Influence of the Polymer Structure and its Crystallization on the Interface Resistance in Polymer-LATP and Polymer-LLZO Hybrid Electrolytes

For many years, composite electrolytes (CEs) consisting of a mixture of inorganic solid electrolytes (ISEs) and polymer electrolytes (PEs) have been investigated as promising materials for the scalable production of solid-state batteries (SSBs). It is believed that CEs can overcome limitations of the single components, namely the low room-temperature conductivity and lithium ion transference number of PEs and the poor mechanical properties and high temperature processing necessary for ISE ceramics. To facilitate ion transport in the CE between the electrodes a low and stable charge transfer resistance between PEs and ISEs is required. In this study, we investigate by means of electrochemical impedance spectroscopy (EIS) how polymer crystallinity influences the charge-transfer resistance of hetero-ionic interfaces between polyethylene oxide (PEO)-based electrolytes and Li$_{1.5}$Al$_{0.5}$Ti$_{1.5}$(PO$_4$)$_3$ (LATP) as well as Li$_{6.25}$Al$_{0.25}$La$_3$Zr$_2$O$_{12}$ (LLZO) as ISEs. Crystallization of PEO based electrolytes below their melting temperature leads to an increased charge-transfer resistance. On the other hand, electrolytes based on the amorphous poly[2-(2-(2-methoxyethoxy)ethoxy)ethyl glycidyl ether (PTG) do not show an increased charge transfer resistance. Finally, the conductivity of ISE-rich CEs is measured as a function of their temperature and composition for elucidating how the interface resistance influences charge transport in ISE-rich composite electrolytes

Designing Cathodes and Cathode Active Materials for Solid‐State Batteries

Solid-state batteries (SSBs) currently attract great attention as a potentially safe electrochemical high-energy storage concept. However, several issues still prevent SSBs from outperforming today\u27s lithium-ion batteries based on liquid electrolytes. One major challenge is related to the design of cathode active materials (CAMs) that are compatible with the superionic solid electrolytes (SEs) of interest. This perspective, gives a brief overview of the required properties and possible challenges for inorganic CAMs employed in SSBs, and describes state-of-the art solutions. In particular, the issue of tailoring CAMs is structured into challenges arising on the cathode-, particle-, and interface-level, related to microstructural, (chemo-)mechanical, and (electro-)chemical interplay of CAMs with SEs, and finally guidelines for future CAM development for SSBs are proposed

A review on the reliability of GaN-based LEDs

We review the degradation mechanisms that limit the reliability of GaN-based light-emitting diodes (LEDs). We propose a set of specific experiments, which is aimed at separately analyzing the degradation of the properties of the active layer, of the ohmic contacts and of the package/phosphor system. In particular, we show the following: 1) Low-current density stress can determine the degradation of the active layer of the devices, implying modifications of the charge/deep level distribution with subsequent increase of the nonradiative recombination components; 2) high-temperature storage can significantly affect the properties of the ohmic contacts and semiconductor layer at the p-side of the devices, thus determining emission crowding and subsequent optical power decrease; and 3) high-temperature stress can significantly limit the optical properties of the package of high-power LEDs for lighting applications

High temperature instabilities of ohmic contacts on Mg-doped gallium nitride

This paper analyzes the high temperature (250 \ub0C) long-term stability of ohmic contacts on p-type gallium nitride. The contributions of the ohmic contacts and semiconductor material degradation are separated by means of suitable test structures adopting the Transfer Length Method (TLM). Before stress, the current vs voltage curves measured at the pads of the TLMs showed linear shape, indicating good ohmic behaviour of the contacts. Average sheet resistance and contact resistivity were found to be equal to 53 kOhm/ and 2.7 mOhm\u2022cm2. Thermal treatment at 250 \ub0C was found to induce the worsening of the electrical characteristics of the contacts: identified degradation modes consist in a shift of the I-V curves towards higher voltages, and strong non-linearity of the characteristics around zero. This paper demonstrates that: (i) the electrical degradation is due to the worsening of the ohmic properties of the metal/semiconductor interface at the p-side of the diodes, implying the non-linear characteristics of the contacts and in a slight increase of the semiconductor sheet resistance; (ii) that degradation is reversible, after passivation removal and subsequent annealing; (iii) that sputtered passivation can be used as an alternative to usually adopted PECVD passivation for improving the high temperature stability of ohmic contacts on p-type GaN

Thermally activated degradation and package instabilities of low power PC-LEDs

The results achieved in an accelerated life-time test on Phosphor-Converted Light Emitting Diodes (PC-LEDs) have been reported. Two different families of commercially available low-flux devices have been widely characterized and a comparative analysis on performances has been carried out. A wide set of devices has been submitted to a combined electrothermal accelerated stress under different aging conditions. The stress induced a luminous flux decay on LEDs from both series. In particular, the lumen decay was found to be thermally activated for one set of devices. The aged devices showed also a degradation of chromatic properties, in terms of a blue or yellow shift for the two different families. The failure modes found have been detected also in devices aged at constant temperature and no bias. The degradation mechanism responsible for lumen decay and chromatic shift was ascribed to the thermally activated package instabilities. A failure analysis has been carried out on failed devices, detecting different failure modes related to the package (chip detachment) and to the chip (generation of low impedance paths that shorted the junction)

Reversible degradation of ohmic contacts on p-GaN for application in high-brightness LEDs

Abstract\u2014This paper analyzes the high-temperature long-term stability of ohmic contacts on p-type gallium nitride (p-GaN). The contributions of the ohmic contacts and semiconductor material degradation are separated by adopting the transmission line method (TLM). Before stress, the current\u2013voltage (I\u2013V ) curves measured at the pads of the TLMs showed a linear shape, indicating a good ohmic behavior of the contacts. Thermal treatment at 250 \u25e6C was found to induce the worsening of the electrical characteristics of the contacts: identified degradation modes consist of a shift of the I\u2013V curves toward higher voltages and strong nonlinearity of the characteristics around zero. This paper shows that the high-temperature instabilities of ohmic contacts on p-GaN are related to the interaction between the device surface and the plasma-enhanced chemical vapor deposition SiN passivation layer. Hydrogen contained in the passivation layer is supposed to play an important role in the degradation process: the interaction with the acceptor dopant at the metal/semiconductor interface induces the decrease of the effective acceptor concentration. As a consequence, both the ohmic contact characteristics and the semiconductor sheet resistance are worsened

Thermal-activated degradation mechanism on Phosphor-Converted Light Emitting Diode

An accelerated lifetime testing on low power Phosphor- Converted Light Emitting Diodes (pcLEDs) has been performed aiming at detecting the failure modes and evaluating the degradation law. The main actors involved in the current Solid State Lighting market are the light sources based on InGaN blue LEDs. Together with yellow phosphors for light conversion, these devices can ideally assure long lifetimes, high robustness and versatility in chromatic yield. Despite the efforts spent on improving the performances of such devices, the scientific community is still involved in obtaining a better thermal management for pcLEDs. The high temperature levels at the junction still limit the reliability of LED sources, most of all for low power devices not optimized for heat removal. A set of accelerated life testing setups for low power pcLEDs has been designed in order to understand the impact of bias and temperature on optical, electrical and thermal properties of such devices, and to identify the degradation mechanisms involved