IMEC demonstrates advanced spray coating technique for polymer solar cells

Almost three years ago, Canadian scientists from the University of Toronto have invented a spray-on solar power cell that can turn sun’s power into electrical energy. The breakthrough has led theorist to predict that these spray-on solar cells could become five times more efficient than current solar cell technology.

This year, IMEC doubles the feat after demonstrating a method for producing cheap and flexible organic (polymer) solar cells using spray coating. The active layer was sprayed onto the substrate with a nitrogen gas-powered airbrush, using various concentrations and spraying techniques. The power conversion rates of the most efficient spray-coated films were similar to the conversion rates of reference spin-coated cells. The spraying technique is also effective to produce solar cells with multiple polymer layers. Clean and cost-effective power

Scientists suggest that if these spray-on solar cells could cover 0.1 percent of the Earth’s surface with high-efficient large-area solar cells, we could replace all our energy habits with a source of power which is clean and renewable.

Today, organic solar cells have not yet reached the energy conversion rates of silicon solar cells. But they have a definite potential to provide low-cost energy production on lightweight, flexible substrates. Organic (polymer) solar cells are built from thin films—typically 100nm of organic semiconductors, such as polymers, and organic compounds such as carbon fullerenes.

To create large, flexible organic solar cells, various deposition techniques have already been tried, such as ink-jet printing and screen or gravure printing. IMEC demonstrated that spray coating is a simple and efficient alternative. It is a cheap, high-rate, large-area deposition technique, which allows coating a variety of surfaces, morphologies, and topographies. Unlike with spin coating, there is only a minimal waste of solvents and polymers. Also, in this demonstration, IMEC has shown that spray coating can be used to fabricate polymer solar cells with layers of polymers.

With a nitrogen gas-powered airbrush, IMEC sprayed layers of poly (3-hexyl thiophene) (P3HT) and the C60-derivative (6,6)-phenyl C61-butyric acid methyl ester (PCBM). Various spraying techniques were tried, including single-pass and multiple-pass spraying. IMEC also varied the concentration of the solutions to find the optimal mix. The effect of thermal annealing of the layers was studied. The resulting quality and morphology of the coated films were analyzed with atomic force microscopy, ellipsometry and absorption measurements. The power conversion rates of the most efficient films were above 2 percent, a performance which is comparable to that of the spin-coated reference solar cells.

The tests with spray-coated films also revealed that the spray coating technique can effectively create polymer cells with distinct layers. This is owing to the difference in the kinetics of the solvent evaporation compared to other solution-based techniques. Thanks to the small size of the droplet (femto-liters) intrinsic to spray coating, the thin films can dry very rapidly. This allows spraying subsequent films with the same solvent. For other techniques such as spin coating and inkjet printing, subsequent layers with the same solvent will dissolve the underlying layers.

Courtesy of IMEC