Efficient utilization of the abundant solar energy that floods our planet will be crucial in reducing carbon emissions. Although silicon solar cells are widely used, their high cost of production makes them impractical compared to traditional fossil fuels. An alternative approach is to use organic polymers in solar cells. Not only are polymers inexpensive to manufacture and physical advantages—such as flexibility—offer a wider range of applications.

Several technical challenges, however, prevent polymer solar cells from replacing silicon based photovoltaics. One important issue is that current polymer devices only respond to a fraction of the visible light spectrum. Now, Bao-Tsan Ko and colleagues at the Industrial Technology Research Institute in Taiwan1 have developed a new polymer to address this shortcoming.

Fig. 1: (a) Molecular structure of polymer that absorbs across the entire visible spectrum. (b) Transmission electron microscopy image of the solar polymer morphology.

The active components of polymer solar cells are donors, which absorbs light and release electrons into the device. Ko and his team utilized two types of complimentary molecular units to improve the ability of donors to collect light (Fig. 1a).

The first unit consists of carbon and sulphur atoms bonded together to form a large, symmetric, flat structure. By lying together in one plane, this unit easily shares electrons, which in turn lowers the energy required to absorb a photon. The second unit contains cyclic molecules with carbon-nitrogen, nitrogen-sulphur, and carbon-sulphur bonds. The multiple bond types help the polymer absorb light across a range of frequencies.

The net result is that the polymer developed by this group absorbs light across the entire visible spectrum—the first example for such materials.

In polymer solar cells, donors must be mixed with another, electron deficient macromolecules termed the acceptors. The acceptors receive the excited electrons and transfer them to the cell, thereby generating electricity. The blend of donor and acceptor polymers affects device performance—smaller distances between the two yielding higher efficiencies.

Transmission electron microscopy imaging (Fig. 1b) shows the acceptor (dark features) and the donor (light features) polymers segregate homogenously into small domains of about 10 nm in width, producing a 4.4% efficient device.

This group are currently investigating how to optimize the nanoscale polymer morphology to boost the device efficiency.