Lending triarylphosphine oxide to phenanthroline: A Facile approach to high-performance organic small-molecule cathode interfacial material for organic photovoltaics utilizing air-stable cathodes

Article abstract of DOI:10.1002/adfm.201401685

Cathode interfacial material (CIM) is critical to improving the power conversion efficiency (PCE) and long-term stability of an organic photovoltaic cell that utilizes a high work function cathode. In this contribution, a novel CIM is reported through an effective and yet simple combination of triarylphosphine oxide with a 1,10-phenanthrolinyl unit. The resulting CIM possesses easy synthesis and purification, a high T _g of 116 C and attractive electron-transport properties. The characterization of photovoltaic devices involving Ag or Al cathodes shows that this thermally deposited interlayer can considerably improve the PCE, due largely to a simultaneous increase in V _oc and FF relative to the reference devices without a CIM. Notably, a PCE of 7.51% is obtained for the CIM/Ag device utilizing the active layer PTB7:PC₇₁BM, which far exceeds that of the reference Ag device and compares well to that of the Ca/Al device. The PCE is further increased to 8.56% for the CIM/Al device (with J _sc = 16.81 mA cm^-2, V _oc = 0.75 V, FF = 0.68). Ultraviolet photoemission spectroscopy studies reveal that this promising CIM can significantly lower the work function of the Ag metal as well as ITO and HOPG, and facilitate electron extraction in OPV devices.

Full text of DOI:10.1002/adfm.201401685

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Lending Triarylphosphine Oxide to Phenanthroline: a Facile
Approach to High-Performance Organic Small-Molecule
Cathode Interfacial Material for Organic Photovoltaics
utilizing Air-Stable Cathodes
Wan-Yi Tan, Rui Wang, Min Li, Gang Liu, Ping Chen, Xin-Chen Li, Shun-Mian Lu,
Hugh Lu Zhu, Qi-Ming Peng, Xu-Hui Zhu,* Wei Chen,* Wallace C. H. Choy,*
Feng Li,* Junbiao Peng, and Yong Cao
Dedicated to 80th birthday of Prof. Xiao-Zeng You
1. Introduction
Cathode interfacial material (CIM) is critical to improving the power conver-
sion efficiency (PCE) and long-term stability of an organic photovoltaic cell
Organic photovoltaic (OPV) cells have been
that utilizes a high work function cathode. In this contribution, a novel CIM
is reported through an effective and yet simple combination of triarylphos-
phine oxide with a 1,10-phenanthrolinyl unit. The resulting CIM possesses
easy synthesis and purification, a high Tg of 116 °C and attractive electron-
transport properties. The characterization of photovoltaic devices involving
Ag or Al cathodes shows that this thermally deposited interlayer can consid-
erably improve the PCE, due largely to a simultaneous increase in Voc and FF
relative to the reference devices without a CIM. Notably, a PCE of 7.51% is
obtained for the CIM/Ag device utilizing the active layer PTB7:PC71BM, which
far exceeds that of the reference Ag device and compares well to that of the
Ca/Al device. The PCE is further increased to 8.56% for the CIM/Al device
(with Jsc = 16.81 mA cm−2, Voc = 0.75 V, FF = 0.68). Ultraviolet photoemission
spectroscopy studies reveal that this promising CIM can significantly lower
the work function of the Ag metal as well as ITO and HOPG, and facilitate
electron extraction in OPV devices.
the subject of current research interest
due to light weight, mechanical flexibility
and the potential to provide a low-cost
solution for harvesting solar energy.[1,2]
Among others, cathode interfacial mate-
rial (CIM) which is placed between the
active layer and cathode becomes a critical
element in determining the power con-
version efficiency (PCE) and durability of
an OPV device.[3] As a result, introducing
a proper CIM has been shown capable
of producing multiple beneficial effects,
including i) facilitating electron extrac-
tion in the presence of air-stable cathodes,
ii) blocking exciton and hole transport to
the cathode, and iii) preventing metal dif-
fusion into the underlying active layer
during thermal evaporation.
W.-Y. Tan, M. Li, G. Liu, Prof. X.-H. Zhu, Prof. J. B. Peng, Prof. Y. Cao
State Key Laboratory of Luminescent Materials and Devices (SKLLMD)
and Institute of Polymer Optoelectronic Materials and Devices
South China University of Technology (SCUT)
P. Chen, Q.-M. Peng, Prof. F. Li
State Key Laboratory of Supramolecular
Structure and Materials
Jilin University, 2699
Guangzhou 510640, China
E-mail: xuhuizhu@scut.edu.cn
Qianjin Avenue, Changchun 130012, China
E-mail: lifeng01@jlu.edu.cn
R. Wang, Prof. W. Chen
Department of Physics
National University of Singapore
3 Science Drive 2
and Department of Chemistry
National University of Singapore
3 Science Drive 3
X.-C. Li, S.-M. Lu, H. L. Zhu, Prof. W. C. H. Choy
Department of Electrical and Electronic Engineering
The University of Hong Kong
Pokfulam Road, Hong Kong, China
E-mail: chchoy@eee.hku.hk
Singapore 117543
E-mail: phycw@nus.edu.sg
DOI: 10.1002/adfm.201401685
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The development of CIMs for organic photovoltaics is mainly
based on several different lines, ranging from inorganic metal
compounds such as halides and oxides[4] to organic small mole-
cules[5,6] and conjugated[3a]/non-conjugated polymers.[7] Organic
small-molecule based CIMs show a number of potentially
attractive characteristics. They possess a well-defined chemical
structure, ease of synthesis with high purity and low-tempera-
ture processability (via thermal evaporation).[1]
Bathocuproine (BCP) was the first organic compound as a
cathode interfacial layer brought into an OPV cell.[8] Following
this preliminary success, various attempts on organic molec-
ular CIMs such as metal complexes,[9,10] substituted 1,10-phen-
anthrolines (BPhen[11] and DMPP,[12] pyridine derivatives,[13,14]
1,4,5,8-naphthalenetetracarboxylic dianhydride,[15] 1,3,5-tris(1-
phenyl-1H-benzimidazol-2-yl)benzene[16] and zwitterions,[6b]
have been reported in order to further improve the device
PCE and/or stability. Consequently, based on these studies, a
wide-band-gap CIM with a deep HOMO level, sufficient elec-
tron transport and morphological stability is favorable for OPV
applications.
a 7.51% of PCE, which far surpasses the reference Ag device
devoid of a CIM and even compares well with that of the Ca/
Al device. In situ ultraviolet photoelectron spectroscopy (UPS)
and X-ray photoelectron spectroscopy (XPS) measurements
were used to reveal the working mechanism of this new CIM
in terms of the interfacial energy level alignment. While Phen-
NaDPO on graphite (HOPG) and ITO possesses low work func-
tion of around 3.0 eV, its strong chelating interaction with Ag
surface may account for the further reduced work function
as low as 2.6 eV on the metal cathode. The PCE was further
increased from 7.51 to 8.56% for the Phen-NaDPO/Al device.
2. Results and Discussion
2.1. Synthesis of Phen-NaDPO
The synthesis of Phen-NaDPO is outlined in Scheme 1.
Monolithiating 2,6-dibromonaphthalene, followed by treat-
ment of chlorodiphenylphosphine leads to the monobromo
compound 1, which further gives compound 2 under oxida-
tion with an excessive of H2O2 (Scheme 1). Subsequently, the
successful boronation of compound 2 using bis(pinacolato)
diboron provides the key precursor compound 3. Finally Phen-
NaDPO is obtained by Suzuki coupling of compound 3 and
3-bromo-1,10-phenanthroline.
The identity and purity of Phen-NaDPO are confirmed by
1H NMR, HRMS and microanalysis (See Figure S2). It is worth
noting that analytically pure compound can be facilely available
through purification based mainly on column chromatography
without further sublimation. This demonstrates the great
advantage over the commonly used electron-transporting CIMs
based on polypyridinyl derivatives, which usually involves extra
sublimation purification steps with high cost.
As a strong electron donor and acceptor, the rigid and planar
chemical structure of 1,10-phenanthroline (Phen) is a versa-
tile chelating agent and stabilizes in particular the low-valent
metals.[17] However, BCP is mainly hampered by its tendency
to crystallize with no detectable glass transition (T ≈186 °C,
c
Tm ≈ 290 °C. See Figure S1 in the Supporting Information),
while bathophenanthroline (BPhen)[8,11] shows a clear glass
transition and yet a low Tg of ≈66 °C. Therefore, the design and
development of morphologically robust CIMs based on Phen
derivatives with preserved effective electron transport charac-
teristic and simplified synthesis approaches to avoid potentially
complicated and/or costly synthesis/purification methods com-
monly associated with the polypyridinyl compounds are highly
desirable.
In this context, we report a novel cathode interfacial mate-
rial Phen-NaDPO (Scheme 1). Introducing the polar bulky
and chelating moiety (2-naphthyl)diphenylphosphine oxide
(NaDPO) provides the resulting CIM with a high glass transi-
tion temperature (Tg ≈ 116 °C) and a high solubility in common
low-polar and polar solvents. Moreover, analytically pure Phen-
NaDPO is facilely available by column chromatography without
sublimation. Noticeably, utilizing a thermally deposited Phen-
NaDPO interlayer provided the photovoltaic device that con-
tained the active layer PTB7:PC71BM and Ag cathode with
In addition to a high solubility in low-polar solvents such as
CH2Cl2, attaching this particular triarylphosphine oxide moiety
to 3-position of 1,10-phenanthroline produces an increase of
solubility in common alcohol solvents, relative to BCP and
BPhen. For instance, 10 mg of Phen-NaDPO can be easily solu-
bilized in 1 mL isopropanol (IPA) at room temperature.
2.2. Thermal and Morphological Properties of Phen-NaDPO
Phen-NaDPO shows a high thermal stability over 350 °C
(Figure 1a). Although it first melted at 250 °C, no crystallization
upon cooling or remelting was observed. Instead, an obvious
glass transition emerged at approximately 116 °C during the
cooling and reheating processes (Figure 1b), which is consid-
erably higher than that of Bphen (Tg ≈ 66 °C) and TmPyPB
(Tg ≈ 79 °C), a widely used electron-transporting layer in organic
light-emitting diodes and more recently as a CIM in OPV
device,[13b,c] whereas no detectable glass transfixion is present
for BCP, apart from melting and crystallization (See Figure S1,
Supporting Information). The stable amorphous morphology of
Phen-NaDPO with a small surface roughness (Rrms = 0.622 nm)
was further witnessed by the atomic force microscopy (AFM)
measurement of the thermally deposited thin film (180 nm),
annealed at 100 °C for 10 min (Figure 1c).
O
P
Br
Br
Br
i, ii
P
iii
Br
1
2
O
P
O
O
O
P
B
N
N
iv
v
3
Phen-NaDPO
Scheme 1. Synthetic route to Phen-NaDPO: i) n-BuLi, THF, –78 ºC; ii)
chlorodiphenylphosphine; iii) H2O2, dichloromethane, room tempera-
ture; iv) Pd(dppf)Cl2, KOAc, DMF, 80 °C; v) Pd(OAc)2, tricyclohexylphos-
phine, 2 M K2CO3 aqueous solution, toluene, ethanol, 90 °C.
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of PCE to 7.51% (Jsc = 18.04 mA cm−2,
Voc = 0.71 V, FF = 0.59), whereas the ref-
erence Ag device showed an invariably
considerably lower PCE of ≈2.19% with
Jsc = 16.46 mA cm−2, Voc = 0.38 V, FF = 0.35
(Figure 3a and Table 2). Notably, the experi-
mental Jsc values agreed well with those
calculated from the incident photon-to-cur-
rent efficiency (IPCE) measurement within
300–800 nm (See Figure S3 and Table S1 in
the Supporting Information).
Voc is influenced by the electrical contact
between the active layer and the electrodes,
as well as the HOMO level of the donor and
LUMO level of the acceptor.[20,21] Similar to
the Phen-NaDPO/Al and Ca/Al devices (as
discussed below, Figure 2b, 3c and Table 1,2),
introducing a Phen-NaDPO interlayer pro-
vides a Voc of ≈0.60 V concerning the active
layer P3HT:PC61BM and ≈0.70 V concerning
PTB7:PC71BM for the Ag cathode, indi-
cating an Ohmic contact between the active
layer and the interfacial layer on the metal
cathodes.
The high FF of the photovoltaic devices
that contained a Phen-NaDPO interlayer can
be understood in terms of the series resist-
ance (Rs) and the shunt resistance (Rsh),
which are calculated from the inverse slope
near Voc and V = 0 V in the photocurrent
density–voltage curves, respectively.[21] The
resulting generally smaller Rs and higher
Rsh values, as compared to those of the ref-
erence devices without a CIM, reveal that
Phen-NaDPO can effectively reduce the con-
Figure 1. a) Thermogravimetric analysis, b) DSC diagrams, and c) Topographic AFM images
of the thermally evaporated thin film (180 nm) of Phen-NaDPO, annealed at 100 °C for 10 min.
2.3. Phen-NaDPO as a Cathode Interfacial Layer in OPV Devices
tact resistance between PC61BM and the Ag cathode.
Furthermore the Phen-NaDPO/Ag device showed a far
better diode characteristic than the reference Ag device in the
dark with a rectification ratio of ≈3.7 × 104 versus 9.6 × 102 at
2 V (Figure 3b). This observation implies that Phen-NaDPO
can effectively block hole injection while meanwhile improving
Phen-NaDPO was first evaluated by thermal deposition
as a cathode interfacial layer in photovoltaic devices (ITO/
PEDOT:PSS/P3HT:PC61BM/CIM(3−15 nm)/cathode). The
Ag and Al metals were used as the cathodes, respectively. The
analogue devices utilizing metal-only (Ag
or Al) or Ca/Al as the cathode without CIM
were also fabricated for comparison.
InsertingathinlayerofPhen-NaDPO(5nm)
led to a largely improved photovoltaic perfor-
mance relative to the corresponding reference
Ag device (Figure 2a and Table 1) with Jsc
=
8.56 mA cm−2, Voc = 0.61 V, FF = 0.69 and
PCE = 3.61%, which is comparable to those
of the analogue devices utilizing a conjugated
polymer electrolyte[18a] or self-assembled
ammonium salt layer.[18b] By contrast, the
reference Ag device without this interlayer
gave a PCE of 2.18% with Jsc = 9.72 mA cm−2,
Voc = 0.48 V, FF = 0.47.
Replacing the active layer with the
PTB7:PC71BM blend[19] in the presence of
Phen-NaDPO produced a further increase
Figure 2. J–V characteristics of the photovoltaic devices (ITO/PEDOT:PSS/P3HT:PC61BM/
CIM(x nm)/cathode). x = 5 nm for Phen-NaDPO/Ag, 3 nm for Phen-NaDPO/Al and 0 nm for
the reference Ag, Al, and Ca/Al devices.
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Table 1. Summary of the photovoltaic data (ITO/PEDOT:PSS/P3HT:PC61BM/CIM(x nm)/cathode), under light intensity of 100 mW cm−2
Cathode = Ag, Al and Ca/Al.
.
CIM/Cathode
CIM
[nm]
PCE
[%]
Jsc
Voc
[V]
FF
Rs
[Ω cm2]
Rsh
[KΩ cm2]
[mA cm−2
]
Ag
0
3
2.18(2.10)
2.84(2.80)
3.61(3.59)
3.37(3.39)
3.42(3.20)
2.84(2.82)
4.18(4.01)
4.14(4.07)
4.04(3.96)
4.06(3.98)
3.63(3.59)
9.72(9.77)
8.57(8.74)
8.56(8.53)
9.29(9.53)
8.49(7.95)
10.06(10.09)
9.93(9.42)
10.23(10.10)
9.64(9.57)
9.95(9.75)
9.58(9.19)
0.48(0.48)
0.60(0.60)
0.61(0.61)
0.59(0.60)
0.61(0.61)
0.54(0.53)
0.61(0.61)
0.61(0.61)
0.62(0.61)
0.61(0.61)
0.61(0.61)
0.47(0.45)
0.55(0.54)
0.69(0.69)
0.61(0.60)
0.67(0.66)
0.53(0.52)
0.69(0.69)
0.66(0.66)
0.68(0.67)
0.67(0.67)
0.62(0.64)
10.60
12.88
5.75
0.29
0.48
0.96
0.75
2.56
0.73
4.32
0.96
0.69
0.98
1.45
Phen-NaDPO/Ag
5
10
15
0
7.20
9.05
Al
13.06
2.93
Ca/Al
0
Phen-NaDPO/Al
3
5.47
5
5.05
10
15
5.12
10.14
The data in parentheses are the average results of three devices.
electron injection over a voltage of ≈0.4 V in the forward
during the electron extraction via these low-lying LUMO
states of the CIM. The evolution of thickness-dependent XPS
core level spectra in Figure S5 reveals the existence of strong
interaction at the interface between Phen-NaDPO fi lm and Ag
(while not existent between CIM and HOPG), apparently via
direction.
2.4. Photoelectron Spectroscopy Measurements
In order to get an in-depth understanding
of the effects of the present CIM Phen-
NaDPO on electron extraction in OPV
devices, the interfacial electronic structures
of Phen-NaDPO on various substrates were
systematically evaluated by UPS in Figure 4.
The UPS spectra at the low kinetic energy
region (Figure 4a) reveal the evolution of
vacuum level and hence the surface work
function of Ag substrate upon the deposi-
tion of CIM. A tremendous vacuum level
downward shift of 1.98 eV can be observed
after the deposition of 10 nm Phen-NaDPO
film on Ag, with the work function of Ag
reduced to 2.64 eV. Such low work function
can greatly facilitate the electron extraction
in OPVs.
From the UPS spectra near the Fermi
level region (Figure 4b), the HOMO edges
for Phen-NaDPO fi lms on Ag locate at the
binding energy of 3.30 eV below the Fermi
level for 10 nm thickness. The optical band
gap of Phen-NaDPO was measured to be
3.36 eV (See Figure S4, Supporting Infor-
mation). Due to the large exciton binding
energy in organic materials, the HOMO-
LUMO electronic band gap is usually a few
hundred meV larger than the optical band
gap. Combining with the deep lying HOMO
Figure 3. a,c) Light and b,d) dark J–V characteristics of the photovoltaic devices (ITO/
PEDOT:PSS/PTB7:PC71BM/CIM(x nm)/cathode). x = 3 nm for Phen-NaDPO/Ag or Al, and
0 nm for the reference Ag, Al and Ca/Al devices. The active layer was spin-cast from a mixture
of chlorobenzene and DIO (100:3 V/V).
levels, the LUMO of Phen-NaDPO can be
deduced to locate right above the Fermi
level, thereby facilitating electron transfer
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Table 2. Summary of the photovoltaic data (ITO/PEDOT:PSS/PTB7:PC71BM/CIM(x nm)/cathode), under light intensity of 100 mW cm−2. Cathode =
Ag, Al and Ca/Al.
CIM/Cathode
CIM
[nm]
PCE
[%]
Jsc
Voc
[V]
FF
Rs
[Ω cm2]
Rsh
[KΩ cm2]
[mA cm−2
]
Ag
0
3
0
0
3
2.19(2.12)
7.51(7.44)
5.43(5.37)
7.31(7.26)
8.56(8.37)
16.46(16.43)
18.04(18.04)
16.69(16.66)
16.04(16.21)
16.81(16.72)
0.38(0.37)
0.71(0.70)
0.60(0.60)
0.72(0.72)
0.75(0.74)
0.35(0.35)
0.59(0.59)
0.54(0.54)
0.63(0.62)
0.68(0.67)
14.82
4.40
7.94
4.54
3.61
0.14
0.76
0.55
0.65
0.49
Phen-NaDPO/Ag
Al
Ca/Al
Phen-NaDPO/Al
The data in parentheses are the average results of three devices.
the coupling between the P = O functional group with Ag. Fur-
thermore the rigid phenanthrolinyl moiety has been shown to
interact with various metals including Al and Ag.[17,22] Such
strong interactions can promote a good electrical contact
between the CIM and Ag electrodes.
On inert substrate of graphite (e.g., HOPG) and the widely
used ITO substrate, the coating of Phen-NaDPO fi lm can
also lead to the significant reduction on surface work func-
tion, i.e., the surface work function can be reduced to 3.02 eV
(Phen-NaDPO on HOPG), and 3.10 eV (Phen-NaDPO on ITO),
respectively, as shown in Figure 4. This observation suggests
that Phen-NaDPO fi lm can be used as a potentially universal
low work function surface modification layers on various
substrates.
We further evaluate the interface between the low work func-
tion CIM and the active acceptor molecules (n-type) by using
C60 as a model system. As shown in Figure 5a, during the
sequential deposition of C60 on Phen-NaDPO modified ITO
substrate, the vacuum level was gradually upward shifted, or
the work function was increased by 1.16 eV. This was accompa-
nied by an obvious upward band-bending like behavior in C60
film, that is, the HOMO of C60 moved towards the Fermi level
(Figure 5b). Based on such observation, the low work function
of the present CIM can induce significant electron transfer
from the substrate via CIM fi lm to C60, leading to the notable
increase of work function (Figure 5c) and the formation of a
large interfacial dipole or the build-in electric field (Figure 5d).
By contrast, the charge transfer between the unmodified ITO
and C60 appears negligible.[23] As a result, the LUMO level
of C60 is situated right above the Fermi level at the interface
(Figure 5c), thereby facilitating the effective electron extraction
from organic acceptor through the Phen-NaDPO interface layer
(via the low-lying LUMO) to the cathode electrode, as well as
minimizing the energy loss during the electron extraction pro-
cess. Moreover, the build-in electric field in favorable direction
can greatly facilitate electron extraction from the active layer to
the cathode (Figure 5d).
2.5. Photovoltaic Devices Based on Phen-NaDPO/Al Cathode
It is expected that Phen-NaDPO should be also effective for the
Al cathode. As a result, the photovoltaic device concerning the
active layer P3HT:PC61BM and Al cathode in the presence of
the Phen-NaDPO interlayer (≈3 nm) yielded an increase of PCE
to 4.14% with Jsc = 10.23 mA cm−2, Voc = 0.61 V, FF = 0.66,
which is comparable to that of the Ca/Al device (Figure 2b and
Table 1). By contrast, the reference Al cathode without a CIM
gives a PCE of 2.84% with Jsc = 10.06 mA cm−2, Voc = 0.54 V,[18]
FF = 0.53.
Further, a high PCE of 8.56% was obtained for the Phen-
NaDPO/Al device based on the active layer PTB7:PC61BM with
Jsc = 16.81 mA cm−2, Voc = 0.75 V, FF = 0.68, which surpassed
that of 5.43% of the reference Al and even 7.31% of the Ca/
Al devices (Figure 3c and Table 2). Notably, very similar PCEs
were found at both SKLLMD, South China University of
Technology and the University of Hong Kong to demonstrate
reproducibility. The PCE is also comparable to those of the
analogue devices achieved by utilizing a conjugated polymer
interlayer.[24]
The photovoltaic devices that consisted of a Phen-NaDPO
interlayer appear to provide a slow decrease of PCE with the
increasing thickness (Table 1 and Supporting Information
Figure S6). For instance, as the interlayer thickness reached
15 nm, the PCE still remained at 3.42% for the Phen-NaDPO/
Ag device and 3.63% for the Phen-NaDPO/Al device. Gener-
ally, a thicker CIM is beneficial to large area device processing,
Figure 4. UPS spectra at a) the low-kinetic energy part and b) valence
band spectra near the Fermi level for 10 nm Phen-NaDPO on Ag, ITO,
and HOPG surface. The work function of pristine substrates was also
provided in (a).
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comparable to that of BPhen.[25] Nevertheless,
the electron mobility of Phen-NaDPO tends
to increase at low electric fields, amounting
to 3.9 × 10−4 cm2 V−1 s−1 at E = ∼8 ×
105 V cm−1 (See the inset of Figure 6).
2.7. Phen-NaDPO vs NaBDPO as a CIM for
OPV Devices
Finally it is worth noting that Phen-
NaDPO generally provides a better photo-
voltaic performance than (naphth-2,6-diyl)
bis(diphenylphosphine oxide) (NaBDPO,
Figure S7, S8 and Table S2, Supporting
Information), especially for the Ag cathode.
The cathode interfacial materials based on
diphenylphosphine oxide(s) have been shown
effective for the Al cathodes.[18c]
3. Conclusions
In summary, lending the triarylphosphine
oxide moiety to 1,10-phenanthroline pro-
vides the present CIM with multiple attrac-
tive characteristics such as easy accessibility
and purification, a high Tg of 116 °C, and
good electron transport properties. The
photovoltaic devices that contained a ther-
mally deposited Phen-NaDPO interlayer
and Ag or Al cathode produced a consider-
ably improved PCE, due largely to a simul-
taneous increase in Voc and FF relative
to the reference devices without a CIM.
Notably, a PCE of 7.51% was obtained
for the Phen-NaDPO/Ag device utilizing
the active layer PTB7:PC71BM, which far
exceeds that of the reference Ag device
and even compares well to that of the Ca/
Al device. The PCE was further increased to
8.56% for the Phen-NaDPO/Al device (with
Jsc = 16.81 mA cm−2, Voc = 0.75 V, FF =
0.68). In situ UPS and XPS experiments
Figure 5. Thickness-dependent UPS spectra at a) the low-kinetic energy part, b) low binding
energy part near the Fermi level, c) the schematic energy diagram of C60 on Phen-NaDPO as
a function of C60 thickness, and d) the schematic of the additional electrical field formed by
inserting CIM.
while certain CIMs based on inorganic and organic salts such
as LiF[4a] and zwitterions[6b] are effective within only a few ang-
stroms to nanometers.
were carried out to explain the functions of Phen-NaDPO
in the OPV devices based on the energy level alignments at
the CIM/metal and C60/CIM interfaces. Phen-NaDPO on Ag
possessed a low work function of 2.64 eV to facilitate effec-
tive electron extraction in OPV devices. It is found that Phen-
NaDPO can also work as a universal and effective cathode
interfacial material for various substrates including ITO and
HOPG. While the photovoltaic polymers were employed to
facilitate the characterization of Phen-NaDPO as a cathode
interfacial layer, it is expected that the present CIM may
afford promising small-molecular OPV devices in both con-
ventional and inverted device configurations through vacuum
thermal deposition. Moreover, the findings reported here
may stimulate further intensive interest in this kind of small-
molecule CIMs for organic optoelectronics.
2.6. Electron-Transport Property of Phen-NaDPO
The electron transport property of Phen-NaDPO was prelimi-
narily evaluated by the time-of-flight (TOF) technique with a
device configuration (ITO/Alq3(20 nm)/Phen-NaDPO(1.5 µm)/
Al). Tris(8-hydroxyquinolinato)Luminal (Alq3) was used as the
charge-generation layer (CGL). A typical double logarithmic
plot of TOF transient photocurrent at E = 1.1 × 106 V cm−1 was
shown in Figure 6. The transit time of electrons was 0.4 µs,
yielding an electron mobility of 3.3 × 10−4 cm2 V−1 s−1, which is
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solid. Yield: 2.8 g (60%). (300 MHz, DMSO, ppm, δ): 1.34 (s, 12H),
7.53–7.71 (m, 11H), 7.79 (d, 1H, J = 8.2 Hz), 8.05 (d, 1H, J = 8.4 Hz),
8.16 (d, 1H, J = 13.7 Hz), 8.39 (s, 1H).
Synthesis of (2-(1,10-phenanthrolin-3-yl)naphth-6-yl)diphenylphosphine
oxide (Phen-NaDPO) : Pd(OAc)2 (4.5 mg, 0.02 mmol) and
tricyclohexylphosphine (11.2 mg, 0.04 mmol) was added to a mixture
of aqueous K2CO3 (2 M, 2 mL, 4 mmol), compound 3 (289 mg,
0.64 mmol), and 3-bromo-1,10-phenanthroline (150 mg, 0.58 mmol) in
toluene (30 mL) and ethanol (4 mL) under N2. The reaction was heated
at 90 °C overnight. After being cooled to room temperature, the mixture
was extracted with CH2Cl2. The organic layer was separated, dried over
anhydrous MgSO4, filtered and concentrated under reduced pressure.
The crude product was subject to column chromatography over silica gel
using CH2Cl2/ethanol (v/v = 50/1) as the elute to afford a white solid.
Yield: 200 mg (68%). m. p.: 250 °C (in the 1st heating run). Tg: 116 °C (in
the 2nd heating run). 1H NMR (300 MHz, DMSO, ppm, δ): 7.56–7.81 (m,
12H), 8.04–8.12 (m, 2H), 8.21–8.31 (m, 3H), 8.44 (d, 1H, J = 13.5 Hz),
8.53 (dd, 1H, J = 8.1, 1.7 Hz), 8.67 (s, 1H), 8.99 (s, 1H), 9.14 (dd, 1H,
J = 4.3, 1.7 Hz), 9.62 (s, 1H). HRMS (ESI) m/z: [M + H]+ calcd. for
C34H24N2OP, 507.1626; found, 507.1630. Anal. calcd. For C34H23N2OP: C
80.62, H 4.58, N, 5.53. Found: C 80.38, H 4.57, N 5.54.
Surface and Interface Analysis: In situ UPS and XPS measurement
were carried out in a custom designed ultrahigh vacuum (UHV) system
with a base pressure better than 2 × 10−10 mbar. A helium discharge
UV lamp with energy of 21.2 eV (He I) was used as the source of UV
light. XPS was performed with the Al Kα line with a photo energy of
1486.6 eV. All data was recorded by the Omicron EA 125 hemisperical
analyzer at normal emission angle under room temperature. Vacuum
level was determined from UPS spectra at the low-kinetic energy onset
(secondary electron cut-off) with −5 V sample bias. The work function
of the substrate Φ was obtained through the equation Φ =hν–W, where
W is the spectrum width of the energy difference between substrate
Fermi level and secondary electron cutoff. The plots in the UPS kinetic
energy figures were offset by 0.7 eV to view the value of the sample
work function directly. The CIM and C60 were thermally evaporated
from separated Knudsen cells onto the substrates in an UHV preparing
chamber with a base pressure better than 3 × 10−9 mbar. The deposition
rate and hence the “nominal” thickness were in situ monitored by
Quartz crystal microbalance during deposition and further calibrated
by XPS by monitoring the peak intensity attenuation of the substrate
core level peaks. The Ag(111) substrate was cleaned by several cycles of
Ar+ ion sputtering and subsequently annealed at 800 K. Freshly cleaved
HOPG was degassed in UHV chamber at around 600 K overnight before
deposition. ITO substrates were cleaned by subsequent sonication in
acetone, isopropyl alcohol and deionized water before loaded to the
chamber.
Fabrication and Characterization of Photovoltaic Devices: ITO-coated
glass substrates were cleaned by sonication in acetone, detergent,
deionized water, and isopropyl alcohol and dried in a nitrogen stream,
followed by an oxygen plasma treatment. In order to fabricate photovoltaic
devices, a thin hole-collection layer (ca. 40 nm) of PEDOT:PSS (Baytron
PVPAI 4083, filtered at 0.45 µm) was spin-cast on the pre-cleaned
ITO-coated glass substrates and baked at 120 °C for 20 min under
ambient conditions. The active layer P3HT:PC61BM was prepared by
spin-casting 1,2-dichlorobenzene solution (20:20 mg mL−1) at 800 rpm
for 36 s in a dry box, dried in covered glass Petri dishes at room
temperature for 30 min, and thermally annealed at 110 °C for 10 min.
The active layer PTB7:PC71BM (10:15 mg mL−1) was prepared by spin-
casting chlorobenzene solution with the addition of a small amount of
DIO (CB:DIO = 100:3, V/V) at 1500 rpm for 30 s in dry box. The thickness
of the PTB7:PC71BM layer was about 100 nm. Phen-NaDPO and
NaBDPO were either deposited by thermal evaporation at 0.2 Å s−1 in
vacuum (<5 × 10−4 Pa or spin-cast atop the active layer from IPA solution
with the same concentration of 0.5 mg mL−1 at 2000 rpm. The Al and Ag
electrode were thermally deposited for 100 nm and 60 nm, respectively,
through a mask in vacuum (<5 × 10−4 Pa). All steps except processing of
PEDOT:PSS were performed in the glove box. The effective device area
was about 0.04 cm2 for the active layer P3HT:PC61BM and 0.06 cm2 for
Figure 6. Typical double logarithmic plot of TOF transient photocurrent
of electrons of Phen-NaDPO at E = 1.1 × 106 V cm−1, showing a clear
“plateau” region. The insert: electron mobilities vs E1/2
.
4. Experimental Section
All manipulations involving air-sensitive reagents were performed under
dry nitrogen. Tetrahydrofuran (THF) was dried with molecular sieves
(3 Å). All other starting materials were purchased commercially and
used as received, unless otherwise specified.
Synthesis of (2-bromo-naphth-6-yl)diphenylphosphine oxide (Compound
2): A solution of 2,6-dibromonaphthalene (3 g, 10.5 mmol) in anhydrous
THF (180 mL) was degassed under N2 for 30 min, and then cooled to
–78 °C. n-Butyllithium (2.4 M solution in hexane, 4.8 mL, 11.55 mmol)
was added dropwise via syringe. The solution continued to be stirred
for 40 min, and then chlorodiphenylphosphine (2.3 mL, 12.6 mmol) was
then added via syringe. The reaction mixture was stirred overnight whilst
gradually warming to room temperature. Upon concentration, the residue
was treated with water and extracted by CH2Cl2. The organic layer was
separated, dried over anhydrous MgSO4, filtered and concentrated under
reduced pressure. The residue was subject to column chromatography
over silica gel using petroleum ether/CH2Cl2 (v/v = 5/1) as the elute to
afford 2-bromo-6-diphenylphos-phinonaphthalene (Compound 1) as a
white solid, which was used directly for oxidation. Hydrogen peroxide
(30%, 10 mL) was then added to a stirred mixture of compound 1 in
CH2Cl2 (30 mL). The reaction stirred at room temperature overnight.
The mixture was treated with aqueous NaHSO3 (2 M) and then extracted
with CH2Cl2. The organic layer was separated, dried over anhydrous
MgSO4, filtered and concentrated under reduced pressure. Yield: 3.3 g
1
(78%). H NMR (300 MHz, DMSO, ppm, δ): 7.53–7.60 (m, 4H), 7.62–
7.77 (m, 8H), 8.03–8.08 (m, 2H), 8.33 (s, 1H), 8.36 (d, 1H, J = 13.9 Hz).
Synthesis of (2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)naphth-6-yl)
diphenylphosphine oxide (Compound 3): Pd(dppf)Cl2 (81 mg, 0.11 mmol)
was added to a mixture of aqueous KOAc (977 mg, 9.95 mmol), compound
2 (1.35 g, 3.32 mmol), and 4,4,5,5-tetramethyl-2-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-1,3,2-dioxaborolane (1.26 g, 4.98 mmol)
in 1,4-dioxane (30 mL) under N2. The reaction was heated at 80 °C for
2 h. After being cooled to room temperature, distilled water was added.
The organic layer was extracted with CH2Cl2, separated, dried over
anhydrous MgSO4, filtered and concentrated under reduced pressure.
The crude product was subject to column chromatography over silica
gel using CH2Cl2/ethyl acetate (v/v = 3/1) as the elute to afford a white
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Adv. Funct. Mater. 2014,
DOI: 10.1002/adfm.201401685
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7

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PTB7:PC71BM. The current density–voltage (J–V) characteristics were
measured using a Keithley 2400 source meter. The photovoltaic devices
were characterized using a calibrated AM1.5 G solar simulator (Oriel
V. Murugesan, A. Kumar, K. Y. Zeng, J. Subbiah, W. W. H. Wong,
D. J. Jones, J. Y. Ouyang, J. Mater. Chem. 2012, 22, 24155–24165;
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model 91192), under light intensity of 100 mW cm−2
.
Mobility Measurements using the Time-of-Flight (TOF) Technique:
Device configuration (ITO/Alq3/Phen-NaDPO/Al). Alq3 (20 nm) and
Phen-NaDPO (1.5 µm) were deposited by thermal evaporation in
vacuum (< 6 × 10-4 Pa) at a rate of 0.2 and 3 Å s−1, respectively. The
Al electrode (15 nm) were thermally deposited through a mask. The
effective device area was about 0.04 cm2. A Nd:YAG laser (λ = 355 nm,
pulse width: 5 ns) was used as the light source and Alq3 as the charge-
generation layer (CGL). The applied voltage was controlled by a Keithley
2400 source meter. The electrons moved toward the Al contact with a
negative voltage. The photocurrent was characterized across the load
resistor using a digital storage oscilloscope (DPO7104, bandwidth:
1 GHz). The electron mobility was deduced from
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Acknowledgments
X.H.Z. is grateful for the financial support of SCUT, NSFC, and MOST of
China (Grant Nos. 2014CB643500, 2014ZG0009, 51173051, U1301243,
and 91333206). W.C. acknowledges the financial support from Singapore
MOE Grants R143–000–505–112, R143–000–530–112, R143–000–542–
112 and R143–000–559–112. W.C.H.C. would like to acknowledge the
financial support of the Research Grants Council (RGC) of Hong Kong
Special Administrative Region for the General Research Fund (grants:
HKU711813 and HKU711612E), the RGC-NSFC grant (N-HKU709/12)
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©
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2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2014,
DOI: 10.1002/adfm.201401685