US chemists have shown that trace impurities - below the sensitivity of standard characterisation techniques - can halve the efficiency of bulk heterojunction (BHJ) solar cells. Their finding means that initially promising materials for device applications may have been written off prematurely owing to their low efficiencies. Trace impurities are difficult to detect owing to their similarity to donor molecules in solar cells. The impurities can significantly influence photovoltaic properties Unlike traditional inorganic solar cells, polymer cells do not immediately create charge carriers but instead create electron-hole pairs called excitons over a donor-acceptor interface, which migrate to the electrodes. In...