Sciencehttp://dx.doi.org/10.1126/science.1235547 (2013)

Semiconducting transition metal dichalcogenides (TMDC) — commonly used as solid-state lubricants and industrial surface coatings — are also being considered for photovoltaic applications owing to their strong optical absorption. It is hoped that such applications could lead to the development of very-thin-film solar cells. However, a major difficulty for their photovoltaic use lies in efficiently separating photogenerated electron–hole pairs within such materials. Liam Britnell from the UK and an international team of co-workers have now demonstrated a possible solution. The researchers placed the photoactive TMDC layer in a stack between two graphene layers and then doped the two layers in different ways to position Fermi levels and enable photocurrent extraction. They used hexagonal boron nitride as the substrate and encapsulating layer, and tungsten sulphide as the TMDC. They note, however, that their results could be generalized to cover other TMDCs. The researchers used flexible photovoltaic devices with an external quantum efficiency of 30%. The layered structure and mechanical strength of graphene and TMDC crystals were also beneficial.