Optimizing theπ-Bridge of Non-fullerene Acceptors to Suppress Dark Current in NIR Organic Photodetectors

Recently,the rapid development of non-fullerene acceptors(NFAs)has laid the foundation for performance improvements in near-infrared(NIR)organic photodetectors(OPDs).However,reducing the bandgap of NFAs to achieve strong absorption in the shorter-wave region usually leads to increased dark current density(J_(d))and decreased responsivity(R),severely limiting the detectivity(D*)of NIR-OPDs.To date,it remains challenging to manipulate the J_(d) of NIR-OPDs through rational structure engineering of NFAs.Herein,three NIR-NFAs,namely bis(2-decyltetradecyl)4,4′-(2′,7′-di-tert-butylspiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2,6-diyl)bis(6-(((Z)-1-(dicyanomethylene)-5,6-difluoro-3-oxo-1,3-dihydro-2H-inden-2-ylidene)methyl)thieno[3,4-b]thiophene-2-carboxylate)(TSIC-4F),bis(2-decyltetradecyl)6,6′-(2′,7′-di-tert-butylspiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2,6-diyl)bis(4-(((Z)-1-(dicyanomethylene)-5,6-difluoro-3-oxo-1,3-dihydro-2H-inden-2-ylidene)methyl)thieno[3,4-b]thiophene-2-carboxylate)(STIC-4F),and 2,2′-((2Z,2′Z)-(((2′,7′-di-tert-butylspiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2,6-diyl)bis(2,3-bis(5-(2-butyloctyl)thiophen-2-yl)thieno[3,4-b]pyrazine-7,5-diyl))bis(metha-neylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile(TPIC-4F),were designed using the thieno[3,4-b]thiophene(TT)and thieno[3,4-b]pyrazine(TPy)derivatives as theπ-bridge.Owing to the intramolecular S-S and S-N interactions,STIC-4F and TPIC-4F exhibited smaller backbone distortions than TSIC-4F.A significantly red-shifted absorption with a peak at 1015 nm was observed in TPIC-4F film,larger than that(ca.960 nm)for TSIC-4F and STIC-4F films.Moreover,OPDs operating in a photovoltaic mode were successfully fabricated,and TPIC-4F-based OPDs achieved the lowest J_(d) of 3.18×10^(-8) A/cm^(2) at-0.1 V.Impressively,although TPIC-4F-based OPDs exhibited the lowest R,higher shot-noise-limited specific detectivity(D_(sh)*)in 1000-1200 nm could be achieved due to its lowest J_(d).This study underscored the effectiveness of optimizing theπ-bridge structure of NFAs to suppress J_(d),ultimately attaining higher D_(sh)*in the NIR region.

Organic X-Ray Image Sensors Using a Medium Bandgap Polymer Donor with Low Dark Current

The development of portable X-ray detectors is necessary for diagnosing fractures in unconscious patients in emergency situations.However,this is quite challenging because of the heavy weight of the scintillator and silicon photodetectors.The weight and thickness of X-ray detectors can be reduced by replacing the silicon layer with an organic photodetectors.This study presents a novel bithienopyrroledione-based polymer donor that exhibits excellent photodetection properties even in a thick photoactive layer(~700 nm),owing to the symmetric backbone and highly soluble molecular structure of bithienopyrroledione.The ability of bithienopyrroledione-based polymer donor to strongly suppress the dark current density(Jd~10−10 A cm^(−2))at a negative bias(−2.0 V)while maintaining high responsivity(R=0.29 A W−1)even at a thickness of 700 nm results in a maximum shot-noise-limited specific detectivity of D_(sh)^(*)=2.18×10^(13)Jones in the organic photodetectors.Printed organic photodetectors are developed by slot-die coating for use in X-ray detectors,which exhibit D_(sh)^(*)=2.73×10^(12)Jones with clear rising(0.26 s)and falling(0.29 s)response times upon X-ray irradiation.Detection reliability is also proven by linear response of the X-ray detector,and the X-ray detection limit is 3 mA.

Highly Efficient Organic Ultraviolet Photodetectors Based on Re(I) Complexes

High response organic ultraviolet photodetectors（UV-PDs） were demonstrated with 4,4＇,4”- tris[3-methyl-pheny（phenyl）amino]triphenylamine（m-MTDATA） and two Re（Ⅰ） complexes, （bathocuproine）- Re（CO）3CI（Re-BCP） and （bathophenanthroline）Re（CO）3Cl（Re-Bphen） to act as the electron donor and acceptor, re- spectively. UV-PDs have the configuration of indium tin oxide（ITO）/m-MTDATA（25 nm）/m-MTDATA：Re-complex （25-35 nm）/Re-complex（20 nm）/LiF（l nm）/Al（200 nm） with different blend layer thicknesses of 25, 30 and 35 nm. The optimized PD based on Re-Bphen offers a corrected-dark photocurrent up to 760μA/cm^2 at -10 V, corresponding to a response of 310 mA/W which is among the best values reported for organic UV-PDs. Excellent electron transport ability makes for such high photo-to-electron conversion.

All-Polymer Solar Cells and Photodetectors with Improved Stability Enabled by Terpolymers Containing Antioxidant Side Chains

It is of vital importance to improve the long-term and photostability of organic photovoltaics,including organic solar cells(OSCs)and organic photodetectors(OPDs),for their ultimate industrialization.Herein,two series of terpolymers featuring with an antioxidant butylated hydroxytoluene(BHT)-terminated side chain,PTzBI-EHp-BTBHTx and N2200-BTBHTx(x=0.05,0.1,0.2),are designed and synthesized.It was found that incorporating appropriate ratio of benzothiadiazole(BT)with BHT side chains on the conjugated backbone would induce negligible effect on the molecular weight,absorption spectra and energy levels of polymers,however,which would obviously enhance the photostability of these polymers.Consequently,all-polymer solar cells(all-PSCs)and photodetectors were fabricated,and the all-PSC based on PTzBI-EHp-BTBHT0.05:N2200 realized an optimal power conversion efficiency(PCE)approaching~10%,outperforming the device based on pristine PTzBI-EHp:N2200.Impressively,the all-PSCs based on BHT-featuring terpolymers displayed alleviated PCEs degradation under continuous irradiation for 300 h due to the improved morphological and photostability of active layers.The OPDs based on BHT-featuring terpolymers achieved a lower dark current at−0.1 bias,which could be stabilized even after irradiation over 400 h.This study provides a feasible approach to develop terpolymers with antioxidant efficacy for improving the lifetime of OSCs and OPDs.

High Responsivity Organic Ultraviolet Photodetector Based on NPB Donor and C60 Acceptor

We report fabrication and characterization of organic heterojunction UV detectors based on N,N＇-bis（naphthalen- 1-y1）-N,N＇-bis （phenyl） benzidine （NPB） and fullerene C60. The effects of different thicknesses of NPB and C60 layers are studied and compared. Notably, the optimal thicknesses of electron acceptor C60 and electron donor NPB are 40 nm and 80 nm, respectively. The J V characteristic curves of the device demonstrate a three-order- of-magnitude difference when illuminated under a 350nm UV light and in the dark at -0.5 V. The device exhibits high sensitivity in the region of 320-380nm with the peak located around 35Onm. Especially, it shows excellent photo-response characteristic with a responsivity as high as 315 mA/W under the illumination of 192μW.cm 2 350nm UV light at -5 V. These results indicate that the NPB/C60 heterojunction structure device might be used as low-cost low-voltage UV photodetectors.

Performance enhancement in organic solar cells and photodetectors enabled by donor phase optimization at the surface of hole transport layer

The domain purity,material crystallinity and distribution at the interface between the active layer and the transport layer have an important impact on the performance of organic solar cells(OSCs)and organic photodetectors(OPDs),while this focal issue has received less attention in previous studies.From this perspective,a new method to simultaneously enhance the performance of OSC and OPD is proposed,namely,using a sequential deposition method to first construct a compact stacking structure of dualdonor(D18-Cl:PTO2)eutectic in the donor layer,and then induce the ordered deposition of the acceptor(Y6).Compared with the conventional bulk heterojunction(BHJ),the active layer realized by this method not only improves the crystallinity and stacking order of the constituent material on the surface of the transport layer,but also regulates a good vertical distribution,which is conducive to improving the charge transport and extraction efficiency,reducing the leakage current,and enhancing the stability of the device.As a result,the OSC device based on the D18-Cl:PTO2/Y6 structure achieves a power conversion efficiency of up to 17.65%and good light-degradation stability,which is much better than that of BHJbased OSC(PCE of 16.37%).For the OPD,the dark current at reverse bias is reduced by more than an order of magnitude,and the maximum responsivity is improved to 0.52 A/W through the optimization of the donor phase at the interface.Moreover,the strategy does not require additional post-processing compared to the BHJ preparation,which reduces the device construction cost and process complexity,providing an effective way for developing high-performance organic optoelectronic devices.

Recent Progress on Organic Near-Infrared Photodetectors:Mechanism,Devices,and Applications

Comprehensive Summary Near infrared light organic photodetectors have attracted tremendous attention due to their tailorable response,ease of processing,compatibility with flexible substrate,room temperature operation and broad applications such as remote sensing,health monitoring,artificial vision,night vision,and so on.Recently,the great improvement obtained on the important figures of merit performances has made organic photodetectors catch up and even surpass those of inorganic photodetectors in some respects.In this review,after a brief illustration of the organic photodetectors'figures of merit performances,we summarize the research progress of panchromatic and narrowband near infrared light organic photodetectors from their working mechanism,strategies to achieve narrowband near infrared light organic photodetectors,to some practical applications.Finally,we discuss the development challenge of the near infrared light organic photodetectors.

Organic near-infrared photodetectors with photoconductivityenhanced performance

Organic near-infrared(NIR)photodetectors with essential applications in medical diagnostics,night vision,remote sensing,and optical communications have attracted intensive research interest.Compared with most conventional inorganic counterparts,organic semiconductors usually have higher absorption coefficients,and their thin active layer could be sufficient to absorb most incident light for effective photogeneration.However,due to the relatively poor charge mobility of organic materials,it remains challenging to inhibit the photogenerated exciton recombination and effectively extract carriers to their respective electrodes.Herein,this challenge was addressed by increasing matrix conductivities of a ternary active layer(D–A–D structure NIR absorber[2TT-oC6B]:poly(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)benzidin[PolyTPD]:[6,6]-phenyl-C61-butyric acid methyl ester[PCBM]=1:1:1)upon in situ incident light illumination,significantly accelerating charge transport through percolated interpenetrating paths.The greatly enhanced photoconductivity under illumination is intrinsically related to the unique donor–acceptor molecular structures of PolyTPD and 2TT-oC6B,whereas stable intermolecular interaction has been verified by systematic molecular dynamics simulation.In addition,an ultrafast charge transfer time of 0.56 ps from the NIR aggregation-induced luminogens of 2TT-oC6B absorber to PolyTPD and PCBM measured by femtosecond transient absorption spectroscopy is beneficial for effective exciton dissociation.The solution-processed organic NIR photodetector exhibits a fast response time of 83μs and a linear dynamic range value of 111 dB under illumination of 830 nm.Therefore,our work has opened up a pioneering window to enhance photoconductivity through in situ photoirradiation and benefit NIR photodetectors as well as other optoelectronic devices.

Solution sequential deposited organic photovoltaics:From morphology control to large-area modules

Organic optoelectronic materials enable cutting-edge,low-cost organic photodiodes,including organic solar cells(OSCs)for energy conversion and organic photodetectors(OPDs)for image sensors.The bulk heterojunction(BHJ)structure,derived by blending donor and acceptor materials in a single solution,has dominated in the construction of active layer,but its morphological evolution during film formation poses a great challenge for obtaining an ideal nanoscale morphology to maximize exciton dissociation and minimize nongeminate recom-bination.Solution sequential deposition(SSD)can deliver favorable p–i–n vertical component distribution with abundant donor/acceptor interfaces and relatively neat donor and acceptor phases near electrodes,making it highly promising for excellent device performance and long-term stability.Focusing on the p–i–n structure,this review provides a systematic retrospect on regulating this morphology in SSD by summarizing solvent selection and additive strategies.These methods have been successfully implemented to achieve well-defined morphology in ternary OSCs,all-polymer solar cells,and OPDs.To provide a practical perspective,comparative studies of device stability with BHJ and SSD film are also discussed,and we review influential progress in blade-coating techniques and large-area modules to shed light on industrial production.Finally,challenging issues are out-lined for further research toward eventual commercialization.

Energy transfer ultraviolet photodetector with 8-hydroxyquinoline derivative-metal complexes as acceptors

We choose 8-hydroxyquinoline derivative-metal complexes（Beq,Mgq,and Znq） as the acceptors（A） and 4,4＇,4”-tri-（2-methylphenyl phenylamino） triphenylaine（m-MTDATA） as the donor（D） respectively to study the existing energy transfer process in the organic ultraviolet（UV） photodetector（PD）,which has an important influence on the sensitivity of PDs.The energy transfer process from D to A without exciplex formation is discussed,differing from the working mechanism of previous PDs with Gaq[Zisheng Su,Wenlian Li,Bei Chu,Tianle Li,Jianzhuo Zhu,Guang Zhang,Fei Yan,Xiao Li,Yiren Chen and Chun-Sing Lee 2008 Appl.Phys.Lett.93 103309）]and REq[J.B.Wang,W.L.Li,B.Chu,L.L.Chen,G.Zhang,Z.S.Su,Y.R.Chen,D.F.Yang,J.Z.Zhu,S.H.Wu,F.Yan,H.H.Liu,C.S.Lee 2010 Org.Electron.111301]used as an A material.Under 365-nm UV irradiation with an intensity of 1.2 mW/cm^2,the m-MTDATA：Beq blend device with a weight ratio of 1：1 shows a response of 192 mAAV with a detectivity of 6.5 × 10^（11） Jones,which exceeds those of PDs based on Mgq（146 mA/W） and Znq（182 mA/W） due to better energy level alignment between m-MTDATA/Beq and lower radiative decay.More photophysics processes of the PDs involved are discussed in detail.

All-solution-processed PIN architecture for ultra-sensitive and ultra-flexible organic thin film photodetectors

An ideal organic thin film photodetectors(OTFPs) should adopt a hierarchical, multilayer p-type/blend-type/n-type(PIN) structure,with each layer having a specific purpose which could greatly improve the exciton dissociation while guarantee efficient charge transport. However, for the traditional layer-by-layer solution fabrication procedure, the solvent used can induce organic material mixing and molecular disordering between each layer. Hence, such architecture for OTFPs can now only be formed via thermal evaporation. In this paper, a contact-film-transfer method is demonstrated to all-solution processing organic PIN OTFPs on flexible substrates. The fabricated PIN OTFPs exhibit high photoresponse and high stability under continuous mechanical bending. Hence,the method we described here should represent an important step in the development of OTFPs in the future.

一种实现任意窄带光谱检测的有机光电探测器通用策略

窄带光电探测器因其优异的综合性能,在光谱仪、激光雷达、光通信和监控等关键应用领域广受欢迎.尽管窄带硅基探测器在当前市场上占据主导地位,但其本征刚性、光学带隙固定和尺寸大的特性极大地限制了其在新兴领域的应用,尤其是在柔性和可穿戴光电子领域.在本工作中,我们报道了一种构筑窄带有机光电探测器(OPD)的简单通用策略.该策略通过溶液法将活性层中的给体和受体材料制备成光学调制层(OAL),并集成在OPD上,可实现对任意光谱的检测.利用该策略,我们成功地构筑了以750 nm为中心的窄带OPD,其半峰宽(FWHM)为40 nm,光谱响应率为3667,-3 dB截止频率为927 kHz,打破了自驱动窄带OPD的性能记录.值得注意的是,这种新颖的OAL策略在OPD中具有良好的通用性,可以实现对600至1000 nm范围内的任意窄光谱检测,具有FWHM可调(最小步长为2 nm)、响应度和探测率高、响应速度快的优势.受益于有机半导体固有的柔性特性,我们构筑了一种柔性且可弯曲的窄带近红外OPD线性阵列,并应用于全向激光雷达上,在无需机械旋转的情况下实现了360°水平扫描.

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 A1 cathode, which can be demonstrated from the EQE spectra and transient photocurrent curves of OPDs with different ICBA doping ratios.

Ternary organic photodiodes with spectral response from 300 to 1200 nm for spectrometer application

The light spectrometer is one of the most widely used optical instruments,allowing users to measure and analyze spectral components.However,to reach a broad spectral response range,most spectrometers have to combine a silicon photodiode with an additional photodiode to account for the limited operating wavelength range of each in isolation.Here,we developed organic photodiodes(OPDs)by combining a novel narrow-bandgap non-fullerene acceptor(COTIC-4 Cl)with a polymer donor(PCE10)in the presence of PC71 BM as the third component.The resulting device based on the ternary blended material showed a wide spectral response from 300 to 1200 nm,beyond the response range of the traditional silicon-based photodiode.Moreover,at the wavelength of 1100 nm,the resulting OPD exhibited a specific detectivity of 5×10^(12) Jones and a responsivity of 0.3 A W-1,with a cut-off frequency of>1 MHz and a linear dynamic range of>120 d B,among the best performances of OPDs.Of particular importance is that,as a proof-of-concept,the resulting ternary OPD was successfully utilized as the key component of a spectrometer to measure the absorption across 300–1200 nm.These results indicate the potential applications of the broadband response OPDs for next-generation spectrometers.

Versatile aza-BODIPY-based low-bandgap conjugated small molecule for light harvesting and near-infrared photodetection

The versatile nature of organic conjugated materials renders their flawless integration into a diverse family of optoelectronic devices with light-harvesting,photodetection,or light-emitting capabilities.Classes of materials that offer the possibilities of two or more distinct optoelectronic functions are particularly attractive as they enable smart applications while providing the benefits of the ease of fabrication using low-cost processes.Here,we develop a novel,multi-purpose conjugated small molecule by combining boron-azadipyrromethene(aza-BODIPY)as electron acceptor with triphenylamine(TPA)as end-capping donor units.The implemented donor–acceptor–donor(D–A–D)configuration,in the form of TPA-azaBODIPY-TPA,preserves ideal charge transfer characteristics with appropriate excitation energy levels,with the additional ability to be used as either a charge transporting interlayer or light-sensing semiconducting layer in optoelectronic devices.To demonstrate its versatility,we first show that TPA-azaBODIPY-TPA can act as an excellent hole transport layer in methylammonium lead triiodide(MAPbI3)-based perovskite solar cells with measured power conversion efficiencies exceeding 17%,outperforming control solar cells with PEDOT:PSS by nearly 60%.Furthermore,the optical bandgap of 1.49 eV is shown to provide significant photodetection in the wavelength range of up to 800 nm where TPA-azaBODIPY-TPA functions as donor in near-infrared organic photodetectors(OPDs)composed of fullerene derivatives.Overall,the established versatility of TPA-azaBODIPY-TPA,combined with its robust thermal stability as well as excellent solubility and processability,provides a new guide for developing highly efficient multi-purpose electronic materials for the next-generation of smart optoelectronic devices.