Charge Carrier Transport in a Polymeric Semiconductor: Poly(3-hexylthiophene-2,5-diyl) (P3HT)

Abstract

Organic semiconductors are now widely employed in electronic and optoelectronic devices due to their solution processability and compatibility with flexible substrates. 1 The Meyer Neldel (MN) Rule or isokinetic (IK) law means that a set of comparable samples exhibit kinetic behavior independent of activation energy at an IK or MN temperature.2 It is obeyed by the electron or hole mobility, µ, in many organic semiconductors, including pentacene3, fullerene4, N-Alkyl Perylene Diimides,5 etc. The existence of MNR is still not widely known. Further, despite considerable evidence in its favor, the multi-excitation entropy (MEE) model of its origin is still frequently contested. We fabricated organic field effect transistors (FETs) based on regioregular poly(3-hexylthiophene-2,5-diyl) (RR P3HT) of three different values of molecular weight, MW, and studied the dependence of the mobility, µ, upon the gate bias, Vg , and temperature, T. For all values of MW, we found that in the range 200 to 290 K (low T), the µ of the FET carriers exhibits an exponential dependence upon 1/T for different Vg , with an MN temperature about 290K. Between this temperature and 350K (high T), µ increases more rapidly than it did below, and is independent of Vg . We believe that, in the low T regime, the polaronic carriers hop along the polymer backbone chain. We attribute the high temperature behavior to the alpha relaxation of the polymers, that is to the movement of the backbone chains. We believe that in this regime, the polaronic carriers move with the mobile chains. We have also observed the effect of source-drain voltage upon µ.Reference 1. F. Cicoira and C. Santato, Organic Electronics: Emerging Concepts and Technology, Wiley-VCH Verlag GmbH & Co. KGaA, 2013, Germany.2. A. Yelon, et al., Multi-excitation entropy: its role in thermodynamics and kinetics, Rep. Prog. Phys., 69, 1145-1194, 2006.3. E. J. Meijer, et al., The Meyer–Neldel rule in organic thin-film transistors, Appl. Phys. Lett., 76, 3433, 2000.4. J. C. Wang and Y. F. Chen, The Meyer–Neldel rule in fullerenes, Appl. Phys. Lett., 73, 948 1998.5. R. J. Chesterfield, et al., Organic Thin Film Transistors Based on N-Alkyl Perylene Diimides: Charge Transport Kinetics as a Function of Gate Voltage and Temperature, J. Phys. Chem. B, 108, 19281-19292, 2004.