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Synthesis of Dithienogermole-Containing π-Conjugated Polymers and Applications to Photovoltaic Cells Joji Ohshita,*,† Yong-Mook Hwang,† Tomonobu Mizumo,† Hiroto Yoshida,† Yousuke Ooyama,† Yutaka Harima,† and Yoshihito Kunugi‡ † ‡
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Department of Applied Chemistry, Faculty of Engineering, Tokai University, 4-1-1 Kitakaname, Hiratsuka 259-1292, Japan
bS Supporting Information ABSTRACT: Dithienogermole-containing π-conjugated polymers were prepared by the Stille coupling reactions of distannyldithienogermole and dibromoarenes. The polymers exhibit optical properties similar to those of the previously reported silicon analogues and are usable as the active materials for bulk heterojunction-type organic photovoltaic cells as blends with PC70BM.
O
ligo- and polythiophenes are currently receiving considerable attention as functional organic materials in the filed of organic electronics.1 In this area, bithiophenes with a heteroatom bridge at the β,β0 -position forming a fused tricyclic system have been well studied, which provides useful building blocks for the preparation of materials with better conjugation than the parent bithiophenes, not only by fixing the bithiophene system in one plane but also by electronic effects of the bridging atoms.25 Previously, we prepared dithienosiloles (DTSs) having a silicon bridge and found that DTSs exhibit smaller HOMOLUMO band gaps than the carbon-bridged analogues (DTCs),6 due to the orbital interaction between the silicon σ* orbital and the bithiophene π* orbital, which lowers the LUMO energy levels (Scheme 1).6b,7 DTS-containing π-conjugated polymers are of current importance as promising host materials for bulk heterojunction (BHJ)-type organic photovoltaic cells (OPVs).711 It has been demonstrated that DTS-polymer DTS-BTA (Scheme 1) forms more crystalline films when blended with PCBM than the carbon analogue (DTC-BTA), leading to better performance of the DTS-based OPVs,9,11 in which longer SiC bonds reduce the intermolecular steric repulsion to facilitate the close molecular packing in the solid state.9 Distortion of the ring structures induced by replacing the bridging carbon by silicon would also exert influence on the packing structures.11 To explore further the scope of group 14 dithienometallolecontaining π-conjugated polymers, we prepared polymers composed of an alternate arrangement of dithienogermole (DTG) and benzothiadiazole or bithiophene units and examined their applications to OPVs. The germole system is known to possess r 2011 American Chemical Society
electronic states similar to those of silole,12 and the replacement of the silicon by a larger germanium atom would lead to stronger interchain interactions. Applications of dibenzogermole-containing polymers as the active components of thin film transistors as well as OPVs have been recently reported,13 but nothing is known for DTG-based polymers.14 A reaction of dilithiobis(trimethylsilyl)bithiophene with dichlorobis(2-ethylhexyl)germane gave bis(trimethylsilyl)dithienogermole (DTGSi2) in 70% yield, whose trimethylsilyl groups were readily replaced with bromine atoms by treatment with NBS to give dibromodithienogermole (DTGBr2) in 96% yield (Scheme 2). The Stille-coupling reactions of bis(trimethylstannyl)dithienogermole (DTGSn2) prepared form DTGBr2 in 96% yield, with dibromobenzothiadiazole and dibromobithiophene at 150 °C, followed by fractional Soxhlet extraction afforded polymers DTGBTA-L and DTG-2T in 40% and 70% yields, respectively, as black solids that are soluble in hot chloroform but insoluble in hot methanol and hexane (Table 1). Purification of DTG-BTA-L by preparative GPC gave higher molecular weight polymer DTG-BTA-H. The polymer structures are verified mainly by 1 H NMR spectra, in which the signal integration ratios are consistent with the regular DTG and benzothiadiazole or bithiophene repeating structures shown in Scheme 2. DTGSi2 possesses a UV absorption maximum at 346 nm, only slightly blue-shifted from that of a similarly substituted DTS (351 nm for R1 = n-Bu, R2 = SiMe3),15 in agreement with the Received: January 28, 2011 Published: May 27, 2011 3233
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Scheme 1. Group 14 Dithienometalloles and the Polymers
Scheme 2. Synthesis of DTG-Containing Polymers Figure 1. UV spectra of DTG-containing polymers in chloroform and as films.
Table 1. Synthesis and Properties of DTG-Containing Polymers UVvis λmax/nm Mnc
Mw/Mnc
film
polymer
yield/%
DTG-BTA-La
40
8000
1.38
650
683
DTG-BTA-Hb
35
15 000
1.67
651
687
DTG-2Ta
70
10 000
2.70
532
562
in CHCl3
a
Polymeric substance that is soluble in hot chloroform but insoluble in hot methanol and hexane was collected. b Purified by preparative GPC. c Determined by GPC, relative to polystyrene standards.
results of DFT calculations that suggest almost the same HOMO and LUMO energy levels for DTG and DTS models (Figure S-2). The UVvis absorption maxima of the present DTG-containing polymers in chloroform appear at 650 and 651 nm for DTG-BTAL and -H, and 532 nm for DTG-2T, as shown in Figure 1, a little blue-shifted from but comparable to those of DTS-BTA (ca. 670 nm)10,11 and DTS-2T (544 nm)16 (Scheme 1) reported previously, again indicating similarity in the electronic states for the DTG- and DTS-containing polymers. As a film, DTG-2T shows a red-shifted absorption maximum by 30 nm from that in chloroform, in marked contrast to DTS-2T (R = hexyl), which has been reported to show no clear changes in the absorption maxima in solution (544 nm) and film (545 nm),16 probably indicating stronger interchain interactions in film for DTG-2T than DTS2T. Measuring the UVvis spectra of DTG-BTA-L and DTGBTA-H as films again moved the absorption maxima by 33 and 36 nm from those in chloroform. In addition, significant enhancement of the shoulders at about 760 nm is observed, as presented in
Figure 2. (a) IV curves and (b) IPCE plots of DTG-polymerbased OPVs.
Figure 1. This is similar to the optical properties of DTS-BTA (R = 2-ethylhexyl) reported previously, although the DTS-polymer exhibits a smaller red-shift of the major band by approximately 1020 nm and larger enhancement of the shoulder peak at 740 nm in film, judging from the literature figure (no spectral data are given in numbers in the literature).10 The electronic states of DTG-BTA were examined with respect to the electrochemical cyclic voltammetry in film (Figure S-3). The onset oxidation potential of the film is at 0.20 V vs Fc/Fcþ, slightly low-voltageshifted from that reported for DTS-BTA (0.25 V).10 We fabricated BHJ-type OPVs with the structure of ITO (150 nm)/PEDOT:PSS (30 nm)/DTG-polymer:PC70BM (100 nm)/LiF (0.5 nm)/Al (80 nm) and the active area of 0.25 cm2. The active layers were prepared by spin-coating of 3234
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Table 2. OPV Performance Based on DTG-Containing Polymers as Host Materialsa polym
Isc/mA cm
2
Voc/V
FF
PCE/%
DTG-BTA-L
2.43
0.58
0.51
0.71
DTG-BTA-H DTG-2T
4.68 2.52
0.61 0.54
0.43 0.62
1.21 0.84
a
OPV structure: ITO (150 nm)/PEDOT:PSS (30 nm)/DTG-polymer: PC70BM (1:3.6 wt, 100 nm)/LiF (0.5 nm)/Al (80 nm).
12 mg/mL chlorobenzene solutions of the blends of the polymers and PC70BM in a wt ratio of 1:3.6. No annealing of the active layers as well as the devices was applied. As expected, the DTG-BTAand DTG-2T-based cells showed clear photovoltaic properties, whose IV curves and IPCE (incident photon-to-current conversion efficiency) plots are depicted in Figure 2. As summarized in Table 2, the cell performance is affected by both the chemical structures and the molecular weights of the polymers. In particular, using higher molecular weight polymer DTG-BTA-H resulted in approximately 2 times higher current density to enhance the power conversion efficiency (PCE) of the OPV. Application of DTS-2T (R = hexyl) as the thin film transistor (TFT) materials has been studied, and the excellent hole mobility of μ = 0.08 cm2/ V s was reported for the bottom-contact-type device.16 DTG-2T also exhibits clear p-type TFT activity in a spin-coated film, although the mobility (Figure S-4, μ = 7.8 105 cm2/V s, Ion/Ioff = 105) is lower than that of DTS-2T (R = hexyl). In conclusion, we prepared novel DTG-containing π-conjugated polymers and demonstrated their applications to BHJ-type OPVs. The highest PCE of 1.21% for the present OPVs is achieved by using DTG-BTA-H. Although the PCE of a similar OPV with DTS-BTA was reported to be higher (5.1%) than those of the present OPVs,10 this is mainly due to the higher current density (12.7 mA/cm2) of the DTS-based OPV. The current density may be significantly affected by the polymer molecular weights, and DTS-BTA possesses a higher molecular weight (Mn = 18 000) than DTG-BTA-H. Studies to optimize the polymer and cell structures and to increase the polymer molecular weights by tuning the reaction conditions are under way.
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental details for the preparation of the present DTG-polymers, HOMO and LUMO profiles of model compounds of group 14 dithienometalloles derived from DFT calculations at the level of B3LYP/LanL2DZ, a CV of the DTG-BTA-H film, and IdVg, Id1/2Vg, and IdVd curves for the bottom-contact-type TFT with a spin-coated film of DTG-2T as the active layer. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: þ81-82-424-7743. Fax: þ81-82-424-5494. E-mail: jo@ hiroshima-u.ac.jp.
’ ACKNOWLEDGMENT This work was supported in part by a Grant-in-Aid for Scientific Research (No. 23350097) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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(14) Preparation of a dithienogermole and its optical properties have been recently reported: Yabusaki, Y.; Ohshima, N.; Kondo, H.; Kusamoto, T.; Yamanoi, Y.; Nishihara, H. Chem.—Eur. J. 2010, 16, 5581. (15) Kim, D.-H.; Ohshita, J.; Lee, K.-H.; Kunugi, Y.; Kunai, A. Organometallics 2006, 25, 1511. (16) Lu, G.; Usta, H.; Risko, C.; Wang, L.; Facchetti, A.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 7670.
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