Synthesis and Properties of Dithiafulvenyl Functionalized Spiro

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Cite This: Org. Lett. 2018, 20, 780−783

Synthesis and Properties of Dithiafulvenyl Functionalized Spiro[fluorene-9,9′-xanthene] Molecules Min Wu,† Juan Li,‡ Ruqin Zhang,† Xia Tian,† Zhaoxiang Han,§ Xiaoqing Lu,§ Kunpeng Guo,*,† Zhike Liu,*,‡ and Zhongqiang Wang† †

Ministry of Education Key Laboratory of Interface Science and Engineering in Advanced Materials, Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, P. R. China ‡ School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China § College of Science, China University of Petroleum, Qingdao, Shandong 266555, P. R. China S Supporting Information *

ABSTRACT: Two spiroannulated molecular structures with dithiafulvenyl units functionalized at the 2,2′,7,7′- (SFX-DTF1) and 2,3′,6,′7- (SFX-DTF2) positions of a spiro[fluorene-9,9′xanthene] core were synthesized. Studies revealed the hole mobility was significantly influenced by the dithiafulvenyl functionalized positions in the molecular structure. To explore their primary applications as hole-transporting materials in perovskite solar cells, SFX-DTF1-based devices exhibited a power conversion efficiency of 10.67% without the use of p-type dopants, yielding good air stability. Many π-extend TTFs, reported in the literature, have been derived from planar aromatic cores or linear olefins to date.5a,12 By comparison, studies to increase the dimensionality of TTFs based on the spiroannulated molecular structure is still rare,13 although the interest in exploring spiro compounds in organic semiconductors for various optoelectronic applications is growing.14 For our purpose, we noticed that a spiro[fluorene9,9′-xanthene] (SFX) core would be particularly well suited as a scaffold, as its facile one-pot synthetic approach developed by the Huang group and at the same time SFX-centered materials have attracted significant attention for developing efficient organic luminophores and semiconductors.15 With this in mind, we designed two molecules SFX-DTF1 and SFX-DTF2 which feature four DTF units functionalized to a SFX core at its 2,2′,7,7′- and 2,3′,6,′7-positions, respectively. Therefore, SFXDTF1 and SFX-DTF2 can be viewed as a class of novel spiroannulated TTFs. We describe here the synthesis and optical/ electrochemical/thermal properties of SFX-DTF1 and SFXDTF2 and their primary applications as dopant-free holetransporting materials (HTMs) in perovskite solar cells (PVSCs). As depicted in Scheme 1, the synthesis of SFX-DTF1 and SFX-DTF2 started from the corresponding 2,2′,7,7′-tetrabromo-spiro(fluorene-9,9′-xanthene) (SFX1) and 2,3′,6,′7-tetrabromo-spiro(fluorene-9,9′-xanthene) (SFX2) that were readily prepared by the reported procedures.15c Lithium−bromine exchange followed by formylation with N,N-dimethyl-

S

ince the synthesis of tetrathiafulvalene (TTF) by Wudl in 1970 and the first “organic metal” tetrathiafulvalenetetracyano-p-quinodimethane (TTF-TCNQ) reported by Ferraris et al. in 1973, the design and synthesis of tetrathiafulvalene (TTF) and its salt derivatives have drawn great attention for electrically conductive materials due to their strong electron donating ability.1Furthermore, due to their unique solid state as well as their solution properties, the utility of TTF derivatives as building blocks in macromolecular and supramolecular structures,2 as molecule-based magnetic compounds,3 and as a donor moiety in charge transfer systems in nonlinear optic materials and photovoltaics has made TTF one of the most extensively studied molecules in recent years.4 Therefore, the increase in the dimensionality of the TTF analogues has drawn great interest, including replacement of sulfur by other chalcogen atoms,5 peripheral substitution or chemical modification of the TTF core,6 and insertion of the central core between the dithiafulvenyl (DTF) units.7In this context, the insertion of π-conjugated spacers between the DTF units would seem to be especially intriguing because this may significantly influence both the electronic and geometric features of the π-extend TTFs. First, the intramolecular charge transfer (ICT) effect or hole/electron transporting characteristics of the resulting TTFs would be modulated through extension of the π-conjugation.8 Second, various spacers can lead to novel molecular configurations, such as planar molecules,9 butterfly shaped molecules,10 concave bowl shaped molecules, etc.,11 thus further allowing for novel functions. Therefore, π-extend TTFs should be explored with altered core units for their next applications. © 2018 American Chemical Society

Received: December 16, 2017 Published: January 18, 2018 780

DOI: 10.1021/acs.orglett.7b03918 Org. Lett. 2018, 20, 780−783

Letter

Organic Letters

combination with the excitation transition energies (E0−0) determined by their absorption onsets and are extracted in Table S1. As shown in Figure 1b, the oxidation peak of SFXDTF2 (0.79 V) is only 0.03 V higher than that of SFX-DTF1 (0.76 V), which indicates that the DTF substituted position in molecular structure has a slight influence on the electrondonating ability. The reduction peak of SFX-DTF2 (−0.56 V) is 0.15 V lower than that of SFX-DTF1 (−0.41 V), suggesting that SFX-DTF1 is easier to be reduced compared to SFXDTF2. The redox peak potential separation (ΔEp) values are 1.17 and 1.35 V for SFX-DTF1 and SFX-DTF2, respectively. The lower ΔEp for SFX-DTF1 implies its better redox behavior than that of SFX-DTF2. The HOMO levels of SFX-DTF1 and SFX-DTF2 were calculated accordingly as −5.16 and −5.19 eV, respectively.4c The LUMO levels of SFX-DTF1 and SFXDTF2, calculated as Eox − E0−0, were −2.53 and −2.57 eV, respectively. This result suggests significant energy conservation when picking out prominent DTF functionalized SFX molecules for applications in optoelectronic devices. Tuning the energy levels of these materials to match with active layers, such as light harvesting materials or carrier transport materials in devices, is of negligible consideration even though DTF substitute positions are being changed. For instance, the HOMO levels of the two materials are close to higher than the valence band (VB) maximum of CH3NH3PbI3 (−5.43 eV),14b indicating the hole injection from the CH3NH3PbI3to the SFXDTF1 or SFX-DTF2 is feasible. Hence, when SFX-DTF1 and SFX-DTF2 are used as HTM in the PVSCs, high open circuit voltages are expected. In addition, their LUMO levels are sufficiently high relative to the conduction band (CB) of CH3NH3PbI3 (−3.91 eV).16 Therefore, the larger energy barrier (1.34−1.38 eV) between the CB of CH3NH3PbI3and the LUMO of SFX-DTF1 and SFX-DTF2 would block the electrons efficiently from perovskites to the metal electrodes and thus largely reduce the charge recombination. The geometrical configuration and frontier molecular orbital distributions of compounds SFX-DTF1 and SFX-DTF2 were theoretically investigated using the Gaussian 09 program at the B3LYP/6-31G* level. As shown in Figure 2, the dihedral angles between the peripheral DTF substitutes and the SFX core

Scheme 1. Synthesis of SFX-DTF1 and SFX-DTF2

formamide (DMF) produced the key intermediated SFXbearing aldehydes SFX-A1 and SFX-A2, respectively. To the best of our knowledge, this is the first report on the synthesis of SFX-based aldehydes, and their applications in material chemistry as important intermediates would be desirable. In this work, compounds SFX-A1 and SFX-A2 were then subjected to a phosphite-induced Horner−Wittig condensation with a DTF derivative 4,5-bis(methylthio)-1,3-dithiole-2-thione (MDT) to yield the target compounds SFX-DTF1 and SFXDTF2, respectively. The structures of the compounds were fully characterized by NMR and mass spectroscopies. Compounds SFX-DTF1 and SFX-DTF2 both have good solubility in common organic solvents such as acetone, dichloromethane, tetrahydrofuran (THF), and toluene. Thermogravimetric analysis (TGA) reveals that SFX-DTF1 possesses a higher decomposition temperature (Td) of 290.6 °C than that of SFX-DTF2 (Td = 259.7 °C) under a nitrogen atmosphere, as illustrated in Figure S1. Meanwhile, the differential scanning calorimetry (DSC) measurement of SFX-DTF1 shows an endothermic peak at 106.4 °C, and that of SFX-DTF2 displays an endothermic peak at 129.1 °C (Figure S1). These results indicate that both SFX-DTF1 and SFX-DTF2 hold good thermal stability. The absorption spectra of SFX-DTF1 and SFX-DTF2 in dilute tetrahydrofuran (THF) at room temperature are shown in Figure 1a, and the corresponding spectroscopic parameters

Figure 1. (a) UV−vis absorption spectra and (b) cyclic voltammograms of SFX-DTF1 and SFX-DTF2 in dilute THF (1 × 10−5 M).

are summarized in Table S1. Both of the two compounds exhibit three strong absorption bands around 360−440 nm, which can be assigned to a more localized π−π* in peripheral dithiole rings and centered aromatic rings. To explore the influence of DTF substituted positions in molecular structure on the electrochemical properties, the molecular orbital energy levels of SFX-DTF1 and SFX-DTF2 have been derived from cyclic voltammetry (CV) in

Figure 2. Calculated geometries and frontier molecular orbital distributions of SFX-DTF1 and SFX-DTF2. 781

DOI: 10.1021/acs.orglett.7b03918 Org. Lett. 2018, 20, 780−783

Letter

Organic Letters

To date, most of the highly efficient PVSCs were using Spiro-OMeTAD as the HTM accompany with p-type dopants (such as lithium salts) addition. However, such dopants in the HTMs contribute to the low stability of devices due to their deliquescent behavior. Therefore, developing hydrophobic dopant-free HTMs has drawn much attention. As shown in Figure 3b, SFX-DTF1 and SFX-DTF2 exhibit much larger contact angles of 93° and 100°, respectively, suggesting these two materials would be more efficient in keeping the perovskite layer from water than Spiro-OMeTAD (contact angle = 79°). Compared to traditional HTL Spiro-OMeTAD, SFX-DTF1 and SFX-DTF2 have advantages such as suitable energy levels, high hole mobility, and good hydrophobicity, and therefore, they could be developed as dopant-free HTLs for stable PVSCs. More importantly, sulfur-containing HTMs have shown good stability and high hole mobility due to the beneficial S−S interactions.14c,17 Based on these, a series of PVSCs, consisting of FTO/TiO2/CH3NH3PbI3/SFX-DTF1 or SFX-DTF2 or Spiro-OMeTAD/Au, without using the p-type dopants were assembled and tested. Their performances were measured under the AM 1.5 G (100 mW cm−2) irradiation, and the current density−voltage (J−V) curves are displayed in Figure 4a; the corresponding photovoltaic parameters are summarized

moieties for SFX-DTF1 and SFX-DTF2 were in the range of 19°−23°, indicating nearly planar xanthene-extended TTF and fluorine-extended TTF moieties in the molecular structure, respectively. However, the fluorine rings in SFX tilt toward one side, significantly reducing the intersection angle between the fluorene and the xanthene to 89.1° and 89.8° for SFX-DTF1 and SFX-DTF2, respectively, indicating the compounds exhibit highly twisted configurations. The nonplanar molecular structures of SFX-DTF1 and SFX-DTF2 endow them with good solubility in common organic solvents, which is indicative of their practicability for solution-processed organic electronic materials. The electron distributions of the highest occupied molecular orbitals (HOMOs) of SFX-DTF1 and SFX-DTF2 are mainly localized on the centered fluorene rings with some extending to the adjacent DTF of the peripheral units, while the lowest unoccupied molecular orbitals (LUMOs) of them just slightly shift from DTF units to the fluorene rings. This result indicates the donor character of these molecules stem from the conjugation between the two DTF units. To study the charge-transport properties of SFX-DTF1 and SFX-DTF2, single charge carrier devices based on SFX-DTF1 and SFX-DTF2 were fabricated by spin coating from their respective o-dichlorobenzene solutions, and the carrier mobilities of the two compounds were calculated using the space charge limited current (SCLC) method. In the present work, the structure of the hole-only device was ITO/PEDOT: PSS/HTM/MoO3/Al, where the HTM referred to SFX-DTF1 or SFX-DTF2, respectively. Light emission was not detected from the hole-only devices during the measurement, indicating that the electron is not injected and recombined with the hole; thus, SFX-DTF1 and SFX-DTF2 are exhibiting hole-transporting characteristics. The mobility was determined by fitting the dark current to the model of a single carrier SCLC, which was described by the Mott−Gurney law. As shown in Figure 3a

Figure 4. (a) Current density−voltage curves of one of the bestperforming devices using undoped SFX-DTF1, SFX-DTF2, and Spiro-OMeTAD as HTMs, respectively. (b) Time-course changes in the PCE of the PVSCs with undoped SFX-DTF1, SFX-DTF2, and Spiro-OMeTAD as HTMs, respectively. The measurements were performed under in air atmosphere.

Table 1. Photovoltaic Performance of the Devices Based on Dopant-Free SFX-DTF1, SFX-DTF2 and Spiro-OMeTAD under AM 1.5G Illumination (100 mW cm−2)

Figure 3. (a) Mobility of the two hole only devices. (b) Water contact angles on SFX-DTF1, SFX-DTF2, and Spiro-OMeTAD films.

HTM

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

SFX-DTF1 SFX-DTF2 Spiro-OMeTAD

1.03 0.98 0.66

19.23 19.05 14.53

53.81 47.05 35.14

10.67 8.78 3.37

in Table 1. The device fabricated with pure Spiro-OMeTAD showed a short-circuit current density (Jsc) and PCE of only 14.53 mA cm−2 and 3.37%, respectively. As expect, SFX-DTF1 and SFX-DTF2 based devices exhibited a dramatically improved performance. The devices based on SFX-DTF1 with the highest hole mobility showed the highest PCE of 10.67%, Jsc of 19.23 mA cm−2, open circuit voltage (Voc) of 1.03 V, and fill factor (FF) of 53.81. The device using SFX-DTF2 exhibited a smaller PCE value of 8.78% due to the smaller FF and Jsc values (FF = 47.05, Jsc = 19.05 mA cm−2, and Voc = 0.98 V). The Jsc change of devices based on different HTMs was further confirmed by the incident photon-to-current conversion efficiency (IPCE) spectrum shown in Figure S2. The durability of the photovoltaic performance of the PVSCs based on SFX-

and Table S1, the SFX-DTF1 exhibits high hole mobility of up to 1.51 × 10−4 cm2 V−1 s−1, which is 4.95- and 2.28-fold higher than that of SFX-DTF2 (3.05 × 10−5 cm2 V−1 s−1) and commercially available HTM Spiro-OMeTAD (6.62 × 10−5 cm2 V−1 s−1),14c respectively. Considering SFX-DTF1 and SFX-DTF2 show quite similar chemical structures to each other, the significant enhancement in the hole mobility of SFXDTF1 can be assigned to the closer DTF groups between 2′,7′positions and 2,7-positions in the molecular structure, which may reduce the hole reorganization energy thus accelerating the hole transfer rate to some extent. 782

DOI: 10.1021/acs.orglett.7b03918 Org. Lett. 2018, 20, 780−783

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Organic Letters

(4) (a) Si, Y.; Yang, G.; Su, Z. J. Mater. Chem. C 2013, 1, 1399. (b) Narayanaswamy, K.; Venkateswararao, A.; Gupta, V.; Chand, S.; Singh, S. P. Chem. Commun. 2016, 52, 210. (c) Guo, K.; Yan, K.; Lu, X.; Qiu, Y.; Liu, Z.; Sun, J.; Guo, W.; Yang, S. Org. Lett. 2012, 14, 2214. (d) Cheng, J.; Zhang, F.; Li, K.; Li, X.; Zheng, J.; Guo, K.; Yang, S.; Dong, Q. Dyes Pigm. 2017, 136, 97. (e) Cao, Y.; Cheng, J.; Zhang, F.; Liang, X.; Guo, K.; Yang, S. Sci. China Mater. 2016, 59, 797. (f) Segura, J.; Martín, N. Angew. Chem., Int. Ed. 2001, 40, 1372. (5) (a) Kobayashi, H.; Cui, H.; Kobayashi, A. Chem. Rev. 2004, 104, 5265. (b) Hudhomme, P.; Blanchard, P.; Sallé, M.; Le Moustarder, S.; Riou, A.; Jubault, M.; Gorgues, A.; Duguay, G. Angew. Chem., Int. Ed. Engl. 1997, 36, 878. (c) Fabre, J. M. Chem. Rev. 2004, 104, 5133. (6) (a) Nafe, J.; Auras, F.; Karaghiosoff, K.; Bein, T.; Knochel, P. Org. Lett. 2015, 17, 5356. (b) Yamashita, M.; Kawano, K.; Matsumoto, A.; Aratani, N.; Hayashi, H.; Suzuki, M.; Zhang, L.; Briseno, A. L.; Yamada, H. Chem. - Eur. J. 2017, 23, 14979. (c) Shoji, T.; Araki, T.; Sugiyama, S.; Ohta, A.; Sekiguchi, R.; Ito, S.; Okujima, T.; Toyota, K. J. Org. Chem. 2017, 82, 1657. (7) (a) Khadem, M.; Zhao, Y. Chem. Commun. 2017, 53, 1821. (b) Khadem, M.; Walsh, J. C.; Bodwell, G. J.; Zhao, Y. Org. Lett. 2016, 18, 2403. (c) Ogi, D.; Fujita, Y.; Mori, S.; Shirahata, T.; Misaki, Y. Org. Lett. 2016, 18, 5868. (8) (a) Brunetti, F. G.; López, J. L.; Atienza, C.; Martín, N. J. Mater. Chem. 2012, 22, 4188. (b) Hu, Y.; Wang, Z.; Zhang, X.; Yang, X.; Ge, C.; Fu, L.; Gao, X. Org. Lett. 2017, 19, 468. (9) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. Rev. 2004, 104, 4891. (10) Wenger, S.; Bouit, P.-A.; Chen, Q.; Teuscher, J.; Censo, D. D.; Humphry-Baker, R.; Moser, J.-E.; Delgado, J. L.; Martín, N.; Zakeeruddin, S. M.; Grätzel, M. J. Am. Chem. Soc. 2010, 132, 5164. (11) (a) Gallego, M.; Calbo, J.; Krick Calderon, R. M.; Pla, P.; Hsieh, Y.-C.; Pérez, E. M.; Wu, Y.-T.; Ortí, E.; Guldi, D. M.; Martín, N. Chem. - Eur. J. 2017, 23, 3666. (b) Pérez, E. M.; Sierra, M.; Sánchez, L.; Torres, M. R.; Viruela, R.; Viruela, P. M.; Ortí, E.; Martín, N. Angew. Chem., Int. Ed. 2007, 46, 1847. (c) Gallego, M.; Calbo, J.; Aragó, J.; Krick Calderon, R. M.; Liquido, F. H.; Iwamoto, T.; Greene, A. K.; Jackson, E. A.; Pérez, E. M.; Ortí, E.; Guldi, D. M.; Scott, L. T.; Martín, N. Angew. Chem., Int. Ed. 2014, 53, 2170. (12) (a) Mulla, K.; Zhao, Y. J. Mater. Chem. C 2013, 1, 5116. (b) Cocherel, N.; Leriche, P.; Ripaud, E.; Gallego-Planas, N.; Frère, P. New J. Chem. 2009, 33, 801. (13) Sandín, P.; Martínez-Grau, A.; Sánchez, L.; Seoane, C.; PouAméRigo, R.; Ortí, E.; Martín, N. Org. Lett. 2005, 7, 295. (14) (a) Yu, W.-L.; Pei, J.; Huang, W.; Heeger, A. J. Adv. Mater. 2000, 12, 828. (b) Wang, Y.-K.; Yuan, Z.-C.; Shi, G.-Z.; Li, Y.-X.; Li, Q.; Hui, F.; Sun, B.-Q.; Jiang, Z.-Q.; Liao, L.-S. Adv. Funct. Mater. 2016, 26, 1375. (c) Wang, Y.; Zhu, Z.; Chueh, C.-C.; Jen, A. K.-Y.; Chi, Y. Adv. Energy Mater. 2017, 7, 1700823. (15) (a) Xie, L.-H.; Liu, F.; Tang, C.; Hou, X.-Y.; Hua, Y.-R.; Fan, Q.L.; Huang, W. Org. Lett. 2006, 8, 2787. (b) Xu, B.; Bi, D.; Hua, Y.; Liu, P.; Cheng, M.; Grätzel, M.; Kloo, L.; Hagfeldt, A.; Sun, L. Energy Environ. Sci. 2016, 9, 873. (c) Liang, X.; Wang, K.; Zhang, R.; Li, K.; Lu, X.; Guo, K.; Wang, H.; Miao, Y.; Xu, H.; Wang, Z. Dyes Pigm. 2017, 139, 764. (16) Xu, B.; Zhu, Z.; Zhang, J.; Liu, H.; Chueh, C.-C.; Li, X.; Jen, A. K.-Y. Adv. Energy Mater. 2017, 7, 1700683. (17) Molina-Ontoria, A.; Zimmermann, I.; Garcia-Benito, I.; Gratia, P.; Roldán-Carmona, C.; Aghazada, S.; Graetzel, M.; Nazeeruddin, M. K.; Martín, N. Angew. Chem., Int. Ed. 2016, 55, 6270.

DTF1, SFX-DTF2, and pure Spiro-OMeTAD was further investigated by storing the devices in air (humidity: 30%) (Figure 4b). After aging for 24 days, the SFX-DTF1, SFXDTF2, and Spiro-OMeTAD based devices, respectively retained 74.8%, 71.5%, and 39.4% of their initial PCEs. The impressive stability of SFX-DTF1 and SFX-DTF2 based PVSCs can be reasonably attributed to their more hydrophobic nature, as demonstrated by the contact angles (Figure 3b). In summary, to increase the dimensionality of TTFs in the spiroannulated molecular structure, two new SFX-centered, four DTF functionalized molecules SFX-DTF1 and SFX-DTF2 have been synthesized from the key SFX-bearing aldehydes. Studies implied DTF functionalized SFX compounds would be promising hydrophobic hole-transporting candidates. Moreover, the hole mobility can be modified by tuning the substituted positions of DTF in SFX. As a primary application, the PVSC based on the dopant-free SFX-DTF1 yields a moderate PCE of 10.67% with good air stability. This work not only paves a new way for developing spiro-annulated TTFs but also sheds light on the future design of simplified and highly efficient HTMs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03918. Experimental details, synthetic procedures, device fabrication, characterization data, TGA, DSC, IPCE, and 1H and 13C NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaoqing Lu: 0000-0002-7553-7131 Kunpeng Guo: 0000-0003-2075-6000 Zhike Liu: 0000-0001-5681-3930 Zhongqiang Wang: 0000-0002-7069-2116 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (61704101), the Fundamental Research Funds for the Central Universities (GK201702003).

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REFERENCES

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DOI: 10.1021/acs.orglett.7b03918 Org. Lett. 2018, 20, 780−783