Despite decades of research into new technologies and materials for solar energy conversion, the vast majority of the photovoltaic cells in use are based on crystalline silicon wafers. With few exceptions, other proposed solar-cell designs incorporate rare and costly chemical elements, require expensive fabrication techniques, or convert little of the incident light to electricity. The perfect balance between low cost and high performance has remained elusive.

Solar cells based on a class of materials called organometal halide perovskites have recently and rapidly emerged as one of the most promising contenders yet. Last year, just four years after the cells’ debut, two groups independently reported perovskite solar cells with power-conversion efficiencies of 15%. (Commercial silicon-wafer cells are about 20% efficient.) There’s no obvious reason to suspect that that’s anywhere near their efficiency limit. But much remains unknown about why the perovskites work so well.

defect physics. 1 104, 063903 (2014). 1. W. Yin, T. Shi, Y. Yan, Appl. Phys. Lett., 063903 (2014). https://doi.org/10.1063/1.4864778 properties that may help shed new light on the cells’ performance and how they can be improved. Now Yanfa Yan, Wanjian Yin, and Tingting Shi, of the University of Toledo in Ohio, have used density functional theory to study the perovskites’physics.They’ve uncovered some unusualthat may help shed new light on the cells’ performance and how they can be improved.

High efficiency Section: Choose Top of page ABSTRACT High efficiency << Point defects Weathering the elements REFERENCES CITING ARTICLES perovskite is any material with the formula ABX 3 , where A and B are cations and X is an anion, and with the cubic crystal structure shown in figure 1 perovskite, the mineral CaTiO 3 , was discovered in the Ural Mountains and named for Russian mineralogist Lev Perovski.) In the solar-cell perovskites, A is a polyatomic organic cation, usually methylammonium (CH 3 NH 3 +); B is a large atomic ion, usually lead; and X is a halogen—either chlorine, bromine, iodine, or some combination of the three. is anywith the formula ABX, where A and B are cations and X is an anion, and with the cubic crystal structure shown in figurea. (The originalthe mineral CaTiO, was discovered in the Ural Mountains and named for Russian mineralogist Lev Perovski.) In the solar-cellA is a polyatomic organic cation, usually methylammonium (CHNH); B is a large atomic ion, usuallyand X is a halogen—either chlorine, bromine, iodine, or some combination of the three. All of those constituent elements are abundant. And the perovskites can be produced by solution processing, one of the cheapest methods available. Films of the iodide perovskite CH 3 NH 3 PbI 3 , for example, can be made from CH 3 NH 3 I and PbI 2 dissolved in a common solvent. perovskites were originally used as a replacement for the dye in a dye-sensitized solar cell, 2 et al. , J. Am. Chem. Soc. 131, 6050 (2009). 2. A. Kojima, J. Am. Chem. Soc., 6050 (2009). https://doi.org/10.1021/ja809598r 3 353, 737 (1991). 3. B. O’Regan, M. Grätzel, Nature, 737 (1991). https://doi.org/10.1038/353737a0 1 perovskite coats a mesoporous film of titanium dioxide. Absorbed sunlight excites the perovskite’s electrons, which are then injected into the TiO 2 conduction band and conveyed to the cell’s anode (made from the optically transparent but electrically conducting fluorine-doped tin oxide). The holes left behind are transported to the cathode. Thewere originally used as a replacement for the dye in a dye-sensitizeda general solar-cell architecture first described in 1991 by Brian O’Regan and Michael Grätzel.As shown in figureb, a thin layer ofcoats a mesoporousof titanium dioxide. Absorbed sunlight excites the perovskite’s electrons, which are then injected into the TiOconduction band and conveyed to the cell’s anode (made from the optically transparent but electrically conducting fluorine-doped tin oxide). The holes left behind are transported to the cathode. perovskite solar cells, as in the original dye-sensitized cells, the hole-transporting material was a liquid electrolyte solution; more recent devices employ an organic semiconductor. In July 2013 Grätzel and his group at the Swiss Federal Institute of Technology in Lausanne reported a 15% power-conversion efficiency from an iodide perovskite dye-sensitized cell. 4 et al. , Nature 499, 316 (2013). 4. J. Burschka, Nature, 316 (2013). https://doi.org/10.1038/nature12340 In the firstas in the original dye-sensitized cells, the hole-transportingwas a liquid electrolyte solution; more recent devices employ an organic semiconductor. In July 2013 Grätzel and his group at the Swiss Federal Institute of Technology in Lausanne reported a 15% power-conversion efficiency from an iodidedye-sensitized cell. charge carriers. The perovskite layer is so thin—between 2 nm and 10 nm—that its charge-transport properties don’t come into play. But in 2012 Henry Snaith and colleagues at Oxford University reported on a cell they’d made in which the mesoporous TiO 2 was replaced with mesoporous alumina, an electrical insulator. 5 et al. , Science 338, 643 (2012). 5. M. M. Lee, Science, 643 (2012). https://doi.org/10.1126/science.1228604 2 O 3 just served as a scaffold to support the perovskite layer (Snaith and company used the mixed halide perovskite CH 3 NH 3 PbI x Cl 3−x ), and electrons had to travel to the anode through the perovskite itself. Not only did the device still work, but its efficiency was slightly improved over the equivalent TiO 2 cell. In that design, the perovskite’s only function is to absorb light and produceThelayer is so thin—between 2 nm and 10 nm—that its charge-transportdon’t come into play. But in 2012 Henry Snaith and colleagues at Oxford University reported on a cell they’d made in which the mesoporous TiOwas replaced with mesoporous alumina, an electrical insulator.The Aljust served as a scaffold to support thelayer (Snaith and company used the mixed halideCHNHPbICl), and electrons had to travel to the anode through theitself. Not only did the device still work, but its efficiency was slightly improved over the equivalent TiOcell. thin-film architecture shown in figure 1 film of mixed-halide perovskite. 6 501, 395 (2013). 6. M. Liu, M. B. Johnston, H. J. Snaith, Nature, 395 (2013). https://doi.org/10.1038/nature12509 film by vapor deposition rather than the cheaper solution processing because of the difficulty in creating a uniform flat film by solution-based methods. 7 et al. , Adv. Funct. Mater. 24, 151 (2014). 7. G. E. Eperon, Adv. Funct. Mater., 151 (2014). https://doi.org/10.1002/adfm.201302090 That success raised the possibility of eliminating the mesoporous layer entirely and using the simpler, potentially cheaperarchitecture shown in figurec. And Snaith and colleagues did just that: In September 2013 they reported a 15%-efficiency cell with a 330-nm-thickof mixed-halideThey’d made theby vapor deposition rather than the cheaper solution processing because of the difficulty in creating a uniform flatby solution-based methods. material to be able to transport charge carriers more than 10 nm, let alone 330. Grätzel (in collaboration with Tze Chien Sum of Nanyang Technological University in Singapore) and Snaith both investigated the perovskite charge-transport properties more directly; they found transport lengths of about 100 nm for the iodide perovskite and a stunning 1 µm for the mixed halide perovskite. 8 et al. , Science 342, 341 (2013); et al. , Science 342, 344 (2013). 8. S. D. Stranks, Science, 341 (2013); https://doi.org/10.1126/science.1243982 G. Xing, Science, 344 (2013). https://doi.org/10.1126/science.1243167 materials so good? And how can the devices be made better? It’s unusual for a solution-processableto be able to transportmore than 10 nm, let alone 330. Grätzel (in collaboration with Tze Chien Sum of Nanyang Technological University in Singapore) and Snaith both investigated thecharge-transportmore directly; they found transport lengths of about 100 nm for the iodideand a stunning 1 µm for the mixed halideWhat makes theso good? And how can the devices be made better?

Point defects Section: Choose Top of page ABSTRACT High efficiency Point defects << Weathering the elements REFERENCES CITING ARTICLES Yan and colleagues had been studying the defect physics of other promising thin-film solar-cell materials, such as cadmium telluride and copper indium gallium selenide. Thin films of those materials are full of point defects that create electron energy levels near the middle of the semiconductor bandgap. Charge carriers that encounter those defects can lose energy, and electrons and holes can recombine, both of which hinder device performance. As Yin explains, “Our previous understanding of inorganic solar-cell materials led us to believe that the halide perovskites must exhibit unusual defect properties.” There are only so many ways to make a point defect in a crystalline material: A lattice site can be vacant; an extra, or interstitial, ion can be present between lattice sites; or one ion can take the place of another. Focusing on the iodide perovskite (because the locations of I and Cl in the mixed-halide perovskite aren’t completely known), the researchers systematically looked at the 12 possible point defects. For each, they sought to calculate both the defect’s formation energy and the electronic energy levels that it creates. defect depends on the chemical potential μ of each of the constituent ions during film growth; the chemical potential depends in turn on the ion’s concentration or partial pressure. Stable growth of the iodide perovskite means that the chemical potentials of its constituent ions add up to the formation energy of the perovskite, so once two of the chemical potentials are specified, the third is known. Furthermore, large swaths of the chemical potential space shown in figure 2 2 or CH 3 NH 3 I—forms before the perovskite does. That leaves a narrow, effectively one-dimensional range of perovskite growth conditions. The formation energy of adepends on theof each of the constituent ions duringgrowth; thedepends in turn on the ion’s concentration or partial pressure. Stable growth of the iodidemeans that theof its constituent ions add up to the formation energy of theso once two of theare specified, the third is known. Furthermore, large swaths of thespace shown in figureare excluded because another phase—either PbIor CHNHI—forms before thedoes. That leaves a narrow, effectively one-dimensional range ofgrowth conditions. Yan and colleagues did their defect calculations at several points along that range. They found that wherever they looked, all the readily formed defects—such as Pb vacancies or interstitial methylammonium ions—created states with energies at the edges of the bandgap. Defects that create states near the middle of the bandgap—such as interstitial Pb ions or one ion substituting for another of the opposite charge—all had prohibitively high formation energies. That’s an unusual coincidence among materials, but it offers an explanation for why the perovskites can conduct so well even when they’re riddled with defects. And based on what’s already known about perovskite conductivity, the result wasn’t unexpected. More surprising, though, was the finding that at one end of the range of formation conditions, all the readily formed defects produced states at the bottom of the bandgap, and at the other end, all the readily formed defects produced states at the top of the bandgap. That is, depending on formation conditions, the perovskite can have either p-type or n-type conductivity. If that variation can be better harnessed, it could pave the way for new device designs.