Polymer solar cells use semiconducting polymers to absorb light, which kicks electrons in these molecules from the ground state to an excited state with a higher energy. An electron-accepting material then channels the excited electrons toward an electrode, thereby generating an electrical current.
Semiconducting polymers have the advantages of being lightweight, flexible and semi-transparent, and potentially they could be used in low-cost photovoltaic panels. But the best polymer solar cells convert only about 10 per cent of the light that falls on them into electrical power—roughly half the efficiency of conventional solar cells, which are based on inflexible silicon.
Now, Itaru Osaka of the RIKEN Center for Emergent Matter Science and his colleagues have shown that incorporating fluorine atoms into a photoactive polymer offers potential improvements. They studied a polymer that contains two types of chemical groups—naphthobisthiadiazole (NTz) and thiophene units. The scientists added two fluorine atoms to some of those thiophene units to make a new polymer dubbed PNTz4TF2 and four fluorine atoms to form PNTz4TF4.
The team found that adding fluorine lowered the energy level of molecular orbitals in the polymers. This increased the energy difference between them and the electron acceptor, resulting in a higher output voltage and reduced energy losses in the system in both cases.
For PNTz4TF2, attractions between fluorine atoms and other atoms in the polymer also made the polymer strands more rigid, helping to produce a more crystalline material, which allowed more rapid movement of electrical charge through it.
The team then blended each polymer with an electron acceptor called PC71BM, which contains a ball of carbon atoms known as a fullerene. In the case of PNTz4TF2, a 230-nanometer-thick layer of this blend achieved an efficiency of 10.5 per cent, which is slightly more than that of the equivalent fluorine-free polymer. “It is a small but significant improvement,” notes Osaka.
The researchers are confident that there is room for further improvement as they anticipate being able to increase the solubility of the materials by tweaking the chemical composition of the fluorinated polymers. This should improve the fabrication process of the solar cells, further boosting the power conversion efficiency and voltage. “The next stage may be developing further high-performance polymers showing efficiencies of 12 per cent, or hopefully even 15 per cent,” says Osaka.