Solution-processed networks of silicon nanocrystals: The role of internanocrystal medium on semiconducting behavior

We have produced networks of surface-oxidized and hydrogen-terminated silicon nanocrystals (Si-NCs), both intrinsic and n-type doped, on flexible plastic foil from nanoparticle inks. The charge transport in these networks was comprehensively studied by means of time-dependent conductivity, steady-state current versus voltage characteristics, and steady-state photocurrent measurements as a function of incident light intensity. These measurements were complemented by surface chemistry and structural/morphological analysis from Fourier transform infrared spectroscopy and electron microscopy. Whereas H-terminated Si-NC networks function as semiconductors (both in air and in vacuum), where conductivity enhancement upon impurity doping and photoconductivity were observed, these characteristics are not present in networks of surface-oxidized Si-NCs. For both network types, the observation of a power law behavior for steady-state current versus voltage and a current decaying with time at constant bias indicate that charge transport is controlled by space-charge-limited current (involving trap states) via percolation paths through the networks. We have also monitored the evolution of the networks (photo)conductivity when the internanocrystal separating medium formed by Si-H bonds is progressively replaced by a native oxide upon exposure to air. Although a decrease in the (photo)conductivity is observed, the networks still behave as semiconductors even after a long-term air exposure. From an analysis of all (photo)current data, we deduce that in networks of oxidized Si-NCs inter-NC charge transfer requires the participation of oxide-related electronic states, whereas in H-terminated Si-NC networks direct inter-NC charge transfer plays a major role in the overall long-range conduction process.