Jieyi Chen

Growth mechanism of Ge-doped CZTSSe thin film by sputtering method and solar cells

Ge-doped CZTSSe thin films were obtained by covering a thin Ge layer on CZTS precursors, followed by a selenization process. The effect of the Ge layer thickness on the morphologies and structural properties of Ge-doped CZTSSe thin films were studied. It was found that Ge doping could promote grain growth to form a compact thin film. The lattice shrank in the top-half of the film due to the smaller atomic radius of Ge, leading to the formation of tensile stress. According to thermodynamic analysis, Sn was easier to be selenized than Ge. Thus, Ge preferred to remain on the surface and increased the surface roughness when the Ge layer was thin. CZTSe was easier to form than Ge-doped CZTSe, which caused difficulty in Ge doping. These results offered a theoretical and experimental guide for preparing Ge-doped CZTSSe thin films for the potential applications in low-cost solar cells. With a 10 nm Ge layer on the top of the precursor, the conversion efficiency of the solar cell improved to 5.38% with an open-circuit voltage of 403 mV, a short-circuit current density of 28.51 mA cm-2 and a fill factor of 46.83% after Ge doping.

Growth of ideal amorphous carbon films at low temperature by e-beam evaporation

Amorphous carbon (a-C) films were prepared by e-beam evaporation on silicon substrates. The effects of substrate temperature between room temperature and 600 °C were mainly studied. Raman spectroscopy, scanning electron microscopy, atomic force microscopy, UV-Vis-NIR and Hall-effect measurements were used to characterize the structure, morphology, roughness, transmittance and conductivity of a-C films, respectively. The results indicated the films obtained by e-beam evaporation were all graphite-like carbon films. The films deposited at 200 °C with a thickness of 75 nm presented the best performance with ID/IG of 4.50, FWHMG of 115.9 cm−1, an optical gap of 0.6 eV, a roughness of 0.825 nm and an electrical resistivity of 3.60 × 10−3 Ω cm. As the thickness reduced to about 8 nm while remaining at the same substrate temperature, the films still exhibited good structural quality, continuity and conductivity. SEM images from Ge/a-C/Si stacks and Ge/Si stacks confirmed the necessity of a-C buffer layer for smooth Ge film growth. Raman analysis of Ge films indicated that the crystalline quality of Ge films obtained from Ge/Si stacks was improved by inserting an a-C layer. The mechanism of a-C films as buffer layers was further explored by X-ray diffraction. The results suggested that a-C films with good properties prepared at such low temperature may be used as buffer layers for growing high quality Ge or GaAs films on Si.

Large resonance magnetoelectric response in Ni(Terfenol-d)/Pb(Zr,Ti)O3 bilayer laminate composites

In this work we report magnetoelectric laminate composites made up of negative magnetostrictive Ni plates, positive magnetostrictive Tb0.3Dy0.7Fe2 (Terfenol-d) plates and piezoelectric Pb(Zr,Ti)O3 (PZT) plates, in which negative and positive magnetostrictive plates are bonded on the piezoelectric plate and conjugate longitudinally with various orders. Both Ni-Terfenol-d-Ni/PZT and Terfenol-d-Ni-Terfenol-d/PZT composites are constructed and compared with Ni/PZT and Terfenol-d/PZT composites. The bias magnetic field and the frequency dependences of the magnetoelectric coefficient suggest that Ni-Terfenol-d-Ni/PZT is an optimal configuration with excellent magnetoelectric effects such as a large magnetoelectric coefficient at a low bias magnetic field and a low resonance frequency at which the maximum ME voltage coefficient appears. Besides, compared with most of other bulk composites configuration with Terfenol-d plates, the Ni-Terfenol-d-Ni/PZT composite reduces the Terfenol-d usage. We believe that the Ni-Terfenol-d-Ni/PZT composite is useful for the application due to its simple configuration and large resonance magnetoelectric response at a low frequency.

Local structure, nucleation sites and crystallization behavior and their effects on magnetic properties of Fe 81 Si x B 10 P 8−x Cu 1 ( x = 0~8)

In this work, an attempt has been made to reveal critical factors dominating the crystallization and soft magnetic properties of Fe81SixB10P8−xCu1 (x = 0, 2, 4, 6 and 8) alloys. Both melt spun and annealed alloys are characterized by differential scanning calorimetry, X-ray diffractometry, Mössbauer spectroscopy, transmission electron microscopy, positron annihilation lifetime spectroscopy and magnetometry. The changes in magnetic interaction between Fe atoms and chemical homogeneity can well explain the variation of magnetic properties of Fe81SixB10P8−xCu1 amorphous alloys. The density of nucleation sites in the amorphous precursors decreases in the substitution of P by Si. Meanwhile, the precipitated nanograins gradually coarsen, but the inhibiting effect of P on grain growth diminishes causing the increase of the crystallinity. Moreover, various site occupancies of Si are observed in the nanocrystallites and the Si occupancy in bcc Fe decreases the average magnetic moment of nanograins. Without sacrificing amorphous forming ability, we can obtain FeSiBPCu nanocrystalline alloy with excellent soft magnetic properties by optimizing the content of Si and P in the amorphous precursors.

Impact of thiourea concentration on the properties of sol–gel derived Zn(O,S) thin films and Cu(In,Ga)Se 2 solar cells

AbstractA novel approach based on sol–gel spin coating method to deposit Zn(O,S) thin film using thiourea(TU) as a sulfur source replacing CdS as buffer layer was developed and the influence of TU concentration on the properties of Zn(O,S) thin films and Cu(In,Ga)Se2(CIGS) solar cells were investigated in this paper. It was found by X-ray diffraction and X-ray photoelectron spectroscopy that sol–gel derived Zn(O,S) thin films were amorphous and composed of ZnS, ZnO as well as Zn(OH)2. The variation of the optical band gap as a function of the S/(S+O) ratio was determined by energy-dispersive spectroscopy and UV-VIS-NIR. The results indicated that the minimum value for band gap of approximate 3.72 eV was obtained when the S/(S+O) = 0.44. Efficiency of up to 7.28% was achieved for a CIGS solar cell with Zn(O,S) buffer layer from 0.2M TU, which was attributed to the optimized conduction band offset (CBO) of +0.45 eV at the CIGS/Zn(O,S) interface. Zn(O,S) thin films prepared in sol–gel route was used to replace traditional CdS buffer layer deposited by chemical bath deposition method in Cu(In,Ga)Se2 solar cells. The best efficiency was achieved for CIGS/Zn(O,S)/i-ZnO/ITO heterostructure solar cell with S/(S+O) = 0.18, which was attributed to the optimized conduction band offset (CBO) of +0.45 eV at the CIGS/Zn(O,S) interface.

Effect of e-beam evaporated elemental metal stack precursors on the property of Cu(InGa)Se2 thin films through two-step process

Two-step process is considered to be more simple than co-evaporation method in Cu(InGa)Se2 (CIGS) thin films preparation process. However, research on CIGS thin films prepared by two-step process based on evaporation of elemental metals Cu, In and Ga is hardly reported. In this work, four types of metal stacks engineered as In/Ga/Cu/Ga/In (type A), Cu/Ga/In (type B), Cu/In/Ga (type C) and Cu/Ga/In/Cu (type D) were prepared and effects of metal stack precursors on properties of CIGS thin films formed by two-step process were studied. All types of precursors consisted of Cu–In, Cu–Ga and In phases. CIGS thin film from type A precursor showed poor compactness and relative high surface roughness due to the ordered defect compounds on the surface. Type B precursor exhibited a better compactness and crystal quality which led to an optimal structural property of films among all types of CIGS thin films. The reduction of Ga/(Ga + In) and the gaps appeared at the Mo/CIGS interface of CIGS thin film from type C precursor were explained by the re-evaporation of Ga element and compensation of In element during selenization. Grain size in CIGS thin film from type D is a little smaller than that from type B. Models of selenization process in different types of precursors were given. CIGS solar cell from type B exhibited the highest conversion efficiency of 9.2%.

Evolution of Structural and Electrical Properties of Carbon Films from Amorphous Carbon to Nanocrystalline Graphene on Quartz Glass by HFCVD

Direct growth of graphene films on glass is of great importance but has so far met with limited success. The noncatalytic property of glass results in the low decomposition ability of hydrocarbon precursors, especially at reduced temperatures (<1000 °C), and therefore amorphous carbon (a-C) films are more likely to be obtained. Here, we report the hydrogen influence on the structural and electrical properties of carbon films deposited on quartz glass at 850 °C by hot-filament chemical vapor deposition (HFCVD). The results revealed that the obtained a-C films were all graphitelike carbon films. Structural transition of the deposited films from a-C to nanocrystalline graphene was achieved by raising the hydrogen dilution ratios from 10 to over 80%. On the basis of systematic structural and chemical characterizations, a schematic process with three steps including sp2 chain aggregation, aromatic ring formation, and sp3 bond etching was proposed to interpret the structural evolution. The nanocrystalline graphene films grown on glass by HFCVD exhibited good electrical performance with a carrier mobility of 36.76 cm2/(V s) and a resistivity of 5.24 × 10-3 Ω cm over an area of 1 cm2. Temperature-dependent electrical characterizations revealed that the electronic transport in carbon films was dominated by defect, localized, and extended states, respectively, when increasing the temperature from 75 to 292 K. The nanocrystalline graphene films presented higher carrier mobility and lower carrier concentration than those of a-C films, which was mainly attributed to their smaller conductive activation energy. The present investigation provides an effective way for direct growth of graphene films on glass at reduced temperatures and also offers useful insights into the understanding of structural and electrical relationship between a-C and graphene.