Novel optoelectronic technique for direct tracking of ultrafast triplet excitons in polymeric semiconductor

Abstract

The exciton physics of organic semiconductors is exotic. It is a domain in which singlet and triplet kinetics both play an important role in determining the performance of various optoelectronic devices. Since triplet excitons are non-emissive, it brings further challenges in the understanding of triplet kinetics. In this work, we have studied the effect of polymer chain packing on triplet diffusion in the polyfluorene based polymeric system, which is known to give efficient organic light emitting diode (OLED) efficiency for display devices. Furthermore, this polyfluorene system exhibits an efficient triplet–triplet fusion process, which provides singlet excitons as delayed fluorescence and becomes a tool to study triplet exciton kinetics. We have developed a unique method to trace the position of the triplet exciton in the emissive layer of OLEDs by analyzing angle-resolved delayed electroluminescence emission patterns as a function of time. This study could provide exciton transport kinetics in the transverse direction from the substrate plane. Furthermore, direct visualization of the delayed photoluminescence imaging technique could provide lateral transport kinetics of triplet excitons. Results suggest that the diffusion is significantly anisotropic in thinner films. As the thickness of the film increases, anisotropy reduces in triplet transport. Moreover, we noticed that in thicker polymeric semiconductor films, diffusivity approaches close to ultrahigh 10−3 cm2 s−1, which is similar to the values that are reported for acene-based molecular crystalline thin films. Our results also provide important insight into efficient electroluminescence in unusually thick (1.2 μm) polyfluorene-based emissive layers of OLEDs.

Understanding the charge dynamics in organic light-emitting diodes using convolutional neural network

“…To train the 2D modulus spectra images effectively, we designed the neural network comprising the convolutional, pooling, and dense layers. The size of the filter was set to (3,3), and rectified linear units were used as an activation function. In the pooling layer, the max-pooling algorithm was used.…”

“…After the first report of bilayer organic light-emitting diodes (OLEDs), tremendous improvement of device performance has been realized by comprehensive research toward fundamental organic photonics based on quantum chemistry and applied physics. [1][2][3][4][5][6] Internal quantum efficiency was enhanced from 25% to 100% because of the triplet harvesting mechanisms through spin-orbit coupling (phosphorescence) and thermally activated reverse intersystem crossing (thermally activated delayed fluorescence). 7,8 The radiative decay of triplets by heavy metal complex and thermal transition of triplets to singlets by minimizing the thermodynamic barrier between both states successfully realized utilization of the entire singlets and triplets formed by polaron recombination.…”

Reduced Singlet–Triplet Annihilation for Low Threshold Amplified Spontaneous Emission from a Blue Polyfluorene Electroluminescent Organic Semiconductor

“…13 Triplets, being long-lived, once generated via intersystem crossing, remain active (100 μs) in the polymer and start annihilating new optically generated singlets via the bimolecular STA process, which in general keeps dominating at high frequencies of optical and electrical excitations . 14,15 Among the various conjugated polymers, the fluorenederivatives have gained much attention due to their tunability, high thermal and oxidative stability, along with their performance as good hosts. Polyfluorenes (F8) have been highly explored in OLED applications 16,17 and as laser gain media.…”

“…This is because high triplet yields and singlet–triplet annihilation (STA) rates compared to small molecules inherently limit the lasing threshold . Triplets, being long-lived, once generated via intersystem crossing, remain active (100 μs) in the polymer and start annihilating new optically generated singlets via the bimolecular STA process, which in general keeps dominating at high frequencies of optical and electrical excitations . , …”

Impact of Photoluminescence Imaging Methodology on Transport Parameters in Semiconductors

“…7 This thickness-dependent increment in D T has been attributed to more ordered polymer chain alignment and improved polymer chain packing in thicker films as compared to thinner films, in previous related studies. 20,26 Having determined the diffusion coefficient and lifetime, we now use them to determine the length over which a triplet can diffuse, L d . The L d for the F8BT film of different thicknesses is calculated by using the lifetimes from PIA and TrPL experiments.…”