Combining steady-state with frequency and time domain data to quantitatively analyze charge transport in organic light-emitting diodes

Typically, organic light-emitting diodes (OLEDs) are characterized only in steady-state to determine and optimize their efficiency. Adding further electro-optical measurement techniques in frequency and time domain helps to analyze charge carrier and exciton dynamics and provides deeper insights into the device physics. We, therefore, first present an overview of frequently used OLED measurement techniques and analytical models. A multilayer OLED with a sky-blue thermally activated delayed fluorescent dopant material is employed in this study without loss of generality. Combining the measurements with a full device simulation allows one to determine specific material parameters such as the charge carrier mobilities of all the layers. The main part of this tutorial focuses on how to systematically fit the measured OLED characteristics with microscopic device simulations based on a charge drift-diffusion and exciton migration model in 1D. Finally, we analyze the correlation and sensitivity of the determined material parameters and use the obtained device model to understand limitations of the specific OLED device

Small-arms and Light-weapons Risk Education in Iraq

MAG (Mines Advisory Group) has one of Iraq’s most established mine-risk education programs. In 2007, MAG identified a regional need to warn people, particularly children, about the risk of handling guns and other weapons. Drawing on more than 16 years of experience and skill in MRE delivery, MAG aimed to adapt existing methodologies and successfully expand the MRE program to include small-arms and light-weapons risks

Opto-electronic characterization of third-generation solar cells

We present an overview of opto-electronic characterization techniques for solar cells including light-induced charge extraction by linearly increasing voltage, impedance spectroscopy, transient photovoltage, charge extraction and more. Guidelines for the interpretation of experimental results are derived based on charge drift-diffusion simulations of solar cells with common performance limitations. It is investigated how nonidealities like charge injection barriers, traps and low mobilities among others manifest themselves in each of the studied cell characterization techniques. Moreover, comprehensive parameter extraction for an organic bulk-heterojunction solar cell comprising PCDTBT:PC70BM is demonstrated. The simulations reproduce measured results of 9 different experimental techniques. Parameter correlation is minimized due to the combination of various techniques. Thereby a route to comprehensive and accurate parameter extraction is identified

Electron Trap Dynamics in Polymer Light-Emitting Diodes

Semiconducting polymers are being studied intensively for optoelectronic device applications, including solution-processed light-emitting diodes (PLEDs). Charge traps in polymers limit the charge transport and thus the PLED efficiency. It is firmly established that electron transport is hindered by the presence of the universal electron trap density, whereas hole trap formation governs the long-term degradation of PLEDs. Here, the response of PLEDs to electrical driving and breaks covering the timescale from microseconds to (a few) hours is studied, thus focusing on electron traps. As reference polymer, a phenyl-substituted poly(para-phenylene vinylene) (PPV) copolymer termed super yellow (SY) is used. Three different traps with depths between approximate to 0.4 and 0.7 eV, and a total trap site density of approximate to 2 x 10(17) cm(-3) are identified. Surprisingly, filling of deep traps takes minutes to hours, at odds with the common notion that charge trapping is complete after a few hundred microseconds. The slow trap filling feature for PLEDs is confirmed using poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene (MEH-PPV) and poly(3-hexylthiophene) (P3HT) as active materials. This unusual phenomenon is explained with trap deactivation upon detrapping and slow trap reactivation. The results provide useful insight to pinpoint the chemical nature of the universal electron traps in semiconducting polymers

Dipole reorientation and local density of optical states influence the emission of light-emitting electrochemical cells

Herein, we analyze the temporal evolution of the electroluminescence of light-emitting electrochemical cells (LECs), a thin-film light-emitting device, in order to maximize the luminous power radiated by these devices. A careful analysis of the spectral and angular distribution of the emission of LECs fabricated under the same experimental conditions allows describing the dynamics of the spatial region from which LECs emit, i.e. the generation zone, as bias is applied. This effect is mediated by dipole reorientation within such an emissive region and its optical environment, since its spatial drift yields a different interplay between the intrinsic emission of the emitters and the local density of optical states of the system. Our results demonstrate that engineering the optical environment in thin-film light-emitting devices is key to maximize their brightness

XGBoost trained on synthetic data to extract material parameters of organic semiconductors

© 2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.The optimization of organic semiconductor devices relies on the determination of material and device parameters. However, these parameters are often not directly measurable or accessible and may change depending on the neighboring materials in the layered stack. Once the parameters are known, devices can be optimized in order to maximize a certain target, e.g. the brightness of a LED. Here, we combine the use of machine learning and a semiconductor device modelling tool to extract the material parameters from measurements. Therefore, we train our machine learning model with synthetic training data originating from a semiconductor simulator. In a second step, the machine learning model is applied to a measured data set and determines the underlying material parameters. This novel and reliable method for the determination of material parameters paves the way to further device performance optimization

Pinpointing the origin of the increased driving voltage during prolonged operation in a phosphorescent OLED based on an exciplex host

We report on the origin of the reduced power efficiency in a red phosphorescent OLED with an exciplex host after prolonged operation. The power efficiency is reduced solely by an increased driving voltage while the radiant flux remains constant. An electrical model describing the driving voltage increase is, thus, sufficient to explain the reduced power efficiency. The electrical model of the fresh OLED and at different stages of degradation was devised from four different measurement methods. Using multiple measurement methods to determine the model parameters results in a rather unique set of model parameters, despite the large number of model parameters (38) as revealed by a correlation analysis. The increase in driving voltage could be reproduced by modifying only 7 out of the 38 model parameters. A sensitivity analysis identified the parameters with the largest effect (66%) on the driving voltage increase to be the trap density and the mobility of the employed hole transporting layer. This work highlights the benefit of using multiple measurement methods to derive reliable model parameters and the use of a sensitivity analysis to pinpoint the origin of the investigated property

Sinusoidal small-signal (AC) and steady-state (DC) analysis of large-area solar cells

Beside fabrication challenges, efficiency loss factors of solar cells such as shunts and an increasing series resistance caused by the sheet resistance of the electrodes, are issues to be tackled when scaling novel photovoltaic devices up from laboratory to industrial size. We present a FEM (Finite Element Method) software that supports the upscaling process from small- to large-area devices. Considering Ohm’s law in the top and bottom electrodes, which are coupled by a vertical current, the software solves for the electric potential distribution in the 2D electrode domains. In addition to steady-state simulations, we introduce a small-signal analysis that allows us to compute the influence of resistive electrodes and defects on the frequency-dependent impedance response. Herein, we describe the implemented numerical model for the AC (alternating current) mode. The steady-state model was validated with measurements using monocrystalline silicon solar cells of several sizes and one cell was intentionally shunted with a laser to demonstrate the fingerprints of these defects in the DC (direct current) and AC response. In a further step, we verify the numerical simulation of the AC model with an analytical solution to a one-dimensional AC model for a simplistic quadratic domain and linearized coupling law. Overall, the presented AC model is able to reproduce and predict the behavior of the measurements of the original and later shunted silicon solar cell. Thereby we have demonstrated that the presented AC model is a powerful tool to study devices in the frequency domain which complements characterization in steady-state

Electro-thermal model for lock-in infrared imaging of defects in perovskite solar cells

The production of uniform layers without defects is crucial for the efficient upscaling of perovskite solar cells. To understand the origin of defects and their impact on efficiency, we compare steady-state (DC) and alternating current (AC) measurements with simulation results obtained by an electro-thermal 2D+1D finite element method (FEM) implemented in the software Laoss. The software supports the upscaling process from small- to large-area devices by solving for the potential and temperature distribution in 2D top and bottom electrode domains, which are coupled by a vertical 1D coupling law. Recently, we extended this FEM model to the frequency domain in order to study both DC and AC characteristics of solar cells. Here, we report on the extension of this frequency-dependent FEM model to the thermal domain, allowing us to calculate amplitude and phase images of solar cells that are voltagemodulated in the dark. We measured and modelled a screen-printed carbon-based hole-transporter-free perovskite solar cell with a defect, appearing as a hotspot in temperature measurements. In contrast to the traditional DLIT method using a large voltage modulation, we introduce a small-signal DLIT (SS-DLIT) imaging method which our model is capable to reproduce. Fitting thermal AC simulations to measured data, allowed to quantify the defect and examine its behaviour and origin

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