Porous Organic Polymer Films with Tunable Work Functions and Selective Hole and Electron Flows for Energy Conversions

Article abstract of DOI:10.1002/anie.201510723

Organic optoelectronics are promising technologies for energy conversion. However, the electrode interlayer, a key material between active layers and conducting electrodes that controls the transport of charge carriers in and out of devices, is still a chemical challenge. Herein, we report a class of porous organic polymers with tunable work function as hole- and electron-selective electrode interlayers. The network with organoborane and carbazole units exhibits extremely low work-function-selective electron flow; while upon ionic ligation and electro-oxidation, the network significantly increases the work function and turns into hole conduction. We demonstrate their outstanding functions as anode and cathode interlayers in energy-converting solar cells and light-emitting diodes.

Full text of DOI:10.1002/anie.201510723

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201510723
German Edition: DOI: 10.1002/ange.201510723
Semiconducting Films
Porous Organic Polymer Films with Tunable Work Functions and
Selective Hole and Electron Flows for Energy Conversions
+
+
+
Cheng Gu , Ning Huang , Youchun Chen , Huanhuan Zhang, Shitong Zhang, Fenghong Li,
Yuguang Ma, and Donglin Jiang*
Abstract: Organic optoelectronics are promising technologies
for energy conversion. However, the electrode interlayer, a key
material between active layers and conducting electrodes that
controls the transport of charge carriers in and out of devices, is
still a chemical challenge. Herein, we report a class of porous
organic polymers with tunable work function as hole- and
electron-selective electrode interlayers. The network with
organoborane and carbazole units exhibits extremely low
work-function-selective electron flow; while upon ionic liga-
tion and electro-oxidation, the network significantly increases
the work function and turns into hole conduction. We
demonstrate their outstanding functions as anode and cathode
interlayers in energy-converting solar cells and light-emitting
diodes.
have a high enough work function for the injection of holes
into, or the collection of hole from, the highest occupied
molecular orbital (HOMO) of active layers. Therefore,
electrode interlayers require the design of work function
into extreme levels that are either sufficiently high or
substantially low. However, materials that meet these require-
ments are chemically unstable, corrosive, or synthetically
[1c]
difficult to prepare.
Porous organic polymers (POPs) are a unique class of
polymers that combine covalent p-networks with inherent
[
2]
porosity. POPs are a class of emerging multifunctional
materials and show outstanding properties and functions in
[3]
[4]
various applications, such as gas storage, light emitting,
[
5]
[6]
[7]
energy transfer, catalysis, and energy storage. They are
highly stable as a result of a cross-linked network structure,
and can be synthesized using a variety of different p-units.
However, POP-based carrier-selective transporting materials
remain to be explored.
O
rganic energy-converting devices possess great economic
potential and have a promising future in leading to a new
generation of consumer devices that combine low cost,
[
1]
lightweight, and mechanical flexibility. However, the elec-
trode interlayer, a key material that determines not only the
charge carrier flow but also the device performance, remains
Herein, we report the development of p-electronic POPs
as a platform for designing electrode interlayers that feature
robust chemical stability, tunable work function, and out-
standing performance for energy conversions. We designed
a basic structure, that is, BC (borane appended with
carbazoles), which bears a tris(2,3,5,6-tetramethylphenyl)
borane (TPB) focal core with three N-substituted carbazole
groups at the periphery as electroactive units (Figure 1a;
Supporting Information). The polyborane carbazole (PBC)
films were prepared and fabricated through electropolymer-
ization with concurrent polymer-film deposition, which has
proven to be a useful method for the preparation of electro-
[1c]
a substantial challenge. An electron-transporting interlayer
should have a sufficiently low work function for either the
injection of electrons into, or the collection of electrons from,
the lowest occupied molecular orbital (LUMO) of active
materials. In contrast, a hole-transporting interlayer should
[
+]
[+]
[
*] Dr. C. Gu, N. Huang, Prof. Dr. D. Jiang
Department of Materials Molecular Science
Institute for Molecular Science
National Institutes of Natural Sciences
[8]
active and conductive polymer films. Upon electropolymer-
5
-1 Higashiyama, Myodaiji, Okazaki 444-8787 (Japan)
ization, the N-substituted carbazole units form cation radical
species that are highly active in coupling with each other, and
form dimers with near-complete efficiency without the
E-mail: jiang@ims.ac.jp
[
+]
Y. Chen, H. Zhang, S. Zhang, Prof. F. Li
State Key Laboratory of Supramolecular Structure and Materials
Jilin University
[
8]
generation of other oligomers. The resulting PBC thin
films were formed on electrodes (ITO, Au, ZnO, and
PEDOT:PSS) through multi-cycle cyclic voltammetry (CV)
Changchun 130012 (P.R. China)
Prof. Y. Ma
À4
State Key Laboratory of Luminescent Materials and Devices
Institute of Polymer Optoelectronic Materials and Devices
South China University of Technology
Guangzhou 510640 (P.R. China)
of the BC monomer (4.4 ” 10 m) in a mixture of acetonitrile
and dichloromethane (1:4 v/v) that contained tetrabutylam-
monium hexafluorophosphate (TBAPF ) electrolyte in air at
6
room temperature. During the positive CV scan, the onset
oxidative potential was observed at 0.88 V, which was
Prof. Dr. D. Jiang
Present address: Academic Field of Energy and Environment
School of Materials Science
[8]
attributed to the oxidation of the carbazole, whereas the
oxidation of the TPB unit was observed at 1.40 V(Supporting
Information, Figure S1). During the negative scan, two
reductive peaks were observed at 0.92 and 0.64 V, which
were assigned to the reduction of the cations of TPB and
dimeric carbazole, respectively. Consequently, we set up
a potential range between À0.8 and 1.0 V for the film
Japan Advanced Institute of Science and Technology
1
-1 Asahidai, Nomi 923-1292 (Japan)
E-mail: djiang@jaist.ac.jp
+
[
] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201510723.
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À1
of 400 mVs (Figure 1c). High-resolution transmission elec-
tron microscopy (HR-TEM) and Kr adsorption isotherm
measurements confirmed that the films consisted of homo-
geneous porous networks (Figure S5), with a Brunauer–
2
À1
Emmett–Teller (BET) surface area of 1074 m g
a pore size of 1.5 nm (Figure S6). Atomic force microscopy
AFM) measurements revealed that a 10-nm-thick PBC film
and
(
uniformly covered the ITO surface and exhibited a smooth
morphology over a large area, with a root-mean-square
(RMS) roughness of only 0.85 nm (Figure S7). The roughness
of the films remained constant, regardless of the film thick-
ness and oxidative state (see below) of the films (Figure S7).
The smooth film morphologies fully satisfy the requirements
for optoelectronic devices.
À
The ligation of F ions to the borane units was conducted
by immersing the PBC Films in THF solutions that contained
various concentrations of (n-C H ) NF, followed by multiple
4
9 4
rinses with THF. The porous structure of the PBC film
allowed for an easy access of fluorine ions to the boron atoms,
and resulted in homogeneous ligation in the network. The
XPS spectra of the resulting FPBC films revealed the
disappearance of the B1s peak at a high binding energy of
1
89.4 eV and the appearance of a 185.5-eV peak at a low
À
binding energy (Figure S8a), indicating the F coordination
and the subsequent reduction in the electron-accepting ability
of the FPBC films. The XPS spectra further revealed that the
content of fluorinated borane units was precisely controlled
À
by adjusting the concentration of F and the immersion time
(
Table S1). The fully fluorinated films were electro-oxidized
Figure 1. a) Representation of the design of monomer and preparation
of POP films through electropolymerization. Polymerization of BC
resulted in a low work function PBC film that possesses an extremely
to transform the dimeric carbazole units into their cations,
which are known to be high work function moieties. When
a positive potential at 0.95 V was applied, the FPBC films
were oxidized to yield dimeric carbazole cations and ClO4
[9]
low HOMO level and serves as an electron-selective electrode. Upon
À
À
F
ligation to the boron atoms and electro-oxidation of the carbazole
counter ions. XPS measurements revealed that the contents of
dimeric carbazole cations in the oxidized FPBC (OFPBC)
films could be tuned by regulating the oxidative potential,
whereas the fluorinated borate units remained unchanged
units, the resulting OFPBC films greatly raise the HOMO level and
function as hole-selective electrodes. b) CV curves of BC recorded for
1
0 scan cycles (potential from À0.2 to 1.0 V). c) The thicknesses of the
À1
PBC films under different numbers of CV cycles (scan rate: 0.4 Vs ).
(Figure S8b–d, Table S1). This solution-process procedure is
widely applicable to different substrates, including ITO, Au,
À
preparation. A positive potential in this range could only
oxidize the carbazole units, whereas a sufficiently low
negative potential ensured the complete reduction of the
dimeric carbazole cations to neutral carbazoles and the
ZnO, and PEDOT:PSS. The F ion coordination and electro-
oxidation significantly changed the absorption and emission
properties of the films (Figure S3b,c).
To investigate the work functions of these POP films, we
measured ultraviolet photoelectron spectroscopy (UPS) of
20 nm-thick PBC, FPBC, and OFPBC films deposited on
different conducting electrodes. Figure 2a summarizes the
UPS results. The PBC films possess an extremely low work
function, whereas the fluorine-ligated FPBC and oxidized
OFPBC films increase their work functions over a wide range
to achieve a substantially higher work function.
À
[8]
removal of their counterions (PF ) from the films. From
6
the second cycle in the continuous CV scan (Figure 1b), a new
oxidative peak appeared at 0.76 V, which is attributed to the
[8]
oxidation of dimeric carbazoles. The current increased as
the cycle number increased, which suggests that more dimeric
carbazoles formed and that the PBC films grew on the
electrodes upon polymerization.
Infrared, electronic absorption, and X-ray photoelectron
spectroscopy (XPS) revealed that the content of monomeric
carbazole units in the films was negligible (Figures S2, S3a,
and S4). One significant feature is that this electropolymeri-
zation-based synthetic method enables control over the film
thickness. Indeed, a series of thin films were successfully
synthesized by simply changing the cycle numbers. From the
plot of film thickness versus CV cycle number, each CV cycle
produced approximately 1 nm of film thickness at a scan rate
ITO is a conducting substrate with a work function of
4.78 eV; this medium work function value renders it incapable
of either electron- or hole-selective conduction. In optoelec-
tronics, electrodes with selective electron and hole conduc-
tions are essential. When ITO was covered with a 20-nm-thick
PBC film, a sufficiently low work function of 4.05 eV was
achieved (Figure 2b). This low work function endows the
electrode with selective electron transporting functions.
Notably, the work function greatly increased to 4.48 and
3
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and conventional organic solar cells
cOSCs) using PBC and OFPBC on
(
ITO as anodes and cathodes, respec-
tively. In both cells, poly(thieno[3,4-b]-
thiophene/benzodithiophene) (PTB7)
and [6,6]-phenyl C -butyric acid
71
methyl ester (PC BM) (1:1.5 by
71
weight) were used as the photoactive
[10]
layer. In iOSCs, the MoO /Al func-
3
tioned as anode; in cOSCs, LiF/Al
acted as cathode. The device config-
urations of iOSCs and cOSCs were
Figure 2. Secondary electron cut-off obtained by UPS for a) PBC, FPBC, and OFPBC films; b) ITO,
Au, and PEDOT:PSS samples with and without 20 nm-thick PBC films; and c) ITO, Au, and ZnO
samples with and without 20 nm-thick OFPBC films. The shift represents the change of the work
function.
ITO/PBC film (15 nm)/PTB7:PC BM
71
(100 nm)/MoO3 (10 nm)/Al (100 nm)
and ITO/OFPBC film (10 nm)/
PTB7:PC BM (100 nm)/LiF (0.5 nm)/
71
5
.25 eV as the POP film was changed to FPBC and OFPBC,
Al (100 nm), respectively (Figure 3a,d). We optimized the
film thickness of PBC and OFPBC films and the dimeric
carbazole cation content of the OFPBC films to yield the best
device performance (Figures S11,S12). Figure 3b and e pres-
ents the J–V characteristics of the iOSC and cOSC in the dark
and under illumination. The J–V characteristics in the dark
reveal large rectification and a small reverse saturation
current, which demonstrates the electron- and hole-selectivity
of the PBC and OFPBC films, respectively. The iOSCs with
PBC-coated ITO electrodes (Figure 3c) yielded a power-
conversion efficiency (PCE) of 6.88% (open-circuit voltage
respectively (Figure S9). Indeed, the OFPBC film on ITO led
to a work function that was high enough to serve as a hole-
selective electrode (Figure 2c). We further investigated the
effect of POP films on other typical conducting substrates,
including Au, ZnO, and PEDOT:PSS. For this purpose, we
prepared PBC, FPBC, and OFPBC films on these substrates,
respectively. A 20 nm-thick PBC film on Au and PEDOT:PSS
substantially decreased their work functions from 4.93 to
4
.14 eV and from 5.05 to 4.05 eV, respectively (Figure 2b,
Table S2). These decrements enabled their use for selective
electron conduction. Therefore, the PBC films sufficiently
reduced the work functions of the conducting substrates to
yield low work function electrodes, which facilitate the
injection or collection of electrons. In contrast, a 20 nm-
thick OFPBC film increased the work functions from 4.93 to
(VOC) = 0.750 V, short-circuit current density (J ) =
SC
À2
15.80 mAcm , and fill factor (FF) = 0.581). These large
VOC and JSC values are attributable to the following two
factors: 1) the low work function of PBC films minimized the
electron-collecting barrier in the iOSCs, and 2) the large
surface area of the porous PBC films vastly improved the
contact with the photoactive layer and minimized the contact
resistance (Table S3). For comparison, we further fabricated
iOSCs using traditional ZnO nanoparticles as cathode
interlayer, while keeping otherwise same cell configurations.
This control iOSC exhibited a PCE of 6.33% (Figure S13a).
Clearly, the PBC films exhibited higher performance
compared to conventional ZnO nanoparticle (6.88% vs.
6.33%).
5
.23 eV for Au and from 4.33 to 5.11 eV for ZnO, respectively
(
Figure 2c, Table S2). These results clearly indicate that the
OFPBC films serve as high work function interlayers that
allow for injection or collection of holes in optoelectronic
devices.
The thermal and air stabilities of PBC and OFPBC films
on ITO substrates were studied by UPS measurements. The
work functions of neither the PBC nor the OFPBC electrodes
exhibited any change up to 2008C (Figure S10a). Both the
PBC- and OFPBC-coated ITO electrodes were fairly stable
under ambient conditions for more than 6 weeks; the work
function variations were less than 0.3 eV (Figure S10b).
To investigate whether the thickness of the thin film
affects the work functions, we prepared a variety of PBC and
OFPBC films with different thicknesses ranging from 2 to
On the other hand, the PCE of cOSC with OFPBC-coated
ITO electrodes was as high as 7.93% (Figure 3 f; V
=
OC
À2
0.775 V, JSC = 16.12 mAcm , and FF = 0.635). This large FF
value also provided evidence of the good hole-selectivity of
the OFPBC films. To date, the PTB7:PC BM-based cOSCs
71
[10]
exhibit a PCE of 5–7%. Our cOSCs give rise to the highest
efficiency for cOSCs. We also prepared control cOSC by using
PEDOT:PSS as the anode, while otherwise keeping the same
cell configurations for comparison. The control cOSC exhib-
ited a PCE of 7.23% (Figure S13b). These results again
indicate that our films are superior to the conventional
interlayer materials. To our knowledge, the PCEs of the
devices are among the highest reported in solar cells with
2
4
0 nm on ITO. The work function decreased to be lower than
.1 eV when the PBC film was thicker than 15 nm, whereas
the work function increased to be greater than 5.2 eV when
the thickness of the OFPBC film was larger than 15 nm
(
Figure S10c). These observations demonstrated that the POP
films with thicknesses of only 15 nm were sufficient to enable
the selective flow of holes and electrons.
[8a]
To investigate the performance of these high and low
work function POPs, we fabricated energy-converting devi-
ces, including four different types of organic solar cells and
light-emitting diodes using PBC and OFPBC on ITO as
interlayers. We fabricated inverted organic solar cells (iOSCs)
porous polymers as interlayers. Significantly, both VOC and
JSC were enhanced by using the PBC and OFPBC films. This
result reflects the improved charge-selectivity of these PBC
and OFPBC films. Organic solar cells usually show a trade-off
[1]
between VOC and JSC
;
our results suggest that the design of
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Figure 3. a) Device structure of the iOSCs based on the PBC film electrode. b) The J–V characteristics in the dark (black) and under illumination
À2
(
red). c) J–V characteristics of the iOSCs under air mass (AM) 1.5 illumination (100 mWcm ). d) Device structure of the cOSCs based on the
OFPBC film electrode. e) The J–V characteristics in the dark (black) and under illumination (red). f) J–V characteristics of the cOSCs under AM 1.5
À2
illumination (100 mWcm ). g) Device structure of the iOLEDs based on PBC film electrode. h) Voltage-current-density-brightness characteristics
of the iOLED. i) Current-density-luminous-efficiency characteristics of the iOLED. j) Device structure of the cOLEDs based on OFPBC film
electrode. k) Voltage-current-density-brightness characteristics of the cOLED. l) Current-density-luminous-efficiency characteristics of the cOLED.
electrode interlayers may have a chance to break this
limitation.
functions in various types of devices, including solar cells and
light-emitting diodes. Our results reveal the enormous
potential of porous organic polymers as an appealing plat-
form for the rational design of work function, a newly
developed aspect of POP materials worthy of further
exploration. With the diversity of building blocks and
tunability of POP structures, we envisage that POPs with
different work functions would be useful not only in energy
conversions but also in photocatalysis and energy storage.
We investigated the performance of PBC and OFPBC
films in organic light-emitting diodes (OLEDs). For this
purpose, we fabricated inverted OLEDs (iOLEDs) and
conventional OLEDs (cOLEDs) using an authentic green-
emitting polymer, poly[2-(4-(3’,7’-dimethyloctyloxy)-phenyl)-
p-phenylenevinylene] (P-PPV), as the light-emitting layer
[
11]
(
Figure 3g,j). The iOLEDs had the configuration of ITO/
PBC films (15 nm)/P-PPV (75 nm)/MoO3 (10 nm)/Al
100 nm), whereas the cOLEDs had the configuration of
ITO/OFPBC films (10 nm)/P-PPV (75 nm)/CsF (1.3 nm)/Al Acknowledgements
100 nm). The PBC films on ITO in iOLEDs serve as the
(
(
cathode, while the OFPBC films on ITO in cOLEDs function
as the anode. Figure 3h and k present the voltage-current-
density-brightness characteristics of the optimal iOLEDs and
cOLEDs, respectively (detailed data sets are summarized in
Table S5). The two devices exhibited largely rectified J–V
curves without any leakage current, and achieved maximum
This work was supported by a Grant-in-Aid for Scientific
Research (A) (24245030) from MEXT of Japan. C. Gu is an
International Research Fellow of the Japan Society for the
Promotion of Science (JSPS).
Keywords: electrode interlayers · energy conversion devices ·
porous organic polymers · thin films · work function
À2
brightnesses of 30000 (iOLED) and 32500 cdm (cOLED).
Figure 3i and l present the current-density-luminous-effi-
ciency characteristics of the iOLEDs and cOLEDs, respec-
tively. The two devices achieved maximum luminous efficien-
How to cite: Angew. Chem. Int. Ed. 2016, 55, 3049–3053
Angew. Chem. 2016, 128, 3101–3105
À1
cies of 16.4 (iOLEDs) and 17.9 cdA (cOLEDs). These
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À1
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H. Zhang, Y. Pan, Y. Lv, Y. Yang, F. Li, S. Zhang, F. Huang, Y.
Angew. Chem. Int. Ed. 2016, 55, 3049 –3053
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3053