Hot Research on Organic Solar Cells: Polymers vs. Small Molecules vs. Hybrids

December 2013
REUTERS/Michaela Rehle

Trapping the energy of the Sun offers the world a limitless source of electricity. What might curtail our access to this abundant energy is the scarcity of rare elements that are currently needed for solar panels, such as gallium and indium. These will be depleted by the end of this century. On the other hand, organic solar cells (OSCs) are made from sustainable resources. But what kind of organic material should we use? Some favor organic polymers, others favor simple molecules, and yet others favor a hybrid combination with silicon, which will always be available.

Searching Thomson Reuters Web of Science for papers on OSCs published in the past two years produced 1,240 titles, with the 10 most cited already having collected 40 or more citations each (see table below). Top of the list are two reviews: #1 is from Huaxing Zhou, Ligiang Yang and Wei You of the Department of Chemistry at the University of North Carolina, and deals with polymer cells, while #2 comes from Peter Bäuerle and Amaresh Mishra of the University of Ulm, Germany, and concerns small-molecule solar cells.

 

Organic Solar Cells:
Ten Highly Cited Papers, 2012-2013

Rank Paper Citations
1 H. Zhou, et al., “Rational design of high performance conjugated polymers for organic solar cells,” Macromolecules, 2012(45), 607-32, 2012. [U. North Carolina, Chapel Hill] 194
2 A. Mishra, P. Bauerle, “Small molecule organic semiconductors on the move: promises for future solar energy technology,” Ang. Chemie Int. Ed., 51(9), 2020-67, 2012. [U. Ulm, Germany] 173
3 X. Li, et al., “Dual plasmonic nanostructure for high performance inverted organic solar cells,” Adv. Mater., 24 (22), 3046-52, 2012. [U. Hong Kong; Inst. of Chemistry, Beijing, China; U. Calif., Los Angeles] 134
4 J. Zhou, et al., “Small molecules based on benzol[1.2-B:4,5-b’]dithiophene unit for high-performance solution-processed organic solar cells,” J. Am. Chem. Soc., 134 (39), 16345-51, 2012. [Nankai U., Tianjin, China] 64
5 T.S. van der Poll, et al., “Non-basic high-performance molecules for solution-processed organic solar cells,” Adv. Mater., 24(27), 3646-9, 2012. [U. Calif., Santa Barbara] 62
6 C. Duan, et al., “Recent development of push-pull conjugated polymers for bulk-hetero-junction photovoltaics: rational design and fine tailoring of molecular structure,” J. Mater. Chem., 22(21), 10416-34, 2012. [South China U. Technology, Guangzhou, China,] 58
7 F.C. Jamieson, et al., “Fullerene crystallisation as a key driver of charge separation in polymer/fullerene bulk heterojunction solar cells,” Chem. Sci., 3(2), 485-92, 2012. [Imperial College London, UK.] 55
8 S. Jeong, et al., “Hybrid silicon nanocone-polymer solar cells,” Nano Lett., 12(6), 2971-6, 2012. [Stanford U. and SLAC National Accelerator Lab., CA] 48
9 M. Kaltenbrunner, et al., “Ultrathin and lightweight organic solar cells with high flexibility,”Nature Commun., 3: No. 770, 2012. [Johannes Kepler U., Linz, Austria; U. of Tokyo, Japan.] 46
10 M. Vosgueritchian, et al., “Highly conductive and transparent PEDOT:PSS films with a fluorosurfactant for stretchable and flexible transparent electrodes,” Adv. Functional Mater., 22 (2), 421-8, 2012. [Stanford U., CA] 40
Source: Thomson Reuters Web of Science

 

The performance of OSCs can be greatly enhanced by the inclusion of traces of metals. Paper #3, from a team led by Wallace Choy of the University of Hong Kong and Jianhui Hou of the Chinese Academy of Science, Beijing, shows just what effect gold nanoparticles can have on an organic polymer cell, raising the energy conversion to around 9%, but still well below that of current solar cells.

Polymer-based solar cells offer cheapness, lightness, flexibility, and sustainability, but are prone to photochemical degradation. Of the cells’ two key components, the electron donor is the polymer with its delocalized π electrons, while the electron acceptor is typically fullerene,  and these are sandwiched between two electrodes.

An alternative to polymer-based OSCs has emerged in the form of small-molecule solar cells (SMSCs). An example of this comes from the group led by Yongsheng Chen of Nankai University, Tianjin, China, as reported in paper #4. This work shows that small molecules based on benzodithiophene (BDT) might be just as good as polymers, and indeed have some advantages, not least of which is their well-defined structure which results in less batch-to-batch variation. These molecules offer excellent film-formation ability, have wide and efficient energy absorption, a planar structure which leads to good hole mobility, and they are soluble and stable chemically. Chen reports on various terminal groups attached to BDT, such as 3-ethylrhodanine, and devices based on this molecule and the fullerene-containing phenyl-C61-butyric acid (PC61BM) were able to achieve energy efficiencies of 7%. A more recent paper of theirs concerns solution-processed SMSCs and was published in the Journal of the American Chemical Society (J. Zhou, et al., 35[23], 8484-7, 2013).

Small molecules and solution processing is also the subject of paper #5, which comes from the University of California, Santa Barbara. Thuc-Quyen Nguyen and her group focused on the molecule p-DTS(FBTTh2)2. (DTS is short for dithienosilole, and FBTTh2 for fluorobenzothiazole.) This molecule has electron-poor regions which are not prone to attracting protons, and this enables it to be used as interlayers between the organic semiconductor and the hole-collecting electrodes. The cells have efficiencies of 7%.

Polymer-based OSCs which incorporate so-called push-pull polymers have been reviewed in #6 by Yong Cao and colleagues, whose own research involves two-dimensional conjugated copolymers (see C. Duan, et al., Sci. China Chem., 54[4]: 685-94, 2011). This work entailed the construction of polymers based on alternating fluorene and triphenylamine units to which various other groups were attached. These were then incorporated into solar cells and in one case attained a modest efficiency of almost 3%.

Polymer/fullerene blend films are the focus of paper #7 from James Durrant’s group at Imperial College London, which is the leading European center for research into polymer OSCs. In #7, Durrant describes how forming relatively pure domains of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) is the key to separating the electrons and holes in heterojunction devices. He based this conclusion on research into various physical, electrochemical, and structural data, deducing that the way the PC61BM aggregates/crystallizes is the crucial factor.

According to Durrant: “Understanding the relationship between blend structure and device function is a key challenge in OSCs. Our paper demonstrated the fundamental role that a material’s aggregation can play in determining the energetic landscape experienced by electrons and holes in a polymer/fullerene blend, and therefore upon their charge separation and recombination processes.”

Durrant has since followed up paper #7 with a study comparing blends with fullerenes exhibiting different aggregation tendencies (S. Shoaee, et al., Adv. Funct. Mater., 23[26]: 3286–98, 2013). He notes that he and his collaborators are now working on ways to control blend structure and aggregation, including the use of additives to limit polymer/fullerene intercalation.

In silicon-based solar cells, the price of the silicon accounts for a third of the cost of these modules. Part of the answer is to reduce the thickness of the silicon layer. Even so, the formation of the p-n junction material requires temperatures of around 1000oC. Various solutions to these problems have been suggested, but none looks more promising than that devised through the collaboration of Michael McGehee of Stanford University and Yi Cui of the SLAC National Accelerator Laboratory, Menlo Park, California (paper #8). Their answer is to combine silicon and organic polymers, and in so doing they have achieved a remarkable energy conversion of 11%. Their solar cells were a hybrid of silicon nanocones and poly(3,4-ethylene dioxythiophere):poly(styrenesulfonate) (PEDOT:PSS). The combination of silicon and polymer was achieved by a simple solution-processed method carried out at 120oC.

Paper #9 concentrates on a different, but nevertheless important, aspect of organic solar cells: the support material, which is usually glass. Martin Kaltenbrunner of Johannes Kepler University, Linz, Austria, and Takao Someya of the University of Tokyo have developed polymer-based devices on plastic film that are less than 2 μm thick, but which are just as efficient as their glass-mounted counterparts. Not only that, they can withstand extremes of deformation—such as being wrapped round a human hair—and still operate. They consist of poly(3-hexylthiophene):PC61BM, also known as  P3HT:PCBM, as the active layer, with PEDOT:PSS as the transparent electrode mounted on poly(ethylene terephthalate) (PET) as the flexible substrate. The solar cells were also stretchable, as demonstrated by their being attached to an elastomer.

This theme of stretchability also features in paper #10 from Zhenan Bao of Stanford University, and again it involves PEDOT:PSS film, this time incorporating a fluoro-surfactant which makes the film amenable to being attached to hydrophobic surfaces. This material can also replace indium tin oxide (ITO) which is currently used as the anode of most solar cells, and it has the same efficiency.

These papers suggest that the goal of high-efficiency, sustainable OSCs will one day be met. The achievements of OSC chemists are already significant, and there is every hope that within a decade or so the fruits of their labors will make a major contribution toward preventing global warming by lessening the need to generate electricity from fossil fuels.


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

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