Exciton-polaron Interactions Research Articles

Crystal structures and photoluminescence characteristics of cesium lead bromide perovskite nanoplatelets depending on the antisolvent and ligand used in their syntheses

Cesium lead bromide (CsPbBr3) nanocrystals (NCs) with nanoplatelet shapes and different crystal structures were synthesized via the ligand-assisted reprecipitation (LARP) method using different pairs of ligands and antisolvents, namely oleic acid (OA) or linoleic acid (LA) as the ligand and toluene or chloroform as the antisolvent. The XRD data revealed that the obtained CsPbBr3 NCs have different crystal structures, namely orthorhombic, tetragonal, and cubic, depending on the ligand and antisolvent pair, which exhibited significantly different photoluminescence (PL) characteristics. From the XPS data, these CsPbBr3 nanoplatelets showed two doublet peaks of the Br-3d orbital at different binding energies, representing two different chemical environments of the Br bonds. The doublet peak apparent at a higher binding energy was associated with the Br chemical states at the crystal surface, which appeared because of the distorted crystal structure resulting from the interaction of the solvent and ligand with Br ions. The PL emission consists of three luminescence centers: a PL band peaked at 520 nm (A band), a PL band peaked at 540 nm (B band), and a PL band tail, which can be discussed in terms of exciton models. Stable and intense luminescence was observed in CsPbBr3 nanoplatelets synthesized using a pair of toluene antisolvent and LA ligand, namely CsPbBr3#(Tl/LA). The orthorhombic crystal structure and distorted crystal surface in this sample may lead to confinement of the photogenerated small exciton-polaron and weak phonon interactions, which effectively hinder exciton dissociation, particularly at the crystal surface, resulting in intense PL. The results of this study may provide additional important insights into the role of the antisolvent and ligand in the formation of CsPbBr3 NCs and the exciton behavior in their PL characteristics, which may also be found in other types of halide perovskites.

Exciton-polaron interactions in metal halide perovskite nanocrystals revealed via two-dimensional electronic spectroscopy.

Metal halide perovskite nanocrystals have been under intense investigation for their promise in optoelectronic devices due to their remarkable physics, such as liquid/solid duality. This liquid/solid duality may give rise to their defect tolerance and other such useful properties. This duality means that the electronic states are fluctuating in time, on a distribution of timescales from femtoseconds to picoseconds. Hence, these lattice induced energy fluctuations that are connected to polaron formation are also connected to exciton formation and dynamics. We observe these correlations and dynamics in metal halide perovskite nanocrystals of CsPbI3 and CsPbBr3 using two-dimensional electronic (2DE) spectroscopy, with its unique ability to resolve dynamics in heterogeneously broadened systems. The 2DE spectra immediately reveal a previously unobserved excitonic splitting in these 15nm NCs that may have a coarse excitonic structure. 2D lineshape dynamics reveal a glassy response on the 300fs timescale due to polaron formation. The lighter Br system shows larger amplitude and faster timescale fluctuations that give rise to dynamic line broadening. The 2DE signals enable 1D transient absorption analysis of exciton cooling dynamics. Exciton cooling within this doublet is shown to take place on a slower timescale than within the excitonic continuum. The energy dissipation rates are the same for the I and Br systems for incoherent exciton cooling but are very different for the coherent dynamics that give rise to line broadening. Exciton cooling is shown to take place on the same timescale as polaron formation, revealing both as coupled many-body excitation.

Comprehensive Model of the Degradation of Organic Light-Emitting Diodes and Application for Efficient, Stable Blue Phosphorescent Devices with Reduced Influence of Polarons

We present a comprehensive model to analyze, quantitatively, and predict the process of degradation of organic light-emitting diodes (OLEDs) considering all possible degradation mechanisms, i.e., polaron, exciton, exciton-polaron interactions, exciton-exciton interactions, and a newly proposed impurity effect. The loss of efficiency during degradation is presented as a function of quencher density, the density and generation mechanisms of which were extracted using a voltage rise model. The comprehensive model was applied to stable blue phosphorescent OLEDs (PhOLEDs), and the results showed that the model described the voltage rise and external quantum efficiency (EQE) loss very well, and that the quenchers in emitting layer (EML) were mainly generated by dopant polarons. Quencher formation was confirmed from a mass spectrometry. The polaron density per dopant molecule in EML was reduced by controlling the emitter doping ratio, resulting in the highest reported LT50 of 431 hours at an initial brightness of 500 cd/m2 with CIEy<0.25 and high external quantum efficiency (EQE) >18%.

Role of Singlet and Triplet Excitons on the Electrical Stability of Polymer Light‐Emitting Diodes

AbstractStability under electrical stress is an important aspect for the function of organic light‐emitting diodes (OLEDs). Degradation is currently one of the key topics in this field, concerning all types of OLEDs, including fluorescent‐, phosphorescent‐, and thermally activated delayed fluorescence‐based OLEDs. For single‐layer polymer light‐emitting diodes (PLEDs) it has recently been found that degradation is the result of hole trap formation due to exciton–polaron interactions. However, whether singlet or triplet excitons are responsible for degradation is an open question. Here, their contributions are disentangled by systematically manipulating the singlet and triplet exciton populations and their effect on PLED degradation is studied. To control singlet excitons the emission of a blue‐emitting PLED is modified to green by adding a small amount of a perylene‐monoimide based green‐emitting dye. The triplet population is manipulated by blending the light‐emitting polymer with a dye that has either a longer or shorter triplet lifetime as compared to the polymer host. The results reveal that the degradation in fluorescent PLEDs is governed by the interaction between polarons and triplet excitons.

Degradation mechanism of blue thermally activated delayed fluorescent organic light-emitting diodes under electrical stress

Abstract The degradation mechanism of blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) has been elucidated, particularly, focusing on the cause of the degradation in the emitting layer (EML) by different host materials. In two different host materials used as the EML host, the operation stability against the constant electrical stress was investigated. The degradation mechanism of blue TADF OLEDs is described in terms of two parameters of the photoluminescence (PL) quantum yield and the exciton generation efficiency. In the early stage, the degradation in the PL quantum yield of the EML is dominant whereas the long-term degradation is attributed to the decrease in the exciton generation efficiency. Before and after the electrical stress, non-destructive measurements on the OLEDs with different host materials are performed to determine the main origin of the decrease in the exciton generation efficiency. Under the electrical stress, charge traps are formed inside the EML and the charge transport is hindered, leading to the charge imbalance. Such charge traps are generated by the exciton-polaron interactions in the EML. It is suggested that degradation products (non-radiative recombination centers or exciton quenchers), formed inside the EML, reduce the exciton generation efficiency.

Electroluminescence Stability of Organic Light-Emitting Devices Utilizing a Nondoped Pt-Based Emission Layer.

We study the effects of using an emitting material (Pt(II) bis(3-(trifluoromethyl)-5-(2-pyridyl)pyrazolate—Pt(fppz)2) characterized by a preferred horizontal dipole alignment and a nearly unitary quantum yield regardless of concentration on the lifetime of organic light-emitting devices (OLEDs). Using such a material as a dopant in increasingly higher concentrations is found to lead to an increase in device stability, a trend that is different from that commonly observed with conventional OLED guests. The results are consistent with the newly discovered exciton–polaron-induced aggregation degradation mechanism of OLED materials. When this emitter is used as a neat emission layer, the material is already in a highly aggregated state, and the device is no longer affected by exciton–polaron interactions. The results demonstrate the potential stability benefits of using such materials in OLEDs.

Exciton–Polaron Interactions in Polyfluorene Films with β-Phase

Fluorescence quenching by electric charges is an important loss mechanism in high-brightness organic light-emitting diodes (OLEDs), but its effect is difficult to quantify in working devices. Here, we combine an electrochemical technique to control the charge density with time-resolved photoluminescence to distinguish between different quenching mechanisms. The material studied was the blue electroluminescent polymer poly(9,9-dioctylfluorene) with β-phase. Our results show that quenching occurs by Forster resonance energy transfer and is mediated by exciton diffusion. We determine the quenching parameters over a wide range of charge concentrations and estimate their impact on the OLED efficiency roll-off at high current density. We find that fluorescence quenching by charges and singlet–triplet exciton annihilation are the two main mechanisms leading to the efficiency roll-off. Our results suggest that hole polarons are not very effective quenchers of singlet excitons, which is important for understanding...

Degradation Mechanisms in Blue Phosphorescent Organic Light-Emitting Devices by Exciton-Polaron Interactions: Loss in Quantum Yield versus Loss in Charge Balance.

We study the relative importance of deterioration of material quantum yield and charge balance to the electroluminescence stability of PHOLEDs, with a special emphasis on blue devices. Investigations show that the quantum yields of both host and emitter in the emission layer degrade due to exciton-polaron interactions and that the deterioration in material quantum yield plays the primary role in device degradation under operation. On the other hand, the results show that the charge balance factor is also affected by exciton-polaron interactions but only plays a secondary role in determining device stability. Finally, we show that the degradation mechanisms in blue PHOLEDs are fundamentally the same as those in green PHOLEDs. The limited stability of the blue devices is a result of faster deterioration in the quantum yield of the emitter.

The Root Causes of the Limited Stability of Solution‐Coated Small‐Molecule Organic Light‐Emitting Devices: Faster Host Aggregation by Exciton–Polaron Interactions

The degradation mechanism is compared in organic light‐emitting devices (OLEDs) fabricated by solution‐coating to that in vacuum‐deposited OLEDs. Devices comprising various host materials made by vacuum‐deposition or solution‐coating are investigated. Changes in devices electroluminescence (EL) spectra during prolonged electrical driving are compared and analyzed. Hole‐only devices are also utilized, and employed to study the effects of charges and excitons, separately and combined. The results reveal that the faster degradation of solution‐processed devices relative to their vacuum‐deposited counterparts under electrical stress is due to a faster aggregation of the host materials. Interactions between excitons and polarons in the emitting layers of the devices induce this aggregation phenomenon. Although this phenomenon affects both vacuum‐deposited and solution‐coated emitting layers, it is found to occur much faster in the later. The findings shed light on the root causes of the limited stability of solution‐processed OLEDs.

Influence of the Guest on Aggregation of the Host by Exciton-Polaron Interactions and Its Effects on the Stability of Phosphorescent Organic Light-Emitting Devices.

The root causes of the differences in electroluminescence stability among phosphorescent organic light-emitting devices (PHOLEDs) utilizing different emitter guests are studied. The results show that the host material plays a more influential role in limiting device stability in comparison to the guest. During the operation of a PHOLED, the host undergoes aggregation as a result of interactions between the excitons and positive polarons. The rate of this aggregation is found to be the limiting factor for device lifetime and is influenced by the choice of the guest material and its concentration. Finally, it is shown that phase segregation between the host and the guest is an important aspect of the aggregation process. As a result of this segregation, energy transfer from the host to the guest becomes increasingly less efficient, resulting in the observed gradual loss in electroluminescence efficiency in the devices over time. The findings explain why PHOLEDs utilizing different guest materials but otherwise identical material systems can have significantly different lifetimes and provide an answer to a long-lasting question in the field.

Magneto-conductance characteristics of trapped triplet-polaron and triplet-trapped polaron interactions in anthracene-based organic light emitting diodes.

The effects of a magnetic field on the dissociation of triplet excitons by free charges (TCI) are well understood. However, the magneto-conductance (MC) characteristics of trapped triplet-polaron interactions (TtPI) and triplet-trapped polaron interactions (TPtI) within organic light emitting diodes (OLEDs) are not well understood. We have studied these interactions in an anthracene-based OLED. The electroluminescence spectra, current-voltage characteristics and magneto-electroluminescence indicated that the anthracene layer contained many defects that could trap either triplet excitons or polarons, which led to TPtI and TtPI. The MC curves at low temperature exhibited a complex line shape, which indicated that intersystem crossing, TPtI, TtPI, and TCI occurred simultaneously in the device. The individual MC characteristics of TPtI and TtPI were extracted from temperature dependant MC curves by fitting them to three empirical Lorentzian functions and one non-Lorentzian function. The MC of TPtI exhibited a negative sign, while that of TtPI exhibited a positive one, with characteristic magnetic fields (B0) of ∼10.5 and ∼15 mT, respectively. Both processes were prominent below 150 K and weakened with increasing temperature. TPtI was neglected above 200 K, while TtPI was observed even at ambient temperature. These results add significant insight into the magnetic field effects on triplet-polaron interactions.

The different influence of singlet and triplet excitons in the degradation of phosphorescent organic light-emitting devices due to exciton–polaron-induced aggregation of host materials

The lifetime of a phosphorescent organic light-emitting device (PhOLED) may range over several orders of magnitude depending on the choice of the phosphorescent emitter guest material and its concentration. This behavior, we find, results primarily from different rates of exicton–polaron-induced aggregation (EPIA) of the host material due to exciton–polaron interactions. We investigate the influence of singlet versus triplet excitons in the EPIA of host materials and the degradation of PhOLEDs. The results show that there is a correlation between device electroluminescence degradation and the concentration of singlet excitons on the host during electrical driving. In contrast, we find no particular correlation between device degradation rate and the concentration of triplet excitons. The results therefore reveal that singlet excitons play the leading role causing the EPIA of host materials and device degradation by this mechanism.

Enhanced stability in inverted simplified phosphorescent organic light-emitting devices and its origins

We study the stability of simplified phosphorescent organic light-emitting devices in standard and inverted architectures. Results show that the inverted devices have higher electroluminescence stability, exhibiting almost three times longer lifetime relative to the widely used standard device architecture, while having the same current efficiency. Investigations reveal that inverted devices have a higher electron/hole ratio in the hole transport layer and a lower concentration of un-recombined positive charges in the emission layer. The results suggest that their higher stability is due to reduced degradation at the emission layer/electron transport layer interface as a result of reduced exciton–polaron interactions.

Simplified Organic Light-Emitting Devices Utilizing Ultrathin Electron Transport Layers and New Insights on Their Roles

The lifetime of organic light-emitting devices (OLEDs) can be limited by exciton-polaron interactions at the organic/organic interfaces. In this work, we show that simplified phosphorescent OLEDs (PHOLEDs) are subjected to this phenomenon. By reducing the exciton concentration at the emission layer (EML)/electron transport layer (ETL) interface by means of increasing the EML thickness, hence broadening the recombination zone, the device lifetime can indeed be improved. Moreover, we report a device that displays the same extended lifetime, but with only 1 nm thin ETL. Studying the roles of this ultrathin ETL in increasing device efficiency reveals that electron injection, hole blocking, and triplet exciton blocking are all important factors. Hole blocking of the ETL can be achieved by highest occupied molecular orbitals level mismatch, where a layer thickness as low as 1 nm is sufficient, or by low hole mobility of the ETL, where a much thicker layer is required (> 5 nm). This ultrathin ETL also enables devices with only 50 nm total organic stacks, which is more than 50% thinner than the typical. This structure opens up opportunities for much shorter processing time and lower fabrication costs in the OLED industry.