Understanding the transport of electrons in films of touching nanocrystals is of central importance for their future use in printed electronic devices such as light emitting diodes, solar cells, or transistors. The research team developed a new theory that describes the transition of the electron conduction in doped nanocrystal films from a semiconducting to a metallic behavior. The theory predicts that the transition occurs under strikingly different conditions from those previously known for bulk semiconductors. Associated experimental studies of the electron conduction in phosphorous-doped silicon nanocrystal films largely support the new theory. This study was reported in Nature Materials.