Polymer Photodetectors Research Articles

Integrated optical microfluidic biosensor using a polycarbazole photodetector for point-of-care detection of hormonal compounds

A picogram-sensitive optical microfluidic biosensor using an integrated polycarbazole photodiode is developed. The photodetector is mainly composed of the blend heterojunction of poly [N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) and the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as the hole transport layer. Analyte detection is accomplished via a chemiluminescent immunoassay performed in a poly(dimethylsiloxane)-gold-glass hybrid microchip, on which antibodies were immobilized and chemiluminescent horseradish peroxidase-luminol-peroxide reactions were generated. Enhanced sensor response to the chemiluminescent light is achieved by optimizing the thickness of PCDTBT: PC70BM and PEDOT:PSS. Using the optimized polycarbazole photodiode for detecting the human thyroid-stimulating hormone as the model target, the integrated biosensor demonstrates an excellent linearity in the range of 0.03 to 10 ng/ml with an analytical sensitivity of 68 pg/ml. The sensor response shows high specificity and reproducibility. Hormone detection in clinical samples is further demonstrated and compared with a commercial enzyme-linked immunosorbent assay. The integrated device reported here has potential to detect other hormonal compounds or protein targets. Moreover, the presented concept enables the development of miniaturized, low-cost but highly sensitive optical microfluidic biosensors based on integrated polymer photodetectors with high potential for point-of-care diagnostics.

Effect of solvent on detectivity of solution-processed polymer photodetectors

We demonstrate a high-performance solution-processed polymer photodetector (PPD) whose detectivity (D*) exceeds 10(11) Jones over almost the entire range of visible light. PPDs processed from a chlorobenzene solution using 1,8-diiodooctane (DIO) as an additive exhibit large crystalline domains with long-range order, in contrast to the featureless morphology of PPDs processed with pure solvent. Moreover, PPDs processed with DIO exhibit higher charge carrier mobility, and thus high photoconductive gain without compromising long carrier lifetime. Consequently, a high-loading ratio of DIO (3%) yielded a PPD whose D* in the visible range was over threefold higher than that of PPD without DIO.

Solution-Processed High-Detectivity Near-Infrared Polymer Photodetectors Fabricated by a Novel Low-Bandgap Semiconducting Polymer

High-detectivity near-infrared (NIR) polymer photodetectors (PDs) fabricated by a novel low-bandgap semiconducting polymer blended with fullerene derivatives are reported. Operating at room temperature, the polymer PDs have a spectral response from 400 to 1100 nm. By incorporation of an alcohol/water-soluble polymer as a cathode interlayer in bulk heterojunction polymer PDs, the polymer PDs exhibit a high detectivity of 1.75 × 1013 cm•Hz1/2/W at 800 nm. These results demonstrated that the NIR polymer PDs are comparable to Si-based PDs.

Water-soluble CdTe quantum dots as an anode interlayer for solution-processed near infrared polymer photodetectors

Water-soluble cadmium telluride (CdTe) quantum dots (QDs) used as an anode interlayer in solution-processed near infrared (NIR) polymer photodetectors (PDs) were demonstrated. Polymer PDs incorporated with CdTe QDs as an anode interlayer exhibited 10-fold suppressed dark current density and analogous photocurrent density relative to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which resulted in enhanced detectivities over 10(11) Jones in the spectral range from 350 nm to 900 nm. Moreover, with the substitution of PEDOT:PSS by CdTe QDs, the stability of unencapsulated NIR polymer PDs was extended up to 650 hours, which is more than 3 times longer than those with PEDOT:PSS as an anode interlayer. These results indicated that CdTe QDs can be utilized as a solution-processable alternative to PEDOT:PSS as an anode interlayer for high performance NIR polymer PDs.

Solution-processed near-infrared polymer photodetectors with an inverted device structure

Solution-processed near-infrared polymer photodetectors with an inverted device structure were designed and fabricated. By introducing ZnOx and MoO3 as an electron extraction layer and a hole extraction layer, respectively, the asymmetric characteristics of the inverted polymer photodetectors was constructed. Operating at room temperature, the inverted polymer photodetectors exhibited the detectivity over 1012cmHz1/2/W from 400 to 850nm, resulting from the enhanced photocurrent and reduced dark current induced by fabricating photoactive layer from solution with processing additive 1,8-diiodooctane. These device performances were comparable to those of inorganic counterparts.

Solution-Processed Ultrasensitive Polymer Photodetectors with High External Quantum Efficiency and Detectivity

Operating at room temperature, polymer photodetectors (PDs) with external quantum efficiency approximately 80%, detectivity over 10(13) Jones, linear dynamic range over 120 dB, and dark current a few decades of nA/cm(2) were demonstrated. All these performance parameters were achieved by combined treatment of active layer with solvent vapor annealing and of polymer PDs with postproduction thermal annealing. These high performance parameters demonstrated that polymer PDs is comparable to or better than inorganic counterparts.

Zinc Oxide Nanowire As an Electron-Extraction Layer for Broadband Polymer Photodetectors with an Inverted Device Structure

We report the fabrication of high-performance broadband polymer photodetectors based on narrow bandgap conjugated polymers with an inverted device structure, where electrons and holes are collected on indium tin oxide and metal contact with high work function. High-quality wide bandgap vertically aligned ZnO nanowire array offers an enhanced surface area and is used as the cathode buffer layer in this device for effectively extracting electrons and blocking holes from the active polymer layer. The room-temperature detectivity of polymer photodetectors with such an inverted device structure is greater than 1010 Jones with the spectral response from 400 to 1450 nm. Our results define a promising pathway for fabrication of high-sensitivity polymer photodetectors with an inverted device structure using ZnO nanowire array cathode buffer layer for a wide range of applications.

Coplanar Asymmetrical Reduced Graphene Oxide–Titanium Electrodes for Polymer Photodetectors

Narrow gaps and a "built-in" potential originating from the different work functions of reduced graphene oxide (RGO) and titanium electrodes are used to explain the improved photosensitivity of the poly(3-hexylthiophene) photodetectors with asymmetrical RGO-Ti electrodes presented here compared to those based on symmetrical electrodes. Easy processing, high photosensitivity, high on/off ratio, and low energy consumption contribute to the promising potential of coplanar asymmetrical electrodes in the field of photoelectric devices.

Extended spectral response in organic photomultiple photodetectors using multiple near-infrared dopants

We demonstrate highly sensitive polymer photodetectors (OPDs) with spectral response extending from the ultraviolet to the near-infrared (NIR) region (∼1200 nm). After doping two NIR dopants, high external quantum efficiencies (∼5500%) and high responsivities (23.0 A/W) are achieved under a low reverse bias (−3.7 V). The high gains could be attributed to unbalanced carrier transport in the photoactive layer arising from the electron traps at the NIR dopants. This approach allows the ready preparation of OPDs exhibiting broad spectral responses and high quantum efficiencies simultaneously.

Fluorene-Type Polymer Photodetectors Doped with Iridium and Platinum Complexes as Opto-Electrical Conversion Devices

ABSTRACTTo improve the conversion efficiency of polymer photodetectors (PDs) fabricated by solution process, the properties of fluorene-type polymer photodetectors doped with iridium (Ir) and platinum (Pt) complexes were investigated. The devices based on poly(dioctylfluorene) and poly(dioctylfluorene-co-benzothiadiazole) (F8BT) had violet and blue sensitivity, respectively. Triplet materials can enhance the incident-photon-to-current conversion efficiency of the devices utilizing the fluorene-type polymers when their triplet levels are lower than the lowest excited singlet states of the host and higher than the lowest excited triplet states of the host. The transmission of a moving picture was successfully demonstrated using the bilayer F8BT device with green Ir complex as an opto-electrical conversion device. We demonstrate that the polymer PDs fabricated by solution process can be applied to short-range optical communication fields, such as opto-electrical conversion devices for optical links.

Highly sensitive, low-voltage, organic photomultiple photodetectors exhibiting broadband response

Highly sensitive polymer photodetectors exhibiting broad spectral responses, ranging from the ultraviolet to the near-infrared (NIR) region, are obtained after incorporating an organic NIR dye into the device active layer. As a result, high external quantum efficiencies (>7000%) and high responsivities (32.4 A/W) are achieved at an extremely low operating voltage (−1.5 V). The high photomultiplation could be attribute to trapping of electrons, originating from the photogenerated electron/hole pairs, at the dye molecules, which effectively facilitates hole injection from the external circuit. The device preparation scheme presented herein opens up the possibility fabricating lost-cost, flexible organic photodetectors.

Semiconducting Polymer Photodetectors with Electron and Hole Blocking Layers: High Detectivity in the Near-Infrared

Sensing from the ultraviolet-visible to the infrared is critical for a variety of industrial and scientific applications. Photodetectors with broad spectral response, from 300 nm to 1,100 nm, were fabricated using a narrow-band gap semiconducting polymer blended with a fullerene derivative. By using both an electron-blocking layer and a hole-blocking layer, the polymer photodetectors, operating at room temperature, exhibited calculated detectivities greater than 1013 cm Hz1/2/W over entire spectral range with linear dynamic range approximately 130 dB. The performance is comparable to or even better than Si photodetectors.

Polymer photodetector with voltage-adjustable photocurrent spectrum

Polymer photodetectors with voltage-adjustable photoresponse from visible to near infrared range are demonstrated. Poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) blend is used as the active layer. The photoresponse can be continuously adjusted by the thickness of the active layer as well as the applied voltage bias. The thickness of the active layer is varied from 250 nm to 16.2 μm. The mechanism for the photoresponse adjusted by the thickness can be attributed to the absorption of the photons in the infrared range by thick PCBM layer. The mechanism for the photoresponse adjusted by the applied bias can be attributed to the carrier recombination reduction when the applied bias increases. The adjustable photodetector also has high operating speed up to 10 kHz.

BROAD-SPECTRUM PHOTOSENSOR

A PROMISING new polymer photodetector material shows an unprecedented ability to detect photons throughout the full range of UV-Vis to near-IR wavelengths. Photosensors are ubiquitous tools in today’s technologies, used for remote sensing, surveillance, and environmental monitoring. The new material, fabricated by a group that includes University of California, Santa Barbara, physics professor Alan J. Heeger, Cbrite Inc. senior scientist Xiong Gong, and their colleagues there and at South China University of Technology (SCUT), in Guangzhou, could be used to make a single device that would replace the three devices—typically based on GaN, Si, and InGaAs—currently required to cover this spectral range. Scientists have been exploring the combination of polymers and fullerenes as photodetectors since the early 1990s. Heeger won the Nobel Prize in Chemistry in 2000 for his work on conductive polymers. The advantages of polymer photodetectors are many. For example, whereas today’s inorganic IR silicon-based pho...

High-Detectivity Polymer Photodetectors with Spectral Response from 300 nm to 1450 nm

Sensing from the ultraviolet-visible to the infrared is critical for a variety of industrial and scientific applications. Today, gallium nitride-, silicon-, and indium gallium arsenide--based detectors are used for different sub-bands within the ultraviolet to near-infrared wavelength range. We demonstrate polymer photodetectors with broad spectral response (300 to 1450 nanometers) fabricated by using a small-band-gap semiconducting polymer blended with a fullerene derivative. Operating at room temperature, the polymer photodetectors exhibit detectivities greater than 10(12) cm Hz(1/2)/W and a linear dynamic range over 100 decibels. The self-assembled nanomorphology and device architecture result in high photodetectivity over this wide spectral range and reduce the dark current (and noise) to values well below dark currents obtained in narrow-band photodetectors made with inorganic semiconductors.

All‐organic optocouplers based on polymer light‐emitting diodes and photodetectors

AbstractIn this work, we demonstrate all organic flexible polymeric optocouplers by utilizing a donor‐acceptor bulk heterojunction polymer photodetector (PD) as the output unit and a polymer light‐emitting diode (PLED) as the input unit. The input unit is a single‐layer PLED on a glass or a plastic (PET) substrate utilizing a green emitting polyfluorene‐benzothiadiazole copolymer in the active layer. The output unit is a single‐layer PD on a glass substrate utilizing a P3HT:PCBM(1:1 by weight) blend, where P3HT is regioregular poly(3‐hexylthiophene) and PCBM is (6,6)‐phenyl‐C61‐butyric acid methyl ester. The electroluminescence spectrum of the PLED peaks at 530 nm and covers a spectral range that coincides quite well with the PD absorption spectrum (between 450 and 650 nm). The current density transfer ratio reaches 0.012% for an optocoupler that operates at 0 V and 15 V for the PD and PLED, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Small molecular weight organic thin-film photodetectors and solar cells

In this review, we discuss the physics underlying the operation of single and multiple heterojunction, vacuum-deposited organic solar cells based on small molecular weight thin films. For single heterojunction cells, we find that the need for direct contact between the deposited electrode and the active organics leads to quenching of excitons. An improved device architecture, the double heterojunction, is shown to confine excitons within the active layers, allowing substantially higher internal efficiencies to be achieved. A full optical and electrical analysis of the double heterostructure architecture leads to optimal cell design as a function of the optical properties and exciton diffusion lengths of the photoactive materials. Combining the double heterostructure with novel light trapping schemes, devices with external efficiencies approaching their internal efficiency are obtained. When applied to an organic photovoltaic cell with a power conversion efficiency of 1.0%±0.1% under 1 sun AM1.5 illumination, devices with external power conversion efficiencies of 2.4%±0.3% are reported. In addition, we show that by using materials with extended exciton diffusion lengths LD, highly efficient double heterojunction photovoltaic cells are obtained, even in the absence of a light trapping geometry. Using C60 as an acceptor material, double heterostructure external power conversion efficiencies of 3.6%±0.4% under 1 sun AM1.5 illumination are obtained. Stacking of single heterojunction devices leads to thin film multiple heterojunction photovoltaic and photodetector structures. Thin bilayer photovoltaic cells can be stacked with ultrathin (∼5 Å), discontinuous Ag layers between adjacent cells serving as efficient recombination sites for electrons and holes generated in the neighboring cells. Such stacked cells have open circuit voltages that are n times the open circuit voltage of a single cell, where n is the number of cells in the stack. In optimized structures, the short circuit photocurrent remains approximately constant upon stacking thin cells, leading to higher achievable power conversion efficiencies, as confirmed by modelling optical interference effects and exciton migration. A 2.5%±0.3% power efficiency under 100 mW/cm2 AM1.5 illumination conditions is obtained by stacking two ∼1% efficient devices. Alternatively, when the contact layers between the stacked cells are eliminated, a multilayer structure consisting of alternating films of donor and acceptor-type materials is obtained. Since the thicknesses of the individual layers (∼5 Å) can be substantially smaller than the exciton diffusion length, nearly 100% of the photogenerated excitons are dissociated, and the resulting free charges are detected. In addition, the ultrathin organic layers facilitate electron and hole transport through the multilayer stack by tunneling. When these devices are operated as photodetectors under applied fields >106 V/cm, the carrier collection efficiency reaches 80%, leading to external quantum efficiencies of 75%±1% across the visible spectrum in cells containing the thinnest layers. We find that due to the fast carrier tunneling process, the temporal response of these multilayer detectors is a direct measure of exciton dynamics. Response times of 720±50 ps are achieved, leading to a 3 dB bandwidth of 430±30 MHz. A summary of representative results obtained for both polymer and small molecule photovoltaic cells and photodetectors is included in this review. Prospects for further improvements in organic solar cells and photodetectors are considered.

Large area, full-color, digital image sensors made with semiconducting polymers

The high performance polymer photovoltaic cells and photodiodes demonstrated recently are of great interests not only for traditional applications but also for large area, digital image sensing. In this presentation, we demonstrate large sensing area, photodiode arrays and matrices made with semiconducting polymers and polymer blends. These photosensors are of high photosensitivity, low dark current, large dynamic range and fast response time. For full-color image sensing applications, large area photosensor arrays were developed to have true-color (24-bits) resolution. The red, green, and blue color primaries were achieved by coupling a set of color filters with the polymer sensor pixels with response to visible light radiation. Image recovery process from the pixel photocurrent data was developed, which is suitable generally to image arrays with power law light intensity dependence and with finite pixel dark current. Large sensing length photodiode arrays (2.5–5 in.) were fabricated in 40–100 dot-per-inch (dpi). They were used as the sensing unit in page-size document scanners. Voltage-switchable polymer photodetectors were developed. Their photosensitivity can be switched on and off with external bias. These devices can be used as the sensing elements in x– y addressable sensor matrices. The high on/off switching ratio and high photocurrent/darkcurrent ratio allow such two-dimensional, passive photosensor matrices to be used for image sensing applications.

Large Area, Full-Color Image Sensors Made with Organic Semiconductors

ABSTRACTLarge area polymer photodiode arrays were designed and fabricated for full-color imaging applications. These sensor arrays are of high photosensitivity, low dark current, large dynamic range and fast response time. The red, green and blue color primaries were achieved by coupling a set of color filters with the polymer sensor pixels with broad response covering entire visible spectrum. Image recovery process from the pixel photocurrent data was developed, which is suitable generally to image arrays with power-law light intensity dependence and with finite pixel dark current. Large sensing length photodiode arrays (2.5”5”) were fabricated with pixel densities from 40 to 100 dot-per-inch. They were used as the sensing elements in page size document scanners. Voltage switchable polymer photodetectors were developed. Their photosensitivity can be switched on and off with external bias. These devices can be used as the sensing elements in x-y addressable two-dimensional sensor matrices. The high on/off switching ratio and high photocurrent/dark-current ratio allow such two-dimensional, passive photosensor matrices be used for image sensing applications.