The development of high efficiency perovskite solar cells (PSCs) has been proved to depend on the stability and optical properties of perovskite materials. A lot of efforts have been applied to improving these properties. Among them, the alternative mixed-metal perovskite composition has been considered as a new solution for photovoltaic device applications to satisfy the demand for exploring efficient photovoltaic performance. Here, we have systematically performed first-principles calculations using density-functional theory (DFT) to study the structural, electronic, magnetic and optical properties of the perovskite CH3NH3(Pb:Fe)I3, and investigated the effect of iron (Fe) metal ion doping on the properties of the perovskites and solar cell performance. The calculated results reveal that the perovskite CH3NH3(Pb:Fe)I3 exhibits half-metallic behavior due to the impurity bands induced by the Fe dopant crossing the Fermi level. Consequently, it is found that the absorption intensities of CH3NH3(Pb:Fe)I3 are slightly higher than those of CH3NH3PbI3 in the near-infrared light region. It is unexpected that the perovskite CH3NH3(Pb:Fe)I3 exhibits a large magnetic moment of 4 μB and its magnetic coupling belongs to the antiferromagnetic (AFM) configuration. Meanwhile, we found that the Fe incorporation can distort the structure due to its small ionic size, which significantly changes the device performance. Our findings provide a reference for exploring the properties of perovskite materials.

Seeking to increase the triboelectric charge density on a friction layer is one of the most basic approaches to improve the output performance of triboelectric nanogenerators (TENGs). Here, we studied the storage mechanism of triboelectric charge in the friction layer and discussed the function of carrier mobility and concentration in the charge-storing process. As guided by these results, a kind of composite structure is constructed in the friction layer to adjust the depth distribution of the triboelectric charges and improve the output performance of TENGs. To further elucidate this theory, a simple TENG, whose negative friction layer is a composite structure by integrating polystyrene (PS) and carbon nanotubes (CNTs) into polyvinylidene fluoride (PVDF), was fabricated, and its performance test was also carried out. Comparing with a pure PVDF friction layer, the composite friction layer can raise the triboelectric charge density by a factor of 11.2. The extended residence time of electrons in the friction layer is attributed to a large sum of electron trap levels from PS.Keywords: charge storage; composite friction layer; nanogenerator; triboelectric charge; triboelectrification

•A growth mode of perovskite based on mixed anti-solvent and solvent vapor annealing is proposed.•The perovskite film quality is improved, with the increased average grain size and crystallinity.•The power conversion efficiency of perovskite planar heterojunction solar cell is greatly enhanced and shows negligible hysteresis.•The stability of the corresponding device is also obviously improved.The key to improve the photovoltaic performance of perovskite solar cells is the quality of perovskite materials. Here, we report a growth mode that the perovskite precursor film is thermal annealed in mixed anti-solvent (IPA) and solvent (DMF) (100:1, v/v) vapor environment to improve the film quality, which enhances the power conversion efficiency of CH3NH3PbI3 based planar heterojunction solar cell device from 12.2% to 15.1%. After 8 days’ storage without encapsulation, the devices retained about 75% of their original efficiency while devices without solvent annealing reduced to 40% of that. It should be mentioned that by further applying compositional engineering such as using CH3NH3PbI3−xClx perovskite precursor and using interface engineering approach, the efficiency of planar heterojunction solar cell could be further enhanced up to 18.9%. The enhancement in photovoltaic performance and stability is due to the perovskite film quality improvement, with the increased average grain size and crystallinity of perovskite.

A new class of Λ-shaped triarylboranes 2-(4-(N,N-dimethylamino)-8-dimesitylboryl-6H,12H-5,11-methanodibenzo[b,f][1,5] diazocine (TBBN) and 2-(4-(N,N-diphenylamino)-8-dimesitylboryl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (TBBN2), incorporating different electron-donating amino groups and an electron-accepting dimesitylboryl group through a rigid Λ-shaped Tröger's base linker were designed and synthesized. The compounds display twisted structures and effective intramolecular change-transfer transitions. The twisted nonplanar arrangement of the chromophores on the one hand suppresses the fluorescence quenching in the aggregated states, and on the other hand produces a through-space donor–acceptor charge transfer. As a result, dual fluorescent pathways, namely through-space charge transfer from the amino group to the dimesitylboryl group, and the π*–π transitions located on the amino groups, are observed to coexist in each molecule. The dual emissions can be selectively switched on or off by addition of fluoride or cyanide ions. Thus the dyes can be used as “switch-on” probes. The complexation of TBBN and TBBN2 with fluoride or cyanide ions induces dramatic blue shifts (about 72–140 nm) and color changes in the fluorescence, making them potential visually colorimetric and ratiometric sensors for fluoride and cyanide ions.

•Formation mechanism of the continuous metal electrodes is investigated.•The introduction of the MoO3 interlayer can improve the wetting of Ag on substrate.•A 2 nm MoO3 layer effectively reduces Ag threshold from 11 to 9 nm.•Good conductivity and transparency are obtained for MoO3 (2 nm)/Ag (9 nm) anode.•The PCE for MoO3 (2 nm)/Ag (9 nm) anode is comparable to that for ITO anode.Organic solar cells (OSCs) using thermally evaporated Ag thin films as the transparent anode are investigated with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films as the active layer. The optimal power conversion efficiency (PCE) of 2.57% is obtained for the device with the 11 nm thick Ag layer, namely the percolation threshold of Ag. Then an interlayer of MoO3 between the Ag anode and glass substrate is introduced. Two different thicknesses of MoO3 (2 nm or 10 nm) are chosen here, corresponding to the unclosed or closed MoO3 layer, respectively. It is observed that the introduction of the MoO3 interlayer can effectively improve the wetting of Ag on the substrate and reduce the percolation threshold of Ag. When the MoO3 thickness is 10 nm, since the surface energy of Ag (γ=1.25 Jm−2) is higher than that of MoO3 (γ=0.06 Jm−2), the Ag–Ag interactions are stronger than the Ag-substrate interactions, weakening the surface-modifying effect. In contrast, since the 2 nm thick unclosed MoO3 layer may create preferred nucleation sites on the substrate to enhance the lateral growth of Ag film effectively, a very low sheet resistance of 9.32 Ω/□ and a relatively high transmittance are obtained for MoO3 (2 nm)/Ag (9 nm) anode. The PCE of corresponding OSCs can be improved to 2.71%, comparable to that of ITO-based reference OSCs (PCE of 2.85%). From our results, it is concluded that the performance of the Ag thin film electrode can be improved by decreasing the percolation threshold of Ag with a MoO3 interlayer (especially when the MoO3 layer is thin). It may be instructive for the further research of metal thin film electrodes.

A modified low-pressure chemical vapor deposition method was proposed to fabricate large-grain single-crystal few-layer graphene domains with distinctive layers by introducing an assembly to the conventional chemical vapor deposition. It was found that the fabricated single-crystal few-layer graphene domains consist of one- to five-layer graphene areas exhibiting layer growth characteristics. Moreover, the first three layers grew by Bernal stacking while the fourth and fifth layers could take on Bernal stacking or turbostratic stacking depending on the magnitude of the stress at the nucleation site. The results implied that the formation of few-layer graphene of good quality was beneficial from the modified low-pressure chemical vapor deposition system because not only could the assembly provide a stable growth condition for the graphene, but it could also accelerate the generation of gaseous activated vapor carbon atoms, which guaranteed the nucleation and growth of few-layer graphene. Furthermore, the growth mechanism of few-layer graphene was analysed.

•Aluminum doped zinc oxide (AZO) was used as the cathode in ITO-free inverted organic solar cells (IOSCs).•A layer of Ca was employed to modify the work function of AZO cathode.•AZO/Ca (5 nm) IOSC shows a PCE of 2.69% and a light soaking issue.•AZO/Ca (1 nm) IOSC shows an improved PCE of 3.17% and no light soaking issue is observed.•AZO based IOSC shows a better air stability than ITO based IOSC.Aluminum doped zinc oxide (AZO) was used to be the cathode instead of indium-tin-oxide (ITO) in the poly (3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) based bulk heterojunction inverted organic solar cells (IOSCs). For the AZO only IOSC, the device shows a poor power conversion efficiency (PCE) of 1.34% and a light soaking issue related to the energy barrier at the AZO/P3HT:PCBM interface. When a 5 nm Ca modifying layer is inserted between AZO and P3HT:PCBM, the obtained AZO/Ca (5 nm) IOSC shows an increased PCE from 1.74% to 2.69% after 15 min illumination. It is thought that the increased photoconductivity of AZO/Ca (5 nm) film upon illumination and the enhanced electron transport across the AZO/Ca interface may be responsible for the light soaking issue. When an ultrathin Ca modifying layer of 1 nm is employed, a further improved PCE of 3.17% is obtained, and remarkably, no light soaking issue is observed in this case. However, this unexpected issue appears after the un-encapsulated AZO/Ca (1 nm) IOSC has been stored in air for several days, which may be due to the energy loss in the electron transport across the interface between partly oxidized Ca and AZO layers induced by the oxidization of Ca. Furthermore, the AZO/Ca (1 nm) IOSC has a comparable PCE to the referenced ITO/Ca (1 nm) IOSC and presents a better air-stability. It is thus concluded that the AZO cathode is a promising alternative of ITO to fabricate the high efficient and long-lifetime IOSCs.Graphical abstract

As a promising group III-nitride semiconductor material, InAlN ternary alloy has been attracted increasing interest and widespread research efforts for optoelectronic and electronic applications in the last 5 years. Following a literature survey of current status and progress of InAlN-related studies, this paper provides a brief review of some recent developments in InAlN-related III-nitride research in Xidian University, which focuses on innovation of the material growth approach and device structure for electronic applications. A novel pulsed metal organic chemical vapor deposition (PMOCVD) was first adopted to epitaxy of InAlN-related heterostructures, and excellent crystalline and electrical properties were obtained. Furthermore, the first domestic InAlN-based high-electron mobility transistor (HEMT) was fabricated. Relying on the PMOCVD in combination with special GaN channel growth approach, high-quality InAlN/GaN double-channel HEMTs were successfully achieved for the first time. Additionally, other potentiality regarding to AlGaN channel was demonstrated through the successful realization of nearly lattice-matched InAlN/AlGaN heterostructures suitable for high-voltage switching applications. Finally, some advanced device structures and technologies including excellent work from several research groups around the world are summarized based on recent publications, showing the promising prospect of InAlN alloy to push group III-nitride electronic device performance even further.

Low-temperature processes are unremittingly pursued in the fabrication of organic solar cells. The paper reports that the highly efficient and “light-soaking”-free inverted organic solar cell can be achieved by using ZnO thin films processed from the aqueous solution method at a low temperature. The inverted organic solar with an aqueous-processed ZnO thin film annealed at 150 °C shows an efficiency of 3.79%. Even when annealed at a temperature as low as 80 °C, the device still shows an efficiency of 3.71%. With the proper annealing temperature of 80 °C, the flexible device, which shows an efficiency of 3.56%, is fabricated on PET. This flexible device still keeps the efficiency above 3.40% after bent for 1000 times with a curvature radius of 50 mm. In contrast, a low annealing temperature leads to an inferior device performance when the ZnO thin film is processed from the widely used sol–gel method. The device with sol–gel processed ZnO annealed at 150 °C only shows a PCE of 1.3%. Furthermore, the device shows a strong “light-soaking” effect, which is not observed in the device containing an aqueous-processed ZnO thin film. Our results suggest that the adopted aqueous solution method is a more efficient low temperature technique, compared with the sol–gel method.Keywords: aqueous solution method; inverted organic solar cell; sol gel method; zinc oxide; “light-soaking” effect;

We report the studies of AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MISHEMTs) using reactive-sputtered La2O3 as the gate dielectric and the surface passivation layer. The device presents a maximum drain current of 829 mA/mm and a maximum transconductance of 105 mS/mm, while the gate leakage current in the reverse direction is reduced by two orders of magnitude and the forward gate voltage swing increased to +5 V without gate breakdown. Compared to HEMT devices of similar geometry, MISHEMTs present a significant drain current recovery form current collapse.

AlGaN/GaN high electron mobility transistors (HEMTs) were exposed to 3 MeV protons at fluences of 6 × 1013, 4 × 1014 and 1 × 1015 protons/cm2. The drain saturation currents decreased by 20% and the maximum transconductance decreased by 5% at the highest fluence. As the fluence increased, the threshold voltage shifted more positive values. After proton irradiation, the gate leakage current increased. The Schottky barrier height changed from 0.63 eV to 0.46 eV, and the ideality factor from 2.55 to 3.98 at the highest fluence. The degradations of electrical characteristics of AlGaN/GaN HEMTs are caused by displacement damages induced by proton irradiation. The density of vacancies at different proton fluence can be calculated from SRIM. Being an acceptor-like defect, the Ga vacancy acts as a compensation center. While N vacancy acts as a donor. Adding the vacancies model into Slivaco device simulator, simulation results match well with the trends of experimental data. Hall measurement results also indicate the concentration and mobility of 2DEG decrease after proton irradiation. It is concluded that the Ga vacancies introduced maybe the primary reason for the degradation of AlGaN/GaN HEMTs performance.

A multi-step rapid thermal annealing process of Ti/Al/Ni/Au was investigated for ohmic contact of AlGaN/GaN high electron mobility transistor (HEMT). The samples were studied by Transmission Line Model (TLM), Scanning Electron Microscopy (SEM), Auger electron spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) measurements. By the multi-step annealing process, the specific contact resistance was decreased from 10−5 Ω cm2 level to 4–3 × 10−6 Ω cm2 and the surface morphology was improved. The AES measurements showed that the limitation indiffusion of Au and outdiffusion of Al were account for the surface morphology improvement and the surface Fermi level towards the conduction-band edge resulted in a lower specific contact resistance.

The effect of reactor pressure on the growth rate, surface morphology and crystalline quality of GaN films grown on sapphire by metalorganic chemical vapor deposition is studied. The results show that as the reactor pressure increases from 2500 to 20000 Pa, the GaN surface becomes rough and the growth rate of GaN films decreases. The rough surface morphology is associated with the initial high temperature GaN islands, which are large with low density due to low adatom surface diffusion under high reactor pressure. These islands prolong the occurrence of 2D growth mode and decrease the growth rate of GaN film. Meanwhile, the large GaN islands with low density lead to the reduction of threading dislocation density during subsequent island growth and coalescence, and consequently decrease the full width at half maximum of X-ray rocking curve of the GaN film.

Fabrication of enhancement-mode high electron mobility transistors on AlGaN/GaN heterostructures grown on sapphire substrates is reported. These devices with 1 μm gate-length, 10 nm recessed-gate depth, 4 μm distance of source and drain exhibit a maximum drain current of 233 mA/mm at 1.5 V, a maximum transconductance of 210 mS/mm, and a threshold voltage of 0.12 V. The threshold voltage of these devices increased to 0.53 V after 500°C 5 min annealing in N2 ambient. The saturation drain current and transconductance of 15 nm recessed-gate depth reduced compared to those of 10 nm recessed-gate depth, but the threshold voltage increased to 0.47 V. The relations between threshold voltage, controlling ability of gate and recess depth were validated by testing C-V structures on AlGaN/GaN heterostructures with different etching depth.

We report on the use of TiN interlayer to reduce the threading dislocation density in nonpolar a-plane GaN material grown by metal organic chemical vapor deposition (MOCVD), where the interlayer was formed by depositing the Ti metal on a GaN template followed by nitridize. By means of high resolution X-ray diffraction, transmission electron microscopy, and atomic force microscopy analyses, we found that the nonpolar a-plane GaN epitaxial grown on 10 nm-thick TiN interlayer, both on-axis and off-axis, exhibits a significant reduction in the full width at half maximum, the basal plane stacking faults (BSF), the threading dislocation density, and the root-mean-square roughness, respectively.Highlights► The TiN interlayer method has been used to grow nonpolar a-plane GaN by MOCVD. ► These results are much better than that with the traditional ELOG method. ► The two BSF decrease mechanisms block and annihilate are proposed.

A new susceptor structure with a ring groove on the conventional column-shaped graphite susceptor is proposed for the vertical and induction heating MOCVD reactor, aimed at dividing the inner heat of susceptor into two parts, one of which accumulates on the upside and the other downside of the groove. The shape of the ring groove that changes the directions of heat conduction in the susceptor is optimized and validated by using finite element method (FEM). Compared with the conventional one, the optimized susceptor improves the uniformity of temperature distribution in the wafer and consequently promotes the growth characteristics.