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Results for “Carrier Recombination”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Influence of radiative recombination on the minority-carrier transport in direct band-gap semiconductors

When a semiconductor sample is irradiated by means of an external source, emitting photons or electrons, excess carriers are produced which distribute themselves throughout the sample. One of the parameters which determine the distribution of the carriers is the surface recombination velocity. The present investigation is concerned with the recombination lifetime tau. The predominant mechanism for recombination in wide band-gap semiconductors is described by the Shockley-Read-Hall (SRH) theory. The transport equations are derived for free carriers and the radiation field. The considered theory is applied to a semiinfinite, one-dimensional semiconductor slab irradiated by light of a given frequency. Some numerical considerations based on n-type GaAs are presented. Attention is given to a determination of the radiation transmitted through the surface of the sample.

Von Roos, O.↗
DOE OSTI2022

Spontaneous carrier generation and low recombination in high-efficiency non-fullerene solar cells

The current research reveals the unique photo physical properties and carrier generation mechanisms of high efficiency non-fullerene acceptor-based organic photovoltaic blends. The analysis of device performance, morphology, and photo physics reveals that the interface carrier generation cannot support the device's current output. It is seen that primary photon excitons can dissociate into charge carriers in Y6, which displays a time scale of ∼0.3 ps with polaron yields above 28% in neat and blended thin films. This characteristic in combination with a large exciton diffusion length enables high morphology tolerance in Y6-overloaded blends. The carrier generation process is also combined with recombination studies. The recombination rate quantified by flux-dependent transient absorption measurements yields less than 5% total recombination and nearly 0% geminate recombination. Furthermore, these results define the advantages of the Y6 non-fullerene acceptor (NFA) and reveal the fundamental importance of developing new NFA materials with superior photo physical properties.

14 SOLAR ENERGY↗
NASA NTRS1977

Determination of minority-carrier lifetime and surface recombination velocity with high spacial resolution

Quantitative analysis of the electron beam induced current in conjunction with high-resolution scanning makes it possible to evaluate the minority-carrier lifetime three dimensionally in the bulk and the surface recombination velocity two dimensionally, with a high spacial resolution. The analysis is based on the concept of the effective excitation strength of the carriers which takes into consideration all possible recombination sources. Two-dimensional mapping of the surface recombination velocity of phosphorus-diffused silicon diodes is presented as well as a three-dimensional mapping of the changes in the minority-carrier lifetime in ion-implanted silicon.

Watanabe, M.↗
DOE OSTI2022

Carrier Dynamics of Polar, Semipolar, and Nonpolar InGaN/GaN LEDs Measured by Small-Signal Electroluminescence

The carrier dynamics in InGaN/GaN light-emitting diodes (LEDs) are directly tied to their efficiency and maximum modulation speed, which are important metrics for solid-state lighting, displays, and optical communication. In this work, we measure the carrier dynamics of a variety of InGaN/GaN LEDs using small-signal electroluminescence methods [1]. A rate equation approach and associated small-signal circuit are used to model carrier injection, recombination in the active region, recombination in the cladding regions, and carrier escape. The model is fit to the measured optical frequency response (S21) and input impedance (S11) of the LEDs to extract the various carrier lifetimes, the carrier density, and the radiative and non-radiative recombination rates. We specifically study planar nonpolar and semipolar LEDs, which show record-high modulation speeds for III-nitride LEDs and present the modulation characteristics of core-shell nanowire-based LEDs. The planar nonpolar m-plane ($101\bar{0}$) micro-LEDs achieve a record-high -3dB modulation bandwidth for a III-nitride LED of 1.5 GHz [2]. The -3dB response of an electrically injected nanowire-based micro-LED with nonpolar facets is also reported, showing a -3dB bandwidth of 1.2 GHz [3]. The high speed is attributed to the shorter carrier lifetime associated with the nonpolar orientation. We also study the carrier dynamics in semipolar ($20\bar{2}\bar{1}$) LEDs for various temperatures [4]. Finally, we present carrier dynamics measurements on commercial-grade c-plane epitaxy for various active region designs, including a wavelength series and a growth quality series. The wavelength series offers insight into the contributions of the quantum confined Stark effect (QCSE) and InGaN material quality on the green gap [5]. The growth quality series investigates the role of non-radiative centers on the LED performance. Extraction of the carrier dynamics using small-signal electroluminescence offers insight into the factors limiting the efficiency and high-speed performance of III-nitride emitters and can be leveraged to ultimately improve the devices.

LEDs, InGaN, electroluminescence, bandwidth, modul↗
DOE OSTI2022

Distinguishing Bulk and Surface Recombination By Measuring and Modeling Ultrafast Carrier Dynamics

Distinguishing Bulk and Surface Recombination By Measuring and Modeling Ultrafast Carrier Dynamics The combination of modeling with TRTS and TRPL under multiple excitation conditions improves accuracy and precision of parameter estimation. Ultrafast carrier dynamics can link materials, processing, and device performance to direct the design of more efficient solar cells.

14 SOLAR ENERGY↗
DOE OSTI2023

Influence of trap-assisted and intrinsic Auger–Meitner recombination on efficiency droop in green InGaN/GaN LEDs

Here, we study the impact of deep-level defects on trap-assisted Auger–Meitner recombination in c-plane InGaN/GaN LEDs using a small-signal electroluminescence (SSEL) method and deep-level optical spectroscopy (DLOS). Carrier dynamics information, including carrier lifetime, recombination rate, and carrier density, is obtained from SSEL, while DLOS is used to obtain the deep-level defect density. Through fitting the nonradiative recombination rates of wafers with different deep-level defect densities, we obtain the Shockley–Read–Hall (SRH) and trap-assisted Auger–Meitner recombination (TAAR) coefficients. We show that defect-related nonradiative recombination, including both SRH and TAAR, accounts for a relatively small fraction of the total nonradiative recombination, which is dominated by intrinsic Auger–Meitner recombination. The interplay between carrier localization and Coulomb enhancement has a different impact on radiative and intrinsic Auger–Meitner recombination. Evidence is presented that the imbalance between the change of radiative and intrinsic Auger–Meitner recombination is the primary cause of the efficiency droop at high carrier densities in the samples studied.

36 MATERIALS SCIENCE↗
NASA NTRS2020

Development of High-Performance Graphene-HgCdTe Detector Technology for Mid-Wave Infrared Applications

A high-performance graphene-based HgCdTe detector technology is being developed for sensing over the mid-wave infrared (MWIR) band for NASA Earth Science, defense, and commercial applications. This technology involves the integration of graphene into HgCdTe photodetectors that combines the best of both materials and allows for higher MWIR(2-5 m) detection performance compared to photodetectors using only HgCdTe material. The interfacial barrier between the HgCdTe-based absorber and the graphene layer reduces recombination of photogenerated carriers in the detector. The graphene layer also acts as high mobility channel that whisks away carriers before they recombine, further enhancing the detector performance. Likewise, HgCdTe has shown promise for the development of MWIR detectors with improvements in carrier mobility and lifetime. The room temperature operational capability of HgCdTe-based detectors and arrays can help minimize size, weight, power and cost for MWIR sensing applications such as remote sensing and earth observation, e.g., in smaller satellite platforms. The objective of this work is to demonstrate graphene-based HgCdTe room temperature MWIR detectors and arrays through modeling, material development, and device optimization. The primary driver for this technology development is the enablement of a scalable, low cost, low power, and small footprint infrared technology component that offers high performance, while opening doors for new earth observation measurement capabilities.

Sood, Ashok K.↗
NASA NTRS1986

Carrier surface generation and recombination effects in photoconduction of HgI2 single crystals

Dc photoconductivity characteristics of HgI2 single crystals have been measured in the range of 1.75-2.75 eV, using CuI as a transparent electrical contact. Hole and electron photocurrents could be separately measured by applying different voltage polarities to the illuminated electrode. It is shown that charge carrier generation near the surface is highly extrinsic, as displayed by differences in the hole and electron photogeneration spectra. Analysis of the photoconductivity versus voltage characteristics indicates that the hole and electron bulk trapping times satisfy greater than 70 and 3 microsec, respectively. The hole and electron surface recombination velocities were 1.0 x 10 to the 4th cm/s and 8 x 10 to the 5th cm/s, respectively.

Burshtein, Z.↗
NASA NTRS1987

Measurement of minority carrier lifetimes and surface recombination velocities in solar cells

A generic comparison has been carried out of all methods for recombination parameter determination that are based on measurement of a current and/or voltage in either steady-state, transient, or frequency-dependent approaches. The theoretical background for the time-dependent methods has been extended from prior work to include drift fields and be applicable to compound (multilayered) regions. The key findings were: (1) all methods have essentially the same limitations to determination of only the dominant recombination parameters; (2) open-circuit measurements have more sensitivity to the recombination parameters than short circuit measurements, but they present much more difficulty in eliminating spurious effects; (3) transient and frequency-dependent methods have exactly the same expressions for the relaxation constants and the breakpoint frequencies; and (4) in compound regions there exists only one set of relaxation constants for all the coupled layers. These constants include all relevant parameters of these layers. It is thus difficult to separate these parameters in time- or frequency-domain measurements. ASLBIC seems still to be the most suitable method for determination of emitter parameters.

Wolf, M.↗
NASA NTRS1991

Evaluation of the minority carrier diffusion length and surface-recombination velocity in GaAs p/n solar cells

The minority carrier diffusion length (Lp) and the surface recombination velocity (Vs) were measured as a function of distance (x) from the p-n junction in GaAs p/n concentrator solar cells. The measured Vs values were used in a theoretical expression for the normalized electron-beam-induced current. A fitting procedure was then used to fit this expression with experimental values to obtain Lp. The results show that both Vs and Lp vary with x. Lp measured in irradiated cells showed a marked reduction. These values were compared to those measured previously which did not account for Vs.

Hakimzadeh, Roshanak↗
DOE OSTI2022

Interplay of structural fluctuations and charge carrier dynamics is key for high performance of hybrid lead halide perovskites

The ability of methylammonium lead triiodide (MAPbI 3 ) to achieve photoelectric conversion efficiency that is on par with crystalline silicon has led to a surge of interest in perovskite photovoltaics. However, an in-depth understanding of how the ubiquitous coupling between the fast rovibrational movements of the organic cations and the phonon vibrations of the inorganic framework affects the relaxation and recombination of hot carriers remains largely elusive. Access to such knowledge is critical to guide design and increase efficiency of new classes of perovskite solar cells. In this report, we report a time-domain ab initio investigation of temperature dependent excited state dynamics in MAPbI 3 , with particular emphasis on nuclear anharmonic effects. The observed strong anharmonicity is attributed to softness of the material and unusual dynamical coupling between the organic and inorganic components. At an elevated temperature, the hydrogen bonding between MA and iodines is weakened, enhancing rotation of MA cations, which become more dynamically disordered. As a result, thermal vibrations of the inorganic Pb–I sublattice are suppressed, and the lattice anharmonicity is decreased. Thermal fluctuations of the electronic energy levels are found to follow the trend of anharmonic motions of Pb and I atoms, with holes relaxing faster to the band edges than electrons, due to higher density of the hole states. While elevated temperature accelerates intraband carrier cooling, it slows nonradiative carrier recombination. The latter result is important for performance, since solar cells and other devices heat up during operation. The reported signatures of coupled structural dynamics of the organic cations and inorganic framework unravel the interplay between anomalous structural fluctuations and charge carrier dynamics, which is of particular importance for fundamental understanding of the structure–property relationships in hybrid metal halide perovskites.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Bias‐Dependent Quantum Efficiency Reveals Recombination Pathways in Thin Film Solar Cells

Identifying where recombination predominantly occurs—whether at the front interface, back interface, or throughout the bulk—is crucial for optimizing CdSeTe solar cells and many other photovoltaic device architectures. Here, a simple and effective diagnostic is demonstrated: measuring external quantum efficiency (QE) under varying forward biases. The drift–diffusion simulations reveal that each recombination pathway leaves a distinct bias-induced signature in the normalized QE: a progressive drop at long wavelengths for back-limited devices, a short-wavelength decline for front-limited devices, and a relatively uniform decrease across all wavelengths for bulk-limited devices. These predictions are validated with experiments on As-doped and Cu-doped CdSeTe devices, with and without passivation layers or different front buffers. In each case, the observed bias-dependent QE spectral changes align with the simulated recombination map. Because this method uses standard QE instrumentation, it offers a broadly accessible and cost-efficient means to diagnose recombination pathways—extending beyond CdSeTe to other thin-film and emerging solar cell technologies. By pinpointing recombination bottlenecks, researchers and manufacturers can strategically refine doping profiles, passivation schemes, and interface designs to further improve device performance.

CdTe↗
NASA NTRS1986

Development of high-efficiency solar cells on silicon web

Work was performed to improve web base material with a goal of obtaining solar cell efficiencies in excess of 18% (AM1). Efforts in this program are directed toward identifying carrier loss mechanisms in web silicon, eliminating or reducing these mechanisms, designing a high efficiency cell structure with the aid of numerical models, and fabricating high efficiency web solar cells. Fabrication techniques must preserve or enhance carrier lifetime in the bulk of the cell and minimize recombination of carriers at the external surfaces. Three completed cells were viewed by cross-sectional transmission electron microscopy (TEM) in order to investigate further the relation between structural defects and electrical performance of web cells. Consistent with past TEM examinations, the cell with the highest efficiency (15.0%) had no dislocations but did have 11 twin planes.

Meier, D. L.↗
DOE OSTI2021

Impact of Photon Recycling, Grain Boundaries, and Nonlinear Recombination on Energy Transport in Semiconductors

Energy carrier transport and recombination in semiconductors can be directly monitored with optical microscopy, revealing key insights into fundamental photophysics and informs efficient optoelectronic device design. Oftentimes, energy transport is measured by monitoring a time-resolved expanding carrier profile after optical excitation under a focused laser beam. Although this approach has gained widespread adoption, the presence of competing dynamic processes, such as diffusion and photon recycling, has made it difficult to accurately extract and understand important semiconductor properties. Here, we develop and apply a new framework for modeling energy transport in both excitonic and free carrier semiconductors. Here, we demonstrate that processes such as nonlinear recombination and photon recycling can have a significant impact on the apparent energy carrier profiles, especially for excitonic materials with short radiative lifetimes. Additionally, we find that film microstructure can lead to unique transport profiles that strongly depends on material boundary behavior and the difference between feature size and diffusion length. Here, these findings provide a deeper understanding of energy transport in semiconducting materials and provide new strategies for the design and optimization of electronic and optoelectronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1986

Comments on determination of bandgap narrowing from activation plots

A determination is made of the temperature-dependence of emitter saturation current in bipolar devices which allows the derivation of a value for bandgap narrowing that is in better agreement with other determinations than previous results based on ohmic contact measurements of temperature dependence. The new values were obtained by varying the surface recombination velocity at the emitter surface. This improves accuracy by varying the minority carrier surface recombination velocity at the emitter contacts of otherwise indistinguishable emitters.

Park, J.-S.↗
DOE OSTI2024

Multiple-carrier-lifetime model for carrier dynamics in InGaN/GaN LEDs with a non-uniform carrier distribution

Here we introduce a multiple-carrier-lifetime model (MCLM) for light-emitting diodes (LEDs) with non-uniform carrier distribution, such as in multiple-quantum-well (MQW) structures. By employing the MCLM, we successfully explain the modulation response of V-pit engineered MQW LEDs, which exhibit an S 21 roll-off slower than –20 dB/decade. Using the proposed model and employing a gradient descent method, we extract effective recombination and escape lifetimes by averaging the carrier behavior across the quantum wells. Our results reveal slower effective carrier recombination and escape in MQW LEDs compared with LEDs emitting from a single QW, indicating the advantages of lower carrier density achieved through V-pit engineering. Notably, the effective carrier recombination time is more than one order of magnitude lower than the effective escape lifetime, suggesting that most carriers in the quantum wells recombine, while the escape process remains weak. To ensure the reliability and robustness of the MCLM, we subject it to a comprehensive threefold validation process. This work confirms the positive impact of spreading carriers into several quantum wells through V-pit engineering. In addition, the MCLM is applicable to other LEDs with a non-uniform carrier distribution, such as micro-LEDs with significant surface recombination and non-uniform lateral carrier profiles.

42 ENGINEERING↗