Thin Film Counterparts Research Articles

Impact of radiation damage on the photoconductor and photodiode properties of GaN core–shell p–n junction microwires

GaN nano- and microwires have gained increasing interest due to their unique geometry and flexibility, and have already been successfully applied in optoelectronics. One important advantage is their superior crystalline quality in comparison to their thin-film counterparts. Beside this, GaN is also known for its high radiation hardness, owed to the large displacement energy of the atoms in its crystal lattice and efficient dynamic annealing properties. In this work, single GaN core–shell p–n junction microwires have been processed into radiation detectors and the induced damage effects on the electrical and optoelectrical characteristics, when irradiating the detectors with 1 and 2 MeV protons, have been studied. By measuring the dark and photocurrent as a function of the irradiation fluence we show that for fluences up to 1×1014 protons/cm2 there is no significant modification of any of the parameters and that serious degradation only occurs for fluences above 1×1015 protons/cm2. Furthermore, we also observe that the dark current decays at a lower irradiation fluence with respect to the photocurrent due to the different impact of the created irradiation defects on the properties of the neutral region and the p–n junction. As a consequence, the signal-to-noise ratio of the detector is strongly enhanced, especially when applying a higher reverse bias. Thermal treatments furthermore show that the degree of recovery of the properties of the detectors is strongly dependent on the irradiation fluence. The detector irradiated with a fluence of 5×1014 protons/cm2 almost completely retrieved its pre-irradiation characteristics whereas the detector irradiated with 5×1015 protons/cm2 only showed a minimal recovery.

Microscopic insights into metal diffusion and ohmic contact formation in delta-doped GaAs/(Al,Ga)As core/shell nanowires

Semiconductor nanowires (NWs) are promising candidates for use in electronic and optoelectronic applications, offering numerous advantages over their thin film counterparts. Their performance relies heavily on the quality of the contacts to the NW, which should exhibit ohmic behavior with low resistance and should be formed in a reproducible manner. In the case of heterostructure NWs for high-mobility applications that host a two-dimensional electron gas, ohmic contacts are particularly challenging to implement since the NW core constituting the conduction channel is away from the NW surface. We investigated contact formation to modulation-doped GaAs/(Al,Ga)As core/shell NWs using scanning transmission electron microscopy, energy dispersive x-ray spectroscopy and electron tomography to correlate microstructure, diffusion profile and chemical composition of the NW contact region with the current–voltage (I–V) characteristics of the contacted NWs. Our results illustrate how diffusion, alloying and phase formation processes essential to the effective formation of ohmic contacts are more intricate than in planar layers, leading to reproducibility challenges even when the processing conditions are the same. We demonstrate that the NW geometry plays a crucial role in the creation of good contacts. Both ohmic and rectifying contacts were obtained under nominally identical processing conditions. Furthermore, the presence of Ge in the NW core, in the absence of Au and Ni, was found as the key factor leading to ohmic contacts. The analysis contributes to the current understanding of ohmic contact formation to heterostructure core/shell NWs offering pathways to enhance the reproducibility and further optimization of such NW contacts.

Ultrastable N-Type Semiconducting Fiber Organic Electrochemical Transistors for Highly Sensitive Biosensors.

Organic electrochemical transistors (OECTs) have attracted increasing attention due to their merits of high transconductance, low operating voltage, and good biocompatibility, ideal for biosensors. However, further advances in their practical applications face challenges of low n-type performance and poor stability. Here, it is demonstrated that wet-spinning the commercially available n-type conjugated polymer poly(benzimidazobenzophenanthroline) (BBL) into highly aligned and crystalline fibers enhances both OECT performance and stability. Although BBL is only soluble in high-boiling-point strong acids, it can be wet-spun into high-quality fibers with adjustable diameters. The BBL fiber OECTs exhibit a record-high area-normalized transconductance (gm,A) of 2.40 µS µm-2 and over 10 times higher figure-of-merit (µC*) than its thin-film counterparts. More importantly, these fiber OECTs exhibit remarkable stability with no noticeable performance attenuation after 1500 cycles over 4h operation, outperforming all previously reported n-type OECTs. The superior performance and stability can be attributed to shorter π-π stacking distance and ordered molecular arrangement in the fibers, endowing the BBL fiber OECT-based biosensors with outstanding sensitivity while keeping a miniaturized form factor. This work demonstrates that, beyond new material development, developing new fabrication technology is also crucial for addressing the performance and stability issues in n-type OECTs.

Can thick metal-halide perovskite single crystals have narrower optical bandgaps with near-infrared absorption?

Metal-halide perovskite (MHP) single crystals are emerging as potential competitors to their polycrystalline thin-film counterparts. These materials have shown the specific feature of extended absorbance towards the near-infrared (NIR) region, which promises further extension of their applications in the field of photovoltaics and photodetectors. This notable expansion of absorbance has been explained by the narrower effective optical bandgap of MHP single crystals promoted by their large thickness over several micrometres to millimetres. Herein, the attributes of the material's thickness and the measurement technique used to estimate these characteristics are discussed to elucidate the actual origins of the extended absorbance of MHP single crystals. Contrary to the general belief of the narrower bandgap of the MHP single crystals, we demonstrate that the extended NIR absorption in the MHP single crystals mainly originates from the combination of unique below-bandgap absorption of MHPs, the thickness of single crystals, and the technical limitation of the spectrophotometer, with the key attributes of (i) significantly large thickness of the MHP single crystals by suppressing the transmitted light and (ii) the detector's limited dynamic range. Combining the theoretical and experimental characterizations, we clarify the significant role of the large thickness together with the limited sensitivity of the detector in promoting the well-known red shift of the absorption onset of the MHP single crystals. The observations evidently show that in some special circumstances, the acquired absorption spectrum cannot reliably represent the optical bandgap of MHP materials. This highlights some misinterpretations in the estimation of the narrower optical bandgap of the MHP single crystals from conventional optical methods, while the optical bandgap is an inherent property independent of the thickness. The proposed broad applications of the MHP single crystals are dictated by their fascinating properties, and therefore, a deep insight into these features should be considered besides device applications, because much of their property-function relationships are still ambiguous and a subject of debate.

Driving fiber diameters to the limit: nanoparticle-induced diameter reductions in electrospun photoactive composite nanofibers for organic photovoltaics

Electrospun photoactive nanofibers hold significant potential for enhanced photon absorption and charge transport in organic photovoltaics. However, electrospinning conjugated polymers with fiber diameters comparable to exciton diffusion lengths for efficient dissociation, is difficult. Previously, spinning sub-100 nm poly(3-hexylthiophene) (P3HT) fibers has required the auxiliary polymer, poly(ethylene oxide) (PEO), and large antisolvent additions. Therefore, its success differs considerably across donor polymers, due to variable antisolvent addition limits before precipitation. Herein, plasmonic nanoparticle infusion into P3HT nanofibers is used to modulate viscosity and deliver a novel and unrivaled strategy to achieve reduced fiber diameters. Following PEO removal, the fibers measure 55 nm in diameter, 30% lower than any previous report – providing the shortest exciton diffusion pathways to the heterojunction upon electron acceptor infiltration. The nanoparticle-containing nanofibers present a 58% enhancement over their pristine thin-film counterparts. ~17% is ascribed to plasmonic effects, demonstrated in thin-films, and the remainder to along-fiber polymer chain alignment, introduced by electrospinning. The anisotropy of light absorbed when polarized parallel versus perpendicular to the fibers increases from 0.88 to 0.62, suggesting the diameter reduction improves the alignment, resulting in greater electrospinning-induced enhancements. Controlled by the electrospinning behavior of PEO, our platform may be adapted to contemporary donor-acceptor systems.Graphical A dramatic reduction in the diameters of electrospun photoactive nanofibers is achieved by introducing nanoparticles, offering shorter exciton pathways towards the heterojunction in nanofibrous OPVs. Thinner fiber diameters enhance the alignment of the polymer chains along the fiber, manifesting in greater photon absorption. Alongside plasmonic effects, the dual-mode enhancement within the fibers offers 58% additional light harvesting versus their thin-film counterparts.

Optimizing LSM-LSF composite cathodes for enhanced solid oxide fuel cell performance: Material engineering and electrochemical insights

In response to pressing environmental concerns and the ever-increasing global energy demand, the pursuit of sustainable energy sources has garnered substantial momentum. A predominant focus of research within this domain revolves around the enhancement of solid oxide fuel cell performance, with particular attention directed toward the intricate oxygen reduction reaction transpiring at the cathode electrode. The present investigation embarks upon the exploration of a composite material comprising La1-xSrxMnO3-δ (LSM) and La1-xSrxFeO3-δ (LSF) as prospective cathode materials. However, the achievement of optimal composite cathodes necessitates the application of advanced materials engineering strategies. The principal aim of this comprehensive study is the development of high-performance composite cathodes, achieved through the deposition of thin film counterparts onto scandium-stabilized zirconia (ScSZ) electrolyte substrate employing the precision sputtering technique. The ensuing findings unequivocally unveil the remarkable performance exhibited by this innovative composite cathode configuration, notably manifesting its excellence at an operating temperature as modest as 780 °C. These promising outcomes imbue optimism regarding the prospective utilization of this composite material in advanced energy conversion applications, marking a significant advancement toward the realization of sustainable and efficient energy systems.

Design and Optical Performance of a Single-Junction GaAs Nanowire-Ge Solar Cell

Solar cells are one of the most effective methods available for energy harvesting and are constructed from a variety of materials. In recent years, the use of novel materials for low-cost, high-efficiency photovoltaics has been one of the most exciting breakthroughs. This study conducted an in-depth investigation into the optical characteristics of GaAs nanowires on a Ge bottom cell. Geometric optimization of nanowires is necessary to increase solar cell performance metrics. The absorption efficiency per unit volume was considerably boosted over its traditional bulk and thin-film counterparts as a result of inherent antireflection, intensive stimulation of resonant modes, and optical antenna effects. A 3D FDTD framework was used to acquire optical properties and incorporate numerical values. Under typical AM 1.5G illumination, the diameter of GaAs nanowires was optimized to 170 nm.

Identification of low coefficient of thermal expansion in Al23Ce4Ni6 via combinatorial sputtering of Al-Ce-Ni-Mn thin films and upscaling to bulk materials

Al100−x-y-zCexNiyMnz thin films were synthesized via combinatorial sputtering from Al, Al80Ce20, Al91Ni9, and Al93Mn7 targets. The resultant as-deposited films exhibit a continuous composition gradient of the various constituents and form a metastable solid-solution over most of the composition space. Subsequent annealing leads to the formation of the thermodynamically stable systems of Al, and α-Al11Ce3, Al8CeMn4, and Al23Ce4Ni6 intermetallics -- where the phases present and fraction depend on the composition. Temperature dependent x-ray diffraction (TDXRD) was used to determine the coefficients of thermal expansion (CTE) for each phase. The thin film Al and Al11Ce3 temperature dependent CTE values are consistent with previously reported results. The thin film Al8CeMn4 phase reveals a slight decrease in the CTE with a range from 27 × 10−6 oC−1 to 23 × 10−6 oC−1 in the temperature range 25–550 °C. The thin film Al23Ce4Ni6 phase exhibits a CTE value of ≈ 9 × 10−6 oC−1 near room temperature and increases to 14 × 10−6 oC−1 at 550 °C. To confirm the thin film results, bulk stoichiometric Al8CeMn4 and Al23Ce4Ni6 samples were prepared and measured under similar conditions. The Al8CeMn4 bulk sample confirmed the negative CTE trend observed in the thin film sample. Compared to its thin-film counterpart, the Al23Ce4Ni6 bulk sample similarly demonstrated a low CTE value near room temperature of 5 × 10−6 oC−1 and an anomalous minimum in the CTE at ≈ 75 °C, which was confirmed via temperature dependent neutron diffraction. Temperature dependent magnetic and electrical measurements were subsequently taken on Al23Ce4Ni6, and the anomalous minimum in the CTE coincided with an electronic phase transition.

Synthesis and physical properties of LaNiO2 crystals

Infinite-layer (IL) nickelates are an emerging family of superconductors whose similarities and differences to cuprate superconductors are under intense debate. To date, the IL phase of nickelates can only be reached via topotactic oxygen reduction of the perovskite phase, using H$_2$ gas or reducing agents such as CaH$_2$. While the topotactic reduction method has been widely employed on thin film and polycrystalline powder samples, the reduction of La$_{1-x}$Ca$_x$NiO$_3$ single-crystals with lateral dimensions up to 150 $\mu$m was achieved only recently, using an indirect contact method with CaH$_2$. Here we report the topotactic transformation of much larger LaNiO$_3$ crystals with lateral dimensions of more than one millimeter, via direct contact with CaH$_2$. We characterize the crystalline, magnetic, and electronic properties of the obtained IL LaNiO$_{2}$ crystals by powder and single-crystal x-ray diffraction (XRD), magnetometry, electrical transport, and x-ray photoelectron spectroscopy (XPS) measurements. The amount of incorporated topotactic hydrogen due to the reduction process is determined by a gas extraction method. In addition, we investigate the evolution of the lattice parameters under hydrostatic pressure up to 12 GPa, using high-resolution synchrotron XRD. Furthermore, we provide a direct comparison of several physical properties of the LaNiO$_{2}$ crystals to their powder and thin film counterparts.

Chiral Lead-Free Double Perovskite Single-Crystalline Microwire Arrays for Anisotropic Second-Harmonic Generation.

Halide double perovskites present a new branch for versatile optoelectronic devices because of their huge structural compatibility and environmental friendliness, whereas nonlinear optics (NLO) devices remain blank for this fascinating family. Simultaneously, the precise patterning of single-crystalline perovskite microwire arrays remains a challenge for the integration of NLO devices. Herein, we designed lead-free chiral 2D double perovskites with the nonsymmetrical structure presenting second-harmonic generation (SHG). Furthermore, perovskite single-crystalline arrays with regulated geometry, pure orientation, and high crystallinity are prepared using the capillary-bridge confined assembly technique. The efficient SHG originates from the asymmetric crystal structure and high crystallinity of the microwire arrays. Compared with their polycrystalline thin-film counterparts, linearly polarized SHG and a higher SHG conversion efficiency are demonstrated based on microwire arrays. The results not only expand the applications of lead-free double perovskites in the NLO-integrated fields but also provide a viable way for lead-free optoelectronic devices.

Probing the Surface and Bulk Electrical Response of MAPbBr3 Single Crystals

Single crystals (SCs) of halide perovskites are rapidly gaining attention over their polycrystalline thin-film counterparts due to their superior optoelectronic properties. One of the various reasons for their improved properties is the reduced defect concentration as compared with thin films. Since their discovery, many efforts have been devoted to discerning the surface and bulk properties of SCs. However, it is difficult to probe the surface and bulk responses of SCs, and only a few reports distinguishing between these properties have been presented. In this study, we distinguished between the surface and bulk electrical responses of methylammonium lead tribromide (MAPbBr3) SCs using impedance spectroscopy (IS). The electrical response of the MAPbBr3 SC was recorded using different optical excitations for the generation of carriers. The Nyquist plots and capacitance–frequency responses observed under blue and red wavelengths differ significantly from each other and were studied systematically. The results obtained suggest that the photocurrent and capacitive response under blue light are higher than those under red light. Moreover, the change in the low-frequency capacitance is lower in the case of blue light. This is because of the polarization of the interface of metal contacts and the surface of SCs or due to defect-mediated conductivity. As the carriers are collected efficiently from the surface following a rapid process, they do not contribute to the capacitance build-up with temperature. To the best of our knowledge, electrical responses such as the impedance and the capacitive response of SCs using different illumination wavelengths and temperatures have been rarely discussed in the literature.

Promoting Single Channel Photon Emission in Copper(I) Halide Clusters for X‐Ray Detection

AbstractMetal clusters with color‐tunable radioluminescence have attracted growing attention as X‐ray detection materials. However, they are generally less scintillation‐efficient due to the coexistence of multiple exciton recombination centers. Herein, two new zero‐dimensional copper(I)‐based clusters (DIET)3Cu3X3 (DIET = 1,3‐Diethyl‐2‐thiourea, X = Cl, Br) are designed, which support the single channel photon emission (Cu cluster centered, CC) to enhance the scintillation performance. Those DIET ligands (L) anchoring with copper(I) ions not only generate direct CuL bonds, but also form a disorder [Cu3X3] cluster, which results in the intense X‐ray absorption. (DIET)3Cu3Br3 with low halogen electronegativity further weakens halogen‐to‐ligand charge transfer (XLCT) emission, enabling enhanced quantum efficiency (≈69%) and scintillation performance comparable to commercially available Lu3Al5O12:Ce. Moreover, the flexible thin film counterpart demonstrates legible X‐ray imaging with high spatial resolution of 11.71 lp mm−1. This study provides a feasible design principle to discover new metal cluster‐based scintillators and to further expand their radiation detection applications.

Femtosecond optical nonlinearities and ultrafast absorption dynamics of colloidal 2D organometal halide ((C12H25–NH3)2PbI4) nanoparticles and thin films

Layered 2D Inorganic-Organic (IO) hybrid semiconductors of the type (R–NH3)2PbI4 have shown tremendous potential applications in various photonics and optoelectronic devices. In this work, femtosecond laser induced optical nonlinearities and excited state charge carrier dynamics of organometal halide (C12H25–NH3)2PbI4 (C12PI) 2D colloidal nanoparticles (NPs) and the thin film counterpart are investigated. These colloidal 2D NPs of sizes ∼9 nm, show third-order optical nonlinearity having two-photon absorption coefficient (2PA) (β∼26-23 cm/GW) and positive nonlinear refractive index (n2∼7.6 × 10−2 cm2/GW), while thin films show β∼78–134 cm/GW and n2∼0.003-0.2 × 10−2 cm2/GW. The origins of two-photon induced absorption and Kerr-type refractive nonlinearity are as a result of induced molecular polarization and charge transfer effects between organic cation (C12H25–NH3)+ and inorganic anion (PbI6 octahedra). The ultrafast transient absorption spectral studies demonstrate the room-temperature Mott-type exciton excited-state charge carrier dynamics and charge carrier recombination features. The laser pump intensity dependent transient absorption spectral (TAS) features reveal wide information about ground state bleaching (GSB∼510 nm, ΔA<0 with decay of 28.2–3.2 ps) along with a photo-induced absorption (PIA∼476 nm, ΔA>0, with decay of 22–6.3 ps) regions. The systematic studies of optical nonlinearities and ultrafast absorption dynamics of 2D IO hybrid semiconductor colloidal nanoparticles and thin film will provide a new platform for advanced photonics and ultrafast optical switching devices applications.

Effect of Sm2O3 addition on the dielectric behaviour of BaTiO3

BaTiO3 is a widely used ferroelectric ceramic having applications in transducers, capacitors, thermistors, non linear optics etc. BaTiO3 exhibits Curie temperature (Tc) of 120 °C. Recent research in BaTiO3 based thin films has resulted in a significant rise in their Tc. The most prominent increase in Tc has been reported in BaTiO3 – Sm2O3 system in which Sm2O3 nanorods were incorporated in BaTiO3 matrix resulting in the Tc ∼ 800 °C. Such a remarkable increase in the Tc was attributed to the high elastic modulus of Sm2O3 (E = 125 GPa) w.r.t. that of BaTiO3 (E = 67 GPa). Higher elastic modulus of the second phase resulted in the generation of strain in the matrix thereby delaying the onset of tetragonal – cubic transition. The improvement in the Tc of BaTiO3 thus opened possibilities of use of BaTiO3 based systems for high temperature piezoelectric applications. Since BaTiO3 based thin films exhibited a remarkable increase in the Tc, it needed to be studied if such a phenomenon could be replicated in bulk ceramics. Hence, BaTiO3 – Sm2O3 ceramics were prepared by the conventional ceramics processing route. Submicron-sized BaTiO3 and Sm2O3 powders were used as starting materials and compositions with Sm2O3 content varying between 5 and 30 wt% were prepared. BaTiO3 and Sm2O3 powders were milled using a jar mill for 48 h in ethanol followed by drying of slurries and room temperature uniaxial pellet pressing. Green pellets were sintered by pressureless sintering at 1350 °C for 4 h. X-ray diffraction (XRD) analysis of BaTiO3 and BaTiO3 – 5 wt% Sm2O3 ceramics confirmed the presence of tetragonal phase. However, further increase in Sm2O3 content resulted in the transformation of tetragonal BaTiO3 to cubic BaTiO3. Since cubic phase is centrosymmetric and cannot exhibit ferroelectricity, compositions with Sm2O3 > 5 wt% became redundant for any ferroelectric application. Moreover, BaTiO3 – 5 wt% Sm2O3 exhibited Tc of just 100 °C with lower dielectric constant thereby making this composition ineffective for high temperature piezoelectric application. While it had been anticipated that incorporation of Sm2O3 in bulk BaTiO3 ceramics would increase their Tc as in thin films, none of these compositions exhibited the anticipated behaviour. This paper discusses why BaTiO3 – Sm2O3 bulk ceramics failed to exhibit similar dielectric behaviour to their thin film counterparts in light of the phase formation, microstructure and dielectric behaviour.

Investigation of light-matter interaction in single vertical nanowires in ordered nanowire arrays.

Quasi one-dimensional semiconductor nanowires (NWs) in either arrays or single free-standing forms have shown unique optical properties (i.e., light absorption and emission) differently from their thin film or bulk counterparts, presenting new opportunities for achieving enhanced performance and/or functionalities for optoelectronic device applications. However, there is still a lack of understanding of the absorption properties of vertically standing single NWs within an array environment with light coupling from neighboring NWs within certain distances, due to the challenges in fabrication of such devices. In this article, we present a new approach to fabricate single vertically standing NW photodetectors from ordered InP NW arrays using the focused ion beam technique, to allow direct measurements of optical and electrical properties of single NWs standing in an array. The light-matter interaction and photodetector performance are investigated using both experimental and theoretical methods. The consistent photocurrent and simulated absorption mapping results reveal that the light absorption and thus photoresponse of single NWs are strongly affected by the NW array geometry and related light coupling from their surrounding dielectric environment, due to the large absorption cross section and/or strong light interaction. While the highest light concentration factor (∼19.64) was obtained from the NW in an array with a pitch of 1.5 μm, the higher responsivity per unit cell (equivalent to NW array responsivity) of a single vertical NW photodetector was achieved in an array with a pitch of 0.8 μm, highlighting the importance of array design for practical applications. The insight from our study can provide important guidance to evaluate and optimize the device design of NW arrays for a wide range of optoelectronic device applications.

Understanding the Role of Grain Boundaries on Charge‐Carrier and Ion Transport in Cs2AgBiBr6 Thin Films

AbstractHalide double perovskites have gained significant attention, owing to their composition of low‐toxicity elements, stability in air, and recent demonstrations of long charge‐carrier lifetimes that can exceed 1 µs. In particular, Cs2AgBiBr6 is the subject of many investigations in photovoltaic devices. However, the efficiencies of solar cells based on this double perovskite are still far from the theoretical efficiency limit of the material. Here, the role of grain size on the optoelectronic properties of Cs2AgBiBr6 thin films is investigated. It is shown through cathodoluminescence measurements that grain boundaries are the dominant nonradiative recombination sites. It also demonstrates through field‐effect transistor and temperature‐dependent transient current measurements that grain boundaries act as the main channels for ion transport. Interestingly, a positive correlation between carrier mobility and temperature is found, which resembles the hopping mechanism often seen in organic semiconductors. These findings explain the discrepancy between the long diffusion lengths &gt;1 µm found in Cs2AgBiBr6 single crystals versus the limited performance achieved in their thin film counterparts. This work shows that mitigating the impact of grain boundaries will be critical for these double perovskite thin films to reach the performance achievable based on their intrinsic single‐crystal properties.

Influence of diameter on temperature dynamics of hot carriers in photoexcited GaAsP nanowires

Nanowires (NWs) with their quasi-one-dimensionality often present different structural and opto-electronic properties than their thin-film counterparts. The thinner they are the larger these differences are, in particular in the carrier-phonon scattering and thermal conductivity. In this work, we present femtosecond transient absorbance measurements on GaAs0.8P0.2 NWs of two different diameters, 36 and 51 nm. The results show that thinner NWs sustain the hot-carriers at a higher temperature for longer times than thicker NWs. We explain the observation suggesting that in thinner NWs, the build-up of a hot-phonon bottleneck is easier than in thicker NWs because of the increased phonon scattering at the NW sidewalls which facilitates the build-up of a large phonon density. The large number of optical phonons emitted during the carrier relaxation processes generate a non-equilibrium population of acoustic phonons that propagates less efficiently in thin NWs. This makes the possible acoustic-to-optical phonon up-conversion process easier, which prolongs the LO phonon lifetime resulting in the slowdown of the carrier cooling. The important observation that the carrier temperature in thin NWs is higher than in thick NWs already at the beginning of the hot carrier regime suggests that the phonon-mediated scattering processes in the non-thermal regime play a major role at least for the carrier densities investigated here (8x1018-4x1019 cm-3). Our results also suggest that the boundary scattering of phonons at crystal defects is negligible compared to the surface scattering at the NW sidewalls.

High Efficiency Luminescent Solar Concentrator based on Organo‐Metal Halide Perovskite Quantum Dots with Plasmon Enhancement

AbstractOrgano‐metal halide perovskites (OMHPs) are currently one of the most exciting candidates for photovoltaics. However, their impact in other areas related to carrier photogeneration, such as in luminescent solar concentrators (LSCs), has been limited. OMHP thin films have demonstrated encouraging results as LSCs, but for a scalable platform with minimal losses, discrete emitters are preferable. Perovskite quantum dots (PQDs) possess higher photoluminescence quantum yield (PLQY) than their thin film counterparts, and have the added advantage of size tunability, which make them well‐suited as LSC active media. However, since PQDs are not amenable to Stokes shift modulation, large‐scale samples will suffer from increased self‐absorption (SA) losses. In this work, a facile dip‐coating approach is established to fabricating large‐scale LSCs with CH3NH3PbBr3 (methylammonium lead bromide) PQDs by leveraging their plasmonic interactions with gold nanoparticles (AuNPs) to offset SA losses. Optical characterization through emission, lifetime, and spatially resolved photoluminescence measurements provide insight into the effect of plasmon resonance on deposited PQDs, and leveraging this AuNP‐PQD coupling, 2.87% efficiency is achieved in a 100 cm2 LSC with a geometric gain of 50.

Robust Protection of III-V Nanowires in Water Splitting by a Thin Compact TiO2 Layer.

Narrow-band-gap III–V semiconductor nanowires (NWs) with a suitable band structure and strong light-trapping ability are ideal for high-efficiency low-cost solar water-splitting systems. However, due to their nanoscale dimension, they suffer more severe corrosion by the electrolyte solution than the thin-film counterparts. Thus, short-term durability is the major obstacle for using these NWs for practical water-splitting applications. Here, we demonstrated for the first time that a thin layer (∼7 nm thick) of compact TiO2 deposited by atomic layer deposition can provide robust protection to III–V NWs. The protected GaAs NWs maintain 91.4% of its photoluminescence intensity after 14 months of storage in ambient atmosphere, which suggests the TiO2 layer is pinhole-free. Working as a photocathode for water splitting, they exhibited a 45% larger photocurrent density compared with unprotected counterparts and a high Faraday efficiency of 91% and can also maintain a record-long highly stable performance among narrow-band-gap III–V NW photoelectrodes; after 67 h photoelectrochemical stability test reaction in a strong acid electrolyte solution (pH = 1), they show no apparent indication of corrosion, which is in stark contrast to the unprotected NWs that fully failed after 35 h. These findings provide an effective way to enhance both stability and performance of III–V NW-based photoelectrodes, which are highly important for practical applications in solar-energy-based water-splitting systems.

Highly controllable synthesis of MAPbI3 perovskite nanocrystals with long carrier lifetimes and narrow band gap for application in photodetectors

Low-dimensional organic-inorganic hybrid perovskite materials have attracted much attention in photoelectric devices due to their exotic performance in optoelectronics compared with their bulk and thin-film counterparts. In this article, using a simple ligand-assisted deposition method, we successfully synthesized uniform methylammonium lead iodide (MAPbI3) perovskite nanorods, nanotriangles and nanocubes, and investigated their photoelectric properties after fabricated into photodetectors (PDs) on SiO2/Si substrates. It is found that the synthesized MAPbI3 nanorods not only have extremely small optical band gap (minimum of ~1.36 eV), but also have long photoluminescence (PL) lifetime (longest of ~7.81 μs) and high absolute PL quantum yields (maximum of ~ 45%), and the PL lifetime is more than about one order of magnitude higher than that the reported previously. The PDs based on MAPbI3 nanorods presented a wide spectral response “window” from 350 to at least 950 nm, and exhibited a photoresponsivity of 12.2 mA/W, a detectivity of 2.67 × 1011 Jones, the on/off ratio of more than 800, the rise/decay time constants of 18/25 ms, under the excitation at 405 nm. Further analysis suggests that the dramatically improved photo-electric conversion properties of the nanorods PDs could be closely related with their morphology dependent properties, including bandgap, PL lifetime, trap density (ntrap) and carrier concentration. These results provide a simple and effective method for preparing different morphology low-dimensional MAPbI3, and discuss the mechanism of morphology-dependent photoelectric performance of PDs, which is of great significance for potential applications in the field of photoelectronic devices.