Challenges and Prospects of Molecular Spintronics

Molecular spintronics,as an emerging field that makes full use of the advantage of ultralong room-temperature spin lifetime and abundant electrical-optical-magnetic properties of molecular semiconductors,has gained wide attention for its great potential for further commercial applications.Despite the significant progress that has been made,there remain several huge challenges that limit the future development of this field.This Perspective provides discussions on the spin transport mechanisms and performances of molecular semiconductors,spinterface effect,and related spin injection in spintronic devices,and current spin-charge interactive functionalities,along with the summarization of the main obstacles of these aspects.Furthermore,we particularly propose targeted solutions,aiming to enhance the spin injection and transport efficiency by molecular design and interface engineering and explore diverse spinrelated functionalities.Through this Perspective,we hope it will help the spintronic community identify the research trends and accelerate the development of molecular spintronics.

高开路电压与低能量损失有机太阳能电池研究进展

有机太阳能电池(organic solar cells,OSCs)是光伏发电和能源功能材料的重要组成部分,因其制作简单、材料来源广泛、轻量化、柔性好等突出优势成为电池材料领域研究的热点.然而,与无机/钙钛矿太阳能电池相比,有机太阳能电池存在较大的电压损失(即能量损失),限制了效率的进一步提升.目前实验室报道最高效率远低于肖克利-奎伊瑟(Shockley-Queisser)理论所定义的极限效率.因此,最大化降低有机太阳能电池的电压损失,是进一步提升电池器件效率的关键.针对上述问题,国家纳米科学中心的相关科研人员做出了很多努力,在高开路电压、低能量损失的有机太阳能电池分子设计、理论计算以及新型器件结构等方面做了非常出色的工作.本文综述了近年来的相关研究进展,希望为有机太阳能电池相关领域的研究者提供借鉴并对有机太阳能电池的发展起到促进作用.

China’s larch stock volume estimation using Sentinel-2 and LiDAR data

Forest Stock Volume(FSV)is one of the key indicators in forestry resource investigation and management on local,regional,and national scales.Limited by the saturation problems of optical satellite remote-sensing imagery in the retrieving of stock volume,and the high cost of Light Detection And Ranging(LiDAR)data,it is still challenging to estimate FSV in a large area using single-sensor remote-sensing data.In this paper,a method integrated multispectral satellite imagery and LiDAR data was developed to map stock volume in a large area.A random forest model was adopted to estimate the stock volume of larch forest in China based on the training samples from the Airborne Laser Scanning(ALS)-derived stock volume and corresponding Sentinel-2 imagery.Validation using National Forest Inventory(NFI)data,ALS-derived stock volume and ground investigation data demonstrated that the estimated stock volume had a high accuracy(R2=0.59,RMSE=59.69 m^(3)/ha,MD=39.96 m^(3)/ha when validated with NFI data;R2 ranged from 0.77 to 0.85,RMSE ranged from 38.68 m^(3)/ha to 67.38 m^(3)/ha,MD ranged from 24.90 m^(3)/ha to 37.27 m^(3)/ha when validated with ALS stock volume;R2=0.42,RMSE=79.10 m^(3)/ha,MD=62.06 m^(3)/ha when validated with field investigation data).Results of this paper indicated the applicability of estimating stock volume of larch forest in a large area by combining Sentinel-2 data and airborne LiDAR data.

Molecular design for enhanced spin transport in molecular semiconductors

Molecular semiconductors(MSCs),characterized by a longer spin lifetime than most of other materials due to their weak spin relaxation mechanisms,especially at room temperature,together with their abundant chemical tailorability and flexibility,are regarded as promising candidates for spintronic applications.Molecular spintronics,as an emerging subject that utilizes the unique properties of MSCs to study spin-dependent phenomena and properties,has attracted wide attention.In molecular spintronic devices,MSCs play the role as medium for information transport,process,and storage,in which the efficient spin inject–transport process is the prerequisite.Herein,we focus mainly on summarizing and discussing the recent advances in theoretical principles towards spin transport of MSCs in terms of the injection of spin-polarized carriers through the ferromagnetic metal/MSC interface and the subsequent transport within the MSC layer.Based on the theoretical progress,we cautiously present targeted design strategies of MSCs that contribute to the optimization of spin-transport efficiency and give favorable approaches to exploring accessional possibilities of spintronic materials.Finally,challenges and prospects regarding current spin transport are also presented,aiming to promote the development and application of the rosy and energetic field of molecular spintronics.

High-performance floating-gate organic phototransistors based on n-type core-expanded naphthalene diimides

In the field of organic phototransistor, achieving both broad-spectral and high photosensitivity has always been a big challenge. The innovation of device structure has previously proven to be a possible solution to this problem. Here in this study, a novel organic phototransistor based on a high mobility n-type small molecule as the conducting layer and an isolated bulk heterojunction light-absorbing layer as the floating gate has been demonstrated in this study. With the special designed device structure, the phototransistor shows extremely high sensitivity to broad spectral and weak light irradiation, and the photoresponsivity and photocurrent/dark-current ratio of the device can reach up to 4840 mA/W and 1.8×10~5 respectively.For conclusion, this study suggests a potential way to obtain high-performance phototransistors at room temperature, which will further promote the commercial application of organic phototransistors.

Organic solar cells based on small molecule donor and polymer acceptor

Small molecule donor/polymer acceptor(SD/PA)-type organic solar cells(OSCs)have attracted widespread attention in recent years due to the continuing power conversion efficiency(PCE)growth,near 10%,and the excellent thermal stability for the practical applications.However,the development of SD/PA-type OSCs lags far behind that of polymer donor/small molecule acceptor(PD/SA)-type OSCs,which are also based on the combination of small molecule and polymer,with the PCEs exceeding 18%.The reasons accounting for this great gap are well worth exploring.In this review,we have analyzed the key factors affecting the photovoltaic performances of SD/PA-type OSCs,systematically summarized the research progress of SD/PA type OSCs in recent years,and put forward our own views on the future development of SD/PA type OSCs.

Enhancing the performances of all-small-molecule ternary organic solar cells via achieving optimized morphology and 3D charge pathways

With the emergence of non-fullerene acceptors(NFAs),the power conversion efficiencies(PCEs)of allsmall-molecule organic solar cells(ASM-OSCs)have been significantly improved.However,due to the strong crystallinities of small molecules,it is much more challenging to obtain the ideal phase separation morphology and efficient charge transport pathways for ASM-OSCs.Here,a high-efficiency ternary ASMOSC has been successfully constructed based on H11/IDIC-4 F system by introduction of IDIC with a similar backbone as IDIC-4F but weak crystallinity.Notably,the addition of IDIC has effectively suppressed large-scale phase aggregation and optimized the morphology of the blend film.More importantly,the molecular orientation has also been significantly adjusted,and a mixed face-on and edge-on orientation has formed,thus establishing a more favorable three-dimensional(3D)charge pathways in the active layer.With these improvements,the enhanced short-circuit current density(JSC)and fill factor(FF)of the ternary system have been achieved.In addition,because of the high lowest unoccupied molecular orbital(LUMO)energy level of IDIC as well as the alloyed structure of the IDIC and IDIC-4F,the promoted open circuit voltage(VOC)of the ternary system has also been realized.

The effect of alkyl chain branching positions on the electron mobility and photovoltaic performance of naphthodithiophene diimide(NDTI)-based polymers

Conjugated polymers are widely used in organic optoelectronic devices due to their solution processability, thermal stability and structural diversity. Generally, alkyl side chains must be utilized to increase the solubility of final polymers in the processing solvent. However, the effects of different type alkyl chains on the properties of n-type photovoltaic polymers have rarely been investigated. In this article, we synthesized three naphthodithiophene diimide（NDTI） based polymers containing bulky alkyl chains with different branching position, named as NDTI-1, NDTI-2 and NDTI-3, respectively. We systematically investigated the effect of different branching point on the molecular packing, charge transport and photovoltaic performance. When moving the branching point away from the backbone, the intermolecular interaction became stronger, which could be proved by 2D grazing incidence wide angle X-ray scattering（GIWAXS） measurement. Therefore, the electron mobilities in organic field-effect transistors gradually increased from 2.11×10-3 cm2 V/-1 s-1 for NDTI-1 to 4.70×10-2 cm2 V/-1 s-1 for NDTI-2 and 9.27×10-2 cm2 V/-1 s-1 for NDTI-3,which are quite high values for polymers with face-on orientation. In addition, the NDTI-2 and NDTI-3 thin films exhibited redshifted absorption spectra compared with NDTI-1. When blending with three classic donor polymers PBDB-T, PTB7-Th and PE61, NDTI-2 based devices always showed the higher power conversion efficiencies（PCEs） than the other two polymers（beside the comparable result of PTB7-Th:NDTI-3 combination） as a result of the high photocurrent response and high fill factor. Our results indicate that bulky alkyl chain with branching point at 2-position should be a good and safe choice for the design of naphthodithiophene diimide-based and even naphthalene diimide-based n-type photovoltaic polymers.

Side chain engineering of quinoxaline-based small molecular nonfullerene acceptors for high-performance poly(3-hexylthiophene)-based organic solar cells

Poly(3-hexylthiophene)(P3HT)is one of the most used semiconducting polymers for organic photovoltaics because it has potential for commercialization due to its easy synthesis and stability.Although the rapid development of the small molecular non-fullerene acceptors(NFAs)have largely improved the power conversion efficiency(PCE)of organic solar cells(OSCs)based on other complicated p-type polymers,the PCE of P3HT-based OSCs is still low.In addition,the design principle and structure-properties correlation for the NFAs matching well with P3HTare still unclear and need to be investigated in depth.Here we designed a series of NFAs comprised of acceptor(A)and donor(D)units with an A2-A1-D-A1-A2 configuration.These NFAs are abbreviated as Qx3,Qx3 b and Qx3c,where indaceno[1,2-b:5,6-b′]dithiophene(IDT),quinoxaline(Qx)and 2-(1,1-dicyanomethylene)rhodanine serve as the middle D,bridged A1 and the end group A2,respectively.By subtracting the phenyl side groups appended on both IDT and Qx skeletons,the absorption spectra,energy levels and crystallinity could be regularly modulated.When paired with P3 HT,three NFAs show totally different photovoltaic performance with PCEs of 3.37%(Qx3),6.37%(Qx3b)and 0.03%(Qx3 c),respectively.From Qx3 to Qx3b,the removing of phenyl side chain in the middle IDT unit results in the increase of crystallinity and electron mobility.However,after subtracting all the grafted phenyl side groups on both IDT and Qx units,the final molecule Qx3 c exhibits the lowest PCE of only 0.03%,which is mainly attributed to the serious phase-separation of the blend film.These results demonstrate that optimizing the substituted position of phenyl side groups for A2-A1-D-A1-A2 type NFAs is vital to regulate the optoelectronic property of molecule and morphological property of active layer for high performance P3HT-based OSCs.