(201c) Plasma-Assisted Aerosol Synthesis of Silicon Nanocrystals for Solar Cells

Theoretical efficiency limits and high production costs imposed by single crystal semiconductors may ultimately restrict the use of photovoltaics for widespread energy generation.  Therefore, decreasing cell fabrication cost and increasing cell efficiency should be targeted simultaneously to make photovoltaics more commercially viable.  One way to achieve such reductions is to use silicon nanocrystals (SiNCs) synthesized from a nonthermal plasma process to replace one or more layers in a p-n (or p-i-n) junction solar cell.  Thin films of SiNCs can be deposited on flexible or inexpensive substrates, and quantum confinement effects associated with nanoscale silicon theoretically hold the potential to surpassing the Shockley-Queisser limit for photovoltaic energy conversion.  Furthermore, by depositing the SiNCs directly from the aerosol phase, extra processing steps associated with liquid-phase ink preparation and film deposition can be circumvented.

In this study, a capacitively-coupled argon-silane nonthermal rf plasma doped with phosphine is employed in a flow-through reactor to generate phosphorus-doped silicon nanocrystals.  In a simple approach to deposition, the doped SiNCs are accelerated towards a substrate via a pressure drop caused by inserting a rectangular nozzle downstream of the plasma synthesis region.  By moving a substrate through the sheet of particles created by the nozzle, a continuous film of doped nanocrystals is formed.  To investigate the potential of this strategy to form SiNC thin films, such films were used as the n-type layer of a p-n junction solar cell.  The SiNC layer is deposited onto a p-type single crystalline wafer and annealed to modify the SiNC surface states.   RF sputtered ITO or doped zinc oxide from atomic layer deposition was used as a top contact, and 200 nm of evaporated aluminum is used for the back contact.

In this study we characterized the effect of in-situ and post-deposition processing conditions.  We studied the effect that the surface chemistry has on the solar cell performance by varying the plasma synthesis conditions.  Increased SiNC diameter is observed to significantly increase the performance of the solar cells.  By adjusting the plasma input power and the precursor gas mixtures, we were able to identify the effect of nanocrystal hydrogen passivation on the solar cell properties.  Furthermore, by increasing the standoff distance of the substrate from the nozzle, we can control the porosity of the SiNC films.  Then, post-deposition surface modification was studied by annealing films in a variety of environments, such as forming gas, argon, nitrogen, and 10% oxygen.

This work was supported partially by the MRSEC Program of the National Science Foundation under Award Number DMR-0819885 and in part by the 3M Science and Technology Fellowship.  Part of this work was carried out in the College of Science and Engineering Nanofabrication Center, University of Minnesota, which receives partial support from NSF through the NNIN program.