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Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Oligomeric DithienopyrroleThienopyrrolodione (DTP-TPD) 

Donor-Acceptor Copolymer for 

Organic Photovoltaics

Preprint

Scott R. Hammond, Wade Braunecker, Andres 

Garcia, Ross Larsen, Zbyslaw Owczarczyk, 

Dana Olson, and David Ginley

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50660

July 2011
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1 

OLIGOMERIC DITHIENOPYRROLE-THIENOPYRROLEDIONE (DTP-TPD) DONORACCEPTOR COPOLYMER FOR ORGANIC PHOTOVOLTAICS

Scott R. Hammond, Wade Braunecker, Andres Garcia, Ross Larsen, Zbyslaw Owczarczyk, Dana Olson, David Ginley

National Renewable Energy Laboratory, Golden, CO, USA

ABSTRACT

A new donor-acceptor copolymer system based upon a 

dithienopyrrole (DTP) donor moiety and a 

thienopyrrolodione (TPD) accepting moiety has been 

designed and synthesized for organic photovoltaic (OPV) 

applications. The TPD accepting moiety has recently 

gained significant attention in the OPV community and is

being incorporated into a number of different polymer 

systems. In contrast, the DTP donor moiety has received 

only limited attention, likely due in part to synthetic 

difficulties relating to the monomer. In our hands, the 

bis(trimethyltin)-DTP monomer was indelibly contaminated 

with ~5% of the mono-destannylated DTP, which limited 

the Stille polymerization with the dibromo-TPD monomer 

(>99% pure) to produce material with Mn ~ 4130 g/mol 

(PDI = 1.10), corresponding to around eight repeat units. 

Despite this limitation, UV-visible absorption spectroscopy 

demonstrates strong absorption for this material with a 

band gap of ~1.6 eV. Cyclic voltammetry indicates a 

highest occupied molecular orbital (HOMO) energy level of 

-5.3 eV, which is much lower than calculations predicted. 

Initial bulk heterojunction OPV devices fabricated with the 

fullerene acceptor phenyl C61 butyric acid methyl ester 

(PCBM) exhibit Voc ~ 700 mV, which supports the deep 

HOMO value obtained from CV. These results suggest the 

promise of this copolymer system.

MOLECULAR DESIGN

We have designed a new copolymer system based upon a 

dithienopyrrole (DTP) donor moiety and a 

thienopyrroledione (TPD) accepting moiety for organic 

photovoltaic (OPV) applications (see Fig. 1). The TPD 

moiety is a relatively new component in donor-acceptor 

polymers for solar cell applications, but it has attracted 

enormous attention due to its powerful electronwithdrawing capabilities, which produce a strong donoracceptor effect when incorporated into polymers with 

electron-rich moieties.[1-3] In contrast, the DTP moiety 

has received relatively little attention for OPV applications, 

due in part to its extremely electron-rich nature. The 

excessive electron density results in relatively shallow 

HOMO energy levels when incorporated into donoracceptor copolymers.[4-6] This in turn limits the opencircuit voltage (Voc) of bulk heterojunction (BHJ) OPV 

devices made from these polymers and fullerene 

acceptors. According to our preliminary density function 

theory (DFT) calculations, however, the TPD acceptor 

moiety has sufficient electron-withdrawing power to 

produce reasonably deep HOMO levels in copolymers 

with DTP.

S

N

O

O

S

N

S

Chemical Formula: C30H36N2O2S3

••

Molecular Weight: 552.81

Figure 1 Structure of TPD-DTP copolymer.

CALCULATIONS

We used the Gaussian 09 electronic structure program[7] 

to perform geometry optimized DFT calculations at the 

B3LYP/6-31G(d) level to predict the energetic properties 

of the DTP-TPD copolymer system. Surprisingly, the 

lowest energy conformation has the thiophene moieties in 

an all-cis configuration (see Fig. 2). We have computed 

the relevant molecular properties for the n=1 and n=2 

monomer cases using the 6-31G(d) basis set (see Table 

1). Furthermore, we have also explored the effect of using 

larger basis sets on the computed molecular properties for 

n=1 (see Table 1).

Figure 2 Optimized DFT geometry of TPD-DTP for n=1.

At the 6-31G(d) level, the computed HOMOs for n=1 and 

n=2 monomers show a strong upward trend in the HOMO 

energy on increasing oligomer size. For the n=2 case, the 

HOMO energy of ~-4.9 eV is already rather shallow for an 

OPV copolymer system, and further changes in the 

HOMO level on increasing oligomer size might be 

expected to result in a relatively poor Voc in BHJ OPV 

devices. Due to the considerable role lone pair orbitals 

might be expected to play in the energetics of the DTPTPD copolymer system, we also explored the use of larger 
2 

basis sets [6-31+G(d) and 6-311+G(d)] that more 

adequately account for the orbitals involved in such lone 

pair states. These larger basis sets result in significantly 

deeper HOMO and unoccupied molecular orbital (LUMO) 

energy levels for the n=1 monomer, with minimal change 

to the calculated energy gap. This effect is likely to extend 

as well to larger oligomer systems (n ≥ 2), suggesting the 

DTP-TPD copolymer may have deep enough HOMO 

energy level to ensure adequate Voc in BHJ OPV devices.

Table 1 DFT Calculated Molecular Properties of TPDDTP Copolymer System.

Structure/Basis HOMO 

(eV)

LUMO 

(eV)

Energy 

Gap (eV)

Optical 

LUMO 

(eV)

n=1 / 6-31G(d) -5.23 -2.05 2.95 -2.28

n=2 / 6-31G(d) -4.92 -2.34 2.18 -2.74

Larger Basis

n=1 / 6-31+G(d) -5.50 -2.40 2.87 -2.63

n=1 / 6-311+G(d) -5.53 -2.45 2.89 -2.64

SYNTHESIS 

We synthesized dibromo(2-ethylhexyl)TPD according to 

the literature procedure.[2] We synthesized (2-

ethylhexyl)DTP following a modified literature procedure[6]

and prepared the bis(trimethyltin) derivative according to 

the published method.[6] The excessive electron density/ 

shallow HOMO of the DTP moiety results in a relatively 

unstable monomer; generally DTP has been converted 

into the bis(trimethyltin) derivative and used directly 

without further purification. However, the resulting 

polymers are often very low molecular weight[4] due to the 

presence of some mono-tin DTP as a result of oxidative 

degradation of the electron-rich monomer. In our case the 

crude monomer was ~95% pure, and using it directly in a 

Stille polymerization with the TPD monomer (≥99% pure) 

resulted in a polymer with a number-averaged molecular 

weight (Mn) of 4130 g/mol and a polydispersity index (PDI) 

of 1.10, corresponding to ~ eight repeat units (see Fig. 3). 

We subsequently developed a purification method for the 

monomer that involves dissolving it in hot hexanes, 

cooling to -78 ̊C in a dry ice/acetone bath, and then 

filtering to remove the less-soluble mono-tin DTP. This 

procedure resulted in monomer ≥98% pure, which was 

used in a Stille polymerization with the TPD monomer. 

The resulting polymer had a significantly higher molecular 

weight (see Fig. 3) but also exhibited poor solubility, 

leading to excessive aggregation in the analytical GPC 

instrument (in CHCl3) and preventing calculation of an 

accurate Mn and PDI. This suggests larger alkyl chains on 

the donor and/or the acceptor might be desirable to 

increase the solubility of the polymer.

Figure 3 GPC chromatograms of oliogmeric and 

polymeric DTP-TPD materials.

MOLECULAR CHARACTERIZATION

While the oligomer is quite soluble in chloroform, as well 

as chlorobenzene and o-dichlorobenzene, the polymeric 

material is comparatively poorly soluble in those solvents. 

In dilute chloroform solutions, DTP-TPD absorbs broadly 

(see Fig. 4), with a λmax of 600 nm for the oligomer and 

610 nm for the polymer. In thin films (as cast from 

chloroform) the λmax is 610 nm for the oligomer and 630 

nm for the polymer. In both cases the oligomeric material 

exhibits a distinct shoulder that is not evident in the 

polymeric material. This suggests the polymeric material 

may exhibit an improved polymer π-π stacking 

morphology that is only present to a small degree in the 

oligomeric material.

In the case of the films, the shoulder results in an identical 

cut-off wavelength of 760 nm for the two materials, 

corresponding to a band gap of 1.61 eV. This strong, 

broad absorption is ideal for PV applications due to 

favorable overlap with the AM1.5 solar spectrum.

Figure 4 Absorption of oliogmeric and polymeric DTPTPD materials in thin films and chloroform solutions.

We have performed cyclic voltammetry (CV) on thin films 

of oligomeric TPD-DTP in an electrolyte solution of 0.1 M 
3 

tetrabutylammonium hexafluorophosphate in acetonitrile to 

explore the energy levels for this copolymer system (see 

Fig. 5). Using platinum working and counter electrodes 

and a Ag/AgCl reference electrode at a scan rate of 50 

mV/s, we obtained an oxidation potential (E1/2(ox)) of 0.918 

V for the TPD-DTP oligomer. Although the peak is 

somewhat poorly reversible (Ipc/Ipa ~ 4.8), it is sufficient for 

calculation of a reasonable E1/2. Under these conditions, 

ferrocene (HOMO = -4.8 eV)[8] exhibited an E1/2(ox) of 0.39 

V. This corresponds to a HOMO of -5.33 eV for DTP-TPD.

The deep HOMO level measured substantiates the larger 

basis set DFT calculations and suggests the TPD acceptor 

moiety is highly effective at withdrawing electron density 

from the electron-rich DTP donor moiety. The oligomer 

also exhibited an irreversible reduction around -0.65 V, 

corresponding to a lowest unoccupied molecular orbital 

(LUMO) energy level of ~-3.76 eV. It is interesting to note 

that the electrochemical band gap of ~1.57 eV is 

extremely close to the optical band gap of 1.61 eV (film), 

which is unusual for polymer systems.

Figure 5 Cyclic voltammetry of TPD-DTP oligomer. 

DEVICE RESULTS

Initial BHJ OPV devices with the TPD-DTP oligomer and 

the fullerene acceptor phenyl C61 butyric acid methyl ester 

(PCBM) were fabricated in order to further evaluate the 

potential of this copolymer system. The devices were 

prepared using a conventional OPV device structure 

consisting of: ITO/PEDOT:PSS/BHJ/Ca/Al. The active 

layer was a 1:2 DTP-TPD:PCBM ratio, deposited via spin 

casting from chlorobenzene at room temperature. A 

representative current density-voltage trace is presented 

in Figure 6. 

The oligomeric devices yielded an average Voc of 699 ± 9

mV, which supports the deep HOMO value measured by 

CV and predicted by the large basis set DFT calculations. 

It may be possible to enhance this voltage further in fully 

optimized devices. The average short-circuit current 

density (Jsc) for these devices was 3.52 ± 0.32 mA/cm2

, 

which is quite low given the low band gap for TPD-DTP. 

This is suggestive of a relatively poor BHJ blend 

morphology in these initial devices preventing complete 

harvesting of photogenerated excitons. This might be 

expected from the low Mn oligomeric nature of this material 

and is consistent with the UV-vis absorption data. The 

devices also exhibit a relatively low average fill factor of 

40.9 ± 1.2%, which is likely due in part to poor 

morphology, but also in part due to an inherently lower 

hole mobility in the oligomer as compared to a high Mn

polymer. The average power conversion efficiency (PCE) 

of the oligomeric devices was 0.985 ± 0.066%.

Figure 6 Representative DTP-TPD BHJ OPV device

results.

Our subsequent attempts to make devices using the 

higher molecular weight polymeric DTP-TPD material 

were hampered by the limited solubility of this material.

We retained the same 1:2 DTP-TPD:PCBM weight ratio as 

in the oligomeric devices. Films cast from chlorobenzene 

at various temperatures indicated the best performance 

was obtained when films were cast from solutions at 

120 ̊C. A representative current density-voltage trace is 

presented in Figure 6.

The polymeric devices yielded an average Voc of just 542 

± 3 mV, which is substantially lower than that of the 

oligomeric devices. We hypothesize that this result is due 

solely to the poor film quality obtained due to limited 

solubility of the polymeric DTP-TPD material. Although it is 

possible that the increased chain length might result in a 

shallower HOMO and thus lower Voc, as there was minimal 

change in the band gap between the oligomeric and 

polymeric materials, we feel this is unlikely. The average 

Jsc for the polymeric devices, 4.37 ± 0.13 mA/cm2

, was 

slightly higher than for the oligomeric devices, which may 

be indicative of an improved BHJ blend morphology, or 

perhaps simply increased absorption coefficient for the 

longer chains. The fill factor for the polymeric devices, 

38.3 ± 0.51%, was even lower than for the oligomeric 

devices, however, suggesting a poorer blend morphology. 

The average PCE of the polymeric devices was 0.874 ± 

0.032%. The poor solubility of the polymeric material 

apparently hampers the DTP-TPD device performance 
4 

0.032%. The poor solubility of the polymeric material 

apparently hampers the DTP-TPD device performance 

even more than the short chain length does in the 

oligomeric material. 

CONCLUSIONS 

In conclusion, we have designed and synthesized a new 

donor-acceptor copolymer system based on DTP-TPD 

and investigated its properties for application in OPV 

devices. The relatively electron-rich DTP moiety results in 

a poorly stable bis(trimethyltin) monomer that, when used 

directly, leads to a strictly oligomeric product. A purification 

method was developed that allowed preparation of a 

higher-molecular-weight polymer, which exhibited limited 

solubility. Both the oligomer and the polymer exhibit strong 

and broad absorption both in solution and thin films, with a 

band gap of 1.61 eV. Electrochemistry experiments 

measured a HOMO of -5.33 eV for the oligomeric material, 

which supports the hypothesis that the strongly electronwithdrawing TPD acceptor moiety is sufficient to remove 

excess electron density from the DTP donor. This is 

further supported by initial BHJ device results that 

demonstrate Voc of around 700 mV for the oligomeric 

material. Although the Jsc and FF for this material are low, 

this is likely due to poor morphology and hole transport 

due to the oligomeric nature of the material. In contrast, 

the high-molecular-weight material exhibits poor device 

performance due to its limited solubility. This suggests the 

need for larger alkyl groups on the donor and/or acceptor 

to improve the solubility of the polymeric material, which 

may allow for optimized device performance. 

ACKNOWLEDGMENTS 

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-08-GO28308 with 

the National Renewable Energy Laboratory. 

REFERENCES 

[1] X. Guo, H. Xin, F. S. Kim, A. D. T. Liyanage, S. A. 

Jenekhe, and M. D. Watson, "Thieno[3,4- c]pyrrole-4,6-

dione-Based Donor−Acceptor Conjugated Polymers for 

Solar Cells," Macromolecules, 2011, 44, 269-277. 

[2] A. Najari, S. Beaupré, P. Berrouard, Y. Zou, J.-R. 

Pouliot, C. Lepage-Pérusse, and M. Leclerc, "Synthesis 

and Characterization of New Thieno[3,4-c]pyrrole-4,6-

dione Derivatives for Photovoltaic Applications," Adv. 

Funct. Mater., 2010, 21, 718-728. 

[3] Y. Zhang, S. K. Hau, H.-L. Yip, Y. Sun, O. Acton, and 

A. K.-Y. Jen, "Efficient Polymer Solar Cells Based on the 

Copolymers of Benzodithiophene and 

Thienopyrroledione," Chem. Mater., 2010, 22, 2696-2698. 

[4] X. Zhang, T. T. Steckler, R. R. Dasari, S. Ohira, W. J. 

Potscavage, S. P. Tiwari, S. Coppée, S. Ellinger, S. 

Barlow, J.-L. Brédas, B. Kippelen, J. R. Reynolds, and S. 

R. Marder, "Dithienopyrrole-based donor–acceptor 

copolymers: low band-gap materials for charge transport, 

photovoltaics and electrochromism," J. Mater. Chem., 

2010, 20, 123-134. 

[5] J. Liu, R. Zhang, I. Osaka, S. Mishra, A. E. Javier, D.-

M. Smilgies, T. Kowalewski, and R. D. Mccullough, 

"Transistor Paint: Environmentally StableNalkyldithienopyrrole and Bithiazole-Based Copolymer ThinFilm Transistors Show Reproducible High Mobilities 

without Annealing," Adv. Funct. Mater., 2009, 19, 3427-

3434. 

[6] J. Liu, R. Zhang, G. Sauvé, T. Kowalewski, and R. 

McCullough, "Highly Disordered Polymer Field Effect 

Transistors: N-Alkyl Dithieno [3, 2-b: 2′, 3′-d] pyrrole-Based 

Copolymers with Surprisingly High Charge Carrier 

Mobilities," J. Am. Chem. Soc., 2008, 130, 13167-13176. 

[7] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. 

Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. 

Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. 

Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, 

G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. 

Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, 

Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. 

Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. 

J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. 

Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. 

C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. 

M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, 

C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. 

Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. 

Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, 

G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. 

D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. 

Cioslowski, and D. J. Fox, "GAUSSIAN 09," Gaussian, 

Inc., Wallingford CT, 2009. 

[8] P. Jörn, V. Horst, G. Werner, F. M. Rainer, B. Heinz, P. 

Michael, and D. Jörg, "Efficient two layer leds on a 

polymer blend basis," Adv. Mater., 1995, 7, 551-554.