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

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

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

Tissue Equivalent Curved Organic X-ray Detectors Utilizing High Atomic Number Polythiophene Analogues

Organic semiconductors are a promising material candidate for X-ray detection. However, the low atomic number (Z) of organic semiconductors leads to poor X-ray absorption thus restricting their performance. Herein, the authors propose a new strategy for achieving high-sensitivity performance for X-ray detectors based on organic semiconductors modified with high –Z heteroatoms. X-ray detectors are fabricated with p-type organic semiconductors containing selenium heteroatoms (poly(3-hexyl)selenophene (P3HSe)) in blends with an n-type fullerene derivative ([6,6]-Phenyl C71 butyric acid methyl ester (PC70BM). When characterized under 70, 100, 150, and 220 kVp X-ray radiation, these heteroatom-containing detectors displayed a superior performance in terms of sensitivity up to 600 ± 11 nC Gy−1 cm−2 with respect to the bismuth oxide (Bi2O3) nanoparticle (NP) sensitized organic detectors. Despite the lower Z of selenium compared to the NPs typically used, the authors identify a more efficient generation of electron-hole pairs, better charge transfer, and charge transport characteristics in heteroatom-incorporated detectors that result in this breakthrough detector performance. The authors also demonstrate flexible X-ray detectors that can be curved to a radius as low as 2 mm with low deviation in X-ray response under 100 repeated bending cycles while maintaining an industry-standard ultra-low dark current of 0.03 ± 0.01 pA mm−2

Molecular Weight Tuning of Organic Semiconductors for Curved Organic-Inorganic Hybrid X-Ray Detectors

Curved X-ray detectors have the potential to revolutionize diverse sectors due to benefits such as reduced image distortion and vignetting compared to their planar counterparts. While the use of inorganic semiconductors for curved detectors are restricted by their brittle nature, organic-inorganic hybrid semiconductors which incorporated bismuth oxide nanoparticles in an organic bulk heterojunction consisting of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) are considered to be more promising in this regard. However, the influence of the P3HT molecular weight on the mechanical stability of curved, thick X-ray detectors remains less well understood. Herein, high P3HT molecular weights (>40 kDa) are identified to allow increased intermolecular bonding and chain entanglements, resulting in X-ray detectors that can be curved to a radius as low as 1.3 mm with low deviation in X-ray response under 100 repeated bending cycles while maintaining an industry-standard dark current of mu C Gy(-1) cm(-2). This study identifies a crucial missing link in the development of curved detectors, namely the importance of the molecular weight of the polymer semiconductors used

A multifaceted ferrocene interlayer for highly stable and efficient lithium doped spiro‐OMeTAD‐based perovskite solar cells

Over the last decade, 2,2″,7,7″-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) has remained the hole transporting layer (HTL) of choice for producing high efficiency perovskite solar cells (PSCs). However, PSCs incorporating spiro-OMeTAD suffer significantly from dopant induced instability and non-ideal band alignments. Herein, a new approach is presented for tackling these issues using the functionality of organometallocenes to bind to Li+ dopant ions, rendering them immobile and reducing their impact on the degradation of PSCs. Consequently, significant improvements are observed in device stability under elevated temperature and humidity, conditions in which ion migration occurs most readily. Remarkably, PSCs prepared with ferrocene retain 70% of the initial power conversion efficiency (PCE) after a period of 1250 h as compared to only 8% in the control. Synergistically, it is also identified that ferrocene improves the hole extraction yield at the HTL interface and reduces interfacial recombination enabling PCEs to reach 23.45%. This work offers a pathway for producing highly efficient spiro-OMeTAD devices with conventional dopants via addressing the key challenge of dopant induced instability in leading PSCs