Photovoltaic cells fabricated by electrophoretic deposition of CdSe nanocrystals

Nathanael J. Smith, Kevin J. Emmett, and Sandra J. Rosenthal

Applied Physics Letters 93, 043504 2008

Purpose: To use Electrophoretic depsosition (EPD) to deposite CdSe nanocrystals onto TiO₂ for use in photovoltaic cells.

Methods: The CdSe nanocrystals were synthesized using a solution of CdO, trioctylphosphineoxide (TOPO), hexadecylamine, and dodecylphosphonic acid which was heated to 320 °C. The Se was injected via a Se in tributylphosphine solution. The nanocrystals were grown, isolated, and finally stored in hexanes. A voltage of 500V was established between the two electrodes and the electrodes were then placed in the solution of nanocrystals in hexanes. The nanocrystals were deposited onto a variety of materials including TiO₂, Indium/Tin Oxide, glass, and Si.

Key Findings:
1. The EPD was extremely sensitive to the preparation conditions of the nanocrystals. Although the same basic procedure was used, the minor differences that are inherent in the synthesis and isolation caused some nanocrystals to deposit while others did not at all.
2. Deposition stops once the population of either the negatively or positively charged nanocrystals is depleted. (In this case is was the negative species)
3. Unreacted precursors (esp. TOPO) cause the nanocrystals to favor staying in solution rather than forming films on the selected material.
4. Formation of films on the materials is complete after 1 minute.
5. More Cd is deposited on the negative electrode.
6. Nanocrystals do indeed play a role in the functioning of a photovoltaic cell, even though the efficiency of the CdSe nanocrystal cells is ~10^-6%