Scalable Solution Processing of Cu(In,Ga)(S,Se)₂ Solar Cells via Slot Die Coating

With high stability and lab-scale efficiencies over 23%, Cu(In,Ga)(S,Se)₂ (CIGSSe) thin-film photovoltaics are a viable alternative to silicon photovoltaics. However, commercial production of CIGSSe devices employs vacuum deposition methods that have relatively low throughput and are inefficient at utilizing materials. The concept of a solution-based approach for semiconductor thin film fabrication has been touted as a route to improved throughput, reduced energy consumption, and enhanced efficiency of materials utilization compared to vacuum-based approaches. Therefore, there has been significant effort to develop solution-processed routes to CIGSSe solar cells. To this end, researchers have developed numerous solution deposition approaches for CIGSSe films using inks of either soluble molecular precursors or suspensions of nanoparticles. However, many solution-deposition approaches are difficult to scale up from a laboratory to industrial scale. Spin-coating, where the precursor ink is dropped onto a spinning substrate, has been used in the record solution-processed CIGSSe solar cells. However, spin coating is not only very materials inefficient, but there is no clear path to integrating spin coating into a production line for large-area, roll-to-roll CIGSSe solar panels. Therefore, there is a need to employ commercially scalable solution deposition methods.<br/>Herein lies the advantage of slot die coating. In this method, the precursor ink is pumped through the slit of a slot-die head that is passed over the substrate to produce a thin film. Not only does slot die coating offer extremely efficient materials utilization, but it can be readily scaled up for large-area, roll-to-roll processes. Researchers have demonstrated that this method can be an effective solution deposition technique for polymer and perovskite photovoltaics. In this work, we study that application of slot die coating for the fabrication of solution-processed CIGSSe absorber layers. We do so using inks of suspended Cu(In,Ga)S₂ nanoparticles and a custom-built, lab-scale slot die coater converted from a 3D printer. As determined by absorbance measurement, we show that tuning of the coating solvent, substrate temperature, and coating speed enable the formation of uniform, high-quality CIGSSe films. With these films, we have synthesized solar cells with efficiencies in excess of 10%. In addition to the deposition of CIGSSe films, we also discuss the potential role of slot die coating in depositing the subsequent layers needed to complete a fully solution processed CIGSSe solar cell. Preliminary results suggest that slot die coating of silver nanowires is a viable alternative to a vacuum-deposited transparent conductive oxide (TCO).<br/>In conclusion this research focuses on expanding slot die coating techniques to the fabrication of CIGSSe solar cells as a route to achieving large-area, solution-processed photovoltaics.