Hot Carrier Devices Research Articles

Geometric Symmetry Breaking and Nonlinearity Can Increase Thermoelectric Power.

Direct thermal-to-electric energy converters typically operate in the linear regime, where the ratio of actual maximum power relative to the ideal maximum power, the so-called fill factor (FF), is 0.25. Here, we show, based on fundamental symmetry considerations, that the leading order nonlinear terms that can increase the FF require devices with broken spatial symmetry. Studying nonlinear, thermoelectric transport across an asymmetric energy barrier defined in a single semiconductor nanowire, we find in both experiment and theory that we can increase the FF as well as maximum power. Geometric symmetry breaking combined with the design of nonlinear behavior thus represents a strategy for increasing the performance of thermoelectric or hot-carrier devices.

Cu2O@Au-CsPbI3 heterostructures for plasmon hot carrier transfer enhanced optoelectronics

Plasmon hot carrier devices have tremendous potential applications in broadband photovoltaics, photodetection, photocatalysis, and intelligent optoelectronics, as they have the ability to generate hot carriers with energy lower than the optical bandgap at Schottky junctions and broaden the spectral range. However, this is mostly challenged by the lack of effective methods for the separation, migration, and extraction of plasmon hot carrier. In this work, a Cu2O@Au-CsPbI3 heterostructure was fabricated using facile low-temperature solution synthesis and spin-coating technique. The proposed heterostructure benefits from the simultaneous transfer of plasmon hot holes to Cu2O and hot electrons to CsPbI3, as well as enhanced charge separation driven by the built-in electric field at the p-n heterojunction interface. The heterostructure realizes remarkably enhanced light absorption, reduces carrier recombination, and further improves conversion efficiency. It exhibits a wide spectrum response of ultraviolet-visible-near infrared and achieves enhanced photodetection performance. The results suggest that Cu2O@Au-CsPbI3 heterostructure is a promising candidate for optoelectronic devices based on the harvesting of plasmonic hot carriers.

Atomistic Theory of Hot-Carrier Relaxation in Large Plasmonic Nanoparticles.

Recently, there has been significant interest in harnessing hot-carriers generated from the decay of localized surface plasmons in metallic nanoparticles for applications in photocatalysis, photovoltaics, and sensing. In this work, we develop an atomistic method that makes it possible to predict the population of hot-carriers under continuous wave illumination for large nanoparticles of relevance to experimental studies. For this, we solve the equation of motion of the density matrix, taking into account both the excitation of hot-carriers and subsequent relaxation effects. We present results for spherical Au and Ag nanoparticles with up to 250,000 atoms. We find that the population of highly energetic carriers depends on both the material and the nanoparticle size. We also study the increase in the electronic temperature upon illumination and find that Ag nanoparticles exhibit a much larger temperature increase than Au nanoparticles. Finally, we investigate the effect of using different models for the relaxation matrix but find that the qualitative features of the hot-carrier population are robust. These insights can be harnessed for the design of improved hot-carrier devices.

Machine Learning Models Capture Plasmon Dynamics in Ag Nanoparticles.

Highly energetic electron-hole pairs (hot carriers) formed from plasmon decay in metallic nanostructures promise sustainable pathways for energy-harvesting devices. However, efficient collection before thermalization remains an obstacle for realization of their full energy generating potential. Addressing this challenge requires detailed understanding of physical processes from plasmon excitation in the metal to their collection in a molecule or a semiconductor, where atomistic theoretical investigation may be particularly beneficial. Unfortunately, first-principles theoretical modeling of these processes is extremely costly, preventing a detailed analysis over a large number of potential nanostructures and limiting the analysis to systems with a few 100s of atoms. Recent advances in machine learned interatomic potentials suggest that dynamics can be accelerated with surrogate models which replace the full solution of the Schrödinger Equation. Here, we modify an existing neural network, Hierarchically Interacting Particle Neural Network (HIP-NN), to predict plasmon dynamics in Ag nanoparticles. The model takes as a minimum as three time steps of the reference real-time time-dependent density functional theory (rt-TDDFT) calculated charges as history and predicts trajectories for 5 fs in great agreement with the reference simulation. Further, we show that a multistep training approach in which the loss function includes errors from future time-step predictions can stabilize the model predictions for the entire simulated trajectory (∼25 fs). This extends the model's capability to accurately predict plasmon dynamics in large nanoparticles of up to 561 atoms, not present in the training data set. More importantly, with machine learning models on GPUs, we gain a speed-up factor of ∼103 as compared with the rt-TDDFT calculations when predicting important physical quantities such as dynamic dipole moments in Ag55 and a factor of ∼104 for extended nanoparticles that are 10 times larger. This underscores the promise of future machine learning accelerated electron/nuclear dynamics simulations for understanding fundamental properties of plasmon-driven hot carrier devices.

The effect of the effective electron mass on the hot electron collection

The dominant factor for hot electron collecting in internally photoemitted hot carrier (IPHC) devices is still not clear under steady-state low intensity light. We here use SnO2 as the electron-collecting layer to replace TiO2 to construct IPHC devices. Almost no photoresponse is observed for the pure SnO2-based IPHC device. However, when an insulating MgO layer or TiO2 covered SnO2, relatively large photocurrent generated from hot electrons can be achieved. The effective electron mass (EEM) is figured out to be the dominate factor in hot electron collection in IPHC devices. The very small EEM of SnO2 results in a small emission cone of hot electrons. Also due to the small EEM of SnO2, the leakage of trapped electrons back to the Au is very large. Because of these two reasons, the SnO2-based IPHC device shows almost no photoresponse. MgO can block the backflow of electrons (leakage), while the larger EEM of TiO2 can increase the emission cone of hot electrons. Our finding is significant for understanding hot electrons collection and will give new directions for hot carrier solar cell applications under low-intensity excitation at steady state.

Deciphering Adverse Detrapped Hole Transfer in Hot-Electron Photoelectric Conversion at Infrared Wavelengths.

Hot-carrier devices are promising alternatives for enabling path breaking photoelectric conversion. However, existing hot-carrier devices suffer from low efficiencies, particularly in the infrared region, and ambiguous physical mechanisms. Inthiswork,the competitive interfacial transfer mechanisms of detrapped holes and hot electrons in hot-carrier devices are discovered. Through photocurrent polarity research and optical-pump-THz-probe (OPTP) spectroscopy, it is verified that detrapped hole transfer (DHT) and hot-electron transfer (HET) dominate the low- and high-density excitation responses, respectively. The photocurrent ratio assigned to DHT and HET increases from 6.6% to over 1133.3% as the illumination intensity decreases. DHT induces severe degeneration of the external quantum efficiency (EQE), especially at low illumination intensities. The EQE of a hot-electron device can theoretically increase by over two orders of magnitude at 10mWcm-2 through DHT elimination. The OPTP results show that competitive transfer arises from the carrier oscillation type and carrier-density-related Coulomb screening. The screening intensity determines the excitation weight and hot-electron cooling scenes and thereby the transfer dynamics.

Charge Transfer Dynamics at the Interface of CsPbX3 Perovskite Nanocrystal‐Acceptor Complexes: A Femtosecond Transient Absorption Spectroscopy Study

AbstractCsPbX3 (X = I–, Br–, Cl–) perovskite nanocrystals (PeNCs) are being explored extensively due to their impressive optoelectronic and photonic properties. Efficient harvest of photogenerated charge carriers from the particle is essential for photon‐to‐current conversion and enhancement of the performance of PeNC‐based photovoltaic and optoelectronic applications. Since photoinduced charge transfer (CT) from the particle usually occurs on sub‐picosecond to nanosecond timescales, time‐resolved transient absorption (TA) spectroscopy is a powerful technique to investigate this transient dynamic photoinduced process. Herein reported investigations are reviewed, primarily employing femtosecond TA spectroscopy, of the CT process in CsPbX3 PeNCs. Based on the interpretation of TA spectral features and the basic model of CT dynamics, the concerns of CT rates or efficiency characterization in TA measurements, including multiple CT pathway effects, surface ligand effects, and hot charge carrier extraction are addressed. An elucidation of plasmon‐induced hot electron transfer is also provided in CsPbX3 PeNC‐noble metal heterostructures for their promising potential to contribute the development of CsPbX3 PeNC‐based hot carrier devices. It is hoped that this review can provide deeper insights into the photovoltaic conversion in order to make the most of the CT process in CsPbX3 PeNCs.

Strain engineering for enhanced hot-carrier photodetection

Hot-carrier devices in metal–semiconductor junctions have attracted considerable attention but still with quantum efficiencies far from expectations. Introducing the lattice strain to the material can effectively modulate the electronic structure, providing a way to control the hot-carrier dynamics. Here, we study how this strain affects the generation, transport, and injection of hot carriers in gold (Au) by using first-principles calculations and evaluate the overall responses of Au-based hot-carrier devices by Monte Carlo simulation. We find that the compressive strain can significantly increase the hot-electron generation from direct transition at E > 1.1 eV for Au. The compressive strain delocalizes the band structure and decreases the electron density of state, which, in turn, reduce electron–electron and electron–phonon scatterings to improve the transport of hot carriers. Taking the Au/TiO2 device as an example, we find that the compressive strain (−6%) can enable a 1.5- to 3-fold enhancement of quantum efficiency and responsivity at a photon energy between 1.2 and 3 eV.

Slow excitonic carrier cooling in Sr-doped PbS nanocrystals for hot carrier devices: an integrated experimental and first-principles approach

Minimizing the carrier–phonon interactions in polar semiconductors is of great importance for designing hot carrier optoelectronic devices, as it is directly related to the excitonic energy dissipation.

Controlling Photocarrier Lifetime in Graphene for Enhanced Photocurrent Generation via Cascade Hot Electron Transfer.

Because of its broad absorption and high carrier mobility, graphene has been regarded as a promising photoactive material for optoelectronics. However, its ultrashort photoexcited carrier lifetime greatly restricts the device performance. Herein, we show that by constructing a graphene/WS2/MoS2 vertical heterostructure with a cascade electron-transfer pathway, the hot electrons in graphene under low-energy photoexcitation can efficiently transfer through WS2 to MoS2 in 180 fs, thus effectively photogating the graphene layer. Because of the spatial separation and energy barrier imposed by the WS2 intermediate layer which retards back electron transfer, the photocarrier lifetime in graphene is significantly prolonged to ∼382.7 ps, more than 2 orders of magnitude longer than in isolated graphene and graphene/WS2 binary heterostructure. The prolonged photocarrier lifetime in graphene leads to dramatically enhanced photocurrent generation and photoresponsivity. This study offers an exciting approach to control photocarrier lifetime in graphene for hot carrier devices with simultaneous broadband and high responsivity.

Hot-carrier optoelectronic devices based on semiconductor nanowires

In optoelectronic devices such as solar cells and photodetectors, a portion of electron-hole pairs is generated as so-called hot carriers with an excess kinetic energy that is typically lost as heat. The long-standing aim to harvest this excess energy to enhance device performance has proven to be very challenging, largely due to the extremely short-lived nature of hot carriers. Efforts thus focus on increasing the hot carrier relaxation time and on tailoring heterostructures that allow for hot-carrier extraction on short time and length scales. Recently, semiconductor nanowires have emerged as a promising system to achieve these aims, because they offer unique opportunities for heterostructure engineering as well as for potentially modified phononic properties that can lead to increased relaxation times. In this review we assess the current state of theory and experiments relating to hot-carrier dynamics in nanowires, with a focus on hot-carrier photovoltaics. To provide a foundation, we begin with a brief overview of the fundamental processes involved in hot-carrier relaxation and how these can be tailored and characterized in nanowires. We then analyze the advantages offered by nanowires as a system for hot-carrier devices and review the status of proof-of-principle experiments related to hot-carrier photovoltaics. To help interpret existing experiments on photocurrent extraction in nanowires we provide modeling based on non-equilibrium Green's functions. Finally, we identify open research questions that need to be answered in order to fully evaluate the potential nanowires offer toward achieving more efficient, hot-carrier based, optoelectronic devices.

Size-Dependent Hot Carrier Dynamics in Perovskite Nanocrystals Revealed by Two-Dimensional Electronic Spectroscopy.

The lifetimes of hot carriers have been predicted to be prolonged in small nanocrystals with an inter-level spacing larger than phonon energy. Nevertheless, whether such a phonon bottleneck is present in perovskite semiconductor nanocrystals remains highly controversial. Here we report compelling evidence of a phonon bottleneck in CsPbI3 nanocrystals with marked size-dependent relaxation of hot carriers by using broadband two-dimensional electronic spectroscopy (2DES). By combining high resolutions in both the time (<10 fs) and excitation energy domains, 2DES allows the clear disentanglement of the thermalization and cooling processes. The lifetime is over doubled for hot carriers when the average edge length of the nanocrystals decreases from 8.2 nm down to 4.6 nm. The confirmation of the phonon bottleneck effect suggests the feasibility of controlling hot carrier dynamics in perovskite semiconductors with nanocrystal size for potential applications of hot carrier devices.

Metallic Alloys: Emergent Opportunities with Metallic Alloys: From Material Design to Optical Devices (Advanced Optical Materials 23/2020)

Marina S. Leite and co-workers present a comprehensive survey of recent advances accomplished in photonics and plasmonics by implementing metal-alloy-based materials: see article number 2001082. Metallic alloys represent a promising pathway to tailor the optical properties of devices, enabling further control of the electromagnetic spectrum. They have found burgeoning applications in nanophotonics such as superabsorbers, hydrogen sensing, hot carrier devices, photocatalysis, and photovoltaics. The cover image highlights all the metal elements in alloy systems covered in this Review.

Abnormal Electron Emission in a Vertical Graphene/Hexagonal Boron Nitride van der Waals Heterostructure Driven by a Hot Hole-Induced Auger Process.

Understanding the scattering process of field injection hot carriers is important for modulating their behaviors, which is the key for improving the efficiency of charge transfer and energy conversion in hot carrier devices. In this work, a significant electron thermalization induced by Auger scattering between a field injection hot hole and a native cold electron has been observed in a vertical single layer graphene/hexagonal boron nitride/few layer graphene (Gr/hBN/FLG) device by measuring the vacuum electron emission characteristics. For the first time, it is found that vacuum electron emission can be measured under both directions of bias within the device. Furthermore, electrons can be emitted even when the applied bias energy is smaller than the work function of the Gr cathode. Further analysis of the emission electron kinetic energy indicates that the low turn-on bias results from the emission of energetic electrons that are ∼3 eV higher than the Fermi level. A semiquantitative model based on hot hole-induced Auger electron emission is established to reproduce the results. All of these findings not only expand our understanding of the hot carrier scattering process in graphene but also provide insights into the applications of hot carrier devices.

Thermalization of photoexcited carriers in two-dimensional transition metal dichalcogenides and internal quantum efficiency of van der Waals heterostructures

Van der Waals semiconductor heterostructures could be a platform to harness hot photoexcited carriers in the next generation of optoelectronic and photovoltaic devices. The internal quantum efficiency of hot-carrier devices is determined by the relation between photocarrier extraction and thermalization rates. Using \textit{ab-initio} methods we show that the photocarrier thermalization time in single-layer transition metal dichalcogenides strongly depends on the peculiarities of the phonon spectrum and the electronic spin-orbit coupling. In detail, the lifted spin degeneracy in the valence band suppresses the hole scattering on acoustic phonons, slowing down the thermalization of holes by one order of magnitude as compared to electrons. Moreover, the hole thermalization time behaves differently in MoS$_2$ and WSe$_2$ because spin-orbit interactions differ in these seemingly similar materials. We predict that the internal quantum efficiency of a tunneling van der Waals semiconductor heterostructure depends qualitatively on whether MoS$_2$ or WSe$_2$ is used.

Efficient hot-electron extraction in two-dimensional semiconductor heterostructures by ultrafast resonant transfer.

Energy loss from hot-carrier cooling sets the thermodynamic limit for the photon-to-power conversion efficiency in optoelectronic applications. Efficient hot-electron extraction before cooling could reduce the energy loss and leads to efficient next generation devices, which, unfortunately, is challenging to achieve in conventional semiconductors. In this work, we explore hot-electron transfer in two-dimensional (2D) layered semiconductor heterostructures, which have shown great potential for exploring new physics and optoelectronic applications. Using broadband micro-area ultrafast spectroscopy, we firmly established a type I band alignment in the WS2-MoTe2 heterostructure and ultrafast (∼60 fs) hot-electron transfer from photoexcited MoTe2 to WS2. The hot-electron transfer efficiency increases with excitation energy or excess energy as a result of a more favorable continuous competition between resonant electron transfer and cooling, reaching 90% for hot electrons with 0.3 eV excess energy. This study reveals exciting opportunities of designing extremely thin absorber and hot-carrier devices using 2D semiconductors and also sheds important light on the photoinduced interfacial process including charge transfer and generation in 2D heterostructures and optoelectronic devices.

Enhanced near-Infrared Photoresponse from Nanoscale Ag-Au Alloyed Films

Alloying of metals provides a vast parameter space for tuning of material, chemical, and mechanical properties, impacting disciplines ranging from photonics and catalysis to aerospace. From an optical point-of-view, pure thin metal films yield enhanced light absorption due to their cavity effects. However, an ideal metal–semiconductor photodetector requires not only high absorption, but also long hot carrier attenuation lengths in order to efficiently collect excited carriers. Here we demonstrate that Ag-Au alloys provide an ideal model system for controlling the optical and electrical responses in nanoscale thin metal films for hot carrier photodetectors with improved performance. While pure Ag and Au have long hot carrier attenuation lengths >20 nm, their optical absorption is insufficient for high efficiency devices. Instead, we find that alloying Ag and Au enhances the absorption by ∼50% while maintaining attenuation lengths >15 nm, currently limited by grain boundary scattering, although the electron attenuation length of pure Au outperforms pure Ag as well as all of the alloys investigated here. Further, our density functional theory analysis shows that the addition of small amounts of Au to the Ag lattice significantly enhances the hot hole generation rate. Combined, these findings suggest a route to high efficiency hot carrier devices based on metallic alloying with potential applications ranging from photodetectors and sensors to improved catalytic materials.

Hot-Carrier Extraction in Nanowire-Nanoantenna Photovoltaic Devices.

Nanowires bring new possibilities to the field of hot-carrier photovoltaics by providing flexibility in combining materials for band engineering and using nanophotonic effects to control light absorption. Previously, an open-circuit voltage beyond the Shockley-Queisser limit was demonstrated in hot-carrier devices based on InAs-InP-InAs nanowire heterostructures. However, in these first experiments, the location of light absorption, and therefore the precise mechanism of hot-carrier extraction, was uncontrolled. In this Letter, we combine plasmonic nanoantennas with InAs-InP-InAs nanowire devices to enhance light absorption within a subwavelength region near an InP energy barrier that serves as an energy filter. From photon-energy- and irradiance-dependent photocurrent and photovoltage measurements, we find that photocurrent generation is dominated by internal photoemission of nonthermalized hot electrons when the photoexcited electron energy is above the barrier and by photothermionic emission when the energy is below the barrier. We estimate that an internal quantum efficiency up to 0.5-1.2% is achieved. Insights from this study provide guidelines to improve internal quantum efficiencies based on nanowire heterostructures.

Hot-Hole versus Hot-Electron Transport at Cu/GaN Heterojunction Interfaces.

Among all plasmonic metals, copper (Cu) has the greatest potential for realizing optoelectronic and photochemical hot-carrier devices, thanks to its CMOS compatibility and outstanding catalytic properties. Yet, relative to gold (Au) or silver (Ag), Cu has rarely been studied and the fundamental properties of its photoexcited hot carriers are not well understood. Here, we demonstrate that Cu nanoantennas on p-type gallium nitride (p-GaN) enable hot-hole-driven photodetection across the visible spectrum. Importantly, we combine experimental measurements of the internal quantum efficiency (IQE) with ab initio theoretical modeling to clarify the competing roles of hot-carrier energy and mean-free path on the performance of hot-hole devices above and below the interband threshold of the metal. We also examine Cu-based plasmonic photodetectors on corresponding n-type GaN substrates that operate via the collection of hot electrons. By comparing hot hole and hot electron photodetectors that employ the same metal/semiconductor interface (Cu/GaN), we further elucidate the relative advantages and limitations of these complementary plasmonic systems. In particular, we find that harnessing hot holes with p-type semiconductors is a promising strategy for plasmon-driven photodetection across the visible and ultraviolet regimes. Given the technological relevance of Cu and the fundamental insights provided by our combined experimental and theoretical approach, we anticipate that our studies will have a broad impact on the design of hot-carrier optoelectronic devices and plasmon-driven photocatalytic systems.

Hot Carriers in Halide Perovskites: How Hot Truly?

Slow hot carrier cooling in halide perovskites holds the key to the development of hot carrier (HC) perovskite solar cells. For accurate modeling and pragmatic design of HC materials and devices, it is essential that HC temperatures are reliably determined. A common approach involves fitting the high-energy tail of the main photobleaching peak in a transient absorption spectrum with a Maxwell-Boltzmann distribution. However, this approach is problematic because of complications from the overlap of several photophysical phenomena and a lack of consensus in the community on the fitting procedures. Herein, we propose a simple approach that circumvents these challenges. Through tracking the broadband spectral evolution and accounting for bandgap renormalization and spectral line width broadening effects, our method extracts not only accurate and consistent carrier temperatures but also other important parameters such as the quasi-Fermi levels, bandgap renormalization constant, etc. Establishing a reliable method for the carrier temperature determination is a step forward in the study of HCs for next-generation perovskite optoelectronics.