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| Fig. 1: Crystal structure of methylammonium lead halide. [12] Methylammonium in the center is surrounded by lead (blue) halide. (Source: Wikimedia Commons) |
As the world economy grows and we approach the exhaustion of fossil fuels, renewable energy resources must be invested in with great priority. Current investment in alternative energy resources is at an all-time high, but more investment will be necessary to compete with the ease of fossil fuel usage. [1] To meet these demands for new alternative energy resources, implementation of solar technology in society has become a promising avenue of investment, as solar technology represented the large share of added power capacity worldwide in 2017. [2] Recently, a new class of materials called perovskites has provided a new impetus towards high-efficiency solar cells, as a possible successor to the silicon-based photovoltaic cell. [3] They are also easily made in solution, as opposed to costlier methods of silicon-based technology, making them attractive from an industrial standpoint. [3] These advancements, however, belie the fundamental issue of their lead composition, which could prohibit their development due to toxicity concerns.
The term perovskite describes only a crystal structure, with ABX3, where A is usually an alkaline or rare-earth metal, B is a transition metal, and X is typically Cl, Br, or I in halide perovskites. The cation centers (B) can be placed with more specialized elements for desired physical properties, but lead (Pb2+) as been found to be essential for the unique semiconductor properties of lead halide perovskites (Fig. 1). [4] Importantly, the geometrical peculiarities of lead halide perovskites make them unique for light-matter interactions, as lattice modes (phonons) interact strongly with optical modes (light), thus producing an energy transfer that kicks electrons into the conduction band. [5] In methylammonium lead halide, for instance, this unique geometry leads to long carrier (conduction electron) lifetimes, likely through large polaron formation and Coulomb shielding (Fig. 1). [5,6] Furthermore, these conductive properties depend only weakly on temperature, indicating a highly robust process independent of any thermal failure for device applications. [5]
In 2009, it was discovered that organometal halide perovskites were successful in light-conversion processes at device scale. [7] In the following 10 years, significant advancements have been made in producing photovoltaic cells based upon lead-halide perovskites, increasing from less than 10% efficiency in 2010 to greater than 20% by 2018 (Fig. 1). [4] Construction depends on using perovskites as photo-absorbers sandwiched between n- and p-type semiconductors, and have already seen initial production in China. [4] There are limits, however, to the operational efficiency of any semiconductor-based solar cell, as described by the Shockley-Queisser limit, which gives a theoretical maximum of ~30% efficiency from photon energy in a single-junction semiconductor, thus relying on mutli-junction designs for improved efficiency in solar cell devices. [8,9] While its taken silicon-based solar cells to reach ~20% efficiency, recent (and quick) advancements in perovskite technology already have reached this level (Fig. 1).
While the unique properties of lead-halide perovskites make them amenable for solar cell applications, there lies a glaring concern that of their soluble lead properties. Elemental lead is very harmful to human biology, acting as a toxin upon the nervous system, resulting in especially detrimental effects during development. [10] To this point, these materials are made in solution, and long-term stability remains an issue when considering environmental effects on degradation, such as moisture, light exposure, and wind. Some tests have been conducted on the stability of these devices, but much remains to be done to standardize such tests. [4,10]
Currently, much research is devoted to exploring perovskite variants and lead replacements that can circumvent the toxicity issue. Recently, it was demonstrated that titanium (IV) could be used as a replacement for lead, but such devices have so far only demonstrated a 3.3% efficiency. [11] With this in mind, it must be made clear that while high- efficiency solar cells are promising, their impact on the environment and human health remain under serious consideration, thus making perovskite solar technology a long way from realization.
© Benjamin Knapp. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] E. Porter "Wind and Solar Power Advance, but Carbon Refuses to Retreat," New York Times, 7 Nov 17.
[2] S. Sengupta, "Solar Power Is Burning Bright. But Its Hardly Twilight for Fossil Fuels," New York Times, 5 Apr 18.
[3] J. Worland, "Inside the New Technology That Could Transform the Solar Power Industry," Time, 4 Jun 18.
[4] Y. Rong et al., "Challenges for Commercializing Perovskite Solar Cells," Science 361, eaat8235 (2018).
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[6] J. S. Manser, J.A. Christians, and P. V. Kamat, "Intriguing Optoelectronic Properties of Metal Halide Perovskites," Chem. Rev. 116, 12956 (2016).
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[9] G. E. Eperon, M. T. Hörantner, and H. J. Snaith, "Metal Halide Perovskite Tandem and Multiple-Junction Photovoltaics," Nat. Rev. Chem. 1, 0095 (2017).
[10] Q. Zhang et al., "Perovskite Solar Cells: Must Lead Ee Replaced - and Can It Be Done?," Sci. Technol. Adv. Mat. 19, 425 (2018).
[11] M. Chen et al., "Cesium Titanium(IV) Bromide Thin Films Based Stable Lead-free Perovskite Solar Cells," Joule 2, 558 (2018).
[12] C. Eames et al., "Ionic Transport in Hybrid Lead Iodide Perovskite Solar Cells," Nat. Commun. 6, 7497 (2015).
