Polymer Light-emitting Electrochemical Cells Research Articles

Polarized polymer light-emitting electrochemical cells using polyfluorene and polycaprolactone blend film by floating-film transfer method

Abstract Sandwich-type structured polarized Polymer Light-emitting Electrochemical Cells (PLECs) of poly(9,9-dioctylfluorene) (PFO) successfully fabricated by combination of floating-film transfer method (FTM) and Post-blending ionic liquid (IL) procedure. An ion-solvating polymer, polycaprolactone (PCL), was blended in PFO for improving IL penetration. The PFO:PCL film showed clear optical dichroism due to the PFO backbone orientation. The IL, 1-Ethyl-3-methylimidazolium (trifluoromethylsulfonyl)imide (EMIm-TFSI), easily post-blended into the PFO:PCL blend FTM film without disordering PFO backbone orientation. The fabricated thickness varied PLECs with ITO anode and aluminum cathode were demonstrated distinct anisotropy, whole device area emission and low turn-on voltage compared to PLEDs.

Oxygen reduction reaction induced electrode effects in polymer light-emitting electrochemical cells

The perceived diminished sensitivity of Polymer Light-Emitting Electrochemical Cells (LECs) to electrode material and active layer thickness compared to Organic Light-Emitting Diodes (OLEDs) due to electrochemical doping, positions them as a focal point in emerging electronic applications. However, empirical evidence reveals the electrode still influences device performance. Simultaneously, electrochemical doping involves side reactions at the cathode, predominantly with oxygen. Nevertheless, the impact of oxygen reduction reaction on the device performance concerning electrode effects remains underexplored. This study centers on the pivotal influence of oxygen reduction reaction on the electrode effect in LECs. The investigation of the doping process for various electrodes and active layer thicknesses was conducted using photoluminescence imaging. Through controlling temperatures and vacuum levels, obtained time-current curves undergo fitting procedures, which enables quantitative analysis of oxygen reduction reaction effects on both electrode and film thickness. The results underscore the impact of oxygen reduction reaction on the performance of the device induced by electrodes, emphasizing the pronounced effect on the activation energy of the reduction reaction dictated by both the electrode work function and oxygen concentration. In addition, this study elucidates that the utilization of a low-work-function bottom contact in conjunction with a thicker active layer exerts a discernible influence on the device's current magnitude.

Electrical current modeling for polymer light-emitting electrochemical cells: Contributions from electrons, ions, and oxygen

A very promising approach to achieving stable polymer P-N junctions is polymer light-emitting electrochemical cells (LECs). In LECs, under a specific voltage bias, the injection of carriers into the polymer occurs through a redox reaction and subsequently gets compensated by opposite ions, resulting in the creation of electrochemical doping. Unlike organic light-emitting diodes, which have numerous mature electrical current models serving as invaluable tools for understanding the underlying mechanism and predicting device performance, LECs lack such modeling. This lack of modeling stems from the greater complexity of LECs, as the electrical current in LECs is composed of not only electronic components but also ionic contributions, along with a side-reaction portion arising from the electrochemical reaction. This work demonstrates an electrical current model for LECs, which is simple and accurate enough for practical applications. The model achieves a quantitative separation of electronic and ionic charge contributions to the electrical currents, as well as provides insights into the distribution of oxygen through operation schemes. Additionally, this paper incorporates the relationships between oxygen level, voltage, temperature, and current into the current model, thereby discerningly formulating expressions for ionic and electronic currents within the model. This demonstrates a precise equation for LEC electric current.

Investigation of shift in fluorescence wavelength in polymer light-emitting electrochemical cells

Investigation of shift in fluorescence wavelength in polymer light-emitting electrochemical cells

Interaction between components of polymeric light emitting electrochemical cells: A DFT case study for MDMO-PPV/KCF3SO3/PEO system

Polymer light emitting electrochemical cells (PLECs) are organic electronic devices that define interesting alternatives to organic light-emitting devices. However, despite their potential application, some of the basic operational mechanisms involved in their operation are still not completely understood. In this context, here we report a theoretical attempt to unravel some basic aspects on this subject considering a model system based on poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4- phenylene vinylene] (MDMO-PPV), potassium(I) trifluoromethanesulfonate (KCF3SO3) and polyethylene oxide (PEO). Structural and reactivity properties of the constituents were evaluated via DFT-based electronic structure calculations. Condensed-to-atoms Fukui indexes were estimated via Hirshfeld partition charge to identify preferred sites for PLEC component’s interactions. The results suggest that the presence of ionic salt induces significant changes on the semiconducting polymer’s electronic properties, which are dependent on the number of salt molecules and their oxidation state, influencing the devices’ charge dynamics (injection and transport), reinforcing previous results obtained for other similar systems.

Understanding of the Intrinsic Difference between n- and p-Doping Propagation in Polymer Light-Emitting Electrochemical Cells: A Numerical Modeling Approach.

Distinct doping propagation characteristics between p-doping and n-doping in light-emitting electrochemical cells (LECs) have been highlighted by intensive reports. Typically, there are significant differences in the doping speeds between p-doping and n-doping, with the former exhibiting a sawtooth frontier and the latter displaying a more uniform frontier profile. In addition, experimental observations demonstrate a uniform motion instead of the theoretically suggested accelerated electrochemical doping frontier propagation. Therefore, there is an urgent need to establish a quantitative model that delves into the underlying mechanisms responsible for doping propagation in LECs. In this study, four variables were selected to investigate the detailed mechanism of electrochemical doping propagation: temperature, voltage, and concentrations of salt and solid electrolyte. Fluorescence imaging revealed that the n-doping and p-doping propagations behaved contrarily with increasing temperature and voltage. By numerically fitting the doping propagation frontier, equations were derived to describe the relationship between the speed of electrochemical doping propagation and temperature/voltage. The underlying mechanisms were elucidated, indicating that anions undergo motion through the cooperative effects of electric field drift and concentration diffusion, while cation transport strongly relies on poly(ethylene oxide) (PEO) segmental motions. In other words, the movement of anions within the electrolyte is characterized by a greater degree of freedom, whereas the motion of cations is significantly dependent on the segmental motions of PEO. The resulting equations were well-fitted with experimental data, providing a solid foundation for further theoretical investigations into electrochemical doping in various devices.

Visualizing the effects of salt concentration in planar polymer light-emitting electrochemical cells

The effects of salt concentration have been visualized in bilayer, planar polymer light-emitting electrochemical cells (PLECs) with a gap size of 2 mm. Electrochemical doping and junction electroluminescence (EL) have been observed in a luminescent conjugated polymer (CP) top layer with a different solid polymer electrolyte (SPE) underlayer. By varying the salt concentration and thickness of the SPE layer, the initial cell current, peak cell current, doping propagation speed and junction formation time can all be altered/controlled. This is explained by a limiting ionic resistance in the SPE layer which is both salt concentration and thickness dependent. This study establishes the role of electrolyte ions to support doping current in PLECs. The unexpected observation of a systematic junction shift towards the cathode suggests the electrolyte ions, when functioning as counter ions for doping, are also affected by the salt concentration. At high salt concentrations, the light-emitting junction overlaps with the cathode edges which is undesirable. Thus, the bilayer PLECs allowed for the study and visualization of functions of mobile ions in PLEC operation. Moreover, the bilayer PLECs can potentially be used to study the SPE layer itself using highly sensitive imaging techniques.

Iontronic Electroluminescence Devices: Comparing Halide Perovskites and Conjugated Polymers

Halide perovskites and polymer light-emitting electrochemical cells exhibit similar iontronic properties, wherein ion migration under an external bias can be coupled with electronic doping of the perovskites and conjugated polymers, leading to efficient electroluminescence devices with an ultrasimplified device architecture. In this work, it is discovered that a major difference exists between halide perovskites and polymer light-emitting electrochemical cells through low-temperature measurements of photovoltaic properties, photoluminescence spectra, and current–voltage characteristics using precharged devices. In perovskites, ion migration is simultaneously accompanied by doping, and both n-type and p-type doping can occur at a bias much lower than the bandgap/e of the perovskite. On the contrary, ion migration in polymer light-emitting electrochemical cells leads to doping only when the bias exceeds the bandgap/e of the conjugated polymer.

Stable Light‐Emitting Electrochemical Cells Using Hyperbranched Polymer Electrolyte

AbstractThe choice of an adequate electrolyte is a fundamental aspect in polymer light‐emitting electrochemical cells (PLECs) as it provides the in situ electrochemical doping and influences the performance of these devices. In this study, a hyperbranched polymer (Hybrane DEO750 8500) blended with a Li salt is used as a novel electrolyte in state‐of‐the‐art Super Yellow (a polyphenylenevinylene) based LECs. Due to the desirable properties of the hyperbranched polymer and the homogeneous and smooth films that it forms with the emitting polymer, PLEC with excellent electroluminescent properties are obtained using a pulsed current bias scheme. The devices are very stable, with lifetimes in excess of 2000 h with initial luminance values above 450 cd m−2, a peak efficiency of 12.6 lm W−1, and sub‐minute turn‐on times. The stability of the devices is also studied by measuring the photoluminescence (PL) of the semiconductor during electroluminescent operation. The findings suggest that it is possible to observe the quenching of the PL in vertically stacked devices due to the advancement of the doped fronts in the film and an immediate PL recovery when the bias is removed.

Temperature Hypersensitive Organic Electroluminescence in a Reverse Biased, Frozen Polymer P–I–N Junction

AbstractSemiconductor junctions are among the most important material interfaces in modern technology. The solid polymer light‐emitting electrochemical cell (PLEC) possesses a semiconducting polymer homojunction that is reminiscent of a conventional p–n junction but also exhibits distinct features that are profoundly intriguing. The PLEC junction is formed under bias when the propagating p‐ and n‐doping fronts make contact. The PLEC junction can be immobilized by cooling after the initial junction formation. Further, the resulting frozen junction can be relaxed, or partially de‐doped into a p–i–n junction by controlled heating/cooling cycles. It is on such a frozen polymer p–i–n junction that the authors observed one of the most puzzling phenomena in organic electroluminescence (EL). The frozen p–i–n junction displays the brightest reverse bias (RB) EL ever imaged in a polymer junction and a large photovoltage. Moreover, the RB EL exhibits hypersensitivity to temperature, increasing in intensity by 22.6% K−1 when the de‐doped cell is cooled from 250 to 200 K. A model based on the tunnel injection of charge carriers with fast transit time across the intrinsic region explains the RB EL's hypersensitivity to temperature and super‐linear dependence on cell current.

Polymer light-emitting electrochemical cells with ultralow salt content: performance enhancement through synergetic chemical and electrochemical doping actions

This work demonstrates that PLECs made with a low AgOTf salt concentration exhibit record luminance, efficiency, lifetime and response speed.

Investigation of the polymer-salt interactions in polymeric light emitting electrochemical cells: Electronic structure calculations and experimental studies

Abstract Polymer light-emitting electrochemical cells (PLECs) are organic electronic devices which operating mechanism depends on the injection and transport of electronic charge carrier and the electrochemical doping of the organic semiconductor. The details of the interactions between the salt (or its ions) and the semiconducting polymer composing the device active layer provide important information about the electronic processes associated to the device operation in steady-state. In this context, the present paper proposes a study where theoretical results from Density Functional Theory (DFT) were obtained for three different steady-state operational regimes: i) without external voltage, in which the undissociated salt molecules interact with uncharged semiconducting polymer; ii) for applied voltages lower than the device turn-on ( V E g / e ), in which dissociated ions interact with uncharged semiconducting polymer; iii) for applied voltages higher than the turn-on ( V > E g / e ), in which the dissociated ions interact with charged semiconducting polymer. In addition, the theoretical results have been confronted with experimental results of PLECs fabricated using different salt concentrations. For both theoretical and experimental approaches, we considered lithium triflate as the salt compound and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-vinylenephenylene)] (F8PV) as the semiconducting polymer. We observed substantial changes in the electronic structure of the systems at the different operating regimes, which were interpreted in terms of the electronic charge injection from the electrodes and the electrochemical doping of the semiconducting polymer.

A Guide for Realizing Efficient Polymer Light‐Emitting Electrochemical Cells in a Single Active Layer Device Structure

AbstractHere, we systematically examine the impact of electrode work function, ionic species, and charge injection layers on the operation of polymer light‐emitting electrochemical cells (PLECs) based on p‐type Super Yellow. To achieve PLECs yielding a more intensive luminance compared to what can be achieved from the non‐ionic light‐emitting device counterpart, i. e., the polymer light‐emitting diode (PLED), in an analogous device structure, the experimental results here guide the following lessons. First, the use of charge injection layer does not necessarily yield a PLEC that outperforms its PLED counterpart but rather complexes the device structure. Second, injection of minority electrons is critical for enhancing the luminescence characteristics of p‐type polymer based LECs, and the enhanced electron injection is achieved more effectively by modifying the work function of the cathode than by changing the cations. Third, for p‐type PLECs with a low‐work function cathode, careful selection of anions can drastically change the device performance, which are involved in the p‐type electrochemically doping process.

Polymer Light‐Emitting Electrochemical Cells with Bipolar Electrode‐Dynamic Doping and Wireless Electroluminescence

AbstractThis progress report reviews the background and recent developments of polymer light‐emitting electrochemical cells (PLECs). The PLECs of interest have a planar configuration and contain bipolar electrodes (BPEs). A BPE is an electrically floating conductor immersed in an electrochemical cell that contains redox species. When the electrochemical cell is polarized with an externally applied voltage bias, coupled redox reactions are induced wirelessly at the extremities of the BPEs due to the development of a sufficient interfacial potential difference. BPEs can have a dramatic effect on the doping pattern and the emission profiles of PLECs. In a bulk homojunction PLEC containing a large number of dispersed micro‐BPEs, the turn‐on response and light output are greatly enhanced when multiple light‐emitting p–n junctions form throughout the active layer. PLECs offer a solid‐state platform on which the bipolar electrochemistry phenomena can be investigated, and the understanding of the complex PLEC processes can be improved. This progress report highlights several new BPE types as well as their potential applications for device performance enhancement and for the visualized screening of functional materials.

Decoding the Polymer p–n Junction: Controlled Dedoping and Reverse Bias Electroluminescence

AbstractThe polymer light‐emitting electrochemical cell (PLEC) is a unique solid‐state device possessing attractive attributes for low‐cost applications, but also a junction structure that is still poorly understood. In a PLEC, the applied voltage causes in situ electrochemical p‐ and n‐doping of the semiconducting polymer and the formation of a dynamic light‐emitting p–n junction. Once the junction is fixed by cooling or chemical manipulation, the “frozen‐junction” PLEC exhibits a unipolar electroluminescence (EL) and photovoltaic response. Repeated thermal cycling, however, can cause the frozen‐junction PLEC to experience drastically enhanced EL under forward bias and the emergence of reverse bias EL. In this study, a combination of transport measurements and direct imaging is used to elucidate the origin of the mysterious reverse bias EL. A model is developed that explains the reverse bias EL as caused by the tunnel injection of electrons and holes from bandgap states into a dedoped “intrinsic” region between the p‐ and n‐doped regions. The model explains the location, relative intensity, and evolution of EL under both forward and reverse bias. The results hint at a junction that is much narrower than previously resolved.

The dynamic bipolar electrode in polymer light-emitting electrochemical cells

The redox doping reactions in polymer light-emitting electrochemical cells (PLECs) have been analyzed to gain insight into a new type of bipolar electrode (BPE) formed within the cells. Planar (lateral) PLECs are constructed on a glass substrate with an exposed active polymer layer contacted by two aluminum (Al) driving electrodes at 11 mm apart. A horizontal array of 13 silver (Ag) microdiscus is deposited in-between the driving electrodes. When a 200 V bias is applied between the driving electrodes, the electrically floating Ag discs functioned as BPEs to induce redox p- and n-doping reactions in polymer layer that initially occur at the extremities of the BPEs. The doping reactions are visualized as strong photoluminescence quenching of the luminescent polymer. Over time, the doped regions grow in size, and the silver disc BPEs transform into dynamic, composite BPEs consisting of a p- and n-doped polymer conductor connected by the initial BPE disc. Unlike a homogenous BPE, the dynamic BPE is not a quasi-equipotential body under operation. Moreover, the n/p area ratios of all 13 BPEs in the array converge to a single value which is strongly affected by the composition of the PLEC layer. The n/p ratios ranging from 0.24 to 2.19 have been observed in PLECs made with Li, Na, and K triflate salts as well as two different luminescent polymers. The dynamic BPE array thus presents a novel essay to evaluate and screen the mixed luminescent conductor for device applications.

Dynamic Bipolar Electrode Array for Visualized Screening of Electrode Materials in Light-Emitting Electrochemical Cells.

Charge injection at a metal/semiconductor interface is of paramount importance for many chemical and physical processes. The dual injection of electrons and holes, for example, is necessary for electroluminescence in organic light-emitting devices. In an electrochemical cell, charge transfer across the electrode interface is responsible for redox reactions and Faradic current flow. In this work, we use polymer light-emitting electrochemical cells (PLECs) to visually assess the ability of metals to inject electronic charges into a luminescent polymer. Silver, aluminum, and gold microdisks are deposited between the two driving electrodes of the PLEC in the form of a horizontal array. When the PLEC is polarized, the individual disks functioned as bipolar electrodes (BPEs) to induce redox p- and n-doping reactions at their extremities, which are visualized as strongly photoluminescence-quenched growth in the luminescent polymer. The three metals initially generate highly distinct doping patterns that are consistent with differences in their work function. Over time, the doped regions continue to grow in size. Quantitative analysis of the n/p area ratio reveals an amazing convergence to a single value for all 39 BPEs, regardless of their metal type and large variation in the size of individual doped areas. We introduce the concept of a dynamic BPE, which transforms from an initial metal disk of a fixed size to one that is a composite of p- and n-doped polymer joined by the initial metallic BPE. The internal structure of the dynamic BPE, as measured by the n/p area ratio, reflects the properties of only the mixed conductor of the PLEC active layer itself when the area ratio converges.

Stress Testing Polymer Light‐Emitting Electrochemical Cells: Suppression of Voltage Drift and Black Spot Formation

AbstractThe stress characteristics of the polymer light‐emitting electrochemical cells (PLECs) are comprehensively evaluated by varying a host of material and operational parameters. PLECs with the lowest salt concentration of less than 2.5% exhibit runaway voltage drift that leads to rapid cell destruction. PLECs with the lowest electrolyte polymer concentration and a 5% salt content exhibit the longest luminance half‐life, but are slow to activate and are plagued by black spots. At moderate‐to‐high electrolyte concentrations, the PLECs are relatively stable but are not immune to black spots and voltage drift. A lifetime figure‐of‐merit is introduced to quantitatively account for all three indicators of cell degradation in luminance decay, voltage drift, and black spot formation. A surprising discovery is that voltage drift and black spot formation can be effectively suppressed by drastically increasing the electrolyte content. A cell with a 70% electrolyte content exhibits the best lifetime of nearly 350 h when operated at a constant current density of 167 mA cm−2. The black spots can also be effectively eliminated by employing silver instead of aluminum as the cathode material.

Laser-Induced Bipolar Electrochemistry-On-Demand Formation of Bipolar Electrodes in a Solid Polymer Light-Emitting Electrochemical Cell.

Bipolar electrochemistry (BPEC) is a versatile and powerful technique that has found applications in sensing, chemical synthesis, catalysis, fuel cells, and batteries, among others. In BPEC, the reactions of interest occur at a wireless, bipolar electrode (BPE). BPEC is most commonly carried out in an electrochemical cell that contains an electrolyte solution, in which a metallic BPE is immersed and polarized when the wired driving electrodes are biased. In this article, we demonstrate BPEC in a solid light-emitting electrochemical cell (LEC) that does not initially contain a BPE. Shining a focused laser beam onto the mixed conductor LEC film causes the illuminated spot to function as a BPE from which redox reactions are induced and visualized. Separate experiments using a photosensitizer (widely used in polymer solar cells) confirm that a BPE is formed on-demand via photoabsorption that causes the illuminated spot to have elevated photoconductivity. The simplicity of laser-induced BPEC offers exciting opportunities to explore sciences and applications of BPEC in the new realm of solid-state organic photonic devices.

Ion Transport beyond the Polyether Paradigm: Introducing Oligocarbonate Ion Transporters for Efficient Light‐Emitting Electrochemical Cells

AbstractThe light‐emitting electrochemical cell (LEC) is fundamentally dependent on mobile ions for its operation. In polymer LECs, the mobile ions are commonly provided by dissolving a salt in an ion transporter, with the latter almost invariably being an ether‐based compound. Here, the synthesis, characterization, and application of a new class of carbonate‐based ion transporters are reported. A polymer LEC, comprising a star‐branched oligocarbonate endowed with aliphatic side groups as the ion transporter, features a current efficacy of 13.8 cd A−1 at a luminance of 1060 cd m−2, which is a record‐high efficiency/luminance combination for a singlet‐emitting LEC. It is further established that the design principles of a high‐performance carbonate ion transporter constitute the selection of an oligomeric structure over a corresponding polymeric structure and the endowment of the oligomer with functional side chains to render it compatible with the polymeric emitter.