Experimental Physics VI

Overview

Semiconducting conjugated polymers and small molecules are interesting candidates for electronics applications such as light emitting diodes (which are already commercially available) and photovoltaics. Their properties differ significantly from inorganic semiconductors, rendering them an interesting field for innovative research. In our group, we investigate the peculiarities of hopping transport of charge carriers, and its impact on charge carrier interaction during transport and recombination. We apply complementary experimental techniques and also implement and use macroscopic and microscopic simulations in order to gain insight into the detailed function of charge transport in these organic semiconductors.

In the following, we will give two examples of our current research, Carrier Concentration Dependent Charge Carrier Mobility and Influence of Air on Trap States in Conjugated Polymers.

Carrier Concentration Dependent Charge Carrier Mobility

Unlike their crystalline counterparts, amorphous semiconducting materials – conjugated polymers and small molecules in particular – show a variation of the mobility with increasing charge carrier density. While the classical case of highly ordered materials exhibits transport of quasi-free electrons and holes in the valence and conduction bands, disordered organic systems exhibit temperature assisted charge carrier hopping between localized states in a gaussian density of states. Therefore, the drift velocity times a given electrical field, namely the charge carrier mobility, does not remain constant; instead it varies with the amount of occupied states. The figure illustrates a temperature dependent measurement of the mobility versus charge carrier density derived from the accumulation of holes within an organic field effect transistor. While theoretical approaches predict a continuous ascent – unless coulomb interaction takes place – the measurement indicates two regions of differing slope. This observation gives rise to an expansion of the model by considering the filling of deep traps.
Understanding of the transport in organic materials remains a current research topic since well-known theories can only partly explain the experimental findings. Therefore, our group performs experiment as well as analytical, numerical and Monte Carlo simulations for the comprehension of these materials and further based on this knowledge, to optimize organic devices.

Influence of Air on Trap States in Conjugated Polymers

Charge transport in disordered organic semiconductors is generally described as thermally activated hopping in a gaussian distribution of localized states. Applying the transport energy concept it is still possible to distinguish between regular transport states and traps: States below the transport energy act as traps for the charge carriers, whereas those above the transport energy are transport states. The presence of deep traps is critical to the performance of organic electronic devices, since trapped charge carriers do no longer contribute to the current flow. With regard to the lifetime of devices such as organic solar cells, the influence of air-related defect states on the charge transport might be decisive with respect to the long-term stability. We investigate the trap states and especially the influence of air on the trap distribution in our devices by applying the thermally stimulated current (TSC) technique. The figure shows a TSC measurement of the conjugated polymer P3HT, which reveals two trap states with gaussian energy distributions. Thereby the shallow trap can be assigned to the tail of the intrinsic density of states, whereas the concentration of the deep trap is strongly affected by exposure to air.

 

Experiments

We apply different methods to investigate charge transport and particularly carrier mobility in different materials classes, such as

• Charge Carrier Mobility
  • Organic Field Effect Transistor Measurements (FET)
  • Space Charge Limited Current Measurements (SCLC)
  • Transient Photoconductivity Measurements (Time of Flight, TOF)
  • Carrier Extraction by Linearly Increasing Voltage (CELIV)
• Defect Spectroscopy
  • Thermally Stimulated Currents

Simulations

We implemented two different kinds of simulations, complementary in view of the applicability to physical problems:

• Macroscopic Simulation of Organic Semiconductors and Semiconductor Blends
  • current–voltage characteristics of organic solar cells in dependence of illumination intensity and temperature
• Microscopic Simulations of Hopping Transport
  • details of hopping transport in disordered systems
  • simulation of TOF, CELIV and FET measurements
  • investigation of carrier–carrier interaction on mobility

Contact

Dr. Carsten Deibel, phone +49 931 888 5894

Maria Hammer, phone +49 931 888 5770

Julia Schafferhans, phone +49 931 888 5770