Near Infrared Self-Powered Organic Photodetectors with a Record Responsivity Enabled by Low Trap Density

High-performance IR organic photodetectors (OPDs) are of great significance for wireless optical communication, light detection and ranging (LiDAR) technology, and wearable electronics. However, high dark current and low responsivity (R) hinder their future commercial application. Herein, fullerene and non-fullerene acceptors-based OPDs are fabricated to understand the relationship between the trap density and photo-responsivity. Impressively, the non-fullerene system (Poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PCE10):BTPV-4F-eC9) based OPDs exhibits a record R-value of 0.56 A W⁻¹ at 900 nm in no gain photodiode-type OPDs, which results in a high detectivity over 10¹³ Jones in 400–1030 nm at room temperature. Mechanistic studies show that the low trap density plays critical role in reducing the trap-assisted recombination and density of thermally generated carriers, thus improving the responsivity and reducing the dark current of the device. These findings provide new insights into the mechanism of high-performance self-powered near-infrared OPDs.     

Hybrid Organic/PbS Quantum Dot Bilayer Photodetector with Low Dark Current and High Detectivity

Owing to their ease of fabrication, low cost, and high flexibility, organic materials have attracted great interests in photodetector (PD) applications. However, suffering from large dark current, small photocurrent, low on–off ratio, and low sensitivity, performances of bare organic‐based PDs are not satisfactory. Integrating organic materials with other novel semiconductor materials offers an opportunity to overcome these drawbacks. Here, a lateral hybrid organic/lead sulfide (PbS) quantum dot bilayer PD is designed and fabricated, which significantly suppresses the dark current and enhances the photocurrent, leading to improved light detecting capability. Meanwhile, the bilayer PD can be made on a flexible polyimide substrate.     

Air-Stable Self-Powered Photodetectors Based on Lead-Free CsBi3I10/SnO2 Heterojunction for Weak Light Detection

Lead halide perovskites (LHPs) have been widely investigated in photodetection applications owing to their intriguing optoelectronic properties. However, the application of LHPs-based photodetectors (PDs) is hindered because of the toxicity of lead and instability in ambient air. Here, an air-stable self-powered photodetector is designed based on all-inorganic lead-free CsBi₃I₁₀/SnO₂ heterojunction. The device exhibits broad spectral response in both UV and visible light, fast response on µs scale, and decent long-term stability. The device holds a faster response speed (t_r/t_d = 7.8/8.8 µs), among the best reported self-powered lead-free perovskites photodetectors. More importantly, the device can display obvious photoresponses even under ultra-weak light intensity as low as 10 pW cm^–2, showing better weak-light sensitivity than previously reported lead-free perovskites photodetectors, to the best of our knowledge. Moreover, the device holds good air stability in the 73 days test without encapsulation. These results suggest that CsBi₃I₁₀/SnO₂-based self-powered PDs with high photodetection capability possess enormous potential in stable and broadband PDs for weak light detection in the future.     

Two-Dimensional Antimony-Based Perovskite-Inspired Materials for High-Performance Self-Powered Photodetectors

The ongoing Internet of Things revolution has led to strong demand for low-cost, ubiquitous light sensing based on easy-to-fabricate, self-powered photodetectors. While solution-processable lead-halide perovskites have raised significant hopes in this regard, toxicity concerns have prompted the search for safer, lead-free perovskite-inspired materials (PIMs) with similar optoelectronic potential. Antimony- and bismuth-based PIMs are found particularly promising; however, their self-powered photodetector performance to date has lagged behind the lead-based counterparts. Aiming to realize the full potential of antimony-based PIMs, this study examines, for the first time, the impact of their structural dimensionality on their self-powered photodetection capabilities, with a focus on 2D Cs₃Sb₂I_9−xCl_x and Rb₃Sb₂I₉ and 0D Cs₃Sb₂I₉. The 2D absorbers deliver cutting-edge self-powered photodetector performance, with a more-than-tenfold increase in external quantum efficiency (up to 55%), speed of response (>5 kHz), and linear dynamic range (>four orders of magnitude) compared to prior self-powered A₃M₂X₉ implementations (A⁺: monovalent cation; M³⁺: Sb³⁺/Bi³⁺; X⁻: halide anion). Detailed characterization reveals that such a performance boost originates from the superior carrier lifetimes and reduced exciton self-trapping enabled by the 2D structure. By delivering cutting-edge performance and mechanistic insight, this study represents an important step in lead-free perovskite-inspired optoelectronics toward self-powered, ubiquitous light sensing.     

Efficient All-Polymer Photomultiplication-Type Organic Photodetectors with Prolonged Crystallization Process

With the progressive development of photomultiplication-type organic photodetectors (PM-OPDs), increasing research efforts are devoted to all-polymer PM-OPDs due to their potential in terms of device stability and stretchability. However, poor polymer-polymer miscibility and entanglement of long polymer chains are still the main challenges to form desirable active layer morphology in such systems. A smooth solidification process is favorable toward realizing a morphology that features ordered molecular orientation and high crystallinity. Herein, morphological control issue in all-polymer PM-OPDs is addressed by modifying film formation kinetics with an insulating polymer blending strategy. The prolonged crystallization process of polystyrene-blended films can form high-ordered molecular arrangements and crystallinity in donor/acceptor phases, leading to improved charge transport properties and suppressed trap states. With boosting the trap-assisted photomultiplication effect, the polystyrene-blended all-polymer PM-OPD with a high specific detectivity of 4.0 × 10¹³ Jones can be achieved due to the accumulation of enhanced photogenerated electrons at the interface and the efficient injection of external holes, which is one of the best detectivity values reported for PM-OPDs. This study not only reveals valuable insights into the effects of insulating polymers on the film formation kinetics mechanism, but also provides novel strategy to fabricate high-performance all-polymer PM-OPDs.     

Light Detection in Open‐Circuit Voltage Mode of Organic Photodetectors

Organic photodetectors (OPDs) are promising candidates for next‐generation light sensors as they combine unique material properties with high‐level performance in converting photons into electrical signals. However, low‐level light detection with OPD is often limited by device dark current. Here, the open‐circuit voltage (V_oc ) regime of OPDs is shown to be efficient for detecting low light signals (<100 µW cm^?2). It is established that the light‐dependence of V_oc exhibits two distinct regimes as function of irradiance: linear and logarithmic. Whereas the observed logarithmic regime is well understood in organic photovoltaic cells (OPVs), it is shown experimentally and theoretically that the linear regime is due to the non‐infinite shunt resistance of the OPD device. Overall, OPDs composed of rubrene and fullerene show photovoltage light sensitivity across nine orders of magnitude with a detection limit as low as 400 pW cm^?2. A photovoltage responsivity of 1.75 V m² W^?1 demonstrates highly efficient performance without the necessity to supress high dark current. This approach opens up new possibilities for resolving low light signals and provides simplified design rules for OPDs.     

End-Group Functionalization of Non-Fullerene Acceptors for High External Quantum Efficiency over 150 000% in Photomultiplication Type Organic Photodetectors

Photomultiplication-type organic photodetectors (PM-OPDs) with high external quantum efficiency (EQE) of over 100% are attracting increasing attention due to their potential importance in detecting weak incident light. Considering that the gain of PM-OPD is determined by the ratio of carrier lifetime over carrier transit time, a systematic study on the effect of the end-functionalization of a new extended aromatic fused-ring non-fullerene acceptor (NFA) on the carrier trap/transit time of the PM-OPD. Photophysical analyses by means of ultraviolet-visible absorption, ultraviolet photoelectron spectroscopy, and photoluminescence combined with structural analyses through grazing-incidence wide-angle X-ray scattering show that fluorination of the NFA with the deepest lowest unoccupied molecular orbital level and non-isotropic molecular ordering can yield the longest carrier lifetime. Furthermore, surface energy study show that fluorination of the NFA can also yield the most hydrophobic nature, which can allow the most efficient injection barrier thinning/lowering of the active layer/cathode interface under illumination due to the localized acceptor distribution toward cathode, maximizing the hole injection efficiency from cathode. As a result, an unprecedentedly high EQE of 156 000% is obtained from the optimized PM-OPD. This work shows the importance of the molecular design of acceptor molecules in fabricating high-performance PM-OPDs.     

MOVCD生长GaAs/AlGaAs量子阱红外探测器暗电流特性研究

用金属有机物化学气相沉积法(MOCVD)生长了GaAs/AlGaAs量子阱材料,分别制备了300μm×300μm台面,峰值波长8.5μm,外电极压焊点面积80μm×80μm,内电极压焊点面积20μm×20μm的单元测试样品。用变温液氦制冷机测试系统对两个样品进行50～300K的变温测试,分析了器件在不同偏压条件下的暗电流特性。发现该量子阱红外探测器的背景限温度为50K。不同生长次序中GaAs与AlGaAs界面的不对称性,以及掺杂元素的扩散导致了正负偏压下的I/V曲线呈不对成性。探测器电极压焊点面积大小与位置的不同对暗电流有一定的影响。     

Photomultiplication-Type Organic Photodetectors with High EQE-Bandwidth Product by Introducing a Perovskite Quantum Dot Interlayer

A photomultiplication (PM)-type organic photodetector (OPD) that exploits the ionic motion in CsPbI₃ perovskite quantum dots (QDs) is demonstrated. The device uses a QD monolayer as a PM-inducing interlayer and a donor–acceptor bulk heterojunction (BHJ) layer as a photoactive layer. When the device is illuminated, negative ions in the CsPbI₃ QD migrate and accumulate near the interface between the QDs and the electrode; these processes induce hole injection from the electrode and yield the PM phenomenon with an external quantum efficiency (EQE) >2000% at a 3 V applied bias. It is confirmed that the ionic motion of the CsPbI₃ QDs can induce a shift in the work function of the QD/electrode interface and that the dynamics of ionic motion determines the response speed of the device. The PM OPD showed a large EQE-bandwidth product >10⁶ Hz with a −3 dB frequency of 125 kHz at 3 V, which is one of the highest response speeds reported for a PM OPD. The PM-inducing strategy that exploits ionic motion of the interlayer is a potential approach to achieving high-efficiency PM OPDs.     

Organic Photodetectors and their Application in Large Area and Flexible Image Sensors: The Role of Dark Current

Organic photodetectors (OPDs) have gained increasing interest as they offer cost‐effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible‐light OPDs are proposed for use in indirect‐conversion X‐ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near‐infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (J_d) is crucial to improve important figures of merits such as the signal‐to‐noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to J_d in OPDs. J_d reduction is typically addressed by fine‐tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental J_d values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.     

Photomultiplication-Type Organic Photodetectors with Fast Response Enabled by the Controlled Charge Trapping Dynamics of Quantum Dot Interlayer

A novel photomultiplication (PM)-type organic photodiode (OPD) that responds much faster (109 kHz bandwidth) than conventional PM-type OPDs is demonstrated. This fast response is achieved by introducing quantum dots (QDs) as a PM-inducing interlayer at the interface between the electrode and the photoactive layer. When the device is illuminated, the photogenerated electrons within the photoactive layer are rapidly transferred and trapped in the trap states of the QD interlayer. The electron trapping subsequently leads to charging of the QD and a consequent shift of the QD energy levels, thereby inducing hole injection from the electrode. This PM mechanism is distinct from that of conventional PM-type OPDs, whose PM usually requires a long time to induce hole (or electron) injection because of the slow transport and accumulation of electrons (or holes) within the photoactive layer. Because of its PM mechanism, the proposed QD-interlayer PM-type OPD achieves high bandwidth and high specific detectivity. In addition, it is demonstrated that the response speed of the proposed device is closely related to the charge trapping/detrapping dynamics of the QDs. This work not only offers a new concept in the design of fast-responding PM-type OPDs but also provides comprehensive understanding of the underlying device physics.     

基于非富勒烯受体的可见光-近红外有机光电探测器模拟研究

高灵敏度的有机光电探测器(OPD)具有从可见光到近红外(NIR)的宽带响应和优异的整体器件性能,在包括高质量生物成像在内的各种应用中起到了非常重要的作用。文章使用Silvaco TCAD模拟了一种活性层由宽带隙聚合物PBDTTT-C-T作为给体以及稠合八烷基小分子FOIC作为受体构成的混合物所制成的宽带有机光电探测器。模拟结果表明,器件的暗电流密度、外量子效益、可探测到的最低光强能力等各项指标达到了很好的水平,与实验数据吻合较好。由此,可认为模拟过程中所使用到的参数具有较好的可信度和使用价值,可以为同类型光电探测器的模拟与研究提供有益借鉴。     

Facile Fabrication of PbS Nanocrystal:C60 Fullerite Broadband Photodetectors with High Detectivity

Hybrid PbS nanocrystal/C₆₀ fullerite photodetectors are fabricated using a simple one‐step drop casting procedure onto pre‐patterned interdigitated electrodes. The devices exhibit a broad spectral response from the near UV through to the near infrared yielding a detectivity, D*, of above 10¹⁰ Jones from 400 nm to ≈1050 nm. The ability to further extend the spectral response to wavelengths ≈1350 nm in the near infrared via tuning of the PbS nanocrystal diameter is also demonstrated. The dynamic responses of the devices are presented, exhibiting a fast photocurrent rise time (<40 ns) followed by a long bi‐exponential decay with characteristic lifetimes of τ₁ = 5.3 μs ± 0.1 μs and τ₂ = 37.8 μs ± 0.7 μs. These devices, which have a detectivity approaching that of commercial detectors, a broader spectral response, and a fast rise time, offer an attractive low‐cost solution for large‐area broadband photodetectors.     

新型GaAs/AlGaAs量子阱红外探测器暗电流特性

对基于 Ga As/ Al Ga As系子带间吸收的一种新型量子阱红外探测器 ,采用 Poisson方程和 Schrodinger方程 ,计算了新器件结构的能带结构、电子分布特性 ,在此基础上采用热离子发射、热辅助遂穿模型对器件的暗电流特性进行了模拟 ,计算结果与器件实测的暗电流特性吻合得很好 ,说明热离子发射、热辅助遂穿机制是形成器件暗电流的主要构成机制 ,增加垒高、降低阱中掺杂浓度及降低工作温度是抑制器件暗电流的主要途径 ,计算结果对进一步优化器件的设计将起到重要的理论指导作用 .     

MoS2/Si Heterojunction with Vertically Standing Layered Structure for Ultrafast,High‐Detectivity,Self‐Driven Visible–Near Infrared Photodetectors

As an interesting layered material, molybdenum disulfide (MoS₂) has been extensively studied in recent years due to its exciting properties. However, the applications of MoS₂ in optoelectronic devices are impeded by the lack of high‐quality p–n junction, low light absorption for mono‐/multilayers, and the difficulty for large‐scale monolayer growth. Here, it is demonstrated that MoS₂ films with vertically standing layered structure can be deposited on silicon substrate with a scalable sputtering method, forming the heterojunction‐type photodetectors. Molecular layers of the MoS₂ films are perpendicular to the substrate, offering high‐speed paths for the separation and transportation of photo‐generated carriers. Owing to the strong light absorption of the relatively thick MoS₂ film and the unique vertically standing layered structure, MoS₂/Si heterojunction photodetectors with unprecedented performance are actualized. The self‐driven MoS₂/Si heterojunction photodetector is sensitive to a broadband wavelength from visible light to near‐infrared light, showing an extremely high detectivity up to ≈10¹³ Jones (Jones = cm Hz^1/2 W^?1), and an ultrafast response speed of ≈3 μs. The performance is significantly better than the photodetectors based on mono‐/multilayer MoS₂ nanosheets. Additionally, the MoS₂/Si photodetectors exhibit excellent stability in air for a month. This work unveils the great potential of MoS₂/Si heterojunction for optoelectronic applications.     

Highly Sensitive Narrowband Photomultiplication-Type Organic Photodetectors Prepared by Transfer-Printed Technology

Narrowband photomultiplication-type organic photodetectors (PMOPDs) are realized with poly(3-hexylthiophene-2,5-diyl) (P3HT) as the optical field adjusting (OFA) layer and transfer-printed P3HT: [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) (50:1, w/w) as the photomultiplication (PM) layer. The thickness of the OFA layers is adjusted to optimize interfacial trapped electron distribution and density, which determines the external quantum efficiency (EQE) and spectral response range of PMOPDs. Narrowband PMOPDs with 2.5 µm thick P3HT as the OFA layer exhibit two narrow response peaks at 350 and 660 nm, and the corresponding EQE values at 350 and 660 nm are 180% and 760% under an applied bias of −20 V. A wide bandgap polymer poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (P-TPD) is deliberately incorporated into OFA layer for adjusting interfacial trapped electron distribution near Al electrode. Narrowband PMOPDs exhibit only one response peak at 660 nm with the enhanced EQE value of 1120% under the same bias. The enhanced EQE of PMOPDs with P-TPD is primarily attributed to the increased hole tunneling injection and transport, which can be ascribed to the enhanced trapped electron density near the Al electrode and the improved hole mobility, respectively. Clearly resolved images can be obtained from the imaging system with the narrowband PMOPDs as sensing pixel without any current preamplifier, indicating the promising potential of PMOPDs in imaging sense.     

High‐Performance SiC Nanobelt Photodetectors with Long‐Term Stability Against 300 °C up to 180 Days

In the present work, high‐performance photodetectors (PDs) based on a single B‐doped 3C‐SiC nanobelt, which are synthesized via catalyst‐free pyrolysis of polymeric precursors of polysilazane, are reported. The as‐built PDs have a high responsivity and external quantum efficiency of 6.37 × 10⁵ A · W^?1 and 2.0 × 10⁸% under 405 nm light with a power density of 0.14 mW · cm^?2 at 5 V, respectively. The detectivity of the PDs is measured to be of 6.86 × 10¹⁴ Jones. Moreover, the B‐doped 3C‐SiC nanobelt PDs exhibit a long‐term stability against 300 °C up to 180 days, suggesting their promising applications to be served under harsh conditions.     

Sn-Based Self-Powered Ultrafast Perovskite Photodetectors with Highly Crystalline Order for Flexible Imaging Applications

Sn-based perovskite materials are promising lead-free alternatives in thin film photodetectors (PDs) for applications such as optical communications, night visions and biomedical near-infrared imaging systems. However, constructing Sn-based photodetectors with high sensitivity, ultrafast response, and good operation stability has been a challenge. Herein, the phenyl-ethyl ammonium (PEA⁺) additive is introduced in pristine FASnI₃, which regulates the thin film growth, passivates the trap/defect states, prevents Sn²⁺/Sn⁴⁺oxidation, and releases the crystal strain. The Resulting FA_0.8PEA_0.2SnI₃ thin films exhibit highly crystalline order and flexibility. A self-powered PD using FA_0.8PEA_0.2SnI₃ as the active layer demonstrates excellent responsivity of 0.262 W⁻¹, detectivity of 2.3 × 10¹¹ Jones. And it possesses the fastest rise and decay time of 25 µs and 42 µs as compared with the state-of-art Sn-based perovskite PDs. The transient absorption spectroscopy analysis validates greatly reduced trapping states and defects of FASnI₃ with the PEA⁺ film for ultrafast response. A flexible Sn-based perovskite PD without any encapsulation in air continuously shows ultrafast responses after 10,000 bending cycles. Meanwhile, a flexible imaging system can be realized by a 5 × 5 PD array with good sensing results. This study shows great potential in nontoxic and ultrafast Sn-based perovskite PDs for flexible imaging applications.     

The Dependence of Device Dark Current on the Active‐Layer Morphology of Solution‐Processed Organic Photodetectors

Organic photodiodes are presented that utilize solution‐processed perylene diimide bulk heterojunctions as the device photoactive layer. The polymer (9,9′‐dioctylfluorene‐co‐benzothiadiazole; F8BT) is used as the electron donor and the N,N′‐bis(1‐ethylpropyl)‐3,4,9,10‐perylene tetracarboxylic diimide (PDI) derivative is used as the electron acceptor. The thickness‐dependent study of the main device parameters, namely of the external quantum efficiency (EQE), the short‐circuit current (I_SC), the open‐circuit voltage (V_OC), the fill factor (FF), and the dark current (I_D) is presented. In as‐spun F8BT:PDI devices the short‐circuit EQE reaches the maximum of 17% and the V_OC value is as high as 0.8 V. Device I_D is in the nA mm^?2 regime and it correlates with the topography of the F8BT:PDI layer. For a range of annealing temperatures I_D is monitored as the morphology of the photoactive layer changes. The changes in the morphology of the photoactive layer are monitored via atomic force microscopy. The thermally induced coalescence of the PDI domains assists the dark conductivity of the device. I_D values as low as 80 pA mm^?2 are achieved with a corresponding EQE of 9%, when an electron‐blocking layer (EB) is used in bilayer EB/F8BT:PDI devices. Electron injection from the hole‐collecting electrode to the F8BT:PDI medium is hindered by the use of the EB layer. The temperature dependence of the I_D value of the as‐spun F8BT:PDI device is studied in the range of 296–216 K. In combination with the thickness and the composition dependence of I_D, the determined activation energy E_a suggests a two‐step mechanism of I_D generation; a temperature‐independent step of electric‐field‐assisted carrier injection from the device contacts to the active‐layer medium and a thermally activated step of carrier transport across the device electrodes, via the PDI domains of the photoactive layer. Moreover, device I_D is found to be sensitive to environmental factors.