Charge carrier transport and trapping phenomena in organic photovoltaic materials and devices

In photonic organic material engineering and device development carrier transport properties are of primary importance as they are directly related with macroscopic material parameters determining device functionality and efficiency. Charge carrier mobility is one of the main factors limiting carrier transport in organic materials which by their nature are highly disordered. On the other hand mobility (at least of one sign carriers in a given material) is often limited by severe carrier trapping. This results in an imbalance of different carrier flows, and requires specific technological measures to be undertaken to eliminate the problem. Therefore fundamental investigations of complex carrier transport phenomena are challenging and practically important task. In this presentation the current activities and achievements in the field of organic solar cell development in Europe will be reviewed from the point of view of the in-depth microscopic transport processes occurring in such materials and devices. We are to demonstrate that carrier transport in investigated systems is in a complex way influenced by the light-, electric field- and thermally- stimulated mobility and trapping effects, depending on the excitation conditions. Carrier mobility measurements were performed by the CELIV (Charge Extraction by Linearly Increasing Voltage) method, carrier traps were analyzed by the Thermally Stimulated Current spectroscopy, and Current-Voltage characterization was used to investigate carrier injection properties. We will show that the complex experimental analysis enables discrimination and evaluation of numerical parameters of the mobility and trapping phenomena at different excitation conditions.