Polymer Solar Cells: Bringing the Sun’s Energy Down to Earth

March 2014

Governments everywhere are anxious to reduce the dependency of their economies on carbon-based energy. They also face pressure to reduce or eliminate nuclear power. For example, Germany and Japan are phasing out nuclear altogether. But in terms of public policy there are major puzzles for all governments. They ask, what is the right mix of nuclear, carbon, and renewables?

There’s no easy answer when other considerations such as energy security, domestic access to natural resources (coal, oil, shale gas), and costs are factored in. But one thing has always been clear, that only renewables can keep the lights on if the nuclear and carbon industries are to tread more lightly on the planet. Hydroelectric is very good for countries with high rainfall and swift rivers. Offshore wind farms sort of work on the North Sea between the British Isles and Norway, but they require massive subsidies from taxpayers. Ironically they are sited over major oil and natural gas fields.

The Sun is ultimate source of natural energy. Photovoltaic conversion of sunlight is environmentally attractive, and can be done on a domestic scale with silicon cells, the efficiency of which is nudging 20%. For the concerned, responsible, consumer there’s now a lot of good news about a different kind of device to harness sunlight: organic or polymer solar cells (PSCs).  These have the potential to provide lightweight flexible films, easy to manufacture, and with the potential to lower costs.

To shed more light on recent research in this area of photonics ScienceWatch searched the Thomson Reuters Web of Science to find the Hot Papers on “Polymer Solar Cells.” The selected research papers were published in the years 2012 and 2013. The search procedure produced 5,068 papers that have been cited 25,639 times in 9,138 articles.

GROWTH OF PSC RESEARCH

The graph below shows the year-by-year growth of papers on polymer solar cells according to a “topic” search of papers indexed in the Web of Science Core Collection between 2000 and 2013, reflecting roughly 14,000 papers.

Increase in Web of Science-indexed Papers on Polymer Solar Cells, 2000 to 2013

TOP PAPERS:
HIGHLY CITED PAPERS ON ORGANIC OR POLYMER SOLAR CELLS, 2012-2013

In the table below, 20 high-impact papers illustrate that the PSC field is highly interdisciplinary. The top third of this list (#1-6) has papers on device design and the quest for efficiency. In the middle of the table there’s a cluster of papers (#9, #10, #14-17) on polymer chemistry. And at #19 there’s an intriguing exercise in futurology that visualizes the PSC Factory of our Dreams.

Polymer Solar Cells:
Hot Papers, 2012-2013

Listed by citations

Rank Paper Citations
1  Z. He, et al., “Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure,” Nature Photonics, 6(9): 591-5, September 2012. [S. China U. Technol., Guangdong] 489
2 L. Dou, et al., “Tandem polymer solar cells featuring a spectrally matched low-bandgap polymer,” Nature Photonics, 6(3): 180-5, March 2012. [U. Calif, Los Angeles; Natl. Renewable Energy Lab, Golden, CO] 475
3  Y. Sun, et al., “Solution-processed small-molecule solar cells with 6.7% efficiency,” Nature Materials, 11(1): 44-8, January 2012. [U. Calif., Santa Barbara] 380
4 C.E. Small, et al., “High-efficiency inverted dithienogermic-thienopyrrolodine-based polymer solar cells,” Nature Photonics, 6(2): 115-20, February 2012. [U. Florida, Gainesville] 266
5 H. Zhou, et al., “Rational design of high performance conjugated polymers for organic solar cells, Macromolecules, 45(2): 607-32, 24 January 2012. [U. North Carolina, Chapel Hill] 253
6 J. You, et al., “A polymer tandem solar cell with 10.6% power conversion efficiency,” Nature Comm., 4: No. 1446, February 2013. [U. Calif., Los Angeles; Sumitomo Chem. Co. Ltd., Ibaraki, Japan; Natl. Renewable Energy Lab., Golden, CO] 195
7 M.M. Lee, et al., “Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites,” Science, 338(6107): 643-7, 2 November 2012. [U. Oxford, UK; Toin U. Yokohama, Japan; Natl. Inst. Adv. Ind. Sci. & Technol., Ibaraki, Japan] 125
8 A.A. Bakulin, et al., “The role of driving energy and delocalized states for charge separation in organic semiconductors,” Science, 335(6074): 1340-4, 6 March 2012. [U. Cambridge, UK; U. Groningen, Netherlands; U. Mons, Belgium] 123
9 Z. Chen, et al., “High-performance ambipolar diketopyrrolopyrrole-thieno[3.2-b]thiophene copolymer field-effect transistors with balanced hole and electron mobilities,” Adv. Materials, 24(5): 647, 2 February 2012. [U. Cambridge, UK; U. London Imperial Coll., UK; U. Copenhagen, Denmark; Riso DTU, Roskilde, Denmark; U. Libre Bruxelles, Belgium] 119
10 L. Dou, et al., “Systematic investigation of benzodithiophene- and diketopyrrolopyrrole-based low-bandgap polymers designed for single junction and tandem polymer solar cells, J. Am. Chem. Soc., 134(24): 10071-9, 20 June 2012. [U. Calif., Los Angeles] 116
11 Y. Zhou, et al., “A universal method to produce low-work function electrodes for organic electronics,” Science, 336(6079): 327-32, 20 April 2012. [Georgia Tech, Atlanta; Princeton U., NJ; Solvay SA, Brussels, Belgium] 115
12 H. Yip, et al., “Recent advances in solution-processed interfacial materials for efficient and stable polymer solar cells,” Energy & Environ. Sci., 5(3): 5994-6011, March 2012. [U. Washington, Seattle; Korean U., Seoul] 113
13 Y. Huang, et al., “Improving the ordering and photovoltaic properties by extending pi-conjugated area of electron-donating units in polymers with D-A structure,” Adv. Materials, 24(25): 3383-9, 3 July 2012. [U. Massachusetts, Amherst; Chinese Acad. Sci., Beijing] 112
14  H. Chen, et al., “Highly p-extended copolymers with diketopyrrolopyrrole moieties for high-performance field-effect transistors,” Adv. Materials, 24(34): 4618-22, 4 September 2012. [Chinese Acad. Sci., Beijing] 106
15 J. Zhou, et al., “Small molecules based on benzo[1,2-b ‘]dithiophene unit for high-performance solution-processed organic solar cells,” J. Am. Chem. Soc., 134(39): 16345-51, 3 October 2012. [Nankai U., Tianjin, China] 101
16 S. Qu, H. Tian, “Diketopyrrolopyrrole (DPP)-based materials for organic photovoltaics,” Chem. Comm., 48(25): 3039-51, 2012. [E. China U. Sci. Technol., Shanghai] 96
17 T.S. van der Poll, et al., “Non-basic high-performance molecules for solution-processed organic solar cells,” Adv. Materials, 24(27): 3646-9, 17 July 2012. [U. Calif., Santa Barbara] 95
18 Z. Tan, et al., “High-performance inverted polymer solar cells with solution-processed titanium chelate as electron-collecting layer on ITO electrodes,” Adv. Materials, 24(11): 1476-81, 15 March 2012. 91
19 N. Espinosa, et al., “Solar cells with one-day energy payback for the factories of the future,” Energy & Environ. Sci., 5(1): 5117-32, January 2012. [Chinese Acad. Sci., Beijing; N. China Elect. Power U., Beijing] 88
20 T. Yang, et al., “Inverted polymer solar cells with 8.4% efficiency by conjugated polyelectrolyte,” Energy & Environ. Sci., 5(8): 8208-14, August 2012. [U. Akron, Ohio; S. China U. Technol., Guangzhou; Bruker Nano Surfaces Div., Goleta, CA] 87
SOURCE: Thomson Reuters Web of Science

PSCs IN A NUTSHELL

Organic photovoltaic devices are based on organic semiconductors, carbon based materials in which the C-C and C=C bonds are the backbone. PSCs are built up by laying films (typically 100 nm) on a transparent substrate. A photovoltaic cell is, in simple terms, a specialized semiconductor diode that converts visible light into direct current electricity. The active layer of the PSC is a polymer that reacts to sunlight by releasing electrons via the photovoltaic effect. This active layer is sandwiched between two layers of electron donor material and electron acceptor material. Typically those three layers are in turn sandwiched between a metal electrode and a cathode made of the transparent, conductive polymer PEDOT:PSS.

PSCs are currently only found in research laboratories. They are new devices significantly less efficient than the silicon cells that have benefitted from decades of research. However, the ScienceWatch analysis suggests that polymers are fast catching up on silicon. In a nutshell, the story here centers on new materials and new architectures.

ECONOMICS OF PSCs

The backstory linking most of these Hot Papers is that plastic-film cells will have a shorter life than silicon wafer cells; there are problems of stability; and above all the efficiency has to be a minimum 15% of the incoming solar energy converted to electrical energy. The key to addressing these three issues is materials innovation.

The goals of polymer designers are to engineer the bandgap and energy levels to achieve a high short-circuit current density and a large open-circuit voltage. The trick is to find polymers that can transfer from the laboratory to industrial facilities using roll-to-roll technology to build up the multilayer films on a flexible substrate. The processes involved will typically include coating and printing, with hot air drying ovens to deposit layers from a solution.

There’s a lesson here from the boom in high-temperature superconductivity a decade ago. The superconducting oxide compounds that incorporate rare-earth elements are brittle ceramics that cannot easily be made into wires. PSC research is strongly focused on low-cost processes that will be scalable from laboratory to factory, suitable for industrial production, and capable of producing solar panels for retail consumer markets.

RECORD HIGH OF 10.6% FOR TANDEM PSCs

Hot Paper #6 is from a group at Department of Materials Science and Engineering, University of California, Los Angeles, led by Yang Yang. Tandem solar cells combine two cells with different absorption bands so that they harvest across a broader spectrum of solar radiation. However, the efficiency doesn't automatically increase by simply combining two cells. The materials for the tandem cells have to be compatible with each other for efficient light harvesting.

The technical breakthrough made by UCLA team was to incorporate a low-band-gap–conjugated polymer specially designed for the tandem structure. Their tandem structure consists of a front cell with a larger (or high) band gap material and a rear cell with a smaller (or low) band gap polymer, connected by a designed interlayer. It has an efficiency of 10.6%, which is currently the record for a PSC.

As Yang tells ScienceWatch, “Everything is done by a very low-cost wet-coating process that is compatible with current manufacturing. I anticipate that this technology will become commercially viable in the near future.”

INVERTING THE STRUCTURE IMPROVES EFFICIENCY

The table’s top paper, by Zhicai He and colleagues, reports a major advance in efficiency by inverting the PSC structure. This can be thought of as swapping the anode and cathode. The first devices, however, were less efficient. What the researchers at the South China University of Technology have achieved is a PSC with a conversion rate of 9%, a record at the time of the paper’s publication.

Unlike previously reported inverted PSCs, based on n-type metal oxides, the device is solution-processed at room temperature, enabling easy processibility over a large surface area. It is fully amenable to high-throughput roll-to-roll manufacturing techniques, and therefore well suited to practical applications in mass production.

High-performance inverted cells that can be fabricated by solution processing are also reported in #18, with an efficiency of 7.4%, and in #20, where the efficiency is 8.4%. Devices with conventional structure perform no better than 6.4%.

The new science reported in the 20 Hot Papers shows that although PSCs are not yet as efficient as silicon solar cells, the cost-benefit analysis in paper #19 suggests that PSCs will soon sweep ahead in terms of mass production at attractive consumer prices.


Dr. Simon Mitton is a Vice President of the Royal Astronomical Society, and a College Fellow at the University of Cambridge, 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.