Polymer Light-emitting Electrochemical Cells Research Articles

Wireless Electroluminescence: Polymer Light-Emitting Electrochemical Cells with Inkjet-Printed 1D and 2D Bipolar Electrode Arrays

One-dimensional (1D) and two-dimensional (2D) arrays of conductive microdisks are printed on glass substrates using a silver nanoparticle (Ag-NP)-based ink. Polymer light-emitting electrochemical cells (PLECs) are then fabricated on top via spin-coating and the thermal deposition of aluminum driving electrodes (DEs). The extremely large planar PLECs, with an interelectrode separation of 11 mm, are driven with a bias voltage of 400 V. This causes in situ electrochemical doping in the polymer from both DEs and the Ag-NP disks. The latter functions as bipolar electrodes (BPEs) to induce and sustain doping reactions at their extremities. Time-lapse photoluminescence and electroluminescence (EL) imaging reveals that p- and n-doping originating from neighboring BPEs can interact to form multiple light-emitting p–n junctions, connected in series by the BPEs. Unexpectedly, the multiple p–n junctions begin to emit well before a continuous pathway of doped polymers is established between the DEs. This observation b...

High Current Density Injection into Polymer Light-Emitting Electrochemical Cells

High Current Density Injection into Polymer Light-Emitting Electrochemical Cells

Ag nanocluster-based color converters for white organic light-emitting devices

The authors present Ag nanocluster-based color converters (Ag NC color converters), which convert part of the blue light from a light source to yellow light so as to create white organic light-emitting devices that could be suitable for lighting systems. Ag NCs synthesized by poly(methacrylic acid) template methods have a statistical size distribution with a mean diameter of around 4.5 nm, which is larger than the Fermi wavelength of around 2 nm. Hence, like free electrons in metals, the Ag NC electrons are thought to form a continuous energy band, leading to the formation of surface plasmons by photoexcitation. As for the fluorescence emission mechanism, the fact that the photoluminescence is excitation wavelength dependent suggests that the fluorescence originates from surface plasmons in Ag NCs of different sizes. By using Ag NC color converters and suitable blue light sources, white organic light-emitting devices can be fabricated based on the concept of light-mixing. For our blue light sources, we used polymer light-emitting electrochemical cells (PLECs), which, like organic light-emitting diodes, are area light sources. The PLECs were fabricated with a blue fluorescent π-conjugated polymer, poly[(9,9-dihexylfluoren-2,7-diyl)-co-(anthracen-9,10-diyl)] (PDHFA), and a polymeric solid electrolyte composed of poly(ethylene oxide) and KCF3SO3. In this device structure, the Ag NC color converter absorbs blue light from the PDHFA-based PLEC (PDHFA-PLEC) and then emits yellow light. When the PDHFA-PLEC is turned on by applying an external voltage, pure white light emission can be produced with Commission Internationale de l'Eclairage coordinates of (x = 0.32, y = 0.33) and a color rendering index of 93.6. This study shows that utilization of Ag NC color converters and blue PLECs is a very promising and highly effective method for realizing white organic light-emitting devices.

Polymer light-emitting electrochemical cells—Recent advances and future trends

Polymer light-emitting electrochemical cells (PLECs) are electroluminescent devices whose operation involves both ionic and electronic charges. A key reaction in PLEC is the electrochemical doping of the light emitter, a luminescent polymer, which eventually leads to the formation of a light-emitting p–n or p–i–n junction in the interior of the cell. This review opens with a general introduction to the operating mechanism of PLECs. This is followed by a summary of key advancements in the field in the last 2 years. The focus of the review is on the chemical mapping and optical probing of the PLEC doping process and junction structures. The results showcased in this review provide a coherent picture of the PLEC operating mechanism and point in new research directions in both device applications and fundamental sciences of mixed ionic/electronic polymer conductors.

Bipolar Electrode Array Embedded in a Polymer Light-Emitting Electrochemical Cell.

A linear array of aluminum discs is deposited between the driving electrodes of an extremely large planar polymer light-emitting electrochemical cell (PLEC). The planar PLEC is then operated at a constant bias voltage of 100 V. This promotes in situ electrochemical doping of the luminescent polymer from both the driving electrodes and the aluminum discs. These aluminum discs function as discrete bipolar electrodes (BPEs) that can drive redox reactions at their extremities. Time-lapse fluorescence imaging reveals that p- and n-doping that originated from neighboring BPEs can interact to form multiple light-emitting p-n junctions in series. This provides direct evidence of the working principle of bulk homojunction PLECs. The propagation of p-doping is faster from the BPEs than from the positive driving electrode due to electric field enhancement at the extremities of BPEs. The effect of field enhancement and the fact that the doping fronts only need to travel the distance between the neighboring BPEs to form a light-emitting junction greatly reduce the response time for electroluminescence in the region containing the BPE array. The near simultaneous formation of multiple light-emitting p-n junctions in series causes a measurable increase in cell current. This indicates that the region containing a BPE is much more conductive than the rest of the planar cell despite the latter's greater width. The p- and n-doping originating from the BPEs is initially highly confined. Significant expansion and divergence of doping occurred when the region containing the BPE array became more conductive. The shape and direction of expanded doping strongly suggest that the multiple light-emitting p-n junctions, formed between and connected by the array of metal BPEs, have functioned as a single rod-shaped BPE. This represents a new type of BPE that is formed in situ and as a combination of metal, doped polymers, and forward-biased p-n junctions connected in series.

High-color-rendering-index white polymer light-emitting electrochemical cells based on ionic host-guest systems: Utilization of blend films of blue-fluorescent cationic polyfluorenes and red-phosphorescent cationic iridium complexes

Abstract The authors present high-color-rendering-index (CRI) white polymer light-emitting electrochemical cells (PLECs) based on ionic host-guest systems. PLECs were fabricated with blend films composed of a blue fluorescent cationic π-conjugated polymer, poly(9,9-bis[6′-(N,N,N,-trimethylammonium)hexyl]fluorine-co-alt-1,4-phenylene)bromide (PFN+Br−), used as a host, and a red phosphorescent cationic iridium complex, [Ir(ppy)2(biq)]+(PF6)− (where (ppy)− = 2-phenylpyridinate and biq = 2,2′-biquinoline), used as a guest. By optimizing the composition of PFN+Br− and [Ir(ppy)2(biq)]+(PF6)− in the active layers, white light emission with Commission Internationale de l'Eclairage (CIE) coordinates of (x = 0.28, y = 0.31) and a very high CRI of 95.8 was achieved through mixing of blue and red light at an applied voltage of 4.0 V. The white light emissions were obtained at a low [Ir(ppy)2(biq)]+(PF6)− concentration (PFN+Br−:[Ir(ppy)2(biq)]+(PF6)− = 1:0.01 (mass ratio)), showing that [Ir(ppy)2(biq)]+(PF6)− acts as a suitable energy acceptor. The emission mechanism of the ionic host-guest PLECs can be explained by Forster resonance energy transfer (FRET) and Dexter energy transfer (DET) from the excited PFN+Br− to [Ir(ppy)2(biq)]+(PF6)−. Utilization of ionic host-guest PLECs is a very promising method for realizing white light-emitting devices with very high CRI.

Visualizing the Bipolar Electrochemistry of Electrochemically Doped Luminescent Conjugated Polymers

A micromanipulated vacuum probe station and fluorescence imaging are used to induce and visualize bipolar electrochemical redox reactions in a solid-state polymer light-emitting electrochemical cell (PLEC). The PLEC is a planar cell consisting of a composite polymer film partially covered by two parallel, vacuum-deposited rectangular aluminum electrodes at a separation of 10.7 mm. A pair of biased metallic probes are placed into direct contact with the exposed polymer surface, causing in situ electrochemical p- and n-doping of the luminescent polymer in the interior of the PLEC. Subsequently, the biased probes are moved to contact the rectangular aluminum electrodes of the PLEC to activate the device. An interesting phenomenon has been observed: in situ electrochemical doping is seen to originate from the previously doped regions that are isolated from the driving electrodes. By analyzing the complex doping patterns generated, it is concluded that the doped polymers have functioned as bipolar electrodes (...

High resolution scanning optical imaging of a frozen planar polymer light-emitting electrochemical cell: an experimental and modelling study

Light-emitting electrochemical cells (LECs) are organic photonic devices based on a mixed electronic and ionic conductor. The active layer of a polymer-based LEC consists of a luminescent polymer, an ion-solvating/transport polymer, and a compatible salt. The LEC p-n or p-i-n junction is ultimately responsible for the LEC performance. The LEC junction, however, is still poorly understood due to the difficulties of characterizing a dynamic-junction LEC. In this paper, we present an experimental and modeling study of the LEC junction using scanning optical imaging techniques. Planar LECs with an interelectrode spacing of 560 μm have been fabricated, activated, frozen and scanned using a focused laser beam. The optical-beam-induced-current (OBIC) and photoluminescence (PL) data have been recorded as a function of beam location. The OBIC profile has been simulated in COMSOL that allowed for the determination of the doping concentration and the depletion width of the LEC junction.

Fabrication of White Light-emitting Electrochemical Cells with Stable Emission from Exciplexes.

The authors present an approach for fabricating stable white light emission from polymer light-emitting electrochemical cells (PLECs) having an active layer which consists of blue-fluorescent poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) and π-conjugated triphenylamine molecules. This white light emission originates from exciplexes formed between PFD and amines in electronically excited states. A device containing PFD, 4,4',4''-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA), Poly(ethylene oxide) and K2CF3SO3 showed white light emission with Commission internationale de l'éclairage (CIE) coordinates of (0.33, 0.43) and a Color Rendering Index (CRI) of Ra = 73 at an applied voltage of 3.5 V. Constant voltage measurements showed that the CIE coordinates of (0.27, 0.37), Ra of 67, and the emission color observed immediately after application of a voltage of 5 V were nearly unchanged and stable after 300 sec.

Charging and discharging of a planar polymer light-emitting electrochemical cell

Planar polymer light-emitting electrochemical cells (LECs) with an interelectrode gap size of 670μm and 495μm have been activated by applying a fixed voltage bias of 20V at a temperature of 340K to enable them to reach different terminal currents before being discharged under short-circuit conditions. These discharging, or dedoping currents have been monitored for an extended period during which they have decreased by five orders of magnitude. The total dedoping charge has been determined by integrating the time-dependent dedoping current, which allows for the calculation of the cell’s doping level. The data suggest that some additional doping has occurred after the formation of the LEC junction. The overall level of doping, however, is not strongly affected by the large amount charges injected after junction formation. In fact, the total dedoping charge can be approximated as the amount of injected charge, up to the moment of junction formation. The total dedoping charge, however, is sensitive to the interelectrode gap size of the planar cells. The results demonstrate the feasibility of detecting the minute discharging current of a large planar LEC down to pA level. The ability to determine the dedoping charge and its dependence on various operational and device-related parameters provides additional tools to facilitate understanding of the complex doping and dedoping processes in a polymer LEC.

High resolution scanning optical imaging of a frozen polymer p-n junction

Semiconductor homojunctions such as p-n or p-i-n junctions are the building blocks of many semiconductor devices such as diodes, photodetectors, transistors, or solar cells. The determination of junction depletion width is crucial for the design and realization of high-performance devices. The polymer analogue of a conventional p-n or p-i-n junction can be created by in situ electrochemical doping in a polymer light-emitting electrochemical cell (LEC). As a result of doping and junction formation, the LECs possess some highly desirable device characteristics. The LEC junction, however, is still poorly understood due to the difficulties of characterizing a dynamic-junction device. Here, we report concerted optical-beam-induced-current (OBIC) and scanning photoluminescence (PL) imaging studies of planar LECs that have been frozen to preserve the doping profile. By optimizing the cell composition, the electrode work function, and the turn-on conditions, we realize a long, straight, and highly emissive p-n junction with an interelectrode spacing of 700 μm. The extremely broad planar cell allows for time-lapse fluorescence imaging of the in situ electrochemical doping process and detailed scanning of the entire cell. A total of eighteen scans at seven locations along the junction have been performed using a versatile, custom cryogenic laser scanning apparatus. The Gaussian OBIC profiles yield an average 1/e2 junction width of only 1.5 μm, which is the smallest ever reported in a planar LEC. The controlled dedoping of the frozen device via warming cycles leads to an unexpectedly narrower OBIC profile, suggesting the presence and disappearance of fine structures at the edges of the frozen p-n junction. The results reported in this work provide new insight into the nature and structure of the LEC p-n junction. Since only about 0.2% of the entire device area is photoactive in response to an incident optical beam, the effective junction width (or volume) must be dramatically increased to realize a more efficient device.

Polymer Light-Emitting Electrochemical Cell Blends Based on Selection of Lithium Salts, LiX [X = Trifluoromethanesulfonate, Hexafluorophosphate, and Bis(trifluoromethylsulfonyl)imide] with Low Turn-On Voltage

Light-emitting electrochemical cell (LEEC) performance is drastically affected by the selection of suitable electrolyte. With the limited n-type doping in MEH-PPV near the cathodic interface, we hypothesized that anions in the electrolyte are critical to attain a higher conductivity p-doped area. Poly(ethylene oxide)-lithium salts electrolyte systems have been investigated in the LEEC devices to determine the influence of anions on device turn-on voltage and operation. The TFSI– anions showed higher resonance states, low turn-on voltages near the optical bandgap of the emissive conjugated polymer, high ionic conductivity in solid state (1.05 × 10–4 S cm–1), and larger electrochemical stability window compared to conventional CF3SO3– anion. Device maximal brightness (∼100 cd/m2) can be achieved. Modulated differential scanning calorimetry (mDSC) studies of the polymer blend films correlate the thermal stability and doping effectiveness during operation and delineate the onset of degradation that entails burnout of the devices.

High-Performance Light-Emitting Electrochemical Cells by Electrolyte Design

Polymer light-emitting electrochemical cells (LECs) are inherently dependent on a suitable electrolyte for proper function. Here, we design and synthesize a series of alkyl carbonate-capped star-branched oligoether-based electrolytes with large electrochemical stability windows, facile ion release, and high compatibility with common light-emitting materials. LECs based on such designed electrolytes feature fast turn-on, a long operational lifetime of 1400 h at >100 cd m–2 and a record-high power conversion efficiency of 18.1 lm W–1, when equipped with an external outcoupling film.

White Polymer Light-Emitting Electrochemical Cells Fabricated Using Energy Donor and Acceptor Fluorescent π-Conjugated Polymers Based on Concepts of Band-Structure Engineering

The authors report on white polymer light-emitting electrochemical cells (PLECs) fabricated with a polymer blend film composed of a blue fluorescent π-conjugated polymer (blue FCP), poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD), and a red-orange FCP, poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), based on concepts of band-structure engineering. Polymer blending is one of the simplest and most promising methods for fabrication of van der Waals interfaces, which convert electricity to light in PLECs. By optimizing the composition of PFD, MEH-PPV, poly(ethylene oxide) (PEO), and salt (KCF3SO3) in the active layer, white-light emission with Commission Internationale de l’Eclairage (CIE) coordinates of (x = 0.33, y = 0.31) can be achieved through light mixing of blue exciton emission from PFD and red-orange exciton emission from MEH-PPV at an applied voltage higher than the threshold voltage, Vthblue-FCP, which corresponds to Egblue-FCP/e, where Egblue-FCP is the band gap of PFD and e i...

White polymer light-emitting electrochemical cells using emission from exciplexes with long intermolecular distances formed between polyfluorene and π-conjugated amine molecules

The authors present white polymer light-emitting electrochemical cells (PLECs) fabricated with polymer blend films of poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) and π-conjugated triphenylamine molecules. The PLECs have bulk heterojunction structures composed of van der Waals interfaces between the PFD segments and the amine molecules. White-light electroluminescence (EL) can be achieved via light-mixing of the blue exciton emission from PFD and long-wavelength exciplex emission from excited complexes consisting of PFD segments (acceptors (As)) and the amine molecules (donors (Ds)). Precise control of the distances between the PFD and the amine molecules, affected through proper choice of the concentrations of PFD, amine molecules, and polymeric solid electrolytes, is critical to realizing white emission. White PLECs can be fabricated with PFD and amine molecules whose highest occupied molecular orbital (HOMO) levels range from −5.3 eV to −5.0 eV. Meanwhile, PLECs fabricated with amine molecules whose HOMO levels are lower than −5.6 eV cannot produce exciplex emission. The distances between the PFD and amine molecules of the exciplexes appear to be larger than 0.4 nm. These experimental data are explained by perturbation theory using the charge-transfer state (A−D+), the locally excited state (A*D), which is assumed to be the locally excited acceptor state in which there is no interaction with the donor molecule; and the energy gap between the HOMO levels of the PFD and the amine molecules. Color-stable white PLECs were fabricated using 4,4′,4″-tris[N-(2-naphthyl)-N-phenylamino]-triphenylamine, which has a HOMO level of −5.2 eV, as the amine molecule, and the color stability of the device is a function of the fact that PFD forms exciplexes with these molecules.

Synthesis of a PPV‐fluorene derivative: Applications in luminescent devices

ABSTRACTSynthesis of a polyfluorene/poly(p‐phenylene vinylene) derivative, the Poly [(9,9′‐di‐hexylfluorenediylvinylene‐alt‐1,4‐phenylenevinylene)‐co‐((9,9′‐(3‐t‐butylpropanoate) fluorene‐1,4‐phenylene)] (LaPPS 42) was performed following Wittig and Suzuki routes. Polyfluorenes and derivatives have been used in electroluminescent devices, and the synthesis described here has the advantage in pave the way to get distinct structures having different emission spectra. An extensive study of its electrochemical, thermomechanical, optical, and structural properties was carried out, as well as its application in electroluminescent devices. Polymer light‐emitting diodes (PLEDs) and polymer light‐emitting electrochemical cells (PLECs) were built using LaPPS 42 as active layer, and their electric and optical characterizations confirm they have a potential as active element in electroluminescent devices. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42579.

A colour-tunable, weavable fibre-shaped polymer light-emitting electrochemical cell

A fibre-shaped light emitting electrochemical cell that can be woven and integrated with textiles offers new opportunities for smart fabrics.

Efficient white polymer light-emitting electrochemical cells

An efficient white polymer light-emitting electrochemical cell has been fabricated with a thin-film sandwich architecture of glass/indium tin oxide/electroluminescent layer/aluminum.

Long-term testing of polymer light-emitting electrochemical cells: Reversible doping and black spots

Polymer light-emitting electrochemical cells (PLECs) have been tested either continuously or intermittently for a long duration. In situ electrochemical doping of the polymer film causes fluorescence quenching and apparent luminance decay when the effect of quenching outweighs the effect on charge injection. The quenching-induced luminance loss, however, is partly recoverable when the cell is allowed to relax without an applied bias. The long test duration causes the appearance of large black spots in both photoluminescence and electroluminescence. Two very startling observations shed light on the nature of the black spots. First, black spot growth was completely suppressed when a cell was tested with a freshly deposited top aluminum electrode, even though the polymer film had been stored for up to nine months. Second, the black spots in photoluminescence gradually faded when the applied bias voltage was removed. The black spots in these PLECs were therefore sites of heavy doping that were promoted by changes that occurred at the cathode/polymer interface.

Scanning photocurrent and PL imaging of a frozen polymer p-i-n junction

Polymer light-emitting electrochemical cells (LECs) are two-terminal, solid state devices with a mixed ionic/electronic conductor as the active layer. Once activated by a DC voltage or current, a doping-induced homojunction dictates the electrical and optical response of the LEC, making it highly unique and attractive among organic devices. However, the depletion width, a fundamental parameter of any semiconductor homojunction, has never been determined experimentally for a static LEC junction. In this study, we apply spatially resolved photocurrent and photoluminescence (PL) scanning to an extremely large planar LEC that had been turned on to emit strongly then subsequently frozen. These concerted scanning and imaging studies depict a p–i–n junction structure in which the peak built-in electric field lies at the interface between the intrinsic region and the p-doped region. The corresponding 18 μm depletion width is very small compared to the 700 μm interelectrode spacing. A 700 μm planar polymer light-emitting electrochemical cell has been turned on, frozen and probed optically. Scanning PL and photocurrent imaging reveals a p–i–n doping profile. Photocurrent is only detected over a region of 18 μm centered at the i/p junction. Peak PL occurs near the center of the intrinsic region. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)