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DOI: 10.1039/C5RA19715E (Communication) RSC Adv., 2015, 5, 98876-98879

Low band-gap weak donor–strong acceptor conjugated polymer for organic solar cell†

Zugui Shia, Ivy Wong Hoi Kaa, Xizu Wanga, Chellappan Vijilaa, Fei Wanga, Gongqiang Lia, Weng Weei Tjiua, Jun Li*a and Jianwei Xu*ab
aInstitute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A* STAR), 3 Research Link, Singapore 117602. E-mail: jw-xu@imre.a-star.edu.sg; j-li@imre.a-star.edu.sg
bDepartment of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543

Received 24th September 2015 , Accepted 10th November 2015

First published on 12th November 2015

Abstract

By insertion of an additional weak acceptor into a donor–acceptor conjugated polymer backbone, a new weak donor–strong acceptor alternating copolymer PTTBOBT-DFBT was synthesized and it showed a low band-gap of 1.64 eV with a deep HOMO energy level (−5.44 eV). The bulk heterojunction (BHJ) solar cell fabricated from polymer PTTBOBT-DFBT displayed a remarkable power conversion efficiency (PCE) of 5.36% (Jsc = 11.04 mA cm−2, FF = 63.65%, Voc = 0.76 V).

The cost-effective solution processability and good compatibility with flexible substrate and high PCE have made organic photovoltaic (OPV) an intriguing research field over decades. However, the efficiency of OPV is still far behind its inorganic counterpart, silicon solar cell. Thus tremendous efforts have been dedicated to improve its efficiency by either sophisticated material design or device structure optimization.1–6 Among them, fine tuning of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of materials has recorded significant progress.7–21 Recently, the weak donor–strong acceptor strategy demonstrated great potential for high performance OPV materials.22–26 The typical methodology for construction of weak donor could be readily achieved by decreasing electron-richness of thiophene unit by fusing with a relatively electron-deficient aryl unit via multiple steps synthesis, and it has been demonstrated that this strategy is effective to lower down the HOMO energy level, yielding high open circuit voltages (Voc) in related BHJ devices (Scheme 1).
image file: c5ra19715e-s1.tif
Scheme 1 Design of low band-gap polymers by weak donor–strong acceptor strategy.

Herein, we report a simple alternative weak donor–strong acceptor approach. Generally, the donor–acceptor conjugated polymers are constructed by linking electron-rich aromatics as donors, such as oligothiophenes, and electron-deficient counterparts as acceptors, such as 2,1,3-benzothiadiazole. Our hypothesis is to introduce a relatively weak acceptor into donor part,27,28 and meanwhile incorporate 5,6-difluoro-2,1,3-benzothiadiazole (DFBT) as a strong acceptor to ensure an appropriate LUMO level (Scheme 1).29–35 Owing to the presence of two electron-donating alkoxy side chains, 5,6-bis(octyloxy)-2,1,3-benzothiadiazole (BOBT) is defined as a weak acceptor in this case. The two side chains could significantly improve the solubility of resultant polymers. Additionally, it is believed that the noncovalent Coulomb interactions S⋯O and weak hydrogen bonding C–H⋯N would minimize the polymer backbone torsional angle, thus improving backbone coplanarity.36–39 Based on these hypotheses, a unique copolymer PTTBOBT-DFBT was designed and synthesized to examine this concept. As depicted in Scheme 2, BOBT was inserted into a bithiophene building block to form a weak donor unit, which then underwent Stille coupling with strong acceptor DFBT. As expected, the polymer PTTBOBT-DFBT exhibited a deep HOMO (−5.44 eV) level, together with narrow band-gap (1.64 eV), indicating that it would be a promising candidate for OPV application.


image file: c5ra19715e-s2.tif
Scheme 2 Synthesis of weak donor–strong acceptor copolymer PTTBOBT-DFBT.

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on PTTBOBT-DFBT. It exhibited excellent thermal stability with 5% weight loss temperature (Td) up to 343 °C (Fig. S2†). DSC revealed no clear thermal transitions from room temperature to 300 °C (Fig. S3†). The optical properties of PTTBOBT-DFBT were studied by UV-Vis-NIR absorption spectra. The solution and thin film absorption spectra are shown in Fig. 1, and the corresponding photophysical data are summarized in Table S1.† In solution, it showed two absorption bands, attributable to π–π* transition at short wavelength region and intramolecular charge transfer at long wavelength region. A small shoulder peak at ∼700 nm was observed in solution, suggesting that a low degree of inter-chain aggregation existed. In its thin film spectrum, the absorption was red-shifted compared to solution (from 576 nm to 632 nm), implying stronger aggregation and higher ordering in the solid state. A significant shoulder peak at ∼688 nm was observed in the absorption of thin film, indicating strong intermolecular interactions (inter-chain packing or aggregation). From the absorption onset of the polymer thin film, its optical band-gap was calculated to be 1.64 eV.


image file: c5ra19715e-f1.tif
Fig. 1 UV-Vis-NIR spectra of PTTBOBT-DFBT in chlorobenzene and as thin film.

The cyclic voltammetric property of PTTBOBT-DFBT thin film is shown in Fig. S4.† The HOMO level was estimated from the onsets of its corresponding oxidative peak. The LUMO level was then calculated based on the difference between the HOMO energy level and optical band-gap. PTTBOBT-DFBT demonstrated a deep HOMO energy level of −5.44 eV, which is lower than these of documented analogous polymers containing either DFBT or BOBT as a single acceptor, namely FBT-Th4(1,4) (−5.36 eV)32,40 and PQT12oBT (−5.18 eV)41 (Scheme S2†). Accordingly, a high Voc could be expected for its bulk heterojunction OPV device. The calculated LUMO levels for PTTBOBT-DFBT is −3.80 eV, matching the energy difference (>0.3 eV) with PCBM (−4.2 eV), thus ensuring efficient charge separation.

BHJ polymer solar cells were fabricated with a general device structure of ITO/PEDOT:PSS/copolymer:PC71BM/Al and the performance was measured under 100 mW cm−2 AM 1.5 G illumination. Fig. 2 shows the typical current–voltage and external quantum efficiency (EQE) curves. The BHJ polymer solar cell device made from PTTBOBT-DFBT with weight ratio to PC71BM (1[thin space (1/6-em)]:[thin space (1/6-em)]1) resulted in 3.86% PCE, and Jsc of 10.95 mA cm−2, Voc of 0.65 V and FF of 54.30%, without thermal treatment. As shown in Table 1, the efficiency of PTTBOBT-DFBT[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM was significantly improved to 4.65% after annealing at 90 °C. The improvement was attributed to higher open circuit voltages and fill factor (Voc = 0.77 V, FF = 56.48%), which should be stemmed from better film morphologies after thermal annealing. The effect of different polymer/PC71BM ratio was then systematically investigated. When the weight ratio of PTTBOBT-DFBT[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM = 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5, the highest efficiency was observed. The improvement could be attributed to higher Jsc and fill factor (Jsc, from 10.63 mA cm−2 to 11.04 mA cm−2; fill factor, from 56.48% to 63.65%), while Voc remained. When more acceptor materials were blended (polymer: PC71BM = 1[thin space (1/6-em)]:[thin space (1/6-em)]2.0 or 1[thin space (1/6-em)]:[thin space (1/6-em)]2.5), Jsc and FF were decreased, although Voc remained.


image file: c5ra19715e-f2.tif
Fig. 2 Current density–voltage of photovoltaic devices using blends of PTTBOBT-DFBT/PC71BM (1[thin space (1/6-em)]:[thin space (1/6-em)]1, 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5, 1[thin space (1/6-em)]:[thin space (1/6-em)]2, 1[thin space (1/6-em)]:[thin space (1/6-em)]2.5) as active layers and EQE curve of photovoltaic device using blend of PTTBOBT-DFBT/PC71BM (1[thin space (1/6-em)]:[thin space (1/6-em)]1.5).
Table 1 Photovoltaic properties of PTTBOBT-DFBT
Ratioa Tb PCE (%) Jsc (mA cm−2) FF (%) Voc (V)
a PTTBOBT-DFBT[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM weight ratio.b T = annealing temperature.
1[thin space (1/6-em)]:[thin space (1/6-em)]1 25 °C 3.86 10.95 54.30 0.65
1[thin space (1/6-em)]:[thin space (1/6-em)]1 90 °C 4.65 10.63 56.48 0.77
1[thin space (1/6-em)]:[thin space (1/6-em)]1.5 90 °C 5.36 11.04 63.65 0.76
1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 °C 4.72 10.36 60.10 0.76
1[thin space (1/6-em)]:[thin space (1/6-em)]2.5 90 °C 3.99 9.40 56.33 0.75

As aforementioned, FBT-Th4(1,4) is the analogous polymer with 5,6-difluoro-2,1,3-benzothiadiazole as a single acceptor, having a higher HOMO energy level (−5.36 eV) compared to PTTBOBT-DFBT (−5.44 eV). Generally, the open circuit voltage (Voc) is correlated with the energy difference between the HOMO of the donor polymer and the LUMO of the acceptor (PCBM), and thus a higher Voc could be expected by PTTBOBT-DFBT. Gratifyingly, with the same device structure, similar fabrication and test conditions, indeed a higher Voc (0.77 eV) was observed for PTTBOBT-DFBT, while FBT-Th4(1,4) gave a lower Voc (0.74 eV).40 This result demonstrated the additional weak acceptor BOBT unit could improve the open circuit voltage. However, the overall performance of FBT-Th4(1,4) (PCE = 6.63%) is better than PTTBOBT-DFBT, which is attributed to its high short circuit current and fill factor (Jsc = 12.56, FF = 71.30). The higher molecular weight of FBT-Th4(1,4) (Mn = 25 kDa) than PTTBOBT-DFBT (Mn = 12.2 kDa) and better blend film morphology may account for these interesting results. It also suggests that improvement of OPV efficiency be sophisticated, systematic considerations and optimizations are prerequisite.

The morphology of polymer[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM blend films has significant influence on exciton separation, charge carrier mobility and photovoltaic performance. As shown in Fig. 3, the transmission electron microscopy (TEM) image of PTTBOBT-DFBT/PC71BM (1[thin space (1/6-em)]:[thin space (1/6-em)]1.5) blend film annealed at 90 °C possess uniform and fine features, suggesting nanoscale phase separation. Clear fibrous networks are believed to lead to large polymer–PC71BM interface area for exciton dissociation, contributing to higher FF and Jsc, respectively. The fibres could also be observed in phase images obtained from atomic force microscopy (Fig. S6†).


image file: c5ra19715e-f3.tif
Fig. 3 TEM image of PTTBOBT-DFBT/PC71BM (1[thin space (1/6-em)]:[thin space (1/6-em)]1.5) blend film.

In conclusion, by insertion of a weak acceptor moiety (BOBT) into a classic donor unit (tetrathiophenes), a new weak donor was formed; it then underwent coupling with a strong acceptor (DFBT), generating a novel weak donor–strong acceptor alternating copolymer with remarkable PCE. Comparing with reported weak donor–strong acceptor design strategy that mainly relies on fusing different aromatics into polycyclic conjugated π–systems through complicated organic synthesis, the present work would offer an alternative feasible approach to achieve low band-gap conjugated polymers with high PCE for solar cell application. More investigations to further explore the potential of this approach are in progress in our group.

Acknowledgements

This study was supported by the financial support from the Agency for Science, Technology and Research (A* STAR) and Ministry of National Development (MND) Green Building Joint Grant (No. 1321760011), Singapore.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19715e
This journal is © The Royal Society of Chemistry 2015
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