Perovskite Solar Cells (PSC) have attracted great attention due to the high efficiencies achieved in the past few years (up to 24.2 %). Perovskite semiconductors show excellent light absorption and large charge-carrier mobilities. In addition, device fabrication is low cost and easily up-scalable. However, the current density-voltage curve (J-V) shows hysteresis and devices suffer from stability issues which are still poorly understood. Among all perovskite materials, mixed-cation lead mixed-halide PSC have become very popular due to their high efficiencies and reasonably good stabilities1,2. On the other hand, Impedance Spectroscopy (IS) is a very valuable non-destructive technique to obtain information about dynamical mechanisms occurring both in the bulk and at the interfaces3 . In this work, J-V curves and the impedance response have been measured for CsFAPbIBr-based PSC from 1 Hz up to 1 MHz, under different illumination levels (from 0.06 mW/cm2 to 100 mW/cm2 ) both at 0 V (short circuit) and at Voc (open circuit). Impedance spectra show two significant arcs, associated to different recombination and charge accumulation mechanisms. IS data have been fitted to a circuital model that consists of a low-frequency RCPE subcircuit in series with a high frequency resistance, all shunted with a high-frequency capacitance. Dependence of the circuital parameters with Voc and Isc will be discussed.
Impedance Spectroscopy (IS) is a non-destructive characterization technique that has been extensively applied to different electronic devices, such as LEDs, photodiodes and solar cells. This technique provides access to valuable information about dynamical mechanisms (minority carrier recombination, diffusion, etc.) taking place in the different layers of the device. Besides, material and device parameters, such as dielectric constant, built-in potential, and carrier mobilities can be extracted. Impedance spectra results from applying a small AC signal over a steady DC bias and measuring the resulting small AC current over a frequency range, typically from 1 Hz to 1 MHz, đť‘Ť(jω) = VAC/1AC. The Nyquist plot of the complex impedance (imaginary part vs real part) generally presents one or more features (mainly semicircles), depending on the number of mechanisms governing the device. Fitting an electrical equivalent circuit to the complete impedance spectra provides parameters related to each feature (resistance and capacitance). In this work, IS has been used to characterize organic and perovskite solar cells (OSCs and PSCs, respectively). Measurements have been performed in dark and under illumination conditions at different bias (from 0 V to VOC). A simple circuital model containing two resistances and two capacitances has been used to fit the measured IS spectra. The interpretation of extracted circuital parameters and its relationship with the physical model of the device will be discussed.
KEYWORDS: James Webb Space Telescope, Space operations, Contamination, Contamination control, Inspection, Observatories, Manufacturing, Foam, Acoustics, Picture Archiving and Communication System
This paper will continue from Part 1 of JWST contamination control implementation. In addition to optics, instruments, and thermal vacuum testing, JWST also requires contamination control for a spacecraft that must be vented carefully in order to maintain solar array and thermal radiator thermal properties; a tennis court-sized sunshield made with 1-2 mil Kapton™ layers that must be manufactured and maintained clean; an observatory that must be integrated, stowed and transported to South America; and a rocket that typically launches commercial payloads without contamination sensitivity. An overview of plans developed to implement contamination control for the JWST spacecraft, sunshield, observatory and launch vehicle will be presented.
The electrical behavior of organic solar cell (OSC) has been analyzed using a simple circuital model consisting on an
ideal diode together with a series and parallel resistances (RS and RP respectively). Applying Kirchhoff's Laws to the
circuit leads to a transcendental equation that can be solved numerically without approximations using the Lambert W
function. Theoretical expression has been fitted to experimental current-voltage (I-V) curves under forward bias,
obtaining fairly accurate values for the electrical parameters. This model has been validated comparing the extracted
parameters for dark and illumination conditions of different devices. Results show good agreement for RS, and ideality
factor (η).
Electrical parameters obtained in this work are also compared to those ones extracted using an approximated method
often employed by other authors 1. We conclude that approximated method leads to reasonable good values for RS, RP
and η. However, in the case of Rp the voltage range chosen to fit the data with the exact method must be constrained to
the fourth quadrant, where the role of parallel resistance is more critical.
To validate the model, a bunch of organic solar cells with structure ITO/ poly(3,4-ethylenedioxythiophene)-poly
(4-styrene sulfonate (PEDOT:PSS)/ poly(3-hexylthiophene) (P3HT):
1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61
(PCBM)/Al has been fabricated in inert atmosphere. Different active layers were deposited varying the P3HT:PCBM
ratio (1:0.64, 1:1, 1:1.55) and the active layer thickness (ranging from 100 to 280 nm). Devices are encapsulated inside
the glove-box prior its characterization outside the glove-box. Electro optical characterization has been performed with a
halogen lamp.
Values extracted for RS range from 142 Ω to 273 Ω, values for RP range from 25 kΩ to 331 kΩ. Ideality factor ranges
from 5 to 17.
In this work, 4x4 organic light emitting diode passive matrices based on new poli(2,7-fluorene phenylidene) (PFP) derivatives have been developed. The fabrication process has involved spin-cast heterostructures that improve charge carrier injection, processing of devices by means of photolithography, together with metallic contact evaporation. Electroluminescent diodes using different polymer derivatives as active layer, and different geometries, have been fabricated and compared. Electrical characterization was carried out in terms of pulsed current-voltage (I-V) measurements. Dependence of the threshold voltage on active material and structural parameters is obtained from the I-V curves, yielding values from 10 V to 25 V. Electroluminescence spectra recorded from the new PFP based devices, as well as commercial polymers, are in good agreement with similar devices found in literature. Finally, experimental data have been fitted using a theoretical model considering several injection and transport mechanisms, including thermionic, reverse, tunnelling, ohmic and space charge limited currents.
A quasi-3D model has been developed with the aim of studying the different factors limiting the performance of high-brightness high-power tapered lasers. The model solves the complete semiconductor and thermal equations, neglecting the flow of carriers and heat along the cavity ax is, together with a 2D Wide-Angle Beam Propagation method solving the optical propagation. The coupling between electrical, thermal and optical equations yields a stable solution which incorporates carrier and temperature induced perturbations of the refractive index. Although tapered lasers have already demonstrated superior beam quality performance in comparison with broad area devices, they still suffer of beam filamentation at high power levels. We analyze the influence of the different competing factors in the self-focusing process for 980 nm lasers with a gain guided taper section. The simulation results indicate that the lasers with the longest taper section provide the highest output power before the filamentation process is triggered, and that the backward propagating field plays a crucial role in the stability of the output beam.
High indium content InGaAs/GaAs laser structures have been grown on (111)B GaAs substrates by Molecular Beam Epitaxy (MBE). The laser devices showed room temperature CW emission, low threshold current densities and emission wavelength up to 1100 nm. The influence of the internal piezoelectric field on the emission properties is studied theoretical and experimentally. A self consistent model was developed in order to simulate the gain and spontaneous emission spectra. Modeling results were compared with measurements of the spontaneous emission spectra, and a good qualitative agreement was obtained. By analyzing these results, we conclude that the piezoelectric field is not completely screened out even for high injection currents, and that the screening level is strongly dependent on the In content. Spontaneous emission measurements in two different configurations (top and edge emission) were compared, yielding similar results for a range of experimental conditions.
Facet heating is an important mechanism limiting the performance of high power laser diodes. The temperature increase at the laser facets contributes to the gradual degradation and can lead to Catastrophical Optical Damage (COD). We present a two-dimensional optoelectronic and thermal model for Fabry-Perot Quantum Well power lasers. The model is applied to 808 nm AlGaAs laser bars, with the aim of identifying the main volume and facet heating sources. The results of the simulation show that the heating caused by free-carrier absorption is the main source that could be minimized by a proper layer design. We present a new physical mechanism leading to COD: optical absorption at the facets produces carrier accumulation that can trigger the thermal runaway process.
Ignacio Esquivias, Beatriz Romero Herrero, S. Weisser, Konrad Czotscher, John Ralston, Eric Larkins, Julia Arias, A. Schoenfelder, Michael Mikulla, Joachim Fleissner, Josef Rosenzweig
The transport of carriers along the confinement region, the carrier capture into, and the carrier escape out of the quantum wells (QWs) are limiting processes affecting the high-frequency properties of QW lasers. The influence of these processes on the laser performance depends mainly on the ratio of the effective carrier transport/capture time and the effective escape time. We present experimental results about the escape times for GaAs/AlGaAs and InGaAs/GaAs high-speed QW lasers with varied geometrical dimensions (cavity width and length), number of QWs, In-concentrations, and p-doping levels in the active region, as extracted from electrical impedance measurements in the sub-threshold regime. In addition to the expected increase of the escape time with increasing QW barrier height, we observe an important increase in the escape time for lasers with p-doping. The escape time dependences on the carrier concentration and on the temperature are determined and discussed.
We describe short-cavity In0.35Ga0.65As/GaAs multiple quantum well (MQW) lasers with undoped and p-doped active regions. The epilayer structure consists of four 5.7 nm QWs separated by 20.1 nm barriers in a GaAs core. The cladding layers consist of Al0.8Ga0.2As. In the case of p-doped devices a 4.5 nm carbon (C)-doped region (2.5 multiplied by 1019 cm-3) was inserted above each QW, separated by a 3.1 nm GaAs spacer, resulting in a modulation-doped core region. Using a CAIBE process, short-cavity ridge-waveguide lasers are fabricated in a triple-mesa geometry suitable for on-wafer probing. The best device (6 multiplied by 130 micrometers squared) with an undoped active region attained a damping-limited direct modulation bandwidth exceeding 40 GHz at a cw bias current of 160 mA. In contrast, the p-doped devices, demonstrating a maximum bandwidth of 37 GHz, are still limited by power dissipation. (alpha) -factors as low as 1.4 and 1.5 for undoped and p-doped devices, respectively, are extracted from measurements of the sub- threshold gain spectra. In addition, we demonstrate eye diagrams at 25 Gbit/s (limited by the pulse pattern generator) for these laser diodes. A complete characterization of dc and rf properties of these lasers is presented.
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