On the exciton profile in OLEDs-seamless optical and electrical modeling

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.     

The Dynamic Emission Zone in Sandwich Polymer Light‐Emitting Electrochemical Cells

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 (μ_a) and cations (μ_c) are not equal, and the direction of the EZ shift dictates μ_c > μ_a. 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.     

Modelling crosstalk through common semiconductor layers in AMOLED displays

High pixel density displays are demanded for active matrix organic light‐emitting diode displays (AMOLED) in applications such as virtual reality headsets, micro‐displays, and high‐end smartphones. Parasitic emission from non‐addressed neighboring pixels (crosstalk) is a common problem in such high pixel density AMOLED, and this crosstalk becomes more severe as the pixel density and fill ratio of the display increases. One of the causes of crosstalk is parasitic currents that travel through common organic semiconductor layers. In this paper, we model and quantify the pixel crosstalk using a 2 + 1D finite element model that is based on the conductivity of the common layer and the luminance–current–voltage curves of the subpixels as measured input parameters. We assess the effect of crosstalk on the pixel current, observed color, and luminance. The 2 + 1D model limits the number of degrees of freedom so that calculations on a standard personal computer are feasible.     

Charge extraction with linearly increasing voltage: A numerical model for parameter extraction

An accurate determination of the charge carrier mobilities is essential to model and improve organic solar cells. A frequently used method to determine charge carrier mobilities is the charge extraction by linearly increasing voltage technique (CELIV). In this technique a voltage ramp is applied to the device in order to extract free charge carriers inside the bulk. The free charge carriers can be created by injection or by a short light flash (photo-CELIV). With a simple analytical formula the mobility is commonly estimated on the basis of the temporal position of the current peak.We simulate the photo-CELIV experiment with a fully-coupled electro-optical model to analyse the accuracy and limitations of the analytical formulas that are used to calculate the mobilities. We show that for thin film solar cells RC-effects are problematic and can lead to inaccurate results. If RC-effects are negligible only the order of magnitude of the fast carrier mobility can be determined using the analytical formula. We measure CELIV currents for several voltage slopes and transient photo-currents of an organic bulk heterojunction solar cell. By fitting our numerical model to the multiple curves we show that important material parameters like the electron mobility, hole mobility, charge generation and recombination efficiency can be determined using numerical parameter extraction.     

Simulating electronic and optical processes in multilayer organic light-emitting devices

A detailed investigation of the device operation of a blue-emitting multilayer organic light-emitting device (OLED) using an electronic device model is presented. In particular, a transient electroluminescence overshoot at turn-on is found to originate from charge and recombination confinement effects at internal interfaces. The location of the emission zone is obtained from the electronic model and its experimental determination exemplified by a sensing layer method. Moreover, the optimization of emission intensity and color is discussed for a red-emitting OLED. The thin-film interference effects are analyzed with help of an optical device model.     

Coupled 3D master equation and 1D drift‐diffusion approach for advanced OLED modeling

A novel simulation approach for excitonic organic light‐emitting diodes (OLEDs) is established by combining a continuous one‐dimensional (1D) drift‐diffusion (DD) model for the charge carrier dynamics with a three‐dimensional (3D) master equation (ME) model describing the exciton dynamics in a multilayer OLED stack with an additional coupling to a thin‐film optics solver. This approach effectively combines the computational efficiency of the 1D DD solver with the physical accuracy of a discrete 3D ME model, where excitonic long‐range interactions for energy transfer can be taken into account. The coupling is established through different possible charge recombination types as well as the carrier densities themselves. We show that such a hybrid approach can efficiently and accurately describe steady‐state and transient behavior of optoelectronic devices reported in literature. Such a tool will facilitate the optimization and characterization of multilayer OLEDs and other organic semiconductor devices.     

Tuning of electrical and optical properties of organic LEDs using combinatorial device fabrication

The capabilities of combinatorial methods are presented in order to get a detailed understanding of the electrical and optical properties of organic light‐emitting devices (OLEDs), to optimize their performance, and to provide reliable data for device modeling. We show results on multilayer OLEDs ranging from the conventional copper‐phthalocyanine (CuPc)/N,N′di‐(naphtalene‐1‐yl)‐N,N′‐diphenyl‐benzidine (NPB) and tris‐(8‐hydroxy‐quinolinato)aluminum (Alq) tri‐layer device to double‐doped deep‐red‐emitting OLEDs.     

Influence of chemically p-type doped active organic semiconductor on the film thickness versus performance trend in cyanine/C60 bilayer solar cells

Simple bilayer organic solar cells rely on very thin coated films that allow for effective light absorption and charge carrier transport away from the heterojunction at the same time. However, thin films are difficult to coat on rough substrates or over large areas, resulting in adverse shorting and low device fabrication yield. Chemical p-type doping of organic semiconductors can reduce Ohmic losses in thicker transport layers through increased conductivity. By using a Co(III) complex as chemical dopant, we studied doped cyanine dye/C₆₀ bilayer solar cell performance for increasing dye film thickness. For films thicker than 50 nm, doping increased the power conversion efficiency by more than 30%. At the same time, the yield of working cells increased to 80%. We addressed the fate of the doped cyanine dye, and found no influence of doping on solar cell long term stability.     

Reliable extraction of organic solar cell parameters by combining steady-state and transient techniques

A method to extract reliable material and device parameters of organic solar cells is presented. We employ a comprehensive numerical device model to simulate the solar cell operation in transient and steady-state condition. Parameter extraction with numerical simulation is error-prone because model parameters are often correlated, their unique determination is very difficult and extracted parameters are likely to be inaccurate. We combine the current–voltage characteristics, the photo-CELIV currents (charge extraction with linearly increasing voltage) and the photocurrent response to a light pulse to reduce parameter correlation and increase accuracy and reliability of the extracted parameters. With a correlation matrix analysis it is shown that parameter correlation is significantly reduced when combining several experimental data sets compared with the analysis of current–voltage curves only. We find a set of parameters to reproduce the complete series of measurements with the numerical simulation. The full electrical behavior can be described using a basic drift–diffusion model with constant mobilities and direct photon-to-charge conversion. With this model we extract charge carrier mobilities in the order of 10⁻⁴ cm²/V s, a Langevin recombination prefactor of 0.08, charge injection barriers equal at both sides in the range of 0.25 eV and further device parameters for a BHJ cell with PT5DPP as donor and PCBM (C70) as acceptor. The solar cell is simulated with the extracted parameters and internal distribution of electrons, holes and the electric field are visualized.