Fuelling the Future with PTB and PCBM

What's Hot in Chemistry 2011

by John Emsley

The chemistry of carbon continues to dominate the current Hot Ten, mainly in the form of carbenes, but one entrant, at #5, is concerned with putting the element to use in the form of polymeric semiconductors. These have the necessary chemistry for organic photovoltaic (OPV) materials for use in solar cells. They require both an electron-rich polymer and an electron-deficient fulleride. Poly(3-hexylthiophene) has proved suitable as the polymer, and this in combination with a C61 fulleride derivative can deliver an energy conversion of 5%.

Paper #5, the result of collaborative research by groups headed by Luping Yu at the University of Chicago, and Gang Li at Solarmer Energy Inc. of El Monte, California, reveals how this was increased to 6% or more.

In the previous issue of Science Watch (March/April 2011), a later paper of Yu and Li’s entered the Hot Ten at #8 and reported an even higher efficiency of more than 7% for the product combination PTB7/PC71BM. The paper is now at #3 in the current list, making Yu one of the few chemists to have two papers in the list at the same time.

Paper #5 describes the synthesis of polymers with alternate thienothiophene and benzodithiophene units along the chain, and these are given the acronym PTB.

Six chemically modified derivatives of PTB were made with various side-chains attached to the carboxylic acid group of the thienothiophene moiety, such as n-dodecyl and 2-ethylhexyl, while groups such as n-octyloxy and 2-ethylhexyloxy were bonded to the benzodithiophene ring of the polymer backbone. In one case a fluorine atom was also attached to the thienolthiophene unit. The object of the research was to adjust the electron density of the polymer.

Luping Yu "Luping Yu Discusses Organic Photovoltaic Research," Fast Breaking Papers commentary, Feb. 2011.

There were limits to what could be achieved because the length and branching of the side-chains had a significant effect on solubility, making it difficult to prepare uniform polymer films in some cases, such as with n-octyl groups. The polymer structures were characterized by means of 1H NMR and their stability assessed with thermogravimetric analysis, which showed they were stable up to 200 °C.

The polymers were then tested in solar cells in combination with the fulleride [6,6]phenyl-C61-butyric acid methyl ester, a.k.a. PC61BM, and charge-transport behavior was measured using conductive atomic force microscopy. Of the several combinations which Lu tried, the best results were obtained with one labeled PTB4 which has an n-octyl ester group and two 2-ethylhexyloxy groups on the benzene ring. This gave a power conversion efficiency of more than 6%.

Yu and Li’s work continues to show impressive results, and other polymers have subsequently exceeded 8%, says Yu. Currently Yu is concentrating on modifying the polymer system and using this as a platform to develop new derivatives for fine-tuning the polymer properties. [Note: for more on polymer solar cells, see the interview with Yu and Li’s frequent collaborator, Yang Yang of UCLA, in this issue.]

Yu: "Our results point to a promising future for OPV research and reinforce the notion that OPV cells will be a vital alternative to using inorganic materials."

Yu tells Science Watch that his goal is ultimately to understand the structure/property relationships as a way to achieve higher efficiency in solar energy conversion. His new designs focus on improving semiconductor behavior by both decreasing the bandgap, and thereby boosting light harvesting, and by increasing charge-transfer by extended pi-conjugated units. His latest work is reported in Journal of the American Chemical Society (F. He, et al., DOI: 10.1021/ja1110915, 2011).

Yu’s team has also been studying the possible photochemical reactions in well-designed polymer systems, providing more stability to the solid state order to increase device lifetime as well as synergistically combining these efforts to enhance solar cell performance. (See H. Son, et al., J. Am. Chem. Soc., 133[6]: 1885-94, 25 January 2011.)

There are problems still to be overcome and Yu acknowledges this: "To achieve true practical applications for OPV solar cells, challenges still exist if we are to push the solar efficiency beyond 10% and achieve long-term stability.  Although we are only less than 2% shy of reaching this goal with our system, it may be the highest hurdle to overcome." Yu admits that radically new ideas might be needed to achieve this goal but he says they are actively exploring different options.End

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

Click the tab above to view Hot Papers.
What's Hot in Chemistry
Rank Paper Cites This Period
Nov-Dec 10
Rank Last Period
Sep-Oct 10
X.S. Li, et al., "Large-area synthesis of high-quality and uniform graphene films on copper foils," Science, 324(5932): 1312-4, 5 June 2009. [U. Texas, Austin]     *453TF 52 1
A. Reina, et al., "Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition," Nano Letters, 9(1): 30-5, January 2009. [MIT, Cambridge]  *395IZ 46 2
3 Y.Y. Liang, et al., "For the bright future—Bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%," Adv. Materials, 22(20): E135-8, 25 May 2010.  [U. Chicago, IL; Solarmer Energy Inc., El Monte, CA]   *612IK 37 8
D.C. Elias, et al., "Control of graphene’s properties by reversible hydrogenation: Evidence for graphane," Science, 323(5914): 610-3, 30 January 2009. [U. Manchester, U.K.; Inst. Microelectronics Tech., Chernogolovka, Russia; Cambridge U., U.K.; U. Nijmegen, Netherlands]   *400JB  36 3
Y.Y. Liang, et al., "Highly efficient solar cell polymers developed via fine-tuning of structural and electronic properties," J. Am. Chem. Soc., 131(22): 7792-9, 10 June 1009. [U. Chicago, IL; Solarmer Energy Inc., El Monte, CA]   *460HD 30
6 D.V. Kosynkin, et al., "Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons," Nature, 458(7240): 872-6, 16 April 2009.  [Rice U., Houston, TX] *433CS 26 6
7 B. Lim, et al., "Pd-Pt bimetallic nanodendrites with high activity for oxygen reduction," Science, 324(5932): 1302-5, 5 June 2009.  [Washington U., St. Louis, MO; Brookhaven Natl. Lab., Upton, NY]  *453TF 25 9
L.Y. Jiao, et al., "Narrow graphene nanoribbons from carbon nanotubes," Nature, 458(7240): 877-80, 16 April 2009. [Stanford U., CA] *433CS 23 5
9 X.R. Wang, et al., "N-doping of graphene through electrothermal reactions with ammonia," Science, 324(5928): 768-71, 8 May 2009. [Stanford U., CA; U. Florida, Gainesville; Lawrence Livermore Natl. Lab., CA]   *442HN 20
10 S.M. Paek, E.J. Yoo, I. Honma, "Enhanced cyclic performance and lithium storage capacity of SnO2/ graphene nanoporous electrodes with three-dimensionally delaminated flexible structure," Nano Letters, 9(1): 72-5, January 2009.  [AIST, Tsukuba, Japan]  *395IZ 19
SOURCE: Thomson Reuters Hot Papers Database. Read the Legend.




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