Carrier Diffusion Length Research Articles

Optical Characterization of 4H-SiC Thick Epitaxial Layer for Particle Detection

In the present work a deep characterization of 4H-SiC epi-layer was done. A thick layer was epitaxially grown through chemical vapor deposition (CVD) process in a horizontal hot-wall reactor in order to obtain a 250 microns thick epi-layer. This sample will be used as particle detector in hostile environments such as neutron detection in a nuclear fusion reactor. Raman and Photoluminescence (PL) spectroscopy have been used in order to evaluate the general status of epitaxy and, with the support of the Time Resolved Photoluminescence, also important properties such as carrier lifetime and diffusion length have been evaluated. Carrier lifetime evaluation before and after a thermal oxidation process at 1400° C for 48h was estimated, by considering a lifetime increment after oxidation process, due to the decrease of carbon vacancies. Finally, the influence of stacking fault (SF) defects on carrier lifetime was evaluated observing a decrease of the lifetime for the defects at 430 nm (2.88 eV) for both oxidated and non-oxidated samples.

Holistic Determination of Optoelectronic Properties using High-Throughput Spectroscopy of Surface-Guided CsPbBr3 Nanowires.

Optoelectronic micro- and nanostructures have a vast parameter space to explore for modification and optimization of their functional performance. This paper reports on a data-led approach using high-throughput single nanostructure spectroscopy to probe >8000 structures, allowing for holistic analysis of multiple material and optoelectronic parameters with statistical confidence. The methodology is applied to surface-guided CsPbBr3 nanowires, which have complex and interrelated geometric, structural, and electronic properties. Photoluminescence-based measurements, studying both the surface and embedded interfaces, exploits the natural inter nanowire geometric variation to show that increasing the nanowire width reduces the optical bandgap, increases the recombination rate in the nanowire bulk, and reduces the rate at the surface interface. A model of carrier recombination and diffusion ascribes these trends to carrier density and strain effects at the interfaces and self-consistently retrieves values for carrier mobility, trap densities, bandgap, diffusion length, and internal quantum efficiency. The model predicts parameter trends, such as the variation of internal quantum efficiency with width, which is confirmed by experimental verification. As this approach requires minimal a priori information, it is widely applicable to nano- and microscale materials.

Physical properties and photocatalytic activity of pulverized Ga-doped La5Ti2Cu0.9Ag0.1O7S5 powder

The photocatalytic activity of rod-like Ga-doped La5Ti2Cu0.9Ag0.1O7S5 (Ga-LTCA) particles exhibiting the one-dimensional conductivity is considered to be determined by the particle length relative to the charge carrier diffusion length (a few micrometers at most). In this work, Ga-LTCA particles with length of approximately 1 µm and a clear rod-like structure were prepared by thermal sulfidation, ball-milling, and post-annealing in sulfur vapor. Despite the small particle size and neat morphology, however, the photocatalytic H2 evolution activity of the resultant Ga-LTCA was lowered, indicating that the degradation of the crystallinity during the ball-milling treatment was detrimental to the activity.

Evaluation of the performance degradation of silicon solar cell irradiated by low-level (

Radiation damage induced by energetic particles such as protons and electrons beams could cause permanent deterioration of electrical characteristics of the semiconductor material, which is one of the primary reasons for failures of the space solar cells. This paper discusses the degradations of carrier lifetime and diffusion length of c-Si solar cell with low-level energy (<1 MeV) electron and proton irradiations using Photocarrier Radiometry (PCR) methodology. A theoretical one-dimensional two-layer PCR model was used to evaluate these two parameters of the c-Si solar cells. Monte Carlo Simulations (MCS) were employed to analyze the radiation damage of c-Si solar cell induced by electron and proton irradiations at different energies. Experimental investigations on PCR signals of c-Si solar cells irradiated by electron and proton beams were presented. The results show that the carrier lifetime and diffusion length are dramatically reduced due to the radiation damage induced by the low-level energy electron and proton irradiation.

Defect Healing in FAPb(I1‐xBrx)3 Perovskites: Multifunctional Fluorinated Sulfonate Surfactant Anchoring Enables &gt;21% Modules with Improved Operation Stability

AbstractFormamidinium lead triiodide‐based perovskite solar cells have emerged as one of the most promising candidates that can be potentially used to develop photovoltaic technologies in the future. The commercial use of perovskite solar cell modules (PSCMs) is limited as it is challenging to fabricate high‐quality, efficient, and stable large‐area perovskite light‐absorbing films. Heptadecafluorooctanesulfonic acid tetraethylammonium salt (HFSTT), containing fluorinated long alkyl chains as hydrophobic tails and sulfonic acid groups (SO3−) as hydrophilic heads, which exhibit a great synergistic potential in large‐area film uniform fabrication, crystallization orientation modulation, defect passivation, and device operation stability enhancement, are introduced. The HFSTT‐modified films exhibit a prominent (100) orientation and lower trap‐state density as well as enhanced carrier mobilities and diffusion lengths, facilitating a champion unit device with an impressive power conversion efficiency (PCE) of 23.88% (0.14 cm2) and 22.52% (1 cm2) with a low voltage deficit around 0.341 V. The unencapsulated device retains ≈70% of its initial efficiency after 1000 h under heat damping test (60 °C and ≈60% RH). Moreover, the PSCMs exhibiting PCEs of 21.05% (with notable fill factor 0.79) and 18.27% are characterized by the active areas of 25.98 and 60.68 cm2, respectively.

Effect of defect-rich Co-CeOx OER cocatalyst on the photocarrier dynamics and electronic structure of Sb-doped TiO2 nanorods photoanode

This work demonstrates the in-depth mechanism of enhanced photoelectrochemical (PEC) water oxidation of Sb-doped rutile TiO2 nanorods (NRs) photoanode coupled with oxygen vacancy defect-rich Co-doped CeOx (Co-CeOx) oxygen evolution reaction (OER) cocatalyst. The defect-rich Co-CeOx cocatalyst modification improves the conductivity, light absorption, charge transfer efficiency, and surface photovoltage generation of the Co-CeOx/Sb-TiO2 hybrid NRs photoanode. The Co-CeOx cocatalyst also serves as the surface passivating overlayer for the Sb-TiO2 photoanode, which suppresses the surface states mediated recombination of electron-hole pairs in the NRs. The PEC studies further indicate that Co-CeOx cocatalyst induces remarkably large band bending at the semiconductor/electrolyte interface and shortens the carrier diffusion length and depletion layer width, facilitating the rapid separation and transportation of the photocarriers for the surface PEC reactions. The experimental and theoretical studies confirm that the Co-doping in CeOx cocatalyst enhances the surface oxygen vacancy defects, which provides active catalytic sites for OH− adsorption and charge transportation for enhanced OER kinetics. The density functional theory (DFT) calculations demonstrate a higher conductivity of the Co-CeOx cocatalyst, advantageous for rapid charge transfer capability during PEC reactions. The synergy between all these merits helps the optimized Co-CeOx/Sb-TiO2 photoanode to deliver a maximum photocurrent density of 1.41 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (VRHE) and an ultra-low turn on potential (Von) of 0.1 VRHE under AM 1.5G solar illumination compared to the Sb-TiO2 NRs (0.96 mA cm−2 at 1.23 VRHE and Von = 0.42 VRHE). This work demonstrates the design of an efficient defect-rich cocatalyst modified photoanode for solar energy harvesting.

Research Progress of High-Sensitivity Perovskite Photodetectors: A Review of Photodetectors: Noise, Structure, and Materials

In recent years, perovskite materials have been widely used in optoelectronic components due to a series of advantages such as a high light absorption coefficient, high carrier mobility, long carrier diffusion length, shallow defect level, and high crystallinity. The photodetector is an important photoelectric device that can convert light radiation signals into electrical signals, so it has significant applications and scientific research value in military, civil, and other fields. Semiconductor materials are an essential part of photodetectors. So far, many semiconductor materials have been used in photodetectors, including silicon, carbon nanotubes, III–V compounds, and quantum dots, and remarkable progress has been made in improving the light detection performance and device structure design. However, photodetectors based on these materials usually require expensive materials, rigorous processes, and complex manufacturing conditions, which hinder their commercial application. Optoelectronic devices based on perovskite materials have the advantages of a simple fabrication process, low cost, and high performance, making them widely considered in optoelectronic applications. This review focuses on the preparation and photoelectric performance of the perovskite photodetector and the effect of noise on the performance of the perovskite detector and how to reduce the detector’s noise to increase the detection rate. In this review, we mainly discuss the following four aspects of work. First, we discuss the various noises of perovskite photodetectors. Then, we explore ways to reduce the noise of perovskite detectors with different structures (photoconductor, photodiode, and phototransistor). We explore corresponding improvement methods for different device structure detectors to reduce noise and improve the detection performance. Next, we discuss the different synthesis methods of all-inorganic perovskites, including solution processing synthesis, vapor-assisted solution synthesis, and chemical vapor deposition. Finally, current challenges of perovskite photodetectors (toxicity, stability, flexibility, and self-powered) are summarized and prospected.

Recent advancements in batteries and photo-batteries using metal halide perovskites

Metal halide perovskites have gained significant interest for use in solar cells and light-emitting devices. Recently, this material has also gained significant interest for its potential in energy storage devices, particularly lithium-ion batteries and photo-batteries, due to their long charge carrier diffusion length, high charge mobility, high light absorption capacity, non-rigid structure, and variable bandgap. This perspective highlights key properties of metal halide perovskites used as electrodes in lithium-ion batteries. The primary discussion is divided into four sections: an explanation of the structure and properties of metal halide perovskites, a very brief description of the operation of a conventional lithium-ion battery, lithium-ion interaction with metal perovskite halides, and the evolution and progress of perovskite halides as electrodes and photo-electrodes. The purpose of this perspective is to build awareness of recent advancements and provide an outlook on this relatively new subfield in order to motivate continued research and development of batteries and photo-batteries containing metal halide perovskites.

Broadband Enhancement of Mid‐Wave Infrared Absorption in a Multi‐Resonant Nanocrystal‐Based Device

AbstractLight management is one of the main challenges to address when designing a sensor from a nanocrystal (NC) array. Indeed, the carrier diffusion length, limited by hopping mechanism, is much shorter than the absorption depth. Several types of resonators (plasmon, Bragg mirror, guided mode, Fabry–Perot cavity) have been proposed to reduce the volume where light is absorbed. All of them are inherently narrow bands, while imaging applications focus on broadband sensing. Here, an infrared sensor in the short and mid‐wave infrared (SWIR and MWIR) that combines three different photonic modes is proposed to achieve broadband enhancement of the light absorption. Moreover, it is shown that these three modes can be obtained from a simple structure where the NC film is coupled only to a grating and a top metallic layer. The obtained device achieves a high responsivity of &gt;700 mA W–1, a detectivity up to 2 × 1010 Jones at 80 K, and a short response time of 11 µs.

Recent Progress on the Performance of Lead‐Based Halide Perovskite APbX3 Detectors

Lead‐based halide perovskites APbX3 are likely to become competitive candidates in applications of photodetectors and X/γ‐ray detectors owing to their excellent optical and electrical characteristics, such as high and balanced carrier mobility, long carrier diffusion length, and adjustable bandgap. Based on this system, the article summarizes the performance parameters, such as responsivity, detectivity, external quantum efficiency, and response time of photodetector, and generalizes the research status of its application in near‐infrared light, visible light, and ultraviolet light photodetectors. In addition to that, it also summarizes the recent progress of X/γ‐ray detectors. What is more, the influence of radii of A‐site cations on the photoelectric performance are discussed. Finally, based on these summary and discussion, a research trend could be raised that more and more researchers tend to fabricate APbX3 perovskite with single‐crystalline thin‐film for high‐performance photodetectors in the future. It is proposed that if the A‐site ion is an organic group with a larger radius than the formamidinium (FA+), it may have better photoelectric properties than FAPbX3. Inorganic lead‐based perovskite single crystals with few defects, fast carrier mobility, and good stability are beneficial for attaining low dose rate and high‐energy resolution X/γ‐ray detectors.

Subwavelength Densely Packed Disordered Semiconductor Metasurface Units for Photoelectrochemical Hydrogen Generation

For most semiconductors, especially the visible-light-absorbing ones, the carrier diffusion length is significantly shorter than the light penetration depth, limiting their photoactivities. This limitation could be mitigated through the use of subwavelength semiconductor-based metasurfaces and metamaterials. In this paper, a large-scale compatible metasurface photocathode, made of densely packed disordered p-type chromium oxide (CrOX), is developed to be utilized in photoelectrochemical (PEC) hydrogen generation. For this purpose, first, tightly packed random Cr nanorods are fabricated using an oblique angle deposition technique. Afterward, an annealing step is applied to the sample to transform these metallic units into a semiconducting p-type CrOX-based metasurface. Based on the experimental characterization results and numerical simulations, the proposed design can provide strong light–matter interactions in an ultra-broadband-wavelength range, mainly due to its multidimensional random geometry and ultrasmall gap sizes. Finally, to substantiate the activity of the CrOX nanorods, a core–crown geometry is developed where the NiOX capping layer catalyzes the hydrogen evolution reaction (HER). The proposed heterostructure metasurface absorber can impose photocurrent values as large as 50 μA cm–2 with a photocurrent spectral response extended up to 500 nm. Moreover, the electrode shows outstanding operation under light irradiation for 9 hours. This work demonstrates a simple, scalable design strategy to fabricate low-cost and stable photocathodes for PEC hydrogen evolution.

Perovskite-based tandem solar cells gallop ahead

Perovskite-based tandem cells as clean photovoltaic conversion devices drive the transition to decarbonized energy. Recently in <i>Nature</i>, Tan and co-workers report a certified efficiency of 26.4%, a record-setting efficiency in all-perovskite tandem solar cells. High photocurrent density is achieved by ammonium cation passivation, addressing the short carrier diffusion length in thick mixed Pb-Sn narrow-bandgap subcell.

Variable temperature probing of minority carrier transport and optical properties in p-Ga2O3

Electron beam-induced current in the temperature range from 304 to 404 K was employed to measure the minority carrier diffusion length in metal–organic chemical vapor deposition-grown p-Ga2O3 thin films with two different concentrations of majority carriers. The diffusion length of electrons exhibited a decrease with increasing temperature. In addition, the cathodoluminescence emission spectrum identified optical signatures of the acceptor levels associated with the VGa−–VO++ complex. The activation energies for the diffusion length decrease and quenching of cathodoluminescence emission with increasing temperature were ascribed to the thermal de-trapping of electrons from VGa−–VO++ defect complexes.

High-efficiency perovskite photovoltaic modules achieved via cesium doping

By using the Cs-doped MAPbI 3 as the model perovskite materials, several effects arising from the doping of Cs + into hybrid perovskites have been summarized. Significantly, a suppression of ion migration effects and an improvement in the diffusion length were observed. The Cs + doped PSCs showed hysteresis-free behavior, device efficiency boosted from 19.73% to 21.72%, and the fabricated modules (6.5 cm × 7 cm) with a high PCE of 21.08% via “non-destructive replicate” of all photovoltaic parameters from small device. • Cs-doped MAPbI 3 acts as the model perovskite materials. • Doping of Cs + into hybrid perovskites significantly suppresses ion migration. • Cs + doping noticeably increases the carrier diffusion length of the perovskite film. • The Cs + doped PSCs showed hysteresis-free behavior. • Fabricated cell efficiency reached 21.72%, and module over 21.08% Perovskite solar modules have been attracting increasing attention due to their market potential, yet publications concerned with theintrinsic scale-up potential of different perovskite compositions remain relatively scarce. On the other hand, while great success is being made towards improving the power conversion efficiency ( PCE ) of perovskite solar cells (PSCs) by cesium cation (Cs + ) doping of the perovskite, more attention is being paid to the perovskite phase stabilization effect of Cs + doping, and less to other properties that are critical to understand and futher improve the PSC’s. In this work, moderately-Cs-doped MAPbI 3 was employed as a model perovskite material in order to exclude the phase stabilization effect. Our systematic study revealed the influence of Cs + in organic–inorganic hybrid perovskites on the crystal structure, crystallization process, trap state density, band structure and charge (i.e. , ions or photo-carriers) transport. Markedly, it has been observed that Cs + doping can greatly increase the carrier diffusion length in the perovskite films, thus improving the potential to scale-up PSC’s.The PCE of small area devices (0.09 cm 2 ) was increased to 21.72% from 19.73%, with decreased hysteresis behavior and increased operational stability (T 85 = 1000 h) after Cs + doped, where T 85 refers to the retention of 85% of the initial PCE . Moreover, a PCE of 21.08% was obtained for a Cs + -containing perovskite module with an active area > 30 cm 2 , which demonstrates a better “reproducibility” than the reference sample (MAPbI 3 -based perovskite modules, PCE = 18.26%).

Probing electron diffusion length in an accumulation layer in a ligand-free lead sulfide colloidal quantum dot monolayer

Developing an understanding of the minority carrier diffusion in the effective channel close to the gate dielectric in a field effect transistor (FET) configuration has proven to be challenging because the minority carriers are affected by the gate electric field. We, for the first time, have investigated the minority carrier (electron) diffusion length in ultrathin lead sulfide (PbS) colloidal quantum dots (CQDs) treated with ammonium sulfide ((NH4)2S) using photocurrent microscopy (PCM). With increasing gate voltage, an electron diffusion length is found to be gate-dependent, which can be explained by the increased hole occupancy in the midgap states.Using the FET mobility in the (NH4)2S-treated PbS CQDs, we calculated the electron lifetime. We present the first demonstration of the electric field -modulated electron diffusion length in a PbS CQD film treated with an atomic ligand using PCM.

Selection of the ultimate perovskite solar cell materials and fabrication processes towards its industrialization: A review

AbstractHybrid organic–inorganic perovskite materials have become one of the most studied classes of light‐harvesting materials due to their exceptional properties such as high light absorption, long carrier diffusion lengths, bandgap tuning and defect tolerance. Since 2009 that the scientific community has been working on improving the power conversion efficiency (PCE) of perovskite solar cell devices, reaching now an impressive value of 25.5%. Moreover, efficiencies over 18% are often reported by several authors. Since the efficiency goal is almost fulfilled, the scientific community is currently addressing five challenges, with the ultimate objective to make this technology competitive and turn it commercial; these challenges are cost, stability, upscaling, safety and environmental impact. Given the astonishing progresses reached during the past decade and the numerous research groups working to the same goal, it is a matter of time until commercial perovskite solar devices become a reality. In this review work, the most recent achievements regarding this purpose are put together and compared, so as to suggest the most suitable perovskite solar fabrication processes and materials to produce commercial devices.

Long carrier diffusion length in two-dimensional lead halide perovskite single crystals

<h2>Summary</h2> Ruddlesden-Popper (RP) perovskites are two-dimensional semiconductors for high-performance optoelectronic devices. In this work, we report a long in-plane carrier diffusion length in 2D RP perovskite single crystals probed by scanning photocurrent microscopy. Carrier diffusion lengths of 7–14 μm are observed when the number of PbI₆⁻² octahedra between organic spacers increases from 1 to 3. Using detailed light intensity and electric-field-dependent photocurrent measurements, we attribute the observed long diffusion length to the dominating dissociated free carrier transport. This is further validated by time-resolved photoluminescence measurements, where the decay lifetime increases in the presence of an electric field. From our experiments, we conclude that the in-plane transport in RP perovskites is efficient because of the partial free carrier generation, which overcomes strong excitonic effects. Our results suggest that semiconducting devices fabricated from RP perovskite single crystals can be as efficient as their 3D counterparts.

Carrier Diffusion in GaN : A Cathodoluminescence Study. III. Nature of Nonradiative Recombination at Threading Dislocations

We investigate the impact of threading dislocations with an edge component ($a$ or $a+c$ type) on carrier recombination and diffusion in $\mathrm{Ga}\mathrm{N}$(0001) layers close to the surface as well as in the bulk. To this end, we utilize cathodoluminescence imaging of the top surface of a $\mathrm{Ga}\mathrm{N}$(0001) layer with a deeply buried (In,$\mathrm{Ga}$)N quantum well. Varying the acceleration voltage of the primary electrons and comparing the signal from the layer and the quantum well enables us to probe carrier recombination at depths ranging from the close vicinity of the surface to the position of the quantum well. Our experiments are accompanied by fully three-dimensional Monte Carlo simulations of carrier drift, diffusion, and recombination in the presence of the surface, the quantum well, and the dislocation, taking into account the dislocation strain field and the resulting piezoelectric field at the dislocation outcrop. Near the surface, this field establishes an exciton dead zone around the dislocation, the extent of which is not related to the carrier diffusion length. However, reliable values of the carrier diffusion length can be extracted from the dipolelike energy shift observed in hyperspectral cathodoluminescence maps recorded around the dislocation outcrop at low acceleration voltages. For high acceleration voltages, allowing us to probe a depth where carrier recombination is unaffected by surface effects, we observe a much stronger contrast than expected from the piezoelectric field alone. This finding provides unambiguous experimental evidence for the strong nonradiative activity of edge threading dislocations in bulk $\mathrm{Ga}\mathrm{N}$ and hence also in buried heterostructures.

Carrier Diffusion in GaN : A Cathodoluminescence Study. II. Ambipolar versus Exciton Diffusion

We determine the diffusion length of excess carriers in $\mathrm{Ga}\mathrm{N}$ by spatially resolved cathodoluminescence spectroscopy utilizing a single quantum well as a carrier collector or carrier sink. Monochromatic intensity profiles across the quantum well are recorded for temperatures between 10 and 300 K. A classical diffusion model accounts for the profiles acquired between 120 and 300 K, while for temperatures lower than 120 K, a quantum capture process has to be taken into account in addition. Combining the diffusion length extracted from these profiles and the effective carrier lifetime measured by time-resolved photoluminescence experiments, we deduce the carrier diffusivity as a function of temperature. The experimental values are found to be close to theoretical ones for the ambipolar diffusivity of free carriers limited only by intrinsic phonon scattering. This agreement is shown to be fortuitous. The high diffusivity at low temperatures instead originates from an increasing participation of excitons in the diffusion process.

Carrier Diffusion in GaN : A Cathodoluminescence Study. I. Temperature-Dependent Generation Volume

The determination of the carrier diffusion length of semiconductors such as $\mathrm{Ga}\mathrm{N}$ and $\mathrm{Ga}\mathrm{As}$ by cathodoluminescence imaging requires accurate knowledge about the spatial distribution of generated carriers. To obtain the lateral distribution of generated carriers for sample temperatures between 10 and 300 K, we utilize cathodoluminescence intensity profiles measured across single quantum wells embedded in thick $\mathrm{Ga}\mathrm{N}$ and $\mathrm{Ga}\mathrm{As}$ layers. Thin ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ and ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ barriers respectively prevent carriers diffusing in the $\mathrm{Ga}\mathrm{N}$ and $\mathrm{Ga}\mathrm{As}$ layers to reach the well, which would broaden the profiles. The experimental cathodoluminescence profiles are found to be systematically wider than the energy loss distributions calculated by means of the Monte Carlo program casino, with the width monotonically increasing with decreasing temperature. This effect is observed for both $\mathrm{Ga}\mathrm{N}$ and $\mathrm{Ga}\mathrm{As}$ and becomes more pronounced for higher acceleration voltages. We discuss this phenomenon in terms of both the electron-phonon interaction controlling the energy relaxation of hot carriers and the shape of the initial carrier distribution. Finally, we present a phenomenological approach to simulate the carrier generation volume that can be used for the investigation of the temperature dependence of carrier diffusion.