For electrophoretic deposition (EPD) to achieve its potential as a method for assembling functional semiconductors, it will be necessary to understand both what governs the threshold voltage for deposition and how to reduce that threshold. Herein we demonstrate that postsynthetic modification of the surface chemistry of all-inorganic copper zinc tin sulfide (CZTS) nanocrystals (NCs) enables EPD at voltages of as low as 4 V, which is a 3-fold or greater reduction over previous examples of nonoxide semiconductors. The chemical exchange of the original surfactant-based NC-surface ligands with selenide ions yields essentially bare, highly surface-charged NCs. Thus, both the electrophoretic mobility and electrochemical reactivity of these particles are increased, favoring deposition. In situ imaging of the reactor during deposition provides a quantitative measure of the electric field in the bulk of the reactor, yielding fundamental insight into the reaction mechanism and mass transport in the low-voltage regime. A crossover from mass-transport-limited to reaction-rate-limited EPD is observed. Under the latter conditions, the influence of gravity can result in boundary-layer instabilities that are severely deleterious to the uniformity of the deposited film, despite the gravitational stability of the colloids in the absence of electric fields. This knowledge is applied to deposit thick, uniform, and crack-free films without sintering, from stable, well-dispersed colloidal starting materials.

Electrophoretic deposition (EPD) of colloidal nanocrystals (NCs) under flow is explored as a general method for the fabrication of semiconducting thin films. For photovoltaic applications, a low process voltage is highly desirable 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 while preserving the electrophoretic velocity of the NCs by utilizing narrow electrode spacing. In a batch reactor, the low ratio of reactor volume to electrode surface area 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.By continuously flowing the colloidal dispersion of NCs this limitation is obviated and thick films can be deposited. Through modeling and experiment we demonstrate the process parameters necessary to completely utilize the NCs in the feed solution, thereby achieving nearly 100% atom economy in the deposition process. The reactor design is 2 compatible with large area substrates and is specifically designed to enable continuous, high-rate fabrication of the active layer of photovoltaic cells.

The solution processability of colloidal nanocrystals (NCs) offers prospective next-generation solar cells significantly reduced fabrication costs, yet methods for scalable deposition of conductive, well-adhered NC films have yet to be realized. Electrophoretic deposition (EPD), a low-waste, high-throughput technique used commercially in other fields, is proposed for depositing solution-processed absorbing layers for solar cells. Here we share methods for fabricating all-inorganic semiconductor-NC films through low-voltage EPD. Through light scattering, absorbance spectroscopy and electric measurements, the fundamental electrochemical reactions and transport processes that occur during EPD are studied. Special emphasis is applied to elucidate the requirements for low-voltage EPD of semiconductor NCs. Through insights gleaned from this study, choice of NC-ligand-solvent combinations and reactor design, film electronic properties and morphology improvements are realized. Low-voltage EPD is demonstrated using both continuous flow and batch reactors in this study.