Multiple Charge Carriers Research Articles

Weak antilocalization and two-carrier electrical transport in Bi1−xSbx single crystals (0%≤x≤17.0%)

Experimental investigation of the weak antilocalization (WAL) effect on the Hall resistivity is quite rare, because the WAL is known to have no influence on the Hall effect in a single-band system. We challenge this view in a system that has both electrons and holes by deriving a two-band model modified by the WAL effect and by applying it to the low-field magnetoresistance (MR) and Hall resistance (HR) of $\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}$ single crystals $(0\ensuremath{\le}x\ensuremath{\le}17.0%)$. Simultaneous occurrence of a dip in MR and nonlinearity in HR suggests that the $\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}$ is a rare three-dimensional system, in which WAL and two distinct charge carriers interplay. The modified two-band model describes all the main features of MR and HR that are not captured by the conventional theory. From the quantitative analysis based on the modified theory, the values of key parameters, such as carrier density and mobility of electrons and holes, are estimated. The modified two-band model provides a solid framework for understanding electrical transport phenomena of a material with strong spin-orbit interaction and multiple distinct charge carriers.

A Coupled Tank-in-Series Electrochemical Engineering and Thermal Model for Lithium Sulfur Batteries

Lithium sulfur (LiS) batteries are a promising option for next-generation energy storage due to their high energy density. Applications where weight is especially important like electric flight and electric vehicles have driven the study of this chemistry because lithium ion batteries are approaching their practical limits. For these applications, the demand for packing more energy into a battery can lead to higher resistive batteries. For lithium sulfur batteries in particular, the resistance is seen to increase with higher capacities per cell (1). As the required energy and subsequently battery packs increase, the importance of heat dissipation for safety and performance also grows, which requires an accurate thermal model for design and control. Physics-based models enable a look into the internal states of the battery, which correspond to physical phenomena that can contribute to decreased performance under certain conditions. The mathematical model for lithium sulfur batteries developed by Kumaresan et al (2) was the first one-dimensional physics-based model for lithium sulfur batteries. This discharge model was developed with porous electrode theory and considers transport, kinetics, thermodynamics, and morphological changes. The LiS chemistry includes a reduction of solid sulfur to different order polysulfides and finally a precipitation of Li2S, which leads to complexity due to multiple charge carriers and dynamic electrode morphology. The corresponding physics-based model results in a numerically stiff set of equations with variables evolving across many orders of magnitude. To alleviate the computational footprint and still maintain a high degree of accuracy, we have previously developed a mass-conserving lumped model through volume-averaging, called the Lithium Sulfur Tank-in-Series Model. This model, with average quantities in each region, eliminates the spatial dependence for increased computational efficiency and fewer model parameters that expedites model use in estimation, control, and optimization. In this work, the LiS tank-in-series model is coupled with a thermal model (3). The thermal model includes heat generation due to internal resistance and entropic contributions, and Stroe et al. used the thermal model predictions with a simple empirical model. Coupling the thermal model with a more accurate predictive model allows further insight into battery performance. The model predictions are compared with experimental data from 19.5 Ah pouch cells. Acknowledgments The authors are thankful for financial support from the Battery500 Consortium, BAE Systems, and the Joint Center for Aerospace Technology Innovation. References J. Offer and M. Wild, Lithium Sulfur Batteries, p. 149, John Wiley & Sons, New Jersey (2019).Kumaresan, Y. Mikhaylik, and R. E. White, J. Electrochem. Soc., 155, A576 (2008).I. Stroe, V. Knap, M. Swierczynski, and E. Schaltz, ECS Trans., 77, 467–476 (2017).

Multiple charge-carrier transfer channels of Z-scheme bismuth tungstate-based photocatalyst for tetracycline degradation: Transformation pathways and mechanism

As a sustainable and cost-efficient technique, photocatalytic technology provides an ideal method for energy utilization and environmental pollution control. The current photocatalyst is commonly based on single charge transfer approach, which cannot meet the demand of rapidly charge transfer to improve the photocatalytic performance. Herein, a novel Ag3PO4/MWCNT/Bi2WO6 Z-scheme heterojunction photocatalyst with multiple charge-carrier transfer channels was successfully prepared by a simple hydrothermal and deposition procedure, which possessed remarkable charge carriers separation efficiency and broad photoabsorption: (i) Z-scheme charge transfer channel was formed by Ag3PO4, Bi2WO6 and Ag; (ii) Ag showed the “electron sink” property and surface plasmon resonance (SPR) effect; (iii) multi-walled carbon nanotube (MWCNT) can act as electron accepter to improve the transfer efficiency of photoinduced electron. Ag3PO4/MWCNT/Bi2WO6 composite shows excellent visible light drive photocatalytic performance for organic pollution degradation. And the degradation pathways of tetracycline (TC) were investigated at length. In addition, the cyclic experiments confirmed that the photocatalytic stability of Ag3PO4/MWCNT/Bi2WO6. The hole (h+) and superoxide radical (O2−) radicals were confirmed that played a key role in the photodegradation system. This work provides inspiration for rational fabrication of excellent photocatalyst with multi-charge carrier transfer channels to meet increasing environmental requirements.

A Tanks-in-Series Electrochemical Engineering Model for Lithium Sulfur Batteries

Lithium sulfur (LiS) batteries are promising for next-generation energy storage. Due to their high energy density, lithium sulfur batteries are attractive for applications like electric vehicles and electric flights where weight is an important constraint. However, LiS batteries suffer from low coulombic efficiency, poor cycle life, and self-discharge. ­­ 1 Physics-based models can give insight into the internal states of the battery, which can be correlated to the mechanisms that contribute to performance degradation under various operating conditions. The first one-dimensional electrochemical discharge model for LiS batteries was developed by Kumaresan et al. using porous electrode theory, and considers transport, kinetics, thermodynamics, and morphological changes.2 The LiS chemistry includes multiple charge carriers and morphology changes as discharge proceeds, which contributes to the complexity and challenges of accurate modeling. The corresponding physics-based model results in a complex and numerically stiff equation system with variables that change across orders of magnitude and require proper initialization. The mathematical complexity of such electrochemical models motivates the use of reformulation and simplification techniques in order to justify their use in estimation, control, and optimization. Even for fast reformulated models, the large number of kinetic and transport parameters resulting from the complex chemistry makes their parameterization a non-trivial task. Several groups have used lumped and one-dimensional models to simplify modeling and study specific characteristics, such as self-discharge, rate-dependence of precipitation, or charging.3–9 This work demonstrates a mass-conserving tank-in-series model based on the original model developed by Kumaresan et al.2 The tank-in-series model is developed through volume-averaging of the model equations. The volume averaging results in overall balance equations governing the evolution of average variables in each region of the cell. The elimination of the spatial dependence from model equations substantially increases computational efficiency and reduces the number of parameters to be estimated. Comparisons of the tank-model simulations with the full model indicate the ability of the averaged model to capture discharge behavior at low to intermediate discharge rates, with deviations mainly observed towards the end of discharge. Other advantages and limitations of this model as compared to the full model are characterized in terms of electrochemical variable predictions, with applications in control and estimation for hybrid electric flights. The model predictions are compared against experimental data from 19.5 Ah pouch cells and have been improved through estimation of parameters. Acknowledgments The authors are thankful for financial support from BAE Systems and the Joint Center for Aerospace Technology Innovation. References X. Zhang, H. Xie, C.-S. Kim, K. Zaghib, A. Mauger, and C. M. Julien, Mater. Sci. Eng. R Reports, 121, 1 (2017).K. Kumaresan, Y. Mikhaylik, and R. E. White, J. Electrochem. Soc., 155, A576 (2008).Y. V. Mikhaylik and J. R. Akridge, J. Electrochem. Soc., 151, A1969 (2004).K. Yoo, M.-K. Song, E. J. Cairns, and P. Dutta, Electrochim. Acta, 213, 174 (2016).Y. X. Ren, T. S. Zhao, M. Liu, P. Tan, and Y. K. Zeng, J. Power Sources, 336, 115 (2016).T. Zhang, M. Marinescu, L. O’Neill, M. Wild, and G. Offer, Phys. Chem. Chem. Phys., 17, 22581 (2015).M. Marinescu, T. Zhang, and G. J. Offer, Phys. Chem. Chem. Phys., 18, 584 (2016).D. N. Fronczek and W. G. Bessler, J. Power Sources, 244, 183 (2013).A. F. Hofmann, D. N. Fronczek, and W. G. Bessler, J. Power Sources, 259, 300 (2014).

Hot Charge Carriers in Quantum Dots: Generation, Relaxation, Extraction, and Applications

AbstractThis article discusses the hot charge carriers in semiconductor quantum dots (QDs): their generation, relaxation, extraction and applications in different technologically relevant areas. It has been reported that the most common ways to generate hot charge carriers are photo‐excitation by energy more than the band gap energy. However, recently the other means to generate hot charge carriers such as doping, and semiconductor plasmon interaction have also been evolved and their advantages over the conventional method have been discussed. Relaxation dynamics of hot charge carriers and different processes related to this such as multiple exciton generation, Auger recombination, phonon vibrations and ligand assisted charge carrier relaxation are mentioned. It was shown that the relaxation mechanism of hot charge carriers (both hot electron and hot hole) follow different pathways and either of the mechanism is playing a role depending on the QDs structure and the surrounding conditions. Studies pertaining to interaction of QDs with other semiconductors, metal nanoparticles or with different molecular adsorbates suggest that the hot charge carrier extraction is possible in these heterojunctions with extraction time in sub‐ps scale. Application of hot charge carriers in different fields such as photovoltaics, photo‐catalysis, infrared detectors, and H2 detection have been shown and it was observed that the efficiency of the above‐mentioned processes is much higher when the hot charge carriers are involved, suggesting their importance in these areas.

Control of simultaneous effects of the temperature, indium composition and the impact ionization process on the performance of the InN/InxGa1-xN quantum dot solar cells

The impact ionization in semiconductor materials is a process that produces multiple charge carrier pairs from a single excitation. This mechanism constitutes a possible road to increase the efficiency of the p-n and p-i-n solar cells junctions. Our study considers the structure of InN/InGaN quantum dot solar cell in the calculation. In this work, we study the effect of indium concentration and temperature on the coefficient θ of the material type parameter of the impact ionization process for a p(InGaN)-n(InGaN) and p(InGaN)-i(QDs-InN)-n(InGaN) solar cell. Next, we investigate the effect of perturbation such as temperature and indium composition on conventional solar cell’s (p(InGaN)-n(InGaN)) and solar cells of the third generation with quantum dot intermediate band IBSC (p(InGaN-i(QD-InN)-n(InGaN)) by analyzing their behaviour in terms of efficiency of energy conversion at the presence of the impact ionization process. Our numerical results show that the efficiency is strongly influenced by all of these parameters. It is also demonstrated that θ decreased with the increase of indium concentration and temperature which contributes to an overall improvement of the conversion efficiency.

Multiple charge carrier transfer pathways in BiOBr/Bi2O3/BiO0.67F1.66 ternary composite with high adsorption and photocatalytic performance

BiOBr/Bi2O3/BiO0.67F1.66 ternary composites were designed and prepared via a facile chemical precipitation method and their photocatalytic performances were assessed by degradation of rhodamine B (RhB) under visible light illumination. The highest activity was achieved over BiOBr/Bi2O3/BiO0.67F1.66 ternary composite with Br/F molar ratio of 1:1, which approximately degraded 98.61% of RhB after just 15min irradiation. Contrast to pure BiOBr, the enhanced photocatalytic performances of the ternary composites could be attributed to their larger specific surface areas and multiple charge carrier transfer pathways, which efficiently promoted separation of charge carriers. Besides, the formation mechanism of the composites was discussed in detail as well.

Unusual Methylenediolate Bridged Hexanuclear Ruthenium(III) Complexes: Syntheses and Their Application.

Three structurally analogous hexanuclear ruthenium(III) complexes were isolated with the general molecular formula of [Ru6III(O)2(μ4-η2-η2-CH2O2)( t-BuCO2)12(L)2] where L = pyridine (1) or 4-dimethylamino pyridine (DMAP; 2) or 4-cyanopyridine (3). Complexes 1 and 3 were solved in the tetragonal I4̅c2 and P41212 space group, respectively, while 2 crystallized in the monoclinic system with P21 /c space group. In all three complexes, two oxo-centered Ru(III) triangles were bridged by a unique and a rare methylenediolate (CH2O2)2-) ligand. This (CH2O2)2- group is reported to be an intermediate, which is not isolated in its metal-free form, to date, as it is unstable. Control experiments performed evidently reveal that the unique reaction condition followed is mandatory to isolate 1-3 and the origin of (CH2O2)2- is unknown at the moment, as no precursor was used to form this intermediate. The presence of (CH2O2)2- identified through X-ray diffraction was further unambiguously confirmed by various 1D (1H and 13C) and 2D-NMR (HSQC, TOCSY, NOESY, and DEPT) spectroscopies. Direct current (dc) magnetic susceptibility measurements performed on 1 and 2 reveal the predominant antiferromagnetic exchange interaction between the Ru(III) centers result in a diamagnetic ground state at 2.0 K. The paramagnetic influence of 1-3 at room temperature evidently felt by the 1H nuclei of the (CH2O2)2- unit predominates compared to other NMR active nuclei in the complexes. The presence of an electron donating or withdrawing substituent on the terminal pyridine results in significant change in the dihedral angle of two oxo-centered triangular (Ru3O-) planes. The change in the structural parameters of 1-3 due to the substituents markedly reflected on the absorption profile and redox behavior, which are systematically investigated. Preliminary galvanostatic charge/discharge cycling experiments performed on a representative complex (3) suggest that 3 can be a promising candidate to employ as an effective multiple electron charge carrier in a nonaqueous redox flow battery.

Charge Carrier Dynamics in Photocatalytic Hybrid Semiconductor-Metal Nanorods: Crossover from Auger Recombination to Charge Transfer.

Hybrid semiconductor-metal nanoparticles (HNPs) manifest unique, synergistic electronic and optical properties as a result of combining semiconductor and metal physics via a controlled interface. These structures can exhibit spatial charge separation across the semiconductor-metal junction upon light absorption, enabling their use as photocatalysts. The combination of the photocatalytic activity of the metal domain with the ability to generate and accommodate multiple excitons in the semiconducting domain can lead to improved photocatalytic performance because injecting multiple charge carriers into the active catalytic sites can increase the quantum yield. Herein, we show a significant metal domain size dependence of the charge carrier dynamics as well as the photocatalytic hydrogen generation efficiencies under nonlinear excitation conditions. An understanding of this size dependence allows one to control the charge carrier dynamics following the absorption of light. Using a model hybrid semiconductor-metal CdS-Au nanorod system and combining transient absorption and hydrogen evolution kinetics, we reveal faster and more efficient charge separation and transfer under multiexciton excitation conditions for large metal domains compared to small ones. Theoretical modeling uncovers a competition between the kinetics of Auger recombination and charge separation. A crossover in the dominant process from Auger recombination to charge separation as the metal domain size increases allows for effective multiexciton dissociation and harvesting in large metal domain HNPs. This was also found to lead to relative improvement of their photocatalytic activity under nonlinear excitation conditions.

Recent advance in multiple exciton generation in semiconductor nanocrystals

The multiple exciton generation (MEG), a process in which two or even more electron-hole pairs are created in nanostructured semiconductors by absorbing a single high-energy photon, is fundamentally important in many fields of physics, e.g., nanotechnology and optoelectronic devices. Many high-performance optoelectronic devices can be achieved with MEG where quite an amount of the energy of an absorbed photon in excess of the band gap is used to generate morei additional electron-hole pairs instead of rapidly lost heat. In this review, we present a survey on both the research context and the recent progress in the understanding of MEG. This phenomenon has been experimentally observed in the 0D nanocrystals, such as PbX (X=Se, S, and Te), InX (X=As and P), CdX (X=Se and Te), Si, Ge, and semi-metal quantum dots, which produce the differential quantum efficiency as high as 90%10%. Even more remarkably, experiment advances have made it possible to realize MEG in the one-dimensional (1D) semiconductor nanorods and the two-dimensional (2D) nano-thin films. Theoretically, three different approaches, i.e., the virtual exciton generation approach, the coherent multiexciton mode, and the impact ionization mechanism, have been proposed to explain the MEG effect in semiconductor nanostructures. Experimentally, the MEG has been measured by the ultrafast transient spectroscopy, such as the ultrafast transient absorption, the terahertz ultrafast transient absorption, the transient photoluminescence, and the transient grating technique. It is shown that the properties of nanostructured semiconductors, e.g., the composition, structure and surface of the material, have dramatic effects on the occurrence of MEG. As a matter of fact, it is somewhat hard to experimentally confirm the signature of MEG in nanostructured semiconductors due to two aspects:i) the time scale of the MEG process is very short; ii) the excitation fluence should be extremely low to prevent the multi-excitons from being generated by multiphoton absorption. There are still some controversies with respect to the MEG effect due to the challenge in both the experimental measurement and the explanation of signal data. The successful applications of MEG in practical devices, of which each is composed of the material with lower MEG threshold and higher efficiency, require the extraction of multiple charge carriers before their ultrafast annihilation. Such an extraction can be realized by the ultrafast electron transfer from nanostructured semiconductors to molecular and semiconductor electron acceptors. More recently, an experiment with PbSe quantum dot photoconductor has demonstrated that the multiple charge extraction is even as high as 210%. It is proved that MEG is of applicable significance in optoelectronic devices and in ultra-efficient photovoltaic devices. Although there are still some challenges, the dramatic enhancement of the efficiency of novel optoelectronic devices by the application of MEG can be hopefully realized with the rapid improvement of nanotechnology.

REVIEW OF DOSE-RATE EFFECTS IN THE THERMOLUMINESCENCE OF Lif:mg,ti (HARSHAW).

The literature describing the experimental investigations of possible dose-rate effects in the thermoluminescence (TL) of LiF:Mg,Ti (Harshaw) is reviewed. The total lack of glow curve analysis, coupled with inclusion of all or part of the high temperature TL and absence of parallel measurements of possible dose-rate effects in the irradiation stage severely limit the scientific and technical level of the experiments. In addition, the experimental procedures are far from sufficient to warrant any conclusion concerning the presence or absence of dose-rate effects in the TL of LiF:Mg,Ti. This decision is contrary to the widely held belief that there are no dose-rate effects in the TL of LiF:Mg,Ti. In addition, the literature on dose-rate effects in the optical absorption (irradiation stage) of LiF is reviewed and is found contradictory. No dose-rate studies have been carried out on optical absorption in LiF:Mg,Ti. Kinetic simulations demonstrating the possibility, even likelihood, of dose-rate effects are also reviewed. Dose-rate effects are shown to be likely due to competition between excitation and recombination in the irradiation stage. Some other possible mechanisms involving multiple charge carrier trapping are suggested. Further definitive experiments are sorely needed, but the interested researcher should beware, it is not an easy task.

Multichannel Charge Transfer and Mechanistic Insight in Metal Decorated 2D-2D Bi2 WO6 -TiO2 Cascade with Enhanced Photocatalytic Performance.

Promising semiconductor-based photocatalysis toward achieving efficient solar-to-chemical energy conversion is an ideal strategy in response to the growing worldwide energy crisis, which however is often practically limited by the insufficient photoinduced charge-carrier separation. Here, a rational cascade engineering of Au nanoparticles (NPs) decorated 2D/2D Bi2 WO6 -TiO2 (B-T) binanosheets to foster the photocatalytic efficiency through the manipulated flow of multichannel-enhanced charge-carrier separation and transfer is reported. Mechanistic characterizations and control experiments, in combination with comparative studies over plasmonic Au/Ag NPs and nonplasmonic Pt NPs decorated 2D/2D B-T composites, together demonstrate the cooperative synergy effect of multiple charge-carrier transfer channels in such binanosheets-based ternary composites, including Z-scheme charge transfer, "electron sink," and surface plasmon resonance effect, which integratively leads to the boosted photocatalytic performance.

Epitaxial thin films of pyrochlore iridate Bi2+xIr2-yO7-\u03b4: structure, defects and transport properties

While pyrochlore iridate thin films are theoretically predicted to possess a variety of emergent topological properties, experimental verification of these predictions can be obstructed by the challenge in thin film growth. Here we report on the pulsed laser deposition and characterization of thin films of a representative pyrochlore compound Bi2Ir2O7. The films were epitaxially grown on yttria-stabilized zirconia substrates and have lattice constants that are a few percent larger than that of the bulk single crystals. The film composition shows a strong dependence on the oxygen partial pressure. Density-functional-theory calculations indicate the existence of BiIr antisite defects, qualitatively consistent with the high Bi: Ir ratio found in the films. Both Ir and Bi have oxidation states that are lower than their nominal values, suggesting the existence of oxygen deficiency. The iridate thin films show a variety of intriguing transport characteristics, including multiple charge carriers, logarithmic dependence of resistance on temperature, antilocalization corrections to conductance due to spin-orbit interactions, and linear positive magnetoresistance.

Photoelectrochemical Properties of Colloidally Synthesized (Cu,Ag)2ZnSnS4 solid Solution nanocrystals

Introduction Multinary semiconductors of Cu2ZnSnS4 (CZTS) composed of earth-abundant and non-toxic elements have been intensively studied as a new type of potential photovoltaic materials for the application to thin film solar cells[1] and photocatalyst for hydrogen evolution[2] due to its promising optical properties for absorbing solar light, such as band gap energy of 1.4-1.5 eV and high absorption coefficient. Recently, Kudo and co-workers reported that the photocatalytic activities for hydrogen evolution can be greatly enhanced by formation of solid solution between CZTS and Ag2ZnSnS4 (AZTS).[2]The hydrogen evolution rate under visible-light irradiation of CZTS-AZTS photocatalyst particles was 150 times higher than that of pure CZTS particles due to an increase in conduction band edge energy by making solid solution. Nanometer-sized crystals of such photoactive semiconductors have also attracted much attention due to their light-harvesting properties and tunable electronic energy structure depending on their size. In addition, semiconductor nanocrystals (NCs) have opened up new ways to generate multiple charge carriers with a single photon. In our previous papers, we successfully synthesized CZTS NCs via thermal decomposition of precursors in hot organic solution and investigated their photoelectrochemical properties depending on the particle size and composition.[3-5] The incorporation of Ag+into CZTS nanocrystal solid solution is expected to enhance photo-properties of nanoparticles, but such attempt has not been carried out. In this study, we prepare CZTS-AZTS NCs uniformly dispersed in organic solution and report their photoelectrochemical properties. Experimental CZTS-AZTS solid solution ((Cu1-x Ag x )2ZnSnS4; CAZTS) NCs were prepared via thermal decomposition of metal diethyl dithiocarbamate precursors in hot oleylamine under N2 atmosphere. After removal of largely aggregated particles, the resulting NCs were dissolved in chloroform. Photoelectrochemical properties of CAZTS NCs immobilized on ITO electrodes were measured in an aqueous solution containing 0.2 mol dm-3 Eu(NO3)3 as an electron scavenger. The Ag/AgCl electrode and Pt wire were used as reference and counter electrodes respectively. Results and discussions TEM measurement revealed that thus-obtained particles were polygonal shape with average diameter of ca. 10 nm regardless of x value. Their chemical composition could be controlled between 0 < x < 1 by varying the Cu/Ag ratio in precursors. The X-ray diffraction patterns of NCs were assignable to kesterite-type crystal structure and each peak was shifted to lower angle with increase in the Ag content in NCs, indicating that the obtained NCs were not a mixture of CZTS and AZTS NCs but a solid solution between CZTS and AZTS. Figure 1 shows absorption spectra of CAZTS NCs in chloroform. The absorption onset of CAZTS NCs was blue-shifted with an increase in Ag content. The bandgap of the NCs was tunable from 1.1 eV (x = 0) to 2.0 eV (x = 1.0) only by changing value of x. To investigate the photoelectrochemical properties of CAZTS NCs, the NCs were immobilized onto ITO electrodes with spin-coating CAZTS NCs chloroform solution at 1000 rpm for 10 second, followed by heating the electrode at 300°C. Irradiation (λ > 350 nm) to thus-obtained films was carried out in an aqueous solution containing Eu(NO3)3 as an electron scavenger. Cathodic photocurrents were observed for CAZTS NC electrodes except for NCs of x = 1.0 (AZTS) under applied electrochemical bias, the magnitude being increased with negative shift of the electrode potential. The observed behavior was similar to that of a p-type semiconductor photoelectrode. On the other hand, AZTS NCs-immobilized ITO electrodes exhibited anodic photocurrent like as an n-type semiconductor as reported in our previous paper.[6]Action spectra of the photocurrent were in good agreement with absorption spectra of CAZTS NPs used, indicating that CAZTS NPs effectively worked as photovoltaic materials driven by irradiation of visible and near-IR light. Reference [1] D. Aldakov, et al., J. Mater. Chem. C, 2013, 1, 3756. [2] A. Kudo, et al., Chem. Mater., 2010, 22, 1402. [3] T. Kameyama, et al., J.Mater. Chem., 2010, 20, 5319. [4] H. Nishi, et al., Phys. Chem. Chem. Phys., 2013, 16, 672. [5] H. Nishi, et al., J. Phys. Chem. C, 2013, 117, 21055. [6] T. Sasamura, et al., Chem. Lett., 2012, 41, 1009. Figure 1

Lead Telluride Quantum Dot Solar Cells Displaying External Quantum Efficiencies Exceeding 120%.

Multiple exciton generation (MEG) in semiconducting quantum dots is a process that produces multiple charge-carrier pairs from a single excitation. MEG is a possible route to bypass the Shockley-Queisser limit in single-junction solar cells but it remains challenging to harvest charge-carrier pairs generated by MEG in working photovoltaic devices. Initial yields of additional carrier pairs may be reduced due to ultrafast intraband relaxation processes that compete with MEG at early times. Quantum dots of materials that display reduced carrier cooling rates (e.g., PbTe) are therefore promising candidates to increase the impact of MEG in photovoltaic devices. Here we demonstrate PbTe quantum dot-based solar cells, which produce extractable charge carrier pairs with an external quantum efficiency above 120%, and we estimate an internal quantum efficiency exceeding 150%. Resolving the charge carrier kinetics on the ultrafast time scale with pump–probe transient absorption and pump–push–photocurrent measurements, we identify a delayed cooling effect above the threshold energy for MEG.

Multiple-exciton generation in lead selenide nanorod solar cells with external quantum efficiencies exceeding 120.

Multiple-exciton generation—a process in which multiple charge-carrier pairs are generated from a single optical excitation—is a promising way to improve the photocurrent in photovoltaic devices and offers the potential to break the Shockley–Queisser limit. One-dimensional nanostructures, for example nanorods, have been shown spectroscopically to display increased multiple exciton generation efficiencies compared with their zero-dimensional analogues. Here we present solar cells fabricated from PbSe nanorods of three different bandgaps. All three devices showed external quantum efficiencies exceeding 100% and we report a maximum external quantum efficiency of 122% for cells consisting of the smallest bandgap nanorods. We estimate internal quantum efficiencies to exceed 150% at relatively low energies compared with other multiple exciton generation systems, and this demonstrates the potential for substantial improvements in device performance due to multiple exciton generation.

A multi-agent quantum Monte Carlo model for charge transport: Application to organic field-effect transistors

We have developed a multi-agent quantum Monte Carlo model to describe the spatial dynamics of multiple majority charge carriers during conduction of electric current in the channel of organic field-effect transistors. The charge carriers are treated by a neglect of diatomic differential overlap Hamiltonian using a lattice of hydrogen-like basis functions. The local ionization energy and local electron affinity defined previously map the bulk structure of the transistor channel to external potentials for the simulations of electron- and hole-conduction, respectively. The model is designed without a specific charge-transport mechanism like hopping- or band-transport in mind and does not arbitrarily localize charge. An electrode model allows dynamic injection and depletion of charge carriers according to source-drain voltage. The field-effect is modeled by using the source-gate voltage in a Metropolis-like acceptance criterion. Although the current cannot be calculated because the simulations have no time axis, using the number of Monte Carlo moves as pseudo-time gives results that resemble experimental I/V curves.

Multiple exciton generation in quantum dots versus singlet fission in molecular chromophores for solar photon conversion.

Both multiple exciton generation (MEG) in semiconductor nanocrystals and singlet fission (SF) in molecular chromophores have the potential to greatly increase the power conversion efficiency of solar cells for the production of solar electricity (photovoltaics) and solar fuels (artificial photosynthesis) when used in solar photoconverters. MEG creates two or more excitons per absorbed photon, and SF produces two triplet states from a single singlet state. In both cases, multiple charge carriers from a single absorbed photon can be extracted from the cell and used to create higher power conversion efficiencies for a photovoltaic cell or a cell that produces solar fuels, like hydrogen from water splitting or reduced carbon fuels from carbon dioxide and water (analogous to biological photosynthesis). The similarities and differences in the mechanisms and photoconversion cell architectures between MEG and SF are discussed.

Recombination in liquid filled ionisation chambers with multiple charge carrier species: Theoretical and numerical results

Liquid-filled ionisation chambers (LICs) are used in radiotherapy for dosimetry and quality assurance. Volume recombination can be quite important in LICs for moderate dose rates, causing non-linearities in the dose rate response of these detectors, and needs to be corrected for. This effect is usually described with Greening and Boag models for continuous and pulsed radiation respectively. Such models assume that the charge is carried by two different species, positive and negative ions, each of those species with a given mobility. However, LICs operating in non-ultrapure mode can contain different types of electronegative impurities with different mobilities, thus increasing the number of different charge carriers. If this is the case, Greening and Boag models can be no longer valid and need to be reformulated.In this work we present a theoretical and numerical study of volume recombination in parallel-plate LICs with multiple charge carrier species, extending Boag and Greening models. Results from a recent publication that reported three different mobilities in an isooctane-filled LIC have been used to study the effect of extra carrier species on recombination. We have found that in pulsed beams the inclusion of extra mobilities does not affect volume recombination much, a behaviour that was expected because Boag formula for charge collection efficiency does not depend on the mobilities of the charge carriers if the Debye relationship between mobilities and recombination constant holds. This is not the case in continuous radiation, where the presence of extra charge carrier species significantly affects the amount of volume recombination.

Highly efficient carrier multiplication in PbS nanosheets.

Semiconductor nanocrystals are promising for use in cheap and highly efficient solar cells. A high efficiency can be achieved by carrier multiplication (CM), which yields multiple electron-hole pairs for a single absorbed photon. Lead chalcogenide nanocrystals are of specific interest, since their band gap can be tuned to be optimal to exploit CM in solar cells. Interestingly, for a given photon energy CM is more efficient in bulk PbS and PbSe, which has been attributed to the higher density of states. Unfortunately, these bulk materials are not useful for solar cells due to their low band gap. Here we demonstrate that two-dimensional PbS nanosheets combine the band gap of a confined system with the high CM efficiency of bulk. Interestingly, in thin PbS nanosheets virtually the entire excess photon energy above the CM threshold is used for CM, in contrast to quantum dots, nanorods and bulk lead chalcogenide materials.