Research Article
www.acsami.org
Toward Efficient Thick Active PTB7 Photovoltaic Layers Using
Diphenyl Ether as a Solvent Additive
Yifan Zheng,† Tenghooi Goh,‡ Pu Fan,† Wei Shi,† Junsheng Yu,*,† and André D. Taylor*,‡
†
State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of
Electronic Science and Technology of China (UESTC), Chengdu 610054, P. R. China
‡
Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States
S Supporting Information
*
ABSTRACT: The development of thick organic photovoltaics
(OPV) could increase absorption in the active layer and ease
manufacturing constraints in large-scale solar panel production.
However, the efficiencies of most low-bandgap OPVs decrease
substantially when the active layers exceed ∼100 nm in thickness
(because of low crystallinity and a short exciton diffusion length).
Herein, we report the use of solvent additive diphenyl ether (DPE)
that facilitates the fabrication of thick (180 nm) active layers and
triples the power conversion efficiency (PCE) of conventional
thienothiophene-co-benzodithiophene polymer (PTB7)-based
OPVs from 1.75 to 6.19%. These results demonstrate a PCE 20%
higher than those of conventional (PTB7)-based OPV devices using
1,8-diiodooctane. Morphology studies reveal that DPE promotes
the formation of nanofibrillar networks and ordered packing of
PTB7 in the active layer that facilitate charge transport over longer distances. We further demonstrate that DPE improves the fill
factor and photocurrent collection by enhancing the overall optical absorption, reducing the series resistance, and suppressing
bimolecular recombination.
KEYWORDS: diphenyl ether (DPE), solvent additive, organic photovoltaic (OPV), thick active layer, low-bandgap polymer
1. INTRODUCTION
Photovoltaic technology offers an environmentally friendly and
sustainable electricity source for surmounting the global energy
crisis.1,2 In particular, solution-processed bulk heterojunction
(BHJ) organic photovoltaics (OPVs) are scalable, lightweight,
and mechanically flexible, which are all attractive features for
the proliferation of solar panels.3,4 Recent notable OPV
breakthroughs include the synthesis of high-carrier mobility
small-bandgap polymers,5 the development of new device
architectures that enlarge the donor/acceptor (D/A) interface,6
and active layer nanoscale morphology improvements by posttreatment methods.7 These improvements have allowed
researchers to demonstrate single-junction OPVs with power
conversion efficiencies (PCEs) as high as 11.7%.8
In both polymer- and small-molecule-based BHJ OPVs, the
morphology of D/A blends plays a vital role in dictating the
device PCE.9 To operate efficiently, the photogenerated
excitons in OPVs must diffuse promptly to the D/A interface
where they can separate into holes and electrons before they
recombine and dissipate as heat.10−12 In addition, high OPV
photocurrents can be obtained only when the dissociated free
charge carriers are able to arrive at the corresponding electrodes
through interconnected D and A phase networks that span
several tens to hundreds of nanometers.13,14 One option for
suppressing electron−hole recombination and promoting high
© 2016 American Chemical Society
carrier mobility is to use highly ordered and purified D and A
organics. By introducing these kind of D and A materials, we
can facilitate the formation of a purified domain area that
reduces the trap density,15,16 but this route may incur high
manufacturing costs. On the other hand, control and
enhancement of the D/A morphology can also be achieved
by employing cheaper processing techniques such as thermal
treatment, solvent annealing, and solvent addition.17−19
Among these techniques, solvent addition is particularly
interesting as it is compatible with extensive polymer systems
and does not require extra processing steps. Previously, solvent
addition was reported to be effective in poly(3-hexylthiophene2,5-diyl) (P3HT)/(6,6)-phenyl-C61/C71-butyric acid methyl
ester (PCBM)-based devices and more recently has been
extended to low-bandgap polymer-based cells.20,21 Some of the
most common additives are 1,8-octanedithiol, 1,8-diiodooctane
(DIO), 1-chloronaphthalene, and N-methyl-2-pyrrolidone.19,21−23 Among these, DIO yields the best results for
thin BHJ devices consisting of benchmarked low-bandgap
polymers such as thieno[3,4-b]thiophene/benzodithiophene
(PTB7).24,25 The addition of DIO to chlorobenzene (CB)
Received: March 21, 2016
Accepted: June 2, 2016
Published: June 2, 2016
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DOI: 10.1021/acsami.6b03453
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Research Article
ACS Applied Materials & Interfaces
Figure 1. Device architecture and active layer materials.
Figure 2. (a) J−V characteristic curves of OPVs under AM 1.5 simulated 1 sun illumination. (b) UV−vis absorption spectra of a PTB7:PC71BM BHJ
film. (c) EQE spectra of devices with different ratios of solvent additives.
characterized by impedance spectroscopy. We reveal using
optical microscopy, grazing-incidence X-ray diffraction (GIXRD), and atomic force microscopy (AFM) imaging tools that
the DPE molecule enhances the organic crystallinity and
induces a nanofibrillar morphology. As a scalable demonstration, we show we can double the thickness of the active layer
and retain 90% of the PCE (compared to those of conventional
thin films) due to the high hole mobility measured using the
space-charge-limited current (SCLC) method.
effectively dissolves the PCBM aggregate and promotes the
formation of smaller acceptor domains (e.g., larger D/A
interface) within the active layer, resulting in a significant
improvement in the device fill factor (FF) and short-current
density (JSC).26 Unfortunately, DIO is an acceptor-selective
additive that does not enhance the crystallinity of the lowbandgap polymer. This drawback leads to a decreased PCE in
thicker DIO-added BHJ devices.2 In fact, residual DIO in the
active layer could form trap centers that inhibit charge
transport.27,28 Moreover, it has been suggested that excess
DIO above 3% can lead to the reaggregation of PCBM and
poor device reproducibility, which can impede further material
system designs and device physics studies.28,29 Despite these
challenges, the development of thicker active layer devices
could overcome many of the issues that prevent scalable
nanomanufacturing (i.e., roll-to-roll or spray coating processing).30 Recent work by the groups of Hwang and Heeger has
opened a new route for realizing high PCEs with the thick (300
nm) active layers based on different low-bandgap polymers by
using diphenyl ether (DPE).30,31 However, to date, only a few
studies have characterized the origin of the structural
improvements resulting from the use of DPE.30 In addition,
the impact of DPE in PTB7-based devices, arguably the
epitome of low-bandgap polymer OPVs, is yet unknown.
In this work, we demonstrate that the incorporation of DPE
in the low-band gap PTB7:PC71BM BHJ system is a facile and
effective strategy for enhancing photovoltaic PCE. We correlate
device performance to a charge extraction model as
2. EXPERIMENTAL SECTION
We show the architecture of the device and chemical structure of
materials used in the active layer in Figure 1. For the inverted OPV
structure, the device configuration is indium tin oxide/zinc oxide
(ITO/ZnO) (40 nm)/PTB7:PC71BM (80−180 nm)/MoO3 (15 nm)/
Ag (100 nm). ITO-coated glass substrates with a sheet resistance of 10
Ω/sq were consecutively cleaned in an ultrasonic bath containing
detergent, acetone, deionized water, and ethanol for 10 min each step
and then dried with nitrogen. Prior to film deposition, the substrate
was treated with UV light for 10 min. The ZnO precursor was
prepared by dissolving zinc acetate dihydrate [Zn(CH3COO)2·2H2O,
Aldrich, 99.9%, 1 g] and ethanolamine (NH2CH2CH2OH, Aldrich,
99.5%, 0.28 g) in 2-methoxyethanol (CH3OCH2CH2OH, Aldrich,
99.8%, 10 mL) under vigorous stirring for 12 h for the hydrolysis
reaction in air; 40 nm ZnO ETL was spin-casted from the precursor
solution on top of the clean ITO glass substrate and annealed at 200
°C for 1 h in air. For the active layer, the mixture of PTB7:PC71BM at
a weight ratio of 1:1.5 with a concentration of 25 mg/mL was
dissolved in a mixed CB/DPE solvent. DPE was added to the CB at
various concentrations of 1−5 vol %. The solution was then spin15725
DOI: 10.1021/acsami.6b03453
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ACS Applied Materials & Interfaces
Table 1. Summary of Photovoltaic Performance Based on PTB7:PC71BM BHJ with Different DPE Concentrations
VOCa (V)
b,c
without DPE
1% DPEc
2% DPEc
3% DPEc
4% DPEc
5% DPEc
3% DIOb,c
0.65
0.66
0.65
0.69
0.69
0.69
0.68
±
±
±
±
±
±
±
0.02
0.01
0.02
0.02
0.02
0.01
0.02
JSCa (mA/cm2)
8.0
13.8
15.0
16.0
16.7
14.5
15.8
±
±
±
±
±
±
±
0.5
0.5
0.6
0.5
0.5
0.6
0.3
FFa (%)
33.7
38.0
43.8
49.9
51.6
47.6
57.3
±
±
±
±
±
±
±
1.5
1.0
1.6
2.0
2.1
1.3
3.0
PCEa (%)
RS (Ω cm2)
RP (Ω cm2)
JPh (mA/cm2)
±
±
±
±
±
±
±
3.48
3.02
2.65
2.11
1.26
1.88
1.77
111.60
90.66
93.79
125.55
127.96
107.73
178.19
7.83
12.97
14.28
15.40
16.04
14.08
15.21
1.75
3.45
4.30
5.57
5.92
4.62
6.19
0.30
0.28
0.44
0.51
0.64
0.55
0.61
a
All the photovoltaic parameters are the average of a batch of six devices. bThe related low PCE compared with that in ref 33 was found because of
the quality of this batch PTB7 polymer without the use of a filter to eliminate the large PC71BM aggregate. cAll the active layers were spin-coated
following the same method, resulting in a similar thickness of ∼110 nm.
thick active layer than in the thin film. At this point, we want to
investigate the effect of DPE in the thick active layer through
detailed characterization of electric and optical parameters,
which will be discussed in section 3.4. Before that, the role of
DPE in the general thickness of the PTB7:PC71BM film should
be determined.
To understand the low series resistance (RS), we fit the data
using the single-diode-lumped circuit model.34 We show that
the addition of DPE to the active layer (1−4%) decreases the
RS from 3.48 to 1.26 Ω cm2 and corresponds to an increase in
both the FF and the JSC (Table 1). We note that increasing the
concentration of DPE to 5% results in an increase in RS that
lowers the JSC and FF, corresponding to a PCE of 4.62%. This
may due to the poor phase separation in the D/A interface that
reduces the number of percolation pathways for transport
charge carriers to the relative electrode.2,35 We show a decrease
in the device photocurrent density (Jph) when the amount of
DPE increases to 5 vol % (Table 1 and Supporting
Information). Reduction of the Jph signifies a decrease in the
charge collection efficiency and leads to the dominance of
bimolecular recombination, corresponding to a lower FF.36,37
We illustrate UV−vis absorption spectra of the
PTB7:PC71BM blend with added DPE (Figure 2b). A domain
absorption peak with a shoulder for PTB7 is observed in the
wavelength range of 600−750 nm, while the absorption from
350 to 500 nm is mainly attributed to the presence of
PC71BM.33,35,38 The intensity peak around 600 nm corresponds
to an S0−S3 π−π* transition and suggests ordered packing of
the PTB7 (Figure S2a).35 In addition, the stronger absorption
intensity at 478 nm indicates the aggregation of PC71BM, which
is different from the conventional DIO, modified active layer
(Figure S2b).19,38 The effect of DPE on light harvesting sheds
light on results obtained from external quantum efficiency
(EQE) measurements (Figure 2c and Figure S3). As expected,
the control device without DPE exhibits a low EQE across the
entire visible spectrum. For 1 vol % addition of DPE, the
average EQE is enhanced dramatically from 18.2 to 56.8%.
OPVs with a 4% DPE show the highest average EQE of 66.4%
around 500−750 nm, resulting in the best JSC of 16.7 mA/cm2,
which is consistent with the Jph results.
3.2. Characterization of the Morphology of the Active
Layer. In general, solvent addition impacts the device
performance by altering the morphology of the active layer.
To understand the influence of DPE on the microstructure of
the PTB7:PC71BM films, we characterize the films via optical
microscopy. We reveal that without DPE, large numbers of
PC71BM aggregates (∼1 μm) are formed in the devices (Figure
S4). These high-density, large PC71BM aggregates not only
reduce the D/A interface area but also lead to the
coated at a rate of 1000 rpm for 60 s in a nitrogen glovebox.
Subsequently, ∼15 nm MoO3 and ∼100 nm Ag were finally deposited
at a pressure of 3 × 10−3 Pa under vacuum. The anode area is 0.02 cm2
for all devices used in this work.
The morphology of active layer was characterized by AFM (MFP3D-BIO, Asylum Research) and GI-XRD (RIGAKU, D/MAX-RC).
Current density−voltage (J−V) curves under illumination were
measured with a Keithley 4200 programmable voltage−current source.
A light source integrated with a Xe lamp (CHF-XM35, Beijing
Trusttech Co. Ltd.) with a power illumination of 100 mW/cm2 was
used as a solar simulator. An Agilent 4294A Precision Impedance
Analyzer was employed for impedance spectroscopy (IS) measurements. The range of measured frequency was 40 Hz to 1 MHz; 50 mV
of modulation voltage without DC bias was used to extract the DC
bias-dependent AC signal. The thicknesses of films obtained from the
solution process were measured with a Dektak 150 stylus profiler. All
the measurements were taken under ambient conditions without any
encapsulation from air.
3. RESULTS AND DISCUSSION
3.1. Performance of OPVs with DPE as a Solvent
Additive. To elucidate the effects of DPE on standard
PTB7:PC71BM BHJ devices,32,33 we first limit the film
thickness to ∼110 nm (110 nm is the optimal thickness of
PTB7:PC71BM BHJ with 3% DIO, which has been widely
proved) and use the plain, additive-free sample as the control.
The DPE concentration varies from 1 to 5 vol % relative to the
CB solvent. We show the J−V characteristics of these OPVs
under 1 sun irradiation (Figure 2a, Table 1, and Figure S1).
The device without DPE exhibits not only a low JSC of 8.0 mA/
cm2 but also a poor FF of 33.7%, resulting in a PCE of 1.75%.
The poor performance of the control cells is attributed to the
unsuitable packing of the polymer blend and large aggregation
of the PC71BM.33 With the addition of DPE, we observe a
significant improvement in JSC from 8.0 to 13.8 mA/cm2,
yielding an enhancement in PCE from 1.75 to 3.45%.33
Compared to those of the control, both the JSC and the FF are
enhanced as DPE increases from 1 to 4 vol %. We obtain the
best device PCE of 5.92% at the optimal DPE loading (4 vol
%), with an open-circuit voltage (VOC) of 0.69 V, a JSC of 14.5
mA/cm2, and an FF of 47.6%. An enhancement of ∼300% in
device performance is enough to demonstrate the efficacy of
DPE in PTB7:PC71BM BHJ.2,33 We note that at an active layer
thickness of 110 nm (Figure S1), the best OPV PCE with 4%
DPE (5.92%) is still 4% lower than those of the devices with
DIO additives (6.19%), the latter measurement being
consistent with the known literature. As far as we know, PCE
will decrease dramatically with an increase in the thickness of
the active layer, especially in the low-bandgap polymer BHJ
system with only the DIO additive, e.g., PTB7:PCBM.
However, DPE has been reported to be more efficient in the
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ACS Applied Materials & Interfaces
Figure 3. AFM three-dimensional exhibition (a and b) and AFM phase images (c and d) of PTB7:PC71BM BHJ blend films without and with DPE.
recommendation of charge carriers.8 With addition of 1% DPE,
we observe that the density of the PC71BM aggregate domain
decreases and the domain size decreases to ∼500 nm. As the
DPE concentration increases from 2 to 4%, small aggregates of
PC71BM reappear again as indicated by the emergence of
darker color spots (Figure S4c−e). This implies that the DPE
facilitates the formation of ordered packing of the PTB7 rather
than just dispersing the PC71BM aggregates into smaller units
as with DIO.29 This is also consistent with the increased
absorption peak at 450 nm (Figure 2b). However, those dots
seem to disappear at a high concentration of DPE at 5% loading
(Figure S4f). This may due to the formation of ordered packing
of PTB7 on the surface that “presses” the PCBM toward the
bottom, so that these PCBM dots are difficult to detect in this
situation.
To probe the nanostructure features of the BHJ films, we
employ AFM (Figure 3). The active layer without DPE exhibits
a more coarse and spiky appearance, with a root-mean-square
(rms) roughness of 2.72 nm (Figure 3a,c). However, when the
active layer is loaded with 1% DPE, its roughness significantly
decreases to 0.94 nm (Figure S5). With the addition of DPE,
the morphology changes from rough, spiky features to a
substantially smoother surface. The changes in morphology are
less drastic between the films with 1−5 vol % DPE; the rms
roughness values are 0.94, 1.00, 1.09, 1.09, and 1.18 nm (Figure
S5). We speculate that in comparison to those with DIO, the
DPE blended films present a nanofibrillar-like pattern,
indicative of PTB7 lamellar formation (Figure 3b,d). In general,
DPE induces a planar and intermixed polymer−fullerene
network that has been associated with efficient charge
transport, permitting the high PCEs.39−41
To investigate the changes in orientation in the bulk, we
utilize GI-XRD to further characterize the packing condition in
PTB7:PC71BM BHJ (Figure 4). The BHJ of PTB7:PC71BM
with or without DIO shares the same peak at 2θ = 22°, with an
intensity of 350. This result agrees with conclusion in earlier
investigations that DIO does not change the crystallinity of
Figure 4. GIXRD image of the active layer with or without a solvent
additive.
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ACS Applied Materials & Interfaces
Figure 5. (a) Equivalent circuit model of the devices. (b) Cole−Cole plots of the devices with different DPE concentrations.
Figure 6. (a) Photovoltaic parameters and (b) extraction time and hole mobility of OPVs based on 4% DPE with a varied BHJ thickness.
PTB7.22,24,42 However, the BHJ of PTB7:PC71BM with 4%
DPE shows a peak intensity increase of ∼450 at 2θ = 19.8°,
implying that the DPE indeed facilitates ordered packing of the
PTB7. These results correlate well with the formation of a
cross-linked D/A network, resulting in the improvement of JSC
and FF.43,44 To further confirm the formation of a cross-linked
D/A network, we use CB to wash the film. The blended films
with 1−2 vol % DPE can be easily washed out, indicating a
weak interaction between the donor and acceptor (Figure S6a−
c). However, as the DPE concentration increases above 2%, the
blended film develops and becomes darker in color and more
difficult to wash (Figure S6d−f). This observation suggests that
the formation of a cross-linked D/A network can provide a
good pathway for charge transport and suppresses strong
PC71BM aggregation.7
3.3. Equivalent Circuit Model and Impedance Analysis. To examine the carrier dynamics in our devices, we show
the impedance spectroscopy results with an equivalent circuit
modeled (Figure 5a). We illustrate Cole−Cole plots, or the
characteristic retarded AC electrical response through the
device in the frequency range of 40 Hz to 1 MHz (Figure 5b).
We show two distinct regions: a large semicircle at low
frequencies associated with the carrier recombination process
and a quasi straight line in the high-frequency region containing
information about the diffusion mechanism.45 Real impedances
Z′ of OPVs decrease significantly with the addition of DPE,
which is related to the increased JSC.46 In this circuit model, the
constant phase element (CPE) suggests a nonideal behavior of
the capacitor. CPE is often used to represent a capacitance-like
element to compensate for inhomogeneity in the interface.
CPE is defined by two values, CPE-T and CPE-P. CPE-T is
capacitance and CPE-P a nonhomogeneity constant. If CPE-P
equals 1, then the CPE is identical to an ideal capacitor without
defects and/or a grain boundary.47 With the increase in DPE
concentration from 2 to 4%, CPE1-P increases from 0.87 to
0.99 (Table S1), which indicates the interface capacitance
between PTB7 and PCBM is electrically ideal in a more
homogeneous manner. Simultaneously, CPE1-T increases from
2.1 × 10−9 to 2.4 × 10−8, which indicates that the large active
layer surface provides more potential pathways for charge
transportation. However, as the DPE concentration increases to
5 vol %, both CPE1-T and CPE1-P begin to decrease, which
means there is an increase in the level of formation of grain
boundary defects that lead to charge recombination. R1 in
Figure 5a corresponds to the active layer resistance, and the
shunt pair R2 and C2 in Figure 5a is associated with the two
electrical contacts of the interfaces between the active layer and
electrodes.41 A high device R2 with 1% DPE suggests that the
interface between the active layer and the buffer layer is not
efficient for charge transport because of the amorphous
polymer ordering. For devices with 4% DPE, we observe that
R2 decreases to 3.0 × 105 Ω cm2, revealing that the loss due to
interfacial resistance between the active layer and the MoO3 is
minimized at optimal DPE loading and thus contributes to
better charge transport performance. The average of the carrier
transition time (τavg) in the active layer is defined by eq 1:
τavg = R1 × CPE − T
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Table 2. Summary of the Best Photovoltaic Performances with a Variety of Active Layer Thicknesses
solvent additive
4% DPE
3% DIO
a
thicknessa (nm)
VOC (V)
JSC (mA/cm2)
FF (%)
PCE (%)
RS (Ω cm2)
RP (Ω cm2)
JPh (mA/cm2)
±
±
±
±
±
±
±
0.71
0.72
0.72
0.72
0.72
0.71
0.65
15.79
16.64
15.69
15.48
16.32
15.79
13.11
48.2
48.3
53.2
52.7
49.2
49.0
54.0
5.30
5.65
6.05
5.83
5.76
5.46
4.60
6.35
1.72
0.65
1.83
1.47
3.71
9.85
165.02
173.62
205.48
213.59
159.78
168.17
285.62
14.33
15.20
14.48
14.24
15.07
14.64
13.49
80
100
120
140
160
180
180
5
5
5
5
10
10
10
The thickness was characterized by a step profiler, which exists with an error of approximately 5−10 nm.
The τavg values are estimated to be 4.5, 13.5, 37.1, and 27.2 μs
for devices with 0, 2, 4, and 5 vol % DPE, respectively (Table
S1). A longer τavg is associated with a lower recombination rate
and hence greater likelihood that carriers reach the electrode.46
Our results suggest that the addition of DPE suppresses the
charge recombination significantly in the bulk film and prolongs
the carrier lifetime by ∼9-fold, from 4.5 μs (devices without
DPE) to 37.1 μs (4% DPE).
3.4. Potential of DPE in Industrial Fabrication of Thick
OPVs. In the following section, we study the efficacy of DPE in
thick OPVs (>100 nm). Because there is ample evidence
showing that 4% DPE is the optimal loading, we fix this additive
concentration and measure the photovoltaic performance of
thick films under 1sun illumination (Figure 6a and Table 2). To
showcase the DPE potency in this scenario, we vary the
thickness of the active layer from 80 to 180 nm every 20 nm.
We demonstrate that DPE outperforms the conventional DIO
as an effective additive for devices with an active layer thickness
of 180 nm. Even though the BHJ thickness is doubled, the
addition of 4% DPE allows devices to maintain 90% of the
PCE. In comparison, the PCE of DIO-modified OPVs
decreases considerably by 66% with respect to that of the
100 nm thick control. Overall, the DPE-modified OPVs possess
similar VOC and JSC values when the thickness of the active layer
increases from 80 to 180 nm. However, the best FF of the
device is obtained at 120−140 nm, which can be ascribed to the
improved ordering of PTB7. Further increases in the BHJ film
thickness over 160 nm compromise the FF to 47%. Although
thicker BHJ films can absorb more light for photocurrent
generation, adverse effects such as an increase in the level of
bimolecular recombination induced by low charge carrier
mobility of the BHJ components reduce the FF.40 Hence, we
deduce that the recombination loss starts outweighing the
benefits in devices with thicknesses of >200 nm.
To substantiate our notion that DPE assists in the long
distance charge transfer in a thick BHJ, we measure the hole
mobility using a space-charge-limited current (SCLC) model
with a configuration of ITO/MoO3 (15 nm)/PTB7:PC71BM
(80−180 nm)/MoO3 (15 nm)/Ag (100 nm) based on the
Mott−Gurney law (eq 2)
J=
9
V2
εε0μ 3
8
d
respectively (Figure 6b). These results indicate that the hole
mobility increase leads to a more balanced charge transport,
resulting in the enhancement of the FF at 120 nm. Although
140 nm exhibits the highest hole mobilities, the increased RS
limits the PCE. The extraction time (Tex) is quantified to
represent the average time carriers extracted at d/2 and by
using the relationship Tex = d/2μE, in which E is the electric
field and d is the thickness of the active layer (Table 3).46 We
Table 3. Hole Mobilities and Charge Extraction Times in a
PTB7:PC71BM BHJ Blend Film with 4% DPE at Different
Thicknesses
BHJ film thickness (nm)
hole mobility (cm2 V−1 s−1)
80
100
120
140
160
180
1.28
1.72
2.88
7.14
4.27
3.68
×
×
×
×
×
×
−5
10
10−5
10−5
10−5
10−5
10−5
Texa (μs)
6.3
5.8
4.2
1.9
3.7
4.9
a
The value of E is approximated to be 5 × 104 V cm−1 obtained at a
forward bias of 0.5 V across a 100 nm thick film, which is close to the
net potential at the maximal power point of the cells.
observe that the short Tex of 4.9 μs in the thick films (180 nm)
reassures us that charge carriers in the DPE-enhanced films
overcome interfacial recombination and can thus be efficiently
extracted by the electrodes.49
4. CONCLUSION
In summary, we demonstrate donor-selective solvent additive
DPE is effective in increasing the active layer thickness in lowbandgap polymer-based OPVs, and a 3-fold PCE enhancement
is obtained (from 2 to 6%). Through detailed characterization
of the morphology and electric parameters, we reveal that DPE
(i) promotes the formation of nanofibrillar structure in the
active layer, (ii) induces efficient charge transport pathways,
and (iii) suppresses bimolecular recombination. Our results
highlight the tremendous potential of DPE to shift the
paradigm of research and industrial focus toward thick OPVs
and pave the way for the realization of scalable commercialization.
■
(2)
where μ is the charge carrier mobility, ε ≈ 3 is the relative
dielectric constant of the organic film, ε0 is the vacuum
dielectric constant of 8.85 × 10−12 F/m, and d is the film
thickness.48 The detailed J−V characteristics of the hole-only
devices are presented in Figure S7. The hole mobilities of
OPVs based on 4% DPE with a BHJ film thickness from 80 to
180 nm (every 20 nm) are 1.28 × 10−5, 1.72 × 10−5, 2.88 ×
10−5, 7.14 × 10−5, 4.27 × 10−5, and 3.68 × 10−5 cm2 V−1 s−1,
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.6b03453.
Parameters employed for the fitting of the impedance
spectra by use of an equivalent circuit model; figures
showing J−V curves, EQE, and UV−vis absorption for
devices based on varied DPE ratios; morphology images
15729
DOI: 10.1021/acsami.6b03453
ACS Appl. Mater. Interfaces 2016, 8, 15724−15731
Research Article
ACS Applied Materials & Interfaces
■
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photogeneration current density used in this work
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. Telephone: +86-28-83207157.
Fax: +86-28-83206123.
*E-mail: [email protected]. Telephone: (203) 432-2217.
Fax: (203) 432-4387.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This research was funded by the Foundation of the National
Natural Science Foundation of China (NSFC) (Grant No.
61177032), and the Foundation for Innovation Research
Groups of the NSFC (Grant No. 61421002). Also, this work
was sponsored by Science & Technology Department of
Sichuan Province via Grant No. 2016HH0027. The authors
gratefully acknowledge the National Science Foundation
(DMR-1410171) and NSF-PECASE award (CBET-0954985),
for partial support of this work. The Yale Institute for
Nanoscience and Quantum Engineering (YINQE) and NSF
MRSEC DMR 1119826 (CRISP) provided facility support.
■
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