Defect-Mediated Electron–Hole Separation in Colloidal Ag2S–AgInS2 Hetero Dimer Nanocrystals Tailoring Luminescence and Solar Cell Properties

Abstract

Nanoscale heterojunctions with type-II band alignment can efficiently separate a photogenerated electron–hole pair, and therefore find applications in solar cells and photocatalysis. Here, we prepare a nanojunction in the form of Ag2S–AgInS2 hetero dimer nanocrystal that does not contain toxic Cd and Pb. A combination of photophysics, cyclic voltammetry, and quantum dot-sensitized solar cell properties shows that the junction/interface has a type-I band alignment, but still electron–hole separation takes place with efficacy across the interface because of defect states. The electron gets localized in a defect state within the AgInS2 part, and the hole resides in the Ag2S part of the hetero dimer nanocrystal. This type-II-like defect-mediated electron–hole separation, irrespective of the nature interfacial band alignment, is an interesting phenomenon, and can be utilized to tune optoelectronic properties of heterostructured nanocrystals. For example, very long (13 μS) photoluminescence lifetime has been observed for Ag2S–AgInS2 hetero dimer nanocrystals because of this defect-mediated spatial separation of electron and hole wave functions, which in turn improve the solar cell efficiency by more than 3 times as compared to that of AgInS2 nanocrystals.