Bis(trialkylsilyl oxide) Silicon Phthalocyanines: Understanding the Role of Solubility in Device Performance as Ternary Additives in Organic Photovoltaics

Article abstract of DOI:10.1021/acs.langmuir.9b03772

The use of ternary additives in organic photovoltaics is a promising route for improving overall device performance. Silicon phthalocyanines (SiPcs) are ideal candidates due to their absorption profile, low cost, and ease of synthesis and chemical tunability. However, to date, only a few examples have been reported and specific strategies for aiding in the design of improved ternary additives have not been established. In this study, we report a relationship between ternary additive solubility and device performance, demonstrating that device performance is maximized when the SiPc additive solubility is similar to that of the donor polymer (P3HT, in this case). This improved performance can be attributed to the favored interfacial precipitation of the SiPc when its solubility matches that of the other components of the thin film. The power conversion efficiency (PCE) varied from 2.4% to 3.4% by using axially substituted SiPcs with different solubilities, where the best ternary additive led to a 25% increase in PCE compared to that of the baseline device.

Full text of DOI:10.1021/acs.langmuir.9b03772

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge
Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage
Bis (tri-alkylsilyl oxide) silicon phthalocyanines:
understanding the role of solubility on device
performance as ternary additives in organic photovoltaics
Mario C. Vebber, Trevor M. Grant, Jaclyn L. Brusso, and Benoît H. Lessard
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b03772 • Publication Date (Web): 24 Feb 2020
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Bis (tri-alkylsilyl oxide) silicon phthalocyanines:
understanding the role of solubility on device
performance as ternary additives in organic
photovoltaics.
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Mário C. Vebber1, Trevor M. Grant1, Jaclyn Brusso2 and Benoît H. Lessard1*
1Department of Biological and Chemical Engineering, University of Ottawa, 161 Louis
Pasteur, Ottawa, Ontario K1N 6N5, Canada
2Department of Chemistry and Biomolecular Sciences, University of Ottawa, 150
Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada;
*
Author to whom correspondence should be addressed: benoit.lessard@uottawa.ca
Keywords: organic photovoltaics, ternary additive, phthalocyanine, solubility.
Abstract
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The use of ternary additives in organic photovoltaics (OPV) is a promising route to
improve overall device performance. Silicon phthalocyanines (SiPcs) are ideal
candidates due to their absorption profile, low cost and ease of synthesis and chemical
tunability. However, to date only a few examples have been reported and specific
strategies to help design improved ternary additives have not been established. In this
study, we report a relationship between ternary additive solubility and device
performance, demonstrating that device performance is maximized when the SiPc
additive solubility is similar to that of the donor polymer (P3HT, in this case). This
improved performance can be attributed to the favored interfacial precipitation of the
SiPcs when its solubility matches that of the other components of the thin film. The
power conversion efficiency (PCE) varied from 2.4% to 3.4% by using axially
substituted SiPcs with different solubilities, where the best ternary additive led to a 25%
increase in PCE compared to the baseline device.
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Introduction
Organic photovoltaics (OPVs) are an emerging solar energy technology, presenting
a number of advantages over their inorganic counterparts, such as low manufacturing
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cost, ease of integration onto flexible substrates, mechanical robustness and
recyclability.1–4 Recently, single-junction OPVs, on a laboratory scale, achieved record
power conversion efficiency (PCE) between 15-17%.5,6 As OPV performance
continues to rise and with the promise of high throughput manufacturing, it is predicted
that their price will plunge below that of conventional inorganic cells.7
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One of the main challenges with bulk heterojunction (BHJ) OPVs is to maximize light
harvesting across the visible and near-infrared regions of the solar spectrum as most
organic compounds tend to demonstrate a relatively narrow absorption band in the
visible spectrum.8,9 The addition of a photosensitizing ternary additive to the
donor/acceptor blend is a straightforward approach to increase the overall absorption
of the active layer. Secondary functionality of these additives can also be achieved
such as enhancement of the film microstructure,10,11 favoring phase separation in the
nanoscale, and compatibilization of energy levels between donor and acceptor,12
which can increase charge separation and even enhance device stability, through the
use of crosslinking groups.13,14
One class of materials that has been widely used for application in electronic devices
are metal/metalloid containing phthalocyanines (MPcs). While divalent MPcs (Zn, Cu,
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etc) are widely reported in the literature as active materials, tetravalent MPcs are less
studied. Tetravalent MPcs, such as silicon phthalocyanines (SiPcs), have two axial
bonds available for chemical modification, imparting broader chemical versatility and
facilitating the relatively easy tuning of electronic and physical properties.15–18 MPcs
have also demonstrated the ability to act as non-fullerene electron acceptors in bulk
heterojunction (BHJ) OPVs.18 Furthermore, these dyes have found application as
commercial photoreceptors for almost 50 years and, prior to that, as a commercial
pigment.15 The chemistry and purification of MPcs is relatively simple and
straightforward compared to conventional organic semiconductors, making them
desirable candidates for commercially viable OPV devices.15–18 SiPcs have a high
molar extinction coefficient and absorb light in the near infrared region, complimenting
well-established donor/acceptor pairs, such as poly-3-hexylthiophene (P3HT) and
phenyl-C61-butyric-acid-methyl-ester (PCBM), that often show poor performance in
that region of the solar spectrum.13,18,19
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Honda et al. first reported the use of bis(trihexyl)silyl oxide SiPc as an additive in a
P3HT / PC61BM BHJ OPV and obtained a 50% increase in current density (Jsc) . 20,21
The same group also found that the SiPc derivative had a high performance as an
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additive in OPVs due to their tendency to migrate to the donor-acceptor interface,
where they improve charge collection and charge transfer between the donor and the
acceptor.22 Silicon napthalocyanines (SiNcs) have also been applied in OPVs, with
similar increases in Jsc, by harvesting near infra-red light.23 Alternatively, Ke et al.
functionalized SiPcs with pyrenes in the axial position, resulting in effective ternary
additives with increased absorption range, which were reported to increase Jsc by 20%
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and PCE by 50%. It has been demonstrated that the absorption coverage can be
further improved by a combination of different SiPcs and SiNcs as ternary and
quaternary additives.20,25 Moreover, SiPc derivatives have also been applied in light-
emitting diodes (OLEDs) 26–28 and in organic thin film transistors (OTFTs).29–33
.
While most reports focus on energy level tuning of ternary additive materials, less
attention is given to the role of other physical properties, such as solubility and crystal
structure.6,34 In the case of SiPcs, it has been shown that axial functionalization has
little to no effect on the frontier orbital energy levels or optical properties; however, it
does lead to significant effects in molecular packing.16,35
The solubility of active materials is often analyzed in the search for greener solvents,
yet studies focused on the relationship between solubility and the performance of the
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resulting devices are surprisingly scarce, especially given the interest in solution
processable OPVs.36,37 Troshin et al. have demonstrated that the solubility of fullerene
acceptors significantly impacts the PCE of P3HT:fullerene devices, and that best
performance is achieved when the solubility of the fullerene acceptor is close to that of
P3HT, leading to favorable nanomorphology.38 However, no similar studies have been
performed on ternary additives. To that end, we present herein the synthesis and
characterization of nine different SiPcs functionalized with a range of axial silanes, and
their employment as additives in P3HT / PC61BM BHJ OPVs. This study demonstrates
that a small change in structure has a significant effect on the solubility and, ultimately,
the performance of the resulting ternary BHJ/OPV devices. This investigation therefore
represents the first report that facilitates the establishment of new design rules for
ternary additives.
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Experimental Section
Materials
N-butyldimethylchlorosilane (>95%), hexyldimethylchlorosilane (>95%), n-
octyldimethylchlorosilane (>95%), n-dodecyldimethylchlorosilane (>95%), tri-n-
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ethylchlorosilane (>95%), tri-n-butylchlorosilane (>95%), tri-n-hexylchlorosilane
(>95%) and n-octyldiisopropylchlorosilane (>95%) were purchased from Gelest and
used as received. P3HT (MW = 75 kDa, 96% regioregular) was purchased from Rieke
Metals and PC61BM from Nano-C, respectively. All solvents were purchased from
commercial suppliers and used without further purification. Dichlorosilicon
phthalocyanine (Cl2-SiPc) 39 was synthesized according to reported procedures in the
literature.
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Synthesis of silicon phthalocyanine ternary additives
The general synthetic procedure for SiPc axial functionalization was performed as
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follows : Cl2-SiPc (1.6 mmol), sodium hydroxide (8.6 mmol), trialkylchlorosilane (6
mmol), Aliquat HTA-1 (0.015 g) and chlorobenzene (10 ml) were added to a 100 ml
flask and were stirred under reflux (132 °C). After 1 h, additional trialkylchlorosilane
(1.9 mmol) was added to the reaction medium, which was left stirring at reflux for
another hour. At this point, additional sodium hydroxide (3.4 mmol) and
trialkylchlorosilane (1.9 mmol) were added to the mixture and refluxed for further 4 h
before removing from heat. The dark-blue solution was then filtered to remove a purple
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residue, and the solvent was removed under reduced pressure. Methanol was added
to the resulting residue affording the desired product, which was isolated upon filtration,
washed with methanol and dried in a vacuum oven. Purification was carried out by
train sublimation (180 °C to 250 °C, 130 mTorr) before their application in OPVs. Using
this procedure, the soluble X2-SiPcs shown in Fig. 1 were produced:
(1) Bis(thexyldimethylsilyl oxide) silicon phthalocyanine: 1H NMR 9.60 – 9.66 (m, 8
H), 8.28 – 8.34 (m, 8H), -0.95 – -1.0 (m, 2 H), -1.0 – -1.05 (d, 12 H) and -1.52 – -1.44
ppm
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13
(s, 12 H). C NMR 148 , 136 , 130 , 123 (Aromatic); 32 , 22 , 17 , 16 and -5 ppm
(Alkyl). MS (EI) expected mass: 858.37, obtained mass: 858.3. Yield: 0.45 g (30%).
(2) Bis(n-octyldimethylsilyl oxide) silicon phthalocyanine: 1H NMR 9.60 – 9.66 (m, 8
H), 8.28 – 8.34 (m, 8H), 1.11 – 1.27 (m, 4 H), 0.92 – 1.04 (m, 4 H), 0.85 – 0.94 (t,
6H), 0.68 – 0.80 (m, 4H), 0.25 – 0.37 (m, 4H), -0.23 – -0.10 (m, 4H) and -1.17 – -1.04
ppm (m, 4H). 13C NMR 148 , 136 , 130 , 123 (Aromatic); 32 (2), 29 , 23 , 21 , 15 , 14
n, -4 ppm (Alkyl). MS (EI) expected mass: 914.43, obtained mass: 914.4. Yield: 0.45
g (30%).
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(3) Bis(n-dodecyldimethylsilyl oxide) silicon phthalocyanine: H NMR 9.60 – 9.66
(m, 8 H), 8.28 – 8.34 (m, 8H), 1.20 – 1.39 (m, 20 H), 1.10 – 1.19 (m, 4 H), 0.95 – 1.02
(m, 4H), 0.88 – 0.95 (t, 6H), 0.64 – 0.73 (m, 4H), 0.19 – 0.31 (m, 4H), -0.29 – -0.18
(m, 4H) and -1.42 – -1.31 ppm (m, 4H). 13C NMR 148 , 136 , 130 , 123 (Aromatic); 32
(2), 29 (5), 23 , 21 , 15 , 14 n, -4 ppm (Alkyl). MS (EI) expected mass: 1026.56,
obtained mass: 1026.5. Yield: 0.50 g (28%).
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(4) Bis(tri-n-ethyl oxide) silicon phthalocyanine: 1H NMR 9.60 – 9.66 (m, 8 H), 8.28
– 8.34 (m, 8H) and -1.30 – -1.20 ppm (t, 12 H). 13C NMR 148 , 136 , 130 , 123
(Aromatic);
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and
3 ppm (Alkyl). MS (EI) expected mass: 802.31, obtained mass: 802.3. Yield: 0.28 g
(20%).
(5) Bis(triisobutylsilyl oxide) silicon phthalocyanine: 1H NMR 9.60 – 9.66 (m, 8 H),
8.28 – 8.34 (m, 8H), -0.58 – -0.50 (d, 12H) and -0.96 – -0.85 ppm (m, 2H). 13C NMR
148 , 136 , 130 , 123 (Aromatic); 25 , 24 and 22 ppm (Alkyl). MS (EI) expected mass:
970.49, obtained mass: 970.5. Yield: 0.49 g (30%).
(6) Bis(tri-n-butylsilyl oxide) silicon phthalocyanine: 1H NMR 9.60 – 9.66 (m, 8 H),
8.28 – 8.34 (m, 8H), -0.12 – 0.12 (m, 30H) and -1.36 – -1.22 ppm (m, 12H). Yield:
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0.52g
13C NMR and mass spec are published elsewhere. 18
(30%).
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(7) Bis(tri-n-hexylsilyl oxide) silicon phthalocyanine: 1H NMR 9.60 – 9.66 (m, 8 H),
8.28 – 8.34 (m, 8H), 0.73 – 0.91 (m, 12H), 0.61 – 0.74 (t, 18H), 0.28 – 0.42 (m, 12H),
-0.12 – 0.06 (m, 12H) and -1.38 – -1.22 ppm (m, 12H). Yield: 0.60g (31%).13C NMR
and mass spec are published elsewhere. 41
(8) Bis(diisopropyloctylsilyl oxide) silicon phthalocyanine: 1H NMR 9.60 – 9.66 (m, 8
H), 8.28 – 8.34 (m, 8H), 1.16 to 1.31 (m, 4H), 1.01 to 1.14 (m, 4 H), 0.77 to 0.95 (m,
8H), 0.33 to 0.45 (m, 4H), 0.05 to 0.22 (m, 4H), -1.05 to -0.90 (m, 4H) and -1.40 to -
1.20
(m,
20H)
and
-2.10 to -2.00 ppm (m, 2H). 13C NMR 148, 136, 130, 123 (Aromatic); 33 , 32, 29, 28,
22, 21, 15, 14, 11 and 10 ppm (Alkyl). MS (EI) expected mass: 1026.56, obtained
mass: 1026.5. Yield: 0.28 g (17%).
(9) Bis(n-butyldimethyl oxide) silicon phthalocyanine: No appreciable amount of this
compound was obtained.
Mass obtained from MS was defined according to the fragment corresponding to the
entire molecule.
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Figure 1. Axially substituted X2-SiPcs synthesized and characterized in the present
study.
Materials Characterizations
1H NMR spectra were obtained on a Bruker Avance II spectrometer in CDCl3 and
operating frequency of 400 MHz. Molecular absorption spectra in the ultraviolet and
visible regions (UV-Vis) were recorded on an Ocean Optics Flame spectrophotometer,
using a DH-2000 light source and a quartz cuvette with a path length of 10 mm. Cyclic
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voltammograms were obtained using a VersaSTAT 3 potentiostat, a polished platinum
disk as working electrode, a coiled platinum wire as counter electrode, and an Ag/AgCl
electrode as reference. Three cycles from 0 V to 1.6 V were recorded at a scan rate of
0.1 V/s. Such experiments were carried out in dichloromethane (DCM) solutions with
tetrabutylammonium perchlorate as the supporting electrolyte. Highest occupied
molecular orbital (HOMO) energy levels were estimated according to the empirical
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correlation EHOMO (eV) = -(Eox,onset
- Eox Fc/Fc+,onset) - 4.80 eV, where Eox,onset and
EoxFc/Fc+,onset are the onset oxidation potentials of the sample and of the ferrocene
standard, respectively. Solubility measurements were conducted by mixing 10, 20 or
40 mg of the axially substituted SiPcs in 0.15 ml of 1,2-dichlorobenzene and stirring
the mixture for 1 h at 70°C, before allowing these mixtures to cool down to room
temperature. The samples were then filtered through a 0.25 μm Teflon syringe filter
and 100 μL samples of filtrate were collected, dried at 150°C under vaccum for 2 h and
then weighted to calculate the solubility.
Device Preparation
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The OPV devices were fabricated on indium tin oxide (ITO) coated glass slides
(1 in x 1 in), which were cleaned in an ultrasonic bath with soapy water, deionized
water, acetone then methanol, before being treated with air plasma for 15 min. 150 uL
of a zinc solution constituted of zinc acetate (0.195 g), ethanolamine (0.055 ml) and
ethanol (6 ml) was spincoated on the ITO substrate at 2000 RPM for 1 min. These
films were annealed at 180°C for 1 h, leading to the formation of the ZnO electron
transport layer. The active layer solutions were prepared by dissolving P3HT and
PC61BM in DCB at a 1:0.8 ratio. Different amounts of X2-SiPcs were added to the
aforementioned mixture to form solutions with P3HT concentration of 20 mg ml-1 and
additive ratios of 2.7 wt %, 3.7 wt % and 4.8 wt %. These solutions were spincoated at
1000 RPM, for 35s inside a N2 glovebox. The films were dried at room temperature,
open to the glovebox atmosphere. Finally, the devices were moved into an evaporator
chamber where MoO3 (7 nm) and Ag (70 nm) films were evaporated at a pressure of
10-6 torr to make 5 devices per substrate, with areas of 0.32 cm2, as defined by shadow
masks. The device structure and its components’ energy levels are depicted in Fig. 1.
Current density vs. voltage curves were obtained with a Kiethley 2400 equipment,
under 1000 W m-2 illumination from an AM1.5 xenon lamp solar simulator, inside a
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glovebox. Devices were encapsulated using an optical adhesive (Norland NOA61),
which was cured under an UV lamp (6 W), and a glass coverslip before EQE
measurements in air, in a Newport Quantx-300 equipment.
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Results and Discussion
Synthesis and Characterization of SiPcs
The X2-SiPcs described here were all synthesized using the same experimental
procedure, in which various trialkylchlorosilanes were used to afford the nine different
derivatives outlined in Fig. 1. The overall yields from diiminoisoindoline to axially
substituted SiPcs varied from 20 to 40% by weight (see Experimental Section for
details). The reactions affording the lowest yields were associated with low MW
silanes, such as (4) and (9), which may be attributed to the preferential formation of
siloxane (i.e., R–O-Si-O–R) byproducts. In fact, this reoccurring impurity may be
present even after sublimation, which can be seen in the 1H NMR spectrum as a strong
singlet with a chemical shift of approximately 0.5 ppm (see ESI figures S1-S19 for
NMR). Nonetheless, running the sublimation overnight or washing the solids with
hexanes are both effective ways to remove this impurity. Unfortunately, no appreciable
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amounts of compound (9) were obtained due to the formation of this byproduct, which
inhibited its incorporation into devices or further characterization. Nevertheless, this is
a simple and straightforward method to obtain various X2-SiPcs that does not require
dry solvents or oxygen free environments.
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UV-Vis Spectroscopy and Cyclic Voltammetry
The eight X2-SiPc compounds were characterized optically and electrochemically to
estimate their optical bandgap (Eg), highest occupied molecular orbital energy levels
(EHOMO) and lowest unoccupied molecular orbital energy levels, ELUMO (Table 1).
Table 1. Optical and electrochemical (CV) characterization of silicon phthalocyanine
derivatives
a)
a)
b)
c)
c)
Compound
λmax
671
672
670
671
Eg
EOx
EHOMO
-5.31
-5.29
-5.29
-5.30
ELUMO
-3.50
-3.49
-3.48
-3.49
(1)
(2)
(3)
(4)
1.81
1.80
1.81
1.81
0.98
0.96
0.96
0.97
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(5)
672
669
669
672
1.81
1.82
1.82
1.81
0.95
0.98
0.98
0.95
-5.28
-5.31
-5.31
-5.28
-3.47
-3.49
-3.49
-3.47
(6) 18
(7) 18
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(8)
a)
peak absorbance determined in DCM solution, Eg = energy bandgap determined
from the onset of UV-Vis absorption spectrum.
b) half wave potential of oxidation measured using an Ag/AgCl reference electrode
c)
and platinum rod working electrode in DCM solution. EHOMO = highest occupied
molecular orbital energy level determined using the empirical correlationEHOMO (eV) =
-(Eox,onset
- Eox Fc/Fc+,onset) - 4.80. Lowest unoccupied molecular orbital energy level
calculated from ELUMO = EHOMO +Eg.
As expected, all SiPc derivatives have effectively the same energy levels and optical
bandgap. The UV-Vis and CV spectra are presented in Fig. 2, except for compounds
(6) and (7), which have been published elsewhere.18 These values are identical to our
previously published bis(tri-n-hexylsilyl) SiPc and bis(tri-n-butylsilyl) SiPc derivates (6
and 7 from Fig. 1).18 For the current study these results indicate that the axial
substituents have no effect on the absorption profiles, or frontier molecular orbitals,
which is key when comparing their performance in OPVs with respect to their physical
properties, such as solubility.
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Figure 2. a) UV-Vis spectrum of the X2-SiPcs and b) CV spectra and X2-SiPcs for
HOMO level estimation.
Solubility
The solubility of P3HT, PC61BM and all eight X2-SiPcs was determined in
dichlorobenzene (see experimental section for details) and plotted in Fig. 3. It is clear
that small changes in the alkylsilanes functional groups leads to a large difference in
the solubility of the compounds, which varies from 8 to 190 mg mL-1. It would appear
that an increase in roughly one order of magnitude in solubility was observed for SiPc
derivatives with alkyl chains going from 6 to 8 carbons (Fig. 3). This type of significant
increase in solubility has been observed in other small molecules such as fullerenes for example,
where changing the alkyl solubilizing group from ethyl to butyl, increases the solubility from
<1 mg ml-1 to >70 mg ml-1.38 Unsubstituted SiPcs are very insoluble due to strong π-π
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intramolecular interactions. Since the SiPc radius (0.86 nm) 15 is roughly equivalent to
the length of a hexyl group (0.82 nm), one can hypothesize that alkyl groups longer
than 6 carbons in the axial position will efficiently disrupt ring-ring interactions and
cause the observed jump in solubility.
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P3HT and PC61BM were characterized to have a solubility of 100 and 140 mg
mL-1, respectively. It is important to note that the previously reported solubilities for
PC61BM and P3HT are within 50 to 80 mg mL-1,38 which are obtained by UV-Vis
absorption spectroscopy. Nevertheless, all solubilities in this study were measured
with the same procedure to ensure reliable comparisons can be made. These results
further suggest that the changes in axial silane groups on the SiPcs has little to no
effect on optical, electrochemical or hydrophobicity properties but has a significant
impact on the solubility.
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Figure 3. Solubility of different X2-SiPcs, P3HT and PCBM in DCB.
Contact Angle
Contact angle measurements were carried out for X2-SiPc, P3HT and PC61BM
films, spincoated on octylctrichlorosilane (OTS) treated glass slides and the surface
energy was estimated using the iterative Neumann’s method 42. The results are shown
in Fig. 4. The X2-SiPc films have intermediate surface energies to those of P3HT and
PC61BM. These results are in accordance with previous reports that found that X2-
SiPcs compatibilize the surface energy of P3HT and PCBM and tend to migrate to the
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donor/acceptor interface 12. The measured surface energy was also very similar for all
X2-SiPcs films (22.5 - 24.5 N mm-1), which indicates that similar surface
compatibilization effects can be expected when X2-SiPcs are added to P3HT/PC61BM
BHJ. Previously18 we have demonstrated via atomic force microscopy (AFM) that the
addition of X2-SiPcs (at concentrations relevant to the current study), does not lead to
significant changes in P3HT/PC61BM BHJ morphology. The discrepancy observed for
compound (4) is attributed to bad film formation, resulting from its poor solubility (8 mg
ml-1) which led to nonhomogeneous films.
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Figure 4. Contact angle and corresponding surface energy measurements of X2-SiPc
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OPV performance
The eight X2-SiPc derivatives were employed as ternary additives in P3HT/PC61BM
BHJ OPV devices in three different concentrations: 2.7 wt%, 3.7 wt% and 4.8 wt%.
The results are presented on Table 2. The Jsc was measured by both current density
vs. voltage (I-V) curves and EQE measurements. In general, Jsc values obtained with
EQE are greater than those measured with the I-V curve. This is a common feature of
thin-film devices due to photo current barrier effects.43 Nevertheless, the difference is
within 5-10%, which is consistent and acceptable for the purposes of this comparative
work.
Table 2. Current density – voltage results for all the assembled OPVs and Jsc
estimation from EQE measurements.
Jsc
P3HT:PCB
M:X2-SiPc a)
Compound
Vocb)
Jsc b)
FFb)
PCE b)
(EQE) c)
Baseline
1:0.8:0
0.53 ± 0.00
-9.44 ± 0.31
10.33
0.54 ± 0.00 2.70 ± 0.07
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1:0.8:0.05
1:0.8:0.07
1:0.8:0.09
1:0.8:0.05
1:0.8:0.07
1:0.8:0.09
1:0.8:0.05
1:0.8:0.07
1:0.8:0.09
1:0.8:0.05
1:0.8:0.07
1:0.8:0.09
1:0.8:0.05
1:0.8:0.07
1:0.8:0.09
1:0.8:0.05
1:0.8:0.07
1:0.8:0.09
1:0.8:0.05
1:0.8:0.07
1:0.8:0.09
1:0.8:0.05
1:0.8:0.07
1:0.8:0.09
0.50 ± 0.01
0.51 ± 0.01
0.52 ± 0.01
0.53 ± 0.00
0.54 ± 0.01
0.54 ± 0.01
0.57 ± 0.01
0.54 ± 0.01
0.56 ± 0.01
0.52 ± 0.00
0.53 ± 0.00
0.52 ± 0.01
0.52 ± 0.00
0.54 ± 0.01
0.54 ± 0.01
0.52 ± 0.00
0.54 ± 0.01
0.56 ± 0.01
0.55 ± 0.00
0.55 ± 0.00
0.56 ± 0.00
0.55 ± 0.00
0.55 ± 0.00
0.56 ± 0.00
-9.80 ± 0.75
9.84
0.48 ± 0.03 2.38 ± 0.37
0.50 ± 0.02 2.37 ± 0.14
0.51 ± 0.02 2.50 ± 0.26
0.56 ± 0.01 3.07 ± 0.09
0.57 ± 0.01 3.27 ± 0.08
0.56 ± 0.02 3.32 ± 0.12
0.59 ± 0.01 3.40 ± 0.12
0.57 ± 0.01 3.16 ± 0.05
0.57 ± 0.01 3.38 ± 0.09
0.50 ± 0.01 2.74 ± 0.10
0.53 ± 0.01 3.08 ± 0.11
0.51 ± 0.00 2.87 ± 0.13
0.53 ± 0.01 3.02 ± 0.11
0.56 ± 0.02 3.24 ± 0.07
0.56 ± 0.01 3.28 ± 0.08
0.54 ± 0.00 3.11 ± 0.06
0.57 ± 0.01 3.27 ± 0.08
0.57 ± 0.01 3.38 ± 0.09
0.57 ± 0.01 3.18 ± 0.08
0.58 ± 0.01 3.43 ± 0.05
0.58 ± 0.01 3.43 ± 0.11
0.58 ± 0.01 3.13 ± 0.12
0.59 ± 0.01 3.25 ± 0.12
0.58 ± 0.00 3.08 ± 0.07
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
-9.18 ± 0.19
-9.34 ± 0.48
-10.35 ±0.34
-10.62 ±0.28
-10.98 ± 0.27
-10.13 ± 0.30
-10.32 ±0.22
-10.65 ± 0.41
-10.64 ± 0.29
-11.00 ±0.29
-10.81 ± 0.27
-10.78 ± 0.33
-10.78 ± 0.10
-10.71 ±0.28
-10.97 ± 0.27
-10.51 ± 0.25
-10.65 ± 0.41
-10.12 ±0.23
-10.80 ± 0.16
-10.55 ± 0.27
-9.75 ± 0.24
-10.05 ± 0.24
-9.54 ± 0.16
8.80
9.51
11.49
11.81
11.76
12.07
11.73
11.69
12.79
11.60
11.14
12.16
12.56
12.00
10.97
11.79
11.88
11.10
11.78
11.14
10.34
10.96
10.63
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a) Made with 20 mg ml-1 P3HT concentration, at 1000 RPM, dried in N2 atmosphere,
no annealing.
b) Measured from I-V curves, average of 5 devices, irradiation power of 1000 W m-2.
c)
Calculated by integrating the EQE spectra.
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With the exception of compound (1), the inclusion of all the X2-SiPcs improve the
overall device performance, through increased Jsc and FF. Jsc is increased due to
extended light harvesting at long wavelengths (>600 nm), which can be observed in
the EQE spectra and overlaps with the absorption of SiPc chromophore (Fig. 2). The
additives also effectively collect and transport charges at the P3HT and PC61BM
interface, which causes the P3HT:PC61BM quantum efficiency contribution (300 nm to
600 nm) to increase as well. Fig. 5 displays the EQE spectra of every device listed on
Table 2, where these features can be observed. In most devices, the X2-SiPc EQE
contribution (≈675 nm) is almost a perfect match to the absorption profile of X2-SiPc in
solution, which indicates little to no solid state aggregation of the X2-SiPc,44 further
suggesting good dispersion of the SiPc derivative in the P3HT/PC61BM blend. We
observed this type of behavior for all SiPc derivatives except for (4), (Figure 5d) the
least soluble of the X2-SiPc. At greater concentration of (4) we observe a red-shift and
broadening of the EQE band, characteristic of solid-state interactions of SiPc
aggregates. In fact, Honda et al.12 demonstrated, through transient absorption
spectroscopy, that bis(tri-n-hexylsilyl)-SiPc normally migrates to the amorphous
P3HT/PC61BM interface, where its efficiency is greatest. Taking into consideration the
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chemical similarities between all SiPc derivatives and the similar loading
concentrations, we expect similar migration mechanism when blended in ternary films.
We surmise the use of low solubility X2-SiPcs as a ternary additive lead to poor OPV
performance due to the formation of tertiary semicrystalline domains. These
aggregates form a third independent phase that is not found at the P3HT and PC61BM
interface and therefore disrupts efficient charge transfer, leading to no appreciable
increase in EQE between 300 nm and 600 nm. While the devices containing compound
(1) (second least soluble) do not show the same red-shift of the SiPc peak (Fig. 5a), a
decrease in the P3HT/ PC61BM EQE contribution (300 nm to 600 nm) is observed,
which indicates that the preferred morphology is also not achieved.
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Figure 5. EQE spectra of all the devices produced in the present worked divided by X2-
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In some instances, such as compounds (2) and (6), the greatest X2-SiPc contribution
to the photocurrent (e.g., EQE at 675 nm ≈ 42%) is achieved at the highest additive
concentration (4.8%, pink line in Fig. 5b and 5f), but, overall, is approximately the same
when compared to the medium concentration (3.7%, blue line Fig. 5). However, the
best P3HT:PC61BM photocurrent generation is observed at the 3.7% concentration, as
it is the case for compounds (2), (5), (6), (7) and (8) (Fig 5b, 5e, 5f, 5g and 5h) that
display up to 10% EQE increase in the 300 – 600nm region. At lower additive loading
concentrations, not enough SiPc is located at the interface; however, at greater
concentrations, it is expected that SiPc is located not only at the interface leading to
trapped charges. Therefore, the 3.7% concentration is often the one with the highest
efficiency because it maximizes the amount of SiPc at the interface without diffusing
into the bulk of the film.
By simply changing the X2-SiPc, which only differ in solubility, the overall BHJ OPV
PCE varies from 2.4% to 3.4%, compared to PCE = 2.7% for the P3HT:PC61BM
baseline. Therefore, even though all the X2-SiPc solubilities are above processability
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(minimum
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8 mg ml-1), the choice of additive can lead to improved or reduced OPV performance.
Fig. 6 shows the correlation between solubility PCE, Jsc, Voc and fill factor, for the
ternary BHJ OPV devices with the addition of 3.7 wt% X2-SiPc.
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The greatest PCE was obtained using compound (7), whose solubility is the closest
to that of P3HT. We surmise this matching of solubility leads to more SiPc additive to
be located at the donor:acceptor interface when the film dries. The same trend was
found by Troshin et al.38 where the authors reported that the best performance for
P3HT/fullerene BHJ is achieved when the solubility of the donor/acceptor pair is
similar, which leads to better nanomorphology. It would appear that in the case of the
X2-SiPcs, matching the solubility of P3HT has a stronger effect than matching
PC61BM’s solubilty. We expect this to be a result of X2-SiPc’s stronger interaction with P3HT
than with PC61BM 12,22, i.e. they do not tend to solubilize in the PC61BM domains. Nonetheless,
this is an area for further investigation.
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Figure 6. Solubility correlation with OPV performance parameters a) PCE, b) Voc, c)
Jsc and d) Fill Factor.
The solubility of the X2-SiPc ternary additive will determine the type of nanostructure
formed during solvent evaporation, as a result of coprecipitation thermodynamics.22,38
More specifically, a low solubility additive, such as (4) and (1), will tend to precipitate
before P3HT and PC61BM and may be forced beyond the P3HT and PC61BM interface,
or even form its own crystalline domains. When the solubility of the additives is similar
or higher than that of the other components of the BHJ, the precipitation kinetic will
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favor interfacial coverage. The proposed mechanism can be better visualized in Fig.
7. Honda et al.12 found that the P3HT:PC61BM nanostructure is composed of relatively
pure crystalline phases and an amorphous interfacial phase (Fig.7 a), and that the
SiPcs performance is greatest when it is found at the amorphous interface where it can
mediate the charge transfer between P3HT and PC61BM, thereby contributing to the
photogeneration of charges at higher wavelengths. Obviously, the concentration of X2-
SiPc additive will also play a role in the drying mechanics, as previously stated. A low
additive concentration can prevent the scenario illustrated in Fig. 7 b) from happening;
however, it will also limit the possible performance increase.
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