(223a) Electrophoretic Deposition of Nanocrystals Under Flow to Synthesize Semiconducting Thin Films with High Atom Economy

Electrophoretic deposition (EPD) of colloidal nanocrystals (NCs) under flow is explored as a general method for the fabrication of semiconducting thin films that is amenable to high throughput, low cost operation. EPD is readily employed for depositing ceramic or metallic coatings, which typically require hundreds of volts for deposition to occur. However, for photovoltaic applications, a low process voltage is critical to avoid damaging the accreting semiconductor. Here we report a continuous flow reactor design that can operate at reduced voltage compared to a traditional batch reactor. This design preserves the required high electrophoretic velocity of the NCs at low process voltages by utilizing narrow electrode spacing. In a batch reactor, the low volume-to-surface-area ratio dictated by such a narrow spacing of the electrodes would impose a limit on the mass of nanocrystals that are resident in the reactor and therefore the thickness of the films that can be deposited. This limitation is removed by continuously flowing the colloidal dispersion of NCs and thus thick films are deposited. Through modeling and experiment we elucidate the process parameters that control the uniformity of the resulting coating and the utilization of the NCs in the feed solution. The reactor design is compatible with large area substrates and is specifically designed to enable continuous, high-rate fabrication of the active layer of photovoltaic cells with high atom economy.