Carrier Multiplication Research Articles

Characterization of thin carbonated LGADs after irradiation up to 2.5· 1015 n1 Mev eq./cm2

EXFLU1 is a new batch of radiation-resistant silicon sensors manufactured at Fondazione Bruno Kessler (FBK, Italy).The EXFLU1 sensors utilize thin substrates that remain operable even after extensive irradiation. They incorporate Low-Gain Avalanche Diode (LGAD) technology, enabling internal multiplication of charge carriers to boost the small signal produced by a particle crossing their thin active thicknesses, ranging from 15 to 45 μ m.To address current challenges related to acceptor removal, the EXFLU1 production incorporates improved defect engineering techniques. This includes the so called carbonated LGADs, where carbon doping is implanted alongside boron in the gain layer. This contribution focuses on evaluating the performances of thin sensors with carbonated gain layer from the EXFLU1 production, before and after irradiation up to 2.5· 1015 n1 Mev eq./cm2.The conducted tests involve static and transient characterizations, including I-V and C-V measurements, as well as laser and β-source tests.This work aims to present the state of the art in LGAD sensor technology with a carbonated gain layer and shows the characterization of the most radiation-resistant LGAD sensors produced to date.

Study of Laser-Induced Multi-Exciton Generation and Dynamics by Multi-Photon Absorption in CdSe Quantum Dots.

Multi-exciton generation by multi-photon absorption under low-energy photons can be thought a reasonable method to reduce the risk of optical damage, especially in photoelectric quantum dot (QD) devices. The lifetime of the multi-exciton state plays a key role in the utilization of photon-induced carriers, which depends on the dynamics of the exciton generation process in materials. In this paper, the exciton generation dynamics of the photon absorption under low-frequency light in CdSe QDs are successfully detected and studied by the temporal resolution transient absorption (TA) spectroscopy method. Since the cooling time of hot excitons extends while the rate of auger recombination is accelerated when incident energy is increased, the filling time of defect states is irregular, and exciton generation experiences a transition from single-photon absorption to multi-photon absorption. This result shows how to change the excitation. Optical parameters can prolong the lifetime of excitons, thus fully extracting excitons and improving the photoelectric conversion efficiency of QD optoelectronic devices, which provides theoretical and experimental support for the development of QD optoelectronic devices.

Photo‐Induced Electrical Gating with the Inserting of an Interfacial Carrier Blocking Layer for Organic/Graphene Phototransistors

AbstractAlthough ultrahigh photoconductive gain is achieved in organic semiconductor‐sensitized graphene phototransistors, the response speed is limited by the photo‐induced carrier injection at the organic/graphene interface. In this work, an insulating hexagonal boron nitride layer is inserted between the organic heterostructure and graphene to block the interfacial carrier transport. Above the h‐BN layer, the organic heterostructure is adopted as the light‐absorbing layer, and the photoresponse of graphene is realized through electrical gating of the atop h‐BN dielectric layer, by the accumulated carriers in the organic heterostructure. Under this “photo‐induced electrical gating” mechanism, the recombination of nonequilibrium carriers takes place only within the organic heterostructure. Furthermore, monolayer perylene‐3,4,9,10‐tetracarboxylic dianhydride (PTCDA) is deposited on graphene before transfer of the h‐BN, by which the interfacial encapsulated bubbles are effectively reduced. As a step further, the trap states are pre‐filled with background light illumination, then the carrier recombination follows the band‐to‐band pathway. Finally, the response time of the organic/h‐BN/graphene phototransistor is reduced to 7.15 ms, and a high photoresponsivity is achieved even though the carrier multiplication is sacrificed, which is reconciled by the improvement of the device quantum efficiency.

New Theoretical Model to Describe Carrier Multiplication in Semiconductors: Explanation of Disparate Efficiency in MoTe2 versus PbS and PbSe.

We present a theoretical model to compute the efficiency of the generation of two or more electron-hole pairs in a semiconductor by the absorption of one photon via the process of carrier multiplication (CM). The photogeneration quantum yield of electron-hole pairs is calculated from the number of possible CM decay pathways of the electron and the hole. We apply our model to investigate the underlying cause of the high efficiency of CM in bulk 2H-MoTe2, as compared to bulk PbS and PbSe. Electronic band structures were calculated with density functional theory, from which the number of possible CM decay pathways was calculated for all initial electron and hole states that can be produced at a given photon energy. The variation of the number of CM pathways with photon energy reflects the dependence of experimental CM quantum yields on the photon energy and material composition. We quantitatively reproduce experimental CM quantum yields for MoTe2, PbS, and PbSe from the calculated number of CM pathways and one adjustable fit parameter. This parameter is related to the ratio of Coulomb coupling matrix elements and the cooling rate of the electrons and holes. Large variations of this fit parameter result in small changes in the modeled quantum yield for MoTe2, which confirms that its high CM efficiency can be mainly attributed to its extraordinary large number of CM pathways. The methodology of this work can be applied to analyze or predict the CM efficiency of other materials.

Cryptographic transistor for true random number generator with low power consumption.

We design a cryptographic transistor (cryptoristor)-based true random number generator (tRNG) with low power consumption and small footprint. This is the first attempt to use irregular and unpredictable operation-induced randomness of a cryptoristor as an entropy source. To extract discrete random numbers with a binary code from the cryptoristor, we developed a noise-coupling analog-to-digital converter. This converter not only converts analog signals to digital random bits but also improves the randomness of the entropy source with low power consumption. The randomness of the cryptoristor is attributed to the random carrier multiplications with the creation and stochastic carrier escape as destruction, which occurs iteratively as long as the input current is fed. The cryptoristor-based tRNG passed 15 randomness test suites of NIST Special Publication 800-22. It is robust to iterative operational stresses and to ambient temperature changes, making it an attractive option for hardware-based security solutions in the Internet of Things due to its low power consumption and small size.

Boosting exciton dissociation and charge transfer in CsPbBr3 QDs via ferrocene derivative ligation for CO2 photoreduction

Photo-catalytic CO2 reduction with perovskite quantum dots (QDs) shows potential for solar energy storage, but it encounters challenges due to the intricate multi-electron photoreduction processes and thermodynamic and kinetic obstacles associated with them. This study aimed to improve photo-catalytic performance by addressing surface barriers and utilizing multiple-exciton generation in perovskite QDs. A facile surface engineering method was employed, involving the grafting of ferrocene carboxylic acid (FCA) onto CsPbBr3 (CPB) QDs, to overcome limitations arising from restricted multiple-exciton dissociation and inefficient charge transfer dynamics. Kelvin Probe Force Microscopy and XPS spectral confirmed successfully creating an FCA-modulated microelectric field through the Cs active site, thus facilitating electron transfer, disrupting surface barrier energy, and promoting multi-exciton dissociations. Transient absorption spectroscopy showed enhanced charge transfer and reduced energy barriers, resulting in an impressive CO2-to-CO conversion rate of 132.8 μmol g-1 h-1 with 96.5% selectivity. The CPB-FCA catalyst exhibited four-cycle reusability and 72 h of long-term stability, marking a significant nine-fold improvement compared to pristine CPB (14.4 μmol g-1 h-1). These results provide insights into the influential role of FCA in regulating intramolecular charge transfer, enhancing multi-exciton dissociation, and improving CO2 photoreduction on CPB QDs. Furthermore, these findings offer valuable knowledge for controlling quantum-confined exciton dissociation to enhance CO2 photocatalysis.

Efficient Single-Phase Tunable Dual-Color Luminescence with High Quantum Yield Greater than 100% for Information Encryption and LED Applications.

In modern society, the investigation of highly efficient photoluminescent bulk materials with excitation-induced tunable multicolor luminescence and multiexciton generation (MEG) is of great significance to information security and the application of optoelectronic devices. In this study, two bulk Cu-based halide crystals of (C4H10NO)4Cu2Br5·Br and (C4H10NO)4Cu2I5·I·H2O, respectively, with one-dimensional structures were grown by a solvent evaporation method. Unexpectedly, (C4H10NO)4Cu2I5·I·H2O displayed excitation-induced tunable dual-color luminescence; one band is a brilliant green-yellow emission centered at 547 nm with a high photoluminescence quantum yield (PLQY) of up to 169.67%, and the other is a red emission at 695 nm with a PLQY of 75.76%. Just as importantly, (C4H10NO)4Cu2Br5·Br exhibits a strong broadband green-yellow emission at 561 nm under broad band excitation ranging from 252 to 350 nm, a long PL decay lifetime of 106.9 μs, and an ultrahigh PLQY of 198.22%. These materials represent the first two examples of 1D bulk crystals and Cu(I)-based halides that have a PLQY exceeding 100%. Combining the unusual luminescence characteristics with theoretical calculations reveals that MEG contributes to the green-yellow emission with ultrahigh PLQY > 100%, and that the red emission can be ascribed to [Cu2I5]3- cluster-centered emission. Additionally, an information encryption method was designed based on the Morse Code. The high luminescence characteristics of LED devices fabricated using the (C4H10NO)4Cu2Br5·Br and (C4H10NO)4Cu2I5·I·H2O crystals appear to lead to promising applications in solid-state lighting. This work extends the catalog of high-performance luminescent materials and also promotes application prospects of low-dimensional copper-based halides in optoelectronics.

Ligand-mediated exciton dissociation and interparticle energy transfer on CsPbBr3 perovskite quantum dots for efficient CO2-to-CO photoreduction

Perovskite quantum dots (PQDs) hold immense potential as photocatalysts for CO2 reduction due to their remarkable quantum properties, which facilitates the generation of multiple excitons, providing the necessary high-energy electrons for CO2 photoreduction. However, harnessing multi-excitons in PQDs for superior photocatalysis remains challenging, as achieving the concurrent dissociation of excitons and interparticle energy transfer proves elusive. This study introduces a ligand density-controlled strategy to enhance both exciton dissociation and interparticle energy transfer in CsPbBr3 PQDs. Optimized CsPbBr3 PQDs with the regulated ligand density exhibit efficient photocatalytic conversion of CO2 to CO, achieving a 2.26-fold improvement over unoptimized counterparts while maintaining chemical integrity. Multiple analytical techniques, including Kelvin probe force microscopy, temperature-dependent photoluminescence, femtosecond transient absorption spectroscopy, and density functional theory calculations, collectively affirm that the proper ligand termination promotes the charge separation and the interparticle transfer through ligand-mediated interfacial electron coupling and electronic interactions. This work reveals ligand density-dependent variations in the gas-solid photocatalytic CO2 reduction performance of CsPbBr3 PQDs, underscoring the importance of ligand engineering for enhancing quantum dot photocatalysis.

The Development of Carbon/Silicon Heterojunction Solar Cells through Interface Passivation.

Passivating contactsin heterojunction (HJ) solar cells have shown great potential in reducing recombination losses, and thereby achieving high power conversion efficiencies in photovoltaic devices. In this direction, carbon nanomaterials have emerged as a promising option for carbon/silicon (C/Si) HJsolar cells due to their tunable band structure, wide spectral absorption, high carrier mobility, and properties such as multiple exciton generation. However, the current limitations in efficiency and active area have hindered the industrialization of these devices. In this review, they examine the progress made in overcoming these constraints and discuss the prospect of achieving high power conversion efficiency (PCE) C/Si HJ devices. A C/Si HJ solar cell is also designed by introducing an innovative interface passivation strategy to further boost the PCE and accelerate the large area preparationof C/Si devices. The physical principle, device design scheme, and performanceoptimization approaches of this passivated C/Si HJ cells are discussed. Additionally, they outline potential future pathways and directions for C/Si HJ devices, including a reduction in their cost to manufacture and their incorporation intotandem solar cells. As such, this review aims to facilitate a deeperunderstanding of C/Si HJ solar cells and provide guidance for their further development.

Rational ligand design for enhanced carrier mobility in self-powered SWIR photodiodes based on colloidal InSb quantum dots.

Solution-processed colloidal III-V semiconductor quantum dot photodiodes (QPDs) have potential applications in short-wavelength infrared (SWIR) imaging due to their tunable spectral response range, possible multiple-exciton generation, operation at 0-V bias voltage and low-cost fabrication and are also expected to replace lead- and mercury-based counterparts that are hampered by reliance on restricted elements (RoHS). However, the use of III-V CQDs as photoactive layers in SWIR optoelectronic applications is still a challenge because of underdeveloped ligand engineering for improving the in-plane conductivity of the QD assembled films. Here, we report on ligand engineering of InSb CQDs to enhance the optical response performance of self-powered SWIR QPDs. Specifically, by replacing the conventional ligand (i.e., oleylamine) with sulfide, the interparticle distance between the CQDs was shortened from 5.0 ± 0.5 nm to 1.5 ± 0.5 nm, leading to improved carrier mobility for high photoresponse speed to SWIR light. Furthermore, the use of sulfide ligands resulted in a low dark current density (∼nA cm-2) with an improved EQE of 18.5%, suggesting their potential use in toxic-based infrared image sensors.

One-Pot Colloidal Synthesis Enables Highly Tunable InSb Short-Wave Infrared Quantum Dots Exhibiting Carrier Multiplication.

Colloidal quantum dots (CQDs) are emerging materials for short-wave infrared (SWIR, ≈1100-3000 nm) photodetectors, which are technologically important for a broad array of applications. Unfortunately, the most developed SWIR CQD systems are Pb and Hg chalcogenides; their toxicity and regulated compositions limit their applications. InSb CQD system is a potential environmentally friendly alternative, whose bandgap in theory, is tunable via quantum confinement across the SWIR spectrum. However, InSb CQDs are difficult to exploit, due to their complex syntheses and uncommon reactive precursors, which greatly hinder their application and study. Here, a one-pot synthesis strategy is reported using commercially available precursors to synthesize-under standard colloidal synthesis conditions-high-quality, size-tunable InSb CQDs. With this strategy, the large Bohr exciton radius of InSb can be exploited for tuning the bandgap of the CQDs over a wide range of wavelengths (≈1250-1860nm) across the SWIR region. Furthermore, by changing the surface ligands of the CQDs from oleic acid (OA) to 1-dodecanthiol (DDT), a ≈20-fold lengthening in the excited-state lifetime, efficient carrier multiplication, and slower carrier annihilation are observed. The work opens a wide range of SWIR applications to a promising class of Pb- and Hg-free CQDs.

The anisotropic nature of singlet fission in single crystalline organic semiconductors

The escalating global energy predicament implores for a revolutionary resolution—one that converts sunlight into electricity—holding the key to supreme conversion efficiency. This comprehensive review embarks on the exploration of the principle of generating multiple excitons per absorbed photon, a captivating concept that possesses the potential to redefine the fundamental confines of conversion efficiency, albeit its application remains limited in photovoltaic devices. At the nucleus of this phenomenon are two principal processes: multiple exciton generation (MEG) within quantum-confined environments, and singlet fission (SF) inside molecular crystals. The process of SF, characterized by the cleavage of a single photogenerated singlet exciton into two triplet excitons, holds promise to potentially amplify photon-to-electron conversion efficiency twofold, thereby laying the groundwork to challenge the detailed balance limit of solar cell efficiency. Our discourse primarily dissects the complex nature of SF in crystalline organic semiconductors, laying special emphasis on the anisotropic behavior of SF and the diffusion of the subsequent triplet excitons in single-crystalline polyacene organic semiconductors. We initiate this journey of discovery by elucidating the principles of MEG and SF, tracing their historical genesis, and scrutinizing the anisotropy of SF and the impact of quantum decoherence within the purview of functional mode electron transfer theory. We present an overview of prominent techniques deployed in investigating anisotropic SF in organic semiconductors, including femtosecond transient absorption microscopy and imaging as well as stimulated Raman scattering microscopies, and highlight recent breakthroughs linked with the anisotropic dimensions of Davydov splitting, Herzberg–Teller effects, SF, and triplet transport operations in single-crystalline polyacenes. Through this comprehensive analysis, our objective is to interweave the fundamental principles of anisotropic SF and triplet transport with the current frontiers of scientific discovery, providing inspiration and facilitating future ventures to harness the anisotropic attributes of organic semiconductor crystals in the design of pioneering photovoltaic and photonic devices.

Monopolarity of Hot Charge Carrier Multiplication in AIIIBV Semiconductors at High Electric Field and Noiseless Avalanche Photodiodes (a Review)

Monopolarity of Hot Charge Carrier Multiplication in AIIIBV Semiconductors at High Electric Field and Noiseless Avalanche Photodiodes (a Review)

High‐Efficiency Infrared Sulfide Lead Quantum Dot Solar Cells via Mixed Halide Ions Ligand Engineering

Tandem solar cells are regarded as an effective way to break through the theoretical efficiency of the Shockley–Queisser limit, and large‐size lead sulfide quantum dots (PbS QDs) are considered ideal infrared (IR) photovoltaic materials for absorbing low‐energy IR photons in the bottom subcells of tandem solar cells due to their tunable bandgaps and multiexciton generation effects. However, the exposed (100) surface of large‐size PbS QDs leads to poor passivation during the ligand‐exchange process, resulting in degraded device performance. Herein, a mixed halide ions ligand passivation strategy is developed to synergistically passivate the (111) and (100) facets of large‐size QDs. Particularly, a gradual dropwise addition of lead chloride salts dissolved in dimethyl sulfoxide for achieving precisely proportional surface passivation of Cl ions in the ligand exchange is proposed, which enhances the quality of the QD solids and obtains excellent control of the surface trap states. Additionally, optical and electrical characterization results show that the passivated IR PbS QD solids have higher carrier mobility, longer carrier lifetime, and lower defect density, which increases the IR efficiency of IR PbS QD solar cells to 1.25%.

Investigations on Multiexciton Generation and Hot Electron Transfer Process in CdTe QD-Decorated TiO2 Nanostructures for Effective Photocatalytic Activity

Investigations on Multiexciton Generation and Hot Electron Transfer Process in CdTe QD-Decorated TiO₂ Nanostructures for Effective Photocatalytic Activity

Advancing perovskite solar cell performance: Enhanced efficiency and stability through superimposed PbS QDs

This study investigates enhancing efficiency in thin-film photovoltaic devices by incorporating varying-sized superimposed PbS colloidal quantum dots (CQDs). These CQDs offer tunable near-infrared absorption, multiple exciton generation effects, and solution processability, rendering them promising for improved performance. Superimposing QDs of different sizes extends the device's absorption spectrum beyond conventional limits, utilizing tailored absorption coefficients for broader sunlight wavelength utilization and increased solar energy conversion efficiency. Each QD group exhibits a unique absorption coefficient, with a peak at its central wavelength from the Schrödinger equation solution. Spectral analysis reveals strategic enhancement in light absorption across visible and near-infrared regions, broadening the perovskite absorption range beyond 800 nm. These QDs enhance both infrared and visible spectra, improving light harvesting. Summing the absorption of single QD groups creates a continuous spectrum, enhancing perovskite film stability and morphology. Rigorous optical analysis and comprehensive electrical modeling investigate charge carrier interactions within the photovoltaic framework. The study verifies around 24 % efficacy enhancement compared to QD-absent configurations, surpassing uniformly dispersed QDs by 11 %. Optical and electrical analyses yield a short-circuit current of 26.7 mA/cm2, open-circuit voltage of 1.13 V, fill factor of 0.88, and overall efficiency of 25.12 % in superimposed configurations. Results underscore the significance of varied QD dimensions and layered structures for enhanced absorption, offering a promising route for advancing thin-film solar technology. The study showcases the potential of PbS CQDs with varying radii in photovoltaics, contributing to improved efficiency and stability, with implications for the future of solar energy technology.

Templated synthesis of ZnO quantum dots via double solvents method inside MOF-801 as emerging photocatalyst for photodegradation of Acid Blue 25 under UV light

Templated synthesis of ZnO quantum dots (QDs) is one of the most important techniques for controlled synthesis of it without need of surfactants and advanced laboratory equipment. MOF-801 was used as a template for the synthesis of ZnO QDs with controlled shape and size. The “ship-in-a-bottle” approach was applied for templated synthesis of ZnO QDs into the MOF-801 porous matrix via double solvents method (DSM) combined with an in situ reduction route (ISRR). The formation of ZnO QDs inside MOF-801 template was monitored by PXRD, FT-IR, EDX, ICP-OES, XPS, BET, TEM, DRS, PL, EIS, and Mott − Schottky analyses. The ZnOQDs@MOF-801 nanocomposite as emerging photocatalyst exhibited better photocatalytic performance for degradation of Acid Blue 25 with a rate constant of 0.00179 min−1 which is 2.35 higher than MOF-801 (0.00076 min−1) under UV irradiation, due to ZnO QDs embedded inside MOF-801 template. The unique properties of ZnO QDs, including quantum confinement, multiple exciton generation, and efficient separation of charge carriers through decrease in electron-hole recombination at the quantum scale, contribute to the photocatalytic degradation of Acid Blue 25.

Fluoride passivation of ZnO electron transport layers for efficient PbSe colloidal quantum dot photovoltaics

Lead selenide (PbSe) colloidal quantum dots (CQDs) are suitable for the development of the next-generation of photovoltaics (PVs) because of efficient multiple-exciton generation and strong charge coupling ability. To date, the reported high-efficient PbSe CQD PVs use spin-coated zinc oxide (ZnO) as the electron transport layer (ETL). However, it is found that the surface defects of ZnO present a difficulty in completion of passivation, and this impedes the continuous progress of devices. To address this disadvantage, fluoride (F) anions are employed for the surface passivation of ZnO through a chemical bath deposition method (CBD). The F-passivated ZnO ETL possesses decreased densities of oxygen vacancy and a favorable band alignment. Benefiting from these improvements, PbSe CQD PVs report an efficiency of 10.04%, comparatively 9.4% higher than that of devices using sol-gel (SG) ZnO as ETL. We are optimistic that this interface passivation strategy has great potential in the development of solution-processed CQD optoelectronic devices.Graphical

Slow Auger Recombination in Ag2Se Colloidal Quantum Dots.

Efficient Auger recombination (AR) presents a significant challenge for the advancement of colloidal quantum dot (QD)-based devices involving multiexcitons. Here, the AR dynamics of near-infrared Ag2Se QDs were studied through transient absorption experiments. As the QD radius increases from 0.9 to 2.5 nm, the biexciton lifetime (τ2) of Ag2Se QDs increases from 35 to 736 ps, which is approximately 10 times longer than that of comparable-sized CdSe and PbSe QDs. A qualitative analysis based on observables indicates that the slow Auger rate is primarily attributed to the low density of the final states. The biexciton lifetime and triexciton lifetime (τ3) of Ag2Se QDs follow R3 and R2.6 dependence, respectively. Moreover, the ratio of τ2/τ3 is ∼2.3-3.2, which is markedly lower than the value expected from statistical scaling (4.5). These findings suggest that environmentally friendly Ag2Se QDs can serve as excellent candidates for low-threshold lasers and third-generation photovoltaics utilizing carrier multiplication.

Semiconductor quantum dots as a mechanism to enhance charge transfer processes in polymer solar cells

The light trapping capability of thin film polymer solar absorber, composed of poly (3-hexylthiophene) and [6,6]–phenyl C61- butyric acid methyl ester (P3HT:PC60BM) blend, is improved using ZnS semiconductor quantum dots (QD) as third donor-acceptor (D:A) component. The inherent characteristics of the microwave-assisted synthesized ZnS QD, such as quantum size effect, and multiple exciton generation were leveraged in harvesting high energy photons, which resulted in a better exciton generation, dissociation, and effective charge transport in the polymer medium. The synthesized QD exhibited good phase purity, effective kinetic enhancement, and control of the aggregation process. Hence, the impact of ZnS QD on the performance of thin film polymer solar cells (TFPSC) is evident by a remarkable improvement in the measured photovoltaic parameters. Nonetheless, it is observed that the device performances are generally dependent on the concentration of the QD in the absorber layer. Consequently, the power conversion efficiency has increased by 58% at 3% concentration of QDs by weight. This is an interesting development of TFPSC fabricated under an ambient environment.