# Tuning Thermal Stability through Dopant Size in Chemically Doped DPP–Thiophene Polymers

## TL;DR

This study shows that using larger dopants improves the thermal stability of doped polymers used in organic thermoelectric devices.

## Contribution

The paper introduces the use of dopant size as a strategy to enhance the thermal stability of doped conjugated polymers.

## Key findings

- F4TCNQ-doped DPP-T loses conductivity significantly after high-temperature annealing.
- Mo(tfd-CO2Me)3-doped DPP-T maintains stable conductivity under the same thermal stress.
- Dedoping in F4TCNQ-doped DPP-T is attributed to dopant phase separation, not sublimation.

## Abstract

Molecular doping
of conjugated polymers (CPs) is a key
strategy
for improving the performance of organic electronics devices, particularly
thermoelectrics. Doped donor–acceptor (D–A) conjugated
polymers, characterized by a tunable energy gap between the Fermi
level and the transport band, show great promise in achieving high
electrical conductivity (σ) while preserving a favorable Seebeck
coefficient (S). Despite the promising performance
enhancement of chemically doped D–A polymers, their thermal
stability remains largely underexplored, a crucial consideration for
the long-term operation of organic thermoelectric devices. In this
study, we investigated the dopant size-dependent thermal stability
of a diketopyrrolopyrrole-thiophene (DPP-T) D–A copolymer,
utilizing two p-dopants: 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane
(F4TCNQ) and Mo­(tfd-CO2Me)3. Temperature-dependent
UV–vis–NIR spectroscopy revealed that DPP-T/F4TCNQ is more prone to dedoping under a high temperature thermal stress
than DPP-T/Mo­(tfd-CO2Me)3. Although the F4TCNQ doped polymer shows higher initial in-plane conductivity
than its Mo­(tfd-CO2Me)3 counterpart, it undergoes
a conductivity loss of more than an order of magnitude after annealing
at 120 °C for 30 min. In contrast, the in-plane conductivity
of DPP-T/Mo­(tfd-CO2Me)3 remains stable under
the same thermal conditions. Thermogravimetric analysis ruled out
dopant sublimation as a primary contributor to dedoping, leading us
to attribute the conductivity loss in F4TCNQ-doped DPP-T
to dopant phase separation and migration. This observation was further
confirmed by X-ray scattering studies and nanoscale infrared microscopy
and spectroscopy studies. This work could provide further insights
into the thermal stability of doped conjugated polymers and suggests
that incorporating bulkier dopants is an effective strategy to enhance
the thermal robustness of doped DPP-type systems.