Experimental Physics VI

Overview

The need for the conversion and storage of information and energy has become a general one nowadays. On the one hand, the current generation fluctuations of renewable energies call for the provision of short to medium term energy storage. Supercapacitors made of carbon aerogels supply storage of a high power density due to their high surface area and small distance between the electrodes. On the other hand, the modern lifestyle requires affordable access to electronic media. Organic field effect transistors – the major building block in printable plastic electronics – can be a promising and competitive alternative. In order to develop efficient storage and plastic electronics devices, it is essential to understand the underlying transport mechanism by performing suitable experiments.

SuperCaps

Electrochemical double layer capacitors, better known as supercapacitors, are able to store about a million times more charges than conventional capacitors. Right now the energy density is comparable to lead acid batteries. The high power density allows for charging within some seconds, not hours, like in batteries.
The charge is stored in the so called Helmholtz double layer. When voltage is applied, ions of inverse polarization accumulate at the electrodes and the double layer is formed. Its thickness is in the range of several angstroms. In addition highly porous carbon is used as electrode material. After activation-procedures the surface can reach up to 2000 m² (half of a soccer field) per gram electrode material. Due to this very high specific electrode surface and the very small distances within the double layer, capacities of several hundreds farads can be achieved. As charge storage is only physical and not chemical, like in batteries, the lifetime of supercaps in nearly unlimited.

Ambipolar Organic Field Effect Transistors

Organic field effect transistors (OFETs) are the basic building blocks of plastic integrated circuitry. The working principle for a p-type OFET is applying a negative gate-source voltage and cause an accumulation of holes near the semiconductor-insulator interface. Applying a negative drain-source voltage will cause a current to flow across the channel, which is dominated by the charges closest to the semiconductor-dielectric interface. It is known that two complementary transistors one with hole and the other one with electron conducting source to drain channels can be interconnected to an inverter switch which is sufficient for the assembly of logical circuits. Many of the well known semiconducting polymers show electron as well as hole transport. The performance of each charge carrier type can -to a certain extent- be affected by the dielectric surface and the dielectric constant as well as by the work function of the injecting material. Using ambipolar semiconductors the transistor characteristics are modified at low gate-voltages: If both carrier types are located in the transistor channel recombination can take place and lead to relatively high currents. Suppressing one carrier type by applying higher gate voltages leads to usual field effect characteristics for the majority charge carrier. The measurement of ambipolar transport is especially interesting for blend materials which are commonly used in solar cells. The affect of the blend ratio on the mobility can be calculated and the result can be applied for better solar cell performaces. The figure shows the output characteristics of a P3HT:PCBM OFET (blend ratio 1:3)

 

Experiments

We apply different experimental methods, such as

• Field Effect Transistor Measurements (FET)
• Cyclic Voltammetry (CV)
• Impedance Spectroscopy
• Galvanostatic charging and discharging

Contact

Prof. Dr. Vladimir Dyakonov, phone +49 931 888 3111

Dr. Carsten Deibel, phone +49 931 888 3119

Volker Lorrmann (SuperCaps), phone +49 931 888 3115

Maria Hammer (OFETs), phone +49 931 888 3113