Perovskite Solar Cells Go Lead Free - Cell Press

119 downloads 0 Views 1MB Size Report
lar cells (PSCs) can convert the energy of the solar light directly into electric power with the highest efficiency.1 In addition to established technologies, halide ...
Please cite this article in press as: Abate, Perovskite Solar Cells Go Lead Free, Joule (2017), https://doi.org/10.1016/j.joule.2017.09.007

Perspective

Perovskite Solar Cells Go Lead Free Antonio Abate1,*

Halide perovskites are one of the most promising materials for delivery of the next generation of solar cells. Perovskite solar cells have enabled power conversion efficiency comparable with established technologies, such as silicon and cadmium telluride. Their stability is constantly improving and it is not difficult to believe that stability will soon reach the market requirements. There remains a main concern about the toxicity of lead, a constituent of all the better performing perovskite solar cells so far demonstrated. Here, we discuss the environmental and toxicological implications of lead, paying particular attention to the existing regulations. Both regulation and common sense suggest that perovskite solar cells have to become lead free to deliver a sustainable technology. Thus, we provide a critical overview of the current research and an outlook of the paths toward lead-free perovskite solar cells. Introduction Fighting climate change demands a boost in the development of renewable energies. ABX3 halide perovskites, depicted in Figure 1, are newly discovered photovoltaic materials with potential to provide a disruptive new solar cell technology. As with their analogs silicon and cadmium telluride solar cells (CdTe), perovskite solar cells (PSCs) can convert the energy of the solar light directly into electric power with the highest efficiency.1 In addition to established technologies, halide perovskites are prepared from inexpensive materials, which are compatible with highly productive deposition methods already in use for organic electronics.2 After the first demonstration by Kojima et al.,3 the later work of Kim et al.4 and Lee et al.4 demonstrated the effective potential of halide perovskite in solar cells. Since these seminal studies, the reported power conversion efficiency of PSCs has risen to over 22%,5 which makes ABX3 halide perovskites one of the most serious contenders for the design of the next generation of solar cells.

Context & Scale The solution for global sustainability may come from one of the most abundant and cheap materials on Earth: the perovskites. These materials can convert sunlight directly into electricity with the highest efficiency. After only a few years of research, it is clear that perovskites will play a major role in the future energy scenario. The last step is the removal of lead from the perovskite composition to meet the worldwide-adopted restriction of the use of hazardous substances. Here, we provide a critical overview of the current research and an outlook of the paths toward lead-free perovskite for solar energy.

A currently intensely debated topic in PSCs concerns the use of lead, a constituent of most of the halide perovskites so far demonstrated as effective photovoltaic materials.6,7 Here, we discuss the environmental and toxicological implications of lead, paying particular attention to the existing regulations. Both regulation and common sense suggest that PSCs have to become lead free to deliver a sustainable technology. Thus, we provide a critical overview of the current research and an outlook of the paths toward lead-free PSCs. Environmental and Toxicological Implications of Lead Perovskites A thorough life cycle assessment, which can include materials, manufacturing, and recycling analysis aiming to estimate the potential impact of PSC technology, is premature at this stage.8 However, we can already point out that lead-based perovskites are a major issue that may prejudice implementation of any PSC technology.9,10 In fact, the concentration of lead in the currently working PSCs falls short of the limit adopted in all the countries that regulate the use of heavy metals in

Joule 1, 1–6, December 20, 2017 ª 2017 Elsevier Inc.

1

Please cite this article in press as: Abate, Perovskite Solar Cells Go Lead Free, Joule (2017), https://doi.org/10.1016/j.joule.2017.09.007

Figure 1. ABX3 Perovskite Crystal Structure A is an organic or inorganic cation filling the empty space in the middle of the unit cell highlighted in blue, B is a divalent metal depicted as a sphere in the corners of the unit cells in blue, and X is a halide depicted as purple sphere. The halide perovskite structure was reported for the first time by Weber in 1978.

electronics. For example, the European Union adheres to the ‘‘Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment,’’ RoHS1 in 2003 and the subsequent RoHS2 in 2011, which are also known as the ‘‘lead-free directives.’’11 The RoHS takes into account the more recent understanding of long-term risks associated with continuous exposure to low levels of toxic heavy metals. Specifically, it restricts to 0.1% in weight the maximum concentration of lead in each homogeneous material contained in any electronic devices, i.e., the perovskite within a PSC. Unfortunately, all the halide perovskites that have been so far demonstrated as effective photovoltaic materials contain more than 10% lead in weight, and are indeed banned from the European energy market.1 According to the same RoHS, the high content of cadmium should also prohibit CdTe from the European energy market. However, CdTe benefits from an exception of the RoHS directive, which will expire in 2018 if not renewed. While an additional ad hoc exception may qualify PSCs for the market, this auspice looks foolish as lead halide perovskites have a dramatically higher water solubility compared with CdTe.12 While many factors influence the effective risk of toxicity of a chemical, solubility in water is one of the most hazardous. In contact with water or humid air, lead halide perovskites form water-soluble compounds of lead, which can slowly, but inevitably, accumulate within the food chain and therefore into the human body, as schematically depicted in Figure 2. Extensive studies have documented the catastrophic impact, in particular

2

Joule 1, 1–6, December 20, 2017

1Helmholtz-Zentrum

Berlin fu¨r Materialien und Energie, Kekule´strasse 5, 12489 Berlin, Germany *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2017.09.007

Please cite this article in press as: Abate, Perovskite Solar Cells Go Lead Free, Joule (2017), https://doi.org/10.1016/j.joule.2017.09.007

Figure 2. Toxic Heavy Metals Entering the Food Chain (1) Industrial activates release metals in the environment. (2) Water transports metals into plants, which small fishes consume. (3) Larger fishes consume small contaminated fish. (4) Humans consume contaminated fish. Heavy metals accumulate up the food chain; thus, more in human than in any species lower down in the chain. Source: US Environmental Protection Agency.

for children, of only up to a few micrograms of heavy metals accumulated in the human body.13 Therefore, we have thoroughly addressed any risk associated with potential lead contamination beyond any regulation and economic argument. While a cautious design of PSC life cycle could reduce the risk of lead contamination, we aim for the ultimate solution of eliminating lead from PSCs. Current Research and Outlook toward Lead-free Perovskites Since the first demonstrations of lead halide perovskite as photovoltaic materials, several works have proposed alternative lead-free compositions.14–20 From computational studies, it is evident that the electronic configuration of Pb2+ comprising the ABX3 perovskites is responsible for their exceptional photovoltaic behavior. The isoelectronic s2p2 elements contained in group IV are thus the most obvious candidates to replace lead.21 To form 2+ ions, the elements in group IV need to keep the s2 pair and lose only their p electrons. The so-called inert s pair is more common in heavier elements such as lead, where the relativistic contraction stabilizes the s orbitals.22 Consequently, lead is more stable in a 2+, while the upper group elements such as tin and germanium are more stable in a 4+ oxidation state. Indeed, any attempts to prepare lead-free PSCs using Sn/Ge ABX3 perovskites have shown critical stability issues, which directly correlate with the Sn/Ge oxidation and the consequent formation of Sn/Ge lattice vacancies.23 One of the highest-performing lead-free Sn PSCs so far reported has been recently demonstrated by Zhao et al. using the mixed organic cation composition (FA)0.75(MA)0.25SnI3, which enabled achievement of an initial power conversion efficiency of around 8%.20 While there were no specific insights into the role of the organic cations, the authors convincingly showed that this particular mixed composition results in a more compact and uniform perovskite film, which seems important to enhance Sn2+ stability. We speculate that Sn2+ ions are more prone to oxidize when exposed at the surface than in the bulk of the perovskite crystals. Therefore, reducing the grain boundaries to prepare a more compact and uniform perovskite film will indirectly enhance the stability of Sn2+.

Joule 1, 1–6, December 20, 2017

3

Please cite this article in press as: Abate, Perovskite Solar Cells Go Lead Free, Joule (2017), https://doi.org/10.1016/j.joule.2017.09.007

Figure 3. Potential Lead-free Perovskites (A) Schematic showing the perovskite structure with two divalent metals (M 2+ : Ge, Sn, Pb) replaced by a combination of monovalent (M +: Na, K, Rb, Cu, Ag, Au, In, Tl) and trivalent (M 3+: Sb, Bi) metals. (B) Matrix displaying the calculated properties relevant for photovoltaic performances such as decomposition enthalpy (DH), band gap, carrier effective masses (m e *, m h *), and exciton binding energy (DE b ). Source: Zhao et al. 28

Together with controlling the perovskite film morphology, reducing atmosphere during the device preparation or additives in a completed device significantly inhibits Sn2+ oxidation. For example, Song et al. used vapors of hydrazine to lower the content of Sn4+ by about 20% during the perovskite film preparation,24 while Lee et al., Chung et al., and Kumar et al. added SnF2 to the perovskite precursor solution to obtain working tin-based PSCs.25–27 Both of these approaches look promising, and it would be interesting to explore them with the previously discussed morphology control. Combinations of monovalent and trivalent cations can replace divalent ones from group IV to form the so-called double perovskites, which exhibit extended three-dimensional structures resembling ABX3 perovskites, as schematically described in Figure 3A. This approach disclosed a potentially large number of new halide perovskites potentially suitable as photovoltaic material. Zhao et al.28 identified 64 new potential compounds, combining different monovalent and trivalent metals with halides, as summarized in Figure 3B. Among these, 11 compositions are theoretically stable and suitable for photovoltaics. However, only indium-based compositions, such as Cs2InSbCl6 and Cs2InBiCl6, seem to have the potential to achieve photovoltaic performances comparable with that of ABX3 perovskite. Whether these compounds can be effectively synthesized and to what extent they will work in PSCs remains thus far unknown and calls for more research along these lines.

4

Joule 1, 1–6, December 20, 2017

Please cite this article in press as: Abate, Perovskite Solar Cells Go Lead Free, Joule (2017), https://doi.org/10.1016/j.joule.2017.09.007

The Highest-Performing and Safe Perovskite Solar Cells May Be Tin Based We are only at the very early stage of the investigation of alternative lead-free perovskites. The PSC community have explored several approaches with encouraging preliminary results.23,29 Double perovskites constitute a theoretically large library of new materials, although a lack of experimental studies makes it difficult to predict their effective potential. On the other hand, tin-based ABX3 perovskites have been more extensively explored with gradually improving results. Tin perovskites offer the possibility to capture the optimum band gap for the highest power conversion efficiency, thus outperforming their lead-based analog. The stability of Sn2+ in a working device is a lasting challenge, but we believe that the current research activities are quickly moving toward a solution. More likely, this will encompass a combination of different approaches including: i. New perovskite formulations made by multiple cations and halides ii. Morphological control of the perovskite film aiming to reduce the detrimental effect of the grain boundaries iii. Use of reducing atmosphere during the material and device preparation iv. Chemical doping of the perovskite v. Passivating the perovskite surface vi. Exploring new selective contact materials vii. Encapsulation to ensure water and oxygen-free conditions during the whole lifetime of the device There remains the debated question about the effective lower toxicity of tin as compared with the lead analog ABX3 perovskite.30 Babayigit et al. clearly showed that the risk of toxicity of the halide perovskite is linked not only to the heavy metal but also to other side-degradation products released in water.9 They reported that the relatively high concentration of iodidric acid released by CH3NH3SnI3 in water is actually more lethal than lead released from the same amount of CH3NH3PbI3. Interestingly, they found that the toxicity of tin-based perovskite is not in any way linked to the presence of the heavy metal. In fact, Sn2+ rapidly oxidized to toxicologically inactive Sn4+ metal oxides. We believe this to be a key point in assessing the effective risk of toxicity associated with tin-based PSCs. Iodidric acid or other soluble side products released by CH3NH3SnI3 in water are certainly unhealthy but become toxic only upon direct exposure to high concentration. Given that most of these side products rapidly degrade into toxicologically inactive compounds, such a risk is effectively restricted only to persons directly working with PSCs, as often happens for many other technologies considered safe. From this perspective, the oxidative instability of tin perovskite may be an advantage: the potentially toxic component encapsulated in the solar cell will spontaneously degrade into toxicologically inactive compounds as soon as it comes into contact with air.

REFERENCES 1. Saliba, M., Correa-Baena, J.-P., Graetzel, M., Hagfeldt, A., and Abate, A. (2017). Perovskite solar cells from the atomic to the film level. Angew. Chem. Int. Ed. https://doi.org/10. 1002/anie.201703226.

3. Kojima, A., Teshima, K., Shirai, Y., and Miyasaka, T. (2009). Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 6050–6051.

2. Tzounis, L., Stergiopoulos, T., Zachariadis, A., Gravalidis, C., Laskarakis, A., and Logothetidis, S. (2017). Perovskite solar cells from small scale spin coating process towards roll-to-roll printing: optical and morphological studies. Mater. Today 4, 5082–5089.

4. Kim, H.-S., Lee, C.-R., Im, J.-H., Lee, K.-B., Moehl, T., Marchioro, A., Moon, S.-J., Humphry-Baker, R., Yum, J.-H., Moser, J.E., et al. (2012). Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2, 591.

5. Yang, W.S., Park, B.-W., Jung, E.H., Jeon, N.J., Kim, Y.C., Lee, D.U., Shin, S.S., Seo, J., Kim, E.K., Noh, J.H., et al. (2017). Iodide management in formamidiniumlead-halide–based perovskite layers for efficient solar cells. Science 356, 1376–1379. 6. Williams, S.T., Rajagopal, A., Chueh, C.-C., and Jen, A.K.Y. (2016). Current challenges and prospective research for upscaling hybrid perovskite photovoltaics. J. Phys. Chem. Lett. 7, 811–819.

Joule 1, 1–6, December 20, 2017

5

Please cite this article in press as: Abate, Perovskite Solar Cells Go Lead Free, Joule (2017), https://doi.org/10.1016/j.joule.2017.09.007

7. Lyu, M., Yun, J.H., Chen, P., Hao, M., and Wang, L. (2017). Addressing toxicity of lead: progress and applications of low-toxic metal halide perovskites and their derivatives. Adv. Energy Mater. 7, https://doi.org/10.1002/ aenm.201602512. 8. Ibn-Mohammed, T., Koh, S.C.L., Reaney, I.M., Acquaye, A., Schileo, G., Mustapha, K.B., and Greenough, R. (2017). Perovskite solar cells: an integrated hybrid lifecycle assessment and review in comparison with other photovoltaic technologies. Renew. Sustainable Energy Rev. 80, 1321–1344. 9. Babayigit, A., Thanh, D.D., Ethirajan, A., Manca, J., Muller, M., Boyen, H.-G., and Conings, B. (2016). Assessing the toxicity of Pband Sn-based perovskite solar cells in model organism Danio rerio. Sci. Rep. 6, 18721. 10. Benmessaoud, I.R., Mahul-Mellier, A.-L., Horva´th, E., Maco, B., Spina, M., Lashuel, H.A., and Forro´, L. (2016). Health hazards of methylammonium lead iodide based perovskites: cytotoxicity studies. Toxicol. Res. 5, 407–419. 11. European Parliament and Council (2003). Directive 2002/95/EC on the restriction of the use of certain hazardous substances in electrical and electronic equipment. Official J. Eur. Union 46, 19–23.

15. Noel, N.K., Stranks, S.D., Abate, A., Wehrenfennig, C., Guarnera, S., Haghighirad, A.-A., Sadhanala, A., Eperon, G.E., Pathak, S.K., Johnston, M.B., et al. (2014). Lead-free organic–inorganic tin halide perovskites for photovoltaic applications. Energ. Environ. Sci. 7, 3061–3068. 16. Hao, F., Stoumpos, C.C., Cao, D.H., Chang, R.P.H., and Kanatzidis, M.G. (2014). Lead-free solid-state organic-inorganic halide perovskite solar cells. Nat. Photon. 8, 489–494. 17. Hao, F., Stoumpos, C.C., Chang, R.P.H., and Kanatzidis, M.G. (2014). Anomalous band gap behavior in mixed Sn and Pb perovskites enables broadening of absorption spectrum in solar cells. J. Am. Chem. Soc. 136, 8094–8099. 18. Liao, W., Zhao, D., Yu, Y., Grice, C.R., Wang, C., Cimaroli, A.J., Schulz, P., Meng, W., Zhu, K., Xiong, R.G., et al. (2016). Lead-free inverted planar formamidinium tin triiodide perovskite solar cells achieving power conversion efficiencies up to 6.22%. Adv. Mater. 28, 9333– 9340. 19. Wang, F., Ma, J., Xie, F., Li, L., Chen, J., Fan, J., and Zhao, N. (2016). Organic cationdependent degradation mechanism of organotin halide perovskites. Adv. Funct. Mater. 26, 3417–3423.

12. Babayigit, A., Ethirajan, A., Muller, M., and Conings, B. (2016). Toxicity of organometal halide perovskite solar cells. Nat. Mater. 15, 247–251.

20. Zhao, Z., Gu, F., Li, Y., Sun, W., Ye, S., Rao, H., Liu, Z., Bian, Z., and Huang, C. (2017). Mixedorganic-cation tin iodide for lead-free perovskite solar cells with an efficiency of 8.12%. Adv. Sci. https://doi.org/10.1002/advs. 201700204.

13. van der Voet, E., Salminen, R., Eckelman, M., Norgate, T., Mudd, G., Hisschier, R., Spijker, J., Vijver, M., Selinus, O., Posthuma, L., et al. (2013). Environmental Challenges of Anthropogenic Metals Flows and Cycles (United Nations Environment Programme).

21. Meng, W., Wang, X., Xiao, Z., Wang, J., Mitzi, D.B., and Yan, Y. (2017). Parity-forbidden transitions and their impacts on the optical absorption properties of lead-free metal halide perovskites and double perovskites. J. Phys. Chem. Lett. 2999–3007.

14. Chen, Z., Wang, J.J., Ren, Y., Yu, C., and Shum, K. (2012). Schottky solar cells based on CsSnI3 thin-films. Appl. Phys. Lett. 101, 093901.

22. Drago, R.S. (1958). Thermodynamic evaluation of the inert pair effect. J. Phys. Chem. 62, 353–357.

6

Joule 1, 1–6, December 20, 2017

23. Konstantakou, M., and Stergiopoulos, T. (2017). A critical review on tin halide perovskite solar cells. J. Mater. Chem. A 5, 11518–11549. 24. Song, T.-B., Yokoyama, T., Stoumpos, C.C., Logsdon, J., Cao, D.H., Wasielewski, M.R., Aramaki, S., and Kanatzidis, M.G. (2017). Importance of reducing vapor atmosphere in the fabrication of tin-based perovskite solar cells. J. Am. Chem. Soc. 139, 836–842. 25. Chung, I., Lee, B., He, J., Chang, R.P.H., and Kanatzidis, M.G. (2012). All-solid-state dyesensitized solar cells with high efficiency. Nature 485, 486–489. 26. Kumar, M.H., Dharani, S., Leong, W.L., Boix, P.P., Prabhakar, R.R., Baikie, T., Shi, C., Ding, H., Ramesh, R., Asta, M., et al. (2014). Lead-free halide perovskite solar cells with high photocurrents realized through vacancy modulation. Adv. Mater. 26, 7122–7127. 27. Lee, S.J., Shin, S.S., Kim, Y.C., Kim, D., Ahn, T.K., Noh, J.H., Seo, J., and Seok, S.I. (2016). Fabrication of efficient formamidinium tin iodide perovskite solar cells through snf2– pyrazine complex. J. Am. Chem. Soc. 138, 3974–3977. 28. Zhao, X.-G., Yang, J.-H., Fu, Y., Yang, D., Xu, Q., Yu, L., Wei, S.-H., and Zhang, L. (2017). Design of lead-free inorganic halide perovskites for solar cells via cation-transmutation. J. Am. Chem. Soc. 139, 2630–2638. 29. Liao, Y., Liu, H., Zhou, W., Yang, D., Shang, Y., Shi, Z., Li, B., Jiang, X., Zhang, L., Quan, L.N., et al. (2017). Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance. J. Am. Chem. Soc. 139, 6693–6699. 30. Serrano-Lujan, L., Espinosa, N., Larsen-Olsen, T.T., Abad, J., Urbina, A., and Krebs, F.C. (2015). Tin- and lead-based perovskite solar cells under scrutiny: an environmental perspective. Adv. Energy Mater. 5, 1614–6840.