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Panchromatic All‐Polymer Photodetector with Tunable Polarization Sensitivity

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... Each FR consists of N layers of polymer retarder films (r 1 to r N ), with equal thicknesses L and fast axes oriented in a folded manner, i.e., alternating between an angle of ±, as illustrated in Fig. 1F. The polarization sensitivity of the OPV cells can then be introduced by orienting the polymer semiconductors in the plane of the film and exploiting the anisotropic optical character of the polymers (25)(26)(27). Referring to Fig. 1E, OPV1 and OPV2 sense polarization, while OPV3 to OPV6 sense color and also provide a single polarization channel. The light's polarization state is detected by OPV1 and OPV2, which are nominally sensitive to 0° and +45° polarized light relative to the x axis, respectively. ...
... The intrinsic P-OPV cells were realized through orienting the active polymer semiconductors in the plane of the film, as previously described (25,27,33,34). This exploits the optical anisotropy of the polymers, which have their primary optical transition dipole moment (-*) aligned parallel to their backbone (35,36). ...
... The chemical structure of these two polymers is depicted in Fig. 3A. These polymers were chosen because they are both ductile at room temperature and have complimentary spectral absorption, enabling panchromatic P-OPV cells (27,38). However, the strain alignment approach limited the peak dichroic ratios to approximately 2 at the maximum possible strain before film tearing (27). ...
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Combining hyperspectral and polarimetric imaging provides a powerful sensing modality with broad applications from astronomy to biology. Existing methods rely on temporal data acquisition or snapshot imaging of spatially separated detectors. These approaches incur fundamental artifacts that degrade imaging performance. To overcome these limitations, we present a stomatopod-inspired sensor capable of snapshot hyperspectral and polarization sensing in a single pixel. The design consists of stacking polarization-sensitive organic photovoltaics (P-OPVs) and polymer retarders. Multiple spectral and polarization channels are obtained by exploiting the P-OPVs’ anisotropic response and the retarders’ dispersion. We show that the design can sense 15 spectral channels over a 350-nanometer bandwidth. A detector is also experimentally demonstrated, which simultaneously registers four spectral channels and three polarization channels. The sensor showcases the myriad degrees of freedom offered by organic semiconductors that are not available in inorganics and heralds a fundamentally unexplored route for simultaneous spectral and polarimetric imaging.
... In this section, the radiometric performance of the Solc-based OPV detectors is quantified using realistic polarization-sensitive OPV materials as described in [29]. Additionally, an optimization scheme for the OPV's transmittance properties is provided in order to enhance the SNR across all five OPVs. ...
... The parameters used in the simulations include a pixel area of 2.5 × 10 −3 cm 2 , a fill factor of 0.9, a detector integration time of 10 ms, an integration capacitance of 100 pf, a holdswitch resistance of 1.5 k , and a reset-switch resistance of 1000 G . The OPV's parameters were obtained from [29] for the OPV cells under applied strain of 60% and included (i) a dark current density of 10 −9 A/cm 2 at 0 V bias, (ii) responsivity values between 0.1 and 0.2 A/W from 400 to 700 nm, and (iii) transmittances (T x and T y ) values as provided in Table 1. ...
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Using organic photodetectors for multispectral sensing is attractive due to their unique capabilities to tune spectral response, transmittance, and polarization sensitivity. Existing methods lack tandem multicolor detection and exhibit high spectral cross talk. We exploit the polarization sensitivity of organic photodetectors, together with birefringent optical filters to design single-pixel multispectral detectors that achieve high spectral selectivity and good radiometric performance. Two different architectures are explored and optimized, including the Solc-based and multitwist-retarder-based organic photodetectors. Although the former demonstrated a higher spectral resolution, the latter enables a more compact sensor as well as greater flexibility in device fabrication.
... A rotating AQWP modulator is placed at the entrance of the pixel, followed by 4 P-OPV cells (OPV0 -OPV3) that alternate with 3 LC-based MTR units (MTR1 -MTR3) in series. The OPVs' anisotropy is obtained by aligning the polymer backbone chains uniaxially along the plane of the film [36,37]. In the current configuration of Fig. 1 (d), all OPV cells have their transmission axes parallel to the x-axis. ...
... These materials also have complimentary absorption spectra ( Figure 2a) and appropriate energy level offsets for efficient exciton dissociation and charge collection. 22 In addition to the optoelectronic characteristics, these polymers are found to be ductile, which has been shown to be a good screening tool to asses toughness, 23 as discussed further below. Our primary interest is to explore the impact of the polymer MW and film morphology on mechanical behavior for an All-PSC. ...
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The development of flexible and physically robust organic solar cells requires detailed knowledge of the mechanical behavior of the heterogeneous material stack. However, in these devices there has been limited research on the mechanical properties of the active organic layer. Here, two critical mechanical properties, stiffness and ductility, of a widely studied organic solar cell active layer, a blend film composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) are reported. Processing conditions are varied to produce films with differing morphology and correlations are developed between the film morphology, mechanical properties and photovoltaic device performance. The morphology is characterized by fitting the absorption of the P3HT:PCBM films to a weakly interacting H-aggregate model. The elastic modulus is determined using a buckling metrology approach and the crack onset strain is determined by observing the film under tensile strain using optical microscopy. Both the elastic modulus and crack onset strain are found to vary significantly with processing conditions. Processing methods that result in improved device performance are shown to decrease both the compliance and ductility of the film.
Article
Polymer conductors that are solution processable provide an opportunity to realize low-cost organic electronics. However, coating sequential layers can be hindered by poor surface wetting or dissolution of underlying layers. This has led to the use of transfer printing where solid film inks are transferred from a donor substrate to partially fabricated devices using a stamp. This approach typically requires favorable adhesion differences between the stamp, ink, and receiving substrate. Here, we present a shear-assisted organic printing (SHARP) technique that employs a shear load on a post-less polydimethylsiloxane (PDMS) elastomer stamp to print large-area polymer films that can overcome large unfavorable adhesion differences between the stamp and receiving substrate. We explore the limits of this process by transfer printing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films with varied formulation that tune adhesive fracture energy. Using this platform, we show that the SHARP process is able to overcome a 10-fold unfavorable adhesion differential without the use of a patterned PDMS stamp enabling large area printing. The SHARP approach is then used to print PEDOT:PSS films in the fabrication of high performance semitransparent organic solar cells.
Article
The synthesis of an acceptor polymer PIDT- 2TPD, comprising indacenodithiophene (IDT) as the electron-rich unit and an interconnected bithieno[3,4-c]- pyrrole-4,4′,6,6′-tetrone (2TPD) as the electron-deficient unit, and its application for all-polymer photodetectors is reported. The optical, electrochemical, charge transport, and device properties of a blend of poly(3-hexylthiophene) and PIDT-2TPD are studied. The blend shows strong complementary absorption and balanced electron and hole mobility, which are desired properties for a photoactive layer. The device exhibits dark current density in the order of 10−5 mA/ cm2, external quantum efficiency broadly above 30%, and nearly planar detectivity over the entire visible spectral range (maximum of 1.1 × 1012 Jones at 610 nm) under −5 V bias. These results indicate that PIDT-2TPD is a highly functional new type of acceptor and further motivate the use of 2TPD as a building block for other n-type materials
Article
Organic solar cells (OSCs) have been dominated by donor:acceptor blends based on fullerene acceptors for over two decades. This situation has changed recently, with non-fullerene (NF) OSCs developing very quickly. The power conversion efficiencies of NF OSCs have now reached a value of over 13%, which is higher than the best fullerene-based OSCs. NF acceptors show great tunability in absorption spectra and electron energy levels, providing a wide range of new opportunities. The coexistence of low voltage losses and high current generation indicates that new regimes of device physics and photophysics are reached in these systems. This Review highlights these opportunities made possible by NF acceptors, and also discuss the challenges facing the development of NF OSCs for practical applications.
Article
Polymer semiconductors are an attractive material system for flexible and stretchable electronic devices owing to their potentially favorable mechanical attributes. Establishing the thermomechanical behavior of polymer semiconductors is thus an important consideration to ensure successful operation in these applications. One of the most common mechanical characterization methods for these materials is to manipulate the thin films while on an elastomer substrate. A primary measurement with this approach is the film’s crack onset strain (COS), a measure of ductility. It is simple and effective; however, it is a highly qualitative view of film mechanical stability, particularly in flexible device applications. Alternatively, cohesive fracture energy (Gc) provides a direct quantitative measure of the mechanical integrity of the film. While fracture energy provides important insight into mechanical stability, it typically requires a more complex measurement method than the film on elastomer tests. Here, we compare the COS using film on elastomer testing, with cohesive fracture energy measured using four-point bending for a range of polymer semiconductor films. The polymers considered have a range of molecular structures and molecular packing characteristics providing a broad representative sample set. The values of Gc ranged from 0.4 to 18 J/m² while COS ranged from 2% to over 100%. We show that COS of the films can be correlated with Gc providing support that COS is a valuable measurement to probe the mechanical toughness of polymer semiconductor films. We also discuss the physical characteristics each measurement highlights and the complementary nature of these measurements.
Article
The ability to use infrared imaging systems with multicolor capabilities, high photoresponsivity and polarization sensitivity, is central to practical photodetectors and has been demonstrated with conventional devices based on Ⅲ-Ⅴ or Ⅱ-Ⅵ semiconductors. However, the photodetectors working at room temperature with high responsivity for polarized infrared light detection remains elusive. Here, we first demonstrate a broadband photodetector using a vertical photogate heterostructure of BP-on-WSe2 (black phosphorus-on-tungsten diselenide) in which BP serves as the photogate and WSe2 as the conductive channel. Ultrahigh visible and infrared photoresponsivity at room temperature can reach up to ~10³ A/W and ~5×10⁻¹ A/W, respectively, and ultrasensitive visible and infrared specific detectivity is obtained up to ~10¹⁴ and ~10¹⁰ Jones respectively at room temperature. Moreover, the high sensitivity to infrared polarization is about 40 mA/W with incident light polarized along the horizontal axis (defined as 0° polarization). This performance is due to the strong intrinsic linear dichroism of BP and the device design which can sufficiently collect the photoinduced carriers isotropically, as well as the influence from the orientation of the edge of the BP-on-WSe2 overlapped area which is the same for all polarizations. The high responsivity, good sensitive detectivity and highly polarization-sensitive infrared photoresponse suggest that the photodetectors based on photogate structure afford new opportunities for infrared detecting or imaging at room temperature by using two-dimensional materials.
Article
A series of electron-acceptor polymers, copolymers and blends were used in all-polymer BHJ photodetectors and the effect of acceptor compositions on the key device parameters of dark current density and photocurrent was investigated. Compared with acceptor polymers and polymer blends, the devices based on acceptor copolymer showed lowered dark current and higher photocurrent, due to optimal molecular stacking and morphology of the BHJ active layer. The acceptor blends tend to cause a large phase separation and rough surface of the active layer, thus leading to a low detectivity of the device. Among all the acceptor compositions studied in this work, the all-polymer BHJ photodetector based on a donor polymer (PolyD) and an acceptor copolymer (PolyAA′50) exhibited the highest specific detectivity of over 10¹² Jones in the spectral region of 320–980 nm under -0.1 V bias.
Article
Low noise current is critical for achieving high-detectivity organic photodetectors. Inserting charge blocking layers is an effective approach to suppress the reverse-biased dark current. However, in solution-processed organic photodetectors, the charge transport material needs to be dissolved in solvents that don't dissolve the underneath light-absorbing layer, which is not always possible for all kinds of light absorbing materials developed. Here, we introduce a universal strategy of transfer-printing a conjugated polymer poly(3-hexylthiophene) (P3HT) as the electron-blocking layer to realize highly sensitive photodetectors. The transfer-printed P3HT layers substantially and universally reduced the reverse-biased dark-current by about three orders of magnitude for various photodetectors with different active layers. These photodetectors can detect the light signal as weak as several picowatt per square centimeter, and the device detectivity is over 1012 Jones. The results suggest that the strategy of transfer-printing P3HT films as the electron-blocking layer is universal and effective for the fabrication of sensitive organic photodetectors.
Article
An intrinsic coincident full-Stokes polarimeter is demonstrated by using strain-aligned polymer-based organic photovoltaics (OPVs) that can preferentially absorb certain polarized states of incident light. The photovoltaic-based polarimeter is capable of measuring four Stokes parameters by cascading four semitransparent OPVs in series along the same optical axis. This in-line polarimeter concept potentially ensures high temporal and spatial resolution with higher radiometric efficiency as compared to the existing polarimeter architecture. Two wave plates were incorporated into the system to modulate the S3 Stokes parameter so as to reduce the condition number of the measurement matrix and maximize the measured signal-to-noise ratio. Radiometric calibration was carried out to determine the measurement matrix. The polarimeter presented in this paper demonstrated an average RMS error of 0.84% for reconstructed Stokes vectors after normalized to S0. A theoretical analysis of the minimum condition number of the four-cell OPV design showed that for individually optimized OPV cells, a condition number of 2.4 is possible.
Article
The mechanical properties of organic electronic materials and interfaces play a central role in determining the manufacturability and reliability of flexible and stretchable organic electronic devices. The synergistic effects of mechanical stress and deformation, together with other operating parameters such as temperature and temperature cycling, and exposure to solar radiation, moisture, and other environmental species are particularly important for longer-term device stability. We review recent studies of basic mechanical properties such as adhesion and cohesion, stiffness, yield behavior, and ductility of organic semiconducting materials, and their connection to underlying molecular structure. We highlight thin-film metrologies to probe the mechanical behavior, including when subjected to simulated operational conditions. We also report on strategies for improving reliability through interface engineering and tailoring material chemistry and molecular structure. These studies provide insights into how these metrologies and metrics inform the development of materials and devices for improved reliability.
Article
Power conversion efficiency (PCE) has surpassed 10% for single junction organic solar cells (OSCs) mainly through the design and synthesis of novel donor materials, the optimization of film morphology and the evolution of the devices. However, the development of novel acceptor materials is relatively sluggish compared with the donor compounds. Nowadays, fullerene derivatives, such as PC61BM and PC71BM, are still the dominant acceptors due to their superior charge transporting properties. Unfortunately, these two acceptors suffer from some intrinsic shortcomings such as limited absorption, difficult functionalization, and high production cost. Therefore, developing novel non-fullerene acceptors that can overcome the above-mentioned disadvantages is highly desirable. As a matter of fact, research on non-fullerene acceptors has made considerable progress in the last two years and a highest PCE of around 12% has been achieved. In this review, we will summarize recent research progress in non-fullerene small molecule acceptors and compare these molecules' performances in OSCs employing the same donor materials. Moreover, the acceptors with excellent photovoltaic performance are highlighted and the reasons are elaborated. Finally, the implications and the challenges are proposed.
Article
Low-cost organic photodetectors have shown sensitivity levels comparable to those of inorganic photodetectors, but with response speeds generally limited to the megahertz range due to the low mobility of organic semiconductors. Here, we integrated organic-inorganic hybrid perovskite (OIHP) photoactive layers with low-bandgap organic bulk-heterojunction (BHJ) layers to produce a device that combined the advantages of the two types of photodetectors. Integrating methylammonium lead triiodide (CH3NH3PbI3) with a low-bandgap BHJ layer extended the response of perovskite photodetectors to a wavelength of 1000 nanometers without deteriorating the responsivity and specific detectivity of either type of photodetector. The high mobility of charge carriers in CH3NH3PbI3 allowed the constraints of the resistance-capacitance constant to be relieved so that the device response speed could be increased dramatically. A response time of five nanoseconds was measured for incident infrared light from the device with an active area of 0.1 square millimeters, which represents the state-of-the-art performance for organic-based photodetectors.
Article
Three acceptor–acceptor (A–A) type conjugated polymers based on isoindigo and naphthalene diimide/perylene diimide are designed and synthesized to study the effects of building blocks and alkyl chains on the polymer properties and performance of all-polymer photoresponse devices. Variation of the building blocks and alkyl chains can influence the thermal, optical, and electrochemical properties of the polymers, as indicated by thermogravimetric analysis, differential scanning calorimetry, UV–vis, cyclic voltammetry, and density functional theory calculations. Based on the A–A type conjugated polymers, the most efficient all-polymer photovoltaic cells are achieved with an efficiency of 2.68%, and the first all-polymer photodetectors are constructed with high responsivity (0.12 A W−1) and detectivity (1.2 × 1012 Jones), comparable to those of the best fullerene based organic photodetectors and inorganic photodetectors. Photoluminescence spectra, charge transport properties, and morphology of blend films are investigated to elucidate the influence of polymeric structures on device performances. This contribution demonstrates a strategy of systematically tuning the polymeric structures to achieve high performance all-polymer photoresponse devices.
Article
The reduction of dark current is required to enhance the signal-to-noise ratio and decrease the power consumption in photodetectors. This is typically achieved by introducing additional functional layers to suppress carrier injection, a task that proves to be challenging especially in printed devices. Here we report on the successful reduction of dark current below 100 nA cm−2 (at −1 V bias) in an inkjet printed photodetector by the insertion of an electron blocking layer based on poly[3-(3,5-di-tert-butyl-4-methoxyphenyl)-thiophene], while preserving a high quantum yield. Furthermore, the electron blocking layer strongly increases the surface energy of the hydrophobic photoactive layer, therefore simplifying the printing of transparent top electrodes from water based formulations without the addition of surfactants.
Article
Broad-response and high-detectivity for all-polymer photodetectors based on p- and n-type semiconducting polymers have been achieved through optimization of polymer property and film microstructure. The electron-donating units in the p-type polymers affect a great deal of the polymer properties such as solubility, absorption spectra, and electronic energy levels, which in turn can influence the device performance. The polymer (P3) based on dithienopyrrole and diketopyrrolopyrrole is most promising for photodetector applications, as it possesses the suitable energy level with regard to the n-type polymer and exhibits appropriate film morphology and molecular stacking with aid of 1,8-diiodooctane as an additive during film processing. The photodetector based on P3/poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (PNDI) exhibits good response from 300 to 1100 nm and nearly constant detectivity (D*) above 10¹² Jones at 330–980 nm, rendering a great potential of all-polymer photodetectors for practical applications. (Figure presented.).
Article
The specific optical absorption of an organic semiconductor is critical to the performance of organic optoelectronic devices. For example, higher light-harvesting efficiency can lead to higher photocurrent in solar cells that are limited by sub-optimal electrical transport. Here, we compare over 40 conjugated polymers, and find that many different chemical structures share an apparent maximum in their extinction coefficients. However, a diketopyrrolopyrrole-thienothiophene copolymer shows remarkably high optical absorption at relatively low photon energies. By investigating its backbone structure and conformation with measurements and quantum chemical calculations, we find that the high optical absorption can be explained by the high persistence length of the polymer. Accordingly, we demonstrate high absorption in other polymers with high theoretical persistence length. Visible light harvesting may be enhanced in other conjugated polymers through judicious design of the structure.
Article
Polymer semiconductors based on donor-acceptor monomers have recently resulted in significant gains in field effect mobility in organic thin film transistors (OTFTs). These polymers incorporate fused aromatic rings and have been designed to have stiff planar backbones, resulting in strong intermolecular interactions, which subsequently result in stiff and brittle films. The complex synthesis typically required for these materials may also result in increased production costs. Thus, developing methods to improve mechanical plasticity while lowering material consumption during fabrication will significantly improve opportunities for adoption in flexible and stretchable electronics. To achieve these goals, we consider blending a brittle donor¬-acceptor polymer poly[4(4,4dihexadecyl4Hcyclopenta[1,2b:5,4b’]dithiopen2yl)alt[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT) with ductile poly(3hexylthiophene). We find that the ductility of the blend film is significantly improved compared to neat PCDTPT films, and when employed in an OTFT, the performance is largely maintained. The ability to maintain charge transport character is due to vertical segregation within the blend, while the improved ductility is achieved due to intermixing of the polymers throughout the film thickness. Importantly, applying large strains to the ductile films is then shown to orient both polymers, which further increases charge carrier mobility. These results highlight a processing approach to achieve high performance polymer OTFTs that are electrically and mechanically optimized.
Chapter
This chapter reviews the status of heterogeneous integration of silicon waveguides and photodetectors. The most commonly used available fabrication technologies, namely germanium growth and III/V-silicon wafer bonding, are described. Design constraints common to both platforms, as well as the key differences between them, are discussed. An overview of demonstrated devices on both platforms is presented.
Article
High efficiency all-polymer solar cells with less thickness-dependent behavior are demonstrated, by using low bandgap n-type conjugated polymer N2200 as acceptor and an absorption-complementary difluorobenzotriazole-based medium bandgap polymer J51 as donor.
Article
All-printed organic photodiode arrays on plastic are reported with average specific detectivities of 3.45 × 10(13) cm Hz(0.5) W(-1) at a bias of -5 V. The blade-coated polyethylenimine cathode interlayer and active layer and screen-printed anode enable precise device performance tunability and excellent homogeneity at centimetric scales. These devices' high operational reverse bias, good linear dynamic range, and bias stress stability make them attractive for implementation in imaging systems.
Article
Bulk heterojunction (BHJ) organic solar cells are fabricated with the polymer semiconductor aligned in the plane of the film to probe charge recombination losses associated with aggregates characterized by varying degrees of local order. 100% uniaxial strain is applied on ductile poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) BHJ films and characterize the resulting morphology with ultraviolet-visible absorption spectroscopy and grazing incidence X-ray diffraction. It is found that the strained films result in strong alignment of the highly ordered polymer aggregates. Polymer aggregates with lower order and amorphous regions also align but with a much broader orientation distribution. The solar cells are then tested under linearly polarized light where the light is selectively absorbed by the appropriately oriented polymer, while maintaining a common local environment for the sweep out of photogenerated charge carriers. Results show that charge collection losses associated with a disordered BHJ film are circumvented, and the internal quantum efficiency is independent of P3HT local aggregate order near the heterojunction interface. Uniquely, this experimental approach allows for selective excitation of distinct morphological features of a conjugated polymer within a single BHJ film, providing insight into the morphological origin of recombination losses.
Article
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.
Article
A novel method of strain-aligning polymer films is introduced and applied to regioregular poly(3-hexylthiophene) (P3HT), showing several important features of charge transport. The polymer backbone is shown to align in the direction of applied strain resulting in a large charge-mobility anisotropy, where the in-plane mobility increases in the applied strain direction and decreases in the perpendicular direction. In the aligned film, the hole mobility is successfully represented by a two-dimensional tensor, suggesting that charge transport parallel to the polymer backbone within a P3HT crystal is strongly favored over the other crystallographic directions. Hole mobility parallel to the backbone is shown to be high for a mixture of plane-on and edge-on packing configurations, as the strain alignment is found to induce a significant face-on orientation of the originally highly edge-on oriented crystalline regions of the film. This alignment approach can achieve an optical dichroic ratio of 4.8 and a charge-mobility anisotropy of 9, providing a simple and effective method to investigate charge-transport mechanisms in polymer semiconductors.
Article
Anisotropic charge transport, or polarized photo- and electroluminescence are possible applications for materials combining the optical properties of arylenevinylene polymers with the characteristic orientational order of the liquid-crystalline state. Thermally stale, soluble materials with a broad mesophase, which can be oriented macroscopically using conventional processing techniques and when exhibit highly anisotropic luminescence behavior of use in polarized LEDs are presented.
Article
While organic electronics is mostly dominated by light-emitting diodes, photovoltaic cells and transistors, optoelectronics properties peculiar to organic semiconductors make them interesting candidates for the development of innovative and disruptive applications also in the field of light signal detection. In fact, organic-based photoactive media combine effective light absorption in the region of the spectrum from ultraviolet to near-infrared with good photogeneration yield and low-temperature processability over large areas and on virtually every substrate, which might enable innovative optoelectronic systems to be targeted for instance in the field of imaging, optical communications or biomedical sensing. In this review, after a brief resume of photogeneration basics and of devices operation mechanisms, we offer a broad overview of recent progress in the field, focusing on photodiodes and phototransistors. As to the former device category, very interesting values for figures of merit such as photoconversion efficiency, speed and minimum detectable signal level have been attained, and even though the simultaneous optimization of all these relevant parameters is demonstrated in a limited number of papers, real applications are within reach for this technology, as it is testified by the increasing number of realizations going beyond the single-device level and tackling more complex optoelectronic systems. As to phototransistors, a more recent subject of study in the framework of organic electronics, despite a broad distribution in the reported performances, best photoresponsivities outperform amorphous silicon-based devices. This suggests that organic phototransistors have a large potential to be used in a variety of optoelectronic peculiar applications, such as a photo-sensor, opto-isolator, image sensor, optically controlled phase shifter, and opto-electronic switch and memory.
Article
Homogeneous alignment of poly(9,9-dioctylfluorene) films on thin layers of rubbed precursor-route poly(p-phenylenevinylene) allows the construction of light-emitting diodes that emit highly polarized blue light (λem = 458 nm). The rubbed poly(p-phenylenevinylene) acts as an effective hole-injecting alignment layer. Annealing of poly(9,9-dioctylfluorene) in its nematic phase followed by rapid quenching orients the polymer as a glassy monodomain on the alignment layer and gives devices with a polarization ratio of 25:1 and a luminance of up to 250 cd/m2. © 2000 American Institute of Physics.
Article
Field-effect transistors fabricated from semiconducting conjugated polymers are candidates for flexible and low-cost electronic applications. Here, we demonstrate that the mobility of high molecular weight (300 kDa) regioregular, poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] can be significantly improved by introducing long-range orientation of the polymer chains. By annealing for short periods, hole mobilities of 6.7 cm2/Vs have been demonstrated. The transport is anisotropic, with higher mobility (approx. 6:1) parallel to the polymer backbone than perpendicular to the polymer backbone.
Article
Highly oriented films of an electron accepting polymer semiconductor, poly{[N,N´-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5´-(2,2´-bithiophene)} (PNDI2OD-T2), are obtained by two different methods, namely directional epitaxial crystallization (DEC) on 1,3,5-trichlorobenzene (TCB) and epitaxy on friction transferred poly(tetrafluoroethylene) (PTFE) substrates. Two distinct polymorphs with unprecedented intra-chain resolution are identified by high resolution transmission electron microscopy (HR-TEM). Form I is obtained by DEC on TCB, whereas highly oriented films of form II are obtained on PTFE substrates after melting at T=300°C and cooling at 0.5K/min. In form I, both electron diffraction and HR-TEM indicate a segregated stacking of bithiophene (T2) and naphthalene diimide (NDI) units forming separate columns. In form II, a ∼c/2 shift between successive π-stacked chains leads to mixed π-overlaps of T2 and NDI. Form I can be transformed into form II by annealing at T>250°C. The different π-stacking of NDI and T2 in the two polymorphs have characteristic signatures in the UV-vis spectra, especially in the charge transfer band around 750nm which is also observed in spin coated films.
Article
Understanding the complex interplay between the 3D structural hierarchy within thin films of conjugated polymers and the properties of devices based thereon is starting to be recognized as an important challenge in the continued development of these materials for a range of applications. As a result, for example, accurate measurements of molecular orientation and elucidation of its influence on optical characteristics are of significant interest. Here we report an improved optical method to determine both the order parameter and the angle between the polymer backbone director and the optical transition dipole moment for the lowest energy π–π* absorption peak in uniaxially aligned thin films of conjugated polymers. The method uses a combination of polarized Raman spectroscopy and UV-vis spectroscopy and is based on a general theoretical treatment to describe the expected Raman and optical absorption anisotropies of such films. It is applied to study the orientation within thermotropically aligned films of the electroluminescent fluorene-based copolymer poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). A more highly axial transition dipole moment is found for the dominant long wavelength absorption peak of F8T2 compared to that of other fluorene-based (co)polymers. The angle between the polymer backbone director and the transition dipole is estimated to be β ≤ 3°, a deduction that helps to explain the relatively large optical dichroism for aligned films of F8T2 and that offers the prospect of highly polarized electroluminescence from F8T2-based light-emitting diodes.
Article
The organic electroluminescent devices emitting polarized light show potential as backlights in conventional liquid-crystal displays. The necessary high polarization ratios require, in turn, highly aligned emitters. This article discusses the various methods of orientation together with the materials to which they are applicable.