(39d) Electrostatic Stabilization of Silicon Nanocrystal Colloids for Solution-Processed Photovoltaics

The tunable optoelectronic
properties of semiconductor nanocrystals (NCs) have garnered significant attention for photovoltaic
device integration.  Additionally,
solution processing of semiconductor nanocrystals (NCs) provides a low-cost,
scalable technique for the fabrication of third-generation solar cells.  Group II-VI and IV-VI compound
semiconductors have dominated the field of NC photovoltaics, as solution-based
synthesis processes are well established. The process results in a sterically
stabilized colloid from which ligand-capped NCs will naturally self-assemble
when cast into a film.  The ligands
can then be removed or exchanged to form a dense, electronically-coupled NC
film.

Solution synthesis of Silicon NCs
has proven difficult due to the high temperatures needed.  Nonthermal flow-through plasma
synthesis offers an effective alternative.  Typical plasma processes using silane as the silicon
precursor lead to free-standing, hydrogen-terminated Si NCs that will not,
however, stabilize in solution. A hydrosylilation reaction will attach alkyl
ligands to the surface to achieve a stable colloid similar to group II-VI and
IV-VI NCs, but post-processing of cast films to remove or exchange the ligands
is problematic.   

Here we report nonthermal plasma
synthesis of Si NCs with an alternative silicon precursor, silicon
tetrachloride.  The presence of Cl
during synthesis allows for tuning of the surface chemistry of the Si NCs.  FTIR analysis reveals Cl present on the
surface, which facilitates a novel colloidal stabilization in aprotic polar
solvents.  The Si NCs are readily
dispersed and stabilized in aprotic dipolar solvents.  The solutions are optically transparent and have remained
stable for over 8 months.  We
believe the electronegativity of the chlorine surface leads to solvation of the
NCs by electrostatic interaction with solvent molecules.

Solutions of Cl-terminated Si NCs
are spin- or drop-cast into self-assembled films that do not require a post-treatment
to remove ligands.  Electron
microscopy and Scanning Probe Microscopy (SPM) reveal the smooth and continuous
film morphology needed for photovoltaic device structures.  Electrical characterization reveals photoconductive
Si NC films with differential dark conductivities of 1.09×10^-7
S/cm.   This is competitive with nano-and micro-crystalline Si films
deposited using high-vacuum techniques such as PECVD.

This work was supported in parts
by the National Science Foundation under grant CBET-0756326 and by DOE under
the EFRC Center for Advanced Solar Photophysics.