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The light-emitting electrochemical cell (LEC) is an emerging electroluminescent technology, which is attractive since it can deliver bright area emission at low voltage from thin and flexible devices that can be cost- and energy-efficiently fabricated by scalable printing techniques. This opens up for applications where current light emitting diode (LED) and organic light emitting diode (OLED) technologies fall short, primarily in terms of cost, sustainability and form factor. Many projected LEC applications are portable (and powered by an integrated energy storage device or by integrated wireless energy harvesting) in the realm of, for instance, MedTech, packaging, signage, security and wearables, where the available input energy is limited. Thus, achieving a high brightness at high emission efficiency, i.e. the efficiency at which the device converts electric energy to emitted light from the device structure, is critical. However, this requires a detailed understanding of the complex operational mechanism of the LEC.

When the LEC is electrically powered, the mobile ions in the active material reorganize in situ to form injection-enabling electric double layers followed by electrochemical p- and n-type doping and the formation of a p-n junction region. The emission zone (EZ), located in the p-n junction region where the injected electrons and holes meet and light is generated, is dynamic in the LEC due to the in situ electrochemical doping. This complicates the analysis of the device performance since the EZ position strongly influences the outcoupling efficiency of the light generated in the p-n junction and since the EZ width plays an important role in the internal quenching mechanisms of the LEC. 

In this thesis, we present a characterization method for the measurement of the dynamic EZ in an LEC, and we add to the understanding of how an efficient LEC should be designed. The EZ position is determined with the aid of the optical microcavity effect by fitting simulated to measured angle-resolved electroluminescence spectra. By using this method, we could show that the EZ position can be shifted from close to the anode towards the center of the active material in a common LEC through the inclusion of an appropriate additive into the active material, which is very attractive since it resulted in a 60 % improvement of the emission efficiency. Secondly, we combined this method with so-called “efficiency roll-off” measurements (i.e. the drop in emission efficiency with increasing current density) to derive the internal loss factors. Specifically, by quantifying the change of the light outcoupling efficiency with the extracted shift of EZ position, we could identify and quantify the losses due to exciton quenching. We find that the efficiency roll-off in common singlet-exciton emitting LECs is mainly due to the singlet-exciton:polaron quenching (with the polarons being the electrons and holes in the organic semiconductor). Finally, we improved the EZ measurement method by considering the doping-dependent refractive index in the optical model, by enhancing the measured emission spectra intensity at large angles by using a half-cylinder lens, and by removing the influence from the emitter’s anisotropy with a polarizer. With this improved method, the EZ width can also be extracted, and we find that the EZ width decreases during the initial operation and that the EZ width at steady-state is ~20 % of the thickness of the active material for a common LEC device. In summary, the findings presented in this thesis contribute to a deeper understanding of the complex LEC doping structure, thus paving the way for brighter and more efficient LECs for practical applications.

En ljusemitterande elektrokemisk cell (LEC) är en ny typ av ljusemissionsteknologi som är attraktiv eftersom den kan leverera icke-bländande ljus vid låg spänning från tunna och flexibla filmer och tillverkas kostnads- och energieffektivt med olika skalbara tryckmetoder. Detta öppnar upp för tillämpningar där dagens ljusemitterade dioder (LED) och organiska ljusemitterade dioder (OLED) faller kort, främst vad gäller kostnad, hållbarhet och formfaktor. Många potentiella LEC-tillämpningar är portabla (och drivna av en energilagringskomponent som ett batteri eller genom integrerad trådlös energiutvinning), och ett antal tillämpningsexempel finns inom medicinteknik, förpackningar, annonsering, säkerhetstryck och ”wearables”, där den tillgängliga tillförda energin är begränsad. I sådana tillämpningar är det därför viktigt med hög emissionseffektivitet, det vill säga den effektivitet med vilken en LEC omvandlar elektrisk energi till ljus. 

När en spänning appliceras mellan LEC elektroderna börjar de mobila jonerna i det aktiva materialet migrera för att först bilda så kallade elektriska dubbel-lager som möjliggör effektiv elektron- och hål-injektion följt av elektrokemisk dopning som resulterar i skapandet av en pn-övergång. Emissionszonen (EZ), som ligger i pn-övergången där de injicerade elektronerna och hålen möts och ljus genereras, är dynamisk i en LEC på grund av den elektrokemiska dopningen. Detta komplicerar analysen av en LECs prestanda eftersom EZ-positionen starkt påverkar utkopplingseffektiviteten och eftersom EZ-bredden spelar en viktig roll i de interna konverteringsprocesserna. Det är med andra ord centralt att kunna mäta EZ positionen noggrant under driften av en LEC för att därigenom kunna designa LEC komponenter med hög emissionseffektivitet.

I denna avhandling presenterar vi en karakteriseringsmetod för att mäta den dynamiska EZ hos en LEC, och vi bidrar med ny förståelse för hur en energieffektiv LEC ska designas. EZ-positionen bestäms med god noggrannhet genom att anpassa simulerade till uppmätta vinkelupplösta elektroluminiscensspektra och genom att utnyttja den optiska mikrokavitetseffekten. Med denna metod har vi visat att EZ-positionen i en LEC kan flyttas från nära anoden till mitten av det aktiva materialet genom att inkludera ett lämpligt additivmaterial till det aktiva materialet; vilket var attraktivt då det resulterade i en 60 % förbättring av energieffektiviteten. För det andra har vi använt denna metod för att kunna extrahera de interna förlustfaktorerna från mätningar av ”emissions-roll-off” (dvs hur emissions-effektiviteten avtar vid ökad strömdrivning). Mer specifikt, genom att bestämma hur ljusutkopplingseffektiviteten beror på den uppmätta EZ-positionen kunde vi separera och kvantifiera excitonförlusterna. Vi fann att emissions-roll-off i vanliga singlet-emitterande LEC-komponenter huvudsakligen beror på singlet-exciton:polaron-förluster (polaroner är benämningen för elektroner och hål i organiska halvledare). Slutligen så har vi utvecklat EZ-metoden genom att inkludera: det dopningsberoende brytningsindexet i den optiska modellen, en halvcylinderlins för att höja intensiteten hos emissionsspektra, och en polarisator för att eliminera eventuellt okänd inverkan av emitterar-anisotropi. Med vår förbättrade metod kan vi bestämma även EZ-bredden, och vi finner att EZ-bredden initialt minskar med tiden samt att EZ-bredden vid steady-state motsvarar ~20 % av den totala tjockleken hos det aktiva materialet för en vanlig LEC. Sammanfattningsvis bidrar resultaten som presenteras i denna avhandling till en förbättrad mätning av, och kunskap om, den komplexa dopningsstrukturen hos LEC-komponenter, vilket lovar att bana väg för ljusstarka och mer effektivare LECar för praktiska tillämpningar.

The light-emitting electrochemical cell (LEC) forms a p-n junction doping structure by bipolar electrochemical doping during its initial operation. The light emission originates from the p-n junction region through the formation and radiative decay of excitons. The width of this emission zone (EZ) is important since it strongly affects the emission losses by exciton quenching, the outcoupling efficiency, and the drive voltage. The challenge is that it has proven very difficult to determine the width of the dynamic EZ in LECs. Here, this issue is addressed through the presentation of a method that fits simulated angle-resolved emission spectra to measured spectra, using the EZ width and position as the two free parameters. For improved accuracy, a linear polarizer is employed for the selective detection of s-polarized emission and a half-cylinder outcoupling structure for enhanced spectral output. The method is finally employed on a common conjugated-polymer LEC, and it is derived that its EZ width decreases during the initial operation, that the steady-state EZ width is equal to ≈20% of the active-material thickness at a current density of 10 mA cm−2, and that the steady-state EZ width appears to decrease with increasing current density.

Light-emitting electrochemical cells (LECs) are promising candidates for fully solution-processed lighting applications because they can comprise a single active-material layer and air-stable electrodes. While their performance is often claimed to be independent of the electrode material selection due to the in situ formation of electric double layers (EDLs), we demonstrate conceptually and experimentally that this understanding needs to be modified. Specifically, the exciton generation zone is observed to be affected by the electrode work function. We rationalize this finding by proposing that the ion concentration in the injection-facilitating EDLs depends on the offset between the electrode work function and the respective semiconductor orbital, which in turn influences the number of ions available for electrochemical doping and hence shifts the exciton generation zone. Further, we investigate the effects of the electrode selection on exciton losses to surface plasmon polaritons and discuss the impact of cavity effects on the exciton density. We conclude by showing that we can replicate the measured luminance transients by an optical model which considers these electrode-dependent effects. As such, our findings provide rational design criteria considering the electrode materials, the active-material thickness, and its composition in concert to achieve optimum LEC performance.

The p-i-n junction structure that develops dynamically under an applied bias via electrochemical doping (ECD) is of key importance for the performance of light-emitting electrochemical cells (LECs). While the complex electronic and ionic processes that govern its transient formation have been extensively studied by both experiments and drift-diffusion modeling, less attention has been given to the steady-state junction as a function of voltage. Here we study the formed p-i-n structure of a polymer LEC at the steady state by measuring and analyzing its most distinctive feature: the current density-voltage-luminance characteristics. Unexpectedly, we find that the effective conductance of the p-i-n structure exhibits a positive correlation with the applied bias, a behavior not predicted by existing LEC drift-diffusion models. We attribute this discrepancy to the assumption in these models of a constant density of mobile ions. Hence, we present a modified model in which the ECD level - represented by the number of ions with which the organic semiconductor is doped - scales with the applied voltage, implying a voltage-dependent doping efficiency. We validate this hypothesis using electron spin resonance spectroscopy and drift-diffusion modeling, while additionally establishing that only a small fraction of the available ions in our system, which increases from 1% to 3% with increasing bias, contributes to the ECD and is necessary for efficient LEC operation. These findings not only provide fundamental insights into the operational mechanism of LECs but also have direct implications for the broader organic mixed ionic and electronic conductor community.

Understanding “efficiency roll-off” (i.e., the drop in emission efficiency with increasing current) is critical if efficient and bright emissive technologies are to be rationally designed. Emerging light-emitting electrochemical cells (LECs) can be cost- and energy-efficiently fabricated by ambient-air printing by virtue of the in situ formation of a p-n junction doping structure. However, this in situ doping transformation renders a meaningful efficiency analysis challenging. Herein, a method for separation and quantification of major LEC loss factors, notably the outcoupling efficiency and exciton quenching, is presented. Specifically, the position of the emissive p-n junction in common singlet-exciton emitting LECs is measured to shift markedly with increasing current, and the influence of this shift on the outcoupling efficiency is quantified. It is further verified that the LEC-characteristic high electrochemical-doping concentration renders singlet-polaron quenching (SPQ) significant already at low drive current density, but also that SPQ increases super-linearly with increasing current, because of increasing polaron density in the p-n junction region. This results in that SPQ dominates singlet-singlet quenching for relevant current densities, and significantly contributes to the efficiency roll-off. This method for deciphering the LEC efficiency roll-off can contribute to a rational realization of all-printed LEC devices that are efficient at highluminance.

The position of the emission zone (EZ) in the active material of a light-emitting electrochemical cell (LEC) has a profound influence on its performance because of microcavity effects and doping- and electrode-induced quenching. Previous attempts of EZ control have focused on the two principal constituents in the active material—the organic semiconductor (OSC) and the mobile ions—but this study demonstrates that it is possible to effectively control the EZ position through the inclusion of an appropriate additive into the active material. More specifically, it is shown that a mere modification of the end group on an added neutral compound, which also functions as an ion transporter, results in a shifted EZ from close to the anode to the center of the active material, which translates into a 60% improvement of the power efficiency. This particular finding is rationalized by a lowering of the effective electron mobility of the OSC through specific additive: OSC interactions, but the more important generic conclusion is that it is possible to control the EZ position, and thereby the LEC performance, by the straightforward inclusion of an easily tuned additive in the active material.

Temporal filtering and spectral broadening are simultaneously achieved, allowing the compression of 20 fs laser pulses down to sub-4 fs duration through the method of nonlinear elliptical polarization rotation in an argon filled hollow-core fiber. The sub-4 fs source provides ~ 35-µJ energy with an internal efficiency >50%, which is more than from the most commonly used pulse-cleaning methods. Further, the contrast is improved by 3 orders of magnitude when measured after amplifying the pulses to 16 TW using non-collinear optical parametric chirped pulse amplification with a prospect to even further enhancement.