The Electrical Behaviors of Grain Boundaries in Polycrystalline Optoelectronic Materials

Ultrasensitive Deep-Ultraviolet Photodetectors Based On Band Engineering and Ferroelectric Modulation

Wide bandgap semiconductors have emerged as a class of deep-ultraviolet sensitive materials, showing great potentials for next-generation integrated devices. Yet, to achieve a high photoresponse of deep-ultraviolet detector without complicated designs at low supply voltage and weak light intensity is challenging. Herein, a new way is designed to fabricate an ultrasensitive vertical-structured photodetector with epitaxial 7 nm BaTiO3 interlayer and 10 nm Ga2O3 photosensitive layer, realizing the detection to a rare weak deep UV light intensity (0.1 µW cm- 2) at a voltage under 4.8 V and demonstrating a surge in responsivity up to 1.1 A W-1 with ultrafast response of 0.24 µs/33.4 µs (rise/decay). A responsivity of 3.8 mA W-1 at 0 V also has been reached. The dark current is suppressed by enlarged conduction band offset and meanwhile the photocurrent is enhanced by unidirectional conducting valance band offset, which formed by BaTiO3 interlayer. BaTiO3 also contributes most to the photoresponse at 0 V through its ferroelectric depolarization electric field. These results provide a path toward high-sensitive, low-power-consumption, and highly-integrated deep-ultraviolet detection, beyond conventional ones.

Emerging Thermal Detectors Based on Low-Dimensional Materials: Strategies and Progress

Thermal detectors, owing to their broadband spectral response and ambient operating temperature capabilities, represent a key technological avenue for surpassing the inherent limitations of traditional photon detectors. A fundamental trade-off exists between the thermal properties and the response performance of conventional thermosensitive materials (e.g., vanadium oxide and amorphous silicon), significantly hindering the simultaneous enhancement of device sensitivity and response speed. Recently, low-dimensional materials, with their atomically thin thickness leading to ultralow thermal capacitance and tunable thermoelectric properties, have emerged as a promising perspective for addressing these bottlenecks. Integrating low-dimensional materials with metasurfaces enables the utilization of subwavelength periodic configurations and localized electromagnetic field enhancements. This not only overcomes the limitation of low light absorption efficiency in thermal detectors based on low-dimensional materials (TDLMs) but also imparts full Stokes polarization detection capability, thus offering a paradigm shift towards multidimensional light field sensing. This review systematically elucidates the working principle and device architecture of TDLMs. Subsequently, it reviews recent research advancements in this field, delving into the unique advantages of metasurface design in terms of light localization and interfacial heat transfer optimization. Furthermore, it summarizes the cutting-edge applications of TDLMs in wideband communication, flexible sensing, and multidimensional photodetection. Finally, it analyzes the major challenges confronting TDLMs and provides an outlook on their future development prospects.

Grain-Boundary-Rich Pt/Co3O4 Nanosheets for Solar-Driven Overall Water Splitting

Interfacial engineering is considered an effective strategy to improve the electrochemical water-splitting activity of catalysts by modulating the local electronic structure to expose more active sites. Therefore, we report a platinum-cobaltic oxide nanosheets (Pt/Co3O4 NSs) with plentiful grain boundary as the efficient bifunctional electrocatalyst for water splitting. The Pt/Co3O4 NSs exhibit a low overpotential of 55 and 201 mV at a current density of 10 mA cm-2 for the hydrogen evolution reaction and oxygen evolution reaction in 1.0 M potassium hydroxide, respectively. A negligible degradation of 1.52 V at a current density of 10 mA cm-2 after continuous operation for 100 h, demonstrates the long-term stability of the catalyst. Furthermore, the overall water-splitting performance of the Pt/Co3O4 NSs surpasses that of the commercial Pt/C||RuO2. The density functional theory calculation results explain that the improvement of catalyst activity is attributed to the moderate adsorption/desorption energy of *H and the low reaction energy barrier of the rate-determining step. This work presents a novel vision to design bifunctional catalysts for the storage and conversion of hydrogen energy.

Grain-Boundary Elimination via Liquid Medium Annealing toward High-Efficiency Sb2Se3 Solar Cells

Suppression of charge recombination caused by unfavorable grain boundaries (GBs) in polycrystalline thin films is essential for improving the optoelectronic performance of semiconductor devices. For emerging antimony selenide (Sb2Se3) materials, the unique quasi-1D structure intensifies the dependence of GB properties on the grain size and orientation, which also increases the impact of defects related to grain structure on device performance. However, these characteristics pose significant challenges in the preparation of thin films. In this study, a novel annealing approach using ammonia-thiourea is developed mixed solution as the liquid medium (LM) to finely regulate the crystallization of Sb2Se3 films, resulting in micron-sized large grains with enhanced [hk1] orientation and fewer defects. Mechanistic studies indicate that the intermediate phase formed at the GBs promotes the growth of large grains. Moreover, LM creates a closed and uniform environment for thin-film annealing, suppressing the volatilization of Se and reducing the types of deep-level defects. Consequently, the film delivers a device efficiency of 9.28%, the highest efficiency achieved for Sb2Se3 solar cells fabricated via thermal evaporation. Hence, this study provides a facile and effective annealing method for controlling the crystallization of low-dimensional materials and offers valuable guidance for the development of chalcogenide materials.

Surface Defects Control Bulk Carrier Densities in Polycrystalline Pb-Halide Perovskites

The (opto)electronic behavior of semiconductors depends on their (quasi-)free electronic carrier densities. These are regulated by semiconductor doping, i.e., controlled "electronic contamination". For metal halide perovskites (HaPs), the functional materials in several device types, which already challenge some of the understanding of semiconductor properties, this study shows that doping type, density and properties derived from these, are to a first approximation controlled via their surfaces. This effect, relevant to all semiconductors, and already found for some, is very evident for lead (Pb)-HaPs because of their intrinsically low electrically active bulk and surface defect densities. Volume carrier densities for most polycrystalline Pb-HaP films (<1 µm grain diameter) are below those resulting from even < 0.1% of surface sites being electrically active defects. This implies and is consistent with interfacial defects controlling HaP devices in multi-layered structures with most of the action at the two HaP interfaces. Surface and interface passivation effects on bulk electrical properties are relevant to all semiconductors and are crucial for developing those used today. However, because bulk dopant introduction in HaPs at controlled ppm levels for electronic-relevant carrier densities is so difficult, passivation effects are vastly more critical and dominate, to first approximation, their optoelectronic characteristics in devices.

Scanning Probe Microscopy of Halide Perovskite Solar Cells

Scanning probe microscopy (SPM) has enabled significant new insights into the nanoscale and microscale properties of solar cell materials and underlying working principles of photovoltaic and optoelectronic technology. Various SPM modes, including atomic force microscopy, Kelvin probe force microscopy, conductive atomic force microscopy, piezoresponse force microscopy, and scanning near-field optical microscopy, can be used for the investigation of electrical, optical and chemical properties of associated functional materials. A large body of work has improved the understanding of solar cell device processing and synthesis in close synergy with SPM investigations in recent years. This review provides an overview of SPM measurement capabilities and attainable insight with a focus on recently widely investigated halide perovskite materials.

On the grain boundary charge transport in p-type polycrystalline nanoribbon transistors

Grain boundaries (GB) profoundly influence charge transport, and their localized potential barrier with a high density of defect states plays a crucial role in polycrystalline materials. There are a couple of models to estimate the density of states (DoS) of nanostructured materials in field-effect transistors (FETs) that probe interface traps between the semiconductor and dielectric but not at the grain boundaries. Here, we report on utilizing Levinson's and Seto's models of grain boundary transport and correlate them with the temperature-dependent hopping transport in copper iodide (CuI) polycrystalline nanoribbon (PNR) FETs. Experimentally, PNRs are obtained by e-beam lithography and thermal evaporation of CuI. To investigate the impact of GB, the devices are fabricated with different channel aspect ratios by varying widths (80, 260, and 570 nm) and lengths (20 to 90 μm). Owing to the high hole concentration, PNR FETs operate in depletion mode at 300 K. At various low temperatures (80-300 K), the figures-of-merits of FETs are estimated to understand device performance. We determine GB barrier heights, activation energy, and density of GB trap states and find equivalence between the two models. Furthermore, we calculate temperature-dependent hopping and trap-limited transport parameters to obtain DoS at the Fermi energy, trapped and free charge carrier density, localization length, hopping distance, hopping energy, etc. at various channel lengths. Based on this quantitative analysis, we propose a channel length-dependent GB barrier height variation due to the in-plane electric field and elucidate CuI energy band levels.

Study of rutile TiO2(110) single crystal by transient absorption spectroscopy in the presence of Ce4+cations in aqueous environment. Implication on water splitting

Ce4+cations are commonly used as electron acceptors during the water oxidation to O2reaction over Ir- and Ru-based catalysts. They can also be reduced to Ce3+cations by excited electrons from the conduction band of an oxide semiconductor with a suitable energy level. In this work, we have studied their interaction with a rutile TiO2(110) single crystal upon band gap excitation by femtosecond transient absorption spectroscopy (TAS) in solution in the 350-900 nm range and up to 3.5 ns. Unlike excitation in the presence of water alone the addition of Ce4+resulted in a clear ground-state bleaching (GSB) signal at the band gap energy of TiO2(ca. 400 nm) with a time constantt= 4-5 ps. This indicated that the Ce4+cations presence has quenched the e-h recombination rate when compared to water alone. In addition to GSB, two positive signals are observed and are attributed to trapped holes (in the visible region, 450-550 nm) and trapped electrons in the IR region (>700 nm). Contrary to expectation, the lifetime of the positive signal between 450 and 550 nm decreased with increasing concentrations of Ce4+. We attribute the decrease in the lifetime of this signal to electrostatic repulsion between Ce4+at the surface of TiO2(110) and positively charged trapped holes. It was also found that at the very short time scale (<2-3 ps) the fast decaying TAS signal of excited electrons in the conduction band is suppressed because of the presence of Ce4+cations. Results point out that the presence of Ce4+cations increases the residence time (mobility) of excited electrons and holes at the conduction band and valence band energy levels (instead of being trapped). This might provide further explanations for the enhanced reaction rate of water oxidation to O2in the presence of Ce4+cations.

Tuning Surface Electronics State of P-Doped In2.77S4/In(OH)3 toward Efficient Photoelectrochemical Water Oxidation

Indium sulfide with a two-dimensional layered structure offers a platform for catalyzing water oxidation by a photoelectrochemical process. However, the limited hole holders hinder the weak intrinsic catalytic activity. Here, the nonmetallic phosphorus atom is coordinated to In2.77S4/In(OH)3 through a bridge-bonded sulfur atom. By substituting the S position by the P dopant, the work function (surface potential) is regulated from 445 to 210 mV, and the lower surface potential is shown to be beneficial for holding the photogenerated holes. In2.77S4/In(OH)3/P introduces a built-in electric field under the difference of Fermi energy, and the direction is from the bulk to the surface. This band structure results in upward band bending at the interface of In2.77S4/In(OH)3 and P-doped sites, which is identified by density functional theory calculations (∼0.8 eV work function difference). In2.77S4/In(OH)3/P stands out with the highest oxidation efficiency (ηoxi = 70%) and charge separation efficiency (ηsep = 69%). Importantly, it delivers a remarkable water oxidation photocurrent density of 2.51 mA cm-2 under one sun of illumination.