Impact of Charge-Trapping Effects on Reliability Instability in AlxGa1-xN/GaN High-Electron-Mobility Transistors with Various Al Compositions

In this study, we present a detailed analysis of trapping characteristics at the Al_xGa_1-xN/GaN interface of Al_xGa_1-xN/GaN high-electron-mobility transistors (HEMTs) with reliability assessments, demonstrating how the composition of the Al in the Al_xGa_1-xN barrier impacts the performance of the device. Reliability instability assessment in two different Al_xGa_1-xN/GaN HEMTs [x = 0.25, 0.45] using a single-pulse I_D-V_D characterization technique revealed higher drain-current degradation (∆I_D) with pulse time for Al_0.45Ga_0.55N/GaN devices which correlates to the fast-transient charge-trapping in the defect sites near the interface of Al_xGa_1-xN/GaN. Constant voltage stress (CVS) measurement was used to analyze the charge-trapping phenomena of the channel carriers for long-term reliability testing. Al_0.45Ga_0.55N/GaN devices exhibited higher-threshold voltage shifting (∆V_T) caused by stress electric fields, verifying the interfacial deterioration phenomenon. Defect sites near the interface of the AlGaN barrier responded to the stress electric fields and captured channel electrons-resulting in these charging effects that could be partially reversed using recovery voltages. The quantitative extraction of volume trap density (N_t) using 1/f low-frequency noise characterizations unveiled a 40% reduced N_t for the Al_0.25Ga_0.75N/GaN device, further verifying the higher trapping phenomena in the Al_0.45Ga_0.55N barrier caused by the rougher Al_0.45Ga_0.55N/GaN interface.

A Morphologically Stable Li/Electrolyte Interface for All-Solid-State Batteries Enabled by 3D-Micropatterned Garnet

Morphological degradation at the Li/solid-state electrolyte (SSE) interface is a prevalent issue causing performance fading of all-solid-state batteries (ASSBs). To maintain the interfacial integrity, most ASSBs are operated under low current density with considerable stack pressure, which significantly limits their widespread usage. Herein, a novel 3D-micropatterned SSE (3D-SSE) that can stabilize the morphology of the Li/SSE interface even under relatively high current density and limited stack pressure is reported. Under the pressure of 1.0 MPa, the Li symmetric cell using a garnet-type 3D-SSE fabricated by laser machining shows a high critical current density of 0.7 mA cm^-2 and stable cycling over 500 h under 0.5 mA cm^-2 . This excellent performance is attributed to the reduced local current density and amplified mechanical stress at the Li/3D-SSE interface. These two effects can benefit the flux balance between Li stripping and creep at the interface, thereby preventing interfacial degradation such as void formation and dendrite growth.

Mitigating the Interfacial Degradation in Cathodes for High-Performance Oxide-Based Solid-State Lithium Batteries

Solid-state lithium batteries (SSLBs) are the promising next-generation energy storage systems because of their attractive advantages in terms of energy density and safety. However, the interfacial engineering and battery building are of huge challenges, especially for stiff oxide-based electrolytes. Herein, we construct SSLBs by a cosintering method using Li₃BO₃ as a sintering agent to bind the cathode materials LiNi_0.6Mn_0.2Co_0.2O₂ (NMC) and solid-state electrolytes Li_6.4La₃Zr_1.4Ta_0.6O₁₂. Small NMC primary particles are compared with large secondary particles to study the effects on interfacial adhesion, mechanical retention, internal resistance evolution, and electrochemical performance. Our results reveal that the interfacial resistance decreases during charging and increases during discharging, resulting in an overall increase in the interfacial resistance after one cycle. The main reason is attributed to the microcracks induced by the volumetric changes of NMC during the electrochemical process. The mechanical degradations at the interfaces accumulated upon cycling can cause capacity decay and low Coulombic efficiency. The SSLB constructed from small NMC primary particles shows regulation of particle distribution, mitigation in local volumetric change, and alleviation in mechanical degradation at the interfaces, leading to smaller resistance change and better electrochemical performance. The findings shed lights on designing SSLBs with good mechanical retention and electrochemical performance.

Interfacial Degradation of Planar Lead Halide Perovskite Solar Cells

The stability of perovskite solar cells is one of the major challenges for this technology to reach commercialization, with water believed to be the major degradation source. In this work, a range of devices containing different cathode metal contacts in the configuration ITO/PEDOT:PSS/MAPbI3/PCBM/Metal are fully electrically characterized before and after degradation caused by steady illumination during 4 h that induces a dramatic reduction in power conversion efficiency from values of 12 to 1.8%. We show that a decrease in performance and generation of the S-shape is associated with chemical degradation of the metal contact. Alternatively, use of Cr2O3/Cr as the contact enhances the stability, but modification of the energetic profile during steady illumination takes place, significantly reducing the performance. Several techniques including capacitance-voltage, X-ray diffraction, and optical absorption results suggest that the properties of the bulk perovskite layer are little affected in the device degradation process. Capacitance-voltage and impedance spectroscopy results show that the electrical properties of the cathode contact are being modified by generation of a dipole at the cathode that causes a large shift of the flat-band potential that modifies the interfacial energy barrier and impedes efficient extraction of electrons. Ionic movement in the perovskite layer changes the energy profile close to the contacts, modifying the energy level stabilization at the cathode. These results provide insights into the degradation mechanisms of perovskite solar cells and highlight the importance to further study the use of protecting layers to avoid the chemical reactivity of the perovskite with the external contacts.