Advanced Functional Materials and Devices (AFMD) Group

Abstract:

In this contribution the effect of changes in the activematerial system and modification of contact properties in organic solar cells is investigated at the example of ZnPc:C60 small molecule solar cells. Devices with different blend mixing ratios and a variation of hole transport layer are prepared by vacuum deposition and compared to drift-diffusion simulation data. It is shown that the open circuit voltage is mainly defined by themixing ratio,whereas the fill factor is strongly influenced by the choice of hole transport layer. Extraction barriers for photogenerated holes lead to S-shaped IV curves. The strength of the S-shape scales with the height of the extraction barrier. The slope of the IV curves at open circuit suggests that the observed increase in open circuit voltage with a higher amount of C60 in the blend might be due to a downshift of the highest occupied molecular orbital of ZnPc. © 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

Abstract:

Organic solar cells (OSC) have attracted growing attention in recent years and their development has reached a stage at which several companies are preparing to make them commercially available either as standalone products or integrated into other device. There are different production routes for OSC: one very promising approach uses thermal evaporation of small organic molecules in vacuum, i.e. the same approach that is used in all current commercial manufacturing of organic LEDs. We use vacuum processing to create an organic stack in the p-i-n concept. In this concept the intrinsic absorber layers are sandwiched between p- and n-doped wide gap transport layers which leads to a nearly ideal solar cell structure and offers a stable platform both for investigation of fundamental processes and device optimization. In recent years it was found that the device operation crucially depends on the morphology of the bulk heterojunction and that optical interference effects in the organic stack play an important role for light absorption. We show here how the morphology of the organic layers can be controlled in vacuum deposited layers, and describe the optical optimization of tandem solar cells, for which an efficient recombination contact and current matching are essential requirements. Applying these principles and subsequently combining two complementary absorbing subcells lead to a tandem organic solar cell with an independently certified efficiency of 6.07% on 2cm2 device area, i.e. the first OSC over 6% on module relevant dimensions. © 2010 IEEE.

Abstract:

The ZnPc/C60 solar cells with a reasonable energy conversion efficiency of -1.5% were investigated. The samples were aged for 1269 hours upon illumination by the blue LED with peak emission at 475nm. Upon ageing, the devices have shown a strong and fast degradation of the efficiency, short circuit current and of the fill factor within several hours followed by a much slower decrease thereof. The carrier mobility dependences on electric field strength at different temperatures were measured by the Charge Extraction by Linearly Increasing Voltage method. The observed drop of device current cannot be explained be only mobility decrease. The increase of the effective barrier height by about 0.1eV from -0.55eV up to -0.65eV was observed in the aged samples. Meanwhile, thermal activation energy values of the electrical conductivity grew from about 0.28eV prior to degradation up to about 0.34eV after ageing. © 2010 - STC "Institute for Scientific Cruystals".

Abstract:

Recently, much effort has been put in the simulation of charge transport in organic materials and more and more sophisticated models with an increasing number of parameters are developed. In this paper, we show which models are necessary and applicable for the drift-diffusion modeling of IV curves of multilayer solar cells. For comparison to experiment, a data series of small molecule organic solar cells comprising several materials, architectures, and thicknesses of layers is used. The model is chosen as simple as possible to reduce the number of parameters and to allow for the identification and accurate investigation of the key processes in detail. As exemplary result, the spatial exciton generation profile is examined within the active blend layer: It does not only influence the short circuit current, but also the fill factor and open circuit voltage at constant short circuit current, especially in the case of imbalanced mobilities. In another example where interfacial barriers are present, broadening of the energy levels due to disorder has to be taken into account to describe the IV curves showing S-Shape like behavior with realistic parameters. Transient measurements of extraction barrier devices show indeed an accumulation of charge carriers as predicted by simulation. ©2010 IEEE.

Abstract:

A series of oligothiophenes that are end-capped with dicyanovinyl (DCV) and 1,3,2-(2H)-dioxaborine (DOB) moieties has been prepared using standard procedures. Their optoelectronic properties have been investigated by cyclic voltammetry and optical absorption. The optical absorption has been measured both in solution and thin film state. © 2010 Elsevier Ltd. All rights reserved.