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

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

Double charge transfer dominates in carrier localization in low bandgap sites of heterogeneous lead halide perovskites

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The dynamic emission zone in sandwich polymer light‐emitting electrochemical cells

In light‐emitting electrochemical cells (LECs), the position of the emission zone (EZ) is not predefined via a multilayer architecture design, but governed by a complex motion of electrical and ionic charges. As a result of the evolution of doped charge transport layers that enclose a dynamic intrinsic region until steady state is reached, the EZ is often dynamic during turn‐on. For thick sandwich polymer LECs, a continuous change of the emission color provides a direct visual indication of a moving EZ. Results from an optical and electrical analysis indicate that the intrinsic zone is narrow at early times, but starts to widen during operation, notably well before the electrical device optimum is reached. Results from numerical simulations demonstrate that the only precondition for this event to occur is that the mobilities of anions (µa) and cations (µc) are not equal, and the direction of the EZ shift dictates µc > µa. Quantitative ion profiles reveal that the displacement of ions stops when the intrinsic zone stabilizes, confirming the relation between ion movement and EZ shift. Finally, simulations indicate that the experimental current peak for constant‐voltage operation is intrinsic and the subsequent decay does not result from degradation, as commonly stated

Carrier motion in as-spun and annealed P3HT:PCBM blends revealed by ultrafast optical electric field probing and Monte Carlo simulations

Charge transport dynamics in solar cell devices based on as-spun and annealed P3HT:PCBM films are compared using ultrafast time-resolved optical probing of the electric field by means of field-induced second harmonic generation. The results show that charge carriers drift about twice as far during the first 3 ns after photogeneration in a device where the active layer has been thermally annealed. The carrier dynamics were modelled using Monte-Carlo simulations and good agreement between experimental and simulated drift dynamics was obtained using identical model parameters for both cells, but with different average PCBM and polymer domain sizes. The calculations suggest that small domain sizes in as-spun samples limit the carrier separation distance disabling their escape from geminate recombination