Unraveling the mechanism behind air instability in thin semiconducting polymer layers p-doped with molybdenum dithiolene complexes

Abstract

Doping efficiency and stability are crucial requirements for the integration of doped organic semiconductors in optoelectronic devices. This work presents a detailed experimental study on the air stability of p-doped systems based on a low-bandgap polymer, poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b’)dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene-)-2-6-diyl) (PBDTTT-c) doped with a strong molecular p-dopant, i.e. molybdenum tris[1-(trifluoroethanoyl)-2-(trifluoromethyl) ethane-1,2-dithiolene] (Mo(tfd-COCF3)3). The electrical conductivity and the optical absorption are measured for different dopant concentrations in argon atmosphere, and their variations monitored as a function of the air exposure time. The results indicate a clear instability under ambient air related to a dedoping process, which is particularly pronounced in ultra-thin (< 50 nm) doped layers. By evaluating the stability of the p-doped polymer layers under different atmospheres (ambient air, anhydrous air and argon), the detrimental impact of moisture and/or O2(H20)n complexes is highlighted. X-ray Photoelectron Spectroscopy (XPS) revealed that the p-doping instability in ambient air can be assigned to changes in the oxidation state of the metallic center as well as to an intrinsic degradation of the dopant molecule. This study unravels an important degradation mechanism with this class of dopants that should be taken under consideration and solved for future integration of ultrathin p-doped layers in printed electronic devices.