Layer-by-layer nanoarchitecture of ultrathin films of PEDOT-PSS and PPy to act as hole transport layer in polymer light emitting diodes and polymer transistors

In this paper, we report a controlled architectural growth of ultrathin films of conducting polymers via layer-by-layer (L-b-L) self-assembly with poly(3, 4-ethylenedioxythiophene), poly(styrenesulfonate) (PEDOT-PSS), and polypyrrole (PPy) as alternating layers. A typical step of the film growth was 2.3±0.1nm for every other bilayer. Linear growth of thin films has been observed by annealing each layer, while super-assembly was observed without annealing. The conductivities obtained range from 0.037S/cm at room temperature to 0.13S/cm at 120°C. The improved conductivity may be attributed to either the increase in mobility of charged carriers due to less carrier scattering in the self-assembled layer, or the increased inter-chain hopping between two polymers due to closely packed polymer-chains. The charged carriers in the hole transport layer (HTL) increase the recombination rate of electrons and holes in the electroluminescent layer thus increasing the external quantum efficiency of the polymer light emitting diodes (PLEDs). Polymer field effect transistors have been fabricated using L-b-L assembled PEDOT-PSS and PPy. Polymer field-effect transistors (PFETs) with a number of different gate lengths were used to obtain source/drain contact resistance and channel mobility. The overall mobility from the L-b-L assembled PEDOT/PPy is calculated to be 8.8×10^-3cm²/Vs at the linear regime. This mobility is five times higher than the spin coated device in linear regime. The I_on/I_off current ratio is 210. Thus confinements of holes in L-b-L assembled conducting polymer films improve the overall performance of polymer thin film transistors.