Organic Photodetectors Research Articles

An Efficient Narrowband Near-Infrared at 1040nm Organic Photodetector Realized by Intermolecular Charge Transfer Mediated Coupling Based on a Squaraine Dye.

A highly sensitive short-wave infrared (SWIR, λ> 1000 nm) organic photodiode (OPD) is described based on a well-organized nanocrystalline bulk-heterojunction (BHJ) active layer composed of a dicyanovinyl-functionalized squaraine dye (SQ-H) donor material in combination with PC61 BM. Through thermal annealing, dipolar SQ-H chromophores self-assemble in a nanoscale structure with intermolecular charge transfer mediated coupling, resulting in a redshifted and narrow absorption band at 1040 nm as well as enhanced charge carrier mobility. The optimized OPD exhibits an external quantum efficiency (EQE) of 12.3% and a full-width at half-maximum of only 85 nm (815 cm-1 ) at 1050 nm under 0 V, which is the first efficient SWIR OPD based on J-type aggregates. Photoplethysmography application for heart-rate monitoring is successfully demonstrated on flexible substrates without applying reverse bias, indicating the potential of OPDs based on short-range coupled dye aggregates for low-power operating wearable applications.

(Invited) Polymer-Based Electronics: Infrared Detectors and Supercapacitors

The shortwave infrared spectral region is particularly powerful for health monitoring and medical diagnostics, because biological tissues show low absorbance and minimal infrared auto-fluorescence, enabling greater penetration depth and improved resolution in comparison to visible light. Here I will discuss my lab’s development of organic infrared photodetectors using a new generation of semiconducting polymers extending to 1600 nm. Several demonstrations will show the various potential applications including blood pulse measurements, spectroscopic identification, and image reconstruction. Furthermore, we have implemented upconversion imagers that combine photo-sensing and display in a compact structure, to extend the capability of human and machine vision to 1400 nm. In addition, the same infrared polymers are narrow bandgap and present opportunities to create energy dense supercapacitors. Here we present promising n-type polymers that retained 90% of initial capacitance after 2000 charge-discharge cycles. Through current-voltage and spectroscopic measurements, we infer the mechanisms that lead to capacitance fade and suggests structural and electrochemical strategies to realize high-endurance energy storage devices for flexible printed systems.

(Invited) Integration of Organic Photodiode with Low Temperature Poly Silicone Thin Film Transistors for a Conformable Image Sensor

With the rapid aging of Japanese society, how to increase quality of life (QoL) while controlling rising medical expenses has become an urgent issue. To solve this difficult issue, the acquisition and utilization of biological information using new technologies such as wearable devices is more expected. In particular, self-care and home medical care, in which patients and their families are responsible for their own health, are considered as one of the way to solving the issues of a super-aging society. Indeed, wearable sensors that can constantly monitor health conditions and home-use blood pressure monitors with communication functions are being introduced to the market one after another to prepare for the advent of a self-care era.On the other hand, when designing a new insurance system or incentive system using biometric information from wearable sensors, it is important issue how to confirm whether data measured at home is the patient's own. Furthermore, the risk of patient mix-ups must be reduced as more wearable devices come to be used in hospitals and welfare facilities in the future. Therefore, measuring vital signs simultaneously with biometric authentication of the user is an urgent issue.We have developed a sheet-type image sensor that enables high-resolution and high-speed reading. This sheet-type image sensor can take the high-resolution image of fingerprints and veins used for biometric authentication. In addition, the same sheet-type imager can measure the pulse wave which is one of the vital signs, and its distribution.Although there are many reports of sheet-type image sensors, it has not been achieved both high-resolution imaging and high-speed readout, and static biometric data and dynamic vital signs cannot be measured by one sheet-type image sensor. This is because high-sensitivity photodetectors and high-speed switching elements could not be integrated on a polymer substrate without damaging of the switching elements.The developed sheet-type image sensor is fabricated by densely integrating a high-efficiency readout circuit using an active matrix of a low-temperature polysilicon thin film transistor and a photodetector that uses a highly efficient organic semiconductor as a photosensitive layer. The resolution of the image sensor achieves 508 dots per inch (dpi) required for fingerprint authentication, and the organic photodetector consists of bulk hetero structure organic layer which has high photosensitivity to near infrared light with a wavelength of 850 nanometers (external quantum efficiency of 50% or more). It it easy to integrate the sheet-type image sensor into equipment and attach it to curved surfaces due to the thickness of the polymer base material is 10 micrometers, and the total thickness of the sheet-type image sensor is 15 micrometers (Figure 1). By developing a process technology that integrates photodetectors and thin-film transistors without damage to each other, it has become possible to realize a sheet-type image sensor that achieves both high-resolution imaging and high-speed reading.Evaluation of vein and fingerprint images taken by this sheet-type image sensor showed that the contrast difference of the veins was less than 5% compared to images using a general CMOS imager. We confirmed that the conformable imager has high image quality equivalent to that of conventional CMOS imagers.Since the sheet-type image sensor is thin and bendable, it can be easily integrated into wearable devices, and it is possible to measure health condition and perform biometric authentication at the same time. As a result, it is expected that prevention of “spoofing” and patient from being mixed up.

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

AbstractA 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.

Carrier Blocking Layer Materials and Application in Organic Photodetectors.

As a promising candidate for next-generation photodetectors, organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using solution processes and a tunable spectral response range, making them particularly attractive for large area image sensors on lightweight flexible substrates. Carrier blocking layers engineering is very important to the high performance of OPDs that can select a certain charge carriers (holes or electrons) to be collected and suppress another carrier. Carrier blocking layers of OPDs play a critical role in reducing dark current, boosting their efficiency and long-time stability. This Review summarizes various materials for carrier blocking layers and some of the latest progress in OPDs. This provides the reader with guidelines to improve the OPD performance via carrier blocking layers engineering.

Enhanced Static and Dynamic Properties of Highly Miscible Fullerene-Free Green-Selective Organic Photodetectors.

We developed p-n junction organic photodetectors (OPDs) composed of a polymer donor and a nonfullerene acceptor (NFA) to increase both the responsivity (R) and detectivity (D*) while maintaining a narrow wavelength selectivity. The selection of the polymer donor and NFA with similar green (G) absorption is important for achieving G-wavelength selectivity in these OPDs, which differentiates them from current fullerene-based OPDs and NFA-based panchromatic absorption OPDs. In addition, mixing the polymer donor and asymmetric NFA was efficient toward increasing the miscibility and decreasing the interfacial energy difference of the blended films, resulting in the formation of a uniform and well-mixed nanomorphology in the photoconductive layer. Two-dimensional (2D) grazing incidence X-ray diffraction and Fourier-transform infrared spectroscopy revealed that the lamellar ordering of the polymer donor was enhanced in the blend film prepared with an asymmetric NFA, whereas the aggregation of a symmetric NFA in the blend films did not increase the lamellar ordering of the polymer donor. Consequently, we achieved an R value of 0.31 A/W and D* value of 2.0 × 1013 Jones with a full width at half-maximum value of 230 nm at -2 V and fast response time of 27 μs without any external bias in the asymmetric NFA-based OPDs. The enhancement in the lamellar ordering and miscibility of the blended films are crucial toward increasing the static and dynamic properties of OPDs.

Probing the metal/conducting polymer interface and implications of the metal diffusion in two-terminal sandwich devices

In this article, we have investigated the role of polymer regio-regularity and film fabrication technique in reducing metal diffusion from top electrodes in sandwich type devices. Poly(3-hexyl thiophene)(P3HT) polymer of varying regio-regularity was chosen for investigation and its corresponding thin films were fabricated by spin-coating(SC) and floating film transfer method(FTM). Sandwich devices with a top interface between gold and i) FTM coated regio-regular (rr) P3HT, ii) FTM coated non-regiocontrolled (nr) P3HT, iii) SC nr-P3HT and iv) SC rr-P3HT were fabricated. The current density vs voltage (J-V) characteristics of the devices revealed non-ohmic characteristics. Estimation of interfacial hole barrier heights using Fowler Nordheim tunneling model revealed a small barrier of 1.12 eV for the isotropic spin-coated nr-P3HT film which increased to 1.37 eV with an increase in film anisotropy for the FTM nr-P3HT film. The barrier height also gradually declined from 1.37 eV to 1.05 eV with increase in regioregularity from FTM nr-P3HT to FTM rr-P3HT. These observations were confirmed through optical and structural characterizations along with depth profiling of metal diffusion using XPS. Pronounced Au diffusion to a higher depth was observed in the XPS results for the highly anisotropic FTM film that decreased subsequently with decrease in anisotropy from FTM to spin-coated film. It was thus deduced that a judicious selection of fabrication technique and polymer aids in reducing metal diffusion leading to improved device performance. The results hold crucial significance towards the fabrication of efficient organic photodetectors that have a sandwich type device architecture.

Optical Distance Measurement Based on Induced Nonlinear Photoresponse of High-Performance Organic Near-Infrared Photodetectors

Extraction barriers are usually undesired in organic semiconductor devices since they lead to reduced device performance. In this work, we intentionally introduce an extraction barrier for holes, leading to nonlinear photoresponse. The effect is utilized in near-infrared (NIR) organic photodetectors (OPDs) to perform distance measurements, as delineated in the focus-induced photoresponse technique (FIP). The extraction barrier is introduced by inserting an anodic interlayer with deeper highest occupied molecular orbital (HOMO), compared to the donor material, into a well-performing OPD. With increasing irradiance, achieved by decreasing the illumination spot area on the OPD, a higher number of holes pile up at the anode, counteracting the built-in field and increasing charge-carrier recombination in the bulk. This intended nonlinear response of the photocurrent to the irradiance allows determining the distance between the OPD and the light source. We demonstrate fully vacuum-deposited organic NIR optical distance photodetectors with a detection area up to 256 mm2 and detection wavelengths at 850 and 1060 nm. Such NIR OPDs have a high potential for precise, robust, low-cost, and simple optical distance measurement setups.

6‐1: Invited Paper: Measuring Health Parameters with Large‐Area Organic Photodetector Arrays

Solution‐processed organic photodetectors (OPDs) have witnessed great progress in the past few years. With silicon‐like performance achieved, OPDs enable new medical imaging applications by offering cost‐effective light detection over large area. Herein, we illustrate the compelling advantages of high spatial and temporal resolution OPD arrays for medical X‐ray detectors and wearable pulse oximeters.

6‐2: Invited Paper: An Imager with Organic Photodetectors Based on LTPS‐TFT Technology

We have developed a high‐resolution 508 ppi conformable imager using low‐temperature polysilicon thin‐film transistors (LTPS TFTs) on a plastic substrate together with organic photodetectors which has high sensitivity in wavelength of visible and near‐infrared regions (NIR). Using a NIR light source, both images of fingerprints/veins and pulse waves are successfully obtained. By adding red light source, both red pulse wave and NIR pulse are measured to calculate percutaneous oxygen saturation.

6‐3: Fingerprint‐on‐Display Module Based on Organic Optical Sensors for 1‐to‐4‐Finger Authentication in Next‐Generation Smartphones

Fingerprint on Display (FoD) Modules based on organic photodetectors have been realized and have achieved high performances for integration within smartphones. Such modules allow full display size enabling multiple fingers authentication at once and increasing security level.

70‐2: Sheet‐type Image Sensor with Near Infrared Sensitive Organic Photodiode

We have developed an ultra‐flexible organic photodetector with high sensitivity in near infrared regions. The photodetector exhibits good mechanical durability. The change in dark and photocurrent was approximately 10% after 500 cycles at a bending radius of 1.6 cm. Furthermore, the photodetector also shows good air stability. By combining the organic photodetector and low‐temperature polycrystalline silicon thin‐film transistor backplane, we developed a conformable imager with a high resolution of 508 dpi.

Improving the Sensitivity of an Organic Photodetector by Adding a Polar Solvent to the Hole-Transport Layer for Indirect X-ray Detection.

In this work, we investigated how the performance enhancement of an organic X-ray detector was improved by adding a dimethyl sulfoxide (DMSO) polar solvent to poly(3, 4-ethylene dioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) hole-transport layer. The changes in the properties, such as surface roughness, chemical structure, sheet resistance, and absorbance, of the PEDOT:PSS film caused by the DMSO treatment were examined. The application of DMSO treatment lowered the resistance of the PEDOT:PSS film because of the removal of PSS and the chemical structure change after DMSO treatment, and thus the transport of light-induced carriers was increased. The organic detector treated with 10 vol% DMSO showed the highest collected current density (CCD) of 357.42 nA/cm² and highest sensitivity of 2.58 mA/Gy ·cm², which were 31.88% and 32.31% higher than the CCD and sensitivity of the detector without DMSO treatment.

Scalable Single-Crystalline Organic 1D Arrays for Image Sensor.

Optoelectronic applications of organic semiconductors demand single-crystalline structures with long-range order and suppressed defects for sustaining efficient carrier transport and long photocarrier lifetime, which are pivotal in photodetection, photovoltaic, and light emission. For integrated devices, an additional requirement of precise patterning is imposed, whereas the patterning of single-crystalline organic microstructures is still challenging because the molecular stacking is easily perturbed by disordered fluids in microdroplets. Herein, a capillary-bridge lithography is developed for driving the directional transport of capillary flows to control the confined crystallization of organic 1D single-crystalline arrays with aligned positioning and pure orientation. Through tuning the concentration and pressure, the size of organic 1D arrays in three dimensions can be controlled with 2.9-5.8µm in width and 1.2µm to 110nm in height. Organic 1D array photodetectors exhibit a stable performance with on/off ratio of 180 and responsivity of 4.99mA W-1 . Based on the scalable fabrication of 1D array photodetectors, 20 × 20 multiplexed image sensors with high accuracy are demonstrated for capturing the light signals of capital letter "A," "B," and "C." This research will open opportunities for the large-scale fabrication of organic single-crystalline semiconductors toward the integrated optoelectronic modules.

Self-powered ultraflexible photonic skin for continuous bio-signal detection via air-operation-stable polymer light-emitting diodes

Ultraflexible optical devices have been used extensively in next-generation wearable electronics owing to their excellent conformability to human skins. Long-term health monitoring also requires the integration of ultraflexible optical devices with an energy-harvesting power source; to make devices self-powered. However, system-level integration of ultraflexible optical sensors with power sources is challenging because of insufficient air operational stability of ultraflexible polymer light-emitting diodes. Here we develop an ultraflexible self-powered organic optical system for photoplethysmogram monitoring by combining air-operation-stable polymer light-emitting diodes, organic solar cells, and organic photodetectors. Adopting an inverted structure and a doped polyethylenimine ethoxylated layer, ultraflexible polymer light-emitting diodes retain 70% of the initial luminance even after 11.3 h of operation under air. Also, integrated optical sensors exhibit a high linearity with the light intensity exponent of 0.98 by polymer light-emitting diode. Such self-powered, ultraflexible photoplethysmogram sensors perform monitoring of blood pulse signals as 77 beats per minute.

Shortwave infrared-absorbing squaraine dyes for all-organic optical upconversion devices

ABSTRACT Shortwave infrared (SWIR) optical sensing and imaging are essential to an increasing number of next-generation applications in communications, process control or medical imaging. An all-organic SWIR upconversion device (OUC) consists of an organic SWIR sensitive photodetector (PD) and an organic light-emitting diode (OLED), connected in series. OUCs directly convert SWIR to visible photons, which potentially provides a low-cost alternative to the current inorganic compound-based SWIR imaging technology. For OUC applications, only few organic materials have been reported with peak absorption past 1000 nm and simultaneous small absorption in the visible. Here, we synthesized a series of thermally stable high-extinction coefficient donor-substituted benz[cd]indole-capped SWIR squaraine dyes. First, we coupled the phenyl-, carbazole-, and thienyl-substituted benz[cd]indoles with squaric acid (to obtain the SQ dye family). We then combined these donors with the dicyanomethylene-substituted squaraine acceptor unit, to obtain the dicyanomethylene-functionalized squaraine DCSQ family. In the solid state, the absorbance of all dyes extended considerably beyond 1100 nm. For the carbazole- and thienyl-substituted DCSQ dyes, even the peak absorptions in solution were in the SWIR, at 1008 nm and 1014 nm. We fabricated DCSQ PDs with an external photon-to-current efficiency over 30%. We then combined the PD with a fluorescent OLED and fabricated long-term stable OUCs with peak sensitivity at 1020 nm, extending to beyond 1200 nm. Our OUCs are characterized by a very low dark luminance (<10−2 cd m−2 at below 6 V) in the absence of SWIR light, and a low turn-on voltage of 2 V when SWIR light is present.

Highly durable organic photodetector for complementary metal oxide semiconductor image sensors

Utilizing organic electronics compatible with conventional semiconductor fabrication processes is extremely difficult because of their low chemical resistivity and poor environmental durability. To preserve the intrinsic functionality of organic materials, only a few fabrication processes can be used. Moreover, it is essential to achieve process expandability and silicon-process compatibility to develop high-resolution electronics suitable for mass production. Therefore, we developed wet-process-compatible organic photodetectors by replacing the conventional shadow-mask process with photolithography. This suppresses particle deposition during the serial fabrication processes, providing high operational stability. The fabricated green organic photodiodes exhibit a low dark current (1.0 × 10−11 A/cm2) with high photon–electron conversion efficiency (EQE = 65%). The charge collection and charge separation efficiencies are stable (ηcc = 84.6% and ηcs = 97.7%, respectively). Moreover, the organic semiconductors are compatible with conventional wet- and dry-etching processes owing to thin-film encapsulation layers. Finally, the novel organic image sensor can withstand 500 h under 85 °C/85% relative humidity and 1000 thermal cycles (−55–125 °C). Because of its robustness and strong barrier properties, the novel process architecture reported herein can be extended to any organic electronic devices, including widely commercialized organic light-emitting diodes and organic photovoltaic devices.

Lowering the Contact Barriers of 2D Organic F16 CuPc Field-Effect Transistors by Introducing Van der Waals Contacts.

2D organic crystals exhibit efficient charge transport and field-effect characteristics, making them promising candidates for high-performance nanoelectronics. However, the strong Fermi level pinning (FLP) effect and large Schottky barrier between organic semiconductors and metals largely limit device performance. Herein, by carrying out temperature-dependent transport and Kelvin probe force microscopy measurements, it is demonstrated that the introducing of 2D metallic 1T-TaSe2 with matched band-alignment as electrodes for F16 CuPc nanoflake filed-effect transistors leads to enhanced field-effect characteristics, especially lowered Schottky barrier height and contact resistance at the contact and highly efficient charge transport within the channel, which are attributed to the significantly suppressed FLP effect and appropriate band alignment at the nonbonding van der Waals (vdW) hetero-interface. Moreover, by taking advantage of the improved contact behavior with 1T-TaSe2 contact, the optoelectronic performance of F16 CuPc nanoflake-based phototransistor is drastically improved, with a maximum photoresponsivity of 387 AW-1 and detectivity of 3.7×1014 Jones at quite a low Vds of 1V, which is more competitive than those of the reported organic photodetectors and phototransistors. The work provides an avenue to improve the electrical and optoelectronic properties of 2D organic devices by introducing 2D metals with appropriate work function for vdW contacts.

Status and Prospects of Aggregation-Induced Emission Materials in Organic Optoelectronic Devices.

Aggregation induced emission (AIE) luminogens (AIEgens) have great potential in the field of organic optoelectronic devices because of their highly efficient emission property in solid state. For example, high efficiency organic light-emitting diodes (OLEDs) based on AIEgens have been developed successfully. Some AIEgens also show good photovoltaic response properties for organic solar cells (OSCs) and organic photodetectors (OPDs), and lasing properties for optically pumping organic lasers (OLs). The review will cover the status and prospects of AIEgens in OLEDs, OLs, OSCs and OPDs. It is expected that AIEgens will become an important organic optoelectronic material system in the future.

Ultra-Narrow-Band NIR Photomultiplication Organic Photodetectors Based on Charge Injection Narrowing.

Ultra-narrow-band NIR photomultiplication organic photodetectors (PM-OPDs) were realized in ITO/PEDOT:PSS/active layers/Al based on an interfacial-trap-induced charge injection narrowing (CIN) concept. The rather less Bod Ethex-Hex (BEH) is imbedded in a polymer donor matrix to form large amounts of isolated electron traps. Trapped electrons in BEH close to an Al electrode will enforce hole-tunneling injection induced by interfacial band bending, resulting in a photomultiplication phenomenon. PM-OPDs with P3HT:BEH as the active layer exhibit a narrow response peak at 850 nm with a full-width at half-maximum (fwhm) of 27 nm as well as a rather weak response from 650 to 800 nm. The EQE of 29 700% at 850 nm was achieved in PM-OPDs by incorporating 0.02 wt % of F6TCNNQ under -13 V of applied voltage. The rejection ratio (RR) of the optimized PM-OPDs with F6TCNNQ is 11 for EQE850nm/EQE700nm and 10 for EQE850nm/EQE750nm, respectively. An EQE of 15 300% at 850 nm was achieved in the ternary PM-OPDs under -13 V of applied voltage, with markedly enhanced RRs of 44 for EQE850nm/EQE700nm and 30 for EQE850nm/EQE750nm.