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The Importance of Domain Size and Purity in High-Efficiency Organic Solar Cells Print
Wednesday, 27 March 2013 00:00

The efficiency of polymer/organic photovoltaic cells hinges on excitons—electron/hole pairs energized by sunlight—getting to the interfaces of donor and acceptor domains quickly, before recombining. At the interfaces, they become free charges that must then reach device electrodes. With the discovery of mixed domains of donor and acceptor molecules, many have pictured the excitons' journey as easy (interfaces are everywhere) but the charges' journey as precarious (interfaces are everywhere). Instead, using a combination of x-ray scattering and microscopy techniques, researchers have found that excitons may actually not fare so well in mixed domains but need access to pure aggregates to efficiently convert into charges. The smaller the aggregates, the better, allowing increased interfacial area and dramatic increases in device performance.

A New Path to Higher Efficiency

Why are efficient and affordable solar cells so highly coveted? Volume. The amount of solar energy lighting up Earth's land mass every year is nearly 3,000 times current usage. But to compete with energy from fossil fuels, photovoltaic devices must convert sunlight to electricity with a certain measure of efficiency. For polymer-based photovoltaic cells, which are far less expensive to manufacture than silicon-based solar cells, scientists have long believed that the key to high efficiencies rests in a difficult-to-engineer nanostructure like that of two interlocked combs. These "combs" would each be purely composed of the two organic molecules—one that donates electrons and one that accepts them. Now, however, an alternate and possibly easier route forward has been shown. Collins et al. have demonstrated that a nanoscale mixture of pure domains in an impure matrix can also lead to improved performance in polymer-based organic photovoltaic cells. A sweet spot of sorts in the interplay between domain purity and size could be found that should be much easier to achieve than engineered structures and ultrahigh purity.

Molecular view of a polymer/organic solar film showing an interface between donor and acceptor domains. Red dots are organic fullerene molecules and blue lines represent polymer chains. Excitons (yellow dots) need to reach the fullerene aggregates to be separated into electrons (purple dots) and holes (green dots).

Organic photovoltaics are of great interest as a potential source of renewable and economically viable electric energy. Materials that blend polymer chains with organic fullerenes, processed from solution, have achieved power conversion efficiencies surpassing 8%, with the record being held by blends of PTB7 (a polymer) and PC71BM (a C70-based fullerene). The addition of a solvent, diiodooctane (DIO), increases efficiency, but the precise origin of the performance enhancement remains elusive. In general, the lack of information to date has been attributable to a paucity of characterization tools having the required sensitivity, resolution, and quantitative nature.

At the ALS, complementary techniques offer the capability to measure, for the first time, the domain size, composition, and crystallinity of a given organic solar-cell sample. This is made possible by grazing-incidence wide-angle x-ray scattering (GIWAXS) at Beamline 7.3.3, scanning transmission x-ray microscopy (STXM) at Beamline 5.3.2, and resonant soft x-ray scattering (RSoXS) at Beamline 11.0.1. These techniques enabled the researchers to obtain nanoscale to mesoscale pictures of the morphology of a polymer-based organic photovoltaic film, which until now had been unattainable.

The team studied thin films of PTB7:PC71BM, made from chlorobenzene (CB) solution, with and without the addition of the solvent DIO. At Beamline 7.3.3, GIWAXS confirmed that the crystallinity of the material is very low and changed very little with the addition of DIO, suggesting that this cannot be the reason for dramatic changes in device performance. At Beamline 5.3.2, STXM images revealed ~200-nm pure fullerene aggregates for the CB samples and a much finer texture in the CB+DIO sample.

Left: STXM composition map of a polymer/fullerene film made without the solvent additive (CB only). Center: STXM composition map of a polymer/fullerene film made with the additive (CB+DIO). The domains in this second film are too small for compositional analysis. Right: Normalized histogram of scattering intensity versus fullerene domain size from calcluations based on RSoXS data. The dominant domain size of 177 nm in the CB sample is reduced to 34 nm in the CB+DIO sample.

To probe below the STXM resolution limit, the researchers conducted RSoXS experiments at Beamline 11.0.1 on the same two films. There are two primary ways that structure in the film causes scattering: contrast between the material and vacuum through roughness and contrast between material domains. The researchers conducted the scattering experiment at an energy where contrast between molecular species dominates vacuum contrast. From the scattering data, the researchers were able to extract the domain size in each film, which was an order of magnitude smaller in the presence of the DIO additive. The scattering profiles could also be processed further to obtain the domain purity. Remarkably, the results reveal that the relative domain compositions are nearly identical in both samples.

These results clearly show that the DIO additive does not affect crystallinity or composition, but rather helps to better disperse excess fullerene in a "saturated" polymer/fullerene matrix. Because the decrease in fullerene aggregate size increases the interfacial area between the phases, one explanation for the measured increase in photocurrent in the polymer is that direct access of the polymer chains to a fullerene agglomerate facilitates efficient electron/hole charge separation—the dispersed fullerenes just can't do the job. In the future, increasingly quantitative measurements such as these will be required for probing organic blend films in general and organic solar cells in particular, helping to advance the rational design of highly efficient polymer-based organic photovoltaic devices.

 


 

Research conducted by: B.A. Collins, J.R. Tumbleston, E. Gann, and H. Ade (North Carolina State University); Z. Li (University of Cambridge, UK); and C.R. McNeill (Monash University, Australia).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES); U.S. Department of Education; Engineering and Physical Sciences Research Council (UK); and the Australian Research Council. Operation of the ALS is supported by DOE BES.

Publication about this research: B.A. Collins, Z. Li, J.R. Tumbleston, E. Gann, C.R. McNeill, and H. Ade, "Absolute measurement of domain composition and nanoscale size distribution explains performance in PTB7:PC71BM solar cells," Advanced Energy Materials 3, 65 (2013).

ALS Science Highlight #266

 

ALSNews Vol. 340