Jul 2, 2012
Intra-chain conduction boosts polymer diode performance
Researchers from Bogazici University, Turkey, have built solution-state polymer diodes with nanogap electrodes that support intra-chain-dominant conduction, which is hard to obtain with solid-state counterparts. The behaviour of polymer chains in solution prevents piling and aggregation, and an electrode separation comparable to the polymer chain length allows charge carrier conduction without the need for inter-chain hopping. As a result, higher charge carrier mobilities can be obtained, which leads to better current densities.
When a polymer is used in solution state, ambulatory chains prefer to remain separate rather than entangling each other. Also, certain polymer chains, such as regioregular polythiophene (rr-P3HT), form nanorod-like structures over distances comparable to their persistence length when dissolved in certain organic solvents such as dichlorobenzene (DCB). This arrangement benefits intra-chain mobility thanks to the lack of bends and twists, which act as impurities and impede π-bond conduction.
Configuring the device
The polymer diodes built by the group have silicon and aluminium electrodes separated by a 40 nm thick SiO2 insulation layer. A solution of P3HT:DCB is prepared and the diode is realized by dipping the electrode structure into the mixture.
In measurements, a current density of 300 mA/cm2 is obtained, which corresponds to at least a six-fold improvement in carrier mobility compared with solid-state polymer counterparts. Solution concentration and hysteresis effects were also investigated.
The next step is to exploit the various advantages of a liquid interface. Solution-state diodes are relatively easy to integrate with microfluidic devices, which could be highly beneficial for biological applications. Also the microchannel structure can be designed so that the polymer solution can be refreshed or changed as desired, to provide multifunctional devices.
About the author
The work was performed at Bogazici University in Istanbul, Turkey. Diodes were built and tested in the Micro Electro Mechanical Systems Laboratory (BUMEMS). Senol Mutlu is an associate professor in the Department of Electrical & Electronics Engineering at Bogazici University and is the supervisor of the project. Bedri Gurkan Sonmez is a PHD student in the same department and works on microfluidic systems and applications. This work is fully supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under project EEEAG 110E063.