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.