Enhancement of efficiency in organic photovoltaic devices containing self-complementary hydrogen-bonding domains

Article abstract of DOI:10.3762/bjoc.9.122

Self-complementary hydrogen-bonding domains were incorporated as the electron deficient unit in "push-pull", p-type small molecules for organic photovoltaic active layers. Such compounds were found to enhance the fill factor, compared with similar non-sel

Full text of DOI:10.3762/bjoc.9.122

Enhancement of efficiency in organic photovoltaic
devices containing self-complementary
hydrogen-bonding domains
Rohan J. Kumar*1,2, Jegadesan Subbiah1 and Andrew B. Holmes1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 1102–1110.
1School of Chemistry, Bio21 Institute, University of Melbourne, VIC
3010, Australia and 2CSIRO Materials Science and Engineering,
Private Bag 10, Clayton South, Victoria 3169, Australia
doi:10.3762/bjoc.9.122
Received: 12 February 2013
Accepted: 07 May 2013
Published: 06 June 2013
Email:
Rohan J. Kumar* - jkumar@unimelb.edu.au
Associate Editor: P. J. Skabara
* Corresponding author
© 2013 Kumar et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
hydrogen-bonding; organic photovoltaics; self-assembly
Abstract
Self-complementary hydrogen-bonding domains were incorporated as the electron deficient unit in “push–pull”, p-type small mole-
cules for organic photovoltaic active layers. Such compounds were found to enhance the fill factor, compared with similar non-self-
organized compounds reported in the literature, leading to higher device efficiencies. Evidence is presented that the ability of these
molecules to form one-dimensional hydrogen-bonded chains and subsequently exhibit hierarchical self-assembly into nanostruc-
tured domains can be correlated with improved device efficiency.
Introduction
The efficient generation of energy without producing environ- designed supramolecular interactions that control the spatial and
mentally detrimental side-products is an ongoing challenge for temporal distribution of materials are of interest [9-12].
science [1,2]. Incident solar radiation offers the largest source
of energy [1]; however, efficient photovoltaic devices with low Polymeric donor materials in blends with solution-processable
manufacturing costs and rapid energy payback times are fullerenes have produced the highest efficiency organic photo-
required [3]. The use of organic materials in photovoltaic voltaic bulk heterojunction (BHJ) devices to date [13,14]. The
devices is attractive owing to the abundance of the elements critical role of supramolecular interactions together with ma-
used, the possibility to use low-cost manufacturing techniques, terial behaviour at interfaces has been significant in achieving
and the diversity of molecular structure that is accessible. There these high efficiencies, as has been highlighted by the use of
is a growing understanding of the control of hierarchical inter- thermal annealing and solvent additives to produce dramatic
actions to produce functional behaviour in molecular materials efficiency increases [15]. In their role as systems for studying
[4-8]; however, much remains to be investigated. In particular, supramolecular interactions, polymers suffer from the added
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Beilstein J. Org. Chem. 2013, 9, 1102–1110.
complexity of compositional factors such as molecular weight a known self-complementary hydrogen-bonding domain. The
and polydispersity [16,17], as well as the technical difficulty of self-assembly properties of this unit have been extensively
studying morphology changes in thin films. Whilst high-effi- studied in a variety of systems [21-26], but this approach has
ciency BHJ devices based on solution-processed small mole- not yet been applied to improving organic photovoltaic device
cules have also relied on the use of solvent additives in order to performance. Solubility could be maintained by introduction of
achieve high efficiencies [18,19], they offer a relatively simple alkyl substituents at the β-position of the thiophenes, and the
basis for designing supramolecular interactions. Such interac- NH-cyanopyridone unit could be unmasked for hydrogen-
tions can be readily investigated in solution and the results can bonding. In turn, this may allow access to higher order supra-
be translated to the solid state. As such, these supramolecular molecular structures, promoting charge transport and increasing
interactions may offer a means to control bulk morphology the fill factor and efficiency of devices based on this compound.
without reliance on post-processing, leading to cost and energy
Results and Discussion
input advantages in device manufacture.
Synthesis
Recently, a donor–acceptor small molecule with a cyanopyri- The synthesis of compound 2 was completed in a simple two-
done moiety as the acceptor motif and displaying moderate step procedure by using the strategy of Gupta et al. [20]
photovoltaic efficiency in a BHJ device with a fullerene was (Scheme 1). A Suzuki coupling of the triarylaminoboronic acid
reported [20]. The efficiency of these optimized devices was 3 to the bromo-dihexylbithiophene-carboxaldehyde 4 [27] fol-
primarily limited owing to a low fill factor, which was attrib- lowed by a Knoevenagel condensation provided the target com-
uted to poorly interconnected domains for charge transport in pound 2 in excellent yield. The solubilizing strategy of
the cyanopyridone. The N-2-ethylhexyl substituent was incorporating alkyl groups on the thiophene unit provided the
employed to enhance the solubility of compound 1 in organic compound with excellent solubility in a wide range of common
solvents (Figure 1). However, alkylation on the nitrogen masks solvents.
Figure 1: N-ethylhexyl-substituted (1) [20] and target free N–H (2) cyanopyridone structures.
Scheme 1: Synthesis of the cyanopyridone 2, reagents and conditions: (i) Pd(PPh3)4, Cs2CO3, toluene, reflux, 16 h, 84%; (ii) EtOH, reflux, 2 h, 82%.
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Beilstein J. Org. Chem. 2013, 9, 1102–1110.
Optoelectronic properties
pound 1 [20] and are suitable for charge transfer to take place to
In chloroform solution the UV–vis absorption spectrum of com- fullerene-based electron acceptors, such as PC61BM. The NH
pound 2 exhibited a peak absorbance of 586 nm (Figure 2a), cyanopyridone unit can presumably exist as a number of
compared with 590 nm for the N-alkylated analogue 1. This tautomers; however, if present, these do not appear to affect the
transition is assigned to an intramolecular charge-transfer state electronic properties in solution significantly compared with the
from the electron-rich triarylamine moiety to the electron-defi- N-alkylated compound.
cient cyanopyridone. The absorption peak of 2 displays a 30 nm
hypsochromic shift in the solid state, with the maximum absorp- Density functional theory (DFT) calculations, at the B3YLP-
tion at 556 nm, and the absorption is also broadened with a 61G*(d,p) level of theory using the Gaussian ‘09 suite of
shoulder at approximately 614 nm. When compound 2 in chlo- programs [28], result in the observation of a polar distribution
roform solution is excited at the absorption maximum, the emis- of the calculated molecular orbital densities for 2, as expected
sion spectrum shows a weak peak at 782 nm. The emission from the dipolar absorption transitions observed (Figure 2b). A
maximum of a thin film of 2 is hypsochromically shifted by conformational search followed by DFT optimization of the
31 nm to 751 nm. Cyclic voltammetry and photoelectron spec- resultant structures showed that the monomer has an energetic
troscopy in air (PESA) were used to determine the HOMO and preference for the flat structure shown in Figure 2b, with a
LUMO levels of compound 2 and these are summarized in barrier to rotation for the thiophene alkene bond of
Table 1. These values are similar to those reported for com- 272 KJ mol−1. This is supported by the crystal structure of a
Figure 2: (a) Chloroform (solid line) and thin-film (dashed line) UV–vis absorption and emission spectra and (b) optimized DFT structure and calcu-
lated orbital densities of compound 2.
Table 1: Optoelectronic and electrochemical (CV) data for compounds 1 and 2.
Absorption (nm)
Emission (nm)
E½a(ΔEp)b mV
HOMOc
(eV)
Egd
(eV)
LUMOe
(eV)
λmax
λmax
λmax
λmax
R/R−
R+/R
R2+/R
(CHCl3)
(film)
(CHCl3)
(film)
1f
590
586
584
556
800
782
—
—
—
—
−5.4
1.6
−3.8
−5.26
(−5.55)
−3.66
(−4.05)
2
751
−1136g
464 (60)
786 (70)
1.6 (1.5)
aHalf wave potential determined as the average of the anodic and cathodic peak potentials, V versus ferrocene 10 mM in CH2Cl2, 0.1 M Bu4N(PF6).
bDifference between the anodic and cathodic peak potentials, V, v = 0.1 mV s−1. cDetermined from EHOMO = −(Eox + 4.80) eV, data in brackets
measured by PESA on thin films. dDetermined from the difference in electrochemically measured HOMO and LUMO, data in brackets estimated from
the onset of absorption in thin films. eDetermined from ELUMO = −(Ered + 4.80) eV, data in brackets measured by PESA on thin films and absorption
onset ELUMO = EHOMO + Eg. fData from [20]. gIrreversible reduction, value in the peak reduction potential.
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Beilstein J. Org. Chem. 2013, 9, 1102–1110.
related NH-cyanopyridone structure determined by Würthner et pounds show that both dimers [29], higher order n-mers [31],
al. [29], which exhibited a similarly oriented, flat thiophene and chains of hydrogen-bonded molecules are possible [32].
cyanopyridone structure in a hydrogen-bonded dimer. The
barrier for rotation, as determined by temperature-dependent
(1)
1H NMR analysis, was reported to be at least 60 KJ mol−1 [29].
Self-assembly of cyanopyridone 2
The hydrogen-bond-mediated self-assembly behaviour of com- An additional observation from the 1H NMR experiments was
pound 2 was monitored by 1H NMR spectroscopy. The chem- gel formation at the highest concentrations, indicating the for-
ical shift of the proton on nitrogen in chloroform was observed mation of high-aspect-ratio nanofibers in solution [33]. Films
to shift downfield in a concentration-dependent manner. drop cast from 1 mg/mL and 10 mg/mL solutions of 2 in chloro-
Regression analysis of the concentration-dependent equilibrium form were investigated by AFM in order to directly observe any
was not possible in chloroform as the maximum 1H chemical nanostructures present. A representative scan at each concentra-
shift was not reached at concentrations as high as 30 mg/mL tion is shown in Figure 4. The films were found to contain inter-
(see Supporting Information File 1). However, in d6-benzene connected fibrous nanostructures approximately 85–110 nm in
observation of the maximum and minimum chemical shift of width at both concentrations; however, the structures appeared
the NH proton was possible in an accessible concentration to be more regular in films cast from solutions at higher concen-
range (Figure 3a). It was also observed that other chemical trations.
shifts within the molecule exhibit a concomitant concentration-
dependent downfield shift, notably the vinyl proton (Figure 3b). Hydrogen-bond-mediated self-association of molecules bearing
These experiments were replicated in the presence of an excess a self-complementary imide motif has been well studied by
amount of PC61BM and no significant difference in concentra- 1H NMR, as reported in the literature [21-26]. Given this prece-
tion-dependent change of chemical shift was observed (see dent, the observed concentration-dependent downfield change
Supporting Information File 1).
in the NH chemical shift is assigned to intermolecular hydrogen
bonding. The downfield chemical shift of other protons in mole-
The variation of chemical shift with concentration for both the cules bearing a cyanopyridone motif is unprecedented and at
cyanopyridone NH and the vinyl proton could be fitted by a least three scenarios may be invoked to explain this observation.
theoretical expression for dimerization derived from a two-state Since the 1H chemical shift data can support either a dimeriza-
equilibrium (Equation 1, fit shown in Figure 3) [25]. It should tion or isodesmic self-association model as the initial self-
be noted that Equation 1 describes the simplest two-state model assembly step, both models must be considered.
of association, namely dimerization, but can also be used to fit a
two-state isodesmic self-association interaction; the two cases The hydrogen-bond-mediated centrosymmetric dimerization of
are indistinguishable using this model [30]. Crystal structures two molecules of 2, leading to an alteration of the charge distri-
and scanning tunnelling microscopy studies of related com- bution and thus the 1H chemical shift of a number of protons is
Figure 3: The concentration dependence of (a) the NH 1H NMR chemical shift and (b) the vinyl proton chemical shift for compound 2 in d6-benzene.
The line is the regression fit of Equation 1 to the data.
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Beilstein J. Org. Chem. 2013, 9, 1102–1110.
Figure 4: AFM height image of observed nanostructures of films drop cast from (a) 10 mg/mL and (b) 1 mg/mL chloroform solution of 2.
the simplest explanation of the NMR data (Figure 5). This does natively, isodesmic self-association to form one dimensional
not explain the formation of nanostructures as observed by hydrogen-bonded chains of 2 may incur conformational
AFM and the gelation behaviour at high concentration. Alter- changes in 2 for steric reasons, which could result in changes to
Figure 5: Dimeric structures of 2. (a) centrosymmetric dimer of 2; (b) bond-rotated centrosymmetric dimer of 2 as predicted by DFT calculations.
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Beilstein J. Org. Chem. 2013, 9, 1102–1110.
the 1H chemical shifts of 2. The final scenario considered here change in the chemical shift of the alkene proton resonance
is that the hydrogen-bonding-mediated interaction is immedi- (Δδ 0.05 ppm) in contrast with the downfield change observed
ately followed by a kinetically faster second supramolecular (Δδ 0.89 ppm). Bond rotation of the alkene thiophene bond and
interaction, leading to the observation by 1H NMR of only the subsequent calculation of the 1H NMR chemical shifts gives a
rate-limiting first step. π–π stacking interactions are observed downfield change in chemical shift for the same proton
for planar aromatic systems and dimerized nucleobases; (Δδ 1.01 ppm), in agreement with experimental observations
however, in these cases an upfield change in chemical shift is (Δδ 0.89 ppm). Thus, of the scenarios considered, that which
generally observed, [5,30] in contrast to the downfield move- best fits the data is hydrogen bonding of the NH proton with a
ment in chemical shift observed for the vinyl proton in com- concomitant rotation of the alkene thiophene bond. Due to the
pound 2. Non-specific van der Waals interactions are typically empirically demonstrated barrier to bond rotation for these com-
weaker than hydrogen-bonding interactions and insufficient to pounds [20], this implies the need to accommodate the steric
explain the changes in chemical shift at the concentrations bulk of the molecule, possibly in extended one-dimensional
observed here. However, given the strongly dipolar nature of hydrogen-bonded arrays formed under isodesmic kinetic
the compound, the role of dipole-based interactions cannot be control.
ruled out. We then turned to molecular modelling to shed
further light on the possible modes of self-organization.
The formation of interconnected nanostructures by 2, as
observed in the AFM experiments, also leads to the interpreta-
In order to further investigate the plausibility of each scenario, tion of the 1H NMR self-association data as the formation of
GIAO shielding calculations were used to predict the 1H NMR one-dimensional chains of 2. One possible mode of self-
chemical shifts of (i) the optimized structure of 1 (Figure 2), (ii) complementary interaction leading to one-dimensional chains is
an anti-symmetric dimer of 2 (Figure 5a) and (iii) the same represented in Figure 6. The nanostructures formed are in the
dimer structure in which the C–C single bond connecting the region of 85–100 nm in size and this is an order of magnitude
alkene to the adjacent thiophene ring is rotated by 90° larger than the width of two molecules (8 nm). The UV–vis
(Figure 5b). This bond rotation is the major conformational absorption spectrum of a thin film of 2 shows an asymmetric
change possible for 2 in order to accommodate the steric bulk of hypsochromic shift of the absorption peak (Figure 2a) indica-
the molecule in hydrogen-bonded chains. It is also likely to tive of the formation of weak H-aggregates in the solid state
produce significant changes in the alkene proton chemical shift. [34,35]. Thus, a hierarchical model for association can be devel-
These calculated chemical shifts are compared with those oped in which the formation of hydrogen-bonded chains of 2 is
obtained experimentally (Table 2).
followed by interchain H-aggregation, although this requires
further experimental verification.
The calculated 1H chemical shifts for the optimized structure of
2 are in close agreement with the experimentally observed Organic photovoltaic device performance
values at low concentration. This close agreement between Finally, organic photovoltaic (OPV) BHJ devices with active
experimental observation and calculated value is observed for layers comprising 1:1 mixtures of 2 and PC61BM spin-coated
both the NH proton resonance (δ 7.50 versus 6.92, respectively) from chloroform solutions of various concentrations were fabri-
and the alkene proton resonance (δ 7.47 and 7.50, respectively). cated in order to determine the functional consequences of the
The calculated 1H NMR chemical shifts for the optimized dimer observed self-assembly behaviour. The architecture of the
of compound 2 shows a large downfield shift (Δδ 4.14 ppm) for devices was glass/ITO/poly(3,4-ethylenedioxythio-
the NH proton resonance, which is in approximate agreement phene)poly(styrenesulfonate)/active layer/ZnO nanoparticles/
with the experimental value (Δδ 4.04 ppm), and a small upfield Al. Zinc oxide nanoparticles were used as an electron transport
Table 2: Experimental versus calculated proton chemical shifts in the dimers shown in Figure 5.a
Condition
NH proton chemical shift (ppm) alkene proton chemical shift (ppm)
0.14 mM 2 in d6-benzene
7.50
7.47
42 mM 2 in d6-benzene
11.54
8.36
2 in Figure 2 (calculated)
6.92
7.50
Optimized dimer of 2 in Figure 5a (calculated)
Bond-rotated dimer of 2 in Figure 5b (calculated)
11.06, 10.97
10.79, 10.70
7.45, 7.48
8.51, 8.24
aDFT NMR calculations where performed using the Gaussian 09 suite of programs and the BYLP 631G(d) basis set. Solvation was not considered.
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Beilstein J. Org. Chem. 2013, 9, 1102–1110.
Figure 6: Schematic representation of self-complementary interactions leading to one-dimensional chains.
layer. The spin-coating velocity was varied to control film the electrodes is optimized, and compounds that self-assemble
thickness. The average results from ten devices are summarized into arrays have been observed to display high fill factors [36].
in Table 3 and the devices show significant performance varia- Thus, the formation of one-dimensional chains of 2 via
tion with concentration. The open-circuit voltage (Voc) varies hydrogen bonding may create networks that facilitate hole
between 0.90 and 0.96 for the four conditions tested. The fill percolation to the electrodes, resulting in an increase in fill
factor (FF) varies from 49% to 37% and the short circuit current factor [37].
(Jsc) varies between 5.1 and 3.05 mA/cm2. The devices fabri-
cated from 10 mg/mL, 1:1 solutions of 2:PC61BM show the The variation of performance with the concentration of the solu-
highest efficiency and also the highest Voc, FF and Jsc.
tions used to deposit the active layer gives insight into the
correlation of supramolecular interaction and functional perfor-
A primary observation is that optimized devices using com- mance. It is seen that the performance of the devices increased
pound 2 exhibit marginally higher average performances than with the concentration of active layer components in the deposi-
any of the devices reported by Gupta et al. using the N-alky- tion solution, (e.g., 6 mg/mL and 10 mg/mL) and the efficiency
lated compound 1 (2.40% versus 2.25% when 1 was deposited dropped when deposition was carried out from solutions at
from chlorobenzene and 1.64% when 1 was deposited from higher concentration of the active layer. This was primarily due
chloroform) [20]. The primary parameter for which compound to the poor film-forming ability of the more concentrated solu-
2 outperforms its N-alkylated analogue 1 is the fill factor. Fill tions as they approached the concentration at which organogel
factor is believed to be optimized when charge conduction to formation occurred. This is a similar effect to that observed for
Table 3: OPV device performance.a
Compound
Concentration (mg/mL)b
Jsc (mA/cm2)c
Voc (V)d
FF (%)e
η(%)f
2
6
4.40 (4.55)
5.10 (5.30)
3.90 (4.00)
3.05 (3.20)
6.77
0.94 (0.94)
0.96 (0.96)
0.92 (0.92)
0.90 (0.90)
0.87
48 (49)
49 (51)
40 (41)
37 (38)
38
2.02 (2.10)
2.40 (2.58)
1.42 (1.50)
1.02 (1.10)
2.25
10
15
20
20
1g
aData are the mean for n = 10 devices, and best device data is shown in brackets. bThe active layers were spin coated from 1:1 (w/w) solutions of
2:PC61BM at the concentrations shown. cShort circuit current. dOpen circuit voltage. eFill Factor. fDevice energy conversion efficiency (AM 1.5). gData
is from the best device.
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Beilstein J. Org. Chem. 2013, 9, 1102–1110.
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A self-complementary hydrogen-bonding domain has been
incorporated as the electron-deficient moiety in a “push–pull”
p-type molecule 2 for organic solar cells. Self-association of
this domain could be monitored by 1H NMR, and at higher
concentrations the compound was found to form nanostructures
and organogels. In bulk heterojunction organic photovoltaic
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Full experimental details and compound characterization.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-122-S1.pdf]
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and the Victorian State Government (DBI-VSA and DPI-ETIS)
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