Numerical analysis of steady-state and transient charge transport in organic semiconductor devices

A one-dimensional numerical model for the simulation of organic semiconductor devices such as organic light-emitting devices and solar cells is presented. The model accounts for the energetic disorder in organic semiconductors and assumes that charge transport takes place by a hopping process between uncorrelated sites. Therefore a Gaussian density of states and the use of the Fermi-Dirac statistics are introduced. The model includes density-, field- and temperature- dependent mobilities as well as the generalized Einstein relation. The numerical methods to solve the underlying drift-diffusion problem perform well in combination with the novel physical model ingredients. We demonstrate efficient numerical techniques that we employ to simulate common experimental characterization techniques such as current-voltage, dark-injection transient and electrical impedance measurements. This is crucial for physical model validation and for material parameter extraction. We also highlight how the numerical solution of the novel model differs from the analytical solution of the simplified drift-only mode

Analysis of negative capacitance and self-heating in organic semiconductor devices

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

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

Analysis of self-heating and negative capacitance in organic semiconductors devices

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

Quantitative analysis of pixel crosstalk in AMOLED displays

The resolution of organic light-emitting diode (OLED) displays is increasing steadily as these displays are adopted for mobile and virtual reality (VR) devices. This leads to a stronger pixel crosstalk effect, where the neighbors of active pixels unintentionally emit light due to a lateral electric current between the pixels. Recently, the crosstalk was quantified by measuring the current flowing through the common hole transport layer between the neighboring pixels and comparing it to the current through the active pixel diode. The measurements showed that the crosstalk is more crucial for low light levels. In such cases, the intended and parasitic currents are similar. The simulations performed in this study validated these measurement results. By simulations, we quantify the crosstalk current through the diode. The luminous intensity can be calculated with the measured current efficiency of the diodes. For low light levels, the unintended luminance can reach up to 40% of the intended luminance. The luminance due to pixel crosstalk is perceivable by humans. This effect should be considered for OLED displays with resolutions higher than 300 PPI

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

Impact of light scattering for efficiency enhancement in organic solar cells

Further efficiency enhancements in organic solar cells require a deeper understanding of the charge generation and transport in the cell as well as the employment of advanced light trapping mechanisms. Both electronic and optical device models for organic solar cells have been developed already in the past. This paper, however, for the first time presents a simulation tool that combines a state of the art driftdiffusion electrical model with a complex optical model able to simulate wave propagation in thin film optics but also ray-based light propagation in incoherent layers and scalar scattering. The combination of the light-scattering algorithm and this driftdiffusion model leads to a coupled opto-electronic cell model which represents a powerful design tool for cell characterization and optimization. This tool is then used to evaluate the gain of efficiency introduced by a light scattering interface made of a rough TCO in a bulk heterojunction (BHJ) solar cell. The results were found to be in good qualitative agreement with previously published experimental results

Numerical analysis of steady-state and transient charge transport in organic semiconductor devices

A one-dimensional numerical model for the simulation of organic semiconductor devices such as organic light-emitting devices and solar cells is presented. The model accounts for the energetic disorder in organic semiconductors and assumes that charge transport takes place by a hopping process between uncorrelated sites. Therefore a Gaussian density of states and the use of the Fermi-Dirac statistics are introduced. The model includes density-, field- and temperature-dependent mobilities as well as the generalized Einstein relation. The numerical methods to solve the underlying drift-diffusion problem perform well in combination with the novel physical model ingredients. We demonstrate efficient numerical techniques that we employ to simulate common experimental characterization techniques such as current-voltage, dark-injection transient and electrical impedance measurements. This is crucial for physical model validation and for material parameter extraction. We also highlight how the numerical solution of the novel model differs from the analytical solution of the simplified drift-only model

Numerical simulation of charge transport in disordered organic semiconductor devices

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.