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Page 18 of 33 Girase et al. Energy Mater. 2025, 5, 500132 https://dx.doi.org/10.20517/energymater.2025.14

Doping mechanism and method
The enhancement of σ in CPs is highly dependent on the doping strategy employed. Redox doping and
acid/base doping are the two main doping mechanisms that have been commonly recognized as shown in
Figure 6A. Electron transfer between the CP and a dopant species governs the redox doping process and in
+
case of acid/base doping the CP interacts with acidic or basic species that can donate or accept protons (H )
-
or hydride ions (H ). Depending on the direction of charge movement, this electron exchange may result in
either n-type or p-type doping. In particular, n-type doping takes place when electrons transport from the
dopant’s HOMO to the polymer’s LUMO, whereas p-type doping happens when electrons move from
HOMO of CP to LUMO of the dopant. The efficiency of this doping process is critically influenced by the
relative alignment of the energy levels between the dopant and the polymer.

The selection of an appropriate dopant is one of the most important factors in optimizing the TE properties
of CPs because it directly influences critical parameters such as σ, S, and overall PF. For effective p-type
doping, the EA of the dopant must be greater than the ionization energy (IE) of the polymer (EA dopant > IE )
CP
for efficient charge transfer through the integer charge transfer (ICT) mechanism. Similarly, n-type doping
requires dopants with ionization energies lower than the EA of the polymer (EA > IE dopant ). This energy
CP
level compatibility not only enables efficient doping but also prevents the formation of trap states that
would hinder charge mobility.

The method by which dopants are introduced into TE polymers significantly influences doping efficiency,
[152,153]
morphology, and the resulting σ . Among the commonly employed methods, the mixing doping and
sequential doping methods are common techniques as shown in Figure 6B, in the mixing doping the dopant
and CP are individually dissolved in suitable solvents, then mixed together in specific ratios to produce a
homogeneous mixture. This one-step, simple process enables easy control of the doping level by simply
adjusting the concentration of the dopant. At higher doping concentration of the dopant, dopant molecules
form aggregates, inducing phase separation and disturbing the polymer microstructure. Such aggregation
can disturb the packing of polymer chains, reduce film crystallinity, and form trap states that eventually
hinder the mobility of charge carriers . Apart from that, miscibility of the dopant with the polymer and
[154]
dopant solubility in the chosen solvent system are major concerns to ensure a homogeneous doping effect.
For instance, the low solubility of high-electron-affinity dopants such as F4TCNQ in the majority of organic
solvents can restrict their applicability for the mixing approach. To address the limitations associated with
mixing doping, sequential doping methods have been developed. The sequential doping method has several
advantages over the mixing doping method [155,156] . In this method, the CP film is first cast and dried, then a
solution of dopant is deposited on top of it. In spin-coating, a minute quantity of the dopant solution
dissolved in a semi-orthogonal solvent that swells but doesn’t dissolve the polymer is dispensed on the
polymer surface and left to interact for a measured period of time. It is then spun rapidly to remove excess
solution, giving a controlled and homogeneous doping layer. This method minimizes disruption of the
native polymer morphology and helps preserve the chain packing and order, even at higher doping levels.
The spin-coating process enables accurate control over the doping level by adjusting key parameters such as
dopant concentration and contact time, resulting in enhanced electrical properties without compromising
film integrity. Overall, mixing and spin-coating-based SQD both have advantages and challenges, and their
selection should be guided by factors that include polymer-dopant compatibility, doping precision needed,
and process scalability.

p-type dopants
To improve the TE performance of CPs, a variety of p-type dopants have been used; each one has unique
benefits depending on its electronic characteristics and interactions with the polymer as shown in
Figure 7A. The EA of p-type dopants determines their efficiency in CP, which further governs charge