Solution-processed polymer photodetectors with trap-assisted photomultiplication

Here, we report a trap-assisted photomultiplication(PM) phenomenon in solution-processed organic photodetectors(OPDs) using poly(3-hexylthiophene)(P3HT): indene-C60 bisadduct(ICBA) as the active layer. The maximum external quantum efficiency(EQE) is 685% for the device with 2% ICBA doping ratio, which is much higher than that of OPDs with P3HT:ICBA(1:1) as the active layer. The PM phenomenon is attributed to the hole tunneling injection assisted by trapped electron in ICBA near Al cathode, which can be demonstrated from the EQE spectra and transient photocurrent curves of OPDs with different ICBA doping ratios.     

Device simulation of lead-free CH_3NH_3SnI_3 perovskite solar cells with high efficiency

The lead-free perovskite solar cells(PSCs) have drawn a great deal of research interest due to the Pb toxicity of the lead halide perovskite.CH_3NH_3SnI_3 is a viable alternative to CH_3NH_3PbX_3,because it has a narrower band gap of 1.3 eV and a wider visible absorption spectrum than the lead halide perovskite.The progress of fabricating tin iodide PSCs with good stability has stimulated the studies of these CH_3NH_3SnI_3 based cells greatly.In the paper,we study the influences of various parameters on the solar cell performance through theoretical analysis and device simulation.It is found in the simulation that the solar cell performance can be improved to some extent by adjusting the doping concentration of the perovskite absorption layer and the electron affinity of the buffer and HTM,while the reduction of the defect density of the perovskite absorption layer significantly improves the cell performance.By further optimizing the parameters of the doping concentration(1.3 × 10~(16) cm~3) and the defect density(1 × 10~(15) cm~3) of perovskite absorption layer,and the electron affinity of buffer(4.0 eV) and HTM(2.6 eV),we finally obtain some encouraging results of the J_(sc) of 31.59 mA/cm~2,V_(oc) of 0.92 V,FF of 79.99%,and PCE of 23.36%.The results show that the lead-free CH_3NH_3SnI_3 PSC is a potential environmentally friendly solar cell with high efficiency.Improving the Sn~(2+) stability and reducing the defect density of CH_3NH_3SnI_3 are key issues for the future research,which can be solved by improving the fabrication and encapsulation process of the cell.     

Performance and Stability of Polymer Solar Cells Based on the Blends of Poly(3-Hexylthiophene) and Indene-C_(60) Bis-Adduct

The performance and morphology stability of polymer bulk heterojunction solar cells based on poly(3-hexylthiophene)(P3HT) as the donor and indene-C_(60) bisadduct(ICBA) or methanofullerene[6,6]-phenyl C_(61)-butyric acid methyl ester(PCBM) as the acceptor are compared.Effect of the different donor and acceptor weight ratios on photovoltaic performance of the P3HT:ICBA device is studied.The optimal device achieved power conversion efficiency of 5.51%with J_(sc) of 10.86 mA/cm~2,V_(oc) of 0.83 V,and fill factor(FF) of 61.1%under AM 1.5G(100mW/cm~2)simulated solar illumination.However,the stability measurement shows that cells based on P3HT:ICBA are less stable than those of the device based on P3HT:PCBM.Atomic force microscope results reveal that the morphology of the P3HT:ICBA film changed considerably during the storage periods due to unstable interpenetrating D-A network.This observation can be explained by the fact that there is lack of intermolecular hydrogen bonds in the P3HT:ICBA system.However,in the P3HT:PCBM system the molecules in the blend film are firmly held together in the solid state by means of intermolecular hydrogen bonds originating from C-H… Os bonds(where Os comes from the singly-bonded O atom of PCBM),forming a stable three-dimensional network.The measured PL decay lifetimes for P3HT:PCBM and P3HT:ICBA systems are 33.66 ns and 35.34 ns,respectively,indicating that the P3HT:ICBA system has a less efficient exciton separation efficiency than that of P3HT:PCBM,which may result in the interfacial photogenerated charges accumulated on the D:A interface.Such progressive phase segregation between P3HT and ICBA eventually leads to the degradation in performance and deteriorates the stability of the device.We also present an approach to enhance the stability of P3HT:ICBA systems by adding PCBM as the second acceptor.Our results show that by carefully tuning the contents of PCBM as the second acceptor,more stable polymer solar cells can be obtained.     

Electronegativity explanation on the efficiency-enhancing mechanism of the hybrid inorganic–organic perovskite ABX_3 from first-principles study

Organic–inorganic hybrid perovskites play an important role in improving the efficiency of solid-state dye-sensitized solar cells. In this paper, we systematically explore the efficiency-enhancing mechanism of ABX_3(A = CH_3NH_3; B = Sn,Pb; X = Cl, Br, I) and provide the best absorber among ABX_3 when the organic framework A is CH_3NH_3 by first-principles calculations. The results reveal that the valence band maximum(VBM) of the ABX_3 is mainly composed of anion X p states and that conduction band minimum(CBM) of the ABX_3 is primarily composed of cation B p states. The bandgap of the ABX_3 decreases and the absorptive capacities of different wavelengths of light expand when reducing the size of the organic framework A, changing the B atom from Pb to Sn, and changing the X atom from Cl to Br to I. Finally, based on our calculations, it is discovered that CH_3NH_3 Sn I_3has the best optical properties and its light-adsorption range is the widest among all the ABX_3 compounds when A is CH_3NH_3. All these results indicate that the electronegativity difference between X and B plays a fundamental role in changing the energy gap and optical properties among ABX_3 compounds when A remains the same and that CH_3NH_3 Sn I_3 is a promising perovskite absorber in the high efficiency solar batteries among all the CH_3NH_3BX_3 compounds.     

Performance Improvement of GaN Based Schottky Barrier Ultraviolet Photodetector by Adding a Thin A1GaN Window Layer

We propose a new structure of GaN based Schottky barrier ultraviolet photodetector, in which a thin n-type A1GaN window layer is added on the conventional n^--GaN/n^＋-GaN device structure. The performance of the Schottky barrier ultraviolet photodetector is found to be improved by the new structure. The simulation result shows that the new structure can reduce the negative effect of surface states on the performance of Schottky barrier GaN photodetectors, improving the quantum efficiency and decreasing the dark current. The investigations suggest that the new photodetector can exhibit a better responsivity by choosing a suitably high carrier concentration and thin thickness for the A1GaN window layer.     

Multiple Fano resonances in metal–insulator–metal waveguide with umbrella resonator coupled with metal baffle for refractive index sensing

A single baffle metal–insulator–metal(MIM) waveguide coupled with a semi-circular cavity and a cross-shaped cavity is proposed based on the multiple Fano resonance characteristics of surface plasmon polaritons(SPPs) subwavelength structure. The isolated state formed by two resonators interferes with the wider continuous state mode formed by the metal baffle, forming Fano resonance that can independently be tuned into five different modes. The formation mechanism of Fano resonance is analyzed based on the multimode interference coupled mode theory(MICMT). The finite element method(FEM) and MICMT are used to simulate the transmission spectra of this structure and analyze the influence of structural parameters on the refractive index sensing characteristics. And the transmission responses calculated by the FEM simulation are consistent with the MICMT theoretical results very well. The results show that the figure of merit(FOM)can reach 193 and the ultra-high sensitivity is 1600 nm/RIU after the structure parameters have been optimized, and can provide theoretical basis for designing the high sensitive refractive index sensors based on SPPs waveguide for high-density photonic integration with excellent performance in the near future.     

In-plane oriented CH_3NH_3PbI_3 nanowire suppression of the interface electron transfer to PCBM

One-dimensional nanowire is an important candidate for lead-halide perovskite-based photonic detectors and solar cells. Its surface population, diameter, and growth direction, etc., are critical for device performance. In this research,we carried out a detailed study on electron transfer process at the interface of nanowire CH_3 NH_3 PbI_3(N-MAPbI_3)/Phenyl C61 butyric acid methyl-ester synonym(PCBM), as well as the interface of compact CH_3 NH_3 PbI_3(C-MAPbI_3)/PCBM by transient absorption spectroscopy. By comparing the carrier recombination dynamics of N-MAPbI_3, N-MAPbI_3/PCBM,C-MAPbI_3, and C-MAPbI_3/PCBM from picosecond(ps) to hundred nanosecond(ns) time scale, it is demonstrated that electron transfer at N-MAPbI_3/PCBM interface is less efficient than that at C-MAPbI_3/PCBM interface. In addition, electron transfer efficiency at C-MAPbI_3/PCBM interface was found to be excitation density-dependent, and it reduces with photo-generation carrier concentration increasing in a range from 1.0 × 1018 cm~(-3)–4.0 × 1018 cm~(-3). Hot electron transfer,which leads to acceleration of electron transfer between the interfaces, was also visualized as carrier concentration increases from 1.0 × 10~(18) cm~(-3)–2.2 × 10~(18) cm~(-3).     

A reconfigurable terahertz polarization converter based on metal–graphene hybrid metasurface

A metal-graphene hybrid metasurface polarization converter is designed in this Letter.The unit cell of the hybrid metasurface is composed of a butterfly-shaped structure whose branches are connected by multi-layer graphene sheets.The proposed device can be reconfigured from linear-to-circular polarization to cross-polarization by changing the Fermi energy of graphene.The simulation results show that for three-layer graphene,the device acts as a linear-to-circular polarization converter when EF=0 eV and switches to a cross-polarization converter when EF=0.5 eV.Compared with single-layer graphene,the device with three-layer graphene can maintain the cross-polarization conversion performance under low Fermi energy.Furthermore,two equivalent circuits in the x and y directions are developed to understand the working mechanism of the device.     

Magnetocaloric effect in the layered organic–inorganic hybrid(CH_3NH_3)_2CuCl_4

We present a study of magnetocaloric effect of the quasi-two-dimensional(2D) ferromagnet(CH_3NH_3)_2CuCl_4 in ab plane(easy-plane). From the measurements of magnetic field dependence of magnetization at various temperatures,we have discovered a large magnetic entropy change associated with the ferromagnetic–paramagnetic transition. The heat capacity measurements reveal an abnormal adiabatic change below the Curie temperature T_c~8.9 K, which is caused by the nature of quasi-2D layered crystal structure. These results suggest that perovskite organic–inorganic hybrids with a layered structure are suitable candidates as working substances in magnetic refrigeration technology.     

Localized electric-field-enhanced low-light detection by a 2D SnS visible-light photodetector

Due to their excellent carrier mobility, high absorption coefficient and narrow bandgap, most 2 D IVA metal chalcogenide semiconductors(GIVMCs, metal = Ge, Sn, Pb;chalcogen = S, Se) are regarded as promising candidates for realizing high-performance photodetectors. We synthesized high-quality two-dimensional(2 D) tin sulfide(Sn S) nanosheets using the physical vapor deposition(PVD) method and fabricated a 2 D Sn S visible-light photodetector. The photodetector exhibits a high photoresponsivity of 161 A·W~(-1) and possesses an external quantum efficiency of 4.45 × 10~4%, as well as a detectivity of 1.15 × 10~9 Jones under 450 nm blue light illumination. Moreover, under poor illumination at optical densities down to 2 m W·cm~(-2), the responsivity of the device is higher than that at stronger optical densities. We suggest that a photogating effect in the 2 D Sn S photodetector is mainly responsible for its low-light responsivity. Defects and impurities in 2 D Sn S can trap carriers and form localized electric fields, which can delay the recombination process of electron-hole pairs,prolong carrier lifetimes, and thus improve the low-light responsivity. This work provides design strategies for detecting low levels of light using photodetectors made of 2 D materials.     

Exploring photocurrent output from donor/acceptor bulk-heterojunctions by monitoring exciton quenching

In this work,a series of polymer bulk-heterojunctions is fabricated based on the combinations of different donors(Ds)(P3HT and PCPDTBT) and acceptors(As)(PCBM,ICBA,and F8BT).Exciton quenching efficiencies of the D–A pairs are obtained in order to quantify charge-transfer between the donor and the acceptor via a modified approach developed in conjunction with experimental results of optical absorption and photoluminescence spectra.It is discovered that the exciton quenching efficiency in the combination of PCPDTBT:PCBM and P3HT:PCBM reaches 70% and over,but in PCPDTBT:ICBA it is about 12%.A relatively high △LUMOdonor-acceptorresults in a relatively high exciton quenching efficiency,which is responsible for better charge separation.The results agreed well with the photocurrent effect of the heterojunction layers.The work offers a convenient way to predict a potentially promising photovoltaic material with a selected D–A pair.     

Closed-form breakdown voltage/specific on-resistance model using charge superposition technique for vertical power double-diffused metal–oxide–semiconductor device with high-κ insulator

An improved vertical power double-diffused metal–oxide–semiconductor(DMOS) device with a p-region(P1) and high-κ insulator vertical double-diffusion metal–oxide–semiconductor(HKP-VDMOS) is proposed to achieve a better performance on breakdown voltage(BV)/specific on-resistance(Ron,sp) than conventional VDMOS with a high-κ insulator(CHK-VDMOS).The main mechanism is that with the introduction of the P-region,an extra electric field peak is generated in the drift region of HKP-VDMOS to enhance the breakdown voltage.Due to the assisted depletion effect of this p-region,the specific on-resistance of the device could be reduced because of the high doping density of the N-type drift region.Meanwhile,based on the superposition of the depleted charges,a closed-form model for electric field/breakdown voltage is generally derived,which is in good agreement with the simulation result within 10% of error.An HKP-VDMOS device with a breakdown voltage of 600 V,a reduced specific on-resistance of 11.5 m?·cm~2 and a figure of merit(FOM)(BV~2/Ron,sp)of 31.2 MW·cm~(-2) shows a substantial improvement compared with the CHK-VDMOS device.     

Improved performance of polymer solar cells by using inorganic,organic, and doped cathode buffer layers

The interface between the active layer and the electrode is one of the most critical factors that could affect the device performance of polymer solar cells. In this work, based on the typical poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester(P3HT:PCBM) polymer solar cell, we studied the effect of the cathode buffer layer(CBL) between the top metal electrode and the active layer on the device performance. Several inorganic and organic materials commonly used as the electron injection layer in an organic light-emitting diode(OLED) were employed as the CBL in the P3HT:PCBM polymer solar cells. Our results demonstrate that the inorganic and organic materials like Cs_2CO_3, bathophenanthroline(Bphen), and 8-hydroxyquinolatolithium(Liq) can be used as CBL to efficiently improve the device performance of the P3HT:PCBM polymer solar cells. The P3HT:PCBM devices employed various CBLs possess power conversion efficiencies(PCEs) of 3.0%–3.3%, which are ca. 50% improved compared to that of the device without CBL. Furthermore, by using the doped organic materials Bphen:Cs_2CO_3 and Bphen:Liq as the CBL, the PCE of the P3HT:PCBM device will be further improved to 3.5%, which is ca. 70% higher than that of the device without a CBL and ca. 10% increased compared with that of the devices with a neat inorganic or organic CBL.     

High performance silicon waveguide germanium photodetector

High-performance Ge-on-SOI p–i–n waveguide photodetectors with different sizes were fabricated. The performances, in terms of dark-current, photo current responsivity and 3-d B bandwidth, were well studied. A responsivity of 0.842 A/W at 1550 nm and dark current of 70 n A was measured from this detector at-1 V. The detector with a size of4 μm×10 μm demonstrated an optical band width of 19 GHz at-5 V for 1550 nm. Both the experimental results and the finite-difference time domain simulation show that, when the device size is above a certain threshold, the absorption is not sensitively dependent on such designing parameters as the width and length of the photodetector.     

Flexible electrically pumped random lasing from ZnO nanowires based on metal–insulator–semiconductor structure

Flexible electrically pumped random laser(RL) based on ZnO nanowires is demonstrated for the first time to our knowledge. The ZnO nanowires each with a length of 5 μm and an average diameter of 180 nm are synthesized on flexible substrate(ITO/PET) by a simple hydrothermal method. No obvious visible defect-related-emission band is observed in the photoluminescence(PL) spectrum, indicating that the ZnO nanowires grown on the flexible ITO/PET substrate have few defects. In order to achieve electrically pumped random lasing with a lower threshold, the metal–insulator–semiconductor(MIS) structure of Au/SiO_2/ZnO on ITO/PET substrate is fabricated by low temperature process. With sufficient forward bias, the as-fabricated flexible device exhibits random lasing, and a low threshold current of ～ 11.5 m A and high luminous intensity are obtained from the ZnO-based random laser. It is believed that this work offers a case study for developing the flexible electrically pumped random lasing from ZnO nanowires.     

Improving nonlinearity tolerance of 112-Gb/s PDM OFDM systems with coherent detection using BLAST algorithm

A novel long wavelength photodetector with dual-wavelength spectral response is designed and fabricated using a step-shaped Fabry-Prot (F-P) filter structure.The step-shaped GaAs/AlGaAs distributed Bragg reflectors and the InP PIN photodetector are grown on a GaAs substrate using low pressure metal organic chemical vapor deposition.High quality GaAs/InP heteroepitaxy is realized by employing a thin low temperature buffer layer.The photodetector structure is optimized by theoretical simulation.This device has a dual-peak distance of 19 nm (1 558 and 1 577 nm).The 3-dB bandwidth of 16 GHz is simultaneously obtained with peak quantum efficiencies of 8.5% and 8.6% around 1 558 and 1 577 nm,respectively.     

First-principles analysis of the structural,electronic, and elastic properties of cubic organic-inorganic perovskite HC(NH_2)_2PbI_3

The structural, electronic, and elastic properties of cubic HC(NH_2)_2PbI_3 perovskite are investigated by density functional theory using the Tkatchenko–Scheffler pairwise dispersion scheme. Our relaxed lattice parameters are in agreement with experimental data. The hydrogen bonding between NH_2 and I ions is found to have a crucial role in FAPbI_3 stability. The first calculated band structure shows that HC(NH_2)_2PbI_3 has a direct bandgap(1.02 eV) at R-point, lower than the bandgap(1.53 eV) of CH_3NH_3PbI_3. The calculated density of states reveals that the strong hybridization of s(Pb)–p(I) orbital in valence band maximum plays an important role in the structural stability. The photo-generated effective electron mass and hole mass at R-point along the R–Γ and R–M directions are estimated to be smaller: m_e~*= 0.06 m0 and m_h~*= 0.08 m0 respectively, which are consistent with the values experimentally observed from long range photocarrier transport. The elastic properties are also investigated for the first time, which shows that HC(NH_2)_2PbI_3 is mechanically stable and ductile and has weaker strength of the average chemical bond. This work sheds light on the understanding of applications of HC(NH_2)_2PbI_3 as the perovskite in a planar-heterojunction solar cell light absorber fabricated on flexible polymer substrates.     

Graphene/SrTiO_3 interface-based UV photodetectors with high responsivity

Strontium titanate(SrTiO3),which is a crucial perovskite oxide with a direct energy band gap of 3.2 eV,holds great promise for ultraviolet(UV)photodetection.However,the response performance of the conventional SrTiO3-based photodetectors is limited by the large relative dielectric constant of the material,which reduces the internal electric field for electron-hole pair separation to form a current collected by electrodes.Recently,graphene/semiconductor hybrid photodetectors by van-der-Waals heteroepitaxy method demonstrate ultrahigh sensitivity,which is benefit from the interface junction architecture and then prolonged lifetime of photoexcited carriers.Here,a graphene/SrTiO3 interface-based photodetector is demonstrated with an ultrahigh responsivity of 1.2×106 A/W at the wavelength of 325 nm and～2.4×104 A/W at 261 nm.The corresponding response time is in the order of～ms.Compared with graphene/GaN interface junctionbased hybrid photodetectors,～2 orders of magnitude improvement of the ultrahigh responsivity originates from a gain mechanism which correlates with the large work function difference induced long photo-carrier lifetime as well as the low background carrier density.The performance of high responsivity and fast response speed facilitates SrTiO3 material for further efforts seeking practical applications.     

Band-stop optical nanofilters with split-ring resonators based on metal–insulator–metal structure

Novel band-stop filters with circular split-ring resonators based on the metal–insulator–metal(MIM) structure are presented, with their transmission properties of SPPs propagating through the filter simulated by the finite-difference timedomain(FDTD) method. The variation of the gap of the split ring can affect the transmission characteristics, i.e., the transmission spectrum of SPPs exhibiting a shift, which is useful for modulating the filter. Linear and nonlinear media are used in the resonator respectively. By varying the refractive index of the linear medium, the transmission properties can be changed obviously, and the effect caused by changing the incident intensity with a nonlinear medium is similar.Several resonant modes that are applicable can be enhanced by changing the position of the gap of the split ring. Thus, the transmission properties can be modulated by adjusting the size of the gap, varying the refractive index, and changing the incident intensity of the input light. These methods may play significant roles in applications of optical integrated circuits and nanostructural devices.