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Low-Cost Synthesis of Hole Transporting Materials for Efficient Perovskite Solar Cells
6. Collins, D.B., Bertram, T.H., Sultana, C.M., Lee, C., Axson, J.L., and Prather, K.A. (2016). Geophys. Res. Lett. 43, 9975–9983. 7. DeMott, P.J., Hill, T.C.J., McCluskey, C.S., Prather, K.A., Collins, D.B., Sullivan, R.C., Ruppel, M.J., Mason, R.H., Irish, V.E., Lee, T., et al. (2016). Proc. Natl. Acad. Sci. USA 113, 5797–5803.
8. Cochran, R.E., Laskina, O., Trueblood, J.V., Estillore, A.D., Morris, H.S., Jayarathne, T., Sultana, C.M., Lee, C., Lin, P., Laskin, J., et al. (2017). Chem 2, this issue, 655–667. 9. Collins, D.B., Zhao, D.F., Ruppel, M.J., Laskina, O., Grandquist, J.R., Modini, R.L., Stokes, M.D., Russell, L.M., Bertram, T.H.,
Grassian, V.H., et al. (2014). Atmos. Meas. Tech. 7, 3667–3683. 10. Ault, A.P., Moffet, R.C., Baltrusaitis, J., Collins, D.B., Ruppel, M.J., Cuadra-Rodriguez, L.A., Zhao, D., Guasco, T.L., Ebben, C.J., Geiger, F.M., et al. (2013). Environ. Sci. Technol. 47, 5603–5612.
Preview
Low-Cost Synthesis of Hole Transporting Materials for Efficient Perovskite Solar Cells Valentina Mirruzzo1 and Aldo Di Carlo1,* Given their high efficiency and easy fabrication procedures, perovskite solar cells are one of the most promising third-generation photovoltaic technologies. In this issue of Chem, Sun and colleagues identify a smart synthetic strategy for achieving efficient and low-cost hole transporting materials, a step forward for the success of this technology.
Nowadays, with the continuously increasing energy demand and the effects of global warming on climate change, the transition to environmentally sustainable energy is no longer deferrable. Energy can be defined as renewable if it is derived from natural process (e.g., sunlight and wind) that are replenished at a higher rate than that consumed. Solar, wind, geothermal, hydropower, bioenergy, and ocean power are sources of renewable energy.1 Among all of the renewable energy technologies, photovoltaic (PV) technology, which directly converts solar energy into electricity, is considered the most promising. In fact, even though solar power is reduced by atmospheric absorption and scattering, the earth benefits from the sun’s great power supply. For these reasons, it is not surprising that solar PV technology is the fastest-growing energy technology, and the PV market has increased dramatically during recent decades. As discussed by Han and collabo-
612
rators,2 the vast majority of PV plants are dominated by silicon solar modules, and although it has decreased dramatically, the cost of electricity produced by PV technology is still higher than that of electricity supplied by conventional fossil fuels. Efforts to lower the cost have resulted in the development of many emerging solar technologies based on cheap materials and low-cost processes, including thin-film silicon solar cells and organic PV and dye-sensitized solar cells. Recently, perovskite solar cells (PSCs) have attracted wide attention because of their easy solution-processable fabrication and high performance and have reached a certified power conversion efficiency (PCE) of 22.1%. In order to make PSCs appealing for the market, it is of paramount importance to move toward materials that guarantee both high efficiency and stable performance. Among all of the PSC components, hole transporting material (HTM) plays a significant role in that it is responsible for the extrac-
Chem 2, 610–620, May 11, 2017 ª 2017 Elsevier Inc.
tion of photogenerated holes from the perovskite layer and their transport toward the electrode.3 For this purpose, an optimal HTM should have energy levels well matched with those of the absorber material, high thermal and chemical stability, good solubility for solution-processed devices (e.g., by spin-coating deposition), and nonplanar 3D structure.3,4 In fact, the HTM architecture is important for reducing electronic coupling and charge recombination and also forming a compact film over the absorber layer. One of the key methods for achieving such properties is the utilization of a spiro carbon in the central core of the HTM chemical structure, and 0 0 0 0 N2,N2,N2 ,N2 ,N,7N,7N7 ,N7 -octakis(40 methoxyphenyl)-9,9 -spirobi[9H-fluorene]2,20 ,7,70 -tetramine (spiro-OMeTAD), where two fluorene units are orthogonally interconnected, is still the most widely and successful used HTM. However, the large-scale production of spiro-OMeTAD still remains prohibitive because it involves several synthetic steps with consequent purification stages, has low yields (total yield less than 30%), and uses expensive materials such as Pd catalyst and phenylboronic acid.5 For this reason, the use of this HTM negatively affects the entire PSC cost balance, limiting the commercialization of this technology. Moreover, the stability of spiro-OMeTAD to
1Center
for Hybrid and Organic Solar Energy, University of Rome Tor Vergata, via del Politecnico 1, 00133 Rome, Italy *Correspondence: [email protected] http://dx.doi.org/10.1016/j.chempr.2017.04.005
thermal stress required for certifying PV panels is limited, and crystallization processes can occur for temperatures exceeding 60 C.6 To overcome such problems and the complexity of spirobifluorene (SBF) core synthesis, a few groups of scientists have recently focused their interest on spiro[fluorene-9,9’-xanthene] (SFX), a complex that can be easily synthesized by the one-pot method.7 These preliminary studies have shown that replacing SBF with SFX yields similar results in terms of redox potential, hole mobility, and PSC efficiency, as well as a significant reduction in the synthetic cost by a factor 30!3,4 Along these lines, Sun and colleagues in this issue of Chem8 have created new HTM molecules based on the use of SFX moiety. In this work, the first appealing aspect is undoubtedly the simplicity of the synthesis: using the one-pot approach, Sun and coworkers built two SFX-based oligomers (called X54 and X55) consisting of SFX and an anisidine (para-methoxyaniline) donor group connected in different ways. Starting from a dibromine SFX derivative, the authors achieved the two oligomers by Buchwald-Hartwig cross-coupling with p-methoxyaniline in a simple one-pot reaction with a strategy similar to that
used for the SFX core construction. In this way, they obtained the desired oligomers with low synthetic effort and high yields (exceeding 90%) and, at the same time, dramatically reduced the cost of a hypothetical scaled-up production. A thorough physicochemical characterization showed that both X54 and X55 had HOMO (highest occupied molecular orbital) levels deeper than those of spiro-OMeTAD, making these HTMs very promising for PSCs with high open-circuit voltage. This very appealing characteristic pushed the authors to fabricate several X54and X55-based PSCs to compare their photovoltaic performances with those of conventional cells based on spiroOMeTAD. As a final result, the X55based solar cells showed a maximum PCE of 20.8%, which outperformed that of the spiro-based PSC (18.8%) used as a reference. Beyond this intriguing finding, the authors obtained remarkable results for the stability of the solar cell. This work shows that after 6 months of aging in low-humidity atmosphere, X55-based PSCs retain 93% of their initial efficiency. In addition, under thermal stress at high temperatures (up to 100 C) and 60% relative humidity, X55 shows slightly better behavior than spiro-OMeTAD. Therefore, these results confirm that the X55 design is well thought out: the tailor-made 3D molecular structure
allows good film formability and fast hole transport. In summary, with these preliminary and already satisfying results, Sun and coworkers have demonstrated that with a rational synthetic approach, it is possible to cut the costs of one of the most expensive materials in PSCs without sacrificing efficiency and stability of the device. This represents an important step toward large-scale production and commercialization of perovskite PV technology. 1. International Energy Agency. Solar Energy. https://www.iea.org/topics/renewables/ subtopics/solar/. 2. Cai, M., Wu, Y., Chen, H., Yang, X., Qiang, Y., and Han, L. (2017). Adv. Sci. 4, 1600269. 3. Xu, B., Bi, D., Hua, Y., Liu, P., Cheng, M., Gratzel, M., Kloo, L., Hagfeldt, A., and Sun, L. (2016). Energy Environ. Sci. 9, 873–877. 4. Maciejczyk, M., Ivaturi, A., and Robertson, N. (2016). J. Mater. Chem. A Mater. Energy Sustain. 4, 4855–4863. 5. Murray, A.T., Frost, J.M., Hendon, C.H., Molloy, C.D., Carbery, D.R., and Walsh, A. (2015). Chem. Commun. (Camb.) 51, 8935–8938. 6. Malinauskas, T., Tomkute-Luksiene, D., Sens, R., Daskeviciene, M., Send, R., Wonneberger, H., Jankauskas, V., Bruder, I., and Getautis, V. (2015). Appl. Mater. Interfaces 7, 11107–11116. 7. Xie, L.H., Liu, F., Tang, C., Hou, X.Y., Hua, Y.R., Fan, Q.L., and Huang, W. (2006). Org. Lett. 8, 2787–2790. 8. Xu, B., Zhang, J., Hua, Y., Liu, P., Wang, L., Ruan, C., Li, Y., Boschloo, G., Johansson, E.M.J., Kloo, L., et al. (2017). Chem 2, this issue, 676–687.
Chem 2, 610–620, May 11, 2017
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8. Cochran, R.E., Laskina, O., Trueblood, J.V., Estillore, A.D., Morris, H.S., Jayarathne, T., Sultana, C.M., Lee, C., Lin, P., Laskin, J., et al. (2017). Chem 2, this issue, 655–667. 9. Collins, D.B., Zhao, D.F., Ruppel, M.J., Laskina, O., Grandquist, J.R., Modini, R.L., Stokes, M.D., Russell, L.M., Bertram, T.H.,
Grassian, V.H., et al. (2014). Atmos. Meas. Tech. 7, 3667–3683. 10. Ault, A.P., Moffet, R.C., Baltrusaitis, J., Collins, D.B., Ruppel, M.J., Cuadra-Rodriguez, L.A., Zhao, D., Guasco, T.L., Ebben, C.J., Geiger, F.M., et al. (2013). Environ. Sci. Technol. 47, 5603–5612.
Preview
Low-Cost Synthesis of Hole Transporting Materials for Efficient Perovskite Solar Cells Valentina Mirruzzo1 and Aldo Di Carlo1,* Given their high efficiency and easy fabrication procedures, perovskite solar cells are one of the most promising third-generation photovoltaic technologies. In this issue of Chem, Sun and colleagues identify a smart synthetic strategy for achieving efficient and low-cost hole transporting materials, a step forward for the success of this technology.
Nowadays, with the continuously increasing energy demand and the effects of global warming on climate change, the transition to environmentally sustainable energy is no longer deferrable. Energy can be defined as renewable if it is derived from natural process (e.g., sunlight and wind) that are replenished at a higher rate than that consumed. Solar, wind, geothermal, hydropower, bioenergy, and ocean power are sources of renewable energy.1 Among all of the renewable energy technologies, photovoltaic (PV) technology, which directly converts solar energy into electricity, is considered the most promising. In fact, even though solar power is reduced by atmospheric absorption and scattering, the earth benefits from the sun’s great power supply. For these reasons, it is not surprising that solar PV technology is the fastest-growing energy technology, and the PV market has increased dramatically during recent decades. As discussed by Han and collabo-
612
rators,2 the vast majority of PV plants are dominated by silicon solar modules, and although it has decreased dramatically, the cost of electricity produced by PV technology is still higher than that of electricity supplied by conventional fossil fuels. Efforts to lower the cost have resulted in the development of many emerging solar technologies based on cheap materials and low-cost processes, including thin-film silicon solar cells and organic PV and dye-sensitized solar cells. Recently, perovskite solar cells (PSCs) have attracted wide attention because of their easy solution-processable fabrication and high performance and have reached a certified power conversion efficiency (PCE) of 22.1%. In order to make PSCs appealing for the market, it is of paramount importance to move toward materials that guarantee both high efficiency and stable performance. Among all of the PSC components, hole transporting material (HTM) plays a significant role in that it is responsible for the extrac-
Chem 2, 610–620, May 11, 2017 ª 2017 Elsevier Inc.
tion of photogenerated holes from the perovskite layer and their transport toward the electrode.3 For this purpose, an optimal HTM should have energy levels well matched with those of the absorber material, high thermal and chemical stability, good solubility for solution-processed devices (e.g., by spin-coating deposition), and nonplanar 3D structure.3,4 In fact, the HTM architecture is important for reducing electronic coupling and charge recombination and also forming a compact film over the absorber layer. One of the key methods for achieving such properties is the utilization of a spiro carbon in the central core of the HTM chemical structure, and 0 0 0 0 N2,N2,N2 ,N2 ,N,7N,7N7 ,N7 -octakis(40 methoxyphenyl)-9,9 -spirobi[9H-fluorene]2,20 ,7,70 -tetramine (spiro-OMeTAD), where two fluorene units are orthogonally interconnected, is still the most widely and successful used HTM. However, the large-scale production of spiro-OMeTAD still remains prohibitive because it involves several synthetic steps with consequent purification stages, has low yields (total yield less than 30%), and uses expensive materials such as Pd catalyst and phenylboronic acid.5 For this reason, the use of this HTM negatively affects the entire PSC cost balance, limiting the commercialization of this technology. Moreover, the stability of spiro-OMeTAD to
1Center
for Hybrid and Organic Solar Energy, University of Rome Tor Vergata, via del Politecnico 1, 00133 Rome, Italy *Correspondence: [email protected] http://dx.doi.org/10.1016/j.chempr.2017.04.005
thermal stress required for certifying PV panels is limited, and crystallization processes can occur for temperatures exceeding 60 C.6 To overcome such problems and the complexity of spirobifluorene (SBF) core synthesis, a few groups of scientists have recently focused their interest on spiro[fluorene-9,9’-xanthene] (SFX), a complex that can be easily synthesized by the one-pot method.7 These preliminary studies have shown that replacing SBF with SFX yields similar results in terms of redox potential, hole mobility, and PSC efficiency, as well as a significant reduction in the synthetic cost by a factor 30!3,4 Along these lines, Sun and colleagues in this issue of Chem8 have created new HTM molecules based on the use of SFX moiety. In this work, the first appealing aspect is undoubtedly the simplicity of the synthesis: using the one-pot approach, Sun and coworkers built two SFX-based oligomers (called X54 and X55) consisting of SFX and an anisidine (para-methoxyaniline) donor group connected in different ways. Starting from a dibromine SFX derivative, the authors achieved the two oligomers by Buchwald-Hartwig cross-coupling with p-methoxyaniline in a simple one-pot reaction with a strategy similar to that
used for the SFX core construction. In this way, they obtained the desired oligomers with low synthetic effort and high yields (exceeding 90%) and, at the same time, dramatically reduced the cost of a hypothetical scaled-up production. A thorough physicochemical characterization showed that both X54 and X55 had HOMO (highest occupied molecular orbital) levels deeper than those of spiro-OMeTAD, making these HTMs very promising for PSCs with high open-circuit voltage. This very appealing characteristic pushed the authors to fabricate several X54and X55-based PSCs to compare their photovoltaic performances with those of conventional cells based on spiroOMeTAD. As a final result, the X55based solar cells showed a maximum PCE of 20.8%, which outperformed that of the spiro-based PSC (18.8%) used as a reference. Beyond this intriguing finding, the authors obtained remarkable results for the stability of the solar cell. This work shows that after 6 months of aging in low-humidity atmosphere, X55-based PSCs retain 93% of their initial efficiency. In addition, under thermal stress at high temperatures (up to 100 C) and 60% relative humidity, X55 shows slightly better behavior than spiro-OMeTAD. Therefore, these results confirm that the X55 design is well thought out: the tailor-made 3D molecular structure
allows good film formability and fast hole transport. In summary, with these preliminary and already satisfying results, Sun and coworkers have demonstrated that with a rational synthetic approach, it is possible to cut the costs of one of the most expensive materials in PSCs without sacrificing efficiency and stability of the device. This represents an important step toward large-scale production and commercialization of perovskite PV technology. 1. International Energy Agency. Solar Energy. https://www.iea.org/topics/renewables/ subtopics/solar/. 2. Cai, M., Wu, Y., Chen, H., Yang, X., Qiang, Y., and Han, L. (2017). Adv. Sci. 4, 1600269. 3. Xu, B., Bi, D., Hua, Y., Liu, P., Cheng, M., Gratzel, M., Kloo, L., Hagfeldt, A., and Sun, L. (2016). Energy Environ. Sci. 9, 873–877. 4. Maciejczyk, M., Ivaturi, A., and Robertson, N. (2016). J. Mater. Chem. A Mater. Energy Sustain. 4, 4855–4863. 5. Murray, A.T., Frost, J.M., Hendon, C.H., Molloy, C.D., Carbery, D.R., and Walsh, A. (2015). Chem. Commun. (Camb.) 51, 8935–8938. 6. Malinauskas, T., Tomkute-Luksiene, D., Sens, R., Daskeviciene, M., Send, R., Wonneberger, H., Jankauskas, V., Bruder, I., and Getautis, V. (2015). Appl. Mater. Interfaces 7, 11107–11116. 7. Xie, L.H., Liu, F., Tang, C., Hou, X.Y., Hua, Y.R., Fan, Q.L., and Huang, W. (2006). Org. Lett. 8, 2787–2790. 8. Xu, B., Zhang, J., Hua, Y., Liu, P., Wang, L., Ruan, C., Li, Y., Boschloo, G., Johansson, E.M.J., Kloo, L., et al. (2017). Chem 2, this issue, 676–687.
Chem 2, 610–620, May 11, 2017
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