The best plastic solar cells are made from a light-absorbing polymer containing soccer ball-shaped carbon molecules called fullerenes. The fullerenes provide stepping stones in the plastic film for charge to hop across. By carefully heating finished cells to around 150ºC, Carroll and colleagues made the fullerene molecules form whiskers of crystal. These trigger crystallisation in the surrounding polymer as well. The fullerene-and-polymer crystal creates a network across the cell, allowing charge to move easily and preventing space charge blockages.
“Getting around space charge is a big step,” says Carroll. The heating process can increase efficiency from 5% to 6%. “Some performed as well as 7%,” Carroll told New Scientist. “We think we can probably push it up to 10%.”