Solution-processed boron subphthalocyanine derivatives as acceptors for organic bulk-heterojunction solar cells

Article abstract of DOI:10.1039/c5ta00715a

We report the fabrication of solution-processed bulk heterojunction devices from subphthalocyanine (SubPc) units as the acceptor component and conventional polymeric donor materials such as MEH-PPV, P3HT and PTB7. The high solubility of the SubPc derivatives facilitated the formation of efficient donor/acceptor networks and provided power conversion efficiencies of 0.4% with MEH-PPV, 1.1% with P3HT and 3.5% with PTB7. A clear contribution of photon harvesting by the acceptor was identified from the external quantum efficiency spectra. Analysis of the current-voltage characteristics and photoluminescence quenching revealed trap-assisted and geminate recombination as a loss mechanism. Our results show that solution-processable SubPcs are a promising alternative to fullerenes for polymer solar cells.

Full text of DOI:10.1039/c5ta00715a

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Materials Chemistry A
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Solution-processed boron subphthalocyanine
derivatives as acceptors for organic bulk-
heterojunction solar cells†
Cite this: DOI: 10.1039/c5ta00715a
a
b
c
b
*
Bernd Ebenhoch,‡ Nor B. A. Prasetya,‡ Vincent M. Rotello, Graeme Cooke
a
*
and Ifor D. W. Samuel
We report the fabrication of solution-processed bulk heterojunction devices from subphthalocyanine
(SubPc) units as the acceptor component and conventional polymeric donor materials such as MEH-
PPV, P3HT and PTB7. The high solubility of the SubPc derivatives facilitated the formation of efficient
donor/acceptor networks and provided power conversion efficiencies of 0.4% with MEH-PPV, 1.1% with
P3HT and 3.5% with PTB7. A clear contribution of photon harvesting by the acceptor was identified from
the external quantum efficiency spectra. Analysis of the current–voltage characteristics and
photoluminescence quenching revealed trap-assisted and geminate recombination as a loss mechanism.
Our results show that solution-processable SubPcs are a promising alternative to fullerenes for polymer
solar cells.
Received 29th January 2015
Accepted 21st February 2015
DOI: 10.1039/c5ta00715a
www.rsc.org/MaterialsA
physical properties, but also can prevent aggregation in the
solution and solid state.9 These features coupled with their
Introduction
convenient synthesis have made SubPcs attractive building
blocks for organic electronics,10 photonics11–13 and photovol-
taics.14,15 Their incorporation into the active layer of solar cells
is particularly attractive due to their high absorption coeffi-
cients in the visible region.10 Whereas SubPc molecules were
originally used as donor materials in vacuum deposited solar
Solution-processed organic solar cells are attractive candidates
for low-cost and large area light to electricity converters. The
deposition of the active layer from solution has signicant
advantages over other methods, due to the high-speed
production of roll-to-roll fabrication.1 Fullerenes have become
the archetypal acceptor in efficient solution-processed bulk
heterojunction (BHJ) devices. However, fullerenes have impor-
tant limitations, such as high synthesis costs and a low
absorption coefficient in the visible region of the solar spec-
trum.2 To address the issues associated with fullerene based
acceptors, efforts have been directed towards the development
of new acceptor systems and were thoroughly discussed in
recent reviews.3–5 These systems are generally less efficient than
fullerenes, principally due to strong aggregation, which
increases the phase separation and lowers the power conversion
efficiency (PCE) of their resulting devices.6,7
14,16,17
18,19
cells, with C60
,
or C70
as acceptor, their electron
accepting properties have also recently been investigated20–22
and a high electron mobility of 8 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1 has been
reported.23 Solar cells containing SubPc molecules achieved
the highest efficiency of 8.2% for fullerene-free solar cells.24
Whilst vacuum-processing has been effectively used to
produce solar cells from SubPc derivatives,25 the good solu-
bility characteristics of halogenated SubPc derivatives also
makes these materials viable candidates for solution-pro-
cessed BHJ solar cells.26,27 In this paper we report the synthetic
procedures, chemical and optical properties and a comparison
of the device properties of BHJ solar cells of halogenated
SubPc derivatives 1 and 2 (Scheme 1).
Boron subphthalocyanine (SubPc)8 derivatives have
received considerable interest in recent years due to their
cone-shaped structure, which not only confers interesting
aOrganic Semiconductor Centre, SUPA, School of Physics & Astronomy, University of St
Andrews, St. Andrews, KY16 9SS, UK. E-mail: idws@st-andrews.ac.uk
bGlasgow Centre for Physical Organic Chemistry, WestCHEM, School of Chemistry,
University of Glasgow, Glasgow, G12 8QQ, UK. E-mail: graeme.cooke@glasgow.ac.uk
cDepartment of Chemistry, University of Massachusetts Amherst, 710 Nt. Pleasant
Street, Amherst, MA 01003, USA
Results and discussion
Synthesis
We have synthesized SubPc acceptors 1 and 2 according to
Scheme 1. SubPc precursor derivatives 3 and 4 were prepared
according to literature procedures described by Sullivan et al.20
Reaction with the appropriate phenol28 provided derivatives 1
and 2 in good yields as purple solids.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ta00715a
‡ These authors contributed equally.
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Scheme 1 Synthesis of SubPc 1 and 2.
previously reported DFT investigations, the HOMO is evenly
distributed across the SubPc core, whereas the LUMO is
Electrochemical properties
The electrochemical properties of the SubPc derivatives 1 and 2
were investigated using cyclic voltammetry (CV) and square
wave voltammetry (SWV). Their SWVs are provided in Fig. 1 and
the redox potentials and the derived estimates of the HOMO
and LUMO energies are provided in Table 1. Both compounds 1
and 2 have a LUMO level of around ꢁ3.5 eV, which lies between
typical donor materials (about ꢁ3.0 eV) and PCBM-based
acceptors (about ꢁ3.7 eV). This LUMO level makes these
materials interesting for applications as either donor or
acceptor. We have therefore investigated both modes for solar
cells. The HOMO levels are at ꢁ5.7 eV and ꢁ5.6 eV for 1 and 2
respectively, resulting in an electrochemical band gap of 2.2 eV
and 2.1 eV, respectively.
less
evenly
distributed.29
Likewise,
only
limited
contribution towards the LUMO/HOMO occurs from the
peripheral halogens, whereas the axial phenoxy and the
boron linker unit appear to provide no contributions
whatsoever.
Optical properties
The absorption spectra in chlorobenzene show a pronounced
peak at 553 nm and 571 nm for 1 and 2, respectively, which is
red-shied to 560 nm (1) and 582 nm (2) in lms (see Fig. 2).
This bathochromic shi is an indicator for intermolecular
interactions.30 As the shi is rather small, these interactions are
assumed not to play a signicant role in lm formation. The
optical gap estimated from the absorption onset agrees well
with the band gap estimated by voltammetry. The absorption
coefficient reaches high values up to 3.5 ꢀ 105 cmꢁ1 at the peak
of 2 and 1.5 ꢀ 105 cmꢁ1 at the peak of 1. In lms a slight
amount of scattering is observed from the offset at wavelengths
above the band gap. An interesting aspect of these materials is
the complete transparency in the region of 400 to 425 nm as
observed by absorption measurements of chlorobenzene solu-
tions and by probing the uorescence emission for varying
excitation wavelengths. Complete transparency can be bene-
cial for many measurement techniques, for example probing
the dynamics of light absorption and charge generation where
excitation of specically the donor or acceptor is required. Both
materials have a moderate photoluminescence quantum yield
of about 13–16% in solution and low-loaded polystyrene blends
(Table 2). In neat lms the quantum yield dropped to around
2.5%. A considerable red shi is observed for the emission in
lms compared to that in solution, which could indicate
aggregation, see Fig. 3.
The DFT modelled structures of compounds 1 and 2 are
provided in ESI Fig. S2.† The DFT-predicted HOMO and
LUMO energies are in reasonable accordance with electro-
chemically determined values (Table 1). In line with
Fig. 1 Square wave voltammogram of 1 (black line) and 2 (red line).
The potential is referenced to ferrocene/ferrocenium redox couple ¼
0.0 V.
Table 1 Electrochemical and DFT data for compounds 1 and 2. SWV indicates values calculated using square wave voltammetry
Compound
E(ox) (V)
E(red) (V)
HOMO(SWV) (eV)
LUMO(SWV) (eV)
HOMO(DFT) (eV)
LUMO(DFT) (eV)
1
2
0.85
0.81
ꢁ1.29
ꢁ1.27
ꢁ5.7
ꢁ5.6
ꢁ3.5
ꢁ3.5
ꢁ5.8
ꢁ5.8
ꢁ3.0
ꢁ3.1
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Fig. 2 (a) Absorption of 1 and 2 in chlorobenzene solution (solid lines) and fluorescence excitation spectra (symbols) showing complete
transparency in the region of 400 to 425 nm. (b) Absorption spectra of films of 1 and 2 spin coated from chlorobenzene.
Table 2 Photoluminescence quantum yield (PLQY) of 1 and 2 in neat
dissolved and no particles were found in a spin-coated lm
films, polystyrene (PS) blends and solution
from this solution. Hence the solubility is clearly higher than
100 mg mLꢁ1. The uorinated compound 1 needed to be further
PLQY (%)
1
2
diluted to dissolve completely. A quantitative measurement
revealed a solubility of 45 mg mLꢁ1
.
Neat lm
PS blend
Solution
2.4
13.0
14.1
2.6
14.5
16.3
By investigating the lms spin-coated from an incompletely
dissolved solution of compound 1, large and homogenous
crystals in the size of tens of micrometres were found. These
also grew over time in initially homogenously spin-coated lms
in a period of about two days when le in air or nitrogen. We
attribute this aggregation behaviour being due to the well-
documented ability of organouorine derivatives to undergo
self-assembly through a range of non-covalent interactions such
as C–F/H and C–F/p bonding.31 A representative image of the
crystalline structures formed is shown in Fig. S3.†
SubPcs as acceptor
Devices with three different donor polymers, MEH-PPV, P3HT
and PTB7, were fabricated to explore the usability of the
SubPcs as acceptors. The results of the external quantum
efficiency (EQE) of these devices are shown in Fig. 4. A
signicant contribution of acceptor absorption to the photo-
current is evident from the additional peak at 560 and 580 nm
for acceptor 1 and 2, respectively. Absorption of light by the
acceptor initially leads to the formation of singlet excitons
which can then lead to charge generation by hole transfer to
the donor. The lack of absorption of these acceptors in the
deep blue region, between 350 nm and 450 nm, causes quite
an unusual shape in the EQE spectra, where all curves reveal a
Fig. 3 Normalized fluorescence of 1 (squares) and 2 (circles) in solu-
tion (open symbols) and films (filled symbols) at 360 nm excitation.
Solubility and crystallization behaviour
A solution with a high concentration of 100 mg mLꢁ1 of 1 or 2 in
1,2-dichlorobenzene was prepared and stirred under moderate
heating of 50 ꢂC to estimate the solubility of the two
compounds. The chlorinated compound
2 quickly fully
Fig. 4 EQE spectra of MEH-PPV (squares), P3HT (circles) and PTB7 (diamonds) blended with compound 1 (a) and compound 2 (b).
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valley. In the MEH-PPV solar cells the acceptor extends the blend ratio is identical to the one typically used for
EQE response of the solar cells beyond the absorption edge of PTB7:PC71BM.32 The spectra show that the peak from the
MEH-PPV to a longer wavelength up to 600 nm with compound acceptor absorption strongly increases with increasing
1 and 650 nm with compound 2. To gain the full potential of concentration, whereas the contribution from PTB7 decreases.
these materials a further reduction of the band gap would be As a trade-off between both contributions the ratio of 1 : 1.5 was
benecial to achieve better coverage of the sun-spectrum. The found best, gaining the maximum EQE over the entire
PTB7 devices clearly resulted in the highest EQE of up to 43%, spectrum.
a surprising result given the small energy offset of the LUMO
between PTB7 (3.3 eV) and the SubPc (3.5 eV). In all blends 2 temperature varied between 100 ꢂC and 160 ꢂC with a constant
was found to outperform 1, with a higher EQE. time of 10 minutes. The efficiency for devices of PTB7:2,
The PTB7 solar cells were optimized with the annealing
The JV-characteristics, shown in Fig. 5 indicate a very promising annealed at 160 ꢂC improved by a factor of 1.25 compared to the
high open circuit voltage of 0.95 V for MEH-PPV, 0.6 V for P3HT non-annealed devices, due to an improvement of JSC and FF,
and 0.90 V for PTB7 blended with compound 2. The efficiency thus whereas the performance of PTB7:1 remained fairly constant.
reached 0.4% with MEH-PPV, 1.1% with P3HT and 3.5% with Annealing generally gives the molecules free energy to assemble
PTB7. With compound 1 the efficiencies were lower, as will be in the lm and leads to a stronger phase separation. This phase
explained below, but showed the same trend. In all blends it was separation can give rise to better charge transport of electrons
found that the device performance improved when the active layer and holes to the electrodes. To obtain the optimum conditions,
was thermally annealed before deposition of the Ca/Al electrode. we varied the duration of annealing from 0 to 20 minutes and
Details of the device fabrication and performance parameters are kept the temperature of 160 ꢂC constant. A clear optimum of the
given in the Experimental section and ESI.†
power conversion efficiency was achieved for the previously
used 10 minutes, where the VOC and ll factor have a maximum.
Additionally the morphology can be altered by solvent
additives. Previous studies on PTB7:PC71BM solar cells show
Optimisation of PTB7:SubPc BHJ cells
The morphology plays a critical role in obtaining high efficiency that adding the solvent additive 1,8-diiodooctane (DIO) strongly
in BHJ solar cells. It is strongly inuenced by the mass ratio of enhances the power conversion efficiency.32 In the case of
the donor and the acceptor, with the optimum depending on PTB7:PC71BM blends, it was found that the amount of 3 vol% of
the materials used. Fig. 6 shows the EQE spectra for different the additive gives the strongest improvement, but for other
blend ratios. For the devices of PTB7 : 1 and PTB7 : 2 the materials systems this amount can vary signicantly.33 The
highest EQE had about 40% at a blend ratio of 1 : 1.5. This concentration of DIO was thus varied between 0 vol% and 8
Fig. 5 JV-characteristics of acceptors 1 (a) and 2 (b) blended with MEH-PPV (squares), P3HT (circles) and PTB7 (diamonds).
Fig. 6 Influence of the blend ratio of PTB7 : 1 (a) and PTB7 : 2 (b).
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Fig. 7 (a) Power conversion efficiency of PTB7:1 and PTB7:2 as a function of the amount of solvent additive DIO. (b) JV-curves of optimized solar
cells in comparison to PTB7:PC71BM.
vol% to nd the optimum conditions for our system. The effi- Fig. 8 shows the variation of JSC and VOC on light intensity. From
ciency of PTB7:1 had a maximum PCE of 1.9% at a concentra- the exponent of JSC versus intensity the charge density depen-
tion of 2 vol% DIO and PTB7:2 gave a maximum PCE of 3.5% at dent recombination can be qualitatively explored. An exponent
a concentration of 1 vol% DIO as shown in Fig. 7.
of unity indicates that no bimolecular losses occur. The peak in
the exponent for 1 vol% of DIO shows that for compound 2 a
minimum of recombination occurs at the optimized concen-
tration, whereas for PTB7:1 the trend is not clear. In devices
fabricated using compound 2, the slope is higher for all
concentrations of DIO indicating less intensity dependent
recombination losses than for compound 1.
A similar method applies for the slope of VOC versus the
logarithm of intensity (I), a lower slope indicates less trap-
assisted recombination.35 In both blends a relatively high
amount of trap assisted recombination is found with a slope of
Analysis of the loss mechanisms of the optimized devices
Through the device optimisation we reached a power conver-
sion efficiency of 3.5% with PTB7:2 and 1.9% with PTB7:1. This
is a very promising result for devices with a non-fullerene
acceptor. To facilitate further developments of these acceptors,
we investigated the prominent loss mechanisms in these
blends. Comparing the optimized JV-curves of the present
systems with PTB7:PC71BM shows a lower short circuit current
density (JSC), which can be attributed to a loss of exciton
dissociation, which will be discussed below. The fact that
compound 2 absorbs further into the red part of the spectrum
compared to compound 1 gives rise to a slight increase of 14%
of the maximum obtainable short circuit current, determined
from the integral of the normalized EQE spectra.
V
OC versus ln(I) between 1.5 to 1.8kT/e, where kT/e is the thermal
voltage. This behaviour shows that traps, which may originate
from impurities or incomplete percolation pathways, have a
signicant impact on these solar cells. The recombination los-
ses give rise to a reduced ll factor of about 45% compared to
70% for PTB7:PC71BM, which may originate from the lower
charge carrier mobility observed for the P3HT:2 system using
charge extraction measurements (ESI†).
PTB7:2 has a high open circuit voltage (VOC) of 0.94 V
compared to PTB7:1 and PTB7:PC71BM, with both 0.76 V. The
VOC is related to the effective band gap of the active layer, but as
the HOMO and LUMO levels for compound 1 and 2 are very
similar, the VOC should also be similar. Additionally the VOC is
Photoluminescence quenching
inuenced by recombination in the active layer, a process that The study of photoluminescence can reveal geminate recom-
can be investigated by varying the light intensity of JV-curves.34,35 bination losses of inefficient exciton dissociation, as the charge
Fig. 8 (a) The exponent of the short circuit density vs. light intensity (I) as a function of content of DIO. The inset shows an example at the
optimized conditions. (b) Slope of the open circuit voltage as a function of content of DIO. The inset shows an example at the optimized
conditions.
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can be improved by slightly increasing the LUMO offset in
future acceptor materials. The second limitation is a high
degree of trap-assisted recombination. This recombination can
be potentially overcome by further reducing the aggregation
and crystallization behaviour by appropriate peripheral (and
axial) substitution, as it was observed that the chlorinated
compound 2 gave a better performance than the uorinated
compound 1. The third issue is the relatively poor coverage of
the solar spectrum that can be improved by reducing the band
gap by utilising related derivatives such as sub-
naphthalocyanines.24,37,38 Following these directions gives a
promising route for efficient fullerene-free and solution-
processable polymer solar cells.
Fig. 9 Fluorescence emission spectra of PTB7 : 2 for varying blend
ratio. The excitation wavelength was 280 nm. The emission at 650
originates from compound 2 and the emission at 800 nm from PTB7.
Experimental
Chemical analysis
transfer between donor and acceptor deactivates the radiative
decay, causing a decrease of the uorescence. The steady state
emission spectra for different blend ratios of PTB7:2, prepared
with the optimized content of DIO and annealing conditions,
are shown in Fig. 9. Two peaks of emission are present, the rst
at 650 nm which comes from compound 2 and the second at
800 nm from PTB7. For the blend ratio of 1 : 0.5 the emission
from compound 2 is completely quenched, indicating that all
excitons created in 2 are efficiently dissociated. For the same
blend a high emission of PTB7 is observed which shows inef-
cient exciton harvesting. Upon increasing the concentration of
2 the emission at 650 nm steadily increases. In contrast, the
emission from PTB7 has a minimum at the blend ratio of 1 : 1.5
and then increases again. This clearly shows that for the blend
ratio of 1 : 1.5 most excitons are dissociated into free charges,
however complete quenching is not observed at any of the blend
ratios. Incomplete quenching of PTB7 can be caused by the
rather low LUMO offset to the acceptor of only 0.3 eV and
aggregation of the compound 2 at higher concentrations. For
further developments of these acceptors we thus suggest to
further tune the energy levels, as a slightly higher LUMO offset
might be benecial to improve exciton harvesting from the
donor material.
Melting points were determined in a Stuart Scientic Melting
Point apparatus and are uncorrected. MS spectra were recorded
using a Bruker microTOF Instrument. NMR spectra were
recorded using a Bruker Ultrashield Spectrometer. Elemental
analyses were obtained using an Exeter CE 440 Analytical
Elemental Analyzer. Column chromatography was carried out
using silica gel Si-60 (40–63 mm), and thin layer chromatography
was performed on aluminium sheets pre-coated with silica gel
(Merck).
Synthesis
SubPc 1. SubPc 3 (0.3 g, 0.56 mmol), pentauorophenol
(0.5 g, 2.8 mmol), and toluene (5 mL) were stirred under reux
for 24 h. The solution was cooled to room temperature and the
product was then puried by ash column chromatography
(petroleum ether/toluene 40 : 60) to afford SubPc 1 as a purple
solid (0.38 g, 100%). mp > 300 ꢂC, 1H NMR (CDCl3, 500 MHz) d
(ppm) 8.62 (t, J ¼ 7.9 Hz, 6H); 13C NMR (CDCl3, 101 MHz) d
(ppm) 152.8 (dd, J ¼ 257.4 Hz, 16.6 Hz), 150.3 (s), 127.2 (d, J ¼
2.9 Hz), 110.7 (m); MS (ESI) m/z: 687.1 [M + H]+; anal. calcd for
C
30H6BF11N6O: C 52.50%, H 0.88%, N 12.24%, found: C 52.56%,
H 0.83%, N 12.14%.
SubPc 2. SubPc 4 (0.3 g, 0.47 mmol), phenol (0.2 g, 2.3 mmol)
and toluene (3 mL) were stirred under reux for 7 days. The
solution was cooled to room temperature and the product was
then puried by ash column chromatography (petroleum
ether/toluene 30/70) to afford SubPc 2 as a purple/gold solid
(0.3 g, 96%). mp > 300 C; H NMR (CDCl3, 400 MHz) d (ppm)
8.86 (s, 6H), 6.79 (m, 2H), 6.68 (m, 1H), 5.39 (m, 2H); 13C NMR
(CDCl3, 101 MHz) d (ppm) 151.8, 150.2, 135.0, 129.7, 129.1,
123.9, 122.1, 119.0; MS (ESI) m/z: 694.9 [M + H]+; anal. calcd for
Conclusions
We have reported solution-processed subphthalocyanine
acceptors with up to 3.5% power conversion efficiency which is
very favourable over previous reports of 0.1%.26 Other solution-
processed non-fullerene acceptor systems typically give results
between 2–4% PCE,3–5 although recently a PCE of 6.8% was
reported.36 A strong contribution to the photocurrent by
absorption in the acceptor phase was observed in all blends
with MEH-PPV, P3HT and PTB7. This is a promising direction
to enhance the absorption of the thin active layers oen
required for efficient charge transport and to provide comple-
mentary absorption with the donor component to cover a broad
1
ꢂ
C
30H11BCl6N6O: C 51.85%, H 1.60%, N 12.09%, found: C
51.64%, H 1.50%, N 11.94%.
Cyclic and square wave voltammetry
band of absorption. Aer a full optimization of the PTB7:SubPc CV and SWV measurements were performed on CH Instru-
devices, we identied three loss mechanisms limiting their ments 440A electrochemical workstation. Voltammograms were
solar cell efficiency. The rst one is limited exciton dissociation recorded as DCM solutions (1 ꢀ 10ꢁ3 M) containing 0.1 M of
especially from excitons created in the PTB7 phase. This loss tetrabutylammonium hexauorophosphate as the supporting
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electrolyte. A platinum disc working electrode, a platinum wire
As donor materials for the active layer MEH-PPV (poly[2-
counter electrode and a silver wire pseudo-reference electrode methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] from Sigma
were used in all measurements. The redox potentials were Aldrich), P3HT (poly(3-hexylthiophene-2,5-diyl) from Rieke
referenced to the ferrocene/ferrocenium redox couple (adjusted metals) and PTB7 (poly[[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-
to 0.0 V). The HOMO/LUMO energies were estimated from the b0] dithiophene-2,6-diyl][3-uoro-2-(2-ethylhexy)carbonyl] thieno
redox potentials derived from SWV data, and were calculated [3,4-b]thiophenediyl]), from 1-Material) were used. The solutions
using the relationships: E(HOMO) ¼ (ꢁ4.8) ꢁ (E(ox)) and E(LUMO)
¼
contained MEH-PPV : 1 (1 : 5) and MEH-PPV : 2 (1 : 3) with a
(ꢁ4.8) ꢁ (E(red)); where ꢁ4.8 eV is the E(HOMO) for Fc against the concentration of 12 mg mLꢁ1 in toluene, P3HT : SubPc (1 : 1)
vacuum.39
with a concentration of 20 mg mLꢁ1 in 1,2-dichlorobenzene and
PTB7 : SubPc (1 : 0.5 to 1 : 4) with a concentration of 20 mg mLꢁ1
in 1,2-dichlorobenzene. MEH-PPV devices were annealed at
160 ꢂC for 10 min. P3HT devices were annealed at 140 ꢂC for 20
min. PTB7 devices shown in Fig. 4 and 5 were prepared from
DFT calculations
DFT calculations were performed using the Spartan '14 (64 bit)
soware suite.40 Geometries were initially optimized semi-
empirically (AM1) and then re-optimized using DFT (B3LYP/6-
31G*). The resulting structures were shown to be local minima
by inspection of their vibrational frequencies.
ꢂ
1 : 1.5 blend ratio and annealed at 160 C for 10 minutes with
vol% of the additive DIO (1,8-diiodooctane from Sigma Aldrich).
Fabrication details of the device optimisation are given in the
main text. Devices of PTB7 blended with PC71BM ([6,6]phenyl
C71 butyric acid methyl ester, from Solenne) were fabricated
from a 1 : 1.5 blend ratio, 20 mg mLꢁ1 of 1,2-dichlorobenzene,
Solubility and lm quality
The quantitative solubility was measured according to the with 3 vol% DIO and without annealing.
method described in the literature.41 Films were observed with a
Nikon Eclipse LV100ND optical microscope.
JV-Characterisation
Solar cell performance was measured under simulated sun-light
Absorption and uorescence spectroscopy
from a Sciencetech class A simulator. The intensity was cali-
brated with an ORIEL reference cell (Model 91150V) with KG5
lter. A shadow mask was positioned in front of each device
with the same size as the electrode overlap in order to avoid
photocurrent contribution from outside the device area. The
current–voltage characteristic was then measured with a
Keithley 2400 sourcemeter. The spectral mismatch (of 0.950 for
MEH-PPV:1, 0.952 for MEH-PPV:2, 1.002 for P3HT:1, 0.982 for
P3HT:2, 0.917 for PTB7:1 and 0.901 for PTB7:2) was uncor-
rected. This means that the real power conversion efficiency of
e.g. PTB7:2 is a factor of 1.1 higher than the quoted values in the
text. A Cree LED was used as a dimmable light source and
calibrated for linearity. The absolute intensity, at a setting of
one sun, was obtained from the ratio of the short circuit current
when the sample was illuminated by the LED and the short
circuit current when illuminated with the solar simulator.
Absorption spectra of low-concentrated solutions of chloro-
benzene were recorded using a Cary 300 spectrometer and are
corrected for the absorption of the solvent. Films were spin-
coated from 20 mg mLꢁ1 solutions of chlorobenzene at 1000
rpm and measured with the same instrument. The lm thick-
ness (d) was measured with a Dektak surface prolometer. The
absorption coefficient, is estimated by A/d ꢀ ln(10), where A is
the optical density of the lm. Errors in the estimation of
absorption coefficient are substantial because reection and
thin lm interference effects are neglected, and because errors
in the lm thickness measurement are up to 20%. Emission
spectra were recorded using an Edinburgh Instruments uo-
rimeter at 360 nm excitation. To obtain the quantum yield in
solution the emission was compared to a reference of quinine
sulphate following the description by Crosby et al., eqn (16).42
Film PLQY was measured with a Hamamatsu U6039-05 inte-
grating sphere.
EQE measurement
Device fabrication
To obtain the external quantum yield, the devices were illumi-
nated with monochromatic light through an optical bre with
dimensions smaller than the active area. The short circuit
current of the test solar cell was then compared to an NPL
calibrated reference photodiode.
Glass substrates with a 4 mm wide central stipe of ITO (Xinyan
Technology) were cleaned in acetone and isopropanol and
plasma treated at 80
organic contamination.
W
for
3
minutes to remove
(poly(3,4-ethyl-
PEDOT:PSS
enedioxythiophene):poly(styrenesulfonate) from Heraeus, Cle-
vios VP AI 4083) was then spin-coated at 4000 rpm and
ꢂ
baked at 120 C for 10 minutes. The active layer was then spin
Acknowledgements
coated in a nitrogen glove box at 1000 rpm. Annealing was
performed in the glove box on a hot plate. The cathode con- GC and IDWS thank the EPSRC for funding (Grant EP/I00243X).
sisting of 20 nm Ca and more than 100 nm Al was thermally NBAP thanks Directorate General of Higher Education (DGHE)
evaporated at a rate lower than 0.5 nm sꢁ1 and a pressure lower of Republic Indonesia for PhD Scholarship Grant (948/E4.4/K/
than 3 ꢀ 10ꢁ6 mbar. The active area, as the overlap of the ITO 2013). IDWS also acknowledges a Royal Society Wolfson
stripe and the cathode, was 8 mm2.
Research Merit Award.
This journal is © The Royal Society of Chemistry 2015
J. Mater. Chem. A

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Journal of Materials Chemistry A
Paper
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A. S. Paton, Z.-H. Lu, T. P. Bender and T. S. Jones, J. Phys.
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