Xinchen Li

Low-temperature solution-processed hydrogen molybdenum and vanadium bronzes for an efficient hole-transport layer in organic electronics.

A simple one-step method is reported to synthesize low-temperature solution-processed transition metal oxides (TMOs) of molybdenum oxide and vanadium oxide with oxygen vacancies for a good hole-transport layer (HTL). The oxygen vacancy plays an essential role for TMOs when they are employed as HTLs: TMO films with excess oxygen are highly undesirable for their application in organic electronics.

Post‐treatment‐Free Solution‐Processed Non‐stoichiometric NiOx Nanoparticles for Efficient Hole‐Transport Layers of Organic Optoelectronic Devices

DOI: 10.1002/adma.201405391 In the early stage, Marks and co-workers [ 13 ] reported a striking performance improvement of OSCs by replacing PEDOT:PSS with NiO x fi lm using a pulsed-laser deposition technology. From then on, NiO x HTLs have been reported for organic optoelectronics by various preparation methods, such as thermal evaporation, [ 14 ] sputtering, [ 9a ] and solution process. [ 9d , 15 ] Among them, solution process method is desirable for low-cost, large-scale and roll-to-roll production. Olson and co-workers [ 16 ] proposed a solution-processed NiO x fi lm as highly effi cient HTL in OSCs. The functional NiO x HTL was fabricated through annealing the precursor fi lm at a temperature of 275 °C. So and co-workers [ 17 ] also presented a NiO x fi lm by using monoethanolamine (MEA) to react with Ni in ethanol solution followed by thermally converting (275 °C) coordination complexes ions [Ni(MEA) 2 (OAc)] + into high-quality NiO x . Meanwhile, solution-processed NiO x at 150 °C has also been realized. Ma and co-workers [ 18 ] reported a solution-processed NiO x fi lm for OSCs using oxygen-plasma treatment and annealing treatment simultaneously. Zhang et. al. reported that the colloidal NiO nanoparticles are used as the anode buffer layer in OSCs without high temperature post-annealing to induce decomposition and crystallization. [ 9f ] For a long period, the studies of NiO x HTLs were focused on utilizing sol–gel methods with thermally converting the precursor solution to NiO x thin fi lms. In the process of device fabrications, thermal annealing process and oxygen-plasma treatment may be simultaneously required, which hinders the applications of NiO x in fl exible optoelectronic devices. Instead of precursor method, an approach to signifi cantly reduce the processing temperature of TMO HTLs is to directly use high-quality colloidal nanoparticles (NPs). Jin and co-workers demonstrated a facile and general strategy based on ligand protection for the synthesis of unstable colloidal NiO nanocrystals. [ 19 ] Fattakhova-Rohlfi ng and co-workers described the preparation of ultrasmall, crystalline, and dispersible NiO nanoparticles, which are promising candidates as catalysts for electrochemical oxygen generation. [ 9e ]

Room-temperature solution-processed molybdenum oxide as a hole transport layer with Ag nanoparticles for highly efficient inverted organic solar cells

While metal oxide films are typically formed by high-temperature and sputtering processes, we report an approach with the features of a room-temperature, water-free and solution-based process for the formation of a molybdenum oxide (MoOx) film for inverted organic solar cells (OSCs) by proposing a vacuum treatment at room temperature and selecting an appropriate solvent. Remarkably, our results indicate that the vacuum treatment can introduce oxygen vacancies in the molybdenum oxide film and modify its work function for functioning as an efficient hole transport layer. To further improve OSC performances, a silver nanoparticle–molybdenum oxide (Ag NP–MoOx) composite film is prepared by the introduction of Ag nanoparticles into the solution. Evidence and explanations confirm that OSC performance enhancement is mainly due to the improvement of the electrical properties of the Ag NP–MoOx composite film. With the optimized composite film, inverted OSCs with a power conversion efficiency (PCE) of 7.94% are achieved. Through the demonstration of high performance inverted OSCs with different polymer materials, the water-free, room-temperature and solution-processed MoOx can contribute to the evolution of high performance OSCs such as inverted and tandem OSCs and other optoelectronic devices.

High Efficiency Organic Solar Cells Achieved by the Simultaneous Plasmon-Optical and Plasmon-Electrical Effects from Plasmonic Asymmetric Modes of Gold Nanostars

The plasmon-optical effects have been utilized to optically enhance active layer absorption in organic solar cells (OSCs). The exploited plasmonic resonances of metal nanomaterials are typically from the fundamental dipole/high-order modes with narrow spectral widths for regional OSC absorption improvement. The conventional broadband absorption enhancement (using plasmonic effects) needs linear-superposition of plasmonic resonances. In this work, through strategic incorporation of gold nanostars (Au NSs) in between hole transport layer (HTL) and active layer, the excited plasmonic asymmetric modes offer a new approach toward broadband enhancement. Remarkably, the improvement is explained by energy transfer of plasmonic asymmetric modes of Au NS. In more detail, after incorporation of Au NSs, the optical power in electron transport layer transfers to active layer for improving OSC absorption, which otherwise will become dissipation or leakage as the role of carrier transport layer is not for photon-absorption induced carrier generation. Moreover, Au NSs simultaneously deliver plasmon-electrical effects which shorten transport path length of the typically low-mobility holes and lengthen that of high-mobility electrons for better balanced carrier collection. Meanwhile, the resistance of HTL is reduced by Au NSs. Consequently, power conversion efficiency of 10.5% has been achieved through cooperatively plasmon-optical and plasmon-electrical effects of Au NSs.

Efficient hole transport layers with widely tunable work function for deep HOMO level organic solar cells

Hole transport layers (HTLs) with large work function (WF) tuning ability for good energy level alignment with deep highest occupied molecular orbital (HOMO) level donor materials are desirable for high-performance and high open-circuit voltage (VOC) organic solar cells (OSCs). Here, a novel low-temperature and solution-process approach to achieve WF tuning in HTLs is proposed. Specifically, the HTLs made from 2,3,4,5,6-pentafluorobenzylphosphonic acid (F5BnPA) incorporated graphene oxide (GO) and molybdenum oxide (MoOx) solution (representing two possible classes of HTLs where carriers transport via valence and conduction bands, respectively) offer continuous WF tuning (the tuning range as large as 0.81 eV) by controlling F5BnPAs concentration. By employing a deep HOMO donor material, OSCs using the composite HTLs can achieve improved performances with largely increased VOC (0.92 V for GO:F5BnPA versus 0.65 V for pristine GO; 0.91 V for MoOx:F5BnPA versus 0.88 V for pristine MoOx). The enhanced performance can be experimentally and theoretically explained by the decreased hole injection barrier (HIB) for GO or equivalent HIB (i.e. electron extraction barrier) for MoOx and enhanced surface recombination velocity, which contribute to eliminating S-shaped current–voltage characteristics. Consequently, the incorporation of F5BnPA can efficiently tune HTL WF for high VOC OSCs and extend HTL applications in organic electronics.

Spin-torque diode with tunable sensitivity and bandwidth by out-of-plane magnetic field

Spin-torque diodes based on nanosized magnetic tunnel junctions are novel microwave detectors with high sensitivity and wide frequency bandwidth. While previous reports mainly focus on improving the sensitivity, the approaches to extend the bandwidth are limited. This work experimentally demonstrates that through optimizing the orientation of the external magnetic field, wide bandwidth can be achieved while maintaining high sensitivity. The mechanism of the frequency- and sensitivity-tuning is investigated through analyzing the dependence of resonant frequency and DC voltage on the magnitude and the tilt angle of hard-plane magnetic field. The frequency dependence is qualitatively explicated by Kittels ferromagnetic resonance model. The asymmetric resonant frequency at positive and negative magnetic field is verified by the numerical simulation considering the in-plane anisotropy. The DC voltage dependence is interpreted through evaluating the misalignment angle between the magnetization of the free laye...

Performance Optimization of Spin-Torque Microwave Detectors with Material and Operational Parameters

Sensitivity, bandwidth, and noise equivalent power (NEP) are important indicators of the performance of microwave detectors. The previous reports on spin-torque microwave detectors (STMDs) have proposed various approaches to increase the sensitivity. However, the effects of these methods on the other two indicators remain unclear. In this work, macrospin simulation is developed to evaluate how the performance can be optimized through changing the material (tilt angle of reference-layer magnetization) and operational parameters (the direction of magnetic field and working temperature). The study on the effect of magnetic field reveals that the driving force behind the performance tuning is the effective field and the equilibrium angle between the magnetization of the free layer and that of the reference layer. The material that offers the optimal tilt angle in reference-layer magnetization is determined. The sensitivity can be further increased by changing the direction of the applied magnetic field and the operation temperature. Although the optimized sensitivity is accompanied by a reduction in bandwidth or an increase in NEP, a balance among these performance indicators can be reached through optimal tuning of the corresponding influencing parameters.

Comprehensive noise characterisation of magnetic tunnel junction sensors for optimising sensor performance and temperature detection

Abstract Noise performance of magnetic tunnel junction (MTJ) sensors is impacted by various factors including junction structure, post-deposition treatment, and operating parameters. The optimisation of these factors can lead to a better MTJ sensor design with minimised noise level and enhanced detectivity for functioning as a magnetometer. In this paper, the authors studied the influence of several parameters (bias voltage, temperature, magnetic field, and junction area) on the noise performance of MTJ sensors. Relatively high bias voltage and low ambient temperature were suggested to be helpful in reducing the electronic 1/f noise. A mechanism of utilising MTJ as a temperature sensor by making use of the mid-frequency noise (from 10.0 kHz to 22.8 kHz) was proposed. The relation between temperature and noise power was obtained by numerically fitting the measured noise power with an equation composing of three components representing background noise, intertwined thermal and shot noise, and non-linear noise source, respectively. Temperature of the junction could be determined by measuring the mid-frequency noise power at certain bias voltage and substituting it into the equation. This provides a possible route of using a MTJ as a multifunctional sensor for sensing both magnetic field and temperature.

Reduced magnetic coercivity and switching field in conetic-alloy-based synthetic-ferrimagnetic nanodots

The coercivity (H_c) and switching field (H_sw) of free layers increase remarkably with shrinking size, which reduces the sensitivity of spintronic devices. Conetic-alloy-based synthetic ferrimagnetic (SyF) trilayers are proposed to show reduced H_c and H_sw than single-layer in nanodots. The investigation on the thickness dependence reveals linear reliance of H_c and H_sw on amplification factor. H_c and H_sw are further reduced after field annealing at 200 °C. This work provides an approach to reduce the H_c and H_sw in nanomagnets.

Exchange bias study of CoFeB/IrMn antidot and nanodot arrays fabricated by nanosphere lithography

Exchange bias effect in nanostructures are widely investigated for applications in nanometric spintronic sensors. In this work, nanosphere lithography was adopted to pattern CoFeB/IrMn antidot and nanodot arrays. The exchange bias and coercivity of the nanostructures and continuous films exhibit similar exponential dependence on CoFeB layer thickness. High temperature annealing results in decreased exchange bias and coercivity. This work provides physical insights on magnetization reversal response in nanosized spintronic devices.