A bioinspired porous-designed hydrogel@polyurethane sponge piezoresistive sensor for human-machine interfacing.

Conductive coating sponge piezoresistive pressure sensors are attracting much attention because of their simple production and convenient signal acquisition. However, manufacturing sponge-structure pressure-sensing materials with high compressibility and wide pressure detection ranges is difficult because of the instability of rigid and brittle conductive coatings at large strains. Herein, a tough conductive hydrogel@polyurethane (PU) sponge with a porous design is prepared via immersion of a polyurethane sponge in a low-cost and biocompatible polyvinyl alcohol (PVA)/glycerin (Gl)/sodium chloride (NaCl) solution. The sensor based on the hydrogel/elastomer sponge composite material exhibits a compressible range of 0-93%, a pressure detection range of 100 Pa-470.2 kPa, and 10 000-cycle stability (80% strain) because of the compressibility, flexibility, and toughness of the porous hydrogel coating. Benefiting from the resistance change mechanism of microporous compression, the sensor also exhibits a wide range of linear resistance changes, and the corresponding sensitivity and gauge factor (GF) are -0.083 kPa-- (100 Pa-10.0 kPa) and -1.33 (1-60% strain), respectively. Based on its flexibility, compressibility, and wide-ranging linear resistance changes, the proposed sensor has huge potential application in human activity monitoring, electronic skin, and wearable electronic devices.

Poly(vinyl alcohol)/phosphoric acid gel electrolyte@polydimethylsiloxane sponge for piezoresistive pressure sensors.

Piezoresistive pressure sensors based on flexible, ultrasensitive, and squeezable conductive sponges have recently attracted significant attention. However, the preparation of cost-effective conductive sponges with good stability and wide strain range for pressure sensing remains a challenge. Herein, a conductive poly(vinyl alcohol)/phosphoric acid gel electrolyte@polydimethylsiloxane (PVA/H3PO4@PDMS) composite was fabricated by impregnating a PDMS sponge into a PVA/H3PO4 gel electrolyte. The conductivity of the as-prepared sponges was determined using a gel electrolyte polymer film. The sponge exhibited good sensitivity of 0.1145 kPa-1 in the low-pressure range (0-6.5 kPa), short response time (70 ms), and durability for over 2700 s (6000 cycles). The gauge factor of the PVA/H3PO4@PDMS sponge was 5.51, 1.49, and 0.33 at the strain range of 0-10%, 10-30%, and 30-80%, respectively. Based on these outstanding sensing performances, the sponges were applied for the detection of various human motions, such as vocal cord vibration, joint bending, respiratory rate, and pulse signal detection. Further, the sponge demonstrated their great potential in the fabrication of electronic skin and high-performance flexible wearable electronics. Therefore, the obtained PVA/H3PO4 gel electrolyte used as a sponge conductive coating material is a readily available and inexpensive material that can reduce the cost of composite materials for pressure sensing.

Effects of working pressure and power on photovoltaic and defect properties of magnetron sputtered Sb2Se3 thin-film solar cells.

Antimony selenide (${\text{Sb}_2}{\text{Se}_3}$Sb2Se3) is an emerging material with potential applications in photovoltaics, while magnetron sputtering is an important method in material growth. In this study, ${\text{Sb}_2}{\text{Se}_3}$Sb2Se3 thin films, prepared by the magnetron sputtering technique with varied working pressures and sputtering powers, were fabricated into solar cells with a structure of $\text{glass}/\text{ITO}/\text{CdS}/{\text{Sb}_2}{\text{Se}_3}/\text{Au}$glass/ITO/CdS/Sb2Se3/Au. The current density versus voltage measurements and x-ray diffraction were introduced to compare the photovoltaic and structural properties of the cell samples. Characterization and identification of the defects in ${\text{Sb}_2}{\text{Se}_3}$Sb2Se3 thin films were investigated by admittance measurements. The ${\text{Sb}_2}{\text{Se}_3}$Sb2Se3 cell samples prepared with appropriate sputtering power (about 60 W) or working pressure (about 0.4 Pa) were found to own better crystal qualities and lower defect densities, which may be the reason for better efficiency.

Diagnosis of electrically active defects in CH3NH3PbI3 perovskite solar cells via admittance spectroscopy measurements.

The defect properties of CH3NH3PbI3 solar cells with efficiencies ranging from 7.70% to 12.51% were investigated using admittance spectroscopy measurements. Trap levels of the same kind with activation energies varied in the range of 0.16-0.23 eV above the valence band were found for different samples and identified as an interface-type defect. Moreover, the defect parameters, including the capture cross section of the holes, capture lifetime of the holes, and defect density, were extracted, and their relationships with the cell efficiencies were investigated. The results indicated that, compared with other parameters, defect density is a critical factor for CH3NH3PbI3 solar cell performance.

Enhanced photovoltaic response of lead-free ferroelectric solar cells based on (K,Bi)(Nb,Yb)O3 films.

Herein, we firstly present the (K,Bi)(Nb,Yb)O3 inorganic ferroelectric photovoltaic (FPV) film, in which a nearly ideal bandgap of ∼1.45 eV in the center of the solar spectrum and the co-existence of oxygen vacancies as well as ferroelectric polarization were confirmed. Furthermore, a novel cell structure is successfully fabricated by combining charge-transporting TiO2 nanoparticles, the perovskite sensitizer and a light-absorbing oxide hole p-type NiO conductor to realize a 1 V open circuit voltage, which can be increased to 1.56 V by adjusting the test bias near the coercive voltage. Additionally, under simulated standard AM 1.5G illumination, a fill factor of 86% and a power conversion efficiency of 0.85% are achieved via oxygen vacancy electromigration and polarization switching modulation. It is shown that the obtained power conversion efficiency is one to three orders of magnitude higher than those of pure BiFeO3 and Pb(Zr,Ti)O3. The enhanced PV effects are well elucidated using the transformation from a Schottky-like barrier to Ohmic contacts caused by polarization switching and oxygen vacancies. Building upon the above studies, deep insights into the bandgap tunability and PV effects in ferroelectric films with high oxygen vacancy concentration are provided and will facilitate a new versatile route for exploring high PV performance based on inorganic ferroelectric films.

Effect of film thickness and evaporation rate on co-evaporated SnSe thin films for photovoltaic applications.

SnSe thin films were deposited by a co-evaporation method with different film thicknesses and evaporation rates. A device with a structure of soda-lime glass/Mo/SnSe/CdS/i-ZnO/ITO/Ni/Al was fabricated. Device efficiency was improved from 0.18% to 1.02% by a film thickness of 1.3 µm and evaporation rate of 2.5 Å S-1 via augmentation of short-circuit current density and open-circuit voltage. Properties (electrical, optical, structural) and scanning electron microscopy measurements were compared for samples. A SnSe thin-film solar cell prepared with a film thickness of 1.3 µm and evaporation rate of 2.5 Å S-1 had the highest electron mobility, better crystalline properties, and larger grain size compared with the other solar cells prepared. These data can be used to guide growth of high-quality SnSe thin films, and contribute to development of efficient SnSe thin-film solar cells using an evaporation-based method.

Effects of substrate temperature on material and photovoltaic properties of magnetron-sputtered Sb2Se3 thin films.

We studied the material and photovoltaic properties of Sb2Se3 thin films fabricated by a magnetron-sputtering method at different substrate temperatures. The films had good crystallinity at substrate temperatures over 300°C. The band-gap energies between 1.1 and 1.5 eV of the films, which were obtained by transmittance measurements, initially decreased and then increased slowly with increasing temperature. Solar cells based on the films with structures of ITO/CdS/Sb2Se3/Au were fabricated, and the substrate temperature had significant effects on the device performance. Low crystal quality at low temperature resulted in a low short-circuit current (Jsc), while high temperature caused Se deficiency due to evaporation, which decreased the open-circuit voltage (Voc). The best solar cell performance achieved an efficiency of 0.84% with a Voc of 0.27 V and Jsc of 9.47 mA/cm2 when the substrate temperature was 325°C.

Efficient and Hole-Transporting-Layer-Free CsPbI2 Br Planar Heterojunction Perovskite Solar Cells through Rubidium Passivation.

Recently, inorganic perovskite CsPbI2 Br has gained much attention for photovoltaic applications owing to its excellent thermal stability. However, low device performance and high open-voltage loss, which are the result of its intrinsic trap states, are hindering its progress. Herein, planar CsPbI2 Br solar cells with enhanced performance and stability were demonstrated by incorporating rubidium (Rb) cations. The Rb-doped CsPbI2 Br film exhibited excellent crystallinity, pinhole-free surface morphology, and enhanced optical absorbance. By using low-cost carbon electrodes to replace the organic hole-transportation layer and metal electrode, an excellent efficiency of 12 % was achieved with a stabilized efficiency of over 11 % owing to the suppressed trap states and recombination in the CsPbI2 Br film. Additionally, the annealing temperature for the Rb-doped CsPbI2 Br film could be as low as 150 °C with a comparable high efficiency over 11 %, which is one of the best efficiencies reported for hole-transporting-layer-free all-inorganic perovskite solar cells. These results could provide new opportunities for high-performance and stable inorganic CsPbI2 Br solar cells by employing A-site cation substitution.

A sputtered CdS buffer layer for co-electrodeposited Cu2ZnSnS4 solar cells with 6.6% efficiency.

Cu2ZnSnS4 thin films with thicknesses ranging from 0.35 to 1.85 µm and micron-sized grains (0.5-1.5 µm) were synthesized using co-electrodeposited Cu-Zn-Sn-S precursors with different deposition times. Here we have introduced a sputtered CdS buffer layer for the development of CZTS solar cells for the first time, which enables breakthrough efficiencies up to 6.6%.

Co-electroplated Kesterite Bifacial Thin-Film Solar Cells: A Study of Sulfurization Temperature.

Earth-abundant material, kesterite Cu2ZnSnS4 (CZTS), demonstrates the tremendous potential to serve as the absorber layer for the bifacial thin-film solar cell. The exploration of appropriate sulfurization conditions including annealing temperature is significant to gain insight into the growth mechanism based on the substrates using transparent conductive oxides (TCO) and improve device performance. The kesterite solar absorbers were fabricated on ITO substrates by sulfurizing co-electroplated Cu-Zn-Sn-S precursors in argon diluted H2S atmosphere at different temperatures (475-550 °C) for 30 min. Experimental proof, including cross-section scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, UV-vis-NIR transmission spectrum, and Raman and far-infrared spectroscopy, is presented for the crystallization of CZTS on an ITO substrate and the interfacial reaction between the ITO back contact and CZTS absorber. The complete conversion of precursor into CZTS requires at least 500 °C sulfurization temperature. The aggressive interfacial reaction leading to the out-diffusion of In into CZTS to a considerable extent, formation of tin sulfides, and electrically conductive degradation of ITO back contact occurs at the sulfurization temperatures higher than 500 °C. The bifacial devices obtained by 520 °C sulfurization exhibit the best conversion efficiencies and open circuit voltages. However, the presence of non-ohmic back contact (secondary diode), the short minority lifetime, and the high interfacial recombination rates negatively limit the open circuit voltage, fill factor, and efficiency, evidenced by illumination/temperature-dependent J-V, frequency-dependent capacitance-voltage (C-V-f), time-resolved PL (TRPL), and bias-dependent external quantum efficiency (EQE) measurements.

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.