Organic solar cells: Absorbing solution

Scientists in Korea have developed a method to make organic solar cells more sensitive to the full spectrum of the sun’s light.

In spite of the increasing prevalence of blue-paneled rooftops on solar-powered office buildings and homes, the widespread use of silicon solar cells is still prohibitively expensive. Researchers have therefore been working to develop solar cells based on organic materials, which are cheaper than silicon to produce and more flexible.

A promising technology is the dye-sensitized organic solar cell, which contains optically active dye molecules supported on a highly porous semiconductor (typically TiO₂). The dye molecules yield charge when they absorb sunlight. Unfortunately, most dye molecules are not as sensitive to the full solar spectrum as silicon and tend to have a lower efficiency.

An obvious way to extend the optical range of the dye-sensitized solar cell would be to use multiple dye molecules in a single cell. However, properly positioning each type of molecule in the TiO₂ to most efficiently capture sunlight requires a high-temperature treatment that degrades the cell. Now, researchers at Sungkyunkwan University and the Korea Institute of Science and Technology¹ have developed a method for optimally layering up to three different dye molecules without heating.

The researchers coated the TiO₂, which has the porous structure of a sponge, with one type of dye molecule at a time and then partially removed the molecules using a desorbing solution before adding the next type of dye molecule.

Fig. 1: A solar cell consisting of layers of dye molecules with different wavelength sensitivities allows more of the sun’s light to be captured for power generation. Molecules that absorb short wavelengths are placed toward the bottom, closer to where the sun’s light enters, to improve the overall efficiency of the cell.

To do this with a high level of control, however, it was necessary to coat the pores of the TiO₂ with a polymer that slows down the desorbing solution — a mixture of NaOH and polypropylene glycol. This allowed the team to layer the dye molecules so that those molecules that absorb short-wavelength, yellow light were near the surface where the sun’s light enters the matrix (Fig. 1).

This approach is an important step toward capturing a greater proportion of the sun’s spectrum for generating power using organic solar cells. Nam-Gyu Park, a lead author on the research, says, “With improvements to the optomechanical and structural properties of the cells, we expect they can have an efficiency that is comparable with that of current silicon solar cells.”