Twice as Light
Double layering boosts organic-solar-cell efficiency
Context: Solar cells offer the promise of clean, renewable energy. But most cells in use today use silicon to absorb light, which makes them brittle and so expensive to manufacture that it takes years before they produce enough electricity to recoup their cost. Enter organic solar cells, which use carbon-based molecules such as “buckyballs” rather than silicon. In theory, cells made from these cheap, flexible materials could be rolled out in sheets of translucent plastic and stuck on everything from cell phones to clothing. But while commercial silicon-based cells can convert about 20 percent of the sunlight that strikes them into electricity, organic cells typically convert only 1 or 2 percent. Stephen Forrest’s team at Princeton University has developed a new way to boost that average.
Methods and Results: When light interacts with a solar cell, different wavelengths deliver more or less energy at different depths. By varying the composition of their organic solar cells, the Princeton researchers tuned some to more efficiently absorb reddish light and others to absorb bluish light. They then layered the red cells onto the blue, to capture those wavelengths of light where they are most intense. The result: an efficiency of almost 6 percent, the highest published for such cells to date. The researchers speculate that with a third layer (say, for infrared wavelengths), the cell could reach double-digit efficiency.
Why it Matters: Organic solar cells may be cheap and widely applicable, but with their current efficiencies, even plastering the side of a skyscraper would not yield enough power to be practical. The jump in efficiency reported by the Princeton team—plus innovations in manufacturing and integration—makes the prospect of generating municipal power from these renewable solar sources more likely.
Source: Xue, J., et al. 2004. Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions. Applied Physics Letters 85:5757–5759.