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In the last years, perovskite solar cells have attracted great interest in photovoltaic (PV) research due to their possibility to become a highly efficient and low-cost alternative to silicon solar cells. Cells based on the widely used Pb-containing perovskites have reached power conversion efficiencies (PCE) of more than 20 %. One of the major hurdles for the rapid commercialization of perovskite photovoltaics is the lack of deposition tools and processes for large areas. Chemical vapor deposition (CVD) is an appealing technique because it is scalable and furthermore features superior process control and reproducibility in depositing high-purity films. In this work, we present a novel showerhead-based CVD tool to fabricate perovskite films by simultaneous delivery of precursors from the gas phase. We highlight the control of the perovskite film composition and properties by adjusting the individual precursor deposition rates. Providing the optimal supply of precursors results in stoichiometric perovskite films without any detectable residues.
The 2D transition metal dichalcogenide (TMDC) tungsten disulfide (WS2) has attracted great interest due to its unique properties and prospects for future (opto)electronics. However, compared to molybdenum disulfide (MoS2), the development of a reproducible and scalable deposition process for 2D WS2 has not advanced very far yet. Here, we report on the systematic investigation of 2D WS2 growth on hydrogen (H2)-desorbed sapphire (0001) substrates using a hydrogen sulfide (H2S)-free metal-organic vapor phase epitaxy (MOVPE) process in a commercial AIXTRON planetary hot-wall reactor in 10 × 2" configuration. Tungsten hexacarbonyl (WCO, 99.9 %) and di-tert-butyl sulfide (DTBS, 99.9999 %) were used as MO sources, nitrogen (N2) was selected as carrier gas for the deposition processes (standard growth time 10 h). In an initial study, the impact of growth temperature on nucleation and growth was investigated and an optimal value of 820 °C was found. The influence of the WCO flow on lateral growth was investigated. The aim was to maximize the edge length of triangular crystals as well as the total surface coverage. Extending gradually the growth time up to 20 h at optimized WCO flow conditions yields fully coalesced WS2 samples without parasitic carbon-related Raman peaks and with only sparse bilayer nucleation. After substrate removal, a fully coalesced WS2 film was implemented into a light-emitting device showing intense red electroluminescence (EL).
Pb-based organometal halide perovskite solar cells have passed the threshold of 20 % power conversion efficiency (PCE). However, the main issues hampering commercialization are toxic Pb contained in these cells and their instability in ambient air. Therefore, great attention is devoted to replace Pb by Sn or Bi, which are less harmful and - in the case of Bi - also expected to yield enhanced stability. In literature, the most efficient hybrid organic-inorganic methylammonium bismuth iodide (MBI) perovskite solar cells reach PCE up to 0.2 %. In this work, we present spin-coated MBI perovskite solar cells and highlight the impact of the concentration of the perovskite solution on the layer morphology and photovoltaic (PV) characteristics. The solar cells exhibit open-circuit voltages of 0.73 V, which is the highest value published for this type of solar cell. The PCE increases from 0.004 % directly after processing to 0.17 % after 48 h of storage in air. 300 h after exposure to air, the cells still yield 56 % of their peak PCE and 84 % of their maximum open-circuit voltage.
We have investigated organic light emitting diode (OLED) backside contacting for the enhancement of luminance uniformity as a superior alternative to gridlines. In this approach, the low-conductivity OLED anode is supported by a high-conductivity auxiliary electrode and vertically contacted through via holes. Electrical simulations of large-area OLEDs have predicted that this method allows comparable luminance uniformity while sacrificing significantly less active area compared to the common gridline approach.
The method for fabricating backside contacts is comprised of five steps: (1) Thin-film encapsulation of the OLED, (2) Patterning of the OLED surface with lithography (resist mask defining via hole positions), (3) Via hole formation to the bottom anode by a plasma etching process, (4) Organic residues removal and sidewall insulation. (5) Contacting of the anode with a high-conductivity auxiliary electrode.
Backside-contacted OLEDs processed by organic vapor phase deposition show high luminance uniformity. Scanning electron microscopy pictures and electrical breakthrough measurements confirm efficient sidewall insulation.
We demonstrate Ag-free transparent OLED (TOLED) fabricated by organic vapor phase
deposition (OVPD) using thin Au contacts. Three types of TOLED devices have been
studied. The first one has been deposited on ITO substrates to compare thin Ag
and Au films as top cathodes. A 6-fold increase in operational lifetime
(LT50, 4 mA/cm2) from 27 h to 172 h can be observed
when replacing Ag by Au while maintaining similar electro-optical
characteristics. Furthermore, a second type of TOLED on thin Au films, replacing
ITO and suppressing laterally guided modes , has been studied. TOLED on ITO
substrates and on thin Au films exhibit very low onset voltages of 2.2 V. Both
types show about 30% transparency in the VIS light region and emit orange light
with a peak wavelength of 608 nm from either side with a total EQE of about 9%
(measured at 1000 cd/m2 in sum). The third type of TOLED was
fabricated with an inverted structure, with the aim to further increase
operational lifetime by burying the reactive LiF/Al electron injection layer
(EIL). This will make the EIL less accessible for oxygen and moisture. Our
results show difficulties in electron injection when depositing the organic
stack on Al/LiF, which may be attributed to an insufficient thermal activation
of the EIL.
The development of efficient large-area organic light emitting diodes (OLED) requires reliable and easily processable charge generation layers (CGL) with low excess voltage drop and high optical transparency. OVPD offers the advantage of a precise control of layer morphology, composition and thickness and is a powerful method for the deposition of advanced OLED designs. In this work, electrical doping of organic semiconductors using OVPD is investigated and applied to stacked OLED utilizing inorganic/organic CGL. The organic p-type dopant NDP-9 of Novaled GmbH is used for doping the hole transport material N,N‘-diphenyl-N,N‘-bis(1-naphthylphenyl)-1,1‘-biphenyl-4,4‘-diamine (α-NPD) in an AIXTRON OVPD tool. A doping concentration of 8 vol.% of NDP-9 in α-NPD is found optimal for hole injection as well as conductivity. This dopant concentration was employed in CGL with the structure: electron transport material/LiF/Al/α-NPD:8 vol.% NDP-9. External quantum efficiencies (EQE) of 15%, 35% and 50% and luminous efficiencies of 37 lm/W, 45 lm/W and 45 lm/W at 1000 cd/m2 are demonstrated for single, double- and triple-unit green phosphorescent OLED, respectively.
We improve the efficiency of a bottom-emitting red phosphorescent organic light emitting diode (OLED) by the suppression of wave-guided modes in the bottom contact. ITO as bottom contact layer has been substituted by a thin Ti/Au layer. Electromagnetic simulation results of both devices predict the absence of TE polarized guided contact modes by the use of 10 nm Au as bottom electrode. We measured an improved outcoupling of light which overcompensates absorption losses of the Ti/Au layer in the measured emission cone. By the use of 1 nm Ti as undercoat, a continuous Au film of 8 nm thickness could be realized with an improved transmittance for long wavelengths (λ > 550 nm). As a consequence of fewer lateral guided modes, the external quantum efficiency (EQE) has been enhanced from 11.5 % of the standard device to 14 % of the device with the Ti/Au electrode.
The light out-coupling potential of introducing a semitransparent Ag layer between the anode and the organic layer stack of monochrome bottom-emitting organic light emitting diodes (OLED) is examined. Red and green phosphorescent as well as deep-blue fluorescent resonant-cavity OLED (RC-OLED) comprising a semitransparent Ag layer are processed by means of organic vapor phase deposition (OVPD). An enhancement of the luminous efficiency of up to 81% can be observed.
The impressive efficiency enhancement can be explained by a reduced formation of substrate modes in combination with a strong narrowing of the emission spectrum leading to an increased true luminous efficiency.
As global energy demand is steadily growing, renewable energy generation by solar cells is becoming increasingly important. The use of mono- and polycrystalline silicon solar cells, which nowadays dominate the market, is limited by wafer size, rigidness of substrates and the requirement of large energy amounts for manufacturing. Organic solar cells (OSC) have the potential to overcome these limitations; especially organic vapor phase deposition (OVPD) technology offers the possibility of reproducible, large-scale production at low temperatures and on flexible substrates.
We report on planar heterojunction OSC utilizing an active layer of pentacene/N, N’- ditridecylperylene-3, 4, 9, 10-tetracarboxylic diimide (PTCDI) fabricated by an Aixtron Gen-1 OVPD tool. The influence of substrate temperature was studied using atomic force microscopy (AFM) on single layers and bilayers. In addition electrical characterization with and without illumination of fully processed solar cells which utilize different cathode layers was carried out.
AFM images indicate that crystallization of pentacene layers can be widely influenced by substrate temperature, a PTCDI-C13H27 layer atop of these covers the crystallites. Open-circuit voltage was found to be 0.47 V and short-circuit current densities beyond 0.8 mA/cm2 were measured under a spectrum close to AM 1.5 with 100 mW/cm2. Fill factors were determined to be as high as 44 %.
In the past few years, organic vapor phase deposition (OVPD) has been demonstrated to be an effective deposition method for high-performance monochrome and white organic light emitting diodes (OLEDs) [1-4]. OVPD provides good material utilization efficiency and large achievable deposition rates.
An application of p-type doping is the improvement of hole injection either from the anode contact or from a charge generation layer in stacked OLEDs . Nevertheless, no reports on p-type doping using OVPD can be found in literature, in part due to the thermal instability and high chemical sensitivity of organic dopants.
In this work, p-type doping using an AIXTRON Gen-1 OVPD tool with two different show-erhead designs is examined. NDP-2 (NOVALED) and N,N‘-diphenyl-N,N‘-bis(1-naphthylphenyl)-1,1‘-biphenyl-4,4‘-diamine (NPB) were used as p-type dopant (guest) and hole-conducting host, respectively. p-Type doped hole-only devices were fabricated and compared with undoped ones.
Two different showerhead designs (made either of aluminum or stainless steel) were investi-gated with respect to OLED performance to determine possible side reactions.
Highly efficient monochrome red OLEDs including a p-type doped hole transport layer were demonstrated exhibiting a current efficiency of 31 cd/A, a power efficiency of 26 lm/W and a driving voltage of 3.7 V without improved light outcoupling (all values at 1000 cd/m2).
The precise control of organic thin film processing by
organic vapor phase deposition (OVPD®) is presented and
analyzed on device level. OVPD® offers accurate and individual control of
deposition layer properties like mixing of several materials (co-deposition)
and the control of various morphologies by a wide process parameter space
given by, e.g. substrate temperature, deposition rate and pressure. The
benefit of precise co-deposition is demonstrated by an OLED with a sensitive
twofold-doped emissive layer and revealed a doping level of 0.26% for the
red dopant with a std. dev. of 0.38%. The effect of the various
morphologies is investigated by optimizing the efficiency of molecular
organic solar cells consisting of copper phthalocyanine (CuPc) and C60.
With defined process parameters efficiencies of up to 3.0% were
OLED with non-constant dopant concentration profiles have been processed by means of organic vapour phase deposition (OVPD) and were compared with regard to their luminous current efficiencies. Especially when driven at ultra-high luminance (>10,000 cd/A), OLED with a dopant concentration profile starting with a rather high dopant concentration on the anode side of the emissive layer showed improved luminous current efficiencies compared to their conventional counterparts.
To further investigate this effect, the width and location of the recombination zone have been simulated for all investigated concentration profiles by numerical solution of the semiconductor device equations using experimentally determined doping-dependent charge carrier mobilities. The obtained theoretical results are discussed with regard to the accomplished experiments.
The current and luminous efficacy of a red phosphorescent organic light emitting diode (OLED) with sharp interfaces between each of the organic layers can be increased from 18.8 cd/A and 14.1 lm/W (at 1,000 cd/m2) to 36.5 cd/A (+94%, 18% EQE) and 33.7 lm/W (+139%) by the introduction of a layer cross-fading zone at the hole transport layer (HTL) to emission layer (EL) interface. Layer cross-fading describes a procedure of linearly decreasing the fraction in growth rate of an organic layer during deposition over a certain thickness while simultaneously increasing the fraction in growth rate of the following layer. For OLED processing and layer cross-fading organic vapor phase deposition (OVPD) is used. The typical observation of a roll-off in current efficacy of phosphorescent OLED to higher luminance can be reduced significantly. An interpenetrating network of a prevailing hole and a prevailing electron conducting material is created in the cross-fading zone. This broadens the recombination zone and furthermore lowers the driving voltage. The concept of layer cross-fading to increase the efficacies is suggested to be useful in multi-colored OLED stacks as well.
White OLED consisting of a fluorescent blue emissive layer combined with a phosphorescent green and a phosphorescent red emissive layer were processed by means of Organic Vapor Phase Deposition (OVPD). Different concepts to tune the color coordinates of the device are discussed with respect to the luminous efficiency. Furthermore, the influence of device aging on the emitted spectrum is being investigated by means of spectrally resolved lifetime measurements.
We report on deposition and properties of m-plane GaN/InGaN/AlInN structures on LiAlO2 substrates grown by metal organic vapor phase epitaxy (MOVPE). At first, two different buffer structures, one of them including an m-plane AlInN interlayer, were investigated concerning their suitability for the subsequent coalesced single-phase m-plane GaN growth. A series of quantum well structures with different well thickness based on one of these buffers showed absence of polarization-induced electric fields verified by room temperature photoluminescence (RT PL) measurements at different excitation intensities. Furthermore, polarization-resolved PL measurements revealed a high degree of polarization (DoP) of the emitted light with an intensity ratio of 8:1 between light polarized perpendicular and parallel to the c-axis.
Organic light emitting diodes (OLED) are efficient light sources based on
organic semiconductors. Unlike inorganic LEDs which are more or less point
sources, OLED are planar light sources with up to 1 m2 in area.
By using organic materials, they are cheap to produce and economical to use.
The determination of triplet exciton energy levels is of interest for the
development of efficient OLED, based on the fact that electrical excitation
usually creates three times as many triplets as singlets. Additionally, the
knowledge of these energy levels is crucial for the design and choice of
emitter matrix materials and exciton blocking layers. These values are
normally determined by photoluminescence (PL) measurements in solution for
materials which show intersystem crossing (ISC) between singlet and triplet
states. For some materials, the triplet levels cannot be measured this way
because some materials prohibit ISC. In this work, a method is presented
which allows the determination of the energy levels using low-temperature
electroluminescence (EL) spectroscopy. The dependence on ISC is avoided by
creating triplets directly with electrical excitation and this allows to
measure a large class of organic materials. A low-temperature EL spectrum is
N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (TPD) in
a 3-phenyl-4-(1‘-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) matrix (TPD/TAZ
1:3) at 77 K. Triplet emission is only observed at very low charge carrier
density (0.5 μA/mm2). Quenching processes are analyzed using
combined EL and PL measurements and unipolar devices. Two factors can be the
cause of the quenching: A strong quenching based on a low concentration of
electrically activated impurities could explain the dependency. The other
explanation points to a quenching based on electrons in the emitting layer.
This might be explained with triplet-polaron quenching (TPQ). TPQ is
proportional to the charge carrier density and contributes the dominant part
to the quenching at low current densities.
Results on the preparation and properties of AlGaN/GaN HEMTs on silicon substrates are presented and selected issues related to the material structure and device performance devices are discussed. Virtually crack-free AlGaN/GaN heterostructures (xAlN ≅ 0.25), with low surface roughness (rms of 0.64 nm), ns ≅ 1×1013 cm−2 and μ ≅ 1100 cm2/V s at 300 K, were grown by LP-MOVPE on 2-inch (111)Si substrates. HEMT devices with Lg = 0.3–0.7 μm were prepared by conventional device processing steps. Photoionization spectroscopy measurements have shown that a trap level of 1.85 eV, additional to two levels of 2.9 and 3.2 eV found before on GaN-based HEMTs on sapphire, is present in the structures investigated. Self-heating effects were studied by means of temperature dependent dc measurements. The channel temperature of a HEMT on Si increases with dissipated power much slower than for similar devices on sapphire substrate (e.g. reaches 95 and 320 °C on Si and sapphire, respectively, for 6 W/mm power). Prepared AlGaN/GaN/Si HEMTs exhibit saturation currents up to 0.91 A/mm, a good pinch-off, peak extrinsic transconductances up to 150 mS/mm and static heat dissipation capability up to ∼16 W/mm. Unity current gain frequencies fT up to 21 and 32 GHz were obtained on devices with gate length of 0.7 and 0.5 μm, respectively. The saturation current and fT values are comparable to those known for similar devices using sapphire and SiC substrates. Properties of AlGaN/GaN/Si HEMTs investigated show that this technology brings a prospect for commercial application of high power rf devices.
Co on AlGaN is expected to form a large barrier Schottky contact due to its high work function. We have used this material combination with 18 % of Al in AlxGaN for the study of transient photoresponse in the photovoltaic mode and in secondary photocurrent measurements after pulsed laser excitation. In reverse bias and in short- circuit mode a fast decay with a characteristic time of a few microseconds is dominant at room temperature. This mode is appropriate for UV detector operation. At elevated temperature, a much slower tail extending to several milliseconds is also observed. In forward bias operation the slow tail is dominating at any temperature. We discuss this asymmetry with respect to fast minority carrier collection within the space charge region for primary photocurrents and the slower majority carrier transport in forward bias.
We report on recent results obtained using an AIX 2400G3HT production type Planetary Reactor® in the 5×3 inch configuration for growth of typical group-III nitride layer structures consisting of GaN, InGaN and AlGaN. The optimum reactor geometry has been found by extensive modeling of the reactor design. Increased thermal management allows maximum reactor temperatures above 1400°C. As a consequence of extensive reactor modeling, the process transfer from 6×2 inch to 5×3 inch configuration was carried out by simple scaling of the corresponding process parameters of the 6×2 inch configuration. The scaling factor is calculated with respect to the changed reactor geometry. We used optical reflectrometry for in-situ growth control during this process development and could confirm the theoretical scaling requirements for obtaining identical growth conditions as compared to the 6×2 inch reactor configuration. This is verified by the generation of identical reflectance spectrum features. This important issue of in-situ control is discussed in detail. The TMGa efficiency could be kept at about 17%. Switching to the 8×3 inch configuration the efficiency increases up to about 27%, which is an improvement of 63% as compared to the 6×2 inch configuration