Page 251 - Read Online

Girase et al. Energy Mater. 2025, 5, 500132 https://dx.doi.org/10.20517/energymater.2025.14 Page 21 of 33

In contrast to molecular redox dopants, acid dopants such as (tridecafluoro-1,1,2,2-
tetrahydrooctyl)trichlorosilane (FTS), PTSA (p-toluenesulfonic acid), 4-ethylbenzenesulfonic acid (EBSA),
and dodecylbenzenesulfonic acid (DBSA) operate via protonic (acid/base) doping methods and do not rely
on EA-driven redox process. Successful p-type doping of the high-mobility polymer poly[2,5-bis(3-
tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) with FTS and EBSA has been demonstrated,
[162]
providing an insight into non-redox doping mechanisms . FTS, specifically, functions via a protonation
mechanism facilitated by acidic silanol groups produced upon hydrolysis, which form a self-assembled
monolayer at the polymer surface and donate protons to the polymer backbone. This process enhances
doping efficiency without compromising the structural integrity and stability of the film. Among, FeCl has
3
emerged as the most useful and powerful p-type dopant for DPP-based TE materials, due to its strong
oxidative ability and the ability to enhance charge carrier concentration without disturbing the polymer’s
semi-crystalline order. In contrast to other molecular dopants such as F4TCNQ or CN6-CP, which are
prone to solubility or miscibility limitations in certain backbones, FeCl boasts remarkable compatibility in a
3
broad variety of DPP structures. For instance, FeCl -doped PDPP-4T-EDOT, PDPSS-12T, PDPPSe-12T,
3
and P29DPP-BTOM exhibit excellent TE performance. The improved performance is attributed to the
ability of the dopant to penetrate deep-lying HOMO levels of the DPP units and introduce holes effectively,
with the order of packing of the backbone for effective charge transfer.

n-type dopants
Effective n-type doping of OTE is essential to enhance σ, S, and thus the PF of CP. The chemical structure of
common n-type dopants is shown in Figure 7B. Among the extensively studied dopants, N-DMBI is the
most widely used n-type dopant. The E HOMO of N-DMBI is -4.44 eV, which is lower in energy than the E LUMO
of typical n-type conjugated polymers, making direct electron transfer unfavorable thermodynamically.
Instead, it follows other mechanisms of doping, mainly the hydrogen radical/electron transfer and hydride
transfer mechanisms . Thermal activation in the radical transfer mechanism enables cleavage of the C-H
[120]
bond of N-DMBI to form a hydrogen radical and a neutral N-DMBI radical. The electron from the energy
of singly occupied molecular orbitals E SOMO (-2.23 eV) of the N-DMBI radical is subsequently transferred to
the E LUMO of the polymer, leading to the formation of a stable N-DMBI cation. For the hydride transfer
+
mechanism, a hydride ion (H ) is heterolytically cleaved and transferred to the polymer. The two
-
mechanisms are energetically demanding, with enthalpy changes of 80.2 and 74.6 kcal mol for the
-1
formation of neutral hydrogen and hydride, respectively. Thus, thermal treatment (typically 80-180 °C) is
required to initiate doping and the efficiency is highly sensitive to the chemical structure of the
polymer [121-123] . In an attempt to overcome the solubility and miscibility problem of N-DMBI with polymers,
scientists have synthesized many molecular derivatives. One of these is N-DPBI (4-(1,3-dimethyl-2,3-
dihydro-1H-benzimidazol-2-yl)-N,N-diphenylaniline), wherein the dimethylamino group in N-DMBI is
[163]
replaced with a diphenylamino substituent . This modification was performed to stabilize the radical
species via resonance by phenyl rings. However, despite structural modification, N-DPBI showed similar
-3
-1
-1
-3
conductivity (σ ~4 × 10 S cm ) to that of N-DMBI (~8 × 10 S cm ) when used to dope N2200, largely due
to poor miscibility of dopants in the polymer matrix. TP-DMBI is another novelty where a 3,4,5-
trimethoxyphenyl unit is introduced into the N-DMBI backbone to enhance miscibility as well as electron-
[124]
donating capability . Further, a dimeric species of N-DMBI, (N-DMBI) , which had the advantages of
2
strong reducing ability and small cation size, was synthesized to deliver two electrons directly to the
polymer and form two N DMBI cations. This pathway significantly enhances doping efficiency with σ
+
~8 S cm in flouro-benzodifurandione-phenylenevinylene (FBDPPV) using only 10.7 mol% of (N-DMBI) ,
-1
2
[164]
while 43 mol% monomeric N-DMBI is required to achieve similar performance . Other notable n-type
dopants include Tetrakis(dimethylamino)ethylene (TDAE), a highly volatile compound with four electron-
donating amine groups connected by a central C=C bond with strong π-electron donating nature. The high
reducing ability of TDAE is advantageous for efficient doping in the vapor phase with conductivities of 2.4-