Behavior Of Carriers Research Articles

Organic cations promote exciton dissociation in Ruddlesden-Popper lead iodide perovskites: a theoretical study.

Two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs) are a class of quantum well (QW) materials showing large exciton binding energy owing to quantum confinement. The existence of localized edge states was proposed to accelerate exciton dissociation into long-lived charge carriers in 2D RPPs, but recent experimental reports suggested that highly efficient internal exciton dissociation is achievable in 2D RPPs despite the absence of edge states. Herein, we adopt first-principles calculations to unveil the physical origin of the high internal quantum efficiency in the bulk region of widely familiar (BA)2(MA)n-1PbnI3n+1 (BA = butylammonium; MA = methylammonium) materials. We discover that the dipolar nature of MA cations provides the driving force for the separation of photoexcited electron-hole pairs inside QWs as the inorganic layer thickens from n = 1 to n = 3. Concurrently, electronic coupling between organic spacer layers and QWs is enhanced in the energetically favorable configurations where MA cations orient with their CH3 groups towards the exterior PbI2 layers of QWs in the n = 3 structure. Consequently, hole delocalization is promoted along the out-of-plane direction of QWs, which in turn facilitates exciton dissociation into free charge carriers despite large exciton binding energy. Our simulations reveal that the hydrogen bonding between organic species (including both MA and BA cations) and iodine atoms, which is subtly interconnected, engineers the response of morphology in QWs and electronic interactions at organic-inorganic interfaces, providing novel insights for the exciton-free carrier behavior in the bulk area of 2D RPPs.

Hollow cubic CuSe@CdS with tunable size for plasmon-induced Vis-NIR driven photocatalytic properties.

The rational design of the dimension and geometry of a plasmonic semiconductor cocatalyst is vitally important for efficient utilization of near-infrared (NIR) light and superior photocatalytic hydrogen generation. Herein, hollow cubic CuSe@CdS composites with different sizes and strong localized surface plasmon resonance (LSPR) were prepared by selenizing size-tunable Cu2O templates and loading CdS nanoparticles. The size of hollow cubic CuSe can affect the surface area and the conduction band potential through the size effect, regulating the carrier behavior of the CuSe@CdS heterojunction. The CuSe@CdS composites show enhanced and wide absorption in the full spectrum due to the LSPR effect of CuSe. Meanwhile, the composites show excellent photocatalytic hydrogen capacity in the full spectrum in a 0.35 M Na2S/0.25 M Na2SO3 sacrificial reagent solution. The best hydrogen production rate of CSCE2 is 1.518 mmol g-1 h-1 (5.54 times higher than that of CdS) under Vis light (780 > λ > 420 nm) irradiation and 0.28 mmol g-1 h-1 under NIR light (λ > 780 nm) illumination. Interestingly, the photocatalytic activity for H2 under Vis-NIR light (λ > 420 nm) is about 3 times (up to 4.45 mmol g-1 h-1) higher than that without NIR light assistance, due to the photothermal effect. Various analyses and DFT calculations demonstrate that the p-n heterojunction formed in the composites consists of p-type CuSe and n-type CdS, which achieves efficient carrier transfer and separation under the synergistic effect of the size effect and the photothermal effect. In addition, the expansion of the photocatalytic performance to the NIR range is mainly due to the "hot-electron" injection mechanism induced by the LSPR effect of CuSe. The reasonable design coupled with the plasmonic materials offers a new path to achieving the highly efficient conversion of solar energy to hydrogen energy.

Electric effects reinforce charge carrier behaviour for photocatalysis

Recent studies on enhancing charge carrier behavior through electric effects for efficient photocatalysis are summarized, evaluating the in-depth function of these effects. This provides unique perspectives to optimize photocatalytic processes.

Interfacial fixed charges and dipoles to boost carrier selectivity for crystalline silicon solar cells

<p>With the rapid evolution of passivating contact, the carrier transport layer in crystalline silicon (<i>c</i>-Si) solar cells has expanded beyond the doped silicon films to non-silicon films with lower parasitic absorption and manufacturing cost. However, using the emerging carrier transport layers in direct contact with <i>c</i>-Si suffers from the insufficient carrier selectivity, which limits the efficiency of the device. Thus, to make such new strategies practical, it becomes particularly important to enhance the passivation quality and thus minimize the recombination losses at <i>c</i>-Si surfaces. By introducing interfacial fixed charges (<i>Q</i><sub>f</sub>) or dipoles on the <i>c</i>-Si surface, the local electric field can be tuned to affect the recombination and charge carrier transport dynamics. This perspective highlights the significance of interfacial <i>Q</i><sub>f</sub> and dipoles for carrier selectivity, and points out the challenges in a comprehensive understanding of the mechanism and precisely controlling the carrier behavior as well as the solar cell stability.</p>

Temperature-dependent charge carrier behavior in phosphorene quantum dots probed by terahertz time-domain spectroscopy.

Although phosphorene quantum dots (PQDs) have gained significant attention in optoelectronics and physics due to their unique optical responses, the low-frequency electromagnetic properties of PQDs and the effects of temperature still remain largely unexplored. Herein, we investigate the temperature-dependent terahertz (THz) response of PQDs by using THz time-domain spectroscopy. Effective THz conductivity of the PQD sample is extracted based on THz measurements to analyze the charge carrier behavior. It is shown that the carriers in the PQDs can be approximated as a weakly confined Drude gas of classical and noninteracting charge particles, which are described by the modified Drude-Smith formula. Then, we also obtain the temperature dependences of the effective characteristic parameters for the charge carriers. As the temperature increases, the plasma frequency linearly enhances whereas both of the carrier diffusion time and the momentum scattering time decrease, which are akin to conventional semiconductors to a large extent. In addition, the confinement factor is closed to 1 and nearly insensitive to temperature. These results are helpful to gain an in-depth understanding of the low-frequency electromagnetic response of charge carriers in PQDs and to explore new applications in photonics and optoelectronics.

Carrier behavior of a carbon material assisted TIPS-pentacene composite film for improvement of electrical conductivity

Organic semiconductor devices have a lower intrinsic carrier density than inorganic semiconductors, and improving their electrical conductivity is important for organic electronic devices.

Unraveling the origin of facet-dependent photocatalytic H2O2 production over anatase TiO2

A typical photocatalytic H2O2 production reaction primarily comprises three processes: photoabsorption, electron–hole separation and transfer, and surface oxygen reduction reaction. Previous researches have focused on revealing the origin of the facet effect in photocatalysis. However, a comprehensive understanding of the structure–activity relationship for facet effect at atomic scale remains limited. Herein, we provide fundamental insights into photocatalytic H2O2 production performance of three types of TiO2 with predominantly exposed (101), (100), or (001) facets. The TiO2 nanosheets with exposed (001) facet exhibit the highest H2O2 yield rate of 649.2 μmol g-1 h-1, which is 29.2 and 5.2 times higher than those of TiO2 with (101) facet and (100) facet, respectively. Band alignment, charge carrier behavior, and surface reaction are studied systematically and correlated with unique surface atom arrangement. Even with narrowed light-response range, short charge carrier lifetime, inferior charge separation, and inconspicuous specific surface area, the (001) facet exposed TiO2 still performs best. The experimental results and density functional theory (DFT) calculations reveal that the under-coordinated Ti atoms on (001) surface serve as active sites and have strong interaction with oxygen molecule, while the Ti atoms on (101) and (100) surface can hardly adsorb and activate oxygen molecule. Hence, we suggest that the significantly lowered barrier for oxygen molecule adsorption and activation results from intrinsic surface sites rather than band-alignment, and the charge carrier behavior plays the principal role in this case. This work may provide semiquantitative insights into the origin of facet effect in photocatalysis.

Unification of rheology of thixotropic yield stress fluids and screen printing properties of pastes

At present, the electrode width of solar cells has been successfully reduced to less than 20 µm. Ensuring its morphology and aspect ratio remains a major challenge to further reducing the electrode width, which puts higher requirements on the performance of the paste. Previous studies have focused less on selecting organic carrier components and the relationship between carrier and paste printing suitability. Therefore, in this study, considering the wettability of the solvent to the silicon wafer, the solubility to the binder and the boiling point, the selection method of the mixed solvent for the organic carrier was given. The Herschel-Buckley model was introduced to characterize the flow behavior of the organic carrier. Furthermore, the model parameters were unified with the printability of the paste and the organic carrier model parameters corresponding to the paste with good printability were contributed. The results showed that when the consistency index (K) was 21.883 and the exponent (n) was 0.1545, the aspect ratio of the electrode was the largest. When the yield stress (5.38 Pa) and thixotropic loop area (707.71 Pa s−1) were the largest, the morphology of the electrode was the most regular.

Investigation on the attrition behavior and reactivity characteristic of Fe-based oxygen carriers in long-term chemical looping catalytic oxidation process

Human activities and industrial processes result in substantial methane emissions, predominantly at low concentration. This is particularly evident in coal mining where the gas concentration is notably low, posing challenges for effective utilization. Chemical looping catalytic oxidation (CLCO) is believed to be a potential approach for the disposal of ultra-low concentration methane (UCM) and the capture of CO2. Unlike pure air, which is generally used to oxidize OC in air reactor, UCM typically contains 0.1 ∼ 1 %CH4 and less than 60 ppm H2S, therefore, the introduction of UCM into the air reactor would potentially threaten oxygen carrier (OC). In this study, the evolution of OC in CLCO process was investigated in terms of reaction performance and physicochemical parameters compared to traditional chemical looping process. XRD showed that the phase compositions of OC remained stable over 300 cycles and no sulfur-containing compounds were detected. The comparison showed that CLCO process negatively affected the performance of OC, especially for attrition. The attrition behavior of OC can be summarized in three stages: fines blowing out, steady-state attrition and intense-attrition stage. The main reason for increased attrition was attributed to methane oxidation, which released a large amount of heat and would impose a heat shock on OC. Contrary to the commonly reported “L”-shape, our data showed a “U”-shaped attrition rate trend in long-term cycles. The difference was that the attrition rate would suddenly increase after a certain number of cycles, indicating that the actual service life of OC may be significantly lower than predicted.

Efficient Perovskite Light‐Emitting Diodes Enabled by Nickel Acetate Interlayer

AbstractMetal halide perovskite light‐emitting diodes (Pero‐LEDs) have gained great attention due to their promising applications in lighting and displays. However, in their conventional three‐layered sandwich structure, some undeserved carrier behaviors, such as imbalanced carrier injection, severe carrier loss, and unstable recombination zone, limit the device's performance. Herein, a four‐layered design by inserting a nickel acetate (Ni(OAc)2) interlayer between the emitter and hole‐transport layer(HTL) to manage carrier behavior and improve radiative recombination efficiency is proposed. Specifically, the Ni(OAc)2 interlayer is poorly conductive and can partially block the hole injection, making the hole‐electron injection more balanced. And the Ni(OAc)2 interlayer avoids the direct contact between the perovskite emitter and hole transporter, reducing the interfacial carrier quenching. Moreover, the Ni(OAc)2 interlayer inhibits the electron‐migrated recombination at the hole transporter interface, confining the carrier recombination zone in the emitter layer. As a result, the corresponding Pero‐LEDs achieve a maximum external quantum efficiency (EQEmax) of 24.6% with good reproducibility, showing an average EQEmax of over 20%.

First-principles study of electronic, optical, magnetic, and thermoelectric properties of novel Sr2UXO6 (X = Mn, Zn) double perovskites

Based on Density Functional Theory, the first-principles investigation is carried out to calculate the optoelectronic, magnetic, and thermoelectric transport properties of novel Sr2UXO6 (X = Mn, Zn) double perovskites in their monoclinic structural phase. The electronic band structures and DOS analysis investigations suggest that these materials have a semiconducting nature. To investigate the magnetic behavior spin-polarized calculations have been carried out. The magnetic studies predict the antiferromagnetic (AFM) nature of Sr2UMnO6 and the non-magnetic (NM) nature of Sr2UZnO6. The electrical conductivity analysis demonstrated semiconducting behavior for both materials, with both materials exhibiting n-type conductivity. The Seebeck coefficient indicated the shift in carrier behavior, while thermal conductivity revealed distinct temperature-dependent trends. These results contribute to understanding and optimizing the thermoelectric performance of these materials for energy conversion applications. The optical parameters including both the real and the imaginary part of the complex dielectric function, absorption coefficient, refractive index, energy loss, and reflectivity in the energy 0–14 eV have been calculated and discussed. The optical results show that the materials demonstrate great potential for applications in optoelectronics due to their notable energy absorption characteristics.

Operando dynamics of trapped carriers in perovskite solar cells observed via infrared optical activation spectroscopy

Conventional spectroscopies are not sufficiently selective to comprehensively understand the behaviour of trapped carriers in perovskite solar cells, particularly under their working conditions. Here we use infrared optical activation spectroscopy (i.e., pump-push-photocurrent), to observe the properties and real-time dynamics of trapped carriers within operando perovskite solar cells. We compare behaviour differences of trapped holes in pristine and surface-passivated FA0.99Cs0.01PbI3 devices using a combination of quasi-steady-state and nanosecond time-resolved pump-push-photocurrent, as well as kinetic and drift-diffusion models. We find a two-step trap-filling process: the rapid filling (~10 ns) of low-density traps in the bulk of perovskite, followed by the slower filling (~100 ns) of high-density traps at the perovskite/hole transport material interface. Surface passivation by n-octylammonium iodide dramatically reduces the number of trap states (~50 times), improving the device performance substantially. Moreover, the activation energy (~280 meV) of the dominant hole traps remains similar with and without surface passivation.

Stabilized Huang-Rhys factor in non-doped exciplex-based Firpic phosphorescent organic light emitting diodes via quantum well-like multiple emissive layer structure and investigation of carrier behaviors by transient electroluminescence

Stabilized Huang-Rhys factor in non-doped exciplex-based Firpic phosphorescent organic light emitting diodes via quantum well-like multiple emissive layer structure and investigation of carrier behaviors by transient electroluminescence

Optical tuning of the terahertz response of black phosphorus quantum dots: effects of weak carrier confinement

Abstract In contrast to few-layer black phosphorus (BP) with a relatively larger area, BP quantum dots (BP-QDs) are expected to have distinctive electromagnetic response and carrier behaviors, especially in low-frequency range such as in the THz regime. Herein, we experimentally investigate the THz properties of BP-QDs as well as the optical control of these properties. It is demonstrated that the effects of weak carrier confinement, which is associated with diffusive restoring current in each BP-QD, contribute significantly to the effective THz conductivity of BP-QDs. Instead, spectral features of discretely spaced energy levels as shown for many kinds of semiconductor QDs in UV-visible range are not observed in the THz regime. This indicates an insignificant contribution of strong quantum confinement here. Based on the modified Drude–Smith formula, we show that the optical excitation/pump of a CW laser can induce photogenerated carriers and enhance the effects of weak carrier confinement in BP-QDs. Thus, a nonlinear enhancement of THz absorption can be observed by increasing the power of the excitation laser. These results not only deepen our understanding of the fundamental physics of BP nanomaterials but also provide an alternative approach to realize active control of BP-based THz devices.

Quantitative characterization of built-in potential profile across GaAs p–n junctions using Kelvin probe force microscopy with qPlus sensor AFM

The electrostatic potential distribution in materials and devices plays an important role in controlling the behaviors of charge carriers. Kelvin probe force microscopy (KPFM) is a powerful technique for measuring the surface potential at a high spatial resolution. However, the measured surface potential often deviates from the potential deep in the bulk owing to certain factors. Here, we performed KPFM measurements across the p–n junction, in which such factors were eliminated as much as possible by selecting the sample, force sensor, and measurement mode. The measured surface potential distribution agrees well with the line shape of the simulated bulk potential. Our results demonstrate that KPFM is capable of quantitatively characterizing potential distributions whose changes occur on the order of 10 nm.

Probing Charge Carrier Behavior in Engineered Photocatalysts with Time-Resolved Visible to Mid-IR Absorption Spectroscopy

Probing Charge Carrier Behavior in Engineered Photocatalysts with Time-Resolved Visible to Mid-IR Absorption Spectroscopy

Some features of the behavior of energy carriers during transients in an open thermodynamic system

Some features of the behavior of energy carriers during transients in an open thermodynamic system

Self-assembly tourmaline@BiFeO3 composites with enhanced polarization for dual-selective C3H6O and H2S detection

Ferroelectric oxides with great polarization have attracted much attention due to the polarization modulated carrier behaviors causing high gas sensitivity in detecting hazardous toxic gases. In this work, tourmaline@BiFeO3 (T@BFO) composites were fabricated by the sol-gel method based on the electrostatic self-assembly strategy for the C3H6O and H2S detection. It is found that BFO nanoparticles are uniformly distributed on tourmaline due to the strong electrostatic field of tourmaline. In comparison with pure BFO, the T@BFO composites showed an improved gas sensitivity. More importantly, the sensor based on 3.0-T@BFO can effectively detect C3H6O at 150 °C and H2S at 210 °C in limits of detection as low as 50 and 7 ppb, respectively. The results exhibit that 3.0-T@BFO composites have lower activation energies (Eact) of C3H6O at 150 °C and H2S at 210 °C, which endow 3.0-T@BFO with dual-selectivity for C3H6O and H2S. The Ex-situ XPS and density functional theory calculations further reveal that tourmaline activates the sensing site of BFO to achieve selective adsorption of C3H6O at Bi, Fe, and oxygen vacancies sites and H2S at surface lattice oxygens, Fe and oxygen vacancies sites. These results provide an efficient strategy to design practice multifunctional and highly sensitive gas sensors for detecting hazardous toxic gases.

Application of running fringes technique for measurement of photocarriers transport parameters in perovskite films

This study aims to contribute to the development of theoretical and experimental tools for characterizing the transport properties of perovskite semiconductors. In the context of existing transport characterization methods for perovskites, there is a need for techniques that can accurately assess the critical transport parameters, such as diffusion lengths, given the specific challenges posed, such as their inherent instabilities. The novel methodology employed involves the development of a theoretical model to describe the running fringes-induced photo-electromotive force (RF photo-EMF) effect in bipolar photoconductors with a rather general type of photoconductivity relaxation behaviors for both carriers. This model is founded on the theory of photoinduced space charge grating formation in semiconductors. Subsequently, RF photo-EMF experiments were conducted on methylammonium lead iodide (CH3NH3PbI3 or MAPI) polycrystalline films of varying grain sizes. By utilizing the RF photo-EMF technique, we successfully elucidated crucial transport and recombination characteristics, notably the ambipolar diffusion length and relaxation times of the charge carriers. Significantly, the developed theoretical model exhibited a remarkable agreement with the experimental results, highlighting its ability in explaining and predicting the behavior of charge carriers in perovskite semiconductors. The results of this study make a substantial contribution to the field of perovskite semiconductors by offering a novel theoretical and experimental approach to characterization of perovskites’ transport properties.

Theoretical Dissection of Cation-Deficient Layered Ruddlesden-Popper Oxysulfide Perovskites with a High-Efficiency Carrier Transport Channel.

The search for lead-free perovskite materials has triggered intensive interest. Here, we study the electronic structures and optical properties of cation-deficient Ruddlesden-Popper oxysulfide perovskites Ln2Ti2O5S2 (Ln = Sc, Y, or La), with a tunable band gap of 1.45-2.1 eV and a small exciton binding energy of ∼0.1 eV, among which Y2Ti2O5S2 has been synthesized experimentally. Sc2Ti2O5S2 possesses the largest light absorbance in the visible region. We further rationalize the light absorption via the transition dipole moment and suggest potential applications of Sc2Ti2O5S2 in solar cells and Y2Ti2O5S2 and La2Ti2O5S2 in water splitting. In addition, this family exhibits small effective masses within the x-y plane and large ones along the z direction. Most importantly, electron gas-like carrier behaviors are observed within the Ti-O bond region, offering a diffusion channel for electron transport. These findings greatly advance our understanding of lead-free perovskites and offer a novel material platform for future optoelectronic devices.