Head-to-tail monomers improve solar cell efficiency

A team of British and South Korean researchers has improved the efficiency of polymer-based solar cells. 

Polymer cells, like all solar cells, comprise an electron-rich layer on top of an electron-poor layer, producing an electric field at the junction between them. When light hits the cell it releases free electrons, which pass through the cell in response to the electric field, generating an electric current.

Polymer cells are cheaper to produce than silicon-based solar cells, but they are less efficient. Efficiency is the ratio of electric power output to light power input. 

Jenny Nelson from Imperial College London and colleagues investigated polymer-based solar cells in which the electron-rich layer comprises poly(3-hexylthiophene) (P3HT) and the electron-poor layer comprises a fullerene derivative called 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6) C₆₁ (PCBM). Previous research suggested that P3HT films with a high degree of regioregularity (RR; the percentage of monomers adopting a head-to-tail configuration rather than head-to-head) could absorb more light.

Nelson and colleagues tested three P3HT films with different levels of RR and showed that solar-cell efficiency increased in line with RR levels. But the increase was too large to be fully explained by enhanced light absorption. 

The team found that the P3HT polymer chains stacked together more tightly in high RR films, which made them absorb more light and let electrons travel more easily through them.

Nelson’s team is now investigating other ways to increase the efficiency of this type of polymer-based solar cell. 

Jon Evans