LAUSR

Assemblies of colloidal semiconductor nanocrystals (NCs) in the form of thin solid films leverage the size-dependent quantum confinement properties and the wet chemical methods, vital for the development of the emerging solution-processable electronics, photonics and optoelectronics technologies. The ability to control the charge carrier transport in the colloidal NC assemblies is fundamental for altering their electronic and optical properties for the desired applications. Here we demonstrate a strategy to render the solids of narrow-bandgap NC assemblies exclusively electron-transporting by creating type-II heterojunction via shelling. Electronic transport of molecularly cross-linked PbTe@PbS core@shell NC assemblies is measured using both conventional solid gate transistor and electric-double-layer transistor, as well as compared with those of core-only PbTe NCs. In contrast to the ambipolar characteristics demonstrated by many narrow bandgap NCs, the core@shell NCs exhibit exclusive n-type transport; i.e., drastically suppressed contribution of holes to the overall transport. The PbS shell that forms type-II heterojunction assists the selective carrier transport by heavy doping of electrons into the PbTe-core conduction level, and simultaneously strongly localize the holes within the NC core valence-level. This strongly enhanced n-type transport makes these core@shell NCs suitable for applications where ambipolar characteristics should be actively suppressed; in particular, for thermoelectric and electron transporting layer in photovoltaic devices.