Abstract
Organic solar cells (OSC) have evolved to a promising thin film technology for integrated applications and outperform their competitors like hydrogenated amorphous silicon (a-Si:H) or other flexible thin film technologies already in terms of performance to weight, energy balance or flexibility. The efficiency of OSC has now reached 14% with a rapidly increasing trajectory targeted at 20% efficiency. If this efficiency goal can be realized, OSC will become a highly competitive photovoltaics technology that is efficient, cheap and environmentally friendly. However, the main concern of the general photovoltaics community on organic materials is their instability as materials and in devices. Therefore, understanding and improving the stability of OSC become critically important for the commercialization of OSC.
While many factors could affect their stability, the morphology of the photoactive layer is probably the most important issue. The performance of OSC and photodetectors relies on a fairly complex blend of n- and p-type semiconductors (called bulk heterojunction), which typically consists of several phases with a few nm to a few tens of nm dimensionality. The identical two materials can show a quantum efficiency of up to 100% or less than 10%, depending on their morphology and microstructure. More specifically, the properties of the crystalline and amorphous domains of the OSC blends can be vital parameters in determining the performance of the blends.
While the property of crystalline domains have been widely studied using a series of X-Ray and electron microscopy techniques with nm resolutions, all these methods are only sensitive to crystalline domains and fail to give an insight into the amorphous regimes. This is especially unsatisfying, as the scientists could recently show that the amorphous regimes have a detrimental influence on the stability of a composite. A theoretical understanding of the microstructure of organic semiconducting polymers and blends and especially to their amorphous regimes is therefore vital to further advance the optoelectronic device performance of organic electronics. In this lecture, the speaker would outline the theoretical framework of a combined numerical approach based on polymeric solution theory to study the microstructure of polymer: small molecule blends. He feeds the results of ab-initio density functional theory quantum chemistry calculations into an artificial neural network for the determination of solubility parameters. These solubility parameters are used to calculate Flory-Huggins intermolecular parameter, which is one of the most dominant steady-state thermodynamic material properties characterizing miscibility. He would further show that the theoretical values are in line with experimentally determined data. Based on the Flory-Huggings parameters, he establishes a relative stability determination model as a metric for assessing the phase diagrams of organic semiconducting blends in thin films. This combined model informs design criteria and processing guidelines for existing and new high performance semiconducting blends for organic electronics applications and ultimately will allow the ab-initio design of novel materials with the ideal and stable solid state morphology.
In the outlook, the speaker will show how this concept of predicting the performance of organic semiconductors will be transferred onto a robot-based combinatorial high throughput system for device fabrication, which ultimately will be able to autonomously search for the most efficient materials.
About the speaker
Prof Christoph J Brabec was appointed the Assistant Professor of Kepler University of Linz in 1995. He joined Siemens in Germany in 2001 as Principal Research scientist and became the CEO of Konarka Technologies (Austria and Nuremberg) in 2005. In 2009, Prof Brabec moved to University of Erlangen-Nuremberg, where he is currently the Professor of Materials Science.
Prof Brabec’s research interests include renewable energies; photovoltaics and solar energy; production of semiconductor devices by printing/coating methods; and combinatorial and high throughput research. He has been the member of the advisory board of Energy Technology; Progress in Photovoltaics and Journal of Photonics for Energy.
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