Low-Voltage Electrophoretic Deposition of Nanocrystal-Based Copper-Chalcogenide Thin-Films
Authors:   , Dillon Andrew , Fafarman Aaron T. 
Published: July 2021

Photovoltaic (PV) devices are a clean and renewable source of energy, yet their widespread adoption is hindered by their cost, much of which is dominated by fabrication of the devices themselves. Current methods for PV device fabrication are slow and energy intensive. While much work has been done to engineer solution-processable precursors to thin-film electronic materials, particularly for PV applications, relatively little has been done in developing scalable methods for depositing these "inks". In this work, electrophoretic deposition (EPD) of colloidal nanocrystals (NCs) is explored as a method for the fabrication of semiconducting thin films. For photovoltaic applications, a low process voltage is highly desirable to avoid damaging the accreting semiconductor. Herein is reported 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 and removing film thickness limitations by continuously flowing the colloidal dispersion of NCs. Through modeling and experiment, the process parameters necessary to completely utilize the NCs in the feed solution, thereby achieving nearly 100% atom economy in the deposition process, are demonstrated. For electrophoretic deposition (EPD) to achieve its potential as a method for assembling functional semiconductors, however, it is necessary to understand both what governs the threshold voltage for deposition and how to reduce that threshold. Post-synthetic modification of the surface chemistry of all-inorganic copper-zinc-tin-sulfide (CZTS) nanocrystals (NCs) enables EPD at voltages below 2V-a six-fold or greater improvement over previous examples of non-oxide 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, resulting in thick, uniform and crack-free films without sintering from stable, well-dispersed colloidal starting materials. In-situ imaging of the reactor during deposition provides a quantitative measure of the electric field in the bulk of the reactor; this, coupled with chronoamperometry, reveals the fundamental reaction and mass transport limitations of low-voltage EPD, and a crossover from mass transport-limited to reaction rate-limited EPD is observed. In order to fully realize an all-solution-processed PV device, every aspect of the device must be fabricated by solution-processing methods. Consequently, solution-processed transparent conductors are also studied. 2D transition metal carbides and nitrides, known collectively as MXenes, are highly conductive and water-dispersible, suggesting their utility as solution-assembled optoelectronic and plasmonic materials. Here, 2D Ti₃C₂ is assembled from solution into optical quality, nanometers-thin films that, at 6500 Siemens-per-centimeter, surpass the conductivity of other solution-processed 2D materials due to their metal-like free-electron densities. Simultaneously they transmit >97% of visible light per-nanometer-thickness, constituting the first example of a new class of solution-processed, carbide-based 2D optoelectronic materials.