Thermally Activated And Field Dependent Hole Transport In Poly(3-hexylthiophene)

¹Organic & Hybrid Solar Cells Group, Physics of Energy Harvesting Division and CSIR-Network of Institutes for Solar Energy, CSIR-National Physical Laboratory (CSIR-NPL), Dr. K. S. Krishnan Marg, New Delhi 110012, India

²Academy of Scientific and Innovative Research (AcSIR), CSIR-NPL Campus, Dr. K. S. Krishnan Marg, New Delhi110012, India

Adv. Mater. Lett., 2016, 7 (6), pp 430-435

DOI: 10.5185/amlett.2016.5991

Publication Date (Web): May 30, 2016

Copyright © IAAM-VBRI Press

E-mail: apatra@nplindia.org

Abstract

Here, we investigate the hole transport mechanism in poly(3-hexylthiophene) (P3HT). First, ohmic contact has been established at indium tin oxide (ITO)/P3HT interface by molybdenum oxide (MoO_x) hole injection layer. Thickness of MoO_x layer is observed to play a crucial role with ohmic contact being observed even for 1 nm layer. However, device with less than 5 nm layer are found to be extremely unstable. A device with a 5 nm layer of MoO_x is found to be stable and ohmic injection at ITO/P3HT layer enabled to observe ohmic conduction at low voltages (< 3 V), trap free space charge limited conduction (SCLC) for > 3 V. At higher voltages, effect of field on charge carrier mobility is also observed. Observation of SCLC enabled us to directly evaluate the hole mobility in P3HT which is calculated to be 5.4 × 10^-5 cm²/Vs. Conductivity is calculated from the low voltage region and found to be 6.85 × 10^-8 S/cm. Temperature dependent mobility is used to study the charge transport behavior and it has been observed that mobility is thermally activated with an extremely low activation energy of 39 meV. 

Keywords

Hole transport, p-type polymer, activation energy, field dependent mobility