Performing Plastics: New Marks for Promising Polymer Solar Cells

Barely two months into the new year, we've seen a crop of new performance spanning the spectrum of solar cell technologies: thin-film (including CIGS, CdTe, and other), c-Si, some unknown combination, and even some with some nanoscale assistance.

Just in the past couple of weeks we've seen numbers trotted out for a different set of solar tech: organic (aka polymer aka plastic) solar cells, approaching and exceeding 10 percent conversion efficiency. That's a far cry from the high-teens of crystalline silicon or even low-teens for other thin-film options—but it's a magical number to spur further interest in the technology beyond lab-scale tinkering, notes Keith Emery, who manages NREL's cell and module performance characterization group. And it's significant for a technology which promises more simplified manufacturability, and widened applications where rigid modules cannot dream of going, from building integration (BIPV) to niche markets like being sewn onto travel gear.

• UCLA researchers have unveiled a nearly 9-percent-efficient (NREL-confirmed) tandem polymer solar cell. A single-layer device topped out at around 6 percent; adding a new infrared-absorbing polymer from Sumitomo Chemical to the group's already 8.6 percent efficient cells spiked efficiency to 10.6 percent. The promise of tandem solar cells is in stacking layers of solar cells with sensitivity to different absorption bands, so that the overall device capture a wider set of the solar spectrum than single-junction solar cells, and thus harvest more energy. The key in this case, the researchers explain, was creating a specific low-band-gap–conjugated polymer for the solar cell structure. "Everything is done by a very low-cost wet-coating process," and the process "is compatible with current manufacturing," says Yang Yang, UCLA prof. of materials science and engineering and principal investigator on the research. He thinks the cells could reach 15 percent efficiency in the next few years. (More details are in this issue of Nature Photonics.)

• Konarka, long a pusher of organic PV, says Newport Corp. has certified its next-generation solar cells with 9 percent single-junction efficiency. (Konarka points out that Newport's PV lab is accredited by the American Association for Laboratory Accreditation.) The technology is its "newest proprietary blue-grey polymer system," the firm points out.

• A four-year, €14.2 million European research project under the European Commission's Seventh Framework Program (FP7) aims to develop better flexible plastic solar panels. The "SUNFLOWER" project ("SUstainable Novel FLexible Organic Watts Efficiently Reliable"), led by the Swiss Center for Electronics and Microtechnology with more than a dozen partners, started work in Oct. 2011 to increase the cells' efficiency and lifetime, and decrease production costs. Goals for an initial prototype include a "tandem" multilayer structure to increase efficiency, better-performing barrier layers and getters, and created on a roll-to-roll atmospheric printing process. "We have the chance to develop a technology that is ideally suited to manufacturing in the EU due to its high level of automation, need for highly trained personnel, low energy consumption, and close proximity to suppliers and markets," says project coordinator Giovanni Nisato from CSEM.

• Eschewing efficiency numbers for sheer brawn, New Energy Technologies recently built a 170-square-centimeter organic PV device that's 14 times larger than its predecessors. The technology spray-deposits tiny solar cells a quarter the size of a grain of sand onto a substrate, without high-temperature or high-vacuum methods.

While breaking through the 10 percent efficiency mark for plastic solar cells is an important milestone, equally important is perspective, notes Emery. It's one thing to make them harvest energy and conduct electricity, but it's another thing to make them stable (inconveniently, "these things are unstable in air and water," he notes) and then another to figure out how to package them into a commercial-scale alternative. Thin-film solar tech options such as copper-indium-(gallium)-selenide (CIS/CIGS) similarly boast the option to be made flexible, but as yet it hasn't worked out at a commercial scale; CIS still gets rigidly sandwiched between glass. "It's not a done deal to compete with costs because of packaging needs," he emphasizes.

Still, while commercial-scale power generation from polymer solar cells may be years away (if ever), there could be lower-hanging fruit: powering consumer electronics devices, where efficiency and stability are less of a concern. Assuming the technology adopts improved packaging and maintains its simplified manufacturability, it should be attractive enough to power all sorts of personal gadgetry. (CIS technology got its start in the early 1980s on solar calculators, Emery points out.) Few digital gadgets last longer than a few years before being upgraded out of necessity or more likely must-have-it-ness — no 20-year warranty or PPA required.

Tandem solar cell structure. (Source: UCLA)