The formation of polymer-dopant aggregates as a possible origin of limited doping efficiency at high dopant concentration

Abstract

The 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) p-doped with the molecular dopant tris[1-(trifluoroethanoyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd-COCF3)3) exhibits a decline in transport properties at high doping concentrations, which limits the performance attainable through organic semiconductor doping. Scanning Electron Microscopy is used to correlate the evolution of hole conductivity and hopping transport activation energy with the formation of aggregates in the layer. Transmission Electron Microscopy with energy-dispersive X-ray analysis along with liquid-state Nuclear Magnetic Resonance experiments are carried out to determine the composition of the aggregates. This study offers an explanation to the limited efficiency of doping at high dopant concentrations and reinforces the need to increase doping efficiency in order to be able to reduce the dopant concentration and not negatively affect conductivity. (A. Revaux). ! 1Molecular p-dopants as tetracyano-quinodimethane (TCNQ) derivatives or dicyanodichloro-quinone (DDQ) have been introduced in various polymers, showing effective p-doping ability [9,10]. Increased conductivity, carrier mobility and hole density have been reported for multiple polymer-dopant mixtures [7,11]. However, to reach sufficient doping impact, high concentrations of molecular dopants (several % in molar ratio) need to be added to the polymer matrix, in contrast to inorganic semiconductors where concentrations of the order of 10 -6 -10 -3 dopant per semiconductor atom are required [12]. High concentrations of organic molecules can lead to a degradation of the transport properties through the formation of defects [13,14]. The conductivity saturation or decline above a certain dopant concentration threshold has been widely reported in the literature [15][16][17]. As organic electronic devices require efficiently doped layers, it is crucial to understand the origins of the limited doping efficiency in order to unlock the boundaries of organic semiconductor doping.In this study, we use a soluble derivative of Mo (tfd)3, i.e. tris[1-(trifluoroethanoyl)-2-(trifluoromethyl)ethane-1,2dithiolene] (Mo(tfd-COCF3)3), to p-dope the polymer Poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5b')dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene-)-2-6-diyl)] (PBDTTT-c). Electrical characterizations are carried out as a first step to determine the evolution of the hole conductivity and hopping transport activation energy with the doping concentration, highlighting the lower doping efficiency at high doping concentration. Scanning Electron Microscopy (SEM) is used to correlate the electrical characteristics with changes in the layer morphology. Finally, Transmission Electron Microscopy (TEM) with energy-dispersive X-ray (EDX) analysis and liquid state Nuclear Magnetic Resonance (NMR) are carried out to further understand the morphology evolution. Experimental method...

Molecular Doping of a Water‐Soluble Polythiophene Derivative

“…We assume that, at least no detectable, CT occurs in solution. The precursor solution can be filtrated by nanoporous (200 nm) syringe filters indicating that no aggregates or non‐soluble compounds are formed, as it was reported for other polythiophene derivatives . However, the absorptions of the thin films (thickness 40–60 nm) change distinctly with increasing MDR.…”

“…The high concentration of impurities and the charged species in the polymer matrix can cause defects, distortion or aggregation, which disturb the self‐assembly behavior. As a result, traps and scattering centers for the free charge carriers are present in the polymer matrix, which decreases the charge carrier mobility …”

Toward a better understanding of the doping mechanism involved in Mo(tfd-COCF3)3 doped PBDTTT-c

“…2. In a previous study, 28 we have provided evidence of the effective p-doping of PBDTTT-c with this p-dopant and highlighted the limited doping efficiency at high doping concentration.…”

Synthesis of luminescent polymers in the UV light region from dimethacrylate monomer using novel quinoline dyes

“…Mid‐infrared spectroscopy (MIR) has been used to indicate the degree of conversion and rate of polymerization in the photopolymerization process when using different photoinitiators . The morphological characteristics of polymers are widely analyzed by scanning electron microscope (SEM), and solid UV–vis analysis is used to determine optical band gap and modifications in absorption bands …”

“…7,10,15,16 Midinfrared spectroscopy (MIR) has been used to indicate the degree of conversion and rate of polymerization in the photopolymerization process when using different photoinitiators. [6][7][8][9][10][11][12] The morphological characteristics of polymers are widely analyzed by scanning electron microscope (SEM), [17][18][19] and solid UV-vis analysis is used to determine optical band gap and modifications in absorption bands. 20,21 The main objective of this research is to calculate the degree of conversion in the photopolymerization using new quinoline derivatives and verify the thermal and fluorescence properties of final polymers.…”