Luminescence and solar cell from ligand-free colloidal AgInS2 nanocrystals

Abstract

Ligand-free AgInS2 nanocrystals display significant defect-related sub-band gap photo-absorption in addition to the excitonic absorption. The post-synthesis annealing of the nanocrystal dispersion at 150 °C reduces the intrinsic defect density, and also increases the nanocrystal size (5.8 nm) thereby reducing surface related defects. The AgInS2 nanocrystals exhibit a broad emission spectra, with a large Stokes shift compared to the excitonic absorption, which is attributed to two radiative electron–hole recombination processes: the first one involves the localized defect-state and the delocalized valence/conduction band with a lifetime of ~20 ns, and the second one involves two localized donor and acceptor states with a lifetime >250 ns. The faster decay dominates the higher energy end of the emission spectrum. Quantum dot sensitized solar cells using these ligand-free AgInS2 nanocrystals exhibit a maximum energy conversion efficiency of 0.8% and an open circuit voltage of 0.45 V. The device performance is better compared to previous AgInS2 nanocrystal based solar cells, due to both the use of the ligand-free nanocrystals that improve the charge transport and the post-synthesis annealing which partially removes the mid-gap defect states.