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Polymer Solar Cells with 90% External Quantum Efficiency Featuring an Ideal Light‐ and Charge‐Manipulation Layer

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Rapid progress in the power conversion efficiency (PCE) of polymer solar cells (PSEs) is beneficial from the factors that match the irradiated solar spectrum, maximize incident light absorption, and reduce photogenerated charge recombination. To optimize the device efficiency, a nanopatterned ZnO:Al2O3 composite film is presented as an efficient light- and charge-manipulation layer (LCML). The Al2O3 shells on the ZnO nanoparticles offer the passivation effect that allows optimal electron collection by suppressing charge-recombination loss. Both the increased refractive index and the patterned deterministic aperiodic nanostructure in the ZnO:Al2O3 LCML cause broadband light harvesting. Highly efficient single-junction PSCs for different binary blends are obtained with a peak external quantum efficiency of up to 90%, showing certified PCEs of 9.69% and 13.03% for a fullerene blend of PTB7:PC71BM and a nonfullerene blend, FTAZ:IDIC, respectively. Because of the substantial increase in efficiency, this method unlocks the full potential of the ZnO:Al2O3 LCML toward future photovoltaic applications.
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Polymer Solar Cells with 90% External Quantum Efficiency
Featuring an Ideal Light- and Charge-Manipulation Layer
Jing-De Chen, Yan-Qing Li, Jingshuai Zhu, Qianqian Zhang, Rui-Peng Xu, Chi Li,
Yue-Xing Zhang, Jing-Sheng Huang, Xiaowei Zhan, Wei You, and Jian-Xin Tang*
J.-D. Chen, Prof. Y.-Q. Li, R.-P. Xu, C. Li, Y.-X. Zhang, J.-S. Huang,
Prof. J.-X. Tang
Institute of Functional Nano & Soft Materials (FUNSOM)
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices
Collaborative Innovation Center of Suzhou Nano Science and
Technology
Soochow University
Suzhou 215123, China
E-mail: jxtang@suda.edu.cn
J. Zhu, Prof. X. Zhan
Department of Materials Science and Engineering
College of Engineering
Key Laboratory of Polymer Chemistry and Physics of Ministry of
Education
Peking University
Beijing 100871, China
Q. Zhang, Prof. W. You
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599-3290, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201706083.
DOI: 10.1002/adma.201706083
efficiency (PCE) exceeding 13% by
employing new designed donor/acceptor
materials,[12–15] sophisticated morphology
engineering,[9,16–18] or novel device archi-
tectures.[19,20] The external quantum
efficiency (EQE), (or the incident-photon-
to-converted-current efficiency, IPCE)
is defined as the ratio of the number of
collected photogenerated charges to the
number of incident photons of a given
wavelength, which represents the light
harvesting capability in PSCs. Neverthe-
less, it is noteworthy that the peak EQE in
the majority of single-junction PSCs con-
sisting of either binary or ternary donor/
acceptor bulk-heterojunction is still con-
siderably below 80% due to limited light
absorption in a thin active layer (<200 nm)
and serious monomolecular/bimolecular
charge recombination loss induced by
low carrier mobility of organic materials
and poor interface contact for charge collection.[16,21–24] In
this respect, promoting charge collection and improving light
absorption become important issues for further enhancing the
efficiency of PSCs.
A variety of interface engineering methods have been
explored to enhance the EQE with an ultrathin polyelectrolyte
interlayer or a modified metal oxide layer (e.g., ZnO) between
the electrode and active layer.[23–27] The improved charge col-
lection toward the electrodes has been achieved along with the
reduced nonohmic contact losses, charge recombination losses
and exciton quenching at the interfaces. Meanwhile, light
manipulation has also been becoming a popular strategy to
boost light absorption efficiency and increase the short-circuit
current (JSC). For example, the grating, nanobowls, or metal
nanoparticles have been incorporated into substrate surface,
charge extraction layers, or the active layers, which can induce
the light trapping due to the antireflection, light scattering, or
plasmonic resonance effect.[28–33] Specially, our previous work
has been demonstrated that the deterministic aperiodic nano-
structure (DAN) can function as an efficient wavelength-/angle-
independent light trapping scheme in PSCs.[6]
Here, we demonstrate an effective method for the fabrication
of highly efficient single-junction PSCs by using a nanostruc-
ture-patterned Al2O3-doped ZnO (denoted as ZnO:Al2O3) com-
posite film as a light- and charge-manipulation layer (LCML) to
simultaneously guide the incident light within the active layers
for sufficient light absorption as well as to improve the electron
Rapid progress in the power conversion efficiency (PCE) of polymer solar
cells (PSEs) is beneficial from the factors that match the irradiated solar
spectrum, maximize incident light absorption, and reduce photogenerated
charge recombination. To optimize the device efficiency, a nanopatterned
ZnO:Al2O3 composite film is presented as an efficient light- and charge-
manipulation layer (LCML). The Al2O3 shells on the ZnO nanoparticles offer
the passivation effect that allows optimal electron collection by suppressing
charge-recombination loss. Both the increased refractive index and the pat-
terned deterministic aperiodic nanostructure in the ZnO:Al2O3 LCML cause
broadband light harvesting. Highly efficient single-junction PSCs for different
binary blends are obtained with a peak external quantum efficiency of up to
90%, showing certified PCEs of 9.69% and 13.03% for a fullerene blend of
PTB7:PC71BM and a nonfullerene blend, FTAZ:IDIC, respectively. Because of
the substantial increase in efficiency, this method unlocks the full potential
of the ZnO:Al2O3 LCML toward future photovoltaic applications.
Polymer Solar Cells
Polymer solar cells (PSCs) hold great promise as clean and
renewable energy sources due to their superior features, such
as simple fabrication process, light weight, and potential appli-
cation in flexible devices.[1–11] In recent years, rapid progress in
PSCs has been achieved with the certified power conversion
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collection with suppressed recombination loss (Figure 1a). The
Al2O3 shells on ZnO nanoparticles offer the passivation effect
and the formation of an interface dipole layer to optimize the
electron collection with dramatically suppressed charge recom-
bination loss. The increased refractive index and the incorpora-
tion of nanoimprinted DAN in the ZnO:Al2O3 LCML result in
a broadband self-enhanced light absorption without scarifying
the fill factor (FF) and open-circuit voltage (VOC). Highly effi-
cient single-junction PSCs are obtained with peak EQE values
approaching 90%, and certified PCEs of 9.69% and 13.03%
for the fullerene-based active layer of thieno[3,4-b]thiophene/
benzodithiophene:[6,6]-phenyl C71-butyric acid methyl ester
(PTB7:PC71BM) and the nonfullerene FTAZ:IDIC active layer,
respectively (see the chemical structures in Figure 1b).
The ZnO:Al2O3 films were fabricated via a sol–gel method
on indium-tin oxide (ITO) glass substrate by first preparing the
ZnO precursor as reported in the literature,[34] and subsequently
adding a defined amount of Al(NO3)3·9H2O into the ZnO pre-
cursor. Then the modified ZnO precursor solution was stirred
at 80 °C for 2 h. The film properties of various ZnO:Al2O3(x%)
films were systematically characterized, where x is the molar
concentration of Al(NO3)3·9H2O in Zn(CH3COO)2·2H2O and
Al(NO3)3·9H2O. As characterized by X-ray photoelectron spec-
troscopy (XPS) in Figure 1c, the peak intensity of Al 2p core
level increases with the increasing Al2O3 content, and its peak
position at 74.3 eV corresponds to the Al3+ cation of Al2O3.[35]
Meanwhile, the Zn 2p3/2 peak remains at the almost same posi-
tion. These results reveal the fact that the Al2O3 is only the pro-
duction of Al(NO3)3·9H2O.[36] Figure 1d displays the second
electron cut-off (SECO) region of ultraviolet photoelectron spec-
troscopy (UPS) spectra of various ZnO:Al2O3(x%) films, where
the work function (
φ
) of ZnO:Al2O3(x%) films can be extracted
Adv. Mater. 2018, 1706083
Figure 1. a) Device structure of PSCs incorporating a DAN-patterned ZnO:Al2O3 LCML. b) Chemical structures of FTAZ, IDIC, PTB7, and PC71BM.
c) XPS Al 2p and Zn 2p3/2 core level spectra, d) SECO region of UPS spectra, e) PL spectra, and f) SEM images of ZnO:Al2O3(x%) films with various
molar concentrations.
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from the SECO as summarized in Table S1 (Supporting Infor-
mation). It is found that the increase of Al2O3 content from 0%
to 15% causes a continuous decrease in
φ
from 4.2 to 3.9 eV.
The
φ
change in ZnO:Al2O3(x%) films can be ascribed to the for-
mation of an interface dipole layer between ZnO and Al2O3 due
to their difference in oxygen areal density (OAD) that drives the
oxygen ion diffusion from high OAD oxide of Al2O3 to the ZnO.[37]
As presented in Figure S1 (Supporting Information) of the sche-
matic energy level diagrams, in which the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) of PC71BM were extracted from photoemis-
sion measurements,[38] it can be found that the Schottky barrier
height is minimized for electron collection from PC71BM to the
ITO with respect to the
φ
decrease in ZnO:Al2O3. Meanwhile, the
photoluminescence (PL) spectra obtained from ZnO:Al2O3(x%)
films in Figure 1e shows that the ZnO trap-state-induced emis-
sion decreases with respect to the increase in Al2O3 content. The
suppressed trap-state emission implies that the Al2O3 coating on
ZnO nanoparticles can effectively passivate these traps, which is
in good agreement with the previous results.[39] To understand
the morphology of Al2O3 and ZnO, scanning electron microscopy
(SEM) measurements were conducted as displayed in Figure 1f.
The morphologies of various ZnO:Al2O3(x%) films show that
the pinhole density on ZnO:Al2O3 surface decreases with the
increasing Al2O3 content, indicating that the Al2O3 coating on
ZnO nanoparticles fills the gaps between ZnO nanoparticles.
These results are consistent with the atomic force microscopy
(AFM) measurements in Figure S2 (Supporting Information),
showing the continuous morphology with increasing Al2O3 con-
tent. The X-ray diffraction (XRD) results of various ZnO:Al2O3
films in Figure S3 (Supporting Information) show the decrease
in peak intensity as compared to bare ZnO (that is x = 0), con-
firming a possible increase in Al2O3 coverage on ZnO nanopar-
ticles with the increasing Al2O3 content.[40] On the other hand, it
is noted in Figure 1f that some big bulges can be formed on the
surface of ZnO:Al2O3(20%). According to the high-magnification
SEM image in Figure S4 (Supporting Information), the signifi-
cant morphology difference in ZnO:Al2O3(20%) is speculated to
be related to the Al2O3 aggregation. As shown in the AFM image
in Figure S2e (Supporting Information), the ZnO:Al2O3(20%)
film has the Al2O3 aggregation with a bulge size of 450 nm in
width and 120 nm in height. To evaluate the electron trans-
porting property of the ZnO:Al2O3 films, the current density–
voltage (I–V) characteristics of electron-only devices with a con-
figuration of glass/ITO/ZnO:Al2O3(x%)/Ag were measured. The
almost identical device performances as plotted in Figure S5
(Supporting Information) indicate the negligible influence of the
incorporation of Al2O3 on the electron transport property of the
ZnO:Al2O3 films, which may be due to the presence of the con-
tinuous ZnO scaffold in the doped layers.
Single-junction PSCs were fabricated to assess the feasi-
bility of ZnO:Al2O3 in devices. As depicted in Figure 1a, the
PSCs were constructed with an inverted architecture of glass/
ITO cathode/ZnO:Al2O3 LCML/Active layer/MoO3/Ag anode,
where the ZnO:Al2O3 LCML without or with DAN patterns was
used for electron collection and light manipulation. Figure 2a
Adv. Mater. 2018, 1706083
Figure 2. Device performance of PTB7:PC71BM-based PSCs on various ZnO:Al2O3 layers. a) I–V characteristics recorded under 100 mW cm2 AM 1.5G
illumination. b) EQE spectra, c) absorption spectra, and d) IQE spectra of various devices.
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plots the I–V characteristics of PSCs constructed with a spin-
coated PTB7:PC71BM active layer, which were recorded under
100 mW cm2 air mass 1.5 global (AM 1.5G) illumination. The
detailed photovoltaic parameters deduced from the I–V curves
are summarized in Table 1. It is obvious in Figure 2a that the use
of the ZnO:Al2O3 layers causes a remarkable improvement in JSC
and PCE as compared to the case of bare ZnO. Especially, JSC is
increased to a value of 16.70 mA cm2 and the PCE is enhanced
to 9.52% with respect to an optimal ZnO:Al2O3(15%) layer.
It has been demonstrated that PSCs with corrugated ZnO
layer can achieve the significantly enhanced absorption and
EQE without scarifying the FF and VOC.[6] To further optimize
the PSCs with a PTB7:PC71BM active layer, the DAN pattern
with a period of 400 nm was duplicated onto the ZnO:Al2O3
layer by adopting soft nanoimprint lithography (SNIL) tech-
nique. In previous reports,[41] it has been demonstrated that the
DAN pattern is advantageous for broadband light trapping over
the full solar spectral range, while the SNIL techique is com-
patibile with the large-area manufacturing of organic devices
without material damage. The AFM image in Figure S6a
(Supporting Information) displays that the quasi-periodic sub-
wavelength nanostructures have been effectively patterned on
ZnO:Al2O3 surface with an average period of 400 nm and a
groove depth of 50–60 nm. It is also obvious in Figure S6b (Sup-
porting Information) that corrugated surface morphology is
well maintained by the subsequently spin-coated PTB7:PC71BM
layer. As presented in Figure 2a and Table 1, significant per-
formance improvement can be achieved in the PSC with a
DAN-patterned ZnO:Al2O3(15%) layer, exhibiting the further
enhanced JSC and PCE up to 18.02 mA cm2 and 10.11%,
respectively. To verify the variation of JSC, the EQE curves were
measured and shown in Figure 2b for comparison. The cal-
culated JSC from the EQE spectra for the whole spectrum are
14.73, 15.88, 16.09, 16.18, 15.71, and 17.88 mA cm2 for PSCs
based on ZnO:Al2O3(0%), ZnO:Al2O3(5%), ZnO:Al2O3(10%),
ZnO:Al2O3(15%), ZnO:Al2O3(20%), and DAN-patterned
ZnO:Al2O3(15%), respectively. The measured JSC are 3% error
to the calculated values. To further confirm the device perfor-
mance, the optimal cells were tested by the National Center
of Supervision and Inspection on Solar Photovoltaic Products
Quality of China (CPVT), and a certified PCE of 9.688% was
obtained for the PTB7:PC71BM-based device with the DAN-
patterned ZnO:Al2O3(15%) layer (Table 1; Figure S7, Supporting
Information). This is in agreement with the results obtained in
our laboratory with 4% mismatch. In addition, it is noted that
the PSC based on DAN-patterned ZnO:Al2O3(15%) achieved a
peak EQE of 89% at 690 nm and an average EQE of 77.9% at
the visible region (390–700 nm), suggesting that this device has
a superior light harvesting capability.
To verify the origin of the performance improvement, the
absorption spectra of PTB7:PC71BM-based PSCs on various
ZnO:Al2O3(x%) layers were extracted by measuring the reflec-
tion spectra with an integrated sphere equipped UV-visible
spectroscopy. As shown in Figure 2c, the absorption of the
devices based on ZnO:Al2O3 is remarkably enhanced at the
whole visible region as compared to that on bare ZnO. In
particular, the use of a DAN-patterned ZnO:Al2O3(15%) layer
leads to the further enhanced absorption over a broadband
wavelength range. These results are consistent with the EQE
enhancement as shown in Figure 2b, implying that more inci-
dent light can be trapped in the active layer with the use of a
ZnO:Al2O3 LCML. To verify the nature of the improved absorp-
tion in the ZnO:Al2O3 LCML, the film refractive index (n) was
measured via an ellipsometer (Figure S8, Supporting Informa-
tion) and summarized in Table S1 (Supporting Information).
It is found that the n of ZnO:Al2O3(x%) films is significantly
increased from 1.63 to 1.72 with respect to the increase in x
from 0 to 15, although bare Al2O3 film has a low n of 1.52. We
speculate that the increased n is because the ZnO:Al2O3(x%)
films became denser with the increasing Al2O3 content.[42]
Theoretical simulation was carried out to estimate the n influ-
ence of ZnO:Al2O3(x%) on the photogeneration in PSCs by
using the finite-difference-time-domain (FDTD) method (see
the “Experimental Section” for the details of the optical mod-
eling calculations). Figure 3 presents the normalized cross-
sectional near-field profiles of field distributions in PSCs for a
transverse electric (TE) polarized incident light at a wavelength
of 550 nm. The optical field distribution in the active layer of
the ZnO:Al2O3(15%)-based device is apparently stronger than
the case of ZnO:Al2O3(0%)-based device, implying that more
Adv. Mater. 2018, 1706083
Table 1. Photovoltaic parameters of PSCs using various ZnO:Al2O3 layers without and with DANs. The average PCE values were obtained from four
cells.
Devices Active layer VOC [V] JSC [mA cm2]FF [%] PCE[best] [%] PCE[avg.] [%]
ZnO PTB7:PC71BM 0.735 15.18 73.01 8.15 8.05
ZnO:Al2O3(5%) 0.74 16.40 73.67 8.94 8.82
ZnO:Al2O3(10%) 0.75 16.49 75.52 9.34 9.17
ZnO:Al2O3(15%) 0.75 16.70 76.01 9.52 9.35
ZnO:Al2O3(20%) 0.73 16.09 70.75 8.31 8.19
ZnO:Al2O3(15%) w/DAN 0.75 18.02 74.84 10.11 9.82
ZnO:Al2O3(15%) w/DANa) 0.755 17.02 75.35 9.688
ZnO:Al2O3(0%) FTAZ:IDIC 0.84 18.54 70.82 11.03 10.95
ZnO:Al2O3(15%) w/DAN 0.85 20.87 72.77 12.91 12.67
ZnO:Al2O3(15%) w/DANa) 0.850 21.58 71.02 13.03 -
a)The devices were certificated by the National Center of Supervision and Inspection on Solar Photovoltaic Products Quality of China (CPVT).
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incident light can be trapped and thus absorbed in the active
layer. Moreover, the modeled results show that the use of a
DAN pattern in the ZnO:Al2O3 layer allows a further localiza-
tion of photon flux in the active layer, which has been ascribed
to the collective effects of light scattering and localized surface
plasmonic resonance for the enhanced light harvesting.[28–30]
Consequently, the simulation results offer the theoretical
understanding of the optical effect on the performance
enhancement in PSCs based on the ZnO:Al2O3 layers without
and with a DAN pattern. The internal quantum efficiency
(IQE) spectra of PSCs based on various ZnO:Al2O3(x%) layers
can be deduced from IQE = EQE/Absorption. As displayed in
Figure 2d, all the ZnO:Al2O3-based PSCs show higher IQE
values than the device with bare ZnO, indicating that the
enhanced EQE for ZnO:Al2O3-based PSCs originates not only
from the enhanced light absorption but also the improved
electron collection.
Figure 4a plots the photocurrent density (Jph) versus effec-
tive voltage (Veff) characteristics on a double logarithmic scale.
Here, Jph is determined by Jph = JLJD, where JL and JD are the
current densities under AM 1.5G 100 mW cm2 illumination
and in dark, respectively. Veff is defined as Veff = V0Vb, where
Vb is the bias voltage and V0 is the compensation voltage at
Jph = 0. The JphVeff curves in Figure 4a clearly show three
regimes: a linear increase of Jph with Veff for small fields, a
square-root dependence for middle fields, and then a gradual
transition to saturation of Jph at high fields. It is obvious that
ZnO:Al2O3-based PSCs have higher saturation current density
(Jsat) at the region of Veff > 0.2 V. As Jsat is mainly dependent
on the absorbed incident photon flux and independent of the
bias and temperature, a noticeable enhancement in Jsat indi-
cates that the ZnO:Al2O3 layer indeed improves the absorption
of incident light in the active layer, which is in good agreement
with the absorption measurements in Figure 2c and the optical
modeling in Figure 3. On the other hand, the ZnO:Al2O3-
based PSCs achieve higher Jph at the low Veff region, indicating
that the electron collection property of the ZnO:Al2O3 layer
is better than that of bare ZnO. The proportion of swept-out
charges can be calculated by dividing Jsat by JSC. Accordingly,
the device with a ZnO:Al2O3(15%) layer exhibits a relatively
high JSC, indicating that 94.5% generated charges are swept
out of the device prior to recombination and contribute to the
photocurrent.
Adv. Mater. 2018, 1706083
Figure 3. Optical simulation of normalized cross-sectional near-field profiles of TE polarized light at a wavelength of 550 nm for PSCs based on
ZnO:Al2O3(0%), ZnO:Al2O3(15%), and DAN-patterned ZnO:Al2O3(15%) layers, respectively.
Figure 4. a) Photocurrent density (Jph) as a function of the effective
voltage (Veff) for PSCs under 100 mW cm2 AM 1.5G illumination. b) The
experimental data (open symbols) of VOC and JSC of ZnO:Al2O3-based
PSCs as a function of incident light intensity, together with the fits to the
data (dashed lines).
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To gain further insight into the influence of ZnO:Al2O3 on
electron collection property, light intensity dependence of VOC
and JSC were measured to verify the recombination processes in
devices. As shown in Figure 4b, the VOC varies logarithmically
with incident light intensity (ln(I)) by following the relationship
/ln
OC B
()
()
Vk
Tq I (1)
where kB is the Boltzmann constant, T is the absolute tempera-
ture in kelvin, q is the electron charge, and I is the light inten-
sity.[43] The curve fitting show the slop values of 1.55 kBT/q,
1.42 kBT/q, 1.31 kBT/q, 1.13 kBT/q and 1.17 kBT/q for devices
based on ZnO:Al2O3(0%), ZnO:Al2O3(5%), ZnO:Al2O3(10%),
ZnO:Al2O3(15%), and ZnO:Al2O3(20%), respectively. The
ZnO:Al2O3-based devices have a decreased slope with the
increasing of Al2O3 content, indicating a weak dependence
of VOC on the I. As reported previously,[44,45] a slope close to
kBT/q means that the dominant recombination mechanism
in ZnO:Al2O3-based device is bimolecular recombination. In
other words, the monomolecular recombination, also termed
the trap-assisted Shockley–Read–Hall (SRH) recombination,
is significantly suppressed in the ZnO:Al2O3-based devices. In
parallel, it is evident in the dark I–V characteristics (Figure S9,
Supporting Information) that the reverse current is obviously
reduced for the cases of incorporating Al2O3. It has been
reported that the electron collection layer with low trap den-
sity in PSCs will presumably decrease the possibility of trap-
assisted recombination and enhance the values of JSC and FF,
thereby improving the PCE.[46] It is thus reasonable to infer
that trap-assisted SRH recombination loss in ZnO:Al2O3-based
PSCs is dramatically suppressed by taking into account the evi-
dences of the light intensity dependent VOC spectra (Figure 4b),
the decrease in trap-induced PL emission (Figure 1d) and the
reduced reverse current (Figure S9, Supporting Information).
Thus, the suppressed charge recombination loss is attributed to
the passivation effect due to the Al2O3 coating on ZnO, which
decreases the trap density in the ZnO film.[47] In addition, the
weaker performance in ZnO:Al2O3(20%)-based device likely
originates from the Al2O3 aggregation on ZnO:Al2O3(20%)
film, which acts as insulating islands at the ZnO/PC71BM inter-
face and thus blocks the electron collection from the PC71BM.
The incident light dependence of JSC was recorded to fur-
ther clarify the charge collection behaviors in ZnO:Al2O3-based
PSCs. Figure 4b shows that the light intensity dependence
of JSC follows the power law as described as JSC I
α
, where
α
is the coefficient of the power law. The fitting of experi-
mental data yields the
α
values of 0.950, 0.956, 0.962, 0.965,
and 0.945 for devices with ZnO:Al2O3(0%), ZnO:Al2O3(5%),
ZnO:Al2O3(10%), ZnO:Al2O3(15%), and ZnO:Al2O3(20%),
respectively. The increased
α
value close to the ideal unity indi-
cates that the electron collection property of the ZnO:Al2O3-
based devices can be effectively improved by optimizing the
Al2O3 content. An increasing
α
could result from the bal-
anced mobility of charge carriers, the uniformed distribution
of the density of states, the suppressed bimolecular recom-
bination.[48] In order to verify the influence of ZnO:Al2O3 on
such issues, AFM measurement was carried out to study the
morphology of the active layer. Figure S10 (Supporting Infor-
mation) shows that height and phase images of PTB7:PC71BM
active layer on various ZnO:Al2O3 layers exhibit the similar
morphology, implying no influence of ZnO:Al2O3 on the mor-
phology of active layer. Given that the active layers are almost
identical in each case, the improved electron collection property
in ZnO:Al2O3-based PSCs may be attributed to the suppression
of bimolecular recombination loss, which is related to the elec-
tric field formed by the interfacial dipole with negative charge
in ZnO and positive charge in Al2O3 (see Figure 1c) that can
promote the electron drift from PC71BM to ZnO and refrain
the electron diffusion from ZnO to PTB7 at the short-circuit
condition.[49]
To verify the potential of the ZnO:Al2O3 layer as a LCML
for various material systems, nonfullerene PSC based on a
FTAZ:IDIC active layer was also fabricated (see the chemical
structures in Figure 1b). Figure 5 presents the I–V characteris-
tics and EQE of the corresponding devices under the illumi-
nation, and the detailed parameters are listed in Table 1. It is
noteworthy that the JSC of the FTAZ:IDIC device with a DAN-
patterned ZnO:Al2O3(15%) LCML is increased to 20.87 mA cm2
Adv. Mater. 2018, 1706083
Figure 5. Device performance of nonfullerene FTAZ:IDIC-based PSCs.
a) I–V characteristics and b) EQE spectra recorded under 100 mW cm2
AM 1.5G illumination.
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with a peak EQE of 92% at 610 nm. Furthermore, a certified
PCE of 13.03% was obtained for a representative device as
tested by CPVT (see Table 1; Figure S11, Supporting Infor-
mation), which is among the highest reported results for
single-junction PSCs. These results confirm that the use of a
ZnO:Al2O3 LCML is a simple and universal way to achieve the
high-efficiency PSCs.
In conclusion, an efficient EQE enhancement strategy
for PSCs has been demonstrated by using a sol–gel-derived
ZnO:Al2O3 composite layer as an LCML for unique light and
charge manipulation. Film characterization and electrical meas-
urement reveal that the optimal Al2O3 content in ZnO:Al2O3
films offers the passivation effect to suppress the trap-assisted
charge recombination loss during electron collection from the
active layer to the contact. Optical measurement and theoret-
ical simulation indicate that the combination of DAN pattern
in the ZnO:Al2O3 LCML allows a broadband enhancement of
light absorption. Highly efficient single-junction PSCs with dif-
ferent binary blends are achieved with a peak EQE up to 90%,
showing certified PCEs of 9.69% and 13.03% for the fullerene
blend of PTB7:PC71BM and nonfullerene FTAZ:IDIC blend,
respectively. Due to the ease of use and substantial increase in
efficiency shown here, we anticipate that our method unlocks
the full potential of the ZnO:Al2O3 LCML toward the future
photovoltaic applications.
Experimental Section
Solution Preparation: The sol–gel-derived ZnO precursor solution
was prepared by dissolving 220 mg zinc acetate dehydrate in 61 µL
ethanolamine and 2 mL 2-methoxyethanol with overnight vigorous
stirring. Especially, for ZnO:Al2O3 precursor solutions, different amount
of Al(NO3)3·9H2O were added into the prepared ZnO precursor with
2 h stirring at 80 °C. PTB7 and PC71BM were purchased from 1-material
Chemscitech. The PTB7:PC71BM blend solution was prepared by
dissolving 10 mg PTB7 and 15 mg PC71BM in 1 mL 1,2-dichlorobenzene
with 3 vol% 1,8-iodooctane and stirring overnight at 60 °C. FTAZ and
IDIC were synthesized as the previous reports.[50] The FTAZ:IDIC
solution was prepared by dissolving 6 mg FTAZ and 8 mg IDIC in 1 mL
chloroform with 0.3 vol% 1,8-iodooctane and stirring 2 h at 60 °C.
Device Fabrication: ITO-glass substrates were in turn ultrasonic-
cleaned in detergent, DI water, acetone, and isopropanol, and finally
dried in an oven at 110 °C. ZnO and ZnO:Al2O3 precursor solutions were
spin-coated onto UV–ozone-cleaned ITO-glass substrate at 3000 rpm
for 40 s, and then annealed at 150 °C for 20 min, resulting in a film
with a thickness of 40 nm. For the DAN-patterned ZnO:Al2O3 film, soft
nanoimprint lithography with a polydimethylsiloxane mold was used
to duplicate the pattern to ZnO:Al2O3 as our previous report.[6] The
size and height of DANs can be controlled by varying the imprinting
conditions, e.g., the imprinting pressure and the annealing time. The
active layers were formed by spin-coating PTB7:PC71BM and FTAZ:IDIC
blend solutions onto ZnO and ZnO:Al2O3 films at 1500 rpm for 120 s
and 2500 rpm for 30 s, respectively, in the nitrogen-filled glove box. The
devices were completed by subsequently evaporated 8 nm-thick MoO3
and 100 nm-thick Ag on the active layer to form the top anode for hole
collection, which were performed in an interconnected high vacuum
evaporation chamber (base pressure: 2 × 106 Torr). Deposition rate
and layers thickness were monitored by a quartz crystal microbalance.
A shadow mask was used to define the Ag anode with an active area of
0.0725 cm2.
Film and Device Characterization: The film properties of various
ZnO:Al2O3(x%) films were systematically characterized, where x
is the molar concentration of Al(NO3)3·9H2O in the mixture of
Zn(CH3COO)2·2H2O and Al(NO3)3·9H2O. Chemical compositions
and electronic structures of the samples were measured by X-ray and
ultraviolet photoemission spectroscopy (XPS and UPS) in a Kratos AXIS
Ultra-DLD system, in which a monochromatic Al K
α
source (1486.6 eV)
with a resolution of 0.5 eV and an unfiltered HeI excitation line (21.22 eV)
from a Helium discharge lamp with a resolution of 0.1 eV were used
for XPS and UPS, respectively. Optical properties were measured using
a UV–vis–near-IR spectrometer (Perkin Elmer Lambda 750) with an
integrating sphere. Surface morphology of the samples was characterized
by scanning electron microscopy (SEM) (FEI, Quanta 200FEG) and AFM
(Veeco MultiMode V) in tapping mode. The refractive index (n) and layer
thickness of the films were determined by the alpha-SE Spectroscopic
Ellipsometer (J. A. Woollam Co., Inc). The crystal structure of samples
was characterized by XRD (a PANalytical diffractometer) with Cu K
α
radiation (
λ
= 0.15406 nm). The photovoltaic characteristics of PSCs
were examined in air at room temperature using a programmable
Keithley 2612 source measurement unit under 100 mW cm2 AM 1.5G
illumination generated by a solar simulator (Oriel model 91160). The
illumination intensity of the solar simulator was calibrated using a
standard Si photodiode with a known spectral response. To avoid the
shadow effect on the area derivation of the devices during the electrode
deposition, a shadow mask with an area of 0.0725 cm2 was used to
determine the actual illumination area of the devices during the current–
voltage measurements. The EQE measurement was carried out with
a photomodulation spectroscopic setup (Newport monochromator),
including a xenon lamp, an optical chopper, a monochromator, and
a lock-in amplifier operated by a computer, whose power-density
calibration was performed by a traceable silicon photodiode. For EQE
measurement, monochromatic light with profile area much smaller than
the device area was chose to illuminate the device without any mask.
Optical Modeling: The in-plane waveguide modes in PSCs were
simulated through FDTD approach by Lumerical FDTD Solutions. The
optical properties, i.e., polarized incident light, reflection, absorption,
and surface plasmonic resonance were fully taken into account in the
device modeling. For simplicity, the DAN pattern was constructed
by using hexagonal closely packed nanostructures with sinusoidal
tapered profile. The wavelength-dependent refractive index (n) and
extinction coefficient (k) of ITO, ZnO, ZnO:Al2O3, and PTB7:PC71BM
films were experimentally determined by the ellipsometer. All the films
were constructed on precleaned silicon wafer except for ITO. The
recorded ellipsometric data and fitting results were shown in Figure S8
(Supporting Information). Cauchy model was employed to fit the
recorded Psi and Delta of ITO, ZnO, and ZnO:Al2O3, while basis spline
model was used to fit PTB7:PC71BM data.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was financially supported by the National Basic Research
Program of China (Grant No. 2014CB932600), the National Natural
Science Foundation of China (Grant Nos. 91633301, 61520106012,
91433116, 61522505, 11474214, and 61722404), the National Key R&D
Program of China (Grant Nos. 2016YFB0401002 and 2016YFB0400700),
Qing Lan Project, and the project of the Priority Academic Program
Development (PAPD) of Jiangsu Higher Education Institutions.
Conflict of Interest
The authors declare no conflict of interest.
Adv. Mater. 2018, 1706083
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1706083 (8 of 8)
www.advmat.dewww.advancedsciencenews.com
Keywords
charge extraction, external quantum efficiency, light manipulation,
polymer solar cells
Received: October 19, 2017
Revised: November 26, 2017
Published online:
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Adv. Mater. 2018, 1706083
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A new polymer donor (PBDB-T-SF) and a new small molecule acceptor (IT-4F) for fullerene-free organic solar cells (OSCs) were designed and synthesized. The influences of fluorination on the absorption spectra, molecular energy levels and charge mobilities of the donor and acceptor were systematically studied. The PBDB-T-SF:IT-4F-based OSC device showed a record high efficiency of 13.1%, and an efficiency of over 12% can be obtained with a thickness of 100–200 nm, suggesting the promise of fullerene-free OSCs in practical applications.
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Fabricating organic solar cells (OSCs) with a tandem structure has been considered an effective method to overcome the limited light absorption spectra of organic photovoltaic materials. Currently, the most efficient tandem OSCs are fabricated by adopting fullerene-derivatives as acceptors. Here, we design a new NF-acceptor with an E(opt)g of 1.68 eV for the front sub-cells and optimize the phase separation morphology of a fullerene-free active layer with an E(opt)g of 1.36 eV to fabricate the rear sub-cell. The two sub-cells show a low Eloss and high external quantum efficiency (EQE), and their photo response spectra are complementary. In addition, an interconnection layer composed of ZnO and a pH-neutral self-doped conductive polymer PCP-Na and with a high light transmittance in the near-IR range was developed. Based on the highly optimized sub-cells and ICL, solution-processed fullerene-free tandem OSCs with an average PCE over 13% are demonstrated.
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A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71BM (PCE = 5.22%).
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Triarylamine (TAA) and related materials have dramatically promoted the development of organic and hybrid photovoltaics during the past decade. The power conversion efficiencies of TAA-based organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), and perovskite solar cells (PSCs) have exceeded 11%, 14%, and 20%, which are among the best results for these three kinds of devices, respectively. In this review, we summarize the recent advances of the high-performance TAA-based materials in OSCs, DSSCs, and PSCs. We focus our discussion on the structure–property relationship of the TAA-based materials in order to shed light on the solutions to the challenges in the field of organic and hybrid photovoltaics. Some design strategies for improving the materials and device performance and possible research directions in the near future are also proposed.
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We demonstrate the contact resistance reduction for III-V semiconductor-based electrical and optical devices using the interfacial dipole effect of ultrathin double interlayers in a metal-interlayers-semiconductor (M-I-S) structure. An M-I-S structure blocks metal-induced gap states (MIGS) to a sufficient degree to alleviate Fermi-level pinning caused by MIGS, resulting in contact resistance reduction. In addition, the ZnO/TiO2 interlayers of an M-I-S structure induce an interfacial dipole effect that produces Schottky barrier height (ΦB) reduction, which reduces the specific contact resistivity (ρc) of the metal/n-type III-V semiconductor contact. As a result, the Ti/ZnO(0.5 nm)/TiO2(0.5 nm)/n-GaAs metal-double interlayers-semiconductor (M-DI-S) structure achieved a ρc of 2.51×10-5 Ω·cm², which exhibited a ∼×42000 reduction and a ∼×40 reduction compared to the Ti/n-GaAs metal-semiconductor (M-S) contact and the Ti/TiO2(0.5 nm)/n-GaAs M-I-S structure, respectively. The interfacial dipole at the ZnO/TiO2 interface was determined to be approximately −0.104 eV, which induced a decrease in the effective work-function of Ti, and therefore reduced ΦB. X-ray photoelectron spectroscopy (XPS) analysis of the M-DI-S structure also confirmed the existence of the interfacial dipole. On the basis of these results, the M-DI-S structure offers a promising non-alloyed ohmic contact scheme for the development of a III-V semiconductor-based applications.
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While it has been argued that field-dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBT-type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p-DTS(FBTTh2)2 layer as donor material and C60 as acceptor. The authors measure JV-characteristics as a function of intensity and charge-extraction-by-linearly-increasing-voltage-type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness (Lp) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor–acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor–acceptor interface. This effect is absent in the p-DTS(FBTTh2)2 diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s-shape of the JV-curves and the concomitant additional decrease of the FF with increasing layer thickness.
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Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.