Impact of Traps on Charge Transport in Organic Semiconductors

• Applicants: Professor Dr.-Ing. Niels Benson; Professor Dr. Carsten Deibel; Professor Dr. Roland Schmechel
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2015 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 263376198

• Final Report Year: 2020

Final Report Abstract

In this project we have investigated traps in a range of organic semiconductors relevant for polymer-based light emitting diodes and solar cells, with respect to universal trap levels postulated by Nicolai et al. and Zuo et al., as well as their influence on the charge carrier transport for electrons and holes. With our experiments based on a combination of MIS-TSC, t-SCLC and sub-gap EQE measurements, we were able to find a direct semiconductor dependent oxygen- and water-based ageing influence. However, our findings do not confirm the singular trap levels proposed by Nicolai et al. and Zuo et al. Instead, we have found an energetically broad trap state distribution for all investigated materials in their pristine as well as their aged state, and suggest that the proposed universal trap levels are a part of this trap distribution. The influence of the ageing experiment typically energetically widens the trap distribution and results into Ea values of up to 1 eV, as well as trap densities in the 1016 cm3 range. Yet, as the intrinsic trap levels are high for the semiconductors P3HT and MDMO-PPV, even in their pristine state, the impact of the ageing on the charge carrier transport, as well as on the result of the sub-gap EQE experiments is limited. A strong impact of the ageing experiment was found for PCPDTBT, which exhibits an intrinsically low trap concentration in its pristine state, and consequently results in a significantly reduced charge carrier mobility on the account of the ageing. Unfortunately, due to the delays and required re-scheduling in particular at the TU Chemnitz, we were not able to test if the oxygen related trap states can be healed or compensated by doping. However, we were able to go beyond the scope of the original proposal by investigating the influence of oxygen and water ageing on electron as well as hole related trap states and transport.

Publications

• MIS-TSC: A combination of the thermally stimulated current method and a metal-insulator-semiconductor device for unipolar trap spectroscopy Appl. Phys. Lett., 15, 114 (2019)
  K. Rojek, R. Schmechel, and N. Benson
  
• Ultra-fast measurement circuit for transient space charge limited current in organic semiconductor thin films Meas. Sci. Technol., 1, 31 (2020)
  K. Rojek, R. Schmechel, and N. Benson