He Xi

Enhanced efficiency of planar perovskite solar cells via a two-step deposition using DMF as an additive to optimize the crystal growth behavior

The performance of perovskite solar cells (PSCs) is extremely dependent on the morphology and crystallization of the perovskite film. However, the complete conversion of PbI2 to perovskite and controlling the perovskite crystal size as well as its surface morphology are challenging in the conventional two-step sequential deposition method. We herein present a facile method involving the use of a polar solvent additive in an inter-diffusion two-step sequential deposition method to achieve a high-quality perovskite film. The results showed that the addition of a small amount of DMF solvent into the MAI precursor solution could help the complete conversion of PbI2 to perovskite, and at the same time could also reduce the pinholes, improve the film morphology, increase the grain sizes and enhance the film absorption ability. The improved perovskite film quality results in the boosting performance of PSCs. Consequently, an optimized device with power conversion efficiency as high as 19.2% is obtained. This current method provides a highly repeatable route for enhancing the PSC performance with the inter-diffusion sequential solution deposition method.

Performance Enhancement of Planar Heterojunction Perovskite Solar Cells through Tuning the Doping Properties of Hole-Transporting Materials

Chemical doping has been widely used to finely tune the electrical properties of organic hole-transporting materials (HTMs) that find widespread applications in perovskite solar cells (PSCs). Here, to shed light on the precise role of chemical p-doping in affecting the charge-transport properties of HTMs and photovoltaic performance of PSCs, two kinds of representative dopants, including lithium bis(trifluoromethane)sulfonimide (LiTFSI) and two Co(III) complexes tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine]cobalt(III)tris[bis(trifluoromethylsulfonyl)imide] (FK209) and tris[2-(1H-pyrazol-1-yl)pyridine]cobalt(III)tris[bis(trifluoromethylsulfonyl)imide] (FK102), are employed as the p-type dopant models to dope the 2,2′,7,7′-tetrakis[N,N-di-p-methoxyphenylamine]-9,9′-spirobifluorene (spiro-OMeTAD) HTM. Both dopants can facilitate the generation of oxidized spiro-OMeTAD radical cation and improve hole mobility. Co-doping of FK209 and LiTFSI is necessary to achieve an optimal doping property and best device performance with power conversion efficiency of 17.8% compared to that of the FK209-doped device (13.5%) and the LiTFSI-doped device (15%). UV–vis absorption, space-charge-limited current measurements, and steady-state and time-resolved photoluminescence measurements have confirmed that with the co-doping of the two kinds of p-dopants in a proper ratio the doped spiro-OMeTAD exhibits a high charge carrier mobility and charge carrier transfer/collection capability.

Dissolvable and Biodegradable Resistive Switching Memory Based on Magnesium Oxide

In this letter, dissolvable and biodegradable resistive switching devices with a cell structure of Mg/MgO/Mg were demonstrated. Electrical tests demonstrated good memory characteristics for nonvolatile application. The dissolution rate of Mg and MgO is characterized in deionized (DI) water and in phosphate-buffered saline solution, with clear difference, 0.36, 1.25, 0.057, and 0.13 nm/s, respectively. The Mg/MgO/Mg devices on silk fibroin substrates are able to be completely dissolved as fast as 30 min while immersed in DI water. The Mg/MgO/Mg devices have excellent prospects for the applications in transient electronics, secure memory systems, and implantable medical therapy devices.

Effects of Annealing Conditions on Mixed Lead Halide Perovskite Solar Cells and Their Thermal Stability Investigation

In this work, efficient mixed organic cation and mixed halide (MA0.7FA0.3Pb(I0.9Br0.1)3) perovskite solar cells are demonstrated by optimizing annealing conditions. AFM, XRD and PL measurements show that there is a better perovskite film quality for the annealing condition at 100 °C for 30 min. The corresponding device exhibits an optimized PCE of 16.76% with VOC of 1.02 V, JSC of 21.55 mA/cm2 and FF of 76.27%. More importantly, the mixed lead halide perovskite MA0.7FA0.3Pb(I0.9Br0.1)3 can significantly increase the thermal stability of perovskite film. After being heated at 80 °C for 24 h, the PCE of the MA0.7FA0.3Pb(I0.9Br0.1)3 device still remains at 70.00% of its initial value, which is much better than the control MAPbI3 device, where only 46.50% of its initial value could be preserved. We also successfully fabricated high-performance flexible mixed lead halide perovskite solar cells based on PEN substrates.

Efficient Bifacial Semitransparent Perovskite Solar Cells Using Ag/V2O5 as Transparent Anodes

Bifacial semitransparent inverted planar structured perovskite solar cells (PSCs) based on Cs0.05FA0.3MA0.7PbI2.51Br0.54 using an Ag thin film electrode and V2O5 optical coupling layer are investigated theoretically and experimentally. It is shown that the introduction of the cesium (Cs) ions in the perovskite could obviously improve the device performance and stability. When only the bare Ag film electrode is used, the PSCs show a bifacial performance with the power conversion efficiency (PCE) of 14.62% illuminated from the indium tin oxide (ITO) side and 5.45% from the Ag film side. By introducing a V2O5 optical coupling layer, the PCE is enhanced to 8.91% illuminated from the Ag film side, which is 63% improvement compared with the bare Ag film electrode, whereas the PCE illuminated from the ITO side remains almost unchanged. Moreover, when a back-reflector is employed, the PCE of device could be further improved to 15.39% by illumination from the ITO side and 12.44% by illumination from the Ag side. The devices also show superior semitransparent properties and exhibit negligible photocurrent hysteresis, irrespective of the side from which the light is illuminated. In short, the Ag/V2O5 double layer is a promising semitransparent electrode due to its low cost and simple preparation process, which also point to a new direction for the bifacial PSCs and tandem solar cells.

Evolution of resistive switching and its ionic models in Pt/Nb-doped SrTiO3 junctions

Charge-trapping or ionic mechanisms of the resistive switching (RS) at metal/Nb-doped SrTiO3 (NSTO) interfaces are still unclear. Here, the electrical properties and RS evolution at Pt/NSTO interfaces are investigated. A volatile RS in the fresh junctions complies with Schottky theory involving an interfacial layer and electrically dependent permittivity. The RS is interpreted by a redox-reaction modulated barrier model. A nonvolatile RS emerges and evolves with increasing the forward voltage. I–V and C–V characteristics imply different conductive filament (CF) configurations in high and low resistance states. An in-barrier ionic CF model is established for the nonvolatile RS. The coherent ionic models are beneficial for understanding the interfacial role in RS and for regulating RS characteristics or realizing high quality metal/oxide diodes.

High performance transient organic solar cells on biodegradable polyvinyl alcohol composite substrates

Physically transient electronics represent an emerging class of technology which can disappear in a controlled manner when triggered by stimuli. Transient power supply devices are essential components in meeting the power demands of transient systems. Here we report on the first demonstration of transient organic solar cells (OSCs) fabricated on polyvinyl alcohol (PVA) composite substrates. PVA is employed as a substrate material because of its high transparency, good water solubility and versatility in chemical and physical properties by appropriate choice of composition. The dissolution and transparency of PVA substrates can be programmed by addition of sucrose or gelatin at different ratios, which further define the photovoltaic performance and transiency behavior of the transient OSCs. Based on the poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) system, a power conversion efficiency up to 2.05% is obtained for OSCs fabricated on a PVA/sucrose substrate with a ratio of 2 : 1 (PS2), which is comparable to that of the reference devices on glass substrates (2.37%). Once triggered by deionized water, the devices disconnect within only 10 s exposure to stimuli. When using a PVA/gelatin composite with a composition of 2 : 1 (PG2) as substrate, the disintegration is significantly prolonged. The devices can maintain their integrity even after an hour of exposure, however, in forms of individual particles. Efficient and easy fabrication of transient OSCs on the PVA-based substrates is expected to open the door for transient organic photovoltaic technology.

An acidic pH fluorescent probe based on Tröger's base

A novel pH fluorescent probe 2,8-(6H,12H-5,11-methanodibenzo[b,f]diazocineylene)-di(p-ethenyl-pyridine) (TBPP) incorporating an electron-donating amine moiety and electron-accepting pyridine group through Trogers base linker was designed and synthesized. TBPP exhibits an intramolecular charge transfer effect caused by the donor–acceptor interaction between its amine and pyridine units. Its emission can be reversibly switched between blue and dark states by protonation and deprotonation. Such behavior enables it to work as a turn-off fluorescent pH sensor in solution state. 1H NMR spectroscopy analysis suggests that the change in electron affinity of the pyridinyl unit upon protonation and deprotonation is responsible for such sensing processes.

Passivating ZnO Surface States by C60 Pyrrolidine Tris-Acid for Hybrid Solar Cells Based on Poly(3-hexylthiophene)/ZnO Nanorod Arrays

Construction of ordered electron acceptors is a feasible way to solve the issue of phase separation in polymer solar cells by using vertically-aligned ZnO nanorod arrays (NRAs). However, the inert charge transfer between conducting polymer and ZnO limits the performance enhancement of this type of hybrid solar cells. In this work, a fullerene derivative named C60 pyrrolidine tris-acid is used to modify the interface of ZnO/poly(3-hexylthiophene) (P3HT). Results indicate that the C60 modification passivates the surface defects of ZnO and improves its intrinsic fluorescence. The quenching efficiency of P3HT photoluminescence is enhanced upon C60 functionalization, suggesting a more efficient charge transfer occurs across the modified P3HT/ZnO interface. Furthermore, the fullerene modified hybrid solar cell based on P3HT/ZnO NRAs displays substantially-enhanced performance as compared to the unmodified one and the devices with other modifiers, which is contributed to retarded recombination and enhanced exciton separation as evidenced by electrochemical impedance spectra. Therefore, fullerene passivation is a promising method to ameliorate the connection between conjugated polymers and metal oxides, and is applicable in diverse areas, such as solar cells, transistors, and light-emitting dioxides.

Theoretical analysis of proton irradiation effects on AlGaN/GaN high-electron-mobility transistors

To study radiation damage, the authors irradiated AlGaN/GaN high-electron-mobility transistors with 3 MeV protons at various fluences. This irradiation caused displacement damage, which decreased the saturated drain current, maximum transconductance, cutoff frequency, and maximum frequency of oscillation. The authors extracted the damage factors of the threshold voltage, two-dimensional electron gas (2DEG) surface density, and mobility, which are usually used to simulate and estimate device performance in radiation environments. Calculations based on the charge control model show that the acceptor defects induced in the GaN layer play a leading role, while defects induced in the AlGaN barrier layer rarely matter. The removal rate of carriers from the 2DEG is unrelated to the thickness of undoped AlGaN layer, the conduction band discontinuity, and the doping concentration of AlGaN barrier layer; it only depends on the concentration of acceptor defects induced.