Pingxiong Yang

7.1% efficient co-electroplated Cu2ZnSnS4 thin film solar cells with sputtered CdS buffer layers

Cu2ZnSnS4 (CZTS) thin films with fine control over composition and pure phase were fabricated by sulfurization of co-electroplated Cu–Zn–Sn–S precursors. We have systematically investigated that the concentration of Cu(II) ions can influence the properties of CZTS absorber layers and the photovoltaic performance of the resulting solar cell devices. The results indicate that an increase in Cu(II) concentration almost linearly increases the Cu content in the final CZTS thin films, greatly enhances the (112) preferred orientation, significantly improves the crystallinity of the absorber layer, remarkably reduces the ZnS secondary phase, and hence improves their photovoltaic performance. However, a further increase in the Cu(II) concentration degrades the crystal quality of the absorber layer, and forms the CuSx secondary phase, which is quite detrimental to the device photovoltaic performance. Here we introduce a novel sputtered CdS buffer layer for the CZTS solar cells. For the first time, co-electrodeposited CZTS solar cells exceed the 7% efficiency threshold. These findings offer new research directions for solving persistent challenges of chemical bath deposition of CdS in CZTS solar cells.

Electrochemically-deposited nanostructured Co(OH)2 flakes on three-dimensional ordered nickel/silicon microchannel plates for miniature supercapacitors

Silicon microchannel plates (Si-MCPs) coated with a nickel layer are an excellent substrate in miniature supercapacitors. Nanometer-sized Co(OH)2 flakes serving as the active materials are electrodeposited on ordered three-dimensional (3D) Ni/Si-MCPs and the Co(OH)2 flakes have different structures depending on the solvent used. The cobalt hydroxide synthesized from a de-ionized water solvent is composed of compact nano-flakes, whereas that synthesized from an alcohol containing solvent is composed of loosely packed nano-flakes, and that from acetone are nano-flakes with nano-particles. The three types of electrode materials are investigated from the perspective of electrochemical capacitors by means of cyclic voltammogram, galvanostatic charge–discharge measurements and electrochemical impedance spectroscopy. The highest specific capacitance of 6.90 F cm−2 is achieved from the samples prepared in acetone at a discharge current density of 10 mA cm−2 and it is much better than the 1.46 F cm−2 observed in previous studies, thus demonstrating excellent capacity retention.

Structural phase transition, narrow band gap, and room-temperature ferromagnetism in [KNbO3]1−x[BaNi1/2Nb1/2O3−δ]x ferroelectrics

Structural phase transition, narrow band gap (Eg), and room-temperature ferromagnetism (RTFM) have been observed in the [KNbO3]1−x[BaNi1/2Nb1/2O3−δ]x (KBNNO) ceramics. All the samples have single phase perovskite structure, but exhibit a gradual transition behaviour from the orthorhombic to a cubic structure with the increase of x. Raman spectroscopy analysis not only corroborates this doping-induced change in normal structure but also shows the local crystal symmetry for x ≥ 0.1 compositions to deviate from the idealized cubic perovskite structure. A possible mechanism for the observed specific changes in lattice structure is discussed. Moreover, it is noted that KBNNO with compositions x = 0.1–0.3 have quite narrow Eg of below 1.5 eV, much smaller than the 3.2 eV band gap of parent KNbO3 (KNO), which is due to the increasing Ni 3d electronic states within the gap of KNO. Furthermore, the KBNNO materials present RTFM near a tetragonal to cubic phase boundary. With increasing x from 0 to 0.3, the magnetis...

Synthesis and characterization of Cu2ZnSnS4 thin films by the sulfurization of co-electrodeposited Cu–Zn–Sn–S precursor layers for solar cell applications

Cu2ZnSnS4 (CZTS) absorbers have been successfully deposited on tin-doped indium oxide coated glass (ITO/glass) substrates by sulfurization process of co-electrodeposited Cu–Zn–Sn–S precursor thin films at various annealing temperatures ranging from 500 to 580 °C for 30 min in an atmosphere of Ar–H2S (6.5%). The effects of sulfurization temperature on the structure, morphology, composition and optical properties of CZTS thin films have been investigated in details. XRD and Raman measurements reveal that the intensity of preferential orientation along the (112) direction becomes relatively more intense and sharp with increasing annealing temperature. The morphological and chemical composition studies indicate the formation of compact and homogenous CZTS thin films with Cu-poor and Zn-rich composition at a sulfurization temperature of 560 °C. And its band gap energy is around 1.50 eV. The AZO/i-ZnO/CdS/CZTS/ITO/glass thin-film solar cell is fabricated with the CZTS absorber layer grown at an optimized sulfurization temperature of 560 °C. It shows a power conversion efficiency of 1.98% for a 0.25 cm2 area with Voc = 490 mV, Jsc = 9.69 mA cm−2 and FF = 40.03%.

Al1.3Sb3Te material for phase change memory application

Comparing with Ge2Sb2Te5, Al1.3Sb3Te is proved to be a promising candidate for phase-change memory use because of its higher crystallization temperature (∼210 °C), larger crystallization activation energy (3.32 eV), and better data retention ability (124 °C for 10 yr). Furthermore, Al1.3Sb3Te shows fast phase change speed and crystallizes into a uniformly embedded crystal structure. As short as 10 ns width, voltage pulse can realize reversible operations for Al1.3Sb3Te based phase-change memory cell. Moreover, phase-change memory cell based on Al1.3Sb3Te material also has good endurance (∼2.5 × 104 cycles) and an enough resistance ratio of ∼102.

W-Sb-Te phase-change material: A candidate for the trade-off between programming speed and data retention

W-Sb-Te phase-change material has been proposed to improve the performance of phase-change memory (PCM). Crystallization temperature, crystalline resistance, and 10-year data retention of Sb2Te increase markedly by W doping. The Wx(Sb2Te)1−x films crystallize quickly into a stable hexagonal phase with W uniformly distributing in the crystal lattice, which ensures faster SET speed and better operation stability for the application in practical device. PCM device based on W0.07(Sb2Te)0.93 shows ultrafast SET operation (6 ns) and good endurance (1.8 × 105 cycles). W-Sb-Te material is a promising candidate for the trade-off between programming speed and data retention.

Characteristics of in-substituted CZTS thin film and bifacial solar cell.

Implementing bifacial photovoltaic devices based on transparent conducting oxides (TCO) as the front and back contacts is highly appealing to improve the efficiency of kesterite solar cells. The p-type In substituted Cu2ZnSnS4 (CZTIS) thin-film solar cell absorber has been fabricated on ITO glass by sulfurizing coelectroplated Cu-Zn-Sn-S precursors in H2S (5 vol %) atmosphere at 520 °C for 30 min. Experimental proof, including X-ray diffraction, Raman spectroscopy, UV-vis-NIR transmission/reflection spectra, PL spectra, and electron microscopies, is presented for the interfacial reaction between the ITO back contact and CZTS absorber. This aggressive reaction due to thermal processing contributes to substitutional diffusion of In into CZTS, formation of secondary phases and electrically conductive degradation of ITO back contact. The structural, lattice vibrational, optical absorption, and defective properties of the CZTIS alloy absorber layer have been analyzed and discussed. The new dopant In is desirably capable of improving the open circuit voltage deficit of kesterite device. However, the nonohmic back contact in the bifacial device negatively limits the open circuit voltage and fill factor, evidencing by illumination-/temperature-dependent J-V and frequency-dependent capacitance-voltage (C-V-f) measurements. A 3.4% efficient solar cell is demonstrated under simultaneously bifacial illumination from both sides of TCO front and back contacts.

Oxygen-vacancy-related dielectric relaxation in BiFeO3 films grown by pulsed laser deposition

Two kinds of BiFeO3 (BFO) thin films with different oxygen stoichiometry are fabricated on (La, Sr)CoO3 coated silicon substrates. A Debye-like dielectric relaxation was observed in the samples thermally treated at 3 Pa oxygen. The frequency dependence of permittivity of the samples treated at 3 Pa oxygen can be fitted by a model containing the Debye-like dielectric response and the universal dielectric response. According to the model, the dielectric relaxation can be ascribed to the oxygen vacancy, and the possible influences from the interfacial polarization between BFO and electrodes have been excluded by the measurement of the dielectric responses of BFO films at different dc biased voltages. The calculated value of dc electric conductivity in BFO films from this model has the same order of magnitude as the published results. These results indicate that the existence of oxygen vacancy not only influences the leakage performance of BFO films but also affects the dielectric properties of BFO. The electrical performance of BFO films and devices can be improved by decreasing the density of oxygen vacancy.

Co-electrodeposited Cu2ZnSnS4 thin-film solar cells with over 7% efficiency fabricated via fine-tuning of the Zn content in absorber layers

A simple and cost-effective co-electrodeposition process has been demonstrated to fabricate high-performance Cu2ZnSnS4 (CZTS) photovoltaic materials with composition tunability and phase controllability. Here we report a systematic investigation of the effects of the Zn(II) concentration on the properties of CZTS thin films and thus the performance of the as-resulted solar cells. These results indicate that increasing the concentration of Zn(II) linearly increases the Zn content in the final composition of CZTS thin films, significantly improves the grain size and morphology of the absorber layers, and consequently improves their photovoltaic properties, especially the response to the medium wavelength. In contrast, further increase of the Zn(II) concentration degrades the crystal quality of the absorber layer, and more ZnS phase appears on the surface of the CZTS thin film, forming a rather rough morphology, which is harmful to the photovoltaic performance of the device. When the concentration of Zn(II) is optimized to 30 mM, a power conversion efficiency of 7.23% is achieved, which, to the best of our knowledge, is the highest efficiency for a co-electrodeposited CZTS solar cell with a sputtered CdS buffer layer to date. Our findings offer a promising alternative approach towards the industrialization of CZTS solar cell modules.

Magnetism switching and band-gap narrowing in Ni-doped PbTiO3 thin films

Ions doping-driven structural phase transition accompanied by magnetism switching and band-gap narrowing effects has been observed in PbTi1−xNixO3−δ (xPTNO, x = 0.00, 0.06, and 0.33) thin films. With the increase of x, the xPTNO thin films exhibit not only a phase transition from the pseudotetragonal structure to a centrosymmetric cubic structure but also a drastic decrease of grain size. Moreover, the as-grown Ni-doped PbTiO3 (PTO) thin films show obvious room-temperature ferromagnetism and an increased saturation magnetization with increasing the Ni content, in contrast to undoped PTO, which shows diamagnetism. A bound magnetic polaron model was proposed to understand the observed ferromagnetic behavior of PTO-derived perovskite thin films. Furthermore, the 0.33PTNO thin film presents a narrowed band-gap, much smaller than that of PTO, which is attributed to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence ...