A comparison of consistent UV treatment versus inconsistent UV treatment in horticultural production of lettuce

UV radiation is an underrated radiation currently missing in many horticultural production systems of vegetables in protected cultivation. It can be added e.g., in LED light sources. Using lettuce as a model plant, this study determined whether the use of UVB LEDs is suitable (1) for use in consistent systems (indoor farming) or (2) inconsistent systems (greenhouse). Blue and red LEDs were selected as additional artificial lighting to UVB LEDs. Both approaches led to a reproducible increase of desired flavonol glycosides, such as quercetin-3-O-(6′′-O-malonyl)-glucoside or quercetin-3-O-glucuronide and the anthocyanin cyanidin-3-O-(6′′-O-malonyl)-glucoside in lettuce. The impact of the consistent UVB treatment is higher with up to tenfold changes than that of the inconsistent UVB treatment in the greenhouse. Varying natural light and temperature conditions in greenhouses might affect the efficiency of the artificial UVB treatment. Here, UVB LEDs have been tested and can be recommended for further development of lighting systems in indoor farming and greenhouse approaches

Comparison of Ultraviolet B Light‐Emitting Diodes with Single or Triple Quantum Wells

Light-emitting diodes (LEDs) with an emission wavelength of 310 nm containing either a single or a triple quantum well are compared regarding their efficiency and long-term stability. In addition, the influence of the thickness of the lower quantum well barrier and the quantum well thickness in single quantum well (SQW) LEDs is investigated. Electroluminescence measurements show a 28% higher initial output power for the SQW LEDs compared with the triple quantum well (TQW) LEDs because of larger spatial overlap of the carriers in the SQW as revealed by electro-optical simulations of the LED heterostructures. However, TQW LEDs show a higher output power than SQW LEDs after 1 h operation under harsh conditions. For SQW LEDs, it is found that for a thicker lower quantum well barrier (65 nm instead of 25 nm) the initial output power decreases by ≈15%. A thicker SQW (3 nm instead of 1.6 nm) reduces the initial output power by even 45% but increases the lifetime by a factor of 6 which is attributed to reduced Auger recombination from an enhanced spatial separation of electrons and holes in the quantum wells due to the quantum-confined Stark effect

Increased Light Extraction of Thin-Film Flip-Chip UVB LEDs by Surface Texturing

Ultraviolet light-emitting diodes (LEDs) suffer from a low wall-plug efficiency, which is to a large extent limited by the poor light extraction efficiency (LEE). A thin-film flip-chip (TFFC) design with a roughened N-polar AlGaN surface can substantially improve this. We here demonstrate an enabling technology to realize TFFC LEDs emitting in the UVB range (280-320 nm), which includes standard LED processing in combination with electrochemical etching to remove the substrate. The integration of the electrochemical etching is achieved by epitaxial sacrificial and etch block layers in combination with encapsulation of the LED. The LEE was enhanced by around 25% when the N-polar AlGaN side of the TFFC LEDs was chemically roughened, reaching an external quantum efficiency of 2.25%. By further optimizing the surface structure, our ray-tracing simulations predict a higher LEE from the TFFC LEDs than flip-chip LEDs and a resulting higher wall-plug efficiency

Noninvasive measurement of the 308 nm LED‐based UVB protection factor of sunscreens

The current method for determining the sun protection factor (SPF) requires erythema formation. Noninvasive alternatives have recently been suggested by several groups. Our group previously developed a functional sensor based on diffuse reflectance measurements with one UVB LED, which was previously evaluated on pig ear skin. Here we present the results of a systematic in vivo study using 12 sunscreens on 10 volunteers (skin types [ST] I-III). The relationship of the UVB-LED reflectance of unprotected skin and melanin index was determined for each ST. The spatial variation of the reflectance signal of different positions was analyzed and seems to be mainly influenced by sample inhomogeneity except for high-protection factors (PFs) where signal levels are close to detection noise. Despite the low-signal levels, a correlation of the measured LED-based UVB PF with SPF reference values from test institutes with R-2 = 0.57 is obtained, suggesting a strong relationship of SPF and LED-based UVB-PF. Measured PFs tend to be lower for increasing skin pigmentation. The sensor design seems to be suitable for investigations where a fast measurement of relative changes of PFs, such as due to inhomogeneous application, bathing and sweating, is of interest

Skin tolerant inactivation of multiresistant pathogens using far-UVC LEDs

Multiresistant pathogens such as methicillin-resistant Staphylococcus aureus (MRSA) cause serious postoperative infections. A skin tolerant far-UVC (< 240 nm) irradiation system for their inactivation is presented here. It uses UVC LEDs in combination with a spectral filter and provides a peak wavelength of 233 nm, with a full width at half maximum of 12 nm, and an irradiance of 44 µW/cm2. MRSA bacteria in different concentrations on blood agar plates were inactivated with irradiation doses in the range of 15–40 mJ/cm2. Porcine skin irradiated with a dose of 40 mJ/cm2 at 233 nm showed only 3.7% CPD and 2.3% 6-4PP DNA damage. Corresponding irradiation at 254 nm caused 11–14 times higher damage. Thus, the skin damage caused by the disinfectant doses is so small that it can be expected to be compensated by the skin's natural repair mechanisms. LED-based far-UVC lamps could therefore soon be used in everyday clinical practice to eradicate multiresistant pathogens directly on humans

Spectrally pure far-UVC emission from AlGaN-based LEDs with dielectric band pass filters

AlGaN-based far ultraviolet-C (UVC) light emitting diodes (LEDs) with a peak emission wavelength below 240 nm typically show a long-wavelength tail at >240 nm that is detrimental to the use of the devices for skin-friendly antisepsis. We present the development of far-UVC LEDs with reduced long-wavelength emission using a HfO2/SiO2-based distributed Bragg reflector (DBR) filter. When the DBR filter is directly mounted on an LED package, the long-wavelength emission around 250 nm is reduced by two orders of magnitude while the transmitted output power is reduced down to 18%–27% of the initial value for DBR filters with cut-off wavelengths at 237–243 nm. As the transmission through the DBR filter depends strongly on the angle of incidence of the radiation, the transmitted output power of the spectrally pure far-UVC radiation can be doubled when an additional collimating lens is used on top of the LED package before passing through the filter.BMBF, 03ZZ0146C, Zwanzig20 - Advanced UV for Life - Verbundprojekt: Verhinderung der Infektion mit multiresistenten Erregern über In-vivo-UVC-Bestrahlung, TP3: UVC-LED-Strahler für die In-vivo-AnwendungBMBF, 03ZZ0146D, Zwanzig20 - Advanced UV for Life - Verbundprojekt: Verhinderung der Infektion mit multiresistenten Erregern über In-vivo-UVC-Bestrahlung, TP4: Herstellung von UV LEDs um 230 nm für die In-vivo-UVC-BestrahlungBMBF, 03COV10D, CORONA - CORSA - Verbundvorhaben - Inaktivierung von SARS-CoV-2 durch UVC-Licht und Verträglichkeit für den Menschen; TP4: Epitaxie von 230 nm LEDs für die in vivo UVC-Bestrahlun

Design von ultraviolett-emittierenden III-nitridbasierten Leuchtdioden-Chips mit hoher Effizienz und erhöhter Lichtauskopplung

Ziel dieser Arbeit war es hocheffiziente III-nitridbasierte UV-LEDs für Anwendungen wie z.B. Wasserdesinfektion, medizinische Phototherapie oder Gasdetektion zu entwerfen und herzustellen. Der Schwerpunkt lag dabei auf dem Design der UV-LEDs, wodurch nicht nur das elektrische Verhalten, sondern auch die Lichtauskopplung (engl. light extraction efficiency, LEE) der UV-LED verbessert wird. Der erste Teil dieser Arbeit beschätigt sich mit dem Chipdesign von UV-LEDs und den es beeinflussenden drei Schlüsselparametern. Diese sind neben der Abhängigkeit der Internen Quanteneffizienz (IQE) von der Stromdichte auch der Temperatur Droop und das Phänomen der Stromspreizung in der lateralen Struktur der LEDs. Basierend auf den zuvor gewonnenen Erkenntnissen wurden ineinander greifende Fingerkontakte und Micro-LED-Arrays als effiziente UV-LED-Chip-Geometrien vorgeschlagen und systematisch untersucht. Diese Geometrien führen zu einer gleichmäßigen Strominjektion und dadurch zu einer reduzierten Stromdichte, einem reduzierten seriellen Widerstand und einer reduzierten Eigenerwärmung der Bauelemente im Vergleich zu konventionellen großflächigen quadratischen Kontakten. Designregeln für die nötige Breite der p-Fingerkontakte und die optimale Größe des Mikro-LED-Arrays wurden in dieser Arbeit entwickelt. Der zweite Teil dieser Arbeit beschäftigt sich mit der Lichtauskopplung aus UV-LEDs. Hierbei kann das in der aktiven Schicht der UV-LED erzeugte Licht, aufgrund der hohen Brechungsindizes der verwendeten Halbleitermaterialien und des Saphirsubstrates, aufgrund fehlender transparenter oder reflektierender ohmscher p-Kontakte und des Polarisationszustand des emittierten Lichtes, nicht effizient ausgekoppelt werden. Um die LEE von UV-LEDs zu erhöhen wurden die Verkapslung des Bauelementes, die Strukturierung der Substratrückseite und der Einsatz von Nanopixel-LED-Kontakten untersucht. Es wurde demonstriert, dass Polydimethylsiloxan ein effizientes und stabiles Verkapslungsmaterial für UV-LEDs darstellt, mit dessen Hilfe sich eine Verdopplung der Lichtleistung bei Einsatz in 380 nm-LEDs realisieren lässt. Simulationen und Experimente haben gezeigt, dass eine geeignete Strukturierung der Substratrückseite die LEE erhohen kann. Durch mechanische Aufrauung der glatten Saphirsubstratrückseite wurde bei 323 nm-LEDs eine Erhöhung der Lichtleistung um (16 ± 7)% demonstriert. Durch Ätzen von Mikro-Kegelstümpfen in die Saphirrückseite konnte die Lichtleistung sogar um (20 ± 7)% erhöht werden. Schlussendlich konnte ein neues Nanopixel-Kontakt-LED-Design vorgeschlagen werden. Es besteht aus Nanopixel-Kontakten und p-Kontaktmaterial mit ohmschem Verhalten und niedrigem Widerstand kombiniert mit einem Metallreflektor. Durch diese Kombination können transparente oder reflektierende ohmschen p-Kontakte auf AlGaN-Schichten realisiert werden. Verglichen mit konventionellen großflächigen quadratischen Kontakten konnte eine Erhöhung der Lichtleistung um 90 % bei 390 nm-LEDs, bei Verwendung von Nanopixel-LEDs mit einer Nanopixelgröße von 1x1 µm² und einem Abstand von 1 µm, demonstriert werden. Durch Kombination aller in dieser Arbeit vorgestellten Technologien kann bei UV-LEDs eine LEE von ca. 70 % erreicht werden.The aim of this thesis was the design and fabrication of highly efficient III-nitride based UV LEDs for use in applications such as water disinfection, medical phototheraphy and gas sensing. This work focused on the design of UV LED chip geometries that not only improved the electrical characteristics of the LED but also increased the light extraction efficiency (LEE) of the devices. The first part of this work focused on the design of efficient UV LED chip layouts. Three key physical parameters influencing the geometry of UV LEDs were investigated, namely; the current density dependence of the internal quantum efficiency, the temperature droop and the phenomenon of current crowding in lateral geometry LEDs. Based on the results of these investigations, the use of interdigitated finger contacts and micro-LED arrays as efficient UV LED chip geometries was proposed and systematically studied. These geometries were found to provide uniform current injection, hence a lower current density, reduced series resistance and reduced self heating of the device as compared to conventional large area square contacts. Design rules for determining the necessary width of the p-fingers and the optimum size of the micro-LED arrays were developed in this work. The second part of this thesis focused on the issue of light extraction from UV LEDs. The main reasons for the difficulties in exacting the light generated in the LED are the high refractive indices of the semiconductors and the sapphire substrate, the absence of transparent or reflective p-ohmic contacts and the polarization of the light emitted from the active region. To enhance the LEE of UV LEDs, the encapsulation of the devices, the texturing of the substrate back-surface and the use of nanopixel LED contacts were investigated. Polydimethylsiloxane was proposed as an efficient and stable encapsulant for UV LEDs and a 2-fold increase in the output power of 380 nm LEDs was demonstrated. With the help of simulations and experiments, the texturing of the substrate back-surface was shown to increase the LEE of UV LEDs. A (16 ± 7)% increase in the output power of 323 nm LEDs was demonstrated by mechanically roughening the smooth sapphire substrate back-surface while a (20 ± 7)% increase in the output power of LEDs emitting at 323 nm was obtained with the use of micro-frustums etched in the sapphire back-surface. Finally a novel nanopixel contact LED design, consisting of nanopixel contacts of a low resistivity ohmic p-contact material combined with a metal reflector, was proposed to circumvent the problems in obtaining highly reflective or transparent p-ohmic contacts to p-doped AlGaN layers. A 90 % increase in the light output power of a 390 nm AlInGaN LED was demonstrated for nanopixel LEDs with a nanopixel size of 1x1 µm² and spacing of 1 µm as compared to the conventional square contact geometry. If all the techniques to increase the LEE of UV LEDs investigated in this work are technologically combined, an LEE of 70% is estimated

Thin-film flip-chip UVB LEDs realized by electrochemical etching

We will give an overview of different concepts to increase the light extraction efficiency (LEE) of ultraviolet (UV) light-emitting diodes (LEDs) with a focus on thin-film flip-chip (TFFC) devices. Optical simulations show that a TFFC design can greatly improve the LEE with a transparent p-side, reflective contacts, and optimized surface roughening. We will demonstrate UVB-emitting TFFC LEDs based on our fabrication platform for AlGaN thin films with high aluminum content. The fabrication is compatible with a standard LED process and uses substrate removal based on selective electrochemical etching as the key enabling technology

A comparison of consistent UV treatment versus inconsistent UV treatment in horticultural production of lettuce

Abstract UV radiation is an underrated radiation currently missing in many horticultural production systems of vegetables in protected cultivation. It can be added e.g., in LED light sources. Using lettuce as a model plant, this study determined whether the use of UVB LEDs is suitable (1) for use in consistent systems (indoor farming) or (2) inconsistent systems (greenhouse). Blue and red LEDs were selected as additional artificial lighting to UVB LEDs. Both approaches led to a reproducible increase of desired flavonol glycosides, such as quercetin-3- O -(6′′- O -malonyl)-glucoside or quercetin-3- O -glucuronide and the anthocyanin cyanidin-3- O -(6′′- O -malonyl)-glucoside in lettuce. The impact of the consistent UVB treatment is higher with up to tenfold changes than that of the inconsistent UVB treatment in the greenhouse. Varying natural light and temperature conditions in greenhouses might affect the efficiency of the artificial UVB treatment. Here, UVB LEDs have been tested and can be recommended for further development of lighting systems in indoor farming and greenhouse approaches.Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347Georg-August-Universität Göttingen 50110000338

Improved Efficiency of Ultraviolet B Light‐Emitting Diodes with Optimized p‐Side

The effects of design and thicknesses of different optically transparent p-current spreading layers [short-period superlattice, superlattice (SL), and bulk p- (Formula presented.)] as well as the type and thickness of the p-GaN cap layer on the electrical and optical characteristics of 310 nm ultraviolet light-emitting diodes (LEDs) are investigated. Scanning transmission electron microscopy measurements display self-organized composition variations in the nonpseudomorphically grown SLs, reducing the effect of increased hole injection efficiency of a SL. In addition, the effect leads to an increased operation voltage. In contrast, the bulk p-AlGaN layer has a uniform composition and the corresponding LEDs show only a slightly lower output power along with a lower operating voltage. If the thickness of the p-AlGaN bulk layer in the LED is reduced from 150 nm to 50 nm, the output power increases and the operating voltage decreases. Finally, LEDs with a nonuniform (Formula presented.) -GaN cap layer from a 3D island-like growth mode feature the highest output power and operating voltage. In contrast, the output power and operating voltage of LEDs with a smooth and closed cap depend on the thickness of (Formula presented.) -GaN. The highest output power and lowest operating voltage are achieved for LEDs with the thinnest (Formula presented.) -GaN cap. © 2020 The Authors. Published by Wiley-VCH Gmb