Effects of heteroatom substitution in conjugated heterocyclic compounds on photovoltaic performance: From sulfur to tellurium

Article abstract of DOI:10.1039/c4cc01862a

We report a general strategy for fine-tuning the bandgap of donor-acceptor-donor based organic molecules by modulating the electron-donating ability of the donor moiety by changing the benzochalcogenophene donor groups from benzothiophenes to benzoselenophenes to benzotellurophenes. These molecules show red-shifts in absorption and external quantum efficiency maxima from sulfur to selenium to tellurium. In bulk heterojunction solar cell devices, the benzoselenophene derivative shows a power conversion efficiency as high as 5.8% with PC₆₁BM as the electron acceptor. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01862a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Effects of heteroatom substitution in conjugated
heterocyclic compounds on photovoltaic
performance: from sulfur to tellurium†
Cite this: Chem. Commun., 2014,
50, 7964
Received 12th March 2014,
Accepted 30th April 2014
Y. S. Park,a T. S. Kale,a C.-Y. Nam,a D. Choib and R. B. Grubbs*ab
DOI: 10.1039/c4cc01862a
www.rsc.org/chemcomm
We report a general strategy for fine-tuning the bandgap of donor– the acceptor, is often lowered in these molecules due to
acceptor–donor based organic molecules by modulating the elevation of the HOMO energy level of the donor. In order to
electron-donating ability of the donor moiety by changing the obtain simultaneous enhancements in Voc and Jsc, designing
benzochalcogenophene donor groups from benzothiophenes to organic electron donors and fine-tuning their energy levels is a
benzoselenophenes to benzotellurophenes. These molecules show useful approach towards maximizing OPV efficiency.4,5 Seferos
red-shifts in absorption and external quantum efficiency maxima and coworkers have demonstrated that energy levels can be
from sulfur to selenium to tellurium. In bulk heterojunction solar cell controlled through atomistic bandgap engineering in conjugated
devices, the benzoselenophene derivative shows a power conversion donor–acceptor polymer backbones by changing the heteroatom
efficiency as high as 5.8% with PC61BM as the electron acceptor.
in the acceptor benzochalcogenodiazole unit from sulfur to
selenium to tellurium.6,7 In comparisons with analogous
Understanding structure–property relationships in conjugated thiophene-containing polymers, smaller bandgaps have been
organic materials is essential for improving the characteristics observed in the tellurophene-based polymers.8,9
of optoelectronic devices.1,2 The rational design of organic
From this perspective, isoelectronic substitution,10 or sys-
semiconducting materials has led to notable improvements in tematic replacement of heteroatoms in aromatic heterocycles of
the efficiencies of organic photovoltaic (OPV) devices. Bandgap electron donors would modulate electron-donating ability and
engineering strategies, in which the frontier molecular orbital potentially enhance properties such as optical absorption and
energy levels of both the electron donor and electron acceptor carrier mobility that positively affect OPV device power conver-
components of bulk heterojunction OPV cells are adjusted to sion efficiency.11
optimize performance, have proven effective in enabling improve-
To this end, we have chosen the benzofuran end-capped
ments in efficiency. Here, we describe systematic studies on diketopyrrolopyrrole (DPP) derivative, DPP(TBFu)2 (1, Fig. 1)
benzochalcogenophene-containing donor–acceptor–donor type developed by Nguyen and co-workers, which is a high efficiency
molecules by changing the donor moieties from benzothiophene molecular system boasting a power conversion efficiency (PCE)
to benzoselenophene and benzotellurophene, and demonstrate of 4.4% with PC71BM,12,13 as a parent structure and investigated
their performance as efficient electron donors in OPV cells.
the effect of changing the benzochalcogenophene heteroatom
Conjugated organic light-absorbing molecules or macro- from sulfur to selenium to tellurium on optoelectronic and
molecules with alternating electron-rich and electron-deficient
moieties have narrower bandgaps, which enhances light harvesting
and results in higher short circuit current densities ( Jsc).3 However,
open circuit voltage (Voc), which is related to the energy offset
between the highest occupied molecular orbital (HOMO) of the
donor and the lowest unoccupied molecular orbital (LUMO) of
a Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton,
NY 11973, USA
b Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA.
E-mail: robert.grubbs@stonybrook.edu
† Electronic supplementary information (ESI) available: Experimental details. See
DOI: 10.1039/c4cc01862a
Fig. 1 Structures of DPP(TBFu)2, DPP2T(BTh)2, DPP2T(BSe)2, and DPP2T(BTe)2.
7964 | Chem. Commun., 2014, 50, 7964--7967
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
UV-Vis absorption spectra in solution and in film show slight
but consistent red-shifts in absorption maxima from DPP2T(BTh)2
(lmax = 584 and 624 nm) to DPP2T(BSe)2 (lmax = 588 and 629 nm) to
DPP2T(BTe)2 (lmax = 596 and 633 nm) (Fig. S2, ESI†). These strong
absorption bands from 480 to 680 nm can be attributed to the
intramolecular charge transfer between electron acceptor (DPP)
and electron donor (thienyl and benzochalcogenophene) moiety,
where atomic changes in the remote benzochalcogen units affect
optical properties.
Compared to optical spectra in solution, absorption bands
of all three compounds in thin films (Fig. 2B) become broad
and further red-shifted as a result of intermolecular interactions.
While DPP2T(BTh)2 and DPP2T(BSe)2 show similar intensities for
the two absorption maxima in films, DPP2T(BTe)2 shows a weaker
relative absorbance at the longer wavelength maximum (694 nm),
suggesting that tellurium substitution has significant effects on
molecular packing. In film, similar red-shifts on absorption
maxima are observed from DPP2T(BTh)2 (lmax = 611 and
673 nm) to DPP2T(BSe)2 (lmax = 618 and 683 nm) to DPP2T(BTe)2
(lmax = 618 and 694 nm). From onsets of absorption in films, optical
bandgaps are estimated as 1.74 eV for DPP2T(BTh)2, 1.71 eV for
DPP2T(BSe)2, and 1.66 eV for DPP2T(BTe)2, confirming that the
bandgap can be fine-tuned by varying the heteroatom within the
same group of the periodic table.
Fig. 2 (A) Synthesis of 2, 3, and 4. (i) Pd(PPh3)4/toluene (110 1C); (ii) Pd(OAc)2,
PCy3, Cs2CO3/o-xylene (140 1C). (B) Optical absorption spectra in thin films
cast from chloroform.
Electrochemical bandgaps were calculated by cyclic voltam-
metry (Fig. S3, ESI†) as 1.83 eV for DPP2T(BTh)2, and 1.80 eV for
device properties. Because solubility of this series of molecules DPP2T(BSe)2, which are in good agreement with the optical
in common solvents decreases as furan groups are replaced bandgaps. However, a smaller relative electrochemical bandgap
with other heterocycles, the use of longer 2-butyloctyl solubiliz- for DPP2T(BTe)2 (1.60 eV) was calculated due to the elevated
ing chains was necessary.14,15
HOMO energy level of DPP2T(BTe)2 (À5.55 eV), as has been
Compounds DPP2T(BTh)2 (2) and DPP2T(BSe)2 (3) contain- observed in benzoditellurophene due to the lower ionization
ing benzothiophene and benzoselenophene units were synthe- energy of tellurium.19
sized efficiently (isolated yields of 79% for 2 and 85% for 3)
Solution-processed bulk heterojunction solar cell devices
using a Stille coupling methodology with 3,6-bis(5-bromo-2- were fabricated in air with DPP2T(BTh)2, DPP2T(BSe)2, and
thienyl)-2,5-dihydro-2,5-di(2-butyloctyl)pyrrolo[3,4-c]pyrrolo-1,4- DPP2T(BTe)2 as the electron donor. Current–voltage ( J–V)
dione DPP2TBr2 (5)16 and 2-tributylstannylbenzothiophene (6) curves and incident photon conversion efficiency (IPCE) measure-
or 2-tributylstannylbenzo-selenophene (7) (Fig. 2A). For the ments for the best performing devices are shown in Fig. 3 and all
benzotellurophene-containing compound DPP2T(BTe)2 (8), device parameters are summarized in Table 1. The superior
the Stille coupling route offered low yields, as have previously performance for devices prepared from DPP2T(BSe)2 resulted
been observed with other tellurophene derivatives.6 Because mainly from a higher Jsc, which originates from a higher external
the ipso-arylative coupling of diphenylcarbinol-substituted het- quantum efficiency across the absorption spectrum, with a peak of
erocycles has previously been reported,17 it was examined as an 61% around 600 nm (Fig. 3B). As we observed in film absorption
alternative strategy. Benzo[b]tellurophene18 was lithiated in the spectra of the donor molecules, the IPCE curves show red-shifted
2-position followed by addition of benzophenone to afford absorption from DPP2T(BTh)2 to DPP2T(BTe)2, which contributes
(2-benzotellurophenyl)diphenylmethanol (8). The palladium- to photocurrent generation. Optimized devices prepared from
catalyzed ipso-arylation reaction between benzotellurophene compound DPP2T(BSe)2 show power conversion efficiencies
derivative 8 and DPP core 5 allowed us to obtain compound (PCE) as high as 5.8%. Devices prepared from DPP2T(BTe)2
DPP2T(BTe)2 (4) in a moderate yield (42% isolated yield).
showed lower Voc values than devices prepared from the other
In 1H-NMR spectra of these compounds, singlets arising from molecules. This can be explained by the higher HOMO level of
the 2-benzochalcogenophene protons (Ha) shifted downfield DPP2T(BTe)2 since Voc is correlated to energy offset between
from DPP2T(BTh)2 (d 7.59 ppm) to DPP2T(BSe)2 (d 7.74 ppm) to HOMO of the donor and the LUMO of PC61BM.
DPP2T(BTe)2 (d 8.01 ppm), whereas thienyl doublets (Hb) shifted
In order to further investigate the better device performance
upfield from DPP2T(BTh)2 (d 7.48 ppm) to DPP2T(BSe)2 (d 7.38 ppm) from DPP2T(BSe)2, carrier mobilities for each compound were
to DPP2T(BTe)2 (d 7.24 ppm) (Fig. S1, ESI†). These shifts are measured by analyzing J–V characteristics of hole-only ohmic
indicative of changes in heterocycle electron density as the devices with the space-charge-limited current (SCLC) model,20,21
electronegativity of the heteroatom decreases down the period. which is relevant to OPV devices with an out-of-plane charge
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7964--7967 | 7965

View Article Online
ChemComm
Communication
key step in obtaining high efficiency devices.22,23 The optical band-
gap was observed to narrow down the period from DPP2T(BTh)2 to
DPP2T(BSe)2 to DPP2T(BTe)2, and the highest efficiency was
observed for DPP2T(BSe)2 devices. These results confirm that an
atomistic bandgap engineering approach can be adopted as rational
molecular design strategy for OPV devices since solar cell para-
meters, such as Voc and Jsc, are correlated, however, control of
other properties such as morphology24 and mobility25 are crucial
for obtaining high efficiency devices. Even though DPP2T(BTe)2
devices did not show the highest Jsc in the series, they are a rare
example of tellurium containing organic solar cells and the
red-shifted light absorption when compared to compounds
DPP2T(BTh)2 and DPP2T(BSe)2 suggests that in depth studies of
morphology and charge separation in these systems could lead to
further increases in efficiency. We are working to improve the
efficiency of these compounds, especially DPP2T(BTe)2, through
further structural variation, using PC71BM26 as an electron
acceptor, and by optimizing the cathode interlayer.27
This research was carried out at the Center for Functional
Nanomaterials, Brookhaven National Laboratory, which is sup-
ported by the U.S. Department of Energy, Office of Basic Energy
Sciences, under Contract No. DE-AC-02-98CH10886. This research
was also supported by the BNL Laboratory Directed Research and
Development Award 09-003. Mass spectrometry was performed at
the Proteomic Center, Stony Brook University, Shared Instrumen-
tation Grant NIH/NCRR 1 S10 RR023680-1.
Fig. 3 (A) J–V curves and (B) IPCE measurements for solar cells prepared
from compounds 2–4 (device structure: indium tin oxide (ITO)/MoOx/
Donor:PC61BM/Al).
Notes and references
´
1 P. M. Beaujuge and J. M. J. Frechet, J. Am. Chem. Soc., 2011, 133,
20009–20029.
2 Y. Li, Acc. Chem. Res., 2012, 45, 723–733.
3 Y.-J. Cheng, S.-H. Yang and C.-S. Hsu, Chem. Rev., 2009, 109,
5868–5923.
4 H. Zhou, L. Yang, A. C. Stuart, S. C. Price, S. Liu and W. You, Angew.
Chem., Int. Ed., 2011, 50, 2995–2998.
5 G. Dennler, M. C. Scharber and C. J. Brabec, Adv. Mater., 2009, 21,
1323–1338.
Table 1 Solar cell device parameters
PCE (%)
Jsc
Donor
Voc (V)
(mA cmÀ2
)
FF
Max
Avg
DPP2T(BTh)2
DPP2T(BSe)2
DPP2T(BTe)2
0.94
0.98
0.88
9.2
13.2
6.6
0.49
0.45
0.52
4.2
5.8
3.0
3.9a
5.1a
2.8b
6 G. L. Gibson, T. M. McCormick and D. S. Seferos, J. Am. Chem. Soc.,
2012, 134, 539–547.
7 G. L. Gibson, T. M. McCormick and D. S. Seferos, J. Phys. Chem. C,
2013, 117, 16606–16615.
a
b
Averaged over 25 devices. Averaged over 5 devices.
8 A. A. Jahnke, B. Djukic, T. M. McCormick, E. Buchaca Domingo,
C. Hellmann, Y. Lee and D. S. Seferos, J. Am. Chem. Soc., 2013, 135,
951–954.
9 M. Kaur, D. Seul Yang, J. Shin, T. Wan Lee, K. Choi, M. Ju Cho and
D. Hoon Choi, Chem. Commun., 2013, 49, 5495–5497.
10 Z. B. Henson, K. Mullen and G. C. Bazan, Nat. Chem., 2012, 4,
699–704.
11 K. Takimiya, Y. Kunugi, Y. Konda, N. Niihara and T. Otsubo, J. Am.
Chem. Soc., 2004, 126, 5084–5085.
12 B. Walker, A. B. Tamayo, X.-D. Dang, P. Zalar, J. H. Seo, A. Garcia,
M. Tantiwiwat and T.-Q. Nguyen, Adv. Funct. Mater., 2009, 19,
3063–3069.
transport orientation. Molecule DPP2T(BSe)2 displays a hole
mobility of B5 Â 10À5 cm2 VÀ1
s
À1, comparable to that of
DPP(TBFu)2 (B1 Â 10À5 cm2 VÀ1
s
À1),12 an order of magnitude
higher than that of DPP2T(BTh)2 (B7 Â 10À6 cm2 VÀ1
s
À1) and
B60% higher than DPP2T(BTe)2 (B3 Â 10À5 cm2 VÀ1 sÀ1
)
(Fig. S4 and S5, ESI†). This is consistent with the overall superior
IPCE and Jsc observed in DPP2T(BSe)2.
In conclusion, we have prepared a series of benzochalco-
genophene–DPP–benzochalcogenophene-based molecules with
13 B. Walker, A. Tamayo, D. T. Duong, X.-D. Dang, C. Kim, J. Granstrom
and T.-Q. Nguyen, Adv. Energy Mater., 2011, 1, 221–229.
the benzochalcogenophene heteroatom changed from sulfur to 14 C. H. Woo, P. M. Beaujuge, T. W. Holcombe, O. P. Lee and
´
J. M. J. Frechet, J. Am. Chem. Soc., 2010, 132, 15547–15549.
selenium to tellurium and evaluated each molecule as the
electron donating material in bulk heterojunction solar cell
15 J. Liu, B. Walker, A. Tamayo, Y. Zhang and T.-Q. Nguyen, Adv. Funct.
Mater., 2013, 23, 47–56.
devices. DPP(TBTh)2, the 2-ethylhexyl analogue of DPP2T(BTh)2, 16 L. Dou, J. Gao, E. Richard, J. You, C.-C. Chen, K. C. Cha, Y. He, G. Li
and Y. Yang, J. Am. Chem. Soc., 2012, 134, 10071–10079.
has been reported to show a power conversion efficiency of 1.4%
´ ´
17 A. B. Bıro and A. Kotschy, Eur. J. Org. Chem., 2007, 1364–1368.
(1 : 1 blend with PC71BM),15 so the use of 2-butyloctyl side chains
to improve solution processability of these molecules was also a
18 H. Sashida, K. Sadamori and T. Tsuchiya, Synth. Commun., 1998, 28,
713–727.
7966 | Chem. Commun., 2014, 50, 7964--7967
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
19 K. Takimiya, Y. Konda, H. Ebata, N. Niihara and T. Otsubo, J. Org. 24 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger and
Chem., 2005, 70, 10569–10571. G. C. Bazan, Nat. Mater., 2007, 6, 497–500.
20 C.-Y. Nam, D. Su and C. T. Black, Adv. Funct. Mater., 2009, 19, 3552–3559. 25 P. W. M. Blom, V. D. Mihailetchi, L. J. A. Koster and D. E. Markov,
21 C.-Y. Nam, J. Phys. Chem. C, 2012, 116, 23951–23956. Adv. Mater., 2007, 19, 1551–1566.
22 A. T. Yiu, P. M. Beaujuge, O. P. Lee, C. H. Woo, M. F. Toney and 26 J. C. Bijleveld, A. P. Zoombelt, S. G. J. Mathijssen, M. M. Wienk,
´
J. M. J. Frechet, J. Am. Chem. Soc., 2011, 134, 2180–2185.
M. Turbiez, D. M. de Leeuw and R. A. J. Janssen, J. Am. Chem. Soc.,
2009, 131, 16616–16617.
23 C. Cabanetos, A. El Labban, J. A. Bartelt, J. D. Douglas, W. R.
´
Mateker, J. M. J. Frechet, M. D. McGehee and P. M. Beaujuge, 27 V. Gupta, A. K. K. Kyaw, D. H. Wang, S. Chand, G. C. Bazan and
J. Am. Chem. Soc., 2013, 135, 4656–4659.
A. J. Heeger, Sci. Rep., 2013, 3, 1965, DOI: 10.1038/srep01965.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7964--7967 | 7967