Thin films of copper indium selenide fabricated with high atom economy by electrophoretic deposition of nanocrystals under flow

Abstract

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.

Influence of Compact, Inorganic Surface Ligands on the Electrophoretic Deposition of Semiconductor Nanocrystals at Low Voltage

“…Several researchers have explored electrophoretic deposition (EPD) as a possible scalable alternative. − Electrophoresis is the movement of a charged particle suspended in solution due to an externally applied electrical field. Once reaching one of the electrodes, a separate deposition process must occur, possibly through different mechanisms depending on the conditions.…”

“…These depositions are typically performed at 10 1 –10 3 V but occasionally at voltages below 10 V. − , In contrast, for the chalcogenide semiconductors [CdTe, CuIn­(Ga)­Se 2 , etc. ], the most common absorber layers in commercially relevant thin film solar cells, the applied voltages in previous studies exclusively range from tens to hundreds of volts. − ,, In addition to generating unwanted heat and convective flows in the EPD bath, high voltage poses the risk of driving poorly controlled electrochemical reactions that can alter the properties of the deposit. For example, high deposition voltage degrades the photoluminescence quantum efficiency of CdTe nanocrystals .…”