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A dumbbell-like A–D–A molecule for single-component organic solar cells† Jiamin Cao, Xiaoyan Du, Shan Chen, Zuo Xiao* and Liming Ding*
Received 2nd November 2013, Accepted 27th November 2013 DOI: 10.1039/c3cp54642j www.rsc.org/pccp
A dumbbell-like A–D–A molecule, triad 1, containing a dithienosiloledibenzothiadiazole donor unit and two fullerene acceptor units was designed and synthesized. Triad 1 shows good solubility, thermostability, light absorbance, and film-forming capability. Solar cells using 1 as the active layer gave a PCE of 0.4%.
Organic solar cells (OSCs) possess unique advantages like low cost, lightweight, flexibility, and roll-to-roll fabrication, thus attracting great attention from academic institutions and industry.1 In bulk heterojunction (BHJ) solar cells, the polymer (or small molecule) donor and fullerene acceptor form a nanoscale interpenetrating network to allow efficient free charge carrier generation and transport.2 The power conversion efficiency (PCE) for polymer or small molecule solar cells has surpassed 8%.3 To obtain an ideal nano-morphology for the active layer, much effort has to be paid on the fabrication conditions, such as donor/acceptor (D/A) ratios, solvents, annealing, and additives. Tedious work in device optimization and low reproducibility of device performance retard OSC commercialization.4 Besides, the optimized morphology of the active layer is metastable and evolves with time due to unavoidable fullerene aggregation, leading to D/A interface reduction and device performance deterioration.5 To deal with the stability issue in bi-component cells, single-component OSCs using only one photoactive material, which possesses multi-functions like light absorption, exciton dissociation, and electron and hole transport, were proposed.4 The strategy is to covalently link the donor and acceptor units to make dyad or triad molecules. A star-shaped molecule based on a triphenylamine donor and dicyanovinyl acceptors showed a PCE of 0.40%.6 A dyad based on a oligo(p-phenylenevinylene) (OPV) donor and a fullerene acceptor showed a PCE of 1.28%.7 A dyad containing a fluorene-alt-bithiophene oligomer donor and a
National Center for Nanoscience and Technology, Beijing 100190, China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Experimental details including synthesis, measurements, and instruments. See DOI: 10.1039/c3cp54642j
3512 | Phys. Chem. Chem. Phys., 2014, 16, 3512--3514
perylene diimide acceptor afforded a 1.5% PCE.8 A diketopyrrolopyrrole (DPP)–fullerene dyad and a DPP-containing oligothiophene– fullerene triad gave 1.1% and 0.5% PCEs, respectively.9 Recently, Hashimoto et al reported an OPV–C70 dyad giving a PCE record of 1.92%.10 Conjugated polymer and small molecule donors made of dithienosilole (DTS) and benzothiadiazole (BT) units demonstrated outstanding performance in BHJ solar cells due to their good light-absorbing and charge carrier transporting properties.11 So we used DTS and BT units in developing the target molecule. We designed a dumbbell-like triad 1 consisting of a dithienosilole-dibenzothiadiazole (DTSDBT) core and two fullerenes. The synthesis, physical properties, and photovoltaic performance of triad 1 were studied. Solar cells using triad 1 as the active layer gave a PCE of 0.4%. Sonogashira coupling of but-3-yn-1-ol and 4,7-dibromo2,1,3-benzothiadiazole afforded compound 3 in 52% yield (Scheme 1). Stille coupling of 3 and bis(trimethylstannyl)substituted-DTS afforded compound 2 in 31% yield. Finally, triad 1 was obtained in 53% yield by esterification of compound 2 with fullerene carboxylic acid, PCBA, which was derived from hydrolysis of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). NMR spectra indicate Cs symmetry of 1 (Fig. S5 and S6, ESI†). 13C NMR spectrum shows 57 signals for 186 carbons of 1, e.g., fullerene sp3 carbons at 79.81 ppm, fullerene sp2 carbons at 130–150 ppm, carbonyl carbons at 172.76 ppm, alkyne carbons at 93.15 and 78.52 ppm. MALDI-TOF mass spectrum for 1 gives the expected molecular ion peak at 2580.7 m/z (Fig. S7, ESI†). Triad 1 shows good solubility in common organic solvents and good thermostability, with 5% weight loss at 375 1C (Fig. S8, ESI†). As shown in Fig. 1, the absorption of triad 1 is the integration of the respective absorption of 2 and PC61BM. The peaks at 328 and 432 nm for 1 originate from fullerene absorption (329 and 431 nm), and the broad band at 544 nm from DTSDBT unit (2), which exhibits an absorption maximum at 541 nm. The cyclic voltammogram of triad 1 shows characteristics of fullerene and 2 (Fig. 2). The first reduction potential for 1 is identical to that of
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Scheme 1
PCCP
Fig. 2
Cyclic voltammograms for compound 2, PC61BM and triad 1.
Fig. 3
J–V curve for solar cells based on triad 1.
Synthesis of triad 1.
a Jsc of 1.75 mA cm 2, and an FF of 0.27 (Fig. 3). Triad 1 possesses an electron mobility of 5.7 10 4 cm2 V 1 s 1 and a hole mobility of 3.6 10 5 cm2 V 1 s 1 (Fig. S9, ESI†). Unbalanced charge carrier transport in the device accounts for low FF. External quantum efficiency (EQE) and internal quantum efficiency (IQE) spectra for triad 1 solar cells, and absorption spectrum of triad 1 film are shown in Fig. 4. The EQE spectrum shows three peaks at 349, 441, and 556 nm, Fig. 1
Absorption spectra for compound 2, PC61BM and triad 1 in CHCl3.
PC61BM ( 1.05 V), while the second one ( 1.54 V) shows a negative shift from that of PC61BM ( 1.44 V). The characteristic oxidation (0.51 V) and two-electron reduction ( 1.77 V) waves for compound 2 display in the cyclic voltammogram of triad 1 (0.54 and 1.72 V). LUMO levels estimated from the first reduction potentials are 3.75 and 3.03 eV for fullerene and 2, respectively.12 0.72 eV difference between LUMO levels of donor and acceptor units provides the driving force for exciton dissociation.1a Single-component solar cells with a structure of ITO/PEDOT: PSS/1/Ca/Al were fabricated to evaluate the performance of triad 1. A solution of 1 in chlorobenzene (15 mg mL 1) gave good films on PEDOT with a root-mean-square roughness of 0.25 nm. The cells afforded a PCE of 0.4%, with a Voc of 0.79 V,
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Fig. 4 EQE and IQE spectra for triad 1 solar cells (absorption spectrum of triad 1 film added as a reference).
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respectively. EQE response in 300–400 nm and 500–650 nm regions originates from the absorption of fullerene and DTSDBT, respectively. Though the absorbance of triad 1 in 400–500 nm region is weak, the EQE response is comparable to that in 500–600 nm region, which might be due to the relatively high IQE in 400–500 nm region. The integrated photocurrent from EQE is 1.65 mA cm 2, consisting with JSC from J–V measurements. In summary, we designed and synthesized a dumbbell-like A–D–A molecule consisting of a DTSDBT core and two fullerenes. This molecule possessed good solubility, thermostability, and filmforming capability, and gave a 0.4% PCE in single-component solar cells. More efforts will focus on modifying the molecular structure to improve electron/hole mobilities and light absorbance to get significantly enhanced photocurrent and FF. Highly extended conjugation and balanced charge carrier transport will be the target of our future work.
Communication
4 5
6 7
Acknowledgements 8 This work was supported by the ‘‘100 Talents Program’’ of Chinese Academy of Sciences, National Natural Science Foundation of China (21374025, 21372053, and 21102028) and Ministry of Science and Technology of China (2010DFB63530).
Notes and references ´chet, Angew. Chem., Int. 1 (a) B. C. Thompson and J. M. J. Fre Ed., 2008, 47, 58; (b) Y.-J. Cheng, S.-H. Yang and C.-S. Hsu, Chem. Rev., 2009, 109, 5868; (c) J. L. Delgado, P.-A. Bouit, ´. Herranz and N. Martı´n, Chem. Commun., S. Filippone, M. A ¨sel, 2010, 46, 4853; (d) F. C. Krebs, N. Espinosa, M. Ho R. R. Søndergaard and M. Jørgensen, Adv. Mater., 2014, 26, 29. 2 G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789. 3 (a) C. Cabanetos, A. E. Labban, J. A. Bartelt, J. D. Douglas, ´chet, M. D. McGehee and W. R. Mateker, J. M. J. Fre P. M. Beaujuge, J. Am. Chem. Soc., 2013, 135, 4656;
3514 | Phys. Chem. Chem. Phys., 2014, 16, 3512--3514
9
10 11
12
(b) I. Osaka, T. Kakara, N. Takemura, T. Koganezawa and K. Takimiya, J. Am. Chem. Soc., 2013, 135, 8834; (c) M. Zhang, Y. Gu, X. Guo, F. Liu, S. Zhang, L. Huo, T. P. Russell and J. Hou, Adv. Mater., 2013, 25, 4944; (d) J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li, G. He, C. Li, B. Kan, M. Li and Y. Chen, J. Am. Chem. Soc., 2013, 135, 8484; (e) A. K. K. Kyaw, D. H. Wang, D. Wynands, J. Zhang, T.-Q. Nguyen, G. C. Bazan and A. J. Heeger, Nano Lett., 2013, 13, 3796. J. Roncali, Adv. Energy Mater., 2011, 1, 147. (a) X. Yang, J. K. J. van Duren, R. A. J. Janssen, M. A. Michels and J. Loos, Macromolecules, 2004, 37, 2151; (b) X. Yang, J. Loos, S. C. Veenstra, W. H. J. Verhees, M. M. Wienk, J. M. Kroon, M. A. J. Michels and R. A. J. Janssen, Nano Lett., 2005, 5, 579. ´ve ˆque, S. Roquet and J. Roncali, A. Cravino, P. Leriche, O. Ale Adv. Mater., 2006, 18, 3033. T. Nishizawa, H. K. Lim, K. Tajima and K. Hashimoto, Chem. Commun., 2009, 2469. L. Bu, X. Guo, B. Yu, Y. Qu, Z. Xie, D. Yan, Y. Geng and F. Wang, J. Am. Chem. Soc., 2009, 131, 13242. (a) S. Izawa, K. Hashimoto and K. Tajima, Chem. Commun., 2011, 47, 6365; (b) T. L. Chen, Y. Zhang, P. Smith, A. Tamayo, Y. Liu and B. Ma, ACS Appl. Mater. Interfaces, 2011, 3, 2275. S. Izawa, K. Hashimoto and K. Tajima, Phys. Chem. Chem. Phys., 2012, 14, 16138. (a) J. Hou, H.-Y. Chen, S. Zhang, G. Li and Y. Yang, J. Am. Chem. Soc., 2008, 130, 16144; (b) T. S. van der Poll, J. A. Love, T.-Q. Nguyen and G. C. Bazan, Adv. Mater., 2012, 24, 3646. (a) C. Zhang, S. Chen, Z. Xiao, Q. Zuo and L. Ding, Org. Lett., 2012, 14, 1508; (b) G. Ye, S. Chen, Z. Xiao, Q. Zuo, Q. Wei and L. Ding, J. Mater. Chem., 2012, 22, 22374; (c) S. Chen, G. Ye, Z. Xiao and L. Ding, J. Mater. Chem. A, 2013, 1, 5562; (d) Z. Xiao, G. Ye, Y. Liu, S. Chen, Q. Peng, Q. Zuo and L. Ding, Angew. Chem., Int. Ed., 2012, 51, 9038; (e) J. Cao, Q. Liao, X. Du, J. Chen, Z. Xiao, Q. Zuo and L. Ding, Energy Environ. Sci., 2013, 6, 3224.
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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 3512
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A dumbbell-like A–D–A molecule for single-component organic solar cells† Jiamin Cao, Xiaoyan Du, Shan Chen, Zuo Xiao* and Liming Ding*
Received 2nd November 2013, Accepted 27th November 2013 DOI: 10.1039/c3cp54642j www.rsc.org/pccp
A dumbbell-like A–D–A molecule, triad 1, containing a dithienosiloledibenzothiadiazole donor unit and two fullerene acceptor units was designed and synthesized. Triad 1 shows good solubility, thermostability, light absorbance, and film-forming capability. Solar cells using 1 as the active layer gave a PCE of 0.4%.
Organic solar cells (OSCs) possess unique advantages like low cost, lightweight, flexibility, and roll-to-roll fabrication, thus attracting great attention from academic institutions and industry.1 In bulk heterojunction (BHJ) solar cells, the polymer (or small molecule) donor and fullerene acceptor form a nanoscale interpenetrating network to allow efficient free charge carrier generation and transport.2 The power conversion efficiency (PCE) for polymer or small molecule solar cells has surpassed 8%.3 To obtain an ideal nano-morphology for the active layer, much effort has to be paid on the fabrication conditions, such as donor/acceptor (D/A) ratios, solvents, annealing, and additives. Tedious work in device optimization and low reproducibility of device performance retard OSC commercialization.4 Besides, the optimized morphology of the active layer is metastable and evolves with time due to unavoidable fullerene aggregation, leading to D/A interface reduction and device performance deterioration.5 To deal with the stability issue in bi-component cells, single-component OSCs using only one photoactive material, which possesses multi-functions like light absorption, exciton dissociation, and electron and hole transport, were proposed.4 The strategy is to covalently link the donor and acceptor units to make dyad or triad molecules. A star-shaped molecule based on a triphenylamine donor and dicyanovinyl acceptors showed a PCE of 0.40%.6 A dyad based on a oligo(p-phenylenevinylene) (OPV) donor and a fullerene acceptor showed a PCE of 1.28%.7 A dyad containing a fluorene-alt-bithiophene oligomer donor and a
National Center for Nanoscience and Technology, Beijing 100190, China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Experimental details including synthesis, measurements, and instruments. See DOI: 10.1039/c3cp54642j
3512 | Phys. Chem. Chem. Phys., 2014, 16, 3512--3514
perylene diimide acceptor afforded a 1.5% PCE.8 A diketopyrrolopyrrole (DPP)–fullerene dyad and a DPP-containing oligothiophene– fullerene triad gave 1.1% and 0.5% PCEs, respectively.9 Recently, Hashimoto et al reported an OPV–C70 dyad giving a PCE record of 1.92%.10 Conjugated polymer and small molecule donors made of dithienosilole (DTS) and benzothiadiazole (BT) units demonstrated outstanding performance in BHJ solar cells due to their good light-absorbing and charge carrier transporting properties.11 So we used DTS and BT units in developing the target molecule. We designed a dumbbell-like triad 1 consisting of a dithienosilole-dibenzothiadiazole (DTSDBT) core and two fullerenes. The synthesis, physical properties, and photovoltaic performance of triad 1 were studied. Solar cells using triad 1 as the active layer gave a PCE of 0.4%. Sonogashira coupling of but-3-yn-1-ol and 4,7-dibromo2,1,3-benzothiadiazole afforded compound 3 in 52% yield (Scheme 1). Stille coupling of 3 and bis(trimethylstannyl)substituted-DTS afforded compound 2 in 31% yield. Finally, triad 1 was obtained in 53% yield by esterification of compound 2 with fullerene carboxylic acid, PCBA, which was derived from hydrolysis of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). NMR spectra indicate Cs symmetry of 1 (Fig. S5 and S6, ESI†). 13C NMR spectrum shows 57 signals for 186 carbons of 1, e.g., fullerene sp3 carbons at 79.81 ppm, fullerene sp2 carbons at 130–150 ppm, carbonyl carbons at 172.76 ppm, alkyne carbons at 93.15 and 78.52 ppm. MALDI-TOF mass spectrum for 1 gives the expected molecular ion peak at 2580.7 m/z (Fig. S7, ESI†). Triad 1 shows good solubility in common organic solvents and good thermostability, with 5% weight loss at 375 1C (Fig. S8, ESI†). As shown in Fig. 1, the absorption of triad 1 is the integration of the respective absorption of 2 and PC61BM. The peaks at 328 and 432 nm for 1 originate from fullerene absorption (329 and 431 nm), and the broad band at 544 nm from DTSDBT unit (2), which exhibits an absorption maximum at 541 nm. The cyclic voltammogram of triad 1 shows characteristics of fullerene and 2 (Fig. 2). The first reduction potential for 1 is identical to that of
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Scheme 1
PCCP
Fig. 2
Cyclic voltammograms for compound 2, PC61BM and triad 1.
Fig. 3
J–V curve for solar cells based on triad 1.
Synthesis of triad 1.
a Jsc of 1.75 mA cm 2, and an FF of 0.27 (Fig. 3). Triad 1 possesses an electron mobility of 5.7 10 4 cm2 V 1 s 1 and a hole mobility of 3.6 10 5 cm2 V 1 s 1 (Fig. S9, ESI†). Unbalanced charge carrier transport in the device accounts for low FF. External quantum efficiency (EQE) and internal quantum efficiency (IQE) spectra for triad 1 solar cells, and absorption spectrum of triad 1 film are shown in Fig. 4. The EQE spectrum shows three peaks at 349, 441, and 556 nm, Fig. 1
Absorption spectra for compound 2, PC61BM and triad 1 in CHCl3.
PC61BM ( 1.05 V), while the second one ( 1.54 V) shows a negative shift from that of PC61BM ( 1.44 V). The characteristic oxidation (0.51 V) and two-electron reduction ( 1.77 V) waves for compound 2 display in the cyclic voltammogram of triad 1 (0.54 and 1.72 V). LUMO levels estimated from the first reduction potentials are 3.75 and 3.03 eV for fullerene and 2, respectively.12 0.72 eV difference between LUMO levels of donor and acceptor units provides the driving force for exciton dissociation.1a Single-component solar cells with a structure of ITO/PEDOT: PSS/1/Ca/Al were fabricated to evaluate the performance of triad 1. A solution of 1 in chlorobenzene (15 mg mL 1) gave good films on PEDOT with a root-mean-square roughness of 0.25 nm. The cells afforded a PCE of 0.4%, with a Voc of 0.79 V,
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Fig. 4 EQE and IQE spectra for triad 1 solar cells (absorption spectrum of triad 1 film added as a reference).
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respectively. EQE response in 300–400 nm and 500–650 nm regions originates from the absorption of fullerene and DTSDBT, respectively. Though the absorbance of triad 1 in 400–500 nm region is weak, the EQE response is comparable to that in 500–600 nm region, which might be due to the relatively high IQE in 400–500 nm region. The integrated photocurrent from EQE is 1.65 mA cm 2, consisting with JSC from J–V measurements. In summary, we designed and synthesized a dumbbell-like A–D–A molecule consisting of a DTSDBT core and two fullerenes. This molecule possessed good solubility, thermostability, and filmforming capability, and gave a 0.4% PCE in single-component solar cells. More efforts will focus on modifying the molecular structure to improve electron/hole mobilities and light absorbance to get significantly enhanced photocurrent and FF. Highly extended conjugation and balanced charge carrier transport will be the target of our future work.
Communication
4 5
6 7
Acknowledgements 8 This work was supported by the ‘‘100 Talents Program’’ of Chinese Academy of Sciences, National Natural Science Foundation of China (21374025, 21372053, and 21102028) and Ministry of Science and Technology of China (2010DFB63530).
Notes and references ´chet, Angew. Chem., Int. 1 (a) B. C. Thompson and J. M. J. Fre Ed., 2008, 47, 58; (b) Y.-J. Cheng, S.-H. Yang and C.-S. Hsu, Chem. Rev., 2009, 109, 5868; (c) J. L. Delgado, P.-A. Bouit, ´. Herranz and N. Martı´n, Chem. Commun., S. Filippone, M. A ¨sel, 2010, 46, 4853; (d) F. C. Krebs, N. Espinosa, M. Ho R. R. Søndergaard and M. Jørgensen, Adv. Mater., 2014, 26, 29. 2 G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789. 3 (a) C. Cabanetos, A. E. Labban, J. A. Bartelt, J. D. Douglas, ´chet, M. D. McGehee and W. R. Mateker, J. M. J. Fre P. M. Beaujuge, J. Am. Chem. Soc., 2013, 135, 4656;
3514 | Phys. Chem. Chem. Phys., 2014, 16, 3512--3514
9
10 11
12
(b) I. Osaka, T. Kakara, N. Takemura, T. Koganezawa and K. Takimiya, J. Am. Chem. Soc., 2013, 135, 8834; (c) M. Zhang, Y. Gu, X. Guo, F. Liu, S. Zhang, L. Huo, T. P. Russell and J. Hou, Adv. Mater., 2013, 25, 4944; (d) J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li, G. He, C. Li, B. Kan, M. Li and Y. Chen, J. Am. Chem. Soc., 2013, 135, 8484; (e) A. K. K. Kyaw, D. H. Wang, D. Wynands, J. Zhang, T.-Q. Nguyen, G. C. Bazan and A. J. Heeger, Nano Lett., 2013, 13, 3796. J. Roncali, Adv. Energy Mater., 2011, 1, 147. (a) X. Yang, J. K. J. van Duren, R. A. J. Janssen, M. A. Michels and J. Loos, Macromolecules, 2004, 37, 2151; (b) X. Yang, J. Loos, S. C. Veenstra, W. H. J. Verhees, M. M. Wienk, J. M. Kroon, M. A. J. Michels and R. A. J. Janssen, Nano Lett., 2005, 5, 579. ´ve ˆque, S. Roquet and J. Roncali, A. Cravino, P. Leriche, O. Ale Adv. Mater., 2006, 18, 3033. T. Nishizawa, H. K. Lim, K. Tajima and K. Hashimoto, Chem. Commun., 2009, 2469. L. Bu, X. Guo, B. Yu, Y. Qu, Z. Xie, D. Yan, Y. Geng and F. Wang, J. Am. Chem. Soc., 2009, 131, 13242. (a) S. Izawa, K. Hashimoto and K. Tajima, Chem. Commun., 2011, 47, 6365; (b) T. L. Chen, Y. Zhang, P. Smith, A. Tamayo, Y. Liu and B. Ma, ACS Appl. Mater. Interfaces, 2011, 3, 2275. S. Izawa, K. Hashimoto and K. Tajima, Phys. Chem. Chem. Phys., 2012, 14, 16138. (a) J. Hou, H.-Y. Chen, S. Zhang, G. Li and Y. Yang, J. Am. Chem. Soc., 2008, 130, 16144; (b) T. S. van der Poll, J. A. Love, T.-Q. Nguyen and G. C. Bazan, Adv. Mater., 2012, 24, 3646. (a) C. Zhang, S. Chen, Z. Xiao, Q. Zuo and L. Ding, Org. Lett., 2012, 14, 1508; (b) G. Ye, S. Chen, Z. Xiao, Q. Zuo, Q. Wei and L. Ding, J. Mater. Chem., 2012, 22, 22374; (c) S. Chen, G. Ye, Z. Xiao and L. Ding, J. Mater. Chem. A, 2013, 1, 5562; (d) Z. Xiao, G. Ye, Y. Liu, S. Chen, Q. Peng, Q. Zuo and L. Ding, Angew. Chem., Int. Ed., 2012, 51, 9038; (e) J. Cao, Q. Liao, X. Du, J. Chen, Z. Xiao, Q. Zuo and L. Ding, Energy Environ. Sci., 2013, 6, 3224.
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