Organic Photodetectors Research Articles

High-performance organic photodetectors enabled by a refined fibrillar multiphase morphology

Film morphology of donor:acceptor blend layers plays a critical role in photon-to-current efficiency and dark/noise current of organic photodetectors (OPDs). One effective approach to manipulate crystallization and mesoscale phase separation of such blend layers is sequential casting (SC). However, the guiding strategies to control the morphology and the impacts on OPD performance of the SC films remain elusive, as the film structural evolution during SC is different from conventional blend casting (BC). Here, a refined fibrillar multiphase morphology is demonstrated by SC in an NT812:IEICO-4F blend film, where the high-quality IEICO-4F crystallites infiltrate the robust NT812 fibril network from the surface to the bulk. Such a morphology leads to improved charge generation and collection, reduced trap states, and enhanced charge block capability of resultant device, enabling simultaneous achievement of high external quantum efficiency and low dark/noise current. A maximum special detectivity of 5 × 1013 Jones is achieved at 860 nm under –0.1 V, which is among the highest detectivities for vis-to-NIR OPDs. The linear dynamic range and response speed are also improved. Such enhancements are parallelly observed from OPD devices based on other blend systems with similar fibrillar refinement, which provides guidelines for film structure manipulation towards high OPD performance.

Thieno[3,4-b]pyrazine-containing D-A conjugated polymer-based photodetectors with high performance and UV–visible dual-band response for color temperature sensor and encrypted optical communication

Three thieno[3,4-b]pyrazine (TP)-containing D-A type conjugated polymers are successfully used to develop multifunctional high-performance organic photodetectors (OPDs). The TP acceptor units are respectively coupled with three different donor units to effectively regulate their molecular energy levels and carrier mobilities. The resultant polymers exhibit dual-band absorbance in the UV–visible region, intramolecular charge transfer (ICT) effect and efficient ambipolar transport characteristics. They are evaluated as the single photosensitive layers in OPDs for the first time. The optimal device based on TP-Si presents a peak external quantum efficiency (EQE) over 40% and a high specific detectivity (D∗) of 3.35×1012 Jones with fast temporal response under 560 nm at −1.5 V, which are superior to the most of reported single-active-layer OPDs. The high performance could be ascribed to the well-balanced photogenerated carriers and the efficient charge separation and transport in the device. Noteworthy, this dual-band response device can be used as a novel color temperature (CT) sensor for complementary white light illumination, and an encrypted UV–visible light communication scheme has been successfully demonstrated. The results show that these ambipolar D-A polymers have a broad application prospect in secure optical communication in the future.

Dual-Band, Efficient Self-Powered Organic Photodetectors with Isotype Subphthalocyanine-Based Heterojunctions toward Secure Optical Communications

High-performance heterojunction organic photodetectors (OPDs) are of great significance in optical detecting technology due to their tailorable optoelectronic properties. Herein, we designed and synthesized three n-type subphthalocyanine (SubPc) derivatives PhO-BSubPcF12, CHO-PhO-BSubPcF12, and NO2-PhO-BSubPcF12 via axial nonhalogen substitution on fluorinated SubPc. These SubPc derivatives exhibit improved intramolecular charge transfer, high electron mobilities, optimized energy levels, and good thermal stability. The novel isotype p-n SubPc heterojunctions are evaluated as photosensitive layers in OPDs, which show a UV-visible dual-band response and self-powered effect. The optimal OPD with Br-BSubPc/NO2-PhO-BSubPcF12 presents stable and superior performances with a high responsivity (R) of 0.14 A W-1, a peak external quantum efficiency (EQE) of 30.6%, and an extremely low dark current of 0.92 nA cm-2 under a 570-595 nm illumination without a bias voltage. It has outperformed most of the reported SubPc-based OPDs. The better interfacial contact of p-n SubPc derivatives leads to a large depletion region with decreased trap densities as well as a low carrier recombination rate, which is conducive to the photoinduced carriers' separation and well-balanced transport, resulting in high device performances. Moreover, a secure communication strategy is successfully demonstrated by dual-band optimal OPD. This work is expected to provide some guidance for molecular engineering and device performance toward multifunctional electronics.

Photocurrent diffusion of organic photodetector between pixels for fingerprint sensor by using interdigitated electrodes

The organic photodetectors (OPDs) with an interdigitated electrodes structure are analyzed to evaluate the optical crosstalk between neighboring pixels for the organic image sensor used in the fingerprint-on-display authentication system. The shorter exciton diffusion length of organic semiconductors in the OPD can promote the high resolution of the organic image sensor for full-display sensing applications. We demonstrated the photocurrent diffusion from the exposed area to the hidden electrodes by changing the distance to predict the minimum pixel pitch for the organic image sensor. Unexpectedly, the photocurrent diffusion was observed, even at 10 μm length, as more significant than the expected exciton diffusion length.

Organic Photodetectors with Extended Spectral Response Range Assisted by Plasmonic Hot-Electron Injection.

Organic photodetectors (OPDs) have aroused intensive attention for signal detection in industrial and scientific applications due to their advantages including low cost, mechanical flexibility, and large-area fabrication. As one of the most common organic light-emitting materials, 8-hydroxyquinolinato aluminum (Alq3) has an absorption wavelength edge of 460 nm. Here, through the introduction of Ag nanoparticles (Ag NPs), the spectral response range of the Alq3-based OPD was successfully extended to the near-infrared range. It was found that introducing Ag NPs can induce rich plasmonic resonances, generating plenty of hot electrons, which could be injected into Alq3 and then be collected. Moreover, as a by-product of introducing Ag NPs, the dark current was suppressed by around two orders of magnitude by forming a Schottky junction on the cathode side. These two effects in combination produced photoelectric signals with significant contrasts at wavelengths beyond the Alq3 absorption band. It was found that the OPD with Ag NPs can stably generate electric signals under illumination by pulsed 850 nm LED, while the output of the reference device included no signal. Our work contributes to the development of low-cost, broadband OPDs for applications in flexible electronics, bio-imaging sensors, etc.

Electron-donating amine-interlayer induced n-type doping of polymer:nonfullerene blends for efficient narrowband near-infrared photo-detection

Inherently narrowband near-infrared organic photodetectors are highly desired for many applications, including biological imaging and surveillance. However, they suffer from a low photon-to-charge conversion efficiencies and utilize spectral narrowing techniques which strongly rely on the used material or on a nano-photonic device architecture. Here, we demonstrate a general and facile approach towards wavelength-selective near-infrared phtotodetection through intentionally n-doping 500–600 nm-thick nonfullerene blends. We show that an electron-donating amine-interlayer can induce n-doping, resulting in a localized electric field near the anode and selective collection of photo-generated carriers in this region. As only weakly absorbed photons reach this region, the devices have a narrowband response at wavelengths close to the absorption onset of the blends with a high spectral rejection ratio. These spectrally selective photodetectors exhibit zero-bias external quantum efficiencies of ~20–30% at wavelengths of 900–1100 nm, with a full-width-at-half-maximum of ≤50 nm, as well as detectivities of >1012 Jones.

Organic bulk heterojunction enabled with nanocapsules of hydrate vanadium pentaoxide layer for high responsivity self-powered photodetector

This article demonstrates the fabrication of organic-based devices using a low-cost solution-processable technique. A blended heterojunction of chlorine substituted 2D-conjugated polymer PBDB-T-2Cl, and PC71BM supported nanocapsules hydrate vanadium penta oxides (HVO) as hole transport layer (HTL) based photodetector fabricated on an ITO coated glass substrate under ambient condition. The device forms an excellent organic junction diode with a good rectification ratio of ~200. The device has also shown excellent photodetection properties under photoconductive mode (at reverse bias) and zero bias for green light wavelength. A very high responsivity of ~6500 mA/W and high external quantum efficiency (EQE) of 1400% have been reported in the article. The proposed organic photodetector exhibits an excellent response and recovery time of ~30 and ~40 ms, respectively.

Photomultiplication type organic photodetectors with different response characteristics under forward or reverse bias

Photomultiplication type organic photodetectors (PM-OPDs) with a sandwich structure of ITO/PNDIT-F3N/P3HT:[71]PCBM(100:1, wt/wt)/Al were successfully developed, exhibiting different response characteristics under forward or reverse bias. The rather less [71]PCBM can act as isolated electron traps to capture photogenerated electrons. The trapped electrons near ITO (Al) electrode can make interfacial band bending for hole tunneling injection from external circuit under forward (reverse) bias. The injected holes can be efficiently transported along the channel provided by P3HT and then collected by the corresponding electrode under bias. The external quantum efficiency (EQE) spectral shape of PM-OPDs is primarily determined by the trapped electron distributions near ITO or Al electrode under forward or reverse bias. The PM-OPDs exhibit flat EQE spectra under forward bias and ‘M-shaped’ EQE spectra under reverse bias, which is consistent with the curves of trapped electron distribution near ITO or Al electrode. The champion EQE values of PM-OPDs arrive to 2550% at 425 nm under 10 V bias and 2360% at 365 nm under −10 V bias. The PM-OPDs exhibit distinct response speed under forward or reverse bias, which is primarily attributed to the different trapped electron density near ITO or Al electrode under incident light from ITO side. • The PNDIT-F3N interfacial layer makes PM-OPDs work well under forward or reverse bias. • The response speed of PM-OPDs was investigated under distinct polarity bias, resulting from trapped electrons near electrode. • The champion EQE values of PM-OPDs arrive to 2550% at 425 nm under 10 V bias and 2360% at 365 nm under −10 V bias.

Transparent organic photodiodes for high-detectivity CMOS image sensors

The demand for high-definition complementary metal-oxide–semiconductor (CMOS) image sensors has increased considerably over the past few years in industry as well as in academia. Here we propose transparent green-sensitive organic photodetectors (TG-OPDs) with both dark-current-based high detectivity (over 10 14 c m H z 1 / 2 W − 1 at a wavelength of 550 nm under 3 V) and high responsivity ( 0.34 A W − 1 under 3 V) for organic–silicon hybrid CMOS image sensors. A b a t h o c u p r o i n e : C 60 electron-transporting layer provided the fabricated TG-OPDs with a minimal junction resistivity, smooth morphology, and desirable energy level modulation, resulting in exceptional light sensitivity, a low dark current (below 10 − 11 A c m − 2 ), and a high rectification ratio spanning 10 orders of magnitude. The TG-OPDs had high-temperature endurance (up to 150°C for 2 h) and operational stability under intense heat (above 85°C) for 50 d. We expect this performance to enable the industrialization of these TG-OPDs for optoelectronic sensor applications, such as photoplethysmography, fingerprint recognition, proximity sensing, and imaging.

Improvements of Organic Photodetectors for VLC Using a New Active Layer, Focal Lens, and a Transimpedance Amplifier

Beyond 5G networks, and for short distance but high-speed communication systems, there is an incipient growing interest in visible light communications (VLC). Furthermore, it is an interesting alternative within the framework of the Internet of Things (IoT). However, there is a problem on the part of the receiver in these systems, motivated by the non-existence of photovoltaic elements used particularly for this purpose. Because of their simplicity and small investment, organic-type photodetectors have a high capacity for use in VLC systems. With a structure of polymers represented by an organic photodetector fabricated with poly (3-hexylthiophene) (P3HT) and a bulk heterojunction active layer of phenyl-C61-butyric acid methyl ester (PCBM) and commercial LEDs, we show our system in this paper. The main novelty of this work is in reception. With different upgrade using a new active layer or adding a focal lens or inserting a transimpedance amplifier, we have achieved improved performance results related to voltage and distance, with an increase of more than 40 %, and Bit Error Rate (BER) by modifying the active layer concentration with respect to results obtained in a former work. Also, we have tested the use of a transimpedance amplifier to obtain the best results in distances of 12 cm to 20 cm and a focal lens with the same objective of improving the BER in critical environments, where the Signal-to-Noise Ratio (SNR) is close to zero. To conclude, these modifications show that it is possible to increase the main parameters of our system to be useful in VLC systems.

Ambient‐Stable Near‐Infrared Organic Photodetectors with Ultrahigh Detectivity and Ultrafast Response for Biometric Monitoring

AbstractPM6:Y6 is adopted as the active layer to prepare highly efficient organic photodetectors (OPDs), which is applied to realize a biometric sensor for real‐time heart‐rate monitoring. Here, the effects of the device structure, thickness, deposition process, and solvent on the subsequent performance of the OPDs are examined. Through judicious choice of the fabrication conditions, an OPD is obtained displaying a dark current density of less than 8.77 × 10−10 A cm−2, a responsivity of greater than 0.516 A W−1, and a high detectivity of 3.1 × 1013 Jones, measured at bias of −2 V (870 nm). This OPD featured a wide linear dynamic range (LDR, 530 nm) of 104 dB at −2 V, an ultrahigh cutoff frequency (750 kHz) and rise/fall times of less than 220 ns under a reverse bias of −2 V. This sub‐microsecond response and high detectivity is comparable with, and in some cases better than, those of the state‐of‐the‐art perovskite‐PDs and Si‐PDs. The OPD displays consistent responses, good repeatability, and stability [ambient: 50% humidity, 25 °C)]. The air‐stable and portable OPD‐based photoplethysmogram system provides clear signal resolution, potentially benefitting its applicability to blood pressure monitoring.

Highly Sensitive Broad Light Response Organic Photodetectors from Pyrrolo[3,4-c]pyrrole-1,4-dione-Based Conjugated Polymer with Noncovalent Conformational Locks

Highly Sensitive Broad Light Response Organic Photodetectors from Pyrrolo[3,4-<i>c</i>]pyrrole-1,4-dione-Based Conjugated Polymer with Noncovalent Conformational Locks

Low Dark Current Organic Photodetectors Utilizing Highly Cyanated Non-fullerene Acceptors.

Organic materials combining high electron affinity with strong absorption in the visible spectrum are of interest for photodetector applications. In this study, we report two such molecular semiconductors, based upon an acceptor–donor–acceptor (A-D-A) approach. Coupling of an acceptor end group, 2,1,3-benzothiadiazole-4,5,6-tricarbonitrile (TCNBT), with a donor cyclopentadithiophene core affords materials with a band gap of 1.5 eV and low-lying LUMO levels around −4.2 eV. Both materials were readily synthesized by a one-pot nucleophilic displacement of a fluorinated precursor by cyanide. The two acceptors only differ in the nature of the solubilizing alkyl chain, which is either branched 2-ethyl hexyl (EH-TCNBT) or linear octyl (O-TCNBT). Both acceptors were blended with polymer donor PTQ10 as an active layer in OPDs. Significant device differences were observed depending on the alkyl chain, with the branched acceptor giving the optimum performance. Both acceptors exhibited very low dark current densities, with values up to 10–5 mA cm–2 at −2 V, highlighting the potential of the highly cyanated cores (TCNBT) as acceptor materials.

Visible‐Blind Deep Ultraviolet Photomultiplication Organic Photodetectors with Ultrahigh Gain for UVB and UVC Light Detection

AbstractPhotomultiplication‐type organic photodetectors (PM‐OPDs) have developed rapidly in UVA, visible and near‐infrared ranges, but they face great challenges in the UVB and UVC ranges, mainly due to the difficulty of simultaneously realizing deep UV light‐transmittance electrode and high gain structure. Here, the copper and silver bilayer composite electrodes are developed to overcome the problem of silver island growth, thus ensuring high deep UV light transmittance and good matching with electrical gain mechanism. The fabricated device shows high external quantum efficiency as high as 195,665% at 255 nm in UVC region. Furthermore, the above electrodes are integrated with low‐cost UV filter glass ZWB1 by simple vacuum evaporation. Finally, low‐cost visible‐blind deep UV PM‐OPDs with a rejection ratio of up to 4788 is achieved through an additional strategy of adjusting the absorption spectrum of the active layer. Benefiting from its fast response speed, the device can be directly applied to faint flame monitoring in real‐time.

Bulk-Heterojunction Adjustment Enables a Self-filtering Organic Photodetector with a Narrowband Response.

Image-sensor technology is the foundation of many emerging applications, where the photodetector is designed to interact with incoming photons that have specific colors or wavelengths. A color filter is therefore crucial to enable the selective spectral response of the photodetector and to eliminate the crosstalk interference resulting from ambient lights. Unfortunately, a reduced detection sensitivity of the photodetector is inevitable due to an imperfect light filtering, which greatly limits the practical applications of selective-response photodetectors. Herein, we demonstrate a bulk-heterojunction (BHJ) organic composite featuring a self-filtering light responsive characteristic. Through a careful optimization of the BHJ film, the organic photodetector (OPD) demonstrates a high-selective spectral response to the infrared (IR) radiation without the need of applying a color filter. As a result, the self-filtering top-illuminated OPD exhibits a narrowband external quantum efficiency (EQE) of 53% with a narrow full width at half-maximum (fwhm) of 56 nm centering at 1080 nm. A high responsivity of 0.46 A W-1 is also achieved at 1080 nm wavelength due to the self-filtering characteristic.

Multifunctional Organic Vertical Photodiodes for Photo‐Detection and Photo‐Synapse Enabled by Modulation of the Interface Energy Barrier

AbstractMultifunctional organic optoelectronic devices are of great significance for the development of low‐energy consumption and space‐constrained systems. Herein, multifunctional organic photodiodes integrating transient light detection and photo‐synaptic functions in one device are unprecedentedly achieved upon readily changing the direction of the external bias. Under negative bias, the devices exhibit excellent photodetection performance with high shot‐noise‐limited specific detectivity ( &gt; 1013 Jones) in the range from ultraviolet, visible, to near infrared and large linear dynamic range (188 dB at 915 nm), which are among the best organic photodetectors. Interestingly, the photodiode devices change to work as organic photosynapses upon tuning the external bias to be positive. Associative learning behavior is successfully simulated in an all‐optical way, while high image recognition accuracy (&gt;80%) is realized based on artificial neural network. The mechanism studies reveal that the interface energy barrier is critical for achieving the multi‐function in one device. Finally, the universality of electrically switchable transient light detection and photo‐synaptic functions is further studied. This work provides a novel method for constructing vertical diode‐type organic optical synapses and developing multifunctional organic electronics.

Large near-infrared lateral photovoltaic effect in an organic egg albumin/Si structure

Previously reported photoelectric devices have mainly been limited to inorganic materials. Even though preparing high-performance photoelectric devices with organic biomaterials is an inevitable trend in commercialization, fabricating organic photoelectric devices based on naturally occurring materials with high sensitivity remains a great challenge due to the high resistivity of and few free electrons in these materials. Herein, high-performance photoelectric devices based on an egg albumin (EA)/Si structure are proposed, and a new, to the best of our knowledge, perspective is provided on photodetection in naturally occurring materials by utilizing the surface state of p-Si to separate light-induced carriers effectively. The free electrons of metal atoms restrain the surface states, leading to a sensitivity of 5 mV/mm for metal/Si devices, while the sensitivity of the EA/Si device in the near-infrared region is greatly promoted to 357 mW/mm, which is intimately related to the lack of effect of EA on the dangling bonds of the surface. The EA/Si device is among the most sensitive organic near-infrared photoelectronic device to date. This work opens up new avenues to overcome the obstacle of the low sensitivity of organic photodetectors, indicating that the EA/Si device has great potential for future applications in flexible photovoltaic devices.

Synthesis and characterization of a copper(II) phthalocyanine‐based dye for organic photodetectors

AbstractNovel copper(II) 4,4′,4″,4‴‐tetra(hexylthienyl)phthalocyanine (CuPc‐HT) was synthesized via Stille coupling reaction between 2‐trimethylstannyl‐5‐hexylthiophene and tetra‐brominated copper(II) phthalocyanine (CuPc). The incorporation of hexylthiophene to CuPc increased organic solubility, Q‐band absorption and p‐type characteristics. The bulk heterojunction organic photodetectors (OPDs) were developed by blending CuPc‐HT with n‐type [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM). CuPc‐HT:PC71BM‐based devices showed the strong green light detection in the range of 400–600 nm. The measured specific detectivity (D*) was 3.1 × 1011 Jones under 530 nm light‐emitting diode (LED) light of 0.1 mW/cm2 at −2 V, which was approximately three times higher than those under 680 nm red LED irradiation. Dynamic characteristics of OPDs were investigated, and the −3 cut‐off frequency of CuPc‐HT:PC71BM devices was extremely high as 0.1 MHz. Notably, the signal response time at −1.0 V was ultrafast to be 9 μs. Even though CuPc‐HT:PC71BM devices showed insufficient static properties, their dynamic characteristics were comparable with those of state‐of‐the‐art OPDs.

Localized Surface‐Plasmon Enhanced Near‐Infrared Organic Photodetector

AbstractNear infrared (NIR) organic photodetector (OPD) has exhibited tremendous potential in research due to mechanical flexibility, optoelectronic tunability, and low cost. However, the limited NIR light absorption, low carrier mobility, and relatively low photon‐to‐electron conversion efficiency in organic photoactive layer have restricted further development of OPD. To address these challenges, localized surface‐plasmon resonance (LSPR) enhanced D18:Y6 OPD is systematically designed and fabricated with uniform Ag nanocubes (AgNCs) of 60 nm deposited right on D18:Y6 organic photoactive layers, realizing a 25‐fold enhancement in responsivity and detectivity which reach 2200 mA W−1 and 2.8 × 1012 Jones, respectively, upon 850 nm laser excitation compared with pristine device without AgNCs. The detailed COMSOL Physics simulation of as‐obtained devices with AgNCs indicates that electric filed is strongly amplified and concentrated within the D18:Y6 film. The enhancement of photoluminescence intensity in D18:Y6 films with AgNCs further verifies the resonant enhancement of light absorption in the D18:Y6 film embedded with AgNCs. To get in‐depth knowledge of the LSPR enhancement effect on OPD, the photocurrent enhancement of devices with different laser power densities and plasmonic modes have been systematically explored. Therefore, this work has opened a new window to enhance the photoresponse for next generation NIR OPDs.

A Transparent Electrode Based on Solution-Processed ZnO for Organic Optoelectronic Devices

Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the development of high-conductivity and high-transparency materials for being used as the front electrode. Herein, sol-gel-grown zinc oxide (ZnO) films with high conductivity (460 S cm−1) and low optical absorption losses in both visible and near-infrared (NIR) spectral regions are realized utilizing the persistent photoinduced doping effect. The origin of the increased conductivity after photo-doping is ascribed to selective trapping of photogenerated holes by oxygen vacancies at the surface of the ZnO film. Then, the conductivity of the sol-gel-grown ZnO is further increased by stacking the ZnO using a newly developed sequential deposition strategy. Finally, the stacked ZnO is used as the cathode to construct ITO-free organic solar cells, photodetectors, and light emitting diodes: The devices based on ZnO outperform those based on ITO, owing to the reduced surface recombination losses at the cathode/active layer interface, and the reduced parasitic absorption losses in the electrodes of the ZnO based devices.