both the traditional magnetic agitation and the frequent substrate vibration by hand were done simultaneously during the deposition. Various properties of the deposited films such as surface morphology, crystallinity, and optical properties were investigated. The CdS films deposited from cadmium acetate show a good crystallinity and exhibit a preferential orientation along the hexagonal (002) direction. Their surface morphologies are more homogeneous with smaller grains than those from cadmium chloride. The CdS films prepared from both cadmium ion sources present a high optical transmission (more than 80%) in the visible range with the band gap value of about 2.4 eV. The substrate vibration reduces the formation and residence of gas bubbles on the glass substrate during growth and resulted in deposition of CdS thin films with high quality. XRD, SEM, and UV–Vis measurements have provided the supported data. The fundamental CBD growth mechanisms involving different cadmium salts are discussed.

Stability of transparent conducting oxide films deposited by sputtering for solar cells applications

The stability of transparent conducting oxide (TCO) films with bi-layered structures has been tested in air at temperatures up to 550 °C. Aluminum-doped ZnO and Sn-doped In2O3 (AZO/ITO) transparent conducting oxide (TCO) films were deposited on glass substrates by a home made radio-frequency (RF) magnetron sputtering system at room temperature in pure argon ambient. A typical commercial ITO and AZO films by mid-frequency (MF) magnetron sputtering at a substrate temperature of 350 °C were also investigated for comparison. A strong decrease of electrical conductivity was observed after testing at temperature above 350°C for RF deposited bi-layered TCO as well as for the commercial ITO, while the AZO films deposited by MF magnetron sputtering showed a quite stable electrical property at temperature not greater than 500 °C. The optical absorption edge was found to shift to the longer wavelength with an increase in testing temperature for all the TCO. MF sputtered AZO films were used as transparent front contacts for the fabrication of copper indium gallium selenide (CIGS) superstrate thin film solar cells.

PREPARATION OF INDIUM TIN OXIDE FILMS BY RADIO FREQUENCY MAGNETRON SPUTTERING UNDER LOW VACUUM LEVEL

Indium tin oxide (ITO) thin films were prepared by radio frequency (RF) magnetron sputtering and under a quite low vacuum level of 2.3 × 10-3 Pa. The sputtering was done in an Ar and O2 gas mixture at a temperature of 200°C. A ceramic In2O3:SnO2 target (10 wt% SnO2) was used. The microstructures of the films were investigated by a field emission scanning electron microscope (FESEM) and an X-ray diffractometer (XRD). X-ray photoelectron spectroscopy (XPS) was performed to characterize the composition of the films. ITO films with a high transparency in the visible wavelength range (80–95%) were obtained. The dependency of the electrical, optical and structural properties of ITO films on both the O2 flow ratio (O2/(O2 + Ar)) and the working pressure was investigated. In the case of low working pressure (1 Pa), the more highly transparent and conducting films were produced at the lower O2 flow ratio. High working pressure (2 Pa) gave rise to low quality, low transparency and amorphous films. Under RF sputtering at low vacuum level, the main contribution to the chamber atmosphere is due to water vapor. Oxygen originating from water vapor dissociation induced by plasma plays the same role as an oxygen or water vapor flux usually intentionally introduced in the system in order to have good quality films.

STUDY OF THE STOICHIOMETRIC RATIO OF ONE-STEP ELECTRODEPOSITED CuInSe2 FILMS ON ITO/SODA-LIME GLASS

Copper indium diselenide (CuInSe2) thin films were grown on indium–tin oxide (ITO)/soda-lime glass using a one-step cathodic electrodeposition process at potentials lower than -0.6 V vs SCE, and in the presence of a large excess of In3+. The source solution contained CuCl2, InCl3, and H2SeO3 complexed by citric acid. The concentration of InCl3 in the electrochemical bath affected the structure, composition, stoichiometric ratio, and morphological properties of electrodeposited films. CuInSe2 films with a chalcopyrite structure and quite good stoichiometry were directly electrodeposited from a solution of 20 mM InCl3, 5 mM CuCl2, and 8 mM H2SeO3. Annealing of these CuInSe2 films in the temperature range from 300°C to 500°C improves their crystallinity and increases their grain size. Good chalcopyrite CuInSe2 films with a (112) preferential orientation suitable for the production of efficient solar cells are obtained after annealing at 500°C. The formation mechanism of the ternary CuInSe2 compound during the electrodeposition process was discussed.

DEPOSITION OF SINGLE-PHASE CuInSe2 THIN FILMS UNDER LOW VACUUM LEVEL BY A TWO-STAGE GROWTH TECHNIQUE

Single-phase CuInSe2 films were grown by high vapor selenization of CuIn alloy precursors within a partially closed graphite box. The CuIn precursors were prepared using CuxIny alloy targets with different composition rates under low vacuum level by a homemade sputtering system. The Cu and In composition rates of the used targets are 11:9, 10:10, and 9:11, respectively. The metallic precursor films were selenized using a two-step temperature profile, i.e. at 250°C and 400–500°C, respectively. The influence of the temperature at the second selenization step on the quality of the CIS absorbing layers was investigated. The CIS films were characterized by X-ray diffractometry, scanning electron microscopy, energy dispersive X-ray analysis, and Raman spectroscopy. The deposited CIS absorbers selenized at a high temperature of 500°C for 30 min exhibited a single-phase chalcopyrite structure with a preferential orientation in the (112) direction. These layers display uniform, large, and densely packed crystals with a grain size of about 3–5 μm. Cadmium sulfide buffer layer was manufactured by chemical bath deposition method. Bilayers ZnO/ZnO:Al were prepared by RF magnetron sputtering deposition. CIS solar cells with an efficiency of about 6.5% were produced without antireflective films. The method to fabricate CIS solar cells by a combination of the low vacuum sputtering deposition and the graphite box selenization process has provided a simple control process and shown a promising potential for developing high efficient and low-cost CuInSe2 solar cells.

Open-circuit voltage improvement by using TiO 2 nanotubes as a working electrode of dye-sensitized solar cell

The titanium dioxide nanotubes (TiNTs) were directly fabricated from commercial P25 TiO2 via alkali hydrothermal transformation. The optimized synthesis, thermal and hydrothermal stability, and consequent optical properties of the titanate nanotubes were systematically studied. The TiO2 nanotubes were characterized by transmission electron microscopy. Dye-sensitized solar cells (DSSCs) were constructed with films made of grown TiO2 nanotubes as working electrodes. The nanocrystalline TiO2 pastes were prepared with PEG (Mw 20000) and as made TiO2 nanotubes. The titania thin films were grown by screen printing method in order to efficiently control the DSSC fabrication process. The microstructures of nanoporous films in solar cells were characterized by scanning electron microscope (SEM) and Brunauer Emmett Teller (BET) analysis. A metal-free organic dye (referred to as D102 dye) was used as a sensitizer. A high conversion efficiency of light-to-electricity of around 6% under illumination of simulated AM1.5 sunlight (65mW/cm2) was achieved with the TiO2 nanotube cell. Compared with the case of DSSCs with TiO2 nanoparticles, the open-circuit voltage and fill factor of DSSCs with TiO2 nanotubes increased significantly. The related mechanisms are discussed.