Polymeric solar cells have an efficacy of around 3 %, compared to 15–20 % of other solar technologies. Harald Ade and co-workers, North Carolina State University, USA, report that this is due to the structure of the solar cell themselves.

By using resonant soft X-ray scattering, the team studied the interfacial width of the heterojunction at a variety of annealing temperatures. Photoluminescence quenching measurements showed a three-fold increase in the number of excitons dissociated with annealing at over 140 °C. However, device performance was observed to systematically decrease with annealing.

Monte Carlo simulations revealed that a significant portion of the energy is lost due to the inefficient charge separation associated with increased interface roughness. The team conclude that heterojunction sharpness is a requirement for high performance devices.

• Influence of Annealing and Interfacial Roughness on the Performance of Bilayer Donor/Acceptor Polymer Photovoltaic Devices
  H. Yan, S. Swaraj, C. Wang, I. Hwang, N. C. Greenham, C. Groves, H. Ade, C. R. McNeill,
  Adv. Funct. Mat. 2010, 20.
  DOI: 10.1002/adfm.201001292