Reliabiltiy of devices and technologies for solid-state lighting

The role of high-power white LEDs in general lighting applications is becoming, day-by-day, increasingly important. The reliability of these devices, compared to that of conventional light sources, represents one of the keys for their development and their market penetration. It is then of fundamental importance to deeply understand the various degradation mechanisms that affect the operation of LEDs, in terms of lifetime, chromaticity characteristics and efficiency. This thesis reports the results of a research activity focused on several issues related to the reliability of LED-based lighting systems. After an initial overview on the most important theoretical concepts necessary for the understanding of the physical results, three main sections can be identified in this thesis, concerning the presentation of research activity: • The first section reports an extensive study on one of the most critical Electrical Over Stress (EOS) phenomena, called “Hot-plugging”, which occurs when an LED module is directly connected to an energized power supply and can generate current spikes up to several tens of amperes that can potentially destroy or damage the LEDs. The aim of this section is to analyze, for the first time, the nature of the current spikes generated during hot-plugging and to present a simplified model to explain the hot plugging phenomenon. The study is based on transient electrical measurements, carried out on several LED modules (fabricated by different manufacturers), connected to three different power supplies. Results reveal that the amplitude and the time constants of the current spikes are directly determined by the number of LEDs connected in series and by the output capacitance of the current driver. • Afterwards, the second section presents an extensive study on the effects of Electrostatic Discharges (ESD) on state-of-the-art GaN based LEDs, based on optical and electrical measurements carried out during the ESD events. ESD events were simulated through a Transmission Line Pulser (TLP), which generates voltage pulses with a duration of 100ns and increasing amplitude: during each pulse, spatially resolved electroluminescence measurements were carried out through a high speed EMCCD camera. These measurements allowed to identify the chip region where the discharge is localized and the change in the damaged area induced by consecutive ESD events. In addition, the current and voltage waveforms at the LED terminal were monitored during the tests; this analysis provided important information about modifications the impedance of the devices. The analysis was carried out on different types of commercially available low-power GaN-based LEDs with several differences in the manufacturing technology. Thanks to these tests, we have identified two different failure behaviors during a destructive ESD event, clearly related to the different defects in the semiconductor lattice and to structure of the chip. • The last section investigates the thermal stability of remote phosphor plates to be used in solid-state lighting systems, for the conversion of the blue light emitted by GaN-based LEDs into white light. A preliminary thermal characterization revealed that in normal conditions of blue light irradiance the phosphor plates could reach temperature levels higher than 60°C, which can affect both performance and reliability. The results of accelerated thermal stress tests indicate that high temperature levels can trigger a relevant degradation mechanism (estimated activation energy is 1.2 eV), that drastically reduces the phosphor conversion efficiency and modifies the photometric and colorimetric characteristics of the emitted white light

Dopaminergic Neurotransmission in Patients With Parkinson's Disease and Impulse Control Disorders: A Systematic Review and Meta-Analysis of PET and SPECT Studies

Background: Around 30% Parkinson's disease (PD) patients develop impulse control disorders (ICDs) to D2/3 dopamine agonists and, to a lesser extent, levodopa. We aim to investigate striatal dopaminergic function in PD patients with and without ICD.Methods: PubMed, Science Direct, EBSCO, and ISI Web of Science databases were searched (from inception to March 7, 2018) to identify PET or SPECT studies reporting striatal dopaminergic function in PD patients with ICD (ICD+) compared to those without ICD (ICD–). Studies which included drug naïve patients, explored non-pharmacological procedures (e.g., deep brain stimulation), and those using brain blood perfusion or non-dopaminergic markers were excluded. Standardized mean difference (SDM) was used and random-effect models were applied. Separate meta-analyses were performed for dopamine transporter level, dopamine release, and dopamine receptors availability in the putamen, caudate, dorsal, and ventral striatum.Results: A total of 238 studies were title and abstract screened, of which 19 full-texts were assessed. Nine studies (ICD+: N = 117; ICD–: N = 175 patients) were included in the analysis. ICD+ showed a significant reduction of dopamine transporter binding in the putamen (SDM = −0.46; 95% CI: −0.80, −0.11; Z = 2.61; p = 0.009), caudate (SDM = −0.38; 95% CI: −0.73, −0.04; Z = 2.18; p = 0.03) and dorsal striatum (SDM = −0.45; 95% CI: −0.77, −0.13; Z = 2.76; p = 0.006), and increased dopamine release to reward-related stimuli/gambling tasks in the ventral striatum (SDM = −1.04; 95% CI: −1.73, −0.35; Z = 2.95; p = 0.003). Dopamine receptors availability did not differ between groups. Heterogeneity was low for dopamine transporter in the dorsal striatum (I2 = 0%), putamen (I2 = 0%) and caudate (I2 = 0%), and pre-synaptic dopamine release in the dorsal (I2 = 0%) and ventral striatum (I2 = 0%); heterogeneity was high for dopamine transporter levels in the ventral striatum (I2 = 80%), and for dopamine receptors availability in the ventral (I2 = 89%) and dorsal (I2 = 86%) striatum, putamen (I2 = 93%), and caudate (I2 = 71%).Conclusions: ICD+ patients show lower dopaminergic transporter levels in the dorsal striatum and increased dopamine release in the ventral striatum when engaged in reward-related stimuli/gambling tasks. This dopaminergic imbalance might represent a biological substrate for ICD in PD. Adequately powered longitudinal studies with drug naïve patients are needed to understand whether these changes may represent biomarkers of premorbid vulnerability to ICD

Premessa: Nuove forme di sorveglianza

La non-deperibilità dei dati accumulati dalla sorveglianza in rete, combinata con la confusione giurisdizionale e normativa, pone un serio problema di sicurezza persino per i sorveglianti. In altre parole, anche semplicemente garantire che i dati verranno usati solo per gli scopi originari che si era prefissato chi sorvegliava può rivelarsi alla lunga estremamente difficile. Gli apparati governativi sono colpiti da fughe di notizie, le aziende private possono essere acquisite da terzi o fallire, gli hacker – spalleggiati o meno da Stati esteri – sono sempre una minaccia incombente. L’archivio informatico, più che risorsa di potere, dovrebbe essere considerato alla stregua di un rifiuto tossico, difficilmente smaltibile

Reliability oriented design of LED-based light sources

The performance of LED-based light sources is increasing day- by-day, thanks to the continuous efforts of manufacturers and researchers on the developments of this technology. As a consequence, solid-state light sources have higher efficiency and similar color rendering compared to conventional light sources, and have costs that are rapidly decreasing as the market penetration increases. Despite the excellent characteristics of modern power LEDs, several factors complicate the development of robust and efficient LED-based light sources, including a non-trivial thermal management, problems related to chemical compatibility, the need of designing suitable optical and mechanical design, and developing efficient electrical configuration

Innovative methodology for testing the reliability of LED based systems

With this work we propose an innovative method for the analysis of the reliability of LED and Laser devices and systems. The basic idea of the proposed method is the separation of the different degradation forces that lead to the decrease of LED performances during ageing. By using a specific reliability analysis procedure it is possible to separately evaluate the effects of the single accelerating factors: temperature, current intensity, applied signal waveform, voltage overstress, optical and mechanical solicitation. To individually determine the degradation kinetics it is fundamental to separate the effects of temperature and current. For these reasons we carried out iso-currents reliability tests, where several devices have been stressed with the same current at different junction temperatures, and iso-thermal stresses, where junction temperature is instead constant for different applied currents. The result of the analysis will be a multivariable law that relates the several degradation parameters in the form of degradation kinetics. This will allow the estimation of the devices lifetime for a very wide operating conditions region. During degradation an extensive set of measurements have been carried out at fixed steps in the form of photometric, optical, electrical, capacitive, mechanical and thermal characterization. The combination of these results allows the understanding of what degradation mechanisms are taking place and therefore it is a fundamental tool to improve system reliability. Degradation has also been studied by analyzing catastrophic damages by means of failure analysis; the failure investigation is useful for the catastrophic damage: melting of bonding wire, contacts evaporation, facet melting (for laser diodes)

The role of operating conditions in the chip-level degradation of white LEDs

The aim of this work is to present the results of an extensive reliability analysis carried out on two different types of commercial white LEDs. In order to analyze the effects of operating current and storage temperature on the electro-optical properties of these devices, isothermal, iso-current and pure thermal stress tests were performed for several thousands hours; we used storage and junction temperatures in the range 60-200\ub0C, and current levels in the range 0.5 A - 1.5 A. The two types of LEDs showed marked differences in the degradation kinetics especially for the electro-optical modifications of the chip

Degradation Mechanisms of High Power LEDs for Lighting Applications: an overview

This paper reports on the degradation mechanisms that limit the reliability of high-power light-emitting diodes (LEDs) for lighting applications. The study is based on the experimental characterization of state-of-the-art LEDs fabricated by leading manufacturers. We demonstrate that, despite high potential reliability, high-power LEDs may suffer from a number of degradation mechanisms that affect the stability of the blue semiconductor LED chip and of the phosphor layer used for the generation of white light.More specifically, we describe the following relevant mechanisms: 1) the optical degradation of LEDs, due to an increase in the nonradiative recombination rate, which can be correlated to modifications in the forward-bias current\u2013voltage characteristics; 2) the variation in forward voltage, due to the increase in series resistance; 3) the optical degradation of phosphor layers used for blue-to-white light conversion; and 4) the failure of LEDs submitted to \u201chot plugging,\u201d which is the direct connection of an LED chain to an energized power supply, due to the generation of high current spikes. Results provide an overview on the failure mechanisms that limit the reliability of state-of-the-art LEDs and on the role of current and temperature in determining the failure of the devices

Thermally Activated Degradation of Remote Phosphors for Application in LED Lighting

This paper reports on an extensive analysis of the degradation of remote phosphors for solid-state lighting applications. The study is based on combined optical and thermal measurements, carried out before and during long-term stress tests, and provides the following results: 1) During normal operation, phosphors can show significant self-heating; 2) as a consequence of self-heating, the conversion efficiency of the phosphors decreases; and 3) exposure to long-term stress tests at moderate/high temperature levels (in the range of 85 \ub0C-145 \ub0C) can lead to remarkable degradation of the phosphors. Degradation mainly consists in a decrease in conversion efficiency and in worsening of the chromatic properties of the light-emitting diode-phosphor system. Finally, an activation energy value of 1.2 eV was extrapolated for the thermally activated degradation of the phosphors