Chemistry Top Ten: The "Light" Side of Lead

March 2015

Lead is a metal that we want to get rid of; it damages our health and the environment. For many years it has been of little interest to chemists. Now it appears to have found a role that promises to benefit us all by reducing our dependency on fossil fuels. It does this by boosting the efficiency of solar panels and is highlighted in the current Chemistry Top Ten, in the adjoining table.

Eight of the papers in the table are about chemicals which can absorb solar energy, and lead features in four of them (#1, #3, #4 and #6). It is referred to as organometallic perovskite (OMP), which can either be methylammonium lead bromide (CH3NH3PbBr3) or the corresponding iodide (CH3NH3PbI3). These organo-lead molecules adopt a crystal structure known as perovskite. This term describes a particular array of atoms that can confer remarkable properties on a material, such as superconductivity in the case of barium yttrium cuprate Ba2YCu2O7. In OMP the structure has the ability to trap solar energy. The name perovskite comes from a calcium titanium oxide mineral which was discovered in the Urals in 1839 by Lev Perovski.

This effect of OMP was reported in 2009 by a group of Japanese chemists headed by Tsutomu Miyasaka of the University of Tokyo. Their definitive paper, entitled “Organometal halide perovskites as visible-light sensitizers for photovoltaic cells,” was published in Journal of the American Chemical Society, 131 (17), 6050-6051, 2009. They had deposited a layer of OMP on to a layer of titanium dioxide (TiO2) in such a cell, and it generated a voltage of around 1.0 V with an energy conversion efficiency of 3.8%. This groundbreaking paper has since been cited 308 times.

What’s Hot in Chemistry

Rank Paper Citations This Period (Jul-Aug 14) Rank Last Period (May-Jun 14)
1 J. Burschka, et al., “Sequential deposition as a route to high-performance perovskite-sensitized solar cells,” Nature, 499 (7458): 316-9, 18 July 2013. [Swiss Fed. Inst. Technol., Lausanne; Max Planck Inst. Solid State Res., Stuttgart, Germany] 97 2
2 J.B. You, et al., “A polymer tandem solar cell with 10.6% power conversion efficiency,” Nature Communications, 4: no. 1446, 5 February 2013. [Univ. California, Los Angeles; Sumitomo Chem. Co. Ltd, Tsukuba, Japan; Natl. Renewable Energy Lab., Golden, CO] 69 1
3 G.C. Xing, et al., “Long-range balanced electron- and hole-transport lengths in organic-inorganic CH3NH3PbI3,” Science, 342 (6156): 344-7, 18 October 2013. [5 Singaporean and European institutions] 34 3
4 L. Etgar, et al., “Mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells,” J. Amer. Chem. Soc., 134 (42): 17396-9, 24 October 2012. [Swiss Fed. Inst. Technol., Lausanne; Natl. Univ. Singapore; Hebrew Univ. Jerusalem, Israel] 31 +
5 O. Lopez-Sanchez, et al., “Ultrasensitive photodetectors based on monolayer MoS2,” Nature Nanotechnology, 8 (7): 497-501, July 2013. [Swiss Fed. Inst. Technol. Lausanne] 28 6
6 N.G. Park, et al., “Organometal perovskites light absorbers toward a 20% efficiency low-cost solid-state mesoscopic solar cell,” J. Phys. Chem. Lett., 4 (15): 2423-9, 1 August 2013. [Sungkyunkwan Univ., Suwon, South Korea] 26 +
7 H. Uoyama, et al., “Highly efficient organic light-emitting diodes from delayed fluorescence,” Nature, 492 (7428): 234-8, 13 December 2012. [Kyushu Univ., Fukuoka, Japan.] 24 7
8 J.B. You, et al., “10.2% power conversion efficiency polymer tandem solar cells consisting of two identical sub-cells,” Advanced Materials, 25 (29): 3973-8, 7 August 2013. [Univ. California, Los Angeles; Sumitomo Chem. Co. Ltd., Tsukuba, Japan] 23 +
9 C.Z. Yuan, et al., “Ultrathin mesoporous NiCo2O4 nanosheets supported on Ni foam as advanced electrodes for supercapacitors,” Advanced Functional Materials, 22 (21): 4592-7, Nov. 7, 2012. [Anhui Univ. Technol., China; Nanyang Technol. Univ., Singapore; Nanjing Univ. Aeronaut. & Astronaut, China ] 22 +
10 S. Mathew, et al., “Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers,” Nature Chemistry, 6 (3): 242-7, March 2014. [Swiss Fed. Inst. Technology, Lausanne] 22 +

SOURCE: Thomson Reuters Web of Science

NB. Only papers indexed by Thomson Reuters since September 2012 are tracked. The “+” sign indicates that the paper was not ranked in the Top Ten during the last period. In the event that two or more papers collected the same number of citations in the most recent bimonthly period, total citations to date determine the rankings

Paper #1 in the Hot Ten comes from the group headed by Michael Grätzel of the École Polytechnique Fédérale de Lausanne (Swiss Federal Institute of Technology, Lausanne) in Switzerland, which is now a leading center for solar energy thanks to his inspired research. Grätzel made the first dye-based/TiO2 solar cell back in 1991. His paper reporting this appeared in Nature [353 (6346), 737-740] and has since been cited more than 13,000 times.

Now he has brought his expertise to bear on the problems that affect the formation of OMP when this is deposited in a single step on to the TiO2 film. The normal procedure for doing this is to use a mixture of lead iodide and methylammonium iodide, but the photovoltaic performance of the resulting cell is somewhat variable. What Grätzel has done is to introduce the lead iodide solution first, followed by the ammonium iodide solution. The resulting cell gives more reproducible results with a power conversion of 15%.

Grätzel’s influence is to be seen in another paper in the Hot Ten, namely #3. This comes as a result of collaboration with scientists at Nanyang Technology University in Singapore headed by Guichuan Xing. There they sought to answer the question of why OMP photovoltaics perform so well. They deduce that this is due to the lengths of the diffusion paths along which the charge-carrying entities, the electrons and the holes, have to move once a photon has activated them. What makes OMPs different is that these paths are long and well-matched to one another and to the 100 nanometer optical-absorption path of the material itself.

Another paper (#4) involves the Lausanne scientists collaborating with Zhaosheng Xue and Bin Lui of the University of Singapore, and Lioz Etgar of the Hebrew University of Jerusalem. It deals with a device made by depositing OMP from a solution of methylammonium iodide and lead iodide in γ-butyrolactone, and then covering it with a thin gold film. Power conversion was 5.5% under normal light conditions, rising to 7.3% under lower intensity light.

The fourth OMP paper in the Hot Ten (#6) comes from chemist Nam-Gyu Park of Sungkyunkwan University, South Korea. He has reviewed recent progress in this field and concluded that a power conversion efficiency of 20% should be possible. Recently his group have published their achievements in a paper in Nano Letters, [13 (6), 2412-2417, 2013]. In this they report a power conversion efficiency of 9.4%. Their work focused on controlling the growth of the sub-surface TiO2 nanorods; the longer these rods, the less efficient was the solar cell.

While OMP seems to be dominating the current citations listings, there are other solar chemistry papers in the Hot Ten which merit attention—in particular, the high-ranking paper #2. This is the result of collaborative work between academia and industry. Its lead authors are Jingbi You, of the University of California Los Angeles, and Letian Dou, of the Sumitomo Chemical Company of Tsukuba City, Japan. Also involved were Tom Moriarty and Keith Emery of the National Renewable Energy Laboratory, Golden, Colorado.

The paper reports easy-to-make organic photovoltaics which cost little to produce and are lightweight and flexible. It says that the efficiency of so-called tandem solar cells can be improved by making use of a broader part of the spectrum down to the near infrared region (800-900 nanometres) with a notable 10.6% power conversion and it is based on polymers with a low band-gap (1.38 eV). The semiconducting polymers are polycyclopenta-dithiophene (PCPDT) and benzothiadiazole (BT) which were modified by the attaching of fluorine atoms to the BT moiety. This paper was featured as a Hot Paper in Chemistry in May 2014, having accumulated 124 citations at that time; now the total is more than 470.

Paper #8 comes from the same group and reports a solar cell with similar conversion efficiency and this has two layers of polymer between which is sandwiched an interconnecting layer based on molybdenum oxide. This achieves an efficiency of 10.2%.

Paper #10 also reports improved solar efficiency, in this case for dye-sensitized solar cells whose performance is increased to 13% by the use of a specially-engineered porphyrin dye. This paper comes from groups headed by Simon Mathew and Aswanti Yella of the Swiss Federal Institute of Technology at Lausanne. An earlier paper on a porphyrin-containing cell that achieved 12% efficiency was published in Science [334 (6056), 629-634, 4 November 2011] and was highlighted as a Hot Paper in Chemistry in April 2014.  This work involved collaboration with researchers at the National Chiao Tung University of Hsinchu, Taiwan, and the paper then had a citations total of 1,077. This now exceeds 2,200.

Chemists are turning their hands to solving one of the world’s most pressing problems: generating enough sustainable electricity without the need to burn fossil carbon. Can we really devise solar cells that are cheap to make and with efficiencies of 20%? The answers may come from the use of that much-maligned metal, lead. (It must be acknowledged, however, that preliminary research from 2014 has pointed to tin as a possible replacement for lead in solar cells, perhaps ultimately circumventing the problem of lead’s toxicity.)

So is it now “come back, all is forgiven” for lead? Have we misjudged it all these years? Clearly not when it contaminates our water supply, our food, and the air we breathe. However, when it comes to alternative energy, perhaps it can redeem itself.

Dr. John Emsley is based at the Department of Chemistry, Cambridge University, UK.

The data and citation records included in this report are from Thomson Reuters Web of ScienceTM. Web of ScienceTM is a registered trademark of Thomson Reuters. All rights reserved.