Thermally Activated n-Doping of Organic Semiconductors Achieved by N-Heterocyclic Carbene Based Dopant

Article abstract of DOI:10.1002/anie.202011537

Molecular doping plays an important role in the modification of carrier density of organic semiconductors thus enhancing their optoelectronic performance. However, efficient n-doping remains challenging, especially owing to the lack of strongly reducing and air-stable n-dopants. Herein, an N-heterocyclic carbene (NHC) precursor, DMImC, is developed as a thermally activated n-dopant with the excellent stability in air. Its thermolysis in situ regenerates free NHC and subsequently dopes typical organic semiconductors. In sequentially doped FBDPPV films, DMImC does not disturb the π–π packing of the polymer and achieves good miscibility with the polymer. As a result, a high electrical conductivity of up to 8.4 S cm^?1 is obtained. Additionally, the thermally activated doping and the excellent air stability permit DMImC to be noninteractively co-processed with polymers in air. Our results reveal that DMImC can be served as an efficient n-dopant suitable for various organic semiconductors.

Full text of DOI:10.1002/anie.202011537

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Accepted Article
Title: Thermally Activated n-Doping of Organic Semiconductors
Achieved by N-heterocyclic Carbene Based Dopant
Authors: Yi-Fan Ding, Chi-Yuan Yang, Chun-Xi Huang, Yang Lu, Ze-
Fan Yao, Chen-Kai Pan, Jie-Yu Wang, and Jian Pei
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Thermally Activated n-Doping of Organic Semiconductors
Achieved by N-heterocyclic Carbene Based Dopant
Yi-Fan Ding, Chi-Yuan Yang, Chun-Xi Huang, Yang Lu, Ze-Fan Yao, Chen-Kai Pan, Jie-Yu Wang and
Jian Pei*
[*]
Y.-F. Ding, C.-Y. Yang, C.-X. Huang, Y. Lu, Z.-F. Yao, C.-K. Pan, Prof. J.-Y. Wang, Prof. J. Pei
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft
Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University,
Beijing 100871 (China)
E-mail: jianpei@pku.edu.cn
Supporting information for this article is given via a link at the end of the document.
[
7]
Abstract: Molecular doping plays an important role in the modification
of carrier density of organic semiconductors thus enhancing their
optoelectronic performance. However, efficient n-doping remains
challenging, especially due to the lack of strongly reducing and air-
stable n-dopants. Herein, an N-heterocyclic carbene (NHC) precursor,
DMImC, is developed as a thermally activated n-dopant with the
excellent stability in air. Its thermolysis in situ regenerates free NHC
and subsequently dopes typical organic semiconductors. In
sequentially doped FBDPPV films, DMImC doesn’t disturb the π-π
packing of the polymer and achieves good miscibility with the polymer.
semiconductors. However, some of them are gradually oxidized
[
8]
or decomposed in the solution under ambient condition. Hence,
more efforts are needed to develop air-stable yet strong reducing
n-dopants for the development of n-doped organic semiconductor
devices.
As strong σ donors, N-heterocyclic carbenes (NHCs) have
become essential ligands in organic chemistry, with numerous
applications in coordination chemistry and organocatalysis.
Würthner and co-workers demonstrated free NHC reacted with
chlorinated perylene bisimide to yield delocalized zwitterion
[
9]
-
1
As a result, a high electrical conductivity of up to 8.4 S cm is obtained.
Additionally, the thermally activated doping and the excellent air
stability permit DMImC to be noninteractively co-processed with
polymers in air. Our results reveal that DMImC can be served as an
efficient n-dopant suitable for various organic semiconductors.
[
10]
radical. This implies like hydride dopants that offer nucleophilic
[
11]
and electron-donating hydride, NHCs with strong nucleophilic
and electron-donating ability can act as reactive species to dope
organic semiconductors. Herein, we develop an air-stable NHC
precursor,
namely
1,3-dimethylimidazolium-2-carboxylate
(
DMImC) (Scheme 1a), as a thermally activated n-dopant.
Molecular doping has been a powerful tool to improve the
performance of organic semiconductor devices, in particular to
modulate the electrical conductivity.[1] The intrinsic organic
conjugated materials are typically the insulators or
semiconductors with low carrier density, molecular doping is
usually employed to enhance their carrier density and
conductivity.[ P-doped conjugated polymers, such as poly[2,5-
bis(3-alkylthiophen2-yl)thieno(3,2-b)thiophene] (PBTTT) and
poly(3,4-ethylenedioxythiophene) (PEDOT), have exhibited
impressive electrical conductivities up to 1000 S cm . However,
n-doped conjugated polymers still show far more inferior
conductivities than their p-type counterparts. One of the major
limitations is the scarcity of air-stable n-dopants with strong n-
doping ability. To realize an efficient n-doping, the ionization
energy (IE) of dopants should be close to or lower than the
electron affinity (EA) of organic semiconductors.[3] However, the
shallow IE of n-dopants results in their high sensitivity to air, which
represents a major challenge to develop air-stable n-dopants for
organic semiconductors.[3a, 4]
DMImC, an adduct of carbon dioxide and NHC, regenerates the
corresponding NHC (1,3-dimethylimidazolylidene) via thermal
activation, which subsequently in situ dopes organic
semiconductors. DMImC exhibits excellent stability and good
solubility in common polar solvents, facilitating its practical
solution-processability. The small molecular volume of DMImC is
beneficial for less perturbation of the polymer morphology and
efficient incorporation into the underlying polymer layers in
1a]
[
12]
sequential casting.
Further investigations demonstrate that
-
1 [2]
NHC released from DMImC after activation can effectively n-dope
representative n-type organic semiconductors (Figure S1) to
obtain high electrical conductivity.
The reaction between 1-methylimidazole and dimethyl
carbonate yielded the desired product, DMImC (Scheme 1b).
Thermal gravity analysis (TGA) of DMImC afforded a moderate
decarboxylation temperature of 159 oC (Figure S2). DMImC
showed good air stability and solution stability at room
temperature (Figure S3). Upon heating, DMImC reacted with
N,N’-dibutyl-1,6,7,12-tetrachloroperylene-3,4:9,10-
Recently, organic precursors have been proposed to
tetracarboxylic acid bisimide (PBI-Cl
4
) to yield the PBI-centered
[
5]
circumvent the stability limitation of n-dopants.
These
radical anion (Scheme 1c), which was the same with the reported
[
10]
precursors that liberate strong reducing species in situ upon
external stimulus avoid using low IE n-dopants directly, increasing
their air stability. Organic salts (including o-MeO-DMBI-I and PyB)
exhibited good n-doping ability for organic semiconductors but
reaction of free NHC with PBI-Cl
4
.
The absence of hyperfine
splitting in the electron paramagnetic resonance (EPR) spectrum
and calculated spin density plot suggested that the unpaired
electron was delocalized over the whole PBI skeleton (Figure S4).
Above all, DMImC can be used as a promising thermally activated
n-dopant to in situ generate NHC for n-doping.
[
6]
were processed via vacuum deposition in most cases. Besides,
a series of organic radical dimers and organic hydrides showed
the excellent capability to n-dope
a variety of organic
1
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showed that after doping, FBDPPV presented electron density of
2
0
2
1
.33  10 cm-3 at DMImC concentration of 0.4 mg mL-1 (Figure
-
1
c). Further increasing DMImC concentration to 1.0 mg mL led
2
0
-3
to an increased carrier density of 7.26  10 cm . Based on
ultraviolet photoelectron spectroscopy (UPS) measurement, the
Fermi level of doped FBDPPV films shifted toward the lowest
unoccupied molecular orbital (LUMO) (Figure 1d), confirming the
[
11]
effective occurrence of n-doping. When the concentration of
-
1
DMImC reached 0.6 mg mL , the Fermi level shift seemingly
saturated, which perhaps arose from the pinning effect.[14] Hence,
we prove that the doping level of FBDPPV film can be controlled
by varying the DMImC concentration in the casting solution.
Scheme 1. a) Decarboxylation of DMImC to NHC and b) the synthetic approach
to DMImC. c) The reaction between PBI-Cl
4
and DMImC under heating. A
delocalized PBI-centered radical anion was yielded, in which the cationic
imidazolium was introduced in the bay position of PBI-Cl
of polymer FBDPPV.
4
. d) Chemical structure
Due to the poor solubility of DMImC in the commonly used
halogenated solvent, sequential casting was applied to prepare
the DMImC-doped systems, in which DMImC was deposited from
acetonitrile (an orthogonal solvent for organic semiconductors)
onto the underlying polymer layer. FBDPPV was chosen as the
host polymer for the subsequent studies (Scheme 1d), which was
[
13]
renowned for its high n-type field-effect mobility. The activation
o
temperature of 160 C was used as an external stimulus to trigger
the decarboxylation of DMImC, followed by the n-doping process
(
Figure S5).
The doping process of FBDPPV with DMImC was firstly
characterized by the UV-vis-NIR absorption spectra (Figure 1a).
The intrinsic FBDPPV film displayed two absorption bands (band
I: 600-900 nm; band II: 400-600 nm) and no absorption features
exceeding 1000 nm. Upon doping, a new absorption band around
Figure 1. a) UV-vis-NIR absorption spectra and b) EPR spectra of the intrinsic
and doped FBDPPV films after thermal activation of DMImC. c) Molar fractions
of dopant relative to FBDPPV repeat unit (calculated from the N(1s) XPS
results) and the carrier density measured by the AC Hall effect as a function of
DMImC concentration in the casting solution; d) UPS spectra around the
secondary electron cutoff (SECO) regions of FBDPPV films.
1100 nm, which was attributed to the (bi)polaron absorption, was
observed, accompanied by the bleaching of the neutral
absorption. Polarons carrying unpaired electrons can be directly
[
1a]
observed by the EPR spectra. Doped FBDPPV film yielded an
obvious paramagnetic signal with a characteristic electron g-
factor of about 2.0039 (Figure 1b), indicating the generation of
polarons. All these results suggest that NHC released in situ from
DMImC can efficiently dope FBDPPV.
To demonstrate the practical application of DMImC in organic
electronics, electrical conductivity (σ), Seebeck coefficient (S) and
2
power factor (푃퐹 = 푆 휎) of doped FBDPPV films were measured
(
Figure 2a and 2b). Upon doping, the electrical conductivity of
FBDPPV dramatically increased with a maximal film conductivity
of 8.4 ± 0.6 S cm-1 at the DMImC concentration of 0.5 mg mL-1.
Additionally, a high conductivity was also obtained in FBDPPV
films doped by another NHC based n-dopant (DPImC) with N-
isopropyl substituent (Figure S9), indicating that the strategy of
thermally activated NHC based n-dopant was universally
N(1s) X-ray photoemission spectroscopy (XPS) showed that
after the thermal activation of DMImC, a quadrivalent nitrogen
signal from the dopant cation arose at 402.7 eV (Figure S6), which
was distinct from the original lactam nitrogen signal of FBDPPV
(
400.9 eV). The peak area ratio between these two peaks was
-
1
used to monitor the DMImC incorporation upon sequential casting. applicable. At DMImC concentration of 0.4 mg mL , doped
DMImC with concentrations of 0.4, 0.6 and 1.0 mg mL-1 in casting
solution resulted in 0.36, 0.40 and 0.52 dopants per repeat unit
FBDPPV exhibited a negative Seebeck coefficient of -182 µV K−1
and the coefficient further decreased with higher DMImC
concentrations. Due to the high electrical conductivity and
moderate Seebeck coefficient, an optimized power factor as high
(
Figure 1c), respectively. In a word, the higher DMImC
concentration permitted more DMImC to infiltrate into FBDPPV
films and offered an improved doping level. This conclusion was
also supported by the increased carrier density and Fermi level
shift in doped FBDPPV films. The AC Hall effect measurements
as 16 W m K-2 at room temperature was achieved. The
conductivities of another two famous n-type semiconductors
N2200 and PCBM with higher LUMO levels than FBDPPV were
-1
2
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also investigated to demonstrate the doping capability of DMImC
scattering of lamellar packing were still clearly observed (Figure
3b). Along with the increased DMImC concentrations, the lamellar
packing distance of FBDPPV gradually increased from 36.5 Å to
38.3 Å, but the π-π packing distance stayed at 3.44 Å (Figure
S14). The unchanged π-π packing distance which played a vital
role in interchain carrier transport prevented the Hall mobility of
doped films from a dramatic decrease as the concentration of
DMImC increased (Figure S15b). The surface morphology of
doped FBDPPV films was investigated by atomic force
microscopy (AFM). The doped FBDPPV films displayed uniform
morphology and fiber-like networks, which were the same as
those undoped films. Moreover, no aggregated particles were
observed on the surface of annealed films even at the high
(
Figure 2c). After doping, N2200 and PCBM presented the
maximum conductivities of (5.4 ± 0.6)  10 S cm-1 and 0.10 ±
-
3
-
1
0
.02 S cm , respectively. The conductivities of DMImC-doped
systems were among the highest reported values doped by the
best n-dopants nowadays (Figure S1 and S12), assuring that
DMImC was a high-performance n-dopant for commonly used n-
type organic semiconductors.
In most cases, the n-doped organic semiconductor devices
were prepared in an inert atmosphere, increasing the complexity
of device fabrications. The n-doping by thermally activated
dopants involves a two-step process, deposition and post-
deposition annealing.[15] Given the excellent air stability and non-
interaction with semiconductors before heating, DMImC can be
noninteractively sequentially deposited onto FBDPPV films in air.
The air stability of the n-doped films, which is seriously affected
-
1
DMImC concentration of 1.0 mg mL . The homogeneous
morphology of doped FBDPPV films indicated the good miscibility
between dopant and polymer, which was beneficial to high doping
[
17]
efficiency.
Since the electrical conductivity depends on both
2 2
by H O or O in air, is more dependent on the properties of n-type
mobility and carrier density, the undisrupted π-π packing and
good miscibility contribute to the high electrical conductivity of
doped FBDPPV.
semiconductors, especially their electron affinity and crystallinity
[
16]
of polymer films. Since the n-doped FBDPPV thin films were
sensitive to air (Figure S13),[7h] the subsequent annealing of
doped FBDPPV thin films was performed in an inert atmosphere.
FBDPPV films sequentially deposited with DMImC in air showed
a maximal conductivity of 7.0 ± 0.7 S cm-1 (Figure 2d), which was
comparable to those prepared under inert gas. The combination
of DMImC and post-deposition annealing provide the feasibility of
deposition and handling the n-doped films in air, e.g., lithography
and spin coating.
Figure 3. Microstructure and morphology study of the intrinsic and doped
FBDPPV films. a,b) GIWAXS images and c,d) surface topography mappings by
AFM.
In summary, we have developed an air-stable thermally
activated NHC based n-dopant, DMImC, for solution-processed
n-doping. This thermally activated n-dopant provides several
advantages: (1) simultaneously satisfying the requirement of
excellent air stability and high n-doping efficacy; (2) easily
triggered n-doping process by thermal activation; and (3) avoiding
the formation of polymer/dopant ions when mixed directly and
allowing to be noninteractively deposited with polymers even in
air. Contribution from the good miscibility with polymer and
unchanged π-π stacking distance in sequentially doped FBDPPV
films benefit to high doping efficiency and efficient interchain
carrier transport, which results in a high film conductivity.
Additionally, FBDPPV films sequentially deposited with DMImC in
Figure 2. Thermoelectric performance characterization. a) Electrical
conductivity, Seebeck coefficient and b) power factor of doped FBDPPV with
various DMImC concentrations. c) Conductivities of doped N2200 and PCBM
with various DMImC concentrations. d) Conductivities of doped FBDPPV films
sequentially deposited with DMImC in air.
Grazing incidence wide-angle X-ray scattering (GIWAXS) was
performed to analyze the solid-state microstructures of doped
-
1
FBDPPV films. When the concentration (1.0 mg mL ) of DMImC
far exceeded the optimum concentration for obtaining the
maximal conductivity, the signs of π-π packing (010) and multiple
3
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Keywords: n-dopant • N-heterocyclic carbene precursor •
thermally activated n-doping • n-type conductivity
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Angewandte Chemie International Edition
10.1002/anie.202011537
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Based on N-heterocyclic carbene, a thermally activated n-dopant with excellent air stability and high n-doping efficacy is developed,
which can in situ regenerate N-heterocyclic carbene by decarboxylation for the subsequent n-doping. The strategy of thermally activated
n-doping not only increases the air stability of n-dopants, but also permits n-dopants to be co-processed with polymer FBDPPV
noninteractively before heating in air.
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