Optimizing Performance and Operational Stability of CsPbI3 Quantum-Dot-Based Light-Emitting Diodes by Interface Engineering

Perovskite light-emitting diodes (PeLEDs) have emerged as a promising candidate for next-generation display technology and lighting applications owing to their high current efficiency, low operating voltage, narrow spectral emission, and tunable emission color. Keys to achieving efficient PeLEDs are, besides an emitter layer with high optical quality, a negligible charge injection barrier between charge injecting layers (CILs) and an optimized thickness of these CILs for a controlled flow of charge carriers through the device. In this study, we systematically optimized hole transport layers and electron transport layers (ETLs) in PeLEDs employing CsPbI3 quantum dots as an emitter layer. We also investigated two bilayer cathodes (Liq/Ag and LiF/Al) with the various ETLs employed in our study and observed that 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) as an ETL improves the band alignment, leading to better electron injection. The improved electron/hole current balance results in ∼63% higher external quantum efficiency (EQE) in PO-T2T-based devices compared to PeLEDs employing other ETLs. In addition, we tracked the operational stability of the different devices observing a correlation with the EQE, where samples with higher EQE (PO-T2T-based devices) also present the highest stable operation at elevated current densities

Dissociation of charge transfer states and carriers separation in bilayer organic solar cells - A time-resolved electroabsorption spectroscopy study

Ultrafast optical probing of the electric field by means of Stark effect in planar heterojunction cyanine dye / fullerene organic solar cells enables to directly monitor the dynamics of free electron formation during the dissociation of interfacial charge transfer (CT) states. Motions of electrons and holes is scrutinized separately by selectively probing the Stark shift dynamics at selected wavelengths. It is shown that only charge pairs with an effective electron-hole separation distance of less than 4 nm are created during the dissociation of Frenkel excitons. Dissociation of the Coulombically bound charge pairs is identified as the major rate-limiting step for charge carriers’ generation. Interfacial CT states split into free charges on the time-scale of tens to hundreds of picoseconds, mainly by electron escape from the Coulomb potential over a barrier that is lowered by the electric field. The motion of holes in the small molecule donor material during the charge separation time is found to be insignificant

Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic solar cells

Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges. However, the use of D-A blends intrinsically limits the photovoltage and introduces morphological instability. Here, we demonstrate that polycrystalline films of chemically identical molecules offer a promising alternative and show that photoexcitation of α-sexithiophene (α-6T) films results in efficient charge generation. This leads to α-6T based homojunction organic solar cells with an external quantum efficiency reaching up to 44% and an open-circuit voltage of 1.61 V. Morphological, photoemission, and modelling studies show that boundaries between α-6T crystalline domains with different orientations generate an electrostatic landscape with an interfacial energy offset of 0.4 eV, which promotes the formation of hybridised exciton/charge-transfer states at the interface, dissociating efficiently into free charges. Our findings open new avenues for organic solar cell design where material energetics are tuned through molecular electrostatic engineering and mesoscale structural control

Asmens reikšmė valstybei: Juozo Brazaičio svarstymai apie tautą ir kūrybą

Straipsnio tikslas atskleisti literatūros profesoriaus Juozo Ambrazevičiaus-Brazaičio (1903–1974) vaidmenį Lietuvos politinės minties istorijos žemėlapyje, kuriame jis užima didelę ir nepelnytai užmirštą dalį. Brazaitis – universali asmenybė, kurios veikla apima platų veiklų spektrą tiek praktinėje, tiek idėjinėje plotmėse. Darbe plačiau analizuojama viena pagrindinių veiklos gijų – asmens laisvės samprata. Šio siekio įgyvendinimas susijęs su valstybine struktūra, kurios atskiros sritys pajungtos asmens ugdymui. Straipsnyje atskleidžiama Brazaičio politinė pasaulėžiūra, detaliai aptariamos asmens, kūrybos, tautos ir valstybės sąvokos. Taip pat aptariami jo samprotavimai apie tautos charakterį ir valstybės valdymą – korporacinę valstybės santvarką. Apibendrinant Brazaičio idėjas, straipsnio išvadose teigiama, kad tautos gyvybingumui stiprinti būtina plėtoti tautinę kultūrą, kuri yra valstybės pagrindas. Siekiant ugdyti dvasinį žmogaus pradą, reikalinga atitinkama valstybės santvarka, kuri neprimestų valios iš viršaus. Tokius valstybės metmenis Brazaitis išsakė deklaracijose ir daugelyje straipsnių, kuriuose valstybė apibrėžiama kaip organiškas darinys. Valstybė nėra baigtinis kūrinys, o nuolatos laisvų asmenybių perkuriama pagal konkretaus laiko reikalavimus. Jo koncepcijai būdingi krikščioniškos pasaulėžiūros elementai – ji sutelkta į individą, kuris turi išskleisti savo prigimtį. Unikali asmens realizacija vyksta per kūrybos aktą. Kūryba yra laisvos asmenybės išraiška.The aim of the article is to reveal the role of the literary professor Juozas Ambrazevičius-Brazaitis (1903-1974) on the map of the history of Lithuanian political thought, in which he occupies a large and unjustly forgotten part. J. Brazaitis is a universal personality, whose activities cover a wide range of activities in both practical and conceptual areas. In this work, one of the main thread of activities is analysed more broadly. It is the concept of individual freedom. The implementation of this goal relates to the structure of the State, which in individual areas are intended for the education of the person. The article reveals the political worldview of J. Brazaitis, the concepts of personality, creativity, nation, and state are also discussed in detail. His thoughts on the character of the nation and state governance - the corporate state system - are also discussed. When summarising the ideas of J. Brazaitis, in the conclusions of the article, it is stated, that in order to strengthen the viability of the nation, it is necessary to develop national culture, which is the basis of the State. In order to nurture the spiritual rudiment of a person, the new system of a state is necessary, which would not impose a will from the above. The mentioned outlines of the State were expressed by J. Brazaitis in most of the declarations and articles, in which a state is defined as an organic formation. The state is not a finite creation, but rather is constantly redeployed according to the requirements of a specific time by free persons. The elements of the Christian outlook are characterised by his concept - it is focused to an individual, which has to expand his own nature. The unique realisation of a person takes place through a creative act. Creativity is the expression of a free personality

Charge carrier transport in conjugated polymer films revealed by ultrafast optical probing

Conjugated polymers are promising candidates for applications in all kinds of organic optoelectronic devices: OLEDs, organic field-effect transistors (OFETs) and organic photovoltaic cells. The main goal of this work was to investigate transport features of photogenerated electrical charge in pi-conjugated polymers by means of novel technique based on time-resolved electric field-induced second harmonic generation (TREFISH). TREFISH measurement setup was implemented in the laboratory of Molecular compounds physics, and applicability of the method has been verified. Measurements were performed on three different model polymers: methyl substituted ladder-type poly(para-phenylene) (MeLPPP), poly(fluorene-co-benzothiadiazole) (F8BT) and poly(spirobifluorene-co-benzothiadiazole) (PSF-BT), having different morphological and chemical structure. It has been found that motion of photogenerated charge carriers in π-conjugated polymer films experiences rapid dynamics after excitation. Different time domains of charge transport were distinguished. Initial fast transport of photogenerated charge carriers corresponds to the carrier motion along the single polymer chain or conjugated segment of the polymer chain. Slowest carrier motion phase is well described by the stochastic drift, which is attributed to interchain jumps and determines the macroscopic equilibrium mobility. Thus, the equilibrium mobility value is not applicable to the transport on nanometer scale up to tens of nanometers. These findings provide knowledge about charge transport mechanisms in polymers. Also, they may give guidelines for the designers of organic electronic devices