Transport In Semiconductors Research Articles

Exciton Formation Dynamics and Band‐Like Free Charge‐Carrier Transport in 2D Metal Halide Perovskite Semiconductors

AbstractMetal halide perovskite (MHP) semiconductors have driven a revolution in optoelectronic technologies over the last decade, in particular for high‐efficiency photovoltaic applications. Low‐dimensional MHPs presenting electronic confinement have promising additional prospects in light emission and quantum technologies. However, the optimisation of such applications requires a comprehensive understanding of the nature of charge carriers and their transport mechanisms. This study employs a combination of ultrafast optical and terahertz spectroscopy to investigate phonon energies, charge‐carrier mobilities, and exciton formation in 2D (PEA)2PbI4 and (BA)2PbI4 (where PEA is phenylethylammonium and BA is butylammonium). Temperature‐dependent measurements of free charge‐carrier mobilities reveal band transport in these strongly confined semiconductors, with surprisingly high in‐plane mobilities. Enhanced charge‐phonon coupling is shown to reduce charge‐carrier mobilities in (BA)2PbI4 with respect to (PEA)2PbI4. Exciton and free charge‐carrier dynamics are disentangled by simultaneous monitoring of transient absorption and THz photoconductivity. A sustained free charge‐carrier population is observed, surpassing the Saha equation predictions even at low temperature. These findings provide new insights into the temperature‐dependent interplay of exciton and free‐carrier populations in 2D MHPs. Furthermore, such sustained free charge‐carrier population and high mobilities demonstrate the potential of these semiconductors for applications such as solar cells, transistors, and electrically driven light sources.

Gate-Tunable Negative Differential Conductance in Hybrid Semiconductor–Superconductor Devices

Negative differential conductance (NDC) serves as a crucial characteristic that reveals various underlying physics and transport process in hybrid superconducting devices. We report the observation of gate-tunable NDC outside the superconducting energy gap on two types of hybrid semiconductor–superconductor devices, i.e., normal metal–superconducting nanowire–normal metal and normal metal–superconducting nanowire–superconductor devices. Specifically, we study the dependence of the NDCs on back-gate voltage and magnetic field. When the back-gate voltage decreases, these NDCs weaken and evolve into positive differential conductance dips; and meanwhile they move away from the superconducting gap towards high bias voltage, and disappear eventually. In addition, with the increase of magnetic field, the NDCs/dips follow the evolution of the superconducting gap, and disappear when the gap closes. We interpret these observations and reach a good agreement by combining the Blonder–Tinkham–Klapwijk (BTK) model and the critical supercurrent effect in the nanowire, which we call the BTK-supercurrent model. Our results provide an in-depth understanding of the tunneling transport in hybrid semiconductor–superconductor devices.

Rapid Self‐Assembly Process at Air/Water Confined Interface for Highly Aligned Crystalline Polymeric Semiconductor Films

AbstractHighly aligned aggregation state structures have great significance for effective charge‐carrier transport in organic semiconductors. Several methods have been developed to provide organic semiconducting molecules with highly oriented aggregation state structure; among these, using a water surface to form organic semiconductor films is a widely implemented strategy, wherein solutions are spread on the surface of water. However, common techniques of film formation on water surfaces generally result in a nonuniform orientation of the film owing to the isotropic spread process of droplets on the water surface. In this study, a spatially confined air/water interfacial assembly method is proposed to obtain uniformly aligned monolayer and multilayer poly(diketopyrrolopyrrole‐thieno[3,2‐b]thiophene) thin films with controlled thickness. The structural and morphological characterizations obtained using atomic force microscopy, high‐resolution transmission electron microscopy, and grazing incidence wide‐angle X‐ray scattering indicates the crystalline structure of the thin films and high alignment of the molecular chains. The maximum mobility of the thin films reaches up to 2.06 and 0.5 cm2 V−1 s−1 in the parallel and perpendicular direction, respectively, indicating apparent anisotropic electrical properties. Furthermore, an inverter based on these thin films exhibits a voltage gain of up to 70, demonstrating the potential of applying the proposed technique to logic circuits.

Jumping Kinetic Monte Carlo: Fast and Accurate Simulations of Partially Delocalized Charge Transport in Organic Semiconductors.

Developing devices using disordered organic semiconductors requires accurate and practical models of charge transport. In these materials, charge transport occurs through partially delocalized states in an intermediate regime between localized hopping and delocalized band conduction. Partial delocalization can increase mobilities by orders of magnitude compared to those with conventional hopping, making it important for the design of materials and devices. Although delocalization, disorder, and polaron formation can be described using delocalized kinetic Monte Carlo (dKMC), it is a computationally expensive method. Here, we develop jumping kinetic Monte Carlo (jKMC), a model that approaches the accuracy of dKMC for modest amounts of delocalization (such as those found in disordered organic semiconductors), with a computational cost comparable to that of conventional hopping. jKMC achieves its computational performance by modeling conduction using identical spherical polarons, yielding a simple delocalization correction to the Marcus hopping rate that allows polarons to jump over their nearest neighbors. jKMC can be used in regimes of partial delocalization inaccessible to dKMC to show that modest delocalization can increase mobilities by as much as 2 orders of magnitude.

Coherent Phonon‐Induced Modulation of Charge Transfer in 2D Hybrid Perovskites

AbstractElectron–phonon interactions play an essential role in charge transport and transfer processes in semiconductors. For most structures, tailoring electron–phonon interactions for specific functionality remains elusive. Here, it is shown that, in hybrid perovskites, coherent phonon modes can be used to manipulate charge transfer. In the 2D double perovskite, (AE2T)2AgBiI8 (AE2T: 5,5“‐diylbis(amino‐ethyl)‐(2,2”‐(2)thiophene)), the valence band maximum derived from the [Ag0.5Bi0.5I4]2– framework lies in close proximity to the AE2T‐derived HOMO level, thereby forming a type‐II heterostructure. During transient absorption spectroscopy, pulsed excitation creates sustained coherent phonon modes, which periodically modulate the associated electronic levels. Thus, the energy offset at the organic–inorganic interface also oscillates periodically, providing a unique opportunity for modulation of interfacial charge transfer. Density‐functional theory corroborates the mechanism and identifies specific phonon modes as likely drivers of the coherent charge transfer. These observations are a striking example of how electron–phonon interactions can be used to manipulate fundamentally important charge and energy transfer processes in hybrid perovskites.

Experimental demonstration of electrical spin injection into semiconductor employing conventional three-terminal and non-local Hanle devices using spin gapless semiconductor as ferromagnetic injector

Here, the deposition of a polycrystalline thin film of a noble promising alloy Ti2CoSi (TCS) on a p-Si substrate has been reported, and its spin gapless semiconducting characteristics have been investigated experimentally. The structural, magnetic, and electronic transport features of the TCS film have been investigated in detail followed by its implementation as a ferromagnetic tunnel contact for proficient spin accumulation into a semiconductor employing both conventional three-terminal and non-local (NL) Hanle measurements. As we can avoid noticing erroneous effects like anisotropic magnetoresistance of the ferromagnetic electrodes, the NL-Hanle experiment has been established to be the most effective method for demonstrating true spin transport in semiconductors.

Mechanism of Delocalization-Enhanced Exciton Transport in Disordered Organic Semiconductors.

Large exciton diffusion lengths generally improve the performance of organic semiconductor devices, because they enable energy to be transported farther during the exciton lifetime. However, the physics of exciton motion in disordered organic materials is not fully understood, and modeling the transport of quantum-mechanically delocalized excitons in disordered organic semiconductors is a computational challenge. Here, we describe delocalized kinetic Monte Carlo (dKMC), the first model of three-dimensional exciton transport in organic semiconductors that includes delocalization, disorder, and polaron formation. We find that delocalization can dramatically increase exciton transport; for example, delocalization across less than two molecules in each direction can increase the exciton diffusion coefficient by over an order of magnitude. The mechanism for the enhancement is 2-fold: delocalization enables excitons to hop both more frequently and further in each hop. We also quantify the effect of transient delocalization (short-lived periods where excitons become highly delocalized) and show that it depends strongly upon the disorder and transition dipole moments.

Carrier transport and photoconductivity properties of BN50/NiO50 nanocomposite films

BN50/NiO50 and Au-loaded BN50/NiO50 nanocomposite films were separately fabricated on the glass substrates for carrier transport and photoconductivity properties. X-ray diffraction pattern of the films show the hexagonal structure of BN and presence of defect states by Nelson Riley factor analysis. Morphological images show spherical shaped particles with highly porous structure. The incorporation of NiO may hindered growth of BN layers and resulted in spherical particles. Temperature-dependent conductivity describes semiconductor transport behaviour for deposited nanocomposite films. Thermal activation conduction with low activation energy (∼0.308 eV) may be responsible for the resulting conductivity. Further, the light intensity dependent photoelectrical properties of BN50/NiO50 and Au-loaded BN50/NiO50 nanocomposites have been explored. The effect of Au nanoparticles loading on enhanced photo-conductivities (∼22% increase) than bare nanocomposite film has been elaborated by proposed mechanism. This study provided the insightful information for carrier transport and photoconductivity of BN-based nanocomposites.

An ordered, self-assembled nanocomposite with efficient electronic and ionic transport.

Mixed conductors-materials that can efficiently conduct both ionic and electronic species-are an important class of functional solids. Here we demonstrate an organic nanocomposite that spontaneously forms when mixing an organic semiconductor with an ionic liquid and exhibits efficient room-temperature mixed conduction. We use a polymer known to form a semicrystalline microstructure to template ion intercalation into the side-chain domains of the crystallites, which leaves electronic transport pathways intact. Thus, the resulting material is ordered, exhibiting alternating layers of rigid semiconducting sheets and soft ion-conducting layers. This unique dual-network microstructure leads to a dynamic ionic/electronic nanocomposite with liquid-like ionic transport and highly mobile electronic charges. Using a combination of operando X-ray scattering and in situ spectroscopy, we confirm the ordered structure of the nanocomposite and uncover the mechanisms that give rise to efficient electron transport. These results provide fundamental insights into charge transport in organic semiconductors, as well as suggesting a pathway towards future improvements in these nanocomposites.

On the Einstein relation between mobility and diffusion cofficint in an active Nanostructured Materials

It is commonly known that the speed at which contemporary switching semiconductors work much depends on the carrier degeneracy of the band. Furthermore, the relation of the diffusivity to mobility ratio of the carriers in semiconductors (referred to as DMR) is very helpful because it is more accurate than any of the individual relations for diffusivity to mobility ratio, which is thought to be the two most frequently used parameters in carrier transport in semiconductors. With n-InSb and Hg1-xCdxTe as examples of numerical calculations, we will examine DMR under strong magnetic quantization of III-V in line with the three- and two-band models of Kane, respectively, along with parabolic energy bands.

Charge transport in mixed metal halide perovskite semiconductors.

Investigation of the inherent field-driven charge transport behaviour of three-dimensional lead halide perovskites has largely remained challenging, owing to undesirable ionic migration effects near room temperature and dipolar disorder instabilities prevalent specifically in methylammonium-and-lead-based high-performing three-dimensional perovskite compositions. Here, we address both these challenges and demonstrate that field-effect transistors based on methylammonium-free, mixed metal (Pb/Sn) perovskite compositions do not suffer from ion migration effects as notably as their pure-Pb counterparts and reliably exhibit hysteresis-free p-type transport with a mobility reaching 5.4 cm2 V-1 s-1. The reduced ion migration is visualized through photoluminescence microscopy under bias and is manifested as an activated temperature dependence of the field-effect mobility with a low activation energy (~48 meV) consistent with the presence of the shallow defects present in these materials. An understanding of the long-range electronic charge transport in these inherently doped mixed metal halide perovskites will contribute immensely towards high-performance optoelectronic devices.

Two-phonon scattering in nonpolar semiconductors: A first-principles study of warm electron transport in Si

The ab-initio theory of charge transport in semiconductors typically employs the lowest-order perturbation theory in which electrons interact with one phonon (1ph). This theory is accepted to be adequate to explain the low-field mobility of non-polar semiconductors but has not been tested extensively beyond the low-field regime. Here, we report first-principles calculations of the electric field-dependence of the electron mobility of Si as described by the warm electron coefficient, $\beta$. Although the 1ph theory overestimates the low-field mobility by only around 20%, it overestimates $\beta$ by over a factor of two over a range of temperatures and crystallographic axes. We show that the discrepancy in $\beta$ is reconciled by inclusion of on-shell iterated 2-phonon (2ph) scattering processes, indicating that scattering from higher-order electron-phonon interactions is non-negligible even in non-polar semiconductors. Further, a ~20% underestimate of the low-field mobility with 2ph scattering suggests that non-trivial cancellations may occur in the perturbative expansion of the electron-phonon interaction.

Role of Temperature on Charge Carrier Transport in Cadmium Lead Sulfide Ternary Semiconductors

Role of Temperature on Charge Carrier Transport in Cadmium Lead Sulfide Ternary Semiconductors

Symmetry-Guaranteed High Carrier Mobility in Quasi-2D Thermoelectric Semiconductors.

Quasi-2D semiconductors have garnered immense research interest for next-generation electronics and thermoelectrics due to their unique structural, mechanical, and transport properties. However, most quasi-2D semiconductors experimentally synthesized so far have relatively low carrier mobility, preventing the achievement of exceptional power output. To break through this obstacle, a route is proposed based on the crystal symmetry arguments to facilitate the charge transport of quasi-2D semiconductors, in which the horizontal mirror symmetry is found to vanish the electron-phonon coupling strength mediated by phonons with purely out-of-plane vibrational vectors. This is demonstrated in ZrBeSi-type quasi-2D systems, where the representative sample Ba1.01 AgSb shows a high room-temperature hole mobility of 344 cm2 V-1 S-1 , a record value among quasi-2D polycrystalline thermoelectrics. Accompanied by intrinsically low thermal conductivity, an excellent p-type zT of ≈1.3 is reached at 1012 K, which is the highest value in ZrBeSi-type compounds. This work uncovers the relation between electron-phonon coupling and crystal symmetry in quasi-2D systems, which broadens the horizon to develop high mobility semiconductors for electronic and energy conversion applications.

A Green Binary Solvent Method to Control Organic Semiconductor Crystallization

AbstractWhile solution‐processed small‐molecular organic semiconductors have been reported with significant research progress, their random crystal orientations and abundant defects at grain boundaries remain as a major obstacle to achieving high performance in organic electronic devices. In this paper, we demonstrate a binary solvent method that comprises a green solvent to manipulate the crystal alignment, morphology and charge transport of organic semiconductors. 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was studied as an example to showcase the enlargement of grain width and millimeter‐scale alignment of organic crystals based on the toluene/n‐amyl acetate binary solvents. At an optimized volume ratio of 3 : 1, the toluene/n‐amyl acetate binary solvent gave rise to a 4‐fold reduction in misorientation angle and approximately 40 % increase in the grain width. UV‐visible spectroscopy confirmed strong H‐aggregations with more favorable intrachain interaction and charge transport in TIPS pentacene. X‐ray diffraction showed enhanced peak intensities in (00 l) type of reflections and improved film crystallinity. Finally, thin film transistors were demonstrated to test the charge transport in the TIPS pentacene organic crystals. This work demonstrates the binary green solvent method can effectively regulate the organic semiconductor crystallization and alignment even in the absence of external alignment techniques such as solution shearing, and thereby sheds light on advancing facile organic electronic applications.

Definition of electron polar scattering angle on ionized impurities for Monte Carlo simulation of charge carrier transport in semiconductors

Procedures of polar scattering angle simulation for electron scattering on ionized impurities are examined for Brooks--Herring, Conwell--Weisskopf and Ridley models as the most frequently used in Monte Carlo simulation of charge carrier transport in semiconductors. A more correct procedure for polar scattering angle simulation is proposed for Ridley model. Peculiarities of scattering angle distribution densities calculated in the framework of regarded models are analyzed taking silicon as an example. Comparison of electron mobility calculated by ensemble Monte Carlo method using considered models has been done for doped silicon at 300 K and for constant electric field strength F = 7· 104 V/m. Keywords: ionized impurity, electron scattering, Monte Carlo method.

Разыгрывание полярного угла рассеяния электронов на ионах примеси при моделировании процессов переноса заряда в полупроводниках методом Монте-Карло

Procedures of polar scattering angle simulation for electron scattering on ionized impurities are examined for Brooks-Herring, Conwell-Weisskopf and Ridley models as the most frequently used in Monte Carlo simulation of charge carrier transport in semiconductors. A more correct procedure for polar scattering angle simulation is proposed for Ridley model. Peculiarities of scattering angle distribution densities calculated in the framework of regarded models are analyzed taking silicon as an example. Comparison of electron mobility calculated by ensemble Monte Carlo method using considered models has been done for doped silicon at 300 K and for constant electric field strength F = 7∙104 V/m

Insight into the effect of side chains on thermal transport of organic semiconductors.

Understanding the correlation mechanism of side chains on thermal transport of organic semiconductors is crucial for functionalized organic electronics. In this study, phenyl and alkyl side chains, two representatives of side chain engineering, are chosen to modify dinaphtho-[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) to synthesize Ph-DNTT and C10-DNTT. The thermal conductivities of the three organic semiconductors exhibit obvious anisotropy, and the corresponding relationships are along-chain > inter-chain > cross-chain. The phenyl side chains enhance the thermal conductivity in the along-chain direction and degrade it in the inter-chain direction, while the alkyl side chains hinder thermal transport. In the cross-chain direction, side chains have a slight effect on thermal transport. The structure orientation consistency between the phenyl side chains and the main chains in Ph-DNTT leads to phonon coupling in the along-chain direction, which improves phonon transport. In the inter-chain direction, the combined effect of the phonon group velocity and phonon participation ratio causes the thermal conductivity degeneracy of Ph-DNTT. For C10-DNTT, the vibrational mismatch between the alkyl side chains and the main chains results in the degradation of thermal transport in the along-chain and inter-chain directions. In the cross-chain direction, the nonbonding interaction dominates the energy transfer in the three organic semiconductors, which induces inferior phonon transport properties and weak effects of side chains.

Binary solvent engineering for small-molecular organic semiconductor crystallization

This article reviews the synergistic effects of engineering binary solvents on the crystallization, morphology and charge transport of organic semiconductors.

Relating band edge DOS occupancy statistics associated excited state electron entropy generation to free energy loss and intrinsic Voc deficit of solar cells.

Ever since the invention of solar cells, thermodynamics has been used to assess their performance limits and investigate advances in materials science and photovoltaic technology to reduce the gap between practical efficiencies and thermodynamic limits to photovoltaic energy conversion. By systematically addressing thermodynamic efficiency losses in current photovoltaics, ultrahigh efficiency photovoltaic can be expected. Currently, the non-radiative recombination of some ultrahigh efficient solar cells is almost completely suppressed, and the radiative recombination loss is the key to restricting the further improvement of device performance. This work relates the energy band edge electronic density of states (DOS) of semiconductor absorber and transport layer, excited/transfer state electronic entropy to thermodynamically inevitable energy loss during photoelectric conversion in solar cells. Through comprehensive theoretical analysis and device simulation, it is revealed why solar cells based on semiconductor material with a low DOS have higher Voc. On account of the basic limitations of thermodynamic laws on the energy conversion process, this work reveals a hidden variable that affects the photovoltaic performance and puts forward the band edge DOS engineering as a new dimension in performance optimization of solar cells apart from the traditional material and defect passivation engineering, etc. This work highlights the great importance of DOS engineering for further improving the performance of solar cell devices.