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In recent years, halide perovskite solar cells (HPSCs) have attracted a great attention due to their superior photoelectric performance and the low‐cost of processing their quality films. In order to commercialize HPSCs, the researchers are focusing on developing high‐performance HPSCs. Many strategies have been reported to increase the power conversion efficiency and the long‐term stability of HPSCs over the past decade. Herein, we review the latest efforts and the chemical‐physical principles for preparing high‐efficiency and long‐term stability HPSCs in particular, concentrating on the perovskite materials, technologies for perovskite films, charge transport materials and ferroelectric effect to reduce the carrier loss, and photon management via plasmonic and upconversion effects. Finally, the key issues for future researches of HPSCs are also discussed with regard to the requirements in practical application. This review mainly reported the recent efforts to develop the high‐performance halide perovskites solar cells with high‐efficiency and long‐term stability by perovskite engineering (including crystal structure, dimension, and components of halide perovskites), film engineering (including preparation technology and defect passivation), carrier engineering (including charge transport materials and ferroelectric effect), and photon engineering (including plasmonic and upconversion).

We study the carrier dynamics in planar methyl ammonium lead iodide perovskite films using broadband transient absorption spectroscopy. We show that the sharp optical absorption onset is due to an exciton transition that is inhomogeneously broadened with a binding energy of 9 meV. We fully characterize the transient absorption spectrum by free-carrier-induced bleaching of the exciton transition, quasi-Fermi energy, carrier temperature and bandgap renormalization constant. The photo-induced carrier temperature is extracted from the transient absorption spectra and monitored as a function of delay time for different excitation wavelengths and photon fluences. We find an efficient hot-phonon bottleneck that slows down cooling of hot carriers by three to four orders of magnitude in time above a critical injection carrier density of ∼5 × 10 17  cm −3 . Compared with molecular beam epitaxially grown GaAs, the critical density is an order of magnitude lower and the relaxation time is approximately three orders of magnitude longer. Hot carriers in perovskites experience slow cooling.

In recent years, the investigation of the complex interplay between the nanostructure and photo-transport mechanisms has become of crucial importance for the development of many emerging photovoltaic technologies. In this context, Kelvin probe force microscopy under frequency-modulated excitation has emerged as a useful technique for probing photo-carrier dynamics and gaining access to carrier lifetime at the nanoscale in a wide range of photovoltaic materials. However, some aspects about the data interpretation of techniques based on this approach are still the subject of debate, for example, the plausible presence of capacitance artifacts. Special attention shall also be given to the mathematical model used in the data-fitting process as it constitutes a determining aspect in the calculation of time constants. Here, we propose and demonstrate an automatic numerical simulation routine that enables to predict the behavior of spectroscopy curves of the average surface photovoltage as a function of a frequency-modulated excitation source in photovoltaic materials, enabling to compare simulations and experimental results. We describe the general aspects of this simulation routine and we compare it against experimental results previously obtained using single-point Kelvin probe force microscopy under frequency-modulated excitation over a silicon nanocrystal solar cell, as well as against results obtained by intensity-modulated scanning Kelvin probe microscopy over a polymer/fullerene bulk heterojunction device. Moreover, we show how this simulation routine can complement experimental results as additional information about the photo-carrier dynamics of the sample can be gained via the numerical analysis.

Developing multijunction perovskite solar cells (PSCs) is an attractive route to boost PSC efficiencies to above the single-junction Shockley-Queisser limit. However, commonly used tin-based narrow-bandgap perovskites have shorter carrier diffusion lengths and lower absorption coefficient than lead-based perovskites, limiting the efficiency of perovskite-perovskite tandem solar cells. In this work, we discover that the charge collection efficiency in tin-based PSCs is limited by a short diffusion length of electrons. Adding 0.03 molar percent of cadmium ions into tin-perovskite precursors reduce the background free hole concentration and electron trap density, yielding a long electron diffusion length of 2.72 ± 0.15 µm. It increases the optimized thickness of narrow-bandgap perovskite films to 1000 nm, yielding exceptional stabilized efficiencies of 20.2 and 22.7% for single junction narrow-bandgap PSCs and monolithic perovskite-perovskite tandem cells, respectively. This work provides a promising method to enhance the optoelectronic properties of narrow-bandgap perovskites and unleash the potential of perovskite-perovskite tandem solar cells. Tin-based perovskites possess the suitable narrow-bandgap for tandem solar cells but their short carrier diffusion lengths limit device efficiency. Here Yang et al . add cadmium ions to increase diffusion length to above 2 µm by reducing the background free hole concentration and electron trap density.

In this work, we demonstrated the successful construction of metal-free zero- dimensional/two-dimensional carbon nanodot （CND）-hybridized protonatedg=C3N4 （pCN） （CND/pCN） heterojunction photocatalysts b; means of electrostatic attraction. We experimentally found that CNDs with an average diameter of 4.4 nm were uniformly distributed on the surface of pCN using electron microscopy analysis. The CND/pCN-3 sample with a CND content of 3 wt.% showed thehighest catalytic activity in the CO2 photoreduction process under visible and simulated solar light. This process results in the evolution of CH4 and CO. Thetotal amounts of CH4 and CO generated by the CND/pCN-3 photocatalyst after 10 h of visible-light activity were found to be 29.23 and 58.82 molgcatalyst-1, respectively. These values were 3.6 and 2.28 times higher, respectively, than thearn＊ounts generated when using pCN alone. The corresponding apparent quantum efficiency （AQE） was calculated to be 0.076%. Furthermore, the CND/pCN-3 sample demonstrated high stability and durability after four consecutive photoreaction cycles, with no significant decrease in the catalytic activity.

Topological semimetals have revealed a wide array of novel transport phenomena, including electron hydrodynamics, quantum field theoretic anomalies, and extreme magnetoresistances and mobilities. However, the scattering mechanisms central to the fundamental transport properties remain largely unexplored. Here, we reveal signatures of significant phonon-electron scattering in the type-II Weyl semimetalWP2via temperature-dependent Raman spectroscopy. Over a large temperature range, we find that the decay rates of the lowest energyA1modes are dominated by phonon-electron rather than phonon-phonon scattering. In conjunction with first-principles calculations, a combined analysis of the momentum, energy, and symmetry-allowed decay paths indicates this results from finite momentum interband and intraband scattering of the electrons. The excellent agreement with theory further suggests that such results could be true for the acoustic modes. We thus provide evidence for the importance of phonons in the transport properties of topological semimetals and identify specific properties that may contribute to such behavior in other materials.

Van der Waals heterojunctions are fast‐emerging quantum structures fabricated by the controlled stacking of two‐dimensional (2D) materials. Owing to the atomically thin thickness, their carrier properties are not only determined by the host material itself, but also defined by the interlayer interactions, including dielectric environment, charge trapping centers, and stacking angles. The abundant constituents without the limitation of lattice constant matching enable fascinating electrical, optical, and magnetic properties in van der Waals heterojunctions toward next‐generation devices in photonics, optoelectronics, and information sciences. This review focuses on the charge and energy transfer processes and their dynamics in transition metal dichalcogenides (TMDCs), a family of quantum materials with strong excitonic effects and unique valley properties, and other related 2D materials such as graphene and hexagonal‐boron nitride. In the first part, we summarize the ultrafast charge transfer processes in van der Waals heterojunctions, including its experimental evidence and theoretical understanding, the interlayer excitons at the TMDC interfaces, and the hot carrier injection at the graphene/TMDCs interface. In the second part, the energy transfer, including both Förster and Dexter types, are reviewed from both experimental and theoretical perspectives. Finally, we highlight the typical charge and energy transfer applications in photodetectors and summarize the challenges and opportunities for future development in this field. As a fast‐emerging platform, van der Waals heterojunctions have exhibited exotic carrier dynamics in the quantum limit, including charge and energy transfer. Based on the recent experimental and theoretical progress, this review summarizes the state‐of‐art understanding, followed by the representative applications in optoelectronic devices. We also summarize the remaining challenges and opportunities for future development in this field.

Simultaneous optimization of surface and interlayer characteristics of graphite-phase carbon nitride (g-C 3 N 4 ) is crucial for enhanced photogenerated-carrier separation efficiency. Integration of distinct strategies with specific merits for constructing efficacious charge carrier transport pathways from bulk to surface faces challenges. Herein, we proposed a novel carboxyl functional group and potassium (K) ions co-modified g-C 3 N 4 for steering dynamic charge transfer processes. Specifically, carboxyl functional groups were grafted to the surface to substantially improve charge carrier dynamics through the driving force induced by its electron-withdrawing effects. Concurrently, K ions were inserted into the interlayers of g-C 3 N 4 to facilitate interlayer carrier transport by bridging adjacent layers. Such a bi-functional photocatalyst achieves 8.68-fold increase in CO yield compared wtih the pristine g-C 3 N 4 without any cocatalyst or sacrificial agent. This work provides a profound discernment into the directional transport of charge carriers within the surface and interlayers, and presents a promising approach for rational design of photocatalysts with remarkably efficient solar energy conversion.

We demonstrate ultrafast terahertz (THz) field emission from a tungsten nanotip enabled by local field enhancement. Characteristic electron spectra which result from acceleration in the THz near-field are found. Employing a dual frequency pump-probe scheme, we temporally resolve different nonlinear photoemission processes induced by coupling near-infrared (NIR) and THz pulses. In the order of increasing THz field strength, we observe THz streaking, THz-induced barrier reduction (Schottky effect) and THz field emission. At intense NIR-excitation, the THz field emission is used as an ultrashort, local probe of hot electron dynamics in the apex. A first application of this scheme indicates a decreased carrier cooling rate in the confined tip geometry. Summarizing the results at various excitation conditions, we present a comprehensive picture of the distinct regimes in ultrafast photoemission in the near- and far-infrared.

Time-resolved photoluminescence (TRPL) has been extensively used to measure the carrier lifetime in lead halide perovskites. The TRPL curves of perovskite materials are usually fitted with a multi-exponential function, instead of a single exponential one. This was considered to be a result of the surface and the bulk recombination or the additional radiative recombination caused by the high excited carrier density. Here, a new model considering the diffusion and the trap-assisted recombination of carriers is proposed to explain the TRPL curves. The expressions of the TRPL curves and the transient absorption (TA) dynamic curves are theoretically derived, demonstrating that the TRPL curve is an infinite exponential series, regardless of the presence of surface recombination or not. Our newly developed highly sensitive nanosecond TA and TRPL were employed to measure the carrier dynamics of the same sample under low illumination in the linear response region of TA, thereby experimentally verifying our model. These results suggest that the decay of the TRPL is not only a consequence of the carrier recombination but also the carrier diffusion. TRPL cannot provide a direct measurement of the carrier lifetime, whereas TA spectroscopy can. Furthermore, the surface and the bulk recombination can be resolved and the average diffusion coefficient ( D ¯ ) can also be correctly obtained by combining TRPL and TA measurements. We also propose an approximate method for calculating the carrier lifetime and diffusion coefficient of high-quality perovskite films. Our model provides not only a new interpretation of the dynamics of the PL decay but also a deep insight into the carrier dynamics in the nanosecond time scale under working condition of perovskites solar cells.