Increasing demands for highly efficient sources of renewable energy is motivating scientists to explore novel approaches. Polymer solar cells represent a promising route, but the efficiency in converting sunlight into useable electric power is still relatively low compared with inorganic technology based on silicon semiconductors.

A team of researchers at Gwangju Institute of Science and Technology, Gwangju, Korea, and colleagues at University of California at Santa Barbara have made significant advances to improve the efficiency and cost of solar cells based on polymer and fullerene composites.1

In solar cells, photons are absorbed with the creation of electrons and holes, which are then spatially separated giving rise to an electric voltage, and effectively collecting the solar radiation. It has been proposed that the efficiency of power conversion could be improved using so‑called ‘tandem cell’ architecture, where two cells are linked in series in order to absorb light from complementary wavelength ranges of the solar spectrum.

Fig. 1: Device structure of the all solution processible polymer tandem solar cell. The back cell is a composite of poly(3- exylthiophene) (P3HT) and [6,6]-phenyl-C71 butyric acid methyl ester (PC70BM). The front cell is a composite of composite of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b'] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM).

Tandem solar cells combining different types of materials are already available. But, this report by Lee and colleagues is the first demonstration of a device in which both cells were polymer–based and processed from solution (Fig.1).

“The achievement of an all solution‑processed polymer tandem cell implies that the promise of a high efficiency printable solar cell can be realized as an alternative to inorganic solar cells because they can be fabricated in large areas using low cost printing and coating technologies,” says Lee.

The polymers used for producing the two cells were chosen to absorb complementary absorption spectra. A layer of titanium oxide was placed between the two cells to collect electrons from the first cell, and as this material is transparent, absorption by the back cell was not reduced—which is essential for high conversion efficiency.

The devices showed peak efficiencies of 6.5%, which decreased to 5.5% after six months—a small reduction considering that the average efficiency of previous results has a maximum of 5%. And there is room for improvement: “Polymer tandem cells utilizing both new materials with tuned absorption spectra as well as high mobility and series stacking of the sub‑cells will yield results approaching 15% efficiency,” says Lee.