Numerical simulation and optimisation of organic light emitting diodes and photovoltaic cells

• Main Author: Kozlowski, Fryderyk
• Other Authors: Technische Universität Dresden, Mathematik und Naturwissenschaften, Physik, Institut für Angewandte Physik
• Format: Doctoral Thesis
• Language: English
• Published: Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden 2005
• Subjects: Dotierung; Heteroübergang; Modellierung; Transportschichten mit großer Bandlücke; organische Leuchtdioden; organische Solarzellen; Organic light emitting diode; doping; heterojunction; modelling; organic solar cells; wide-gap transport layers; ddc:530; rvk:VN 6057; OLED; Solarzelle; Transportschicht
• Online Access: http://nbn-resolving.de/urn:nbn:de:swb:14-1134592504212-65990; http://nbn-resolving.de/urn:nbn:de:swb:14-1134592504212-65990; http://www.qucosa.de/fileadmin/data/qucosa/documents/1413/1134592504212-6599.pdf

A numerical model and results for the quantitative simulation of multilayer organic light emitting diode (OLED) and organic solar cell (OSC) are presented. In the model, effects like bipolar charge carrier drift and diffusion with field-dependent mobilities, trapping, dopants, indirect and direct bimolecular recombination, singlet Frenkel exciton diffusion, normal decay and quenching effects are taken into account. For an adequate description of multilayer devices with energetic barriers at interfaces between two adjacent organic layers, thermally assisted charge carrier hopping through the interface, interface recombination, and formation of interface charge transfer (CT) states have been introduced in the model. For the simulation of OSC, the generation of carrier pairs in the mixed layer or at the interface is additionally implemented. The light absorption profile is calculated from optical simulations and used as an input for the electrical simulation. The model is based on three elements: the Poisson equation, the rate equations for charge carriers and the rate equations for singlet Frenkel excitons. These equations are simultaeously solved by spatial and temporal discretisation using the appropriate boundary conditions and electrical parameters. The solution is found when a steady state is reached, as indicated by a constant value of current density. The simulation provides a detailed look into the distribution of electric field and concentration of free and trapped carriers at a particular applied voltage. For organic light emitting diodes, the numerical model helps to analyze the problems of different structures and provides deeper insight into the relevant physical mechanisms involved in device operation. Moreover, it is possible to identify technological problems for certain sets of devices. For instance, we could show that ? in contrast to literature reports - the contact between Alq3 and LiF/Al did not show ohmic behaviour for the series of devices. The role of an additional organic blocking layer between HTL and EML was presented. The explanation for the higher creation efficiency for singlet excitons in the three-layer structure is found in the separation of free holes and electrons accumulating close to the internal interface 1-Naphdata/Alq3. The numerical calculation has demonstrated the importance of controlled doping of the organic materials, which is a way to obtain efficient light emitting diodes with low operating voltage. The experimental results has been reproduced by numerical simulation for a series of OLEDs with different thicknesses of the hole transport layer and emitting layer and for doped emitting layers. The advantages and drawbacks of solar cells based on flat heterojunctions and bulk heterojunctions are analyzed. From the simulations, it can be understood why bulk-heterojunctions typically yield higher photocurrents while flat heterojunctions typically feature higher fill factors. In p-i-n ?structures, p and n are doped wide gap materials and i is a photoactive donor-acceptor blend layer using, e.g,. zinc phthalocyanine as a donor and C60 as an acceptor component. It is found that by introducing trap states, the simulation is able to reproduce the linear dependence of short circuit currents on the light intensity. The apparent light-induced shunt resistance often observed in organic solar cells can also be explained by losses due to trapping and indirect recombination of photogenerated carriers, which we consider a crucial point of our work. However, these two effects, the linear scaling of the photocurrent with light intensity and the apparent photoshunt, could also be reproduced when field-dependent geminate recombination is assumed to play a dominant role. First results that show a temperature independent short circuit photocurrent favour the model based on trap-mediated indirect recombination.