Solution processable diketopyrrolopyrrole (DPP) cored small molecules with BODIPY end groups as novel donors for organic solar cells

Article abstract of DOI:10.3762/bjoc.10.283

Two novel triads based on a diketopyrrolopyrrole (DPP) central core and two 4,4-difluoro-4-bora-3a,4a-diaza- s - indacene (BODIPY) units attached by thiophene rings have been synthesised having high molar extinction coefficients. These triads were characterised and used as donor materials in small molecule, solution processable organic solar cells. Both triads were blended with PC 71 BM as an acceptor in different ratios by wt % and their photovoltaic properties were studied. For both the triads a modest photovoltaic performance was observed, having an efficiency of 0.65%. Moreover, in order to understand the ground and excited state properties and vertical absorption profile of DPP and BODIPY units within the triads, theoretical DFT and TDDFT calculations were performed.

Full text of DOI:10.3762/bjoc.10.283

Solution processable diketopyrrolopyrrole (DPP) cored
small molecules with BODIPY end groups as novel donors
for organic solar cells
Diego Cortizo-Lacalle1, Calvyn T. Howells2, Upendra K. Pandey2,3, Joseph Cameron1,
Neil J. Findlay1, Anto Regis Inigo1, Tell Tuttle1, Peter J. Skabara*1
and Ifor D. W. Samuel*2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 2683–2695.
1WestCHEM, Department of Pure and Applied Chemistry, University
doi:10.3762/bjoc.10.283
of Strathclyde, Glasgow, G1 1XL, UK, 2Organic Semiconductor
Centre, SUPA, School of Physics & Astronomy, University of St.
Andrews, St. Andrews, KY16 9SS, UK and 3Interdisciplinary Centre
for Energy Research, Indian Institute of Science, Bangalore 560012,
India
Received: 18 July 2014
Accepted: 07 November 2014
Published: 18 November 2014
Associate Editor: H. Ritter
Email:
Peter J. Skabara* - peter.skabara@strath.ac.uk; Ifor D. W. Samuel* -
idws@st-andrews.ac.uk
© 2014 Cortizo-Lacalle et al; licensee Beilstein-Institut.
License and terms: see end of document.
*
Corresponding author
Keywords:
BODIPY; diketopyrrolopyrrole; organic semiconductors; organic solar
cells; thiophene
Abstract
Two novel triads based on a diketopyrrolopyrrole (DPP) central core and two 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
(
BODIPY) units attached by thiophene rings have been synthesised having high molar extinction coefficients. These triads were
characterised and used as donor materials in small molecule, solution processable organic solar cells. Both triads were blended with
PC71BM as an acceptor in different ratios by wt % and their photovoltaic properties were studied. For both the triads a modest
photovoltaic performance was observed, having an efficiency of 0.65%. Moreover, in order to understand the ground and excited
state properties and vertical absorption profile of DPP and BODIPY units within the triads, theoretical DFT and TDDFT calcula-
tions were performed.
Introduction
The discovery of photoinduced electron transfer from conju- conjugated polymers for bulk heterojunction organic photo-
gated polymers to fullerene (C60), and the favourable interpene- voltaics (OPVs) [1-3]. In these devices, the conjugated polymer
trating network they form within a bulk heterojunction (BHJ), acts as an electron donor and a soluble fullerene, most
has led to intense research directed towards the synthesis of commonly phenyl-C61-butyric acid methyl ester (PCBM), as
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the electron acceptor [4-9]. However, there is growing interest BODIPY derivatives have demonstrated superior performance
in the use of small molecules as donor materials in OPVs [10- in OPVs. Roncali and Ziessel developed a series of small mole-
1
7]. This interest derives from advantages and properties that cules based on BODIPY derivatives by substitution of the fluo-
small molecules show over conjugated polymers, such as rine atoms with ethynylglycol chains, achieving PCEs higher
(
(
i) synthetic reproducibility, (ii) higher structural versatility, than 2% [40-42]. Recently, a new series of BODIPY deriva-
iii) ease of purification by recrystallisation and/or chromatog- tives grafted with bis-vinylthienyl groups exceeded 4.5% [43].
raphy and therefore monodispersity, (iv) higher degrees of crys-
tallinity and (vi) the possibility of vacuum deposition or solu- Although, a few dyads and triads containing both the DPP and
tion processing during device fabrication. The difference in BODIPY core have been prepared [44-46], here we present the
power conversion efficiencies (PCEs) between polymer and synthesis and characterisation of two novel BODIPY-DPP-
small-molecule based OPVs is decreasing and PCEs over 7% BODIPY triads linked by thiophene bridges. These materials
have been realised in the case of the latter [18,19].
were tested in bulk heterojunction OPVs with moderate power
conversion efficiencies.
Small molecules used in OPVs are most commonly based on
oligothiophenes and their derivatives (e.g., selenophene) [20-
Results and Discussion
2
3], often in combination with other heterocyclic units; the best Synthesis
performing systems are push–pull molecules or dyes [14]. In the Our synthetic approach was to prepare BODIPY derivatives
last few years the diketopyrrolopyrrole (DPP, 1, Figure 1) core bearing a brominated thiophene on the meso-position and
has been widely incorporated in conjugated polymers for both coupling these derivatives via Suzuki coupling to the central
OPVs and organic field-effect transistors [24,25]. The DPP- DPP core 8 (Scheme 1). Compound 6 was prepared by acid-
based conjugated polymers usually show good electron and hole catalysed condensation of 5-bromothiophene-2-carbaldehyde
mobility and promising PCE values in OPVs due to large inter- with 3-ethyl-2,4-dimethylpyrrole, followed by oxidation with
molecular interactions through π–π stacking. Nguyen et al. have DDQ. Deprotonation with triethylamine and subsequent treat-
investigated the DPP core in small molecules for OPVs with ment with boron trifluoride diethyl etherate yielded 6. The
excellent results [26-28]. A PCE greater than 4% was achieved entire synthesis was carried out as a one-pot reaction.
in combination with phenyl-C71-butyric acid methyl ester
(
PC71BM) [29]. Interestingly, a small molecule based on a DPP Extending the conjugated π-system of a compound leads to a
core substituted with electron-withdrawing units was also used narrower HOMO–LUMO gap and a bathochromic shift of the
as an acceptor in OPVs as a substitute for fullerene with PCEs absorption spectrum. Both effects are usually desirable to
of 1% when combined with poly(3-hexylthiophene) [30,31].
enhance the solar absorption. Therefore, an extended analogue
of compound 6 with an additional thiophene ring was synthe-
sised (compound 7). The synthesis of 7 was achieved from the
α-brominated derivative of bithiophene carbaldehyde 4. This
synthetic route to prepare 7 has been described previously in the
literature [47]. Formylation of the bithiophene was carried out
using Vilsmeier–Haack conditions by treatment with phos-
phoryl chloride and N,N-dimethylformamide to give 3, which in
turn was brominated with NBS (1.05 equiv) to yield compound
4
. Both reactions proceeded in high yields: 90% for the formy-
lation step and 88% for bromination. Compound 7 was prepared
following the same procedure used to synthesise 6. Whereas the
synthesis of 6 was achieved in 33% yield, the yield decreased to
Figure 1: Chemical structures of DPP core 1 and BODIPY core 2.
4
,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY, 2), and 12% when the derivative with two thiophenes was prepared
its derivatives have been widely used in the last two decades [47].
due to their outstanding chemical and optoelectronic properties
[
32-34]. BODIPY derivatives are promising compounds to be The BODIPY-DPP-BODIPY triads (9 and 10) were synthe-
used in the active layer of OPV devices as they show high sised via Suzuki–Miyaura cross-coupling by reaction of the
absorption coefficients, good photostability and chemical functionalised DPP core 8 [48] with the brominated BODIPY
robustness. Although the BODIPY unit has been incorporated derivatives 6 and 7, respectively. The compounds were purified
in conjugated polymers [35-37] and tested in OPVs with by standard silica gel column chromatography, but further
moderate PCEs [38,39], several small molecules containing purification using HPLC was required to isolate compound 10
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Scheme 1: Synthesis of triads 9 and 10. Reagents and conditions: (i) phosphoryl chloride, N,N-dimethylformamide, 50 °C, 16 h, 90%; (ii) NBS, N,N-
dimethylformamide, rt, 16 h, 88%; (iii) trifluoroacetic acid, dichloromethane, rt, 16 h; DDQ, rt, 24 h; triethylamine, BF3·OEt2, rt, 24 h, 33% and 12% for
6
and 7, respectively; (iv) DPP 8, Pd2(dba)3, tri-tert-butylphosphonium tetrafluoroborate, THF/water, tripotassium phosphate, reflux, 48 h, 55% and
34% for 9 and 10, respectively.
in sufficiently high purity. Compounds 9 and 10 were thus them to the oxidation of the second terthiophene unit, followed
isolated in 55% and 34% yields, respectively.
by the oxidation of the BODIPY fragment. In 9 these two
processes coalesce, albeit at a higher potential.
Electrochemical and optical properties
The oxidation and reduction processes of 9 and 10 in solution The reduction processes of the triads are difficult to interpret
are shown in Figure 2 and Table 1 summarises the corres- accurately due to the occurrence of multiple reduction
ponding electrochemical data. Upon oxidation, 9 shows two re- processes. Compound 9 shows two sequential quasi-reversible
versible peaks at +0.42 and +0.73 V and 10 shows three revers- peaks at −1.39 V and −1.58 V and one irreversible peak at
ible processes at +0.37, +0.57 and +0.73 V. In both cases, the −2.01 V. The reduction behaviour of 10 is more complex with
first oxidation wave is assigned to the formation of the radical several processes overlapping. Compound 10 displays three
cation on one of the bi/terthiophene segments of the molecule. quasi-reversible peaks at −1.48, −1.55 and −1.90 V and an irre-
The lower oxidation potential for 10 compared to 9 is consis- versible peak at −2.09 V. It is difficult to assign each of these
tent with the tendency to decrease the oxidation potential when processes accurately, but it is reasonable to assume that the first
the oligothiophene chain is extended. There is then a marked two reduction processes are due to the reduction of the DPP and
difference in the oxidation behaviour of the two compounds. In BODIPY moieties. By analogy, the reduction waves at higher
1
0, we observe two sequential oxidation processes and ascribe negative potentials can be due to the reduction of the oligothio-
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Figure 2: Cyclic voltammetry of 9 (black) and 10 (red) in solution (left) and thin-film (right). The experiments in solution were carried out in
dichloromethane (0.1 mM) using a glassy carbon electrode. A film was deposited from a solution of the triads in dichloromethane on a glassy carbon
electrode and experiments were carried out in acetonitrile. In both cases, a Ag wire reference electrode and a Pt counter-electrode, in the presence of
Bu4NPF6 (0.1 M), were used. All the values are quoted versus the redox potential of the ferrocene/ferrocenium couple.
Table 1: Electrochemical data for compounds 9 and 10 in solution and solid state.a
Solution state
Eox [V]
Ered [V]
HOMO [eV]
LUMO [eV]
HOMO–LUMO gap [eV]
1.63
−
1.44/1.34qr
+
+
0.45/+0.39
0.80/+0.66
9
−1.61/1.54
2.01ir
−5.13
−3.50
−
−
1
1.51/−1.45qr
.58/−1.52qr
+
0.40/+0.33
10
+0.60/+0.54
−5.10
−3.40
1.70
−1.93/−1.87qr
+0.76/+0.69
−2.09ir
Solid state
Eox [V]
Ered [V]
HOMO [eV]
LUMO [eV]
HOMO–LUMO gap [eV]
−
−
1.30ir
1.34ir
+
0.64ir
9
−1.44ir
−5.31
−3.57
1.74
+0.79 ir
−1.92ir
2.24ir
−
−1.41ir
−1.49ir
−1.84ir
−2.04ir
+
0.56ir
+0.64ir
0.75ir
10
−5.25
−3.44
1.81
+
aqr represents a quasi-reversible process; ir is an irreversible process.
phene units, with 10 having a lower reduction potential for the films dissolving in the electrolytic medium in their highly
reduction of the thiophenes because of the extended conjugated charged states.
chain.
The HOMO and LUMO energy levels were calculated from the
The electrochemical study of the triads in the solid state was onset of the first oxidation and reduction waves in both solu-
also analysed. Although a similar pattern of redox processes is tion and solid state and used to determine the HOMO–LUMO
observed for oxidation and reduction, the reversibility of these gap. Due to the close interactions between molecules in the
peaks is lost compared to the solution state studies due to the solid state, the HOMO–LUMO gap is expected to be lower
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compared to the HOMO–LUMO gap calculated from the unit. Both triads 9 and 10 show absorption maxima at 542 nm,
studies in solution. Interestingly though, 9 and 10 show higher which is ascribed to the absorption of the BODIPY units. Inter-
HOMO–LUMO gaps in the solid state. The film formation estingly, the absorption peak of BODIPY in these compounds is
stabilises significantly the HOMO level of both triads (see exactly the same value found for a series of BODIPY unit
Table 1), presumably through the interaction of the donor derivatives substituted with oligothiophenes at the meso-pos-
components of the molecules with the corresponding acceptor ition [47]. Thus, the incorporation of two extra thiophene rings,
units via aggregates. Although, the HOMO and LUMO energy and connection to the DPP core, does not affect the absorption
levels are lower in the solid state, the stabilisation of the LUMO peak associated with the BODIPY units. On the other hand, the
is not as large as the HOMO and therefore leads to an increase
of the HOMO–LUMO gap. Due to the extended conjugated
system, it was also expected to obtain a lower HOMO–LUMO
energy gap value for 10. On the contrary, compound 9 displays
a slightly lower energy gap both in solution and in the solid
state. This difference is unusual for a system with extended
conjugation and is addressed later.
The optical properties of compounds 9 and 10 were charac-
terised by UV–vis absorption spectroscopy in both solution and
solid state (drop-cast on ITO) and the corresponding spectra are
shown in Figure 3, with the data summarised in Table 2. For
comparison, the absorption spectrum of the dithieno-DPP
(
Figure 4, 11) core in solution is also shown in Figure 3. All the
Figure 4: Structure of the dithieno-DPP (11) core.
spectra are normalised to the absorption band for the BODIPY
Table 2: UV–vis absorption data for compounds 9 and 10 in solution (dichloromethane) and solid state.
Solution
Solid
Absorption peaks [nm]
Absorption peaks [nm]
HOMO–LUMO
gap [eV]
Ε at 542 nm
[dm3 mol−1 cm−1]
HOMO–LUMO
gap [eV]
9
355, 400 (br), 542, 584, 622
386, 542, 607, 645
1.86
1.77
390,000
252,000
423, 554, 591, 643
395, 554, 619, 668
1.71
1.67
10
Figure 3: Normalised UV–vis absorption spectra of 9 (black), 10 (red) and DPP core (11, green) in dichloromethane solution (left); UV–vis absorption
spectra of 9 (black) and 10 (red) core in the solid state, drop-cast from a dichloromethane solution onto ITO (right).
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Beilstein J. Org. Chem. 2014, 10, 2683–2695.
extension of the π-conjugated system bathochromically shifts
the wide absorption band associated with the DPP core and
thiophene rings. The DPP core substituted with two thiophenes
(
compound 11) showed two intense peaks at 512 and 548 nm.
These peaks are red-shifted 72 and 74 nm, respectively, for 9
584 and 622 nm). The shift is even larger for 10 as the conju-
(
gation increases (95 and 97 nm), with compound 10 giving two
peaks at 607 and 645 nm. Theoretical calculations support the
assignment of these absorption bands to the π–π* transition
which is localised on the DPP core (vide infra). Optical
HOMO–LUMO gaps in solution were calculated from the onset
of the longest wavelength absorption peaks and are summarised
in Table 2.
Figure 5: BOD-T4 structure reported by Harriman et al. [50].
In the solid state, the triads show the same spectral profile as in
solution. In both cases, the main absorption peak occurs at
5
54 nm. This value is red-shifted 12 nm in comparison with the However, despite each BODIPY twisting out of the conjuga-
same peak in solution. The bi/terthiophene-DPP component of tion plane, these accepting units play an interesting role in the
the molecule is also red-shifted. A higher degree of order in the distribution of electrostatic potential charge in the molecules.
solid state is expected compared to the experiments carried out Shown below are the values of the CHelpG [51] electrostatic
in solution, which shifts both the absorption wavelength and the potential charge for the component units in compounds 9 and 10
HOMO–LUMO gaps towards lower energies. The relative (Figure 6), compared to (2Th)2DPP and (3Th)2DPP synthe-
intensity of the peak ascribed to the BODIPY unit is dimin- sised by Nguyen et al. (Figure 7) [27].
ished compared to the rest of the spectrum. This is due to the
higher absorptivity of the thiophene units in the aggregated The compounds (2Th)2DPP and (3Th)2DPP in their neutral and
state. The peaks characteristic of the thieno-DPP sections for 9 radical anion geometries show that the DPP core becomes
and 10 appear at 591 and 643 nm and at 619 and 668 nm, res- slightly less negative with increased conjugation, whilst the
pectively. The HOMO–LUMO gaps of the triads were calcu- DPP core becomes less positive with increased conjugation in
lated from the onset of the longest wavelength absorption peaks the radical cation form. The increase in conjugation allows
and are summarised in Table 2. The exact calculation of the charge to be more evenly distributed across the whole molecule.
HOMO–LUMO gap of 9 is difficult as the onset is diffuse. However, the inclusion of BODIPY accepting units presents a
Nevertheless, the estimated HOMO–LUMO gap of 9 is wider more complex picture. The BODIPY units act as a stronger
(
(
estimated at 1.71 eV) compared to the energy gap of 10 acceptor than the DPP core, causing the overall electrostatic
1.67 eV) as the extension of the conjugated system leads to a potential charge of the DPP-oligothiophene unit to be positive.
lower HOMO–LUMO gap. Interestingly, whereas in solution This is observed in all three redox states, with the more conju-
the HOMO–LUMO gaps of the triads differ significantly (see gated compound 10 more positive in each case. Thus, the elec-
Table 2), in the solid state the incorporation of two extra thio- tron-accepting ability of the BODIPY units dominate over the
phene rings does not decrease the HOMO–LUMO gap to the effect of increasing the conjugation when comparing com-
same extent. The aggregation of 9 in the solid state results in a pounds 9 and 10 to (2Th)2DPP and (3Th)2DPP, with an
decrease of the energy gap making it appear similar to 10, even increase in conjugation resulting in more negatively charged
if the conjugation length is shorter [49].
BODIPY units.
Theoretical calculations
Using the electrostatic potential charges, along with analysis of
DFT optimisations were carried out for compounds 9 and 10, the molecular orbitals in compounds 9 and 10, can help to deter-
with the optimised structures for the compounds showing a mine why the reduction wave of the cyclic voltammogram is
twist between the BODIPY and thiophene units of 80°, compa- more negative for compound 10, despite the increase in conju-
rable to the 81° twist witnessed in the crystal structure of the gation.
BOD-T4 structure (Figure 5) reported by Harriman et al. [50].
The twist of the BODIPY units in these triads suggests that the The SOMO of the radical anion of each compound is localised
conjugation extends to the thiophene–DPP central component, on a BODIPY unit (Figure 8), meaning reduction of com-
isolating the terminal BODIPY moieties.
pounds 9 and 10 should occur at the BODIPY unit. The electro-
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Figure 6: Electrostatic potential charges for each unit in compounds 9 and 10: radical anion (blue), neutral (black) and radical cation (red) geometries.
static potential calculations in Figure 6 show the BODIPY units the lowest energy peaks in compounds 9 and 10 determined by
in compound 10 to be more negatively charged than in com- TDDFT is 29 nm (exp = 23 nm), whilst the BODIPY peaks are
pound 9. Increased negative charge on the BODIPY would in identical positions; this is also evident experimentally. The
result in a higher energy barrier for reduction, resulting in a good agreement between the computational and experimental
greater reduction potential in compound 10. This in turn results shows that wB97XD/TDDFT can be a useful tool in
contributes to the larger HOMO–LUMO gap determined by predicting the absorption of BODIPY-based triads and com-
cyclic voltammetry for compound 10 with respect to compound pounds with multiple absorbing units, which are normally diffi-
9
.
cult to describe computationally.
In addition to the DFT calculations, TDDFT was also carried Device characterisation
out in order to investigate the vertical absorption profile of the Thin films of compound 9 and 10 have favourable absorption
triads in more depth and the results are shown below in Table 3. when used as a donor for organic photovoltaic applications,
absorbing in the region 500–700 nm (Figure S1, Supporting
The lowest energy transition is attributed to the excitation of the Information File 1). PC71BM, which absorbs in the range
thiophene–DPP portion of the molecule (Figures S17 and S20, 300–500 nm, was used as an acceptor with these compounds as
Supporting Information File 1), whilst the excitation that it gives favourable energy level matching for efficient devices.
appears at 510 nm in the TDDFT data for both compounds is Spin-coated films of the donor and acceptor blend allows excel-
the result of absorption by the BODIPY units (Figure S18 and lent absorption from 300–700 nm. The absorption spectra of
S21, Supporting Information File 1). The difference between 9:PC71BM and 10:PC71BM with a 1:1 ratio are shown in Figure
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Beilstein J. Org. Chem. 2014, 10, 2683–2695.
S2 (Supporting Information File 1). Here we see an enhanced
absorption in the region where 9 and 10 have poor absorption
(
300–500 nm). Absorption is an important process in the func-
tion of an organic solar cell but, as previously discussed, it is
not the sole process in the operation of an organic solar cell.
The dissociation of the coulombically bound electron–hole pair
or exciton into free charge and their transport to the electrodes
are critical for device operation. Dissociation and transport are
two processes that are strongly linked with the morphology of
the active layer. Therefore, controlling the morphology can lead
to improved device performance. A common technique to opti-
mise morphology is to vary the donor/acceptor ratio. A
screening of various donor/acceptor ratios revealed that the
most promising performance was evident with the ratio 1:3
(
Figures S15 and S16, Supporting Information File 1). As the
concentration of the acceptor is increased from 2:1 to 1:3 an
increase in short-circuit current (Jsc), open circuit voltage (Voc)
and fill factor (FF) was observed. With further increase in the
concentration of the acceptor to 1:4 we observed a decrease in
Jsc, Voc and FF. Larger concentrations of the acceptor in the
active layer enhance absorption in the region 300–500 nm
Figure 7: Electrostatic potential charges for each unit in (2Th)2DPP
and (3Th)2DPP radical anion (blue), neutral (black) and radical cation
(
Figure S3, Supporting Information File 1). This is not ideal for
(red) geometries, as analogues of compounds 9 and 10.
photovoltaic operation as the majority of the solar spectrum is
Figure 8: Frontier orbitals for radical anion SOMO (top), neutral HOMO (bottom) of 9 (left) and 10 (right).
Table 3: TDDFT results.
Compound
Calculated absorption
peaks [nm]
Transitions
9
612
HOMO→LUMO (75%); HOMO−3→LUMO+2 (12%); HOMO−3→LUMO+3 (13%)
510
HOMO−2→LUMO+1 (32%); HOMO−2→LUMO+2 (12%); HOMO−1→LUMO (9%);
HOMO−1→LUMO+1 (15%); HOMO−1→LUMO+2 (32%)
370
HOMO−4→LUMO (20%); HOMO−4→LUMO+2 (13%); HOMO−3→LUMO+1 (24%);
HOMO−3→LUMO+3 (7%); HOMO→LUMO+2 (27%); HOMO→LUMO+4 (9%)
10
641
HOMO→LUMO (83%); HOMO−3→LUMO+3 (17%)
5
10
03
HOMO−3→LUMO+1 (35%); HOMO−2→LUMO+1 (23%); HOMO−1→LUMO+2 (42%)
4
HOMO−4→LUMO (32%); HOMO−3→LUMO+1 (14%); HOMO−3→LUMO+3 (23%);
HOMO→LUMO+2 (10%); HOMO→ LUMO+4 (21%)
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Beilstein J. Org. Chem. 2014, 10, 2683–2695.
above 500 nm. However, considering the incident photon to
current conversion efficiency (IPCE) (Figure 9) of our fully
optimised devices (Figures S13 and S14, Supporting Informa-
tion File 1) we see for both DPP core derivatives an EQE
response from 500–700 nm. The EQE spectra indicate that the
small molecules are indeed contributing to the overall photocur-
rent of the devices. Furthermore, the peak at 550 nm is easily
identified as the BODIPY core.
Figure 10: J–V for 9:PC71BM (1:3) and 10:PC71BM (1:3) in the dark.
Figure 9: Incident photon to converted electron (IPCE) ratio or
external quantum efficiency (EQE) for 9:PC71BM (1:3) (black) and
10:PC71BM (1:3) (red).
The dark and illuminated J–V curves corresponding to these
EQE spectra are shown in Figure 10 and Figure 11, respective-
ly. It is important to mention that we noticed an issue
concerning the photodegradation of these materials. We have
not provided any quantitative analysis of this phenomenon as it
is beyond the remit of this study. However, it is noticed when
we compare the calculated and measured Jsc. The calculated Jsc
for DPP derivatives is greater than 3 mA cm−2.
Figure 11: J–V for 9:PC71BM (1:3) and 10:PC71BM (1:3) under illumi-
nation at 100 mW cm−2 with an AM1.5 G source.
10:PC71BM than 9:PC71BM. Firstly, 10:PC71BM when
compared with 9:PC71BM has a shallow HOMO and the theo-
The performance of compounds 9 and 10 as the active layer in retical maximum Voc out of a device is defined as the energy
OPV devices are shown in Table 4. We observe a dramatic difference between the LUMO of the acceptor and the HOMO
difference in Voc between 9:PC71BM and 10:PC71BM of 0.71 of the donor. However, this difference is small. A more
and 0.53 V, respectively. A favourable dark current is observed compelling reason is that 10:PC71BM has a propensity to
with 9:PC71BM when compared with 10:PC71BM (Figure 10) aggregate. As such, an investigation into the morphology of the
for the same donor acceptor concentration ratio. There are optimised blends was carried out. Wide-field images indicate
several possible reasons why we observe a lower Voc with more aggregates in the blend 10:PC71BM (1:3) compared to
Table 4: Performance of 9:PC71BM (1:3) and 10:PC71BM (1:3) under illumination at 100 mW cm−2 with an AM1.5 G source.
Active layer
Jsc [mA/cm2]
Voc [V]
FF [%]
PCE [%]
9
:PC71BM (1:3)
3.39
4.55
0.71
0.53
27
26
0.65
0.64
10:PC71BM (1:3)
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Beilstein J. Org. Chem. 2014, 10, 2683–2695.
that in 9:PC71BM (1:3). These aggregates are contained within the acceptor material is also vital for efficient photovoltaic
red circles (Figures S9 and S10, Supporting Information File 1). devices. Therefore, we have prepared two novel molecular
triads bearing the highly absorptive DPP and BODIPY units,
AFM images (Figure 12) show bead-like aggregates for which have been used for OPVs. The OPVs demonstrated PCEs
9
:PC71BM (1:3) and 10:PC71BM (1:3). The aggregates are greater than 0.65%. Although low when compared to devices
larger for 10:PC71BM (1:3) than 9:PC71BM (1:3), while the fabricated with well-studied solution-processable DPP cored
maximum height observed with 10:PC71BM (1:3) is 4.3 nm small molecules [24,30,52], these devices do show a compa-
compared to 2.2 nm for 9:PC71BM (1:3). These small aggre- rable photoinduced spectral response. A comparatively low fill
gates are interesting, but are not of serious concern. However, factor of 26–27% was observed as a result of poor energy level
the aggregates observed in the wide-field (Figures S9 and S10, alignment at the anode side leading to low efficiency. Other
Supporting Information File 1) and from the Dektak profiler parameters governing performance such as short-circuit current
(
Figures S7 and S8, Supporting Information File 1) are of density (Jsc), and open-circuit voltage (Voc) of these devices, are
concern for device performance, since the active layer is remarkable, considering the size of aggregates observed within
approximately 70 nm thick (Figure S7, Supporting Information the active layer, for example an open-circuit voltage of 0.71 V,
File 1). Figures S11 and S12 (Supporting Information File 1) which is greater than that reported for OPVs with poly(3-
show tapping mode AFM images of height and phase for hexylthiophene) [53]. These aggregates, observed by means of a
1
0:PC71BM (1:3). Here we see an aggregate of 5.1 nm height, profiler and wide-field microscopy, are detrimental to device
which is more than double the size of that for 9:PC71BM (1:3). performance [54]. Hence, the development of similar com-
To further investigate aggregation we prepared films of neat 9 pounds with improved solubility and a more favourable
and 10 for PLQY measurements. At an excitation of 550 nm we morphology would hopefully lead to more efficient OPVs.
acquired a PLQY of 1.5% for 9 and 0.8% for 10. The spectra Finally, we acknowledge the very recent work of Ziessel et al.,
from the integrating sphere are shown in Figures S5 and S6 who have reported solar cells incorporating a hybrid
(
Supporting Information File 1). This difference in PLQY is a thiophene–benzothiadiazole–thiophene–BODIPY derivative
firm indication of the tendency for 10 to aggregate to a greater with power conversion efficiencies of ca. 1.25% [55]. Whilst
degree than 9. We believe these two DPP cored derivatives to the open circuit voltage of our best device was higher than
be of interest for photovoltaic applications. Moreover, we are theirs (0.62 V), in Ziessel’s work the fill factor (35%) and short
synthesising some new materials which have superior solubility circuit current density (5.8 mA cm−2) were higher.
and will not be prone to aggregate on the microscopic scale.
Experimental
All the chemicals were purchased from Aldrich and Alfa-Aesar
Conclusion
Undoubtedly, the importance of having highly absorptive cores and used without further purification. For reactions under anhy-
in the organic compounds is essential in order to increase the drous conditions, the glassware was dried in an oven at 130 °C.
light harvesting for organic photovoltaics. However, other para- Apart from dry DMF, dry solvents were collected through a
meters such as good alignment of HOMO–LUMO levels with PureSolv purification system.
Figure 12: Tapping mode AFM height images for 9:PC71BM (1:3) (left) and 10:PC71BM (1:3) (right) on fused silica.
2692

Beilstein J. Org. Chem. 2014, 10, 2683–2695.
1
H and 13C NMR spectra were recorded at room temperature on 0.04 mmol) and tri-tert-butylphosphonium tetrafluoroborate
a Bruker DRX500 at 500 and 125 MHz or a Bruker Avance 400 (40 mg, 0.1 mmol) were dissolved in dry THF (20 mL). To the
instrument at 400 and 100 MHz; chemical shifts are given in previous solution, a solution of tripotassium phosphate (84 mg,
ppm and all J values are in Hz. MALDI–TOF–MS were 0.4 mmol) in water (3 mL) was added. The reaction was re-
recorded on a Shimadzu Axima-CFR spectrometer (mass range fluxed for 48 hours under nitrogen. Dichloromethane (50 mL)
1
–150,000 Da). Column chromatography was carried out on was added to the reaction mixture and washed with water
VWR silica gel (40–63 µm mesh). Solvents were removed (50 mL), brine (50 mL) and water (50 mL). The organic layer
using a rotary evaporator (vacuum supplied by low vacuum was dried over MgSO4, filtered and the solvents evaporated.
pump) and, where necessary, a high vacuum pump was used to The resulting solids were loaded onto a silica column (eluent
remove residual solvent.
mixture, hexane/dichoromethane, 2:1). The product was
subjected to further chromatographic columns in silica (eluent
Compounds 3 [56], 4 [56], 6 [47], and 7 [47] were prepared mixture, hexane/ethyl acetate, 7:3). Preparative HPLC
according to the literature.
(isocratic) was then carried out (eluent mixture, hexane/
dichoromethane, 2:1) to obtain 10 as a dark purple solid
1
0,10'-(5',5'''-(2,5-Bis(2-octyldodecyl)-3,6-dioxo-2,3,5,6- (120 mg, 34%). 1H NMR (CDCl3) 8.93 (d, J = 4.1, 2H), 7.35
tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis([2,2'-bithio- (m, 4H), 7.27–7.24 (2H, (partially masked by CDCl3 peak)),
phene]-5',5-diyl))bis(2,8-diethyl-5,5-difluoro-1,3,7,9-tetra- 7.19 (d, J = 3.8, 2H), 6.93 (d, J = 3.6, 2H), 4.06 (d, J = 7.5, 4H),
methyl-5H-dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-4-ium- 2.56 (s, 12H), 2.35 (m, 8H), 1.98 (br s, 2H), 1.66 (s, 12H),
5
-uide) (9): DPP 8 (100 mg, 0.09 mmol, 3 equiv), BODIPY 6 1.42–1.16 (m, 64H), 1.02 (t, 12H), 0.85 (m, 12H); 13C NMR
124 mg, 0.3 mmol, 1 equiv), Pd2(dba)3 (20 mg, 0.02 mmol) (CDCl3) 161.1, 154.2, 141.5, 138.8, 138.1, 138.0, 136.7, 136.1,
and tri-tert-butylphosphonium tetrafluoroborate (20 mg, 134.9, 134.7, 132.8, 131.1, 130.4, 128.5, 127.9, 125.3, 124.6,
.06 mmol) were dissolved in dry THF (10 mL). A solution of 124.4, 123.6, 108.1, 45.8, 37.4, 31.4, 30.8, 29.5, 29.1, 29.0,
(
0
tripotassium phosphate (84 mg, 0.4 mmol) in water (3 mL) was 28.9, 28.8, 25.9, 22.1, 16.6, 14.0, 13.6, 12.1, 10.8; MALDI–MS
added to the previous solution. The reaction was refluxed for m/z: 1794.2 [M+]; Anal. calcd for C104H138B2F4N6O2S6: C,
4
8 hours under nitrogen. Dichloromethane was added to the 69.62; H, 7.75; N, 4.68; S, 10.72; found: C, 67.33; H, 7.60; N,
reaction mixture and washed with water (50 mL), brine (50 mL) 4.87; S, 11.00; MP: 109–111 °C.
and water (50 mL). The organic layer was dried over MgSO4,
filtered and the solvents evaporated. Column chromatography Device fabrication
on silica (eluent mixture, hexane/dichloromethane, 1:1) was Indium tin oxide (ITO) coated glass substrates from Xin Yan
carried out. The main fractions were recrystallised by dissolving Technology Ltd. (15 Ω /□) were masked and etched in
in dichloromethane and precipitating with methanol. The hydrochloric acid (37%) for 20 minutes in order to get 4 mm
precipitate was dissolved in hot hexane and the beaker was left wide strips. The substrates were then cleaned using an ultra
in the fridge. The precipitate was filtered and a dark purple solid sonicator in deionised water, acetone and isopropanol succes-
was obtained (83 mg, 55%). 1H NMR (CDCl3) 8.88 (d, J = 4.1, sively. The substrates were then dried with nitrogen and oxygen
2
2
1
H), 7.37 (m, 4H), 6.96 (d, J = 3.6, 2H), 4.05 (d, J = 6.8, 4H), plasma treated for 3 minutes. Poly(3,4-ethylenedioxythio-
.55 (s, 12H), 2.35 (m, 8H), 1.98 (br s, 2H), 1.66 (s, 12H), phene):poly(styrenesulfonate) (PEDOT:PSS) from Clevios
.40–1.15 (m, 64H), 1.02 (t, 12H), 0.85 (m, 12H); 13C NMR (AI4083) was spin-coated at 4000 RPM in order to obtain a
(
CDCl3) 161.1, 154.4, 141.1, 138.9, 137.9, 137.6, 135.9, 135.8, 20 nm thin layer on top of the ITO. The PEDOT:PSS coated
1
3
2
32.9, 131.0, 130.1, 128.7, 128.1, 124.7, 124.6, 108.2, 45.8, ITO samples were then placed on a hotplate inside a nitrogen
7.5, 31.4, 31.3, 30.8, 29.5, 29.1, 29.09, 29.04, 28.8, 28.7, 25.8, filled glove box (O2 < 0.1 PPM, H2O < 0.5 PPM) and baked at
2.1, 16.6, 14.0, 13.5, 12.1, 10.8; MALDI–MS m/z: 1628.3 120 °C for 20 min in order to remove residual solvents. Films
[M+]; Anal. calcd for C96H134B2F4N6O2S4: C, 70.74; H, 8.29; containing various donor (9 and 10)/acceptor ratios (1:2, 1:3
N, 5.16; S, 7.87; found: C, 68.78; H, 8.05; N, 5.56; S, 8.09; MP: and 1:4) were spin-coated from a 20 mg mL−1 chlorobenzene
1
65–167 °C.
solution. [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM)
from Solenne B. V. Company was used as the acceptor. The
1
0,10'-(5'',5'''''-(2,5-Bis(2-octyldodecyl)-3,6-dioxo-2,3,5,6- devices were then annealed at 140 °C for 20 minutes before
tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis([2,2':5',2''- being placed into an evaporator for back electrode deposition.
terthiophene]-5'',5-diyl))bis(2,8-diethyl-5,5-difluoro-1,3,7,9- 20 nm of calcium and 200 nm of aluminium were thermally
tetramethyl-5H-dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-4- evaporated at a base pressure of 2 × 10−6 mbar. Devices were
ium-5-uide) (10): BODIPY 7 (278 mg, 0.5 mmol, 2.5 equiv), then encapsulated with a glass cover slip and a UV curable
DPP 8 (218 mg, 0.2 mmol, 1 equiv), Pd2(dba)3 (40 mg, optical adhesive from Thorlabs. The active area of the devices
2693

Beilstein J. Org. Chem. 2014, 10, 2683–2695.
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Absorption and emission spectra of compounds 9 and 10
and their fullerene blends; film thickness measurements;
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License and Terms
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