External Conversion Efficiency Research Articles

High-Performance Infrared Detectors Based on Black Phosphorus/Carbon Nanotube Heterojunctions.

Infrared detectors have broad application prospects in the fields of detection and communication. Using ideal materials and good device structure is crucial for achieving high-performance infrared detectors. Here, we utilized black phosphorus (BP) and single-walled carbon nanotube (SWCNT) films to construct a vertical van der Waals heterostructure, resulting in high-performance photovoltaic infrared detectors. In the device, a strong built-in electric field was formed in the heterojunction with a favored energy-band matching between the BP and the SWCNT, which caused a good photovoltaic effect. The fabricated devices exhibited a diode-like rectification behavior in the dark, which had a high rectification ratio up to a magnitude of 104 and a low ideal factor of 1.4. Under 1550 nm wavelength illumination, the 2D BP/SWCNT film photodetector demonstrated an open-circuit voltage of 0.34 V, a large external power conversion efficiency (η) of 7.5% and a high specific detectivity (D*) of 3.1 × 109 Jones. This external η was the highest among those for the photovoltaic devices fabricated with the SWCNTs or the heterostructures based on 2D materials and the obtained D* was also higher than those for most of the infrared detectors based on 2D materials or carbon materials. This work showcases the application potential of BP and SWCNTs in the detection field.

Controlled Preparation and Device Application of Sub-5 nm Graphene Nanoribbons and Graphene Nanoribbon/Carbon Nanotube Intramolecular Heterostructures

Narrow graphene nanoribbons (GNRs) and GNR/single-walled carbon nanotube (SWNT) intramolecular heterojunctions are ideal candidates to construct next-generation electronic and optoelectronic devices. However, the fabrication of high-quality long sub-5 nm wide GNRs and GNR/SWNT heterojunctions is a great challenge. Here, we report a method to produce high-quality sub-5 nm wide GNRs with smooth edges and GNR/SWNT intramolecular heterostructures via palladium-catalyzed full and partial unzipping of SWNTs, respectively. The resulting GNRs could be as narrow as 2.2 nm and had an average length of over 1 μm. By adjusting the unzipping time and the deposited positions of palladium nanoparticles, controlled multiple GNR/SWNT heterostructures were also fabricated on an individual parent SWNT. A GNR field-effect transistor (FET) constructed by a 3.1 nm wide GNR could simultaneously achieve a high on/off current ratio of 1.1 × 104 and a large mobility of 598 cm2 V-1 s-1. The photovoltaic device based on a single GNR (2.4 nm in width)/SWNT (0.8 nm in diameter) heterojunction exhibited a large open-circuit voltage (Voc) of 0.52 V and a high external power conversion efficiency (η) of 4.7% under the 1550 nm wavelength illumination of 931 mW cm-2. Our method provides a pathway to controllably prepare high-quality sub-5 nm GNRs and GNR/SWNT heterojunctions for fundamental studies and practical applications in the electronic and optoelectronic fields.

Highly-doped lanthanide nanomaterials for efficient photothermal conversion – selection of the most promising ions and matrices

Materials doped with rare earth ions are widely used in luminescence applications. However, alternative uses of these materials, such as photothermal conversion, have not yet been systematically investigated. In this work, we systematically and quantitatively compare internal and external photothermal conversion efficiency of materials doped with neodymium, thulium and ytterbium ions, and co-doped with samarium and dysprosium ions. Furthermore, we investigate influence of various matrix materials on photothermal conversion efficiency. These include fluoride, oxide, vanadate and perovskite materials. This was done to determine if photothermal conversion efficiency is dependent on dopant and matrix selection. Our research indicates that the highest internal and external photothermal conversion efficiency is achieved with neodymium ions. Moreover, NdVO4 was found to be the most promising host for future applications. Although the internal photothermal conversion efficiency was relatively low (19%), NdVO4 exhibited significant external photothermal conversion efficiency of 0.08 L/g·cm at 806 nm, which is ca. 14 times higher than that of NaNdF4 at the same wavelength.

Approaching the External Quantum Efficiency Limit in 2D Photovoltaic Devices.

2D transition metal dichalcogenides (TMDs) are promising candidates for realizing ultrathin and high-performance photovoltaic devices. However, the external quantum efficiency (EQE) and power conversion efficiency (PCE) of most 2D photovoltaic devices face great challenges in exceeding 50% and 3%, respectively, due to the low efficiency of photocarrier separation and collection. Here, this study demonstrates photovoltaic devices with defect-free interface and recombination-free channel based on 2D WS2 , showing high EQE of 92% approaching the theoretical limit and high PCE of 5.0%. The high performances are attributed to the van der Waals metal contact without interface defects and Fermi-level pinning, and the fully depleted channel without photocarrier recombination, leading to intrinsic photocarrier separation and collection with high efficiency. Furthermore, this study demonstrates that the strategy can be extended to other TMDs such as MoSe2 and WSe2 with EQE of 92% and 94%, respectively. This work proposes a universal strategy for building high-performance 2D photovoltaic devices. The nearly ideal EQE provides great potential for PCE approaching the Shockley-Queisser limit.

Quantitative Comparison of the Light-to-Heat Conversion Efficiency in Nanomaterials Suitable for Photothermal Therapy.

Functional colloidal nanoparticles capable of converting between various energy types are finding an increasing number of applications. One of the relevant examples concerns light-to-heat-converting colloidal nanoparticles that may be useful for localized photothermal therapy of cancers. Unfortunately, quantitative comparison and ranking of nanoheaters are not straightforward as materials of different compositions and structures have different photophysical and chemical properties and may interact differently with the biological environment. In terms of photophysical properties, the most relevant information to rank these nanoheaters is the light-to-heat conversion efficiency, which, along with information on the absorption capacity of the material, can be used to directly compare materials. In this work, we evaluate the light-to-heat conversion properties of 17 different nanoheaters belonging to different groups (plasmonic, semiconductor, lanthanide-doped nanocrystals, carbon nanocrystals, and metal oxides). We conclude that the light-to-heat conversion efficiency alone is not meaningful enough as many materials have similar conversion efficiencies—in the range of 80–99%—while they significantly differ in their extinction coefficient. We therefore constructed their qualitative ranking based on the external conversion efficiency, which takes into account the conventionally defined light-to-heat conversion efficiency and its absorption capacity. This ranking demonstrated the differences between the samples more meaningfully. Among the studied systems, the top-ranking materials were black porous silicon and CuS nanocrystals. These results allow us to select the most favorable materials for photo-based theranostics and set a new standard in the characterization of nanoheaters.

Frequency Conversion Interface towards Quantum Network: From Atomic Transition Line to Fiber Optical Communication Band

Quantum repeater is a key component of quantum network, and atomic memory is one of the important candidates for constructing quantum repeater. However, the atomic transition wavelength is not suitable for long-distance transmission in optical fiber. To bridge atomic memory and fiber communication, we demonstrate a frequency conversion interface from rubidium D1 line (795 nm) to the optical communication L-band (1621 nm) based on difference frequency generation. To reduce broadband noise of spontaneous Raman scattering caused by strong pumping light, we use a combination of two cascaded etalons and a Fabry-Perot cavity with low finesse to narrow the noise bandwidth to 11.7 MHz. The filtering system is built by common optical elements and is easy to use; it can be widely applied in frequency conversion process. We show that the signal-noise ratio of the converted field is good enough to reduce the input photon number below 1 under the condition of low external device conversion efficiency (0.51%) and large duration of input pulse (250 ns). The demonstrated frequency conversion interface has important potential application in quantum networks.

19‐2: Design Heuristics of Color Conversion Films in Micro LED Displays

Scaling challenges and design requirements needed to support high external color conversion efficiency (ECCE) for light emitters based on blue emitting Micro LEDs and QD color conversion films are quantified using a novel physical model. The model is shown to reproduce published data of ECCE dependence on quantum dot (QD) film thickness. Using this model, it is shown that scaling the QD film thickness from 10μm to 1μm in order to minimize cross talk and reduce manufacturing cost results in ECCE reductions of ~2x and ~10x for red and green QD films, respectively. Moreover, the blue leakage is found to be ~1% for QD film thickness of ~2.5μm.

Efficient second harmonic generation of a high-power picosecond CO2 laser.

A comparative analysis of AgGaSe2, GaSe, CdGeAs2, and Te for second harmonic generation (SHG) of a picosecond CO2 laser at intensities up to 50 GW/cm2 is presented. We demonstrate external energy conversion efficiency of >20% in AgGaSe2. Conversion efficiency >5% is measured in GaSe and CdGeAs2. Self-focusing and multifilamentation are found to severely limit the SHG process in CdGeAs2 and Te at such high fields. Demonstration of ≥150 MW SH pulses for a 10 μm picosecond pump, in combination with femtosecond CO2 laser development, will open new strong-field applications in the 4.5-5.5 μm range.

Quantum frequency conversion from ultraviolet to visible band through waveguides in a period-poled MgO:LiTaO3 crystal.

In research on hybrid quantum networks, visible or near-infrared frequency conversion has been realized. However, technical limitations mean that there have been few studies involving the ultraviolet band, and unfortunately the wavelengths of the rare-earth or alkaline-earth metal atoms or ions that are used widely in research on quantum information are often in the UV band. Therefore, frequency conversion of the ultraviolet band is very important. In this paper, we demonstrate a quantum frequency conversion between ultraviolet and visible wavelengths by fabricating waveguides in a period-poled MgO:LiTaO3 crystal with a laser writing system, which will be used to connect the wavelength of the dipole transition of 171Yb+ at 369.5 nm and the absorption wavelength of Eu3+ at 580 nm in a solid-state quantum memory system. An external conversion efficiency of 0.85% and a signal-to-noise ratio of greater than 500 are realized with a pumping power of 3.28 W at 1018 nm. Furthermore, we complete frequency conversion of the classical polarization state by means of a symmetric optical setup based on the fabricated waveguide, and the process fidelity of the conversion is (96.13 ± 0.021)%. This converter paves the way for constructing a hybrid quantum network and realizing a quantum router in the ultraviolet band in the future.

Determination of the Wavelength-Dependent Photothermal Conversion Efficiency of Photosensitizers for Photothermal Therapy: Application to Ag2S-Glutathione Quantum Dots

Nanoparticles have become popular photosensitizers for photothermal therapy (PTT), as they can be targeted to specific cancer tissues and deliver a chemotherapeutic drug, providing a multimodal therapeutic approach. Photothermal conversion efficiency of nanoparticles is critical in the assessment of their therapeutic use in PTT. We describe an accurate calorimetric method for the determination of the photothermal conversion efficiency of nanoparticles in solution. A tightly focused laser beam was used to irradiate a cuvette containing a solution of silver sulfide-glutathione quantum dots (Ag2S-GSH QDs), and the maximum steady-state temperature rise was measured with an infrared camera. The data were analyzed using two different photothermal conversion efficiencies, the intrinsic and external conversion efficiencies, to relate the induced heating power of the nanoparticles to the absorbed and incident optical powers, respectively. Measurements with a tunable Ti3+:sapphire laser showed that the intrinsic photothermal conversion efficiency of Ag2S-GSH QDs exceeded 91% over the 720-810 nm wavelength range. The method was also used to analyze poly(acrylic acid)-coated superparamagnetic iron oxide nanoparticles (PAA/SPIONs), and the intrinsic photothermal conversion efficiency was determined to be 83.4% at 810 nm. This approach is useful for the evaluation of various potential nanoparticles for photothermal therapy applications.

Key photovoltaic parameters of organohalide lead perovskite quantum dot intermediate band solar cell: A numerical investigation

In this work, a p-i-n junction type intermediate band solar cell (IBSC) model based on metal halide perovskite nanocrystals, specifically methylammonium lead iodide (MAPbI3) quantum dot (QD) and methylammonium lead bromide (MAPbBr3) barrier is designed analytically for the revolutionary next-generation photovoltaic applications. TiO2 and Spiro-OMeTAD are considered as transport layer for electron and hole, respectively. The impact of QD size and dot spacing on the key parameters of MAPbI3 QD-IBSC is illustrated by showing the wave nature of charge carriers within the QDs and their barrier. In order to identify the number of miniband in a single regime, Schrödinger equation is solved as a function of host energy gap using Kronig–Penney model. From the characteristics of electronic wave functions, it is evident that varying the QD size and interdot spacing lead to formation of more than one IB. For any case, (100)-oriented fcc crystal lattice structure is assumed. Major performance indicators of the device such as photocurrent intensity Jsc, open circuit voltage Voc, fill factor, absorbance spectra, photoluminescence (PL) intensity, external quantum efficiency (%EQE) and power conversion efficiency (PCE) η have been delineated. Highest PCE of 26.03% is attained for QD size of 5 nm and interdot spacing of 0.5 nm under one sun and AM 1.5 solar irradiation.

Progress on nonreciprocity of acoustic metamaterials

Reciprocity is a generic feature in various physical systems because it is directly associated to the symmetry of physical laws under time reversal. The breaking of the reciprocity is vitally important in numerous fields, such as the control of energy flux, imaging technologies and topological insulators. For instance, electrical diodes, as a typical nonreciprocal representative in electronic systems, have already contributed to substantial scientific revolutions in many aspects, including integrated circuits, lighting and signal indication. Motivated by the electrical diodes, thermal diodes exhibiting the rectifying effect on thermal energy are achieved in nonlinear lattices. Similarly, the nonreciprocity in photonic systems can be realized by many mechanisms, such as the nonlinearity of optical medium and the mode conversion. Synchronously, the concept of the nonreciprocity further extends to phononic systems. The main means to achieve acoustic nonreciprocity are based on the nonlinearity of acoustic material, dynamic element, and magnetoelastic interaction. However, the nonlinear approach introduces the inherently low conversion efficiency, the dynamic scheme has difficulty in application at high-speed modulation or on nanoscale, and the method based on magnetoelastic interaction has small bandwidth and high frequency, which have severe limitations for practical application of the nonreciprocity. From the perspectives of principles, history, and problems, this paper reviews in detail the progress of the nonreciprocity based on nonlinearity, dynamic components, and magnetoelastic interactions. Some existing problems with these methods were pointed out. For example, the nonreciprocal solution based on nonlinear media has low energy conversion efficiency, it is difficult to miniaturize the nonreciprocal device based on dynamic components because of itself complexity, and the bandwidth of nonreciprocity based on magnetoelastic interaction is narrow. Correspondingly, improvement of the energy conversion efficiency based on the nonlinear medium, more reasonable design of a dynamic modulation system, and widening broadband of nonreciprocity in the magnetoelastic system, can be researched in the future. For example, in order to reduce the nonreciprocal frequency of magnetoelastic system, it is worth studying the unidirectional transmission of sound waves in the magnetoelastic system under a dynamic magnetic field. For the sake of realizing a nonreciprocal system with no external power source and high conversion efficiency, the nonreciprocity of acoustic waves in a magnetoelastic system under a zero magnetic field may become a focus of future research. In addition, since each scheme has its advantages, the combined effects of multiple schemes may overcome the shortcomings of a single scheme and achieve more significant nonreciprocal effects. So far, the combined effects between nonlinearity and dynamic components have not received much attention. We expect that research of nonreciprocity will be extended to both nonlinear and spatiotemporally modulated systems in the future. The combined effects between nonreciprocity and magnetoelastic interaction may become an important content of future research. Finally, considering practical engineering application, there are still many aspects worthy of further study, such as the miniaturization of nonreciprocal devices, optimal design of nonreciprocity based on optimization theory, and adjustable nonreciprocity based on smart materials or structures. In short, the nonreciprocity of acoustic metamaterials is an important field of scientific research, which has potential applications, such as rectification, vibration and noise reduction, sensing, and bioimaging.

Experimental and theoretical investigation of 2D nanoplatelet-based conversion layers for color LED microdisplays.

In the field of augmented reality, there is a need for very bright color microdisplays to meet the user specifications. Today, one of the most promising technology to manufacture such displays involves a blue micro-LED technology and quantum dots-based color conversion layers. Despite recent progress, the external power conversion efficiencies (EPCE) of these layers remain under ∼25%, below the needs (>40%) to reach a white luminance of 100,000 cd/m2. In this work, we have synthesized CdSexS1-x nanoplatelet-based conversion layers for red and green conversion, and measured their absorption properties and EPCE performances with respect to layer thickness. On this basis, a model was developed that reliably predicts the layer EPCE while using only few input data, namely the layer absorption coefficients and the photoluminescence quantum yield (PLQY) of color photoresist. It brings a new insight into the conversion process at play at a micro-LED level and provides a simple method for extensive optimization of conversion materials. Finally, this study highlights the outstanding red conversion efficiency of photoresist layers made of core-double shell CdSexS1-x nanoplatelets with 31% EPCE (45% external PLQY) for 8 µm-thick conversion layer.

Highly efficient generation of coherent light at 2128 nm via degenerate optical-parametric oscillation.

Cryogenic operation, in conjunction with new test-mass materials, promises to reduce the sensitivity limitations from thermal noise in gravitational-wave detectors. Currently, the most advanced materials under discussion are crystalline silicon as a substrate with amorphous silicon-based coatings. However, they require operational wavelengths around 2 µm to avoid laser absorption. Here we present a light source at 2128 nm based on a degenerate optical parametric oscillator to convert light from a 1064 nm nonplanar ring-oscillator. We achieve an external conversion efficiency of (87.1±0.4)% at a pump power of 52 mW in periodically poled potassium titanyl phosphate (internal efficiency was 93%). With our approach, light from the established and existing laser sources can be efficiently converted to the 2 µm regime while retaining the excellent stability properties.

Light management using CsPbBr3 colloidal quantum dots for luminescent solar concentrators

CsPbBr3 colloidal quantum dots have been synthesized by hot-injection method showing spherical shape with an average diameter of ∼10.5 nm. UV–vis absorption of CsPbBr3 colloidal quantum dots shows a broad spectrum with an optical bandgap of ∼2.3682 eV. The steady-state photoluminescence measurement reveals a narrow emission peak at 2.352 eV with full-width at half maximum of 0.113 eV. Absolute photoluminescence quantum yield of colloidal quantum dots dispersed in poly(methyl methacrylate) was found to be 60 ± 1%. The time-resolved photoluminescence data recorded at 266 nm excitation were well fitted using a mono-exponential curve with a decay time of 25.36 (5) ns. A luminescent solar concentrator was fabricated using colloidal quantum dots in transparent poly(methyl methacrylate) polymer uniformly coated over glass substrate that shows an external optical conversion efficiency of ∼5.4% under one sun illumination. The experimental results presented in this manuscript reveals that luminescent solar concentrator prepared using colloidal CsPbBr3 quantum dots shows absorption in wide spectral range, high absorption coefficient, high photoluminescence quantum yield, high external optical conversion efficiency, and good photostability, thermal stability and long-term stability under ambient conditions and therefore are in many ways superior to the other luminescent materials explored for LSC devices.

Self-phase modulation and four-wave mixing in a chalcogenide ridge waveguide

Third order optical nonlinear effects relying on the instantaneous Kerr effect are investigated in a straight chalcogenide ridge waveguide. The sample consists of a GeSbSe film deposited on a thermally oxidized silicon substrate. Ridge waveguides were processed using photolithography and dry etching techniques. From a 1.1 cm long integrated GeSbSe device, self-phase modulation with a maximum nonlinear phase shift of 2.02 π for a peak power of 15.8 W and four-wave mixing with an external conversion efficiency of −42.6 dB for a pump power of 28 mW are demonstrated. Experimental results show a good agreement with calculations.

In-plane Intramolecular WSe2 p-n Junction Realized by Combination of Chemically and Electrically Doping

The p-n junction is one of the most important basic structures in the semiconductor field, with vast applications in photodetectors, transistors, light emitting diodes, and solar cells. Here, an new in-plane intramolecular WSe2 p-n junction is realized, in which the n-type region and p-type region are chemically doped by polyethyleneimine and electrically doped by the back-gate, respectively. An ideal factor of 1.66 is achieved, proving the high quality of the p-n junction realized by this method. As a photovoltaic detector, the device possesses a responsivity of 80 mAW-1 (≈20% external quantum efficiency), a specific detectivity of over 1011 Jones and fast response features (200 μs rising time and 16 μs falling time) at zero bias, simultaneously. Moreover, a large open-circuit voltage of 0.38 V and an external power conversion efficiency of ≈1.4% realized by the device also promises its potential in microcell applications.

Long-Distance Distribution of Atom-Photon Entanglement at Telecom Wavelength.

Entanglement between stationary quantum memories and photonic channels is the essential resource for future quantum networks. Together with entanglement distillation, it will enable efficient distribution of quantum states. We report on the generation and observation of entanglement between a ^{87}Rb atom and a photon at telecom wavelength transmitted through up to 20km of optical fiber. For this purpose, we use polarization-preserving quantum frequency conversion to transform the wavelength of a photon entangled with the atomic spin state from 780nm to the telecom S band at 1522nm. We achieve an unprecedented external device conversion efficiency of 57% and observe an entanglement fidelity between the atom and telecom photon of ≥78.5±0.9% after transmission through 20km of optical fiber, mainly limited by decoherence of the atomic state. This result is an important milestone on the road to distribute quantum information on a large scale.

Highly efficient and reproducible evanescent Raman converters based on a silica nanofiber immersed in a liquid

We present highly efficient and reproducible Raman converters built with a silica nanofiber immersed in ethanol. The converters are pumped at 532 nm in the sub-nanosecond regime and the first Stokes order photons are generated in the evanescent field probing the liquid at 630 nm. Two standard fibers (SMF28 and 460HP) are tested and compared. The Raman conversion operating range limited by the damage threshold is optimized, leading to an external Raman conversion efficiency up to 60% with a nanofiber radius of 300 nm and a length of 8 cm. The extracted Stokes energy is 0.29 μJ, which is three times higher than the previous result. We give guidelines for the design of other efficient evanescent Raman converters, opening the way for a new family of all-fibered compact Raman sources.

Energy transfer-enhanced external power conversion efficiency in blended polymeric thin film solar devices

In this paper, the spectral and electrical properties of a conjugated polymer poly [(9, 9-dioctyl-2, 7-divinylenefluorenylene)-alt-co-(1, 4-phenylene)] (PFO–MEH–PPV) with poly[3-(2-ethyl-isocyanato-octadecanyl) thiophene] (PECOD) in thin films have been studied. First, PFO–MEH–PPV and PECOD were dissolved in tetrahydrofuran and chloroform respectively for different concentrations. These solutions were deposited on glass substrates to form thin films with different thicknesses. The absorbance and photoluminescence spectra for each individual pure polymer were recorded and contrasted with those for blended conjugated polymer’s films to determine the effect of blending on the absorption and photoluminescence. Finally, we present a study on the processing and characterization of organic solar cells fabricated by spin coating pure PFO–MEH–PPV, PECOD and their blend as the organic active layer onto indium tin oxide layer (150 nm), followed by the evaporation of silver cathode (110 nm). The current–voltage characteristics of these cells were determined and external quantum efficiency. Upon blending the two polymers in solid forms, it could be seen that the efficiency (6.25%) for the cells based on a blend layer is higher than the ones without blending (4.4%). Finally, we demonstrated here that the combination/blending of conjugated polymers has resulted in optimized solar device function, with reasonably quantum efficiency higher than 10%.