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We present a comprehensive approach to the characterization and modeling of photovoltaic metal-halide perovskite single junction devices and perovskite-silicon tandem solar cells. The framework is based on 1D opto-electronic device simulation in steady-state and transient modes as well as frequency domain including specific features of the perovskite materials such as mobile ions, combined with a broad variety of device characterization experiments. As a salient feature, advanced optimization algorithms are used for reliable parameter extraction and opto-electronic device optimization purposes in both single junction and tandem solar cell architectures.

Magnetic nanoparticles and their ability to convert electromagnetic energy into heat are of explicit interest for various applications. However, precise quantification of their heating efficiency is not always upfront, and several parameters render comparative studies challenging. This paper describes the theory behind lock-in thermography, a new technique for quantifying the heating properties of magnetic nanoparticles. This technique allows the investigation of some of the potential sources of variability: key factors such as magnetic field inhomogeneity and its effects on the heating power are explored in detail. The presented results, obtained from various nanoparticle batches of different origins, highlight the importance of pursuing a standardized and systematic approach when quantifying the heating efficiency of magnetic nanoparticles.

There is increasing evidence that the presence of mobile ions in perovskite solar cells can cause a current-voltage curve hysteresis. However, it is still subject of ongoing debates how exactly mobile ions influence the device operation. We use drift-diffusion simulations incorporating mobile ions to describe IV-curves of preconditioned methylammonium lead iodide perovskite solar cells and compare them with experimental results. Our simulation results show that the hysteresis depends on the extent of surface recombination and on the diffusion length of charge carriers. We provide a detailed explanation for the reduced hysteresis of perovskite solar cells with high power conversion efficiencies. We find that in high-efficiency solar cells ion migration is still present, but does not cause a hysteresis effect. In these devices charge extraction is mainly driven by diffusion of free electrons and holes.

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.

We discuss the numerical optimization of organic and hybrid multijunction solar cell devices, using an integrated optoelectronic device simulation approach that captures the essential properties of the complex multilayer structures, such as, light scattering at both macroscopic and rough thin film textures, resonator effects in coherent stacks, and the impact of organic interface properties for the design of efficient recombination layers. The optimization capabilities of the numerical device simulation framework are illustrated on the examples of textured perovskite-silicon and all-organic tandem solar cells.

Many electron transport layer materials (ETL) employed in state of the art organic light emitting diodes (OLEDs) are known to be polar. We combine for the first time simulations and electrical characterization of OLEDs based on polar ETL, in order to understand the impact of such materials on the device operation. Depending on the orientation of the dipole orientation, simulations predict either a benefit or a disadvantage of the polar ETL for the device performance.

​This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Materials & Interfaces, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/acsami.9b04991, A variety of experiments on vacuum-deposited methylammonium lead iodide perovskite solar cells are presented, including JV curves with different scan rates, light intensity-dependent open-circuit voltage, impedance spectra, intensity-modulated photocurrent spectra, transient photocurrents, and transient voltage step responses. All these experimental data sets are successfully reproduced by a charge drift-diffusion simulation model incorporating mobile ions and charge traps using a single set of parameters. While previous modeling studies focused on a single experimental technique, we combine steady-state, transient, and frequency-domain simulations and measurements. Our study is an important step toward quantitative simulation of perovskite solar cells, leading to a deeper understanding of the physical effects in these materials. The analysis of the transient current upon voltage turn-on in the dark reveals that the charge injection properties of the interfaces are triggered by the accumulation of mobile ionic defects. We show that the current rise of voltage step experiments allow for conclusions about the recombination at the interface. Whether one or two mobile ionic species are used in the model has only a minor influence on the observed effects. A delayed current rise observed upon reversing the bias from +3 to -3 V in the dark cannot be reproduced yet by our drift-diffusion model. We speculate that a reversible chemical reaction of mobile ions with the contact material may be the cause of this effect, thus requiring a future model extension. A parameter variation is performed in order to understand the performance-limiting factors of the device under investigation.

Many electron transport layer materials (ETL) employed in state of the art organic light emitting diodes (OLEDs) are known to be polar. We combine for the first time simulations and electrical characterization of OLEDs based on polar ETL, in order to understand the impact of such materials on the device operation. Depending on the orientation of the dipole orientation, simulations predict either a benefit or a disadvantage of the polar ETL for the device performance. We also show that OLEDs featuring a polar material are perfectly suited for extracting mobility activation energy and Injection barrier from the anode to the ETL.

A numerical model for charge transport in organic semiconductor devices that accounts for self-heating is presented. In admittance spectroscopy this model reproduces the negative capacitance in bipolar, and more importantly, in single carrier devices. We show that self-heating is crucial not only in large-area OLEDs, but also in small-area devices.

We present a new approach to simulate the transport of charges across organic/organic layer interfaces in organic semiconductor devices. This approach combines the drift-diffusion formalism away from the interface with a hopping description of the charge transport in the vicinity of the interface. It has been implemented in the commercial software SETFOS allowing for fast simulations of the complete device. This new model takes into account both recombination and generation mechanisms across the interface enabling the modeling of charge-generation/recombination interfaces for the numerical simulation of tandem devices. Using this approach, it is also possible to simulate devices using 1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile as a hole-injection layer. This particular material has a very deep HOMO level (approximately 9.5 eV), which would seemingly prevent such a layer to be used as a hole-injection material in the framework of traditional drift-diffusion models.

Single-carrier devices are an excellent model system to study charge injection and charge transport properties of (doped) transport layers and to draw conclusions about organic electronics devices in which they are used. By combining steady-state and impedance measurements at varying temperatures of hole-only devices with different intrinsic layer thicknesses, we are able to determine all relevant material parameters, such as the charge mobility and the injection barrier. Furthermore, the correlation and sensitivity analyses reveal that the proposed approach to study these devices is especially well suited to extract the effective doping density, a parameter which cannot be easily determined otherwise. The effective doping density is crucial in organic light-emitting diodes (OLEDs) for realizing efficient injection, charge balance, and lateral conductivity in display or lighting applications. With the fitted drift-diffusion device model, we are further able to explain the extraordinary two-plateau capacitance–frequency curve of these hole-only devices, which originates from charges that flow into the intrinsic layer at zero applied offset voltage. We demonstrate that the observation of this behaviour is a direct indication for ideal charge injection properties and the observed capacitance–frequency feature is linked to the charge carrier mobility in the intrinsic layer. The extracted material parameters may directly be used to simulate and optimize full OLED devices employing the investigated hole-injection and -transport materials.

Organic light-emitting diodes (OLEDs) rely on the use of functional materials with suitable energy levels and mobilities for selective charge carrier injection and transport of one species only at the respective electrode. Until recently, however, the dipolar nature of many organic semiconductors has been largely ignored in this context. In particular, electron transports layers (ETLs) often exhibit spontaneous orientation polarization leading to interfacial charges that modify the electrical potential landscape inside a hetero-layer device. Here we demonstrate that the effect of polar ETLs can be simulated using the well-established Poisson and drift-diffusion formalism, if these interfacial charges are taken into account. Impedance spectroscopy is used in order to validate our approach and to characterize the polarity of the material. Finally, simulations allow to quantify the impact of polar ETLs on device performance.

We present a comprehensive model to simulate organic light-emitting devices (OLEDs) that includes a seamless coupling of charges, excitons and photons. The comprehensive model accounts for the position dependent exciton lifetime due to the optical environment in the multilayer OLED structures. We first study the effect of different charge mobilities and quantum efficiencies of the light-emitting material on the exciton profiles. Moreover, we discuss the extension of an optical model to account for the exciton dynamics. This comprehensive optical model is validated and justified on the basis of consistency checks. Namely, we show that our comprehensive optical model can take the cavity effects as seen in simulation results of the comprehensive electrical model into account. The advantage of the comprehensive optical model is a quick and accurate insight into the exciton physics if applied together with a nonlinear least square (NLSQ) fitting method. Finally, we apply the comprehensive optical model with the NLSQ-method in order to extract the exciton profiles from emission spectra of a blue light-emitting polymer diode (PLED) measured at different current levels.

Effects of ion concentration and active layer thickness play a critical role on the performance of light-emitting electrochemical cells. Expanding on a pioneering materials system comprising the super yellow (SY) polymer and the electrolyte trimethylolpropane ethoxylate (TMPE)/Li+CF3SO3-, it is reported that a slightly lowered salt concentration and layer thickness result in a substantial efficiency increase, and that this increase is confined to a narrow concentration and thickness range. For a film thickness of 70 nm, a blend ratio SY:TMPE:Li+CF3SO3- = 1:0.075:0.0225, and a current of 7.7 mA cm(-2) the current efficacy is 11.6 cd A(-1), on a par with SY light-emitting diodes. The optimized salt content can be explained by increased exciton quenching at higher concentrations and hindered carrier injection and conduction at lower concentrations, while the optical dependence on the layer thickness is due to weak microcavity effects. A comprehensive optical modeling study is presented, which includes the doping-induced changes of the refractive indices and self-absorption losses due the emission-absorption overlap of intrinsic and doped SY. The analysis indicates either a thickness-independent emitter position (EP) close to the anode or a thickness-dependent EP, shifted to the cathode for increased thicknesses.

So far self-heating has only been of concern in large-area devices where the resistive transparent anode leads to a potential drop over the device resulting in inhomogeneous current, brightness and temperature distributions. In this work, we show that even small lab devices suffer from self-heating effects originating from the organic semiconductor layer. In admittance spectroscopy of organic semiconductor devices, negative capacitance values often arise at low frequency and high voltages. In this study we demonstrate the influence of self-heating on organic semiconductor devices with the aid of a numerical 1D drift-diffusion model that is extended by Joule heating and heat conduction. Furthermore the impact of trap states on the capacitance in combination with self-heating is demonstrated. The typical signature of self-heating might be overshadowed depending on the trapping dynamics. In a next step, we compare the negative capacitance vs. frequency for uni- and bipolar devices to quantify the different processes. We emphasize the impact of self-heating and trapping on OLEDs and organic solar cells. To ease the interpretation of the results we investigate simulations in the time domain as well as in the frequency domain. We have provided clear evidence of self-heating of organic semiconductor devices and conclude that a comprehensive model requires the inclusion of heat conduction and heat generation in the drift-diffusion model.

We present a comprehensive numerical impedance spectroscopy analysis of an organic semiconductor device. A physical model that considers localized states is combined with a space- and frequency-resolved numerical framework. We study the details of the frequency-dependent capacitance of an electron-only device and distinguish different trapping regimes depending on the parameters. Depending on the choice of the trapping parameters, a capacitance rise at low frequency is observed. The extraction of the characteristic temperature of the exponential of the trap density of states (DOS) by a simplified method by T. Okachi et al. [Appl. Phys. Lett. 94, 043301(2009)] is investigated.

Our numerical analysis demonstrates that exciton distributions extracted from spectral emission measurements of OLEDs are equivalent to those obtained with charge and exciton transport simulations when optical quenching effects are taken into account.

A numerical small-signal analysis is conducted for organic light-emitting devices with novel model ingredients as the generalized Einstein relation and the Extended Gaussian Disorder Model.

For the design of organic semiconductor devices such as organic light-emitting devices and solar cells, it is of crucial importance to solve the underlying charge transport equations efficiently and accurately. Only a fast and robust solver allows the use of fitting algorithms for parameter extraction and variation. Introducing appropriate models for organic semiconductors that account for the disordered nature of hopping transport leads to increasingly nonlinear and more strongly coupled equations. The solution procedures we present in this study offer a versatile, robust, and efficient means of simulating organic semiconductor devices. They allow for the direct solution of the steady-state drift-diffusion problem. We demonstrate that the numerical methods perform well in combination with advanced physical transport models such as energetic Gaussian disorder, density-dependent and field-dependent mobilities, the generalized Einstein diffusion, traps, and its consistent charge injection model.

A general problem arising in computer simulations is the number of material and device parameters, which have to be determined by dedicated experiments and simulation-based parameter extraction. In this study we analyze measurements of the short-circuit current dependence on the active layer thickness and current-voltage curves in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based solar cells. We have identified a set of parameter values including dissociation parameters that describe the experimental data. The overall agreement of our model with experiment is good, however a discrepancy in the thickness dependence of the current-voltage curve questions the influence of the electric field in the dissociation process. In addition transient simulations are analyzed which show that a measurement of the turn-off photocurrent can be useful for estimating charge carrier mobilities., 10 pages, 12 figures, 2 tables, Accepted for publication in Journal of Applied Physics

In admittance spectroscopy of organic semiconductor devices, negative capacitance values arise at low frequency and high voltages. This study aims at explaining the influence of self-heating on the frequency-dependent capacitance and demonstrates its impact on steady-state and dynamic experiments. Therefore, a one dimensional numerical drift-diffusion model extended by the heat equation is presented. We calculate the admittance with two approaches: a Fourier method that is applied to time domain data and a numerically efficient sinusoidal steady state analysis (S3A), which is based on the linearization of the equations around the operating point. The simulation results coincide well with the experimental findings from reference [H. Okumoto and T. Tsutsui, Appl. Phys. Express 7, 061601 (2014)] where the negative capacitance effect of an organic device becomes weaker with better cooling of the structure. Linking the frequency- and time-domain with the Fourier approach supports an effortless interpretation of the negative capacitance. Namely, we find that negative capacitance originates from self-heating induced current enhancement.

We present an analysis of charge mobility determination methods for the steady as well as the transient state and investigate shallow charge traps with respect to their dynamic behavior. We distinguish between fast and slow trap states in our numerical model corresponding to two characteristic regimes. The two regimes manifest themselves in both impedance spectroscopy and dark injection transient currents (DITC). Further we investigate the charge mobility obtained from dynamic simulations and relate it to the extracted charge mobility from steady-state current-voltage curves. To demonstrate the practical impact of these regimes, we apply our numerical model to the DITC that have commonly been used to determine the charge mobility in organic semiconductor devices. The obtained results from DITC studies strongly depend on the measurement conditions. Therefore we analyze the measurements of reference [Esward et al., J. Appl. Phys. 109, 093707 (2011)] and reproduce the effects of varying pulse off-times on the transient current qualitatively. Thus, our simulations are able to explain the experimental observations with the help of relaxation effects due to shallow traps.

We demonstrate the importance of a comprehensive modeling of the dynamics of excited states in organic optoelectronics devices. Our numerical analysis demonstrates that exciton distributions extracted from spectral emission measurements of OLEDs are equivalent to those obtained by solving charge and exciton transport equations when the position-dependent coupling to optical modes is taken into account. The transport simulations are based on the extended Gaussian disorder model for organic semiconductors. Further, we show that the same numerical modeling framework can be used to accurately simulate bulk-heterojunction organic solar cells with dissociation of charge-transfer excitons. The simulations are compared to experimental data.

Our numerical analysis demonstrates that exciton distributions extracted from spectral emission measurements of OLEDs are equivalent to those obtained with charge and exciton transport simulations when optical quenching effects are taken into account. The transport simulations are based on the Extended Gaussian Disorder Model for organic semiconductors.