Tuning Thermal Stability through Dopant Size in Chemically Doped DPP-Thiophene Polymers.

Area of Science:

• Materials Science
• Organic Electronics
• Polymer Chemistry

Background:

• Molecular doping of conjugated polymers (CPs) is crucial for advancing organic electronics, especially thermoelectrics.
• Donor-acceptor (D-A) CPs offer tunable electronic properties for high electrical conductivity and Seebeck coefficient.
• The thermal stability of doped D-A polymers is critical for device longevity but remains under-investigated.

Purpose of the Study:

• To investigate the impact of dopant size on the thermal stability of diketopyrrolopyrrole-thiophene (DPP-T) D-A copolymers.
• To compare the thermal robustness of DPP-T doped with small F4TCNQ versus bulkier Mo-(tfd-CO2Me)3.
• To elucidate the mechanisms behind thermal degradation and dedoping in these materials.

Main Methods:

• Synthesis and doping of DPP-T with F4TCNQ and Mo-(tfd-CO2Me)3.
• Temperature-dependent UV-vis-NIR spectroscopy to monitor dedoping.
• In-plane conductivity measurements before and after thermal annealing.
• Thermogravimetric analysis (TGA) to assess dopant volatility.
• X-ray scattering and nanoscale infrared spectroscopy to study dopant behavior.

Main Results:

• DPP-T doped with F4TCNQ exhibited significant conductivity loss (>1 order of magnitude) after annealing at 120 °C.
• DPP-T doped with Mo-(tfd-CO2Me)3 maintained stable conductivity under identical thermal stress.
• UV-vis-NIR spectroscopy indicated F4TCNQ-doped polymer was more prone to dedoping.
• TGA ruled out dopant sublimation; phase separation and migration of F4TCNQ were identified as causes for dedoping.

Conclusions:

• Larger dopants, such as Mo-(tfd-CO2Me)3, significantly improve the thermal stability of doped DPP-type conjugated polymers.
• The thermal instability of F4TCNQ-doped DPP-T is attributed to dopant migration and phase separation.
• Incorporating bulkier dopants is a viable strategy to enhance the thermal robustness of organic thermoelectric materials.